Needs report: appendix E

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Appendix EPotential Source Control and SUDS Applications

Annex 1: SUDS Evaluation for Example Areas



100-RG-PNC-00000-900008 Printed 07/09/2010



London Tideway Tunnels ProgrammeThames Tunnel Project Needs Report

Potential source control and SUDS appl ications:Land use and retrofit options

28 th April 2010

Richard Ashley

Virginia StovinSarah MooreLouise HurleyLinda Lewis

Adrian Saul

University of Sheffield



Potential source control and SUDS appl ications:

Land use and retrofit options

Table of Contents

Land use and retrofit options ................................................................................................................. 1

Figures ..................................................................................................................................................... 4

Tables ...................................................................................................................................................... 6

Acknowledgements ................................................................................................................................. 8

Land use and retrofit options ................................................................................................................. 1

Executive summary ................................................................................................................................. 1

1. Introduction and objectives ............................................................................................................ 5

2. Work carried out ........................................................................................................................... 10

3. Methodology for selection of options for retrofitting .................................................................. 12

3.1 Existing frameworks for the selection of options ................................................................. 12

3.2 Framework used in the study............................................................................................... 15 3.2.1 Review of available SUDS options and ranking of performance potential .......................... 15

4. Identification of subcatchments and performance criteria .......................................................... 18

4.1 Identification of subcatchments ........................................................................................... 18

4.2 Performance and performance assessment ......................................................................... 24

4.2.1 Hydraulic and other aspects of the performance of SUDS elements .................................. 25

4.2.2 Sewerage performance assessment .................................................................................... 32

5. Practicalities of retrofitting SUDS ................................................................................................. 34

5.1 Overview of benefits of using SUDS retrofits ....................................................................... 34

5.2 Summary of difficulties in using SUDS retrofits .................................................................... 38

5.2.1 Legal and regulatory issues ........................................................................................... 38

5.2.2 Transfer of burdens and responsibilities ...................................................................... 38

5.2.3 Maintenance, operational needs, risks and health and safety ..................................... 39

5.2.4 Incentivisation ............................................................................................................... 39



5.2.5 Practicalities of construction ........................................................................................ 39

5.2.6 Acceptability and awareness ........................................................................................ 39

6. Catchment scale disconnection strategies ................................................................................... 41

6.1 Initial Approach – ANNEX 1 disconnection scenarios ........................................................... 41

6.2 Land use types and suitability for retrofit ............................................................................. 41

6.3 Method 1 – ‘typical’ disconnection profiles by land ‐use types ............................................ 41

6.4 Method 2 – Land‐use categories within OS MasterMap ...................................................... 43

6.5 Assignment of Impermeable/Pervious Areas ....................................................................... 45

6.6 Results ................................................................................................................................... 46

7. Cost ‐ benefit assessment of detailed retrofit designs ................................................................. 48

7.1 Calculation of Whole Life Costs ............................................................................................ 48

7.2 SUDS Unit costs ..................................................................................................................... 50

7.2.1 Pocket raingardens ....................................................................................................... 54

7.2.2 Permeable road surfaces .............................................................................................. 54

7.2.3 Detention basins ........................................................................................................... 55

7.2.4 Swales ........................................................................................................................... 55

7.2.5 Green roofs ................................................................................................................... 56

7.2.6 Roof downspout disconnection .................................................................................... 56

8. Using exemplar designs to evaluate the disconnection options and cost ‐benefits for the three

subcatchments ...................................................................................................................................... 57

8.1 Lytton Grove subarea (Appendix E.1) ................................................................................... 57

8.2 Carlton Drive Subarea (Appendix E.2) ................................................................................... 59

8.3 Chartfield Avenue Subarea (Appendix E.3) ........................................................................... 60

8.4 Summary of lessons from exemplar designs ........................................................................ 61

9. Upscaling costs .............................................................................................................................. 62

10. Disconnection scenarios for modelling purposes ..................................................................... 63

10.1 Initial results from the modelling of disconnection scenarios .................................................. 63

10.1.1 Overview of global disconnection scenarios...................................................................... 63

10.1.2 Overview of feasible disconnection scenarios ................................................................... 64

10.2 Refined Approach – Final disconnection scenarios ............................................................. 65 10.2.1 Introduction to refinements .............................................................................................. 65





D.4.3 Normative lock‐in; how to transform systems ........................................................... 112

D.5 Implementation of non ‐piped/sewered systems ............................................................... 113

D.6 Non ‐piped/sewer systems and water quality ..................................................................... 113

D.7 Non ‐piped/sewered systems and water quantity .............................................................. 113 D.8 Perceptions of non ‐piped/sewered systems ...................................................................... 114

D.9 Current surface water management initiatives .................................................................. 114

D.10 Use and adoption of SUDS by Sewerage Undertakers .................................................... 115

D.11 Added benefits of ‘joined ‐up’ surface water management ............................................ 116

E.1 Lytton Grove Subarea retrofit SUDS ................................................................................... 117

14. ....................................................................................................................................................... 132

E.2 Carlton Drive Subarea SUDS ............................................................................................... 143 15. ....................................................................................................................................................... 148

E.3 Chartfield Avenue subarea SUDS ........................................................................................ 155

16. ....................................................................................................................................................... 158

Appendix F ‐ photographic record of areas used in exemplar designs ............................................... 159

ANNEX 1 ‐ SUDS Evaluation for Example Areas .................................................................................. 168

Figures

Figure 1‐1 The place of SUDS within the Water Sensitive Urban Design framework (adapted from

Landcom, 2009 and CIRIA, 2007) ............................................................................................................ 7 Figure 3‐1a SNIFFER Framework – Phase I Feasibility Assessment ...................................................... 13 Figure 3‐2 Generic hierarchies for SUDS retrofit Selection proposed by Stovin et al (2007) ............... 14 Figure 3‐3 Approach taken in this study to the selection and evaluation of the effectiveness of

retrofit SUDS ......................................................................................................................................... 15 Figure 4‐1 The three subcatchments investigated (not to scale) (numbers refer to images in Figure

4.5) ........................................................................................................................................................ 18 Figure 4‐2 West Putney Mastermap subcatchment characterisation .................................................. 19 Figure 4‐3 Putney Bridge Mastermap subcatchment characterisation ................................................ 20 Figure 4‐4 Frogmore (Buckhold Road) Mastermap subcatchment characterisation ........................... 21 Figure 4‐5(a) Putney Bridge subcatchment......................................................................................... 22 Figure 4‐6 Pocket rain gardens in Auckland New Zealand .................................................................... 27 Figure 4‐7 Permeable block paving (from Interpave website) ............................................................. 28 Figure 4‐8 Off ‐line local detention basin (Orleans, France) .................................................................. 28

Figure 4‐9 Swales .................................................................................................................................. 29 Figure 4‐10 Downspout connected to pervious area in Seattle, USA ................................................... 30



Figure 4‐11 Green roofs in Rotherham, Yorkshire ................................................................................ 31 Figure 4‐12 Slimline RWH tanks in an access pathway to a property in Melbourne ............................ 32 Figure 6‐1 Comparison of drainage/disconnection land ‐use categories by land ‐use category for Sub‐

catchments 1 & 2 .................................................................................................................................. 42 Figure 8‐1 Example of the extensive green areas in municipal housing in the Lytton Grove area that

could accommodate detention basins .................................................................................................. 58 Figure 8‐2 Example of paved hard standing in the Lytton Grove area that could be converted to

permeable pavement ............................................................................................................................ 58 Figure 8‐3 Carlton Drive showing flat roofed apartment blocks and wide road .................................. 59 Figure 8‐4 Chartfield Avenue illustrating the width of the road and potential to fit pocket raingardens .............................................................................................................................................................. 60 Figure 10 ‐1 Examples of municipal housing surrounded by generous grounds ................................... 65 Figure 10 ‐2 Land‐use categories within each of the Frogmore (Buckhold Road) sub ‐catchments,

compared with the exemplar design case, Lytton Grove sub ‐areas 1 to 4 .......................................... 66

Figure 10 ‐3 Comparison between remaining impermeable area for feasible and refined disconnection strategies ....................................................................................................................... 68 Figure 12 ‐1 Water stored temporarily on the highway in Skokie (Carr & Walesh. 2008) .................... 75 Figure B‐0‐1 Criteria for Pocket road SUDS .......................................................................................... 90 Figure B‐0‐2 Example houses deemed suitable for disconnection to gardens ..................................... 91 Figure B‐0‐3 Example of all properties deemed suitable for disconnection to adjacent land ............. 91 Figure E‐0‐1 Lytton Grove sub ‐catchment with sub ‐areas showing roads highlighted ...................... 117 Figure E‐0‐2 Option 1 – detention basins all in the lowest area (1) adjacent to Lytton Grove .......... 119 Figure E‐0‐3 Area 2 – potential for local storage of surface water. .................................................... 122 Figure E‐0‐4 Area 3 – potential for local storage of surface water ..................................................... 123 Figure E‐0‐5 Area 4 – potential for local storage of surface water. .................................................... 124 Figure E‐0‐6 required detention basin area for areas 1‐4 if located entirely in area 1. (Swales and

piped connections not shown). .......................................................................................................... 126 Figure E‐0‐7 Area 5 disconnect back roofs to gardens ....................................................................... 133 Figure E‐0‐8 Area 6 .............................................................................................................................. 134 Figure E‐0‐9 Area 7 .............................................................................................................................. 135 Figure E‐0‐10 Area 8 ............................................................................................................................ 136 Figure E‐0‐11 Area 9 ............................................................................................................................ 137 Figure E‐0‐12 Area 10 .......................................................................................................................... 138 Figure E‐0‐13 Area 11 .......................................................................................................................... 139

Figure E‐0‐14 Area 12 .......................................................................................................................... 140 Figure E‐0‐15 Area 13 .......................................................................................................................... 141 Figure E‐0‐16 Main and local roads .................................................................................................... 142 Figure E‐0‐17 Carlton Drive and adjacent properties ......................................................................... 144 Figure E‐ 0‐18 Carlton Drive Area 1 .................................................................................................... 149 Figure E‐0‐19 Carlton Drive Area 2 ..................................................................................................... 150 Figure E‐0‐20 Carlton Drive Area 3 ..................................................................................................... 151 Figure E‐0‐21 Carlton Drive Area 4 ..................................................................................................... 152 Figure E‐0‐22 Carlton Drive Area 5 ..................................................................................................... 153

Figure E‐0‐23 Carlton Drive Area 6 ..................................................................................................... 153 Figure E‐0‐24 Carlton Drive Roads ...................................................................................................... 154



Figure E‐0‐25 Pocket rain garden in road edge in Brisbane ................................................................ 155 Figure E‐0‐26 On‐street pocket rain gardens (after Smith et al., 2007) ............................................. 155 Figure E‐0‐27 Chartfield Avenue ......................................................................................................... 156 Figure E‐0‐28 Genoa Road leading from Chartfield Avenue (looking south with Chartfield Avenue in

the distance) ....................................................................................................................................... 156 Figure F‐0‐1 A new development in the vicinity of Lytton Grove (Clockhouse Place) with significant

hard standing ...................................................................................................................................... 159 Figure F‐0‐2 Garages associated with the development in Figure F‐1 (it is unlikely that this is

permeable) .......................................................................................................................................... 159 Figure F‐0‐3 Lytton Grove looking west towards the lowest point .................................................... 160 Figure F‐0‐4 Some drives off Lytton Grove are already permeable .................................................... 160 Figure F‐0‐5 Lytton Grove ‐ speed humps may be replaceable by pocket rain gardens .................... 161 Figure F‐0‐6 The Kersfield Estate off Lytton Grove has lots of grassed areas and disused hard standing

areas (looking SW) .............................................................................................................................. 161

Figure F‐0‐7 looking in the opposite direction (NE) to Figure F‐6 ....................................................... 162 Figure F‐0‐8 Kersfield Estate lower end .............................................................................................. 162 Figure F‐0‐9 Kersfield Estate middle area ........................................................................................... 163 Figure F‐0‐10 Top end of Kersfield Estate ........................................................................................... 163 Figure F‐0‐11 Apartments off Carlton Drive ....................................................................................... 164 Figure F‐0‐12 An existing rain garden off Carlton Drive ..................................................................... 164 Figure F‐0‐13 Paved areas for car parking for apartments on Carlton Drive ...................................... 165 Figure F‐0‐14 Speed humps in Carlton Drive ...................................................................................... 165 Figure F‐0‐15 Chartfield Avenue and adjoining roads (no speed humps) .......................................... 166 Figure F‐0‐16 Junction of Chartfield Avenue and Genoa Avenue ....................................................... 166 Figure F‐0‐17 Paved drive on Chartfield Avenue (unlikely to be permeable) ..................................... 167 Figure F‐0‐18 One of a few apartment blocks in the vicinity of Chartfield Avenue (Genoa Avenue) 167

TablesTable 3‐1 Potential SUDS, preference and indicative hydraulic performance ..................................... 16

Table 4‐1 SUDS Hydraulic and overall performance ............................................................................. 25

Table 5‐1 Summary of the benefits of SUDS options (CIRIA, 2007) ..................................................... 36

Table 5‐2 Summary of potential benefits of stormwater separation (Cascade, 2009) ........................ 37

Table 6‐1 Retrofit SUDS options, SQL queries and Hydraulic preferences ........................................... 44 Table 6‐2 Hydraulic modelling options for retrofit SUDS ..................................................................... 45

Table 6‐3 Distribution of impermeable area by MasterMap land ‐use category .................................. 45

Table 6‐4 Disconnection scenarios for InfoWorks modelling ............................................................... 47

Table 7‐1 Outline of hydrologic/hydraulic parameters used in UKWIR/WERF WLC model ................. 49

Table 7‐2 SUDS unit costs ..................................................................................................................... 51

Table 7‐3 Pocket raingarden whole life costings .................................................................................. 54

Table 7‐4 Permeable pavement whole life costing for lighter vehicle loading .................................... 55

Table 7‐5 Whole life costing of detention basins ................................................................................. 55

Table 7‐6 Whole life costing of swales .................................................................................................. 55

Table 7‐7 Capital costs for roof downspout disconnection with some maintenance allowance ......... 56



Table 9‐1 Unit costs of municipal housing areas other than roof, road or manmade land

disconnections ...................................................................................................................................... 62

Table 9‐2 Ball‐park estimates for scheme costs ‐ all areas – nearest £1M ........................................... 62

Table 10 ‐1 Sample performance improvements associated with 50% disconnection of impervious

area for the October 2000 event .......................................................................................................... 64

Table 10 ‐2 Assumed disconnection rates and InfoWorks modelling categories for 'municipal housing'

areas ...................................................................................................................................................... 66

Table 10 ‐3 Assumed uptake levels for the refined disconnection strategies ....................................... 67

Table 10 ‐4 Combined area disconnections ........................................................................................... 68

Table 10 ‐5 Final design scenario allocations of existing impermeable area (%) .................................. 68

Table A‐0‐1 Examples of types of SUDS available for surface water attenuation ................................ 81

Table A‐0‐2 Quantity performance of selected SUDS (source: SUDS manual, CIRIA, 2007, Table 5.7) 83

Table A‐ 0‐3 Effectiveness of source controls on water quantity downstream (adapted from

Weinstein et al, 2006) ........................................................................................................................... 84

Table E‐0‐1potential sizes of drainage units in Area 1 ........................................................................ 120 Table E‐0‐2 potential impervious areas that could be disconnected in areas 1‐4.............................. 120

Table E‐0‐3 present value costs of the detention basins A‐D in Area 1. ............................................. 120

Table E‐0‐4 present value costs for swales and associated pipework in Area 1. ............................... 121

Table E‐0‐5 costs of alternative potential detention storage and connecting swales in areas 2‐4. ... 125

Table E‐0‐6 present value costs of the detention basins A‐C in Area 1, Figure E.6. ........................... 125

Table E‐0‐7 Impervious areas in the remainder of the Lytton Grove Subcatchment ......................... 127

Table E‐0‐8 Options and costs for alternative drainage for subareas 5‐13 and the roads in the Lytton

Grove Subcatchment .......................................................................................................................... 129

Table E‐0‐9 Lytton Grove Subcatchment retrofit SUDS summary ...................................................... 132

Table E‐0‐10 Carlton Drive and adjacent properties – distribution of types of area (m2) .................. 143

Table E‐0‐11 Carlton Drive characteristics and costs of retrofits by Subarea .................................... 145

Table E‐0‐12 Summary of Carlton Drive costs of retrofits by Subarea ............................................... 148

Table E‐0‐13 Chartfield Avenue SUDS units and costs ........................................................................ 157

Table E‐0‐14 Chartfield Avenue areas attenuated and summary of costs ......................................... 158



Acknowledgements

The use and application of the Whole Life SUDS costing tool described in Section 7 and used in the

detailed designs in Section 8, was supported by Bridget Woods ‐Ballard of H R Wallingford under subcontract. The project team are also grateful for the input from and guidance provided by the

project steering group from CH2M Hill, Halcrow and Thames Water.

All mapping reproduced by permission of ordnance survey on behalf of HMSO. © Crown copyright

and database right 2009. All rights reserved. Ordnance Survey Licence number 100019345.

All maps produced using the British National Grid projection.



1

Potential source control and SUDS applications:

Land use and retrofit options

Executive summary

The Thames Tideway Tunnel has been designed to significantly reduce the spill flows from CombinedSewer Overflows (CSOs) and Pumping Stations into the River Thames. The planned interventionstrategy to transfer the flow into the tunnel has, in many cases, resulted in costly diversion structures.However, in some instances the volumes and magnitudes of the spilled flow are relatively small andthis has raised the question as to whether other options such as the introduction of SustainableDrainage Systems (SUDS), could see a potential benefit in the reduction of the spilled flow and hencein a reduced cost or the entire elimination of some diversion or overflow structures. This needs to bebalanced against the cost and practicability of the implementation of SUDS systems within existing

urbanised areas in the London Tideway Tunnels (LTT) catchment.This report provides an assessment of the potential of and options for stormwater disconnection usingretrofit SUDS techniques for 3 of the subcatchments in the London Tideway Tunnels (LTT)catchment. These are the West Putney, Putney Bridge and Frogmore (Buckhold Road) CombinedSewer Overflow catchments. The objective of the study was to determine whether or not there wasscope for stormwater disconnection that would be effective at reducing the frequency and spills fromthe Combined Sewer Overflows (CSOs) and comply with the Urban Waste Water Treatment Directive(UWWTD) at less cost and potentially greater benefit than the traditional sewered approach.

There is growing evidence globally that alternatives to piped or sewered systems, usually hybridresponses to stormwater problems, using combinations of ‘natural’ drainage systems and piped orsewered where necessary, are more flexible, adaptable and ultimately more resilient to on-goingmajor changes such as due to climate. These systems also have the added benefits of being able to

improve the quality of the stormwater running off surfaces and can also be used to enhance theliveability of urban areas by the introduction of water features and more green space; also providingopportunities for cooling and carbon sinks, important responses to climate change.

The retrofit stormwater options considered here are so-called ‘sustainable drainage systems’ (SUDS),defined as: “surface water drainage systems developed in line with the ideals of sustainabledevelopment” (CIRIA, 2007). These systems comprise combinations of source controls, such asgreen roofs; local controls, including collective areas draining to permeable pavements; and end ofsystem controls such as detention basins and ponds. These systems should be seen as part of a‘treatment train’ whereby a number of SUDS are used together in a combined portfolio of responses.

Potential SUDS options that may be applicable in the relatively dense areas of London have beenreviewed and selected within a preference hierarchy of applications based on published guidance onretrofitting. Their overall potential for reducing the CSO discharges was assessed using a computermodel of the London drainage system, the InfoWorks CS© London Tideway Tunnels seweragesimulation model.

Initially ‘global’ figures were used to test whether or not disconnection of stormwater inputs to theLondon Tideway Tunnels sewer network would potentially be useful. Assuming it were possible toremove the flows coming off 50% of the existing impermeable areas it was found that there could besignificant improvements to the CSO spill behaviour. Some, but less significant, improvements inspills were also found for the use of SUDS systems that removed or attenuated the initial 50millimetres of runoff from contributing areas.

The greatest reduction in overflow was seen when 50% of the impermeable area was removed,reducing the overflow volume at West Putney by 55%, at Putney Bridge by 78% and for Frogmore(Buckhold Road) CSO by 77% during the typical year. The number of overflow events during the yearwas also reduced, but only slightly for the West Putney CSO, and 16 and 10 spill events per yearrespectively for the Putney Bridge and Frogmore (Buckhold Road) CSOs. Disconnecting 50% of theimpermeable area from the entire LTT catchment reduced the total overflow volume by 54%,



however, this represents the disconnection of some 10,327 ha of hard surfaces such as roofs, drives,car parks, roads and pavements, a not inconsiderable amount.

Estimation of the potential ‘on the ground’ reduction in contributing impervious areas, as opposed tothe theoretical reductions above, was carried out in two stages. Firstly an assessment was made byassessing the applicability of a retrofit SUDS option by physical, mapped, characteristics of theunderlying surface and local topography by inspection using GIS OS Mastermap. A series of logicalSQL queries were determined that could be used to automatically select areas of land that wereapparently suitable for each potential retrofit option. From this a more realistic set of disconnectionoptions than the global 50% was generated and found by modelling to be potentially valuable forreducing CSO spills.

The assumed global disconnection figure of 50% and the GIS semi-automatic estimation of areas thatcould be disconnected outlined above, were further refined by a detailed investigation of the realpotential for disconnection by undertaking feasibility designs for retrofitting a range of SUDSmeasures. These were in trial areas that included converting road surfaces to porous; fitting shallowbasins in grassed areas to slow down the rate of flow into the sewer network; disconnecting thedownpipes from back roof areas of houses and directing them on to lawns; installing pocket raingardens in streets (which also serve to calm traffic); and converting paved and car parking areas fromimpervious to pervious surfaces.

The opportunity to disconnect stormwater in these detailed areas was considered in terms of thepracticability on the ground. In these areas, although chosen for their good potential, the practical useof SUDS, based on an evaluation of the realistic local opportunities for disconnection, was found to beconsiderably less promising than originally assessed. This more refined disconnection strategyresulted in approximately 37% of the existing impermeable area being disconnectable. Thedisconnected flows were diverted to a mixture of initial loss, pervious area and storage attenuation.The most receptive of the three trial subcatchments, Frogmore (Buckhold Road), was shown bymodelling to spill 10 times in a typical year when the impermeable area was reduced by 50%.Correspondingly spill frequency for a 37% reduction would be more than 10 spills per year.

In parallel with the above, the whole life costs (capital and operational) for the final disconnectionoptions have been determined, at a cost of some £28M-£59M over a lifetime of 50 years for each ofthe 3 subcatchments. This assessment has been based on the methodology and database originallydeveloped for (new) SUDS by UKWIR/WERF in 2005 and subsequently updated by H R Wallingford.This has used data developed in a joint US-UK study with a time base of 2002. In the whole life costassessments the unit costs have been scaled up by an inflation factor of 1.25 (to 2009) and theTreasury discount rate has been taken as 3.5% in the analysis.

A review of the potential intangible and other costs and benefits in non-monetary terms has alsoidentified where these may be significant and add considerable value to the application of retrofitstormwater disconnections. This is particularly significant for mitigating and adapting to future climatechange, for which SUDS are much more resilient than piped/sewered drainage systems and can alsobring added benefits in terms of green space and amelioration of heat island problems.

As there are many associated benefits from using alternatives to piped and sewered drainagesystems for stormwater management; not only benefits relevant to the duties of the SewerageUndertakers, there may be a case for the costs of implementation being shared between the variousbeneficiaries, rather than borne by the Sewerage Undertaker alone.

Overall it is concluded in the present study that it is technically feasible to retrofit stormwaterdisconnection measures using SUDS which could potentially be effective at reducing the CSO spillsinto the River Thames. However, spill frequencies are likely to remain above 10 events per typicalyear even in areas where the strategy is the most practical.

There are also significant logistical, legal and regulatory impediments to the utilisation of SUDS asproposed, in the short to medium term. These include:

It would be necessary to engage with the very wide range and large numbers of individualstakeholders who would become involved directly and indirectly to ensure acceptability of thealternative arrangements;

There may be problems in defining responsibility for the long-term operation and maintenance of theretrofit SUDS. Although under the new Flood and Water Management Act 2010 and delivery of



Surface Water Management Plans , Lead Local Flood and other Authorities will play a main role inadopting and managing any new SUDS in the future;

Using SUDS in the way proposed transfers the burden of stormwater management from theUndertaker to a variety of other stakeholders each of whom may be more or less competent toassume that responsibility and this would potentially increase the risks of poor performance and couldlead to third party problems due to exceedences / overflows when systems fail to perform asexpected;

If the proposals were to go ahead, there would be widespread disruption to local areas as the widenumber and types of SUDS proposed are constructed;

Expenditure by the Sewerage Undertakers in England should be directed only to investment insewerage assets, making investments in SUDS currently problematical;

There would be a need to obtain planning and building regulatory approval for the retrofit SUDS andalso the disconnection arrangements;

Agreements would be needed to access and in some cases, purchase, the land needed for the retrofitSUDS;

There would be a need to reach agreement with the roads/highway authority for the SUDS proposedin the roads and highwaysNotwithstanding the above, it is expected that the passing of the Flood and Water Management Act 2010 could address some of these major impediments for new developments and areas undergoing

redevelopment. The Act, however, does not deal specifically with retrofitting altered stormwater

management measures and how these would be dealt with still remains to be seen. The proposed

approach is also in keeping with current initiatives to pass on the true costs of stormwater

management from Ofwat, and as given in the Walker Review (December 2009) report.





1. Introduction and objectives

The Tideway Tunnels are being designed to significantly reduce the spill flows from Combined Sewer

Overflows (CSOs) and Pumping Stations into the River Thames in London. The planned intervention strategies to transfer the flow into the tunnel have, in many cases, resulted in costly diversion

structures. However, in some instances the volumes and magnitudes of the spilled flow are

relatively small and this has raised the question as to whether other options, for example, the

introduction of Sustainable Drainage Systems (SUDS)1, could see a potential benefit in the reduction

of the spilled flow and hence in a reduced cost, or the entire elimination of some diversion or

overflow structures, balanced against the cost and practicability of the implementation of SUDS

systems within the urbanised areas. The latter not only include the selection and design costs; the

capital and construction costs but also the social, economic and environmental costs and impacts

associated with their acceptance and agreed implementation with all stakeholders, particularly the

public.

This report considers the potential to influence the performance of the stormwater system by

reducing the frequency, volumes and flows of overflow at certain of the CSOs, within three of the

sub ‐catchments – Putney Bridge, West Putney and Frogmore (Buckhold Road), by use of source

control and other SUDS techniques within the sub ‐catchments. The report considers the

performance and cost benefit potential associated with the application of SUDS within the three

areas which collectively cover the highly urbanised inner urban areas and less urbanised, more

peripheral areas that form part of the overall sewer network.

The overall objective of the study has been to review the potential for source control and other

SUDS options to reduce stormwater flows to CSOs in the Thames catchment, by the following:

Reviewing and defining pilot catchments for the evaluation of the above as examples of what may be achievable.

Defining specific opportunities within the pilot catchments in terms of technical,

environmental and practicability criteria and evaluating the performance in terms of flow reduction and consequences for downstream CSO spill flows, volumes and

frequency reduction

Evaluating the social, regulatory and other opportunities and barriers to implementation

of the useable opportunities (as defined above) based on desk studies.

Estimation of the potential cost ‐benefits of the usable options based on readily available

data and tools (e.g. WERF/UKWIR, 2005)

Recommendations for the selection of the primary retrofit options in the studied

catchments and the practicability and value of implementation.

Recommend further work to support the better definition and utilisation of source

control and SUDS measures for the Thames catchment as a whole.

There has been a growing awareness amongst policy makers and regulators that surface water

drainage will become an increasing challenge in the future. In her recent interim report Walker (The

Independent Review of Charging for Household Water and Sewerage Services, 2009), focusing on

householders, points out that: “The surface water drainage charge is on average 9 per cent of the combined household water and sewerage bill, but varies significantly between companies. Surface

1 Defined as: “surface water drainage systems developed in line with the ideals of sustainable development “

(CIRIA, 2007).



water drainage will be an increasingly significant issue in the future, as the likely effect of climate change – bringing shorter and heavier periods of rainfall – has potential implications for the necessary size of sewers, which in turn has implications for future costs .”

The review goes on to say: “Implementing these recommendations will have an impact on the future costs for sewerage companies of surface water and highway drainage. In particular, sustainable drainage systems (SUDS) will reduce the amount of water that runs off into the sewerage system. These recommendations highlight the importance of ensuring the capacity of the sewerage system achieves an acceptably low risk of surface flooding in the future. It will be important to establish the right incentives to encourage SUDS where appropriate, as SUDS offer an alternative to increasing the capacity of the sewerage system; it therefore reduces the need to make investment in the future and helps achieve lower future bills. This would also accord with the fairness principles of complying with the ‘polluter pays’ principle, reflecting in charges the costs that particular customers impose on the system.” (Walker, 2009).

Whilst encouraging to the use of non ‐piped/sewered drainage systems, these sentiments apply to

new developments and the control of extensions to paving gardens and similar areas. Although it

does recognise some of the problems: “For existing households, it is difficult to exclude households from the benefits of rainwater drainage services; whether a property is flooded will depend largely on the drainage services supplied to all neighbouring properties, and not simply to their own property. On the other hand, the costs a particular property imposes on the system depend on its drained area .”

The interim recommendation in relation to households: “looking to the future, it has to be recognised that households will need to be incentivized to consider SUDS as a way of alleviating the likely effects of climate change in rainfall patterns. It is important to start looking now at what can be done to minimise future problems caused by an increase in peak rainwater run ‐off. To achieve this, there needs to be an incentive for existing homes to reduce the amount of water run ‐off from a

property. The review team recommends that Defra, the Assembly government, the Environment Agency, Ofwat and sewerage companies should consider how the future charging system could incentivise householders to minimise the amount of rainwater run ‐off from existing and new households, including incentives to install small ‐scale sustainable drainage systems.”

In the present study the focus is on managing existing dense urban areas using retrofit stormwater

drainage systems and is not restricted only to households, but covers all developed areas within the

London Tideway Tunnels catchment.

The use of retrofit SUDS to materially influence the use of the London Tideway tunnels, has been

variously considered previously and their utilisation discounted for large ‐scale benefits although

there were some initial indications that there may be value in investigating these options further

(Binnie, Black & Veatch, 2002; Jacobs, 2006).

Globally there is widespread agreement that the use of SUDS or non ‐piped/sewered drainage

systems as part of a much broader approach to stormwater management planning wherever feasible

for stormwater management is preferable and generally more sustainable as part of a ‘whole ‐

systems approach’ (e.g. Weinstein et al, 2009) and can add a wide range of multi ‐benefits to urban

environments within a ‘green infrastructure’ approach advocated by the Centre for Neighbourhood

Technology (Wise et al, 2010). Outside the UK the approach is known variously as: Water Sensitive

Urban Design (WSUD) in Australasia (Engineers Australia, 2006); and as Low Impact development

(LID) (and/or Water Sensitive Planning and Design) that includes Best Management Practices (BMPs)

in the USA (e.g. France, 2002). In continental Europe a number of initiatives have taken the same

approach and recent and on ‐going studies are attempting to develop guidance and decision support tools for the application of these approaches – e.g. the pan ‐European DayWater project (Thevenot,



2008) and the Danish 2BG project (Jensen, 2008). Even in the UK, the forthcoming Technology

Strategy Board business case considering opportunities in the priority technology area of Integrated

Urban Water Management (IUWM) has identified the need to manage the water cycle more

holistically. As well as advocating the use of WSUD in land use planning, storm ‐water and

wastewater source control and pollution prevention; storm ‐water flow and quality management and

the use of mixtures of soft (ecological/green) and hard (infrastructure) technologies are identified as three of the main ways of realising the benefits from IUWM (TSB, in print).

In Australia the approach is embedded in policy and practice and is mandatory in all States. It is seen

as attractive to developers especially as it can help to offset drought problems by making

stormwater runoff readily part of the supply chain (e.g. Landcom, 2009). These systems potentially

provide multiple benefits as illustrated in Figure 1.1.

Figure 1‐1 The place of SUDS within the Water Sensitive Urban Design framework (adapted from Landcom, 2009 and CIRIA, 2007)

For the first time a German study has reported the inherent flexibility and adaptability of SUDS

compared with the ability of conventional drainage systems to cope with future climate change

(Seiker et al, 2008); reinforcing the wider value of their utilisation. There are other multiple benefits from alternative stormwater management practices, such as the influence on property values but

not all SUDS provide a positive benefit in this regard. However, a recent study from the USA has

shown that where properties overlook detention basins (ponds) that have been provided for the

single objective of stormwater attenuation, compared with properties adjacent to other multiple ‐

use basins, property values are adversely affected. Nonetheless, in the multi ‐use areas (quantity ‐

quality ‐amenity) property prices actually increased (pre ‐2009) (Lee et al, 2009).

A major (but not the only) component of alternative stormwater management systems are SUDS. In

the UK the Construction Industry Research and Information Association (CIRIA) has been promoting

the use of SUDS through the development of guidance and best practice manuals. The latest of these

is the ‘SUDS manual’ (CIRIA, 2007) and a separate guidance document is also under development for planners (CIRIA, 2009), which states: “Sustainable drainage is becoming the preferred approach to



managing rainfall and surface water in developments. Delivering sustainable drainage systems (SUDS) can provide a number of benefits for those that live, work and play in or around a development. They can help manage flood risk and water quality as well as provide better places to live by providing habitat for wildlife and improved amenity ”. In Figure 1.1 the CIRIA (2007) vision of the use of SUDS, shown in yellow, is related to the larger picture of WSUD as now being practiced

elsewhere in many parts of the world.

Unlike Australia, in the UK the use of SUDS techniques as part of this holistic and more integrated

approach to urban water management, is only now beginning to emerge. In England, there is a

preponderance of interest in the water quantity management aspects of these systems, despite the

coming need to better control urban runoff diffuse pollution as required under the Water

Framework Directive. The difficulties of taking an integrated approach in England relate to the

fragmentation of responsibilities and different scope of the key players in the management of the

urban water cycle. Hence much of the benefits of this approach cannot be readily realised despite

attempts to encourage the use of SUDS in planning guidance. In the study reported here, TW are

concerned to provide a better quality of water in the Tideway Thames by controlling the polluting

impacts from the existing combined sewer overflow (CSO) discharges, a requirement under the

Urban Wastewater Treatment Directive, but also to enhance the ecology and usability of the River.

Longer term there may be a wish to address the potential for potable and other water uses from

stormwater in the Tideway Thames catchments and also to provide a system that will enable

developers, local authorities and others to enhance amenity and other benefits as is now being

promoted in the USA (Wise et al, 2010). This may include addressing climate change problems such

as the better management of the urban heat island through ‘greening’ (e.g. Mitchell et al,2008); the

enhancement of streetscapes using water features and the direct use of stormwater for supply

purposes. However, the immediate need is to manage the stormwater in the Thames Tideway (TT) catchments such that the frequency, volume and rate of spills from the existing CSOs are reduced to

acceptable levels as indicated by the Urban Waste Water Treatment Directive (UWWTD). Therefore

this report concentrates on the potential use of SUDS retrofitted into the Thames catchments as a means of reducing the rate and volume of runoff into the sewer network, through stormwater

disconnection. In this study no consideration is given to the additional value of managing the

stormwater for local flood risk reduction or other purposes other than as possible additional

potential benefits from using SUDS as an alternative.

A difficulty in applying the wide range of guidance for using SUDS, WSUD and LID approaches to the

Thames catchments is that most of the available guidance is aimed at new developments, whereas

what is required in this study is to attempt to retrofit SUDS into the existing urban areas in London.

There is much less guidance available as to how best to do this. In the UK there is no specific

guidance for retrofitting and the only studies have been those by SNIFFER (2006); Stovin & Swan

(2007); Stovin et al (2007). Recently a number of pilot studies conducted by Defra but focused on flood risk management, have showed that the well ‐planned use of retrofit SUDS is a feasible

alternative to conventional piped/sewered systems (Gill, 2008) and retrofitting non ‐piped systems is

a key component of the first generation of Surface Water Management Plans (SWMPs, Defra, 2010)

e.g. Falconer (2009). In the USA retrofitting is virtually ‘standard practice’ employed to reduce the

size of large sewer storage tunnels (Natural Resources Defence Council, 2006; Weinstein et al, 2006),

although much of the approach relies on usable infiltration capacity. US Guidance is also available

from the Center for Watershed Protection (2007).

The present report considers a much wider range of SUDS options than in the other recent studies

looking at their use in London and shows that technically and hydraulically, there is scope for SUDS

to reduce certain CSO spill volumes and frequencies. However, the practicalities of application on



the ground make their implementation difficult and not without considerable risk within current

Governance and regulatory regimes. This report is laid out as summarised below:

In section 2 the work undertaken and methods used are introduced.

In Section 3 the approach to the selection of retrofit SUDS options is set out.

Section 4 deals with the performance criteria for the SUDS units

Section 5 reviews the practicalities of utilisation of retrofit SUDS for the Thames catchment.

Section 6 illustrates by way of example, the detailed application of retrofit SUDS for selected

areas.

Section 7 outlines the approach used to assess the disconnection of stormwater potential at

Thames catchment scale.

Section 8 considers available information about the cost benefits of retrofitting SUDS and

stormwater disconnection.

Section 9 up scales the results from the detailed analyses in Section 6 from the three

catchments investigated in detail to estimate the costs of application.

Section 10 considers the usage of the disconnection findings in Thames catchment hydraulic

modelling.

Section 11 summarises the investigation and gives conclusions.

Recommendations are given in Section 12.

Appendices are provided for:

The definition of the types of SUDS and their potential effectiveness for stormwater

disconnection is given in Appendix A

The SUDS considered in detail are illustrated in Appendix B

An examination of the current cost ‐benefit approaches available from UKWIR studies in

Appendix C

Practicalities of retrofitting in the areas investigated in detail in Appendix D

Details of the designs and costings for the example areas investigated in Appendix E

An illustrative photographic record of the areas examined in detail is given in Appendix F.



2. Work carried out

This study has been undertaken by staff from the Pennine Water Group (PWG) at the University of

Sheffield, supported by HR Wallingford Ltd over an intense period of time from March – June 2009. This work has been carried out in association with CH2M Hill and Halcrow Group. The former

undertaking computational modelling of the TT sewerage network to evaluate the effectiveness or

otherwise of the disconnection options and the latter managing this study.

The core staff from PWG are amongst the leading UK proponents of the use of SUDS systems and

collectively have experience that is particularly relevant in: SUDS modelling and design (flow

quantity and quality); retrofit SUDS specification; accounting and economics of water systems and

related asset management; integrated urban drainage system modelling; design and adaptation to

climate change; futures and scenario analysis; sustainability assessment; local and catchment–wide

flood risk management; water system governance; diffuse pollution modelling and management;

and public

engagement.

Close

working

with

a

number

of

local

authorities

and

water

and

sewerage

managers in each of the countries of the UK and overseas, also ensures a wide vision and

understanding of the potential for the use of SUDS systems and also the barriers to implementation.

The PWG has also been previously involved in the development of the Thames (TTT) schemes

through: involvement in the original Thames Tideway Strategic Study (TTSS) steering group; in the

independent review of the 2007 proposals on behalf of Ofwat, working with Jacobs consulting; in an

initial computational model assessment of the regime proposed for sediment flushing from the

operational tunnels.

Following an initial introduction to the scope and requirements of the study, an overview of the

potential catchments to be examined was provided by the TT project team. From this, 3

subcatchments were selected as potentially providing benefits from retrofit stormwater disconnections as their sewerage system performance were such that relatively modest stormwater

disconnections (removing, slowing down or reducing the volume of flows entering the systems)

could potentially improve CSO spill performance. The subcatchments were also considered as areas

where there may be opportunities for retrofitting in terms of available land, density of properties

and open space. These areas are considered in Section 3.

The work concentrated initially on the technical, environmental and practicability aspects of selecting potential candidate retrofit options within each of the three test subcatchments. The

approach followed a sequence of: reviewing land uses, potential sites for retrofitting following the

framework given in Section 3 and using a GIS platform developed for and with PWG, in conjunction

with CH2M Hill to make a coarse estimate of what disconnections might be feasible at a large scale based on a hierarchy of preferred SUDS options. This provided the information for CH2M Hill to

make a first analysis of the potential effect of disconnection on the performance of the CSOs. A site

visit was able to determine on the ground opportunities for carrying out detailed designs and

costings as examples and then these were related to the wider Thames catchment GIS‐based

analysis to determine a more realistic potential disconnection assessment and overall cost

estimates. Option design had to utilise simple approaches in view of the time available, and outline

SUDS design was provided through the HR Wallingford SUDS whole life costing tool (WERF/UKWIR, 2005) that is continually being enhanced as new cost data become available.

After selection of the feasible options, the other elements of retrofit option selection and design

were considered. These relate to the local implications and practicability of utilisation (and

construction) of each option. An overview of water quality and environmental implications was also

undertaken (without any attempt at quality modelling). This provided a more detailed assessment of



the likely regulatory, social, institutional (inc. adoptability) aspects of the primary candidate retrofit

options and identified key barriers to implementation and potential future changes in regulation

that may assist utilisation. To avoid raising undue adverse community reactions at this stage, the

assessment has been entirely desk ‐based with limited site visits.

It was expected that in order to compare the cost ‐benefits of the retrofit vs the ‘traditional’ options,

it would be necessary to review the way in which the TTT costs and benefits have been determined.

However, it proved not to be possible to interact with the TTT team sufficiently to do this. Hence,

the review of costs and benefits in this report are stand ‐alone. Hence, there may be certain aspects

of the costs ‐benefits and values presented here that are not commensurate with the interpretation

and assumptions made in the TTT studies.

A master list of potentially viable and cost ‐effective retrofit options has been drawn up considered in

terms of their implementation potential, including an assessment of taking a staged adaptive

approach over a period of time – and even potentially using these options in the future to

supplement the TTT as climate and other demands increase with time. Recommendations for

further staged assessments of the need and opportunities for retrofitting have also been defined.

The final conclusions provide recommendations for potential retrofit within the 3 studied

catchments and also the possibilities for scaling up across the London area, draining to the tunnel as

a whole.





MCDM – Multi ‐criteria ‐decision ‐making

Figure 3‐1a SNIFFER Framework – Phase I Feasibility Assessment

In a later study, Stovin et al (2007) built on an earlier methodology developed by Swan (2003) and

devised a hierarchy for selection of SUDS site for retrofit, Figure 3.2. This indicates a preference system for looking for the sites/SUDS options that will provide ‘quick‐wins’ first, in terms of being

the most straightforward to implement and also having a potentially significant impact.



Data review and detaileddata gathering

Outline design andcosting of retrofit options

Hydraulic modelling

Decision making Apply MCDM if

necessary

Consider other options

Consult stakeholders

Are retrofitSUDS best way

to meetobjectives?

Detailed technicalfeasibility and design

Implementation phase

Pursue other optionsno

yes

Figure 3.1bSNIFFER Framework ‐ Outline design and implementation Phase II

*Water quality improvements may be maximised by disconnecting industrial/commercial roofs and/or highways; however

adequate protection against local contamination needs to be ensured in the design of SUDS options.

Figure 3‐2 Generic hierarchies for SUDS retrofit Selection proposed by Stovin et al (2007)



3.2 Framework used in the study In the present study, the approach taken is illustrated in Figure 3.3.

Figure 3‐3 Approach taken in this study to the selection and evaluation of the effectiveness of retrofit SUDS

The stages in the analysis shown in the Framework are considered further below.

3.2.1 Review of available SUDS options and rank ing o f performancepotential

The details of potential SUDS options that may be selected for retrofit are outlined in Appendix A

and illustrated in Appendix B in relation to different categories of use for either roads, buildings or

land. A wide range of alternative and conjunctive use SUDS could in principle be considered for

retrofit for the various land uses and catchments being studied. Ideally, a comprehensive analysis

should be undertaken in which each option and combination of options (including retaining

piped/sewered drainage) is evaluated based on whole life costs and other considerations (e.g.

Weinstein et al, 2006). However, there was not time in this study to do this and hence only single

options were identified in terms of their likely potential applicability and effectiveness. In the



detailed analysis, Section 8, only one option was examined in detail for each land use investigated as

there was not time to compare alternatives. Expert judgement has been used to select what were

considered likely to be the most options.

A preference hierarchy was developed based on expert judgement of the likely effectiveness of application as shown in Table 3.1.

Table 3‐1 Potential SUDS, preference and indicative hydraulic performance

Surface type Primary options Hydraulic performance Preference rank

Roads Pocket street infiltration c) removes first 12 mm of storm runoff with subsequent

drain down into network

3

To adjacent pervious/SEA Streets b) 2

Permeable road surface c) removes first 25 mm of storm runoff & d)

1

Off ‐site – local detention and swale

conveyance d) 4

Non ‐road hard

standing (inc. car

parks) Contiguous

areas of man ‐made

surfaces >200m 2

Permeable surface storage c) & d) 1

Adjacent pervious b) 2

Off ‐site e.g. local detention d) 3

Man ‐made surfaces

other than above Adjacent pervious b) 1

Roofs Green/blue c) blue can remove first 25

mm of storm runoff b)green can act as pervious

storage for smaller storm

events

4

Soakaways a) 1

Disconnect to lawn (Classified as

mixed permeability) b) 2

Water butts/RWHWhere there is adjacent green

space or hard standing to site them

c) can remove first 25 mm of storm runoff if oversized

cistern used

3

Key: see below for explanation of a) ‐d) and 1‐3

The initial starting point for selecting the most preferred option was to choose the option likely to

deliver the best hydraulic performance, i.e. remove, retain or detain stormwater flows most

effectively. Four different hydraulic mechanisms were considered, and these were initially ranked in preference order as follows:

a) Complete removal – 1st

b) Transfer to pervious – 2nd equal

c) Initial losses (x mm) – 2nd equal

d) Storage/attenuation – 3rd

Preference between b) and c) depends on local infiltration (pervious) characteristics, level of initial

losses and on the precise performance criteria being assessed. If there is poor infiltration, compared

with potentially good initial losses, then c) is preferable to b).

This preference order has been applied whenever more than one disconnection option was considered to be technically viable.



The initial evaluation considered source control options only, in keeping with the preference

hierarchy promoted by the Center for Watershed Protection (2007) and Weinstein et al (2006). This

was because ‘regional controls’ (CIRIA, 2007), such as storage ponds, detention basins an inter ‐

linking swales were considered as ‘end ‐of ‐system’ options, collecting runoff from a succession or

number of contributing areas. Given the available guidance, the study concentrated initially on

source controls as the best options as part of potential downstream ‘treatment trains’. This assumption was found subsequently to be too limiting in the subcatchments investigated as these

had considerable areas of green space that was found to be suitable for regional, or near end of system, SUDS (Section 8). This led to a re ‐evaluation of the potential stormwater disconnections

from the initial appraisal and the final analysis (Section 9) and a refinement of the preference order

(Section 4.2.1).



4. Identif ication of subcatchments and performancecriteria

4.1

Identification of

subcatchments

It was intended that the retrofit study would investigate areas within the London Thames catchment

that were as typical as possible of the overall catchment in order to be able to make some

assessment of the overall potential for retrofitting stormwater disconnections across the Thames

catchment. Three subcatchments were selected in the London Thames (LTT) catchment for the study

(Annex 1), located in the west of the London Tideway Tunnels catchment, south of the River Thames

and comprising the subcatchments contributing to the West Putney, Putney Bridge and Frogmore

(Buckhold Road) Combined Sewer Overflows (CSO). These CSOs are thought representative of the

London Tideway Tunnels catchment as a whole. The subcatchments are shown in Figure 4.1, with

more detail of each in 4.2 ‐4.4 from ANNEX 1.

© Crown Copyright and database right 2009. Ordnance Survey

Figure 4‐1 The three subcatchments investigated (not to scale) (numbers refer to images in Figure 4.5)



Figure 4‐2 West Putney Mastermap subcatchment characterisation



20

Figure 4‐3 Putney Bridge Mastermap subcatchment characterisation



Figure 4‐4 Frogmore (Buckhold Road) Mastermap subcatchment characterisation



22

Figure 4.5(a) – 4.5(f) shows illustrative images and a brief overview of the types of areas in the

Putney Bridge, West Putney and Frogmore (Buckhold Road) catchments. The numbers [ ] refer to

locations in the general vicinity in Figure 4.1.

[1] Characterised by high density development, narrow roads, small/no front gardens, on ‐street

parking

Figure 4‐5(a) Putney Bridge subcatchment

Northern and South Western area of catchment is dominated by golf courses [2].

[3] Centre of the catchment dominated by large apartment blocks, medium rise, often set in large

communal grounds.

Figure 4.5(b) West Putney subcatchment



Much of the Frogmore catchment is characterised by large apartment blocks [3], particularly in area

[4]. Medium density housing, often with large back gardens is found in the NE Frogmore catchment

[5]. These areas often have wider roads than other residential areas. Many houses have off ‐street parking with partially paved front gardens.

Figure 4.5(c) Frogmore (Buckhold Road) subcatchment

There are also Institutional Buildings, such as hospitals. Large buildings, often with lots of landscaped

grounds and paved areas

Figure 4.5(d) Frogmore (Buckhold Road) subcatchment



The Southern part of the Frogmore catchment is dominated by apartments [3], [4] as well as high

density housing, often terraced/semi ‐detached with small rear gardens. Typically with on ‐street

parking.

Figure 4.5(e) Frogmore (Buckhold Road) Putney Heath

[8] At the heart of the Frogmore catchment there is a large area of maintained open space woodland

and grass (Putney Heath).

Figure 4.5(f) Frogmore (Buckhold Road) Putney Heath

4.2 Performance and performance assessment Performance of the retrofitted systems has to provide appropriate stormwater management and

also has to perform adequately within the urban landscape. Although TW are interested primarily in

the improvement of the system in relation to the pollution in the River Thames, there are other

aspects of performance that may militate against using SUDS systems or provide additional value to

their use.



The main evaluation of the required performance of the London Tideway Tunnels system has been

based on the potential for improvement to the performance of the individual CSOs. Each of these

has to perform better than at present in relation to the frequency and volume of spills. However, the

precise performance specification is still under development, hence no ‘target’ figures could be

defined for the present study. Instead an evaluation of what was possible in terms of improvements

in CSO performance has been made for various scenarios of retrofit SUDS and stormwater disconnection. In the following sections the performance of the individual SUDS options used in the

study is reviewed.

4.2.1 Hydrauli c and other aspects o f the performance of SUDS elementsThe design of SUDS for new developments is recommended to be based on critical design storms of 1 in 100 year return period (CIRIA, 2007). Ideally, time series rainfall events should also be used

where there is significant storage in the SUDS (Kellagher & Udale ‐Clark, 2008). However, when

retrofitting SUDS, 1 in 100 year standards may not be achievable due to land space limitations.

Consequently, when designing the detailed SUDS under evaluation in the present study a design

storm with a return period of 1 in 30 years has been used as this is typical of the standard currently

used for sewerage asset management planning. An uplift has been applied for climate change of 20% on rainfall intensity (Section 6). The hydraulic design has utilised the HR Wallingford model also

used for Whole Life Cost evaluation and is a simplified approach using a 1‐node InfoWorks CS urban

drainage model with a range of typical rainfall characteristics across the UK and by producing

correlation equations to fit the results (WERF/UKWIR, 2005) (Section 7.1).

The SUDS elements utilised in the study are given in Table 3.1, together with the hydraulic

performance assumed. Table 4.1 reproduces this information, together with more detail and also

indicates the nature of the SUDS design tool available in the HR Wallingford whole life costing model

(Appendix C). The overall performance is also summarised in terms of hydraulic, water quality,

ecological, aesthetic and safety benefits.

Table 4‐1 SUDS Hydraulic and overall performance

Surface type Primary options Hydraulic performance Overall performance HR Wallingford model tool

Roads

Roads

Pocket street

infiltration c) removes first 12 mmof storm runoff with

subsequent drain down

into network

Hydraulic, water

quality, ecological,

aesthetic and safety

benefits

Swales and/or

Detention basin

(normally dry)

To adjacent

pervious/SEA Streets

b) ’converts’

impervious to pervious surfaces

Hydraulic, water

quality, ecological, and aesthetic benefits /SEA

streets not used in

present study

Detention basin

and/or filter drain, although may be

flat grassed area

Permeable road

surface c) removes first 25 mm

of storm runoff & d) Hydraulic and water

quality benefits and

can be aesthetically

attractive.

Permeable

pavement (with

liner)

Off ‐site – local

detention and

swale conveyance

d) maximum outflow

constrained to 5l/s Hydraulic, water

quality, aesthetic but

may be perceived as

unsafe

Swale, detention

basin or retention

pond depending

on local

circumstances Non ‐road Permeable surface c) & d) Hydraulic, water Permeable



Surface type Primary options Hydraulic performance Overall performance HR Wallingford model tool

hard

standing (inc.

car parks)

Contiguous

areas of man ‐made

surfaces

>200m 2

storage quality, ecological,

aesthetic benefits pavement (with

liner)

Adjacent pervious b) Hydraulic, water


aesthetic benefits

Detention basin

and/or filter drain,

although may be

flat grassed area

Off ‐site e.g. local

detention d) Hydraulic and water

quality benefits Swale, detention

basin or retention

pond depending

on local

circumstances

Man ‐made

surfaces

other than

above

Adjacent pervious b) Hydraulic, water


aesthetic benefits

Detention basin


although may be

flat grassed area

Roofs Green/blue c) blue can remove first

25 mm of storm runoff b)green can act as

pervious storage for

smaller storm events

assume 25mm

Green only used:

Hydraulic, water


aesthetic benefits

Not included

Soakaways a) Not used in present

study Filter drain/trench

Disconnect to lawn

(Classified as mixed

permeability)

b) Hydraulic, water


aesthetic and safety

benefits

Detention basin


although may be

flat grassed area

Water butts/RWH

Where there is

adjacent green

space or hard

standing to site

them

c) can remove first 25

mm of storm runoff if oversized cistern used.

Not used in present

study although

considered optionally.

Hydraulic and

ecological benefits

As well as supply

supplement potential.

Not included

Key a) Complete removal – 1st




Only those SUDS units used in the present study have been reviewed in terms of their performance

in the following sections.

4.2.1.1 Pocket Street infiltration (see Appendix E.3) This is illustrated in Figure 4.6. Provided these units in plan are at least 2% of the impervious area

connected (Melbourne Water, 2005), these have recently been shown to provide on average some

33% reduction in inflow volumes and may attenuate peak flow rates by up to 80% (Hatt et al, 2009).

Although tested in Australian conditions, this study also showed no apparent changes to these

quantitative benefits in different seasons. Nonetheless it has been considered prudent here to treat

them as attenuation systems only. The specified outflow has been restricted to 5l/s and growing

media (soil supported by sand and gravel) has been included. The attenuation is conservatively



expected to ensure that at the least the first 12mm of runoff will be stored and only released slowly

even during successions of storm events.

The management of the stormwater which originates in roads and adjacent areas will become more

of a responsibility for the roads and highways agencies. It would be expected that much of the flow

would still eventually reach the public sewerage system, although some would be lost through

evapo ‐transpiration. These systems are potentially very effective at improving runoff water quality

and ecologically and aesthetically beneficial as vegetated street features. There may be minor safety

considerations when in operation as ponding on the surface will occur. This should be similar to

street surface puddles; although in extreme events may have a depth of up to 0.5m. These systems,

which are green, can help with climate change due to carbon sequestration and potential to lower

ambient temperatures. However, plant maintenance is not straightforward and requires skilled

operation. Residents may also object to their use if it impairs car parking outside their properties.

Construction would be dependent upon local conditions, but would be expected to be problematic

in relation to existing buried services, in some cases requiring re ‐routing.

Figure 4‐6 Pocket rain gardens in Auckland New Zealand

4.2.1.2 Permeable road surfaces / permeable paved areas (see Appendix E.1)

An illustration of these systems is shown in Figure 4.7. In the present study these units have been

designed based on standard block paved systems, such as that produced by Interpave2

and are subsurface storage systems. The sub ‐base provides storage for infiltrating runoff and the outflow

can be constrained to a fixed rate (in this case 5l/s). Design in the present study has usually

presumed that only the surface that is currently impervious will be drained to the same plan area

that has been reconstructed as permeable. Hence there may be scope for additional disconnections

into the system over and above the converted impervious area itself. However, this was presumed

to provide a worst case scenario other than for one of the case examples, where properties were

also drained to the permeable local roadway.

2 http://www.paving.org.uk/cost_of_paving.php (accessed 10/07/09)



Figure 4‐7 Permeable block paving (from Interpave website)

The assumption that these systems will remove the first 25mm of runoff is therefore believed to be

conservative in the designs. As there is not a liner in the designs, there is also scope for infiltration into the surrounding soil and a risk that groundwater may infiltrate where there is a high local water

table. Removal of the surface water into the subsurface storage not only attenuates the flow but can

also provide for some improvement in outflow water quality. There are no other potential benefits

except potential attractiveness in housing areas. There will be a transfer of responsibility away from

the sewerage undertaker to either the roads or highway authority or to private or other land owners

currently responsible for road drainage, although, ultimately the restricted outflow will enter the

public sewerage system. Construction of these systems will also be very disruptive and will be

problematic in terms of underground services and traffic disruption.

4.2.1.3 Off–site local detention, storage and swale conveyance (see Appendix E.1)

An illustration of a vegetated detention basin is shown in Figure 4.8. These may or may not contain

vegetation depend upon local need. Figure 4.9(a) shows a typical conveyance swale and 4.9(b) a

vegetated swale.

Figure 4‐8 Off ‐line local detention basin (Orleans, France)



(a) conveyance swale in Dundee, Scotland (b) vegetated swale in Seattle, USA

Figure 4‐9 Swales

In the present study the design of detention basins has assumed a restricted outflow of 5l/s and the

temporary storage is entirely presumed within the basin, with no storage assumed in the

interconnecting swales. The detention basin depth is assumed as 0.6m with 1:4 side slopes. These

systems are presumed to be able to retain the critical design storm, with the outflow not exceeding

5l/s.

Consideration will need to be given to ensuring that there is a clear exceedance flow pathway for

when the basin is overwhelmed, in accordance with the (CIRIA, 2006) guidance on this.

Although providing good hydraulic attenuation and potential ecosystems with plant growth as in

Figure 4.8, these systems may be perceived as hazards by local residents when they are in operation.

Where the basin is dry and laid only to grass, there may also be a lack of awareness that it will at

some point fill with water. When operational, these systems will be a hazard and local residents and

others will need to be included in the operation and maintenance planning to ensure caution and

also to get community involvement. By maintaining natural vegetation (even grass) there will be

benefits for climate change buffering and also if free of plants, opportunity to use the basins as

recreational areas.

There will be considerable changes needed in responsibilities for stormwater management where

these systems are used. They will become the responsibility of the local land owner/operator

despite the outflows subsequently entering the public sewer system. There will also be a need for

the sewerage undertaker to accept these flows – which is contrary to the usual current

arrangement, where they are seen as ‘land drainage’ and not the responsibility of the undertaker.

Construction of these systems should not be too disruptive as they are not in roads, although there

will inevitably be buried service problems. There may also be difficulties due to land ownership and

convincing owners/managers of the need to agree to allow holes to be dug that will occasionally fill with water.



4.2.1.4 Transfer to pervious and roof disconnection (see Appendix E.2) There may be opportunities to redirect existing pervious surface runoff on to adjacent permeable

surfaces. In the present study these adjacent areas have not been modified and so they are only

existing green areas such as gardens, green areas in municipal or other large institutions and public

greenspace. The redirection arrangements will be location specific and may entail some regrading of existing gradients and falls.

Figure 4‐10 Downspout connected to pervious area in Seattle, USA

No cost of design allowance has been made for this as it is too site specific. The disconnection of roof drainage is more straightforward and simply requires an adjustment of downpipes, with an

erosion preventer and redirection on to pervious surfaces, see Figure 4.10. The hydraulic behaviour

of these systems is to ‘convert’ existing impervious areas into pervious. There will still be runoff from

the latter, but at a reduced rate compared with the impervious condition. These systems redirect

the runoff from the sewerage undertaker’s assets onto private and public property and hence will remove the responsibility other than for any pervious surface runoff that finds its’ way into the

public sewerage system. Currently the sewerage undertakers have no responsibility to accept land

drainage. The Floods and Water Management Act 2010 and associated National SUDS Standards will make the use of these types of system more ‘normal’ in the longer term.

In areas with water shortages and as a mitigator of climate change, this redirection of water will be

beneficial for ecosystems and for greening in general. Coupled with water collection in butts or

cisterns, this option could be especially beneficial for supplementing supply.

Construction of these systems should be relatively straightforward other than for the paved areas where there may be a need to regrade to ensure adequate redirection of flows.

4.2.1.5 Green roofs (see Appendix E.2)

Green roofs, Figure 4.11, are used extensively in much of the rest of Europe, especially Germany

where they are even installed on sloping roofs, and are seen as adding significantly to property

values. There are now many installations in the UK although these are mainly on flat roof surfaces

(CIRIA, 2007a). In the present project green roofs have been considered only as an option on existing

flat roofed apartments. In each case the overflow during extreme events is also directed to adjacent

pervious areas or in exceptional circumstances into a permeable pavement area. This is because the

hydraulic performance during more extreme rainfall events is as yet uncertain, although flow

attenuation (loss by retention, evapo ‐transpiration and plant uptake) is known to be good for the

smaller events and over an annual cycle, with runoff coefficients reduced to 70%, even with a thin



roof (20 ‐40mm substrate) (CIRIA, 2007a). It is also claimed (ibid) that the peak rate of runoff from a

green roof can be less than or equal to that from a greenfield site for storm events with total

volumes up to 3x the maximum water retention capacity of the substrate used in the green roof. The

retention will depend on the construction and substrate as well as the installation details. Hence in

the present study a retention rainfall depth of some 25mm has been assumed for all storms, with

the excess routed into adjacent pervious areas as these are expected to be ‘most effective when combined with other SUDS components to form a stormwater management train’ (CIRIA, 2007a).

Green roof retrofitting requires adjustments to the roof structure unless lightweight media are used,

hence, here a lightweight system has been assumed appropriate. Green roofs can add ecological and

aesthetic value, can help mitigate climate change heat and insulation effects, but tend to be subject

to extreme and polarised opinions as to their value. They should not be used where there are plans

to collect roof runoff for use as in water butts and are very dependent on property owner/occupiers

views and expectations as to their acceptability. This option also transfers responsibility away from

the sewerage undertaker to property owners.

Figure 4‐11 Green roofs in Rotherham, Yorkshire

Construction of the retrofit green roofs will depend entirely in the property owner/occupier.

Incentives to allow this may be required over and above the typical £30 current reduction in water

charges for disconnecting stormwater systems. In England and Wales, even a partially connected

system, such as an overflow from an otherwise disconnected system, disqualifies the property

owner from any rebate.

4.2.1.6 Rainwater harvesting (see Appendix E.1) Although this is presented here only as an optional extra (due to added cost) it is outlined in some

detail due to the potential future usage if climate change encourages on ‐site water collection and/or

water companies provide stronger disconnection incentives.

The value of rainwater harvesting (RWH) as a means of attenuating downstream flows is the subject

of current research although this is subordinate to the interest in what the safe yield is from these

systems. It is apparent that time series rainfall is needed to properly evaluate the performance of RWH systems. RWH can be effective at reducing downstream runoff volumes for all storms – with

only some 14% of the original (non ‐intercepted) runoff volume passing downstream for an extreme rainfall time series for a storage volume of 1m 3 per person (equivalent to 29m 2 of roof area per



person in a UK study (Kellagher & Udale ‐Clarke, 2008). Another study found that storage of 0.75m 3

per person reduces the downstream flow volume to 20% of the original (Kellagher and Franco, 2005)

and storage of 1.5m 3 per head showed a 50% reduction in downstream peak flows for most events.

Overall it has been found that retrofitting RWH systems can significantly reduce flood frequencies

and volumes in areas that suffer from frequent flooding (ibid) although it is necessary to increase the

size of RWH tanks above what is required purely for RWH purposes. It can be concluded that RWH

can significantly reduce volumes downstream provided tanks are of the order of 1m 3 per head and

that it is possible to assume an initial loss of 50mm for each event for modelling purposes; although

in the present study this has been reduced to 25mm to account for uncertainty.

To ensure effective flood volume reduction: Assume up to a 1m 3 tank per person, although it could

be smaller – a 2m 3 tank per property (3.3 dwellers). For modelling, an initial loss of 25mm at the

start of the event is feasible in areas where there are a substantial number (>50%) of RWH

installations.

Although there are now lots of types of tank – some that can double as garden fences (Figure 4.12)

or be fitted into small roof spaces or under houses, Figure 4.13, meaning that lack of space may not be an impediment to retrofitting; there is no incentive for UK property owners and dwellers to fit

these at the present time as mains provided water is so cheap.

Figure 4‐12 Slimline RWH tanks in an access pathway to a property in Melbourne

Figure 4‐13 Ecosac (www.ecosac.com. au) flexible rainwater storage system

Retrofit of these RWH systems is best done by a specialist service provider. There are a number in

the UK with experience and construction should not be an impediment.

4.2.2 Sewerage performance assessmentThe London Tideway Tunnels catchment is being modelled using an InfoWorks CS computer

simulation model. Because of the complexity of the sewer network it has not been possible to model

every individual sewer and pipe length and the smallest diameter in the model is 375mm. SUDS

units, especially those used for source control, operate at a very local scale and caution is required

when using large ‐scale sewerage models to represent SUDS. For this study no attempt has been



made to model the SUDS units themselves, rather their effects in terms of changing the current

stormwater runoff and inputs to the sewer network have been modelled by CH2M Hill.

Descriptions of the application of the model to the retrofit SUDS options are given in ANNEX 1. For

this work the London Tideway Tunnels model has been amended to represent the change in

contributing areas produced by the various SUDS options. General disconnection options were

modelled as reductions in impermeable contributing area which was modelled as both lost and

transferred to permeable areas. Initial losses through rainfall capture techniques were also

modelled before the site specific options (produced by PWG) were investigated for the three CSO

test subcatchments.

In the London Tideway Tunnels studies, the December typical year and October 2000 rainfall events

represent the most severe recorded events for the typical year and 154 event rainfall series

respectively. The London Tideway Tunnels catchment model simulations have been carried out with

these and with the various specified SUDS options to produce the most extreme overflows at the

CSOs. The complete typical year rainfall has also been simulated to provide a representation of the

number of spills and total overflow that could be expected at CSOs during the annual series (Annex

1).

The InfoWorks CS model for the London Tideway Tunnels has been modified to provide a better

means of assessing the potential effect of removal and attenuation of stormwater runoff by the

retrofitted SUDS (Annex 1). Assessment of changes in the effective impervious contributing areas

and in initial storage (Table 3.1) from the retrofits was based on the contributing impervious area

changes for each of the Subcatchment InfoWorks node IDs within each of the 3 test subcatchments

Annex 1.

The potential changes to contributing areas were assessed by PWG (Sections 6 & 9) and used in the

London Tideway Tunnels InfoWorks model to assess the changes in the system CSO performance.

The process

is

described

in

LTTD,

2009:

the

model

was

first

amended

so

that

the

areas

for

each

source control option matched the areas provided by PWG. In InfoWorks this was done by adjusting

area percentage in the various subareas contributing to the nodes in each of the subcatchments.

For example in subarea 21757452 (West Putney subcatchment) the impermeable area was reduced

from 22.0% (19.2ha) to 5.61% (4.9ha) based on detailed analysis of surfaces and source control

options by PWG.

Areas with initial losses were also added to the model subcatchments. For example subarea

21757452 (West Putney subcatchment) was estimated to have potential for 4.81% of the subarea to

be subject to initial losses, equivalent to an area of 4.2 hectares.

Any contributing area with storage was represented as being lost in the subcatchment as the flow

would be captured by the SUDS storage volume and returned to the system slowly after the CSO event.



5. Practicalities of retrofi tting SUDS

The opportunities for and difficulties of retrofitting SUDS in urban areas are reviewed by SNIFFER (2006) and Stovin et al (2007). Technically, provided there is adequate land area available, virtually

any of the options in Appendix A and B could be retrofitted. There are limitations as regards

infiltration based SUDS, however, as these require sufficient infiltration capacity in the underlying

soil to operate. They also risk contaminating groundwater and compromising aquifers being used for

supply purposes. Ironically, the guidance available from the USA for retrofitting (Weinstein et al,

2006; Center for Watershed Protection, 2007) promotes the use of infiltration BMPs preferentially

for retrofits and also for the control of water quantity (Table A3, Appendix A). As it is unlikely that

there is significant infiltration potential in the London Tideway Tunnels catchments (Binnie, Black &

Veatch, 2002) this option has been discounted in the present study. As a consequence, all of the

SUDS units used in this study have had any infiltration capacity discounted; although to save on

costs, no impervious liners have been used in any of the designs so there may be some limited

infiltration.

In addition, in the designs the groundwater level has been considered to be low enough (typically

1m below ground level) so as not to impede the performance of the SUDS units.

According to Stovin et al (2007): “The lack of implementation (of retrofit SUDS) (associated with this project) reflects a level of complexity in the legislation and management of urban storm water.”

The report goes on to suggest the need for legislative change in England and Wales:

1. “Simplification of the current system for storm water management in England and Wales. 2. Reviews of both the ‘right to connect’ and ‘permitted development rights’. 3. Greater transparency and flexibility in drainage charging to provide incentives for property

owners to implement full or partial stormwater disconnection or to provide attenuation.”

Since the 2007 report was published there have been initiatives by Government and the main

stakeholders to address some of the factors discouraging the use of retrofit SUDS to deal with

stormwater problems. The Floods and Water Management Act 2010, has confirmed the place of the

Environment Agency as having a supervisory overview function in relation to all flood risk

management in England. The Act has also addressed the right to connect problem, removing this as

an automatic right and therefore preventing the reconnection to sewer of previously disconnected

SUDS. As regards item (3) above, recent moves to implement full cost charging for large

impermeable surface area contributors has been supported by Ofwat, although the implementation

of this, especially to public buildings such as churches and community buildings has proven politically

difficult and a more gradual introduction of full‐cost charges increasing over time is now being

implemented.

However, these are not the only impediments to the widespread introduction of retrofit stormwater

measures. These are considered in Appendix D and summarised in the following sections, based on

Stovin et al, 2007.

5.1 Overview of benefits of using SUDS retrofits The SUDS manual (CIRIA, 2007) provides a table that indicates not only the hydraulic performance of these systems but also the quality and other benefits, Table 5.1.

Many of the benefits accrue to society rather than the owner/operator of the SUDS units.



In addition, the bearer of the costs of retrofitting may not be the beneficiary.

The options used in the present study are shown with blue boxes in Table 5.1. It is apparent that

these options all have potential environmental benefits as well as effective hydraulic and water

quality control.

The whole life costs of the retrofits in this present study are considered in Sections 7 and 9. The

SUDS manual gives some guidance on the assessment of intangible benefits:

“Theoretically, a benefit assessment would account for:

1. The hydraulic benefits, including peak flow rate reductions, storm runoff volume reductions, and enhancements to river baseflow and aquifer recharge;

2. The pollutant loading reductions achieved by the system, and associated benefits to in‐stream ecology, human health, and human value perceptions;

3. The amenity and recreational benefit enjoyed by those who live close to the SUDS scheme; 4. The additional value of properties adjacent or within view of the SUDS scheme;

5.

The ecological

value

of

the

SUDS

systems

themselves.

The Government recognises that although there are techniques available to value the environment, environmental benefits have to be considered within a situation ‐specific context. The Green Book (HM Treasury, 2003) stated that ‘wider social and environmental costs and benefits for which there is no market price need to be brought into any assessment. They will often be more difficult to assess but are important and should not be ignored simply because they cannot be easily costed.’”

Table 5.2 gives a summary of the potential benefits from the use of measures for stormwater

separation from the point of view of the sewerage undertaker from Cascade (2009) which shows a

similar view of the potential benefits as the SUDS manual. In order to monetise the benefits into a

decision support tool, the Cascade study has derived benefit values from WaSC stated preference

surveys or as standard values (e.g. carbon emissions, traffic congestion) (Appendix C). No attempt

has been made in the present study to use this rather tenuous approach.



36

Table 5‐1 Summary of the benefits of SUDS options (CIRIA, 2007)



37

Table 5‐2 Summary of potential benefits of stormwater separation (Cascade, 2009)

Interventionmeasure

Reduction insewer flooding risk

Water qualityimprovement 1

Otherenvironmental

benefits 2

Conventional intervention measures Y

l t e r n a t i v e m e a s u r e s b a s e d o n

d i s c o n n e c t i o n

R e a l i g n i n g s u r f a c e w a t e r c o n n e c t i o n s Foul/ surface separation

Y

Land drainage separation Y

Highway runoff separation Y

Surface sewer separation Y

Watercourse separation Y

River restoration Y + Y

R u n o f f a t t e n u a t i o n m e a s u r e

Green roofs Y Y YBio-retention areas Y Y YSwales Y Y +Balancing Ponds Y Y YDetention basins Y Y +Soakaways Y YInfiltration Basin Y Y +Filter strips Y Y +Filter drains Y YSand filters Y YRainwater harvesting YConstructed wetlands Y Y YPervious surfaces Y Y +

Source reduction

measure

Domestic demand management

Y +

Industrial water efficiency Y +Y= primary benefit, + = secondary benefit1 Excluding water quality benefits associated with a reduction in inflow or increase in capacity in the sewerage

network, common to all intervention measures.2 Categorised as amenity, aesthetics and ecology benefits (CIRIA, 2007b); excluding water quality benefits

Recently with the advent of climate change concerns, there is a growing awareness of the need for

drainage (and other) systems to be adaptable – i.e. to be able to be modified relatively easily as

knowledge about climate change develops. A study has shown that SUDS are inherently more

flexible (a key component of adaptability) than piped/sewered drainage systems and hence more

likely to be useful in the process of adapting (Sieker et al, 2008). As many SUDS are relatively small

scale and locally based, especially source control measures, their use as part of an overall strategy

for coping with climate change is inevitable, most likely in conjunction with existing piped/sewered

systems which will have limited expansion potential.

As stated above, very few of the benefits will be seen as of tangible value to the SUDS owner or

promoter, nonetheless it is still beneficial to highlight potential values (not monetised) as part of the

present study and the assumption has to be made that the benefits will be relevant in the decision

as to whether or not to proceed with retrofitting SUDS for stormwater disconnection. This issue is

further considered in Section 7.2 and Appendix C.



5.2 Summary of difficulties in using SUDS retrofits Appendix C considers the issues involved in assessing the whole life costs of these systems and

Section 7 the application to this present study. Appendix D considers the practicalities of using SUDS, with reference to the limited studies on retrofitting. Here a summary of the practical difficulties of application is provided. The following is not a fully comprehensive list, but includes the most

significant points.

5.2.1 Legal and regulatory issues In the present study, as in all their wastewater related actions, TW are responsible for

‘effectually draining’ the London Tideway Tunnels area. Disconnection of stormwater inputs as

proposed here, may be construed as a dereliction of this duty as the responsibility will be

passed on to others, many of whom will not have the capacity to manage their new

alternative drainage systems.

Even if there is disconnection, the arrangements must comply with the Building Regulations in

force and under Section 106 of the Water Industry Act, there has been a right by the property

owner to subsequently reconnect at any time in the future if they wish.

The Flood and Water Management Act 2010 will, once enacted fully, amend the right to

reconnect. However, compliance with Building Regulations will remain.

The new Act will also provide the means for other players, such as local authorities, to become

more involved, with surface water management plans already encouraging the use of SUDS

and as the SUDS Approval Body (SAB) involved in the delivery and management of SUDS. They

are also expected to take on the role of maintenance and operation although this is still under

consideration. At present, however, this applies only to redevelopment and newly constructed

systems that have passed through the planning and building regulatory approval process.

The transfer of communal private sewer responsibilities to sewerage undertakers in the next 5

year period

will

also

incur

a

new

and

risky

burden

for

local

sewerage

undertakers.

This

may

potentially provide new opportunities to alter the existing drainage arrangements,

disconnecting any areas that are draining stormwater systems and making these into SUDS.

Road and highway drainage authorities have the right to connect their drainage to the public

sewer network. In theory, but not in practice, there could be a charge levied for this. Whilst

this charge is not made explicit there will be no incentive for these authorities to disconnect

and use alternative drainage systems.

5.2.2 Transfer of burdens and responsibil ities There is only limited experience by property owners/managers/occupiers in managing their

own drainage systems. It is unlikely that even where there is experience already, this will not

have included management of SUDS. Hence any transfer of responsibilities by disconnection will need to be supported by a capacity building and engagement process. This will have to be

funded by TW.

If the draft Floods and Water Management Bill is implemented (and this may not be for

several years), there may be a duty on local authorities in particular to take up responsibilities

for managing SUDS in their area. At present, however, any transfer will become the

responsibility of the property/land owner/road or highway authority. There is no apparent

incentive for any of these to take up responsibility for their own drainage systems rather than

rely on TW to provide a service at a very low cost.



5.2.3 Maintenance, operational needs, risks and health and safety There is only limited experience in operating and maintaining SUDS systems in the UK. There

is even less experience in the potentially more complex management of retrofit systems. This

means that retrofitting SUDS may introduce greater risks that the systems will not provide the

expected service.

Surface SUDS, such as open water bodies require special consideration as regards human

health and safety. In the present study there are no SUDS that will have permanent open

water surfaces, although in times of heavy rainfall, the detention basins, swales and rain

gardens will have standing water in them. It is not practical to fence these off and

arrangements will have to be made to ensure that residents and others in the locality are

aware that they will from time to time fill with water and what precautions to take in that

event. This should be the responsibility of TW.

Although the retrofit SUDS in this study have been designed to cope with a 1 in 30 year design

storm in the present study, to match the usual performance of urban sewers, there may be a

greater risk that overland exceedance flows from the retrofit SUDS may be of greater

magnitude than

the

flows

that

could

occur

when

the

underground

sewerage

system

is

no

longer able to cope. Further consideration of the behaviour of SUDS under exceedence

conditions is required to ensure safe systems.

5.2.4 Incentivisation Existing arrangements for charging for stormwater runoff and management are such that the

real cost of this is not transferred to the user of the service. Recent attempts by sewerage

undertakers to charge by impervious area connected, whilst supported by Ofwat, have met

with considerable public resistance. The gradual ramping up of charges over time is to be

welcomed and may provide an incentive, if appropriate reductions in these charges can be

offered, to encourage the take ‐up of disconnection options. Current rebates of circa £30 per

household disconnected do not apply to partial disconnection and are in any case too trivial for the householder to wish to take on the personal responsibility for their own drainage.

Proper charging for highway drainage should be introduced so that this can be used to

incentivise the disconnection of this source from the public sewerage system.

5.2.5 Practicalities of const ruct ion Retrofitting of any large scale measures in urban areas is always problematic. The location of

and need to move underground and buried services can be costly and at times almost

impossible unless at great cost. This is a location specific problem and may if found to be too

costly, completely prohibit an otherwise effective and practicable retrofit.

As well

as

the

usual

problems

of

complying

with

environmental

legislation

and

requirements

in constructing anything, several of the options proposed in the present study entail digging

up and working in roads. This will lead to traffic disruption, noise control problems and other

environmental difficulties. When finalising any retrofit options these problems will need to be

accounted for.

5.2.6 Acceptability and awareness As it is proposed to transfer the responsibility for stormwater management away from TW, it

will be important to engage early on in the planning of this with the key stakeholders in order

to get their agreement. By early engagement, rather than attempts to impose changes, it is

more likely that a consensual approach can be developed.

It will be necessary to engage not only those directly affected (i.e. those whose drainage is to be disconnected) but the wider community. Most of the SUDS proposed here are surface



features and hence will affect the entire neighbourhood and local authority aspirations and

activities in relation to streetscapes. Ideally the SUDS should be seen as an opportunity to

enhance neighbourhoods as part of the new ‘green infrastructure’ initiatives and it may be

necessary to work with local planners, landscape architects and others to take advantage of this.



6. Catchment scale disconnection strategies

This section explains the the disconnection strategies that were initially determined and modelled by

the London Tideway Tunnels Delivery Team (Annex 1), and explains how these have subsequently been refined (Section 10).

6.1 Initial Approach – ANNEX 1 disconnection scenarios This section describes the methodology that was initially adopted to identify potential retrofit SUDS

options, to select preferred retrofit SUDS options and, ultimately, to provide input data for

InfoWorks to enable the hydraulic impacts of proposed disconnections to be modelled. OS

MasterMap data was utilized to associate potential retrofit SUDS options with particular areas of land. Note that the anticipated use of Department of Communities and Local Government GLUD

(Generalised Land Use Database) data, which would have provided additional information about

Land Use

characteristics

than

can

be

gathered

from

OS

Mastermap

data

alone,

was

rejected

at

an

early stage due to software stability and data incompatibility issues. However, given the widespread

use of MasterMap data, the approach developed is expected to be more generic and transferable.

6.2 Land use types and suitability for retrofit In view of the innovative nature of this study, two alternative approaches were initially considered:

1. Assignment of ‘typical’ disconnection profiles to a set of zonal land ‐use types, such as ‘high‐

density terraced housing’, ‘detached houses with large gardens’, ‘municipal medium ‐rise

housing blocks’, etc.

2. Using the land ‐use categories within OS MasterMap, combined with logical SQL queries, to

determine suitability for a range of retrofit SUDS options.

6.3 Method 1 – ‘typical’ disconnection profiles by land -use types The disconnection strategies were initially evaluated by first identifying generic land ‐use types, then

identifying the potential retrofit SUDS options associated with each type. This was done at a

detailed level for two of the subareas within the Frogmore (Buckhold Road) subcatchment:

24743902 and 23749901. The remaining subareas were then classified according to the generic

land ‐use types, and average disconnection opportunity profiles applied. However, this data is not

presented here, as the approach has now been superseded. Two limitations of this approach led to

it being abandoned in favour of the second option:

It was not as straightforward as initially expected to derive classification criteria, and

significant differences were observed between the characteristics of areas subjectively

placed within the same nominal (land use and building type) category. This meant that the

overall distribution of disconnection options by zonal land ‐use type was not as consistent as

might have been hoped for. Five zonal land ‐use types common over the two sub ‐

catchments are compared in Figure 6.1. In most cases there is a reasonable correlation, but

not always. Differences for categories 1 and 2 in (High‐Density, Low‐Rise apartments), and

for category 2 (pervious) in (Flat roofed apartments in grounds) and (Pitched roofed

apartments in grounds) are significant, and therefore raise potential

uncertainties/inconsistencies in the interpolation technique.

The process of classifying areas into the generic land ‐use types is manual, time ‐consuming

and ultimately somewhat subjective.



Hign‐Density, Low‐Rise Apartments

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4 5

Sub ‐catchment 1Sub ‐catchment 2Mean

Mixed Apartments

0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5


Flat Roofed Apartments in Grounds

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2 3 4 5


Pitched ‐Roofed apartments in grounds

0

0.1

0.2

0.3

0.4

0.5

0.6

1 2 3 4 5


High‐Density, Low‐rise apartments

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1 2 3 4 5


1. Impervious

2. Pervious

3. Impervious with initial losses

4. Complete removal

5. Impervious with storage

Figure 6‐1 Comparison of drainage/disconnection land ‐use categories by land ‐use category for Sub ‐catchments 1 & 2



6.4 Method 2 – Land -use categories within OS MasterMap In the second approach, the applicability of a retrofit SUDS technique was determined by physical,

mapped, characteristics of the underlying surface and local topography. For example, hydraulically

effective retrofit SUDS options for roofs are governed by factors such as the pitch of the roof, and

the presence of garden/greenspace. A series of logical SQL queries have therefore been determined

that select areas of land that are suitable for each retrofit option. The main advantage of this

approach is that it is consistent, replicable and less time consuming than the original approach, and

can be readily applied to existing data with minimal preparation. In addition, the SQL queries can

easily be linked to other GIS layers, such as geology, or topography; allowing the method to be

further refined in the future.

OS MasterMap data are classified into 3 main themes: Roads Tracks and Paths (which includes roads,

pavements, paths/tracks); Buildings; and Land (which is subdivided into ‘manmade’, ‘natural’ and

‘mixed permeability’ – predominantly gardens). There is also an ‘other’ category, giving rise to six

land ‐use types in total.

Table 6.1 highlights the options that were considered potentially feasible for retrofit to each of the three predominantly impermeable land ‐use types (i.e. Roads, Buildings and Manmade Land). These

options are described and illustrated more fully in Appendix A and B and also introduced in Section

3.2.1. For each potential disconnection option, relevant SQL queries were generated. These are

itemised in Table 6.1. In some cases additional manual digitization was required; this is also

highlighted in Table 6.1. Detailed information on how suitabilities were evaluated using

MasterMap/SQL may be found in Appendix B. It should be noted that the novelty of the SQL‐based

approach in this context means that there remains considerable scope to refine it, and the following

limitations should be acknowledged:

There is a need to be careful not to ‘disconnect’ too many impermeable areas to the same

area of adjacent land – currently this is checked for manually;

This uses a fairly crude selection of buildings/gardens – more work could be done to increase

the reliability of the selection;

Ideally the assessment would include other datasets, such as geology or topography, to

produce more reliable and practicable representations of the likely disconnection options;

Some options, deemed unlikely for application within this study, have not had the relevant

SQLs developed.

This process generated layers indicating where each option might be feasible. Given that – in some

locations – more than one option might be possible, it was necessary to rank the options according

to the set of preferences given in Section 3.2.1.



44

Table 6‐1 Retrofit SUDS options, SQL queries and Hydraulic preferences

Surface type Retrofit SUDS options

SQL Hydraulic Modelling

Preference rank

Notes

Roads Pocket street

infiltration

Google

Streetview to

assess road

width and

parking

provision,

manual

digitisation

c) 12 mm 3 Assumed small areas of

permeable compared with

drained area

To adjacent

pervious/SEA Streets

Within 2 m of

natural land

(>100 m2

contiguous

area)

b) 2

Permeable road

surface

All roads and

tracks judged

eligible

c) 25 mm &

d)

1 Assumed engineered

surfaces with sub ‐surface

storage. No natural

infiltration.

Off ‐site – local

detention and swale

conveyance

d) 4 Conveyance likely to be

difficult

Man ‐made land (inc.

car parks and

hardstanding)

Permeable surface

storage

> 200m 2 c) & d) 1

Adjacent pervious With 1 m of

natural land

b) 2

Off ‐site e.g. local

detention

d) 3

Man ‐made surfaces

other than above

Adjacent pervious b) 1

Roofs Green/blue Aerial

photography

to identify

flat roofs –

manual

digitization

c) 25 mm 4

Soakaways a) 1

Disconnect to lawn

(Classified as mixed

permeability)

b) 2

Water butts/RWH

Where there is

adjacent green space

or hard standing to

site them

c) 25 mm 3 Needs oversized storage to

ensure performance

(Kellagher & Franco, 2005;

Kellagher et al, 2008)



45

As described in Section 3.2.1, the initial selection of the most preferred option was to choose the

option likely to deliver the best hydraulic performance, i.e. remove, retain or detain stormwater

flows most effectively. Four different hydraulic mechanisms were considered, and these were

initially ranked in preference order as follows:

a) Complete removal – 1st




Preference between b) and c) depends on local infiltration (pervious) characteristics, level of initial

losses and on the precise performance criteria being assessed. If there is poor infiltration, compared

with potentially good initial losses, then c) is preferable to b).

Modelling is then required to determine the ‘best’ option in terms of the overall benefits. In some

cases two hydraulic options are expected to result (see Table 6.2), giving rise to a complete list of six

hydraulic modelling options for disconnection scenarios.

Table 6‐2 Hydraulic modelling options for retrofit SUDS

1 Impervious with Initial Losses (12mm)


3 Complete removal

4 Impervious area with storage/attenuation5 Transfer to pervious

6 Initial Losses and Storage/attenuation

6.5

Assignment of

Impermeable/Pervious

Areas

Specific areas to be disconnected were initially determined using the SQL queries. This generated a

set of data for each InfoWorks node, comprising the six MasterMap land ‐use categories, plus up to

six alternative InfoWorks SUDS hydraulic modelling recommendations. For all areas not deemed

suitable for disconnection, the distributions of impermeable/pervious surface shown in Table 6.3

were assumed.

Table 6‐3 Distribution of impermeable area by MasterMap land ‐use category

MasterMap land ‐use Percentage

Impermeable

Roads Tracks and Paths 70% Buildings 100%

Manmade Land 90%

Natural Land 15%

Gardens 50%

Other 100%

The disconnection scenarios were therefore presented in terms of % impermeable, % pervious, or

one of the six SUDS hydraulic model types indicated in Table 6.2. Table 6.1 highlights the complete

range of disconnection options that have been evaluated for each of the MasterMap land ‐use categories. It also explains how each of these physical options may be modelled (i.e. through a



46

combination of initial losses, transfer to permeable or storage/attenuation). In practice, three of the

categories that could arise as outcomes from the preference hierarchy were not initially used:

Impervious with 12 mm initial losses (i.e. SEA Streets) – the same or better performance can

be achieved by using pervious streets;

Complete removal (i.e. soakaways) – infiltration capacity is generally expected to be poor; Storage/attenuation (e.g. regional detention basin or pond) – lowest in the preference order

as considered as an end ‐of ‐system option of last resort, requiring significant land ‐take

(although see Section 8).

Preliminary modelling (Annex 1) highlighted a problem with the initial disconnection assessments, in

that these were formulated with reference to the (independent) impermeable/permeable

categorization, rather than the types of catchment area used in the calibrated InfoWorks model.

This mismatch resulted in a systematic overestimation of total impermeable area; with the result

that the disconnection strategies generated total land ‐use distributions that did not differ

significantly from the original surface areas. Model runs using these data generated performance

improvements which appeared somewhat marginal, and certainly less impressive than the 25% disconnection found in the global scenarios. This did not provide a fair assessment of the potential

benefits associated with disconnection and was therefore revised for subsequent analysis.

The disconnection area data submitted for subsequent modelling were therefore presented

differently. In this case the disconnection options were calculated as a percentage of the

impermeable area (as calculated from the MasterMap land ‐use data in combination with the

assumed percentage impermeabilities shown in Table 6.4). These were then used to redistribute the

impermeable area for each InfoWorks node into the appropriate impermeable, pervious or newly ‐

defined surface types.

6.6

Results

Table 6.4 summarises the disconnection options suggested for the whole of the Frogmore (Buckhold

Road), West Putney and Putney Bridge subcatchments respectively. Each value is expressed as a

percentage of the existing impervious area within the InfoWorks model.



47

Table 6‐4 Disconnection scenarios for InfoWorks modelling a) Frogmore (Buckhold Road)

Node ID

Disconnection option

2 5 7 4 0 1 0 2

2 4 7 4 9 5 0 1

2 4 7 4 3 9 0 2

2 4 7 3 6 2 9 1

2 3 7 4 9 9 0 1

2 3 7 4 1 1 0 1

2 3 7 3 2 7 0 2

2 3 7 3 1 8 0 1

2 2 7 4 9 8 0 2

Remains Impervious 27.44 17.13 22.43 22.73 20.39 43.87 18.33 30.28 31.16

Impervious with 25mm initial loss 9.56 13.69 8.50 9.34 11.91 0.54 3.72 8.41 6.42

Transfer to Pervious 20.93 17.72 24.14 21.40 21.88 2.96 36.96 25.56 19.33

25mm initial losses with Storage/attenuation 42.07 51.45 44.93 46.53 45.82 52.63 40.99 35.75 43.08

b) West Putney

Node ID

Disconnection option

2 4 7 5 1 4 5 1 a

2 1 7 2 9 6 0 1

2 1 7 5 7 4 5 2

2 1 7 4 5 7 0 1

2 4 7 5 1 4 5 1 b

2 1 7 4 3 2 0 1

2 1 7 4 3 0 0 1

Remains Impervious 26.42 21.58 25.48 49.70 29.57 13.70 82.36

Impervious with 25mm initial loss 4.52 14.78 21.86 1.26 3.78 18.01 1.03

Transfer to Pervious 27.62 9.11 4.37 14.57 26.79 14.55 0.78

25mm initial losses with Storage/attenuation 41.45 54.53 48.28 34.47 39.86 53.74 15.84

c) Putney Bridge

Node ID

Disconnection option 24750701 2475145d 2475145c 24751402

Remains Impervious 23.01 15.51 26.27 11.69

Impervious with 25mm initial loss 18.90 25.41 2.68 19.85

Transfer to Pervious 18.04 24.50 34.11 21.53

25mm initial losses with Storage/attenuation 40.04 34.57 36.93 46.93

The modelling of the London Tideway Tunnels catchment using these data and proportionate

disconnections of 25% and 50% of the impervious contributing areas are reported in ANNEX 1 and

discussed further in Section 10. These results represent a first stage analysis of the potential for

retrofit to remove or reduce the stormwater runoff and the CSO spills. Subsequent analysis as

outlined in the following sections has been used to evaluate in more detail the practical levels of disconnection that may be possible.



48

7. Cost - benefit assessment of detailed retrofi t designs

7.1

Calculation of

Whole

Life

Costs

The SUDS manual (CIRIA, 2007) recommends the use of a Whole Life Cost (WLC) approach to

evaluating SUDS in order that ongoing costs are taken into account at the planning and feasibility

stage. This approach incorporates the present day value of the scheme and the costs associated with

its operation over its expected useful life. The benefits that are likely to accrue over the lifetime of the scheme may not be apparent at installation, therefore a long ‐term view is required. The SUDS

manual lists the benefits of the WLC approach as giving:

• improved understanding of long ‐term investment requirements, in addition to capital costs;

• more robust decision making at project appraisal stage;

• improved assessment of long term risks to drainage system performance and inclusion of

monitoring and management plans to minimise these risks; and

• reduced uncertainties associated with the development of adoption agreements, and

commuted sum contributions.

There are limited sources of reliable data for UK applications that deal with all aspects of the costs of new ‐build SUDS and there are even fewer dealing with retrofitting. Certain studies include direct

costs (e.g. Swan, 2003, Swan & Stovin, 2007), and UKWIR investigations provide comprehensive

attempts to look at Whole Life costs and benefits (WERF/UKWIR, 2005, UKWIR, 2005; UKWIR, 2009).

The 2005 studies have been regularly updated since by HR Wallingford. The 2009 study is intended

to help inform sewerage undertakers about the whole ‐life costs and benefits of stormwater

disconnection especially from combined sewerage systems. In this report, the updated HR

Wallingford methodology has been utilised for the detailed analysis and the results have then been

reviewed in the light of the UKWIR (2009) study.

The UKWIR/WERF (2005) WLC assessment as adapted by HR Wallingford (UKWIR, 2006a) is a model

based approach using spreadsheet tools that allow for the systematic and consistent identification of capital costs and on ‐going expenditure to estimate the whole life costs of individual components of SUDS‐based drainage systems. The 2006 study made attempts to quantify the likely implications of future waste management strategies. The model has two complementary modes of application: site

specific and generic application. It includes separate unit models for:

retention ponds;

detention basins;

swales;

filter drain/trenches, and

permeable pavements.

These models have been matched to the options selected for the study in Table 7.1 (from Stovin et

al, 2009).

The hydrological/hydraulic analysis is simple and uses look ‐up tables. As an example, it uses the

information given in Table 7.1 for detention basin design and costing.



49

Table 7‐1 Outline of hydrologic/hydraulic parameters used in UKWIR/WERF WLC model

Catchment area type

R"‐residential; "C"‐

Commercial; "Ro"‐Roads;

"I"‐Industrial

Parameters for

detention pond

Catchment sediment yield Outlet throttle rate 3/5/10 l/s

Impermeability %

Hydrological parameters

WRAP and SOIL Climate change Volume increase

of 40%

FEH Rainfall

Return period (1/10/30/100)

The detention volumes calculated by the model are given in a look ‐up ‐table and have been

estimated from a simplified approach using a 1‐node InfoWorks CS urban drainage model with a

range of typical rainfall characteristics across the UK and by producing correlation equations to fit

the results. Details of the WLC assessment model and its data sources are given in Appendix C.

The model includes a range of benefits, some of them intangible and ranked by preference, giving a

subjective view of environmental and amenity benefits for example, but a clear distinction between

environmental and social is not made (re costs and benefits). In fact some of what are generally

regarded as ‘social’ are included with ‘environmental’ costs and benefits.

The more recent report (UKWIR, 2009, in conjunction with Cascade Consulting) provides a more

comprehensive mechanism for identifying criteria that produce a positive cost benefit for

stakeholders and encourages stakeholder engagement in the decision making and trade ‐off process.

Optioneering can include comparison of schemes based on a range of policies and obligations such

as environmental and social benefit as well as carbon footprint and cost ‐benefit.

Overall the limitations and assumptions used in the present study in relation to the WERF/UKWIR model may be summarised as:

The hydraulic design in the model is very simple and suitable only for an initial assessment or

feasibility study as undertaken here. If more robust cost estimates are required to reduce

uncertainty, generic simple models, representative of Thames Tideway pilot conditions

should be developed and the look ‐up tables improved accordingly.

The design of the units is restricted to particular sizes and layouts as originally available for

the data base and therefore the costs may not be aligned with the actual sizes and

configurations used to fit into the sites examined.

The cost database is limited by the available accessible data, which is limited especially for

UK conditions.

Construction overhead costs are included as 15% of the capital costs.

The discount rate is taken as 3.5%.

Inflation factor used to scale from 2002 prices is 1.25.

The design life is taken as 50 years.

In the present study capital and other costs have been scaled up from a number of ‘average’

units designed using the model directly and based on the required storage volume or unit

plan area



50

Not all of the elements of the associated infrastructure for each designed unit have been

costed in detail; for example, associated and connecting pipework has been costed based on

aggregated estimates and ‘averaged’ costs from the design of a few of the units

The potential (and relative scaled) environmental and ecological benefit costs provided in

the WERF/UKWIR model have not been used in the assessment, due to their potential

subjectivity, nor has the associated pollution costs or flood damage prevention costs. The benefit cost approach is similar in both HRW and the Cascade models in terms of needing an

intangible ‘unit’ benefit value (Cascade uses a willingness to pay value from the most recent

public surveys) which is scaled up by numbers affected. The values suggested in the HRW

model are those available at the time (in 2003) and are therefore out of date. Both models

require site ‐specific data; otherwise the numbers are of limited value, being indicative only.

Where appropriate, for certain units, the cost of sediment removal and associated

maintenance has also been included.

The analysis in this study draws on both approaches in order to provide a robust and

comprehensive view of the whole life costs and potential benefits of the SUDS schemes

proposed.

7.2 SUDS Unit costs A review of published unit costs of SUDS was undertaken as part of the costing process. This relies

heavily on the HR Wallingford whole life costing model (Appendix C) as shown in Table 7.2. The

costings for the SUDS used are explained in the following Sections.



51

Table 7‐2 SUDS unit costs

Surface type

Primary options

Swan costs (construction)

new rather

than retrofit (2002 prices factored 1.25)

Wallingford costs

Costs used in the present study/comments

Performance and

HR

Wallingford model tool

Roads Pocket streetinfiltration

Infiltrationtrench.Inc.membraneand distributorpipework. £92-£125 per mlength.

WLC: £229/m 2 ofconstructed unit(7 x 5m) £29/m 2 of road surfacedrained(constructioncosts £7.6/m 2 &£112/m 2

respectively)Does not includecosts of roadreinstatement orservicedisruption

£225/m 2 (£112construction cost)of unit areaconstructed (7 x5m x0.9m depth).and £29/m 2 (£7.6/m 2 construction) ofroad area drained.Not used so far inUK

Infiltrationtrench and/orDetentionbasin (normallydry).

To adjacentpervious/SEAStreets

Assume as forpermeable roadsurface, but unitsmay have larger

plan area andsmaller depthmore likedetention basins(see below).

Costs ofredirecting roadand other localdrainage depend

on localcircumstances. Allow WLC of £121per m 2 asequivalent topermeable pavingfor roads below.

Detentionbasin and/orfilter drain,although may

be flat grassedarea

Permeableroad surface

Mediumtrafficked: WLC£121/m 2 and£110/m 2 capital

cost.Potentially usepermeablepavement withhigh loading(5000kg)£130/m 2 and£118/m 2.These do notinclude servicerelocation costs

Not consideredfeasible in shortterm in UK exceptfor certain non-

main or trunkroads. Mediumtrafficked: WLC£121/m 2 and£110/m 2 capitalcost.Potentially usepermeablepavement withhigh loading(5000kg) £130/m 2 and £118/m 2.

Permeablepavement(without liner)



52

Surface type

Primary options


new rather than retrofit (2002 prices factored 1.25)

Wallingford costs


Performance and

HR Wallingford model tool

Roads Off-site –localdetention andswaleconveyance

Swales: inc.costs of localconnections.£22 - £25 inc.reinstatement

Swale asconnections -costs £21-£25/m.

Swale WLC,includingpipework :£247/m length or£40 -£70/m 2

SUDS manual:Swale 15 yeardesign life. Capex- £12.50/m 2.Regular Opex -£0.1/m 2.Occasional Opex -£0.15/m 2.Remedial Opex -£2.0/m 2 Monitoring -£0.05/m 2

Swale,detention basinor retentionponddepending onlocalcircumstances

Non-roadhardstanding(inc. carparks)Contiguousareas ofman-madesurfaces>200m 2

Permeablesurfacestorage

Grasscrete:£78/m 2.

WLC: £121/m 2 based onformpave (2009)data and lightaxle load(2000kg)

Formpave suggest£100-£1000 peryear formaintenance.SUDS manual(2006) for 40 yearlife: Capex £54/m 2 Opex £0.4/m 2.£168/m 2 Scott-Wilson Interpave

SUDS manual :0.8m 3 storedper m 2 ofpervioussurface.Permeablepavement(without liner)

Adjacentpervious

Costs ofredirecting roadand other localdrainage dependon localcircumstances

Detentionbasin and/orfilter drain,although maybe flat grassedarea

Off-site e.g.localdetention

Ponds: £44-£70/m 3 unlined butincludingpipework.

WLC modelgives £39/m 3 fordetention basinwithout pipeworkand with liner

Swale,detention basinor retentionponddepending onlocalcircumstances

Man-madesurfacesother thanabove

Adjacentpervious

Costs ofredirecting roadand other localdrainage dependon localcircumstances –take as the samefor disconnectingroof downpipes£160 per 65m 2 i.e.£2.50 per m 2.




53

Surface type

Primary options


new rather than retrofit (2002 prices factored 1.25)

Wallingford costs


Performance and

HR Wallingford model tool

Roofs Green/blue £150/m 2 greenroof capital costs

Blue roofs notincluded.

Soakaways Soakaway3.6m 2, 9m 3:£560-£690.Infiltrationtrench. Inc.membraneand distributorpipework. £92-

£125 per mlength.

See designdetails fromUKWIR/WERFmodel

Filterdrain/trench

Disconnect tolawn(Classified asmixedpermeability)

£160/ 65m 2 of roofsurface (from costof componentsfrom DIY store)and assumed asone 75mmdownpipe per65m 2 maximumroof surface(Building

Regulations, 2000states that gutterlimited to this). EN12056 (part 3)gives maximumcapacity as 2.2l/sfor 75mmdownpipe. For therainfall required,0.016l/s/m 2 (Building Regs)this give amaximum roof

area of 138 m2

.Lower value takenof 65m 2.


Waterbutts/RWHWhere thereis adjacentgreen spaceor hardstanding

0.3m 3 buttplusinstallationand feederpipe.£125-£302

Downpipedisconnectionincluded in abovecosts. If waterbutts are usedthese will be anadd-on. Allow £50per butt.

Not included inbaseline costs.

Pipework costs £218/m



54

7.2.1 Pocket raingardensDesign using the HR Wallingford cost models (Appendix C) has utilised a combination of analyses.

The hydraulics were assessed using the detention basin model with a maximum outflow of 5l/s.

Design costings were made with the swale model assuming unit side slopes of 1:2. Other assumptions were: Inlet structure ‐ assumed total cost of approx £1250 allowing for kerb

adjustments and some sort of inlet sediment management (arrest) system. The construction

comprises 0.15m of soil, 0.45m of gravel and 0.1m of sand with 0.2m of freeboard. Whilst not being

necessary for hydraulic attenuation (an open storage volume would have more volume), these layers

are needed for plants and will also provide water quality improvements. The small size of the units

may be outside the data base costs but there are no other data available. There are no costs

included for traffic management during construction. Nor land take costs, risk costs, damage costs,

VAT or a wide range of benefits included.

Maintenance: assumed vegetation is 'managed' every 6 months. Regular maintenance costs

estimated at between £350 and £450 a year ‐ which may be high. Irregular maintenance includes

£150 every 5 years for sediment management. The details of the whole life costs for the units used

in the designs in Section 8 are given in Table 7.3.

Table 7‐3 Pocket raingarden whole life costings

Length Width Outflow Abovegrounddepth

Soildepth

Sanddepth

Graveldepth

Totaldepth

CapitalCost

O&M(Whole

Life,PV)

WholeLife, PV

m m litre/s m m m m m £ £ £7 5 5 0.5 0.15 0.1 0.15 0.9 3,942 10,997 14,939

7.2.2 Permeable road surfacesThere are two versions of this, depending on axle loading. For the lighter commercial vehicles

(2000kg) assuming silty clay, with CBR value of 4, and water table not within 600mm of formation

level. There are 2 versions of each model. The original versions (Version 1 in Table 7.4) are based on

using the Scott Wilson datasets on pavement maintenance done for Interpave ‐ which seem high.

The version 2 models are using some more recent, simple annual O&M costs from Formpave (£1000

pa for 2000 m2 pavements reducing to £100 pa for 50 m2 pavements for bulk operation) and these

have been used in the study in preference. The costs are £121 whole life, with capital costs of £110/m 2 of drained impervious paved surface for medium, commercial road traffic. With the heavier

industrial type road traffic the costs increase to £130 and £118 per m2 respectively.



55

Table 7‐4 Permeable pavement whole life costing for lighter vehicle loading

Length Width Area Subbasedepth for

attenuation

Subbasedepth

forloading

CapitalCost

VERSION 1 VERSION 2

O&M(WholeLife, PV)

WholeLife, PV

O&M(WholeLife, PV)

WholeLife, PV

m m m 2 m m £ £ £ £ £

66 22 1452 155 468 165,445 83,389 248,834 17,114 182,559

50 20 1000 155 468 129,935 57,405 187,340 12,813 142,748

25 20 500 155 468 70,061 28,715 98,776 6,494 76,554

20 10 200 155 468 34,226 11,486 45,712 2,111 36,337

10 10 100 155 468 22,071 5,743 27,814 2,160 24,232

10 5 50 155 468 15,237 2,872 18,109 2,148 17,385

6 2 12 155 468 10,552 689 11,241 2,138 12,691

The unit costs for the heavier vehicles with 5000kg axle loading have also been determined using the

same model.

7.2.3 Detention basinsThe costings for these include inlet and outlet structures and maintenance. Table 7.5 illustrates the

relative breakdown of costs.

Table 7‐5 Whole life costing of detention basins

Present Value (£) 8 9 10 11

Basin Capital RegO&M

CorrO&M Waste Total

A: 55m x 15m 22,100 20,135 5,573 6,355 54,163B: 22m x 20m 17,497 17,809 4,077 6,256 45,640C: 55m x 25m 27,463 22,749 7,476 6,256 63,944

Total 163,748

7.2.4 SwalesThe costings shown in Table 7.6, from the HR Wallingford model also include sections of pipework

interconnecting these conveyance swales. The swales have side slopes of 1:4.

Table 7‐6 Whole life costing of swales

Length(m)

top w(m)

Depth(m) Capital

Present Value (£) RegO&M Total

40 7 0.6 11,996 1,507 13,53750 7 0.6 11,688 1,634 13,26610 7 0.6 3,928 1,125 4,72610 7 0.6 3,928 1,125 4,726



56

7.2.5 Green roofsThese are not included in the HR Wallingford model and the costs have been provided by reviewing

a number of sources. The Green Roof Centre estimate costs of £60 ‐100/m 2 for extensive (shallow)

green roof systems. However, these are generic costs, not specifically retrofit, therefore likely to be

low. Alumasc – one of the UK’s main suppliers for retrofitting suggest that costs could range from

£50/m 2 up to £150/m 2 depending on whether the old roof needed to be stripped back and re ‐waterproofed or whether the existing roof was in sufficiently good condition to simply overlay.

Data from Lambeth council, which is based on real implementation case studies in London, suggests

between £120 ‐180/m 2. The Ethelred estate, Kennington installed in 2005; Bauder extensive system

(4000 m2) cost £716,000 or £179/m 2. Portland Grove, 961 m2 – sedum roof £94,673 + scaffolding

£20,300 = £120/m 2.

Lambeth have also conducted an independent study as part of their Decent Homes Programme,

where they estimated that it would generally add 13% (60 yr whole ‐life costs) to incorporate green

roofs compared to non ‐green roof. Hence the estimated cost was taken a £150/m 2 for retrofitting

an extensive green roof. Note that this is capital cost only.

7.2.6 Roof downspout disconnectionThese costs were based on DIY supplied components as illustrated in Table 7.7.

Table 7‐7 Capital costs for roof downspout disconnection with some maintenance allowance

Cost components units Unit cost

Rain water goods Straight pipe

Bend

£10

£8

Benching Precast concrete (similar to

Portland) Gravel

£10 (bulk)

£3

Labour 2 hours £100

Minor maintenance (30

years)

estimated costs £25

Total £156 – round up to £160

Assume 1 downspout per property up to 65m 2 then 1 downspout per 65m 2.

The above do not include costs for blocking off the existing downpipe connection to the existing

drain or sewer as these costs will be entirely site dependent. It is recommended, however, that the

connecting pipe is completely and fully grouted up to the connection point. This is for two reasons: to discourage subsequent reconnection; and also to prevent rodent infestation (UKWIR, 2000).



57

8. Using exemplar designs to evaluate the disconnectionoptions and cost-benefits for the three subcatchments

Following the initial evaluation of the potential benefits of retrofitting SUDS in terms of reductions in CSO spills (Section 6) based on GIS assessments of what might be feasible to disconnect, detailed

design and costings have been carried out for 3 example areas in order to test the feasibility in more

detail. These areas were identified as being appropriate for detailed study during a site visit to the 3

subcatchments on 21 st May 2009. Appendix E provides details of the designs and costings and

Appendix F has photographs of the areas concerned.

It was considered important to evaluate the options for SUDS designs and their costs for different

types of area. The site visit revealed that there were many green spaces in the outer parts of the

subcatchments and a lot of these were associated with what appeared to be municipally owned

estates. In addition, there were many low rise flat roofed apartment blocks, often in areas with

substantial gardens. Many of the roads in the area were also wide enough for either pocket rain

gardens or even SEA streets although it was assumed that the latter are not going to be viable in the

near future.

Three separate subareas were selected that were typical of these types of area: An area draining

down to Lytton Grove (Figure E.1); the area surrounding Carlton Drive (Figure E.17) and the road

itself in Chartfield Avenue (Figure E. 29).

The approach to selection of which retrofit options were appropriate was based on expert

judgement as there was not sufficient time to evaluate the relative performance of a number of alternatives. Hence the overall costs relate only to the particular options selected and should be

further tested should retrofit options be deemed worthwhile in future. The options selected were

also those which were considered to be the most viable in the short term. This meant that the originally proposed conversion of many of the roads to permeable roads (Section 6), was revisited

and has resulted in considerable reductions in the size of the road surfaces considered viable for

this. This option is still considered potentially viable but in the much longer term.

8.1 Lytton Grove subarea (Appendix E.1) Appendix E.1 provides full details of the approach to and design and costing of the retrofit

disconnection options for this area. The analysis has been based on the Mastermap data and the

single site visit and is therefore subject to error and clarification. The type of area investigated in the

first part of this detailed study is illustrated in Figure 8.1.



58

Figure 8‐1 Example of the extensive green areas in municipal housing in the Lytton Grove area that could accommodate detention basins

Only the conclusions from the detailed study (Appendix E.1) are summarised here:

Detention basins were considered viable in part of the area that consisted of considerable

existing green space and hence sufficient room. These provided an exemplar for what could

be achieved in other parts of the 3 subcatchments for these types of area. Therefore this

was used in the scaling up (Section 9).

These areas were also local authority managed and hence retrofitting would require

negotiations with only one landowner/manager and could therefore be expected to be

potentially implementable in the near future.

As there was some uncertainty about the precise extent of the impervious areas as these are

not all given in the MasterMap output, only the roofs and car parking/main paved areas

were disconnected. Overall this resulted in some 80% of the (presumed) original impervious

area of 67387m2

being disconnected and attenuated through 3 detention basins at a whole life and direct construction cost of £684,700. This gives a unit cost of £12.7/m 2 per unit of municipal housing disconnected (attenuated).

Figure 8‐2 Example of paved hard standing in the Lytton Grove area that could be converted to permeable pavement

In the rest of the subarea of Lytton Grove, as illustrated in Figure 8.2, a variety of options were used

(Table E.8):



59

Roof disconnections to adjacent garden areas for the rear roofs of properties only not for

the front roof areas. These would require engagement with individual property owners.

Permeable paved areas converted from impervious surfaces (one for one), mainly in use for

car parking and hard standing. Also for certain local roads, including both medium weight

commercial and heavier industrial vehicles.

These options require agreement of local land and property owners of which there would be

several 100 ‐1000s. Although some of these are larger institutions, such as hospitals and

colleges. Under current legislation, with the right to connect to public sewers, any

disconnections made could be reversed in future by new owners/occupiers unless specific

covenants are placed on the properties.

Overall, this detailed study was able to disconnect some 40% of the original 260591 m2 impervious

area with the measures proposed. This was believed to be a realistic application of retrofit SUDS in

the area. It would be possible to disconnect additional areas, but this would require further and

property specific investigations. Overall the whole life costs were estimated at £7,249,100, with an

additional construction cost only of £84,320, giving an overall unit cost of £58/m 2 of the impervious

area disconnected.

8.2 Carlton Drive Subarea (Appendix E.2) Appendix E.2 gives full details of the approach used and the designs and costings. This area was

selected as it has a large number of flat roofed apartment blocks and a wide road without any traffic

calming measures, as shown in Figure 8.3.

Figure 8‐3 Carlton Drive showing flat roofed apartment blocks and wide road

The analysis has been based on the Mastermap data and the single site visit and is therefore subject to error and clarification. Only the conclusions are summarised here:

The extensive flat roofed apartment blocks, set within their own grounds makes the

utilisation of green roofs an obvious option, with overflows to the surrounding gardens.

There are substantial hard standing and car park areas and drives that could readily be

converted to permeable paving. There may be opportunity to take additional inflows into

these from adjacent properties following a more rigorous analysis using a dynamic

simulation model, but this was not considered in this first stage analysis.

The single and terraced properties are typically substantial in size with extensive

surrounding areas that are either gardens or paved parking. It should be feasible to



60

disconnect at least 50% of the pitched roof areas to the surrounding areas and convert the

hard standing to permeable surfaces.

The road width makes the implementation of pocket rain gardens feasible and would assist

with traffic calming.

As for Lytton Grove above, implementation would require agreement of local land and

property owners, many of these being landlords and of which there would be several 100s.

Inducements would need to be devised to make this attractive. Under current legislation,

with the right to connect to public sewers, any disconnections made could be reversed in

future by new owners/occupiers unless specific covenants are placed on the properties.

Construction of the rain gardens would have to be agreed with the local authority (as roads

authority) but could be promoted as contributing to the greening of the area and the

fulfilment of key sustainability indicators as well as mitigating climate change. Property

owners who currently park in the street (Figure 8.3) may not be happy to see parking

restricted due to the loss of kerbside space.

Overall the retrofits in this area would potentially remove some 27954m 2 or 56% of the existing

impervious area at a whole life cost of £3,690,600 and additional capital cost of £24,960; a unit cost

of £133/m 2 for the removal of impervious area.

In this area it may have been possible to design an alternative mix of retrofit SUDS systems, which

may have altered the distribution of costs, however, the options selected were those considered the

most feasible to implement other than the green roofs, which may not be feasible due to the need

to convince the various landlords of the value of green roofs.

8.3 Chartfield Avenue Subarea (Appendix E.3) Details of the design and costings for this area are given in Appendix E.3. This area was selected as

an example of a wide road that could readily be retrofitted with pocket raingardens, Figure 8.4. If these were more commonly used in the UK, this site would be amenable to the fitting of SEA streets,

although these are known to be very expensive (Seattle Municipality, 2009) they work extremely

well in terms of both quantity and quality management and are an amenity benefit.

Figure 8‐4 Chartfield Avenue illustrating the width of the road and potential to fit pocket raingardens

Chartfield Avenue is typical of many roads in the area (Appendix E.3) and hence the appropriate

retrofits for this site would also be applicable elsewhere. The analysis has been based on the

Mastermap data and the single site visit and is therefore subject to error and clarification. Only the

conclusions are summarised here:



61

Retrofitting pocket raingardens is technically viable and the design here has shown that

some 20 of these (plan size 7 x 5m) would be required over the roughly 1km length

investigated. The precise number and shape of unit is open to adjustment and it would be

expected that for a final design, these would be of varying size to suit the local

circumstances especially the presence of services to be avoided.

There may be opportunity to take additional inflows into the rain gardens from adjacent properties following a more rigorous analysis using a dynamic simulation model, however,

for this feasibility design, this was not included and the adjacent properties were considered

separately.

Only the back roof and hard standing drainage was considered disconnectable to the rear

lawn/gardens for the properties adjacent to the road. The front of the properties – roofs

were not disconnected.

As above, Construction of the rain gardens would have to be agreed with the local authority

(as roads authority) but could be promoted as contributing to the greening of the area and

the fulfilment of key sustainability indicators as well as mitigating climate change. Property

owners who currently park in the street (Figure 8.4) may not be happy to see parking

restricted due to the loss of kerbside space.

Overall the retrofits in this area would potentially remove some 26274m 2, 33% of the original

impervious area at a unit cost £8/m 2 of impervious surface drained with a whole life cost of £156,800 and additional capital cost of £39,100.

8.4 Summary of lessons from exemplar designs It is clearly technically feasible to implement a range of retrofit SUDS to disconnect stormwater in

the three subcatchments examined, based on the detailed investigations described in the subareas

above, although these measures are applicable mainly in the outer, less densely built ‐up areas, as

they require space. It is unlikely, for instance, that even roof downpipe disconnection is going to be viable in the denser housing areas such as northern Southfields. In the future, however, if there is

increasing water stress due to climate or demographic changes, RWH and the imaginative use of innovative RWH collection systems, such as that shown in Figure 4.12, it may even be applicable in

these areas. There may also be areas where green roofs are feasible and with time, the roads in

these areas could potentially be converted to permeable surfaces, providing also the benefit of traffic calming.

The range of costs and disconnection achievements per unit costs as determined above, illustrate

the diversity of options and costs in application of retrofit SUDS. Unlike piped/sewered systems,

where the criteria and design details are largely standardised (e.g. Sewers for Adoption, 6th Ed.),

even the SUDS manual (CIRIA, 2007) does not prescribe which systems should be used or are the most applicable under which circumstances. More experience and knowledge is needed especially in

UK application before it will be possible to define ‘a priori’ which retrofit options should be used

under what conditions. The examples presented above should be taken as just that – example, or

illustrations only, of what is possible, not the definitive and best designs for the applications.



62

9. Upscaling costs

The costings obtained from the examples in Section 8 have been used together with the unit costs in

Section 7, to evaluate the possible costs for retrofitting the 3 subcatchments West Putney, Putney

Bridge and Frogmore (Buckhold Road).

As there are a number of areas like the municipal housing area looked at in detail in the Lytton

Grove study (Section 8.1) and as it is expected that these areas are potentially among the easiest to

target for retrofit, these have been considered separately using the unit costs determined in Section

8.1.

A single lumped whole ‐life cost value of £ 12.7/m 2 (for roof, road or manmade land disconnections)

has been derived from (Section 8.1) (Lytton Grove areas 1 to 4, Appendix E.1) which suggests that a

disconnection of 53,882 m2 is achievable at a cost of some £684,700.

For the ‘remaining areas’, unit costs (£/m 2) as shown in Table 9.1 have been applied, from Table 7.2.

Table 9‐1 Unit costs of municipal housing areas other than roof, road or manmade land disconnections

Roads, Tracks and Paths Construction (£/m 2)

WLC (£/m 2)

Disconnect roads to adjacent pervious 110 121

Pocket infiltration 7.6 29

Pervious roads 118 130

Buildings Area of Flat roofs 150 150+

Disconnect roofs to lawns 160 160+

Water butts (all buildings, irrespective of size) 50 50+

Manmade land Manmade land greater than 200m ‐ pervious paving 110 121

Disconnect small manmade to adj land 2.5 2.5+

Table 9‐2 Ball‐park estimates for scheme costs ‐ all areas – nearest £1M

Cost element Putney bridge West Putney Frogmore

(Buckhold Road) Overall total

Construction (£M) 27 45 56 127

WLC (£M) 28 48 59 135



63

10. Disconnection scenarios for modelling purposes

The InfoWorks model runs initially carried out (Section 6) were based on global amounts of disconnection of impervious areas of 25% and 50% and also initial assessments of the potential

disconnection percentages in the 3 subcatchments: West Putney, Putney Bridge and Frogmore (Buckhold Road). These results were reported in Annex 1. Following the more detailed designs and

assessment of practicalities given in Section 8, the likely percentages of disconnected impervious

area could be revisited. This section reconsiders the potential amounts of disconnection in the 3

subcatchments and proposes percentages that are believed to be more realistic of what could

possibly be achievable in the short to medium term.

10.1 Initial results from the modelling of disconnection scenarios Report (Annex 1) summarises the InfoWorks modelling work undertaken by the London Thames

Delivery Team so far to evaluate the potential improvements to CSO discharges associated with

various retrofit

SUDS

strategies.

The modelling work was divided into three phases. The initial phase considered ‘global’

disconnection scenarios. The second phase focussed on more practicable scenarios, in which

technically ‐feasible mechanisms for achieving specific levels of disconnection were recommended by

PWG. The second phase modelling corresponds to the disconnection scenarios outlined in Section

6.6. The third phase of disconnection scenarios (described in Section 10.2.4) were not specifically

modelled as performance was inferred from the initial phase modelling.

The report includes an overview of the modelled catchments, and the existing system’s hydraulicperformance, which are not reproduced here. For all scenarios, the system performance was

evaluated against two worst ‐case storm events (December typical year and October 2000) and a

typical year rainfall profile (Section 4.2.2).

10.1.1 Overview of global disconnection scenariosSection 2 of the ANNEX 1 report concerned ‘global’ disconnection scenarios, applied to the West

Putney, Putney Bridge and Frogmore (Buckhold Road) CSOs. The purpose of this preliminary

modelling exercise was to assess the extent to which the disconnection of surface water inputs could

impact on CSO discharges, and to get some idea of the level of disconnection required. Clearly there

would be no justification for undertaking detailed design of retrofit SUDS schemes if it was evident

that even high levels of disconnection would have no or limited impact on overall system

performance (Section 6). Given the highly‐interconnected nature of the sewer network, and the fact

that the system runs close to capacity even in dry weather, it would not be appropriate to assume

that widespread disconnection would achieve as much as might be hoped for.

The global disconnection scenarios considered were: 25% impermeable area transferred to

permeable area; 25% impermeable area removed; 50% impermeable area transferred to permeable

area; 50% impermeable area removed; and 5 mm of rainfall lost at the beginning of the storm. As an

example of performance improvements, 50% removal of impermeable area results in the following

improvements in performance for the October 2000 event (approx 1 in 4 yr return period) as shown

in Table 10.1.



64

Table 10 ‐1 Sample performance improvements associated with 50% disconnection of impervious area for the October 2000 event

Subcatchment West Putney CSO

Putney Bridge CSO

Frogmore (Buckhold Road) CSO

Maximum flow (m3/s)

Existing → Disconnection

[% reduction]

0.93 → 0.52

[‐39%]

2.61 → 1.96

[‐31%]

3.03 → 1.63

[‐45%]

Total overflow volume

(m3)

Existing → Disconnection

[% reduction]

13,900 → 8,300

[‐40%]

9,100 → 3,600

[‐60%]

17,700 → 4,700

[‐73%]

The Putney Bridge and Frogmore (Buckhold Road) subcatchments show particularly promising

results for the potential disconnection. For the typical year, the 50% removal option results in

reduced numbers of CSO events, maximum flow rates and total overflow volume. For Frogmore

(Buckhold Road) (which is the best case), for example, the number of events is reduced from 29 to

10 (‐65%), and the total overflow volume from 94,500 m3 down to 21,400 m3 (‐77%). The number of events producing over 1000 m3 is significantly reduced at all three CSOs. The impact of removing the

first 5 mm of rainfall (via storage in blue/green roofs etc) has little impact on the large storms

considered here. However, a greater depth of 50 mm would have been sufficient to contain each of the rainfall events in the typical year.

The impact of removing 50% impermeable area was also assessed for the entire London Tideway

Tunnels model. The number of individual CSOs producing overflow during the typical year reduces

from 39 to 25, and the total overflow volume reduces by 55%. This would, it should be noted, be

contingent on the disconnection of 10,327 ha impermeable area.

10.1.2 Overview of feasible di sconnection scenariosSection 3 of the ANNEX 1 report provides the model results corresponding to the initial feasible

disconnection scenarios (i.e. those outlined in Section 6.6). These were intended to provide a

realistic representation of the level of disconnection that might technically be feasible, although

without reference to costs and/or public acceptability issues. These scenarios included the

introduction of 25 mm initial losses and storage/attenuation hydraulic modelling options, in addition

to the previously ‐considered removal or transfer to pervious. However, these scenarios were shown

not to be as practicable as first assessed.

In general these ‘realistic’ scenarios produced better performance outcomes than the 50% global disconnection scenarios. For example, total overflow volume reductions were 49%, 70% and 83% at

West Putney, Putney Bridge and Frogmore (Buckhold Road) respectively for the October 2000 event.

The December event did not generate any spill at either Putney Bridge or Frogmore (Buckhold

Road), although other events with different rainfall characteristics mean that spills are not

completely eliminated from any of the CSOs in a typical year. The number of spills in a typical year

at the Frogmore (Buckhold Road) CSO was reduced to 2 (‐93%), with the total volume being only

1700 m3 (2% of the existing situation). This could have been significant, as it is possible that consent

might be issued for up to three (small) events per year/bathing season, avoiding the need for

additional CSO control measures. The number of CSO events remains high at West Putney and

Putney Bridge, and SUDS alone may not provide sufficient source control to eliminate the need for

additional CSO facilities.



65

10.2 Refined Approach – Final disconnection scenarios

10.2.1 Introduction to refinementsThe initial preference rankings for retrofit options (Table 3.1) were based on hydraulic performance

only; other factors – such as cost, acceptability, environmental and social cost/benefit – were not

taken into account. The refined approach therefore aimed to generate more ‘realistic’ options

based on the example cases in Section 8. In particular it was apparent that site ‐scale options may

have been discounted too readily in the initial preference ranking, and that further thought should

be given to the feasibility of utilizing site/regional ‐scale controls especially on publicly ‐owned

buildings and land. The refinements here have therefore focused on:

Identification of ‘easy ‐picking’ municipal housing areas;

More realistic assessment of ‘likely’ uptake rates for the remaining catchments

10.2.2 Identif ication of ‘easy p icking’ municipal housing areas.

During a

site

visit

to

the

West

Putney,

Putney

Bridge

and

Frogmore

(Buckhold

Road)

catchments

in

May 2009, it was noted that there were large proportions of municipal housing within the

catchments, especially Frogmore (Buckhold Road) and West Putney. These areas are characterised

by large, often flat roofed buildings set in large communal gardens (Figure 10.1). It was believed

that these areas had the potential to exploit a type of retrofit SUDS approach which had not fully

been considered in the earlier optioneering and generation of preferences. The approach is more

regional in nature, being based on detention ponds and swales and utilizing communal greenspace

(Section 8.1).

Figure 10 ‐1 Examples of municipal housing surrounded by generous grounds

Municipal housing estates may provide one of the easiest and most effective routes for retrofit SUDS

application. The buildings are large, set in large amounts of communal green space, and therefore

have greater potential for treatment train SUDS. The buildings are also likely to be of single or not

too many individual ownerships, notwithstanding the right to buy, which is more practical for

implementation.

Notwithstanding previously ‐stated reservations regarding the use of subjectively ‐defined land ‐use

types (Section 6), areas of similar coverage to Lytton Grove sub ‐areas 1‐4 (Appendix E.1) were

manually identified from the MasterMap data, and treated in isolation from the remaining area in

each sub ‐catchment. Figure 10.2 shows that there is a reasonable degree of correspondence

between their land ‐use distributions. With the exception of InfoWorks node 23741101 which is

atypical in many respects, as it largely serves the Putney Heath/Wimbledon Park area.

The detailed design for Lytton Grove sub ‐areas 1 to 4 generated a retrofit SUDS proposal based on

swales and detention ponds which effectively disconnected all manmade land and roofs and

approximately 25% of the roads (not main routes). It was assumed that 50% of the runoff would

effectively be transferred to pervious, whilst the remaining 50% would experience storage and

attenuation (restricted outfall to sewer). Hence the disconnection profile illustrated in Table 10.2



66

was applied to the digitized ‘municipal housing’ areas within each subcatchment. The remaining

area in each sub ‐catchment was then assessed for disconnection options using a refined version of the SQL‐based area identification and preference framework outlined previously.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

25740102 24749501 24743902 24736291 23749901 23741101 23732702 23731801 22749802 Lytton Grove

Other

Gardens

Natural Land

Manmade Land

Buildings

Roads Tracks and Paths

Figure 10 ‐2 Land ‐use categories within each of the Frogmore (Buckhold Road) sub ‐catchments, compared with the exemplar design case, Lytton Grove sub ‐areas 1 to 4

Table 10 ‐2 Assumed disconnection rates and InfoWorks modelling categories for 'municipal housing' areas

Land use types Percent disconnection

Disconnection strategies

1 2 3 4 5 6

Roads Tracks and Paths 26% 0% 0% 0% 0% 50% 50%

Buildings 100% 0% 0% 0% 0% 50% 50%

Manmade Land 100% 0% 0% 0% 0% 50% 50%

Natural Land 0% 0% 0% 0% 0% 50% 50%

Gardens 0% 0% 0% 0% 0% 50% 50%

Other 0% 0% 0% 0% 0% 50% 50%

Disconnection strategies 1‐6 correspond to (from Table 6.1):



3 Complete removal

4 Impervious area with storage/attenuation

5 Transfer to pervious

6 Initial Losses and Storage/attenuation



67

10.2.3 Disconnection op tions for t he remaining catchment.The main refinement to the earlier framework is the introduction of uptake levels. These were

intended to indicate ‘practical’ levels of uptake of each SUDS retrofit that might realistically be

expected for each option. Table 10.3 shows the adopted uptake levels, with some explanation as to

why these values were chosen.

Table 10 ‐3 Assumed uptake levels for the refined disconnection strategies

Assumed uptake level

Justification

Roads, Tracks and Paths

Disconnect roads to adjacent

pervious

40% Many of the areas identified as suitable may have unfavourable

local site conditions (access, traffic, presence of services etc)

Pocket infiltration 10% Local resistance because of car parking, road width suitability and

the cost of dealing with services

Pervious roads 0% Not generally a practical solution in the short term

Buildings

Green roofs 50% This may be optimistic and will depend on the inducements

available to landlords and owners

Disconnect roofs to lawns 50% Potentially only rear half of roof easily connected to lawn area

Water butts (all buildings,

irrespective of size)

50% As above

Manmade land

Manmade land greater than

200m ‐ pervious paving

75% Potentially feasible although this will be a big task to deal with the

1000s of property and landowners. This will also require

significant inducements.

Disconnect small manmade to

adjacent land

75% As above.

Note that the implementation of the preference hierarchies makes the simplifying assumption that

the disconnected areas associated with second and subsequent priority options are determined

from the eligible area that is remaining after the higher ‐ranked option has been allocated. The

implied assumption here is that the same building was in fact eligible for both options. If it had been

assumed that each option were independent, then higher levels of disconnection would have been

proposed, but at the risk of some double ‐counting. At this stage the degree of overlap has not

explicitly been investigated and no sensitivity analyses have been undertaken.

10.2.4 ResultsTable 10.4 shows that, for the combined area (Frogmore (Buckhold Road) + West Putney + Putney

Bridge), 20% of the roads can be disconnected, 72% of building roofs and 83% of man ‐made land. Despite these impressive disconnection rates, the impact on impermeable area reduction is less

impressive than might have been expected, and notably lower than the feasible disconnection

scenarios (LTTD, 2009), Figure 10.3. This is largely because significant areas of the catchment are

categorized as either gardens or natural land, for which 50% and 15% impermeable has been

assumed respectively due to the Mastermap uncertainties (Section 6 and Table 10.4).



68

Table 10 ‐4 Combined area disconnections

Land ‐use Total area (m 2) Disconnected area (m 2) % DisconnectedRoads Tracks and Paths 1548170 317304 20

Buildings 1457971 1047957 72

Manmade Land 869328 719561 83

0

10

20

30

40

50

60

70

80

90

May 28 values

Current values

Note: Percent of impermeable area remaining; Second phase modelling in blue, final phase modelling in red

Figure 10 ‐3 Comparison between remaining impermeable area for feasible and refined disconnection strategies

Table 10 ‐5 Final design scenario allocations of existing impermeable area (%)

N o d e

2 5 7 4 0 1 0 2

2 4 7 4 9 5 0 1

2 4 7 4 3 9 0 2

2 4 7 3 6 2 9 1

2 3 7 4 9 9 0 1

2 3 7 4 1 1 0 1

2 3 7 3 2 7 0 2

2 3 7 3 1 8 0 1

2 2 7 4 9 8 0 2

P u t n e y B r i d g e

W e s t P u t n e y

Disconnection Strategy

Remains

Impervious

64.77 56.84 55.06 58.50 52.85 82.77 24.43 60.24 68.73 65.71 59.11

2 10.15 10.95 7.86 10.81 7.56 1.36 0.00 10.90 7.86 13.91 6.64

5 11.82 11.78 18.23 13.52 19.05 5.80 38.33 13.18 12.66 11.00 16.33

6 13.25 20.44 18.86 17.17 20.54 10.07 37.25 15.68 10.75 9.38 17.91



69

The refined disconnection breakdown (Table 10.5) would be expected to lead to significantly

reduced levels of improvement in the overall CSO performance of the modelled subcatchments,

compared with those outlined in ANNEX 1.

The refined disconnection strategy corresponds to approximately 37% removal of the existing

impermeable area, with the disconnected flows being diverted to a mixture of initial loss, pervious

area and storage attenuation. Given that none of the impermeable area is completely disconnected,

this suggests that a reasonable estimate of the system response would correspond to the mid ‐point

between the 25% and 50% global scenarios for which disconnected flows were re ‐routed onto

pervious areas. Some key interpolated performance characteristics for the most promising of the

three subcatchments – Frogmore (Buckhold Road) – are presented in the second column of Table

10.6.

An alternative means of estimating the potential performance levels of the refined disconnection

scenario is by reference to Figure 10.3. This indicates that the levels of disconnection are generally

around 50% of the levels associated with the first (less ‐realistic) scenario. Hence the estimates

presented in the third column of Table 10.6 correspond to 50% of the performance expected for the

initial set of disconnection options.

It should be noted that non ‐linearities in system response mean that neither of these interpolation

approaches provides more than a very crude estimate of performance. However, the interpolated

data suggests that the refined disconnection scenarios will nonetheless provide significant

improvements in system performance, potentially halving the number and volume of annual spills.

Table 10 ‐6 Estimated performance levels for Frogmore (Buckhold Road) corresponding to the refined disconnection strategies

Existing

Performance estimatebased on 37%disconnect to pervious(i.e. mid-poin t between25% and 50%disconnect to perviousglobal scenarios)

Performance estimatebased on 50%

reduction in t he level ofdisconnection (i.e. halfthe level ofimprovement modelledfor the initialdisconnection sc enario

December typical year event –total overflow volume (m 3)

4,000 400 2,000 [-50%]

October 2000 event – totaloverflow volume (m 3)

16,900 900 10,100

Typical year event – number of

spills

27 >10* 16

Typical year event – Totaloverflow volume (m 3)

76,700 >21,400* 48,100 [-50%]

* 25% disconnection scenarios were not modelled for the typical year event.



70

11. Summary

This report provides an overview of the potential for stormwater disconnection using SUDS with

particular emphasis on 3 subcatchments within the overall London Tideway Tunnels catchment.

The study has followed a procedure for examining the potential for retrofitting as shown in Figure

3.3. This entailed an initial assessment of the potential benefit of disconnection through retrofitting

SUDS based on global percentage reductions of 25% and 50% in impervious areas connected to the

main sewerage network. In addition, model runs were made to assess whether or not SUDS options

that worked by removing or attenuating the initial 12mm and 25mm of storm events were effective.

This was followed by specific assessments of disconnection possibilities based on a series of logical

SQL queries used in OS Mastermap to determine the selected areas of land that were suitable for

each prioritised retrofit option. At this stage, however, the practicalities of application of the various

SUDS options were not considered in detail. Nonetheless these various options for disconnection

were found to be effective at reducing many of the CSO spill event volumes and numbers of spills;

although not all CSOs benefitted.

In order to evaluate the practicalities of retrofitting SUDS as part of any future disconnection

programme a number of detailed studies were made based on site visits to the 3 subcatchment

areas and on data provided from the OS Mastermap GIS database. This resulted in a revised set of priorities as to which of the SUDS options was likely to be the most useful. This has then been

applied across the 3 test subcatchments, with the identified most likely to deliver option of detention basins, being considered separately. The result has been a significant downward revision

of the potential for disconnection of the existing impervious areas which may result in a much

poorer relative performance as regards CSO operation. This revised, but what is believed to be more

practicable, disconnection rate needs to be re ‐tested using the London Tideway Tunnels InfoWorks

model.

The unit and overall costs of the revised disconnection scenarios have been determined using a

combination of a whole life cost model originally developed jointly between UKWIR and WERF and

since updated by HR Wallingford and unit cost models for additional SUDS units not included in this

model. The overall costs estimated do not include land purchase, service and utility problems during

construction and other location specific costs. Hence the costs for retrofitting, which overall exceed

£100 million, are expected to be conservative.

There are considerable added benefits in using SUDS instead of piped/sewered drainage systems,

including improvements to water quality; amenity and ecological benefits and greater resilience to

climate change, although none of these is really relevant to TW as these benefits accrue to other

stakeholders such as property owners and communities. There are also many difficulties in

implementing these approaches. These include legal and regulatory problems in regard to transfer of ‘ownership’ of the redirected stormwater from TW to a myriad of property and land owners and

road and highway operators. Many of these stakeholders do not have the experience and hence the

capacity to take on this responsibility and would need to be assisted by TW to develop this capacity.

With the revised legislative and regulatory arrangements, any disconnected property or drained area

is expected no longer to have the automatic right to reconnect to the public sewerage system in the

future. Hence, depending on the timing of enacting this, TW may no longer run a risk that in future

an enhanced sewerage capacity is going to be needed to cope with any reconnections. However, it

appears that TW would have to undertake to accept any through or exceedance flows from any new

or retrofitted SUDS, contrary to normal practice whereby land drainage is not accepted into public

sewers.



71

The legislative changes in the Flood and Water Management Act 2010, the Environmental

Regulations 2009 and the Surface Water Management Plans potentially make the utilisation of SUDS

easier and place more of the responsibility for local stormwater management on local authorities.

This could potentially make disconnection more attractive to TW and other sewerage undertakers as

the risks would no longer be as significant to themselves if local authorities were responsible for the

operation and maintenance of SUDS. Although there still would be risks that are as yet poorly quantified as TW would still be obliged to accept through and overflows from any SUDS installed.

If these systems are to be introduced in the near future it is apparent that there will be a need to

engage extensively with a wide range of stakeholders – including individual property owners,

communities, local authorities (several departments), plus the key major stakeholders Ofwat and the

Environment Agency. For successful implementation, retrofit disconnection proposals would need to

be developed in conjunction with these various actors and stakeholders, not imposed upon them.

There have been a number of limitations in the present study, the most significant of which are:

The timescale and resources available have only allowed a feasibility study to be carried out

using readily available data, models and tools; estimates of contributing areas by type and extent have been made fairly crudely using OS

Mastermap and GIS inspection;

the location and practicability of fitting retrofit options into the space available and to

appropriate line and level has only been made using digital maps and Google streetview,

following a single site visit;

the whole life cost estimates have been based on simplistic hydraulic analysis of the units

utilised and on available limited cost databases, albeit developed for the UK water industry;

no trains of units have been considered, with only single options used for the more detailed

design studies;

no attempts have been made in the detailed designs to provide for surface overflows from

the retrofitted units once the design flows (1 in 30 years) have been exceeded;

no attempts have been made to include any increases in paved surfaces within the areas

examined and there has been no attempt to include any aspects of the local development

plans;

climate change has been considered only as a simple 20% scale ‐up on rainfall intensities

used in the hydraulic design of the retrofit systems;

no future socio ‐economic scenarios have been considered in terms of changes to future

behaviour, acceptability or affordability, hence the designs cannot be considered robust in

this context;

simplified and approximate estimates have had to be made in the scaling up across the 3

catchments of the costs and applicability of the units considered in detail as examples of what could be done.



72

12. Conclusions and recommendations

12.1

Conclusions

1. The range of potential retrofit options is broad and diverse. Different options exist for roofs,

roads and man ‐made land. The options differ in their hydraulic effectiveness, their costs,

and their potential to enhance water quality, to provide amenity and other benefits such as

mitigation to climate change and to prove acceptable to stakeholders. This report has

considered all of these issues – to a greater or lesser extent – when evaluating the

necessarily ‐broad range of potential options. Most external authors/case studies recognize

the need for multiple techniques and approaches to generate credible retrofit scenarios.

Indeed, many of the potential benefits associated with SUDS are maximised through the use

of treatment ‐train multiple technique ‐based approaches. In most cases it is likely that a

hybrid source control/pipe/sewer option will be the most sustainable approach. In many

other countries retrofitting non ‐piped systems to reduce CSO spills has now become the norm. This can also provide considerable multi ‐benefits via the provision of green

infrastructure which adds to the quality of urban life; for example the added ‐value monetary

benefits of such an approach being implemented now in the City of Philadelphia USA has

been estimated as some $2.8bn compared with a storage pipe solution, bringing only some

£133m in added ‐value (Wise et al, 2010). There is a need to better account for these added ‐

value benefits in alternative ways of managing surface water in England and Wales and find

ways of incentivisation.

2. Three sub ‐catchments of the whole Tideway InfoWorks model were identified for detailed

investigation: Putney Bridge; West Putney and Frogmore (Buckhold Road) CSOs. These were

selected based on discussions in the steering group and as subcatchments that may

potentially have some obvious benefits from the use of stormwater disconnection.

3. In the first phase of the assessment ‘Global’ disconnection scenarios, 50% impermeable area

removed; 50% impermeable area diverted to pervious; 25% impermeable area removed;

25% impermeable area diverted to pervious; and 5 mm initial losses were modelled in order

to get an initial idea of the potential impacts associated with impermeable area

disconnection. In this case, the ‘global’ disconnection scenarios suggested that all three

CSOs would benefit from removal of 50% impermeable area.

4. The second phase assessment focused on more ‘practical’ disconnection options, i.e. specific

mechanisms that might be utilized to achieve the required levels of impermeable area

disconnection. The project team has developed a preliminary GIS‐based framework, utilizing OS MasterMap data and SQL queries, to generate catchment ‐scale profiles of potential

retrofit SUDS options. This novel approach offers a powerful tool at the preliminary

appraisal stage. During the course of the project this has been refined to some extent, and

there is clearly potential to refine it further.

5. The initial disconnection scenarios tested in the modelling corresponded to options which

were perceived to be physically possible, but without considering cost or acceptability

issues.

6. In one case only, Frogmore (Buckhold Road CSO), the initial modelling scenario suggested

that the retrofit SUDS option might provide sufficient control that no additional CSO



73

intervention would be required; in the two Putney sub ‐catchments any SUDS‐related

improvements would need to be supplemented with additional CSO control measures.

7. The final set of disconnection scenarios presented in this report attempts to include cost and

acceptability issues within the catchment ‐scale assessment framework. This profile offers

levels of disconnection that are significantly reduced when compared with the initially

assumed options. Although they have not been modelled, it is obvious that reduced levels

of CSO reduction would be predicted. Nevertheless it is likely that all three sub ‐catchments

would require additional CSO control measures to be implemented.

8. Within this specific catchment, a site visit revealed the potential advantages of focusing on

municipal housing. There were many low ‐medium rise housing blocks, often flat roofed and

set in extensive grassed grounds. Space and ownership issues would need to be resolved to

make application feasible. Specific designs and costings have been undertaken, which

suggest that this provides a relatively practical and cost ‐effective option at some £12.70 per

m2 of roof and paved surface diverted into detention basins that attenuate the flow rate.

9. Technically the disconnection of impervious areas using SUDS is feasible in the London Tideway Tunnels subcatchments studied. There would appear to be potential benefits in

terms of the performance of the subcatchment CSOs provided that significant proportions

(of the order of 50%) of the impervious areas could be disconnected.

10. The whole ‐life costs of disconnection has been evaluated and found as a minimum to be of the order of £20 ‐£59 million in each subcatchment for a design life of 50 years. In the

absence of information about the costs of implementation of the proposed sewer tunnels it

is not possible to assess whether or not this is comparably cost ‐effective. Having reviewed

the available guidance on assigning value to the benefits of using retrofits, it was concluded

that there is inadequate information to monetise the value of the options considered at this

time and

to

properly

account

for

the

wide

range

of

potential

benefits.

11. Notwithstanding the apparent potential value of retrofit stormwater disconnection, there

are considerable impediments to implementation in the short to medium term. A number of these impediments, such as arrangements for long ‐term maintenance, may be resolved in

the near future due to the passing of the Flood and Water Management Act 2010, although

how this may apply to retrofitting is as yet unclear.

12. There are considerable additional potential benefits that may arise if retrofit SUDS are used

for disconnection although these will not on the whole accrue to TW. These include water

quality improvements, which would assist with delivery of the Water Framework Directive

requirements; enhancements to green spaces in urban areas that would contribute to

ecology, add environmental benefits and help mitigate and adapt to climate change through amenity and heat island mitigation. In addition they will also provide opportunities for water

supplies in areas that become water stressed in the future.

13. Retrofitting stormwater management systems is also invariably much easier to incorporate

in regeneration of urban areas than conventional piped and sewered systems.

14. It is now inevitable with the passing of the Flood and Water Management Act 2010 that

there will be widespread use of SUDS in England as new developments are encouraged to

use them and as existing housing and property stock is renewed. Therefore over time these

systems will become ‘the norm’. The question remains, however, as to whether it is sensible

to wait for more than a century for this to come about as normal turnover of property stocks



74

occurs. The construction of new sewerage is known to require considerable energy use,

emitting significant greenhouse gases and locking ‐in users for long periods and hence where

this can be avoided now there are important opportunities to contribute to the mitigation of climate change and maximise the many multi ‐benefits of using green infrastructure in urban

areas (Mayor of London, 2009).

12.2 Recommendations The following are recommendations:

15. Two phases of disconnection modelling assessment are recommended. An initial phase of ‘global’ disconnection scenario runs provides a useful preliminary indication of the expected

levels of hydraulic improvement to be expected in response to different levels of disconnection. Only if this reveals strong evidence of potential benefits should detailed

assessment based on specific design options be pursued.

16. At this stage it is not recommended that the London Tideway Tunnels model is re ‐run with

the revised disconnection scenarios modelling until there is more certainty that the options being considered are actually practical, affordable and likely to be of value.

17. In view of the crude assumptions embedded in the way that the catchment node profiles

have been generated there is a need for longer ‐term refinement of the GIS‐based design and

decision ‐support tool used in the analysis.

18. As potential quick wins have been identified for the use of detention basins in municipal

housing areas that have extensive green spaces, these could be used to undertake a trial

that would firstly redesign the systems using time series rainfall and dynamic computer

models and in parallel begin the process of stakeholder engagement to test the feasibility of implementation. Ideally a pilot should be identified and actually constructed to gain

experience of the opportunities and barriers.

19. The assumptions used in the WLC assessments should be further tested, by sensitivity

analysis and at the same time alternative retrofit options to those trialled here, should be

examined, taking into account the enhanced opportunities from a ‘stormwater management

train’ approach. The WLC model could be improved by the use of more context specific

assumptions and willingness to pay local survey data which needs to be updated and would

necessitate early and considered stakeholder engagement.

20. The limited value of the guidance so far available to monetise the benefits from non ‐

piped/sewered retrofit stormwater controls have prevented any estimate being made of this

in the present study. It is recommended that the preliminary UKWIR (2009) model be further

evaluated and if necessary, enhanced as part of the more in‐depth assessment of the costs

and benefits of retrofitting stormwater management systems in the Thames catchment.

21. As only ‘conventional’ SUDS systems have been examined in this study, alternative options

should also be considered, such as the potential for storage of stormwater runoff on streets

as is being done in Skokie in the USA, Figure 12.1. This will require computational modelling

of the potential using at least 2D modelling software. Also review of innovative gully inlet

systems that control the inflow rate. Dialogue will be required with road authorities to

assess whether or not this could be a feasible approach.



75

Figure 12 ‐1 Water stored temporarily on the highway in Skokie (Carr & Walesh. 2008)



76

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Appendix A - Types of SUDS and effectiveness at reducingstormwater inputs

The SUDS manual (CIRIA, 2007) defines SUDS as: ‘surface water drainage systems developed in line

with the ideals of sustainable development’ and it lists their attributes as:

Reducing runoff rates, thus reducing the risk of downstream flooding;

Reducing the additional runoff volumes and runoff frequencies that tend to be increased

as a result of urbanisation, and which can exacerbate flood risk and damage receiving

water quality;

Encouraging natural groundwater recharge (where appropriate) to minimise the impacts

on aquifers and river baseflows in the receiving catchment;

Reducing pollutant concentrations in stormwater, thus protecting the quality of the

receiving water body; Acting as a buffer for accidental spills by preventing direct discharge of high

concentrations of contaminants to the receiving water body;

Reducing the volume of surface water runoff discharging to combined sewer systems,

thus reducing discharges of polluted water to watercourses via CSO spills;

Contributing to the enhanced amenity and aesthetic value of developed areas; and

Providing habitats for wildlife in urban areas and opportunities for biodiversity

enhancement.

The manual states that through effective runoff control at source, the need for large flow

attenuation and control structures is minimised, and that the variety of SUDS available allows the

consideration of current and future land use and the needs of local people in design of the schemes. Examples of SUDS types available for surface water attenuation are given in Table A‐0‐1.

Table A‐0‐1 Examples of types of SUDS available for surface water attenuation

SUDS Description

Filter strips* Wide, gently sloping areas of grass or other dense vegetation that treat runoff from adjacent impermeable areas.

Swales* Broad, shallow channels covered by grass or other suitable vegetation that convey

and/or store runoff, and can infiltrate the water into the ground (if ground conditions allow).

Infiltration

basins* Depressions in the surface designed to store runoff and infiltrate water to the

ground; may also be landscaped to provide aesthetic and amenity value

Wet ponds* Basins that have a permanent pool of water for water quality treatment. They

provide temporary storage for additional storm runoff above the permanent water

level; may also provide amenity and wildlife benefits.

Extended

detention

basins*

Usually dry central basins with small permanent pools at the inlet and outlet.

Designed to detain a certain volume of runoff as well as providing water quality

treatment



82

SUDS Description

Constructed

wetlands* Shallow water with wetland plants that improve pollutant removal and provide

wildlife habitat

Filter drains

and

perforated

pipes*

Trenches filled with permeable material; surface water from the edge of paved

areas flows into the trenches, is filtered and conveyed to other parts of the site. A

slotted or perforated pipe built into the base of the trench can collect and convey

the water.

Infiltration

devices* Temporarily store water and allow it to percolate into the ground (where ground

conditions allow)

Pervious

surfaces* Rainwater infiltrates through the surface into an underlying storage layer, where

water is stored before infiltration to the ground for reuse or release to surface

water (can be driveways, pavements, roads, car parks etc.)

Green roofs* Systems which cover a building roof with vegetation. They are laid over a drainage

layer, with

other

layers

providing

protection,

waterproofing

and

insulation.

Soakaways Small areas of (permeable) land dedicated to the percolation of rainwater

Rain gardens Planted areas often acting as traffic calming islands to the sides of wide roads that

collect road runoff, water the plants and provide infiltration

Sea streets (Street Edge Alternatives) roads with adjacent green space that can be used for

detention/retention/swales; can be used in conjunction with permeable paving

Street trees Trees planted in the pavement that are watered by pavement runoff

Pocket SUDS Small scale SUDS implemented opportunistically

Disconnection

Direction of

roof

runoff

to

local

green

space

for

infiltration

to

the

ground

rather

than piped/sewered collection

Rainwater

harvesting Typically the use of water butts to collect rainwater for garden watering, but can

also be done on a much larger scale and rainwater can be used within a building

for non ‐potable use (e.g. toilet flushing)

Blue roofs Collection of rainwater in dedicated (flat) roof reservoirs, providing insulation,

cooling and habitat

General

‘greening’ Restoring green space where ever possible in the local area to provide opportunity

for surface water infiltration

Daylighting of culverted

watercourses

Locating natural watercourses that have been culverted and opening them up with

the potential to provide increased headroom as well as aesthetic and

environmental benefits

Domestic /

industrial

demand

management

The use of various financial or legislative means to reduce potable water use for

non ‐potable purposes and encourage rainwater harvesting

Legislation Such as the recently introduced planning legislation requiring planning permission

to pave over a front garden greater than 5m 2 with a non ‐pervious material

(www.planningportal.gov.uk)

* As described in the SUDS manual (CIRIA, 2007)



83

The precise forms of SUDS, the terminology and the way they are defined varies internationally and

Table A‐0‐1 is not inclusive of all the types of systems classified as ‘SUDS’, ‘BMPs’, ‘LIDs’. In this study

the application of alternative systems has been considered as well as those listed in Table A‐0‐1.

The SUDS manual (CIRIA, 2007) also gives an indication of likely performance of the different types

of SUDS with respect to quality of treatment potential and hydraulic control (Table A‐0‐2).

Table A‐0‐2 Quantity performance of selected SUDS (source: SUDS manual, CIRIA, 2007, Table 5.7)

SUDS group Technique

T o t a l s u s p e n d e d

s o l i d s r e m o v a l

H e a v y m e t a l s

r e m o v a l

N u t r i e n t

( p h o s p h o r u s ,

n i t r o g e n r e m o v a l )

B a c t e r i a r e m o v a l *

C a p a c i t y t o t r e a t f i n e

s u s p e n d e d s o l i d s a n d

d i s s o l v e d p o l l u t a n t s

R u n o f f v o l u m e

r e d u c t i o n

Hydraulic control Suitability for flow

rate control (probability)

0.5 (1/2 yr)

0.1 ‐0.3

(10/30 yr)

0.01 (100 yr)

Retention

Retention

pond H M M M H L H H H

Subsurface

storage L L L L L L H H H

Wetland

Shallow

wetland H M H M H L H M L

Extended

detention


Pond /


Pocket


Submerged

gravel


Wetland

channel H M H M H L H M L

Infiltration

Infiltration

trench H H M H H H H L

Infiltration

basin H H H M H H H H H

Soakaway H H H M H H H H L

Filtration

Surface

sand filter H H H M H L H M L

Sub ‐surface


Perimeter


Bioretention

/ filter strips H H H M H L H M L

Filter trench H H H M H L H H L

Detention Detention

basin M M L L L L H H H

Open

channels

Conveyance

swale H M M M H M H H H

Enhanced

dry swale H H H M H M H H H

Enhanced

wet swale H

H

M

H

H

L

H

H

H



84

SUDS group Technique

T o t a l s u s p e n d e d

s o l i d s r e m o v a l

H e a v y m e t a l s

r e m o v a l

N u t r i e n t

( p h o s p h o r u s ,

n i t r o g e n r e m o v a l )

B a c t e r i a r e m o v a l *

C a p a c i t y t o t r e a t f i n e

s u s p e n d e d s o l i d s a n d

d i s s o l v e d p o l l u t a n t s

R u n o f f v o l u m e

r e d u c t i o n

Hydraulic control Suitability for flow

rate control (probability)

0.5 (1/2 yr)

0.1 ‐0.3

(10/30 yr)

0.01 (100 yr)

Source

control

Green roof N/A N/A N/A N/A H H H H L

Rainwater

harvesting M L L L N/A M M H L

Permeable

pavement H H H H H H H H L

In the USA, stormwater disconnection is often utilised to improve downstream water quality with

less interest in water quantity control. However, the WERF project reported by Weinstein et al

(2006) is also interested in using retrofits to reduce CSO spills. Table A‐0‐3 shows a qualitative

assessment from

this

report

of

the

effectiveness

of

certain

BMP

measures

on

volumes

and

peak

flows.

Table A‐0‐3 Effectiveness of source controls on water quantity downstream (adapted from Weinstein et al, 2006)

Source control Effect on volumes

Effect on peak discharges

Responsibility Maintenance effort

Downspout

disconnection M M owner Minimal

Infiltration

practices M L owner Medium to high

Pocket wetlands H H owner Moderate to high –

removal of debris, vegetation watering,

sediment removal

Porous pavement H H owner As infiltration ‐ may need

vacuuming

Rain

barrels/cisterns M L owner Minimal

Rain gardens H H owner Minimal Vegetation

management

Rooftop storage* H H owner Minimal

Soil amendments M M N/R Included in other

applications Vegetated roofs M H owner Moderate Vegetation

management

Vegetated swales M M owner As above

Key: H‐High impact; M‐Medium impact; L‐Low impact

* This option was missing in the original Table ES‐1 in Weinstein et al (2006).

The options in Table A‐0‐2 are mainly source controls and the performance applies in conditions

found in the USA and therefore need to be considered carefully when applied elsewhere. Also in the

UK, the ‘ownership’ may be private, collective or municipal and responsibilities could change in

future once

the

Flood

and

Water

Management

Act

2010

is

fully

implemented.



85

Appendix B - Illustration of disconnection options

The following sheets aim to show the retrofit potential for a series of current land use

characteristics. All street level images are courtesy of Google Streetview (Copyright 2009). Aerial

photography is Thames’ own.

1. Roads Characteristic Example Retrofit potential

‘Wide’ Roads –

greater than 8m

width

Sea Streets

Pocket street

infiltration

Permeable paving

Swales

‘Narrow’ roads – less than 8m

width

Permeable paving



86

Main (A) Roads

(High traffic

loading)

Little retrofit

opportunity

Other than permeable

surfaces

Roads with

adjacent green

space

Sea Streets

Disconnection to

adjacent land

Potential for

detention/retention

Permeable paving

Swales

2. Buildings Housing –

terraced

Often small

gardens, on ‐

street parking,

often narrow streets

Rainwater

Harvesting/Waterbutts

(garden permitting)



87

Housing – Semi

detached

Often large

back gardens,

off ‐street

parking

Rainwater Harvesting

Water butts

Housing –

detached

Often large

gardens, off ‐street parking.

Rainwater harvesting

Water butts

Garden soakaways

Flats/

apartments Downspout

disconnection to

surrounding land

Potential for detention

basins, ponds and

wetlands

Green/Blue roofs

(flat

roofs)



88

Flat roofs Green/Blue roofs

Downspout

disconnection to

surrounding land

Opportunistic

area

Each

catchment will have several

buildings that

could be

considered as

‘easy pickings’ for retrofit

SUDS –schools,

playing fields

that could be

redesigned for

dual use

Varied retrofit SUDS

potential:

Permeable paving

Downspout

disconnection

Green/ blue roofs

Rainwater Harvesting

Water ‐butts

3. Land Natural Land



89

Playing fields/school

grounds Potential for offsite/

end of pipe

solutions; for

example, wetlands,

retention basins,

ponds.

Manmade land

greater than 200m Permeable paving

Cellular storage

Firstly, to ascertain the total area of roads that could be converted to permeable/pervious paving,

the area of roads and tracks were selected from the total road track and path layer using the ‘Select

by Attributes’ function within ArcView. Sea Streets and Disconnection to adjacent pervious land

were selected by selecting all areas of “roads tracks and paths” that were within 2m of natural land

(greater than 100m 2, in order to ensure that only areas that can take the road runoff are selected.)

In order to select roads that have the potential to retrofit pocket road SUDS/Traffic calming, the

MasterMap data must be modified. Google Streetview was used to get an insight into the width of the roads, and the availability of on street parking (see Fig.B‐0‐1), roads deemed suitable were then

digitised within ArcView. Figure B‐0‐1 also shows the criteria used to select roads for pocket road

SUDS.



90

Pocket road SUDS are deemed to be suitable for roads that are wide enough to take out some of the

car parking and replace it with engineered rain gardens. Figure B‐0‐1 Criteria for Pocket road SUDS

(Image courtesy of Google Streetview Copyright 2009 Google)

Roofs

Several options are available for the disconnection of roofs. Perhaps the simplest option technically

would be to disconnect roof drainage to surrounding gardens. For this option to be feasible, it is

considered that the area of pervious garden must be at least equal to the area of the roof. From

aerial photography of the Frogmore (Buckhold Road) area, there appear to be distinct types of roof within the catchment that may be suitable. Terraced housing with small courtyard gardens, Larger

semi ‐detached and detached properties with large, predominantly turfed/landscaped lawns, large

pitched roofs (likely mixture of residential, commercial and institutional occupation) surrounded by

grounds, and large, flat roofs, also often set in grounds.

In order to discriminate between these distinct types, an SQL query was used to select out all

buildings from MasterMap. The statistical distribution was then consulted, and the smallest area for

a house was selected (for Frogmore (Buckhold Road) the mean building area was 60m 2). This area

was then used to select all gardens (mixed permeability land) that were greater than or equal to

60m 2.

In the absence of AddressPoint/Address layer data, it is necessary to create a series of assumptions

about the use of the buildings. Studying the MasterMap data in conjunction with Aerial

photography, residential housing (terraced, semi ‐detached, detached and flats) were seen to

approximately fall within 20m2 (terraced) ‐590m

2 (Flats). These buildings were selected out from the

whole layer.

The ‘Select by Location’ tool within ArcView (v9.3) was then used to select any houses within 1m of the gardens greater than 60m 2 (this removed any houses within gardens less than 60m 2). 60m 2 was

selected, as although this figure is quite large, it ensured that as many properties as possible in

gardens with an area significantly less than the building footprint area were left out of the selection.

As there is often a correlation between house size and garden size, and the smaller the garden the

less likely it is to be permeable enough for infiltration, this selection provides an indication of the

roofs that could be disconnected to gardens. As communal grounds were included within the

‘natural land’ layer, the same process was repeated as above to give all buildings assumed to be

residential within 1m of land deemed to be permeable.



91

Key:

= Buildings greater than 20m 2 but less than

590m 2

= Gardens suitable for infiltration

© Crown Copyright and database right 2009. Ordnance Survey Figure B‐0‐2 Example houses deemed suitable for disconnection to gardens

Key:

= All land (natural or’gardens’) greater than 60m 2

= All properties greater than 2m 2 but less than 590m 2, within 1m of suitable land.

= Manmade surface

© Crown Copyright and database right 2009. Ordnance Survey Figure B‐0‐3 Example of all properties deemed suitable for disconnection to adjacent land

From this, the resulting area of disconnectable roofs must be checked for ‘double counts’; that is, two or more roofs, both within 1m of the same area of land. These should be removed from the



92

total area to give a more representative area. A further option for retrofitting roofs, is also the

installation of green roofs to suitably pitched roofs.

To get an estimate of the likely number of green roofs possible in the catchment requires

modification of the existing MasterMap data. Unfortunately, data on the pitch of the roof is not

available within MasterMap, therefore, by using aerial photography, buildings appearing to be flat were digitised within ArcView to create a layer of flat ‐roofed buildings. This ignores any

practicalities of application.

Land

MasterMap separates land into 3 groups; manmade (comprising; hardstanding, carparks); natural

(parks, open space, golf courses, fields) and Mixed Permeability (gardens, accounting for

homeowner’s individual landscape choices).

Natural and Mixed permeability land surfaces have been discussed in roads and roofs above. The

retrofit SUDS options for manmade surfaces are the following; replace impervious surfaces with permeable surfaces, with or without additional cellular storage. For smaller areas of hardstanding it

may be possible to disconnect the drainage to adjacent land, where appropriate.

For this study, areas of contiguous manmade surface greater than 200m 2 were selected from

MasterMap. 200m 2 was deemed the minimum area of hardstanding/carparking that would be

worth the cost and disruption of resurfacing to permeable paving.

The areas of manmade surface smaller than 200m 2, but close to adjacent natural land were

subsequently selected. These areas are deemed suitable for disconnection to adjacent pervious

land.



93

Appendix C - Whole life cost assessment

C.1 Introduction Whole ‐Life Costing (WLC) combines capital and ongoing operational costs to be incurred over a pre ‐

determined life cycle period of individual interventions in order to determine their full cost.

Life cycle periods can be scaled for example to match the five year periodic financial reviews (PR) imposed on Water and Sewerage Companies (WaSCs) by their financial regulator, Ofwat. Longer

term assessments may align with the new Ofwat requirement for WaSCs to produce 25 year

Strategic Direction Statements. A significant feature of WLC is that it is based on the identification of future costs. In order to ensure that costs are comparable over the life cycle of the different

interventions, all costs are discounted to determine their present value at today’s prices. Future

costs during the pre ‐determined life cycle are anticipated and calculated with a discount rate. The

Treasury Green Book (HM Treasury 2003) uses a discount rate of 3.5% for life cycles of 0‐30 years

(rates decline the linger the life cycle as high discount rates make future costs less important (SUDS

Manual, CIRIA, 2007)).

The costs and benefits associated with each intervention are calculated and incorporate the

financial, environmental/ecological and social impacts relevant to each intervention. Many of the

financial capital costs are incurred by the installer (in this case Thames Water (TW)) but a significant

proportion of future operational and maintenance costs are transferred to other stakeholders, this

will primarily be to the Local Authority if the Draft Flood and Water Management Bill is enacted.

Otherwise the distribution of costs will depend on arrangements made with the Local Authority at

the time of designing and installing the interventions.

According to The Green Book (HM Treasury, 2003) “…wider social and environmental costs and

benefits for

which

there

is

no

market

price

need

to

be

brought

into

any

assessment.

They

will

often

be difficult to assess but are important and should not be ignored simply because they cannot easily

be costed”.

Many social and environmental costs and benefits will not accrue directly to the installer of interventions but when considered over the life cycle of the interventions indirect benefits (such as

reputation enhancement) may be offset against initial financial investment. Some costs that are

inevitable with the use of conventional systems may be avoided by the use of innovations and this

should be recognised at the decision making stage.

Monetisation of the environmental/ ecological and social costs and benefits (EESCB) is problematic

but techniques such as willingness to pay (WTP) on the part of stakeholders have been used in other

studies as contingent valuation (e.g. Ashley et al., 2004). Some of the WTP amounts may be ascertained from extant data sources such as preference state surveys conducted by WaSCs. It is

important that if stormwater disconnection is seen as a viable option, the EESCB are identified for

each intervention. In this way, consideration of how to minimise the costs and maximise the

benefits, cost avoidance and long term cost savings become possible. Recognition of EESCB can also

reduce reputational risk.

C.1.1 The need for change in the study areaIn order to understand where the cost burdens of systems of integrated retrofit SUDS lie and to

whom the benefits accrue, it is essential to understand the broad picture of why change to the

existing system is necessary now and in the future. The Drivers, Pressures, State, Impacts, Responses

(DPSIR) framework is a technique used by the European Environment Agency (www.eea.europa.eu)



94

and enables insight into complex environmental problems. Table C‐0‐1 sets out some of the issues

related to the implementation of retrofit SUDS in Frogmore (Buckhold Road), Putney and West

Putney.

Table C‐0‐1 Drivers, Pressures, State, Impact, Responses analysis of the need to upgrade surface water drainage in Frogmore (Buckhold Road), Putney and West Putney

Drivers ‐Need to comply

with EU Directives –

specifically the

urban wastewater

treatment directive

‐Need to justify

pricing to Ofwat

‐Need to provide

high level of service

Increased

population in the

capital

Increased use of the car

Climate change

Pressures ‐ Increased surface

water pollution

‐Customers

demand more and

high quality

drinking water,

reduced leakage &

affordability

‐ Current systems

not equipped to

manage high

intensity rainfall

events or to comply

fully with

environmental

directives

‐Financial climate

reducing credit

availability.

‐High density

housing

‐Reduced green

space

‐Infill development

and building of extensions,

conservatories etc.

‐Existing sewer

system and storm

drainage not

designed for

current population

density

‐Paving over

gardens to enable

car parking

‐Increased

impermeable road

surface area

Increased levels of pollutants in

surface runoff

‐Increased number

of high intensity

rainfall events and

uncertainty

regarding future

extreme weather

events, including

periods of water

stress.

‐ Existing low

rainfall in the area.

Impacts ‐Planning for the future has become

heavily dependent on compliance with

directives and financial constraints

‐Need for balanced planning decisions

Greatly reduced stormwater infiltration

and increased surface water runoff leading to:

‐River pollution; reduced

aesthetic/amenity/environmental quality

‐Increased basement flooding (DG5

registered properties)

‐Potentially greater impacts of all the

above in the future due to further changes

in demographics and climate

Responses ‐2010 to 2015 Business Plan

‐Strategic Direction Statement for 2010 to

2035

Responses available to the Sewerage

Undertaker are limited to: containing and

redirecting surface water flows (the

conventional response); and reducing

surface water runoff at source,

attenuating its flow and reducing its

pollution burden (the SUDS response)



95

A SUDS approach would address a greater number of the pressures and impacts in Table C‐0‐1 than

would the conventional piped/sewered solution and would also provide systems that were more

flexible and adaptable to future uncertainties (Seiker et al, 2008), although it clearly involves the co ‐

operation of a greater number of organisations and individuals.

C.1.2 Stakeholder involvement In‐depth WLC analysis requires the identification of all stakeholders affected by each of the SUDS

measures proposed for each catchment, that is: “any group or individual who can affect or is

affected by the achievement of the organisation’s objectives” (Freeman, 1984). Different

stakeholders have different interest in, influence over and need for work to be carried out. These

differences also need to be recognised in a transparent stakeholder analysis as part of the WLC

approach.

Once identified, the means of involvement of stakeholders in the implementation of the scheme

should be considered. The North Brent Integrated Urban Drainage (IUD) pilot study (Defra, 2008 1)

for example established a Steering Group for the project comprising representatives of local

stakeholder groups that was independent but able to review issues relevant to the area and recommend what it considered to be beneficial solutions. Stakeholder involvement in the scheme is

beneficial for its acceptance and long term success and whilst incurring an initial cost can greatly

enhance the reputation of organisations involved in the scheme’s implementation.

Due to the distribution of responsibilities there will be a distribution of costs and benefits. Many of the solutions to excess surface water runoff lie within different uses of the land which is the

responsibility of the landowner or the local authority rather than the Sewerage Undertaker.

Furthermore the installation of pipework incurs costs to the Sewerage Undertaker but that work

becomes a tangible asset belonging to the company and one that requires only routine and

definable maintenance as defined in the new Sewerage Risk Manual. Implementation of a SUDS

scheme on

land

belonging

to

the

local

authority

places

the

burden

of

upkeep

of

the

scheme

with

the

local authority. A major part of the SUDS scheme is likely to be located in public open space and has

the potential to help promote the sustainable development agenda, providing the public are aware

of and are in support of the scheme, which requires their involvement from an early stage in the

process of implementation. Although many open SUDS systems are potentially problematic as

regards real and perceived health and safety.

Table C‐0‐2 lists financial costs (Capex and Opex) associated with roof disconnection and the likely

bearer of the costs.



96

Table C‐0‐2 Generalised financial costs and their distribution due to roof disconnection and infiltration via detention basin

Action Cost bearer Costs Study of area Thames Water ‐Data gathering

‐Land use analysis

‐Hydraulic modelling ‐Plans and permissions

‐Stakeholder identification and analysis

Involvement of stakeholders Thames Water ‐Hire of communication specialists

‐Hire of SUDS experts

‐Stakeholder consultation (e.g.

householders, conservation groups,

local authority)

‐Compliance costs (health & safety)

‐Compensation costs (disruption to

residents)

Redirection of downspout

water via pipe to detention

basin

Thames Water ‐Design of system ‐Land excavation (labour)

‐Materials: pipes, connections

‐Potential increased sewer

maintenance due to reduced flow

Creation of detention basin Thames Water ‐Land allocation

‐Land excavation (labour)

‐Soil and seeding

Maintenance of guttering and

downspouts Wandsworth Borough

Council Maintenance programme

Maintenance of connection

pipes Thames Water Maintenance programme

Maintenance of detention

basin Wandsworth Borough

Council Mowing, litter gathering, notices of danger when full

Monitoring of SUDS scheme Thames Water,

Wandsworth Borough

Council

Water quantity and quality monitoring

Ongoing communication with

stakeholders Thames Water,

Wandsworth Borough

Council,

Dedicated helpline/website

Non ‐monetary costs of this part of the scheme would be borne by the local population and

environment and may include:

Social:

Temporary disruption and noise

Perceived threats to health and safety

Change in use of public area

Environmental:

Short term habitat disturbance



97

The benefits, both financial and non ‐financial of this part of the SUDS scheme are outlined below:

Table C‐0‐3: Generalised benefits and their distribution due to roof disconnection and infiltration via detention basin

Action Benefitted stakeholder

Benefits

Study of area Thames Water ‐Increased local knowledge ‐Data capture, of enormous benefit for future

schemes

‐Informed decision making

Involvement of stakeholders Thames Water ‐Enhanced reputation as an organisation

involved in the local community

‐Working in line with principles of sustainable

development

Redirection of downspout

water via pipe to detention

basin

Thames Water ‐Small amount of piped assets

‐Reduced sewer inflow thereby reducing

carbon footprint (assuming flow would be

treated if piped)

Creation of detention basin Wandsworth Borough Council

‐Involvement in SUDS scheme to help with local sustainability targets

‐Preparedness for extreme weather events

Local residents ‐Reduced basement flooding

‐Reduced overland flow of stormwater

‐Potential positive educational impact

‐Reduced river pollution

Maintenance of guttering and

downspouts Wandsworth Borough

Council Opportunity to communicate with residents

regarding SUDS scheme

Maintenance of detention basin Wandsworth Borough

Council Opportunity to communicate with public

regarding SUDS scheme

Monitoring of SUDS scheme Thames Water,

Wandsworth Borough

Council

Feedback and data collection regarding an

implemented scheme, valuable for future

schemes

Ongoing communication with

stakeholders Thames Water,

Wandsworth Borough

Council,

Reputation enhancement and channel for

feedback

All stakeholders including

local residents

‐Increased knowledge about SUDS and the

local collective response to flooding and

pollution

‐Ongoing work in line with more sustainable

development

C.2 Evaluation of Costs Costs are applied to each technology individually before the consideration of any cross ‐subsidisation

when more than one technology is adopted. An assumption is that most, but not all (see

maintenance costs) financial costs are incurred by Thames Water (TW).

Financial Costs:

Assumption: installation costs for each technology are incurred exclusively by TW including any

construction costs necessary for each intervention.



98

Some of the tangible costs will be offset not only by the financial benefits associated with cost

savings as a result of surface water management but by the avoidance costs that will arise in

relation to some of the environmental and social benefits.

Capex: Direct costs include:

materials,

labour and

disconnection from existing pipes underground.

Indirect costs include:

gaining planning permission,

access/acceptance costs of any kind which may require negotiation/ consultation

(engagement) with stakeholders (eg. householders, conservation groups, local authority);

communication costs, compliance costs (health & safety);

compensation costs (disruption to residents),

inspection costs during installation;

development of a monitoring system as the technologies are installed;

setting up a customer information / help line during construction;

financing costs and

landfill costs for waste disposal if digging up concrete (rain gardens/ sea streets) where

waste materials cannot be reused/ recycled (this will include transport to landfill and costs

per ton of waste).

Many of these costs will be offset by the financial benefits identified below.

Opex: Dependent on arrangements made, the installed interventions may become the property of the local

authority. However, maintenance and monitoring costs may be borne in the short term at least by

TW, particularly if there is a desire to minimise reputational risk by ensuring that the interventions

become established and work as intended. Longer term costs to the local authorities may influence

the nature of the agreement made between the organisations and the size of any commuted sums.

Such sums must reflect the cost of expertise in different types of SUDS as well as future maintenance

costs to be borne by the local authority.

Other relevant costs: Thames Water may pay commuted sums to the local authority for the responsibility of future

maintenance of the interventions.

Thames Water may suffer a loss of assets due to disconnection from existing pipe and sewer

networks. This has implications for the asset base of the company and could affect the future

funding of TW and its ability to raise finance as well as to its future revenue generation via the five

year price review.



99

Environmental/ ecological and Social Costs In most cases environmental/ecological and social costs (EESC) will be incurred by stakeholders

within (or immediately outside) the catchment area and include disruption and inconvenience

during installation as well as potential future malfunction or inundation. These costs are difficult to

monetise from the point of view of the affected stakeholders, but they are significant for TW and

should not be ignored. Whilst these costs may appear intangible, they could lead to tangible

‘damage limitation’ costs in order to minimise reputational risk.

Some EESC will impact directly on TW, for example in the form of potential fines for non ‐compliance

of pollution consents (where untreated water enters the Thames from surface water runoff due to

the disconnections undertaken).

Other environmental factors to consider are noise pollution during installation, including the traffic

impacts on both noise and carbon footprint in the area; the potential impact on flora and

fauna/habitat given certain technologies involve change to existing green areas (though over the

longer term positive impacts could outweigh initial negative impacts on habitats).

During installation, disruption that could lead to negative perceptions of changes to the area

particularly when residents do not perceive there to be problems with surface water management

or fail to appreciate the benefit and significance of the interventions.

Health and safety impacts could include midges or mosquitoes around standing water and the risk of drowning in ponds and swales for example. To minimise the actual safety risk and at the same time

to reduce stakeholder perceptions of such risks, TW could incur fencing costs around ponds at the

time of installation and any other costs to comply with health and safety regulations in agreement

with the LA.

The reduction in certain types of recreational green areas or the perceived change of use may lead

to resistance from residents and lead to reputational damage.

Complaints to TW that arise in relation to the negative perceptions of residents has financial

implications if ‘customers’ express these in the form or written complaints. To avoid the negative

reactions by stakeholders, communication/ engagement and awareness campaigns should be

undertaken, the costs of which should be considered as part of the interventions. Consultation with

the Consumer Council for Water (CCW) should also be accounted for. The Draft Flood and Water

Management Bill has for example already given this body cause for complaint by proposing that

council tax payers cover the costs of transfer of asset ownership and responsibility from WaSCs to

local authorities. Thames Water has conducted ‘willingness to pay’ surveys which can provide some

data for analyses of EESC and is likely to have a database of costs for awareness campaigns etc.

C.3 Evaluation of benefits There are benefits to TW, in the form of cost savings once the installation is complete and more

intangible ones in the form of enhancing reputation if stakeholder perceptions are managed

effectively and the benefits of the interventions are communicated and experienced by the

stakeholders.

There are also environmental/ ecological and social benefits (EESB) which will accrue more to the

stakeholders in the catchment and may counterbalance any negative perceptions on their part. This

could lead to a reduction in the engagement and negotiation costs identified above.



100

Identification and promotion of the financial benefits to stakeholders will minimise the potential for

resistance from the residents, the LA and to groups such as the CCW.

Financial benefits to TW: The potential asset management benefits as a result of the reduction in surface water runoff to TW

are contained in the UKWIR (2009) report.

A significant financial benefit will be achieved if attenuated flows to the Thames are reduced such

that fewer connections to the Thames Tideway Tunnel (TTT) will be required.

There will be a reduction in sewer flooding risk (internal and external) which will reduce repair and

clean ‐up costs.

Attenuating flows into the Thames will also lead to environmental river quality improvements

resulting in cost savings for TW which is currently under criticism for the condition of parts of the

river and has incurred prosecution costs in the form of legal fees and fines.

There will be benefits in increased headroom and a reduction in CSOs which will reduce the risk of flooding and untreated water overflows into the Thames, leading to further costs savings as a result

of the suspension of prosecution if pollution to the Thames is reduced.

The interventions will lead to WaSC efficiency savings in the form of reduced energy (therefore

carbon) and chemical costs in treatment works.

The interventions have the qualities of flexibility and adaptability. By their nature, the retrofit

interventions offer ‘no regret’ solutions which are easy to reverse and which will not involve

significant costs in the future. Long‐term resilience will reduce costs of opex in short, medium ‐ and

long ‐term.

The environmental/ecological and social benefits (EESB) identified below may lead to cost savings to

TW in the amount of engagement activity and initiatives it will need to undertake.

The interventions also reduce the risk of drought orders and hosepipe bans which has financial

consequences for TW when scrutinised by Ofwat e.g. for leakage rates.

There will also be cost savings by not incurring DG5s.

Financial benefits to residents: The long ‐term cost savings generated to TW by the interventions and cost efficiencies through the

positive impact downstream could result in lower water prices.

The impact of lower prices for water would be offset, potentially, by increased council tax charges

from the LA when the future maintenance of assets such as ponds and swales become its

responsibility. However, there should still be a net financial benefit to property owners as a result of enhanced property values resulting from improvements in amenity value of the area. For residents

who have experienced problems with surface water runoff, there will be a reduction in basement

flooding incidents resulting in savings in clean ‐up costs and insurance charges. The reduction in

flooding incidents will further enhance property values.

Environmental/ecological benefits: As well as the financial efficiency savings there will be reductions in chemical usage (conserving

resources) and TW’s carbon footprint.



101

A number of the interventions, particularly the ponds and swales, will have a positive impact on flora

and fauna in the long ‐term and have the potential to enhance the amenity value of the area.

The reduction in CSOs means there will be a reduced negative impact on river water quality

downstream.

Savings in water usage through rainwater harvesting can reduce the risk of hose pipe bans and avoid

the potential negative impact on TW’s reputation and written complaints which are recorded in June

returns to Ofwat. Conservation of water also increases groundwater replenishment which has

implications for water availability in the longer term and will lead to cost savings in water

abstraction.

In addition, the nature of certain interventions will lead to traffic calming which will reduce carbon

emissions in the area.

Social: The reduced risk of basement flooding in some properties (see above) reduces health and safety

risks to residents. Health and safety is further enhanced by a reduction in the potential for accidents

as a result of reduced ponding on roads.

The increased amenity value of the area is a social benefit as it impacts on the health and well being

of the community. The nature of the interventions will enhance community and social cohesion

(around ponds for example) and lead to enhanced stakeholder relationships.

A focus on the environmental/ecological attributes of the interventions has educational benefits as

they could become the focus of school projects on ecology, water cycle, drought and flood risk

management as well as an area of interest for a variety of other local groups and societies.

The EESB to the area is likely to be perceived favourably as it will enable the LA to demonstrate it is

meeting Agenda 21 commitments. At the same time, such benefits will also enhance the reputation

of TW.

A more direct social benefit as a result of installing and maintaining the interventions is the

development of transferable skills in the area, particularly in relation to future maintenance which

has the potential to create long ‐term business and employment opportunities.

The distribution of many of the costs and benefits will depend on the means by which the SUDS

schemes are implemented and agreements made between the Sewerage Undertaker and the local

authority. There is great opportunity for enhanced communication and co ‐operation between

organisations and the local community in line with the principles of sustainable development. The

benefits of this are not made clear by calculations of financial costs alone. The organisations responsible have choices regarding cost and benefit distribution over the short and long term and

these choices must be weighed against reputational risk.

C.4 Data sources for HR Wallingford’s WLC spreadsheets Site specific data are used which include: agreed design criteria, pre ‐calculated system dimensions,

agreed system characteristics, known unit costs and agreed operation and maintenance frequencies.

(A generic application is also possible for which minimal information is required from the model user

and crude estimates of generic system types are made and entered in the model including default



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entries for: dimensioning, system characteristics, ‘best practice’ design, construction and

maintenance activities, and unit costs that reflect average values of the costs collated in Phase 1 of the project and presented in the cost database.)

The costing model allows costs for different SUDS components to be built as it highlights all the

important materials and processes and allows the user to incorporate costs where and when

required. Site specific costs and characteristics should be entered wherever possible. As a minimum,

the assumptions and costs components should be reviewed for appropriateness prior to the generic

model application. Cells that are essential for user entry to achieve a model result are:

‐ Essential user entry (no model default value available)

‐ User entry (data in these cells will over ‐ride model default values)

‐ Selected option (data to be taken forward in the calculations (i.e. either user or model

default value)

Default values are given where user (specific) values are unavailable and these have been used in

the spreadsheets.

Separate models are available for components:

Retention pond

Detention basin (can be adapted to represent an infiltration basin model)

Swale (can be used to form an appropriate bioretention model)

Filter/drainage

Permeable pavement

The spreadsheet for each model consists of 11 sheets included in the WLC model user guide

(WERF/UKWIR, 2005)

with

an

additional

sheet

(8)

for

Waste

Management

Options

included

which

is

not referred to in the WLC user guide (Table C‐0‐4).





Sheet number and description Information category Data required and units

(if any) Comments on data (taken from Lytton Grove anal

4. Associated

capital costs Planning and design costs,

construction, overheads ,

(plus initial site investigation costs if significant)

30% of total construction

costs (£)

The SUDS manual (p.411) suggests that design, cona percentage of total construction costs and that 3

suggested that if initial site investigation costs areModel default values are 15% of total capital costsand design and construction (so 30% in total as pe

For the detention basin component on which this

relevant/identified though they might be relevant

land costs Land costs will vary between sites. Whilst in some

the site has dual use or where the scheme forms psettings the land value may outweigh the constructtype of drainage option. Inclusion of the cost of lanstakeholders involved and the purpose of the asse

risk management costs Risk management costs (p. 23 of WLC model user

liability for risks associated with the drainage syste

costs indirectly associated with implementation ofdownstream sewerage infrastructure to accommodinfrastructure.







Sheet number and description Information category Data required and units

(if any) Comments on data (taken from Lytton Grove anal

8. Additional Waste

management

options

Waste accumulation and

removal frequency Excavation

Hazardous waste

treatment

Pre ‐treatment to

non ‐hazardous waste

Non ‐hazardous waste

management under

exemption

Non ‐hazardous waste

management

Supersedes material in sheet 7 (which is the origin

9. Cost and benefit

summary sheet Full summary of costs and

benefits entered in the model Allows the user to flag which of the costs/ benefits9. This facilitates any sensitivity analyses or scenar

As indicated

above,

in

this

component

(which

is

cothis model) though environmental benefits are ide

determines the present values of annual costs incui.e. the present value of total costs.

10. Whole Life Costs Time series of costs likely to be

required for the system and

Net Present Value (NPV) of these costs

Identifies annual costs incurred in every category (over a 50 year period to determine the cumulative

11. NPV Charting the NPV of cost with

time plus cumulative cost curve

12. Hydraulic

calculation tables

(UK only)

A very simple hydraulic appraisal routine that allowRequired as part of the calculation tool for this pro



108

C.5 Summary of differences between the two WLC models used The following is a list of the major differences between the UKWIR (2009) approach to WLC, devised

by Cascade consulting (C) and HR Wallingford’s model (HRW)

C refers to environmental and social benefits, HRW to only environmental benefits. C indicates that it produces a CBA for all stakeholders; HRW is from the perspective of the

owner/operator. This difference explains some of the other variations between the two.

C uses a literature review to identify more case specific values to overwrite default values in

spreadsheet tool, HRW has more databases from actual case studies to do this, so arguably,

could be more robust.

C recommends a partnership approach to scheme developments, as many intervention

measures may be better applied either through partnership with others (e.g. local

authorities) or through an integrated framework.

C explores adverse environmental and social impacts. The HRW spreadsheet model only

refers to benefits in the context of improvements (in amenity and ecological values + hydraulic and water quality). As a result in the pilot studies conducted it has been possible to

incorporate environmental and social costs to arrive at NPV of total costs.

C includes broader consideration of sustainability, in that it explores the change in carbon

emissions – stated in terms of CO2 and included in monetised impacts (though there may not

be a difference, given the nature of the disconnection components used by HRW). HRW has

a restricted view of environmental benefits.

C refers to financial benefits and generally all benefits are more easily identifiable being

based on: reduction on sewer flooding risk, water quality benefits (direct economic, public

health, recreational, aquatic ecology), other environmental benefits, operational cost

savings, carbon footprint reduction.

C also refers to financial costs avoided. HRW recognises only actual costs incurred by the

owner/operator. In the Cascade approach this would be because it considers all stakeholders. As a result, C acknowledges that there may be a wide range of non ‐

monetisable benefits of interest to WaSCs and to stakeholders.

C also considers adverse monetisable environment and social impacts avoided.

C suggests that its approach is based on both conventional upgrades to the sewerage

network and disconnections. HRW is based on new build solutions..

HRW is based on new build solutions. C maintains that its pilot studies are based on both

retrofit and those for new developments. C’s report then acknowledges that no suitable site

for retrofit could be found so it undertook two pilot studies based on new developments

because the two had site differences. C then went on to conduct a third study based on a

theoretical pilot site – which was again focused on a ‘new developments’.

C constructs the spreadsheet model to separate and identify the disconnection measures

into ‘realignment of surface water connection’ and ‘run ‐off attenuation measures’. HRW

covers this separation in its design and maintenance options as additional pipework costs

but no realignment costs are included.

C purports that its decision support tool (DST) allows an integrated set of interventions to be

evaluated in one spreadsheet. This allows a range of interventions to be identified on a

spreadsheet. HRW requires a spreadsheet model for each component (intervention).

C provides a template of what can be included to derive a total costs which can then be

discounted over 25 years. However, the data used is not robust (which is acknowledged). It



109

also does not convincingly include costs incurred beyond initial construction and it is

therefore less convincing as a WLC approach.

C 5.1 Similarities between the two modelling approaches1. Both undertake a CBA.

2. Both use reported methodologies, datasets, guidance and default values for derivation of the construction and operating costs.

3. C considers monetisable benefits of improvement drivers. HRW does the same where

possible within environmental benefits included within the four categories of amenity,

ecology, hydraulic and water quality. Both exclude non ‐monetisable benefits/ impacts from

the CBA.

4. Both identify non ‐monetisable benefits to allow them to be considered in decision making

even if not included in the CBA. The user is left to decide how relevant these factors are to

their decisions.

5. Neither provides guidance on the selection of intervention measures appropriate to a

particular study area. They provide the CBA methodology and allow either experienced engineers (C) or operators to select among alternatives.

6. Neither addresses issues of ownership and adoption.

7. Both explore financial construction and operating costs.

8. Both have a form of advanced user input to allow overwriting of default values – including

the addition of new measures and changing parameters (e.g. the assessment period and

discount rate) or when more specific data becomes available (rather than default mode).

C 5.2 SummaryThe Cascade approach goes further than HRW in relation to an integrated approach to WLC with

a much broader range of environmental and social impacts (benefits and costs avoided), but it is not obvious from the report that this has actually been delivered. The HRW model was

developed as a costing tool, to try to build up generic costs for individual systems. The

quantification of benefits was not an aim of the original model. The Cascade report does deliver

DST guidance and a data capture proforma which is acknowledged as only tested with small ‐

scale intervention measures and readily identified benefits (see p.17) – and these are not

interventions based on retrofit but for new development sites.

The UKWIR (2009) Cascade report also acknowledges the limitations of being reliant on default

unit values used in the intervention measures and for willingness to pay study values in the

simple benefits tool.

Overall the Cascade approach provides a decision tool that has not been sufficiently tested and

is described as “providing the mechanism for identifying the criteria that produce a positive cost

benefit for all stakeholders”.

Generic, simple models indicative of pilot conditions should be developed for more robust cost

estimates or to reduce uncertainties.



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Appendix D - Practicalities of retrofitting SUDS

D.1 Ownership and maintenance The implementation of comprehensive SUDS schemes is still relatively new in the UK even for new

developments, although it is an important component of planning for the management of flood risk in new developments, described in Planning Policy Statement 25 (CLG, 2006). In practice the

approach has faced barriers, particularly the issue of who is to take responsibility for ownership and

maintenance (EA, 2007). The Construction Industry Research and Innovation Association (CIRIA) website summarises the situation as follows:

“The ownership and maintenance of conventional piped drainage systems is clearly defined in Sewers for Adoption (Water Services Association, 1994). However, by their nature, many SUDS can be considered either drainage or landscape features, and there is no clear guidance on who is responsible for the operation and maintenance of such facilities. A trial framework agreement has been drawn up in Scotland to remove this barrier to the greater use of SUDS. Due to the different

legal duties, a country ‐wide agreement of this kind will take time to evolve in England and Wales. However there is scope for individual maintenance agreements to be negotiated on a site ‐by ‐site basis. ” (http://www.ciria.org.uk/suds/planning_england_and_wales.htm )

Furthermore, SUDS are not legally described as ‘sewers’ in ‘Sewers for Adoption’ (6th Edition) (Water

UK and WRc, 2006), therefore water utilities have been unable to build or adopt them. Because of this, the national SUDS working group in England and Wales produced an Interim Code of Practice

for Sustainable Drainage Systems (NSWG, 2004) as well as model maintenance agreements (CIRIA, 2004) in order to help overcome these issues (Stovin and Swan, 2007).

The Flood and Water Management Act 2010 was passed in April 2010. The Act will end the

automatic right to connect to sewers for surface water drainage and require developers to put SUDS in place in new developments wherever practicable in accordance with new ‘National SUDS

Standards’. Connection will be conditional on meeting these new national standards on SUDS and

drainage to the satisfaction of the ‘SUDS Approval Body’. The water industry is currently involved in

developing these standards and it is expected that new SUDS will be adopted and maintained by

local authorities (Defra, 2009). Although there are concerns about how this will work with multiply ‐

owned SUDS trains and critical drainage systems and also about how existing systems will be

managed retrospectively. The advent of a new Government has, however, delayed the rolling ‐out of the new legislation and the precise details of how the responsibilities will be delivered.

D.2 Retrofitting

New developments only account for around one percent per year of housing stock and SUDS implemented there will make only a limited contribution overall to reducing flood risk and improving

water quality, although there are also opportunities in regenerating areas. There is an obvious need

for a combined strategy to install SUDS in new developments as well as to retrofit them into existing

ones (EA, 2007).

The philosophy of SUDS is to replicate as closely as possible the natural drainage from a site before

development (CIRIA, 2007), and SUDS should be considered as part of the whole water cycle within a

catchment area. This is an easier task for a new development than retrofitting into an area where

the drainage may already be far removed from its natural state. However, SUDS can be used to

augment (or replace) existing drainage systems in a developed catchment and studies have



111

suggested (Swan et al., 2001) that SUDS could provide cost ‐effective hydraulic improvement, either

as fully SUDS‐based or partially SUDS‐based rehabilitation strategies (Stovin and Swan, 2007).

Arrangements for adoption and maintenance can be more complex for retrofit SUDS. The

Environment Agency summarise issues for some of the stakeholders involved in retrofit SUDS as

follows: “Many of the decisions on SUDS retrofits are the responsibility of property owners. This includes private domestic and commercial properties, and public properties such as schools, leisure centres, hospitals and the associated hard ‐surfaced areas and roads. For instance, the responsibility to retrofit permeable paving resides in organisations that sit outside the formal regulatory process of water management; these include local authorities in a variety of roles as highway authorities, as planning authorities and as property managers in their own right, but also property developers and property owners and managers. A consequence of this is that incentives for change will need to be directed at changing the behaviour of property owners .” (EA, 2007).

D.3 Costs Whilst there is little practical experience of the implementation of wholesale SUDS schemes in

England and each is very context specific, it is possible to cost the schemes. The SUDS manual (CIRIA, 2007) recommends the use of Whole Life Costs (WLC) because many of the benefits of SUDS will accrue over the long term and to a broader range of stakeholders than those of the alternative

piped/sewered drainage. This necessarily includes some evaluation of intangible benefits making

comparison of costs less direct. It does, however, highlight and allow for the inclusion of the multiple

benefits of SUDS in terms of the social and environmental as well as the economic; thus emphasising

their potential sustainability value. A methodology for the calculation of WLC of the retrofit case

study examples in this report is given in section 7.

D.4 Institutional/individual ‘Lock -in’; a resistance to change to a more

resilient approach

D.4.1 Regulation / governanceThe move towards more sustainable management of water resources in the UK and particularly in

England is slow for a number of reasons. This can be attributed in part to the complex way in which

the interconnected parts of the water cycle are managed, with separate and diverse organisations

involved at each stage, often with unclear responsibilities (Rouse, 2007). Public drainage

responsibilities are divided between local authorities and internal drainage boards, highways

authorities, sewerage undertakers and the Environment Agency. Regulatory bodies and other

interest groups such as Ofwat, the Consumer Council for Water, Natural England and the Royal Society for the Protection of Birds also have an influence on water management, mainly by direct

contact with Government departments (Ashley and Brown, 2008). There is a reluctance to innovate due to lack of incentives and barriers (UKWIR, 2006; Cave, 2008) and major difficulties in working

across organisations to deliver the best response to extreme events; especially as the private

Sewerage Undertakers do not have to share vital information about both water resources and flood

risk from sewers. European Directives also provide challenges to planning for security of water

supplies in England (Rouse, 2007) in that they are apt to constrain best practice and compel

investment into areas that may not be a priority when considered in the perspective of society’s

overall needs.

Differences in arrangements in Scotland appear more conducive to the ‘social learning’ required to

deliver transitions, regime change and total water quality management (Ison & Watson, 2007).

Similarly, Dŵ r Cymru Welsh Water can take a wider and more holistic view of the way in which it



112

manages the water cycle, supported closely by the Welsh Assembly Government. This is because

although technically private, Welsh Water has been handed back to the people of Wales and is a

not ‐for ‐profit organisation that has less incentive to increase the value of its’ assets for capitalisation

purposes than privatised Sewerage Undertakers. Welsh Water is showing signs of transition in its

aspirations, including: motive (people not capitalisation); Governance (support and national

identity); vision (champions at the heart of the organisation) (Blackmore, 2004; Page & Bakker, 2005; Saal et al, 2007).

D.4.2 Cognit ion; sticking to the traditional approach in the face of newproblems

The conventional regime, very often seen as the ‘common sense’ approach (Ashley and Brown,

2008), essentially delivers a ‘one ‐size ‐fits ‐all’ design concept ‐ or ‘big‐pipes ‐in’ and ‘big‐pipes ‐out’

(Newman, 2001), which is largely independent of contexts and tends to optimise control of the most

likely future condition (for example ‐ water supply scarcity or excess runoff) without consideration of the rest of the total water cycle; leaving systems vulnerable to future changes (Pahl ‐Wostl et al,

2007). This approach is grounded in the tradition of rational planning, which formulates design

problems as trade ‐offs of costs, risks, and benefits that are dependent on variables such as climatic conditions that change only slowly over time. Trade ‐offs are then evaluated by optimisations or

simulations based on historical data such as rainfall records, which are no longer reliable (sometimes

called stationarity philosophy (Milly et al, 2008)). This minimises the opportunity to create city

specific urban water services and systems that could be integrated into the urban landscape in

alternative and relatively radical ways that allow for substantially higher resilience and other

benefits such as greener landscapes and heat ‐island sinks (McEvoy, 2007). Ashley and Brown (2008)

suggest a critical and highly explicit reflection on these fundamental stationarity design and

operational philosophy, assumptions and design as a starting point for transformation in the

conventional urban water management approach.

D.4.3 Normative lock-in; how to transform systemsThere have been many advances towards alternative options and processes for designing adaptation

opportunities to uncertain future conditions in recent years. These can be integrated within,

superimposed upon and/or able to replace the existing urban water infrastructure. Yet despite this

substantial investment in developing technological and assessment alternatives, there is an

increasing and overwhelming despondence within the urban water research community with the

lack of change in conventional practice (Ashley and Brown, 2008). Many observers highlight how the

compartmentalisation of infrastructure and service provision is being reinforced and leaving the

sector ill‐equipped to respond and adapt to complex sustainability challenges (Marsalek et al. 2001;

Newman, 2001; Brandes and Kriwoken, 2006; Wong, 2006). Emerging research in areas such as

institutional capacity building (Brown et al, 2006), organisational development (Brown, 2008), public

engagement (Sefton, 2008) and, urban water professional receptivity and leadership (Taylor, 2008) has not been drawn upon to help bridge system boundaries. A vision to reverse this situation is now

being implemented in a number of EU projects: SWITCH3 ; MARE4 ; SKINT and FloodResileinCity 5, where as wide a range of stakeholders as needed in order to facilitate change and to engage in the

active learning form a Learning (and action) Alliance. The core alliance can act to engage in both

policy processes and on the ground delivery of responses needed to cope with future challenges. It also forms a vehicle for providing knowledge, information, techniques and consensual learning.

3 SWITCH ‐ Managing Water for the City of the Future, http://www.switchurbanwater.eu/la_switch.php 4 http://www.mare ‐project.eu/ 5 http://www.floodresiliencity.eu/en/about/



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D.5 Implementation of non - piped/sewered systems

Pilot projects (e.g. those conducted under Defra’s auspices) have shown that alternatives to

piped/sewer drainage are a feasible option, although the difficulties of delivering these given

the complex relationships between stakeholders involved in implementation, should not be

underestimated.

The best mechanisms of effecting a paradigm shift to the use of non ‐piped/sewer drainage

systems is still under debate. Local authorities have a key role; in particular the planning

authority is central in the promotion and specification of non ‐piped/sewer systems for new

developments. However, the place of WaSCs is still also crucial due to the need for excess

flows from non ‐piped/sewer systems to be directed into their piped assets; therefore WaSCs

need to become statutory consultees in the planning process as is proposed in the F&W Bill and in the Ofwat recommendations for amendments to the regulatory and legal options for

reducing surface water in sewers.

In the UK, typically drainage considerations are a long way down the priority list for

developments and developers. This needs to change. In the USA, responsibilities are usually at municipal level, or shared with a stormwater utility; hence this approach is easier to

implement there than in England and Wales.

Each individual situation (catchment) needs to be looked at specifically where disconnection

of non ‐foul flows from foul or combined flows is being considered. There is no ‘magic ‐bullet’

approach.

In order for stormwater disconnection measures to offer an effective solution, change is

required to facilitate the uptake; such as the refinement of separate stormwater charging to

provide increased financial incentive. Furthermore, the requirement for public and

stakeholder engagement is critical to the establishment of novel water supply and sanitation

systems.

D.6 Non - piped/sewer systems and water quality

In general planning and implementing the technical aspects of planning, design, adoption

and operation of non ‐piped/sewered systems is now quite clear but their

quantity/quality/amenity performance is still not properly understood.

There is proportionate variability, but generally urban diffuse runoff pollution is potentially

significant. Stormwater can be more polluting than foul flows discharging from combined

sewer overflows.

Interest in the quality performance of non ‐piped/sewered non ‐foul flow systems in England

and Wales is not as prevalent as in Scotland. This represents a failure to provide the

complete nexus of quantity ‐quality and amenity that makes alternative drainage systems

potentially more valuable (in both a tangible and intangible sense) than conventional

systems. However, there is concern that implementation of the WFD will potentially add to

other environmental impacts as illustrated by carbon footprint studies showing the potential

transfer of polluting burdens from the water sector to the energy and waste sectors.

D.7 Non - piped/sewered systems and water quantity

Predicting even the likely changes in non ‐foul flow rates and volumes in the future is

increasingly uncertain, which supports the essential need to take an adaptive and flexible



114

approach. Emerging studies are now showing objectively that non ‐piped/sewered drainage

systems are inherently more flexible and adaptable.

Urban area form and layout need to be better managed to ensure that there are clear

exceedence flow pathways as part of the synergy of the management of water with a multi ‐

functional benefit

for

urban

areas.

Retrofitting rainwater harvesting systems could potentially significantly reduce flood

frequencies and volumes in areas that suffer from frequent flooding provided the RWH tanks

are larger than is required simply to satisfy water demand.

D.8 Perceptions of non - piped/sewered systems

The likely impacts of climate change are still not being considered by most stakeholders.

Stakeholders have expressed concern about the performance of SUDS systems in the flood

plain when they become inundated and new guidance is needed to deal with this.

Utilisation of non ‐piped/sewered non ‐foul flow systems is still seen by many as a greater risk

than piped/sewer systems. There is greater confidence in their performance regarding water

quantity than water quality.

There is confusion amongst stakeholders at all levels regarding the effectiveness of SUDS; the draft Flood and Water Management Bill appears to promote the use of non ‐

piped/sewered drainage systems in preference to piped/sewered, with often an unrealistic

expectation of the likely success.

Misdirected funding based on preconceptions about the best return for investments can be

inefficient when retrofitting to alleviate stormwater pollution problems.

D.9 Current surface water management initiatives

The current initiatives in England regarding surface water management show a lack of appreciation of the need to allow properly for adaptation to changing circumstances.

The lack of provision for more flexible types of regulatory, technical and economic

approaches for the future may compromise future resilience to flooding.

There is still a degree of incompatibility across the various non ‐piped/sewered drainage

approaches to performance standards, although most now advocate a risk‐based approach,

this is still often only qualitative.

There is little indication that joining up the water cycle is seen as the best way to manage

surface water and flooding despite the emergence of Water Cycle Studies.

There is an apparent inability of surface water management deliverers to take a catchment

wide perspective rather than adopting (many) localised measures

Reducing the quantity of stormwater at source through strategies to reduce imperviousness

and maximize infiltration and filtration can be more cost effective than capturing and

improving the quality of vast quantities of urban stormwater runoff.

At municipal level, costs can be decreased when such techniques are incorporated into

redevelopment and ongoing replacement efforts.



115

D.10 Use and adoption of SUDS by Sewerage Undertakers

SUDS do not comply with the definition of a sewer (i.e. having a proper outfall) as stated in

Sewers for Adoption (6th edition) therefore they cannot be included as part of a Sewerage

Undertaker’s asset base. This will be perceived negatively by shareholders and Ofwat.

However, it does not preclude a sewerage undertaker from deciding to ‘take care’ of the

‘structure’ nonetheless.

The absence of formal mechanisms to enable Sewerage Undertakers to adopt SUDS (as

assets recognised by OFWAT) is probably the most significant deterrent to the proper

consideration of SUDS in new developments by developers and in retrofit applications to

solve problems such as sewer flooding under DG5. Although SUDS could be adopted by

parties other than the Sewerage Undertakers (e.g. local authorities) there is no obvious

incentive for others to take on this responsibility unless for example a management

company can charge fees to beneficiaries.

The debate surrounding which organisation might be best placed to manage stormwater is

ongoing, and is one of the key issues being tackled as part of the DEFRA IUD Pilot programme (Gill, 2008), but has come out in favour of local authorities having the

responsibility under the Flood and Water Management Act 2010, although there is strong

pressure by a number of sewerage undertakers for them to be given the operation and

maintenance role in the enabling legislation for the new Act. In any case, the new Act makes

provision for the taking of previously excluded land drainage into sewers inevitable.

Sewerage Undertakers are reluctant to implement SUDS structures because their

maintenance requirements are not something they consider to fall within their normal line

of business. Also, there is a perception that the lack of knowledge about the long term

performance of SUDS makes their adoption very risky (WERF/UKWIR, 2005). However, the

option of brokering agreements with third parties (e.g. local authorities, independent contractors) to undertake maintenance is being considered by several Sewerage

Undertakers.

Sewerage Undertakers might be wary of using SUDS (especially above ‐ground components)

in retrofit applications because this may then establish a precedent for future adoptions

associated with new developments.

There is nothing to prevent a Sewerage Undertaker from retrofitting either surface or

subsurface SUDS on land that they own. However, both the SNIFFER (2006) project and

Stovin et al (2007) showed that constraining options to publicly ‐owned land would lead to

only marginal improvement in the catchment as a whole. Mechanisms/incentives to include

privately owned land within SUDS retrofit schemes are needed.

Where SUDS options are proposed on privately ‐owned land, a distinction needs to be made

between surface and sub ‐surface SUDS. In some cases it might be feasible to consider

purchase of land deemed suitable for the retrofitting of surface SUDS, such as ponds, which

make the land unsuitable for further development. Alternatively, a local authority may agree

to designate land for dual use, such as playing fields which are also designed as detention

basins for low frequency events. In the case of sub ‐surface SUDS, one ‐off ‘deals’ to

compensate landowners for disruption could be brokered to enable a Sewerage Undertaker

to retrofit a SUDS structure on private land. However, the uncertainties surrounding the

responsibilities for long ‐term maintenance of such structures would still need to be

addressed.



116

At the present time there appear to be inconsistencies in the way private land ‐owners are

encouraged to implement source controls. Some Sewerage Undertakers offer to waive the

stormwater management component of customers’ water bills if they disconnect their

stormwater from the sewer system. However, this is usually a yes/no condition, with no

scope to recognise partial disconnection. The waived charges are also very small (circa £30

p.a.). Banded charging schemes reflecting the actual level of connected impermeable area would be welcomed. The definition of disconnection is unclear, and it is understood that

present charging schemes may imply that in situations where flow may enter the sewer

under extreme events (exceedance flows) then strictly the landowner should be charged for

connection. Legislation to support disconnection up to ‘reasonable’ design events would be

welcomed, although it is clear that strategies for dealing with the exceedance flows would

need to be formulated and funded in parallel.

The system is complex, and there are conflicts within current advice. For example, PPS25

suggests design to 100 year return periods, whereas Sewers for Adoption uses 30 year

return period criteria. The forthcoming take over of (communal) private sewers by Sewerage

Undertakers may help to integrate certain aspects of sewer network management, but it

may also act as a disincentive to developers who are considering the use of SUDS techniques

as they would preferentially wish to install piped/sewered drainage systems – as has

happened in Scotland.

Stovin et al (2007) proposed a solution to a DG5 flooding problem in Anglian Water’s region.

This was achievable using a realistic budget and also appropriate timescale. However, it also

resulted in a conflict between the ‘normal’ engineering process (and time constraints) and

the practical need to address the many novel issues associated with the use of SUDS.

The case study above also highlighted problems associated with integrating teams within the

sewerage undertaker. This showed that implementation and detailed design could not be

managed effectively by people remote to the site nor the organisations that will ultimately be responsible for implementing and managing the new SUDS facilities. Scheme success is

contingent on local champions, particularly where new approaches, such as SUDS retrofit

are being trialled.

D.11 Added benefits of ‘joined -up’ surface water management

The introduction of multiple ‐use basins for stormwater management and the use of ‘greening’ approaches have had a positive effect on property prices because of the premium

that buyers place on vegetation and conservation development.

Studies have shown that the restoration of river corridors can actually lead to a reduction in

local crime although badly thought out FRM measures can create spaces that can be

conducive to criminal activity.

There are a number of other benefits accruing to society as a whole that are increasingly

being recognised, especially in the USA, where a major project is enhancing the scope of assessment of the monetary value of green infrastructure (GI), implemented by retrofitting

stormwater management measures to relieve CSO pollution in major cities, such as

Philadelphia (Wise et al, 2010). This has shown that GI delivered from disconnecting

stormwater from the combined sewers can have multiple US$ billion benefits to urban areas

as a whole. This is important in the US context as there the sewerage systems are managed

by the local authorities, who can therefore promote multi ‐beneficial outcomes for the good

of their cities.



117

Appendix E - Design detai ls for SUDS retrofits in exemplarareas

E.1 Lytton Grove Subarea retrofit SUDS The overall Subcatchment being examined is that bounded by Lytton Grove to the North, Putney Hill to the West and West Hill to the East, Figure E‐0‐1. For the purposes of the analysis this has been

subdivided into 13 subareas each with different characteristic land uses.

Figure E‐0‐1 Lytton Grove sub ‐catchment with sub ‐areas showing roads highlighted

The first areas considered were areas 1‐4 which were considered to be suitable to be collectively

drained northwards, with areas 2‐4 draining into area 1, as shown in Figure E‐1, and shown to a

larger scale in Figures E‐0‐2 – E‐0‐5. Three options have been considered for these areas, all based

on detention basins and interconnecting swales and sewers between these and ultimately into a

sewer assumed to run in Lytton Grove (Figure E‐0‐2).

Main slope

direction



118

Lytton Grove Option 1: drain disconnected impervious areas from areas 1‐4 to detention basins situated in the lowest part of the Subcatchment in area 1.

Subarea 1 is the lowest area of the overall Subcatchment, which drops some 20m over the 1km

length from the southern end (Tibbet’s Corner) to Lytton Grove. Shown in Figure E.2 are the

maximum potential areas for locating detention basins. This is Option 1 and has been used initially

to assess whether or not these would be adequate to drain all the impervious surfaces for areas 1‐4.

The potential areas for surface storage are shown as A‐D for option 1. These are linked by swales

and pipes. The outflow from the system is controlled to a maximum of 5 l/s from each of the two

connections to the main sewer in Lytton Grove for a 1 in 30 year storm. The initial design considers

the potential for this area to also drain the runoff from the disconnected impervious areas from the

surrounding areas 2 – 4 (Figures E‐0‐3 – E‐0‐5).

The sizes of the units in Figure E‐0‐2 are given in Table E‐0‐1.

Legend for all following OS Mastermap Figures:



119


Figure E‐0‐2 Option 1 – detention basins all in the lowest area (1) adjacent to Lytton Grove

A

B

C

D

Piped connections to

main sewer

Linking swales with

pipes under roads

Swale from area 4 (included

in data for 4)



120

Table E‐0‐1potential sizes of drainage units in Area 1

Sewers Swales Stepped detention basins

35m 40m

50m

A 80m long x 42m wide 3360 m2

22m B 42m x 6m

+48m x 25 x 0.33m

+ π x 14^2/4

802m 2

10m linking swales C 65m x 26m

+ 135m x 20m

+ 65m x 22m

5820m 2

10m linking swales

Smaller drains not included

D 35m x 0.33 x 27m 315 m2

This provides a maximum pond plan area of 10,308 m2.

The potential contributing impervious disconnected areas from each of areas 1 – 4 are given in Table

E‐0‐2. The intermediate roads are not included in this table. It is also assumed that the runoff from

garden areas will not be directly connected.

Table E‐0‐2 potential impervious areas that could be disconnected in areas 1‐4.

Land use Local plots (m 2) (1)

Catchment to west (m 2) (2)

To south west (3)

Immediate south (4)

buildings 7356 4615 9587 5121

Internal roads 3435 1729 0 0

man made land (car parks) 6000 2945 7909 5175

Total impervious areas

(m2) 16,792 9,291 17,499 10,300

The detention basins will have a maximum depth of 0.6m as will the conveyance swales. The total

collective storage volume of the four units A‐D is approximately 4000m 3 ignoring any additional

swale volume storage. Using the HR Wallingford whole life cost model, which includes a simple SUDS

analysis tool, this indicates a critical storm duration of 4 hours and a required storage volume of 1,400m 3. The whole life costs are given in Table E‐0‐3, totalling some £300,000.

Table E‐0‐3 present value costs of the detention basins A‐D in Area 1.

Present Value (£) Basin Capital Reg O&M Corr O&M Waste TotalA 42,000 29,900 12,600 6,400 90,900

B 17,300 17,800 3,900 6,300 45,300

C 61,200 39,800 18,400 6,300 125,700

D 13,300 15,700 2,700 6,300 38,000

The swale plus associated pipework costs are given in Table E‐0‐4.



121

Table E‐0‐4 present value costs for swales and associated pipework in Area 1.

Present Value (£)

Swale (m) Capital

Reg O&M £ Total

50 12,400 1,600 14,00040 12,000 1,700 13,70010 4,000 1,200 4,90010 4,000 1,200 4,900

These costs allow only for standard piped connections where crossing roads within the catchments

and do not allow for smaller interconnections or complications with services. The costs also do not

include any land purchase. The connecting swales for the other areas, 2‐4, are not included in these

costs.

In Option 2 below, it can be seen that the total length of swales and local sewers is some 1500m and

100m (Table E‐0‐5) respectively. Using the proportionate costs as for the swales and sewers above,

this gives additional whole life costs of £423,000. Therefore the total costs of disconnecting the

impervious parts of the four areas, 1‐4, draining through area 1, as shown in Figure E‐2 is some

£723,000. This will drain these areas comfortably and the sizes of the storage units could actually be

reduced as described below.



122

Option 2: Potential to utilise green space in the other areas (2‐4), Figure E‐0‐1.


Swales (m) Basins (m2) Drains (m)

150m, 40, 52, 59, 18, 65, 35,120.Total 499m

30 x 18

540 m2

3 No. at 10m = 30m

Figure E‐0‐3 Area 2 – potential for local storage of surface water.

Figures E‐0‐3 – E‐0‐5 show where there is potential for storage further up the system in areas 2‐4,

rather than siting it all in the lower sub ‐area (1) as shown in Figure E‐0‐2.



123



50, 40, 48, 26, 20, 44, 45, 40, 29, 89, 42. Total

473m

36 x 12, 40 x 15, 26 x 10, 31 x 10. Total

1602 m2

40m. Plus local

connections

Figure E‐0‐4 Area 3 – potential for local storage of surface water



124



44, 180, 64, 45, 40, 40, 18. Total 431 m 40 x 15, 30 x 10. Total 900m 2 2 x 12. Total 24m

Figure E‐0‐5 Area 4 – potential for local storage of surface water.



125

Table E‐0‐5 shows the potential surface areas for the detention basins, together with connecting

swales.

Table E‐0‐5 costs of alternative potential detention storage and connecting swales in areas 2‐4.

Area No. Basin area

(m 2) Swale length

(m)

Total impervious area

disconnected (m 2)

Drains (m)

Basin Costs (£)

Swale costs (£)

2 540 499 9289 30 46,800 153,700

3 1602 473 17496 40 178,100 145,100

4 900 431 10296 24 90,700 124,500

Total costs 305,600 423,300

From this, it is apparent that the sizes of the detention basins in Area 1 (Figure E‐0‐2) could be

reduced accordingly by a total of 3,042m 3 as the total available in the other areas as shown in Table

E‐0‐5, i.e. the original 10,297m 2 of basin area in Area 1 could be reduced to 7,255m 2 (saving

£171,500) and still achieve a similar level of control. However, there would be additional costs due

to the dispersed location of the storage. The swales shown in Figures E‐0‐3‐E‐0‐5 would be required

whichever option was selected.

Option 3: minimise the storage areas/volumes required to drain the disconnected areas 1‐4.

For this option the storage required to drain each of the four areas 1‐4, is known to be

approximately 1400m 3 (from above, Option 1). Hence the detention pond volume can be reduced

overall to this figure; i.e. the costs for the storage units can be reduced from approximately £300,000 to some £163,700; the plan area for the basins required also reduces from the maximum

available of 10,308m 2 to 2,650 m2 taking up a smaller area in Figure 6.6 if all of the storage is located

in area 1. The costs of the swales and sewers will, however, increase because the swale length will increase by the shortened pond lengths of some 300m at a cost of £97,700. Resulting in an overall

cost of £163,700 + £97,700 + 423,300 = £684,700. This option has the advantage that the required

detention basin plan areas as shown in Figures E‐0‐2‐ E‐0‐5 can be reduced by two thirds.

Table E‐0‐6 present value costs of the detention basins A‐C in Area 1, Figure E.6.

Present Value (£)Basin Capital Reg O&M Corr O&M Waste Total

A 22,100 20,135 5,573 6,355 54,163

B 17,497 17,809 4,077 6,256 45,640

C 27,463 22,749 7,476 6,256 63,944

Total 163,748



126

© Crown Copyright and database right 2009. Ordnance Survey Figure E‐0‐6 required detention basin area for areas 1‐4 if located entirely in area 1. (Swales and piped connections not

shown).

The WLC for these alternative detention basins are given in Table E‐0‐6.

B

A

C



127

Areas 5‐13 and roads: options for disconnection

The other Subareas numbered 5‐13 comprising the Lytton Grove Subcatchment are summarised in

Table E‐0‐7 and shown in Figures E‐0‐7 – E‐0‐15. Table E‐0‐7 shows the different impervious

disconnection areas, gardens and also the total road surface area in the Subcatchment (Figure E‐0‐

1).

Table E‐0‐7 Impervious areas in the remainder of the Lytton Grove Subcatchment

Land use (m 2) (5) terraced housing to SE

Catchment to SE (m 2) (6)

Eastern corner (7) East of (7) (8)

buildings 4340 5717 3714 3891

roads 0 0 0 0


gardens 11255 14709 11181 9119

comments Drain to gardens Also 930m 2

green triangle

Land use to be drained Small area on Putney Heath Lane (m 2) (9)

SW edge (m 2) (10)

Just North of area 10 (11)

South of Putney Heath Lane (12)

buildings 643 7478 1347 2618

roads 0 0 0 0


gardens 1379 5451 + 10097 1610 291+5392

Land use to be drained Small area to south of (12) (13) (m 2)

Total area roads (m 2)

buildings 1812

roads 0 88010

man made land (car parks) 477

gardens 3719

Table E‐0‐8 summarises the subareas 5‐13 and the roads and the potential retrofit options as

alternatives to piped drainage within each discrete area itself. For example, it is proposed to

disconnect as much as possible of the 4340m 2 of roof drainage in Area 5 and reconnect it on to the

garden areas. This may also include use of water butts (enlarged cisterns) installed to the rear of the

properties.

There may be other options to consider draining some of these subareas into adjacent areas,

however, for this first level analysis, stormwater has been considered to be managed only locally

within each sub ‐area.

The assumptions used for the following SUDS retrofit designs are summarised below.



128

1. New pervious paved areas to replace existing impervious areas:

a. Only a maximum of 75% of the paved area can be converted to pervious pavements

– to allow for the practicality of fitting the pervious area not too close to buildings

b. Not all paved surfaces are shown in OS Mastermap and google. Hence areas

connected to the pervious pavements also include an additional 5% assumed for

minor impervious areas such as property drives etc.

2. Disconnecting the property downpipes from roof areas and reconnecting these to adjacent

pervious areas:

a. Most disconnected roof areas to gardens are assumed only from the back (pitched)

roof areas

b. No water butts/cisterns have been included for cost reasons, however, unit costs for

installing these are provided as they may be added later or at a property owners’

wish.

A summary of costs and areas disconnected for Lytton Grove subcatchment is given in Table E‐0‐9.



129

Table E‐0‐8 Options and costs for alternative drainage for subareas 5‐13 and the roads in the Lytton Grove S

Sub Area No.

Characteristics Options for draining disconnected areas

Requirements HR Wallingford model

Impervious

5 Terraced Housing

with rear

gardens

and small front

gardens

Disconnect roofs and

drain to

garden

areas.

Simplest to do this with

back roofs and maybe use

rainwater cisterns

Downpipe

disconnection.

Optionally install

rainwater barrels or

over ‐sized

rainwater cisterns

(at additional cost)

N/R Disconnect

back gardei.e. 2170 m

40 properti

6 As above but semi ‐

detached and with

some paved hard

standing.

Disconnect back roofs.

Consider altering paved

areas to subsurface

storage.

Replacement of hard standing with

permeable surfaces

and subsurface

storage.

Infiltration with

subsurface

storage

Remove 50gardens: 2properties

Convert 751096m 2

As: 19 x 24m; 2.5 x 19x 0.5m

7 As above Disconnect back roofs.


areas to subsurface

storage.


permeable surfaces

and subsurface

storage.

Infiltration with

subsurface

storage

Disconnectroofs to gaeach).

75% Pervio159m 2: 6 x

2.5m 8 As above but larger

properties some

detached



areas to subsurface

storage.


permeable surfaces

and subsurface

storage.

Infiltration with

subsurface

storage

32 propertiback roofs1986m2 (e75% Pervio25 x 2.5m



Sub Area No.



Impervious

9 Separate property

with limited land to

disconnect to.



areas to subsurface

storage. Partial disconnection only

for this one property.


permeable surfaces

and subsurface storage.

Infiltration with

subsurface

storage

50% of one321m 2.

50% of roostanding. 375% Permeplus the abarea into:

x 4m; 15 x10 Mixed large

residential

properties and

institutional.

Institutional properties

replace hard standing

with permeable

pavement draining to

subsurface storage.

Connect roof drainage

also.

Permeable paving

and subsurface

storage for

institutional

properties.

Disconnect roofs of private houses to

gardens.

Infiltration with

subsurface

storage

16 buildingwhole of r7478m 2.

Convert 75pervious: 515m; 45 x

52 x 38m;

x 11m; 80

2m

11 Terraced Housing

with rear gardens

and small front

gardens

Disconnect to rear

gardens only and

consider cisterns

Downpipe

disconnection.

Consider Installing

over ‐sized

rainwater barrels

(cisterns)

Permeable paved

areas

Infiltration with

subsurface

storage

Disconnectgardens. 5

Make 75%permeable:37 x 3m; 23m; 6 x 6m



Sub Area No.



Impervious

12 Mixed housing

density with paved

hard standing

Consider options:

disconnect to gardens;

pervious pavements in

hard standing areas; detention basins.

Keep water on the

surface if possible.

Then below ground

options.

Infiltration with

subsurface

storage

Disconnectgardens: 2

Convert 75pervious 120 x 4m; 78m; 5 x 60x6m

13 Discrete

development of semis

Make access road

pervious with subsurface

storage. Disconnect roofs

both back and front and

add cisterns.

Infiltration with

subsurface

storage but

with stronger

construction

Disconnectareas to ga

Convert ro477m 2. 14m diame36 x 9m

Include res(906m 2).

Roads Mixture of internal

access and through

roads and

peripheral major

trunks.

Look for opportunities

for: diverting road

drainage to adjacent

permeable surfaces; then

in quieter roads where

the width allows, street

gardens. Long‐term

promote porous road surfaces.

Mixture of options

taken up in the

sequence shown.

Infiltration with

subsurface

storage but

with stronger

construction

Assume 50(not trunkconvertedinto adjace88,010m 2

above & 525% for ea

1‐4 and 13and alreadyConversionsurfaces w20,592m 2



Table E‐0‐9 Lytton Grove Subcatchment retrofit SUDS summary

Subarea


(m 2)

Impervious area removed entirely

Flow attenuation/reduction

Impervious area directed to pervious WLC

(m 2) % (m 2) % (m 2) % (£) 1* 21130 16792 79 627,3002* 11903 9291 78

3* 21814 17499 80

4* 12540 10300 82

5 9968 2170 22

6 13072 1096 8 2858 22 145,9007 12768 159 1 1857 15 32,2008 8486 68 1 1986 23 21,100

9 4270 472 11 321 8 76,40010 21063 5789 27 7478 36 715,70011 2704 414 15 674 25 63,10012 4640 1110 24 1964 42 147,600

13* 4657 1383 30 906 19 102,400Roads

other than

in areas 1‐

4 & 13

164736 20592 25 20592 22 2,658,702,658,70

Totals

260591m2

Overall 125771

m

2

(40%) impervious

area

attenuated

at

a unit

cost

of

£58/m 2 £7,249,10

*roads included in these areas



133


Figure E‐0‐7 Area 5 disconnect back roofs to gardens

Disconnect 50% of roof areas

to back gardens.

i.e. 2170 m2.



134


Figure E‐0‐8 Area 6

Take 50%

of

roof

drainage to gardens: 2858m 2

Convert hard standing

to 75% pervious

paving 1096m 2

19 x 24m

16 x 26m

2 x 19 m

2.5 x 19 m

2.5 x 41m 9 x 8.5 x 0.5m



135



Disconnect 28

property back

roofs: 1857m 2.

75% Pervious

paved areas,

159m 2:

6 x 7m

9 x 7.5m

20 x 2.5m



136



32 properties

disconnect 50% ‐ back roofs:

1986m 2.

75% Pervious

pavement

25 x 2.5m



137



50% of single roof to garden:

321m2

. Multiple

downspouts.

50% of roof to permeable

hard standing. 321m 2.

75% Permeable (472m 2)

roads plus the above

connected roof area into:

50 x 5m

10 x 4m

10 x 4m

15 x 2m

40 x 3m



138



16 buildings disconnect to

grassed areas. 7478m 2.

Convert 75% of hard standing

to pervious :

30 x 3m

7 x 15m

45 x 3m

30 x 3m

15 x 2m

52 x 38m

32 x 30m

23 x 8m

15 x 11m

80 x 6m 60 x 24m

65 x 2m



139



Disconnect 14 back roofs to gardens.

50% is 674m 2.

Make 75% of paved areas

permeable: 414m 2

37 x 3m

27 x 3m

20 x 6m

22 x 3m

6 x 6m



140



Disconnect 75% of roof areas

to gardens: 24 properties

1964m 2.

Convert 75% of paved areas to

pervious 1110m 2:

20 x 4m 7 x 10m

70 x3m

20 x 8m

5 x 60m

7 x 28m

9 x6m

7 x6m



141



Disconnect 50% of roof

areas to gardens: 906m 2.

Convert road to pervious:

477m 2.

14m diameter: 154m 2

36 x 9m



142


Figure E‐0‐16 Main and local roads



143

Parts of the main roadway network, Figure E‐0‐16, can be connected into the porous paved areas in

the adjacent Subareas (1‐13) analysed above and also retrofitted with porous pavement – taking

high axle loads (5000kg). This is included in Tables E‐0‐8 and E‐0‐9.

E.2 Carlton Drive Subarea SUDS

Carlton Drive, Figure E‐0‐17 is illustrative of a number of roads in Putney that have large houses and

inter ‐mixed low rise flat ‐roofed apartment blocks. This area has been selected as representing what

might be done to retrofit the drainage systems in these types of areas. The northern part of the

Drive slopes quite steeply down towards Upper Richmond Road as shown in Figure E‐17. Each

Subarea (1‐6) has been considered individually. The distribution of the surfaces in the subareas is

given in Table E‐0‐10 determined from OS Mastermap and Google.

Table E‐0‐10

Carlton

Drive

and

adjacent

properties

–

distribution

of

types

of

area

(m

2

)

Subarea Number 1 2 3 4 5 6

Total Buildings 1658 3320 4727 1963 3536 105

Of which Flat roofed* 1140 1689 0 1117 934 0

Manmade 1263 5258 4012 532 2020 0

Gardens 931 1865 4794 4156 7826 693

Natural 80 77 983 0 0 0

Roads tracks and paths 0.0 348 1035 18 83 0

Water 245 0.0 0 0 0 0

Other Paved Frontage* 0.0 0.0 573 0 389 0 * Manually digitised from aerial photography

Total Roads Tracks and Paths 11338Of which Roads 7015




Figure E‐0‐17 Carlton Drive and adjacent properties



Table E‐0‐11 Carlton Drive characteristics and costs of retrofits by Subarea

Sub

Area No. Characteristics Options for draining

disconnected areas Requirements Impervious area removed/attenuated

1 Mixed low ‐rise

apartment blocks with flat roofs and

pitched roofs. With

other paved areas, a

pond, street gardens

and limited garden

areas.

Green roofs on flat roofed

apartments with connected overflows to the pond. Paved

areas around pond connected

to pond and street gardens.

Pervious pavement to replace

the hard standing. Back roofs

connected to garden area.

Thin green roof 4 Flat roof areas conroofs and overflowspond: 1140m 2.

Only feasible if relative

levels are appropriate Impervious paved areconnected to plan440m 2 disconnected.

Could be built for

lighter axle loading

units

Front paved area copervious: 550m 2 , al270m 2.

Only back roof areas

considered feasible 50% pitched roof argardens/paved perv

2 Mixed low ‐rise flat

roofed apartments

with car parking and

large properties also

with car parking.

Limited green garden

areas.

Green roofs on flat roofed

apartments. Overflows to

grassed areas and to pervious

paved areas – replacements for

existing hard standing.

Thin construction

green roof 7 Flat roofs converte1689m 2. Overflows

and pervious pavem

Porous pavement 75% of paved areaspervious: 4205m 2. 22 x 30m; 9 x 22m;

8 x 15m; 8 x 15m; 4

28 x 9m; 30 x 6m; 2062 x 8m; 43 x 7m; 18



Sub



3 Mixed properties

including residential

and institutional.

Significant

proportion of paved

surfaces for the

latter and drives for

some of the former

Disconnect half of the pitched

roofs to garden areas. Convert

hard standing

to

pervious.

36 individual

downpipes 50% of pitched roof drained to gardens

paved areas:

2364m

Porous pavement 75% of paved areaspervious: 4215m 2: 228m; 55 x 5m; 35 x

8m; 45 x 8m; 66 x 27m.

4 Several flat roofed

apartment blocks. Significant green

spaces surrounding.

One large property.

Some hard standing.

Use green roofs on

apartments, with overflows to grassed areas. Replace hard

standing with pervious

pavement. Additional paved

areas drained to adjacent

grass.

Thin construction Convert 6 flat roofs

1117m2

with overflo

14 downpipes 50% of 1 pitched roo423m 2.

Porous pavement 75% of paved area cpervious 298m 2: 2 x5m; 19 x 3m; 9 x 6m

Direct connection assuming ground levels

are appropriate

25% of paved area dadjacent gardens: 1



Sub



5 Flat roofed

apartments, several

pitched roof

properties. Some

driveways and

significant green

space.

Use green roofs on

apartments, with overflows to

grassed areas.

Replace

hard

standing with pervious

pavement. Additional paved

areas drained to adjacent

grass.

Thin construction 3 flat roofs made groverflows to gardens

20 downpipes 50% of 7 pitched roogardens: 1301 m2.

Porous pavement 75% of paved areas1869m 2: 22 x 6m; 8

4 x 9m; 15 x 6m; 12

x 10m; 18 x 38m; 359 downpipes 25% of paved areas

adjacent gardens: 6

6 Two pitched roof properties. With

large grassed areas

to rear.

Disconnect rear half of the roof drainage. Any paved areas not

seen in Mastermap can drain

to green areas.

1 downpipe Disconnect 50% of 2gardens: 53 m2.

Roads Flat west ‐east but

with a gradient for

south ‐north leg

running down to

main road

Pocket rain gardens also useful

for traffic calming. Road wide enough and

has no speed bumps at

present

Disconnect 7015 m2

drainage and replacebumps with a minim12m 2 pocket rain ga



Table E‐0‐12 Summary of Carlton Drive costs of retrofits by Subarea

Subarea


(m2

)



Impervious area directed to pervious WLC

(m2

) % (m2

) % (m2

) % (£) 1 3399 1580 46 830 24 259 8 270,200

2 9522 1689 18 4205 44 762,200

3 11283 4215 37 2364 21 510,000

4 4573 1117 24 298 7 561 12 203,600

5 9469 934 10 1869 20 623 7 366,200

6 452 395 87

Roads and

tracks

11138 7015 63 1,578,400

Overall impervious

area

attenuated

27954

(56%)

–

unit

cost

£133/m

2

3,690,600



149


Figure E‐ 0‐18 Carlton Drive Area 1

4 Flat roof areas converted to

green roofs and overflows

connected to pond: 1140m 2.

Paved area

in

courtyard

connected to planters and pond:

440m 2 disconnected.

Front paved area to pervious:

550m 2 also rear pathway 270m 2.

50% pitched roof area

connected to gardens: 259m 2.



4 50% Impermeable Area Reduction for the Entire LTT Model

Page 150

Appendix E SUDS report_corrected_18 Jun 10 v3.doc Printed 26/07/2010


Figure E‐0‐19 Carlton Drive Area 2

7 Flat roofs converted to green:

1689m 2. Overflows to grassed

areas and pervious pavements.

75% of paved areas converted

to pervious: 4205m 2. 22 x 30m

9 x 22m

43 x 6m

16 x 8m

8 x 15m

8 x 15m

4 x 4m

30 x 8m 28 x 9m

30 x 6m

20 x 4m

32 x 26m

62 x 8m

43 x 7m

18 x 18m




Page 151




50% of pitched roof buildings drained to

gardens or pervious paved

areas: 2364m 2. 75% of paved areas

converted to pervious:

4215m 2: 26 x 5m

33 x

28m

55 x 5m

35 x 9m

15 x 4m

27x 8m

45 x 8m

66 x 22m

62 x 5m

25 x 7m




Page 152




Convert 6 flat roofs to green:

1117m 2 with overflows to

gardens.

50% of 1 pitched roof to

gardens: 423m 2.

75% of paved area converted to pervious: 298m 2

2 x 6m

27 x 5m

8 x 5m

19 x 3m

9 x 6m

25% of paved area drained to

adjacent gardens: 138m 2




Page 153






3 flat roofs made green and overflows

to gardens: 934 m2. 50% of 7 pitched roofs connected to

gardens: 1301 m2. 75% of paved areas made pervious:

1869m 2: 22 x 6m; 8 x 26m; 11 x 14m; 4 x 9m; 15

x 6m; 12 x 6m; 29 x 9m; 9 x 10m; 18 x

38m; 35 x 4m

25% of paved areas connected to

adjacent gardens: 623 m2.

Disconnect 50% of 2 roof

areas to gardens: 52.5 m2. Also local paved areas (not

seen on Mastermap.




Page 154



Figure E‐0‐24 Carlton Drive Roads

Convert existing speed

humps to pocket street

rain gardens: 7015m 2

road requires minimum

of 14 pocket rain

gardens of 12m 2.




Page 155


E.3 Chartfield Avenue subarea SUDS This is a typical wide road that currently has no traffic calming measures. This has been selected

to investigate options for introducing on ‐street pocket rain gardens. There may also be opportunities to connect a number of adjacent properties to these. The plan size is potentially

variable. Figure E‐0‐25 shows an Australian system in Brisbane, with a plan area of 20 m2, which

is 2% of the contributing impervious area.

Figure E‐0‐25 Pocket rain garden in road edge in Brisbane

It is recommended that these units have a plan area which is at least 2% of the contributing area,

although effective performance has been observed for smaller sizes.

Figure E‐0‐26 On‐street pocket rain gardens (after Smith et al., 2007)




Page 156


Chartfield Avenue is of a width suitable to install to install these units as both drainage systems

and also to assist with traffic calming. Although the general slope of the land is from south to

North (Figure E‐0‐29), there are also local low points as shown in the distance in the photograph

(Figure E‐0‐27).

Figure E‐0‐27 Chartfield Avenue

Others adjacent roads in the area have similar characteristics, although they may be slightly

narrower as illustrated in Figure E‐0‐28.

Figure E‐0

‐28

Genoa

Road

leading

from

Chartfield

Avenue

(looking

south

with

Chartfield

Avenue

in

the

distance)




Page 157


Chartfield Avenue is approximately 9.6m wide and 1030m long. The total road area is 10375m 2. 2% of this is some 208m 2. For the rainfall in London, the HR Wallingford model shows that a 12m 3

unit volume is required. A stepped unit (7m long x 5m wide at the surface), gives the required

storage volume of total depth 0.9m, requires a minimum of 20 individual units located along the

length of the Avenue.

If more than this minimum number is installed, then some of the adjacent properties could also

be partly drained into them. For drainage purposes each of the units could be installed on the

northern (lowest) side of the Avenue, however, if they are considered as traffic calming measures,

then they would need to be on either, possibly opposite, sides. It may also be necessary to install

2 units at low points in the road, such as adjacent to the junction with Genoa Road. The SUDS

units and costs are given in table E‐0‐13.

Table E‐0‐13 Chartfield Avenue SUDS units and costs

Subarea Characteristics Options for draining disconnected areas

Requirements Impervious area removed

Costs (£)

Road

North side The land has a

fall from South

to North.

Pocket rain

gardens Designed as an

infiltration system

without an

impervious

membrane with

flow through of 5 l/s

drains 10375m 2

20 No. 12m 2

pocket

raingardens.

WLC: 78,400

Road

South side WLC: 78,400

Adjacent

properties

– houses

75 individual

properties in

own gardens

Disconnect

roofs to

garden areas

Disconnect back

roof drainage (50%)

to gardens

9448m 2 plus

6451 m2

Potential to

also connect

adjacent

property front

roof and drive

drainage to

rain gardens

in some

locations

39,100




Page 158


Table E‐0‐14 Chartfield Avenue areas attenuated and summary of costs

Subarea

Total impervious

area (m 2)



Impervious area directed

to pervious

WLC Direct cost estimates

only

Reduced flow

(m 2) % (m 2) % (m 2) % (£) (£) 16.

Roads,

tracks &

paths

14667 10375 71 156,800 5 l/s

through

flow

from rain

gardens Adjacent

houses 15899 15899 100 39,100

Other

imperv

areas

48041

Overall impervious area attenuated 26274m 2 (33%) – unit cost £8/m 2 of surface drained


Figure E‐0‐29 Chartfield Avenue and adjacent properties




Page 159


Appendix F - photographic record of areas used inexemplar designs

(All photographs were taken by Richard Ashley and are copyright)

Figure F‐0‐1 A new development in the vicinity of Lytton Grove (Clockhouse Place) with significant hard standing

Figure F‐0‐2 Garages associated with the development in Figure F‐1 (it is unlikely that this is permeable)




Page 160


Figure F‐0‐3 Lytton Grove looking west towards the lowest point

Figure F‐0‐4 Some drives off Lytton Grove are already permeable




Page 161


Figure F‐0‐5 Lytton Grove ‐ speed humps may be replaceable by pocket rain gardens

Figure F‐0‐6 The Kersfield Estate off Lytton Grove has lots of grassed areas and disused hard standing areas (looking SW)




Page 162


Figure F‐0‐7 looking in the opposite direction (NE) to Figure F‐6

Figure F‐0‐8 Kersfield Estate lower end




Page 163


Figure F‐0‐9 Kersfield Estate middle area

Figure F‐0‐10 Top end of Kersfield Estate




Page 164


Figure F‐0‐11 Apartments off Carlton Drive

Figure F‐0‐12 An existing rain garden off Carlton Drive




Page 165


Figure F‐0‐13 Paved areas for car parking for apartments on Carlton Drive

Figure F‐0‐14 Speed humps in Carlton Drive




Page 166


Figure F‐0‐15 Chartfield Avenue and adjoining roads (no speed humps)

Figure F‐0‐16 Junction of Chartfield Avenue and Genoa Avenue




Page 167


Figure F‐0‐17 Paved drive on Chartfield Avenue (unlikely to be permeable)

Figure F‐0‐18 One of a few apartment blocks in the vicinity of Chartfield Avenue (Genoa Avenue)



100-RG-MDL-00000-000002 | AF | 28 April 2010

ANNEX 1

SUDS Evaluation forExample Areas



SUDS ANNEX 1_100-RG-MDL-00000-000002-AF-SUDS-Evaluation-for-Example-Areas-Report.doc Printed 28/07/2010

THAMES TUNNEL

SUDS EVALUATION FOR EXAMPLE AREAS

LIST OF CONTENTS

Page Number

1 INTRODUCTION 1

2 SUDS IMPERMEABLE AREA MODELLING 2

2.1 Introduction 2 2.2 Method 2 2.3 Results 2 2.4 Conclusion 10

3 PWG DISCONNECTION STRATEGY MODELLING 23

3.1 Introduction 23 3.2 Method 23 3.3 West Putney 23 3.4 Putney Bridge 28 3.5 Frogmore (Buckhold Road) 32 3.6 Typical Year Rainfall Analysis 37 3.7 Design Storm Storage 38 3.8 Conclusion 42

4 50% IMPERMEABL E AREA REDUCTION FOR THE ENTIRE LTT MODEL 48

4.1 Introduction 48 4.2 Method 48 4.3 Results 48 4.4 Conclusion 51

5 CONCLUSION 52




LIST OF FIGURES

Page Number

Figure 2.1 CSO flow during the December typical year event for the existing system and 25%alteration to impermeable area in the contributing subcatchments ................................................. 11

Figure 2.2 CSO flow during the October 2000 – 154 event for the existing system and 25%alteration to impermeable area in the contributing subcatchments ................................................. 12

Figure 2.3 CSO flow during the December typical year event for the existing system and 50%alteration to impermeable area in the contributing subcatchments ................................................. 14

Figure 2.4 CSO flow during the October 2000 – 154 event for the existing system and 50%alteration to impermeable area in the contributing subcatchments ................................................. 15

Figure 2.5 CSO flow during the December typical year event for the existing system and with 5mminitial rainfall lost in the contributing subcatchments ........................................................................ 17

Figure 2.6 CSO flow during the October 2000 – 154 event for the existing system and with 5mminitial rainfall lost in the contributing subcatchments ........................................................................ 18

Figure 2.7 CSO volume during the typical year for the existing system and with 50% impermeablearea disconnected in the contributing subcatchments ..................................................................... 20

Figure 2.8 CSO event flow during the typical year for the existing system and with 50%impermeable area disconnected in the contributing subcatchments ............................................... 21

Figure 3.1 West Putney Mastermap subcatchment characterisation ............................................. 25

Figure 3.2 CSO event flow during the typical year for the existing system and West Putney PWGdisconnection strategy ...................................................................................................................... 28

Figure 3.3 Putney Bridge Mastermap subcatchment characterisation ........................................... 29

Figure 3.4 CSO event flow during the typical year for the existing system and Putney Bridge PWGdisconnection strategy ...................................................................................................................... 31

Figure 3.5 Frogmore (Buckhold Road) Mastermap subcatchment characterisation ...................... 34

Figure 3.6 CSO event flow during the typical year for the existing system and Frogmore (BuckholdRoad) PWG disconnection strategy ................................................................................................. 37

Figure 3.7 Putney Bridge event storage volume comparison for each subcatchment ................... 40

Figure 3.8 West Putney PWG disconnection strategy – December typical year and October 2000event results with existing system results for comparison ............................................................... 43

Figure 3.9 Putney Bridge PWG disconnection strategy – December typical year and October 2000event results with existing system results for comparison ............................................................... 44

Figure 3.10 Frogmore (Buckhold Road) PWG disconnection strategy – December typical year andOctober 2000 event results with existing system results for comparison ........................................ 45

Figure 3.11 Total typical year overflow volume comparison for the existing system and PWGdisconnection strategy system ......................................................................................................... 46

Figure 3.12 Number of typical year overflow events for the existing system and PWGdisconnection strategy system ......................................................................................................... 47




LIST OF TABLES

Page Number

Table 2.1 Existing system model results – December typical year event ......................................... 3

Table 2.2 Existing system model results – October 2000 event ....................................................... 3

Table 2.3 Existing system model results – typical year .................................................................... 3

Table 2.4 West Putney CSO - December typical year event – 25% area change ........................... 4

Table 2.5 Putney Bridge CSO – December typical year event – 25% area change ........................ 4

Table 2.6 Frogmore (Buckhold Road) CSO – December typical year event – 25% area change ... 4

Table 2.7 West Putney CSO – October 2000 event – 25% area change ......................................... 5

Table 2.8 Putney Bridge CSO – October 2000 event – 25% area change ...................................... 5

Table 2.9 Frogmore (Buckhold Road) CSO – October 2000 event – 25% area change ................. 5

Table 2.10 West Putney CSO - December typical year event – 50% area change ......................... 6

Table 2.11 Putney Bridge CSO – December typical year event – 50% area change ...................... 6

Table 2.12 Frogmore (Buckhold Road) CSO – December typical year event – 50% area change . 6

Table 2.13 West Putney CSO – October 2000 event – 50% area change ....................................... 7

Table 2.14 Putney Bridge CSO – October 2000 event – 50% area change .................................... 7

Table 2.15 Frogmore (Buckhold Road) CSO – October 2000 event – 50% area change ............... 7

Table 2.16 West Putney CSO - December typical year event – initial 5mm abstraction .................. 8

Table 2.17 Putney Bridge CSO – December typical year event – initial 5mm abstraction ............... 8

Table 2.18 Frogmore (Buckhold Road) CSO – December typical year event – initial 5mmabstraction .......................................................................................................................................... 8

Table 2.19 West Putney CSO - October 2000 event – initial 5mm abstraction ................................ 8

Table 2.20 Putney Bridge CSO – October 2000 event – initial 5mm abstraction ............................. 9

Table 2.21 Frogmore (Buckhold Road) CSO – October 2000 event – initial 5mm abstraction ........ 9

Table 2.22 West Putney CSO – typical year – 50% impermeable area removed ............................ 9

Table 2.23 Putney Bridge CSO – typical year – 50% impermeable area removed .......................... 9

Table 2.24 Frogmore (Buckhold Road) CSO – typical year – 50% impermeable area removed ..... 9

Table 3.1 InfoWorks existing model subcatchment areas contributing to the West Putney CSO .. 24

Table 3.2 PWG West Putney disconnection strategies .................................................................. 26

Table 3.3 Comparison of CSO overflow results for the existing system and West Putney PWGdisconnection strategy ...................................................................................................................... 26




Table 3.4 Comparison of CSO overflow results during the typical year for the existing system andWest Putney PWG disconnection strategy ...................................................................................... 27

Table 3.5 InfoWorks existing model subcatchment areas contributing to the Putney Bridge CSO 28

Table 3.6 PWG Putney Bridge disconnection strategies ................................................................ 30

Table 3.7 Comparison of CSO overflow results for the existing system and Putney Bridge PWGdisconnection strategy ...................................................................................................................... 30

Table 3.8 Comparison of CSO overflow results during the typical year for the existing system andPutney Bridge PWG disconnection strategy .................................................................................... 31

Table 3.9 InfoWorks existing model subcatchment areas contributing to the Frogmore (BuckholdRoad) CSO ....................................................................................................................................... 33

Table 3.10 PWG Frogmore (Buckhold Road) disconnection strategies (% area allocated to eachsurface type) ..................................................................................................................................... 35

Table 3.11 Comparison of CSO overflow results for the existing system and Frogmore (BuckholdRoad) PWG disconnection strategy ................................................................................................. 36

Table 3.12 Comparison of CSO overflow results during the typical year for the existing system andFrogmore (Buckhold Road) PWG disconnection strategy ............................................................... 36

Table 3.13 SUDS subcatchment typical year and October 2000 event rainfall analysis ................ 38

Table 3.14 West Putney SUDS storage requirements based on the PWG disconnection strategyareas ................................................................................................................................................. 39

Table 3.15 Putney Bridge SUDS storage requirements based on the PWG disconnection strategyareas ................................................................................................................................................. 40

Table 3.16 Frogmore (Buckhold Road) SUDS storage requirements based on the PWGdisconnection strategy areas ........................................................................................................... 41

Table 4.1 CSO Results during the typical year for the existing system and 50% impermeable arearemoved ............................................................................................................................................ 48

LIST OF ABBREVIATIONS

CSO Combined sewer overflows

FEH Flood Estimation Handbook

LTT London Tideway Tunnels

PWG Pennine Water Group

SUDS Sustainable drainage systems

STW Sewage treatment works



1 Introduction

Page 1SUDS ANNEX 1_100-RG-MDL-00000-000002-AF-SUDS-Evaluation-for-Example-Areas-Report.doc Printed 28/07/2010

1 INTRODUCTION

This report evaluates the implementation of Sustainable Drainage Systems (SUDS) on threesubcatchments in the London Tideway Tunnels (LTT) catchment. The three example areas arelocated in the west of the LTT catchment, south of the River Thames and represent thesubcatchments contributing to the West Putney, Putney Bridge and Frogmore (Buckhold Road)Combined Sewer Overflows (CSO). These CSOs are thought the most suitable in the LTTcatchment for SUDS selection.

The December typical year and October 2000 events represent the most severe recorded rainfallevents for the typical year and 154 event rainfall series respectively. The LTT catchment modelsimulations uses these with each of the SUDS options to produce the most extreme overflows atthe CSOs. The complete typical year rainfall was also simulated to provide a representation of thenumber of spills and total overflow that could be expected at CSOs during the annual series.

The LTT model was amended to represent the change in contributing areas produced by thevarious SUDS options. General disconnection options were modelled as reductions inimpermeable contributing area which was modelled as both lost and transferred to permeableareas. Initial losses through rainfall capture techniques were also modelled before a site specificoption (produced by PWG) was investigated for the Frogmore (Buckhold Road) CSO catchment.

Model flow results less than 0.001m 3/s are treated as zero flow.

The population contributing foul flow to the system was unaltered throughout the SUDS modellingas were all other model parameters.



2 SUDS Impermeable Area Modelling


2 SUDS IMPERMEABLE AREA MODELLING

2.1 Introduction

The LTT Model of the existing system was used to represent the impact of SUDS on three CSOs in

the west of the LTT catchment (an area contributing to the Crossness Sewage Treatment Works(STW)). The three CSOs are West Putney (CSO5X), Putney Bridge (CSO6X) and Frogmore(Buckhold Road) (CSO7B) and they are, based on review of LTT catchment and local area landuse, the most suitable for SUDS selection in the entire LTT catchment.

2.2 Method

The system model splits the LTT catchment into individual subcatchments which are connected tomodel nodes. Each subcatchment produces flow estimates based on rainfall, subcatchmentcharacteristics and baseflow values. Subcatchment characteristics include the total area of thesubcatchment that is connected to the modelled sewer network and the percentage of connectedarea which is impermeable and permeable.

The existing LTT model was refined by splitting a large subcatchment to better represent thesystem and to allow the impact of SUDS related changes to be calculated.

The basins were simulated with the December typical year event and the October 2000 eventwhich relate to a one in two year return period and a one in four year return period respectively andrepresent the most severe events (in term of CSO volume) of the typical year and 154 eventrainfall series. The typical year is rainfall from October 1979 to September 1980.

The model subcatchments upstream of the three CSOs were adjusted as follows to provide aninitial evaluation of global SUDS impacts on peak flow and CSO volume for the two selectedevents:

25% impermeable area transferred to permeable area

25% impermeable area removed

50% impermeable area transferred to permeable area

50% impermeable area removed

5mm of rainfall lost at the beginning of the storm events

The typical year was also simulated for the existing system and with 50% impermeable arearemoved from the subcatchments upstream of the three CSOs. This provides a comparison ofoverflow volumes, flow rates and number of spills for each of the three CSO during the typical year.

The transfer of impermeable area to permeable area changes the magnitude and how simulatedflows reach the sewer system. With this change more of the rainfall over the subcatchments willfall onto a permeable area. The permeable area is still connected to the sewer network andtherefore will, depending primarily on rainfall intensity, provide some inflow to the sewer during

rainfall events.The removal of impermeable area represents a change where the removed area is no longerconnected to the sewer network and no inflow is generated during rainfall events.

The 5mm rainfall initial abstraction is representative of a local storage system (such as a blue-roof)which stores the first 5mm of rainfall and only when full does the flow contribute to the sewernetwork.

2.3 Results

The existing system model runs produced the following result for the two selected events.





Table 2.1 Exist ing sys tem model results – December typic al year event

Result West PutneyCSO

PutneyBridge CSO

Frogmore (BuckholdRoad) CSO

Maximum flow (m 3/s) 0.47 0.50 0.52 Average flow during event (m 3/s) 0.10 0.11 0.11Total overflow volume (m 3) 10,000 2,300 4,000

Table 2.2 Exist ing sys tem model results – October 2000 event


Putney BridgeCSO


Maximum flow (m 3/s) 0.93 2.61 3.03 Average flow during event(m3/s) 0.18 0.35 0.39

Total overflow volume (m 3) 13,900 9,100 17,700

Tables 2.1 and 2.2 show that all three of the CSOs experience overflow during the December andOctober events and that this is far greater during the more severe October 2000 event. PutneyBridge and Frogmore (Buckhold Road) experience the greatest increases between the two stormsand this is due to backing up of the system preventing flow from travelling downstream and forcingmore overflows at these CSOs.

Table 2.3 Exist ing syst em model result s – typical year


Putney BridgeCSO


Maximum flow (m3

/s) 0.71 1.71 2.75Number of spill events 59 33 29Total overflow volume (m 3) 94,800 54,800 94,500

Table 2.3 shows the model results for the entire typical year and shows the three CSOs experiencea significant number of overflows and overflow volume during the 12 month period. West Putneyexperiences over twice the number of overflow events compared to the two other CSOs and theFrogmore (Buckhold Road) CSO experiences the lowest number of events. West Putney andFrogmore (Buckhold Road) produce approximately twice the overflow volume as that of the PutneyBridge CSO.

2.3.1 25% subc atchment alteration result s for the December typ ical year eventThe results of transferring 25% of the impermeable area to permeable and removing 25%impermeable area for each of the three CSOs during the December typical year event are shownin the following tables.





Table 2.4 West Putney CSO - December typical year event – 25% area change

Result Existingsystem

25% impermeabletransferred to

permeable area

25% impermeablearea removed



Table 2.5 Putney Bridge CSO – December typical year event – 25% area change



permeable area



Total overflow volume (m 3) 2,300 900 700

Table 2.6 Frogmore (Buckhold Road) CSO – December typical year event – 25% areachange



permeable area




Tables 2.4, 2.5 and 2.6 show the impact of transferring or removing impermeable area in thesubcatchments contributing to the three CSOs during the December typical year event. Theimpact of transferring 25% impermeable area to permeable area in the model subcatchments is toreduce the total volume overflow at West Putney by 13%, Putney Bridge by 63% and Frogmore(Buckhold Road) by 80%.

In each case the CSO would remain if 25% of the impermeable area is removed. The total volumeoverflow reduction during the event is 25% at West Putney, 69% at Putney Bridge and 86% atFrogmore (Buckhold Road). Figure 2.1 represents these model flow results.

The tables 2.4 to 2.6 also show that maximum flow rates and average flow rates through the CSOduring the event are also reduced by transferring or removing impermeable area from thesubcatchments.

2.3.2 25% subc atchment alteration result s for the October 2000 event

The results of transferring 25% of the impermeable area to permeable and removing 25%impermeable area for each of the three CSOs during the October 2000 event are in tables 2.7 to2.9.





Table 2.7 West Putney CSO – October 2000 event – 25% area change



permeable area


Maximum flow (m 3/s) 0.93 0.84 0.76 Average low during event(m3/s) 0.18 0.17 0.15


Table 2.8 Putney Bridge CSO – October 2000 event – 25% area change



permeable area




Table 2.9 Frogmor e (Buckhold Road) CSO – October 2000 event – 25% area change



permeable area




Tables 2.7, 2.8 and 2.9 show the impact of transferring or removing impermeable area in thesubcatchments contributing to the three CSOs during the October 2000 event. The impact oftransferring 25% impermeable area to permeable area in the model subcatchments is to reducethe total volume overflow at West Putney by 5%, Putney Bridge by 27% and Frogmore (BuckholdRoad) by 35%.

In all cases the CSO would remain if 25% of the impermeable area is removed. The total volumeoverflow reduction during the event is 20% at West Putney, 32% at Putney Bridge and 43% atFrogmore (Buckhold Road). Figure 2.2 represents these model flow results.

The three tables also show that maximum flow rates and average flow rates through the CSOduring the event are also reduced by transferring or removing impermeable area from thesubcatchments.

2.3.3 50% subc atchment alteration result s for the December typ ical year event

The results of transferring 50% of the impermeable area to permeable and removing 50%impermeable area for each of the three CSOs during the December typical year event are shownin tables 2.10 to 2.12.





Table 2.10 West Putney CSO - December typical year event – 50% area change



permeable area




Table 2.11 Putney Bridge CSO – December typical year event – 50% area change



permeable area




Table 2.12 Frogmore (Buckhold Road) CSO – December typical year event – 50% areachange



permeable area




Tables 2.10, 2.11 and 2.12 show the major impact of transferring or disconnecting 50% of theimpermeable contributing area. Transferring 50% of the impermeable area to permeable alsoremoves all flows at Frogmore (Buckhold Road) and 96% of overflow volume at Putney Bridge.West Putney still has overflow during this event, but the transfer of 50% to permeable area reducesthe total overflow volume by 35%. Disconnecting 50% of the impermeable area again shows theremoval of discharge at Frogmore (Buckhold Road) CSOs during the December typical year eventand further impacts Putney Bridge so that 98% of the overflow is removed. The removal of 50% ofimpermeable area at West Putney reduces the total volume by 50%. Figure 2.3 represents these

model flow results.The three tables also show that maximum flow rates and average flow rates through the CSOduring the event are also reduced by transferring or removing impermeable area from thesubcatchments.

2.3.4 50% subc atchment alteration result s for the October 2000 event

The results of transferring 50% of the impermeable area to permeable and removing 50%impermeable area for each of the three CSOs during the October 2000 event are shown in tables2.13 to 2.15.





Table 2.13 West Putney CSO – October 2000 event – 50% area change



permeable area




Table 2.14 Putney Bridge CSO – October 2000 event – 50% area change



permeable area




Table 2.15 Frogmore (Buc khold Road) CSO – October 2000 event – 50% area change



permeable area




Tables 2.13, 2.14 and 2.15 show the significant impact of transferring or removing 50% of theimpermeable area in the subcatchments contributing to the three CSOs during the October 2000event. However CSOs still occur at the three sites for the October 2000 event. The impact oftransferring 50% impermeable area to permeable area in the model subcatchments is to reducethe total volume overflow at West Putney by 22%, Putney Bridge by 52% and Frogmore (BuckholdRoad) by 64%. If 50% of the impermeable area is completely removed from the subcatchmentsthe total volume overflow during the event is 40% at West Putney, 61% at Putney Bridge and 74%at Frogmore (Buckhold Road). Figure 2.4 represents these model flow results.

The three tables also show that maximum flow rates and average flow rates through the CSOduring the event are also reduced by transferring or removing impermeable area from thesubcatchments.

2.3.5 5mm ini tial rainfall loss results for the December typi cal year event

Tables 2.16, 2.17 and 2.18 show the impact of removing the initial 5mm of the rainfall from thestorm event. The model shows that during the December typical year event an initial 5mmabstraction of rainfall reduces the volume of overflow at West Putney by 10%, Putney Bridge by4% and Frogmore (Buckhold Road) by 2%. This small reduction in overflow volume isrepresentative of the severity of the December typical year event, where the initial 5mm of rainfallrapidly fills and then the model reacts to the rest of the storm rainfall as in the unaltered existingsystems model. Figure 2.5 represents these model flow results in graphical form.





Table 2.16 West Putney CSO - December typical year event – init ial 5mm abstract ion

Result Existing system 5mm rainfall lost

Maximum flow (m 3/s) 0.47 0.47 Average flow during event(m3/s) 0.10 0.10

Total overflow volume (m 3) 10,000 9,000

Table 2.17 Putney Bridge CSO – December typical year event – init ial 5mm abstract ion




Table 2.18 Frogmor e (Buck hold Road) CSO – December typ ical year event – initi al 5mmabstraction




The three tables also show that maximum flow rates remain very similar to the existing situation

and the average flow rates increase at West Putney and Putney Bridge due to the loss of initial lowflows.

2.3.6 5mm ini tial rainfall loss results for the October 2000 event

Tables 2.19, 2.20 and 2.21 show the impact of removing the initial 5mm of the rainfall from thestorm event. The model shows that during the October 2000 event this initial abstraction wouldreduce the total overflow at West Putney by 2% and no reduction in total overflow volume atPutney Bridge and Frogmore (Buckhold Road) CSOs. The minimal reduction in overflow againshows that the removal of the first 5mm of rainfall during severe rainfall events has little impact onmodel overflow volumes. Figure 2.6 represents this in graphical form.

Table 2.19 West Putney CSO - October 2000 event – init ial 5mm abstraction

Result Existing system 5mm rainfall lostMaximum flow (m 3/s) 0.93 0.93

Average flow during event(m3/s) 0.18 0.18






Table 2.20 Putney Bridge CSO – October 2000 event – init ial 5mm abstraction




Table 2.21 Frogmore (Buckhold Road) CSO – October 2000 event – ini tial 5mm abstraction




The three tables also show that maximum and average flow rates remain very similar to theexisting situation.

2.3.7 50% subc atchment alteration result s for the typ ical year

Tables 2.22, 2.23 and 2.24 show the impact of removing 50% of the contributing impermeable areafrom the three CSO catchments during the typical year. This reduces the total volume overflow atWest Putney by 55%, Putney Bridge by 78% and Frogmore (Buckhold Road) by 77%.

Table 2.22 West Putney CSO – typ ical year – 50% impermeable area removed

Result Existing system 50% impermeablearea removed

Maximum flow (m 3/s) 0.71 0.47Number of spill events 59 52Total overflow volume (m 3) 94,800 42,300

Table 2.23 Putney Bridge CSO – typ ical year – 50% impermeable area removed


Maximum flow (m 3/s) 1.71 0.62Number of spill events 33 16


Table 2.24 Frogmore (Buckhold Road) CSO – typic al year – 50% impermeable area removed


Maximum flow (m 3/s) 2.75 1.26Number of spill events 29 10Total overflow volume (m 3) 94,500 21,400

In all cases the CSO would still operate during the typical year; however the number of spills isreduced. Figure 2.7 represents these model flow results.





Tables 1.22, 1.23 and 1.24 also show that maximum flow rates through the CSO during the typicalyear are also reduced by disconnecting impermeable area in the subcatchments.

Figure 2.8 compares each CSO event overflow volume based on volume range, during the typicalyear for the existing system and the 50% impermeable area removed system. The impact ofremoving 50% impermeable area in the West Putney CSO catchment is to significantly reduce thenumber of events which produce overflows greater than 1,000m 3 and increase the number ofevents with lower overflow volume. The Putney Bridge CSO results show all ranges of overflowvolume are reduced and overflows greater than 5,000m 3 are removed. The Frogmore (BuckholdRoad) CSO results show that all ranges of event overflow volumes are reduced, only one eventproduces an overflow volume greater than 5,000m 3 and all events with less than 100m 3 areremoved.

2.4 Conclusion

The December typical year event and October 2000 event represent the most severe recordedrainfall events for the typical year and 154 event rainfall series respectively. The LTT catchmentmodel simulations of these events therefore produce the most extreme overflows at the CSOs.

The complete typical year rainfall was also simulated to provide a representation of the number ofspills and total overflow that could be expected at CSOs during the annual series and with variousSUDS assumptions.

The December event has a return period of approximately one in two years and the results showthat the disconnection of impermeable area has a far greater impact than the transfer ofimpermeable to permeable area. When 25% of the impermeable area is removed or transferredduring the December event the volume and duration of spills is reduced at all three of the CSOs,but overflow is still observed. When 50% impermeable area is transferred to permeable there is nooverflow recorded at Frogmore (Buckhold Road) for this event but there is still a small overflow atPutney Bridge and a significant overflow remains at West Putney. When the impermeable area isdisconnected the impact is greater with overflow from Putney Bridge reduced by 98% and the WestPutney overflow reduced by 50%.

The October event has a return period of approximately one in four years and again shows theoverflows reducing the most significantly when the impermeable area is disconnected rather thantransferred to permeable area. The removal or transfer of 25% of impermeable area has an impacton overflow volume, with the greatest reduction of 43% seen at Frogmore (Buckhold Road), but allthe CSOs produce overflow during the event. When 50% of the impermeable area is disconnectedthe reduction in overflow is far greater, however; all three CSOs still produce during the October2000 event.

The impact of removing the first 5mm of rainfall (via storage in blue-roofs etc) has little effect onsevere storm overflows at the three selected CSO sites in the model. The rainfall quickly fills the5mm depth of storage and the catchment then reacts in the same way as the existing system. It islikely that a far greater impact would be observed on lower intensity or intermittent storms wherethe 5mm rainfall removal represents a more significant proportion of the total rainfall event.

The typical year series contains the December event and over 50 other rainfall events that aretypical to the LTT catchment. The typical year results show the impact of disconnecting 50% ofimpermeable area is to reduce maximum flow rates, total overflow volume and the number of spillevents recorded in the annual series. West Putney shows a small reduction in spill events but asignificant reduction in total overflow volume, while Putney Bridge and Frogmore (Buckhold Road)show a significant reduction in both volume and number of overflow events.

The individual event overflow volumes during the typical year are also greatly affected by theremoval of 50% impermeable area, with the number of events producing over 1,000m 3 reduced atall three CSOs. The overflow volume shifts in West Putney from larger to smaller overflows with aslight reduction in total overflow events, while the volume and number of events significantlyreduces at both Putney Bridge and Frogmore (Buckhold Road) CSOs.





Figure 2.1 CSO flow during the December typic al year event for the existi ng sys tem and25% alteration to impermeable area in the cont ributing subcatchments

0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

R a i n f a l l ( m m )

F l o w

( m 3 / s )

Date and Time

CSO Flow during the December Typical Year Event ‐West Putney CSO

Rainfall

Existing System

25% Impermeable Transfered to Permeable Area

25% Impermeable Area Removed

0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

CSO Flow during the December Typical Year Event ‐Putney Bridge CSO

Rainfall

Existing System







0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

CSO Flow during the December Typical Year Event ‐Frogmore (Buckhold Road) CSO

Rainfall

Existing System



Figure 2.2 CSO flow during t he October 2000 – 154 event for the exis ting sys tem and 25%

alteration to impermeable area in the c ontributi ng subc atchments

0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w ( m 3 / s )

Date and Time

CSO Flow during the October 2000 ‐154Event ‐West Putney CSO

Rainfall

Existing System







0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w

( m 3 / s )

Date and Time

CSO Flow during the October 2000 ‐154Event ‐Putney Bridge CSO

Rainfall

Existing System



0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w

( m 3 / s )

Date and Time

CSO Flow during the October 2000 ‐154Event ‐Frogmore (Buckhold Road) CSO

Rainfall

Existing System







Figure 2.3 CSO flow during the December typic al year event for the existi ng sys tem and50% alteration to impermeable area in the cont ributing subcatchments

0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time


Rainfall

Existing System



0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

CSO Flow during the December Typical Year Event ‐Putney Bridge CSO

Rainfall

Existing System







0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time


Rainfall

Existing System



Figure 2.4 CSO flow during t he October 2000 – 154 event for the exis ting sys tem and 50%alteration to impermeable area in the c ontributi ng subc atchments

0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w

( m 3 / s )

Date and Time

CSO Flow during the October 2000 ‐154Event ‐West Putney CSO

Rainfall

Existing System







0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w

( m 3 / s )

Date and Time


Rainfall

Existing System



0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w

( m 3 / s )

Date and Time

CSO Flow during the October 2000 ‐154Event ‐Frogmore (Buckhold Road) CSO

Rainfall

Existing System







Figure 2.5 CSO flow during the December typic al year event for the existi ng sys tem andwith 5mm initial rainfall lost in the contributing subcatchments

0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time


Rainfall

Existing System

5mm Rainfall Lost

0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

CSO Flow

during

the

December

Typical

Year

Event

‐Putney Bridge CSO

Rainfall

Existing System

5mm Rainfall Lost





0

4

8

12

16

20

24

28

320.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time


Rainfall

Existing System

5mm Rainfall Lost

Figure 2.6 CSO flow dur ing the October 2000 – 154 event for the exist ing system and wit h5mm initial rainfall lost in the contribu ting subcatchments

0

5

10

15

20

25

30

35

40

45

500

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


F l o w

( m 3 / s )

Date and Time

CSO Flow during the October 2000 ‐154Event ‐

West Putney CSO

Rainfall

Existing System

5mm Rainfall Lost





0

5

10

15

20

25

30

35

40

45

500

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


F l o w

( m 3 / s )

Date and Time


Rainfall

Existing System

5mm Rainfall Lost





Figure 2.7 CSO volume during the typical year for the existi ng syst em and wi th 50%impermeable area disconnected in the cont ributing subcatchments

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

V o l u m e ( m 3 )

Date

CSO Event Volume during the Typical Year ‐West Putney CSO

Existing System


0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

V o l u m e ( m 3 )

Date

CSO Event Volume during the Typical Year ‐Putney Bridge CSO

Existing System






0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

V o l u m e ( m 3 )

Date

CSO Event Volume during the Typical Year ‐Frogmore (Buckhold Road) CSO

Existing System


Figure 2.8 CSO event flow during the typical year for the exist ing sys tem and with 50%impermeable area disconnected in the cont ributing subcatchments

0

5

10

15

20

25

N u m b e r o f E v e n t s )

Total Event Volume Range (m 3)

CSO Volume during the Typical Year Event ‐West Putney CSO

Existing System


Large events reduced in volume, hence increase in the number of lower event volumes





0

5

10

15

20

25



CSO Volume during the Typical Year Event ‐Putney Bridge CSO

Existing System


0

5

10

15

20

25



CSO Volume during the Typical Year Event ‐Frogmore (Buckhold Road) CSO

Existing System




3 PWG Disconnection Strategy Modelling


3 PWG DISCONNECTION STRATEGY MODELLING

3.1 Introduction

The London Tideway Tunnels (LTT) Model of the existing system was used to represent the impact

of the disconnection strategy options (supplied by PWG in May 2009) for the West Putney, PutneyBridge and Frogmore (Buckhold Road) Combined Sewer Overflow (CSO) catchments. Thedisconnection options are defined in the PWG Report ‘Potential source control and SUDSapplications: Land use and retrofit options’.

Although the results of the modelling of these options are included here, PWG subsequentlydeveloped a refined approach and identified ‘final disconnection scenarios’ which had a reducedimpact on impermeable area reduction. The refined disconnection strategy corresponds toapproximately 37% removal of the existing impermeable area rather than the 70 to 80% reported inthis section. However the methodology described would apply to the lower figures which as yethave not been modelled.

3.2 MethodThe disconnection strategy for the three CSO catchments used the “MasterMap” mapping layer tosplit the contributing subcatchments into areas of permeable and impermeable. The impermeablearea is then further spilt into areas where initial loss of rainwater (eg, with the use of blue-roofs)can be applied, areas where storage attenuation can be implemented, areas of impermeablesurfaces which can be transferred to permeable and areas which will remain impermeable. Noareas were identified by PWG for complete removal (through storm water separation).

The contributing area with initial losses is representative of a SUDS retention device that provides25mm of storage across its associated area. This was modelled as having 25mm depth of initialstorage which when exceeded would then contribute to the sewer network in the same way as theimpermeable area.

The disconnection strategy was applied to each of the InfoWorks Model subcatchments whichcontribute to the three CSOs by modifying the subcatchment parameters (areas). The followingsection outlines the specific changes made to each of the three CSO areas. The typical yearresults for each of the three CSOs are shown alongside the “Mastermap” subcatchments in figures3.11 and 3.12 at the end of this section.

3.3 West Putney

3.3.1 Catchment Analys is

The InfoWorks Model has seven subcatchments upstream of the West Putney CSO with theproperties shown in Table 3.1. The “MasterMap” characterisation of the West Putneysubcatchment is shown in Figure 3.1.

This exercise was undertaken for all of the seven subcatchments contributing to the West PutneyCSO and the PWG disconnection strategy results are shown in Table 3.2.





Table 3.1 InfoWorks existing model subcatchment areas contribut ing to t he West PutneyCSO

Subcatchment InfoWorks node ID

24751451a 21729601 21757452 21745701 24751451b 21743201 21743001 Totals

Total area (ha) 19.9 95.2 87.2 24.6 28.4 84.0 86.0 425ha

Impermeable

% 48.0 2.7 22.0 1.7 48.0 4.3 2.0 12%

ha 9.6 2.6 19.2 0.4 13.6 3.6 1.7 51ha

Permeable % 52.0 0.0 78.0 0.0 52.0 0.0 98.0 42%

ha 10.3 0.0 68.0 0.0 14.8 0.0 84.3 177ha

Areas may not add to totals due to rounding and permeable area not connected to the system.





Figure 3.1 West Putney Mastermap subcatchment characterisation

Mapping reproduced by permission of Ordnance Survey on behalf of HMSO. © CrownCopyright and database right 2009. All rights reserved. Ordnance Survey Licence number100019345





Table 3.2 PWG West Putney disc onnect ion st rategies

(% area allocated to each surface type)


24751451

a

2172960

1

2175745

2

2174570

1

24751451

b

2174320

1

2174300

1

Totals

Total area (ha) 19.9 95.2 87.2 24.6 28.4 84.0 86.0 425ha

Impermeable

% 12.68 0.58 5.61 0.83 14.19 0.59 1.65 3%

ha 2.5 0.6 4.9 0.2 4.0 0.5 1.4 14ha

Permeable % 65.26 0.25 78.96 0.24 64.86 0.62 98.02 44%

ha 13.0 0.2 68.9 0.1 18.4 0.5 84.3 185ha

Impermeable with initial

losses

% 2.17 0.40 4.81 0.02 1.81 0.77 0.02 1%

ha 0.4 0.4 4.2 0.0 0.5 0.7 0.0 6ha

Impermeable with

storage

% 19.90 1.47 10.62 0.58 19.13 2.30 0.32 5%ha 4.0 1.4 9.3 0.1 5.4 1.9 0.3 22

Areas may not add to totals due to rounding and permeable area not connected to the system.

The model was first amended so that the areas for each source control option matched the areasprovided by PWG. In InfoWorks this is done by adjusting area percentage in the ninesubcatchments. For example in subcatchment 21757452 the impermeable area was reduced from22.0% (19.2ha) to 5.61% (4.9ha) based on detailed analysis of surfaces and source controloptions.

Areas with initial losses were also added to the model subcatchments. For example subcatchment21757452 was estimated to have potential for 4.81% area subject to initial losses or an area of 4.2hectares.

The contributing area with storage was represented as lost in the subcatchment as the flow wouldbe captured by the SUDS storage volume and returned to the system after the CSO event.

All other current model parameters were left unchanged.

3.3.2 Results

The West Putney disconnection strategy model results are shown in Table 3.3 and graphicalrepresentation is shown in Figure 3.8.

Table 3.3 Comparison of CSO overflow results fo r the existing system and West PutneyPWG disconnection strategy

West Putney CSO –existing system

West Putney CSO – PWGdisconnection str ategy

Percentage(PWG vs existi ng)

December typ ical yearevent – volum e (m 3) 10,000 3,300 33%

October 2000 event –volume (m 3) 13,900 6,000 43%





Table 3.3 shows the reduction in CSO volume caused by implementing the PWG disconnectionstrategy in the West Putney catchment. During both the December typical year and October 2000event the overflow from the CSO is significantly reduced. This is expected given the reduction inimpermeable area contributing runoff directly to the sewer network.

When compared to the 50% removal of impermeable area (sections 2.3.3 and 2.3.4) the PWGdisconnection strategy produces approximately 2,000m 3 less overflow during both events.

Table 3.4 Comparison of CSO overflow results during the typical year for the existi ngsystem and West Putney PWG disconnection strategy

West Putney CSO –existing system

West Putney CSO –disconnection str ategy

Percentage(PWG vs existing )

Typical year event –total volume (m 3) 94,800 21,900 23%

Typical year event –number of overflowevents (m 3)

59 39 66%

Table 3.4 shows the effect of the disconnection strategy on West Putney overflows during themodelled typical year. The total overflow volume is reduced by 77%, while the number ofoverflows is reduced by 34%. Figure 3.2 shows the range of overflow volumes during the typicalyear and compares these to the existing system in West Putney. It shows a reduction in thenumber of event overflows that produce more than 1000m 3 and the same number of events whichproduce less than 1000m 3. This is representative of events which originally experienced greateroverflow but which have fallen 1000m 3 due to the PWG disconnection strategy reducing eventoverflows.





Figure 3.2 CSO event flow during t he typical year for the existing system and West PutneyPWG disconnection strategy

0

5

10

15

20

25



CSO Volume during the Typical Year Event ‐West Putney CSO

Existing SystemPWG Disconnection Strategy

3.4 Putney Bridge

3.4.1 Catchment analysis

The InfoWorks Model has four subcatchments upstream of the Putney Bridge CSO with theproperties shown in Table 3.5. The “MasterMap” characterisation of the Putney Bridgesubcatchment is shown in Figure 3.3. Putney Bridge also is influenced by flows originating in WestPutney.

This exercise was undertaken for all of the four subcatchments contributing to the Putney BridgeCSO and the PWG initial disconnection strategy results are shown in Table 3.6.

Table 3.5 InfoWorks existing model subc atchment areas contributi ng to t he Putney BridgeCSO


24750701 2475145d 2475145c 24751402 TotalsTotal area (ha) 74.9 29.6 20.6 17.1 142ha

Impermeable

% 42.0 48.0 48.0 53.0 45%

ha 31.5 14.2 9.9 9.1 65ha

Permeable % 58.0 52.0 52.0 47.0 55%

ha 43.4 15.4 10.7 8.0 78ha




Page 29SUDS ANNEX 1_100-RG-MDL-00000-000002-AF-SUDS-Evaluation-for-Example-Areas-Report.doc

Figure 3.3 Putney Bridge Mastermap subc atchment characterisation

Mapping reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown Copyright and database right2009. All rights reserved. Ordnance Survey Licence number 100019345





Table 3.6 PWG Putney Bridge disconnection strategies

(% area allocated to each surface type)


24750701 2475145d 2475145c 24751402 Totals

Total area (ha) 74.9 29.6 20.6 17.1 142ha

Impermeable

% 9.66 7.44 12.61 6.20 9%

ha 7.2 2.2 2.6 1.1 13ha

Permeable % 65.58 63.76 68.37 58.41 65%

ha 49.1 18.9 14.1 10.0 92ha

Impermeable with initial

losses

% 7.94 12.20 1.29 10.52 8%

ha 5.9 3.6 0.3 1.8 12

Impermeabl

e withstorage

% 16.82 16.59 17.73 24.87 18%

ha 12.6 4.9 3.7 4.3 26

Areas may not add to totals due to rounding.

The model was first amended so that the areas for each source control option matched the areasprovided by PWG. In InfoWorks this is done by adjusting area percentage in the ninesubcatchments. For example in subcatchment 24751402 the impermeable area was reduced from53.0% (9.1ha) to 6.20% (1.1ha) based on detailed analysis of surfaces and source control options.

Areas with initial losses were also added to the model subcatchments. For example subcatchment24751402 was estimated to have potential for 10.52% area subject to initial losses or an area of1.8 hectares.



3.4.2 Results

The Putney Bridge disconnection strategy model results are shown in Table 3.7 and graphicalrepresentation is shown in Figure 3.9.

Table 3.7 Comparison of CSO overflow results for the existing syst em and Putney BridgePWG disconnection strategy

Putney Bri dge CSO –existing system

Putney Bridge CSO – PWGdisconnection str ategy

Percentage

(PWG vs existi ng)

December typ ical yearevent – volume (m 3) 2,300 0 0%






Table 3.7 shows the impact of implementing the PWG disconnection strategy in the Putney Bridgecatchment. During the December typical year event the overflow from the CSO is removed andduring the October 2000 event the CSO is reduced by 75%. This is expected given the reductionin impermeable area contributing runoff directly to the sewer network.

When compared to the 50% removal of impermeable area (section 2.3.3 and 2.3.4) the PWGdisconnection strategy removes the December Event overflow completely when the 50% removalstill left a small overflow at the CSO. 1,300m 3 less overflow is recorded during the October 2000event than in the 50% removal of impermeable area model.

Table 3.8 Comparison of CSO overflow results during the typical year for the existi ngsystem and Putney Bridge PWG disconnection st rategy

Putney Bridge CSO –existing system

Putney Bridge CSO –disconnection str ategy

Percentage

(PWG vs existi ng)



33 8 24%

Table 3.8 shows the effect of the disconnection strategy on Putney Bridge overflows during themodelled typical year. The total overflow volume is reduced by 84%, while the number of overflowsis reduced by 76%. Figure 3.4 shows the range of overflow volumes during the typical year andcompares these to the existing system in Putney Bridge. It shows the reduction in the number ofevents and event volumes. For example there are only five CSO events with a total volume lessthan 1,000m 3 and no events with an overflow volume greater then 5,000m 3 during the typical yearwith the PWG disconnection strategy.

Figure 3.4 CSO event flow during the typic al year for t he existi ng system and Putney BridgePWG disconnection strategy

0

5

10

15

20

25



CSO Volume during the Typical Year Event ‐Putney Bridge CSO

Existing System

PWG Disconnection Strategy





3.5 Frogmore (Buckhold Road)

3.5.1 Catchment analysis

The InfoWorks Model has nine subcatchments upstream of the Frogmore (Buckhold Road) CSOwith the properties shown in Table 3.9. The “MasterMap” characterisation of the Frogmore

(Buckhold Road) subcatchment is shown in Figure 3.5.This exercise was undertaken for all of the nine subcatchments contributing to the Frogmore(Buckhold Road) CSO and the PWG initial disconnection strategy results are shown in Table 3.10.





Table 3.9 InfoWorks existing model subcatchment areas cont ributing to the Frogmore (Buckhold Road) C


25740102 24749501 24743902 24736291 23749901 23741101

2373

a2373

Total area (ha) 69.2 38.5 21.8 95.3 88.7 72.9 10.4

Impermeable % 40.0 53.0 46.0 43.0 44.0 1.1 39.5

ha 27.7 20.4 10.0 41.0 39.0 0.8 4.1

Permeable % 60.0 47.0 54.0 57.0 56.0

88% in parks(permeable)

but area is notconnected to

system

6

ha 41.5 18.1 11.8 54.3 49.7 6.3

Areas may not add to totals due to rounding. Subcatchment 23741101 has a large proportion of permeable area which is not connected to the





Figure 3.5 Frogmore (Buckhold Road) Mastermap subcatc hment characterisation

Mapping reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown Copyright and database right 20 All rights reserved. Ordnance Survey Licence number 100019345





Table 3.10 PWG Frogmore (Buckhold Road) disconnection s trategies (% area alloc ated to each surface typ


25740102 24749501 24743902 24736291 23749901 23741101 23732702 23731801

Total area (ha) 69.2 38.5 21.8 95.3 88.7 72.9 1.4 9.

Impermeable% 10.98 9.08 10.32 9.77 8.97 0.49 7.24 11.9

ha 7.6 3.5 2.2 9.3 8.0 0.4 0.1 1

Permeable% 68.37 56.39 65.10 66.20 65.63 0.03 75.10 70.6

ha 47.3 21.7 14.2 63.1 58.2 0.0 1.1 6

Impermeablewith initial

losses

% 3.82 7.26 3.91 4.02 5.24 0.01 1.47 3.3

ha 2.6 2.8 0.9 3.8 4.6 0.0 0.0 0

Impermeablewith storage

% 16.83 27.27 20.67 20.01 20.16 0.59 16.19 14.1

ha 11.6 10.5 4.5 19.1 17.9 0.4 0.2 1

Areas may not add to totals due to rounding. Subcatchment 23741101 has a large proportion of permeable area which is not connected to the





The model was first amended so that the areas for each source control option matched the areasprovided by PWG. In InfoWorks this is done by adjusting area percentage in the ninesubcatchments. For example in subcatchment 25740102 the impermeable area was reduced from40.0% (69.2ha) to 10.98% (7.6ha) based on detailed analysis of surfaces and source controloptions.

Areas with initial losses were also added to the model subcatchments. For example subcatchment25740102 was estimated to have potential for 3.82% area subject to initial losses or an area of 2.6hectares.



3.5.2 Results

The Frogmore disconnection strategy model results are shown in Table 3.11 and graphicalrepresentation is shown in Figure 3.10.

Table 3.11 Comparison of CSO overflow results fo r the existing system and Frogmore(Buckhold Road) PWG disconnection strategy

Frogmore (BuckholdRoad) CSO – existing

system

Frogmore (Buckhold Road)CSO – PWG discon nectio n

strategy

Percentage

(PWG vs existi ng)

December typ ical yearevent – volume (m 3) 4,000 0 0%


Table 3.11 shows the impact of implementing the PWG disconnection strategy in the Frogmore(Buckhold Road) catchment. During the December typical year event the overflow from the CSOis totally removed and during the October 2000 event the CSO volume is reduced by 86%. This isexpected given the reduction in impermeable area contributing runoff directly to the sewer network.

When compared to the 50% removal of impermeable area (section 2.3.3 and 2.3.4) the PWGdisconnection strategy controls the December typical year event and results in 2,200m 3 lessoverflow during the October 2000 event.

Table 3.12 Comparison of CSO overflow results during the typical year for the existi ngsystem and Frogmore (Buckhol d Road) PWG disconnection s trategy

Frogmore (BuckholdRoad) CSO – existin g

system

Frogmore (Buckhold Road)CSO – disconnection

strategy

Percentage

(PWG vs existi ng)



29 2 7%

Table 3.12 shows the effect of the disconnection strategy on Frogmore (Buckhold Road) overflows

during the modelled typical year. The total overflow volume is reduced by 98%, while the numberof overflows is reduced from 29 to two for the existing and PWG models respectively. Figure 3.6shows the range of overflow volumes during the typical year and compares these to the existing





system in Frogmore (Buckhold Road). It shows the original system produces 6 CSO events duringthe typical year which are greater than 5,000m 3, while the PWG disconnection strategy systemproduces no events with more than 5,000m 3 volume.

The impact of the PWG disconnection strategy on the Frogmore (Buckhold Road) CSO is due tothe reduction (approximately 75%) in impermeable area draining directly to the combined sewersystem during periods of rainfall.

Figure 3.6 CSO event flow during t he typical year for the existing system and Frogmore(Buckhold Road) PWG disconnection strategy

0

5

10

15

20

25



CSO Volume during the Typical Year Event ‐Frogmore (Buckhold Road) CSO

Existing System

PWG Disconnection Strategy

3.6 Typical Year Rainfall Analys is

In the InfoWorks LTT Model a single rain gauge (Gauge 14) covers all of the Putney Bridge andFrogmore (Buckhold Road) catchments and the majority of the West Putney catchment. Anotherrain gauge (Gauge 43) covers the remaining proportion of the north-western area of the WestPutney catchment. The five minute typical year rainfall and October 2000 event rainfall data atRain Gauge 14 is assessed in Table 3.13:





Table 3.13 SUDS subcatchment typ ical year and October 2000 event rainfall analysi s

Typical year December typicalyear event

October 2000event

Number of events 63 3 1

Number of hours w ithrainfall

855 31 12

Total rainfall depth (mm) 565 44 43

Peak (5-minute) rainfallintensity (mm/hr)

67.7 10.4 23.6

Total rainfall volume in t hePWG catchment area (m 3)

5,773,700 449,600 439,400

*Separate events are categorised as having a preceding 24hr of no rain. The data is based on the5 minute rainfall data series and the peak intensity (67.7mm/hr) was recorded at 23:10 on the 25 th July 1980 in the typical year rainfall series.

The peak typical year event was recorded in late December and this lasts for approximately threedays. The InfoWorks model runs the December event for 14 days starting on the 18th of themonth and although producing a total of 44mm, there are extended periods at the start of the 14days where no rain is observed. The main rainfall event in December falls over the 26 th and 27 th with a maximum rainfall intensity (for a 5-minute period) of 10.4mm/hr. The October 2000 eventstarts on the 29 th, lasts just over one day and produces 43mm of rainfall with a maximum rainfallintensity of 23.6mm/hr.

Overflows are a direct consequence of rainfall volume and intensity and the ability of the collectionsystem to carry the flow. This is why, for the typical year results, the December event (large

rainfall volume but over a long time) CSOs has been significantly reduced at Putney Bridge andremoved entirely at Frogmore (Buckhold Road), but other overflows still occur at these locationsduring the year due to more intense storm events.

During the typical year there are 63 individual rainfall events (characterised by a preceding periodof at least 24 hours of no rain). Five of these events have a total rainfall depth greater than 25mm,which would exceed the initial loss SUDS storage depth and contribute runoff to the sewernetwork. However; only a relatively low number of the individual events have a preceding dryperiod of more than three days which means that for a significant proportion of the typical year theinitial loss SUDS devices in the LTT catchment may not have time to completely empty betweenrainfall events. When the subsequent rainfall event occurs over the catchment there would be lessthan the total 25mm of storage depth available so events with lower rainfall depths could alsoexceed the SUDS storage and contribute flow to the local sewers.

3.7 Design Storm Storage

The storage option was modelled separately in InfoWorks so that the peak runoff and total storagevolume for each subcatchment in the PWG disconnection strategy could be assessed for aselection of design storm events.

The contributing area assigned to the storage option for each subcatchment was taken from thePWG disconnection strategy. The storage option also has an initial loss of 25mm applied to runoff,which when exceeded would feed into the SUDS storage device. In reality this would be a numberof multiple storage devices, but for the purpose of modelling this is shown as total volume requiredfor the entire subcatchment.

The results for West Putney, Putney Bridge and Frogmore (Buckhold Road) are shown in the

following tables. The results shown are for the December typical year event, the October 2000event and the two design storms on which the storage requirements could be based.





Table 3.14 West Putney SUDS storage requirements based on the PWG disconnection str ategy areas

PWG SUDS subcatchment st orage volume requirements

24751451a 21729601 21757452 21745701 24751451b 2Total impermeable areawith 25mm initial loss

and storage4.0ha 1.4ha 9.3ha 0.1ha 5.4ha

Decembertypical year

event

Volume(m3) 210 90 770 10 290

October 2000event

Volume(m3)

470 150 350 10 640

15yr designstorm

Volume(m3) 340 140 900 10 490

30yr designstorm

Volume(m3) 490 220 1,350 20 710





Table 3.15 Putney Bridge SUDS storage requir ements based on the PWG disconnectionstrategy areas

PWG SUDS subcatchment st orage volume requirements

24750701 2475145d

2475145c

24751402

Total impermeable area with25mm init ial loss and storage 12.6ha 4.9ha 3.7ha 4.3ha

December typicalyear event

Volume(m3)

820 270 190 280

October 2000 event Volume(m3)

1,340 580 440 450

15yr design st orm Volume(m3) 1,290 440 310 430

30yr design st orm Volume(m3) 2,060 650 460 730

Figure 3.7 Putney Bridge event storage volume comparison for each subcatchment

0

500

1,000

1,500

2,000

2,500

24750701 2475145d 2475145c 24751402

V o l u m e ( m

3 )

Subcatchment

Comparison of Event Storage Volume Requirements for each Subcatchment in Putney Bridge

December Typical Year Event

October 2000 Event

15yr Design Storm

30yr Design Storm





Table 3.16 Frogmore (Buckhold Road) SUDS storage requirements based on the PWG disconnectio n strategy a

PWG SUDS subcatchment s torage volume requirements

25740102 24749501 24743902 24736291 23749901 23741101 23732702 23

Total impermeable areawith 25mm initi al loss and

storage11.6ha 10.5ha 4.5ha 19.1ha 17.9ha 0.4ha 0.2ha

December typicalyear event

Volume(m3)

740 700 280 1,110 1,020 30 10

October 2000event

Volume(m3)

1,270 1,110 500 2,190 2,070 50 30

15yr design st orm Volume(m3) 1,190 1,060 450 1,840 1,710 40 20

30yr design st orm Volume(m3) 1,830 1,770 700 2,740 2,530 60 30





Tables 3.14 to 3.16 show the required storage is proportional to the contributing area. The initial25mm loss during the event reduces the required storage but depending on the design storm usedto size the SUDS storage device the required volume could be several thousand cubic meters.This could be split between a number of sites and provides a significant reduction in peak flowvolume entering the sewer network and contributing to the CSO events.

3.8 Conclusion

The PWG disconnection strategy has a positive impact on all three CSOs and reduces the volumeoverflowing for all modelled events and reduces the volume and number of overflow events duringthe typical year. The reduction in overflow is greater than the 50% impermeable area removal inSection 2 of this report, which is due to the disconnection of approximately 75% of thesubcatchments impermeable area into permeable, initial loss and storage areas. However, asshown by the development of the final disconnection scenarios, this level of disconnection isunlikely to be achievable in practice.

The number of CSO events remains high at West Putney and Putney Bridge and SUDS alone maynot provide sufficient source control to eliminate the need for additional CSO facilities.

Source controls in the Frogmore catchment has the most significant impact at the CSO with thenumber of events reducing to two in the typical year. This potentially has a significant effect on theCSO control options for Frogmore (Buckhold Road). Source controls assessed for Frogmore(Buckhold Road) would appear to be sufficient to avoid additional CSO control facilities. PWG’srefined approach concluded that the disconnection strategies reported in this section were notfeasible in practice. Their refined disconnection strategy corresponds to approximately 37%removal of the existing impermeable area. This scenario was not modelled specifically butperformance was estimated by PWG using the results for 25% and 50% area reduction asreported in Sections 2 and 4.





Figure 3.8 West Putney PWG disconnection strategy – December typical year and October 2000event results with existing system results for comparison

0

4

8

12

16

20

24

28

320

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

West Putney CSO Flow during the December Typical Year Event ‐Existing System and PWG Disconnection Strategy

Rainfall

West Putney ‐Existing System Results

West Putney ‐Disconnection Strategy Results

0

5

10

15

20

25

30

35

40

45

500

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


F l o w

( m 3 / s )

Date and Time

West Putney CSO Flow during the October 2000 Event ‐Existing System and PWG Disconnection Strategy

Rainfall

West Putney ‐Existing System Results

West Putney ‐Disconnection Strategy Results





Figure 3.9 Putney Bridge PWG disconnection strategy – December typi cal year andOctober 2000 event results w ith existing system results f or comparison

0

4

8

12

16

20

24

28

320

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

Putney Bridge CSO Flow during the December Typical Year Event ‐Existing System and PWG Disconnection Strategy

Rainfall

Putney Bridge ‐Existing System Results

Putney Bridge ‐Disconnection Strategy Results

0

5

10

15

20

25

30

35

40

45

500

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5


F l o w

( m 3 / s )

Date and Time

Putney Bridge CSO Flow during the October 2000 Event ‐Existing System and PWG Disconnection Strategy

Rainfall

Putney Bridge ‐Existing System Results

Putney Bridge ‐Disconnection Strategy Results





Figure 3.10 Frogmor e (Buckhold Road) PWG disconnection strategy – December typi calyear and October 2000 event results with existing system results f or comparison

0

4

8

12

16

20

24

28

320

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8


F l o w

( m 3 / s )

Date and Time

Frogmore (Buckhold Road) CSO Flow during the December Typical Year Event ‐Existing System and PWG Disconnection Strategy

Rainfall

Frogmore (Buckhold Road) ‐Existing System Results

Frogmore (Buckhold Road) ‐Disconnection Strategy Results

0

5

10

15

20

25

30

35

40

45

500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


F l o w

( m 3 / s )

Date and Time

Frogmore (Buckhold Road) CSO Flow during the October 2000 Event ‐Existing System and PWG Disconnection Strategy

Rainfall

Frogmore (Buckhold Road) ‐Existing System Results

Frogmore (Buckhold Road) ‐Disconnection Strategy Results





Figure 3.11 Total typical year overflow volume comparison for the existing syst em and PWG disconnection strate






Figure 3.12 Number of t ypical year overflow events for the existing sys tem and PWG disconnection strategy s






4 50% IMPERMEABLE AREA REDUCTION FOR THE ENTIRE LTT MODEL

4.1 Introduction

The LTT model (March 2009) of the existing system was used to represent the impact at eachCSO location of removing 50% impermeable area from all subcatchments in the system. Nodiscussion on how this could be implemented or if practical is included.

4.2 Method

The contributing impermeable area at each of the InfoWorks model subcatchments was reducedby 50%. This reduced the total contributing impermeable area in the LTT model from 20,654ha to10,327ha.

4.3 Results

The results for the existing system with 50% impermeable area removed are shown in Table 4.1.

Table 4.1 CSO Results during t he typical year for the existi ng sys tem and 50% impermeablearea removed

CSO Existing system 50% impermeablearea removed Percentag

e changein overflow

volumeReference no Name

Totaloverflowvolume

(m 3)

Numberof

events

Totaleverflowvolume

(m 3)

Numberof events

CS01X Acton StormRelief

611,700 40 135,400 21 ‐78%

CS02X Stamford BrookStorm Relief 2,200 3 - - ‐100%

CS03X North WestStorm Relief

- - - - ‐

CS04X HammersmithPumping Stn

2,027,300 57 425,700 43 ‐79%

CS05X West PutneyStorm Relief

94,800 59 42,300 52 ‐55%

CS06X Putney Bridge 54,800 33 11,800 16 ‐78%

CS07A Frogmore SR -

Bell Lane Creek

18,300 36 4,200 21 ‐77%

CS07B Frogmore SR-Buckhold Rd

94,500 29 21,400 10 ‐77%

CS08AJews Row -Wandle ValleySR

4,700 2 - - ‐100%

CS08BJews Row -Falcon BrookSR

7,100 2 - - ‐100%

CS09X Falcon BrookPumping Stn

703,900 43 190,900 32 ‐73%

CS10X Lots RdPumping Stn 1,093,500 39 325,000 28 ‐70%






e changein overflow


Totaloverflowvolume

(m 3)

Numberof

events

Totaleverflowvolume

(m 3)

Numberof events

CS11X Church Street - - - - ‐

CS12X Queen Street - - - - ‐

CS13X Smith Street 1,400 4 - - ‐100%

CS14X Ranelagh 278,300 27 29,500 15 ‐89%

CS15X WesternPumping Stn

2,154,900 39 863,300 27 ‐60%

CS16X HeathwallPumping Stn

718,900 35 197,500 22 ‐73%

CS17X South WestStorm Relief

218,700 12 19,300 2 ‐91%

CS18XKings ScholarsPond StormRelief

100 3 - - ‐100%

CS19X Clapham StormRelief

5,800 5 - - ‐100%

CS20X Brixton StormRefief

263,800 30 55,600 16 ‐79%

CS21X Grosvenor Ditch 700 3 - - ‐100%

CS22X Regent Street 8,700 4 - - ‐100%

CS23X NorthumberlandStreet

45,700 13 1,400 1 ‐97%

CS24X Savoy Street 99,800 47 29,500 34 ‐70%

CS25X Norfolk Street - - - - ‐

CS26X Essex Street 900 3 - - ‐100%

CS27X Fleet Main 500,800 20 35,300 6 ‐93%

CS28X Shad ThamesPumping Stn

101,000 15 3,400 3 ‐97%

CS29X North EastStorm Relief

763,200 31 122,400 18 ‐84%

CS30X Holloway StormRelief

529,800 49 173,000 41 ‐67%

CS31X Earl PumpingStn

605,900 28 142,700 18 ‐76%

CS32X Deptford StormRelief 1,525,400 38 505,700 31 ‐67%






e changein overflow


Totaloverflowvolume

(m 3)

Numberof

events

Totaleverflowvolume

(m 3)

Numberof events

CS33X GreenwichPumping Stn

7,026,300 45 3,780,400 37 ‐46%

CS34X Charlton StormRelief

24,900 3 - - ‐100%

CS35X Abbey MillsPumping Stn

19,340,600 55 10,302,900 53 ‐47%

CS36X Wick Lane - - - - ‐

CS37X LL1 BrookGreen

100 1 - - ‐100%

CS39X Horseferry 800 3 - - ‐100%

CS40X Wood Street 1,300 4 - - ‐100%

CS42X Pauls Pier - - - - ‐

CS44X Beer Lane - - - - ‐

CS45X Iron Gate 200 2 - - ‐100%

CS46X Nightingale - - - - ‐

CS49X Cole Stairs - - - - ‐

CS50X Bell Wharf - - - - ‐

CS51X Ratcliffe - - - - ‐

CS52X Blackwall Sewer - - - - ‐

CS53X Henley Road - - - - ‐

CS55X London Bridge 8,800 10 100 1‐99%

CS56X Isle of dogsPumping Stn

12,800 7 1,300 2 ‐90%

CS57X Canning TownPumping Stn

- - - - ‐

Beckton Treatment Works 328,981,400 326,587,200 ‐1%

Crossness Treatment Works 151,007,200 147,518,000 ‐2%





Table 4.1 shows the impact of removing 50% of the impermeable area from all subcatchments inthe LTT model. The number of individual CSOs producing overflow during the typical year reducesfrom 39 to 25 and the total overflow volume reduces from 38,952,000m 3 to 17,420,000m 3, areduction of 55%. All CSOs experience less overflow volume, however the flow into bothtreatment works is only slightly affected as the primary influence on both STWs is dry weather flow.

The original model showed the greatest number of overflow events at West Putney Storm Relief(59 events), Hammersmith Pumping Station (57 events) and Abbey Mills Pumping Station (55events). After the removal of 50% impermeable area the CSOs with the most overflow events arethe same but the number of overflows has reduced: West Putney Storm Relief (52 events),Hammersmith Pumping Station (43 events) and Abbey Mills Pumping Station (53 events).

4.4 Conclusion

Reducing the contributing impermeable area by 50% for the entire LTT model removes 55% of theCSO overflow volume during the typical year. The number of CSOs producing overflow reducesby approximately 36% and the CSO record less overflow events during the typical year.

Removing 50% of the LTT catchment’s impermeable area runoff from the system would have avery positive impact on the CSOs, however this requires the disconnection of 10,327ha. Theinstitutional requirements needed to disconnect this scale of area and how such a program couldbe implemented and accomplished will need evaluation.



5 Conclusion

5 CONCLUSION

The three subcatchments of West Putney, Putney Bridge and Frogmore (Buckhold Road) are themost suitable for SUDS selection in the LTT catchment and were modelled to show the impact ofSUDS on CSO.

The impermeable contributing area in each of the three subcatchments was first amended torepresent a proportion both completely disconnected from the system and transferred intopermeable area. This was done for both 25% and 50% of the impermeable areas. While thetransfer of impermeable to permeable area reduces the overflow seen at the CSO, a far greaterreduction was seen when the impermeable area was totally disconnected.

The greatest reduction in overflow was seen when 50% of impermeable area is removed and thisresulted in the reduction of overflow volume at West Putney by 55%, Putney Bridge by 78% and

Documents

Needs report: appendix E