Needs report: appendix D

8/13/2019 Needs report: appendix D

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100-RG-PNC-00000-900008 | Summer 2010

Appendix DSewer Separation Feasibility Study Final Report

Annex 1: Sewer Separation Total Costs FinalReport


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Thames Tunnel Needs Report Sewer Separation Feasibility StudyFinal Report

TABLE OF CONTENTS

GLOSSARY I

EXECUTIVE SUMMARY 1

1. STUDY OBJECTIVE 4

2. STUDY AREAS 6

3. CATCHMENT CHARACTERISATION 10

4. SEPARATION SELECTION METHODOLOGY 17

5. SPINE SYSTEM PREFERRED OPTION 21

6. CONSTRUCTABILITY & BARRIERS TO IMPLEMENTATION 27

7. COST ESTIMATING 38

8. CARBON COUNTING 49

9. SOCIAL IMPACT ASSESSMENT 51

10. INSTITUTIONAL ISSUES 56

11. DELIVERY PROGRAMME 57 12. OPPORTUNITIES AND ALTERNATIVE OPTIONS 59

13. SUMMARY 60

APPENDIX A FIGURES

APPENDIX B SUB CATCHMENT SPINE SYSTEM PIPE SIZES AND LENGTHS

APPENDIX C COSTING GRAPHS

APPENDIX D CARBON COUNTING

APPENDIX E SOCIAL IMPACT ASSESSMENT SUPPORTING INFORMATION


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GLOSSARY

Term Description

Catchment The term is commonly used to describe an area of land where precipitation drainsto a body of water. More commonly in this report an area draining to a spinesewer/interceptor or outfall.

Combined sewer A sewer conveying both wastewater of domestic or industrial origin and stormwater runoff.

Combined seweroverflow (CSO)

A structure, or series of structures, designed to allow spillage of excesswastewater from a combined sewer under high rainfall conditions. Flows maydischarge by gravity or by pumping.

Dendritic

Descriptive word to define sewers which are tree l ike in layout with many smallbranches merging into larger sewers reaching a single or small number of trunksewers. Generally this type of drainage system only has a single pathway as the

water flows away from its origin.Design Rainfall andTypical Year Rainfall

Design rainfall is a synthetic series of storms to represent the peak intensity for agiven return period storm. The storms do not represent how real storms functionbut will reflect their intensity. This type of rainfall is frequently used to design newsewerage systems.Typical year rainfall is based on recorded rainfall. The rainfall data has beencollected over a period of several years and analysed to establish a typical yearof data. Typical year data is frequently used to analyse overflow spills to watercourses.

Foul flowsWaste water from residential settlements and services which originatespredominantly from the human metabolism and from household, commercial andindustrial activities.

Foul sewer A sewer conveying wastewater of domestic and/or industrial origin, but little or norain water.

GIS data A geographic information system (GIS), or geographical information system, isany system that captures, stores, analyses, manages and presents data that arelinked to location.

Infiltration Is the downward movement of water through the ground. It is also used as a termto describe water that is in the sewers but not attributed to foul flows or directlyfrom surface water drainage.

InfoWorks TM CS Urban drainage network modelling software from MWHSoft.

Interceptor Sewer The London Interceptor Sewers date back to the work carried out by JosephBazelgette. They capture flows from the relief sewers and deliver them to the


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Term Description

Crossness, Mogden, Riverside and Long Reach.

London Tideway TunnelDelivery Team (LTTD)

The team concerned with the investigations and construction of the LondonTideway Tunnels.

London TidewayTunnels (LTT)

The LTT comprises two separate projects: the Lee Tunnel and the ThamesTunnel.

PAS 2050 This Publicly Available Specification (PAS) is a method for measuring theembodied greenhouse gas emissions from goods and services across theirlifecycle. This has been developed upon the request of The Carbon Trust andDefra (Department for Environment, Food and Rural Affairs).

Pervious/Imperviousand Permeable andImpermeable

A pervious surface describes land such as a park, where precipitation would notnormally enter the sewerage system. These are also described as permeablesurfaces. Impermeable surfaces (such as paved roads) do not allow rainfall todrain into them and the runoff drains to the sewerage system. These types ofsurface are described as impervious

Public sewer A sewer that is owned and maintained by one of the UK water companies.

Sewage PumpingStation (SPS)

Sewage pumping stations area facilities including pumps and equipment forpumping fluids from one place to another. They are used to lift flows from lowlying sewers to higher lying sewers.

Sewage TreatmentWorks (STW)

Sewage treatment works are used to remove the contaminants from wastewaterand household sewage, both runoff effluents and domestic. It includes physical,chemical and biological processes to remove the contaminants. The objective ofthe STW is to produce a waste stream (treated effluent) and a solid waste orsludge suitable for discharge or reuse back into the environment.

Sewerage system A system of pipes and drains and pumping stations for the collection andtransportation of domestic and industrial wastewater to the STW

Site of Specific ScientificInterest (SSSI)

A Site of Special Scientific Interest is a conservation designation denoting aprotected area in the United Kingdom. SSSIs are the building blocks of site-based nature conservation legislation and most other legal nature/geologicalconservation designations in Great Britain are based upon them.

Special Area ofConservation (SAC)

Special Areas of Conservation (SACs) are strictly protected sites designatedunder the EC Habitats Directive. Art icle 3 of the Habitats Directive requires the

establishment of a European network of important high-quality conservation sitesthat will make a significant contribution to conserving the 189 habitat types and788 species identified in Annexes I and II of the Directive (as amended). Thelisted habitat types and species are those considered to be most in need ofconservation at a European level (excluding birds). Of the Annex I habitat types,78 are believed to occur in the UK. Of the Annex II species, 43 are native to, andnormally resident in, the UK.


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Term Description

along the Thames Tideway.

Tideway The tidal area of River Thames (i.e. from Teddington to the Thames Estuary).

Transport for London(TfL)

Transport for London (TfL) is the local government body responsible for mostaspects of the transport system in Greater London. Its role is to implement thetransport strategy and to manage transport services across London.

UK Water IndustryResearch (UKWIR)

UKWIR facilitates research for UK water operators, encouraging a collaborativeto generate sound science to allow sound regulation of the industry

Urban Waste WaterTreatment Directive(UWWTD)

This European directive related to the collection, treatment and discharge ofurban waste water. The purpose of the directive is to protect the environmentfrom adverse effects of waste water discharge.

Water FrameworkDirective (WFD)

This European directive is a substantial piece of legislation designed to improvethe way water bodies are managed throughout Europe.


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EXECUTIVE SUMMARY

The purpose of this study is to investigate the potential for sewer separation as a means ofmeeting the requirements of the UWWTD and in doing so improve the water quality in theRiver Thames. At present when flows discharge to the River Thames it is a mixture ofsurface water and foul sewage. Thames Water has been requested by Government toaddress the discharges from the combined sewer overflows in the Beckton and Crossnesscatchments.

Using sample sub drainage areas this study investigated separating foul sewage fromrainwater (surface water) runoff in Londons sewers to reduce combined sewage spill intothe River Thames. This was an alternative approach to intercepting the spills close to theRiver Thames and conveying them to treatment via a storage and transfer tunnel.

Five study areas, provided by LTTD, have been considered for the purpose of this study.The sub drainage areas provide a spectrum of land uses and sewer types across theBeckton and Crossness catchments. The areas include overflows from a total of ten CSOsall of which would be controlled by the Thames Tunnel. All five study areas are located in

west London and have less complex interaction with the interceptor sewers than areas ineast London. The selection of the study areas with less complex interactions was to allowfor a more detailed analysis to take place, as the five areas could be looked at asindividual catchments. The complexities of the sewer network to the east would not haveenabled a clear analysis or trend analysis to take place.

In this study the feasibility of reducing the number of spills at CSOs, to four or less spillsper annum, by using separation was reviewed. The scope defines that the study is basedon the three options below. Combinations of options, SUDS, source control andinvestigation of the full Beckton and Crossness catchments is beyond the scope of this

project.

The brief provided three separation options to consider:

Retain the existing combined system as foul only and install a separate surfacewater system complete with new outfalls and pumping stations as necessary tothe River Thames.

Adapt the existing combined system to become a surface water system only withnew outfalls and pumping stations as appropriate. Construct a new separate foulwater system with connections to the existing interceptor sewers.

Construct a separate system to collect drainage from highways and other largeimpermeable areas, retaining the existing combined system to carry residualflows.

The sewers in the five study areas were defined using three categories; spine, local andproperty level.


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requirements, of four spills per annum, a large percentage of the study areas existingconnected area would need to be separated.

The initial selection process for the separation technique was based upon an existingmodel of the spine sewer and the percentage reduction of connected area. The types ofarea and the amount of reduction needed to reduce spills to four per annum werecalculated. The preferred separation technique was based on the following process:

A detailed analysis of the existing drainage system, land uses, connected areaand existing assets.

If the reduction in connected area required to reduce spills to four or less is lessthan 100% of the area of the roads, pavements and driveways and less than 50%of the area of the roofs, then a new surface water system would be less intrusiveat a property level and the most appropriate system to select. This option wouldbe less intrusive at a property level as the new system would not require access tothe rear of the property.

If the required level of separation is 100% of the area of roads, pavements,driveways and greater than 50% of roof area, then a new foul system would be themost appropriate system to select.

The preferred options for all five study areas were selected using this methodology. Theoutcome was the selection of a new surface water system in three catchments and a newfoul system in two catchments.

A review of the practical issues regarding construction was investigated assuming theselected separation technique. The desktop approach highlighted the major issues thatwould need to be addressed if separation was to occur.

Following the selection and design of the separation sewers required in each of the study

areas a costing exercise was undertaken. This took into account the possible constructionissues to establish an idea of cost for these projects. The estimates were specific to eachof the study catchments. However, generic costing tables to allow the costs to be scaledup across London were developed to allow for an estimated cost of separation across theBeckton and Crossness catchments.

The study has recognised that a number of issues integral to large construction projectswould be applicable to any separation project. A carbon assessment was undertaken forthe study areas to assess the approximate carbon impact of construction. A desktop studyof likely social impacts was also carried out, identifying some of the key benefits and

problems that construction work of this scale would entail. The impact on the largeinstitutional bodies was also discussed.

The study details a number of alternatives to traditional gravity sewers to establish if lessconventional methods would be of benefit in separating Londons foul and storm water Thestudy provides a brief explanation of the methods and their history, but does not evaluatethem in detail as this was beyond the scope of this exercise. The alternatives do have aproven history but have tended to be used for small areas and there would be a number of


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In conclusion, when compared to the cost of the Thames Tunnel the cost for separationbased on this investigation is shown to be significantly higher when meeting the four spillsper typical year performance criteria. The timescale to deliver improvements is also shownto be significantly longer than the Thames Tunnel option.


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1. Study Objective

In this study MWH have investigated the potential for sewer separation as an alternative tointercepting the combined sewer overflows into the proposed Thames Tunnel, as asolution to reduce CSO discharges to the River Thames in order to secure compliancewith UWWTD requirements. Other alternatives including SUDs and source control werebeyond the scope of the study and not investigated as part of this study.

Five pilot study areas (sub drainage areas) were assessed within the Beckton and

Crossness catchments to provide an indication of the practicability of separation for thewhole of the two catchments.

General assumptions on which the study is based are:

Improvements to the sewage treatment works (STW) are complete. The Lee Tunnel is complete. The existing interceptor sewers will be retained as the main carriers of foul flow to

the sewage treatment works at Beckton and Crossness. Existing catchment areas and sewerage system with a projected 2021 population.

To represent these assumptions and to determine the impact of separation, the macromodel of the Beckton and Crossness catchments was used. This model does not takeaccount of the Thames Tunnel but does take account of the Lee Tunnel and upgrades atBeckton and Crossness STWs. The model also represents a design horizon of 2021 withpredicted future population and developments.

The brief provided three separation options to consider:

Retain the existing combined system as foul only and install a separate surfacewater drainage system complete with new outfalls and pumping stations asnecessary to the River Thames.

Adapt the existing combined system to become a surface water only system withnew outfalls and pumping stations as appropriate. Construct a new separate foulwater system with connections to the existing interceptor sewers.

Construct a separate system to collect drainage from highways and other largeimpermeable areas, retaining the existing combined system to carry reducedflows.

In assessing the proposed options, a hydraulic model was used, containing the spinesystem level of detail. The two guiding principles followed were:

Any new sewerage system would need to meet Thames Waters designspecification i.e. minimum velocities for new foul sewers and the critical 30-yeardesign rainfall flow capacity for new storm water systems.

The annual spill frequency at the target combined sewer overflows in the pilotareas should not exceed four This value was provided as guidance by Thames


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Spine system costing. The spine system includes the main trunk sewers,manholes, shafts and pumping stations that convey the flows to the terminal STW.The costs are based on the provided hydraulic model results, costing pipe sizes,depths, gradients, location, manholes and existing system modifications.

Local (street) level costing. The local system includes the smaller sewers,manholes and road gullies within the local roads. The costs are based onassessing existing sewer length, typical depths number of manholes and pipesizes.

Property connection level costing. The property level includes the connectingfrom an individual property into the new local sewer system. The costs account forpipe material, location, default size based on property type, density, land use,typical depths, inspection chambers and typical lengths.

To provide a realistic cost estimate additional pumping stations, outfall locations,diversions and the route of the separation works are also accounted for in the costing.

The costing analysis will provide a number of cost curves to enable the results to bescaled up across the whole of the Beckton and Crossness catchments. The study willprovide the tools to scale up. However the actual scaling up is outside of the scope of thisreport.

As part of the costing, non construction costs including service diversions, highwayrestrictions and transport links are assessed and discussed alongside various barriers toimplementation including:

Construction work, restrictions and issues, highlighting construction methodsappropriate to the construction work and the barriers likely to be encountered

Social Impact Assessment - investigating the ramifications and consequences anengineering project of this size would have on society

Carbon Accounting - providing an indication of the notional energy use toconstruct projects of this scale.

The study includes an indication of the programme of delivery and the duration of theconstruction period based on realistic annual spend.

Finally the report highlights a number of alternatives to conventional sewers, assessingtheir practicality in London.


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2. Study Areas

The study areas are all located within either the Beckton or Crossness catchments. TheBeckton catchment is located to the north of the River Thames and drains from north tosouth towards the river with interceptor sewers located from west to the east of thecatchment to convey flows to the Beckton STW. The Crossness catchment is located tothe south of the River Thames and drains from south to north towards the river, again withinterceptor sewers located to convey flows from the west to the east to the CrossnessSTW.

Both catchments are primarily combined draining both foul and storm runoff. Thesewerage systems, when they become overloaded during periods of heavy rainfall, spillexcess flow to the River Thames at various Combined Sewer Overflows (CSOs). TheBeckton and Crossness sewerage systems both contain loops where the flow can takeseveral paths to a downstream point depending upon flow and level conditions in thesewer system. There are parts that are dendritic, where the flow is generally restricted toone route. The dendritic parts of the sewer system are more straightforward to analyse asthey are not influenced by flows from the wider catchment.

Five areas within London were specified by the LTTD. The five areas were selected asthey gave a spectrum of land uses and sewer types across the Beckton and Crossnesscatchments. The five study areas were utilised as a solid representation of the land usesand sewer types found within the whole of the Beckton and Crossness catchments andwere used to create a cost model for scaling up across the whole area. The selected areasare to the west of the catchment and have a less complex interaction with the interceptorsewers than further east in the catchment. The less complex the interaction the greater theopportunity the study has to analyse and potentially find separation opportunities. Thelower complexity of the five study areas enabled conclusions to be drawn on each of thestudy areas and these conclusions were then used to create the cost models for scalingup across the other catchments within Beckton and Crossness.

The areas specified were:

Crossness catchment upstream of Frogmore Buckhold Road this area is 40-70% combined and is largely residential with light industrial premises in the lowerreaches, close to the River Thames.

Crossness catchment upstream of South West Storm Relief this area is 40-70%combined and is largely residential with light commercial and industrial premisesin the lower reaches, close to the River Thames.

Beckton catchment upstream of Regent Street this area is 70-100% combinedand is largely residential with significant areas of business and commercialpremises.

Beckton catchment upstream of Lots Road Pumping Station this area is 70-100% combined and is largely residential and commercial properties.

Beckton catchment upstream of Northumberland Avenue this area is 70-100%


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The five pilot areas and relative locations are shown in Figure 1 in Appendix A.

2.1. Frogmore Buckhold Road

The study area upstream of Frogmore Buckhold Road CSO is to the south of the RiverThames and covers the areas of Wimbledon Park and Putney Heath. The study area is amixture of high density housing, including several blocks of flats, and open park land, withlarge semi detached properties to the south of the catchment. The whole of this studyarea was used for the study. The study area is approximately 454 hectares in size anddrains to the Frogmore Buckhold Road CSO. The asset data shows the majority of the

sewer network in the Frogmore study area to be combined with some small separateareas which all drain into the combined system. The continuation sewer drains to theCrossness STW and two CSOs spilling into the River Wandle and the Bell Lane Creek jointhe River Thames to the north of the Frogmore study area. The current CSO spillperformance is in section 2.6 CSO Spill Data.

The Frogmore Buckhold Road schematic is Figure 2 in Appendix A.

2.2. South West Storm Relief

The South West study area is to the south of the River Thames and covers the areas ofBalham, Streatham, and Peckham. The land use alters significantly, as you progress fromthe north to the south of the study area. To the north of the study area, blocks of flats andcommercial premises dominate. Large terraces, low-rise flats and semi detachedsuburban properties are more prevalent in the south of the study area. Fifty percent of theSouth West study area was used for this study; the sewer network allowed the study areato be split easily as the east and west of the study area convey flows independently fromone another until the system reaches the downstream area close to the overflow. Theeastern branch of the sewer network was chosen as this provided a better synergy withother areas of London, showing a similar land use change from the north to the south of

the catchment. The study area from this point forward will only be in reference to theeastern branch. The study area typified the land use of numerous catchments in bothnorth and south London and therefore would be representative of several areas acrossLondon. The study area is served by a combined sewer system. However, it also includessome areas of separate sewer network, which drain back into the combined system. Thereare several areas in London where small separate systems have been constructed.However, the lack of a watercourse to drain to means they frequently discharge back intothe combined system. The study area is approximately 1,404 hectares and drains to twoCSOs at the north of the catchment; South West Storm Relief and Heathwall SPS. Thecontinuation sewer drains to Crossness STW and the CSOs spill directly to the River

Thames. The current CSO spill performance is in section 2.6 CSO Spill Data.The South West Storm Relief schematic is Figure 3 in Appendix A.

2.3. Regent Street

The study area upstream of Regent Street CSO is to the north of the River Thames and


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properties. The study area from this point forward will only be in reference to the southernregion. The Regent Street and Northumberland Avenue study areas are adjacent to eachother and both contain high density residential and commercial properties. The study areais approximately 1,015 hectares and drains to Regent Street CSO. The continuationsewer drains to Beckton STW and the CSO spills directly to the River Thames. Thecurrent CSO spill performance is in section 2.6 CSO Spill Data.

The Regent Street schematic is Figure 4 in Appendix A.

2.4. Lots Road Pumping Station

The Lots Road study area is to the north of the River Thames and covers Hammersmith,Fulham, Harlesden and Acton and is located to the west of the Regent Street study area.The study area contains industrial, commercial, residential and parkland zones. Fiftypercent of this study area was used; the southern area of the catchment was selected as itcovers a variety of land uses, including industrial, parkland and areas of high densityproperties. The Grand Union canal also provided a geographical break in the network.The North West area, which is largely separately sewered, was excluded as it is notrepresentative of the Beckton and Crossness catchments being studied, which arepredominantly served by a combined system. From this point forward reference to the

study area will only relate to the selected part. The selected study area is approximately3,276 hectares and drains to five CSOs, Acton Storm Relief, Stamford Brook Storm Relief,North West Storm Relief, Hammersmith SPS and Lots Road SPS. The continuation sewerfrom this study area drains to Beckton STW and the CSOs all spill to the River Thames.The current CSO spill performance is in section 2.6 CSO Spill Data.

The Lots Road Pumping Station schematic is Figure 5 in Appendix A.

2.5. Northumberland Avenue

The Northumberland Avenue study area is to the north of the River Thames and coversthe area to the East of Soho, including Oxford Street and Tottenham Court Road. Thestudy area is a mixture of high density housing and commercial properties and showssimilarities to the Regent Street study area. The whole of this study area was used for thestudy. The study area is predominantly combined and has four interceptor sewersdraining from west to east across the catchment. The study area is approximately 93hectares in size and overflows to the Northumberland Avenue CSO located on the VictoriaEmbankment near Charing Cross. The continuation sewer from this study area drains toBeckton STW via the interceptors and the CSO drains to the River Thames. The current

CSO spill performance is in section 2.6 CSO Spil l Data.The Northumberland Avenue schematic is Figure 6 in Appendix A.

2.6. CSO Spill Data

Spill data for the CSOs has been obtained from the Beckton and Crossness Model (2021design horizon model, provided by LTTD). The results are summarised in Table 2.1.


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Table 2.1 Typical Year CSO Spill Data

Study Area CSO NameAnnualSpills(No.)

Total SpillDuration

(hrs/annum)

Total SpillVolume

(m/annum)

Frogmore

Buckhold Road

Frogmore

Buckhold Road29 84 94,600

South WestStorm Relief

South WestStorm Relief 13 40 238,600

Heathwall SPS 40 248 748,100

Regent Street Regent Street 7 15 11,800

Lots RoadPumping Station

Acton StormRelief 27 125 255,500

Stamford BrookStorm Relief 3 4 1,200

North WestStorm Relief 1 1 250

Lots Road SPS 43 398 1,231,800

HammersmithSPS 59 716 2,457,900

NorthumberlandAvenue

NorthumberlandAvenue 13 38 58,000

Results are based on the 2021 design horizon model (with Lee Tunnel and STWimprovements) and not current spills. (February 2010, LTTD).


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3. Catchment Characterisation

3.1. Data Source

LTTD provided the following basic data sets:

MasterMap DataMasterMap Data contains detailed mapping of the roads, buildings, parks andwatercourses, from which land use type, distribution and prevalence can be

determined for each study area. Macro Model of the Catchment AreasThe Macro Model is a computer based mathematical representation of thecatchment areas. The model is run in a specialist computer software programmecalled InfoWorks CS. The model used was the combined macro model of theBeckton and Crossness catchments. This model does not include the ThamesTunnel but does include the Lee Tunnel and the upgrades at Beckton andCrossness STW. The model also represents a design horizon of 2021 withpredicted future population increases.

Address Point Data

Address point data contains the number of properties in a particular location. Thisinformation can be used with the population taken from the model to understandpopulation density for a given area. The benefit of using this data is highlighted byflats and other multiple occupancy buildings, where it is unclear how manyproperties there are in a single block.

Asset DataThe macro model contains only the main sewers (spine system) and not the localdetail. The asset data contains all the Thames sewer data but it does not includeproperty level (private) sewers. This data is important to establish whereseparately sewered areas exist.

3.2. Classification of Areas

Each of the five pilot study areas has been classified to understand the nature of thecatchment using the following criteria:

Quantity of impermeable area (storm response in the model) Potential quantity of impermeable area (storm response) based on MasterMap

data Land Use

Property density Sewer size and length Population in each area (population density) Sewer Type (Combined, or Separate)

Within the model each study area is split into smaller areas commonly termedsubcatchments. Correlations between the modelled subcatchments in a study area using


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3.2.1. Quantity of impermeable area in the modelThe Beckton and Crossness model included percentages of impermeable area within eachsubcatchment. The percentage of impermeable area varied between subcatchments fromless than 10% to 99% of the total contributing area.

The percentage of modelled impermeable area across the Beckton and Crossnesscatchments can be seen in Figure 7 in Appendix A.

3.2.2. Quantity of impermeable area based on MasterMap data

The available MasterMap Mapping data was used to calculate subcatchment parameters,including proportions of impermeable area (road areas, roof areas and additional hardstanding areas) and permeable area (grass, park and trees). These values were thencompared to the modelled impermeable percentages across the five study areas and werefound to match closely. This provided confidence in the use of the model for the separationassessment.

The percentage of impermeable area within each subcatchment, according to theMasterMap data, can be seen in Figure 8 in Appendix A.

The percentage of impermeable area in each subcatchment was considered the mostconsistent parameter that could be used to make a decision on the new sewer systemrequired in each study area. The results from the analysis showed a strong correlationbetween the five study areas and therefore the quantity of impermeable area could beused to make comparisons between the study areas.

3.2.3. Land Use

The MasterMap data was utilised to classify the surface types in each of the study areas.

An example of the MasterMap surface types can be seen in Figure 9 in Appendix A.

Within the study areas these attributes were grouped into five main land use types. Thegrouping was specific to the areas investigated in the study and could be different for otherparts of London. The land use types identified in the study areas were:

High density housing Medium density housing Mixture of high density housing and industrial/commercial Industrial/commercial Mixture of parkland and high density housing

The general similarities between the study areas showed the land use changed from highdensity housing close to the river to medium density housing (semi-detached anddetached properties) towards the upper ends of the study areas. Commercial andi d t i l ti l t d i h t d H th t f id ti l


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Graph 3.2 Total Sewer Length plotted against Subcatchment Size.

Further investigations were carried out for a selection of subcatchments across the fivestudy areas where the properties were densely packed. The results of this investigationare shown in Graph 3.3. The analysis indicates a positive correlation between the totallengths of sewer sizes of 150mm to 375mm diameters compared to the size of thesubcatchment they are situated within. However, the correlation appears to becomeweaker as the subcatchment size increases.


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Graph 3.3 High Density Subcatchments Comparison of Total SewerLength of Pipe Sizes 150mm to 375mm diameter plotted againstSubcatchment Size.

3.2.6. Population Density

The population data, (taken from the Beckton and Crossness macro model, 2021population), has been used to calculate population densities in each study area. Therewere general similarities between the five study areas with the property types changingfrom high density blocks of flats and terraced properties close to the river to less denselypacked semi detached and detached properties within the upper portions of the studyareas.

An example of the address point per building breakdown can be seen in Figure 11 inAppendix A.

The population data was compared to the subcatchment region size and showed apositive correlation that can be seen in Graph 3.4.


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Graph 3.4 Population Data plotted against Subcatchment Size.

3.2.7. Sewer Type (Combined or Separate)

Sewer types were investigated across the study areas for correlations between differentregions. The modelled spine sewers in all five study areas were combined and the asset

data showed various proportions of foul, surface and combined sewers for the localsewers in each area. The majority of the local sewers (76%) were combined in all fivestudy areas and none of the study areas contained an extensive separate foul or surfacewater network. If the study area had an existing extensive foul sewer network then itwould have been logical and less disruptive to design a new foul sewer system that couldconnect into the current sewer network. Therefore the existing system did not dictatewhich option would be the most suitable for the new sewer system and this parameter wasnot investigated further.


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3.2.8. Property Types

Based upon visual evidence, the majority of residential properties have pitched roofs.There would be a clear difference in effort between separating only the front of theproperties compared to both the front and back. Sample areas were selected toinvestigate how properties are drained. In general residential terraced style propertieshave one roof connection for the front of the property. Semi detached and detachedproperties often have a number of connections. However this often varies dependent onstyle and if additional work to the property has taken place. Commercial property in the

city often has flat roofs and internal drainage from the roof, therefore analysis of theseproperties was not possible.

The assessment of property type indicated that the buildings could be separated intodifferent size categories. The smallest buildings, such as garages and small propertieswould have one roof drainage point, larger semi-detached and detached properties wouldhave two roof drainage points and larger buildings were assumed to have three roofdrainage points. A further assumption at the property level was that the foul connection ina property is regularly at the back and would be more challenging to re-connect than thefront of the properties surface water.

The data analysis detailed in section 3 was carried out to gain a clear understanding of thefive study areas. This understanding was vital in order for a separation methodology to bedeveloped. The understanding of the five study areas was also required in order to provideconfidence in the methodology and in the scaling up factors that were to be produced.


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4. Separation Selection Methodology

Before selecting the sewer separation option, the macro model was used to confirm thecurrent spill frequency performance of the combined sewer overflows within the studyareas.

The contributing areas in the macro model include a total area and the percentage of theconnected impermeable area (road, driveway, pavement and building roof) thatcontributes storm runoff to the combined sewers.

Three separation options were considered as part of the brief:

Option 1Retain the existing combined system as foul only and install a separate surfacewater system complete with new outfalls and pumping stations as necessary tothe River Thames.

Option 2Adapt the existing combined system to become a surface water system only withnew outfalls and pumping stations as appropriate. Construct a new separate foul

water system with connections to the existing interceptor sewers. Option 3

Construct a separate system to collect drainage from highways and other largeimpermeable areas, retaining the existing combined system to carry reducedflows.

For options 1 and 2, complete separation of Beckton and Crossness catchments wouldmeet the design target spill frequency of four spills per annum. The macro model was setup for 100% separation of the study areas and the results confirmed zero spills at thecombined overflows. However a number of the overflows related to the study areas will

spill even with 100% separation upstream, if the separation is only in the study areas. Forthese overflows, the mechanism of spill was a combination of the surface runoff and theincapacity of the interceptor sewers to receive the continuation flows. A more detaileddiscussion is included in the preferred approach discussion for each pilot area.

For option 3, the level of separation required to meet the spill standard was ascertained byrunning a range of reductions to identify the target separation percentage. This reductionprocess was carried out for all of the main CSOs located in the five study areas. Thenumber of spills for the range of reductions and the separation required for each of theselected CSOs to reach four spills per annum can be seen in Table 4.1.


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Table 4.1 Percentage of Separation Required for each CSO.

Study Area CSO NameNo. of spills for a percentage separation *Percentage

SeparationRequired0% 25% 50% 75% 100%

Frogmore Buckhold Road

Frogmore Buckhold Road 29 18 11 4 0 75%

South WestStorm Relief

South West

Storm Relief13 7 2 0 0 38%

Heathwall SPS 40 35 25 12 0 95%

Regent Street Regent Street 7 4 1 0 0 25%

Lots RoadPumpingStation

Stamford BrookStorm Relief 3 1 0 0 0 0%

*Acton StormRelief 28 19 13 3 0 75%

North WestStorm Relief 1 0 0 0 0 0%

Lots Road SPS 43 39 35 22 0 96%

HammersmithSPS 59 58 53 39 0 99%

NorthumberlandAvenue

NorthumberlandAvenue 13 7 3 0 0 42%

*The percentage separation required column is based on Graph 4.1 for Acton StormRelief. The graph uses the model results to form a line of best fit. Acton Storm Reliefshows a 75% separation reduces spills to three per annum. However, to reach four spillsthe accuracy of the best fit line is such that the separation for four spills remains at 75%.


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Graph 4.1 Results for Sizing Separation Percentage

0

5

10

15

20

25

30

0% 20% 40% 60% 80% 100%

A n n u a

l S p i l l F r e q u e n c y

Percentage Separation

Acton Storm Relief

The separation analysis (Graph 4.1 provides an example output for Acton Storm Relief)provided the overall area percentage reduction of runoff surface required to meet the fourspills requirement. Next this was related to the actual storm water contributions. Using theMasterMap data in GIS the area of the four main contributors to surface water runoff(roads, pavements, driveways and building roofs) was calculated. This was then related tothe contributing area included in the macro model so for each subcatchment the areaneeded to be removed from the combined system, to meet the overall separationpercentage, was identified.

Example for Frogmore

The model suggests that the Frogmore Buckhold Road CSO is estimated to spill 29 timesduring typical year rainfall.

Percentage reduction analysis of the upstream catchment indicates a 75% reduction in theconnected surface water is required to meet a four spill frequency at the CSO.

The assumption is that the hierarchy for separation (Option 3) would be roads first,pavements, driveways and then building roofs. This is to minimise the disruption toproperty. This hierarchy has been applied to all five study areas,

Figure 12 in Appendix A characterises the surface water contribution for each of the macromodel contributing areas as pie charts showing the area split between roads, pavements,driveways and building roofs. It also shows the actual percentage reduction required foreach surface water contribution type, that would be needed to be separated (values ineach pie chart) to meet the 75% total reduction

TargetReduction


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Graph 4.2 Separation Requirements (Pie Chart)

Red RoadsGreen PavementYellow DrivewaysBlue Building Roofs

In the example above (Graph 4.2) the analysis shows that the required level of separationis achieved by separating all the road and pavement areas, 80% of the driveways and50% of the building roofs.

Where possible we have tried to limit the roof area to 50% (front half of the roof), avoidinghaving to modify the drainage at the rear of the property. When the separation of an arearequired more than 50% of the roof, construction of a surface water system was proposed.

Figures 12 to 16 in Appendix A provide the analysis as described above for all the studyareas.

4.1. Local Sewer Selection

The spine system selection methodology also dictates the selection of system used todrain the local system. The local system connects the residential streets to the spinesewers and therefore it would be logical to install the same system type in each area forboth the local and spine sewers.

4.2. Property Level Selection

The property connections are also driven by the spine system methodology. The samesewer system type would be installed at each level of the new sewer system.


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5. Spine System Preferred Option

Using the characterisation and required separation analysis the preferred separationoption for each study area was based on the following general principles:

If the required level of separation is less than 100% of the roads, pavements anddriveways and less than 50% of the roofs, then a new surface water system wouldbe less intrusive at a property level and the most appropriate system to select.This option would be less intrusive at a property level as the new system would

not require access to the rear of the property. If the required level of separation is 100% of roads, pavements, driveways andgreater than 50% of roof area, then a new foul system would be the mostappropriate system to select as the new sewer system would require access to therear of the property. A foul system would be selected as it would require smallerpipe sizes than a surface water system.

In addition to the above, where the required separation is significantly less than100% of the roads, pavements and driveways and less than 50% of roofs, thecatchment can be looked at in terms of land use to indicate that not all of thecatchment would need to be separated. In this case the separation would require

a new surface water system but only in parts of the catchment.

The existing Beckton and Crossness macro model was used to generate the flows andpipe design information to size the sewers for each proposed option. The initial feasibilityassumed the new system would follow the same route as the existing system. The newsewers have been designed to meet a 1 in 30 year critical storm condition as per currentguidelines.

Once sized, a reality check was carried out to review the buildability of the new system.The initial design was reviewed for connections required to the existing system, any

obvious alternative (shorter routes) and any alternative discharge points (e.g. for surfacewater sewers discharging to local watercourses). The initial design was also looked at forthe most appropriate construction methodology that would be used and any environmentalor archaeological land use constraints in the area.

There were a number of other important factors that were considered in the analysis.Much of the existing sewer system is interconnected. Any proposed new foul systemwould need to be localised and discharge into the existing combined interceptors. Utilisingthe existing interceptor sewers, to convey the foul flows to the STW, would remove theneed to construct either a new foul system all the way to the STW or a new storm system.

The existing interceptor sewers would convey foul flow during dry weather and duringsmall storms. However, during larger storms any excess surface water will need todischarge into the interceptor sewers to ensure the current capacity in the existing sewersystem is maintained.

For some of the larger study areas the proposed new surface water system would requireparallel systems due to the excessive pipe sizes required. The new surface water


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5.1. Frogmore Buckhold RoadThe proposed scheme for Frogmore Buckhold Road, based upon the separationmethodology, would be to build a new surface water system and utilise the existing systemfor the foul flows. Four pumping stations would be required for this new surface watersystem; two would be located at the outfalls to the south west of Wimbledon Common, onewould be required to lift flows into the River Thames and the final pumping station wouldbe required along Melrose Road to lift the flows into Bell Lane Creek after they havepassed under the railway line. Frogmore is dendritic and does not form part of theinterconnected system within the Beckton Crossness model and therefore could beinvestigated without any influence from other areas affecting the results for this area.

The investigations into this study area showed all of the roads, pavements and themajority of the driveways in the area would need to be separated in order to reach the spilltarget. A maximum of fifty percent of the roof runoff would need to be separated andtherefore a new surface water system was selected as the solution for this area. Pumpingstations would be required at the downstream ends of the new surface water sewernetwork to lift the surface water flows up into the River Thames and Bell Lane Creek.

The separation analysis for Frogmore is shown in Figure 12 in Appendix A.

A new surface water system in this area would only require access to the front of allproperties in order to convey the roof runoff flows in to the new system whereas a new foulwater system could require access to the rear of each property. This rear access wouldcause more disruption than installing a new surface water system in the area. Theinstallation of a new surface water system would minimise the risks of cross contaminationbetween the foul and surface water systems. The existing system would be utilised forconveying foul flows and the surface water flows would be conveyed via new connectionsinto the new sewer network.

The Frogmore Buckhold Road area includes several over ground and underground railwayroutes and some of the new proposed sewer network would require crossing these railwaylines. The new sewer network would cross the railway line from Southfields undergroundstation to East Putney underground station at least once and could cause disruptions tothe train service during the construction phase. The new sewer system could requireadditional tunnel diversions to avoid causing too many disruptions to the major railwayroutes. The design of a new spine system was investigated and found that a single outfall(similar to the existing layout) would not be practical in terms of size, gradients andadditional rail and road crossings. Therefore modifications were investigated and tworoutes to the Thames were found to be the most practical, this included a route

discharging to a similar location to the current overflow and another through Putney,tunnelled through an urban area outside of the study area. The new proposed surfacewater network would cause disruption in a large number of streets in addition to navigatingalong the A219 Putney Hill Road for approximately seven hundred metres and wouldcause traffic disruptions along this route. The constructability issues for the FrogmoreBuckhold Road area are detailed in section 6.2.


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the surface water. The new foul spine system would require one pumping station, locatedat the junction of Croxted Road and Thurlow Park Road, to lift the flows from the southeast of the study area so they can continue to the interceptor sewers. The South Weststorm relief area does not form part of the interconnected system, and does not containsewers that convey flows from outside of the study areas therefore it could be investigatedwithout any influence from other areas affecting the results for this area. The catchmentdoes have more than one sewer leaving the catchment, this is a benefit to creating a newfoul system as they form breaks in the catchment.

The investigations into this area showed all of the roads, pavements and the majority of

the driveways in the area would need to be separated in order to reach the spill target. Anaverage of over 80% of the roof runoff would need to be separated and therefore a newfoul water system was selected as the solution for this area.

The separation analysis for the South West Storm Relief study area can be seen in Figure13.

For this study area a new foul or a new surface water system would require access to therear of the properties in order to connect the flows to the new system. The pipe sizesrequired for the foul network would be smaller than a new surface water system and

therefore a new foul system would be a more constructible option. The new foul systemhas been designed to follow the same route as the existing system as the pipe sizes aresmall and in most cases there should be enough space for the new system. In a moredetailed design, the sewer system route could be modified further but this is beyond thestudy scope. The foul network has been designed to utilise the existing interceptor sewersto convey flows downstream to the Crossness STW. The existing combined system wouldbe used to carry surface water to the River Thames at the downstream end of the studyarea with overflows into the interceptor sewers to be used in extreme storm events. Thereason for this overflow is to ensure no change in the level of service. By disconnectingthe existing interceptors the capacity for surface water would be reduced. Therefore during

larger rainfall events surface water flows would need to overflow into the existinginterceptors to balance flows at existing conditions. The interceptor sewer would thereforeonly carry foul flows during dry weather and minor storm events but may convey somesurface water flow during extreme weather events when the existing system could notcope with the surface runoff.

The South West Storm Relief area covers the areas of Clapham Common, Streatham andBalham. There are several railway lines across the area, both over ground andunderground. The underground stations include Tooting, Clapham North and Brixton.Over ground stations include Clapham High Street, Streatham Common and Herne Hill

Rail and there is also a train depot to the north of the catchment. The constructabilityissues for the South West Storm Relief area are detailed in section 6.3.

The proposed spine sewer network for the South West Storm Relief area can be seen inFigure 18.

The required sewer sizes and lengths and pumping station requirements for the spine


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the Regent Street CSO can be caused by backing up from the Low level no. 1 interceptorsewer as well as the flows from the upstream catchment. Separation of the Regent Streetarea alone would not meet the four spills per annum as the CSO would still spill due to theincapacity of the system downstream backing up and spilling.

The investigations into this area showed all of the roads in the area, up to 100% of thepavements but less than 50% of the roof area would need to be separated in order toreach the spill target and therefore a new surface water system was selected as thesolution for this area. A pumping station would be required at the downstream end of thenew sewer network to lift the surface water flows up into the River Thames.

The separation analysis for the Regent Street study area can be seen in Figure 14 inAppendix A.

The Regent Street area covers the West End, Soho and Belgravia including key roadssuch as Westway and Marylebone Road. There are numerous tourist areas and theatresin the study area and the installation of the new surface water system would requirestrategic tunnelling to the River Thames to minimise disruptions to the downstream areasof the catchment. The surface water system for this area would require large diametersewers requiring tunnelling and parallel systems in order to convey the surface water flows

to the River Thames. The selected catchment is likely to require tunnelled surface watersewers that would be routed to the Thames through areas not analysed in thisassessment. The constructability issues for the Regent Street area are detailed in section6.4.

The proposed spine sewer network for the Regent Street area can be seen in Figure 19.

The required sewer sizes and lengths and pumping station requirements for the spinesystem are detailed in Appendix B.

5.4. Lots Road Pumping StationThe proposed scheme for the Lots Road area, based upon the separation methodology,would be a new foul system. The existing sewer network in the Lots Road area is verycomplex with many overflows and weirs connecting the sewer network and therefore theflows can take many different routes to reach the relief sewers or the CSOs. Theproposed new foul network in this area would connect to the interceptor sewers runningwest to east to reduce the flow volume being conveyed to the downstream end of thesystem. Two pumping stations would be required to convey the flows to the interceptorsewers; one would be located along Holland Road, opposite Kensington Olympia and the

second would be located along Finborough Road.The investigations into this area showed all of the roads, pavements, the driveways andthe majority of the roof runoff in the area would need to be separated in order to reach thespill target. An average of over 90% percent of the roof runoff would need to be separatedand therefore a new foul water system was selected as the solution for this area.


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flows to the river at several points at the downstream end of the catchment. A newsurface water system would therefore require tunnelling in order to cope with the flowvolume within the catchment and significant pumping to discharge to the Thames. Incomparison the new foul network design is smaller with sewer diameters of up to 1.5metres able to cope with the foul flows and a much lower pumping requirement. Theconstructability issues for the Lots Road Pumping Station area are detailed in section 6.5.

In the Lots Road study area, the main overflows are Acton CSO, Hammersmith CSO andLots Road CSO. The continuation flows from each of these CSOs can interact and impacton one another. The CSO at Acton spills once the Acton Storm Tanks are full. Acton CSO

operates independently from the other CSOs in the catchment as it is the most westerlyCSO. However, continuation flow from the Acton CSO passes into the network controlledby Hammersmith and Lots Road CSOs so what is done at Acton has a direct impact onthe downstream catchment. If Acton was treated independently it would require a 75%reduction in storm response. However, the separation required for Lots Road would be96% and 99% for Hammersmith. If the continuation flows from Acton were decreasedfurther by separating a higher percentage, this would potentially lower the separationrequired downstream.

The proposed spine sewer network for the Lots Road Pumping Station area can be seen

in Figure 20.



The proposed scheme for Northumberland Avenue, based upon the separationmethodology, would be to build a new surface water system and utilise the existing systemfor the foul flows. A single pumping station would be required at the downstream end ofthe new spine system, along Northumberland Avenue, to lift the flows into the RiverThames. The selection of a new surface water system ties in with the selection for theRegent catchment which is adjacent to this area. The Northumberland Avenue areaconnects into the low level interceptor sewer and spills at the CSO can be caused bybacking up from the interceptor sewer as well as the flows from the upstream catchment.Separation of the Northumberland Avenue area alone would not meet the four spills perannum as the CSO would still spill due to the incapacity of the system downstreambacking up via the Low Level 1 interceptor and spilling at Northumberland Avenue CSO.

The investigations into this area showed all of the roads and up to 85% of the pavements

would need to be separated in order to reach the spill target. It also indicated that roofarea would not require separating. It could therefore be possible to carry out a largerpercentage of separation in the upstream area (including roof area) of this study area toavoid having to carry out any work in the busy Trafalgar Square and Leicester Squareareas.

The separation analysis for the Northumberland Avenue study area can be seen in Figure


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The proposed spine sewer network for the Northumberland Avenue area can be seen inFigure 21.


5.6. General Design Commentary

The Beckton and Crossness sewer systems are very complicated, containing numerous

interceptors, relief sewers and diversions, resulting in an interconnected system. Due tothe complexities of the catchment it was observed that to separate discrete areas withinthe Beckton and/or Crossness catchment would result in a limited improvement at some ofthe CSOs in the study areas. Northumberland Avenue is a clear example, as removing100% of the storm response in the defined catchment would not stop the CSO fromspilling. This CSO would continue to spill due to its connection into the Low Level No 1Main Line interceptor sewer that collects flows from further west of the catchment.However, if the flows to the west were addressed and the interceptor sewer only containedfoul flow, then all or a significant proportion of the Northumberland Avenue area could beleft as combined and still meet the target of four spills per typical year of rainfall.

If a new surface water system was developed the discharges to the water courses shouldnot contain any foul flow. However, hydrocarbons and debris build up on roads andpavements would be flushed into the surface water system during times of rainfall,especially after a prolonged dry spell. Misconnections will often mean a system will neverbe completely separate and therefore there would always be the opportunity for foul flowsto be in a surface water system. The modelling for this study cannot assess these issueswithin the scope. If a new surface water system was created the combined system wouldbecome foul and be significantly oversized (compared to the foul only flow it would carry).Converting combined sewers to foul sewers risks failure to meet self cleansing velocity.This could create an increased risk of odour problems and blockages.

[In reality if only selected areas were addressed by separation, the number of spills at theCSOs does not significantly change, as many of the systems are linked through theinterceptor systems. Although the separated catchment upstream has significantlyreduced flows, flows from other catchments are drawn down to the area and utilise theadditional capacity and still cause the spills, but with reduced volume. I

5.7. Water Quality CommentaryWater Quality analysis has been undertaken by LTTD using a water quality model of theRiver Thames and the model spill flow with EMC values to assess impact and derive thetarget spill frequency. This study is not addressing all the CSOs and in some cases doesnot meet the target spill frequency due to the interconnections. Given the scope of thisstudy a full water quality analysis is not possible. The study can only state that separation

ld b d h f ill i i If l i hi h i i


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6. Constructability & Barriers to Implementation6.1. Introduction

A desktop study investigating a number of construction methods and likely problems toimplementation has been carried out for each study area. This has been carried out toidentify likely construction techniques required for the new sewerage systems and theproblems. Within the scope of the study the separate systems have been developed toprovide an understanding of construction requirements, but not to the point of beingoptimal solutions. The constructability assessment is also only an outline of some of thekey issues and implementation barriers to provide an idea of the issues that are likely tobe faced. This is not an exhaustive or detailed list of all the issues likely to beencountered, but it does try to identify the main issues to be faced.

The constructability has been designed on the basis of the following assumptions:

Removal of excess sewer lengths and duplicate sewers and/or manholes Re-route sewers within carriage way to avoid tunnelling under buildings where

possible Provide a maximum spacing of 100m between manholes Avoid strategic roads, intersections and traffic hot spots where possible. Provide a more efficient system to remove or reduce large size pipes or deep

pipes where possible.

A number of the construction techniques and possible barriers are generic to all studyareas. New gully connections are applicable to all the catchments where a new surfacewater system is proposed. Local sewers, local pump stations and property connectionswill be generic to both separated foul and surface water systems.

6.1.1. New Local SewersThe construction of the local sewers will require short term rolling road closures throughoutthe study areas. It is anticipated that the local level sewers will be installed via open cut,with only a few sections of tunnelling where required to pass under obstacles such as railcrossings. This will impact every street in each catchment, causing disruption to localbusinesses, residents and traffic. However, as the pipes will generally be of smallerdiameters and at shallower depths than the spine sewers, construction is anticipated to befor only short durations at any one location. Works will need to be programmed to ensuremaximum access is provided for residents and traffic and mindful of local schoolsschedules. Access for emergency vehicles will have to be maintained throughout theworks.

6.1.2. New Local Pump Stations

Small pump stations along the local sewer system may be required in some locations to liftthe flow from the local level sewers into the spine system. This requirement is related totopography but could also relate to local construction issues Therefore an exact number


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6.1.3. New Gulley ConnectionsAs part of the new local surface water sewer system, road gullies will require reconnection.It is assumed that the existing gullies will be adequate, and only reconnection from theexisting gulley to the local surface water system will be required.

The existing gullies are likely to require cleaning at the same time; however, this has notbeen considered as part of this study. It is assumed that the connections will beundertaken within the same rolling road closure as the local sewers detailed above.

6.1.4. New Property Connections

A new connection from every roof area within the catchment to the surface water systemwill be required. For foul systems a new connection from the foul pipe from the propertywould be necessary.

Both types of connection will involve access to private land and temporary closing of somefootpaths. In the Frogmore study area the majority of connections will be from residentialproperties. Access into private front gardens will be required, which is assumed to beconsidered intrusive by the general public. Considerable public relations and consultationwill be required.

6.1.5. Construction Methods and Issues

The majority of the sewers in the five study areas are expected to be constructed via opencut methods with rolling road closures. Some of the sewers will however, requiretunnelling in order to be constructed. It has been assumed that sewers at a depth of lessthan six metres will be constructed via open cut and those at six metres deep or more willrequire tunnelling. Tunnelling would also be required for sewers that are in areas whereconstruction would prove difficult, such as Piccadilly Circus, or to pass under railway linesand rivers.

The availability of space to build has been considered, especially for the surface watersystems where the pipe sizes are large and will require large trenches in order forconstruction to take place. The spine sewers are designed to follow the existing sewernetwork and so there will be limited space within the road due to the existing pipe workand the utilities already situated within the study areas. There could be a requirement fordiversion of utilities whilst the construction takes place.

There are other service tunnels, and abandoned tunnels and stations across London;these would need to be located prior to construction and any necessary consultation would

need to take place with the responsible body.

The works would include rolling road closures and would have a significant impact on localbusinesses, schools and traffic, including numerous bus routes throughout the five studyareas. There are a number of railway and underground stations and lines across the fivestudy areas and construction could have a significant impact on these areas. Consultationwith the local Highways Authority will be required to assess restrictions placed on any road


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the west to Shenfield and Abbey Wood in the east. A separation project would need toavoid clashing with the Crossrail Tunnel, which could have major implications to sewerdepths and potentially pumping requirements. The construction of Crossrail is planned tolast until 2017 and therefore careful, strategic planning of the separation constructionwould be required to avoid any detriment to either project.

6.2. Frogmore

The new surface water spine sewer proposed divides the Frogmore study area into foursub-catchments, see Figure 17. To the north two branches, one from Putney Heath and

the other from Hazelwell Road, join at Putney High Street discharging at a new outfall intothe River Thames.

To the east the new spine sewer runs north along Sutherland Grove from Southfields tubestation, crossing under the railway tracks at the junction with Melrose Road anddischarging into Bell Lane Creek via the existing outfall.

The southern section of the catchment runs along Augustus Road and Inner Park Road toa new outfall, at a local watercourse in Wimbledon Common.

To the west of the catchment the new spine sewer runs south along Putney Park Lane andwest along Putney Heath and Roehampton High Street to a new outfall into a localwatercourse.

Splitting the surface water network into these four sub-catchments reduces the pipe sizesand depths required across the catchment.

Buildability Issues & Existing Site Restrictions Available space to build within the road is expected to be limited, particularly for

the 6km of new sewer that is greater than 800mm diameter.

Tunnelling will be required at the following road locations Putney High Street Beneath the London Underground railway, at the junction of

Sutherland Grove and Melrose Road The connection from Melrose Road into the Bell Lane Creek

outfall as open cut construction would be difficult The A3 is strategic dual carriageway connecting to the M25. Any significant road

closures proposed for this road are unlikely to be granted easily. London Underground and National Rail have two main lines within the Frogmore

area, namely:

London Underground District Line - the majority of the tube line inthis section is either at or above ground level. National Rail: runs east-west from Clapham Junction, through

Putney Rail Station, parallel to Upper Richmond Road.The proposed surface water sewer crosses these rail lines in three locations,

junction of Sutherland Avenue and Melrose Place, Putney High Street andadjacent to Putney East Tube Station Tunnelling would be required at these


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impact assessment and permission from the Environment Agency would berequired in order to convey flows into the watercourse. The existing discharge at Bell Lane Creek will continue to be utilised for the flows

from the eastern part of the Frogmore study area and the existing combined sewerwill continue to function but as a foul only sewer to Crossness STW. Three newsurface water overflows will be constructed at the following locations:

Putney High Street into the River Thames Wimbledon Park Side into a local watercourse in Wimbledon

Common Roehampton High Street into a local watercourse

6.3. South West Storm Relief

The new foul system proposed for this area would utilise as much of the existing sewerageas possible, see Figure 18. The new foul system would be dendritic and would join intothe interceptor sewers at six locations to avoid the flows having to be conveyed from thevery top of the catchment to the bottom.

The six discharge locations from the proposed foul water spine sewer into the interceptorsewers are located at East Dulwich, Brixton Station, Londor Road, Vauxhall Bridge andtwo along Nine Elms Road. The interceptor sewers discharge at Crossness STW.

The existing combined sewers which currently overflow to the River Thames will beutilised as surface water sewers. The existing overflows at East Dulwich, Brixton Station,and London Road would be modified to include a high level weir. These would preventfoul flows from reaching the sewers that overflow to the River Thames but will allow flowsfrom the surface water network to overflow into the interceptor sewers during high rainfallevents. This would ensure there is no reduction in existing capacity in the network. Theweir configuration will be set at a level that the foul water network will not back flow andcontaminate the surface water network.

In addition the two existing CSOs at Nine Elms Road will be modified and used todischarge the surface water into the River Thames.

Buildability Issues & Existing Site Restrictions Available space to build within the road is expected to be limited for the 2km of

new sewer that is greater than 800mm diameter. Tunnelling will be required along the following road locations

Kings Hill, near West Norwood rail station under the rail lines Streatham Hill, near Streatham Hill rail station under the rail lines

London Underground have a two tube lines within the South West Storm ReliefCatchment, namely:

Northern Line Victoria Line

A number of national rail stations are located across the South West Storm Reliefcatchment, with at least five rail lines running through the area. Clapham Junctioni l t d i th t f th t h t d i th b i t il t ti i th


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Clapham Junction

6.4. Regent Street

The new surface water system proposed for this study area has been designed to followmain roads down to the River Thames, see Figure 19. The new system would require twinsewers at some locations due to the large volume of flow that needs to be conveyed to theriver.

There is one discharge location required for this study area on Victoria Embankmentapproximately 100m south of Hungerford Bridge into the River Thames. A new outfall willbe constructed to accommodate the surface water discharge into the River Thames. Theexisting CSO will be modified and used as the overflow device into the existing interceptorsewers.

Two new intermediate pump stations are required at 30m/s each. Located close to theBayswater Road, these pumps pick up the flows outside of the study area to the west andwould have a significant impact on the design.

One new pump station is required at the downstream end of the new surface waternetwork, on the Victoria Embankment. This is a significantly congested area of thecatchment, located between Charing Cross Train Station and Hungerford Bridge,Embankment Tube Station, Victoria Embankment road and the River Thames. The Circleand District Underground lines run directly under Victoria Embankment, between thecatchment and the River Thames, and the Bakerloo line runs between Charing CrossStation, through Embankment Station, to Waterloo Station. In addition, Embankment Pier(a central Riverboat stop) is located directly opposite Embankment Tube Station on theRiver Thames. Therefore there is limited space to install a new pump station and landpurchase is likely to be required.

The new pump station is necessary to lift the surface water up approx 3.0m from the newsewers into the River Thames. Based on a 1 in 30 year peak flow rate from the surfacewater system a rate of 96m/s. would be required to pump all flows. However this is not arealistic rate and is unlikely to be required or constructed. The rate of 96m 3 /s has beenprovided by the model. The model provided is a high level design tool and thereforeconfident estimation of flow rates and overflow operation at 1 in 30 years design has notbeen possible. A more realistic design would have the outfall discharging via gravity to theRiver Thames at an appropriate level with a flap valve to control flows during high tide. Asmaller pump station of approximately 20-30m/s will still be required to fully empty thesurface water tunnels.

Buildability Issues & Existing Site Restrictions: Available space to build within the road is expected to be limited. All 19km of the

new sewers will be larger than 800mm diameter and therefore will requiresignificant space.T lli g ill b i d t b f l ti i l di g


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Circle, Hammersmith & City, Metropolitan lines Circle & District Lines Jubilee Line Northern Line Piccadilly Line Victoria Line Crossrail (under construction): crosses all proposed new surface

water sewers and intermediate pumping stations. Paddington, Marylebone and Charing Cross Train Stations are located within the

Regent Street catchment. Although specific works are not proposed directly

outside these stations, works nearby by, including the new local sewers, are likelyto have an impact and possible mitigation measures should be considered.Consultation must be undertaken prior to works which may be close to any of therailway tracks, bridges or other structures.

There a number of hot spots, or areas where works are likely to have high impactand disruption to traffic, local business and tourists spots. These hot spots include:

Trafalgar Square Leicester Square Piccadilly Circus Oxford Street &

Oxford Circus Regent Street Baker Street Marylebone High

Street

Paddington Hyde Park Regents Park Marble Arch

St. Marys Hospital Lords Cricket Ground Nationally Gallery University of Westminster

Buckingham Palace, Covent Garden, St James Park and Victoria Train station are alsolocated nearby. Some areas are Crown land especially in and around Regents Park,managed by Crown Estates who have the ability to veto the design and access to the land,although this is unlikely. Negotiation would be required.

The southern section of the area is known for a central hub of restaurants, bars, shopsand theatres around Oxford and Regent Streets and the West End. Almost any works inthis area is highly likely to have impact on local businesses, tourist and generalpedestrians. A number of A-roads are located in and around the proposed area of theworks. Any works or road closures on these roads will have a significant impact to traffic,including a considerable number of bus routes and red routes.

6.5. Lots Road Pumping Station

The new foul spine sewer for the Lots Road study area, see Figure 20, utilises the existing

interceptor sewers to form smaller dendritic systems. The design of the area splits thelarge study area into smaller systems feeding into the interceptor sewers that cross thearea. Splitting the area into smaller areas has resulted in smaller pipe sizes and lowerdepths being required for the new spine sewer system.

There are four new overflow locations from the proposed foul water spine sewer into theinterceptor sewers at Lots Road SPS Hammersmith SPS Acton SPS and Cromwell


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In addition the existing CSOs at Action SPS, Stamford Brook, Northwest Storm Relief,Hammersmith SPS and Lots Road SPS will be modified and used as the discharge devicefor the surface water into the River Thames. The existing overflow connection into theinterceptor sewer at Lots Road will be abandoned .

Two new intermediate foul water pump stations are required. The one on Kensington HighStreet will require a discharge rate of 1m/s and the other on Finborough Road/ RedcliffeSquare will require a discharge rate of 0.18m/s.

Buildability Issues & Existing Site Restrictions:

Available space to build within the road will be limited especially for the 11km ofsewers that are at a diameter of 800mm or more. Tunnelling will be required at the following locations:

Connection from Hammersmith roundabout to the continuation sewerat Park Royal Road, under train lines, and A40:

Horn Lane, under train lines The Vale, under train lines Old Oak Road, under train lines Chamberlayne Road, under train lines near Kensal Rise Station Chamberlayne Road, under train lines near Kensal Green Station

Ladbroke Grove, under train lines Hammersmith Road, under train lines Harwood Road, under train lines Munster Road, under train lines Kings Road, under train lines

London Underground has a number of tube lines within the Lots Road Catchment,namely:

Bakerloo Line Central Line Circle Line District Line Hammersmith & City Line Piccadilly Line

A number of national rail stations are located across the Lots Road catchment,with at least six rail lines running through the area. Works are proposed in thevicinity of a number of these rail stations, and under existing railway bridgesacross the area. Some tunnelling under existing railway lines is also required. Theworks will inevitably have an impact on a number of these stations and anypossible mitigation measures should be considered. Consultation with NetworkRail must be undertaken prior to works which may be close to any of the railwaytracks, bridges, stations or any other associated structures.

A number of A-roads are located in and around the proposed area of the works.Any works or road closures on these roads will have a significant impact to traffic,including a considerable number of bus routes.

There are a number of hot spots or areas where works are likely to have a highimpact and disrupt local traffic and businesses. These hot spots include:


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Hammermsith Hospital Chelsea Hospital Middlesex Hospital

There are also a number of large cemeteries across the Lots Road catchment,where construction could be limited or require negotiation or facility consent.

There are a number of large green spaces across the catchment, these include:Gunnersbury Park, South Park, Hurlingham Park, Walham Green, Parsons Green,Bishops Park, Shepherds Bush Green, Ravenscourt Park, Wormwood ScrubsPark, Kensal Green, Hyde Park to the east. Work through some of these parkscould reduce cost and disruption, but the potential for environmental effects would

have to be assessed.


The proposed new surface water spine sewer runs down Tottenham Court Road, parallelto existing system, see Figure 21. The new sewer may either run down Charing CrossRoad or along High Holborn and down Monmouth Street to Trafalgar Square. Anadditional surface water sewer is proposed down Bemers and Wardour Streets to join atTrafalgar Square, and then along Northumberland Avenue to a pumped outfall into theRiver Thames.

The route down Charing Cross Road is more suitable hydraulically, avoiding a sharp turnat the top of Monmouth Street. However, it is likely to be more disruptive to traffic andbusiness during construction and any subsequent maintenance. For the purpose of thisstudy, it has been assumed that the new sewer will follow the existing spine sewer alongHigh Holborn and down Monmouth Street.

The existing combined sewer system will be utilised as the foul water sewer system,requiring only minor modifications at the overflows and outfalls.

There is one discharge location for this area at the end of Northumberland Avenue into theRiver Thames. A new outfall will be constructed to accommodate the surface waterdischarge into the River Thames. The existing CSO will be modified and used as theoverflow device into the existing interceptor sewers.

One new pump station is required at the downstream end of the new surface waternetwork. This is a significantly congested area of the catchment, located between CharingCross Train Station and its associated rail bridge, Embankment Tube Station, VictoriaEmbankment Road and the River Thames. The Circle and District Underground lines rundirectly under Victoria Embankment, between the catchment and the River Thames, and

the Bakerloo line runs between Charing Cross Station, through Embankment Station, toWaterloo Station. In addition, Embankment Pier (a central Riverboat stop) is locateddirectly opposite Embankment Tube Station on the River Thames. Therefore there islimited space to install a new pump station and land purchase is likely to be required.

The new pump station is necessary to lift the surface water up approx 3.0m from the newsewers into the River Thames at a peak flow rate of 15m/s. There is potential to


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Cockspur Street A number of A-roads are located in and around the proposed area of works.These include the following strategic roads

Oxford Street Tottenham Court Road Charing Cross Road Shaftsbury Avenue Haymarket Long Acre

T

Documents

Needs report: appendix D