river forth aberfoyle optioneering - stirling council ... · project title river forth aberfoyle...

River Forth Aberfoyle Optioneering

June 2013

Produced for

Stirling Council Environmental Services

Roads, Transport and Open Spaces

Springkerse Depot

Stirling

FK7 7SZ

Prepared by

Mouchel Ltd

Lanark Court, Tannochside Park

Uddingston, Glasgow

G71 5PW

T 01698 802 850

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© Mouchel 2013 iii

Document Control Sheet

Project Title River Forth Aberfoyle Optioneering

Report Title River Forth Aberfoyle Optioneering Report

Version 3

Status Final

Record of Issue

Version Status Author Date Check Date Authorised Date

1 Draft

J Jones

P Ghimire

M Biesta

R McEvan

P Lambert

October 2012 O Drieu 12/10/12 S. McGee 17/01/13

2 Final P Ghimire

M Biesta March 2013 O Drieu 09/04/13 S. McGee 17/04/13

3 Revision A M Biesta June 2013 S McGee 28/06/13 S McGee 28/06/13


© Mouchel 2013 iv

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

This report is presented to Stirling Council in respect of River Forth Aberfoyle

Optioneering 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 Limited is

obliged to exercise reasonable skill, care and diligence in the performance of the

services required. Stirling Council and Mouchel Limited 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 Limited. 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.


© Mouchel 2013 v

Contents Figures ................................................................................................................... vii Tables .................................................................................................................... viii Appendices ............................................................................................................. ix Abbreviations and Glossary ................................................................................... x Executive Summary .............................................................................................. xii 1 Introduction .................................................................................................... 1

1.1 Background ............................................................................................. 1 1.2 Project Brief and Aims ............................................................................. 1 1.3 Relevant Legislation ................................................................................ 1 1.4 Presentation of the Report ...................................................................... 2

2 Study Area ...................................................................................................... 4 2.1 Catchment Overview ............................................................................... 4 2.2 Study Extents .......................................................................................... 5

3 Data Collection and Review ........................................................................... 6 3.1 Existing Information ................................................................................. 6 3.2 Planning and Development ..................................................................... 6 3.3 Summary of Previous Studies ................................................................. 7 3.4 Site Visits ................................................................................................ 8 3.5 Topographical Survey ............................................................................. 8 3.6 Historic Flooding ..................................................................................... 9 3.7 Flood Maps and Hydrometric Data provided by SEPA .......................... 11

4 Hydrological Assessment............................................................................ 14 4.1 Introduction ........................................................................................... 14 4.2 Sub-catchment Delineation ................................................................... 14 4.3 Hydrological Points of Interest (POIs) .................................................... 15 4.4 FEH Catchment Descriptors .................................................................. 16 4.5 Flow Estimation ..................................................................................... 16 4.6 FEH Rainfall Runoff Method .................................................................. 16 4.7 FEH Statistical Method .......................................................................... 18 4.8 Assumptions and Limitations of the Hydrological Methods .................... 20 4.9 Choice of Hydrological Method .............................................................. 21

5 Hydraulic Modelling ..................................................................................... 22 5.1 Modelling Approach .............................................................................. 22 5.2 Hydraulic Model Development............................................................... 22 5.3 Model Calibration and Verification ......................................................... 24 5.4 Sensitivity Analysis ................................................................................ 32 5.5 Modelling Results .................................................................................. 36

6 Flood Mitigation Optioneering .................................................................... 40 6.1 Potential Options ................................................................................... 40 6.2 Option Appraisal / Impact Assessment .................................................. 47 6.3 Potential Option Constraints .................................................................. 53 6.4 Demountables / Temporary Defences ................................................... 53

7 Cost-Benefit Analysis .................................................................................. 58 7.1 Economic Appraisal Policy and Guidance ............................................. 58 7.2 Present Values (PV) .............................................................................. 59 7.3 Optimism Bias ....................................................................................... 59 7.4 Benefits Methodology ............................................................................ 59 7.5 Summary of Cost-Benefit Methodology ................................................. 63 7.6 Flood Damages in Aberfoyle ................................................................. 63 7.7 Option Costing ...................................................................................... 71 7.8 Cost-Benefit Appraisal .......................................................................... 75 7.9 Further Benefit / Cost Discussion .......................................................... 76


© Mouchel 2013 vi

8 Environmental Feasibility and Constraints ................................................ 78 8.1 Introduction ........................................................................................... 78 8.2 Summary of the Environmental Baseline ............................................... 78 8.3 Summary of Key Constraints and Potential Environmental Opportunities78

9 Geomorphological Assessment .................................................................. 80 9.1 River Reconnaissance Survey .............................................................. 80 9.2 Key Features ......................................................................................... 81 9.3 Potential Impacts of the Explored Flood Mitigation Options ................... 82

10 Conclusions and Recommendations .......................................................... 84 10.1 Conclusions .......................................................................................... 84 10.2 Recommendations ................................................................................ 85

11 Appendices ................................................................................................... 87


© Mouchel 2013 vii

Figures Figure 1 - Catchment overview .............................................................................................. 5

Figure 2 – Location of proposed housing development ..................................................... 7

Figure 3 - Historic flooding locations – Map 1 ................................................................... 10

Figure 4 - Historic flooding locations – Map 2 ................................................................... 10

Figure 5 – Location of gauges ............................................................................................. 12

Figure 6 - Sub-catchments for the hydrological assessment .......................................... 14

Figure 7 – Extents of the hydraulic model .......................................................................... 23

Figure 8 – Schematic of the hydrological and hydraulic model ....................................... 23

Figure 9 - Modelled outline for September 2009 event and trash line survey ................ 30

Figure 10 – Location of the cross sections used for comparison of results of sensitivity

analysis ................................................................................................................ 32

Figure 11 – Locations of the overflow from the Duchray Water near the confluence ... 37

Figure 12 – Flood outline in Aberfoyle centre for the 200 year design event ................. 38

Figure 13 – Flood outline downstream of Aberfoyle for the 200 year design event ...... 38

Figure 14 – Flood outline along the Loch Ard for the 200 year design event ................. 39

Figure 15 – Option 1 – potential flood defence measures ................................................ 40

Figure 16 - Option 2 – potential flood defence measures ................................................. 41

Figure 17 – Option 3 – potential flood mitigation measures............................................. 42

Figure 18 - Natural flood management approaches (from SNIFFER, 2011) ................... 43

Figure 19 – Option 1 - comparison of water levels between baseline and Option 1 (200

years) ................................................................................................................... 47

Figure 20 – Hydrographs of the Duchray Water and Avondhu with and without the flow

structure (200 years event) ............................................................................... 49

Figure 21 – Flood extent and water depth upstream of the flow control structure (200

year event) ........................................................................................................... 50

Figure 22 – Option 2 - comparison of water levels between baseline and Option 2 (200

year event) ........................................................................................................... 51

Figure 23 – Option 3 - comparison of water levels between baseline and ideal Option 3

(200 year) ............................................................................................................. 52

Figure 24 – Potential demountable / temporary defence alignment ............................... 55

Figure 25 - Change in river peak flow to account for climate change in the future ....... 62

Figure 26 - Approximate return period of property flood risk (Main Street) ................... 64

Figure 27 - Approximate return period of property flood risk (Lochard Road) .............. 64

Figure 28 – Locations where flood damages occur (larger symbols indicate higher

damage) ............................................................................................................... 65

Figure 29 – Depth/damage curves for detached residential properties of various ages

.............................................................................................................................. 70


© Mouchel 2013 viii

Tables

Table 1 - SEPA rain gauges for the study area .................................................................. 12

Table 2 - Aberfoyle sub-catchment locations and areas ................................................... 15

Table 3 - Points of Interest ................................................................................................... 16

Table 4 - Peak flows estimated using the FEH rainfall runoff method ............................ 17

Table 5 - Peak flows estimated with FEH rainfall runoff method with catchment wide

storm duration of 12 hours ................................................................................ 18

Table 6 - QMED values .......................................................................................................... 19

Table 7 - Peak flows estimated using the FEH statistical method ................................... 20

Table 8 - Comparison and design flows for the inflow of the Duchray catchment into

the model ............................................................................................................. 21

Table 9– Design flows near the sewage works in Aberfoyle ............................................ 21

Table 10 – Manning’s ‘n’ values used in the hydraulic model .......................................... 24

Table 11 – Hydrometric data availability for each of the three selected events ............. 26

Table 12 – Adjustments made to model parameters during model verification ............. 27

Table 13 – Comparison of recorded and modelled flows and water levels .................... 29

Table 14 – Brief description of the cross sections used for the comparison of

sensitivity results ............................................................................................... 33

Table 15 – Results of the sensitivity analysis following changes in roughness values 33

Table 16 - Results of the sensitivity analysis following changes in the downstream

boundary .............................................................................................................. 34

Table 17 - Results of the sensitivity analysis on a 20% increase of the flows (climate

change) ................................................................................................................ 34

Table 18 – Key features of Option 1 .................................................................................... 41



Table 21 - Social class categories for Stirling Council area ............................................. 60

Table 22 - Present value damage (PVd) for a range of annual exceedance probabilities

.............................................................................................................................. 65

Table 23 – Capped present value damage (PVd) for a range of annual exceedance

probabilities without climate change allowance ............................................. 66

Table 24 – Capped present value damage (PVd) for a range of annual exceedance

probabilities including climate change allowance .......................................... 66

Table 25 - Relative contribution of areas of Aberfoyle to total PVd (with climate change

allowance) ............................................................................................................ 67

Table 26 - Data Quality Scores (DQS) from MCH (Penning-Rowsell et al., 2010) ........... 67

Table 27 – Highest contributors to total PVd for Aberfoyle (without climate change

allowance) ............................................................................................................ 69

Table 28 – Cost breakdown for Option 1 ........................................................................... 72

Table 29 – Cost breakdown for Option 2 ............................................................................ 73

Table 30 – Cost breakdown for Option 3 ............................................................................ 74

Table 31 – PV costs for the three options........................................................................... 75

Table 32 - Benefit cost ratio (without climate change) ...................................................... 76

Table 33 - Benefit cost ratio (with climate change) ........................................................... 76


© Mouchel 2013 ix

Appendices

Appendix A – FEH Catchment Descriptors

Appendix B

Appendix B 1 – Rainfall Graphs

Appendix B 2 – Verification Graphs

Appendix C

Appendix C 1 – Modelled Water Levels for Various Return Period

Appendix C 2 – Water Levels for Sensitivity Analysis

Appendix C 3 – Water Levels for Options

Appendix D

Appendix D 1 – Upper Ard 200 Year Flood Map

Appendix D 2 – Lower Ard 200 Year Flood Map

Appendix D 3 – Milton 200 Year Flood Map

Appendix D 4 – Aberfoyle 200 Year Flood Map

Appendix E

Appendix E 1 – Option 1 Map



Appendix F

Appendix F 1 – Service Map 1


Appendix G

Appendix G 1 – Environmental Appraisal Report

Appendix G 2 – Environmental Constraints Map

Appendix H

Appendix H 1 – Fluvial Geomorphology Assessment Map

Appendix H 2 – River Reaches and Morphological Characteristics

Appendix H 3 – Photographs of Key Morphological Features

Appendix I

Appendix I 1 – Condition Assessment Map 1



Appendix J

Appendix J 1 – Natural Flood Management Review

Appendix J 2 – Duchray Catchment Natural Flood Risk Management Options


© Mouchel 2013 x

Abbreviations and Glossary

1D – 2D 1 Dimensional – 2 Dimensional (integrated hydraulic model)

AEP Annual Exceedance Probability

AMAX Annual Maximum Flow Series

AOD Above Ordnance Datum

BGS British Geological Survey

CAR Controlled Activities Regulations

CDM Construction Design and Management

CEH Centre for Ecology and Hydrology

CFMP Catchment Flood Management Plan

Defra Department for Environment, Food and Rural Affairs

DTM Digital Terrain Model

DQS Data Quality Score

FCERM-AG Flood and Coastal Erosion Risk Management Appraisal Guidance

FEH Flood Estimation Handbook

GIS Geographic Information System

GPS Global Positioning System

InfoWorks RS River modelling software produced by Wallingford Software Ltd

ISIS River modelling software produced by Wallingford Software Ltd & Halcrow Ltd

LiDAR Light Detection and Ranging

Manning‟s n Coefficient used to represent surface roughness along a river reach of for a hydraulic structure

MCH Multi-coloured Handbook

MCM Multi-coloured Manual

NFM Natural Flood Management

NSA National Scenic Area

OS Ordnance Survey

POI Point of Interest

PV(d) Present Value (damages)

QMED Median annual flood (m³/s)

SAAR Standard Annual Average Rainfall

SAC Special Area of Conservation

SAM Scheduled Ancient Monument

SCROL Scotland‟s Census Results Online


© Mouchel 2013 xi

SEPA Scottish Environmental Protection Agency

SNH Scottish Natural Heritage

SPP Scottish Planning Policy

SPA Special Protection Area

SPR Standard Percentage Runoff

SSSI Site of Special Scientific Interest

SuDS Sustainable Drainage System

TBR Tipping Bucket Rain Gauge

Tp Time to peak of a unit hydrograph

TUFLOW Hydrological and hydraulic modelling software which includes a two dimensional (2D) component, useful for modelling overland flow

URBEXT FEH index of fractional urban extent

WINFAP-FEH FEH frequency analysis software package


© Mouchel 2013 xii

Executive Summary

In September 2011, Stirling Council appointed Mouchel to conduct a flood

optioneering study for the village of Aberfoyle with the aim of gaining a better

understanding of the flooding issues affecting the area and developing potential flood

mitigation options. The key issues in the study area are the flooding of properties,

roads and services in Aberfoyle and the isolation of villages upstream of Aberfoyle

which are cut off during a flood event.

The study catchment is located in the headwaters of the River Forth, approximately

30km west of Stirling. The catchment, which covers an area of 140km², is very rural

and heavily forested, and has steep mountainous tributaries. The catchment

contains two lochs which have an attenuating effect on flood peaks. Aberfoyle is a

small village located in a valley alongside the River Forth and is economically

supported mainly by tourism.

A large amount of data was collected for use in this study in order to complete a

detailed assessment of the flooding issues and potential flood mitigation solutions.

This included a review of previous flood studies carried out for Aberfoyle.

A hydrodynamic model was developed to assess the extent of flooding in and around

the village of Aberfoyle and to help develop potential flood mitigation measures. This

model is based on a model developed previously by Atkins in 2009, with appropriate

updates made to the hydrology and hydraulic model for the specific purposes of this

study. The model was verified using various data collected for this study and

sensitivity tests were carried out on the model. Model calibration was limited by lack

of data availability; improvements to the hydrometric network would be valuable in

future model development and flood warning.

Three flood mitigation options were developed for this study, incorporating a

combination of measures including flood defence walls, individual property

protection, flow control and natural flood management (NFM) measures. Options to

protect smaller parts of Aberfoyle and lower standards of protection were also

considered.

The Duchray catchment is the key contributor to flood risk in Aberfoyle. As a result,

any natural flood management measures should focus on this catchment. The

presence of plantation forestry in the catchment creates potential opportunities for

working with the Forestry Commission to implement such measures. Prior to

implementation, however, a detailed study (including hydrological and hydraulic

modelling) will be required to carefully plan the details of any NFM measures and to

assess the downstream benefits. If downstream benefits cannot be quantified, NFM

implementation would be useful as a pilot study only, and funding should be sought

accordingly.

The economic performance of the various options was determined through a cost-

benefit analysis, undertaken for a range of annual exceedance probabilities, and

considering the effects of climate change. It was found that none of the options


© Mouchel 2013 xiii

considered would yield a benefit-cost ratio in excess of unity. Indeed the best ratio

which was achieved was only 0.39.

Although none of the schemes considered were economically viable, there are a

range of intangible benefits (not least access to remote villages and the primary

school) which have not been quantified. The Council may wish to consider a scheme

with a benefit-cost ratio of less than one in cognisance of these factors.

Demountable / temporary defences were investigated as a potential option where a

permanent scheme was not feasible. However, it was found that due to a number of

technical constraints (access to Lochard Road and upstream impacts on flood risk

being the main ones) and high costs associated with installation, a community

demountable scheme is not likely a feasible option. Individual property measures

may be an option and would require individual property assessments to be

considered further as an option.

An environmental appraisal was carried out to identify environmental characteristics

of the study area, along with the potential environmental constraints and

opportunities associated with the three possible flood mitigation options. Although

there are sensitive environmental areas in and around the study area, it was

considered that the potential environmental constraints on the scheme can be

avoided or overcome through careful planning and design. A geomorphological

assessment was also undertaken to identify geomorphological features of the

watercourses and to assess the potential impacts of the potential flood mitigation

options. No major geomorphological constraints were noted in the area, although

any options would need to be design such that the existing geomorphological

conditions are not adversely affected.

It is recommended that the findings of this study are communicated with the local

residents and any other stakeholders. The Council will be able to use the findings of

this study to inform any further work, including NFM studies or further scheme

investigations if considered appropriate.


© Mouchel 2013 1

1 Introduction

1.1 Background

In 2011, Stirling Council appointed Mouchel, through their framework with Scotland

TranServ, to conduct a flood study in and around the village of Aberfoyle. The

Council‟s aim is to gain a better understanding of flooding issues which affect a

number of settlements including Aberfoyle, and roads within the area. In addition, the

study aims to explore potential options to reduce flood risk within the catchment.

There are two key flooding issues in this area. The village of Aberfoyle is periodically

flooded by the River Forth. This has occurred at least six times in the last 60 years, in

1950, 1997, 2005, 2006, 2009, 2011 and 2012. Secondly, the villages of Kinlochard,

Stronachlachar and Inversnaid are regularly (approximately 2-3 times a year) isolated

when Loch Ard flows out of bank or the River Forth floods Loch Ard Road, cutting off

the only access/egress route. The local primary school is also located on Loch Ard

Road and has to be closed once the road begins to flood.

Previous flood investigation studies have been carried out in the area on behalf of

Stirling Council by Atkins, including hydraulic modelling and flood mapping. Mouchel‟s

study builds upon this existing knowledge and data. No formal flood defences have

been identified within the study area.

1.2 Project Brief and Aims

The overall aim of the study is to investigate flooding and explore potential mitigation

options for reducing flood risk and dealing with the specific issues of access and

flooded property and infrastructure. A hydrological and hydraulic model developed by

Atkins in 2009 as part of the studies undertaken on behalf of Stirling Council1 has been

reviewed, updated and extended for the purposes of this study.

The Project Brief was developed in conjunction with Stirling Council. Key deliverables

of the project include flood maps for a range of return periods, flood levels and options

for flood mitigation (described in Section 6). The potential flood mitigation measures

have included not only hard engineering defences but also consideration of „soft‟

options such as upstream storage solutions and natural flood management techniques

in the forested Duchray catchment, which could possibly reduce the frequency and

severity of inundation of Loch Ard Road, the primary school and properties and

infrastructure in Aberfoyle.

1.3 Relevant Legislation

The Flood Risk Management (Scotland) Act was introduced in 2009 and had important

implications for local authorities. The purpose of the Act is to improve the assessment

and sustainable management of flood risk across Scotland. This is supported by a new

duty on local authorities to exercise their flood risk related functions with a view to

reducing overall risk.

1 Aberfoyle River Forth Flood Study Hydraulic Modelling Report (Atkins on behalf of Stirling Council, 2009)

and Aberfoyle Flood Study – River Forth Hydrodynamic Model Update and Recalibration Report (Atkins,

on behalf of Stirling Council, 2010)


© Mouchel 2013 2

Local authorities are to deliver these broad aims which include traditional flood

defence works, surface water management, raising awareness and wherever possible,

promoting natural approaches to managing the sources and pathways of flood waters.

Scottish Planning Policy (SPP)2 sets out the Scottish Government‟s planning policy on

development and flooding. To provide a basis for planning decisions related to flood

risk, SPP introduces a risk-based framework that characterises areas by their annual

probability of flooding for planning purposes. The main emphasis of SPP is to prevent

further development which would have a significant probability of being flooded or

which would increase the flood risk elsewhere. SPP highlights that flood risk

management measures should avoid detrimental effects on the environment and

ecology, with opportunities for habitat restoration or enhancement being sought.

As part of this study, an environmental assessment has been carried out to assess the

key constraints and potential opportunities associated with the potential flood

mitigation options. There are several designated sites located in the vicinity of the

study area (SSSI/SAC). Any works potential 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). Loch Ard and

Aberfoyle are located within the Loch Lomond and The Trossachs National Park which

consists of two National Scenic Areas (NSA), one being Loch Lomond NSA and the

other The Trossachs NSA.

1.4 Presentation of the Report

After an Introduction of the study in Section 1, Section 2 describes the study area, its

physical and hydrological characteristics and the extents of the study.

A large amount of data has been collated for this study and site visits have been

undertaken to collect additional information. This is detailed in Section 3, which also

includes a review of local planning policies and previous studies carried out within the

study area.

A review of hydrological calculations undertaken previously by Atkins was carried out

and updates made for this study using standard FEH methods. The hydrological

analysis is detailed in Section 4. The FEH catchment descriptors used for the

hydrological analysis are contained within Appendix A.

Hydraulic modelling has been undertaken using ISIS-TUFLOW software. The

modelling approach is described in Section 5, along with the model verification,

sensitivity analysis and model results. Appendix B presents the model verification

graphs. The outputs from the model such as water levels across the study area and

flood maps are presented in Appendix C and Appendix D respectively.

Section 6 describes the three potential flood mitigation options and presents a

technical appraisal of each. Appendix E contains maps of the layout of the options.

Service plans have been obtained to investigate any potential utility crossings with the

2 Scottish Planning Policy (The Scottish Government, February 2010)


© Mouchel 2013 3

potential flood mitigation options. Maps of the service locations are contained within

Appendix F.

Section 7 details the cost - benefit analysis carried out for the explored options, along

with results.

Section 8 details the environmental appraisal carried out to identify the environmental

characteristics of the study area and to assess potential constraints or opportunities

presented by the scheme. The full environmental appraisal report is included in

Appendix G.

Section 9 contains information on the geomorphological assessment carried out along

the watercourses to identify key features and any constraints of the potential options.

Supporting information (maps etc.) is included in Appendix H. Maps showing the visual

condition assessment of the watercourses and structures in the study area are

presented in Appendix I.

Section 10 presents the conclusions and recommendations of the study, including key

findings and potential future areas of exploration.


© Mouchel 2013 4

2 Study Area

2.1 Catchment Overview

The study catchment is located in the headwaters of the River Forth, approximately

30km west of Stirling. The catchment is very rural and heavily forested with steep

mountainous tributaries and has a catchment size of 140km². The catchment bedrock

is Pre-Cambrian impermeable strata and is relatively drift free with some boulder clay3.

The main soil types within the study area are alluvial soils (typically confined to river

valleys, gleys (naturally poorly drained soils) and podzols (typically free draining soils).

The average annual rainfall within the catchment is 2106mm4.

The majority of land cover in the study area is plantation woodland, with some areas of

grassland and mountain / heath / bog5. There are a few rural villages within the study

catchment, which have historically been the centre of industries including slate

quarries, ironworks and wool spinning. These industries have since died out and the

area is now economically supported mainly by forestry operations and tourism6.

The village of Aberfoyle has a population of 5767 and is located on the River Forth

downstream of the confluence of two watercourses: the Avondhu Burn and the

Duchray Water. Lochs are important hydrological features of the catchment for this

study, as storage capacity in the lochs have an attenuating effect on flood peaks

downstream. Two lochs are located on the Avondhu branch: Loch Ard and Loch Chon.

Loch Ard is located approximately 3km west of Aberfoyle. It is approximately 5km long

(including the narrows at the east end) and up to 1.5km wide. Approximately 3km

northwest is Loch Chon, which is 2km long, and less than 1km wide.

At the eastern end of Loch Ard the topography forms a narrow floodplain bordered by

steep, forested valley sides. The loch outfalls into the Avondhu Burn through a narrow

steep cascade section. The Avondhu flows through Milton and then to the confluence

with Duchray Water approximately 400m downstream of Milton.

Downstream of the Avondhu / Duchray confluence, the River Forth meanders through

large floodplain areas around Aberfoyle. South of Aberfoyle, the river channel widens

and meanders through a forested area, past Cobleland campsite before flowing under

the A81 at the southern extents of the study area. An overview of the study catchment

is shown in Figure 1.

3 British Geological Survey, 2012

4 Standard Average Annual Rainfall for the catchment, taken from FEH CD ROM version 3

5 Based on Centre for Ecology and Hydrology: NRFA spatial data 2012 at Milton gauge

(http://www.ceh.ac.uk/data/nrfa/data/spatial.html?18022)

6 Aberfoyle Feature Page on Undiscovered Scotland

(http://www.undiscoveredscotland.co.uk/aberfoyle/aberfoyle/index.html)

7 Scotland‟s Census Results Online

(http://www.scrol.gov.uk/scrol/browser/profile.jsp?profile=Population&mainArea=Aberfoyle&mainLevel=Lo

cality )


© Mouchel 2013 5

© Crown Copyright 2012. All rights reserved. Ordnance Survey Licence number 100020780

Figure 1 - Catchment overview

Access to Aberfoyle from the east is provided by the A821 road which runs through the

village. The B829 runs out of the village to the west, providing access to Milton,

Kinlochard, Stronachlachar and Inversnaid. These villages rely on Aberfoyle for

services and facilities,

Loch Ard and Aberfoyle are located within the Loch Lomond and the Trossachs

National Park which contains two National Scenic Areas (NSA).

2.2 Study Extents

On behalf of Stirling Council, Atkins has previously carried out a flood study of the area

and undertook hydrological and hydraulic modelling of the watercourses of interest

(Avondhu, Duchray Water8 and River Forth) using InfoWorks RS software9. This model

has been reviewed by Mouchel, converted into ISIS – TUFLOW software and has

been updated and extended for the specific purposes of this study.

The upstream boundary of the study extent is at the western end of Loch Ard (grid

reference 248650, 701350), at the same location as in the previous model developed

by Atkins. However, the model has been extended to downstream of Cobleland Bridge

(grid reference 25330, 698550) to the south of Aberfoyle, to ensure that there is

minimum influence of the downstream boundary on areas of interest. The downstream

reach of the Duchray Water has also been included in the model as a short reach.

Further details of the model developed for this study are included in Sections 4 and 5

of this report. Figure 7 in Section 5.2 shows the extents of the hydraulic model in the

study area.

8 Only the downstream parts of these two watercourses were included in the hydraulic model.

9 InfoWorks RS simulates flows in rivers, channels and on floodplains.


© Mouchel 2013 6

3 Data Collection and Review

3.1 Existing Information

A large amount of data has been collated from a number of sources for use in this

study including Stirling Council, SEPA and Scottish Water. This data includes the

following:

Ordnance Survey mapping at various scales (Stirling Council),

Aerial photography (2m resolution) (Stirling Council),

Digital Terrain Model (DTM) and Digital Surface Model (DSM) data (5m grid) and LiDAR data (1m grid) (Stirling Council),

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

Stirling Council biennial flood reports (Stirling Council),

Scottish Water sewerage network GIS data (Scottish Water),

Flood mapping (SEPA),

Hydrometric data (SEPA, Stirling Council),

Photographs (Stirling Council),

Geology maps (British Geological Survey),

Service plans (electricity, gas, telecommunications, water & wastewater) (Scottish Water), and

Environmental information (Stirling Council, Scottish Natural Heritage (SNH) and various mapping websites).

3.2 Planning and Development

In the context of possible future pressures on flooding / floodplains resulting from

development, a brief assessment of Stirling Council‟s current planning information was

undertaken to identify the overall strategy and any key development sites on or near

floodplains within the study area.

The Development Plan for Stirling Council‟s administrative area currently comprises of

two documents: Stirling Council Local Plan (1999) and the Clackmannanshire and

Stirling Structure Plan (2002). Stirling Council is currently producing a new Local

Development Plan which will replace the existing Local Plan and Structure Plan. This

will be introduced later in 2012 and will cover a period of 20 years to 2032. In 2011,

The Loch Lomond and The Trossachs National Park developed its own Local Plan10.

The Council‟s current strategy for Aberfoyle is to restrict new development outside of

the rural centre (particularly housing), whilst supporting appropriate rural development,

agriculture, forestry and tourism in line with Aberfoyle‟s role as an established

shopping and tourism centre. The Council‟s strategy also seeks to secure the retention

of remaining undeveloped floodplains.

The Loch Lomond and The Trossachs National Park Local Plan states that „future

10 Loch Lomond & The Trossachs National Park Local Plan, 2011.


© Mouchel 2013 7

development opportunities in Aberfoyle are constrained due to potential flood risk,

topographical constraints and nature conservation designations. One small formal

housing development site is identified (shown as „H1‟ in Figure 2). There may also be

scope for infill housing development and appropriate tourism and visitor infrastructure

that enhances the role of Aberfoyle and the Trossachs as a visitor destination‟.

Figure 2 – Location of proposed housing development

(From The Loch Lomond and the Trossachs National Park Local Plan (2011))

A search was undertaken on Stirling Council‟s online Planning Portal11 to identify any

planning applications submitted within the last year within the study area. No planning

applications were found.

3.3 Summary of Previous Studies

A recent study investigating flooding in and around Aberfoyle was undertaken by

Atkins in 2009/10 on behalf of Stirling Council. The Council provided the following

information pertaining to the Atkins‟ study:

Aberfoyle River Forth Flood Study – Hydraulic Modelling Report (Atkins, August 2009 for Stirling Council),

River Forth Hydrodynamic Model Update and Recalibration Report (Atkins, June 2010 for Stirling Council), and

All final hydraulic modelling files.

A one dimensional (1D) InfoWorks RS hydraulic model was developed and used by

Atkins to define flood extents for a range of design flows and to investigate flood

mechanisms. The main conclusions from this study were as follows:

11 http://pabs.stirling.gov.uk/online-applications/


© Mouchel 2013 8

There are a number of areas at risk of flooding, in particular; the western end of Main Street, Loch Ard Road and Kinlochard Road between Milton and Kinlochard,

The major hydraulic influence are the Duchray Water and the antecedent water levels in Loch Ard prior to a storm event, and

The bed level in The Narrows has a very minor effect on maximum stage in either Loch Ard or at Aberfoyle.

One limitation of this study was that the calibration of the model was limited by the

availability of good quality rainfall data. The report makes recommendations for

improvements in hydrometric monitoring.

The report recommended that if the model was to be used to evaluate and develop

flood mitigation options, then a review of the hydrology and hydraulic components

should be undertaken and updated to include a greater level of detail. This would

involve undertaking additional river channel surveys and two dimensional (2D)

modelling to refine out of bank flows.

3.4 Site Visits

After an initial site visit in October 2011, Mouchel undertook a detailed visual

inspection of the study watercourses in November 2011 in order to gain a detailed

understanding of the study area and especially of the watercourses. The purposes of

the inspection were:

Modeller familiarisation with the watercourses,

Looking for evidence of flooding (flood marks, trash lines etc.),

Identification of potential areas of flooding and likely flooding mechanisms,

Identification of key hydraulic structures such as bridges, culverts and weirs,

Channel roughness estimation and supportive photographs,

Development of detailed specification for additional topographical survey,

Assessment of the watercourse and floodplain geomorphology, and

Visual assessment of the condition of banks and structures along the watercourses (this assessment is documented in Appendix I).

3.5 Topographical Survey

To update and refine the existing model and make it fit for the purposes of this study,

additional topographical survey data was required. During the original topographical

survey undertaken by Atkins in 2008, the survey was limited to 13 river cross sections

in the River Forth and Avondhu, with the other cross sections being interpolated using

modelling software. Additional cross sections were surveyed to enhance and extend

the previously modelled reaches. New data collected included:

New cross sections in Loch Ard,

4 cross sections downstream of the existing model extent (between Aberfoyle and Gartartan - downstream of Cobleland). This was undertaken to extend the existing model downstream, hence minimising the influence of the downstream boundary on the model results in the key flood risk areas,

4 river cross sections along the Duchray Water,


© Mouchel 2013 9

Re-survey (survey check) of 3 existing cross sections to check accuracy of the existing cross sections (originally surveyed by Atkins in 2008),

Key spot levels along the margins of Loch Ard including road levels along the B829 and property flood thresholds (inside the 1 in 200 year flood outline), and

A survey of Cobleland Bridge (located within the extended reach) including invert, soffit, springing, opening, parapet and road levels.

3.6 Historic Flooding

Following significant periods of flooding in 2005 and 2006, Strathard Community

Council12 investigated and collated information on historic flooding13. A summary of

their findings is described in the following sub-sections.

3.6.1. Areas Prone to Flooding

Several areas have been identified by Strathard Community Council as being

particularly prone to flooding including:

Aberfoyle – low-lying stretches of road (closing the B829), car park behind Forth Inn and Wool Centre, Main Street (west), A81 on low lying ground,

Milton – low ground at Dundarroch, riverside to west of bridge, and

Several locations along the B829 – low-lying stretches of road from Tigh-na-Traigh to Forest Hills, Kinlochard village, intermittent flooding at various other points when drainage gullies blocked.

In addition to areas affected directly by the flooding in 2005 and 2006, when the B829

is blocked residents from villages upstream are cut off by floodwaters, becoming

isolated from all services. This is a major issue affecting Lochard Road, Milton,

Kinlochard, Inversnaid and Stronachlachar. In addition to disruption, this isolation

poses a risk to health and safety to the most vulnerable residents.

The main areas of historic flooding are shown in Figure 3 and Figure 4.

12 Local Council covering Aberfoyle, Kinlochard, Inversnaid and Stronachlachar.

13 Flooding - Summary of 2007 Investigations (Strathard Community Council, 22 May 2008).


© Mouchel 2013 10


Figure 3 - Historic flooding locations – Map 1


Figure 4 - Historic flooding locations – Map 2

3.6.2. Nature of Flooding

The study catchment is relatively large and includes several lochs and steep

mountainous watercourses. There are two main factors that largely influence flooding

in Aberfoyle: heavy persistent rainfall and the land use in the Duchray catchment.


© Mouchel 2013 11

Local information suggests that the following changes to watercourse and land

management could have contributed to exacerbating historical flooding:

It was locally perceived that a possible bottleneck at The Narrows could increase flooding issues downstream by reducing available loch storage. Hence The Narrows have been dredged previously by landowners and the local authority. However, the sensitivity analysis carried out by Atkins on the bed level at The Narrows has shown that the influence on the water levels in Aberfoyle is negligible.

Changes in land use by the Forestry Commission (particularly relevant for the Duchray Water catchment) with open spaces creating more runoff, and

Changes to river management as a result of construction e.g. at Milton and Aberfoyle.

3.6.3. Actions to Reduce Flooding identified by Strathard Community Council

The Strathard Community Council report identifies two critical long term solutions

which would improve the situation: lowering the „normal‟ loch levels and reducing the

volume and speed at which water impacts the river system. This information has been

taken into account in Mouchel‟s analysis and considered during the development of

suitable options.

3.7 Flood Maps and Hydrometric Data provided by SEPA

3.7.1. SEPA Strategic Flood Map

SEPA publish a strategic flood map showing areas that may be affected by fluvial or

coastal flooding, which is available on their website14. The flood map shows the

possible extent of flooding and is an important tool for managing flood risk in line with

Scottish Planning Policy (SPP). GIS versions of these indicative SEPA flood maps

were supplied by the Council for the study area which enabled the initial identification

of flood risk areas.

The flood map shows that large parts of Aberfoyle, Milton and Kinlochard lie within the

flood extent for a 1 in 200 year return period flood event. Other key areas of concern

are the B829 along the north of Loch Ard and through Milton and Aberfoyle. The A821

and A81 roads are also at risk of flooding.

It should be noted that SEPA flood mapping is based on a digital terrain model with a

vertical accuracy in the range 0.7 – 1.0m, on a grid spacing of 5m. It is also not

relevant to catchments smaller than 3km2. 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 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. A more detailed assessment was therefore required.

3.7.2. Rain Gauge Data

To facilitate detailed hydrological analysis of the study area, Mouchel contacted SEPA

in order to obtain rainfall data from nearby rain gauges for three recent notable flood

14 (http://www.sepa.org.uk/flooding/flood_map.aspx).

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


© Mouchel 2013 12

events (December 2006, January 2007 and September 2009). The way in which this

data has been used to verify the hydrological and hydraulic model is described in

Section 5.3. There is one active tipping bucket rain gauge (TBR) within the Aberfoyle

catchment at Duchray Castle. Several other rain gauges are located in adjacent

catchments (Figure 5). The TBRs record rainfall data automatically every 15 minutes.

A summary of the SEPA rain gauges is presented in Table 1. These are the same as

the rain gauges that were used for developing the original model by Atkins.

SEPA station name

Catchment Gauge type

Grid reference Period of record used for each

verification events

Duchray Castle

Loch Ard Burn TBR 246800, 698700 14/11/2006 – 01/02/2007

10/11/2009 – 10/12/2009

Loch Katrine Eas Gobhain TBR 249100, 706700 14/11/2006 – 01/02/2007

10/11/2009 – 10/12/2009

High Corrie Keltie Water TBR 248100, 695400 14/11/2006 – 31/01/2007

10/11/2009 – 10/12/2009

Table 1 - SEPA rain gauges for the study area


Figure 5 – Location of gauges

3.7.3. River Gauge Data

The River Forth is not gauged within the study catchment, however the Avondhu

branch is gauged upstream of Aberfoyle at Milton (grid reference 250348 701341).

This is a velocity-area station located immediately downstream of Loch Ard and has an

upstream catchment area of 42km². Data is available from 1992 to the present. SEPA

has advised that the station is designed for low to medium flows, with high flows being

extrapolated. SEPA is currently working to improve the rating curve for this gauge by

taking additional measurements, particularly for high flows. Data for the Milton gauge

was obtained from SEPA for the three events in December 2006, January 2007 and

September 2009 and has been used to verify the hydraulic model.


© Mouchel 2013 13

A river level gauge was installed at Manse Road Bridge in Aberfoyle (grid reference

252010, 700920) by Stirling Council in 2008 to provide a flood warning. River level

data from this gauge has been used in this study for validation of the hydraulic model

for the September 2009 event.

The location of these gauges is also shown on Figure 5.

3.7.4. Hydrometric Data for Verification of the Hydraulic Model

Based on the limited hydrometric data available for large flooding events in Aberfoyle

area, three events were identified as potential verification events and were routed

through the hydraulic model: December 2006, January 2007 and September 2009.

The 2006 and 2007 events have been used for calibration in Atkins‟ previous study.

September 2009 was chosen as an additional recent event. Some anecdotal evidence,

as described in Section 5.3.5, was also available for these events.

3.7.5. Climate Change

Climate change within the UK over the next few decades is likely to result in changes

to observed weather patterns, which will be subject to regional variations. This could

include milder wetter winters and hotter drier summers. Short duration, high intensity

rainfall and more periods of long duration rainfall are expected, in addition to rising sea

levels. These factors lead to an increased risk of flooding and so the consequences of

climate change need to be anticipated and mitigated for.

The importance of climate change with regards to flooding and development is

highlighted in SPP which states that planning authorities must take into account the

probability of flooding from all sources and the risks involved when preparing

development plans. The impacts of climate change need to be taken into account

when considering flood mitigation measures.

For issues related to fluvial flooding, SEPA currently generally recommends that a

climate change allowance of + 20% on peak river flows is made in addition to the 1 in

200 year return period event appraisals and designs. This is the approach that has

been adopted to incorporate climate change in this study.


© Mouchel 2013 14

4 Hydrological Assessment

4.1 Introduction

A hydrodynamic model is required to assess the extent of flooding in and around the

village of Aberfoyle and to facilitate the exploration of suitable flood mitigation

measures. Hydrological analysis has been undertaken to derive design flow estimates

as inputs to the hydraulic model developed for this flood scheme. A review of

hydrological calculations carried out previously by Atkins has been undertaken and

only minor adjustments were required for Mouchel‟s model.

For this catchment, design event hydrographs were generated for events with return

periods of 2, 5, 10, 20, 50, 100, 200, 500 and 1000 years. The standard approach

taken for this hydrological analysis was to use, as the previous Atkins studies, the

hybrid method (i.e. a combination of FEH rainfall runoff and statistical techniques).

For the analysis, the FEH CD-ROM version 3 has been used.

4.2 Sub-catchment Delineation

To produce a detailed hydrological model of the catchment, delineation of sub-

catchments was undertaken using data from the FEH CD-ROM 3. The FEH catchment

boundaries were checked against Ordnance Survey maps. Figure 6 shows the various

sub-catchments identified, and these catchment details are outlined in Table 2.


Figure 6 - Sub-catchments for the hydrological assessment


© Mouchel 2013 15

Site

code Watercourse Site Easting Northing

Area (FEH

CD-ROM 3)

(km2)

% of

catchment

total area

1 Loch Ard Direct inflow to Upper

Loch Ard 248650 701350 37.8 27.1

2 Duchray

Water

Direct inflow from

Duchray Water at

confluence with

Avondhu

250600 701300 70.3 50.4

3 Lower Ard

Lateral area between

Lower Loch Ard at the

Narrows and Milton

gauge

249100 701450 4.4 3.1

4a Lateral

Lateral area

downstream of Milton

gauge

251550 700850 2.9 2.1

4b Lateral

Lateral area to

downstream boundary

of the model

253300 698550 1.9 1.4

5 The Pow

Direct inflow for this

tributary of the River

Forth

252200 700500 6.6 4.7

6 Allt a‟

Mhangam

Direct inflow for this

tributary of the River

Forth

252600 700650 6.7 4.8

7 Trib A Inflow from tributary of

River Forth (left bank) 253050 700200 1.3 0.9

8 Park Burn Inflow from tributary of

River Forth 252900 250360 4.9 3.5

9 Trib B Inflow from tributary of

River Forth (left bank) 253050 699900 1.8 1.3

10 Trib C Inflow from tributary of

River Forth (right bank) 252900 699050 1.0 0.7

Table 2 - Aberfoyle sub-catchment locations and areas

Checks of these catchment areas were carried out against catchments which were in

the existing Atkins model. Results were comparable with only very minor differences in

catchment area. It is thought that these minor differences were a result of a version

change in the FEH CD ROM between models15.

4.3 Hydrological Points of Interest (POIs)

Three Points of Interest (POIs) have been selected within the study catchment; the

direct inflow from the Duchray Water, Milton gauging station and the downstream

model boundary south of Cobleland Bridge.

At these points, peak flows calculated by the FEH rainfall runoff and statistical

methods were compared, and if necessary reconciled, to ensure estimated flows in the

hydraulic model were correct. Table 3 provides the details of the POIs used in this

analysis.

15 The FEH CD-ROM version 2 was used by Atkins during the previous studies


© Mouchel 2013 16

Site

code Site Description Easting Northing

Area from FEH

CD-ROM 3 (km2)

2* Duchray Direct inflow from Duchray Water

at confluence with Avondhu 250600 701300 70.3

11 Milton gauge Milton gauge on Avondhu 373750 427000 42.2

12

Downstream

boundary of the

model

Downstream boundary of

model on the River Forth 253300 698550 139.6

Table 3 - Points of Interest

*Note that the Duchray Water inflow is also a POI.

4.4 FEH Catchment Descriptors

FEH catchment descriptors were obtained from the FEH CD-ROM 3 for each direct

inflow and POI. Catchment descriptors for the lateral inflows (sub-catchments 3, 4a

and 4b) were derived by using an area ratio adjustment which enables FEH catchment

descriptors to be derived for a lateral area using weighted averages of descriptors for

surrounding sub-catchments. FEH catchment descriptors for each sub-catchment are

included in Appendix A.

For the analysis, the URBEXT2000 values from the FEH CD-ROM 3 were adjusted with

the UEF equation16 to account for projected future urbanisation. For the baseline,

URBEXT2000 values were adjusted to year 2012 and for flood mitigation options,

URBEXT2000 values were adjusted to year 2112. However, due to the rural nature of

the catchment, changes to URBEXT values were negligible.

4.5 Flow Estimation

The Flood Estimation Handbook (FEH) provides the two main approaches to flood

frequency estimation in the UK: the rainfall runoff method and statistical analysis. The

rainfall runoff method has been used to generate hydrographs for input to the hydraulic

model, whilst statistical estimates have been generated at each POI in order to check

estimates of peak flows for each return period.

4.6 FEH Rainfall Runoff Method

Catchment descriptors for each direct inflow, lateral inflow and POI were entered into

ISIS FEH boundary units (FEHBDY) and hydrographs were generated for each return

period. Hydrographs for each sub-catchment were routed through the hydraulic model

to provide rainfall runoff flow estimates throughout the modelled reaches at each of the

POIs. For each sub-catchment the FEH 75% winter rainfall profile was used.

The critical storm durations vary across the catchment. The peak flow estimates using

the FEH rainfall runoff method and the critical storm duration for each sub-catchment

are shown in Table 4.

16 UEF = 0.7851 + 0.2124 tan-1[(Year – 1967.5) / 20.32)] The equation used is taken from CEH

document named „The use of LCM2000 to provide improved definition of the FEH catchments descriptor URBEXT in Northern Ireland; Stage 2- Calculation and dissemination of URBEXT2000 values‟ CEH, March 2006


© Mouchel 2013 17

Site code

Watercourse

Critical storm

duration (hours)

Flood peak (m³/s) for following return periods (years)

2.33 5 10 20 50 100 200

1 Loch Ard 8.75 31.8 46.1 55.4 65.9 80.5 91.9 105.4

2 Duchray 11.75 59.7 86.6 103.5 122.5 149.0 169.5 193.9

3 Lower Ard 7.25 3.2 4.6 5.6 6.7 8.3 9.6 11.1

4a Lateral 4.25 2.5 3.5 4.2 5.1 6.4 7.3 8.5

4b Lateral

downstream 4.75 1.9 2.9 3.6 4.3 5.3 6.1 7.1

5 The Pow 7.25 4.7 6.6 8.0 9.6 11.8 13.5 15.6

6 Allt a‟

Mhangam 5.75 5.1 7.2 8.7 10.5 13.0 15.0 17.3

7 Trib A 3.75 0.8 1.2 1.4 1.7 2.1 2.5 2.9

8 Park Burn 6.75 3.1 4.3 5.3 6.4 7.9 9.1 10.5

9 Trib B 4.25 1.5 2.2 2.6 3.1 3.9 4.4 5.1

10 Trib C 3.25 1.2 1.7 2.0 2.3 2.9 3.4 3.9

11 Milton gauge 9.25 33.7 48.9 58.7 69.8 85.3 97.3 111.5

12

Downstream

Boundary of

the model

11.8 98.4 143.1 171.1 202.5 246.4 280.2 320.6

Table 4 - Peak flows estimated using the FEH rainfall runoff method

Peak flows calculated were checked against those derived by Atkins for the same sub-

catchments and results were comparable with only very minor differences.

Following the routing of these hydrological inflows through the hydraulic model, it was

decided to use a single catchment wide storm duration which would produce the

highest peak flow in the area of primary interest, in Aberfoyle village. This approach

means that estimates of flooding around Aberfoyle will be conservative. A catchment

wide storm duration provides a more realistic representation of actual rainfall events.

The main location of interest dictates the appropriate model storm duration to use. The

rainfall runoff flows were calculated using a catchment wide critical storm duration of

12 hours. The flood peaks for each sub-catchment using this method are shown in

Table 5.


© Mouchel 2013 18

Site

code Watercourse

Flood peak (m³/s) for following return periods (years)

2.33 5 10 20 50 100 200

1 Loch Ard 31.8 46.2 55.3 65.6 79.9 90.9 104.0

2 Duchray 60.2 87.2 104.3 123.4 150.1 170.7 195.2

3 Lower Ard 3.1 4.6 5.5 6.6 8.1 9.3 10.7

4a Lateral 2.1 3.1 3.8 4.5 5.2 6.3 7.2

4b Lateral

downstream 2.0 2.9 3.6 4.3 5.3 6.1 7.1

5 The Pow 4.5 6.6 7.9 9.4 11.5 13.1 15.0

6 Allt a‟ Mhangam 4.7 6.8 8.2 9.8 12.0 13.7 15.8

7 Trib A 0.7 1.0 1.2 1.4 1.8 2.0 2.4

8 Park Burn 3.0 4.3 5.2 6.2 7.7 8.8 10.1

9 Trib B 1.3 1.9 2.3 2.7 3.3 3.8 4.3

10 Trib C 0.9 1.4 1.6 1.9 2.3 2.7 3.0

11 Milton gauge 33.7 49.1 58.8 69.7 84.9 96.6 110.6

12

Downstream

Boundary of the

model

99.0 144.1 172.3 203.9 248.0 282.1 322.6

Table 5 - Peak flows estimated with FEH rainfall runoff method with catchment wide storm duration of 12 hours

4.7 FEH Statistical Method

4.7.1. Introduction

At the Points of Interest presented in Table 3, peak flows were estimated using the

FEH statistical method as described in the following sub-sections. Results were also

compared against values obtained by Atkins for the same sub-catchments during their

previous study.

4.7.2. QMED Estimation

For ungauged catchments, FEH recommends that a donor catchment is used to

improve QMED estimates from catchment descriptors. A donor catchment is a local

catchment with gauged data particularly relevant to flood estimation at the subject site.

Where QMED has been estimated from catchment descriptors, the pooling method

has been used to derive pooling groups using sites indicated by HiFlows-UK as

suitable for QMED in order to identify sites that are hydrologically similar to the subject

site. The Centre for Ecology and Hydrology (CEH) report on improving the FEH

statistical procedures for flood frequency estimation sets out a new equation for

adjusting QMED from donor sites based on geographical difference (in kilometres

between the centroids of the subject and a donor catchment). The equation places

greater emphasis for adjusting QMED for pooled sites which are geographically close

to the subject site.


© Mouchel 2013 19

At the Milton gauge site, the final value of QMED has been derived from annual

maximum (AMAX) data at that gauge. The annual maximum series consists of the

largest observed flow in each water year17. The index flood is the median annual

maximum flood, QMED. This is a flood that is exceeded on average in exactly half of

all years, thus it has a return period of 2 years. Table 6 outlines the different methods

used for obtaining QMED and the estimated values.

Site code

Watercourse

QMED from catchment descriptors

(Atkins) (m³/s)

QMED from

AMAX (Atkins) (m³/s)

QMED from catchment descriptors (Mouchel)

(m³/s)

QMED from

AMAX (Mouchel)

(m³/s)

QMED from catchment descriptors

adjusted (Mouchel) (m³/s)

Final value of QMED (m3/s)

2 Duchray 83.7 N/A 83.7 N/A 81.6 81.6

11 Milton gauge 10.7 10.3 10.2 10.5 N/A 10.5

12

Downstream

boundary of

the model*

75.0 N/A 78.9 N/A 77.0 77.0

Table 6 - QMED values

*Note – Mouchel‟s downstream boundary is further downstream than Atkins‟

The slight differences in QMED from the previous values obtained by Atkins can be

attributed to differences between WINFAP version 2 and version 3.

4.7.3. Pooling Groups

There are no flow gauges in the Aberfoyle catchment suitable for pooling analysis. The

Milton gauge (NRFA number 18022), located upstream of Aberfoyle, is not in the

HiFlows-UK database and only has a short record. To obtain growth curves for each

POI, as with QMED estimates, an analysis of data pooled from hydrologically similar

catchments was undertaken.

For the FEH statistical pooling method, catchments are pooled on the basis of

hydrological similarity as identified using the AREA18, SAAR19 and BFIHOST20

catchment descriptors from the FEH CD-ROM 3. The pooling groups created for each

point of interest were reviewed and growth curves derived.

4.7.4. Statistical Method Peak Flow Estimates

For each point of interest, the product of the QMED and the growth curve was used to

obtain a peak design flow for each return period. Table 7 shows the statistical

estimates of peak flows based on three different approaches:

Flows calculated by Atkins,

17 In the UK, the water year runs from 1

st October to 30

th September. This is used to avoid splitting the

principal river flood season (winter) between two years).

18 Catchment area (km²)

19 1961 – 90 standard period annual average rainfall (mm)

20 Baseflow index estimated from soil type


© Mouchel 2013 20

Flows calculated by Mouchel by following Atkins‟ methodology (i.e. using same stations in pooling group but with the QMED adjusted by Mouchel), and

Flows calculated directly by Mouchel.

Site code

Watercourse Method

Flood peak (m³/s) for the following return periods (years)

2 5 10 20 50 100 200

2 Duchray

Atkins 83.7 104.5 118.9 133.9 155.6 173.9 194.1

Mouchel

following

Atkins‟

method

81.5 102.8 117.5 132.8 154.8 173.4 194.0

Mouchel 81.5 101.1 114.2 127.3 145.6 160.6 176.8

11 Milton gauge

Atkins 10.3 13.0 14.9 16.8 19.8 22.2 24.9

Mouchel

following

Atkins‟

method

10.1 12.7 14.5 16.4 19.2 21.4 24.0

Mouchel 10.5 13.9 16.4 19.2 23.3 27.0 31.3

Comment

on flows

used

Because of the attenuation and storage taking place

through the lochs and the floodplain in the sub-catchments,

the statistical pooling method may not provide very reliable

results for this particular catchment.

12

Downstream

boundary of

the model

Atkins 75.0 94.2 107.6 121.6 142.0 159.2 178.4

Mouchel

following

Atkins‟

method

77.0 97.2 111.4 126.1 147.7 166.0 186.3

Mouchel 77.0 100.8 118.4 137.7 166.8 192.6 222.3

Comment

on flows

used

The downstream boundary of the two models developed by

Atkins and Mouchel are slightly different, hence a direct

comparison of flows is complex. Also, because of the

attenuation and storage taking place through the lochs and

the floodplain in the sub-catchments, the statistical method

may not provide very reliable results for this particular

catchment.

Table 7 - Peak flows estimated using the FEH statistical method

4.8 Assumptions and Limitations of the Hydrological Methods

There are a number of assumptions and limitations involved in the use of the different

methodologies. Calibration and routing of rainfall overcomes some limitations. Some

assumptions and limitations of the various methods include:

FEH statistical method:

The hydraulic model may need to allow for effects not fully represented by the statistical method (e.g. floodplain storage and attenuation, artificial influences),and

This method only provides peak flow estimates.


© Mouchel 2013 21

FEH rainfall runoff method:

Catchment wide studies may have several different critical storm durations for different tributaries,

Small fast responding catchments may be more sensitive to storm duration than larger catchments, and

The method can only model single peaked events.

4.9 Choice of Hydrological Method

A calibration exercise was carried out to ensure the hydrological model was

representative of the catchment response. Further details can be found in Section 5.

The resulting design flows, calculated using the calibrated rainfall-runoff model, are

shown in Table 8, together with the initial rainfall-runoff flows and statistical flows.

FEH method Flood peak (m³/s) for the following return periods (years)

2 5 10 20 50 100 200 500 1000

Rainfall Runoff

(before

calibration)

65.0 87.2 104.3 123.4 150.1 170.7 195.2 233.4 273.4

Statistical21

(for

comparison) 81.5 102.8 117.5 132.8 154.8 173.4 194.0 NA NA

Design flow (after

calibration)22

100.8 144.2 171.4 201.5 241.6 273.2 312.2 369.9 421.2

Table 8 - Comparison and design flows for the inflow of the Duchray catchment into the model

The final design flows near to the sewage works in Aberfoyle after adjustments due to

the calibration are presented in Table 9.

Flood peak (m³/s) for the following return periods (years)

2 5 10 20 50 100 200 500 1000

Design flow (after

calibration) 111.5 160.2 182.2 217.4 262.1 295.7 333.0 394.0 448.0

Table 9– Design flows near the sewage works in Aberfoyle

Please note that the effects of snowmelt on runoff have not been assessed or

modelled as this is outside of the standard FEH methodologies.

21 Statistical flows derived by Mouchel similarly to Atkins‟ method

22 The design flows derived by Atkins for the Duchray catchment inflow into the model are in the same

range as the values derived by Mouchel.


© Mouchel 2013 22

5 Hydraulic Modelling

5.1 Modelling Approach

For this study, the existing Atkins model was converted from InfoWorks RS to ISIS–

TUFLOW software and was supplemented with additional survey data described in

Section 3.5. An integrated 1D-2D model has been developed for the reach covering

Milton, Duchray and Aberfoyle to assess in more detail the complex flow paths and

water levels. ISIS-TUFLOW has the ability to model a wide range of hydraulic

structures including bridges, culverts, sluices, pumps and weirs, as well as 2D flow

paths and water levels across floodplains.

The Light Detection and Ranging (LiDAR) data was provided by the client in the form

of a Digital Terrain Model (DTM). The DTM data was used to generate the ground

model at a 10m resolution for the 2D component of the model. Additional topographical

survey data including road levels, and bank top levels were also incorporated into the

ground model to increase the accuracy of the 2D model at key locations and features.

The modelling tasks included conversion of the InfoWorks RS model into ISIS-

TUFLOW, incorporation of supplementary topographical and digital terrain model data,

verification of the model using previous flood events, sensitivity analysis and derivation

of design water levels. The model enabled derivation of peak water levels for the 2, 5,

10, 20, 50, 100, 200, 500 and 1000 year return period flood events.

Once converted to ISIS-TUFLOW software, the model was extended downstream of

Cobleland Bridge. Cobleland Bridge was also added into the model as an additional

hydraulic structure near to the downstream boundary.

5.2 Hydraulic Model Development

5.2.1. Model Extents

The ISIS 1D part of the hydraulic model includes approximately 6km length of the

Avondhu including the reach from Upper Loch Ard to the confluence with the Duchray

Water. Similarly the model includes approximately 5.5km length of the River Forth from

the Duchray confluence to the Cobleland Bridge and approximately 0.89km length of

the Duchray Water. The 2D domain includes approximately 5km length of the

Avondhu/River Forth from the Lower Ard to approximately 1.5km downstream of

Aberfoyle. Similarly, the entire 1D reach (0.89km) of the Duchray was integrated in the

2D domain.

The extents of the hydraulic model are described below and shown in Figure 7:

The upstream boundary of the model is located at the western end of Loch Ard (grid reference 248650, 701350) at the same location as the Atkins‟ model,

The upstream boundary of the reach of the Duchray Water is located approximately 900m upstream of its confluence with the River Forth (grid reference 249850, 700800), and

The downstream boundary of the model is located on the River Forth, approximately 250m downstream of Cobleland Bridge (grid reference 25330, 698550) to the south of Aberfoyle. This is an extension from the Atkins‟


© Mouchel 2013 23

downstream boundary to ensure that there is minimum influence of the downstream boundary on areas of interest.


Figure 7 – Extents of the hydraulic model

Additionally, there are nine smaller inflows included in the model; Lower Ard, The Pow,

Allt a‟ Mhangam, Park Burn, three unnamed Tributaries A, B and C, and two lateral

catchments. A schematic of the hydrological and hydraulic model developed for this

study is shown in Figure 8.

Figure 8 – Schematic of the hydrological and hydraulic model

5.2.2. Boundary Conditions

Upstream boundary conditions

There are a total of 11 inflows (8 direct and 3 lateral inflows) in the hydraulic model,

generated with FEH boundary units which use the FEH rainfall runoff methodology to

derive inflow hydrographs, as described in Section 4.


© Mouchel 2013 24

Downstream boundary conditions

The downstream boundary has been represented in the model as a flow-head

boundary node. This represents the flow against stage (water levels above ordnance

datum) relationship (rating curve) derived from the most downstream cross section of

the model (i.e. normal depth).

5.2.3. River Channels and Floodplain Roughness Values

Manning‟s „n‟ roughness values from the Atkins‟ model have been reviewed23 and

verified through the use of site photographs. Table 10 outlines the typical initial values

used in the model before verification of the model.

River Reach River Channel

Manning’s ‘n’

Bank / Floodplain

Manning’s ‘n’

Upstream of Milton and The Narrows 0.05 0.06

Milton to Duchray Water / Avondhu confluence 0.04 0.08

Duchray Water 0.03 0.06

Duchray Water / Avondhu confluence to downstream

boundary 0.03 0.06

Table 10 – Manning’s ‘n’ values used in the hydraulic model

5.3 Model Calibration and Verification

5.3.1. Introduction

A hydraulic model is a simplified representation of complex physical processes, based

on a number of assumptions. In order to increase confidence in the model results, a

verification process was undertaken. Full calibration of the model was not possible due

to the limited hydrometric information available in the study catchment. Verification of

the model has been undertaken to adjust, when appropriate, some of the hydrological

and hydraulic parameters of the model.

5.3.2. Methodology

Event selection

Model verification was carried out with the following three observed events:

December 2006 - the peak of this event occurred on 14th December,

January 2007 - the peak of this event occurred on 15th January, and

September 2009 - the peak of this event occurred on 8th September.

These events were selected as they are recent large flood events and observed water

level data was available for these events at the Milton gauge. Also, flow data derived

by SEPA using their rating curve was provided for these events. The December 2006

and January 2007 events were used for verification in the original Atkins‟ model and

September 2009 was also used by Atkins for the re-calibration of the model.

23 Open Channel Hydraulics (Chow, 1959)


© Mouchel 2013 25

Additionally for the September 2009 event, water level data was also available from

the Manse Road Bridge gauge in Aberfoyle (grid reference 250348 701341) located at

the upstream side of Manse Road bridge in Aberfoyle. This gauge is used as a flood

warning gauge by Stirling Council and records water levels on the upstream face of the

bridge. Some anecdotal evidence was also available for the September 2009 event

which assisted with verification of the model.

Selection of rain gauges

Three nearby SEPA rainfall gauges (Section 3.7.2) were identified in the area for

model verification. The description of the data available at these gauges for each of

the selected verification events is provided below. Data availability is summarised in

Table 11 and a comparison of the data is presented in charts in Appendix B.

December 2006 event: the rainfall recorded at the three gauges is generally consistent. Loch Katrine gauge did not record the intense event on 12th and 13th December 2006. High Corrie gauge recorded less rain than the amount recorded at the Duchray Castle gauge.

January 2007 event: for this event, the Duchray Castle and High Corrie gauges have similar rainfall profile. High Corrie gauge recorded less rain than the Duchray Castle gauge, and the Loch Katrine gauge recorded significantly less rain from 12th to 19th January.

September 2009 event: for this event, the Duchray Castle and Loch Katrine gauges have similar rainfall profiles. The Loch Katrine gauge recorded less rain that the gauge at Duchray Castle. Also, the High Corrie gauge recorded significantly less rain than both the other gauges.

The comparison highlighted that the Duchray Castle rain gauge is more appropriate to

use for model verification for the three selected events. Furthermore, Duchray Castle

gauge being the only gauge located within the study catchment, it is the most

representative of the rainfall over the catchment during the three events selected for

model verification.


© Mouchel 2013 26

Type of data December 2006 event January 2007 event September 2009 event

Observed water level data at

SEPA Milton gauge Yes Yes Yes

Derived flow data at SEPA Milton

gauge

Yes (multi peak event

with maximum peak at

00.45 on 14th

December 2006)

Yes (single peak

event with peak at

02:15 on 15th

January 2007)

Yes (multi peak event

with maximum peak at

12:45 on 8th

September

2009)

Observed water level data24

at

Manse Road Bridge gauge in

Aberfoyle

No No Yes

Rainfall data at SEPA Duchray

Castle rain gauge Yes Yes Yes

Post event trash line survey data No No Yes

Photographic evidence No No Yes

Evidence from media No No Yes

Table 11 – Hydrometric data availability for each of the three selected events

Modelling

The following approach was applied:

The rainfall data from the Duchray Castle gauge has been applied as inflows

to all sub catchments and routed through the model. This has been carried

out initially for the most significant event (December 2006). Modelled and

recorded water levels and flows at Milton gauge were then compared for the

December 2006 event.

Standard Percentage Runoff (SPR) value has been adjusted for two upper

sub-catchments (sub-catchments 1 and 3 in Figure 6 and Table 2) to closely

match the flows and levels at the Milton gauge for the December 2006 event.

With this adjusted SPR, flows and levels of the two other events were

compared at Milton gauge. In addition, the modelled levels were compared

against the observed levels at Manse Road Bridge gauge in Aberfoyle for the

September 2009 event.

Further adjustments to the values of SPR, time to peak (Tp), Manning‟s

roughness and bridge calibration coefficients have been carried out for the

Duchray Water catchment and the eight sub-catchments25 downstream of the

Duchray - Avondhu confluence. This was undertaken to obtain the best

24 The Atkins‟ River Forth Hydrodynamic Model Update and Recalibration Report, 2010 states that “the

recorded levels at the Manse Road Bridge gauge in Aberfoyle had an error of - 211mm during the flood

event of September 2009. Therefore, an adjustment of + 211 mm has been made with the recorded water

levels”. Mouchel has also applied this adjustment (+211mm) to the recorded water levels at the Manse

Road Bridge gauge in Aberfoyle for the September 2009 event.

25 Sub-catchments 4a, 4b, 5, 6, 7, 8, 9 and 10 in Figure 6 and Table 2


© Mouchel 2013 27

possible match between the observed and modelled water levels at Manse

Road Bridge gauge for the September 2009 event.

With the final adjusted parameters, all the event models were re-run and final

comparison made.

The detailed adjustments to the model parameters are presented in Section 5.3.3 and

the results of the verification are in Section 5.3.4.

5.3.3. Adjustments of Model Parameters

The model had been originally calibrated by Atkins in June 2010 as a part of the River

Forth Hydrodynamic Model Update and Recalibration study. However, in order for the

model results to match closely with the recorded water levels and flows at Milton

gauge and recorded water levels at Manse Road Bridge gauge, further adjustments

were made as appropriate to the values of SPR, time to peak (Tp), Manning‟s

roughness and the bridge calibration coefficients.

The final adjusted values by Mouchel are close to Atkins‟ values. Table 12

summarises the final adjusted parameters used by Mouchel and Atkins.

Sub-

catchment

SPR adjustment

coefficient

Tp adjustment

coefficient

Manning’s roughness

value

Bridge calibration

coefficient

Mouchel Atkins Mouchel Atkins Mouchel Atkins Mouchel Atkins

Loch Ard

and Lower

Ard (two

catchments

upstream of

Milton)

1.2 0.7 No

adjustment 0.97

No

adjustment

No

adjustment

No

adjustment

No

adjustment

Duchray

Water 1.8 1.8 1.2 1.2

No

adjustment

No

adjustment

No

adjustment

No

adjustment

8 sub-

catchments

downstream

of Milton

2.2 2.2 1.2 1.2

River

channel:

increase

from 0.04 to

0.05 from

confluence

of Duchray

Water to

downstream

end of

model.

Floodplain:

no

adjustment

River

channel:

increase

from 0.03 to

0.04 from

confluence

of Duchray

Water to

downstream

end of

model.

Floodplain:

no

adjustment

Manse Road

bridge:

increase

from 1 to

3.5.

Two bridges

downstream:

increase

from 1 to 2.5

No

adjustment

Table 12 – Adjustments made to model parameters during model verification


© Mouchel 2013 28

5.3.4. Calibration and Verification Results

Following the adjustment of model parameters, each event was run through the

hydraulic model. The results are described below.

December 2006

The December 2006 event generated the highest peak of the three selected events

(25.6 m³/s at Milton at 00:45 on 14th December). The return period of this event

estimated based on the comparison of the observed flows and design flows of various

return periods at Milton gauge is approximately 135 years. Flows were out of bank at a

number of locations along the Avondhu, Duchray Water and the River Forth.

Initial baseline model results indicated a slight under prediction of the flows at Milton

gauge. After adjustment of the SPR value (20% increase) for the two upstream sub-

catchments, the observed and modelled values matched well (observed and modelled

(adjusted) flows of 25.6 and 25.3 m³/s respectively) (observed and modelled water

levels of 21.87 and 21.73 mAOD respectively). Also, the rising and receding limbs of

the recorded and modelled hydrographs along with the timing of the peak matched

well after the adjustment. Graphs of the comparison of the modelled and observed

flows and levels are presented in Appendix B.

January 2007

The return period of the January 2007 event estimated based on the comparison of the

observed flows and design flows of various return periods at Milton gauge is

approximately 9 years. During this event, out of bank flow occurred in Aberfoyle

village.

The calibrated model slightly under predicted the peak flow of the January 2007 event

at Milton gauge (by approximately 1.8 m³/s). However, the modelled and observed

maximum water level matched closely (modelled and observed levels of 21.56 and

21.55 mAOD respectively). For this event, the rising and receding limbs of the

recorded and modelled hydrographs were not matching as closely as the other two

events. The modelled time to peak was approximately 2 hours before than the

observed time to peak, suggesting that the response of the river at Milton gauge might

not correlate well with the rainfall recorded at Duchray Castle rain gauge. Graphs of

the comparison of the modelled and observed flows and levels are presented in

Appendix B.

September 2009

Similar to the other two events, the return period of the September 2009 event has

been estimated as approximately 3 years. This event caused out of bank flows at a

number of locations along the River Forth.

The calibrated model results at Milton gauge show that the flows were over predicted

in comparison to the observed flows (maximum observed and modelled flows were 7.9

and 13.5 m³/s), and the observed and modelled water levels were 21.41 and 21.61

mAOD respectively. The time to peak and shape of the rising and falling limbs of the

hydrographs were matching very closely after the adjustment.


© Mouchel 2013 29

Unlike the other two selected events, recorded water level data was available for this

event at Manse Road Bridge gauge in Aberfoyle; therefore data has been used for the

model verification. The initial model calibrated with the December 2006 event (with

20% increase in SPR value for the two upper catchments) showed a good match with

the water levels and flows recorded at Milton gauge. However, the modelled water

levels at Manse Road Bridge gauge were initially lower by approximately 1m

compared to the maximum observed level. The timing of the modelled water level peak

was approximately 2 hours before the observed peak, but the shapes of the rising and

falling limbs of the modelled and observed hydrographs were similar. Therefore, a

further adjustment of the parameters were carried out, including an increase of SPR

and Tp values by 80% and 20% respectively for the Duchray Water. Similarly, SPR

and Tp values for the remaining eight downstream sub-catchments have been

increased by 220% and 20% respectively. Manning‟s roughness has been increased

from 0.04 to 0.05 for the reach from the Duchray Avondhu confluence to the

downstream end of the model. The calibration coefficient of the Manse Road bridge

was increased to 3.5 from the default value of 1. Similarly, the calibration coefficients

for the two bridges downstream of the Manse Road were increased to 2.5 from the

original default value of 1. With these adjustments a reasonable match of the observed

and modelled water levels at Manse Road Bridge gauge was achieved for this event.

Graphs of the modelled and observed levels at Manse Road Bridge and Milton gauges

are in Appendix B.

Summary

For the three selected events, the flows and water levels after the adjustments are

outlined in Table 13.

Event

Flow at Milton gauge Water level at Milton

gauge

Water level at Manse Road

Bridge gauge

Ma

xim

um

ob

se

rved

26(m

3/s

)

Ma

xim

um

ad

jus

ted

mo

de

l

(m3

/s)

Dif

fere

nc

e

(m3/s

)

Ma

xim

um

ob

se

rved

(mA

OD

)

Ma

xim

um

ad

jus

ted

mo

de

l

(mA

OD

)

Dif

fere

nc

e

(m)

Ma

xim

um

ob

se

rved

(mA

OD

)

Ma

xim

um

ad

jus

ted

mo

de

l

(mA

OD

)

Dif

fere

nc

e

(m)

Dec.

2006 25.6 25.3 0.3 21.73 21.87 -0.14

Observed levels were not

available for this event

Jan.

2007 13.9 12.1 1.8 21.55 21.56 -0.01

Observed levels were not

available for this event

Sept.

2009 10.35 13.5 -3.15 21.41 21.61 -0.2 20.56 20.44 0.12

Table 13 – Comparison of recorded and modelled flows and water levels

The comparison shows reasonable fit between the observed data and the model

results, therefore the (adjusted) model is considered to satisfactorily represent the

hydrological and hydraulic regime of this catchment.

26 Milton gauge records water levels. Flows are derived using the rating curve. Both observed water

levels and the converted flows were made available to Mouchel by SEPA.


© Mouchel 2013 30

5.3.5. Verification with Historical Records of Flooding

Following the model verification using the hydrometric records at Milton and Aberfoyle

gauges, the model was verified further against historical records of flooding.

In Atkins‟ River Forth Hydrodynamic Model Update and Recalibration Report - 2010,

the September 2009 event was used to verify the model results against trash line

survey data. To allow for a comparison of Mouchel‟s results and due to the lack of data

for other events, this same event was used for verification.

The following data sources of historical flooding records were available for the

September 2009 event:

Trash line survey outline and photographs: a trash line survey was carried out immediately after the September 2009 event by Stirling Council, the outputs of which were presented in the Atkins‟ Report in 2010. A comparison of the flood outline and water levels at a number of locations was undertaken by Mouchel.

Articles and photographs published in the media such as BBC website27 and newspapers.

Trash Line Survey

The outline of the trash line survey adjacent to Manse Road Bridge, taken from Atkins‟

report, was digitised to allow for comparison with model results. The modelled outline

and thrash line of the September 2009 event is shown in Figure 9. In some locations,

particularly Main Street, the outlines match very well, but elsewhere, such as the Forth

Inn Hotel car park, there are some slight differences.

Trash line across

fence 0.91 m above

ground near river

Trash line across

fence 0.91 m above

ground near river

Forth Inn Hotel

Trash line across

fence 0.91 m above

ground near river

Trash line across

fence 0.91 m above

ground near river

Forth Inn Hotel


Figure 9 - Modelled outline for September 2009 event and trash line survey

27 http://news.bbc.co.uk/1/hi/scotland/glasgow_and_west/8244247.stm


© Mouchel 2013 31

The trash line was found by the Stirling Council to be approximately 0.91m above

ground level south of the Forth Inn Hotel. The model results indicate a water depth of

approximately 1.3m in this area.

Two other trash line levels were mentioned in Atkins‟ report:

Upstream face of the Manse Road Bridge: survey result: 20.65m AOD and model result: 20.50m AOD.

Junction between the B829 and the A821: survey result: 20.57m AOD and model result: 20.41m AOD.

There is generally a reasonable correlation between modelled outlines and levels and

the information reported in the trash line survey. However, there are a number of

possible reasons for the slight differences:

Local flood dynamics: there may be localised flood paths which are not displayed in the model. This could be a result of local features such as buildings, kerbs which would block flow.

Flooding not resulting from the river itself (i.e. overland flow) may have contributed to some of the trash lines. 28

There may have been some difficulties in identifying the trash line at a time after the event.

It is also worth noting that debris, when caught in fences, is not an indication of flood outline; rather, it is an indication of flood depth. Hence, it is possible for the trash line to lie within the flood outline. Where debris remains on the ground, this is a better indication of flood outline.

Articles and Photographs published in the Media

A number of articles published in the media have described the flooding which

occurred in Aberfoyle on 8th of September 2009. They commonly reported that:

sand bags have been deployed to protect properties in Aberfoyle,

the B829 was closed to traffic due to flooding - one person had to be rescued

from their car after “floating” in the water over the road, and

Main Street was flooded, as well as the car park.

A BBC article on the event29 presented a photograph of the inundation in Main Street.

The observed flood extent matches very well with that predicted by the model

In addition, anecdotal evidence has suggested that the A81 road flooded to a depth of

approximately 400mm, likely as a result of localised drainage issues (not included in

the hydraulic model) instead of flood water from the River Forth.

28 Flooding may occur from different sources (e.g. fluvial, surface water and groundwater), and/or as a

result of insufficient capacity of the manholes, and sewer channels. This study only deals with fluvial

flooding from the Avondhu, Duchray and the River Forth.

29 http://news.bbc.co.uk/1/hi/scotland/glasgow_and_west/8244247.stm


© Mouchel 2013 32

5.3.6. Summary of the Model Verification

Bearing in mind the aforementioned limitations, the model verification has confirmed

that the model results are reasonably accurate for the purpose of this study in this

hydraulically complex and diverse catchment.

5.4 Sensitivity Analysis

A sensitivity analysis has been undertaken using the verified (baseline) model to

assess the impact on water levels as a result of changes in model parameters and to

ascertain the robustness of the model. The model parameters altered in the model for

the sensitivity runs are typically those which are most influential on the water levels in

the study area: roughness of the river channels and floodplains, downstream boundary

of the model, peak flows (climate change allowance), and water levels in the Loch Ard.

The sensitivity runs have been undertaken for the 1 in 200 year return period flood

event, and the water levels have been compared at five river cross-sections shown in

Figure 10 and briefly described in Table 14. A comparison of the water levels at the

selected cross sections is presented in the following sub-sections, whilst the results at

each cross section of the model are presented in Appendix C


Figure 10 – Location of the cross sections used for comparison of results of sensitivity analysis


© Mouchel 2013 33

Cross section Grid reference Brief description

AA 245322, 701759 Upper Ard

BB 249410, 701564 Lower Ard

CC 250356, 701019 Approximately 380m upstream of the Duchray Water -

Avondhu confluence

DD 252015, 700931 Immediately upstream of Manse Road Bridge in Aberfoyle

EE 252649, 700573 Approximately 980m downstream of Manse Road Bridge

Table 14 – Brief description of the cross sections used for the comparison of sensitivity results

5.4.1. Changes in the Roughness Values in River Channels and Floodplains

A 20% increase and decrease in the roughness values (Manning‟s „n‟) were applied to

the verified model in both the river channels and floodplains. The changes in water

level and the percentages of change (from the 200 year maximum water level) are

presented in Table 15.

Cross section

Roughness increased by 20% Roughness decreased by 20%

Change in water level (m)

% of change in water depth



AA 0.01 0.1% 0.04 0.6%

BB 0.03 0.5% 0.03 0.5%

CC 0.21 7.2% 0.24 8.3%

DD 0.12 2.4% 0.13 2.5%

EE 0.12 2.2% 0.16 2.9%

Table 15 – Results of the sensitivity analysis following changes in roughness values

The results in Table 15 show that the reaches of the Upper Ard and Lower Ard are not

sensitive to changes in Manning‟s values, whilst the reach of the Duchray Water is

slightly sensitive to changes. Among the five selected cross sections, the percentage

change in water level is greatest immediately upstream of the Duchray Water -

Avondhu confluence (cross section CC).

The model reach of Loch Ard and the downstream reach of the confluence of the

Duchray Water with the Avondhu are relatively insensitive to changes in the roughness

values.

5.4.2. Changes in the Downstream Boundary of the Model

The downstream boundary conditions of a model can have an impact on upstream

modelled water levels in the vicinity of the boundary and beyond; therefore sensitivity

runs have been carried out to assess the potential effects of changes in the boundary

on the model results.

The downstream boundary of the model was generated using a normal depth

flow-water level relationship at the cross section at the downstream end of the model

(grid reference 253300, 698550). For the 1 in 200 years design event, the water level

is 19.49 mAOD at the downstream boundary of the verified model.


© Mouchel 2013 34

A 10% increase and decrease in water depth were applied at the downstream

boundary of the verified model to check the sensitivity of water levels at Aberfoyle to

the assumed boundary condition. The changes in water depth and the percentages of

change (from the 1 in 200 year depth) are presented in Table 16.

Cross section

Downstream boundary depth increased by 10%

Downstream boundary depth decreased by 10%





AA 0.00 0.0% 0.00 0.0%

BB 0.00 0.0% 0.00 0.0%

CC 0.01 0.3% 0.00 0.0%

DD 0.04 0.8% 0.02 0.4%

EE 0.11 2.0% 0.06 1.1%

Table 16 - Results of the sensitivity analysis following changes in the downstream boundary

The effects of a 10% change in the water depth at the downstream boundary are

noticeable up to Caile Mullach (cross section EE), approximately 2.7km upstream of

the downstream boundary. This extent is just downstream of the area of interest in

Aberfoyle village. Upstream of cross section EE, changes in water levels are ± 0.00 to

0.04m (0.0 to 0.8%) up to the confluence of Duchray Water with Avondhu. The

maximum change in the water level occurs at the downstream end of the model, near

to Cobleland Bridge, where the difference in level is ± 0.6m (11%) with a 10% increase

and decrease in the level of the downstream boundary.

The changes in water level in the model reach in Aberfoyle village (area of interest)

induced by changes in the downstream boundary are minimal.

5.4.3. Changes in the Flows (Climate Change Allowance)

The potential effects of climate change include an increase in the frequency and

severity of flooding. A 20% increase in fluvial peak flows (associated with climate

change allowance) was applied into the verified model to assess the sensitivity of the

model to changes in the flows. Table 17 presents the changes in water levels

associated with a 20% uplift on the peak flows for the 1 in 200 year return period

event.

Cross section Water level (m

AOD) Water level with a 20% uplift in flows (m AOD)



AA 33.19 33.37 0.18 2.7%

BB 33.08 33.26 0.18 3.1%

CC 21.92 22.14 0.22 7.6%

DD 21.32 21.60 0.28 5.5%

EE 20.81 21.16 0.35 6.4%

Table 17 - Results of the sensitivity analysis on a 20% increase of the flows (climate change)

The results show that the water levels increase between 0.15 and 0.50m in the model

reaches. The increase in water levels is smaller in the upstream reach than in the

downstream reach of the model. The reach of the Duchray Water (cross section CC)


© Mouchel 2013 35

and the reach downstream of its confluence with the Avondhu (cross sections DD and

EE) are more sensitive to the increase in the flows compared to the reaches of the

Upper and Lower Ard. (cross section AA and BB).

The sensitivity of the model to the change in the flows is relatively small and increases

from upstream to downstream.

5.4.4. Changes in the Bed Levels of the Loch Ard

The model results of the 1 in 200 year return period event show that when the bed

levels of the Upper Ard are raised by approximately 1m along approximately 3km of

the length of the Loch (i.e. 85% of the total length of the Loch), the peak water level

does not change compared to the baseline results (33.19 mAOD).

Also, the sensitivity analysis previously carried out by Atkins30 has shown that changes

in bed levels in The Narrows have negligible influence on the water levels in Aberfoyle.

Overall, the results show that changes in bed levels in the Upper Ard and The Narrows

have negligible influence on the water levels in Aberfoyle.

5.4.5. Conclusion of the Sensitivity Analysis

Four parameters have been tested in the sensitivity analysis (Manning‟s

roughness, downstream boundary, flows, and bed levels in Loch Ard). The

reaches of the Upper Ard and Lower Ard (cross sections AA and BB) are less

sensitive to changes to all the four parameters tested than the reach of the

Duchray Water (cross section CC) and the reach downstream of its confluence

with the Avondhu (cross sections DD and EE).

The reach of the Duchray Water (cross section CC) is more sensitive to the

changes in Manning‟s roughness values and flows than the reaches of the

Upper and Lower Ard (cross sections AA and BB) and the reach downstream

of the confluence of the Duchray Water with the Avondhu (cross sections DD

and EE).

The water level of the model is sensitive to the change in the flows. The

increase in water levels with a 20% increase in flow is smaller in the reaches of

Upper and Lower Ard (cross sections AA and BB) compared to the reach

downstream of the confluence of the Duchray Water with the Avondhu (cross

sections DD and EE). The model generally responds uniformly and in line with

expectations to change in the modelled inflows.

The sensitivity carried out with the changes in the bed levels of the Upper Ard

and The Narrows shows negligible influence on the water levels in Aberfoyle.

Aberfoyle experiences a 0.04m increase in water levels (depending upon

location) with a 10% increase in the downstream boundary levels. Water levels

in Aberfoyle village are therefore not significantly sensitive to the downstream

boundary condition.

30 Aberfoyle Flood Study, Hydraulic Modelling Report, Atkins, 2009


© Mouchel 2013 36

Overall, the sensitivity analysis indicates robustness of the hydraulic model and

provides reasonable confidence in the results.

5.5 Modelling Results

Following hydraulic model verification as described in Section 5.3, the model was run

for the baseline scenario for the following design events: 2, 5, 10, 20, 50, 100, 200,

500 and 1,000 years.

For each event, the water level at each of the cross sections is presented in Appendix

C. Flood maps, key flooding issues, locations of flooding and flood mechanisms for the

baseline case are described in the following sub-sections, whilst the key model results

of the flood mitigation options are presented in Section 6.

5.5.1. Flood Maps

The model results were processed in GIS to generate flood maps for all the design

events of interest. The flood maps are presented in Appendix D. The model results in

terms of flood extent and water depth have been verified with different events.

5.5.2. Key Flooding Issues: Locations and Flood Mechanisms

The flood mechanisms, particularly at the Duchray / Avondhu confluence, are relatively

complex with large interactions between main channel and floodplain flows. Inundation

videos are valuable in further understanding flood risk. These videos are available

upon request.

The identification of potential areas prone to flooding in the study area helps to inform

future flood risk management and mitigation options. The three key locations identified

as most prone to flooding are: the B829 road, Lochard Road and Aberfoyle village.

Flooding in the reaches of the Upper Ard and Lower Ard is mainly due to the Loch Ard

inundating the road. Flooding in the area of Milton is mainly due to the overflow from

the Avondhu Burn. The Duchray Water is the largest contributor to the flooding

occurring in Aberfoyle village accounting for roughly 85% of the total flow experienced

at Aberfoyle.

The time to peak of the Duchray Water is 10 hours. The Avondhu Burn peaks between

9 and 14 hours. The model results show that the overflow from the banks of the

Duchray Water enters the Avondhu and the River Forth at various locations.

Therefore, the Avondhu experiences a prolonged time of peak (from 9 to 14 hours

after event start) upstream of the confluence. The locations where the flows from the

Duchray Water enter the Avondhu are illustrated in Figure 11.


© Mouchel 2013 37


Figure 11 – Locations of the overflow from the Duchray Water near the confluence

Any attempt in the Duchray Water to reduce the flow or increase the time to peak (by

more than four hours) to avoid the concomitance of the peaks of the Avondhu and

Duchray Water at the confluence would help to reduce flooding issues in Aberfoyle.

Therefore, two flood mitigation options (Options 2 and 3) described in Section 6

assess the effects of incorporating measures to reduce the peak flows and increase

the time to peak of the Duchray Water. Details of the flooding mechanisms for each of

the three key locations are provided below.

Aberfoyle

The model results show that parts of Aberfoyle village including residential properties

and Lochard Road experience flooding due to the overtopping of the left bank of the

River Forth during a 1 in 200 year design event. For this event, the flood outline is

depicted by the blue line in Figure 12 (Aberfoyle centre).

Downstream of Aberfoyle, sections of the A821 and A81 roads are at risk of flooding

due to the overtopping of the left bank of the River Forth for a 1 in 200 year design

event. The flood outline of this event is depicted by the blue line in Figure 13

(downstream of Aberfoyle). The reach of the River Forth where the out of bank

overflow from the left bank has been illustrated by red arrows in Figure 12 and Figure

13.


© Mouchel 2013 38

Lochard road and the houses

get flooded

Out of bank overflow from

the left bank occurs in this reach

Lochard road and the houses

get flooded

Out of bank overflow from

the left bank occurs in this reach


Figure 12 – Flood outline in Aberfoyle centre for the 200 year design event

A821 and A81 roads

get flooded

Out of bank

overflow from the

left bank occurs in

this reach

A821 and A81 roads

get flooded

Out of bank

overflow from the

left bank occurs in

this reach


Figure 13 – Flood outline downstream of Aberfoyle for the 200 year design event

Milton

Milton is situated approximately 2.5 km upstream of Aberfoyle centre. The model

results show that two houses located on the left bank of the Avondhu in Milton are at


© Mouchel 2013 39

potential risk of flooding during a 1 in 200 year return period design event due to the

overtopping of the left bank of the Avondhu in this reach. Topographical survey

confirmed that these properties are not at significant risk of flooding, with floor levels in

excess of 1 in 1000 year flood levels.

Kinlochard Road (B829)

Kinlochard Road (B829) runs alongside the left bank of Loch Ard. The flow attenuation

provided by the loch amounts to approximately 80% of the peak flow of the 200 year

event. The model results show that the B829 road gets flooded when the return period

of the event exceeds approximately 20 years. Flooding of the B829 road alongside the

loch creates difficulties with access to the villages Kinlochard, Stronachlachar and

Inversnaid.

The flood outline along Loch Ard and Kinlochard Road (B829) is shown in Figure 14

for a 1 in 200 year flood event. The section of Loch Ard where elevated loch levels

threaten the road is illustrated by red arrows in Figure 14.

The B829 road gets

flooded

Upper Ard

The Narrows

Lower Ard

Out of bank

overflow occurs

from this locations Out of bank

overflow occurs

from this locations

The B829 road gets

flooded

The B829 road gets

flooded

Upper Ard

The Narrows

Lower Ard

Out of bank

overflow occurs

from this locations Out of bank

overflow occurs

from this locations


Figure 14 – Flood outline along the Loch Ard for the 200 year design event


© Mouchel 2013 40

6 Flood Mitigation Optioneering

6.1 Potential Options

Three flood mitigation options (Options 1, 2 and 3) have been developed, incorporating

a combination of measures. All three options were developed in close collaboration

with Stirling Council. Two meetings were held with the client on 3rd March and 3rd May

2012 to discuss these options. All three options have been developed for the 1 in 200

years design event using the verified model.

The detailed analysis of the key flooding issues provided in Section 5.5.2 has

highlighted that the reach of Loch Ard and Milton are prone to flooding due to the

overflow from the river banks. Therefore, in all three options, flood walls have been

considered for the reach of Loch Ard and Milton. In addition, with the Duchray Water

being the largest contributor to the flooding in Aberfoyle, two options (Options 2 and 3)

have been developed with the aim of reducing the peak flows of the Duchray Water

downstream of its confluence with the Avondhu. The potential options are outlined

below and the maps are in Appendix E.

6.1.1. Option 1 - Flood defence wall / embankment and property protection on Loch Ard, in

Milton and Aberfoyle

Option 1 comprises a combination of measures aimed at preventing flooding from

Loch Ard, the Avondhu and the River Forth along the three key locations (Loch Ard,

Milton and Aberfoyle) by means of traditional hard flood defence walls / embankments.

This option includes flood defences along the left bank of Loch Ard, along the left bank

of the River Forth through Aberfoyle, and in some areas downstream of Aberfoyle.

Also, localised protection measures for a few individual properties are also required

and included. The map showing the extent of the flood defence measures of Option 1

is presented in Figure 15 and in Appendix E.


Figure 15 – Option 1 – potential flood defence measures


© Mouchel 2013 41

The key features of Option 1 including a 0.6m of freeboard are outlined in Table 18.

Flood mitigation measure Details

Flood defence walls along Loch Ard and Milton

4 flood defence walls (total length: 2.5 km). The height of the wall ranges from 1 to 1.5 m.

Flood defence wall in Aberfoyle centre 1 flood defence wall (total length: 2.1 km), The height of

the wall ranges from 1.5 to 4 m.

Flood defence wall downstream of Aberfoyle centre

3 flood defence walls (total length: 0.9km). The height of the wall ranges from 1.5 to 3.5 m.

Flood protection to individual property (localised flood resistance measures)

Localised flood resistance measures (bunds or walls) to eight individual properties at flood risk.

Table 18 – Key features of Option 1


Milton and Aberfoyle and flow control structure on the Duchray Water

Option 2 is as per Option 1 but also includes a significant flow control structure on the

Duchray Water and associated flood walling approximately 700m upstream of the

confluence of the Duchray Water and the Avondhu Burn. The combination of the flow

control structure and the flood defence walls mobilises storage of water upstream of

the flow control structure and hence reduces the flow and water level downstream of

the structure through attenuation. As in Option 1, this option also includes flood

defences along the left bank of Loch Ard, the River Forth through Aberfoyle and in

some areas downstream of Aberfoyle. With Option 2, the height requirement of the

flood defences walls is slightly lower than the height required in Option 1. Localised

protection measures of a few individual properties are still also necessary. The map of

the extent of the flood defence measures of Option 2 is presented in Figure 16.


Figure 16 - Option 2 – potential flood defence measures


© Mouchel 2013 42



Flood defence walls along Loch Ard and Milton 4 flood defence walls (total length: 2.5 km). The height of

the wall ranges from 1 to 1.5 m.

Flood defence wall in Aberfoyle centre 1 flood defence wall (total length: 2.1 km), The height of the

wall ranges from 1.5 to 3.5 m.


3 flood defence walls (total length: 0.9 km). The height of the wall ranges from 1.5 to 3.3 m.

Flood protection to individual property(localised flood resistance measures)

Localised flood resistance measures (bunds or walls) to eight individual properties at flood risk.

Flow control structure and flood defence wall in the Duchray catchment

A flow control structure on the Duchray Water and flood defence walls along the watercourse (total length: 0.15 km

and height ranging from 7 to 8 m)31

.


6.1.3. Option 3 – Flood defence wall / embankment and property protection on Loch Ard, in

Milton and Aberfoyle, and natural flood management measures

Option 3 is as per Option 1 but also includes potential flow reduction enhancements

from natural flood management measures implemented in the Duchray Water

catchment. The map of the extent of the flood mitigation measures of Option 3 is

presented in Figure 17.


Figure 17 – Option 3 – potential flood mitigation measures

31 Provision of several small size flow control structures in the Duchray Water at various upstream

locations in addition to, or in place of, a single restriction on the Duchray would be more practical option. The use of the single restriction was to provide for a simpler appraisal as the conclusions would be similar.


© Mouchel 2013 43

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

Aberfoyle, 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 in the Duchray Water catchment.

This brief review is more detailed in Appendix J1 and the potential flood management

measures for the Duchray Water catchment are in Appendix J2. The following

information summarises the key points from the brief review.

Natural Flood Management Techniques

Natural Flood Management 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 natural flood management approaches is provided in Figure 18.

Figure 18 - Natural flood management approaches (from SNIFFER, 2011)

Natural flood management 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.

Sustainable Drainage Systems (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.


© Mouchel 2013 44

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.

This can help to reduce the magnitude of flood peaks, reduce bed scour and

increase the time to peak.

Increase 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

environmental 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 evapo-

transpiration, increasing infiltration and reducing surface runoff, hence 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 / catchment

specific study.

Previous Natural Flood Management Studies and Key Findings

Although the effects of natural flood management on a small scale are well known (i.e.

SuDS), there is only a limited evidence base on natural flood management

implementation on a catchment scale. SEPA has been working to develop the natural

flood management 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 J1.

The following is an extract taken from the Environment Agency‟s 2008 study which

examined the role of land use management in delivering flood risk management

benefits (Environment Agency, 2008):


© Mouchel 2013 45

“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 issues 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.

Hydrological Modelling of Natural Flood Management Measures in the Duchray

Water Catchment

The greatest benefit of natural flood management measures would be realised in

heavily modified catchments where there is larger scope for improvement. The

Duchray catchment has been more heavily modified over the years compared say to

the Callander catchment

To test the effects of potential natural flood management measures on downstream

flows and water levels, the hydrological model was modified by decreasing the

standard percentage runoff (SPR) value of the Duchray catchment by 20% and

increasing the time to peak (Tp) by 20%. These are considered optimistic nominal %

reductions to provide an initial idea of potential catchment attenuation and storage

effects.

However, many of the pilot studies undertaken recently on natural flood management

measures state the changes in peak flow that can be achieved using natural flood

management are generally 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 periods. The greatest benefits would also be realised in heavily

modified catchments

A more realistic scenario has been carried out with the model for the Duchray

catchment with a decrease of 7% of the baseline SPR value and an increase of 7% of

the baseline time to peak (Tp). The results of these modifications to the hydrological

model to effect a theoretical NFM catchment response show a reduction of

approximately 9% of the Duchray catchment inflow to the hydraulic model compared to

the baseline.

Applicability of Natural Flood Management Measures to the Duchray Water

Catchment

The modelling results show there are potential benefits to be achieved from natural

flood management measures to the Duchray Water catchment if the theoretical flow

noted above can be achieved in practice. However, NFM implementation in Scotland


© Mouchel 2013 46

has, to date, been limited. The benefit of the NFM is highly dependent on the return

period of the event (i.e. lower return periods bring greater benefit than the higher return

periods).

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 scale 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 it was noted in the Pickering study, in order to fully explore

NFM, the modelling and scoping work can demand a significant amount of funding.

For the Duchray Water catchment, the most practical NFM measures could be:

Flood storage, in the form of ponds, reservoirs, wetlands etc. to attenuate the

peak of river flows and reduce flood levels downstream - this also has

environmental benefits through habitat creation,

Soil management aiming at improving water retention through increasing

infiltration (e.g. protecting and enhancing the soil condition),

Slowing down and reducing runoff through slope management techniques

combined with enhancement of the vegetation and plantations to increase

roughness, interception, evapo-transpiration and infiltration - this also has

environmental benefits, and

Changes in forestry drainage

Details of potential natural flood management measures for the Duchray Water

catchment are provided in Appendix J.2.

It is suggested that if the Council wishes to investigate in more detail the potential

benefits that could be realised in terms of natural flood management 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:

A thorough literature review (including on-going studies) to identify the options

available and their potential benefits particularly for Scottish catchments similar

to the Duchray Water catchment,

A detailed GIS-based assessment of the most appropriate specific locations to

implement natural flood management measures (similar to Upper Allan study),

More detailed hydrological modelling to determine more accurately the benefits

of natural flood management measures, and

Advice on the possible funding streams.

Such a study should take a catchment-wide integrated approach enabling the

assessment of the benefits to Stirling.


© Mouchel 2013 47



Flood defence walls along Loch Ard and Milton

4 flood defence walls (total length: 2.5 km). The height of the wall ranges from 1 to 1.5 m.

Flood defence wall in Aberfoyle centre 1 flood defence wall (total length: 2.1 km). The height of the wall

ranges from 1.5 to 3.5 m.


3 flood defence wall (total length: 0.9 km). The height of the wall ranges from 1.5 to 3.3 m.

Flood protection to individual property (localised flood resistance measures)

Localised flood resistance measures (walls or bunds) to eight individual properties prone to the flooding

Natural flood management measures in the Duchray Water catchment

Flood storage (ponds, reservoirs, wetlands )

Soil management to improve water retention and infiltration

Slope management techniques combined with enhancement of vegetation and plantations


6.2 Option Appraisal / Impact Assessment



Option 1 includes a total of 5.5 km of linear flood defences (walling / embankments)

and some localised individual flood resilience measures (walls or bunds) to 8 individual

properties.

Figure 19 shows the changes in the maximum water levels between the baseline

(without Option 1) and the design scenario (with Option 1 in place) for the 1 in 200

year return period flood event.

Figure 19 – Option 1 - comparison of water levels between baseline and Option 1 (200 years)


© Mouchel 2013 48

The model results show that by including flood defences along the left bank of the

River Forth through Aberfoyle, water levels are predicted to rise by 0.3 to 0.5 m

upstream of Manse Road Bridge compared to the baseline results for the 200 year

flood event. This increase in water level extends approximately 2.2 km upstream of

Manse Road Bridge and also upstream of the extent of the modelled flood defence

wall. No additional properties are at risk of flooding due to these water level increases.

All the existing properties prone to flooding are protected by the Option 1 flood

defences.

For the 200 year flood event, the height of the flood defence wall immediately

upstream of Manse Road Bridge ranges from 3.5 to 4 m. Downstream of Manse Road

Bridge, the maximum increase in water level is 0.06 m with Option 1 in place. The

peak water levels in Loch Ard remain unchanged with the Option 1 defences in place.

In addition, Option 1 has been tested with the 20 year return period event to assess

the decrease in the water level/height of the embankment compared to the 200 year

event. The model results show that with the 20 year event, the water levels / height of

the flood walls in Aberfoyle town decrease by approximately 0.6 to 0.9m compared to

the 200 year defences.



Option 2 is as per Option 1 but also includes a flow control structure on the Duchray

(nominally modelled in the form of a 2.4m diameter pipe restriction) with an addition of

approximately 150m of flood defence walls / embankments being included

approximately 700m upstream of the confluence with the Avondhu.

To assess the changes in flows and water levels induced by the flow control structure,

a model run was carried out which only included the flow control structure for the 200

year flood event. The results show an increase of time of peak in the Duchray Water

by approximately 1.0 hour and a decrease of flow by 23 m³/s (around 11% of the

baseline flow) in the Duchray Water at its downstream end. Figure 20 provides a

comparison of the hydrographs of the Duchray and Avondhu at the confluence with the

baseline.


© Mouchel 2013 49

0

50

100

150

200

250

0.00 5.00 10.00 15.00 20.00 25.00

Time, hrs

Flo

w, m

3/s

Duchray_Initial model hydrograph Duchray_Base line hydrograph Avon Dhu_initial model hydrograph

1 in 200 years maximum flow in Duchray at the

confluence with the flow control structure = 172

m3/s

1 in 200 years maximum flow in Avon Dhu

at the confluence with the flow control

structure = 35 m3/s


confluence without the flow control structure = 193 m3/s

0

50

100

150

200

250

0.00 5.00 10.00 15.00 20.00 25.00

Time, hrs

Flo

w, m

3/s

Duchray_Initial model hydrograph Duchray_Base line hydrograph Avon Dhu_initial model hydrograph


confluence with the flow control structure = 172

m3/s

1 in 200 years maximum flow in Avon Dhu

at the confluence with the flow control

structure = 35 m3/s


confluence without the flow control structure = 193 m3/s

Figure 20 – Hydrographs of the Duchray Water and Avondhu with and without the flow structure (200 years event)

For the 200 year flood event, the model results with the flow control structure in place

show that the peak water level is reduced by 0.10 to 0.19m in the reach of the River

Forth downstream of the Duchray - Avondhu confluence. Also, the time to peak of the

Duchray Water is increased by approximately 2 hours compared to the baseline. To

avoid the concomitance of the Duchray Water and the Avondhu peak flows, the time to

peak of the Duchray Water would ideally increase by more than 2 hours. This would

potentially further reduce the flooding issues downstream of the confluence. This could

potentially be achieved by further flow control optioneering and / or provision of several

small size flow control structures in the Duchray Water at various upstream locations in

addition (which may be more practical / desirable to implement).

Upstream of the modelled flow control structure, the water depth ranges up to 9 m in

the river channel and up to 7 m in the floodplain. Without the flow control structure, the

water depth at these locations ranges up to 3.5 m in the channel up to 2 m in the

floodplain. Figure 21 shows the location of the control structure along with the likely

depth and extents of the flood water upstream of the structure. This modelled flow

control structure is principally intended to illustrate the potential effectiveness of a flow

restriction at the downstream end of the Duchray, rather than being a specific solution

to be taken forward. There would be numerous technical constraints to consider with

such a structure including the height of retention.


© Mouchel 2013 50

legends

0 – 1 m

1 – 2 m

2 – 3 m

3 – 4 m

4 – 5 m

5 – 6 m

6 – 7 m

7 – 8 m

Location of the flow

control structure approx. 700 m

u/s of the confluence

Flood defence wall to retain the water

Colour regions representing

depth of water in metre

legends

0 – 1 m

1 – 2 m

2 – 3 m

3 – 4 m

4 – 5 m

5 – 6 m

6 – 7 m

7 – 8 m

legends

0 – 1 m

1 – 2 m

2 – 3 m

3 – 4 m

4 – 5 m

5 – 6 m

6 – 7 m

7 – 8 m

Location of the flow

control structure approx. 700 m

u/s of the confluence

Flood defence wall to retain the water

Colour regions representing

depth of water in metre


Figure 21 – Flood extent and water depth upstream of the flow control structure (200 year event)

The model results show that the flow control structure would help reduce flows and

water levels downstream; however, the reduction in water level in the River Forth

downstream of the confluence is only 0.10 to 0.19 m. This reduction in water level is

relatively small and would only form part of a solution for Aberfoyle.

With the rest of Option 2 in place (i.e. including the flood defences along the River

Forth in Aberfoyle, Milton and Loch Ard), Option 2 results in increases in water level by

0.30 to 0.35 m in the reach upstream of Manse Road Bridge (up to approximately 2.1

km upstream) compared to the baseline (Figure 22). Similarly, with this option the

water level decreases by approximately 0.10 to 0.12 m downstream of Manse Road

Bridge in comparison to the baseline (Figure 22).

With Option 2 the water levels are lower than those with of Option 1 by approximately

0.20 m in the reach of the River Forth upstream of Manse Road Bridge. Downstream

of Manse Road Bridge, the water level is 0.15 to 0.20 m lower than the levels of Option

1. However, the very large increase in water depths (7 to 9 m) upstream of the flow

control structure makes this option very difficult to implement with one single structure.


© Mouchel 2013 51

Manse Road Bridge

1 in 200 years (Baseline)_Max Water Level

Option 2_Max Water Level

2.2 km Downstream extent

of the model

Water level difference – 0.2 to 0.3 m

Manse Road Bridge

1 in 200 years (Baseline)_Max Water Level

Option 2_Max Water Level

2.2 km Downstream extent

of the model

Water level difference – 0.2 to 0.3 m

Figure 22 – Option 2 - comparison of water levels between baseline and Option 2 (200 year event)

Option 2 – Further Comment

The effect of providing a series of smaller flow control structures further

upstream in the Duchray catchment could not be tested due to the limited

extent of the Duchray Water reach in the hydraulic model. Provision of small

size flow control structures in the Duchray Water at various upstream locations

would potentially reduce the water depths upstream of the structures and

therefore the potential risk to the residential areas located downstream. More

extensive and location specific appraisals would be required to explore such

storage options.

Currently, Option 2 has only been assessed with the existing topography of the

area upstream of the flow control structure. A storage area could be re-graded

upstream of the flow control structure to allow retention of larger volumes of

water than the current volume with Option 2, thereby potentially further

reducing downstream flows and levels. If storage options are to be pursued

further, further investigations will be required.

The flow control structure currently considered in this option is nominally sized

as a pipe restriction of 2.4 m diameter. Due to the large water depths upstream

of the structure, smaller pipe sizes have not been explored. If the general flow

control structure option on the Duchray Water is pursued further, then other

more efficient types of flow control devices (i.e.: sluice gate, weir, orifice and

hydro-brake) and multiple medium to small structures at different locations

along Duchray Water would be advised to be assessed.

In addition, Option 2 has been tested with the 20 year return period event to

assess the water levels and heights of flood defence embankments compared

to the 200 year event. The model results show that with the 20 year event, the

water levels and heights of the flood defence walls in Aberfoyle town decrease

by approximately 0.5 to 0.7 m compared to the results of the 200 year event.


© Mouchel 2013 52


Milton and Aberfoyle, and natural flood management measures in Duchray Water

catchment

Option 3 is as per Option 1 but also includes potential natural flood management

measures in the Duchray catchment. Details of potential natural catchment flood

management techniques for the Duchray Water catchment are described in Section

6.1.3 and Appendix J. The natural catchment flood management measures aim to

reduce runoff through flood storage, soil management and slope management

techniques combined with enhancement of the vegetation and plantations (e.g. trees

instead of pasture), and at enhancing the environmental benefits.

To test the effects of potential natural flood management measures on downstream

flows and water levels, the hydrological model was initially modified by decreasing the

standard percentage runoff (SPR) value of the Duchray catchment by 20% and

increasing the time to peak (Tp) by 20%. For the 200 year flood event, the model

results for Option 3 show a decrease in water level downstream of Manse Road Bridge

by 0.38 to 0.54 m compared to the baseline. Also, the water level decreases up to 0.12

m over a reach extending approximately 2.1 km upstream of Manse Road Bridge.

Figure 23 depicts the changes in the maximum water levels between the baseline

(without Option 3) and the Option 3 scenario in place for the 200 year return period

flood event.

Figure 23 – Option 3 - comparison of water levels between baseline and ideal Option 3 (200 year)

A more realistic scenario was modelled with a decrease of 7% of the baseline SPR

value and an increase of 7% of the baseline time to peak (Tp). These modifications to

the hydrological model to effect a more realistically achievable theoretical NFM

catchment response showed a reduction of approximately 9% of the Duchray

catchment compared to the baseline. For Option 3, a decrease of 0.14 to 0.20 m in

water level downstream of Manse Road Bridge but an increase of 0.30 to 0.35 m in


© Mouchel 2013 53

water level in the reach upstream of Manse Road Bridge compared to the baseline

was predicted (i.e. the water levels in this reach are similar to those of Option 2).

In addition, Option 3 has been tested with the 20 year return period event to assess

the water levels and heights of flood defence embankments compared to the 200 year

event. The model results show that with the 20 year event, the water levels and

heights of the flood defence walls in Aberfoyle town will decrease by approximately

0.35 to 0.55 m compared to the results of the 200 year event.

6.3 Potential Option Constraints

The potential constraints of implementing any of the three flood mitigation options have

been briefly investigated as part of this study. Service Plans showing electricity, gas,

telecommunications, water and wastewater networks have been reviewed to identify

any major conflict with the location of the flood mitigation measures. Maps showing the

location of services within the study area are presented in Appendix F.

The Allt a' Mhangam is a tributary which enters the River Forth near the petrol station

on Main Street in Aberfoyle. In agreement with the Council, this watercourse has not

been included in this assessment at this stage. As this watercourse could potentially

be cut off by flood defence walls, it would be necessary to investigate a localised

solution during future outline and detailed design stages of the scheme. The floodwalls

along this tributary will need to be extended upstream of the confluence by an

appropriate distance to prevent any overtopping from this tributary due to the backing

up from the River Forth.

Potential environmental and geo-morphological constraints have been investigated

and are discussed in Sections 8 and 9 respectively. Some of the key health and safety

considerations that should be taken into account in scheme development include:

ensure safe access and egress in times of flood,

maintenance plans need to be carefully devised and implemented,

ensure that all rights of way along the river banks have safe egress points in

times of flood,

suitable warning measures in close proximity to the construction area,

assess implications of the increased velocities in the watercourses post

scheme implementation,

the impact on the urban drainage systems will need to be assessed, and

impact on services and if any diversions are required.

6.4 Demountables / Temporary Defences

Where permanent defences are not feasible (be it technically, economically,

environmentally or politically), demountable or temporary defences provide an

alternative which can be considered as a flood mitigation option. The guidance which

applies to such flood mitigation options is Defra / EA report Temporary and


© Mouchel 2013 54

Demountable Flood Protection Guide32. This outline assessment of demountable /

temporary defence feasibility follows the key steps in the aforementioned Defra / EA

report.

A temporary flood protection system is a system which is in place only during a flood

event (and is removed fully as soon as flood waters recede). A demountable flood

protection system is one which is pre-installed but requires operation or partial

installation during the flood event. A combination of permanent, demountable and/or

temporary flood protection measures is also possible.

6.4.1. Is the use of demountable / temporary defences and option?

There is a strong community forum for flood risk in Aberfoyle. Where permanent

defences are not a feasible option, it is anticipated that the local community would be

supportive of alternative options. The organisational capacity required to manage and

operate such systems is potentially available at the Council, SEPA and emergency

services.

Flood forecasting and alerts are an integral component of temporary / demountable

defence systems. Without an appropriate lead time, it is not possible to mobilise such

a defence system. SEPA issues flood alerts and warnings to the Aberfoyle area;

typical (nationwide) lead times for flood alerts are up to 24 hours, whereas flood

warnings are typically issued 3-6 hours in advance. In theory, this would be sufficient

time to mobilise defences. However, the limited amount of reliable river gauging in the

area limits the reliability of flood warning measures. This would lead to defences being

mobilised more often than necessary, leading to a waste of money and unnecessary

disruption.

If flood warning systems can be improved, demountable or temporary defences are a

potential option.

6.4.2. Development of Key Criteria

The two areas in the study area which flood most regularly are Lochard Road and

Aberfoyle village centre (Main Street). Lochard Road floods at various locations along

a length of 1.2 km and, as a result, cannot be readily protected by temporary or

demountable defences (due to logistical and cost reasons). The following

investigations therefore focus on protecting Aberfoyle Village centre. Individual

property protection remains an option and is discussed separately below.

The approximate flood depths at the car park for key return periods are as follows:

2 year: 0.5 m

5 year: 0.8 m

10 year: 0.9 m

20 year: 1.1 m

32 Defra / EA, 2011. Temporary and Demountable Flood Protection Guide. EA Project Number

SC080019. Bristol: Environment Agency


© Mouchel 2013 55

50 year: 1.4 m

100 year: 1.6 m

Some allowance should also be made for freeboard. This is typically 600 mm; however

this could potentially be reduced for lower standards of protection due to increased

confidence in flood levels associated with more frequent events.

Figure 24 below shows a possible alignment for temporary / demountable defences in

the village. It would involve tying into the existing wall at the fire station (which would

need upgraded to an appropriate flood retaining structure) at the eastern end; there

are a number of options for tie-in at the western end of the defences. The options,

shown as dotted lines on the figure below involve taking defences to higher ground on

Trossachs Road or Lochard Road or tying into property walls.

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

Figure 24 – Potential demountable / temporary defence alignment (red lines) and 50 year return period flood outline (light blue)

Key constraints include:

The flood warning lead time is unlikely to be sufficient to allow mobilisation of a significant amount of temporary defences and hence demountable defence would appear to be a more appropriate option (perhaps in combination with temporary defences). This would require permanent installation of some demountable system.

Due to the relatively frequent nature of flooding and the uncertainty associated with flood warnings at Aberfoyle, the defences would require to be activated quite regularly and may mean numerous false alarms. This is not only a waste of the resources of the management organisation; it would also cause unnecessary disruption to the local community.

Unless defences are extended 1.2 km onto Lochard Road (which is not considered practical), Lochard Road cannot be kept open whilst defences are


© Mouchel 2013 56

in place. Access along Manse road to Kirkton will also be a problem with localised demountables in Aberfoyle.

Defences would need to be mobilised in advance of a flood event. It is possible for emergency services to drive through relatively shallow depths of flood waters. The presence of local temporary or demountable defences would restrict access for emergency services and arrangements would need to be made to maintain this access somehow. This could feasibly involve some form of ramping over the defences or configuring the defences in such a way as allow emergency traffic around the defences on Trossachs Road, although this will be difficult. An alternative would be extension of the demountable defences onto Lochard Road 300 m upstream.

There are likely to be water level impacts upstream along Lochard Road as a result of defence placement. Such impact would be unacceptable without mitigation.

A partial solution, as discussed here, may be difficult for the community to accept as it leaves a number of properties without protection and would likely raise questions about the fairness of such an option. Individual property protection may need to be provided to account for this.

There are likely to be significant costs involved with demountable defences:

o Installation of defence system

o Staff training

o Defence maintenance / inspection

o Storage of temporary parts

o Mobilisation of defences

o Impact mitigation

o Improvements to flood warning systems (if required)

Demountable / temporary defence systems are typically less reliable as there are more variables which could affect system performance, not least the mobilisation. Demountable defences are also more susceptible to vandalism.

The locations discussed above should allow ample space for mobilisation / installation of defences, although plans would need to be made to remove any vehicles from the car park before a flood event.

Other issues which would need to be resolved include residual surface water flooding and any drainage network connections to the river.

6.4.3. Individual Property Protection

Where community flood schemes are not feasible, property specific proofing and

resilience measures may be the only practical flood mitigation option. The following are

some commonly used measures:

Periphery walls / bunds

Tanking

External water resistant skirting

Periscope air vents

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)


© Mouchel 2013 57

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.

Funding mechanisms would require further investigation.


© Mouchel 2013 58

7 Cost-Benefit Analysis

The economic performance of a flood prevention scheme is determined through its

benefit/cost 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 mitigation works, it is necessary to assess the economic viability of these options.

For any option, benefit/cost appraisals generally consider the following:

„Do nothing‟

This is essentially the „walk away and do nothing‟ option. Although this option can be

considered in a cost-benefit analysis it is not likely to be an acceptable or desirable

option for the Council due to the Council‟s statutory duty to maintain watercourse as

per 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 current statutory obligation). The „do minimum‟ option is used as a

baseline for assessing the potential benefit of a scheme.

„Do something‟

This is the provision and maintenance of a flood risk management scheme. This

includes both structural engineering measures to reduce flood risk and non-structural

alternatives (natural flood management, flood warning, emergency response, land use

planning etc.) (Penning-Rowsell et al., 2010).

For the purposes of the cost-benefit analysis the „do minimum‟ option is considered to

be the baseline for economic assessment. It is generally the net cost (i.e. the

additional spend to „do something‟) which needs to be considered.

7.1 Economic Appraisal Policy and Guidance

Current appraisal guidance supports the Defra/ODPM/HM Treasury/DfT policy on

“Making Space for Water” (Defra, 2005), which encourages a comprehensive

approach to sustainable flood risk management. In Scotland, the relevant policy

document is “Delivering Sustainable Flood Risk Management” (Scottish Government,

2011) which 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 latest guidance in Scotland is Chapter 5 of “Flood Protection Schemes: Guidance

for Local Authorities – Project Appraisal” (Scottish Government, 2012) which is an

update of the 2005 guidance (Scottish Government, 2005). 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. The equivalent

guidance in England and Wales, Flood and Coastal Erosion Risk Management

Appraisal Guidance (FCERM-AG) was produced by the Environment Agency (2010). A

spreadsheet template for preparing the cost-benefit analysis was included, together

with supplementary guidance.

In order to evaluate the net benefits of implementing a scheme, the damage costs

avoided with the preferred schemes in place are compared against those of the „do


© Mouchel 2013 59

nothing and/or do-minimum‟ options. The damages for flood events of a range of

probabilities are calculated 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). This enables the assessment of the

damages to residential properties to be carried out based on property type and age,

social class of residents and depth and duration of inundation.

Damages to non-residential properties are assessed based on property type (i.e. retail,

office, public building etc.), property size and depth and duration of inundation. Clean-

up and emergency services costs (i.e. police, fire, ambulance, Council, military, etc.)

are also estimated from recommendations in the MCM.

The Scottish Government recommends using a level of detail proportional to the scale

of the project (Scottish Government, 2012). For simplicity and to be conservative, only

direct damages have been included in the calculation of flood damage costs for the

study area. Indirect and intangible losses such as consumer/supplier losses, traffic

disruption and effects on human health have not been taken into account. For

Aberfoyle, it is accepted that, indirect and intangible losses could form a significant

part of the decision making process. The potential impacts on the environment are

assessed qualitatively in Section 8.

7.2 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

reflects the physical life of the longest lived asset of the scheme. With the various flood

mitigation options of the scheme involving earthworks, concrete and masonry

structures, a 100 year timeframe was considered to be appropriate with regular

maintenance (i.e. no capital replacement of the flood defence assets is assumed

necessary during that period). 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.

7.3 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.

7.4 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 property losses, non-residential property losses, clean-up

and emergency services costs.


© Mouchel 2013 60

7.4.1. Residential Property

To calculate the losses to residential property the type and age of each affected

property and the social class of the occupants must be known, also, the depth of flood

water in relation to ground floor level and the duration of the flooding must be

estimated.

The property type and doorstep elevation of each affected property was established

from survey work. The depth of flood water and the duration of the flooding were

derived using the hydraulic model. The social classes of the residents were

determined from the April 2001 census data (results for the 2011 census are planned

for the second half of 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 21. 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).

Region No. people Social Class (see below for description)

AB C1 C2 D33

E

Stirling Council 67,368 25.0% 27.3% 13.0% 15.7% 19.0%

Aberfoyle 458 17.7% 34.5% 15.5% 18.6% 13.7%

Table 21 - Social class categories for Stirling Council area

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

7.4.2. Non-Residential Property

The MCH provides flood damage data for non-residential property in terms of area of

premises inundated, depth and duration of inundation and type of business. The depth

of the flood water and the duration of flooding were 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 OS

MasterMap data. Where a single non-residential property had more than one floor

level, the depths, areas and damages were apportioned appropriately.

7.4.3. Emergency Costs and Other

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 costs34 are

33 For cost-benefit purposes, social classes D and E are considered to be the same.

34 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.

http://www.scrol.gov.uk/


© Mouchel 2013 61

calculated as 10.7% of the economic property damage for floods of all annual

probabilities and for all prevention schemes.

Clean-up costs for residential property are included in the depth-damage tables

provided in the MCH. However, for non-residential property, there is no guidance in the

MCH (or in the MCM) regarding the inclusion of clean-up costs, and these have

therefore not been included.

7.4.4. Damage Capping

Economic appraisal guidance states that the total present value of long term economic

flooding losses should not exceed the current capital value of the property. Where

these damage values exceed estimated market value, a cap has been applied.

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:

o previous sold prices and sale of nearby properties,

o building-specific data such as site and location,

o data from estate agents and other website users, and

o data from other organisations (e.g. Environment Agency).

The assessment is more objective and consistent than simply making

judgements based on property sales in the area. There are cases where the

valuation is inaccurate for various reasons. Sensitivity testing is carried out to

determine the impact this could have on the overall outcome of the

assessment.

For non-residential property, 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.

7.4.5. Residual Damage

It is difficult, if not impossible, to design a scheme to protect against flooding from all

possible flood events. Flood defence schemes are typically designed to protect against

events with exceedance probabilities ranging from 2% (1 in 50 years) to 0.5% (1 in 200

years). In the event of a more extreme event occurring, the scheme is likely to 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

as these are damages that will still be incurred, even with the scheme in place.

It is also important to consider whether any options explored could increase the

severity of these exceedance events, in which case these additional damages should

be taken into account in the economic appraisal. For Aberfoyle, it is assumed that

none of the options explored would increase the severity (depth) of flooding for events

which exceed the design threshold (1 in 200 years plus climate change).

Although there is inherent uncertainty regarding climate change throughout the

scheme, it is roughly assumed that some „additional‟ protection would be available due

to the climate change inclusion hence the residual damages notionally being taken

http://www.findaproperty.com/

http://www.rightmove.co.uk/


© Mouchel 2013 62

from 500 year and above rather than 200 year. Freeboard is included to account for

uncertainty in design level predictions. It cannot be relied upon to provide additional

protection.

7.4.6. Depth / Damage Curve

The extent and depth of flooding associated with floods of a range of return periods

was established from extensive hydrological and 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 a correlation between depth and damage, a range of annual

exceedance probabilities are considered together with the calculation of damage

associated with each event. Once the annual average damage value is derived

bringing all future damage costs to a common timeframe is undertaken. In this study,

the annual exceedance probabilities used to derive the depth / damage relationship

were 50% (2 years), 20% (5 years), 10% (10 years), 5% (20 years), 2% (50 years), 1%

(100 years), 0.5% (200 years), 0.2% (500 years) and 0.1% (1000 years). Using a large

number of depth / damage relationships allows for a more accurate / realistic depth /

damage curve to be derived.

Current guidance recommends the use of judgment when considering potential climate

change impacts. SEPA currently recommends that an uplift of 20% to fluvial peak flow

is applied to account for potential impact of climate change on river flow. This figure is

currently valid until around year 2060. The Environment Agency has recently produced

supplementary guidance to FCERM-AG on accounting for possible climate change

scenarios. This guidance is not available for Scotland. To account for the fact that

climate change will occur gradually, the climate change factor applied to fluvial peak

flows was increased over the 100 year assessment period as shown in Figure 25. This

shape results in a 20% uplift around 2060 (the year to which SEPA guidance is

applicable) and is the same as that recommended for the Tweed river basin in the

Environment Agency (2011) guidance. In agreement with SEPA, this is considered to

be a conservative and robust assumption.

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 25 - Change in river peak flow to account for climate change in the future


© Mouchel 2013 63

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). It is conservatively

assumed that all affected properties would be flooded for less than twelve hours.

7.5 Summary of Cost-Benefit Methodology

In summary, the following assumptions have been made in the cost-benefit analysis:

damages based on all latest flood mapping and modelling results,

climate change allowance included 0 - 15% uplift up to 2025, 15% uplift from 2025 to 2040, 20% uplift from 2040 to 2070 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 are ignored,

flooding to land/gardens and agricultural land is ignored,

100 year scheme lifespan with regular maintenance,

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 available option,

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, and

damages capped at estimated property market values.

A set of Excel worksheets developed on behalf of the Environment Agency (2010)

were used to carry out the cost-benefit analysis. The benefit / cost worksheets allow to

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 breach in flood

defence, or repetitive damages, such as that due to overtopping. An evaluation of

scheme viability was then made based on the benefit/cost relationships of the various

options.

7.6 Flood Damages in Aberfoyle

7.6.1. Existing Onset of Flooding

The probability of the onset of flooding to properties is a good indicator of the scale of

flood damages likely to accrue. If a property floods, on average, every two years (i.e.

50% AEP), then damages will quickly accrue over a 100 year scheme life. Conversely,

if a property floods, on average, only once every 100 years (i.e. 1% AEP), then

damages over 100 years will be significantly smaller. A rough indication of the onset of

flooding in Aberfoyle (in relation to property floor level) is shown in Figure 26 and

Figure 27 for the baseline case (i.e.: „do nothing‟ case).


© Mouchel 2013 64

100-200

2-55-10

2-5

5-102-5

500-1000

100-200

20-50

500-1000

20-50

20-50

200-500

200-500

20-5050-100

100-200

5-10

2-5

50-200

5-10

20-50

100-200

5-10

100-200

200-500

50-100

10-20

20-50

50-100

5-10

100-200

100-200

© Crown Copyright. All rights reserved. Ordnance Survey Licence number 100020780 © Getmapping Partnership 2012

Figure 26 - Approximate return period of property flood risk (Main Street)

500-1000

100-200

500-1000

100-200

10-20

20-50

100-200

5-10

2-5

10-20

5-10

10-20

20-50

© Crown Copyright. All rights reserved. Ordnance Survey Licence number 100020780 © Getmapping Partnership 2012

Figure 27 - Approximate return period of property flood risk (Lochard Road)

7.6.2. Flood Damages

The estimated Present Value (PV) of the flood damages have been derived using the

baseline hydraulic model presented earlier in this report as outlined in Table 22. These

damages include emergency services and clean-up costs. The locations where flood

damage occurs are shown in Figure 28.


© Mouchel 2013 65

Scenario Total PVd PVd 0.5% AEP PVd 1% AEP PVd 2% AEP

Without climate change allowance £4,561,000 £3,763,000 £3,322,000 £2,761,000

With climate change allowance £7,655,000 £6,203,000 £5,326,000 £4,230,000

Table 22 - Present value damage (PVd) for a range of annual exceedance probabilities35

The results show that a total of £4.6m (£7.7m with climate change allowance) worth of

damage is estimated to occur as a result of flooding over the next 100 years in the

whole of the Aberfoyle study area. Cost capping was applied where appropriate.


Figure 28 – Locations where flood damages occur (larger symbols indicate higher damage)

7.6.3. Damage Capping

Damage capping was found to be necessary for a number of residential and non-

residential properties in Aberfoyle village centre and along Lochard Road. The

locations which required capping are as follows:

Ground floor properties at Baillie Nicol Jarvie Court, Lochard Road,

Olde Post Cottage, Lochard Road,

Post office, Main Street,

Guyana, Main Street,

Scottish Wool Centre, Main Street,

Bank of Scotland, Main Street,

The Wee But „n‟ Ben Bistro, Main Street,

Clachan Hotel, Main Street,

Forth Inn Hotel and shops on Main Street,

Scottish Co-op, Main Street, and

Chill Out, Main Street.

35 Damages include the total damage likely to accrue over a 100 year scheme life (Total PVd)

and the damages associated with events not exceeding a 0.5%, 1% and 2% AEP.


© Mouchel 2013 66

Capped damages are summarised in Table 23 and Table 24. It can be seen that the

effect of capping means that there is little difference between damages up to the

various return period thresholds.

Scheme area No climate change allowance

35

Total PVd PVd 0.5% AEP PVd 1% AEP PVd 2% AEP

All £3,323,000 £2,799,000 £2,554,000 £2,253,000

Main Street £1,910,000 £1,589,000 £1,450,000 £1,303,000

Lochard Road £1,222,000 £1,120,000 £1,048,000 £921,000

Railway Cottages £91,000 £68,000 £50,000 £28,000

A82 £81,000 £12,000 £0 £0

Foil Cottage £18,000 £11,000 £6,000 £1,000

Upper Loch Ard £1,000 £0 £0 £0

Milton £2,000 £0 £0 £0

Table 23 – Capped present value damage (PVd) for a range of annual exceedance probabilities without climate change allowance

Scheme area With climate change allowance


All £4,340,000 £3,475,000 £3,076,000 £2,693,000

Main Street £2,384,000 £1,838,000 £1,593,000 £1,401,000

Lochard Road £1,611,000 £1,476,000 £1,391,000 £1,252,000

Railway Cottages £158,000 £116,000 £81,000 £37,000

A82 £149,000 £24,000 £0 £0

Foil Cottage £33,000 £20,000 £11,000 £2,000

Upper Loch Ard £1,000 £0 £0 £0

Milton £3,000 £0 £0 £0

Table 24 – Capped present value damage (PVd) for a range of annual exceedance probabilities including climate change allowance

A capped total of £3.3m (£4.3m with climate change allowance) worth of damage is

estimated to occur as a result of flooding over the next 100 years in the whole of

Aberfoyle study area. The capped damage associated with events not exceeding 0.5%

AEP is £2.8m (£3.5m with climate change allowance); this is the figure to compare

with the residual damages and costs of a scheme designed to protect against flood

events not exceeding 0.5% AEP to derive an indicative cost-benefit ratio.

The relative contributions of each of the areas in Aberfoyle to the total PVd (with

climate change allowance) are outlined in Table 25. It is apparent that, from a flood

damage perspective, most benefits would be derived from concentrating efforts on

Lochard Road and Main Street.


© Mouchel 2013 67

Scheme area With climate change allowance


All 100% 100% 100% 100%

Main Street 57% 57% 57% 58%

Lochard Road 37% 40% 41% 41%

Railway Cottages 3% 2% 2% 1%

A82 2% 0% 0% 0%

Foil Cottage 1% 0% 0% 0%

Upper Loch Ard 0% 0% 0% 0%

Milton 0% 0% 0% 0%

Table 25 - Relative contribution of areas of Aberfoyle to total PVd (with climate change allowance)

7.6.4. Sensitivity Checks

The damage values calculated above are based on a large number of variables, each

with its own level of quality. This section investigates the impact of error in the

variables on the total PVd values and hence the overall outcome of the assessment.

Note that all damages noted in the following sub-Sections are capped damages, and

do not include climate change allowance. A description of the data quality scores

(DQS) is shown in Table 26 and the data quality score for each component is

described in the flowing pages.

DQS Description Explanation

1 „Best of Breed‟ No better available; unlikely to be improved on in the near future

2 Data with known deficiencies To be replaced as soon as third parties re-issue

3 Gross assumptions Not invented but deducted by the project team from experience

or related literature / data sources

4 Heroic assumptions No data sources available or yet found; data based on educated

guesses

Table 26 - Data Quality Scores (DQS) from MCH (Penning-Rowsell et al., 2010)

Climate Change

As a result of predicted climate change, the standard of protection of a scheme will

effectively reduce 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 45 years (assuming 30% increase in flow due to

climate change). Conversely, a scheme designed to have a 200 year return period

standard of protection at the end of its 100 year lifespan would need an initial standard

of protection of the order of 800 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 return

period, which will reduce gradually over time as a result of climate change. It is

considered that calculating the additional benefits (and associated residual damages)

would not materially influence the outcome of this assessment. An increase in benefits

of the order of 10-15% would be anticipated if the initial additional benefits at the start

of the scheme life are accounted for (assuming a 200 year return period standard of

protection at the end of the 100 year scheme life).


© Mouchel 2013 68

Land Use (DQS 3)

The land use categories for both residential and non-residential properties were

derived using site visits and a desktop study, and have been given an overall DQS

score of 3.

The residential land use consists of property age, type and inhabitant social class.

Property type can be identified easily and social class is aggregated for the entire

village. These would therefore be scored as DQS 1 and 2 respectively (an updated

census is required to improve the social class estimate). The property age, on the

other hand, could be subject to significant inaccuracies due to difficulties assessing

property age based purely on a visual examination. This is particularly the case for

properties which have been recently renovated or properties recently constructed in a

style sensitive to the local area. This would therefore have a DQS of 3 or 4.

The non-residential property category can be easily identified from field and desktop

surveys. Some of the categories in the FHRC dataset are inherently generalised. No

distinction can therefore be made between, for example, a high street shop selling

jewellery and a high street shop selling household goods. This could only be improved

by carrying out site specific surveys, and is hence given a DQS of 2.

For residential property, the property type which accrues most damage during flooding

is, by some distance, “pre-1919 detached”, followed by “post-1985 bungalow”. There

are currently 3 properties in Aberfoyle classified as “pre-1919 detached”. If all

properties had this class (a significant overestimate) the total PVd in Aberfoyle would

amount to £4.1m; an increase of 23%. If all properties (except the aforementioned

three) were classified as “post-1985 bungalow” (a more reasonable upper bound) the

total PVd in Aberfoyle would amount to £3.7m; an increase of 11%.

The total damage value for non-residential property is likely to be more reliable than for

residential property; most of the error would come from generalisations made in the

FHRC datasets. 84% of the depth/damage data curves used were given a quality

score of 1 or 2, and should therefore be reasonably reliable. A site specific survey

could be carried out during any subsequent studies to improve these estimates. If non-

residential property damage has an error of, for example, 20% then this could affect

the total damage by around 10%.

Floor Area (DQS 2)

Non-residential property floor area was calculated using OS MasterMap maps and site

visits. There could be slight inaccuracies due to out of date maps but none were noted

during site visits. It is likely that any inaccuracies would „average out‟ through the

dataset. A nominal 5% error in floor area would change the total PVd by 0.8%.

Floor Level of Property (DQS 1)

This is, of course, a key variable in this assessment. Floor levels were determined

using detailed topographic survey (total stations and GPS methods). The stated

accuracy of these instruments is of the order of a few centimetres. In reality, the

accuracy could be less than this, particularly in areas where GPS signals could get

reflected before reaching the receiver, a phenomenon known as a “multipath”. This

has been known to cause errors of up to 2.5 m, but is typically much less than that.

The multipath phenomenon would cause any levels taken to appear lower than they

actually are (never higher).


© Mouchel 2013 69

Based on experience, a reasonable upper bound for error due to multipaths would be

of the order of 300 mm. If all floor levels were raised by this amount the total PVd in

Aberfoyle would amount to £1.5m; a decrease of 56%.

Hydrological / Hydraulic Data (DQS 1)

Hydraulic data (water levels) were derived from a well verified hydraulic model.

Changes in hydrology could affect the predicted water levels, but the analysis is

deemed to be robust and conservative. A 100 mm increase in water levels for all AEPs

would cause the total PVd in Aberfoyle to increase by 19% to £4.0m.

Other Sources of Error

Local authority emergency and recovery services and SEPA costs have been taken as

10.7% of economic property losses, which is based on an analysis of the autumn 2000

floods. A more recent analysis of the 2007 floods found these costs to total 5.6% of

economic property losses. It is thought that this figure is lower due to economies of

scale; the 2007 floods occurred in more densely urbanised areas. Hence, 5.6% is

recommended for use in urban areas. A sensitivity check was carried out to determine

the effect of using 5.6% to account for emergency services and recovery and SEPA

costs. This caused the total PVd in Aberfoyle to decrease by 2.7% to £3.3m.

Individual Properties

There are six properties (Table 27) which individually contribute more than 5% of the

total PVd for Aberfoyle (without climate change allowance); all others contribute 2.6%

or less. This sub-Section describes the influence of individual properties on the total

PVd.

Property PVd % of total

Forth Inn Hotel, Main Street £ 465,000 14.0%

Craigsheen Lodge, Lochard Road £ 248,000 7.5%

Guyana, Main Street £ 265,000 8.0%

Scottish Co-op, Main Street £ 255,000 7.7%

Scottish Wool Centre, Main Street £ 244,000 7.3%

Olde Post Cottage, Lochard Road £ 181,000 5.5%

Table 27 – Highest contributors to total PVd for Aberfoyle (without climate change allowance)

The high percentage contribution of the above properties to the total PVd is a result of

the relatively high frequency of flooding at these properties; they have an annual

probability between 20% and 10% of getting flooded. Any significant changes in any of

the above could have an effect on the overall PVd.

A site specific survey could be carried out during any further studies to improve the

accuracy of these figures, however it is deemed unlikely that this would change the

overall outcome of the economic appraisal, and therefore it would be difficult to justify

the additional time and expenditure at this stage.

Forth Inn Hotel, Main Street

Using a low susceptibility depth/damage curve to derive damages at the hotel results

in a decrease of £191k (5.7%) in the total PVd. An error of 100 mm in the floor level (or

water level) would affect the total PVd by around £5k, or 0.15%. Damages for the hotel

are capped at property value; hence there will be no increase in PVd due to


© Mouchel 2013 70

uncertainties in the assessment. A site survey could improve confidence in the

accuracy of damage at the hotel.

Craigsheen Lodge and Olde Post Cottage, Lochard Road

Flood damage at the Olde Post Cottage is capped for the climate change allowance

scenarios; damage at Craigsheen Lodge did not require capping. Flood damage for

residential property is most susceptible to flood depth (floor and flood levels). As

previously discussed, confidence in these variables is relatively good. There is less

confidence in determining property age but, as shown in Figure 29, there is a relatively

small difference in depth/damage curves for various epochs (detached property), with

the exception of „pre-1919 properties‟, for which damages are significantly higher.

0

20000

40000

60000

80000

100000

120000

140000

-0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

Flood depth (m)

Flo

od

dam

ag

e (

£)

Pre 1919 1919-1944 1945-1964 1965-1974 1975-1985 1975-1985 (utility) post 1985

Figure 29 – Depth/damage curves for detached residential properties of various ages

The Olde Post Cottage was estimated to have been built prior to 1919. If it was in fact

built between 1919 and 1944, the total PVd for Aberfoyle would reduce by £82k

(2.43%). For reasons mentioned above, the damage at Craigsheen Lodge is not likely

to be significantly affected by changes in land use category.

Guyana, Main Street

The land use category for Guyana is Garden Centre, with a DQS of 1. The high

damage accrued despite the relatively low damages associated with garden centres

(around 20% of the damage for a shop) is a result of the high frequency of flooding.

Any changes in land use at this location over the next 100 years (such as potential

construction of retail outlet for example) could potentially have a significant effect on

the overall damages. Damages are capped to property value, and hence any error in

the assessment could not increase the damages accrued.

Scottish Co-op, Main Street

The cap applied to the damages at the Co-op is around a third of the total PVd

calculated, hence any error in the calculation of damage will not affect the total PVd,

and the capping value is the key variable. Only a specific property survey could

improve confidence in this figure.

Scottish Wool Centre, Main Street

Damage at the Scottish Wool Centre is generally uncapped (damages only just exceed

the capping value for the 30% climate change scenario). Any error in the variables


© Mouchel 2013 71

could therefore affect the PVd. The Wool Centre has one of the largest floor areas of

all non-residential properties in Aberfoyle which means damages are significant

despite a relatively low annual probability of flooding (~2%). The land use category

used is “high street shop”. If, on the other hand, “café / food court” was used, the total

PVd would increase by £80k or 2.4%.

7.7 Option Costing



Option 1 described in Section 6 has been provisionally costed. Table 28 presents the

breakdown of the total estimated costs of the main elements of Option 1. The flood

walls have been provisionally assumed to be in reinforced concrete with stone facing

and coping to reduce visual impact. Although for simplicity flood-walling has been

costed for protecting roads and properties at Loch Ard and Milton, it is more likely the

case that any built solution would comprise combinations of raised roads and walling

to these areas. There would also be scope for using embankments in some locations

instead of hard flood-walling.

The design life of the flood prevention scheme components have been assumed to be

100 years, therefore no cost of capital replacement has been included in the final PV

cost for any of the components. In addition to the capital construction costs the

following miscellaneous construction costs have been included:

preliminary works,

service diversions,

traffic management, and

accommodation.

The project fees for consultancy and contracting services have also been included in

the overall option costs. These include: project management, site data collection,

detailed design, ground investigations and data collection, topographic and

environmental surveys, contract preparation, tender, CDM, planning application,

environmental report, land owner identification, consultation, site supervision and

structural survey. Costs for client staff and an allowance for compliance with the

Controlled Activities Regulations (CAR) have also been included. The costs included

are:

design and consultation,

geotechnical site investigations, and

site supervision.


© Mouchel 2013 72

Description Quantity Unit Rate36

Cost37

Flood Defence Walls and Flood Gates at 8 properties

1.5m high reinforced concrete flood defence wall (Loch Ard and Milton)

2,500 m £1,100 £2,750,000

3m high reinforced concrete wall (Aberfoyle town centre)

2,100 m £2,200 £4,620,000

2.8m high reinforced concrete wall (downstream of the Aberfoyle town centre)

900 m £2,100 £1,890,000

Flood gates for 8 individual properties (assumed 4 numbers of 0.9m wide gate for each property)

32 £400 £12,800

Sub-total 1: £9,272,800

Project Fee for Consultancy

Design and Consultation (5% of Sub-total 1) £463,600

Geotechnical Site Investigations (lump sum) £80,000

Site Supervision (3% of Sub-total 1) £278,200

Sub-total 2: £821,800

Miscellaneous

Preliminary Works (6% of Sub-total 1) £556,400

Service Diversions (10 % of Sub-total 1) £927,300

Traffic Management (3% of Sub-total 1) £278,200

Accommodation (3% of Sub-total 1) £278,200

Sub-total 3: £2,040,100

Total £12,134,700

Table 28 – Cost breakdown for Option 1

The on-going costs of routine maintenance to clear watercourses of any debris and

blockages, maintain banks, trim vegetation and maintain new flood defence walls to

their current standard of protection have also been included in the overall option costs.

The operational and maintenance costs have been apportioned on an annual basis

according to estimated requirements. The maintenance costs (without accounting for

the present value) for the 100 year scheme lifespan with Option 1 are assumed as:

1 to 25 years = £ 12,080 per annum




The final total PV costs are presented in Table 31.



Option 2 described in Section 6 has been provisionally costed. Table 29 presents the

breakdown of the total estimated costs of Option 2. The flood walls have been

provisionally assumed to be in reinforced concrete with stone facing and coping to

36 The rate has been estimated in August 2012

37 All costs have been rounded up to nearest £100


© Mouchel 2013 73

reduce visual impact. Details of the flow control structure on the Duchray Water

(upstream of its confluence with the Avondhu) are presented in Section 6.2.


Cost37

Flood Defence Walls and Flood Gates at 8 properties


2,500 m £1,100 £2,750,000

2.8m high reinforced concrete wall (Aberfoyle village centre)

2,100 m £2,100 £4,410,000

2.6m high reinforced concrete wall (downstream of Aberfoyle centre)

900 m £2,000 £1,800,000

Flood gates for 8 individual house protection (assumed 4 numbers of 0.9m wide gate for each property)

32 £400 £12,800

5m high reinforced concrete flood defence wall (adjacent to flow control structure in Duchray)

200 m £3,000 £600,000

Flow control structure (lump sum) 1 £200,000 £200,000

Sub-total 1: £9,772,800


Design and Consultation (6% of Sub-total 1) £586,400

Geotechnical Site Investigation (lump sum) £150,000

Site Supervision (4% of Sub-total 1) £390,900

Sub-total 2: £1,127,300

Miscellaneous

Preliminary Works (7% of Sub-total 1) £684,100

Service Diversions (10% of Sub-total 1) £977,300


Accommodation (4% of Sub-total 1) £390,900

Sub-total 3: £2,345,500

Total £13,245,600


In addition, the on-going costs of routine maintenance to clear watercourses of any

debris and blockages, maintain banks, trim vegetation and maintain new flood defence

walls to their current standard of protection have been included in the overall option

costs. The operational and maintenance costs have been apportioned on an annual

basis according to estimated requirements. The maintenance costs (without

accounting for the present value) for the 100 year scheme lifespan with Option 2 are:







Milton and Aberfoyle, and natural flood management measures in Duchray catchment

Option 3 (with optimised natural flood management scenario for the Duchray

catchment with a decrease by 7% of the baseline SPR value and an increase by 7% of

the baseline time to peak (Tp)) described in Section 6 has been provisionally costed.

Table 30 presents the breakdown of the total estimated costs of Option 3. The flood

walls have been provisionally assumed to be in reinforced concrete with stone facing


© Mouchel 2013 74

and coping to reduce visual impact. The natural flood management measures in the

Duchray catchment aiming at reducing runoff through changes in land management

practices are presented in Appendix J. An estimation of the cost has been included in

the breakdown of the total estimated costs of Option 3.


Cost37

Flood Defence Walls and Flood Gates at each of the 8 properties


2,500 m £1,100 £2,750,000

2.8m high reinforced concrete wall (Aberfoyle village centre)

2,100 m £2,100 £4,410,000

2.6m high reinforced concrete wall (downstream of Aberfoyle centre)

900 m £2,000 £1,800,000

Flood gates for 8 individual house protection (assumed 4 numbers of 0.9m wide gate for each property)

32 £400 £12,800

Sub-total 1: £8,972,800

Natural Flood Management in the Duchray catchment

Estimation of works pertaining to the natural flood management measures in the Duchray Water catchment

£450,000

Sub-total 2 £450,000


Design and Consultation (5% of Sub-totals 1+2) £471,100

Geotechnical Site Investigation (lump sum) £80,000

Site Supervision (3% of Sub-totals 1+2) £282,700

Sub-total 3: £825,800

Miscellaneous

Preliminary Works (6% of Sub-totals 1+2) £565,400

Service Diversions (10 % of Sub-total 1) £942,300


Accommodation (3% of Sub-totals 1+2) £282,700

Sub-total 4: £2,073,100

Total £12,221,700


The cost of implementing effective natural flood management measures in the

Duchray catchment would be highly unknown however, a rough nominal cost of

£450,000 has been assumed for costing purposes (nominally taken as 5% of the

capital cost). Similarly to the other two options, the on-going costs of routine

maintenance for Option 3 have also been included in the overall costs. The operational

and maintenance costs have been apportioned on an annual basis according to

estimated requirements. The maintenance costs (without accounting for the present

value) for the 100 year scheme lifespan with Option 3 are:







© Mouchel 2013 75

7.7.4. Present Value (PV) Scheme Costs

The estimated scheme costs of the three options have been brought to a present value

(August 2012) as outlined in Table 31. The on-going maintenance costs of the „do

minimum‟ option include routine activities such as watercourses and structures

clearing of any debris and blockages, river banks maintenance, and vegetation cutting

have also been included in Table 31. Without accounting for the present value, the on-

going maintenance costs for the „do minimum‟ option for the 100 year lifespan have

been taken as:





The total maintenance cost for the „do minimum‟ option when discounting to present

value38 is £462,500 without optimism bias.

The capital costs include the miscellaneous costs and project fees presented in the

scheme costings for each of the three options. Operation and maintenance costs are

for 100 year lifespan of the scheme.

Scenario Description Capital Costs

(PV)

Future

Maintenance

Cost (PV)

Total Cost

(PV)

Total Cost (PV)

with optimism

bias

Do nothing Walkway and no

maintenance £0 £0 £0 £0

Do minimum Maintenance of existing situation

over next 100 years £0 £462,500 £462,500 £740,000

Do something - Option 1

Construction of Option 1 and

maintenance cost £12,134,700 £725,070 £12,859,770 £20,575,600







Table 31 – PV costs for the three options

7.8 Cost-Benefit Appraisal

Using current Defra benefit / cost worksheets, the following PV costs (including 60%

optimism bias) and PV damages (including emergency services, clean-up costs and

capping) were derived:

PV Damage = £3,323,000 (without climate change from Table 23)

PV Damage = £4,340,000 (with climate change from Table 24)

PV Residual Damage = £635,766 (without climate change)

PV Residual Damage = £1,156573 (with climate change)

PV Cost (‘do nothing’) = £ 0

PV Cost (‘do minimum’) = £ 740,000

38The present value cost for the 100 year lifespan is derived by summing up the product of annual cost

and respective discount factor. The discount factor decreases with the year.


© Mouchel 2013 76

PV Cost (‘do something’ - Option 1) = £20,575,600



Since the Council are not legally or politically able to adopt the „do nothing‟ option, the

possible option(s) („do something‟) need to be expressed in terms of net costs. At the

very least, a „do minimum‟ option would need to be continued to be implemented.

Table 32 and Table 33 present the benefit cost ratio derived39 based on net costs.

Benefit Cost Ratio

Option 1 vs Do Minimum Option 2 vs Do Minimum Option 3 vs Do Minimum

0.14 0.12 0.13

Table 32 - Benefit cost ratio (without climate change)

Benefit Cost Ratio

Option 1 vs Do Minimum Option 2 vs Do Minimum Option 3 vs Do Minimum

0.16 0.15 0.16

Table 33 - Benefit cost ratio (with climate change)

Currently, all three options have a benefit/cost ratio of less than unity and represent

„non-viable‟ economic solutions.

7.9 Further Benefit / Cost Discussion

The very low ratios mean that any uncertainty over climate change, benefits, scheme

costs, etc., will have little effect on the overall „non-viable‟ economic outcome. The

reason the ratio is so low, despite the frequent nature of flooding, is partly due to the

fact that damages are spread over such a long reach of the river. As a result, very long

lengths of defences are needed to defend a relatively small number of properties.

The various scheme options have only been outline costed and only include for main

scheme elements. Many ancillary scheme details such as back drainage systems,

variations in defence construction, tying in details and many other design details have

been omitted. Also, no assessment or costing of the potential requirement for sheet

pile groundwater cut-off has been made. The key findings of the benefit / cost analyses

would not be considered to change upon inclusion of such details.

For simplicity, the options have not been spilt up into smaller discrete scheme

elements. However, upon inspection of individual areas such as Aberfoyle, Loch Ard,

Milton, etc., there would still be no viable scheme for any discrete locations. If

assessing for a scheme for Aberfoyle on its own for instance (where damages are

greatest), the damages would be around £3.4M and costs would be around £10.5M.

This would still represent a non-viable scheme (B/C ratio of 0.33).

39 B/C ratio = (PV Damage – Residual Damage)/{PV Cost („do something‟) – PV Cost („do minimum‟)}


© Mouchel 2013 77

Designing to a lower standard of protection can sometimes be a possibility; in

Aberfoyle, damages occur relatively frequently, so it could be possible to prevent a

significant proportion of damages with a smaller scheme. As an example, a scheme

designed to 20 year (plus climate change) standard of protection for Main Street /

Lochard Road would cost approximately £6.8M and prevent around £1.7M of

damages, yielding a ratio of 0.25. Although this is a higher ratio than has been

achieved with any of the other options (and could potentially be refined further), it is

still some way off being economically viable.

A riverside floodwall protecting for the 200 year flood event (plus climate change)

would need to be around 3m high at Aberfoyle. 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, may be locally unacceptable as it

could potentially blight this valuable amenity and also result in an associated loss of

visitor numbers and business for the town.

Intangible benefits of the various options, in particular access to remote villages and

the school, have not been quantified as part of this assessment. As a result, the

Council may wish to consider options with a benefit-cost ratio of less than one in

cognisance of these benefits. The cost of a flood wall alongside the road along Loch

Ard has been costed at around £7.5M (including ancillaries and optimism bias). This

does not include raising of the road which would likely be required. However, the loch

levels do not vary significantly with flood return period, and virtually no damage to

property is expected around Loch Ard (although there may be damages to the road

and other infrastructure). Such an investment would therefore have to be based on

intangible benefits. For the primary school on Loch Ard, property damages are higher;

however still not enough to be able to justify a scheme based on those damages

alone.


© Mouchel 2013 78

8 Environmental Feasibility and Constraints

8.1 Introduction

An environmental appraisal was carried out to identify the environmental

characteristics of the study area and assess any potential environmental constraints or

opportunities in relation to any of the explored options. The environmental appraisal of

the presented options has considered the following environmental constraints:

Ecological designations and wildlife value of the waterbodies and surrounding areas, and potential issues 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.

Baseline information was collated through a desk study which involved the collection of

data from a range of sources including Ordnance Survey maps, local plans and

websites containing up to date environmental information. A brief summary of the

environmental appraisal is summarised in the next two sub-sections. The full

Environmental Appraisal Report is included in Appendix G.

8.2 Summary of the Environmental Baseline

Loch Ard and Aberfoyle are adjacent to four Sites of Special Scientific Interest

(SSSIs) and one Special Area of Conservation (SAC). Ancient woodland is

located along the banks of Loch Ard and the northern bank of the River Forth

between Milton and Aberfoyle. Land south of the Wool Centre is a designated

community wildlife area.

Loch Ard and Aberfoyle are located within the Loch Lomond and the Trossachs

National Park which is designated as a National Scenic Area (NSA).

There is one Scheduled Ancient Monument (SAM) and thirty one listed

buildings within the study area. An area at Milton is designated as a

Conservation Area.

There are a number of paths providing public access to the countryside which

will need considered in the development of any flood mitigation measures.

The main soil types within the study area are alluvial soils (typically confined to

river valleys, gleys (naturally poorly drained soils) and podzols (typically free

draining soils).

8.3 Summary of Key Constraints and Potential Environmental Opportunities

The Environmental Appraisal Report outlines the key constraints and opportunities

associated with the three flood mitigation options described in Section 6. For example,

any works proposed to be undertaken within or adjacent to a SSSI will require consent

from Scottish Natural Heritage (SNH). A Habitats Regulations Assessment may be


© Mouchel 2013 79

required for Natura 2000 sites (SPAs and SACs). However, it is considered that the

potential environmental constraints on any potential scheme could be avoided or

overcome through careful planning and design. The Environmental Appraisal Report

highlights the importance of further consultation with the appropriate statutory and

environmental bodies and makes recommendations for these should any scheme

options be pursued further. There are various opportunities to enhance / improve the

environment of the riparian corridor and improve local ecology and biodiversity within

the study area, which are also detailed in the Environmental Appraisal Report. Full

details of the above points can be found in the Environmental Appraisal Report

contained within Appendix G.


© Mouchel 2013 80

9 Geomorphological Assessment

9.1 River Reconnaissance Survey

A river reconnaissance survey was undertaken on 18th November and 8th December

2011 to identify key geomorphological features and characteristics of the

watercourses. The survey was undertaken in accordance with SEPA‟s Supporting

Guidance (WAT-SG-21)40 and the Stream Reconnaissance Handbook (1998)41.

The aims of the survey are summarised below:

Establish the contemporary morphological points of interest and general

geomorphic characteristics of the watercourses within the survey area,

Provide an evidence base to develop further geomorphological studies if

required as a result of this study, and

Provide an evidence base to support the hydraulic modelling and optioneering

activities and to assist in developing flood mitigation options.

On both survey days, river levels were high and the weather was dominated by

intermittent wintry showers. On the 18th November 2011 flooding occurred on the River

Forth in Aberfoyle and so a second survey was undertaken on the 8th December 2011.

The survey was undertaken from downstream to upstream of the study area along the

River Forth, the lower reach of the Duchray Water and Loch Ard. The survey began

along the River Forth at Finlarich House at the A81 road bridge (grid ref. 253210,

698665), through Aberfoyle and finished at Kinlochard on the northern shore of Loch

Ard (grid ref. 245450, 702320). The survey along the Duchray Water began at its

confluence with the River Forth (grid ref. 250600, 701314) and finished approximately

400m upstream of the confluence (grid ref. 250213, 701026). The map in Appendix I

shows the survey extents.

The channel of the Avondhu tributary is bedrock and is controlled through a series of

boulder cascades. The gradient of the bed is steep, velocities are high and the river

channel is narrow (approximately 3-4m). Downstream of the Milton gauging station the

gradient becomes shallower and a smaller secondary channel has formed creating an

island. The island and secondary river channel flow through a unique habitat of

woodland forest. The main channel has been engineered to flow alongside the B829

road to its confluence with the Duchray Water. At the confluence the secondary

channel of the Avondhu re-joins the main channel and the Avondhu and Duchray

Water join to form the River Forth.

The Duchray Water provides approximately 60% of the flows into the River Forth. The

Duchray Water channel geomorphology for the surveyed section was characterised by

a 6-8m in width, uniform, gravel bed river channel. The channel has a relatively steep

gradient resulting in fast flows. There was no evidence of bank or bed erosion on the

day of the survey. An established channel island divides the flow approximately 200m

upstream of the confluence with the Avondhu. Recent deposition of gravels and silts

40 SEPA, Supporting Guidance (WAT-SG-21) Environmental Standards for River Morphology (February 2011)

41 Thorne C.R, Stream Reconnaissance Handbook: Geomorphological Investigation and Analysis of River

Channels (1998)


© Mouchel 2013 81

were observed on the downstream side of the vegetated island, inside of the meander

bend and silt deposits on the riverbanks. The riverbanks are characterised by grasses

and established trees. Towards the confluence the catchment opens out to a larger

floodplain across the valley floor.

The River Forth begins as an 8-10m uniform, sinuous, stable gravel bed river channel.

However due to the proximity of the B829 road the left bank has been engineered and

therefore the channel has incised and the gradient remains steep. A small tributary

joins the main channel downstream of Milton bringing gravels from the mountain side.

The gradient of the bed becomes shallower in this location, allowing gravel bars and a

pool-riffle feature to develop.

Large meander bends sweep across the floodplain and valley floor, confined by the

B829 road on the left and the high ground on the right of the valley sides. The

floodplains remain mostly undeveloped allowing tall grasses to stabilise the valley

floor. The river channel upstream and downstream of Aberfoyle through the extensive

floodplain areas are sinuous and wander in planform. The river channel maintains a

constant width of between 6-8m in width. Redundant paleo-channels can be viewed

from aerial photographs across the floodplains. Large sections of erosion on the

outside of large meander bends can be viewed upstream and downstream of

Aberfoyle, supplying sands and silts into the river channel. This material is highly

mobile during high flow events.

The river channel through Aberfoyle has been heavily engineered and the river

channel has been straightened to allow development on the left bank. The river

channel has been engineered to accommodate a twin stone arch vehicle access

bridge through the centre of Aberfoyle; however the river channel remains stable at

this point.

The old railway embankment and local geology dictate the planform in the lower

reaches of the study area. Forested riverbanks and a supply of gravels from the

smaller tributaries have helped to stabilise the river channel. The gradient increases

after the extensive floodplain downstream of Aberfoyle, which suggests there is slow

channel incision through the local geology. The river channel has also widened to 10-

12m with stable gravels bars on the inside of meander bends.

At the downstream extents around the Cobleland caravan park, the river channel

accommodates two highway bridges and modified riverbanks, but the river channel is

stable and the topography opens out again to a wide valley floor floodplain.

A condition assessment was carried out along the watercourses in line with the

Environment Agency Condition Assessment Manual42. Maps showing the condition of

features along the watercourses are included in Appendix I.

9.2 Key Features

The river reconnaissance survey identified distinct reaches along the River Forth and

one reach of the Duchray Water within the study area. The river reaches range from

plane-bed / plane riffle river type at the downstream extent of the study to a bedrock

42 Environment Agency Condition Assessment Manual (2006)


© Mouchel 2013 82

and boulder cascade at the upstream extent. The river reaches and morphological

characteristics identified from the walkover survey have been mapped and are

included in Appendix I. Supporting photographs of key features within the study area

are also included in Appendix I and discussed below:

The natural river channel and floodplain have been modified, most likely to

accommodate the two road bridge crossings, residential property and a

caravan site. There is no evidence of erosion or significant morphological

change occurring in Reach 1 (Photograph 1).

In Reach 2, a small tributary on the right bank is providing a significant source

of cobbles and gravels to the River Forth channel, creating mobile gravel bars

and vegetated channel islands.

The River Forth has characteristics of an irregular meander planform, lacking

the symmetry of regular, classic meanders, indicating bend geometry is

influenced by outcrops of erosion resistant materials. In several locations the

River Forth comes in close proximity to the B829 road and the old railway line

at the apex of meander bends (Photographs 2 & 3) in Reaches 1a, 2, 3, 4 and

5. The meanders are currently stable. No observations were made of erosion at

the toe or face of the riverbank. The river flow through the meander bends is

fast flowing. The meander bends are more susceptible to erosion from changes

to the flow or sediment regime.

Reaches 4 and 6 are the main source of fine silt and sandy sediments into the

river channel. There is severe erosion mainly of the right riverbank in the large

floodplain areas. This is a natural process and is not affecting agricultural land,

infrastructure or the urban environment. The riverbanks affected by erosion are

sensitive to changes in flow conditions as the material in the riverbank is highly

mobile (Photographs 4 & 5).

The river channel through Aberfoyle and Milton (Reaches 3, 4, 6 and 7) has

been engineered to accommodate nearby property, businesses and road

infrastructure (Photograph 6). The channel has been artificially constrained for

large sections with a combination or re-profiling the river channel, rock armour

and retaining walls.

In each identified river reach, there are several small tributaries that feed into

the River Forth, transporting gravels and cobbles from the valley sides. These

are essential sources of cobbles and gravels for aquatic and geomorphic

environments of the River Forth in the study area (Photographs 7 & 8).

The Duchray Water provides approximately 60% of the flows to the River Forth

and the channel provides essential gravels and silts to the River Forth.

Reaches 8 and 9 are high hydraulic energy environments with significant drops

in bed elevation. They are bed rock controlled and transport lake sediments to

the lower reaches within the study area. The river channel is aligned parallel

with the B829 road. (Photograph 9).

9.3 Potential Impacts of the Explored Flood Mitigation Options

The explored flood mitigation options for Aberfoyle (as identified in Section 6) are to

protect the village from a fluvial flooding event up to a 1 in 200 year return period.

Potential impacts of the explored options on the watercourses in the study area are

discussed below:


© Mouchel 2013 83

Option 1 - Flood defence wall / embankment and property protection on Loch Ard,


Installing flood walls / embankments along the River Forth in the three potential

locations would not be anticipated to destabilise the river channel or Loch in these

locations. Along the lake margin there is an existing wall. Along the river channel the

remaining floodplain would not be impeded. Any construction works must be sensitive

to the river morphology and flood defence works should avoid reducing the cross-

sectional area where possible.

At Aberfoyle the potential flood defence is likely to force more flow onto the

downstream floodplain, however the impact is likely to be negligible. Whilst the main

channel is protected with hard engineering the downstream channel is currently

migrating across the floodplain. Changes in flow regime could increase channel

erosion and deposition in this location.

Option 2 - Flood defence wall / embankment and property protection on Loch Ard, in

Milton and in Aberfoyle and flow control structure on the Duchray Water

Installing flood walls / embankments along the Loch Ard, Avondhu and River Forth will

be similar to the impacts stated for Option 1 at the locations considered. Construction

of a flow control structure within the river channel will have a significant impact on

sediment transport and channel and floodplain morphology immediately upstream and

downstream of the control structure. A more detailed study will be required to

investigate the geomorphological impacts this will induce in the catchment.

Option 3 - Flood defence wall / embankment and property protection on Loch Ard, in

Milton and Aberfoyle, and natural flood management measures in the Duchray Water

catchment

Installing flood walls / embankments along the Loch Ard, Avondhu and River Forth will

be similar to the impacts stated in Options 1 and 2 at the locations considered. The

natural flood management measures that could be applied for the Duchray Water

catchment should not change the sediment regime in the River Forth. However a

reduction in the supply of sediments or gravels to the Duchray could trigger catchment

wide morphological adjustment, therefore appropriate natural flood management

measures should be carefully considered with the support of further necessary

investigations.


© Mouchel 2013 84

10 Conclusions and Recommendations

10.1 Conclusions

This report describes the hydraulic modelling of watercourses associated with the

Aberfoyle study area and the potential flood mitigation options explored to address the

identified flooding issues in and around Aberfoyle. The process of hydrological and

hydraulic model development has been described and verification and sensitivity

analysis carried out. Design model runs have been undertaken and analysed for a

range of return periods and flood maps generated.

The main limitation on this study has been that calibration of the model has been

limited by the lack of availability of good quality hydrometric data. Improvements to the

hydrometric network (rainfall and river gauges) and flood event data would be

beneficial to improve the calibration of the model.

The main conclusions of this study are as follows:

There are a number of areas and properties at risk of flooding. The main areas

of flood risk are Aberfoyle village, Lochard Road and Kinlochard Road between

Milton and Aberfoyle.

The major hydraulic influence on the flooding in Aberfoyle is from the Duchray

Water.

Feasibility assessments have been carried out for three outline flood mitigation

options protecting to a 1 in 200 year return period design event.

The economic performance of potential scheme options was determined

through a cost-benefit analysis, undertaken for a range of annual exceedance

probabilities, and also considering the effects of projected climate change. The

options considered all yield a benefit/cost ratio significantly less than unity,

hence considered economically „non-viable‟ options.

Additional considerations were made to check if a better benefit-cost ratio could

be achieved through part schemes or using a lower standard of protection,

however all of the alternatives still yielded a benefit-cost ratio of less than one.

Other options such as natural flood management and temporary / demountable

defences were also considered. Temporary or demountable defences are not

considered to be a suitable option at a community scheme level due to

technical and cost constraints. Individual property measures may the only

available feasible option; individual property investigations would be required

together with exploration of potential funding streams.

Although there are benefits to protecting important social assets such as the

local primary school and the key link of Lochard Road / B829, such benefits are

difficult to quantify economically have not been taken into account in the

economical appraisal. There is also a human cost: people can be killed or

injured by floods and the trauma inflicted by the floods or the fear of the floods

on communities and individuals is acute and long lasting. These issues may

require further consideration at a political level; the Council may wish to

consider a scheme with a benefit-cost ratio of less than one in cognisance of

these intangibles.


© Mouchel 2013 85

10.2 Recommendations

The main recommendations to improve flood risk management in and around

Aberfoyle are as follows:

Installation of additional hydrometric stations to improve rainfall, water level and

flow records to enhance calibration of any future model output and for flood

warning purposes.

Flood mitigation measures within the Duchray catchment such as NFM

techniques (if the measures and associated flow reductions are actually

achievable in practice) combined with direct flow controls / storage on the

Duchray Water (including numerous upstream locations) could in theory reduce

flows through Aberfoyle. However, such measures would not, in themselves,

solve the acute flooding problems in Aberfoyle but they could potentially help

reduce flows and the scale of any hard engineered defence solutions.

Such NFM measures could possibly contribute to a reduction in flood risk in the

River Forth further downstream in Stirling which, at the time of writing, is being

explored further in other related studies.

Potential NFM measures that could be implemented in the Duchray catchment

include flood storage, soil management and slope management techniques

combined with enhancement of the vegetation. Environmental enhancements

would be a key driver for the implementation of such measures. 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 benefits to be included in any detailed feasibility

assessments. Further investigations would be required and should include:

Updated literature review considering latest NFM research findings,

particularly for Scottish catchments similar to the Duchray Water

catchment,

A detailed GIS-based assessment of the most appropriate specific

locations to implement natural flood management and storage

measures (similar to Upper Allan study),

More detailed hydrological modelling to determine more accurately the

benefits of NFM measures,

Initiatives (e.g. pilot studies) to implement NFM measures in close

collaboration with the Forestry Commission in the Duchray catchment

should be encouraged, and

Investigations into possible NFM study funding streams.

Review of flood warning and response time of the systems. Proximity to

watercourses carries an inherent risk of flooding which cannot be entirely

eliminated. Community planning, preparedness, adaptation and resilience in

the event of large flood events are very important, especially in the current

climate change context.

On-going consultation should be undertaken with the appropriate stakeholders

to discuss any possible flood alleviation options, the level of risk that the


© Mouchel 2013 86

community is willing to accept and obtain further information that would inform

any further work in developing flood mitigation strategies. This should include

the residents, relevant departments of Stirling Council, SEPA, Loch Lomond

and the Trossachs National Park, the Forestry Commission, and the providers

of services.

Further investigation into individual property flood proofing / resilience

measures may provide further avenues for flood protection in Aberfoyle.

Opportunities to enhance biodiversity and ecology through river restoration and

the creation of wetland areas should be considered any further phases of

study.


© Mouchel 2013 87

11 Appendices

Appendix A – FEH Catchment Descriptors

Appendix B

Appendix B 1 – Rainfall Graphs

Appendix B 2 – Verification Graphs

Appendix C

Appendix C 1 – Modelled Water Levels for Various Return Periods

Appendix C 2 – Water Levels for Sensitivity Analysis

Appendix C 3 – Water Levels for Options

Appendix D

Appendix D 1 – Upper Ard 200 Year Flood Map

Appendix D 2 – Lower Ard 200 Year Flood Map

Appendix D 3 – Milton 200 Year Flood Map

Appendix D 4 – Aberfoyle 200 Year Flood Map

Appendix E




Appendix F



Appendix G

Appendix G 1 – Environmental Appraisal Report

Appendix G 2 – Environmental Constraints Map

Appendix H

Appendix H 1 – Fluvial Geomorphology Assessment Map

Appendix H 2 – River Reaches and Morphological Characteristics

Appendix H 3 – Photographs of Key Morphological Features

Appendix I




Appendix J

Appendix J 1 – Natural Flood Management Review

Appendix J 2 – Duchray Catchment Natural Flood Risk Management Options

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