Callander River Teith Optioneering and Benefit / Cost ... · InfoWorks RS 1D model for the Teith reach through Callander including short sections of the Eas Gobhain and River Leny

Callander River Teith Optioneering and Benefit / Cost

Appraisal

June 2013 Produced for

Prepared by

Mouchel Ltd

Lanark Court, Tannochside Park

Uddingston, Glasgow

G71 5PW

T 01698 802 850

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Callander River Teith Optioneering and Benefit / Cost Appraisal

© Mouchel 2013 iii

Document Control Sheet

Project Title Callander River Teith Optioneering and Benefit / Cost Appraisal

Report Title Callander River Teith Optioneering and Benefit / Cost Appraisal

Version 3

Status Final

Record of Issue

Version Status Author Check Authorised Date

1 Draft M Biesta

S McGee O Drieu A Taylor 08/01/13

2 Draft M Biesta S McGee A Taylor 25/03/13

3 Final M Biesta S McGee A Taylor 04/04/13

Distribution

Date Organisation Contact Copies

8th

January 2013 Stirling Council Claire Elliot Electronic

25th

March 2013 Stirling Council Claire Elliot Electronic

4th

June 2013 Stirling Council Claire Elliott Electronic

The checking process includes text, calculations and figures prepared for this report.

This report is presented to Stirling Council in respect of the Callander River Teith Optioneering and Benefit / Cost Appraisal and may not be used or relied on by any other person or by the client in relation to any other matters not covered specifically by the scope of this report.

Notwithstanding anything to the contrary contained in the report, Mouchel Ltd is obliged to exercise reasonable skill, care and diligence in the performance of the services required. Stirling Council and Mouchel Ltd shall not be liable except to the extent that they have failed to exercise reasonable skill, care and diligence, and this report shall be read and construed accordingly.

This report has been prepared by Mouchel Ltd. No individual is personally liable in connection with the preparation of this report. By receiving this report and acting on it, the client or any other person accepts that no individual is personally liable whether in contract, tort, for breach of statutory duty or otherwise.


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Executive Summary

The aim of this study was to assess flood risk in Callander from the River Teith and to investigate potential flood mitigation strategies. The present study builds upon a significant amount of work already carried out in the Callander catchment.

An extensive review of existing information was carried out. Previous flood mitigation options for Callander included a flood defence wall at Meadows Car Park and upstream loch storage.

Through the review of existing information and consultations with Scottish Water it was considered that there are significant limitations on what could realistically be achieved on the Katrine / Venachar system (bearing in mind this system is already heavily modified and attenuated). Extensive and detailed water supply studies would be required before any options that involve winter draw downs (more than already done by Scottish Water) could be implemented. It was considered that exploring options for flood storage on the Lubnaig / Voil system would be more practical and potentially more fruitful, considering this system has a similar catchment size to Eas Gobhain, has significant loch / floodplain areas, and remains largely unmodified.

A fully linked 1D/2D ISIS-TUFLOW hydraulic model was developed from the original InfoWorks RS 1D model for the Teith reach through Callander including short sections of the Eas Gobhain and River Leny upstream tributaries. The purpose of this model was to assess existing flood risk and investigate possible flood mitigation options. Two routing models were also developed using HEC-RAS; one for the Leny system (including Lochs Voil and Lubnaig) and another for Eas Gobhain system (including Loch Venachar). These models were used to test upstream storage options. A robust hydrological approach was agreed with SEPA.

The key area of flood risk in Callander is Meadows Car Park, which is at least partially inundated for long periods every year. Despite the perceived level of flood risk in Callander, model results indicated that properties don‟t flood until between the 10 and 25 year return period events. For most other properties the onset of flooding is to much higher return periods. For the 200 year flood event, water is predicted to reach a depth of around 1 m on Main Street. Other areas within the 200 year flood outline include parts of Leny Road, Bridge Street and the left bank of the Teith downstream of the road bridge. For the latter, flooding is predicted to extend as far in-bank as Pearl Street, at the junction with South Church Street for the 200 year flood event. Callander Primary School is largely above 200 year levels, although the playing fields start to flood between the 10 and 25 year return periods.

A damage assessment was carried out to estimate the damages due to flooding over the next 100 years (a typical flood scheme lifespan), including an allowance of climate change. Of the total damages accrued (£2,893k), the majority occur around Meadows Car Park (£1,917k). However, more than half the damages are predicted to occur for events in excess of a 200 year return period and hence the damages which could be practically prevented are limited.

Traditional defence options comprising combinations of earthen embankments, sheet pile / concrete walls and other associated works including flood gates, pumping stations, access ramps, drainage pipes, etc. were assessed for Meadows Car Park, Bridgend and Bridge Street to Buchanan Place. At Meadows Car Park a riverside floodwall protecting the entire car park area and adjacent properties on Main Street for the 200 year flood event (plus climate change) would require a wall over 3.5 m high along much of its length. The river front area is a major amenity for the town and attracts many visitors. Protecting to 50 year levels was therefore considered a more technically feasible and practical level of protection that could maintain the river


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amenity and fit better with prevailing ground levels and the general configuration of the car park and surrounding areas. For the other areas, a typical 200 year standard of protection was explored for the options.

Numerous combinations of restriction / storage on the Leny system were also tested. The most effective options in terms of overall flow reductions in the Leny are those which involve combinations of upstream controls. A combination of a restriction at the head of Loch Voil, Strathyre and Loch Lubnaig could achieve a 50% flow reduction in the Leny, although it resulted in a 2.6 m increase in Loch Voil water level for the 200 year event (no net change further downstream in Strathyre or Loch Lubnaig).

Minor flow reductions are possible through the implementation of an electronically actuated outflow control on Loch Venachar, enhancing current manual arrangements for Loch draw-down in advance of a flood. If justified by downstream (Stirling) benefits, further investigation in the Leny system is considered more beneficial than further work on Loch Venachar due to the potential for greater benefits.

In terms of land management / „at source‟ Natural Flood Management (NFM), it was shown that such measures would be most effective if applied only to the Loch Voil catchment due to the potential desynchronisation effects. Importantly, it was found that a catchment wide peak runoff reduction (without loss of total runoff volume) is not followed by an equivalent percentage reduction further downstream at Callander due to the overall attenuating effect of the lochs, serving to negate any beneficial catchment de-synchronisation. Therefore, any NFM measures pursued for the Leny system should therefore focus on runoff delay of the Voil contributing catchment.

Since no flood schemes are economically viable for Callander, individual property flood proofing and resilience measures may be the only practical flood mitigation option available. Individual property assessments would be required to determine the appropriate measures for the particular type of property in Callander and the particular nature of the flooding. However, once the appropriate measures have been specified, there is very little design work required. Installation, carried out by a qualified contractor, can rapidly follow the specification stage.

An outline costing exercise indicated that none of the options considered are economically viable; the highest benefit-cost ratio for the options was 0.14, meaning costs out-weigh benefits 7 to 1. The upstream storage options were not costed but the significant technical challenges which would need to be overcome also mean this would not be economically justifiable. If there are additional benefits further downstream (in Stirling, for example) then such options could be further considered.

A desk-based environmental assessment was carried out in order to identify key constraints and opportunities relating to the flood mitigation measures considered. A range of environmental constraints, particularly the presence of the River Teith SAC, were identified. It is not anticipated that these would necessarily prevent any particular scheme from going ahead, rather it means that further surveys, consultation and close working with the relevant statutory bodies will be required to minimise environmental impacts. A number of opportunities were identified which could potentially bring about environmental benefits as part of any scheme, including the creation of ecological habitats, enhancement of the riparian corridor and improvements in the amenity value.

Despite the lack of a robust economic case for a community flood scheme for the study area, the Council may wish to promote a scheme/s in cognisance of the intangible and indirect benefits not quantified here. Irrespective of that, it is recommended that the council communicates the findings of this study with the local community and incorporates the findings into flood warning schemes and emergency plans for the area.


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Contents Document Control Sheet ....................................................................................... iii

Executive Summary ............................................................................................... iv

Contents ................................................................................................................. vi

Figures ..................................................................................................................... x

Tables ..................................................................................................................... xii

Appendices ........................................................................................................... xiii

1 Introduction ...................................................................................................... 1

1.1 Introduction ................................................................................................ 1 1.1.1 Background ............................................................................................ 1

1.2 Study Scope .............................................................................................. 1 1.2.1 Detailed Data Collection / Review .......................................................... 2 1.2.2 Topographical Survey ............................................................................ 2 1.2.3 Hydrological / Hydraulic Modelling ......................................................... 2 1.2.4 Flood Alleviation Optioneering ............................................................... 2 1.2.5 Benefit / Cost Appraisals ........................................................................ 3

2 Study Area ........................................................................................................ 4

2.1 Study Area Extents .................................................................................... 4 2.2 Study Area Topography ............................................................................. 5

3 Existing Information / Communications ......................................................... 6

3.1 Site Walkovers ........................................................................................... 6 3.1.1 Flooding 29th November 2011 ............................................................... 6

3.2 Existing Information ................................................................................... 9 3.3 Topographical Data .................................................................................... 9

3.3.1 DTM / LiDAR Data ................................................................................. 9 3.3.2 Survey Data ......................................................................................... 10

3.4 Existing Reporting .................................................................................... 11 3.4.1 Flood Management Of Upper Teith Basin, Above Callander (Callander Flood Study Phase 2), Mott Macdonald (1993)................................................. 11 3.4.2 Effects of Upland Afforestation On Water Resources - The Balquhidder Experiment 1981 – 1991, Institute Of Hydrology (1995a) ................................. 13 3.4.3 Stirling Council Flood Prevention Study (Stage 1), Babtie (1998) ......... 14 3.4.4 Stirling Council Flood Prevention Study (Stage 2), Bullen Consultants (2000) 14 3.4.5 Callander Meadows Car Park – Flood Risk Assessment, Atkins (2005) 15 3.4.6 Callander Meadows Car Park – Flood Mitigation Options Assessment, Atkins (2005) .................................................................................................... 16 3.4.7 Callander Floor Level Survey Assessment, Atkins (2007) .................... 17 3.4.8 Stirling Council River Teith Hydraulic Model Update, Atkins (2010). ..... 17 3.4.9 Callander Strategic Flood Risk Assessment, MNV Consulting Ltd (2010). 18

3.5 Scottish Water ......................................................................................... 19 3.5.1 System Overview / Description ............................................................ 19 3.5.2 Operational Information ........................................................................ 22 3.5.3 Abstractions / Transfers / Compensation Flows ................................... 22 3.5.4 Hydrometric Data ................................................................................. 23 3.5.5 Miscellaneous ...................................................................................... 23 3.5.6 Implications for Flood Mitigation Investigations .................................... 24

3.6 SEPA ....................................................................................................... 25 3.6.1 Flood Mapping ..................................................................................... 25 3.6.2 Climate Change ................................................................................... 25


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3.6.3 Flow Records ....................................................................................... 27 3.6.4 Rainfall Records ................................................................................... 27 3.6.5 Time Series Flood Event Data ............................................................. 27

3.7 Loch Bathymetry ...................................................................................... 28 3.8 Scottish Government ............................................................................... 30

3.8.1 Scottish Planning Policy (SPP) ............................................................ 30 3.8.2 Standards of Protection / Grant Assistance .......................................... 30

3.9 Future Planning and Development ........................................................... 30 3.10 Utilities Information .................................................................................. 31

4 Hydrological Review (Callander) ................................................................... 32

4.1 Catchment Analysis ................................................................................. 32 4.2 Eas Gobhain / River Leny – Relative Flow Contributions ......................... 32 4.3 Callander (Teith) Hydrological Update ..................................................... 35

4.3.1 Proposed Approach ............................................................................. 35 4.3.2 QMED Estimation ................................................................................ 35 4.3.3 Growth Curve Derivation ...................................................................... 35 4.3.4 Peak Design Flows .............................................................................. 36

5 Hydrological Assessment (Leny Routing) .................................................... 38

5.1 Hydrological Schematisation .................................................................... 38 5.2 Hydrological Calibration ........................................................................... 38

6 Hydraulic Modelling - Teith (Callander) ........................................................ 40

6.1 Teith (Callander) Model Build (Conversion and Extension) ...................... 40 6.1.1 Model Extents ...................................................................................... 40 6.1.2 Supplementary Model Build Data ......................................................... 41 6.1.3 Model Build .......................................................................................... 41

6.2 Model Calibration / Verification ................................................................ 41 6.3 Model Sensitivity Analysis ........................................................................ 44 6.4 Model Output and Results ........................................................................ 44

6.4.1 Flood Levels and Outlines .................................................................... 44 6.4.2 Flow Velocities and Depths .................................................................. 46 6.4.3 Key Flood Risk Issues .......................................................................... 46

7 Hydraulic Modelling - Loch System Routing ................................................ 47

7.1 Modelling Rationale ................................................................................. 47 7.2 Model Extents .......................................................................................... 47 7.3 Model Build (HEC-RAS) ........................................................................... 48

7.3.1 Model Build Scoping ............................................................................ 48 7.3.2 Topographical Survey .......................................................................... 48 7.3.3 Key Hydraulic Controls ......................................................................... 49 7.3.4 Supplementary Model Build Data ......................................................... 51 7.3.5 Model Boundary Conditions ................................................................. 51 7.3.6 Upstream / Downstream Boundary Conditions ..................................... 51

7.4 Model Calibration / Verification ................................................................ 51 7.4.1 Leny Calibration ................................................................................... 52 7.4.2 Loch Venachar Flow Gauge Calibration ............................................... 54

7.5 Model Sensitivity Analyses ....................................................................... 55 7.5.1 Model Inflow Sensitivity Analysis .......................................................... 55 7.5.2 Model Roughness Sensitivity Analysis ................................................. 55 7.5.3 Initial Loch Levels Sensitivity Analysis ................................................. 55

8 Option Appraisal – Teith (Callander)............................................................. 56

8.1 Options Overview..................................................................................... 56 8.2 Meadows Car Park FAS........................................................................... 56

8.2.1 Technical Feasibility ............................................................................. 56


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8.2.2 Impacts ................................................................................................ 59 8.3 Bridgend West FAS ................................................................................. 59

8.3.1 Technical Feasibility ............................................................................. 59 8.3.2 Impacts ................................................................................................ 59

8.4 Bridgend East FAS .................................................................................. 60 8.4.1 Technical Feasibility ............................................................................. 60 8.4.2 Impacts ................................................................................................ 61

8.5 Bridge Street to Buchanan Place FAS ..................................................... 61 8.5.1 Technical Feasibility ............................................................................. 61 8.5.2 Impacts ................................................................................................ 62

9 Option Appraisal - Upstream Storage ........................................................... 64

9.1 Leny / Lubnaig / Balvag / Voil Optioneering Modelling ............................. 64 9.1.1 Option Selection ................................................................................... 64 9.1.2 Model Output and Findings .................................................................. 66 9.1.3 Impacts ................................................................................................ 72

9.2 Eas Gobhain / Loch Venachar Optioneering Modelling ............................ 74 9.2.1 Model Description / Option Selection ................................................... 74 9.2.2 Option Selection ................................................................................... 76 9.2.3 Model Output and Findings .................................................................. 76 9.2.4 Impacts ................................................................................................ 78

10 Other Optioneering Considerations ......................................................... 80

10.1 Demountable Defences ........................................................................... 80 10.2 Natural Flood Management (NFM) ........................................................... 81

10.2.1 NFM Techniques .................................................................................. 81 10.2.2 Existing NFM Studies and Key Findings ............................................... 82 10.2.3 Modelling of NFM Measures ................................................................ 83 10.2.4 Conclusions and Applicability of NFM to Callander .............................. 84

10.3 Individual Property Flood Proofing ........................................................... 85

11 Environmental and Social Constraints and Opportunities Appraisal .... 88

11.1 Overview .................................................................................................. 88 11.2 Methodology ............................................................................................ 88 11.3 Baseline Information ................................................................................ 88

11.3.1 Ecology and Nature Conservation ........................................................ 88 11.3.2 Local Wildlife Sites ............................................................................... 89 11.3.3 Landscape Assessment ....................................................................... 89 11.3.4 Cultural Heritage .................................................................................. 89 11.3.5 Public Amenity ..................................................................................... 90 11.3.6 Geology ............................................................................................... 90 11.3.7 Soils ..................................................................................................... 90 11.3.8 Land Capability for Agriculture ............................................................. 91 11.3.9 Coal and Mineral Mining ...................................................................... 91 11.3.10 Potential Contaminated Land ............................................................... 92

11.4 Other Social Impacts ................................................................................ 92 11.5 Opportunities for Enhancement................................................................ 92 11.6 Recommendations ................................................................................... 93

11.6.1 General Consultation ........................................................................... 93 11.6.2 Consultation Regarding Contaminated Land ........................................ 93 11.6.3 Consultation Regarding Ecology .......................................................... 93 11.6.4 Consultation Regarding Cultural Heritage ............................................ 93 11.6.5 Consultation Regarding Public Amenity ............................................... 93

11.7 Summary of Key Constraints and Potential Opportunities ........................ 94

12 Health and Safety Appraisal...................................................................... 96

13 Benefit / Cost Appraisal ............................................................................ 97


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13.1 Economic Appraisal Policy and Guidance ................................................ 97 13.2 Present Values (PV) ................................................................................ 98 13.3 Optimism Bias .......................................................................................... 98 13.4 Benefits Methodology .............................................................................. 98

13.4.1 Residential Property ............................................................................. 98 13.4.2 Non-residential property ....................................................................... 99 13.4.3 Emergency and Clean-up Costs .......................................................... 99 13.4.4 Damage Capping ................................................................................. 99 13.4.5 Residual Damage .............................................................................. 100 13.4.6 Depth / Damage Curve ...................................................................... 100 13.4.7 Climate Change ................................................................................. 101

13.5 Summary of Benefit / Cost Methodology ................................................ 101 13.6 Callander Flood Damages (Benefits) ..................................................... 102

13.6.1 Onset of flooding ................................................................................ 102 13.6.2 Flood Damages .................................................................................. 105 13.6.3 Damage Capping ............................................................................... 109 13.6.4 Sensitivity Checks .............................................................................. 109

13.7 Option Costing ....................................................................................... 109 13.7.1 Meadows Car Park FAS ..................................................................... 110 13.7.2 Bridgend West FAS............................................................................ 111 13.7.3 Bridgend East FAS ............................................................................ 111 13.7.4 Bridge Street to Buchanan Place FAS ............................................... 111 13.7.5 Lubnaig / Balvag / Voil Flow Control FAS ........................................... 112 13.7.6 Other Miscellaneous Scheme Costs .................................................. 112 13.7.7 Present Value (PV) Scheme Costs .................................................... 113

13.8 Cost / Benefit Appraisal ......................................................................... 114 13.9 Further Benefit / Cost Discussion ........................................................... 115

14 Summary and Conclusions ..................................................................... 116

14.1 Study Aims and Initial Data Review ....................................................... 116 14.2 Stakeholder Consultations and Additional Data Collection ..................... 116 14.3 Hydrology .............................................................................................. 116 14.4 Hydraulic Modelling ................................................................................ 117 14.5 Flood Alleviation Options Considered .................................................... 117 14.6 Option Feasibility - Traditional Defences and Demountables ................. 118 14.7 Option Feasibility – Upstream Storage ................................................... 119 14.8 Option Feasibility – Natural Flood Management ..................................... 119 14.9 Environmental Constraints and Opportunities ........................................ 120 14.10 Benefit / Cost Appraisals ........................................................................ 121 14.11 Individual Property Flood Proofing ......................................................... 121 14.12 Recommendations ................................................................................. 121

References ........................................................................................................... 122


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Figures Figure 1 - Study Reach (at Callander) ....................................................................... 4 Figure 2 - Study Area Topography ............................................................................ 5 Figure 3 - Loch Voil (29/11/2011) .............................................................................. 7 Figure 4 - Loch Voil at Balquhidder Bridge (29/11/2011) ........................................... 7 Figure 5 - Inundated Floodplains Downstream of Balquhidder (29/11/2011) ............. 8 Figure 6 - High Water at Meadows Car Park, Callander (29/11/2011) ....................... 8 Figure 7 – 1 m LiDAR Coverage (Yellow and Red Shading Only) ........................... 10 Figure 8 - Loch Katrine System ............................................................................... 20 Figure 9 - Loch Katrine Spillway / Fish Pass............................................................ 21 Figure 10 - Subsidiary Dam at Loch Katrine ............................................................ 21 Figure 11 - Loch Venachar Spillway ........................................................................ 21 Figure 12 - 2005 Event Rainfall and Loch Levels .................................................... 23 Figure 13 - Climate Change Uplifts .......................................................................... 26 Figure 14 - Geo-Referenced Bathymetric Survey Maps .......................................... 28 Figure 15 - Loch Bathymetry ................................................................................... 29 Figure 16 – Leny / Eas Gobhain Hydrological Catchments ...................................... 32 Figure 17 - Growth Curves ...................................................................................... 36 Figure 18 - FEH Defined Tributaries on Leny .......................................................... 38 Figure 19 - New Teith (Callander) Hydraulic Model Extents .................................... 40 Figure 20 - Callander Observed Flooding 14th Dec 2006 – Photo 1 (13:00 hrs) ....... 42 Figure 21 - Callander Observed Flooding 14th Dec 2006 – Photo 2 (13:00 hrs) ....... 42 Figure 22 - Callander Observed Flooding 14th Dec 2006 – Photo 3 (13:00 hrs) ...... 43 Figure 23 - Callander Predicted Flooding 14th Dec 2006 (13:00 hrs) ....................... 43 Figure 24 - Callander Predicted Flooding 14th Dec 2006 – Leny Road (13:00 hrs) .. 43 Figure 25 - Callander 200 Year Flood Outline ......................................................... 45 Figure 26 – Routing Model Extents ......................................................................... 48 Figure 27 – Stronvar Bridge .................................................................................... 49 Figure 28 – Lubnaig Outlet ...................................................................................... 49 Figure 29 – Bridge at Creag Dhubh ......................................................................... 50 Figure 30 – Anie Gauge Location ............................................................................ 50 Figure 31 – Venachar Weir and Spillway ................................................................. 50 Figure 32 – Predicted / observed stage at Anie gauge station for 1994 flood event . 52 Figure 33 - Predicted / Observed Stage at Anie Gauge Station for 1997 Flood Event ................................................................................................................................ 52 Figure 34 - Predicted / observed stage at Anie gauge station for 2005 flood event . 53 Figure 35 - Predicted / observed stage at Anie gauge station for 2006 flood event . 53 Figure 36 - Predicted / observed stage at Anie gauge station for 2008 flood event . 53 Figure 37 - Predicted / observed stage at Anie gauge station for 2009 flood event . 54 Figure 38 – Predicted / observed ratings curve (Anie gauge station) ....................... 54 Figure 39 – Predicted / Observed Ratings Curve (Loch Venachar Gauge Station) .. 55 Figure 40 - Meadows Car Park FAS - 50 year protection ........................................ 57 Figure 41 - Flood Gate Illustration ........................................................................... 58 Figure 42 - Bridgend West FAS - 200 Year + CC Protection ................................... 60 Figure 43 - Bridgend East FAS - 200 year + CC Protection ..................................... 61 Figure 44 - Bridge Street to Buchanan Place FAS - 200 year + CC protection (upstream) ............................................................................................................... 63 Figure 45 - Bridge Street to Buchanan Place FAS - 200 year + CC protection (downstream) .......................................................................................................... 63 Figure 46 – Flow Control Structure Locations .......................................................... 65 Figure 47 – Leny HEC RAS Model – Longitudinal Profile (200 year event) ............. 65 Figure 48 - 200 Year Leny Hydrograph - with 5m Lubnaig Flow Control .................. 72 Figure 49 – Flood Outline Sample - Increased Levels on Voil (+2.6m) .................... 73 Figure 50 – Flood Outline Sample - Increased Levels on Lubnaig (+3.3m) .............. 74


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Figure 51 – Location of Loch Venachar Outlet ......................................................... 75 Figure 52 – Schematic of Loch Venachar Outlet (Mott MacDonald, 1993) ............... 76 Figure 53 - NFM Approaches (SNIFFER, 2011) ...................................................... 81 Figure 54 - Property Floodwater Ingress Points (CIRIA, 2007) ................................ 86 Figure 55 - Change in River Flow Due to Climate Change .................................... 101 Figure 56 - Probability of Flooding Onset - Leny Road .......................................... 102 Figure 57 - Probability of Flooding Onset - Meadows Car Park / Main Street ........ 103 Figure 58 - Probability of Flooding Onset - Bridgend (South) ................................. 103 Figure 59 - Probability of Flooding Onset – Bridgend (North) Including School Area .............................................................................................................................. 104 Figure 60 - Probability of Flooding Onset - Bridge Street to Footbridge ................. 104 Figure 61 - Probability of Flooding Onset - Footbridge to Buchanan Place ............ 105 Figure 62 - Damage Assessment Areas ................................................................ 106 Figure 63 - Property Damage Contributions (Pie Chart) ........................................ 108


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Tables Table 1 - Loch Spillway Levels / Operational Zone Depths ...................................... 22 Table 2 - SEPA Flow Gauging Stations ................................................................... 27 Table 3 - Rainfall Gauge Locations ......................................................................... 27 Table 4 - Key Flood Events at Callander ................................................................. 28 Table 5 - Loch Bathymetry Details ........................................................................... 29 Table 6 – Leny / Eas Gobhain Tributary Flow Comparison ...................................... 34 Table 7- Callander Design Flows ............................................................................. 37 Table 8 - Additional Surveyed Data for Updated Callander Model Build .................. 41 Table 9 - Callander Model Calibration Results ......................................................... 42 Table 10 – Manning‟s „n‟ (Leny)............................................................................... 51 Table 11 - Manning‟s „n‟ (Eas Gobhain) ................................................................... 51 Table 12 – Flood Storage Optioneering – Resulting % Flow Change in Leny .......... 67 Table 13 – Flood Storage Optioneering – Water Level Change (+/- m) ................... 68 Table 14 – Storage Option Lag / Flow Reduction (200 Year) ................................... 71 Table 15 – Eas Gobhain Peak Flows (m3/s) – Venachar Weir Optioneering ............ 79 Table 16 –Loch Venachar Water levels (m AOD) – Venachar Weir Optioneering .... 79 Table 17 – Downstream Impact of Theoretical Runoff Reductions .......................... 83 Table 18 - Ecological Designations – Callander Optioneering Study Area ............... 89 Table 19 - Historic Landscape Designations – Callander River Teith ...................... 89 Table 20 - Intangible and Indirect Impacts on Households (Penning-Rowsell et al., 2010) ....................................................................................................................... 92 Table 21 - Key Constraints ...................................................................................... 94 Table 22 - Potential Opportunities ........................................................................... 95 Table 23 - Social Class Categories for Callander .................................................... 99 Table 24 - PV Damages ........................................................................................ 107 Table 25 – PV Damages (Including Climate Change Uplift) ................................... 107 Table 26 - Property Damage Contributions (Percentages) .................................... 109 Table 27 - Meadows Car Park FAS Outline Costing .............................................. 110 Table 28 – Bridgend West FAS Outline Costing .................................................... 111 Table 29 - Bridgend East FAS Outline Costing ...................................................... 111 Table 30 - Bridge Street to Buchanan Place FAS .................................................. 112 Table 31 – Annual Maintenance Cost Breakdown ................................................. 113 Table 32 - Annual Maintenance Costs per Scheme Option ................................... 113 Table 33 - Present Value (PV) Scheme Costs ....................................................... 114 Table 34 - Benefit Cost Ratios ............................................................................... 114


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Appendices Appendix A - Existing Information / Communications

Appendix B - AMAX Flow Data

Appendix C - FEH Catchment Descriptors

Appendix D - Design Flow Derivation

Appendix E - Teith Model Peak Water Levels

Appendix F - Callander (Teith) Flood Outlines

Appendix G - Routing Models Sections

Appendix H - Routing Models Sensitivity

Appendix I - Natural Flood Management Review

Appendix J - Environmental Constraints

Appendix K - Health and Safety Appraisal

Appendix L - Benefit-Cost Assessment


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1 Introduction

1.1 Introduction

1.1.1 Background

In September 2011 Stirling Council appointed Mouchel, through their framework with Scotland TranServ, to undertake an optioneering study on the River Teith for the town of Callander and to carry out a cost / benefit appraisal of potential flood mitigation options, building upon work undertaken in previous studies.

The Stirling Council Local Authority Area contains three key river catchments, namely the River Teith („the Teith‟), the River Forth („the Forth‟) and the Allan Water („the Allan‟), all of which converge on the westerly outskirts of Stirling where they combine into the River Forth. Of these three rivers, the Teith contributes the largest flow to the Forth. There is a long history of flooding in the region; the Teith regularly causes flooding at Callander, and is a key contributor to flooding in Stirling.

Under the Flood Risk Management (Scotland) 2009 Act, Stirling Council has a duty to manage flooding and, as a designated responsible authority, has powers to enable flood mitigation work to be carried out.

The Flood Risk Management (Scotland) 2009 Act also encourages the use of Natural Flood Management (NFM) measures which includes various forms of catchment storage, to manage and contain flood risk. The Council is keen to explore such storage options as part of this study to determine the potential effectiveness of such approaches for Callander. The Council recognises that in addressing flooding to settlements such as Callander using upstream storage methods, there is also potential for these measures to offer some benefit in alleviating on-going flood issues further downstream in Stirling.

This study overlaps with similar concurrent flood optioneering studies being undertaken by Mouchel for Aberfoyle (River Forth), Dunblane (Allan Water) and Bridge of Allan (Allan Water). The Council wants to be in a position to promote viable flood alleviation schemes across the Council region at the earliest opportunity.

The Teith catchment has been subject to a number of flood studies over the years. The most recent study carried out by Atkins in 2005 (updated in 2010) included the construction of a hydraulic model of the Teith at Callander which allowed estimations of flood risk for a range of return periods.

This study uses and builds upon this existing information (including hydrological / hydraulic models) to develop potential options for flood alleviation to Callander, potentially including benefits to Stirling.

1.2 Study Scope

This study consists of the following key components:

Detailed data collection / review

Topographical survey

Hydrological / hydraulic modelling

Flood alleviation optioneering

Benefit / cost appraisals A brief summary of the key tasks / objectives for this study are detailed below.


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1.2.1 Detailed Data Collection / Review

The data collection and review was undertaken to assess all available existing information, various reports and models, including the River Teith hydraulic model (InfoWorks RS). The latest hydrometric information was also collated from SEPA, Stirling Council and Scottish Water.

Although traditional direct defence options are assessed, a key aspect of this study is the exploration of the potential for utilising the various lochs / floodplains upstream of Callander for flood management purposes (in addition to their current water resources role). The review included stakeholder engagement with Scottish Water and SEPA to discuss / agree data availability, collect and collate relevant loch operational information and to identify key constraints.

1.2.2 Topographical Survey

Topographical surveys were undertaken to facilitate extension and refinement of the existing Teith hydraulic model for Callander. Topographical surveys were undertaken to facilitate construction of hydrological / hydraulic routing models for the River Leny and Eas Gobhain upstream of Callander. The surveys included key loch outlet controls, channel cross sections and some bathymetric levels. LiDAR data was generally used to supplement manual surveys wherever feasible. Property threshold surveys were also undertaken to facilitate the benefit / cost analyses.

1.2.3 Hydrological / Hydraulic Modelling

Existing hydrological and hydraulic modelling was reviewed, updated and extended. Two key models needed to be developed:

A hydraulic model of the Teith through Callander suitable for capturing 2D floodplain mechanisms and for assessing flood risk and flood relief options for Callander.

A hydrological / hydraulic routing model of parts of the upper reaches of the Teith catchment (including lochs) to facilitate assessment of catchment based storage options to attenuate flows through Callander.

1.2.4 Flood Alleviation Optioneering

Upon finalisation of the baseline, this study explored both traditional direct protection and upstream storage options (NFM) for flood alleviation. Consideration was given to the use of both permanent and demountable options, particularly at the rear of Meadows Car Park to protect Main Street together with an assessment of the potential impact that these defences may have on the opposite bank or further downstream.

Although this study is focussed primarily at reducing / mitigating flood risk in Callander, the Council wishes to also reduce flood risk at Stirling by potentially utilising storage in the upper reaches of the Teith if possible. There is little flood storage capacity (few floodplains) on the Teith between Callander and Stirling due to the river being contained in a relatively deep incised channel.

Although options for upstream storage were explored using the calibrated routing model, an assessment was also undertaken of the potential consequences of altering flood-wave times to peak. It is important to demonstrate that if any upstream storage options are developed on the upper reaches of the Teith, this doesn‟t result in a problematic altering of the time-to-peak which could then cause a clash of peaks with the other catchments of the Forth and Allan, if these occur later (and vice versa). Some key observations are presented in this report regarding times to peak at Callander however, further detailed analyses are reported separately within a


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concurrent flood study looking at options for the city of Stirling which is situated downstream of the Allan, Teith and Forth rivers.

The option feasibility appraisals include stakeholder liaison and consideration of environmental and health and safety factors.

1.2.5 Benefit / Cost Appraisals

Flood damages (benefits) were evaluated using the water surface / depth grids generated from the extended and refined 1D / 2D Teith hydraulic model for a full range of return periods. The latest FCERM-AG spreadsheets tailored and adopted for this study, and flood damages were based on the most current „Multi Coloured Manual‟ (Penning-Rowsell et al., 2010) data.

Final estimates of the costs associated with each feasible option including optimism bias were made as appropriate.


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2 Study Area

2.1 Study Area Extents

This study focuses mainly on the Teith through Callander but includes an assessment of the upstream tributaries of the Eas Gobhain and the River Leny as far north as the Braes of Balquhidder, west to Loch Lomond and south to near Achray Forest. The Teith through Callander and the main reaches of interest (as modelled) are shown below in Figure 1 below.

The Eas Gobhain emanates from a heavily modified and managed upstream loch system which includes Loch Katrine, Loch Arklet, Loch Drunkie, Glen Finglas Reservoir and Loch Venachar before joining with the River Leny just upstream of Callander. Loch Katrine, Loch Drunkie, Loch Venachar and Glen Finglas Reservoir are all controlled by hydraulic structures and form part of the Loch Katrine Water Supply Scheme, which provides Glasgow with its main source of potable water.

The River Leny emanates from a more natural upstream loch system. Loch Doine / Loch Voil discharges into the River Balvag which then flows through large floodplain areas between Balquhidder and Strathyre. South of Strathyre, the River Balvag flows into Loch Lubnaig. Loch Lubnaig then discharges into the River Leny before it combines with the Eas Gobhain and becomes the River Teith through Callander.

© Crown Copyright. All rights reserved. Ordnance Survey Licence number 100020780 © Getmapping Partnership 2013

Figure 1 - Study Reach (at Callander)

Eas Gobhain

River Leny

River Teith

CALLANDER


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2.2 Study Area Topography

The Teith drains part of the Loch Lomond and the Trossachs National Park. The Teith catchment encompasses Ben Venue, Ben Ledi and the slopes of Stob Binnein. Stob Binnein lies north of Monachyle and the Braes of Balquhidder and is the highest point in the catchment.

Generally, the various reservoirs and lochs are fed by steep mountainous streams coming off the hillsides. The initial catchment response in these areas is therefore relatively rapid. Although steep-sided, the main valley floor of the River Balvag slopes at a relatively shallow gradient. However, downstream of Loch Lubnaig, the River Leny is relatively steep before flattening out again just upstream of the Callander. The Eas Gobhain catchment has been significantly modified due to the various hydraulic control structures present. Loch Katrine is the dominant water body with a surface area of around 12 k m2.

Downstream of Callander the Teith enters a steeper, deep incised channel.

Figure 2 illustrates the general topography (from contour shaded DTM data) of the wider study area. Also shown are the Leny / Eas Gobhain catchment outlines.

Figure 2 - Study Area Topography

River Leny Catchment

Eas Gobhain Catchment


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3 Existing Information / Communications

Supplementary information regarding utilities, planning and development is contained in Appendix A.

3.1 Site Walkovers

Mouchel undertook walkover inspections of the Teith at Callander and other key locations upstream, including the various lochs and reservoirs.

The purpose of the walkover surveys are summarised as follows:

Modeller familiarisation

Identification of potential areas of flooding and likely flooding mechanisms

Evidence of flooding (wrack lines, etc.)

Identification of key hydraulic controls such as bridges, culvert, weirs and narrow reaches

Identification of key changes in channel bed gradient

Development of detailed modelling approach and schematisation

Channel / floodplain roughness appraisal

Development of detailed specification for topographical survey (cross sections, structures, etc.)

Anecdotal information on flooding from locals, particularly fisherman and walkers who are familiar with the local area and have good knowledge of the river.

3.1.1 Flooding 29th November 2011

The study area was visited during the high water levels which occurred on the 29th November 2011. This site visit provided an improved understanding of flood dynamics in the area, and which flood mitigation options may be feasible. Some findings from the site walkover are summarised below.

The heavy rainfall affected much of the Stirling Council area, with the River Devon and the Bannock Burn recording some of their highest levels since records began.

No properties in Callander appeared to be flooded, although some were clearly very close to being affected. Based on observed flood extents, this event is roughly estimated to have return period of around 10 years.

The floodplains between Balquhidder and Strathyre were inundated. The main river channel was difficult to distinguish in places.

A number of low spots were noted on the road along the edge of Loch Lubnaig. In some locations, the loch water level was effectively at road level. Any further rise would have resulted in flooding of the road. Any options exploring raising the level of the lochs would need to take this issue into account.

Selected pictures taken during the visit are shown in Figure 3 to Figure 6 below.


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Figure 3 - Loch Voil (29/11/2011)

Figure 4 - Loch Voil at Balquhidder Bridge (29/11/2011)


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Figure 5 - Inundated Floodplains Downstream of Balquhidder (29/11/2011)

Figure 6 - High Water at Meadows Car Park, Callander (29/11/2011)


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3.2 Existing Information

The following Information was obtained by Mouchel for the purposes of this commission:

Ordnance Survey 1: 2500 MasterMap series mapping (FVGIS)

1:10,000, 1:25,000 and 1:50,000 OS raster mapping (FVGIS)

Aerial photography (FVGIS)

Digital Terrain Model (DTM) data (5m grid) (FVGIS)

Digital Surface Model (DSM) data (5m grid) (FVGIS)

Pre and post war historic mapping (FVGIS)

Loch Bathymetric information (National Library of Scotland)

Previous flood studies (reports and models) (Stirling Council)

Biennial flood reports (Stirling Council)

Topographical survey / threshold data (Stirling Council)

Scottish Water sewerage network GIS data (Scottish Water)

Scottish Water flooding register data (Scottish Water)

Loch control metrics (Scottish Water)

Loch management information (Scottish Water)

Hydrometric data (SEPA)

SEPA flood mapping (Stirling Council / SEPA)

Local info & photographs (Stirling Council / locals)

Geology maps (BGS)

Hydrological catchment data (FEH CD-ROM)

All the “electronic” data collated from the various sources outlined above will be available and provided on CD upon request.

3.3 Topographical Data

3.3.1 DTM / LiDAR Data

DTM and LiDAR data were acquired for the study area. The data provides an image of the first reflective surface, which is subsequently processed to remove vegetation, buildings and other cultural features. LiDAR data was available at a grid resolution of 1m for the Teith corridor (Figure 7), and DTM data at a 5m grid resolution for the remaining Teith catchment areas. The accuracy of the data will vary depending on the ground cover at the time of survey.

The stated accuracy of the LiDAR data is 1m in the horizontal and 0.25m in the vertical direction. There was good correlation between the surveyed data and the LiDAR data in open areas not subject to tree cover or heavy vegetation. The correlation was more variable in urbanised or heavily vegetated areas as would be expected. The data was considered suitable for use in defining floodplain areas for modelling purposes. Key levels associated with structures and embankments would be captured manually using traditional survey methods.

The 5m DTM has been produced using airborne radar technology that provides a one metre vertical resolution for the first reflective surface; this is subsequently interpolated using a bespoke algorithm to derive the underlying „bald earth‟ or terrain model. The bald earth DTM is thus inherently less accurate than the DSM (Data Surface Model) with a vertical accuracy from +/- 60cm. This level of accuracy is often unsuitable for detailed hydraulic modelling, but provides a good overview of the local topography, facilitating a broad assessment of drainage paths and overall overland flow routes.


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The 5m DTM data accuracy was found to be poor in treed areas when compared with some available survey points. In some cases the DTM data is noted to be over 10m higher than the surveyed data.

© Crown Copyright. All rights reserved. Ordnance Survey Licence number 100020780

Figure 7 – 1 m LiDAR Coverage (Yellow and Red Shading Only)

3.3.2 Survey Data

To facilitate extension and refinement of the existing Teith hydraulic model and to develop upstream routing models, topographical surveys were undertaken by Mouchel‟s survey team. The survey specification was developed following review of existing topographical data, examination of LiDAR data and after completion of detailed site walkovers. The specified surveys included for the following:

Channel cross sections (including some floodplain areas)

Railway embankment levels (with attention paid to any low spots)

Hydraulic structures and any other relevant hydraulic controls

Bathymetric loch surveys (supplementing existing historic bathymetry information)

Property threshold levels for Callander

Numerous spot levels

The survey used a combination of Leica System 1200 Real Time Kinematic (RTK) GPS and Leica 1205 Total Stations. The Total Stations were used in areas where the GPS was unable to function (i.e. under trees or near high buildings), but for the bulk of the survey, 2 surveyors, each equipped with a GPS (rover) receiver radio linked to a "master" base station, were used.

The base station is set over a permanent ground marker (PGM) fixed on site, in a suitably safe area. GPS data is logged at 5 second intervals. The master GPS receiver transmits its position and correction parameters to the rover receivers, the positions of which are updated in real-time.

The base receiver GPS data is processed in Leica Geo-office, together with the simultaneous GPS data for the Ordnance Survey National Active GPS Network. The accuracy of the base receiver co-ordinates is affected by the length of occupation


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and the amount of detail recorded. The GPS Rovers are linked in real-time to the Master (i.e. a 3D vector is calculated from the Master for each point). The accuracy is largely controlled by the Master, but is also affected by various factors, including the proximity to high objects (buildings, trees), and the surface type. With hard surfaces, an accuracy of better than +/- 20-25mm is expected.

For the majority of sections, the channel bed was surveyed by wading from the bank. However, where this was not possible a boat was used, particularly for the loch bathymetric surveys. This survey information was required to undertake the various tasks (hydraulic modelling, B/C appraisals and optioneering) as detailed later in this report.

All topographical survey data gathered and used for the study will be provided in digital format.

3.4 Existing Reporting

A number of previous reports were reviewed in order to gain some valuable background information regarding flooding in the upper Teith catchment and also help inform the detail and scope of this current optioneering study. A brief review of key reports together with a summary of the main points and conclusions are outlined as follows:

3.4.1 Flood Management Of Upper Teith Basin, Above Callander (Callander Flood Study Phase 2), Mott Macdonald (1993)

The Mott MacDonald study investigated flooding in Callander and assessed if management of the upper Teith basin could be improved to reduce downstream flooding. It was thought that the method of operation of the various sluices within the system, particularly Loch Katrine and Loch Venachar, may be a contributing to the increase in flooding experienced in Callander.

The study concentrated on the Eas Gobhain catchment as it was considered that any flood control measures on the River Leny catchment would be exorbitantly expensive and have potential adverse environmental impact.

The Mott MacDonald study included the following investigations:

Hydrological analysis of Upper Teith basin.

Review and re-calibration of rating curve at Venachar compensation weir.

Construction of operational models for lochs in Upper Teith basin and associated flood routing.

Assessment of impacts of any proposed operational modifications that could better control downstream flood risk.

Construction and calibration of hydraulic model of River Teith through Callander.

Development of catchment model of Upper Teith Basin to provide hydrological routing component for flood forecasting model.

A key component of the Mott MacDonald study was the operation of Loch Venachar. The aim was to determine whether or not a change in operation at Loch Venachar could be used to alleviate flooding at Callander. The key scenarios tested were:

Draw down Loch Venachar prior to the arrival of a flood to create additional storage.

Maintaining Loch Venachar and / or Loch Katrine and Glen Finglas at lower winter levels.

Closing the gates of Loch Venachar dam during the passage of the River Leny Flood.


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To explore this, observed rainfall corresponding to five floods which occurred between 1989 and 1993 was routed through a basin model. The lochs operational model simulated the operation of the gates and sluices at Loch Venachar incorporating an hourly water balance involving loch inflow, change in loch level and outflow. The following conclusions were presented:

The Venachar gauge was originally calibrated for low flows (for compensation releases). The Venachar ratings curve was re-calibrated (by Mott MacDonald) for higher flows resulting in a 75% increase in the estimated peak flows out of Loch Venachar. Consequently, there was considered more scope for improvements to flood management from the Eas Gobhain system than previously thought.

Drawing down Loch Venachar levels below weir crest prior to the arrival of a flood was beneficial to some degree but was reported to offer limited flood alleviation to Callander. For less extreme floods, the benefits would be greater.

It was considered that there was little danger that the loss of storage from the drawing down of the loch level before the arrival of a flood could not be recovered on the recession of the flood. Thus, such an operation would not likely reduce the reliability of compensation releases from Loch Venachar.

It was predicted that lowering Loch Venachar levels by 1 m during the winter months could have a more significant influence upon flood levels in Callander. In conjunction, a sluice operation regime for Loch Venachar restricting flow to a „target‟ through Callander of 191 m3/s, just below the onset of flooding in Callander, was tested. It was concluded that this would be difficult to achieve from a practical operational point of view and that the lowering of the loch was not a complete solution. A combination of flood defences and the lowering of Loch Venachar levels during the winter were proposed as a more practical potential option.

There were a number of problems reported associated with the lowering of Loch Venachar during the winter months:

Ecological impact on the Black Water SSSI

Maintenance of compensation flows if Loch Venachar was drawn down by 1 m during the winter period (December to February)

Increased responsibilities for Strathclyde Water (now Scottish Water)

Drawing down 1m during the winter period would have caused serious problems during 1969 and 1975 where Loch Venachar would have effectively dried up. A proposal of reducing the compensation flow from 2.6 m3/s to 1 m3/s was put forward as a potential solution.

The possibility of drawing down Loch Katrine was also explored. A drawdown of 0.5 m in Loch Katrine provides approximately 7 million m3 of available storage. A drawdown of 0.5 m was considered to provide adequate storage for floods of the order of the 1993 event. Although operating Loch Katrine for flood regulation was reported to offer a relatively simple means of controlling flooding in the Teith, any such proposals would require an extensive in-depth water supply study of the effect of reduced winter levels on historic and future yields. Recommendations for immediate improvements to downstream flooding were made which concentrate on the setting up of a flood forecasting system and improving the method of flood operation at Loch Venachar by drawing down loch levels prior to the arrival of the floods.

A hydraulic river model was also constructed which extended from the Keltie Water confluence (4km downstream of Callander) to as far upstream as the Anie (River Leny) and Venachar (Eas Gobhain) gauges. This allowed assessment of flood levels


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through Callander and the assessment of potential flood alleviation options in the town.

Dredging of the river by 0.5m over 400m was modelled which reduced peak levels by 160mm. More extensive dredging and future on-going maintenance was considered to be costly and would damage the ecology of the river.

Outline designs and costs were prepared for flood-walls and embankments. The wall would be to the rear of the car park away from the river at around 1m in height. Some ground levels would need to be raised to keep the wall height below 1.6m at low spots. A flood design level of 70.4m AOD was proposed which included a 250mm freeboard. Downstream of the bridge, earthen embankments were proposed to a design height of 70.15m AOD which included a 400mm freeboard.

In the report there was reference to a previous study (Callander Flood Study, Sir M MacDonald and Partners – 1989). The following was reportedly assessed:

Control of River Leny

Reservoir operation at Loch Venachar

Control of the River Leny and Eas Gobhain

Flood storage cells

Channel improvements through Callander

Raising of ground levels at development sites

Of particular note was the assessment of controlling the outlet to Loch Lubnaig. The effects of a control structure at Loch Lubnaig were assessed. It was found that raising levels by 4m would impound a flood event with 10 year return period. However, some low lying sites would apparently continue to be inundated by lesser floods. Control at Lubnaig alone was not considered a complete solution.

3.4.2 Effects of Upland Afforestation On Water Resources - The Balquhidder Experiment 1981 – 1991, Institute Of Hydrology (1995a)

This study, undertaken by the Institute of Hydrology, mainly concerned the effects of upland afforestation on water resource yields. The study centred around two upland catchments; Kirkton and Monachyle, near Balquhidder. These catchments feed into Loch Voil and are similar in topographical aspect. However, a key objective of the study was to determine how land use would affect catchment yields, stream flows and sediment loads. The land uses assessed included forest, heather / grass moorland and clear-felling.

The conclusions cited in the report included the following:

Changes in water use resulting from the planting of 14% of the Monachyle and the progressive felling of half the forest area in Kirkton were not yet detectable.

No significant changes in total flow occurred in the immediate aftermath of the 14% planting in the Monachyle. A small increase in flow was noted from the Kirkton catchment as a result of 50% felling.

From this study alone, it is not possible to draw conclusions regarding the impacts of afforestation on peak flows, since the study focussed on water resource yield. It would appear that there is little solid evidence clearly demonstrating the benefit of afforestation over heather / moorland in terms of reductions in catchment runoff and peak flows for extreme storm events (or at least within the accuracy of any practical on-site measuring regime). Clear-felling does cause some increases in runoff, as would be expected in the absence of any mature vegetation after the felling.


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3.4.3 Stirling Council Flood Prevention Study (Stage 1), Babtie (1998)

This study was undertaken by Stirling Council in compliance with the „Flood Prevention and Land Drainage (Scotland) Act 1997‟ and identified areas with existing and possible flooding problems associated with watercourses in the council region.

The report provides a basic outline assessment of key watercourses across the region and deals with watercourse conditions and flooding issues. A number of photographs of historical flooding are contained in the report. The report also makes general comments on possible flood alleviation measures and approximate costs. Some relevant pieces of information taken from the Babtie report included the following:

Teith Callander – Some reference to work done by previous consultants where flood walling, ramping and access stop logs were proposed.

Strathyre (River Balvag) – Holiday chalets on right bank of River Balvag are occasionally flooded. Flood embankments were proposed. Little risk of property flooding on left bank. Some scour evidence at bridge in Strathyre.

Balquhidder (River Balvag) – The unclassified Balquhidder to Strathyre Road is sometimes impassable due to flooding.

Brig „o‟ Turk – Water levels in the Black Water during times of spate and during times of high water level in Loch Venachar will affect the gardens of properties at the bottom of the lane opposite the Post Office.

3.4.4 Stirling Council Flood Prevention Study (Stage 2), Bullen Consultants (2000)

Subsequent to the Stage 1 Flood Prevention Study, Stirling Council identified a list of priority sites based upon the greatest capital cost implications. The remit of the Stage 2 study was to look at these sites in more detail. The Stage 2 study included a desktop study, hydrological assessment, hydraulic modelling and a basic scheme proposal for each appropriate site.

An outline benefit / cost assessment was also undertaken. A prioritisation formula was developed, based on benefit / cost analysis and risk assessment, which enabled the different sites to be ranked in order of priority for appropriate methods of flood alleviation. Some relevant pieces of information taken from the Bullen report included the following:

Report referred to previous documents including „Flooding in Callander‟ by Richard C Johnson, 1999 and the report entitled „Flood Management of Upper Teith Basin‟ by Mott MacDonald, 1993.

Callander has been affected by flooding from the River Teith on a number of occasions over the years. Numerous properties have been affected to varying degrees.

In 1913 flood waters apparently reached as far as the Dreadnought Hotel on Main Street, Callander.

In 1990, at least 8 properties were affected by flooding on Main Street. A 250mm depth of flooding was experienced in the Callander Woollen Mill shop (8 Main Street) and the Sweet Boutique (6 Main Street).

Meadows Car Park is reported by locals as being usually partly flooded for most of the winter months.

A stage-only gauge was installed in 1994 with a maximum stage of 69.045m AOD recorded during the first year of installation.

The Eas Gobhain accounts for around 25-30% of peak flood flows experienced in Callander. Without the regulation in the Katrine loch system, this would be more like 50%.

Anomalies were reported regarding the upstream gauged flows and associated exceedance probabilities. Issues with gauge accuracy for high flows were


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reported together with comment on previous ratings curve adjustments (Venachar) and the complexity of the lochs and how this serves to attenuate flows.

Access to and from the car parking to the shops is considered vital to the local economy and any flood defence measures need to still allow this ease of access. Similarly, easy access to the river from the car parking is also considered vital to local tourism.

An outline flood alleviation option was presented based on the findings and flood levels taken from the Mott MacDonald report. However, the proposals which reportedly offer protection to a 1 in 100 year standard only work in partnership with a drawdown of 1 m in Loch Venachar over the winter months and a rise of 1 m of the critical weir depth. If changes in the loch operation are not implemented then the level of protection is reportedly only 1 in 23 years. Further and updated studies are recommended to provide better confidence in the various levels and return exceedance probabilities.

The flood alleviation proposals included a combination of waterproofed masonry walling, masonry clad sheet piling, ramping and stop logs.

3.4.5 Callander Meadows Car Park – Flood Risk Assessment, Atkins (2005)

Atkins was commissioned to refine earlier proposals for flood alleviation in Callander to allow Stirling Council to approach the Scottish Government for capital funding for implementation of flood works. A flood risk assessment was undertaken, focussing on the Meadows Car Park. The following key objectives were outlined:

Assess current levels of flood risk by building and calibrating a hydraulic model of the Teith through Callander.

Review previous proposals made by Bullen et al. for compliance with current best practice for flood protection.

Review the flood mitigation measures outlined in the Stirling Flood Prevention Study Stage 2 report.

Make any other recommendations for further flood studies in the area.

Atkins had contacted Scottish Water to discuss the Loch Venachar operational change proposals developed earlier by Mott MacDonald. Scottish Water was reportedly „unreceptive‟ to altering the management of the upstream loch levels for flood mitigation purposes. In addition, potential adverse effects on the SSSI sites around Loch Venachar and also downstream was considered to be significant due to the requirement to lower compensation flows.

The hydrological analysis noted the significant attenuation effects exerted by upstream lochs. Design flows from the two sub-catchments (Leny and Eas Gobhain) were assessed separately. No reservoir routing was undertaken as part of the assessment. Design flow hydrographs were simply derived using a calibrated runoff model with peak flows scaled to those derived using the FEH statistical approach. The effects of the lochs are bundled within the calibrated runoff model. Antecedent loch levels and sluice operation, which would significantly affect flows for a particular storm event, would not be taken into account. The flood frequency relationship from the combined flood hydrographs was consistent with FEH statistical analysis results for the River Teith at Callander.

It is not clear whether the revised ratings curve developed by Mott MacDonald or any other updated ratings curve was used in the flow analyses for Eas Gobhain.

The Teith hydraulic model, a simple 1D model, comprised 14 surveyed cross sections and two bridge structures, supplemented by survey of all buildings, walls, paths, trees, roads and other relevant features within the study area, including


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threshold levels. It was considered that there were no complex overland flow areas or storage depressions that necessitated a more complex 2D analysis.

Calibration was undertaken by taking the recorded flow from the Eas Gobhain and Leny gauges as an inflow to the model (adjusted to account for the catchment area discrepancy as Callander is the location of interest) and comparing the predicted levels at the Callander level gauge. 8 events selected from Annual Maxima were used for model calibration.

A 200 year (+20% climate change) flood level of 70.95m AOD was predicted at the easterly side of the car park with a water depth of 2.8m on the riverside path. Properties adjacent to the car park would be at risk of flooding with a 10% annual probability.

The model does not extend sufficiently far west to facilitate estimations of flood risk to gardens / properties on Leny Road.

Flood mitigation measures outlined in previous reporting was reviewed. Current requirements and considerations for flood mitigation were then outlined. Key considerations included:

Retention of high amenity value of Meadows Car Park and associated river corridor.

Retention of easy access to shops and cafés.

A riverside floodwall would need to be at least 3m high (at the easterly side of the car park) to protect against a 200 year flood event including a climate change allowance. Difficulties include visual / physical obtrusion, technical feasibility and cost.

Although car parking can be planned in flood zones (CIRIA, 2004), frequent flooding may degrade walls, benches, etc., with post flood silt and debris needing to be cleaned up regularly.

Any proposals should work in tandem with current flood warning scheme.

A minimum 600 mm freeboard is advised based on hydrological / hydraulic uncertainty and freeboard guidance.

Increasing channel conveyance at the car park was not considered feasible as it would likely introduce problems with morphology, ecology and increased flood risk to already sensitive areas downstream. Flood storage immediately upstream of Callander was also considered an impractical option.

12 properties at the Meadows Car Park between Caledonian House and No. 22 A84 Road were identified as being at risk from flooding. No property valuations or any cost / benefit analyses was undertaken.

3.4.6 Callander Meadows Car Park – Flood Mitigation Options Assessment, Atkins (2005)

Following the flood risk assessment, a more detailed assessment of flood mitigation options was undertaken, based on sustainability criteria (social, environmental and economic) and practicality. The following conclusions were presented:

Flood Defence Structures – „These could be viable in providing flood defence, although concerns regarding vehicle access may make them unfeasible and un-economic.‟

Flood Storage – „Has potential to provide sustainable flood defence, however size and location within National Park boundaries are considered to make it uneconomic and impractical.‟

Increased Conveyance – „Presumption against implementation, practical difficulties and environmental impacts makes either option unsuitable.‟


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Demountable Defences – „Flexibility, lower environmental impact and fewer regulatory requirements make this a desirable option.‟

Loch Venachar Operational Management – „Change in reservoir management attractive as it uses an existing asset. However, would only provide limited flood risk reduction, and legislative changes may be difficult to achieve. As water levels are already informally managed to reduce high water levels, it is speculative as to what further benefit can be provided.‟

A few other key points of note:

28 properties in Callander are at risk of flooding but none affected until 10% annual probability. Thus, there are implications for economic viability of significant flood schemes.

Roughly 12.5 million m3 would be required to be stored to attenuate flows sufficiently to keep flows below 10% annual probability (the onset of flooding to properties in Callander)

Demountable defences generally only provide protection to around 1 m in height.

Scottish Water had indicated that wherever possible, discharges from Loch Venachar were already managed to keep water levels down in Callander during periods of high rainfall and anticipated flooding.

Most options were not considered suitable for taking forward mostly on economic grounds but also practicality and uncertainty regarding the desired level of protection. Demountable defences were the preferred option and recommended to be taken forward in any further investigations.

3.4.7 Callander Floor Level Survey Assessment, Atkins (2007)

Stirling Council identified 95 properties which are within the High Risk flood area from the River Teith. The survey recorded threshold levels at these 95 addresses. More than one threshold level was recorded at a number of properties. 28 properties were found to be at High Risk (greater than 0.5% annual probability) of flooding, and being affected by indicative flood depths of up to 1m. No properties were predicted to be at risk from a 10% or greater annual probability of flooding. Flood risk cards were produced for each of the 28 properties identified. In most cases there were no structures that would provide any protection from river floodwaters. The identified properties were grouped accordingly:

Main Street – 15 properties near the Meadows Car Park

Bridgend – 7 properties on the western side by the junction at the southern end of Bridgend (not quite as bad as the flooding to properties at Main Street)

North Bank – 6 properties on the Teith‟s north bank between Ancaster Square and Buchanan Place (flood risk more variable).

The survey also included wall top levels, bank levels, ground levels and other identified barriers to flooding. These threshold, spot and bank levels were added to the existing digital terrain model used in the river model simulations.

3.4.8 Stirling Council River Teith Hydraulic Model Update, Atkins (2010).

Atkins undertook an update to their previous work which included:

Update hydrology (using FEH Version 2)

Review and update hydraulic model with LiDAR data

Re-calibrate hydraulic model with three events including the 2006 event

Update the flood risk maps


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The hydrological approach was as follows:

Statistical method estimations of design event peak flows on the River Leny, Eas Gobhain and Teith at Callander.

Analysis of observed hydrometric data to derive parameter values for rainfall runoff models.

Simulation of historical events and calibration adjustments

Review of flood frequency estimates

For both the River Leny and Eas Gobhain, the statistical pooled analysis yielded peak flows which were considered to be on the low side. The peak flow values derived by the single site analyses were considered more reasonable in relation to the gauge data.

Tp was adjusted from an equation based on catchment LAG. The time to peak for the 2006 event (Leny @ Anie) was revised to 15 hours from 4 hours. It was noted that further adjustments to SPR and base flow could improve the calibration plots for the 2006 event. However, the uncertainty in the ratings curves for high flows and the good calibration achieved for other events meant no other parameter changes were made (very high SPR values would have been required).

Design hydrographs for the River Leny were generated using a calibrated rainfall runoff model as they provide full hydrographs and are similar to the single site statistical analysis which provided a more conservative estimate than the statistical pooled analysis. For the Eas Gobhain, the rainfall runoff estimates were considered inappropriate as they do not capture the attenuation present in the upstream lochs system so the calibrated rainfall runoff hydrographs were scaled to fit the peak of the single site statistical analysis.

A 3.5% adjustment was made to the flows to account for the area discrepancy between the gauge locations and the Leny / Eas Gobhain confluence. The 200 year return period flows for the River Leny and Eas Gobhain are 258 m3/s and 153 m3/s respectively.

The calibration events used were from 1996, 2000 and 2006. Observed flows from these events were used as inflows to the model. The observed stage from the SEPA level gauge in Callander was compared to level output from the model. No further adjustments were considered necessary to the existing model as the target accuracy of 150mm (EA model calibration guidance) was still achieved.

3.4.9 Callander Strategic Flood Risk Assessment, MNV Consulting Ltd (2010).

MNV Consulting Ltd undertook a Strategic Flood Risk Assessment for Callander. The purpose of the SFRA was to characterise flood risk throughout Callander, with particular focus on thirteen potential development sites identified by the draft Local Plan. Callander was chosen as a pilot settlement for this SFRA study due in part to the number of flooding occurrences that the community reportedly experienced. The intention is to use the findings of this SFRA to aid the development of Supplementary Planning Guidance (SPG) for the National Park Authority.

The report cites the following main deliverables:

“Collated information on flood risk in Callander in addition to the SEPA 1 in 200 year indicative flood risk maps.

Specific guidance on actual sites and locations for where a more detailed Flood Risk Assessment may be required with planning applications.

Indicative management options for mitigating flood risk on proposed development sites allocated in the Finalised Draft Local Plan (FDLP)”


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The report concludes that the River Teith is the most significant potential source of flooding but that a number of other sources including small watercourses, combined sewer systems and overland flow routes also pose a substantial level of flood risk.

A database collating historic flooding information was developed with a recommendation that the National Park (the planning authority) manages it, in conjunction with Stirling Council (the Responsible Authority for flooding).

The study touches on some potential flood mitigation measures for Callander from settlement level options through to more integrated approaches to flood risk management including Sustainable Flood Management techniques on a wider catchment scale.

3.5 Scottish Water

A key aspect of this study is the exploration of the potential for utilising the various lochs upstream of Callander for flood management purposes. Scottish Water owns and operates the Loch Katrine Water Supply Scheme (which includes Loch Katrine, Loch Arklet, Loch Drunkie, Glen Finglas Reservoir and Loch Venachar) for supplying a significant portion of Glasgow‟s potable water.

Contact was made with Scottish Water‟s relevant Water Resources Strategic Planner and Katrine Senior Operator at an early stage of the project to obtain relevant loch operational information and the key management constraints. The following information was sought:

Operational details and rules for all lochs upstream of Callander and any seasonal variations (winter / summer, wet / dry periods)

Depth / storage relationships for the lochs (bathymetric data)

Existing models that could be utilised for assessing flood management issues

Info or drawings regarding loch overflow arrangements (spillway crests, weirs, sluice gates, channel cross sections, etc.)

Hydrometric info (rainfall gauges, flow gauges and loch levels)

Key operational constraints in relation to any measures that could feasibly be used to address flood risk (lowering levels through winter, increase in storage potential, other dynamic control options)

Typical seasonal loch levels or time series loch level data (i.e. what would typical loch levels prior to flood season?)

Scottish Water‟s involvement is crucial both in developing an understanding of the current loch system and also in determining the feasibility of any potential options for flood mitigation purposes.

3.5.1 System Overview / Description

The Eas Gobhain catchment which is dominated by the Loch Katrine system is shown below in Figure 8.


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Figure 8 - Loch Katrine System

Loch Katrine is the main supply reservoir and is supported by transfers from Glen Finglas Reservoir and Loch Arklet. Arklet is not part of the natural Teith catchment but provides water to Loch Katrine for water supply. Venachar provides the main compensation flow from the whole system into the Eas Gobhain. It is supported by smaller compensation releases from Katrine, Glen Finglas and Drunkie. Loch Achray (between Katrine and Venachar) is a natural loch and is not part of the Scottish Water operation.

Glen Finglas Dam was constructed to further increase capacity to the Loch Katrine Water Supply Scheme. A tunnel was built to divert water from the River Turk (at Glen Finglas) to Loch Katrine and was opened in 1958. The dam was completed in 1965. As part of the dam, a hydroelectric turbine was also installed.

The control structure at Loch Drunkie was constructed in 1859 and is an earth embankment with a puddle clay core. There is a concrete overflow weir at the dam.

The dam on Loch Katrine includes sluices, an overflow weir and a salmon ladder (Figure 9). It was originally opened in 1859 and had four sluices. The number of sluices was increased to nine when the water level of Loch Katrine was raised after 1885. A further phase of modification raised the level again in the period 1919-29. A concrete coffer dam and gates were added during WWII (Figure 10) to help protect the supply should the main dam be damaged by bombing.

The control structure on Loch Venachar was constructed in 1859 (Figure 11) and is located at the east end of Loch Venachar which was a natural loch. The Sluice House is built on the non-overflow section of dam. The dam includes sluices and a fish pass.


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Figure 9 - Loch Katrine Spillway / Fish Pass

Figure 10 - Subsidiary Dam at Loch Katrine

Figure 11 - Loch Venachar Spillway


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3.5.2 Operational Information

Current operational rules are designed to protect water supplies. Long term weather forecasts are monitored, and if dry weather is forecast, then releases are reduced to minimum compensation flow. From a water supply perspective, the reservoirs are managed so they are at top water level in April to provide sufficient storage to get through a dry summer.

In the winter, the level of Venachar is kept to around 0.305 m (1 foot) below TWL, and may be as low as 0.508 m (20 inches) during very wet weather. When the level rises above this, the flow release is increased to bring the level back down while monitoring river levels at Callander Car Park. This process is carried out in consultation with SEPA to determine appropriate releases. It is thought that the current aim of keeping Loch Venachar down through winter may have been influenced by previous flood studies. This regime has been in place since around the 1990s.

The key operational constraint is that the reservoirs are manually controlled. Large changes to flow release can take a long time in sluice operation. The reservoirs are visited daily (Monday to Friday) and adjustments to flow are made as required. The Friday release setting is adjusted to allow for the weekend weather forecast. Due to the difficulties of measuring flows accurately, compensation releases are generally slightly higher than the licensed compensation flow rate to ensure compliance.

The key elevations / dimensions associated with the various loch overflow / release control structures are shown in Table 1. Also shown are the operation zone depths for the various lochs.

Table 1 - Loch Spillway Levels / Operational Zone Depths

Outfall Control Structure Overflow Weir Level

(m AOD) Weir Length

(m) Operational Zone Depth

(m below TWL)

Glen Finglas Reservoir 157.00 58.67 28.27

Loch Drunkie 126.80 8.91 7.62

Loch Katrine 115.19 21.34 5.18

Loch Venachar 82.25 45.72 3.56

3.5.3 Abstractions / Transfers / Compensation Flows

Loch Katrine is the main water supply reservoir and the loch from where abstractions are taken. The abstraction from Loch Katrine amounts to 5.34 m3/s (470 Ml/d). This remains fairly constant throughout the year, although during the last two winters (2009/2010 and 2010/2011) demand rose to 6.37 m3/s (550 Ml/d) due to cold weather mains bursts. Demand returned to normal by late March.

Abstractions from Katrine are compensated by transfers from Finglas and Arklet. Finglas transfers are constant year round and are also used to generate electricity. The flow rate is 1.05 m3/s (91 Ml/d). Transfers from Arklet are generally constant at 1.10 m3/s (95 Ml/d), although this rate is progressively reduced to 0.74 m3/s (63.6 Ml/d) and 0.37 m3/s (31.8 Ml/d) when the water level in Katrine is high.

To compensate for the abstraction from the loch system, compensation flows are released from the reservoirs (in addition to any spills which occur naturally over the spillway for flood events). Compensation releases, which are the licensed rates agreed with SEPA (under CAR licence), are as follows:

Katrine 0.26 m3/s (22.655 Ml/d)

Glen Finglas 0.52 m3/s (45.31 Ml/d)

Venachar 2.61 m3/s (225.48 Ml/d)

Drunkie 0.03 m3/s (2.76 Ml/d)


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The above are minimum releases; actual releases are slightly higher to ensure compliance, and much higher in the winter to prevent / limit spills. The release from Finglas passes through the hydropower system prior to discharge into the River Turk or transfer to Katrine. The reservoirs are visited daily (Monday to Friday) to make manual adjustments to the sluices which control flow.

3.5.4 Hydrometric Data

Scottish Water supplied hydrometric data for the Katrine system. The data includes:

Annual maximum water levels for Lochs Arklet, Katrine, Finglas, Venachar and Drunkie from 1996 to 2011 (data prior to 1996 is not readily available);

Spreadsheet containing loch levels, release rates and rainfall for SW lochs for period 01/07/1996 to 14/11/2011;

Loch levels for Katrine, Finglas and Venachar for period 01/08/2004 to 31/05/2010, as well as occasional levels for Drunkie during this period;

Daily loch levels for Finglas from 03/05/2010 to present;

Data for the lochs system for the 2004 and 2006 flood events, and;

Details of loch sluice gate operation through 2008.

The maximum loch level readings since 1996 are:

Arklet 07/01/2005 0.406 m above TWL

Finglas 09/01/2005 0.84 m above TWL

Venachar 14/12/2006 1.07 m above TWL

Katrine 14/12/2006 0.74 m above TWL (Coincides with highest level recorded at Loch Lomond)

Rainfall and corresponding loch levels for the January 2005 event are shown in Figure 12, illustrating how loch water levels responded to extreme rainfall. This 2005 event was the second largest flood event recorded in Callander in recent years (Scottish Water hydrometric data for the largest event in 2006 was incomplete).

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Arklet level (m above TWL) Finglas level (m above TWL) Vanachar level (m above TWL) Drunkie level (m above TWL)

Figure 12 - 2005 Event Rainfall and Loch Levels

3.5.5 Miscellaneous

Scottish Water noted that a previous study looking into flood mitigation investigated the feasibility of lowering Katrine levels. However, at the time, Scottish Water was reportedly not conducive to these proposals as they could impact significantly upon water supply and operating costs.


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Scottish Water holds an AQUATOR model of the entire Katrine reservoir system which runs on a daily time-step. Scottish Water considers it unlikely that the model would be suitable for flood assessment purposes. In the current version of the model any excess storage above the reservoir capacity will spill at the end of each day. It does not model flow over the spillway. Scottish Water is currently consulting with SEPA regarding compensation releases, although the discussions are unlikely to reach a conclusion within the timescale of this study.

Scottish Water does not own Lochs Lubnaig, Voil and Doine. Land under Scottish Water ownership surrounding Lochs Katrine and Achray has been leased to Forestry Commission Scotland, which is looking to restore a large area of broad-leaved woodland in the area.

3.5.6 Implications for Flood Mitigation Investigations

A detailed investigation into upstream loch control arrangements (as summarised above) and the potential opportunities / constraints for flood management was undertaken in liaison with SEPA / Scottish Water.

There are significant limitations on what can realistically be achieved by undertaking detailed hydraulic modelling of the whole Katrine system as part of this current study. The conclusions reached in previous studies are that modifications to the existing Katrine system could potentially bring some benefits to flooding in Callander but, would only be a partial solution and would be difficult to achieve in practice (bearing in mind this system is already heavily modified and attenuated).

It is considered that to make the Katrine system more efficient, all sluices would need to be computer controlled, be electronically actuated, based on real time loch levels, long and short range weather predictions, abstraction requirements and rainfall data (spatially varying) and linked to flow gauging on the River Leny. This would be the ideal system. Extensive and detailed water supply studies would be required before any options that involve winter draw down (more than already done) could be seriously entertained. Such an extensive study would require Scottish Water to be the leading partner. Although Scottish Water has obligations under the Water Framework Directive which includes flood management duties, and was very helpful in the various communications, Scottish Water‟s priority is always going to be water supply / drought resilience and not primarily fluvial flood risk.

It is considered that within this study, the most practical option worth exploring on the Katrine system are the effects of increasing weir heights on Venachar (increased storage) but maintaining the current winter draw down level arrangements. A significant variable in this exercise is the compensation releases. The only records are in paper format and just states the number of sluices open at each dam on a particular day. No sluice flow data was found to be available. Various modelled weir height / storage scenarios on Venachar have been proposed and covered later in this report.

It is considered that exploring options (through surveying and modelling) for flood storage on the Lubnaig / Voil system would be more practical, cost effective and potentially more fruitful, considering this system has a similar catchment size to Eas Gobhain and remains largely unmodified. The model includes sufficient detail to capture the key controls and potential storages in the system, which also includes significant floodplain areas between Strathyre and Balquhidder. This is explored later in this report.

Historic reporting suggests that an increase of around 4 m would be required on Lubnaig to alleviate 100 year flooding in Callander down to a 10 year flood. In the 1993 report there was reference to a previous study (Callander Flood Study, Sir M. MacDonald and Partners – 1989). This study reportedly assessed the scope for


© Mouchel 2013 25

controlling flows on the Leny. From initial investigation, including site visit during the flooding on 29th November 2011, there are still likely to be major constraints on what can be achieved on the Leny system. Such constrains include flooding to roads, potential property flooding and physical / technical constraints for flow impoundment and regulation.

It should be noted that environmental impacts (as have also been previously reported) will be a significant constraint when considering any options for increasing loch TWLs.

3.6 SEPA

3.6.1 Flood Mapping

SEPA publishes strategic flood maps showing areas potentially at risk from both fluvial and coastal flooding (http://www.sepa.org.uk/flooding/flood_map.aspx). GIS versions of these SEPA flood-maps were supplied by the Council.

SEPA flood maps show parts of Callander lie within the medium to high risk area. It should be noted that SEPA flood-mapping is based on a digital terrain model with a vertical accuracy in the range 0.7m – 1.0m, on a grid spacing of 5m. It is also not relevant to catchments smaller than 3 km2. SEPA flood-mapping also does not provide enough detail to accurately estimate the flood risk associated with individual properties or specific locations. Local factors such as flood defence schemes, structures and other local influences which might affect flooding have not been included. Furthermore, the flood map does not account for flooding from sources such as surface water runoff or surcharged culverts. More detailed assessments are therefore required.

3.6.2 Climate Change

Current guidance recommends the use of judgement when considering potential climate change impacts. SEPA currently recommends an uplift of 20% to peak river flows to account for the potential impact of climate change. However, this figure is currently valid until around 2060 and does not take account of climate change occurring gradually.

The Environment Agency has produced supplementary guidance to FCERM-AG on accounting for possible climate change scenarios. This guidance is not applicable to Scotland. The Environment Agency guidance recommends a stepped uplift to climate change over a 100 year assessment period as shown in Figure 13 below (as per „Adapting to Climate Change: Advice for Flood and Coastal Erosion Risk Management Authorities,‟ Environment Agency (2011)). This particular EA projection is for the Tweed River Basin which has the same 20% allowance around 2060 as per SEPA. This allows for a variety of climate change scenarios to be tested. This is considered to be a conservative and robust assumption, and it was agreed with SEPA that this approach would be acceptable and defensible if applied for benefit / cost assessment in Scotland, instead of the current fixed 20% uplift.

http://www.sepa.org.uk/flooding/flood_map.aspx


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Figure 13 - Climate Change Uplifts

For 1 in 1000 year design levels, SEPA acknowledge the higher uncertainty associated with flow estimation for such extreme return periods and recommend that the estimated 1 in 1000 year flow should be checked against the 200 year + climate change flow to ensure the 1000 year is greater. The 1 in 1000 year flow should then be acceptable without an additional 20% climate change uplift.


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3.6.3 Flow Records

To facilitate detailed hydrological analyses, SEPA was contacted in order to obtain flow data for all relevant gauges within the study area (see Table 2 below).

Table 2 - SEPA Flow Gauging Stations

Flow Gauge Reference Catchment Area

(km2)

Record Length (years)

Max Flow (m3/s)

Leny @ Anie 18008 190 37 168 (16/01/1993)

Achray Water @ Loch Katrine

Unknown 94 18 42 (11/12/1994)

Eas Gobhain @ Loch Venachar

18015 202 32 104 (14/12/2006)

Teith @ Callander Flood warning

only 410 17

Stage only (14/12/2006)

Teith @ Bridge of Teith

18003 518 55 495 (14/12/2006)

All data provided were quality marked as „good‟, and were noted to be suitable for QMED calculation. Annual maxima (AMAX) data were only provided in calendar years. No water year AMAX data was available at the time of request. For the gauges at Loch Katrine and Callander AMAX data was not available. AMAX data were therefore derived using the supplied monthly maxima. The AMAX data for all the gauges listed above are contained in Appendix B.

Flows are calculated using calibrated gauging and an established rating curve. The gauges at Katrine and Venachar are calibrated for low to medium flows to monitor compensation flows. High flows are an extrapolation of the rating relationship. SEPA note that flows at Katrine corresponding to stages above 1.5 - 2m should be treated with caution as flow is not contained, and the rating relationship is not valid. The rating for the Venachar gauge was updated following the study by Mott MacDonald in 1993, and SEPA‟s entire record was recalculated. SEPA has a good level of confidence in the rating for Venachar for low to medium flows but the rating is not considered to be reliable for high flows. Flows calculated at Anie are deemed to be reliable for the full range of flows, including high flows. The rating at Bridge of Teith is also deemed to be reliable for a full range of flows.

The stage / discharge (ratings) curves were also obtained from SEPA to help with model calibration.

3.6.4 Rainfall Records

The details of the rainfall gauges relevant to the study are shown in Table 3 below. Rainfall data is available at a 15 minute intervals for these gauges.

Table 3 - Rainfall Gauge Locations

Rain Gauge NGR

Strathyre NN 561 167

Loch Venachar NN 602 070

Loch Katrine NN 491 067

Deanston NN 712 019

3.6.5 Time Series Flood Event Data

To facilitate hydrological and hydraulic model calibration and verification, six of the largest recorded flood events at Callander were identified from monthly maximum stage data. Time series data was then requested for these flood events including rainfall, flow and stage data at 15 min intervals. The time / peak stage details of the flood „calibration‟ events for Callander are shown in Table 4 below.


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Table 4 - Key Flood Events at Callander

Date / Time Stage at Callander (mAOD)

14/12/2006 (5:30) 69.5

07/01/2005 (13:00) 69.3

11/12/1994 (20:30) 69.1

02/03/1997 (11:30) 68.9

26/01/2008 (12:00) 68.8

20/11/2009 (12:30) 68.7

It should be noted that there were some apparent anomalies in the monthly maxima dataset provided by SEPA; some months were missing from the record and there were also a number of instances where there was more than one entry per month.

3.7 Loch Bathymetry

A key piece of information required for modelling the upper Teith catchment is loch bathymetric data. This data is needed to accurately capture loch volumes associated with a range of loch levels, ensuring available storages are accurately captured in any modelling.

Scottish Water holds some bathymetric information which was obtained for this study. This includes data for the lochs which form part of its water supply operations and provided elevation / volume data for the operational zones of Katrine, Finglas, Drunkie and Venachar.

For lochs out-with Scottish Water‟s operations, bathymetric data was obtained from the Bathymetrical Survey of the Fresh-Water Lochs of Scotland, organised by Sir John Murray and funded by Laurence Pullar in 1897-1909 (Murray & Pullar, 1900). Loch depths were gauged using a sounding machine designed by Pullar for the purposes of the survey.

For the non Scottish Water lochs (Achray, Lubnaig, Voil and Doine), the old bathymetric survey charts were used to determine the loch bathymetry. The maps were digitised and georeferenced (Figure 14). Loch depths were then converted to meters above ordnance datum and a bathymetric model created (Figure 15). Neither the Scottish Water data nor the bathymetric survey data covers zones above the normal loch surface. Some supplementary data including OS mapping, DTM data and survey data was also needed to allow an extended bathymetry to be created above normal loch levels.

M P

Sheep Pens

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Kennels

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Tom a' Bhuachaille

Uamh an Righ

(Bruce's Cave)

Creag a' Bhuilg

Coille Mhor

Bad nan Earb


Figure 14 - Geo-Referenced Bathymetric Survey Maps


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Sheep Pens

Track

129m

BM 132.17m

W a ter

Tom a' Bhuachaille

128m

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Creag a' Bhuilg

Winter water level 126 metres above Newlyn datum 1973

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Kennels

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Uamh an Righ

(Bruce's Cave)

Creag a' Bhuilg

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Bad nan Earb


Figure 15 - Loch Bathymetry

Having generated the loch bathymetry, loch volumes at various elevations were then determined (Table 5). The derived volume / elevation relationships are only approximate but are considered sufficient for the purposes of these assessments.

Table 5 - Loch Bathymetry Details

Normal

loch level (mAOD)

Loch area at normal loch

level (m2)

Operational zone volume

(m3)

Total loch volume according to

Murray and Pullar

(1900) (m3)

Maximum loch depth according

to Murray and Pullar (1900) (m)

Katrine 115.2 13,270,000 64,610,000 772,314,000 151

Finglas 157 1,417,000 19,002,000 Not surveyed Not surveyed

Drunkie 126.8 577,700 3,517,000 6,145,000 30

Venachar 82.3 3,863,000 11,747,000 53,887,000 34

Achray 84.1 742,000 n/a 9,090,000 30

Voil & Doine

126.3 2,725,000 n/a 33,867,000 30

Lubnaig 123.4 2,421,000 n/a 32,394,000 45


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3.8 Scottish Government

3.8.1 Scottish Planning Policy (SPP)

Scottish Government‟s policy on flood risk, Scottish Planning Policy (SPP) (Scottish Government, 2010), contains a risk matrix which describes in general terms how planning is affected by the annual probability of flooding:

Little or no risk area (less than 0.1% annual probability) – generally no constraints.

Low to medium risk area (0.1% to 0.5% annual probability) – suitable for most development but not essential civil infrastructure.

Medium to high risk area (0.5% annual probability or greater) – in built up areas with flood prevention measures most brown-field development should be acceptable except for essential civil infrastructure. Undeveloped and sparsely populated areas are generally not suited for most development.

For planning purposes the functional floodplain will generally have a greater than 0.5% annual probability of flooding in any given year. SPP states that built development should not take place on the functional floodplain other than in specific, exceptional circumstances (subject to determination by the Planning Authority). Sustainable Drainage System (SuDS) features should be located out-with the 200 year flood envelope if possible.

3.8.2 Standards of Protection / Grant Assistance

A number of properties lie within the 200 year flood outline at Callander. Any flood mitigation measures proposed will attempt to remove or reduce the probability of flooding to these properties, subject to economic assessment and other appraisals. Where grant assistance is sought, the Scottish Government provides guidance to local authorities on grant assistance and typical standards of protection expected for any promoted flood schemes - „Flood Prevention Schemes - Guidance for Local Authorities‟ (Scottish Government, 2005a). The following guidance was noted:

“6.4 - To qualify for grant assistance, it is expected that flood prevention schemes in Scotland will continue to be designed to withstand, at least, a 1 in 100 year flood event. As noted in paragraph 3.8, this guideline standard is not intended to prejudge or otherwise constrain the appraisal process. It does not preclude the adoption of a higher standard where justified, nor indeed a lower one. However, experience over many years in Scotland has shown that a 1 in 100 year standard is appropriate for the protection of non-agricultural land. Such a standard reflects reasonable public expectations in relation to residential and other urban property. Further, it lies within the return period range of 50-200 years, which is the indicative standard in England and Wales for the protection of intensively developed urban areas against fluvial flooding. Against that background, a 1 in 100 year standard is a reasonable starting point, and provides a practical benchmark to assist with the administration of the grant scheme. This does not negate the need to appraise different standards in addition to the guideline value.”

3.9 Future Planning and Development

The development strategy for Callander and the Teith catchment upstream of Callander is laid out in the Loch Lomond & the Trossachs National Park Local Plan (Loch Lomond & the Trossachs National Park Authority, 2011), as intended for adoption. At the time of writing, the key areas identified in the plan which may be affected by flood risk are identified in Appendix A.


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3.10 Utilities Information

Utilities information was gathered to allow for the identification of potential utility constraints to any flood risk management options proposed. The gathered utilities information is contained Appendix A. However, a more thorough investigation would be required during any detailed design stage.


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4 Hydrological Review (Callander)

The hydrological parameters derived in Atkins‟ 2010 study including the critical storm duration and time-to-peak for the Leny and Eas Gobhain were used unchanged. These parameters are briefly reviewed in the following sections. Additional information was present for this current study, allowing for a design flow update to be carried out in liaison with SEPA. This section summarises the design flow derivation for Callander.

4.1 Catchment Analysis

There are two main hydrological sub-catchments within the study area which feed the Teith at Callander (Figure 16). The FEH catchment descriptors for the two catchments are contained in Appendix C. The two catchments are almost identical in size, with areas of 205 km2 and 200 km2 for the Leny and Eas Gobhain catchments respectively.


Figure 16 – Leny / Eas Gobhain Hydrological Catchments

4.2 Eas Gobhain / River Leny – Relative Flow Contributions

The Eas Gobhain and River Leny combine to form the Teith at Callander. The relative contribution of flow from these two tributaries defines the magnitude of flow through Callander. Understanding the relative contributions from these two rivers is fundamental to understanding how flooding occurs in Callander and how effective upstream storage options could be on either of these tributaries. Any options pursued on either of the rivers will only affect a proportion of the flow to the Teith through Callander.


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Historical river gauge records for the Leny, Eas Gobhain and Teith have been analysed to ascertain relative timings of peaks, percentage flow splits and relative peak flow return periods. The storm events relating to the Callander stage AMAX events have been used for this assessment. This key flow contribution information is shown in Table 6 below. The following observations are noted:

Peak Timings

The flow response at the Loch Venachar gauge is strongly influenced by the antecedent upstream loch conditions and the magnitude of the storm/s passing through. For some storm events, the peak passes through the system quickly, for other events it is attenuated significantly.

The flow response at the Loch Katrine gauge often shows little sign of a storm passing through. This is thought to be as a result of the relatively large storage potential on Katrine.

The Leny system, being a natural catchment, generally has a more peaky response than the Eas Gobhain system.

Based on the six largest observed flood events at Callander, there is no consistent pattern of which tributary peaks first. The worst flood events at Callander are typically of long duration (of the order of days). This generally means that when one river peaks, the other will not be far off its peak (typically within 10%). This observation is reflected in the design hydrographs used for the modelling; both inflow hydrographs have durations of around five days, and the peaks occur within around 3 hours of one another.

Due to the noted variable coincidence of peaks and long flood durations for extreme events, it is difficult to conclude whether there would be any benefit in delaying or advancing either tributary peak for flood alleviation purposes. It is considered that any upstream flood storage options on the Leny or Eas Gobhain should focus on reducing peak flows rather than aiming to actively modify the relative timing of these peak.

Peak Flow Magnitude / Return Period

The Leny always has a higher peak flow than Eas Gobhain for all the Callander AMAX events, although it is acknowledged that the Eas Gobhain gauge rating is less reliable than that of the Leny for high flows.

On average, the flow split for AMAX is 71% Leny / 29% Eas Gobhain

On this very limited data set (18 years) there is tentative evidence to show that with increasing event rarity, the flow split becomes more equal. It is thought that for long duration extreme events, reservoir attenuation would be limited and the Eas Gobhain catchment would tend towards a more natural response. Similarly, it can be shown that for the more frequent events (i.e. lower return periods) the flow split variation increases. This % flow split variation is partly reflected in the design flows used in the Teith Callander model.

December 2006 flooding was a 61% Leny / 39% Eas Gobhain flow split.

For the 2006 event, the return period on the Teith in Callander was 56 years. The corresponding return period for the Leny flow was 21 years, whereas the return period of the Eas Gobhain flow was 48 years.


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Table 6 – Leny / Eas Gobhain Tributary Flow Comparison


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4.3 Callander (Teith) Hydrological Update

Subsequent to Atkins‟ model update report of 2010 (the hydrology only includes data up to 2009), a number more station-years of hydrological data were available. Furthermore, Atkins‟ study used WINFAP-FEH version 2. Using version 3, which includes some improvements to the methodology and additional flow updates, a review of design flows for Callander was carried out.

The following sections describe the design flow review and update for Callander. The FEH Statistical Approach was used as the basis for design flow estimation. A summary of the hydrological derivations follow however, further details can be found in Appendix D. SEPA has been consulted and have approved this updated design flow derivation.

4.3.1 Proposed Approach

There are issues with using only the Eas Gobhain / Leny gauges for determining design flows in Callander (low reliability for high flows, particularly Eas Gobhain). There is a reliable stage-only gauge in the centre of Callander with 17 years of record. It was considered that this data should be utilised for flow derivation as there is a well calibrated hydraulic model rating curve available to convert Stage AMAX to Flow AMAX. There is also a flow gauge downstream at Bridge of Teith with 56 years of data which is considered reliable for high flows.

It was agreed with SEPA that the data from both the Callander and Bridge of Teith gauges should be utilised for flow derivation in Callander. Flows were estimated using a range of methods and scenarios. The following approach was considered the most appropriate for Callander.

4.3.2 QMED Estimation

The following initial QMED estimations were made:

QMEDCALLANDER = 147 m3/s (17 years of record)

QMEDBRIDGE OF TEITH = 212 m3/s (using full 56 years of record)

QMEDBRIDGE OF TEITH = 258 m3/s (using last 17 years of record)

The record length at Callander was 17 years; whereas the record length at Bridge of Teith is much longer, at 56 years. The record length at Callander was too short to be used on its own and contained a disproportionate number of significant floods (the three highest flows in the Bridge of Teith record have occurred in the last 17 years). Using the Bridge of Teith 56 year / 17 year QMED ratio of 0.82, the following hybrid QMED for Callander was derived.

QMEDCALLANDER = 119 m3/s (hybrid approach)

This QMED value is considered more appropriate for Callander.

4.3.3 Growth Curve Derivation

Two methods are available within the FEH statistical approach; Pooling Group and Single Site approaches. The Single Site approach uses data from a single gauge to derive peak flows for a range of return periods. The reliability of this approach reduces as return periods exceed the period of record. The Pooling Group approach utilises data from other gauged catchments based on catchment similarity to generate a growth curve. The growth curve is then multiplied by QMED at the subject site to derive peak flows for a range of exceedance probabilities. This is the typical approach for assessing flows for extreme events where the record length at the subject site is not long enough or non-existent.


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SEPA has expressed a preference for the use of Single Site analyses for catchments with low FARL1 due to the variability in flow response for catchments containing significant areas of controlled reservoirs / lochs.

Various growth curves were derived using a range of Single Site, Enhanced Single Site (a pooled approach which uses the subject site in the pooling group) and pooled approaches and permutations:

1. Single Site at Callander 2. Single Site at Loch Venachar 3. Single Site at Anie (Leny) 4. Single Site at Bridge of Teith 5. Enhanced Single Site – Callander station included in Pooling Group (given extra

weighting) 6. Default Pooling Group 7. Pooling Group with two upstream stations only (Anie and Loch Venachar) 8. Pooling Group with three stations in Teith catchment (Anie, Loch Venachar and

Bridge of Teith)

The possible growth curve options resulting from the above analyses are shown in Figure 17 below.

0

1

2

3

4

5

6

1 10 100 1000

Return Period (Years)

Gro

wth

Fa

cto

r

1 2 3 4 5 6 7 8

2

4

8/7

3

5

6

1

Figure 17 - Growth Curves

4.3.4 Peak Design Flows

Peak design flows are derived by multiplying QMED by an appropriate growth curve. As shown above, there are a number of possible approaches available for both QMED calculation and growth curve derivation. Following discussions with SEPA, it was agreed that using the Bridge of Teith Single Site growth curve with the adjusted QMED from the Callander stage gauge would yield a conservative and defensible set of design flows for Callander. Table 7 below shows the updated design flows to be adopted for flood modelling and optioneering for Callander. Also shown for comparison are the design flows derived in the previous 2010 study by Atkins.

1 A lower FARL value indicates higher attenuation due to the presence of reservoirs and lakes.


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The existing hydraulic model for Callander comprises two hydrological inflows; one for Leny and one for Eas Gobhain. For modelling purposes the updated peak flows derived for Callander were divided in the same relative proportions as in Atkins‟ study and scaled to ensure the total modelled peak flow at Callander matched that derived using the statistical approach. The existing hydrograph shapes and durations (as contained in the existing hydraulic model) were reviewed and considered acceptable. Since the hydrograph shapes and durations were derived previously, it was only peak flow scaling that was required.

Table 7- Callander Design Flows

Atkins (2009) Updated (2012)

Return Period (Years)

Eas Gobhain

Flow

(m3/s)

Leny Flow

(m3/s)

Total Design Flow at

Callander (m3/s)

Eas Gobhain

Flow

(m3/s)

Leny Flow

(m3/s)

Total Design Flow at

Callander (m3/s)

2 (QMED) 44 103 147 37 82 119

5 59 135 194 51 106 157

10 71 154 225 62 124 186

25 90 183 273 80 151 231

50 108 208 316 96 175 271

100 128 231 359 116 203 319

200 153 258 411 140 236 376

200 + CC 184 310 493 168 283 451

500 180 288 468

1000 218 335 554


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5 Hydrological Assessment (Leny Routing)

As part of this study, upstream storage options on the Leny system are to be assessed. To allow development of a catchment scale routing model which includes the large upstream water bodies and floodplains, hydrological inputs are required for this large scale model.

5.1 Hydrological Schematisation

Between Loch Voil and Callander, a total of 25 tributaries are defined in the FEH CD-ROM (Version 3) within the modelled reach (see Figure 18). The FEH catchment descriptors for each of these FEH sub-catchments are shown in Appendix C. For model accuracy and flexibility, all 25 hydrological catchments are applied as lateral inflows to the model. This was considered to be the most accurate and realistic way of capturing the hydrological dynamics of the system.

Figure 18 - FEH Defined Tributaries on Leny

The total area of all FEH defined tributaries will be slightly less than the total hydrological catchment area at the downstream end of the model. Consequently a small adjustment is required to ensure that any „missing‟ catchment areas (smaller than 0.5 km2) are not lost. A simple global uplift to the sub-catchment areas was made to ensure that the sum of all the individual sub-catchments would be equal to the total upstream catchment area at the lower extent of the model.

5.2 Hydrological Calibration

Design hydrographs for each of the 25 inflow locations, were derived using the FEH Rainfall Runoff method for a range of storm durations and return periods. By applying these inflows into the routing model at the various locations described above, resulting routed flows can then be determined downstream in Callander.


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Hydrological calibration was carried out using observed data (flow and rainfall) for six of the largest flood events recorded. Rainfall data from the Strathyre gauge was input into the hydrological runoff model and the resulting routed flows compared to those recorded at the Anie gauge station. Adjustments were made to SPR, Tp scale factor, storm duration and fixed/variable percentage runoff to achieve as good a match as possible with the observed runoff / flow response.

The calibrated rainfall-runoff model was then used to derive design hydrographs for a range of exceedance probabilities. A global hydrograph adjustment was then applied to ensure that the peak design flow for a range of return periods matched the previously derived design flow for the Leny at Callander at the downstream end (for the appropriate critical duration). Details of the modelling including hydrological / hydraulic calibration is discussed in more detail in section 7.


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6 Hydraulic Modelling - Teith (Callander)

6.1 Teith (Callander) Model Build (Conversion and Extension)

6.1.1 Model Extents

To improve model accuracy and fulfil the purposes of this study, the existing InfoWorks RS 1D hydraulic model (as used in Atkins‟ 2010 study) was updated as follows:

Model extended upstream to include Leny and Eas Gobhain tributaries and the large floodplain area to the west of Callander

Model extended by around 600 m downstream to fill gap between the existing Callander model and the „rural‟ Teith model (being developed by consultant Halcrow at the time of writing).

Model converted from InfoWorks RS to ISIS to allow for potential future linking with the ISIS rural Teith model.

Model converted from 1D to fully linked 1D / 2D (ISIS-TUFLOW).

The development of a full 2D model was considered necessary to capture the complex floodplain dynamics, improve property flooding assessments and to facilitate flood scheme optioneering / impact assessments. The 2D domain would use TUFLOW, the most widely used 2D river modelling package in the UK. The TUFLOW 2D domain is dynamically linked to the ISIS 1D domain. Model extents are shown below in Figure 19.

Figure 19 - New Teith (Callander) Hydraulic Model Extents


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6.1.2 Supplementary Model Build Data

The following supplementary data was used to update the Callander hydraulic model:

Updated hydrological inputs as described earlier in this report. Atkins‟ hydrograph shape and % flow split (Leny / Eas Gobhain) was retained but scaled using updated hydrology.

Additional river cross sections, floodplain spot levels, embankments and other miscellaneous surveyed levels to aid the extended model build.

A topographical survey was commissioned by Mouchel as part of this study. This topographical survey allowed extension of the modelled reaches of the Leny, East Gobhain and Teith. This additional data, summarised in Table 8, supplemented the existing 21 cross sections and 2 bridges in the previous model. The updated hydraulic model extends approximately 1200 metres along the River Leny, 1350 metres along the East Gobhain River and 2400 metres along the River Teith.

Table 8 - Additional Surveyed Data for Updated Callander Model Build

Surveyed Feature Leny Eas Gobhain Teith

River Cross Sections 7 6 5

Bridges 1 0 0

The 2D component (floodplains) of the Callander model was constructed using 1 m LiDAR data but supplemented and refined in certain locations by topographical survey data. Typically, LiDAR data is less accurate around structures, buildings and heavy vegetation so care was taken to ensure accurate hydraulic representation. The topographical survey included the railway embankment (and associated culvert) which traverses the floodplain area to the west of Callander, between the Eas Gobhain and Leny rivers. The LiDAR ground model was manually edited to capture such hydraulic controls.

6.1.3 Model Build

The 1D component of the model (ISIS) includes the river channel cross sections, hydraulic structures and the hydrological inputs. The 2D component (TUFLOW) of the model comprises the ground model and includes features such as the height of road centre lines, walls, embankments and bridge parapets. A 10 m grid cell size was used in the 2D domain as a balance between model accuracy and practical model run times.

6.2 Model Calibration / Verification

Model calibration is important in order to provide confidence in model predictions. Although the previous model was calibrated within acceptable confidence levels, the updated model required recalibration.

To calibrate the updated model, 5 of the largest recent flood events were selected. For these events, the gauged flow records from the Anie (Leny) and Eas Gobhain (Venachar) events were obtained and simply applied as the upstream model inflow boundary. Once routed through the model, the predicted levels in Callander were compared with those recorded at the SEPA gauge. The SEPA level gauge lies approximately 20m upstream of the footbridge in Callander.

Some adjustments were made to model parameters (mainly Manning‟s „n‟ friction values) within sensible bounds to achieve the best overall calibration across the 5 separate events. SEPA's aerial flooding photographs from December 2006 were also used to sense check and verify the model predictions for extreme events.

Calibration results are shown in Table 9. It can be seen that all 5 observed events have been modelled to within 100 mm of gauged levels. This exceeds the typical EA


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specification for model calibration where 3 events should be modelled within 150 mm of observed.

Table 9 - Callander Model Calibration Results

Callander Model Calibration Results

Observed Flood Event

Max Gauged Level (m AOD)

Max Modelled Level (m AOD)

Variation (mm)

14th

December 1994 69.074 69.169 95

2nd March 1997 68.860 68.916 56

7th

January 2005 69.268 69.316 48

14th

December 2006 69.484 69.424 -60

26th

January 2008 68.841 68.880 39

Figure 20, Figure 21 and Figure 22 shows floodplain extents as photographed by SEPA at 13:00 hrs on 14th December. Figure 23 and Figure 24 show the corresponding predicted extents for the same point in time, taken from the model. Observed and predicted extents are similar. The model is therefore considered suitable for predicting flood levels in Callander and is considered suitable for optioneering / impact assessments.

Figure 20 - Callander Observed Flooding 14th

Dec 2006 – Photo 1 (13:00 hrs)

Figure 21 - Callander Observed Flooding 14th

Dec 2006 – Photo 2 (13:00 hrs)


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Figure 22 - Callander Observed Flooding 14th Dec 2006 – Photo 3 (13:00 hrs)

Figure 23 - Callander Predicted Flooding 14th

Dec 2006 (13:00 hrs)

Figure 24 - Callander Predicted Flooding 14th

Dec 2006 – Leny Road (13:00 hrs)


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6.3 Model Sensitivity Analysis

The hydraulic model has been checked in terms of parameter sensitivity to determine the response / stability of the model for changes in flow, roughness and downstream boundary conditions.

There was a sensible and uniform model response to flow variation over all modelled reaches.

The downstream model boundary was checked using a range of downstream water levels. This check was to ensure that conditions within the upstream reach were not significantly sensitive to conditions downstream. There was some sensitivity to variations in downstream water level but this sensitivity did not extend significantly into the reach of interest.

The hydraulic model was tested by varying the roughness conditions (Manning‟s „n‟) by +/- 20% to assess model stability / sensitivity. Generally, the variation of Manning‟s „n‟ roughness yielded a sensible and constant / stable variation in water levels.

6.4 Model Output and Results

6.4.1 Flood Levels and Outlines

From the modelling, water levels and flood outlines have been estimated for a range of return period design events. Peak water levels for all modelled cross sections (taken from the main channel – 1D model domain) have been collated and presented in Appendix E. Full flood outlines are contained in Appendix F. An overview of the predicted 200 year flood outline for Callander is shown below in Figure 25.

The mechanisms of flooding are relatively complex, particularly in floodplain areas. Flood inundation animation videos of all modelled scenarios are available upon request and show a more complete and visually understandable picture of the mechanisms of flooding.


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Figure 25 - Callander 200 Year Flood Outline


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6.4.2 Flow Velocities and Depths

From model output it is possible to extract floodplain flow depths and velocities. Information regarding predicted flow paths, particularly those with significant velocities and depths is important information when considering safety issues, particularly access and egress, during times of flood. This information can be made available in digital format (GIS and / or video) as required.

6.4.3 Key Flood Risk Issues

The modelling has confirmed the flooding extents and mechanisms of flooding in Callander. The frequency of flood risk has also been confirmed. This flood risk information helps focus damage risk appraisals and ultimately inform optioneering for flood risk management within Callander. A summary of the key flood risk issues and comment is provided below:

Leny Road

The A84 Leny Road, adjacent to the junction of Leny Feus, is predicted to inundate for the 100 year return period.

Bridge Street

For extreme events (200 year and greater) flooding is predicted to spill across the southerly end of Bridge Street towards, and across, Castle Grove. This overland flow continues eastward following a topographical depression where it then follows the line of an existing drainage ditch and flows back into the River Teith.

For extreme events (somewhere between 50 and 100 year return period) flooding is predicted to spill across the northerly end of Bridge Street from west to east.

Meadows Car Park

Meadows car park is at least partially flooded for all return periods modelled. This flooding extends as far as Main Street for the 50 year event. For the 200 year event water is up to 1 m deep on Main Street. The on-set of property flooding is around 25 year return period.

Callander Primary School

The playing field areas adjacent to Callander Primary School are predicted to start inundating directly from the Teith for flood events somewhere between 10 and 25 year return period; the school buildings are above 200 year flood levels.

Callander - Left Bank of Teith Downstream Of Main Bridge

Flooding is predicted to encroach inland for increasing return periods along much of the left bank through Callander. Flooding is predicted to extend as far as Pearl Street, at the junction with South Church Street for the 200 year flood event.

Lagrannoch Industrial Estate

The industrial estate is largely free from flooding for most modelled events, other than for the most extreme events of 500 year return period and greater. However, the adjacent Sewage Treatment Works is lower lying and is predicted to be at risk of flooding for the 200 year event.


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7 Hydraulic Modelling - Loch System Routing

7.1 Modelling Rationale

This study explores flood defence options to reduce flooding in Callander and includes assessment of the potential for utilising flood storage in the upper reaches of the Leny and Eas Gobhain. Although Callander is the focal point of this study, the Council are keen to ascertain if upstream flood storage options and natural flood management could also serve to alleviate flooding further downstream in Stirling.

As concluded earlier in this report, modifications to the Katrine system on the upper reaches of the Eas Gobhain could potentially bring some benefits to flooding in Callander but would only be a partial solution and would be difficult to achieve in practice (bearing in mind this system is already heavily modified and attenuated). The Eas Gobhain flows are already subject to some flood management operational rules.

Extensive modelling of the Katrine system as part of this study was not considered an effective use of time and money and would not be likely to yield a significant benefit in terms of practical flood alleviation options to take forward. It was considered worthwhile however to undertake a more limited and targeted assessment of the effects of various weir heights on Venachar (to increase storage potential) but maintaining the current winter draw down level arrangements

It was considered that exploring (through hydraulic modelling) flood storage options on the Lubnaig / Voil system would be more practical and potentially more fruitful, considering this system has a similar catchment size to Eas Gobhain, yields higher flows and remains largely unmodified. The model needed to be of sufficient detail to capture the key controls and potential storages in the system, which also includes significant floodplain areas between Strathyre and Balquhidder.

7.2 Model Extents

Loch routing model extents were taken as far as Loch Voil / Loch Doine and Loch Venachar. The Leny routing model includes several water bodies; Loch Voil / Loch Doine, River Balvag, Loch Lubnaig and the River Leny. The Venachar routing model includes Loch Venachar and the Eas Gobhain. Both systems contribute to the flooding at Callander differently and for simplicity they are modelled separately. Both models terminate at the confluence of the Eas Gobhain and River Leny, just west of Callander. The proposed model extents equate to approximately 3 km for the Eas Gobhain and 22 km for the Leny.

The modelled reach extents are shown in Figure 26.


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Figure 26 – Routing Model Extents

7.3 Model Build (HEC-RAS)

The loch routing models have been developed using HEC-RAS Version 4, a widely used and freely available 1D river analysis system.

7.3.1 Model Build Scoping

Initial investigations (including site walkovers, examination of historical flooding and desk based GIS data analysis) identified the key hydraulic controls and the most appropriate modelling approach and the associated topographical survey and other data requirements. An unsteady (time varying) 1D model was required to accurately assess flood storage and routing along modelled reaches for a range of storm events. The models comprise loch storage areas, floodplain areas, river channels and various hydraulic controls / structures. The ultimate aim was to develop models that allow the assessment of numerous upstream storage option scenarios and assess how these options ultimately attenuate and reduce flows through Callander.

7.3.2 Topographical Survey

Numerous river cross-sections, hydraulic structures (bridges, weirs, etc.), embankments and floodplains were surveyed to facilitate construction of the hydraulic models. Cross sections were taken immediately upstream and downstream of river structures. 45 cross sections were surveyed to develop the Leny model. 8 cross sections were surveyed to develop the Venachar model. Model cross section


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details are shown in Appendix G. A number of interpolated cross sections were also used in the modelling.

7.3.3 Key Hydraulic Controls

A number of critical hydraulic controls were identified. These controls are fundamental to how the system currently operates and heavily influence the attenuation afforded through the loch systems. Care was taken during the scoping and surveys to ensure that sufficient detail at these critical locations was captured to ensure accurate representation in the models. Key locations included:

Stronvar Bridge

Strathyre Bridge

Balvag floodplain pinch points

Lubnaig loch „outfall‟ to Leny (high bed level which forms a natural weir)

Bridge at Creag Dhubh

Anie gauge river section

Venachar gauge river cross section

Venachar weir and spillway

Venachar gauge river section

Some of these key hydraulic control locations are shown below in the following photos.

Figure 27 – Stronvar Bridge

Figure 28 – Lubnaig Outlet


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Figure 29 – Bridge at Creag Dhubh

Figure 30 – Anie Gauge Location

Figure 31 – Venachar Weir and Spillway


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7.3.4 Supplementary Model Build Data

Other existing data were used to facilitate model build, including DTM data and loch bathymetric information (National Library of Scotland). This data was used to derive loch level / volume relationships for the various lochs represented in models. The level / volume relationship for Loch Venachar was obtained directly from Scottish Water.

7.3.5 Model Boundary Conditions

Flow Boundary

Flow boundary conditions comprise a flow hydrograph for all specified inflows. There are 25 specified inflow locations for Leny routing model corresponding to all 25 defined FEH defined catchments within the modelled reach.

There is a single hydrological inflow for the Venachar model which is simply applied into Loch Venachar in the model. Due to the simplicity of the Eas Gobhain model, the FEH rainfall-runoff method was used to derive a design hydrograph. Adjustments were then made to the SPR and Tp factors to ensure the model outflow roughly matched Atkins‟ Eas Gobhain design hydrograph for the Callander model.

Channel Roughness Boundary

Site inspection and photographs taken during the survey enabled estimation of channel / floodplain roughness to be made. Table 10 and Table 11 show the typical values used for each model.

Table 10 – Manning’s ‘n’ (Leny)

River Feature Manning’s ‘n’ Description

Main Channel 0.042 Clean, winding with some pools and shoals

Left Over-bank 0.050 Open fields with some scattered brush or field crops

Right Over-bank 0.050 Open fields with some scattered brush or field crops

Table 11 - Manning’s ‘n’ (Eas Gobhain)

River Feature Manning’s ‘n’ Description

Main Channel 0.030 Clean, straight with more stones

Left Over-bank 0.035 Open fields/pasture with short and high grass

Right Over-bank 0.035 Open fields/pasture with short and high grass

7.3.6 Upstream / Downstream Boundary Conditions

Loch Voil / Doine and Loch Venachar are modelled as storage areas in HEC-RAS. These storage areas are used as an upstream boundary condition for both models. The inflow hydrograph associated with the loch catchment area is applied directly to the storage areas. The downstream slope is used for normal depth calculation on both the River Leny and Eas Gobhain models.

7.4 Model Calibration / Verification

Using observed rainfall events, the corresponding runoff hydrographs were routed through the models and adjustments made to key model parameters including Manning‟s „n‟ friction values and the variables associated with hydrological calibration (within sensible bounds) to achieve the best correlation with observed data. The hydraulic model calibration was undertaken in conjunction with the hydrological calibration as described earlier in this report.

For model calibration, the aim is to achieve as close a correlation across the various storm events for the following key elements as possible:

Hydrograph shape – ensures the correct flood volumes are accounted for

Hydrograph timing – ensures the peak occurs at the right time


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Peak flow - ensures the magnitude of the routed flow is correct

It should be noted that there are numerous variables which can affect calibration success such as rainfall accuracy (spatial variation of rainfall and errors in readings), flow accuracy (particularly if gauges are unreliable for high flows), antecedent conditions (loch levels and catchment wetness), snow melt, debris blockage at structures, etc. Consequently, caution is required when calibrating to avoid force-fitting parameters for a particular event. Model calibration is important, particularly if the model is to be used for any optioneering scenarios involving storage.

7.4.1 Leny Calibration

Calibration of the Leny routing model was undertaken using observed data from the notable flood events in Callander in 1994, 1997, 2005, 2006, 2008 and 2009. Figure 32 to Figure 37 below shows the predicted and observed stages at the Anie gauge station on the Leny.

11 16 21 26 01 06 11 16 21 26Nov94 Dec94

120.5

121.0

121.5

122.0

122.5

123.0Plan: 1994 Var PR Tp 2 River: Leny Reach: Leny RS: 5408.8

Time

Stag

e (m

)

Legend

Predicted

Observed

Figure 32 – Predicted / observed stage at Anie gauge station for 1994 flood event

01 06 11 16 21 26 03 08 13 18Feb97 Mar97

120.5

121.0

121.5

122.0

122.5

123.0Plan: 1997 Var PR TP 2 River: Leny Reach: Leny RS: 5408.8

Time

Stag

e (m

)

Legend

Predicted

Observed

Figure 33 - Predicted / Observed Stage at Anie Gauge Station for 1997 Flood Event


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07 12 17 22 27 01 06 11 16 21Dec2004 Jan2005

120.5

121.0

121.5

122.0

122.5

123.0


Time

Stag

e (m

)

Legend

Predicted

Observed

Figure 34 - Predicted / observed stage at Anie gauge station for 2005 flood event

14 19 24 29 04 09 14 19 24 29Nov2006 Dec2006

120.5

121.0

121.5

122.0

122.5


Time

Stag

e (m

)

Legend

Predicted

Observed


01 06 11 16 21 26 31 05 10 15Jan2008 Feb2008

120.5

121.0

121.5

122.0

122.5

123.0Plan: 2008 Var Pr Tp 2 River: Leny Reach: Leny RS: 5408.8

Time

Stag

e (m

)

Legend

Predicted

Observed



© Mouchel 2013 54

10 15 20 25 30 05 10Nov2009 Dec2009

120.8

121.0

121.2

121.4

121.6

121.8

122.0

122.2

Plan: 2009 Var PR Tp 2 River: Leny Reach: Leny RS: 5408.8

Time

Stag

e (m

)

Legend

Predicted

Observed


There is a reasonable correlation between observed vs. predicted, particularly across the numerous peaks of the entire period of the various observed events. There is a general over prediction noted for the maximum peak, although for the 2005, an under prediction is noted. The discrepancy present in the 2006 chart is due to rainfall gauging instrument failure, as confirmed by SEPA.

Figure 38 below shows the predicted (modelled) and observed (SEPA) ratings curves for the Anie gauge location. There is reasonable correlation.

0 50 100 150 200

121.0

121.5

122.0

122.5

123.0

Plan: 2005 Var PR Tp 2 River: Leny Reach: Leny RS: 5408.8

Flow(m3/s)

Sta

ge (

m)

Legend

Predicted

Observed

Figure 38 – Predicted / observed ratings curve (Anie gauge station)

7.4.2 Loch Venachar Flow Gauge Calibration

The flow gauge at Venachar was built to monitor compensation flows from the loch. Although the rating for high flows has reportedly been improved over recent years, it is still considered relatively unreliable for high flows as the station doesn‟t have a gauging facility for high flows. A theoretical rating is used.

Figure 39 shows the predicted (modelled) rating curve vs. SEPA‟s AMAX data for the Loch Venachar gauge location. A reasonable correlation is achieved for the lower flows (low return periods). However, a poorer correlation is evident for higher flows. There is an outlier noted on the graph which corresponds to the 1993 flood event. It is assumed that this value has not been updated to the current rating.


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These issues may merit further investigation as it has implications for the reliability of the Leny Eas / Gobhain % flow split assessment explored earlier. If the Venachar gauge is actually under-predicting flows, this could mean that the Eas Gobhain / Leny flow spilt could be more equal. The design flow estimates for Callander will not be affected as these estimates were based on the Bridge of Teith and Callander gauges, however the hydraulic model calibration and upstream option effectiveness would be impacted upon.

79

79.5

80

80.5

81

0.00 20.00 40.00 60.00 80.00 100.00 120.00 140.00 160.00

(m3/s)

(m A

OD

)

Predicted

Observed

Figure 39 – Predicted / Observed Ratings Curve (Loch Venachar Gauge Station)

7.5 Model Sensitivity Analyses

The hydraulic routing models have been assessed in terms of parameter sensitivity (flow, roughness and initial loch levels). Details of the sensitivity analysis can be found in Appendix H. The sensitivity analysis is summarised as follows.

7.5.1 Model Inflow Sensitivity Analysis

The models were tested to determine the response / stability of the models for a wide range of flows and identify any parts of the model that were sensitive to flow variation. There was a reasonably stable and uniform response for both models to flow variation over the modelled reaches.

7.5.2 Model Roughness Sensitivity Analysis

The models were tested by varying the roughness conditions (Manning‟s „n‟) by +/- 20 % to assess model stability / sensitivity. The models showed a uniform and stable variation in water levels with changes in channel friction.

7.5.3 Initial Loch Levels Sensitivity Analysis

The sensitivity of the models was checked against the initial loch water level conditions at Loch Voil and Loch Venachar. This was to check the sensitivity to the starting conditions of the model.

For extreme events it can be seen that varying the initial water level at Loch Voil does not noticeably affect the peak flows further downstream. This sensitivity increases though with lower storm return periods and shorter durations, as would be expected. Similar conclusions were reached with the Venachar model.


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8 Option Appraisal – Teith (Callander)

8.1 Options Overview

Flood alleviation options have been assessed for the following „at risk‟ locations (see section 6.4.3 - Key Flood Risk Issues):

Meadows Car Park

Bridgend West

Bridgend East

Bridge Street to Buchanan Place

These feasibility assessments are preliminary in nature and factors such as ground conditions, landowner issues, services and other technical constraints would need to be considered further at the detailed design stage.

Impact assessments have also been undertaken to assess any water level changes elsewhere as a result of implementation.

Consideration has been given to the use of both permanent and demountable options at the rear of Meadows Car Park to protect Main Street.

Options which involve Natural Flood Management (NFM) are explored later in this report.

8.2 Meadows Car Park FAS

8.2.1 Technical Feasibility

A number of options were considered for protecting the properties to the rear of Meadows Car Park. The following summarises the key findings.

A riverside floodwall protecting the entire car park area and adjacent properties on Main Street for the 200 year flood event (plus climate change) would require a wall over 3.5m high along much of its length. The river front area is a major amenity for the town and attracts many visitors. Any flood-walling which effectively cuts this amenity off, particularly from a visual perspective, would not be an acceptable solution as it would blight this valuable amenity and also result in an associated loss of visitor numbers and business for the town.

To reduce effective wall height as far as practically possible and the resulting visual intrusion, flood protection walls should be set as far back into the car park as possible.

Even when set back, designing to 200 year levels (plus climate change) is not considered practical due to excessive wall heights (well over head height). It would also be difficult to accommodate the necessary accesses / ramping through and over the walls if designing to such levels. It should be noted that flooding emanating from downstream of Bridge Street is predicted for the 200 year (plus climate change) event even if a wall at Meadows Car Park was in place.

Protecting to 50 year levels is considered a more feasible and practical level of protection that wouldn‟t totally cut off the river amenity. It ties better into prevailing ground levels and the general configuration of the car park and surrounding areas. 50 year protection roughly coincides with the crest of the existing vehicular exit route of Meadows Car Park (around 70.3m AOD). A maximum wall height of around 1.6 to 1.8m high including 300mm freeboard would be more feasible. Some localised raising of ground levels could also serve to reduce effective wall heights. Glass panels could help mitigate any residual visual impacts. Walling (stone clad concrete


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or sheet pile depending on ground conditions) with sliding floodgates (at the entrance to the car park and at the rear of the bakery) would be the most sensible permanent solution. During times of flood, flood gates would be closed which would restrict access to the car park. Egress from the car park would not be hindered by any floodwalls or gates. The general layout of the 50 year floodwall option is shown in Figure 40. Figure 41 illustrates a typical flood gate, although a sliding version could be proposed for Callander to minimise space requirements.

Tying the floodwall into „Tom ma Chisaig‟ will require consultation with Historic Scotland. This mound is a scheduled historic monument (possibly a „motte‟ and thought to have been erected in memory of St Kessaig). To avoid any impact to this monument, the floodwall may need extended around the monument tying in further east as illustrated in Figure 40.


Figure 40 - Meadows Car Park FAS - 50 year protection


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Figure 41 - Flood Gate Illustration

A number of other considerations are summarised below:

Floodwall schemes typically require a back drainage system to allow existing runoff and drainage emanating from behind the floodwall to continue to be discharged to the river during times of flood. A back drainage system would typically require a pumping station to be constructed as part of the scheme. There is a possible location for a pumping station in a disused patch of ground adjacent to the Meadows Car Park to the rear of 4 – 8 Main Street. This would allow backed up drainage to be pumped over the floodwall in times of spate when any flap valve discharges are closed.

If upstream elevations allow, it may be possible to intercept and „seal‟ some existing surface water culverts and route them straight through the floodwall, negating the need for flap valves or pumping as the available hydraulic head would be higher than the elevation of river levels to which the culverts discharge.

There is currently a Scottish Water pumping station situated in the grounds of Meadows Car Park. Scottish Water will need to be involved in the development of any flood scheme at the car park to ensure that the existing sewerage infrastructure does not compromise any flood defence scheme, and vice versa. The configuration of the Scottish Water pumping station would need to ensure that there was no possibility of river waters flowing into the pumping station and up through incoming sewer pipes. Appropriate non-return features will be required, if not already in place. A number of sewer pipes will also require to be crossed as the line of the floodwall would pass through them.

Any other utilities encountered will require to be dealt with as appropriate.

A 300mm freeboard would be proposed accounting for good model calibration data and model accuracy. SEPA were consulted and agreed that this should be an acceptable proposal.

Consideration needs to be given to the scenario where the floodwall is overtopped. For return periods exceeding the design height, floodwaters may rapidly inundate behind the floodwall once overtopped. An action plan would be required to manage a safe and timely withdrawal of public and vehicles from behind the floodwall in advance of this possible scenario. Measures will also be required which allow floodwaters to be routed safely back to the river during the


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flood ebb phase. Pumping (possibly utilising the back drainage pumping station) may form part of these arrangements.

It is not considered practical to provide permanent protection for flood events of 200 year return period. It could however be technically possible to construct a permanent scheme (say, to 50 year) and allow for additional demountable elements that could offer protection for higher return periods and lessen the day-to-day intrusion of an equivalent permanent scheme. However, there would be significant technical constraints, not least economic viability.

There is considered to be some potential to increase the standard of protection to greater than 50 year if any upstream flood storage options are ever implemented in conjunction with direct flood wall defences in Callander.

Although the 50 year flood wall option is technically feasible and the most practical, economic viability is going to be difficult as the onset of property flooding is only starting between 10 and 25 year return periods.

8.2.2 Impacts

Impacts have been assessed for the 50 year floodwall option at Meadows Car Park. This option does not result in any significant water level impacts elsewhere.

8.3 Bridgend West FAS


To protect the properties to the west of Bridge Street, an earthen flood embankment was considered to be the most practical option. The general layout is shown in Figure 42. A number of key points are summarised below.

Earthen embankments, typically clay core, and between 1:2 (vertical to horizontal) and 1:3 side slopes protecting to 200 year (plus climate change).

Concrete or sheet pile (dependant on ground conditions) floodwall section required at north of Bridgend which connects the existing bridge and the earth embankment.

Overland flow across Bridgend Road (predicted between 100 and 200 year return period) would be stopped.

8.3.2 Impacts

Significant impacts (over 100mm everywhere upstream of Bridge Street) occur for the 200 year (plus climate change) event. The reason for the impact is considered to be the fact that the Bridgend West FAS embankment now blocks flows across Bridgend Road. Implementing a scheme with such impacts over at Meadows Car Park would not be feasible without mitigation.


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Figure 42 - Bridgend West FAS - 200 Year + CC Protection

8.4 Bridgend East FAS


To protect the properties near the school to the east of Bridge Street, an earthen flood embankment was considered to be the most practical option. The general layout is shown in Figure 43. A number of key points are summarised below.

Earthen embankments, typically clay core, and between 1:2 (vertical to horizontal) and 1:3 side slopes protecting to 200 year (plus climate change).

Localised re-profiling works comprising pedestrian ramps are required where the embankment ties into the existing footpath.

Access to the open school yard area immediately to the north of the embankment needs to be maintained.

Existing entrances to the school need to be maintained.

≈ 5% slopes required for pedestrian ramping.


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8.4.2 Impacts

No significant impacts are anticipated from the implementation of this embankment as the large floodplain area occupying the school‟s playing field is retained. This embankment option is included within the impact assessment for more extensive flood wall options on the left bank between Bridge Street and Buchanan Place.


Figure 43 - Bridgend East FAS - 200 year + CC Protection

8.5 Bridge Street to Buchanan Place FAS


To protect the properties on the left bank of the Teith, downstream of Bridge Street, flood walls / embankments were considered. The general layout is shown in Figure 44 and Figure 45. A number of key points are summarised below.

Earthen embankments, typically clay core, and between 1:2 (vertical to horizontal) and 1:3 side slopes protecting to 200 year (plus climate change). Embankments are only shown to illustrate likely footprints. Two embankment positions are shown (one at river and one set back). Due to space and access restrictions, it will be likely more practical for sheet pile / concrete option, depending on ground conditions.


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If implementing the Bridge Street to Buchanan Place FAS, it should be noted that flows emanating from the Meadows Car Park side are predicted to flow across Bridge Street for flood events of 100 year and greater return period. These flows would then become trapped behind the Bridge Street to Buchanan Place floodwall. This problem would need addressed by curtailing the Bridge Street to Buchanan Place floodwall at some point a few metres downstream of the bridge and tying back to higher ground leaving a potential safe flow route back to the river for these flows, just downstream of the bridge. The details of any overland flow cut-off arrangement would be determined at detailed design stage.

Ancillary works are required at the footbridge tie-in location to accommodate FAS design heights, pedestrian access, maintenance access, etc. Ideally any FAS works would be designed and undertaken in conjunction with the proposed new pedestrian footbridge.

≈ 5% slopes required for pedestrian ramping.

Back drainage system will be required (with pumping station).

A number of other considerations are summarised below:

If upstream elevations allow, it may be possible to intercept and „seal‟ some existing surface water culverts and route them straight through the floodwall.

In addition to two known surface water pipes discharging to the river within this floodwall reach there are also two large Scottish Water CSO discharge pipes noted just downstream of the police station. These will need accommodated in any floodwall designs, particularly to ensure that appropriate non-return features are incorporated into the designs.

Any other utilities encountered will require to be dealt with as appropriate.

A 300mm freeboard would be proposed accounting for good model calibration data and model accuracy.

As per the floodwall scheme at Meadows Car Park scheme, consideration of scenarios where the floodwall is overtopped for return periods exceeding the design height need to be considered (exceeding 200 year + climate change in this case). An action plan would be required to manage a safe and timely withdrawal of public and vehicles from behind the floodwall in advance of this possible scenario. Measures will also be required which allow floodwaters to be routed safely back to the river during the flood ebb phase. Pumping (possibly utilising the back drainage pumping station) may form part of these arrangements.

8.5.2 Impacts

Impacts have been assessed for the Bridge Street to Buchanan Place FAS option. For the 200 year (plus climate change) event there are impacts noted upstream of the road bridge of around 40mm at Bridgend and Meadows Car Park. These impacts would be unacceptable without mitigation.


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Figure 44 - Bridge Street to Buchanan Place FAS - 200 year + CC protection (upstream)


Figure 45 - Bridge Street to Buchanan Place FAS - 200 year + CC protection (downstream)


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9 Option Appraisal - Upstream Storage

9.1 Leny / Lubnaig / Balvag / Voil Optioneering Modelling

9.1.1 Option Selection

Modelling was undertaken to evaluate the scope for using upstream storage to reduce peak flows in the Leny and alleviate flooding in Callander (and potentially further downstream).

To achieve this, some possible storage scenarios and locations were investigated. The following potential storage locations were considered:

Flow control at outlet of Loch Lubnaig (≈ 170m upstream of Anie gauge), thus utilising additional storage potential of Loch Lubnaig.

Flow control on north side of Strathyre (where floodplain narrows ≈ 620m upstream of Strathyre Bridge), thus potentially utilising some of the extensive floodplain areas between Strathyre and Balquhidder.

Flow control at outlet of Loch Voil (just upstream of Stronvar Bridge), thus utilising additional storage potential of Loch Voil / Loch Doine.

Combinations of above

Options would typically involve an online restriction comprising a reduction of main river channel width and an accompanying floodplain embankment structure. This arrangement would serve to store peak floodwaters upstream. The channel bed and banks would remain unchanged as far as possible. The structure would be simple, self-controlling and relatively maintenance free, with no moving parts or M&E elements. It was not considered practical to explore more complex structures and devices.

It is important to avoid any flow storage for the lower end of the flood flow spectrum to ensure that storage is mobilised when most needed (i.e. at the most critical time of the flood wave). This minimises the storage volume requirements as much as possible. The flow control restriction would ideally be implemented at a floodplain pinch point where the linear extents of any perpendicular embanked impounding structure would be minimised.

Figure 46 below shows the locations chosen to test flow restrictions in the model. Figure 47 illustrates the same locations on the longitudinal profile of the model (exported from HEC-RAS). It can be seen that downstream of Loch Lubnaig, the river gradient is relatively steep (at around 1 in 100), compared to the upper reaches. The channel bed elevation at the outlet of Loch Voil is around 124.2 mAOD. The channel bed elevation at the outlet of Loch Lubnaig is around 120.3 mAOD. This gives an average gradient between these two locations of around 1 in 4300. Although this relatively flat gradient is advantageous when looking to implement large volumes of floodplain storage, the implications regarding upstream impact extents may also be significant.


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Figure 46 – Flow Control Structure Locations

0 5000 10000 15000 20000 2500060

70

80

90

100

110

120

130

Main Channel Distance (m)

Ele

va

tio

n (

m)

Legend

WS Max WS

Ground

Leny Leny

Figure 47 – Leny HEC RAS Model – Longitudinal Profile (200 year event)

Numerous option scenarios were tested for a range of design return period storm events. Although the design flows for Callander were reviewed and updated earlier in this report, the downstream flows in the Leny routing model have been „calibrated‟ to

Anie Gauge

Strathyre Bridge

Stronvar Bridge


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match the design flows used in the previous InfoWorks RS model, as per Atkins 2010 Callander report. For the purposes of this optioneering, it was not considered essential to update these peak flow values as the magnitudes were sufficiently accurate to allow for option performance testing. Option performance is assessed in terms of resulting increases or decreases in flow and levels.

The six observed events were also tested. However, it should be noted that there are some notable differences in the observed and predicted peak levels and flows for these scenarios (as shown earlier in the model calibration section). This is a consequence of calibrating across a range of events rather than for a single event. However, general option performance is still able to be assessed.

Modelling was limited to a number of key restriction / storage scenarios to keep the modelling within a practical working scope. The key upstream restriction / storage option scenarios modelled and presented are as follows:

1. 20m wide restriction @ Voil 2. 15m wide restriction @ Voil 3. 10m wide restriction @ Voil 4. 5m wide restriction @ Voil 5. 15m wide restriction @ Voil (with soffit @ 127.30) 6. 10m wide restriction @ Voil (with soffit @ 127.53) 7. 5m wide restriction @ Voil (with soffit @ 128.00) 8. 10m wide restriction @ Voil & 15m @ Strathyre 9. 10m wide restriction @ Voil & 10m @ Strathyre 10. 5m wide restriction @ Voil & 10m @ Strathyre 11. 15m wide restriction @ Strathyre 12. 10m wide restriction @ Strathyre 13. 10m wide restriction @ Lubnaig 14. 5m wide restriction @ Lubnaig 15. 10m wide restriction @ Voil & 15m @ Strathyre & 15m @ Lubnaig 16. 10m wide restriction @ Voil & 15m @ Strathyre & 10m @ Lubnaig 17. 10m wide restriction @ Voil & 15m @ Strathyre & 7.5m @ Lubnaig 18. 4m wide restriction @ Voil & 9.5m @ Strathyre & 13m @ Lubnaig

9.1.2 Model Output and Findings

Summarised results from the above scenarios (compared to the existing baseline without the flood storage option) are shown in Table 12 and Table 13 below.


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Table 12 – Flood Storage Optioneering – Resulting % Flow Change in Leny

Design Flood Event

Downstream flow in Leny without option

(m3/s)

20m @

Voil

15m @

Voil

10m @

Voil

5m @ Voil

15m @ Voil (with soffit @

127.30 m AOD)


127.53 m AOD)

5m @ Voil (with soffit @ 128.00m

AOD)

10m @ Voil & 15m @

Strathyre

10m @ Voil & 10m @

Strathyre

Annual 101.9 -2.9 -7.4 -14.5 -25.2 -7.1 -13.7 -25.4 -15.1 -17.0

5 Yr 133.2 -4.2 -8.3 -17.0 -27.4 -9.5 -16.3 -28.2 -17.9 -20.6

10 Yr 153.7 -4.7 -8.7 -18.1 -29.2 -10.7 -17.6 -30.3 -19.2 -22.3

25 Yr 183.4 -6.4 -10.7 -20.1 -31.2 -13.5 -20.3 -32.4 -21.6 -25.8

50 Yr 207.7 -7.3 -11.6 -21.1 -32.1 -15.5 -22.2 -33.7 -22.9 -27.6

100 Yr 230.4 -7.8 -11.9 -21.6 -32.9 -16.8 -23.4 -34.7 -23.7 -28.9

200 Yr 257.9 -7.9 -11.9 -21.8 -33.5 -17.5 -24.5 -35.7 -24.5 -29.9

1994 118.3 -2.2 -5.0 -9.6 -13.9 -5.5 -9.0 -14.3 -10.4 -12.7

1997 121.8 -3.2 -6.3 -12.1 -14.8 -8.2 -11.4 -14.8 -13.2 -17.0

2005 204.5 -5.4 -7.9 -13.5 -18.0 -9.5 -13.1 -18.7 -17.0 -23.4

2006 90.1 -1.7 -3.4 -7.5 -9.9 -4.0 -6.4 -10.0 -7.8 -8.7

2008 81.2 -2.0 -4.6 -11.2 -16.7 -5.4 -9.9 -16.7 -11.3 -11.9

2009 92.8 -3.0 -6.3 -11.2 -15.1 -5.9 -10.3 -15.1 -12.0 -14.0

Table 12 – continuation….

Design Flood Event

Downstream flow in Leny without

option (m3/s)

5m @ Voil & 10m @ Strathyre

15m @

Strathyre

10m @

Strathyre

10m @

Lubnaig

5m @

Lubnaig

10m @ Voil & 15m @

Strathyre & 15m @

Lubnaig

10m @ Voil & 15m @

Strathyre & 10m @

Lubnaig

10m @ Voil & 15m @

Strathyre & 7.5m @ Lubnaig

4m @ Voil & 9.5m @

Strathyre & 13m @

Lubnaig

Annual 101.9 -26.9 -0.9 -4.3 -16.0 -31.4 -19.7 -26.6 -31.4 -35.5

5 Yr 133.2 -30.3 -1.7 -7.1 -18.6 -36.1 -23.1 -30.2 -35.3 -39.3

10 Yr 153.7 -32.3 -2.3 -8.4 -20.4 -38.7 -25.2 -32.1 -37.4 -41.7

25 Yr 183.4 -35.3 -3.4 -11.5 -22.9 -42.4 -28.4 -35.2 -40.5 -44.6

50 Yr 207.7 -37.8 -4.5 -13.4 -24.4 -44.6 -30.2 -37.0 -42.4 -46.6

100 Yr 230.4 -39.3 -5.0 -15.0 -25.6 -46.3 -31.4 -38.2 -43.7 -48.1

200 Yr 257.9 -40.8 -6.3 -16.8 -26.8 -48.0 -32.5 -39.2 -45.0 -49.6

1994 118.3 -16.8 -0.9 -5.3 -14.1 -26.8 -15.1 -20.8 -24.4 -24.4

1997 121.8 -18.6 -2.1 -7.6 -18.9 -29.6 -21.2 -24.7 -27.1 -27.6

2005 204.5 -26.1 -6.2 -13.9 -22.0 -38.7 -25.7 -30.1 -34.4 -35.3

2006 90.1 -12.3 -0.3 -2.0 -9.1 -19.8 -10.2 -14.3 -17.6 -17.4

2008 81.2 -17.9 -0.2 -1.4 -11.8 -27.0 -13.7 -20.0 -25.1 -23.5

2009 92.8 -15.8 -1.2 -3.7 -15.8 -24.0 -16.7 -20.3 -22.4 -20.2


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Table 13 – Flood Storage Optioneering – Water Level Change (+/- m)

Design Flood Event

Water Levels Without Option

(m AOD)

20m @

Voil

15m @

Voil

10m @

Voil

5m @ Voil


127.30 m AOD)


127.53 m AOD)


AOD)

10m @ Voil & 15m @

Strathyre

10m @ Voil & 10m @

Strathyre

Annual - V 127.35 0.05 0.26 0.55 1.19 0.26 0.56 1.20 0.53 0.53

Annual - S 125.38 -0.06 -0.13 -0.25 -0.48 -0.12 -0.25 -0.49 -0.20 0.00

Annual - L 122.82 -0.04 -0.09 -0.17 -0.32 -0.09 -0.17 -0.33 -0.19 -0.21

5Y - V 127.61 0.10 0.34 0.72 1.48 0.38 0.75 1.52 0.71 0.71

5Y - S 125.75 -0.08 -0.14 -0.28 -0.54 -0.18 -0.30 -0.56 -0.20 0.11

5Y - L 123.15 -0.05 -0.11 -0.21 -0.40 -0.13 -0.22 -0.42 -0.24 -0.29

10Y - V 127.75 0.15 0.40 0.83 1.66 0.46 0.87 1.72 0.83 0.83

10Y - S 125.94 -0.09 -0.15 -0.30 -0.56 -0.19 -0.32 -0.59 -0.18 0.20

10Y - L 123.33 -0.06 -0.12 -0.23 -0.43 -0.15 -0.25 -0.45 -0.27 -0.33

25Y - V 127.92 0.20 0.49 0.97 1.90 0.59 1.05 1.99 0.99 0.99

25Y - S 126.19 -0.12 -0.20 -0.35 -0.62 -0.26 -0.39 -0.65 -0.18 0.29

25Y - L 123.56 -0.09 -0.15 -0.27 -0.49 -0.19 -0.30 -0.51 -0.32 -0.39

50Y - V 128.02 0.28 0.59 1.12 2.07 0.74 1.25 2.17 1.15 1.15

50Y - S 126.36 -0.14 -0.22 -0.37 -0.65 -0.30 -0.43 -0.69 -0.15 0.40

50Y - L 123.73 -0.11 -0.17 -0.29 -0.52 -0.23 -0.34 -0.56 -0.35 -0.43

100Y - V 128.13 0.34 0.67 1.24 2.19 0.88 1.43 2.30 1.29 1.29

100Y - S 126.51 -0.16 -0.23 -0.38 -0.68 -0.34 -0.47 -0.73 -0.11 0.51

100Y - L 123.87 -0.11 -0.17 -0.3 -0.55 -0.26 -0.37 -0.59 -0.37 -0.46

200Y - V 128.27 0.40 0.76 1.37 2.30 1.03 1.64 2.46 1.44 1.44

200Y - S 126.67 -0.16 -0.23 -0.39 -0.71 -0.36 -0.51 -0.77 -0.05 0.66

200Y - L 124.04 -0.13 -0.19 -0.32 -0.59 -0.29 -0.41 -0.64 -0.40 -0.50

1994 - V 127.36 0.06 0.29 0.65 1.55 0.29 0.66 1.59 0.63 0.63

1994 - S 125.52 -0.04 -0.08 -0.16 -0.27 -0.09 -0.16 -0.28 -0.08 0.19

1994 - L 122.97 -0.02 -0.04 -0.10 -0.18 -0.05 -0.10 -0.19 -0.13 -0.16

1997 - V 127.49 0.07 0.27 0.59 1.38 0.29 0.60 1.41 0.55 0.55

1997 - S 125.71 -0.07 -0.14 -0.25 -0.36 -0.16 -0.25 -0.37 -0.17 0.10

1997 - L 123.05 -0.06 -0.10 -0.16 -0.23 -0.10 -0.17 -0.24 -0.20 -0.25

2005 - V 128.05 0.21 0.50 1.00 2.03 0.60 1.08 2.09 1.01 1.01

2005 - S 126.45 -0.12 -0.18 -0.29 -0.44 -0.23 -0.32 -0.47 -0.03 0.47

2005 - L 123.73 -0.08 -0.13 -0.20 -0.31 -0.16 -0.22 -0.33 -0.28 -0.41

2006 - V 127.26 0.02 0.20 0.48 1.26 0.20 0.49 1.27 0.44 0.44

2006 - S 125.29 -0.03 -0.06 -0.11 -0.18 -0.07 -0.11 -0.18 -0.05 0.11

2006 - L 122.67 -0.02 -0.03 -0.06 -0.10 -0.04 -0.06 -0.10 -0.07 -0.10

2008 - V 127.07 0.01 0.17 0.41 1.01 0.19 0.42 1.03 0.35 0.35

2008 - S 125.02 -0.03 -0.08 -0.18 -0.33 -0.10 -0.18 -0.33 -0.14 -0.01


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Design Flood Event

Water Levels Without Option

(m AOD)

20m @

Voil

15m @

Voil

10m @

Voil

5m @ Voil


127.30 m AOD)


127.53 m AOD)


AOD)

10m @ Voil & 15m @

Strathyre

10m @ Voil & 10m @

Strathyre

2008 - L 122.55 -0.02 -0.04 -0.10 -0.19 -0.05 -0.10 -0.19 -0.12 -0.13

2009 - V 127.21 0.02 0.20 0.45 1.20 0.20 0.45 1.21 0.39 0.39

2009 - S 125.28 -0.05 -0.12 -0.21 -0.37 -0.11 -0.22 -0.37 -0.18 -0.01

2009 - L 122.72 -0.04 -0.08 -0.13 -0.21 -0.07 -0.13 -0.21 -0.15 -0.18

Table 13 – continuation…..

Design Flood Event

Water Levels

Without Option

(m AOD)

5m @ Voil &

10m @ Strathyre

15m @

Strathyre

10m @

Strathyre

10m @

Lubnaig

5m @

Lubnaig

10m @ Voil & 15m @

Strathyre & 15m @ Lubnaig

10m @ Voil & 15m @


10m @ Voil & 15m @


4m @ Voil & 9.5m @


Annual - V 127.35 1.20 0.00 0.00 0.00 0.00 0.53 0.53 0.53 1.42

Annual - S 125.38 -0.30 0.10 0.37 0.00 0.07 -0.19 -0.19 -0.18 -0.36

Annual - L 122.82 -0.35 -0.01 -0.05 0.84 2.08 0.01 0.56 1.02 -0.16

5Y - V 127.61 1.52 0.00 0.00 0.00 0.00 0.71 0.71 0.71 1.75

5Y - S 125.75 -0.29 0.17 0.59 0.00 0.11 -0.20 -0.20 -0.18 -0.32

5Y - L 123.15 -0.45 -0.02 -0.09 1.07 2.39 0.05 0.70 1.21 -0.15

10Y - V 127.75 1.72 0.00 0.00 0.00 0.00 0.83 0.83 0.83 1.96

10Y - S 125.94 -0.37 0.24 0.74 0.01 0.19 -0.18 -0.18 -0.16 -0.28

10Y - L 123.33 -0.50 -0.03 -0.11 1.22 2.57 0.10 0.80 1.33 -0.13

25Y - V 127.92 1.99 0.00 0.00 0.00 0.00 0.99 0.99 0.99 2.17

25Y - S 126.19 -0.22 0.35 0.91 0.02 0.31 -0.18 -0.17 -0.15 -0.23

25Y - L 123.56 -0.58 -0.04 -0.16 1.40 2.78 0.15 0.91 1.48 -0.12

50Y - V 128.02 2.17 0.00 0.04 -0.01 0.00 1.15 1.15 1.15 2.33

50Y - S 126.36 -0.18 0.44 1.07 0.02 0.45 -0.15 -0.14 -0.11 -0.18

50Y - L 123.73 -0.63 -0.06 -0.19 1.56 2.95 0.20 1.01 1.61 -0.11

100Y - V 128.13 2.30 0.00 0.04 0.00 0.00 1.29 1.29 1.29 2.45

100Y - S 126.51 -0.15 0.53 1.20 0.02 0.58 -0.11 -0.10 -0.07 -0.14

100Y - L 123.87 -0.67 -0.07 -0.22 1.72 3.12 0.27 1.13 1.74 -0.08

200Y - V 128.27 2.46 0.01 0.07 0.00 0.00 1.44 1.44 1.44 2.59

200Y - S 126.67 -0.11 0.62 1.34 0.05 0.76 -0.05 -0.03 0.00 -0.07

200Y - L 124.04 -0.74 -0.10 -0.27 1.89 3.31 0.35 1.26 1.90 -0.06

1994 - V 127.36 1.59 0.00 0.00 0.00 0.00 0.63 0.63 0.63 1.94

1994 - S 125.52 0.00 0.13 0.46 0.01 0.27 -0.08 -0.07 -0.03 0.03


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Design Flood Event

Water Levels

Without Option

(m AOD)

5m @ Voil &

10m @ Strathyre

15m @

Strathyre

10m @

Strathyre

10m @

Lubnaig

5m @

Lubnaig

10m @ Voil & 15m @


10m @ Voil & 15m @


10m @ Voil & 15m @


4m @ Voil & 9.5m @


1994 - L 122.97 -0.22 -0.01 -0.04 1.06 2.52 0.17 0.85 1.41 0.18

1997 - V 127.49 1.41 0.00 0.00 -0.01 0.00 0.55 0.55 0.55 1.75

1997 - S 125.71 -0.09 0.15 0.51 -0.01 0.09 -0.17 -0.17 -0.16 -0.06

1997 - L 123.05 -0.29 -0.03 -0.10 0.91 2.41 0.04 0.71 1.31 0.06

2005 - V 128.05 2.09 -0.01 0.01 0.00 -0.01 1.01 1.01 1.01 2.33

2005 - S 126.45 0.15 0.45 1.03 0.03 0.48 -0.03 -0.02 0.01 0.20

2005 - L 123.73 -0.47 -0.09 -0.24 1.55 3.09 0.26 1.13 1.82 0.18

2006 - V 127.26 1.27 0.00 0.00 0.00 0.00 0.44 0.44 0.44 1.60

2006 - S 125.29 0.01 0.08 0.28 0.01 0.16 -0.05 -0.05 -0.03 0.03

2006 - L 122.67 -0.13 -0.01 -0.04 0.84 2.22 0.10 0.71 1.22 0.24

2008 - V 127.07 1.03 0.00 0.00 0.00 0.00 0.35 0.35 0.35 1.30

2008 - S 125.02 -0.18 0.07 0.24 0.01 0.06 -0.14 -0.14 -0.13 -0.19

2008 - L 122.55 -0.20 0.00 -0.01 0.72 1.86 0.02 0.54 0.95 0.00

2009 - V 127.21 1.21 0.00 0.00 0.00 0.00 0.39 0.39 0.39 1.59

2009 - S 125.28 -0.18 0.08 0.32 0.00 0.04 -0.18 -0.17 -0.17 -0.19

2009 - L 122.72 -0.23 -0.01 -0.05 0.71 2.11 -0.01 0.58 1.10 0.07

V – Water Level on Voil (upstream of Stronvar Bridge)

S – Water Level at Strathyre (620m upstream of Strathyre Bridge)

L – Water Level at outlet of Loch Lubnaig (170m upstream of Anie gauge),


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The most effective and practical options in terms of overall flow reductions achieved at the downstream end of the Leny are those which involve combinations of upstream controls. Additional controls downstream of Loch Voil serve to supplement and enhance storage on the Voil.

No further storage / attenuation options were explored beyond those achieving a nominal 50% flow reduction in the Leny. This was considered a practical limit for optioneering purposes. Any further restrictions, although hydraulically feasible, would be clearly impractical in terms of the associated increase in extreme water levels. However, any further scenarios, including further flow control restrictions, can be tested upon request.

Soffit-based controls were also tested. Soffit levels were set roughly at annual water level for optioneering modelling purposes (i.e. only when water levels reach Annual return period does the channel restriction become an orifice). These soffit options didn‟t appear to offer significant advantages for the scenarios tested. This is assumed to be due to the relatively low heads behind the structures. Soffit controls would also be more expensive to construct and maintain, and would be more prone to potential blockage.

The time lag in the Leny peak flows at Callander resulting from the implementation of upstream storage options was also explored. It can be seen that options implemented on the Voil do not cause a significant lag. This is thought to be due to the fact that the Voil catchment constitutes around half the whole Leny catchment, therefore, the downstream catchment is the main factor determining the time of the peak (although the peak flow is lower). There is a greater lag noted when options further downstream at Strathyre or Lubnaig are implemented. The % flow reduction and resulting time lag for the various options are summarised in Table 14 below. These results are for the 200 year design event.

Table 14 – Storage Option Lag / Flow Reduction (200 Year)

Modelled Storage Option % Flow Reduction in

Leny (200 year)

Leny Peak Flow Lag at Callander

(hrs)

20m @ Voil -7.9 0-2

15m @ Voil -11.9 0-2

10m @ Voil -21.8 0-2

5m @ Voil -33.5 0-2

15m @ Voil (with soffit @ 127.30) -17.5 0-2



10m @ Voil & 15m @ Strathyre -24.5 3



15m @ Strathyre -6.3 2

10m @ Strathyre -16.8 3

10m @ Lubnaig -26.8 7

5m @ Lubnaig -48.0 10

10m @ Voil & 15m @ Strathyre & 15m @ Lubnaig -32.5 9

10m @ Voil & 15m @ Strathyre & 10m @ Lubnaig -39.2 13

10m @ Voil & 15m @ Strathyre & 7.5m @ Lubnaig -45.0 15

4m @ Voil & 9.5m @ Strathyre & 13m @ Lubnaig -49.6 11


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Options involving controls on Lubnaig yield the greatest time lag on peak flows in the Leny. This is due to the fact that this control is the furthest downstream of those tested and has influence upon the largest catchment area. Figure 48 below shows the resulting Leny hydrograph when a 5m flow control on Lubnaig is implemented. However, using flow controls at Lubnaig alone yields the largest increases in water levels. For example, the 5m flow control tested causes a 3.3 m increase on Lubnaig for the 200 year event. A 2.1m increase would occur on Lubnaig for the Annual event.

Figure 48 - 200 Year Leny Hydrograph - with 5m Lubnaig Flow Control

Based on a typical Leny / Eas Gobhain flow split as outlined earlier in this report, a 50% reduction in Leny flows could potentially yield reductions in flood frequency in the Teith at Callander roughly in the order of:

50 year flood in Callander becomes a 10 year flood



9.1.3 Impacts

Generally, increasing water levels on the Lubnaig / Balvag floodplains would be difficult to implement due to extreme loch levels already posing problems to roads and properties for extreme flood events. The site visit on 29th November 2011 (where flooding was evident) and the SEPA photos taken during the 2006 flood event confirmed that flooding issues already exist. There are not thought to be any properties currently at direct flood risk on the banks of the Voil / Doine. However, any flood storage measures that significantly increased flood levels on the Voil / Doine could bring some properties into the flood risk zone.

The model predicts that parts of Strathyre are at flood risk for the 200 year flood event. The areas around sewage works in Strathyre and Immervoulin Caravan Park are predicted to be flooded. Therefore, implementing any storage options which would further increase water levels on Loch Lubnaig (and consequently in Strathyre) would be difficult. There are also properties at risk of flooding from the Balvag between Stronvar Bridge and Strathyre Bridge.

Modelling has shown that storage implemented on the Voil actually serves to reduce peak water levels downstream on the Balvag floodplains. Consequently, attempts were made to test storage scenarios that could re-mobilise the existing storage on the Balvag and Lubnaig floodplains thus maximising storage without creating any net


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increases for extreme flood levels. Modelling has shown it is possible to provide flow control scenarios that do not increase the risk of flooding for the properties along the Balvag (between Stronvar and Strathyre Bridge). The following 3 component flow control option serves to reduce flows in the Leny by around 50%.

4m restriction @ Voil & 9.5m restriction @ Strathyre & 13m restriction @ Lubnaig

This scenario increases levels on the Voil by around 2.6m for the 200 year event but importantly, there was minimal net change in extreme flood levels downstream on the Balvag / Lubnaig floodplains. Indeed, in most cases a slight decrease in downstream levels was achieved.

The consequences of raising loch levels on Voil or Lubnaig were discussed with the Council and the need to assess the extent of additional properties brought into flood risk that weren‟t before. Flood maps were produced for the storage options achieving the nominal target 50% reduction of flows in the Leny at Callander for the 200 year event. These events involved the largest increase in loch levels of the many scenarios tested:

5m restriction @ Lubnaig gives a 3.3m rise on Lubnaig

4m restriction @ Voil plus 9.5m restriction @ Strathyre plus 13m restriction @ Lubnaig gives a 2.6m rise on Voil

200 year flood outlines are shown for two sample areas in Figure 49 and Figure 50 below, illustrating the consequences of a 2.6m and 3.3m rise on Lubnaig and Voil respectively. GIS files for full flood outlines are available on request.


Figure 49 – Flood Outline Sample - Increased Levels on Voil (+2.6m)


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Figure 50 – Flood Outline Sample - Increased Levels on Lubnaig (+3.3m)

These outlines have been generated using 5m DTM data, which is the only ground model data set available for this part of the study area. The DTM accuracy is poor when compared with survey points taken in the area, particularly on the heavily treed loch edges. In some cases the DTM data was noted to be over 10m higher than the surveyed data. Consequently, it is difficult to assess with any degree of confidence the number of flood risk properties from these outlines. Flood risk to individual properties, roads or other infrastructure would require to be assessed using a much more accurate ground model or ideally, a manual survey of property thresholds or road levels along the loch periphery. The outlines produced only give a rough indication of possible flood extents and potential at-risk properties and need to be treated with caution.

Options that would increase loch levels (particularly on Lubnaig / Balvag) would potentially require to be implemented in conjunction with major localised flood defence works to protect a number of individual properties or lands. The vesting of affected properties may also be an option which removes / relocates flood risk properties. Major road raising works would also be required to mitigate the effects of extreme loch level rises.

The environmental impacts associated with upstream flood storage measures which increase existing flood levels are discussed later in this report.

9.2 Eas Gobhain / Loch Venachar Optioneering Modelling

9.2.1 Model Description / Option Selection

Limited modelling was undertaken to evaluate the effects of raising the weir height on Venachar (to increase storage potential), but maintaining the current winter draw down level arrangements using upstream storage to reduce peak flows in the Leny.

A simple routing model of Loch Venachar and Eas Gobhain was developed. The model comprises an elevation / area element describing Venachar, with overflow weir


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and sluice arrangement. A number of surveyed channel sections enabled the downstream Eas Gobhain reach to be modelled which then routes flows to Callander.

A single inflow hydrograph into loch Venachar was used. For simplicity, Atkins‟ design hydrographs for the Eas Gobhain were entered into the upstream end of the model (i.e. into Venachar). A simple „calibration‟ exercise was then undertaken so that the routed hydrograph was similar to Atkins‟ design hydrograph for the Eas Gobhain. This would allow for testing numerous weir arrangement and antecedent loch level scenarios. The aim would be to determine how the various options would alter the resulting Leny hydrograph.

A significant variable in this assessment is the compensation releases. There are no specified release rates for a particular loch level. The only records that Scottish Water holds are in paper format and just state the number of sluice gates open at each dam on a particular day. No sluice flow data is available. The following variables were tested in the model:

Weir / spillway elevation

Antecedent loch levels

Sluice release rates

Figure 51 below shows Loch Venachar‟s weir from aerial photography. Figure 52 shows the schematic of the Venachar dam, weir and sluices.

W aterW ater

W ater

Weir

Coilantogle Ford

(dis)

Sh

ingle

Rain Gauge

Weir

Dam


Figure 51 – Location of Loch Venachar Outlet


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Figure 52 – Schematic of Loch Venachar Outlet (Mott MacDonald, 1993)

The overflow weir is 45.72m long with a crest elevation of 82.25m AOD. The sluice house comprises 11 sluices: four “Salmon” sluices, four “Six Feet” sluices and three “Four Feet” sluices. After discussion with Scottish Water personnel it was concluded that the two Salmon sluices are permanently open (No. 3 and 1 in Figure 52) to provide a free passage for salmon and compensation flows. The other sluices are manually operated in order to keep a water level in the loch at approximately 1 foot below weir level over winter. Scottish Water confirmed that there is no level “trigger point” as such, dictating sluice operation. The sluices are set once daily depending on current loch level and weather forecast. The sluices would not usually be operated over the weekend.

9.2.2 Option Selection

A number of weir / sluice scenarios were tested in order to check the effects on the peak flow attenuation. Model testing has covered the following general scenarios:

Venachar weir raised (2 salmon sluices open – minimum release)

Venachar weir raised (more sluices open to roughly equate to Annual flow)

Venachar weir raised (all sluices open – maximum release)

9.2.3 Model Output and Findings

Model sensitivity analyses have shown that for extreme flood events, initial loch levels have limited impact upon peak flows in the downstream Eas Gobhain. Initial starting loch levels of +/- 1m in relation to existing weir crest have been tested. Due to the relatively long hydrograph duration (5 days), the initially drawn down loch tends to fill to weir crest level well in advance of the peak flow for the 200 year event (i.e. there is no storage available when needed). Initial loch levels have more influence on lower return period events. For smaller storms, there is enough storage to slightly attenuate the flows and reduce the peak.

The effect of raising the weir was tested to ascertain if the additional storage resulting behind the weir could serve to reduce flows for higher return periods. In addition to this, additional sluices were opened in the model in an attempt to release flows throughout the entire flood wave (thus maximising the available storage when the


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peak arrives). Table 15 and Table 16 below shows modelling results for the four basic scenarios outlined above, in terms of resulting changes in peak flow and loch level. The following observations were made:

Raising the weir by 1m and having only the two salmon sluices open results in significant attenuation for the lower return periods (up to 45% reduction in Eas Gobhain peak flows). However, this flow reduction tapers to effectively zero for the 200 year event. Effectively, the volume provided by the addition of 1m on the weir height is not sufficient to store the volumes associated with the higher return period storm events such as the 200 year.

Raising the weir by 2m and having only the two salmon sluices open provides more available storage which results in around a 45% reduction of flows in Eas Gobhain for more return periods up to around 50 year return period. This flow reduction then drops to 11% for the 200 year event.

Scenarios were then tested with all sluices open to their maximum. It became clear that if all sluices were open to their maximum during the routing of the storm events then there would be a significant increase in flooding for the lower return periods (56% increase in flows in the Eas Gobhain for the Annual event). With all sluices open full, this sluice discharge would actually exceed the annual flow for the Eas Gobhain, even with no flow spilling over the weir. Logical controls were required in the model to close sluices if levels in the loch dipped below the current minimum operational level of 82.94m OAD (1 ft. below weir crest level), otherwise the loch would drain to well below this level. There were reductions in Eas Gobhain flows for the higher return periods. For the 200 year event, flow reductions of around 19% and 26% were noted for the 1m and 2m raised weirs respectively.

In cognisance of the model results above where all sluices were open, a more practical scenario was considered to be a sluice regime where the discharge didn‟t exceed that of say, the annual flow in the Eas Gobhain. Consequently, a nominal sluice opening regime that would only discharge around 40 m3/s was adopted. Similar to above, logical controls were used to stop this discharge if levels in the loch dipped below the current minimum operational level (82.94m OAD). Again, there were reductions in Eas Gobhain flows for the higher return periods. For the 200 year event, flow reductions of around 7% and 33% were noted for the 1m and 2m raised weirs respectively. However, importantly, this scenario meant that for the lower return period events there was no increase in flow in the Eas Gobhain.

In summary, it is hydraulically possible to implement measures on Loch Venachar that would serve to attenuate flows in the Eas Gobhain. The most effective solution would be a combination of weir height raising and sluice operation. However, any solution would need to be computer controlled, electronically actuated, based on real time hydrometrics, and part of a wider operational system including Katrine, Finglas, Leny, Teith etc.

The simplest, cheapest and most practical modification to the Venachar weir that could yield some minor reductions in Eas Gobhain flood flows would be the implementation of electronically actuated sluices. The sluices would be controlled by loch level and set to maintain a relatively high constant discharge. Reductions in loch level would result for all return periods (0.2m reduction for Annual and 0.5m reduction for 200 year). These slight reductions would be beneficial for existing flood risk issues around the loch. In many ways, the discharging of flood flows in advance of the peak and the maintenance of loch levels is what Scottish Water is already trying to achieve, but within the constraints of a manually operated system.


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9.2.4 Impacts

Options that utilise storage on Venachar would require major civil engineering works and would need to be implemented in conjunction with major localised flood defence works to protect a number of existing properties, lands and roads from the impacts of increased loch levels. This would be difficult to achieve in practice. The costs of modifying the Venachar dam, weir and spillway structure would also be considerable. The environmental impacts associated with upstream flood storage measures which increase existing flood levels are discussed later in this report.


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Table 15 – Eas Gobhain Peak Flows (m3/s) – Venachar Weir Optioneering

Storm Event

Peak Flow (2 sluices open and existing

weir height)

Peak Flow (2 sluices / and 1m

weir raise)

% flow

change

Peak Flow (2 sluices open / 2m weir raise)

% flow

change

Peak Flow (2 sluices open /

existing weir / controlled 40

m3/s discharge)

% flow

change

Peak Flow (2 sluices open / 1m weir raise / controlled 40

m3/s discharge)

% flow

change


m3/s discharge)

% flow

change

Annual 43.68 26.69 -39% 26.65 -39% 42.62 -2% 43.08 -1% 43.07 -1%

5 Yr 59.17 32.42 -45% 32.89 -44% 48.09 -19% 45.32 -23% 45.31 -23%

10 Yr 71.89 54.33 -24% 38.53 -46% 63.26 -12% 48.64 -32% 48.62 -32%

25 Yr 90.23 79.36 -12% 47.12 -48% 83.91 -7% 56.07 -38% 54.65 -39%

50 Yr 107.52 101.05 -6% 61.16 -43% 102.75 -4% 81.37 -24% 61.33 -43%

100 Yr 128.70 126.25 -2% 99.92 -22% 125.57 -2% 109.05 -15% 70.57 -45%

200 Yr 152.71 153.63 +1% 135.51 -11% 150.98 -1% 141.84 -7% 102.18 -33%

Table 16 –Loch Venachar Water levels (m AOD) – Venachar Weir Optioneering

Storm Event

Peak Flow (2 sluices open and existing

weir height)

Peak Flow (2 sluices / and 1m

weir raise)

Level change (+/- m)

Peak Flow (2 sluices open / 2m weir raise)


Peak Flow (2 sluices open /

existing weir / controlled 40

m3/s discharge)



m3/s discharge)



m3/s discharge)


Annual 82.63 82.97 +0.34 82.97 +0.34 82.10 -0.53 82.12 -0.51 82.12 -0.51

5 Yr 82.74 83.37 +0.63 83.33 +0.59 82.37 -0.37 82.38 -0.36 82.38 -0.36

10 Yr 82.82 83.49 +0.67 83.63 +0.81 82.52 -0.30 82.69 -0.13 82.69 -0.13

25 Yr 82.94 83.61 +0.67 84.04 +1.10 82.67 -0.27 83.23 +0.29 83.14 +0.20

50 Yr 83.04 83.70 +0.66 84.43 +1.39 82.79 -0.25 83.51 +0.47 83.56 +0.52

100 Yr 83.16 83.79 +0.63 84.58 +1.42 82.93 -0.23 83.63 +0.47 84.06 +0.90

200 Yr 83.29 83.88 +0.59 84.69 +1.40 83.07 -0.22 83.76 +0.47 84.51 +1.22


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10 Other Optioneering Considerations

10.1 Demountable Defences

Temporary and demountable flood defences can represent a cost-effective and rapidly achieved method of protection against flood risk. These defences can be used at an individual household level as well as a community level in coordination with the Local Authorities and partnering organisations.

Mobilisation & closure are critical to the performance of temporary and demountable flood defences. Temporary and demountable systems are only functional when the barriers are fully erected (or in a closed position in the case of a demountable system) before the water rises to the lowest safe permanent protection level. Typical demountable / temporary systems include:

Demountable panel systems (e.g. Bauer Inner City / Caro Flood Defence / Flood Control / Task Green Flood Barrier)

Flood barrier with frame (e.g. Portadam)

Flood barriers free standing (e.g. Rapidam)

Pallet Barriers (e.g. Geodesign Barriers)

Water filled tubes (e.g. Aquabarrier, Aqua tube, Aqua dams)

Filled containers – permanent (e.g. Hesco Bastion Concertainer)

The selection of temporary and or demountable flood defences involves a thorough understanding of the flooding problems and associated flood levels in a particular area. Other considerations include:

Suitability of a particular type of system (retained heights, site conditions, etc.)

Availability and dependability of a flood warning system

Mobilisation timescales

Availability and skill of staff for mobilisation operations

Existence and effectiveness of any permanent flood defences

Budgetary constraints

Urgency of flood protection scheme

The Defra / EA R&D Publication 130 (Ogunyoye et al., 2010) provides guidance on selecting an appropriate system. Typical failure modes of such systems include overtopping / seepage and insufficient strength and / or stability.

Demountable options have been considered for use in Meadows Car Park Callander but not considered practical. It would be difficult to determine when demountable defences should be erected as the car park and adjacent yards are flooded to varying degrees for prolonged periods over the winter. There is an inherent risk with demountable options as they involve bringing assets to site for erection and also clearing parts of the car park to facilitate erection. Council plant / staff access may be difficult when the car park is flooded. Although the car park floods relatively frequently, the onset of property flooding is generally around 25 years, so wall heights for demountable defences would need to retain around 1 m of water before starting to offer any protection to properties (assuming defences are set as far back in the car park as possible and based on existing ground levels).


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10.2 Natural Flood Management (NFM)

The Flood Risk Management (Scotland) 2009 Act encourages the use of Natural Flood Management (NFM) measures and promotes a new sustainable approach to managing flood risk. Local Authorities have a responsibility under the Act to prepare local FRM plans which are to include a consideration for NFM at flood protection scheme level.

To explore the potential for using NFM measures for mitigating the flooding issues in Callander, a brief review of NFM policy, guidance and research literature was undertaken. This review provides information on typical NFM measures and the likely effectiveness of such measures for consideration for Callander. This brief NFM review is detailed in Appendix I however, the following information summarises the key points.

10.2.1 NFM Techniques

NFM includes “alteration (including enhancement) or restoration of natural features and characteristics of any river basin or coastal area in a flood risk management district”. The features should be such that they “assist in the retention of flood water, whether on a permanent or temporary basis, (such as floodplains, woodlands and wetlands) or in slowing the flow of such water (such as woodlands and other vegetation), those which, contribute to the transporting and depositing of sediment, and the shape of rivers and coastal areas”.

A description of NFM approaches is shown in Figure 53 below.

Figure 53 - NFM Approaches (SNIFFER, 2011)

NFM techniques include, but are not limited to the following (Environment Agency, 2012):

Managed realignment, the creation of inter-tidal habitat through breaching or removing existing sea wall or embankments. This can reduce both wave height and energy and deliver additional benefits to wildlife.

SuDS, which encompass a range of runoff management techniques to mimic natural processes. This can help to minimise the impact of development on runoff generation and at the same time result in habitat creation and social benefits.

Flood storage, in the form of on-line or off-line storage can attenuate the peak of river flows and reduce flood levels downstream.

Floodplain reconnection can return storage volumes to the river system by breaching artificial barriers to connectivity such as agricultural embankments.


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This can help to reduce the magnitude of flood peaks, reduce bed scour and increase the time to peak.

Increases in channel roughness by planting trees and other vegetation can reduce flow velocities, resulting in increased water levels which will mobilise floodplain storage and potentially reduce flood risk downstream. This also has ecological benefits through habitat creation.

Soil management can improve groundwater recharge and reduce the amount of runoff from soils. This can also help to reduce sediment, pollution and nutrient loading on receiving water-bodies.

The management of sediment transport through source control can reduce the loss of floodplain storage and channel conveyance through deposition.

Woodland creation could be used to increase interception storage and evapotranspiration, increasing infiltration and reducing surface runoff, and slowing down runoff.

The Environment Agency (2012) has demonstrated the benefits of all of the above techniques, but also notes that the benefits vary considerably between catchments (Environment Agency, 2008): the findings of one case study cannot be reliably transferred to another site. This highlights the need for a detailed site specific study.

10.2.2 Existing NFM Studies and Key Findings

Although the effects of NFM on a small scale are well known (i.e. SuDS), there is only a limited evidence base on NFM implementation on a catchment scale. SEPA has been working to develop the NFM knowledge base to allow practical implementation on the ground, on the back of an objective evidence based assessment of economic, social and environmental costs and benefits.

Demonstration projects to address the knowledge gap include the Allan Water, Upper Clyde, Eddleston Water, Tarland Burn and Balmaleedy Burn. The Allan Water, Eddleston Water and Firth of Forth Futurescapes are currently supported by SEPA. Other projects include WWF‟s work on the River Devon catchment and the Pickering catchment in England. There are other on-going studies, however, the findings and conclusions from the studies noted above are shown to be highly variable. Key findings are contained in Appendix I.

The following is an extract taken from the Environment Agency‟s 2008 study which examines the role of land use management in delivering flood risk management benefits (Environment Agency, 2008):

“The lack of robust catchment scale evidence does not necessarily mean that there is no catchment scale effect, but rather may just indicate that effects are difficult to detect and differ between catchments. Some observational examples, such as at Crowlas in Cornwall, do appear to show that flood risk in small catchments responded dramatically to land use changes. However, because flood risk management policy should be based on sound scientific evidence, the lack of robust catchment scale evidence currently provides a major constraint in considering land management as an effective tool to manage flood risk.”

The report also highlights that local measures get diluted at catchment scale. Catchment scale problems require catchment scale solutions. The report also states that it is difficult to transfer findings from one site to another; the various findings are very site specific. Where uncertainty exists over hydrograph peak timings on multiple river systems at a catchment scale, the benefits of catchment scale solutions will also become more difficult to prove or rely upon.


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10.2.3 Modelling of NFM Measures

The benefits of directly utilising and enhancing upstream loch and floodplain storage were investigated in detail in Section 9. It was found that it is possible to achieve substantial reductions in peak flow through mobilisation of significant volumes of additional loch and floodplain storage. However, by inspection, the scale of such measures would present technical, environmental and financial challenges that would be difficult to justify for Callander alone.

Modelling was also undertaken to assess the downstream effects of a theoretical % runoff reduction provided by possible land management measures. This was to assess in theory how a nominal % reduction in runoff on a catchment scale would propagate to Callander once the various attenuated runoff hydrographs were routed through the lochs and floodplains. How such % reductions in runoff could be achieved in practice through land use modifications remains highly uncertain and would warrant further detailed investigation.

Modifications were made to the unit hydrograph time to peak to achieve a nominal and theoretical 5% attenuated reduction in the peak flow for each inflow of the Leny routing model. Different levels of parameter adjustment were required for each of the inflows to achieve a 5% reduction. This highlights the variability in sensitivity for land use change for different sub-catchments.

The scope for significantly increasing interception, infiltration or evapo-transpiration would seem limited for a typical Scottish winter in the relatively steep upper catchment reaches of the Leny. Therefore, care was taken to obtain an attenuated runoff reduction (i.e. the runoff volumes were largely retained) rather than just simply scaling the runoff hydrographs down by a particular percentage with an associated flow volume reduction which may be unrealistic.

The 5% attenuated runoff flows were entered into the Leny routing model to determine how the theoretical peak flow reductions would impact on flows downstream at the Eas Gobhain confluence. The results are shown in Table 17 below.

To test whether any reductions in peak flow were a result of peak flow reduction or catchment desynchronisation, the Voil inflow was delayed by 3 hours (the same delay as for the 5% attenuated hydrograph) without any adjustment of peak flow. These results are also shown in Table 17 below.

Table 17 – Downstream Impact of Theoretical Runoff Reductions

Scenario Original model outflow (m

3/s)

NFM model outflow (m

3/s)

Reduction in peak flow

5% peak runoff reduction for all inflows, 200 year return period

257.9 255.8 0.8%

5% peak runoff reduction for Voil inflow only, 200 year return period

257.9 245.6 4.8%

3 hour delay on Voil inflow only (no peak reduction), 200 year return period

257.9 247.4 4.1%

5% peak runoff reduction for Voil inflow only, annual return period

101.9 97.8 4.0%

The benefits of a catchment wide runoff reduction (attenuated) would appear to be limited despite a theoretical 5% reduction in all peak runoff hydrographs. The peak flow in the Leny was reduced by just 0.8%. The benefits of modifying only the Voil catchment runoff appear far greater. This conclusion holds for lower return periods as well. Simply delaying flows upstream of Loch Voil, without any reduction in peak flow, resulted in similar reductions in peak model outflow. It can be surmised that runoff delay, rather than peak attenuation, would be the most important goal for any NFM measures implemented on the Voil catchment.


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A crude assessment was carried out to determine the impact of a theoretical flow reduction on flood damages in Callander. A 5% reduction in the Leny flow would, based on the Callander gauge rating equation, result in a water level drop of between 37 and 96 mm depending on the return period.

10.2.4 Conclusions and Applicability of NFM to Callander

It is clear that there are potential benefits to be achieved from NFM and there are a range of NFM techniques. However, NFM implementation in Scotland has, to date, been limited. From the various pilot studies, the changes in peak flow which can be achieved using NFM are generally reported as being limited to approximately 10% and this depends highly on the return period of the flood event. The greatest benefits can generally be achieved for the lower return period events.

The greatest benefits would be realised in heavily modified catchments where the scope for restoration and naturalisation would be much greater. The upper catchment of the River Leny is mostly natural with extensive floodplains and no intensive agricultural activity on its relatively steep hill slopes. There would appear to be few opportunities for restoration in this instance and hence NFM would have to be implemented in the form of semi-natural approaches which utilise existing natural processes. This could be in the form of flow restrictions to increase floodplain storage or altered plantation (which may not be in keeping with the existing natural vegetation) to slow down runoff on hill slopes.

NFM techniques should be considered in conjunction with traditional flood protection schemes, not as an alternative to traditional defences. It is frequently stated that it is the secondary benefits such as environmental and social factors which makes NFM an attractive option. The reason is that NFM alone cannot solve acute flooding problems; rather, it is a useful technique to potentially reduce the size of engineered measures required and improve the secondary benefits of flood schemes. NFM may therefore require additional or alternative sources of funding than those normally used for a flood protection scheme. As was noted in the Pickering study, in order to fully explore NFM, the modelling and scoping work can demand a significant amount of funding.

It should also be noted that NFM may not be appropriate in catchments used for water supply purposes. Many of the NFM techniques actively improve evapotranspiration and infiltration rates in an effort to reduce runoff. Inherently, water yields may be reduced and therefore NFM could present a conflict of interests. On the other hand, groundwater recharge could be of benefit in areas where groundwater is a source of water supply. Of course it would be possible to both implement NFM and maintain adequate catchment yield for water supply, however the level of cooperation and detailed investigation required would be greatly increased. Such situations may provide opportunities for greater collaborative working between the various stakeholders, as required by the Act, to work towards achieving sustainable integrated water resources management.

For Callander, the most practical and demonstrably effective NFM options (in terms of substantial reductions in flood risk) have been investigated and comprise utilising and enhancing upstream loch and floodplain storage as presented in section 9. However, key constraints are increased water levels and the associated impacts.

The benefit of a theoretical peak runoff reduction in the Leny catchment was also tested to investigate the benefits of land management measures. It was found that the Loch Voil catchment would be the best location for NFM measures, as sub-catchment de-synchronisation and peak flow reduction yields a reduced flow in the Leny. A theoretical 5% reduction in peak flow for each return period would result in a water level drop of the order of 37 to 96 mm in Callander depending on the return


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period. Whether this 5% could be achieved in practice, particularly for the full range of return periods, would be highly uncertain and would warrant further detailed investigation.

It is suggested that if the Council wishes to investigate in more detail the potential benefits that could be realised in terms of NFM that alternative sources of funding are sought to allow for the various secondary benefits to be included in any feasibility assessments. Further investigations could include the following:

A thorough literature review (including studies currently on-going) to identify the options available and their potential benefit particularly for Scottish catchments similar to the Leny

A detailed GIS-based assessment of the most appropriate specific locations to implement NFM (similar to Upper Allan study)

More detailed hydrological modelling to determine the benefits of NFM

Advice on the possible funding streams.

Such a study should take a catchment-wide approach to be able to incorporate benefits to Stirling.

10.3 Individual Property Flood Proofing

There will be many instances where community flood schemes are not economically viable, although flooding may affect a number of individual properties in that community. This is likely to be the case in Callander, where some properties are affected by flooding from the Teith but not enough properties to justify the capital expenditure of a major flood scheme. In these instances, property specific flood proofing and resilience measures may be the only practical flood mitigation option.

Floodwater will always follow the path of least resistance and will enter a building at the weakest points in the construction, particularly through masonry and construction joints, and any voids and gaps. Figure 54 summarises the typical entry points.


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Figure 54 - Property Floodwater Ingress Points (CIRIA, 2007)

Individual property owners have ways of increasing the resilience of their properties to flood ingress and damage, potentially in partnership with their mortgage provider, insurer, or local authority.

There are a range of methods and products available for keeping floodwater out of properties during flood events. However, many of these methods and products do not keep flood waters out for prolonged periods. Flood boards installed around doors will not often offer protection against flooding lasting many hours or days. Water will eventually enter through the floor and brickwork. Flood protection products will generally give some time to move possessions out of the reach of floodwaters. A pump and sump system may be effective in keeping flood levels down to a minimum. Even when water does get into the house which has flood resilience measures employed, it is usually "cleaner", because much of the mud and silt stays outside the property, thus reducing post flood clean-up costs. For floods deeper than 1m, it is generally recommended that water is allowed to inundate the property as the hydrostatic pressure building up outside could potentially cause structural damage.

Different flood proofing techniques and products are appropriate for different types of buildings and flood hazards. The following are some commonly used individual property flood proofing and resilience measures:

Outside landscaping to divert floodwaters

Periphery walls / bunds

Tanking

Concrete / flood proof floors

External water resistant skirting

Periscope air vents


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External wall waterproof renders and facings, including veneer walling

Lime based plaster for walling, rather than gypsum

Resilient internal walls (e.g. tiled or coated)

Pumps and sumps

Non return valves on waste pipes, vents and other outlets

Flood resilient internal doors (easily removable)

Raised electrical sockets, and electrical goods, etc.

Temporary products (free standing barriers, door boards, flood skirts, airbrick covers, etc.)

Individual property assessments are required to determine the appropriate measures for the particular type of property and the likely nature of the flooding. However, once the appropriate measures have been specified, there is very little design work required. Installation, carried out by a qualified contractor, can rapidly follow the specification stage.


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11 Environmental and Social Constraints and Opportunities Appraisal

11.1 Overview

As part of the development of options for flood alleviation, environmental constraints require to be identified, together with potential opportunities for environmental enhancement. This section also includes social impacts and benefits. The study area covered by this assessment includes the town of Callander and upstream areas as far as Loch Katrine, Glen Finglas Reservoir and Lochs Voil and Doine.

Two main flood mitigation options are being considered for Callander; direct defences by way of flood walling or embankments and upstream storage comprising the utilisation of existing lochs and floodplains. These options are described earlier in this report. The environmental and social appraisal has considered the following key constraints:

Ecological designations and wildlife value of the waterbodies and surrounding area, and potential issues of the proposals for the local and wider ecology/nature conservation;

Landscape/heritage designations and value of the area in which the waterbodies are located;

Public amenity issues, including existing use of the waterbodies and the surrounding area for access, amenity and recreational activities; and

Land use issues, comprising: geology and soils, land capability for agriculture, coal and mineral mining, and potential contaminated land.

Social issues including the impacts of flooding and the impacts / benefits of scheme implementation on society and heritage features

Environmental and social constraints are described in the following sections, with key constraints mapped in GIS. Recommendations are provided which detail potential environmental and social opportunities and further consultation, and more detailed investigation that would be required should any of the options be explored further in more detail.

11.2 Methodology

The baseline information and environmental constraints identified within this report have been collated from desk study only.

The desk study was undertaken to gather information regarding the area in which the possible flood mitigation options are located. A range of information sources were utilised and data reviewed, including OS maps and mapping websites (for mining, soils, and agriculture); the Stirling Council Development Plan, and Local Town and Village Plans; the Loch Lomond and The Trossachs National Park Core Paths Plan; Pastmap website; and Scottish Natural Heritage (SNH) websites (for landscape and ecology).

11.3 Baseline Information

11.3.1 Ecology and Nature Conservation

The SNH database “SiteLink” lists Sites of Special Scientific Interest (SSSIs), Ramsar Sites - Wetlands of International Importance, Special Protection Areas (SPAs), Special Areas of Conservation (SACs), National Nature Reserves and National Scenic Areas (NSAs) in Scotland (SNH, 2012).


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The website indicated that there are twelve designated areas in the study area; nine are designated as SSSIs (national designation) and two as SACs (European designation). These sites are noted in Table 18 and shown in Appendix J.

Any works proposed to be undertaken within or adjacent to a SSSI will require consent from Scottish Natural Heritage. A Habitats Regulations Assessment may be required for Natura 2000 sites (SPAs and SACs).

Table 18 - Ecological Designations – Callander Optioneering Study Area

Name of Designated Area Designation

Ben More - Stob Binnein SSSI Biological

Black Water Marshes SSSI Biological

Ben A'an and Brenachoile Woods SSSI Biological

Brig o' Turk Mires SSSI Biological

Leny Quarry SSSI Geological

Loch Lubnaig Marshes SSSI Biological

Mollands SSSI Geological

Pass of Leny Flushes SSSI Biological

Stronvar Marshes SSSI Biological

River Teith SAC

Trossachs Woods SAC

11.3.2 Local Wildlife Sites

The Callander Town and Village Plan produced by Stirling Council (2012) indicates that there are no local wildlife sites within the Callander area (including upstream).

11.3.3 Landscape Assessment

The historical land use within the study area is described as „Woodland and Forestry‟ and „built up‟ within the Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS) Historic Land-use Assessment (HLAMAP) database (RCAHMS, 2012a). The Landscape Character Assessment commissioned by SNH (1999a and 1999b) shows the site to be a mixture farmed or forested upland glen, glen sides, hills and, strath and glen floors landscape character category.

Callander and the upstream waterbodies are located within the Loch Lomond and the Trossachs National Park. The National Park is designated as two National Scenic Areas (NSA), one being Loch Lomond NSA and the other The Trossachs NSA.

A search of the RCAHMS PASTMAP database (RCAHMS, 2012b) confirms the presence of a historic garden and designated landscape (reference 31), which is noted in Table 19, and its location shown in Appendix J.

Table 19 - Historic Landscape Designations – Callander River Teith

Reference (see Appendix J) Name Size Designation

31 The Roman

Camp 8 ha

Historic Garden and Designated Landscape

11.3.4 Cultural Heritage

A search of the RCAHMS PASTMAP database confirms the presence of the sites and features detailed in Appendix J, where the locations are also shown.

In terms of statutorily designated sites of cultural heritage interest within the study area, there are fourteen Scheduled Ancient Monuments (SAM) and eighty-four Listed Buildings (LB).

SAMs are nationally important sites and monuments that are legally protected under the Ancient Monuments and Archaeological Areas Act 1979.


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The Listed Buildings are divided into three categories based on different levels of interest or importance. Category A Listed Buildings are those of national or international importance; either architectural of historic or particularly good (minimally altered) examples of a specific period, style or building type. Category B refers to buildings of regional or more local importance or major examples of a particular period, style or building type which may have been altered. Category C(S) relates to buildings of local importance or lesser examples of any period, style or building type, as originally constructed or altered and simple, traditional buildings or as part of planned group, such as an estate or industrial complex.

Also located within the study extents are a large number of unscheduled sites identified from the National Monuments Records of Scotland and Scottish Sites and Monument Records.

It should be noted that the Callander Conservation Area is located within the potential scheme extents (shown in Appendix J).

Potential for Unrecorded Heritage Sites

Although no other archaeological sites are known to exist at this time, there is the potential for unrecorded archaeological features within the study area, which may have survived undetected.

11.3.5 Public Amenity

There are a number of core paths which provide public access to the countryside within the study area, several of which could be affected by flood mitigation measures.

There is a long distance cycleway/footpath from Stirling to Killin, which uses the redundant rail track (ref CALL.T1 on the Callander Town and Village Plan).

Core Paths designated within the Loch Lomond and The Trossachs National Park include: two main routes from north and south of Callander denoted as part of the National Cycle Network (NCN) Route 7, the Rob Roy Way, and a series of Core Paths meandering through Callander, with a further route to the east known as NCN76.

These Core Paths are detailed on Maps 27 West, 27A and 27 East on the Core Paths Maps Index, and can be accessed from the Loch Lomond and The Trossachs National Park‟s website (http://www.lochlomond-trossachs.org/corepathmaps).

11.3.6 Geology

No Regionally Important Geological Sites (RIGS) were identified although two geological SSSIs were identified; Leny Quarry and Mollands (Table 18). Mollands has also been identified as a site of national and international importance by Geological Conservation Review (GCR) (SNH, 2012).

11.3.7 Soils

The main soil types in the study area have been classified on the Soil Survey of Scotland Map Eastern Scotland Sheet 5 produced by the Macaulay Institute (1982).

The distribution of soils within the study area is dependent on the geology, topography and drainage regime of the area. The site soils consist of units belonging to a number of soil associations, derived from Old Red Sandstone, Carboniferous and Dalradian bedrock and from various drift materials.

The main soil types within the study area are:

http://www.lochlomond-trossachs.org/corepathmaps


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Alluvial soils: immature soils that are very variable in character, with textures ranging from gravel to silty clay and drainage varying from free to very poor. Typically confined to principal river valleys and stream channels;

Peat: accumulations of organic material that have remained wet to the surface;

Gleys: naturally poorly drained soils that develop under conditions of intermittent or permanent waterlogging. Soils are typically greyish or blue-grey with orange mottling. Peaty gleys have a peat-rich surface horizon; non-calcareous gleys have a low lime content; humic gleys include a humus-rich surface layer;

Podzols: typically free-draining acid soils developed under aerobic conditions. Podzols are generally nutrient-deficient and heavily leached in the upper horizons, with an accumulation of iron/aluminium oxides („ironpan‟) or organic material at lower levels within the soil profile. Peaty podzols have a peat-rich surface horizon; humus-iron podzols have a humus-rich surface layer and a higher concentration of iron oxides within the soil profile. In areas with low slope angles, waterlogging may occur above the ironpan; this can produce a soil intermediate between a podzol and a gley.

Fourteen soil units are found within the environs of the study area and are summarised in Appendix J. Each soil unit consists of varying proportions of the soil types discussed above, with the proportion of each soil type within a soil unit is dictated by the local climatic, topographical and drainage conditions.

11.3.8 Land Capability for Agriculture

Agricultural land within the study area has been identified using Land Capability Map for Agriculture Sheet 57 (scale 1:50,000) produced by the Macaulay Institute (1986).

Between Loch Doine and Callander, much of the land is classed as being capable of use only as rough grazing (class 6.1 and 6.2) with a number of small areas classed as land capable of use as improved grassland (class 5.2 and 5.3). Class 5 land is restricted to grass production which plays an important economic role. Class 6 land has very severe site, soil or wetness limitations which generally prevent the use of tractor-operated machinery for improvement.

Land classed as capable for producing a narrow range of crops has been indicated as present on the banks of the River Balvag at Balquhidder and additional small areas adjacent to the River Balvag and Loch Lubnaig. Class 4 land is suitable for primarily grassland with short arable breaks e.g. barley, oats and forage.

At the south and eastern extents of Callander is land classed as land capable of producing a moderate range of crops (class 31, which is prime agricultural land) such as cereals, grass and vegetables.

11.3.9 Coal and Mineral Mining

The Coal Mining and Brine Subsidence Claims Gazetteer for Scotland website provided by the Coal Authority indicates that a mining report is not required for the study area. The British Geological Survey (BGS) online database Geoindex indicates that there are no opencast coal prospecting or active mining/quarrying sites within the study area.

Arup‟s “Mining Instability in Great Britain” report (Arup, 1991) indicates that mining activities (lead, and zinc with minor silver) have taken place within the 10 km National Grid square (NN50) in the study area.

A review of the OS map of the area revealed there a number of disused quarries and pits located within the site extents. It is not anticipated that these would affect or be affected by the works considered.


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11.3.10 Potential Contaminated Land

A dismantled railway is noted northeast of Balquhidder Station running southwards to the edge of Callander adjacent to the River Balvag. Contaminated land may be associated with the historic use of this land. A review of historical maps will identify any other previous land use which may have acted as a source of contamination.

11.4 Other Social Impacts

In addition to cultural heritage and amenity impacts detailed in this section, there are other social impacts which are not accounted for here, or in the economic damage calculations. The Multi-Coloured Handbook (Penning-Rowsell et al., 2010) summarises such impacts; an extract from the handbook is shown in Table 20 below. Shown in the first column are the impacts which have been quantified as part of the benefit-cost assessment.

The Council may decide to promote a scheme with a benefit-cost ratio of less than unity in cognisance of these important intangible and indirect impacts.

Table 20 - Intangible and Indirect Impacts on Households (Penning-Rowsell et al., 2010)

11.5 Opportunities for Enhancement

Opportunities to enhance and improve biodiversity, ecology and amenity as part of a flood mitigation option for Callander are possible and potentially include the following:

As part of a loch storage option, it is possible to create new habitats and increase biodiversity by providing wetlands, spawning pools and fish passes. This involves selecting herbaceous species that stabilise the substrate and have potential value for fish and wildlife; using species that are adaptable to a broad range of water depths; and providing appropriate vegetated buffer zones.


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As part of a flood wall / embankment option, there may be opportunities to enhance the riparian corridor. This could be achieved by increasing species diversity through established best practice river restoration techniques, such as grazing management; removal and control of non-native species; planting of trees, shrubs and grasses (all of local provenance); and maintaining a diversity of habitat structure.

For all options, there are opportunities to enhance the amenity value of the works through sensitive landscaping, way marking, footpaths, information boards, provision for fishing and water sports.

There may be scope for working with other departments within the council and the Trossachs National Park to achieve joint goals. For example, it may be possible to improve awareness and access to local heritage features.

11.6 Recommendations

It is recommended that if any of the Callander River Teith flood mitigation options are to be taken forward and developed in further detail, site walkovers, ecological surveys and consultation with appropriate bodies should be undertaken to verify the information collated from this desk study and to obtain any other information that would inform further stages of scheme development.

11.6.1 General Consultation

Consultation should include relevant departments of Stirling Council, Loch Lomond and The Trossachs National Park, and Scottish Environment Protection Agency (SEPA).

11.6.2 Consultation Regarding Contaminated Land

A review of coal mining reports and historical maps is required to identify any potential impacts from mining and any previous land use which may have acted as a source of contamination.

11.6.3 Consultation Regarding Ecology

Consultation regarding survey requirements and ecological issues in the surrounding areas of the Callander River Teith flood mitigation options will include discussion with: Stirling Council‟s biodiversity officer and planning officer who deals with Tree Preservation Orders; Scottish Natural Heritage (SNH); and any relevant departments within the Loch Lomond and The Trossachs National Park; the Royal Society for the Protection of Birds (RSPB), any local bat and red squirrel groups; Scottish Badgers; RSPB local office; Scottish Ornithologists' Club (SOC) relevant regional office; and local Biological Records Centre.

The presence of European designated sites will require further consideration under the Habitats Regulations and the need for/extent of more detailed assessment of impacts associated with the works identified in consultation with relevant bodies.

11.6.4 Consultation Regarding Cultural Heritage

Consultation regarding cultural heritage issues in the surrounding areas of the Callander River Teith flood mitigation options will include seeking advice from: Historic Scotland, the West of Scotland Archaeology Service (who advise Loch Lomond and The Trossachs National Park on cultural heritage issues) and Scottish Water (regarding any works in the vicinity of the former Loch Katrine Aqueduct).

11.6.5 Consultation Regarding Public Amenity

Consultation regarding public amenity issues and recreational use in the surrounding areas of the Callander River Teith flood mitigation options will include: Stirling Council‟s Access Officer; Loch Lomond and The Trossachs National Park; SNH;


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Paths for All Partnership; Ramblers Association Scotland; Scottish Rights of Way and Access Society (ScotWays); Cycling Touring Club (CTC) Scotland; Sustrans Scotland; and any local fisheries organisations.

Other issues concerning public amenity may be impacts upon Core Paths, which may require closure during FAS option works, and the visual amenity of any new walls/embankments within the National Scenic Areas of the Loch Lomond and The Trossachs National Park, requiring sensitive design/construction principles.

11.7 Summary of Key Constraints and Potential Opportunities

A summary of the key environmental constraints identified as part of this desk study is given below in Table 21.

Table 21 - Key Constraints

Scheme Key Constraints

Increased upstream loch storage and associated flow control structures

Within NSA – not considered to be significant constraint considering nature of works proposed

Presence of SACs & River Teith SAC – careful construction and pollution control would be required, consultation with SNH and potential need for

Habitats Regulations Assessment(s)

Presence of ancient woodland in proximity – not considered to be significant constraint considering nature of works proposed

Presence of Listed Buildings and Scheduled Ancient Monuments (SAM) within all areas of proposed works – Listed Buildings can be avoided;

possible sensitive design and consultation required in vicinity of the SAMs

Listed Buildings in area part of the former Glasgow Corporation Water Works Loch Katrine Aqueduct – consultation with Scottish Water required,

and consent for works may be required

Core paths close to proposed flood mitigation measures

Potential contaminated land associated with the dismantled railway northeast of Balquhidder Station – not considered to be significant

constraint

Flood walls / embankments around

River Teith at Callander

Within NSA – not considered to be significant constraint considering nature of works proposed

Presence of River Teith SAC – careful construction and pollution control would be required, consultation with SNH and potential need for Habitats

Regulations Assessment

Presence of ancient woodland – could be affected but possible to avoid

Sections of flood wall within Conservation Area, close to Listed Buildings and within a Historic Garden and Designated Landscape – sensitive design and consultation required, Listed Buildings can be avoided with exception of

Roman Camp (also a Designed Landscape) which is considered to be a constraint

Presence of Scheduled Ancient Monuments (SAM) – possible sensitive design and consultation required

Possible loss of / disturbance to prime agricultural land at the south and eastern extents of Callander

Core paths and part of a long distance cycleway/footpath from Stirling to Killin close to proposed flood mitigation measures

It is recommended that further consultation is taken place with the relevant bodies to ensure any proposed scheme is sensitive to the local environment.

The study area is generally natural with limited artificial influences. Nevertheless, there are some potential opportunities to realise environmental benefits with the flood alleviation options (direct flood defences and upstream storage) as shown in Table 22.


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Table 22 - Potential Opportunities

Scheme Potential Opportunities

Increased upstream loch storage and associated flow

control structures

Features such as wetlands, spawning pools and fish passes could be used to enhance and maintain existing ecological features

Flood walls / embankments around River Teith at

Callander

Opportunity to enhance riparian corridor where possible through sensitive design and landscaping – see for example the Manual of

River Restoration Techniques (The River Restoration Centre, 2002)

All options

Sensitive landscaping, creation/enhancement of footpaths, improved access, way marking and public information boards could enhance the

amenity value of the proposed options

Opportunity to work with other departments within the Council and the Loch Lomond & Trossachs National Park to achieve joint goals


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12 Health and Safety Appraisal

An outline health and safety review of the preferred options has been undertaken to identify significant design issues and risks that would need to be addressed in any later design stages. A Designer‟s Hazard Checklist and Hazard Elimination Management Schedule has been completed and is included in Appendix K. It is considered that the majority of hazards identified at this optioneering stage will be addressed during the detailed design. Some key health and safety considerations that should be taken account of in any future scheme development are as follows:

Consideration needs to be given to flood events in excess of the design event, assessing where the flood water may flow and how it will escape.

Ensure no formation of „islands‟ in flood storage areas.

Ensure safe access and egress in times of flood, particularly to properties behind flood walls.

If flood gates are used, ensure access is kept clear at all times and backup keys etc. are available.

The impact on the urban drainage systems will need to be investigated in more detail.

Ensure scenario for failed back drainage is assessed. What would happen and is it safe?

Impact on services and if any diversions are required.

Maintenance and operation plans need to be carefully devised and implemented.

Ensuring that all rights of way along the river bank have safe egress points in times of flood.

Systems to ensure the flood storage areas can be safely evacuated prior to use in a flood event.

Suitable warning measures in close proximity to the storage areas.

Are there any implications for increased velocities in the watercourses post scheme implementation? How would this be mitigated?

Is the timing and speed of any flood-wave being altered by the flood scheme implementation?

Will roads, particularly around the loch, be affected by increased water levels?


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13 Benefit / Cost Appraisal

The economic performance of a flood prevention scheme is determined through its benefit / cost (B/C) ratio. Benefits are measured in terms of the present value (PV) of damages avoided over the life of a scheme, with the present value of capital and maintenance costs being estimated over that period. To justify expenditure on any flood alleviation works, it is necessary to assess the economic viability of these options. For any option, benefit / cost appraisals generally consider the following scenarios:

„Do nothing‟

This is essentially the „walk away and do nothing‟ option. Although this option can be considered in a benefit / cost analysis it is not usually an acceptable or desirable option for Councils due to their statutory duty to maintain watercourses under the Flood Risk Management (Scotland) Act 2009.

„Do minimum‟

This is usually the provision of on-going maintenance of the current situation (as per the Council‟s statutory obligation). The „do minimum‟ option is often used as a baseline for assessing the potential benefit of flood alleviation works, particularly for watercourses which currently require a heavy maintenance regime.

„Do something‟

This is the provision and maintenance of a flood alleviation scheme. This includes both structural engineered ways to reduce flood risk and also non-structural alternatives such as flood warning, emergency response, and land use planning etc.

13.1 Economic Appraisal Policy and Guidance

The policy document entitled “Delivering Sustainable Flood Risk Management” (Scottish Government, 2011) states that the impacts of flooding should be assessed not only in financial terms, but also in terms human health, environment and cultural heritage.

The current appraisal guidance in Scotland is “Chapter 5 - Flood Protection Schemes: Guidance for Local Authorities – Project Appraisal” (Scottish Government, 2012). This document is considered to be interim guidance as the intention is to replace this with guidance on appraisal of the whole flood risk management planning process. In England and Wales, Flood and Coastal Erosion Risk Management appraisal guidance (FCERM-AG) is produced by the Environment Agency (2010). A spreadsheet template for preparing the benefit cost analysis is included, together with supplementary guidance. Current Scottish guidance is largely based on this.

In order to evaluate the net benefits of implementing a scheme, the damage costs avoided with the preferred scheme/s in place are compared against those of the „do nothing‟ and/or „do-minimum‟ options. The damages for flood events of a range of probabilities are estimated and an average annual damage value determined. Damage costs were calculated from 2010 flood loss tables, as detailed in the „Multi-coloured Manual‟ (MCM) prepared by the Flood Hazard Research Centre (FHRC) at Middlesex University (Penning - Rowsell, 2005) and updated in the 2010 „Multi-coloured Handbook‟ (MCH) (Penning - Rowsell, 2010).

Damage to residential properties is based on property type and age, social class of residents and depth and duration of inundation. Damages to non-residential properties (NRPs) are assessed based on property type (i.e. retail, office, public


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building etc.), property size and depth and duration of inundation. Clean-up and emergency services costs (i.e. police, fire, ambulance, Council, military, etc.) were also estimated from recommendations in the MCM.

Guidance recommends using a level of detail proportional to the scale of the project. For simplicity and robustness, only direct damages have been included in the calculation of flood damage costs in this study. More complex indirect and intangible losses such as consumer/supplier losses, traffic disruption and effects on human health have not been taken into account.

Although economic viability is a principal constraint, the final scheme implementation decision will ultimately take due account many complex inter-related factors including social and environmental factors and other indirect and intangible factors as necessary. Many of these factors are not quantifiable in monetary terms. Any known impact which cannot be quantified should not be valued at £0; rather, they should be identified and described so that they can be included in the appraisal process (Scottish Government, 2012).

13.2 Present Values (PV)

The costs (including capital and maintenance) and damages incurred over the entire life of the scheme are discounted to present day values (PV). The appraisal period should reflect the physical life of the longest lived asset of a scheme. With the various scheme options typically involving some form of earthworks, concrete and masonry structures, a 100 year timeframe is considered appropriate with capital replacement of the flood defence assets assumed after 50 years (assuming an appropriate maintenance schedule is in place). Some scheme components with a shorter lifespan may need to be replaced during the lifetime of the wider scheme (e.g. pumps and associated M&E may need replaced after 25 years).

The current test discount rates used (as specified by the Treasury Green Book) are 3.5% for years 0-30, 3% for years 31-75, and 2.5% thereafter.

13.3 Optimism Bias

There is a widely recognised tendency to be overly optimistic when estimating project costs, timescales and benefits compared with actual final outturn costs. This is known as „optimism bias‟. This bias is applied as a percentage uplift of the estimated present value costs, this includes both capital and maintenance costs. For this assessment, an optimism bias of 60% has generally been applied to reflect the preliminary nature of scheme option development. Optimism bias estimates can be refined / reduced in later study stages where scheme details are better known and where risk minimisation strategies have been developed.

13.4 Benefits Methodology

The benefit of a scheme is measured in terms of the present value of the damages avoided over the life of that scheme. Using a range of flood events of different probabilities allows an annual average damage value to be determined for each scheme, which is then discounted to present day values. The damages are categorised into residential losses, NRP losses, clean-up and emergency services costs.

13.4.1 Residential Property

To calculate the residential losses, the type and age of each affected property and the social class of the occupants must be known. The depth of flood water in relation to ground floor level and the duration of the flooding must also be estimated.

The property type and doorstep elevation of each affected property was established from survey work. The social classes of the residents were determined from the April


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2001 census data (results for the 2011 census are planned for late 2012). As the social class variable derived from census data relates to areas as a whole, and not individual properties, the social class of each property has been calculated on the basis of averages. The percentage of the population within each social class is shown in Table 23. The depth / damage data was weighted accordingly. The percentages used were based on the socio-economic classification data for the specific areas, provided on the Scotland's Census Results Online (SCROL) website (www.scrol.gov.uk).

Table 23 - Social Class Categories for Callander

Region No. People Social Class (see below for descriptions)

AB C1 C2 D2 E

Callander 2155 19.7% 32.4% 14.9% 14.5% 18.4%

Social Class descriptions:

AB Higher and intermediate managerial / administrative / professional

C1 Supervisory, clerical, junior managerial / administrative / professional

C2 Skilled manual workers

D Semi-skilled and unskilled manual workers

E On state benefit, unemployed, lowest grade workers

13.4.2 Non-residential property

The MCH provides flood damage data for NRPs in terms of area of premises inundated, depth and duration of inundation and type of business. The depth of the flood water was estimated in the same way as for the residential properties. Information on business type was collected as part of the property survey and the area of each of the premises was calculated from MasterMap data. Where a single NRP had more than one floor level, the depths, areas and damages were apportioned appropriately.

13.4.3 Emergency and Clean-up Costs

Research by the FHRC (Penning-Rowsell et al., 2002) has found that the emergency services costs for the autumn 2000 floods amounted to 10.7% of the total economic property losses. The MCH therefore recommends that the emergency costs are calculated as 10.7% of the economic property damage for floods of all annual probabilities and for all prevention schemes. The data sources used by FHRC for this estimation included District and County Councils, the fire, police and ambulance services, the military, water authorities and voluntary services.

Clean-up costs for residential property are included in the depth-damage tables provided in the MCH. For NRPs there is no guidance in the MCH (or the MCM) regarding the inclusion of clean-up costs, and these have therefore not been included.

13.4.4 Damage Capping

Economic appraisal guidance states that the total present value of long term economic flooding losses for a particular property should not exceed the current capital value of that property. Where these damage values exceed estimated market value, a cap has been applied.

2 For benefit cost purposes, social classes D and E are considered to be the same.

http://www.scrol.gov.uk/


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For residential property the capping value was based on valuations provided by property websites, www.Findaproperty.com and www.Rightmove.co.uk. These valuations are based on the following:

Previous sold prices and sale of nearby properties

Building-specific data such as site and location

Data from estate agents and other website users

Data from other organisations (e.g. Environment Agency)

The above assessment will be more objective and consistent than simply making judgements based on property sales in the area. There will be cases where the valuation is inaccurate for various reasons. Sensitivity testing was carried out to determine the impact this could have on the overall outcome of the assessment.

For NRPs the capping value was based on rateable value which was obtained from the Scottish Assessors Association website (www.saa.gov.uk). As recommended in the MCH, rateable value was multiplied by 10 to derive an approximate valuation.

The above data sources are considered to be the most accurate sources of valuation data short of individual property surveys.

13.4.5 Residual Damage

It is difficult, if not impossible, to design a scheme to protect against flooding from all events. Flood defence schemes are typically designed to protect against events with exceedance probabilities ranging from 2% (1:50) to 0.5% (1:200). In the event of a more extreme event occurring, the scheme will be overtopped, resulting in residual damages. It is important to include these residual damages when undertaking economic appraisals over the design lifespan of a scheme.

It is also important to consider whether any options proposed could increase the severity of these exceedance events, in which case these additional damages should be taken into account during the appraisal stage. With appropriate and careful design, it is not anticipated that any of the options considered for Callander would have a significant effect on residual damages over the design threshold.

It is also possible for a scheme to bring about benefits during events above the design threshold. For example bypass channels or flood storage areas would lower water levels for all events. Flood walls and embankments would not result in such benefits, since they do not lower flood levels but merely retain flooding.

13.4.6 Depth / Damage Curve

The extent and depth of flooding associated with floods of a range of return periods was established from extensive hydraulic modelling. Modelled water level outputs were entered into GIS and compared to surveyed floor level data to estimate the flood depth at each property.

In order to derive depth / damage relationships, a range of annual exceedance probabilities had to be considered together with the calculation of the damages associated with each event. Once the annual average damage value is derived it is possible to bring all future damage costs to a common timeframe. In this study, the annual probabilities used to derive the depth / damage relationship were 1:2, 1:5, 1:10, 1:25, 1:50, 1:100, 1:200, 1:500 and 1:1000. Using a large range of depth / damage relationships allows for a more accurate and realistic depth / damage curve to be derived.

The damages incurred are also dependant on the duration of inundation (i.e. whether properties are flooded for less than or more than twelve hours). For this study, it was assumed that all affected properties would be flooded for a total duration of less than twelve hours.

http://www.findaproperty.com/

http://www.rightmove.co.uk/

http://www.saa.gov.uk/


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13.4.7 Climate Change

Current guidance recommends the use of judgment when considering potential climate change impacts. SEPA currently recommends a simple uplift of 20% to peak flow is applied to account for potential impact of climate change on river flow. This figure is currently valid until around 2060. The Environment Agency has recently produced supplementary guidance to FCERM-AG on accounting for possible climate change scenarios for England and Wales. However, to account for the fact that climate change will occur gradually, the climate change factor applied to flows was increased over the 100 year assessment period as shown in Figure 55 below. This shape results in a 20% allowance around 2060 (the year to which SEPA‟s is applicable) and is the same as that recommended for the Tweed River Basin in the Environment Agency (2011) guidance. This is considered to be a conservative, robust and defensible assumption and agreed with SEPA.

Base year used in

assessment (2012)

0

5

10

15

20

25

30

35

2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110

% c

ha

ng

e in

riv

er

flo

w d

ue

to

clim

ate

ch

an

ge

Factor used in assessment SEPA guidance Extrapolation beyond 2099

Figure 55 - Change in River Flow Due to Climate Change

13.5 Summary of Benefit / Cost Methodology

In summary, the following parameter assumptions have been made in the course of the benefit/cost analyses:

Damages based on all latest flood-mapping and modelling

Climate change allowance included – 0-15% uplift up to 2025, 20% uplift from 2040 and 30% from 2070 onwards

Prices and base year as of March 2012

Optimism bias taken as 60%

Discount rate 3.5% for years 0-30, 3% for years 31-75, and 2.5% thereafter

Indirect/intangible and traffic related losses ignored

Flooding losses to land/gardens and agricultural land ignored

100 year scheme lifespan with complete capital replacement of any flood defence assets with a lower lifespan (e.g. pumps, gates, etc.).

Residual damages included in analyses as appropriate

Net costs are typically used for B/C analysis in instances where „do nothing‟ is not considered an acceptable option


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10.7 % of property damage value added to account for emergency services costs

Clean-up costs included in depth-damage data from MCH for residential property only.

Damages capped at estimated property market values

A set of excel worksheets developed on behalf of the Environment Agency (2010) was used as a basis to carry out the benefit / cost analysis. The benefit / cost worksheets calculate the present value (PV) damages and costs for the options. The damages can be categorised into damages due to a single major event, such as a wall breach, or repetitive damages, such as that due to overtopping. An evaluation of scheme viability can then be made based on the benefit/cost relationships of the various options.

13.6 Callander Flood Damages (Benefits)

13.6.1 Onset of flooding

The probability of the onset of flooding to properties is indicative of the level of flood damage like to accrue. If a property floods, on average, every two years (i.e. 50% AEP), then damages will accrue significantly over a 100 year time period. Conversely, if a property floods, on average, only once every 100 years, then damages over 100 years will be significantly smaller. An indication of the onset of flooding (i.e. the return period of the flood event which results in floodwater higher than floor level) in Callander is shown below in the following figures. It should be noted that based on MCM data, some damage costs start to accrue within 300 mm of floor level for residential properties, although once flood levels are above actual floor level, these damages increase significantly.


Figure 56 - Probability of Flooding Onset - Leny Road


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Figure 57 - Probability of Flooding Onset - Meadows Car Park / Main Street


Figure 58 - Probability of Flooding Onset - Bridgend (South)


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Figure 59 - Probability of Flooding Onset – Bridgend (North) Including School Area


Figure 60 - Probability of Flooding Onset - Bridge Street to Footbridge


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Figure 61 - Probability of Flooding Onset - Footbridge to Buchanan Place

From the above figures it is evident that the most frequent property flooding experienced is at Meadows Car Park. However, the onset of property flooding is between 10 – 25 year return periods. By typical standards, property flooding elsewhere in Callander is not particularly frequent.

13.6.2 Flood Damages

For a number of discrete locations in Callander as shown in Figure 62 (in cognisance with the possible FAS option extents explored earlier), estimated PV flood damages over an assumed 100 year scheme lifespan are summarised in Table 24 and Table 25 below. Damages occurring for all flooding not exceeding 200, 100 and 50 year return period are also shown. This information is particularly useful when considering where most of the damage is occurring in terms of flood return period and the associated residual damage if implementing a scheme which protects to say, 50 year return period (i.e. there will still be flooding damage to account for when these design levels are exceeded).

As a result of climate change, the standard of protection of a scheme effectively reduces during its design life. A scheme designed to have a 200 year standard of protection at the start of its 100 year lifespan would have an eventual standard of protection of the order of 76 years (assuming 30% increase in flow due to climate change). Conversely, a scheme designed to have a 200 year standard of protection at the end of its 100 year lifespan would need an initial standard of protection of the order of 600 years. This issue needs acknowledged within the economic appraisal. However, to limit the volume of model runs required, the benefits (damages) presented here assume an initial standard or protection of 200 years, which will reduce gradually as a result of climate change. By inspection, it is considered that calculating the additional benefits (and associated residual damages) would not materially affect the outcome of this assessment. An increase in damages of the order of 30% would be anticipated if the initial additional benefits at the start of the scheme life are accounted for (assuming a 200 year standard of protection).


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The damages shown also include emergency services and clean-up costs. The damages are associated only with flood originating from the Teith.

It should be noted that the existing car wash building to the north of Meadows Car Park (which is one of the first buildings to flood) has not been included in the damage figures as it is literally a shed with a concrete floor and is also understood to be due for re-development into a residential property with new floor levels being built to appropriate elevation above flood levels.


Figure 62 - Damage Assessment Areas

BRIDGEND B

BRIDGEND A

BRIDGEND C

PRIMARY SCHOOL AREA


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Table 24 - PV Damages

Scheme area

PVd - no climate change

Total PVd PVd 0.5% AEP PVd 1% AEP PVd 2% AEP

All £1,558,139 £ 726,832 £ 436,072 £ 189,705

Meadows Car Park £1,057,286 £ 545,955 £ 336,904 £ 155,757

Bridge Street to Buchanan Place £ 258,933 £ 73,107 £ 31,507 £ 3,400

Bridgend B £ 123,180 £ 82,293 £ 57,204 £ 28,839

Bridgend A £ 50,262 £ 14,041 £ 5,934 £ 1,349

Leny Road £ 22,750 £ 9,241 £ 4,523 £ 361

Primary School Area £ 31,955 £ 2,075 £ - £ -

Bridgend C £ 10,100 £ 120 £ - £ -

Lagrannoch Industrial Estate £ 3,674 £ - £ - £ -

Table 25 – PV Damages (Including Climate Change Uplift)

Scheme area

PVd - with climate change


All £2,839,380 £1,319,946 £ 741,521 £ 260,790

Meadows Car Park £1,916,718 £ 984,279 £ 568,401 £ 214,934

Bridge Street to Buchanan Place £ 483,115 £ 141,926 £ 59,170 £ 4,325

Bridgend B £ 218,880 £ 144,617 £ 94,707 £ 39,357

Bridgend A £ 93,436 £ 26,790 £ 10,662 £ 1,715

Leny Road £ 42,696 £ 17,967 £ 8,581 £ 459

Primary School Area £ 59,072 £ 4,127 £ - £ -

Bridgend C £ 18,619 £ 239 £ - £ -

Lagrannoch Industrial Estate £ 6,844 £ - £ - £ -

It can be seen from the above tables that a total of around £1.56M (£2.48M with climate change uplifts) worth of damage is estimated to occur as a result of fluvial flooding emanating from the Teith over the next 100 years across Callander as a whole. The damage associated with events not exceeding 0.5% AEP is £0.73M, or £1.32M with a climate change uplift included (i.e. £0.83M of damage occurs for events more extreme than 200 year return period, or £1.52M with a climate change uplift).

For economic viability, scheme costs require to be less than the damages saved once the scheme is in place. By simple inspection, economic viability will be difficult to achieve for the main scheme options outlined earlier.

The relative damage contributions from the various properties in Callander, accounting for the total PVd (including climate change) are shown in Figure 63 and Table 26 below. It is apparent that most damages are occurring in the Meadows Car Park area.


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EDINBURGH WOOLLEN MILL, 7 MAIN STREET

1%

RIVERSIDE HOUSE, LENY ROAD

1%

CALLANDER NURSERY SCHOOL (CABIN),

BRIDGEND

2%

CRAIGVIEW, 9 BRIDGE STREET

2%

IDEAS, 1 STATION ROAD

2%

54 BRIDGEND

2%

SWEET BOUTIQUE, 6B MAIN STREET

2%

58 BRIDGEND

2%

WATERSIDE HOUSE, SOUTH CHURCH STREET

2%

AVONBEITH, 16 SOUTH CHURCH STREET

2%

ST KHESSAG HOUSE, 6 BRIDGE STREET

2%

4 LENY ROAD

2%

BENVUE, 56 BRIDGEND

2%

CRAIGROYSTON, 4 BRIDGE STREET

3%

CALLANDER WOOLLEN MILL, 16-18 MAIN

STREET

3%

MHOR BREAD, 8 MAIN STREET

3%

1 MAIN STREET

3%

RIVERSIDE INN, 8 LENY ROAD

4%

UNIT 2 - TREE HOUSE, 4A MAIN STREET

4%

NUTCRACKER CHRISTMAS SHOP, 4B MAIN

STREET

4%

NUTCRACKER CHRISTMAS SHOP, 4C MAIN

STREET

4%

DREADNOUGHT HOTEL (UPPER), LENY ROAD

4%

WEE BUT 'N' BEN BISTRO, 12 MAIN STREET

4%

UNIT 1 - THE HANDY CORNER SHOP, 4A MAIN

STREET

5%

TASTY FRY, 6 MAIN STREET

5%

MHOR BREAD (REAR), 8 MAIN STREET

6%

CALLANDER WOOLLEN MILL, 16-18 MAIN

STREET

7%

DREADNOUGHT HOTEL (LOWER), LENY ROAD

15%

Figure 63 - Property Damage Contributions (Pie Chart)


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Table 26 - Property Damage Contributions (Percentages)

Scheme area

PVd contributions - with climate change


All 100% 100% 100% 100%

Meadows Car Park 68% 75% 77% 82%

Bridge Street to Buchanan Place 17% 11% 8% 2%

Bridgend B 8% 11% 13% 15%

Bridgend A 3% 2% 1% 1%

Leny Road 2% 1% 1% 0%

Primary School Area 2% 0% 0% 0%

Bridgend C 1% 0% 0% 0%

Lagrannoch Industrial Estate 0% 0% 0% 0%

13.6.3 Damage Capping

No property damage capping was found to be necessary for Callander due to the relatively infrequent onset of property flooding and no accrual of damages in excess of likely individual property values, over the 100 year economic appraisal period.

13.6.4 Sensitivity Checks

A sensitivity analysis was carried out to determine the impact of any inaccuracies in the components of the damages on the total PVd. The damages presented above represent the best estimate based on all currently available data, however it is important to consider the potential impact of inaccuracy or error. The key variables identified in the sensitivity analysis which would likely affect damage values the most are computed water levels and the assumptions made for the Dreadnought Hotel.

Hydraulic data (water levels) were derived from a well calibrated hydraulic model. Inaccuracies in the hydrology however could affect the predicted water levels, and this is particularly the case considering the uncertainty associated with the Eas Gobhain @ Loch Venachar rating and the joint probability of Eas Gobhain / Leny flows. Furthermore, the flows derived for Callander are based on a limited dataset, and hence additional data could have a significant impact on the design flows, and hence water levels. A 100 mm increase in water levels for all scenarios would cause the total PVd in Callander to increase by 22% to £1.9M.

No damage capping was required for Callander, and hence the depth/damage data set is a particularly important variable. The Dreadnought Hotel contributed 13% of all damages in Callander; the single largest proportion. Any significant changes in this number could therefore have an effect on the overall PVd. The basement of the hotel has not been included in the assessment as it was considered that floodwaters would have to spill into the basement from street level rather than be directly linked to levels in the river. Including the basement for the lower portion of the hotel would cause the PVd for the hotel to almost double to £399k, which in turn increased the total PVd for Callander by 12%. Using a low susceptibility depth-damage curve for the hotel would cause the PVd for the hotel to decrease by 42% to £121k, which in turn decreased the total PVd for Callander by 5.6%.

Considering the findings of the sensitivity analysis, it can be concluded that the damages presented are as accurate as possible using the available data. The results are a conservative assessment of the damages likely to occur over the next 100 years, particularly considering climate change uplifts.

13.7 Option Costing

The flood defence options for Callander as described earlier have been provisionally costed. These cost estimates are outline in nature and would be refined in later study


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stages if scheme details are further developed. Issues such as ground conditions and riparian ownership will also be significant factors determining scheme feasibility.

13.7.1 Meadows Car Park FAS

A breakdown of the key elements of the Meadows Car Park FAS option is presented in Table 27. For costing purposes, the 50 year return period scheme including 300mm freeboard has been taken forward as it is considered the most feasible and practical scheme option.

It is assumed that the flood wall ties directly into the „Tom ma Chisaig‟ historic monument. The feasibility of this tie-in would need discussed and confirmed with Historic Scotland. Additional flood walling will be needed if the tie-in needs to be located further east. For costing purposes a natural stone clad concrete wall is assumed for construction however, this would be dependent upon achieving sufficient foundation and cut-off. With the old stone bridge adjacent, it is probable that high rock will be encountered in the vicinity. However, depending on prevailing ground conditions a sheet pile option may be the most appropriate solution, particularly if a sheet pile cut-off is required. The susceptibility of nearby properties to vibration will need assessed, particularly if pile driving activities are required. Pre-construction structural surveys of nearby properties would be recommended.

The potential option of a sealed gravity discharging drainage pipe from higher ground at the northerly car park is included in the costing. The feasibility of this option however is dependent on avoiding clashes with existing pipes and services and dealing with any existing connections.

Costs associated with larger schemes protecting to higher return periods will be significantly higher. Such a scheme would involve much more extensive works extending beyond the car park area.

Table 27 - Meadows Car Park FAS Outline Costing

Meadows Car Park Flood Wall No. Unit Rate Cost

Concrete flood wall (1.6 m average height) 120 m

£1,400 £168,000

Natural stone facing 384 m2

£120 £46,080

Natural stone coping 120 m

£100 £12,000

Flood gate (sliding, 5m clear opening, 1.8 m high) 2 - £50,000 £100,000

Porous back drain (assumed 300 mm diam. including bedding and surround)

110 m £130 £14,300

Back drainage pumping station and associated M&E works including discharge pipe run

1 - - £200,000

Sealed pressure tight 750 mm gravity pipeline 250 m £250 £62,500

Sealed pressure tight manhole chambers on gravity pipeline (1.8 m diam.)

6 - £3,000 £18,000

Manhole chambers on back drainage (1.2 m diam.) 6 - £1,600 £9,600

Dual concrete outfall structure for new sealed pipeline (750 mm diam.) and pump discharge

1 - £4,500 £4,500

Flap vales for existing culvert discharge (675 mm diam.)

1 - £750 £750

TOTAL £635,730

It has been assumed that the pumping station would need a complete M&E re-fit every 25 years and that floodgate elements would need replaced every 50 years. These major capital refurbishment costs have been included in the final PV Cost.

Flood gate replacements = £ 100,000

Pumping station M&E replacement = £ 110,000


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13.7.2 Bridgend West FAS

A breakdown of the key elements of the Bridgend West FAS option is presented in Table 28. For costing purposes, the 200 year return period (plus climate change) scheme has been assumed comprising mainly earthen embankments with clay core and 300mm freeboard. For costing purposes, a stone faced concrete floodwall section has been assumed at the north of Bridgend which connects the existing masonry bridge to the earth embankment. This FAS scheme is designed to remove flooding from all the „Bridgend‟ locations as shown in Figure 62, except the primary school area.

Table 28 – Bridgend West FAS Outline Costing

Bridgend West Flood Embankment No. Unit Rate Cost

Concrete flood wall (3m average height) 75 m

£2,500 £187,500


£120 £27,000


£100 £7,500

Clay core earthen flood embankment (1.2 m average height)

300 m £320 £96,000

TOTAL £318,000

13.7.3 Bridgend East FAS

A breakdown of the key elements of the Bridgend East FAS option is presented in Table 29. This FAS scheme is designed to remove flooding from the primary school area as shown in Figure 62. For costing purposes, the 200 year return period (plus climate change) scheme has been assumed comprising mainly earthen embankments with clay core with 300 mm freeboard. A stone faced concrete retaining wall / floodwall section has been included to tie the embankment into higher ground near the school entrance and maintain the existing pedestrian access.

Table 29 - Bridgend East FAS Outline Costing

Bridgend East Flood Embankment No. Unit Rate Cost

Concrete flood wall / U shaped channel retaining wall / ramp (variable height from 0 m to 1 m)

50 m

£1,000 £50,000

Granular fill material 45 m3 £40 £1,800

Access track blacktop surfacing (1.8 m wide) 90 m2

£50 £4,500


£120 £6,000


£100 £10,000

Stainless steel pedestrian guardrail (1.1m high) 100 m £175 £17,500


150 m £450 £67,500

TOTAL £157,300

13.7.4 Bridge Street to Buchanan Place FAS

A breakdown of the key elements of the Bridge Street to Buchanan Place FAS option is presented in Table 30. For costing purposes, the 200 year return period (plus climate change) scheme has been assumed comprising a set-back clay core earthen embankment upstream of the existing footbridge at South Church Street and a sheet pile floodwall downstream of the footbridge (all assuming 300 mm freeboard). The final solution will be dependent on prevailing ground conditions.

There will be a requirement to accommodate existing surface water pipes and CSOs which discharge to the river along this reach. It is understood that the existing pedestrian bridge is due for replacement. Details regarding the tie in arrangements to the new footbridge and the potential flow cut off from bridge Street are unknown at this stage and the costs associated with these details are not included in the outline costing.


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The susceptibility of nearby properties to vibration will need assessed, particularly considering the fact that there will be pile driving activities. Pre-construction structural surveys of nearby properties would be recommended.

The option of a sealed gravity discharging drainage pipe from higher ground, potentially from around the top of South Church Street, is included in the costing. The feasibility of this option however is dependent on avoiding clashes with existing pipes and services and dealing with any existing connections.

Table 30 - Bridge Street to Buchanan Place FAS

Bridge Street to Buchanan Place Floodwall No. Unit Rate Cost


230 m £380 £87,400

Sheet pile flood wall inc. concrete coping (1.3 m average height)

280 m

£450 £126,000

Porous back drain (assumed 300 mm diam. including bedding and surround)

490 m £130 £63,700

Manhole chambers on back drainage (1.2 m diam.) 15 - £1,600 £24,000

Back drainage pumping station and associated M&E works including discharge pipe run

1 - - £200,000

Sealed pressure tight 300 mm gravity drainage pipe 130 m £200 £26,000

Sealed pressure tight manhole chambers on sealed gravity system (1.2 m diam.)

3 - £2,500 £7,500

New concrete outfall structure and flap valve for existing river discharge pipe (150 mm diam.)

1 - £1,500 £1,500

New concrete outfall structure and flap valve for new sealed river discharge pipe (300 mm diam.)

1 - £1,500 £1,500

New concrete outfall structure and flap valve for existing river discharge pipe (960 mm diam. CSO)

1 - £2,750 £2,750

New concrete outfall structure and flap valve for existing river discharge pipe (225 mm diam. CSO)

1 - £1,500 £1,500

TOTAL £541,850

It has been assumed that the pumping station would need a complete M&E re-fit every 25 years. These capital refurbishment costs have been included in the final PV Cost.

Pumping station M&E replacement = £ 110,000

13.7.5 Lubnaig / Balvag / Voil Flow Control FAS

By simple inspection, options that involve extensive upstream flood storage works would not represent an economically viable scheme for Callander. Consequently, no scheme costings have been developed for these upstream storage options at this stage. These upstream storage options may be explored in further detail at a later stage if considered of benefit and potentially economically viable in addressing flooding to the city of Stirling.

13.7.6 Other Miscellaneous Scheme Costs

In addition to the capital construction costs of the key elements outlined above, the following typical miscellaneous construction costs require to be included (expressed as a percentage of the capital construction costs):

Preliminary Works (15%) (site compound, storage areas, traffic management etc.)

Utility Crossings / Diversions (5%)

Accommodation Works / Landscaping / Reinstatement (10%)


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Land purchase may be required depending upon the final scheme footprint and associated land ownership. Therefore, the following item is nominally included:

Land Vesting (5%)

Project fees for consultancy and contracting services also need included in the overall option costs. These include such items as: project management, site data collection, detailed design, ground investigations and data collection, topographic and environmental surveys, contract preparation, tender, CDM, planning application, environmental reports, land owner identification, consultation and site supervision. Costs should also include client staff time and compliance with the Controlled Activities Regulations (CAR). Typical costs for inclusion are:

Design and Consultation, including structural surveys (15%)

GI, Topographical Survey and Environmental (10%)

Site Supervision (10%)

Costs for routine maintenance also need considered and included in the overall option costs and apportioned on an annual basis over the 100 year scheme lifespan. Table 31 shows the annual cost build-up of the key maintenance elements and

Table 32 shows the total annual maintenance cost for each of the Callander Scheme Options.

Table 31 – Annual Maintenance Cost Breakdown

Maintenance Element Occasions

per year

Staff x

Hours

Labour Rate

Materials Cost per Annum

General inspection & maintenance of flood defence assets

2 2x8 £40 £1,500 £2,780

Repair of flood walls – stone cladding - assume £1500 of cladding material per year and 2 days labour

1 2x16 £40 £1,500 £2,780

Maintenance of pumping station 1 2x8 £40 £2,500 £3,140

Table 32 - Annual Maintenance Costs per Scheme Option

Scheme Option Cost per Annum

Meadows Car Park FAS £8,700

Bridgend West FAS £5,560

Bridgend East FAS £2,780

Bridge Street to Buchanan Place FAS £5,920

13.7.7 Present Value (PV) Scheme Costs

The various estimated scheme costs outlined above have been brought to a present value (March 2012) as outlined in Table 33. The PV capital costs include the miscellaneous costs and project fees and also include the capital replacement / refurbishment costs. Operation and maintenance costs are for 100 year lifespan of the scheme.


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Table 33 - Present Value (PV) Scheme Costs

Meadows Car Park

FAS (50 year)

Bridgend West FAS

(200 year + CC)

Bridgend East FAS

(200 year + CC)


FAS (200 year + CC)

Main Scheme Element PV

Capital Costs £635,730 £318,000 £157,300 £541,850

Other Miscellaneous Scheme Costs

(35%)

£222,506 £111,300 £55,055 £189,648

Project Fees for Consultancy and

Contracting (35%) £300,382 £150,255 £74,324 £256,024

Future PV Asset Replacement

Costs £98,335 £0 £0 £78,609

Future PV Maintenance

Costs £259,369 £165,758 £82,879 £176,490

Optimism Bias (60% of all)

£850,792 £447,188 £221,735 £698,407

TOTAL PV COST (inc. optimism

bias) £2,367,114 £1,192,501 £591,293 £1,941,028

13.8 Cost / Benefit Appraisal

Using FCERM-AG excel worksheets, PV costs (including 60% optimism bias) and PV damages (including emergency services, clean-up costs, climate change uplifts and any capping) were calculated. Residual damages have been taken as those damages occurring due to flood events more extreme than the design threshold, over the lifetime of the scheme. These residual damage costs need to be subtracted from the total damage costs to derive the actual damages saved (benefits) over the lifetime of the scheme. These PV benefits are then divided by the PV scheme cost to derive the B/C ratio. PV benefits, costs and B/C ratios are summarised below in Table 34.

Table 34 - Benefit Cost Ratios

Scheme Total PV Damage

PV Residual Damage

PV Net Damage (Benefits)

PV Cost

B/C Ratio

Meadows Car Park FAS (50 year protection)

£1,917k £1,702k £215k £2,367k 0.09

Bridgend West FAS (200 year protection)

£331k £159k £172k £1,192k 0.14

Bridgend East FAS (200 year protection)

£59k £55k £4k £591k 0.01

Bridge Street to Buchanan Place FAS (200 year protection)

£483k £341k £142k £1,941k 0.07

All of the direct flood defence options considered yield a B/C ratio of significantly less than unity and represent non-viable economic solutions. All of the FCERM-AG based excel worksheets used to develop this assessment are available upon request.


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13.9 Further Benefit / Cost Discussion

Damages associated with the existing car wash compound to the rear of Meadows Car Park have not been included in the B/C analysis as it comprises a shed with a concrete floor. It is also understood to be due for re-development into a residential property and as such, should be constructed with floor levels to an appropriate elevation in due course.

In determining residual damage values, it has been simply assumed that the damages associated with events exceeding the design threshold will be of a similar order even with the scheme in place. No additional damage appraisals were undertaken to determine the residual damages based on model results with scheme options in place.

Flooding that exceeds property floor levels is relatively infrequent in Callander. Although flooding in and around the car park area is often visible and is perceived to be severe, the frequency of flooding in excess of property floor levels is less frequent (onset between 10 – 25 years around Meadows Car Park). A floodwall scheme which protects all the properties to the rear of Meadows Car Park therefore struggles with economic viability. It is worth noting that a scheme to protect the Meadows Car Park area to a return period of 200y plus climate change would yield benefits of £0.98M. However, the fact that a 50 year scheme is roughly costed at £2.42M, means that a more extensive scheme involving ramping, gates and more extensive and higher walling would not yield a positive B/C ratio either.

If any of the outline options proposed for Callander were to be taken forward, the noted water level impact issues would need addressed through some form of mitigation works.

Indirect and intangible damages, together with other potential benefits of an implemented scheme (environmental, social, etc.) should always be considered in final scheme appraisal as most of these factors cannot easily be attributed simple monetary values.

It should be noted that if a scheme is designed to account for climate change, the standard of protection at year 0 will be higher than at the end of the scheme life, and consequently the benefits will be higher as well. It is likely that such benefits would diminish relatively quickly. In addition, there is a high level of uncertainty associated with climate change impacts; indeed, such impacts could be higher than the 30% included in the benefit cost assessment.

There is always scope for refining scheme details, costs and associated optimism bias for the options explored for Callander. However, the overall conclusions regarding scheme viability will likely remain.


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14 Summary and Conclusions

14.1 Study Aims and Initial Data Review

The aim of this study was to assess flood risk to Callander from the River Teith and to investigate the feasibility and viability of possible flood mitigation options, including both traditional direct defences (including demountables) and also more natural measures including upstream flood storage options.

A data collection and review was undertaken of available existing information including various reports and models. The Teith catchment has been subject to a number of flood studies over the years. The most recent study was carried out by Atkins in 2005 and updated in 2010.

This study uses and builds upon existing information including principally the hydraulic model initially developed by Atkins for the River Teith through Callander.

14.2 Stakeholder Consultations and Additional Data Collection

The review included stakeholder engagement with Scottish Water and SEPA to discuss / agree data availability, agree hydrological approaches, collect and collate relevant loch operational information and to identify key constraints. The latest hydrometric information was also collated from SEPA, Stirling Council and Scottish Water.

From stakeholder consultations and reviewing existing information, the key findings were that there are significant limitations on what can realistically be achieved on the Katrine / Venachar system (bearing in mind this system is already heavily modified and attenuated). It is considered that to make the Katrine system more effective at attenuating flood flows through Callander, all sluices would require to be computer controlled, be electronically actuated, based on real time loch levels, long and short range weather predictions, abstraction requirements, rainfall data (spatially varying) and linked to flow gauging on the River Leny. Extensive and detailed water supply studies would also be required before any options that involve winter draw downs (more than already done by Scottish Water) could be implemented.

It was considered that exploring options for flood storage on the Lubnaig / Voil system would be more practical and potentially more fruitful, considering this system has a similar catchment size to Eas Gobhain and remains largely unmodified. However, some limited modelling of the flood attenuation effects of various weir height / draw down scenarios for Loch Venachar was also undertaken.

Topographical surveys were undertaken to facilitate extension and refinement of the existing Teith hydraulic model and also to allow construction of the hydrological / hydraulic routing models required to assess upper lochs storage options. Property threshold surveys were also undertaken to facilitate the benefit / cost analyses.

14.3 Hydrology

A hydrological review was carried out in liaison with SEPA.

To derive design flows for the Teith through Callander for a range of return periods, data from the SEPA gauges at Callander and Bridge of Teith were utilised, in conjunction with the FEH Statistical Approach. The 200 year design flow for Callander was estimated at 376 m3/s.

For the Voil / Lubnaig routing model, flow hydrographs for the various tributary inflows were derived using the FEH Rainfall Runoff method. The hydrological model parameters were then calibrated using observed hydrometric data (rainfall from


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Strathyre gauge and river flows from Anie gauge). The calibrated rainfall-runoff model was then used to derive design hydrographs for a range of exceedance probabilities which could then be used for flood storage option testing.

As part of the hydrological review, an assessment was made regarding the relative contributions of the Eas Gobhain and River Leny to the total flow at Callander. Due to the significant attenuation afforded by the lochs in the Eas Gobhain catchment, the flow split was found to be 71% Leny / 29% Eas Gobhain on average for flood events. There is tentative evidence to show that with increasing event rarity, the flow split becomes more equal; the Eas Gobhain system tending towards a more natural response. For example in December 2006 the flow split was 61% Leny / 39% Eas Gobhain. Some uncertainty regarding the suitability of the Eas Gobhain rating for high flows was raised by SEPA.

14.4 Hydraulic Modelling

The existing InfoWorks RS hydraulic model was converted and extended into a fully linked 1D-2D ISIS-TUFLOW model. The development of a full 2D model was considered necessary to capture the complex floodplain dynamics, improve property flooding assessments and to better facilitate flood scheme optioneering and impact assessments. The updated model extends through Callander and also includes the two tributaries and floodplain areas on the westerly side of Callander. There is good confidence in the model results; the model is calibrated to within ±100mm of measured water levels for the five largest observed flood events.

Modelling has confirmed the flooding extents, frequency and mechanisms of flooding through Callander. The key area at risk of flooding is Meadows Car Park, which is at least partially flooded for all return periods modelled. For the 200 year event, water is around 1m deep on Main Street. The onset of property flooding at Meadows Car Park is somewhere within the 10 to 25 year return period range. Other areas within the 200 year flood outline include parts of Leny Road, Bridge Street and the left bank of the Teith downstream of the road bridge. For the latter, flooding is predicted to extend as far in-bank as Pearl Street, at the junction with South Church Street for the 200 year flood event. Callander Primary School is largely above 200 year levels, although the playing fields start to flood between the 10 and 25 year return periods.

The loch routing models have been developed using HEC-RAS Version 4 and comprise upstream loch storage areas, floodplain areas, river channels and various hydraulic controls / structures. The Leny routing model extends from Callander to Loch Voil and the Eas Gobhain model extends to Loch Venachar. The ultimate aim was to develop models that allow the assessment of numerous upstream storage option scenarios and assess how these options ultimately attenuate and reduce flows through Callander.

14.5 Flood Alleviation Options Considered

This study explored both traditional direct protection and upstream storage options for flood alleviation for Callander. Consideration was given to the use of both permanent and demountable options, particularly at the rear of Meadows Car Park to protect Main Street, together with an assessment of the potential impact that these defences may have elsewhere. Other options including individual property flood resilience measures were explored and reviews were undertaken of the potential scope for natural flood management options.

Traditional defence options comprising combinations of earthen embankments, sheet pile / concrete walls and other associated works including flood gates, pumping stations, access ramps, drainage pipes, etc. have been assessed for the following discrete locations in Callander:


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Meadows Car Park

Bridgend West

Bridgend East


Upstream flood storage scenarios and locations on the Leny / Balvag loch system were also investigated. The following potential storage locations were considered:

Flow control at outlet of Loch Lubnaig, thus utilising additional storage potential of Loch Lubnaig.

Flow control on north side of Strathyre, thus potentially utilising some of the extensive floodplain areas between Strathyre and Balquhidder.

Flow control at outlet of Loch Voil, thus utilising additional storage potential of Loch Voil / Loch Doine.

Upstream storage options involve an online restriction comprising a reduction of main river channel width and an accompanying floodplain embankment structure. This arrangement would serve to store peak floodwaters upstream.

Limited option modelling was also undertaken to mainly evaluate the effects of raising the weir height on Venachar (to increase storage potential. The following option variables were tested in the model:

Weir / spillway elevation

Antecedent loch levels

Sluice release rates

14.6 Option Feasibility - Traditional Defences and Demountables

At Meadows Car Park a riverside floodwall protecting the entire car park area and adjacent properties on Main Street for the 200 year flood event (plus climate change) would require a wall over 3.5m high along much of its length. The river front area is a major amenity for the town and attracts many visitors. Any flood-walling which effectively cuts this amenity off, particularly from a visual perspective, would not be an acceptable solution as it would blight this valuable amenity and also result in an associated loss of visitor numbers and business for the town. Even when set back, designing to 200 year levels (plus climate change) is not considered practical due to excessive wall heights. Protecting to 50 year levels was considered a more technically feasible and practical level of protection that wouldn‟t totally cut off the river amenity and ties better into prevailing ground levels and the general configuration of the car park and surrounding areas. The Meadows Car Park scheme would also involve the use of floodgates and a back drainage pumping station.

Scheme options to protect properties around Bridgend typically included earthen embankments (clay core) and between 1:2 (vertical to horizontal) and 1:3 side slopes protecting to 200 year (plus climate change). Some concrete walling and ramping would be required to tie into the existing features.

To protect properties on the left bank between Bridge Street and Buchanan Place, a combination of sheet pile flood walls and earthen embankments was considered the most appropriate option. If implementing the Bridge Street to Buchanan Place FAS, it should be noted that flows emanating from the Meadows Car Park side are predicted to flow across Bridge Street for flood events of 100 year and greater return period. These flows would then become trapped behind the Bridge Street to Buchanan Place floodwall. This problem would need addressed by curtailing the Bridge Street to Buchanan Place floodwall at some point a few metres downstream of the bridge and tying back to higher ground leaving a potential safe flow route back to the river for these flows, just downstream of the bridge. Ancillary works are required at the


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footbridge tie-in location to accommodate FAS design heights, pedestrian access, maintenance access, etc. Ideally any FAS works would be designed and undertaken in conjunction with the proposed new pedestrian footbridge. A back drainage system will also be required (with pumping station).

Minimal water level impacts were noted for the Meadows Car Park and Bridgend East FAS options once implemented. However, impacts for the Bridge Street to Buchanan Place FAS and Bridgend West FAS would be unacceptable without some form of mitigation.

Demountable options have been considered for use in Meadows Car Park Callander but none of the options explored were considered practical.

14.7 Option Feasibility – Upstream Storage

Numerous combinations of upstream restrictions on the Leny system were tested. The most effective options in terms of overall flow reductions achieved in the Leny are those which involve combinations of upstream controls. The following scenario achieved a 50% flow reduction in the Leny.

4 m wide restriction at the head of Voil & 9.5 m restriction at Strathyre & 13 m restriction at the head of Lubnaig

A 50% flow reduction was considered a practical limit for optioneering purposes. Any further restrictions, although hydraulically feasible, would be clearly impractical in terms of the associated increase in extreme water levels. This scenario increases levels on the Voil by around 2.6 m for the 200 year event but importantly, there was minimal net change achieved in extreme flood levels downstream on the Balvag / Lubnaig floodplains.

For Venachar, it is hydraulically feasible to implement measures that would serve to attenuate flows in the Eas Gobhain. The simplest, cheapest and most practical modification to the Venachar weir that could yield some minor reductions in Eas Gobhain flood flows would be the implementation of electronically actuated sluices. The sluices would be controlled by loch level and set to maintain a relatively high constant discharge and a nominal drawdown. The discharging of flood flows in advance of the peak and the maintenance of loch levels is what Scottish Water is already trying to achieve, but within the constraints of a manually operated system.

Options that would increase loch levels (particularly on Lubnaig / Balvag floodplains) would require fairly major civil engineering works and would likely require to be implemented in conjunction with major localised flood defence works to protect a number of individual properties or lands. The vesting of affected properties may also be an option which removes / relocates flood risk properties. Major road raising works would also be required to mitigate the effects of extreme loch level rises.

14.8 Option Feasibility – Natural Flood Management

To explore the potential for using land management NFM options for mitigating the flooding problems in Callander, a brief review of NFM policy, guidance and research literature was undertaken. From the various pilot studies, the changes in peak flow which could be achieved using NFM are generally reported as being limited to around 10%, and this depends highly on the return period of the flood event. The greatest benefits can generally be achieved for the lower return period events. The greatest benefits would also be realised in heavily modified catchments. However, the upper catchment of the River Leny is mostly natural with extensive floodplains and no intensive agricultural activity on the hill slopes. There will be fewer opportunities for restoration in this instance.


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NFM techniques should be considered in conjunction with traditional flood protection schemes, not as an alternative to traditional defences. It is frequently stated that it is the secondary benefits such as environmental, ecological and social factors which make NFM an attractive option. It should also be noted that NFM may not be appropriate in catchments used for water supply (except where groundwater recharge would be of benefit) as many of the NFM techniques actively improve evapotranspiration and infiltration rates in an effort to reduce runoff.

Methods involving catchment de-synchronisation can be difficult to prove and in many instances could be dangerous to rely upon, especially in large catchments (due to the spatial variability in storms) or those with artificial influences (where the response to any particular event is governed by antecedent operational controls). It is therefore considered important to also reduce peak flow, rather than relying solely upon the modification of the timing of the peak, and to ensure all synchronisation scenarios are considered.

For Callander, the most practical and demonstrably effective NFM options (in terms of substantial reductions in flood risk) have been investigated; these comprise utilising and enhancing upstream loch and floodplain storage. However, a key constraint is the impact associated with increased water levels at a number of locations.

In terms of land management / „at source‟ NFM, it was shown that NFM measures would be most effective if applied only to the Loch Voil catchment due to the potential desynchronisation effects. Importantly, it was found that a catchment wide peak runoff reduction (without loss of total runoff volume) is not followed by a similar percentage reduction further downstream at Callander due to the overall attenuating effect of the lochs and lack of desynchronisation. Therefore, any NFM measures pursued for the Leny system should therefore focus significantly on runoff delay of the Voil contributing catchment.

It is suggested that if the Council wishes to investigate in more detail what benefits could be realised in terms of NFM that alternative sources of funding are sought to allow for the various secondary benefits to be included in any feasibility assessments, especially since the flooding benefits to Callander are considered to be limited. Further investigations could include the following:

A thorough literature review (including studies currently on-going) to identify the options available and their potential benefit particularly for Scottish catchments similar to the Leny

A detailed GIS-based assessment of the most appropriate specific locations to implement NFM (similar to Upper Allan study)

More detailed hydrological modelling to determine the benefits of NFM

Advice on the possible funding streams

Such a study should take a catchment-wide approach to be able to incorporate benefits to Stirling.

14.9 Environmental Constraints and Opportunities

A desk-based environmental assessment was carried out in order to identify key constraints and opportunities relating to the flood mitigation measures considered. A range of environmental constraints, particularly the presence of the River Teith SAC, were identified. It is not anticipated that these would necessarily prevent any particular scheme from going ahead, rather it means that further surveys, consultation and close working with the relevant statutory bodies will be required to minimise environmental impacts.


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A number of opportunities were identified which could potentially bring about environmental benefits as part of any scheme, including the creation of ecological habitats, enhancement of the riparian corridor and improvements in the amenity value.

14.10 Benefit / Cost Appraisals

A full benefit-cost assessment using FCERM-AG methods was carried out to determine the economic feasibility of the options considered. The onset of flooding in Callander is, despite the perceived level of flood risk, relatively infrequent. As a result, damages are relatively limited. The total damages predicted over the next 100 years for the whole of Callander amounts to around £1,558k (£2,839k with climate change), the majority of which is accrued in the Meadows Car Park area. However, around half of the damages occur as a result of events exceeding a 200 year event, meaning the damages which could be prevented are more limited. The biggest contributors to damages are the Dreadnought Hotel, at 15% of total damages, and Callander Woollen Mill, at 7%.

All of the costed direct flood defence options considered for Callander yield a B/C ratio of significantly less than unity and represent non-viable economic solutions. The highest B/C ratio achieved for any of the outlined scheme options is only 0.14 for Bridgend West FAS.

By simple inspection, options that involve extensive upstream flood storage works would not represent an economically viable scheme for Callander (or a part scheme as it would be required to be). Consequently, no B/C ratios have been developed for upstream storage options. These upstream storage options may be explored in further detail at a later stage if considered of benefit and potentially economically viable in addressing flooding to the city of Stirling.

14.11 Individual Property Flood Proofing

Where community flood schemes are not economically viable and the capital expenditure of a major scheme cannot be justified (as is the case for Callander), property specific flood proofing and resilience measures may be the only practical flood mitigation option available.

Individual property assessments would be required to determine the appropriate measures for the particular type of property in Callander and the particular nature of the flooding. However, once the appropriate measures have been specified, there is very little design work required. Installation, carried out by a qualified contractor, can rapidly follow the specification stage.

14.12 Recommendations

Due to the potential lack of a robust economic case for a community flood scheme for the study area, the Council may wish to consider the following:

The intangible and indirect benefits of a flood scheme not quantified here may out-weigh the unfavourable benefit-cost ratio calculated

Funding mechanisms (grants, etc.) for individual property flood protection

Further investigation into individual property flood protection

Review of flood warning measures for the area Review of evacuation plans and emergency procedures for the area

Public engagement regarding flood risk in the area and the outcome of this study


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Appendix A - Existing Information / Communications

Appendix B - AMAX Flow Data

Appendix C - FEH Catchment Descriptors

Appendix D - Design Flow Derivation

Appendix E - Teith Model Peak Water Levels

Appendix F - Callander (Teith) Flood Outlines

Appendix G - Routing Models Sections

Appendix H - Routing Models Sensitivity

Appendix I - Natural Flood Management Review

Appendix J - Environmental Constraints

Appendix K - Health and Safety Appraisal

Appendix L - Benefit-Cost Assessment

Documents

Callander River Teith Optioneering and Benefit / Cost ... · InfoWorks RS 1D model for the Teith reach through Callander including short sections of the Eas Gobhain and River Leny