Cambridgeshire Flood Risk Management Partnership … · This report has been prepared for the Cambridgeshire Flood Risk Management Partnership in accordance ... 4 Model Development

Cambridgeshire Flood Risk Management Partnership

Ely Surface Water Management Plan

Detailed Assessment and Options Appraisal Report

Final

Ely Surface Water Management Plan—Detailed Assessment and Options Appraisal Report

Hyder Consulting (UK) Limited-2212959 c:\tga74494\documents\cambs swmp\ely\draft outputs\draft report issued\5301-ua002163-bmr-10_ely_swmp_report_rg.docx

Hyder Consulting (UK) Limited

2212959

Aston Cross Business Village 50 Rocky Lane Aston Birmingham B6 5RQ United Kingdom

Tel: +44 (0)121 333 4466

Fax: +44 (0)121 333 4275

www.hyderconsulting.com

Cambridgeshire Flood Risk Management Partnership

Ely Surface Water Management Plan

Detailed Assessment and Options Appraisal Report

Draft

Authors T Lester

Checkers L Foster

Approver R. Gunasekara

Report No 5301-UA002163-BMR-01

Date April 2012

Front Cover: Surface Water Drain near Tennyson Place, Ely, Renuka Gunasekara, Hyder Consulting PLC

This report has been prepared for the Cambridgeshire Flood Risk Management Partnership in accordance

with the terms and conditions of appointment for the Surface Water Management Plan dated March 2011.

Hyder Consulting (UK) Limited (2212959) cannot accept any responsibility for any use of or reliance on the

contents of this report by any third party.


Hyder Consulting (UK) Limited-2212959 Page ic:\tga74494\documents\cambs swmp\ely\draft outputs\draft report issued\5301-ua002163-bmr-10_ely_swmp_report_rg.docx

CONTENTS

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

1.1 Terms of Reference .......................................................................... 1

1.2 Surface Water Management Plans .................................................... 1

1.3 Partnership Establishment ................................................................. 2

1.4 Policy Framework ............................................................................. 3

1.5 Surface Water Flooding ..................................................................... 4

1.6 Sustainable Drainage Systems (SuDS) .............................................. 4

2 Scope of the SWMP .......................................................................... 7

2.1 Aims and Objectives ......................................................................... 7

2.2 Drivers for Change ............................................................................ 9

2.3 Geographic Extent ............................................................................ 9

2.4 Methodology ................................................................................... 10

3 Evidence Base ................................................................................ 12

3.1 Previous Studies ............................................................................. 12

3.2 Historical Flooding........................................................................... 15

3.3 Flood Incident Register ................................................................... 16

3.4 Sources of Flooding ........................................................................ 17

3.5 Potential Indicators of Surface Water Flood Risk .............................. 22

3.6 Maintenance Regimes ..................................................................... 25

4 Model Development ........................................................................ 27

4.1 Model Evolution .............................................................................. 27

4.2 Hydraulic Modelling - Common Principles ........................................ 28

4.3 Stage 1 - Hydrological Analysis/Bare Earth Modelling ....................... 30

5 Wetspot Selection & Prioritisation .................................................... 33

5.1 Approach ........................................................................................ 33

5.2 Stage 2 – Identification of Potential Wetspot Areas........................... 33

6 Detailed Assessment....................................................................... 34

6.1 Stage 3 Detailed Modelled Development.......................................... 34

6.2 Model Results Analysis ................................................................... 35

6.3 Model Verification ........................................................................... 39

6.4 Model Assumptions ......................................................................... 40

6.5 Model Sensitivity ............................................................................. 40

6.6 Model Outputs ................................................................................ 42

7 Engineering Options Identification & Assessment ............................ 44

7.1 Measures Identification ................................................................... 44

7.2 Ely Engineering Measures and Options ........................................... 46


Hyder Consulting (UK) Limited-2212959 Page iic:\tga74494\documents\cambs swmp\ely\draft outputs\draft report issued\5301-ua002163-bmr-10_ely_swmp_report_rg.docx

7.3 Ely Engineering Option Combinations .............................................. 51

8 Economic Appraisal ........................................................................ 53

8.1 Introduction ..................................................................................... 53

8.2 Damages Assessments - Assumptions ............................................ 61

8.3 Damages Assessment – Exclusions ................................................ 62

8.4 Option C2 – Rain Gardens .............................................................. 63

9 Summary ........................................................................................ 64

9.1 Key Surface Water Flooding Issues ................................................. 64

9.2 Preferred Options For Further Investigation...................................... 65

9.3 Benefits of SWMP ........................................................................... 66

10 Next Steps ...................................................................................... 68

10.1 Surface Water Management Action Plan – Preparation,

Implementation and Monitoring ........................................................ 68

10.2 Engage with Stakeholders ............................................................... 69

11 References ..................................................................................... 71

Appendices

Appendix A Glossary

Appendix B Partnership Arrangements and Stakeholder Engagement

Appendix C Data Collection Process and Data Register

Appendix D Flood Incident Register

Appendix E Hydrological Parameters

Appendix F Engineering Option Details

Appendix G Detailed Modelling Results

Appendix H Schematics and Preferred Option Modelling

Appendix I Sensitivity Analysis Results


Hyder Consulting (UK) Limited-2212959 Page iic:\tga74494\documents\cambs swmp\ely\draft outputs\draft report issued\5301-ua002163-bmr-10_ely_swmp_report_rg.docx

Report Version Control Schedule

Version Date of

Issue

Document Reference Status

0.1 23/01/2012 5301-UA002163-BMR-00_Ely Detailed SWMP Report

Draft

0.2 05/04/2012 5301-UA002163-BMR-01_Ely_SWMP_Report

Final


Hyder Consulting (UK) Limited-2212959 Page 1c:\tga74494\documents\cambs swmp\ely\draft outputs\draft report issued\5301-ua002163-bmr-10_ely_swmp_report_rg.docx

1 Introduction

1.1 Terms of Reference

Hyder Consulting (UK) Limited (HCL) was appointed to produce detailed Surface Water

Management Plans (SWMP) for the entire county, which are to be completed by April 2015. The

first phase of work was the Countywide Strategic Assessment report1, which was completed in

April 2011.

HCL was then asked to undertake the detailed surface water management assessment of Ely,

as it was identified as being one of the highest priority wetspots for Cambridgeshire County

Council. This SWMP is formed from the outputs of all the stages of the study, from a strategic

assessment of the overall study area through to flood risk mitigation measures optioneering of

the prioritised wetspots. The options assessed at this stage provide a theoretical assessment of

how best to mitigate flood risk in the wetspot subject to overcoming key engineering, financial

and environmental constraints. This provides an analysis of where investment could be directed

in the future if finance is available.

The Ely Detailed SWMP was completed in April 2012, and the findings are detailed in this

report. It is recommended that this report be read alongside the Countywide Strategic

Assessment report mentioned above.

1.2 Surface Water Management Plans

The wide scale flooding experienced during 2007 precipitated the publication of the Pitt Review2

which contained a large number of recommendations for Government to consider. The key

recommendation in the Pitt Review with respect to surface water management is

Recommendation 18, reproduced below, which in turn refers to Planning Policy Statement 25

Development and Flood Risk (PPS25)3.

Recommendation 18: “Local Surface Water Management Plans, as

set out in PPS25 and coordinated by local authorities, should

provide the basis for managing all local flood risk.“

Surface Water Management Plans (SWMPs) are referred to in

Planning Policy Statement 25 (PPS25) as a tool to manage surface

water flood risk on a local basis by improving and optimising

coordination between relevant stakeholders. SWMPs will build on

Strategic Flood Risk Assessments (SFRAs) and provide the vehicle

for local organisations to develop a shared understanding of local

flood risk, including setting out priorities for action, maintenance

needs and links into local development frameworks and emergency

plans.

Guidance on the production of SWMPs was published in March 20104 informed by the

Integrated Urban Drainage (IUD) Pilot Studies carried out under the Government’s Making

Space for Water (MSfW)5 strategy.

A SWMP outlines the preferred strategy for the management of surface water in a given location

and the associated study is carried out in consultation with local partners having responsibility

for surface water management and drainage in that area. The goal of a SWMP is to establish a

long term action plan and to influence future strategy development for maintenance, investment,

planning and engagement.



The framework for undertaking a SWMP is illustrated using a wheel diagram, reproduced from

the Defra Guidance³ as shown in Figure 1-1.

Figure 1-1 SWMP Wheel (source Defra Guidance³)

The SWMP process is formed of four principal phases;

� preparation,

� risk assessment,

� options, and

� implementation and review.

This report contains the findings from the preparation stage and the strategic and intermediation

elements of the risk assessment phase. Text boxes at the start of each chapter summarise the

elements of the guidance addressed within the subsequent text.

1.3 Partnership Establishment

The formation of partnerships has an important role in the undertaking of a SWMP, and is

required under Defra’s SWMP guidance documentation. The SWMP guidance details the



identification of those partners / organisations that should be involved and what their roles and

responsibilities should be.

It recommends the formation of an engagement plan, which should include objectives for the

individual partners, and detail how and at what stages of the SWMP the engagement with

stakeholders should take place.

Appendix B describes the partners, their roles and responsibilities and their objectives as

required by the SWMP guidance.

1.4 Policy Framework

1.4.1 Flood Risk Regulations 2009

The Flood Risk Regulations 2009 (FRR) transpose the European Floods Directive 2007/60/EC

into English and Welsh law and bring together key partners to manage flood risk from all

sources and in doing so reduced the consequences of flooding on key receptors. Local

authorities are assigned responsibility for management of surface water flooding.

As part of the ongoing cycle of assessments, mapping and planning, the FRR requires the

undertaking of a PFRA. National guidance was published by the Environment Agency (EA) in

December 20106. The requirements of the FRR have also been used to shape this report and to

inform the content of the Council’s PFRA report to the Government produced by HCL.

Under Flood Risk Regulation 19-1 a Lead Local Flood Authority must prepare a flood hazard

map and a flood risk map in relation to each relevant Flood Risk Area, if identified by the PFRA

process. No significant Flood Risk Area has been identified by the EA nationally within

Cambridgeshire, nor the first cycle of the Cambridgeshire PFRA at a local level. However,

depth, velocity and hazard maps (Section 6.6) have been prepared for the Ely SWMP study

area and they will inform Cambridgeshire’s Local Flood Risk Management Strategy

development (see Section 2.1.3) and the second cycle of the PFRA process in six years time.

1.4.2 Flood and Water Management Act 2010

The Flood and Water Management Act places the responsibility for managing the risk of local

floods on the Upper Tier or unitary authorities, as their role as Lead Local Flood Authorities

(LLFAs), but allows for the delegation of flood risk management functions to other statutory

authorities. The Act also seeks to encourage the uptake of Sustainable Drainage Systems

(SuDS) by agreeing new approaches to the management of drainage systems and allowing,

where delegated, for district councils and Internal Drainage Boards (IDBs) to adopt SuDS for

new developments and redevelopments.

1.4.3 Planning Policy Statement 25

Planning Policy Statement 25 (PPS25) requires that new development should not increase flood

risk, and requires developers to design, build and fund the maintenance of SuDS; a SWMP will

support this by informing the Local Planning Authority (LPA) of areas at risk of surface water

flooding ‘and by providing an evidence base to aid the consideration of future development

options.



1.4.4 New Planning Policy Framework (NPPF)

The NPPF proposes to review all existing planning policies and to restructure the planning

process7. The aim of this new framework is to make planning more streamlined and

transparent. The NPPF also aims to give local councils more control over local planning with

more emphasis being placed on sustainable local growth.

The consultation period ended on the 17th of October 2011 and the Government’s response to

consultation and the final version due to be published in April 2012.

1.5 Surface Water Flooding

In the context of SWMPs, the technical guidance4 defines surface water flooding as:

� Surface water runoff; runoff as a result of high intensity rainfall when water is ponding or

flowing over the ground surface before it enters the underground drainage network or

watercourse, or cannot enter it because the network is full to capacity, thus causing

flooding (known as pluvial flooding);

� Flooding from groundwater where groundwater is defined as all water which is below the

surface of the ground and in direct contact with the ground or subsoil;

� Sewer flooding; flooding which occurs when the capacity of underground systems is

exceeded due to heavy rainfall, resulting in flooding inside and outside of buildings. Note

that the normal discharge of sewers and drains through outfalls may be impeded by high

water levels in receiving waters as a result of wet weather or tidal conditions;

� Flooding from any watercourse not designated a “Main River”, including culverted

watercourses which receive most of their flow from inside an urban area and perform an

urban drainage function;

� Overland flows from the urban/rural fringe entering the built-up area; and

� Overland flows resulting from groundwater sources.

This report aims to consider surface water flooding issues in Ely as above but it does not

address sewer flooding where it is occurring as a result of operational issues, i.e. blockages and

equipment failure. It should also be noted that the compilation of all historical flooding within the

area does include some flooding due to main rivers, although further investigation of these

occurrences is outside the remit of this report.

Information on Main River Flooding is covered under other strategic planning documents such

as Strategic Flood Risk Assessments, produced by district councils.

1.6 Sustainable Drainage Systems (SuDS)

Sustainable drainage systems are used to manage rainfall run-off from impermeable surfaces.

SuDS encompass a range of techniques which aim to mimic the natural processes of runoff and

infiltration as closely as possible. These techniques can include green roofs, ponds, permeable

paving and soakaways. Any SuDS scheme should integrate with existing drainage systems and

be easily maintainable.

SuDS schemes should be based on a hierarchy of methods termed the ‘SuDS treatment train’

as illustrated in Figure 1-2.



Figure 1-2 SuDS Treatment Train

Guidance recommends that the management of surface water runoff should use a combination

of site specific and strategic SuDS measures, encouraging source control where possible to

reduce flood risk and improve water quality. Table 1-1 describes some of the SuDS techniques

that will be considered in the development of the Ely SWMP.



Table 1-1 SuDS Techniques (source Ciria8)

Type Description

Balancing Pond A pond designed to attenuate flows by storing runoff during the peak flow and releasing

it at a controlled rate during and after the peak flow has passed. The pond always

contains water. Also known as wet detention pond.

Brown Roof A roof covered with a locally sourced material, its main aim is to partly mitigate any loss

of habitat when new developments are constructed.

Detention Basin A vegetated depression, normally dry except after storm events constructed to store

water temporarily to attenuate flows. May allow infiltration of water to the ground

Filter Strip A vegetated area of gently sloping ground designed to drain water evenly off

impermeable areas and filter out silt and other particulates.

Green Roof A roof with plants growing on its surface, which contributes to local biodiversity. The

vegetated surface provides a degree of retention, attenuation and treatment of

rainwater, and promotes evapotranspiration. Sometimes referred to as a “living” roof.

Infiltration Basin A dry basin designed to promote infiltration of surface water to the ground.

Road Side Rain

Gardens

Where space allows, these can be constructed alongside roads to allow run-off from

roads or pavements to filter slowly through the root system of plants, rather than

entering underground drainage systems.

Permeable

Surface

A surface formed of material that is itself impervious to water but, by virtue of voids

formed through the surface, allows infiltration of water to the sub-base through the

pattern of voids, e.g. concrete block paving.

Rainwater

Harvesting

A system that collects rainwater from where it falls rather than allowing it to drain away.

It includes water that is collected within the boundaries of a property, from roofs and

surrounding surfaces. The harvested water is then re-used in applications where

potable water is not essential.

Swale A shallow vegetated channel designed to conduct and retain water, but may also permit

infiltration; the vegetation filters particulate matter.

SuDS techniques can be divided into two main groups; infiltration based or attenuation based.

Infiltration based SuDS facilitate the discharge of water directly into the ground through soil and

rocks; this is only possible where the underlying geology is permeable enough to allow the

passage of water downwards. Attenuation based SuDS retain water on a site and allow it to

discharge at a prescribed and controlled rate into a watercourse or sewer.

The feasibility for the use of any SuDS technique should be investigated prior to their

installation.



2 Scope of the SWMP

Flood Risk Regulations 2009

Define the aims, objectives and purpose of the report

Describe the overall approach and methodology applied

2.1 Aims and Objectives

2.1.1 Study

The final aim of the SWMP study is to produce a long term surface water management Action

Plan for Ely, which, once in place, will be reviewed every 6 years at a minimum.

The objectives of this study are to:

� Map historical flood incident data

� Engage with partners and stakeholders

� Map surface water influenced flooding locations

� Identify surface water flooding wetspot areas

� Assess, compare and prioritise wetspot areas for detailed assessment

� Identify measures, assess options and confirm preferred options for the prioritised

‘wetspots’

� Make recommendations for next steps

A wetspot is defined as being an area susceptible to Surface Water flooding following analysis

of Modelled Surface Water outputs or historical records.

These objectives will be met following the progression of a number of project stages. The first

stage is data collection, involving contact with the varying partner organisations to obtain all



relevant information. During this stage the collation of historical and future flooding along with

information on flood receptors and flood consequences will take place.

Once the data collection stage is complete, the surface water flooding information will be

analysed to identify wetspots that have a history of flooding incidents or potentially could be at

risk of future flooding. Those wetspots identified as being at higher risk or priority through

agreed local assessment criteria will then progress forward to the next stages, detailed

assessment and optioneering.

Following the optioneering stage, recommendations for flood alleviation or mitigation will be

considered.

2.1.2 Partnership Working

The Cambridgeshire Flood Risk Management Partnership comprises all the flood risk authorities

in Cambridgeshire, including Cambridgeshire County Council, Cambridge City Council, East

Cambridgeshire District Council, the Environment Agency, Ely Group of Internal Drainage

Boards and Anglian Water. A SWMP Project Management Board was formed as a sub group of

CRMP to steer the production of SWMPs, and they are discussed in more detail in Appendix B.

The CFRMP has developed a Stakeholder Engagement Plan, which will aid in communicating

the work of the partnership to the key stakeholders, and is discussed in further detail in

Section 2.4 of the Countywide SWMP. It is of great importance that collaborative working of this

nature is undertaken in order to share experience and expertise.

2.1.3 Context

Alongside the legislative requirements discussed above this SWMP will support the following

initiatives.

Local Flood Risk Management Strategies

Local Flood Risk Management Strategies9 came into force as part of the Flood and Water

Management Act 2010. As LLFA, CCC must develop a strategy for local flood risk management.

The strategy must be consistent with the National Flood and Coastal Erosion Risk Management

Strategy for England, the regional CFMPs and River Basin Plans, and should be developed and

maintained with consultation from other stakeholders, such as the public and other risk

management authorities.

The strategy must specify:

� the risk management authorities in the authority's area,

� the flood and coastal erosion risk management functions that may be exercised by those

authorities in relation to the area,

� the objectives for managing local flood risk (including any objectives included in the

authority's flood risk management plan prepared in accordance with the Flood Risk

Regulations 2009),

� the measures proposed to achieve those objectives,

� how and when the measures are expected to be implemented,

� the costs and benefits of those measures, and how they are to be paid for,

� the assessment of local flood risk for the purpose of the strategy,

� how and when the strategy is to be reviewed, and

� how the strategy contributes to the achievement of wider environmental objectives.



Catchment Flood Management Plan (CFMP)

The Ely study area falls within the area covered by the Great Ouse CFMP, as discussed in

Section 3.1.1. The Action Plan associated with the Great Ouse CFMP, in conjunction with

district wide SFRA’s and this SWMP, will assist in informing the Local Development Framework

process and future flood risk management.

Anglian River Basin Management Plan (RBMP)

The Ely study area falls within the Anglian RBMP. The plan has been prepared under the Water

Framework Directive and is designed to protect, improve and ensure the sustainable use of the

water environment within the Anglian Basin.

2.2 Drivers for Change

The CFRMP are undertaking this SWMP in order to:

� Better understand the risks and consequences of surface water flooding in Ely;

� To meet or significantly assist in meeting some of the requirements on CCC as Lead

Local Flood Authority under the Flood Risk Regulations 2009;

� To meet a number of the requirements of the Flood and Water Management Act

specifically in terms of developing an asset register and producing a local flood risk

management strategy.

It is worth noting that the developed area of Ely is steadily increasing due to a number of

residential developments already constructed, and further developments are planned to North of

Ely. These will have had significant impacts on the natural environment, as greener rural areas

have been replaced in part by housing and commercial developments, roads and other forms of

community infrastructure.

The SWMP process allows the opportunity to enhance the condition of these urbanised

catchments helping to improve the water quality. Additionally, the implementation of the SWMP

and Action Plan can help to provide significant economic and environmental benefits to the

community through better preparation against these potential extreme rainfall events, which to a

large extent has not occurred since this development has occurred.

2.3 Geographic Extent


Define the geographic extent of the report and relate to the relevant river basin district and

relevant maps

This SWMP has been undertaken for the cathedral city of Ely. The city lies approximately 14

kilometres to the north east of Cambridge City Centre. Ely is situated on an island of higher

ground within the low lying Fens. Ely has a population of approximately 15,200.

Ely is located within the Anglian River Basin District and the River Great Ouse catchment. It sits

in the north of the East Cambridgeshire District Council area. The study area/ 2D extent

incorporates all of the Ely urban area and incorporates approximately 16.2km², as shown in

Figure 2-1.



Study Area /2D Model Extent

Figure 2-1 Ely SWMP Study Area © Crown Copyright and database right 2012. All rights

reserved. Ordnance Survey licence number 100026380

The land surrounding Ely is predominantly arable agricultural land. A ridge of higher land

approximately 20-30 m Above Ordnance Datum Newlyn runs through the centre of Ely. The city

is bypassed by the A10 which is a main trunk road connecting several large urban areas within

Cambridgeshire. The urban area to the east ranges from 5 – 8 m above ordnance datum (AOD)

and the area to the west ranges from 1 – 3m AOD. Ordnance Datum or is a vertical datum used

by an ordnance survey as the basis for deriving altitudes on maps.

2.4 Methodology

The methodology used to carry out this SWMP follows the advice set out in the Defra SWMP

guidance4 for the preparation stage and the strategic risk assessment phase. Figure 2-2

illustrates the process carried out to inform this detailed assessment and options appraisal

report, a key output of Ely SWMP. It should be noted that this figure only shows the steps

subsequent to the formal identification of the Ely settlement as a priority wetspot discussed in

the Countywide Strategic SWMP.

Further details on the methodology are discussed throughout the report in the relevant sections.

The work undertaken for the study is also informed by the EA’s PFRA guidance in order to

assist in meeting the obligations of CCC as the Lead Local Flood Authority (LLFA). Information

on the methodology for subsequent phases of the SWMP is set out in Section 10 of this report.



Figure 2-2 Overall Approach to Study Methodology

The specific methodology adapted for the Ely study is further explained in Sections 4 to 8.

Collate and map Historic Flood Incident Data

Map surface water influenced historic and future flood locations,

mechanisms and consequences

Undertake Strategic Assessment

Produce List of Initial Wetspot Areas

Consult stakeholders for local knowldge and to obtain missing

data and prioritise wetspots

Recommend Priority Wetspot Areas for Detailed Assessment

Undertake Detailed Assessment and Options Appraisal

Confirm Preferred Options

County wide SWMP

EA – Surface Water

Map and

Areas susceptible to

Surface Water

Flooding Maps

BGS – Groundwater

Vulnerability Maps

National and Local

Information on flood

receptors and

consequence



3 Evidence Base

3.1 Previous Studies

As part of this study, it has been critical to identify the links to other local and regional delivery

plans which may influence or be influenced by the SWMP. The SWMP will seek to integrate and

align these plans and processes to provide a clear and robust path to delivering flood risk

management objectives throughout Ely. The studies listed below have already been completed,

however the information from this SWMP and the emerging Local Flood Risk Management

Strategy can be used to inform any updates to these studies.

Appendix C provides further information on data collection and review process.

3.1.1 Great Ouse CFMP10

The Great Ouse Catchment Flood Management Plan (CFMP) was published by the

Environment Agency (EA) in July 2010. The catchment covers approximately 8,600 km2, and is

predominantly rural, with the larger population centres of Milton Keynes, Cambridge, Bedford

and King’s Lynn.

Ely falls within the Fens Policy Unit and South Level sub-catchment. Ely was not considered as

a “Main Area” for detailed investigation as it is situated at a much higher elevation than the

surrounding area with minimal flood risk from the Ely Ouse River (see Figure 2-1). The main

source of flood risk across the whole sub-catchment were identified as the overtopping or

breaching of small drainage channels due to restricted discharges into the Main Rivers and

limited flood storage capacities.

A number of flood risk management policy options were identified across the whole catchment,

and the policy option covering Ely was Policy Option 4 - areas of low, moderate or high flood

risk where the EA are already managing the flood risk effectively but where they may need to

take further actions to keep pace with climate change.

3.1.2 East Cambridgeshire SFRA 200511

The final Level 2 SFRA for East Cambridgeshire was completed by Atkins in 2005. The aim of

the study was to produce a data set to allow potential development areas to be graded by flood

risk. The study focussed on 12 growth areas highlighted by East Cambridgeshire DC.

The study identified that the greatest flood risk in the area was posed by a failure or breach of

defences along the Bedford Ouse and Ely Ouse river systems. The majority of the area is

protected by defences designed to protect against fluvial flooding up to a return period of

100 years and tidal flooding to 200 years. The River Lark upstream of Isleham is the only area

not protected but this is outside the Ely SWMP study area. The predicted flood risk of the 12

growth areas is tabulated in Table 3-1 below; Ely is within a Category 2 predicted flood risk

area.



Table 3-1 Findings of East Cambridgeshire SFRA

Growth Area Predicted Flood Risk

Stretham, Haddenham,

Little Downham,

Witchford, Isleham

Category 1 - Settlements within an area protected to an appropriate minimum

standard of defence and not flooded from a breach inundation.

Ely, Sutton, Littleport Category 2 - Settlements within an area protected to an appropriate minimum

standard of defence and partially inundated by flooding from a breach.

Bottisham, Burwell,

Fordham, Soham

Category 3 – Undefended

As discussed in Section 3.1.3 below, this SFRA was updated by Scott Wilson in 2011.

3.1.3 East Cambridgeshire Level 1 SFRA12

Scott Wilson completed a Level 1 SFRA for East Cambridgeshire District Council in 2011. This

study builds on the findings of the previous SFRA completed in 2005.

The SFRA states that East Cambridgeshire District has significant areas which lie within the

fluvial and/or tidal flood zone but Ely is located in an ‘island’ on the high ground above the

floodplain.

The SFRA suggests that the specific Drift Deposits (sedimentrary deposits of glacial origin)

within East Cambridgeshire that prevent signiciant groundwater flow. The SFRA concludes that

Ely could utilise Attenuation Systems such as basins, ponds and green roofs rather than

Infiltration Systems when considering suitable SUDS schemes in the area.

3.1.4 Ely Drainage Area Study13

A drainage area plan was undertaken by Mott MacDonald in 1998 on behalf of Anglian Water.

The aim of the study was to assess known operational, flooding and pollution problems

associated with the foul and combined sewer system in Ely. As such the model built for this

study focused on the foul and combined system, and no modelling of the surface water system

was undertaken.

We have been advised by Anglian Water Services that this model will have little relevance to the

current Surface Water issues in Ely and that there is no current hydraulic model available for

this area.

Table 3-2 Hydraulic Modelling of Ely Sewer System

Asset Type Description

Foul The foul system is maintained and operated by AWS, with no current hydraulic

model available of the system. A 1998 model of the network exists . The which

contains asset data, sewer network data and manhole locations, was received

from AWS

Storm Water The storm water system is predominantly maintained and operated by AWS;

however no current hydraulic model exists of the system. They maintain a GIS

database, which lists the locations of the manholes and the sewer network.




There are also numerous areas of development where AWS have no records

of the storm network. This is could be due to these sewers not yet being

adopted by AWS or are under private ownership. The transfer of sewers

serving more than one property and connected to a public sewer to water

company ownership occurred on the 1st of October 2011.

3.1.5 River Basin Management Plan 14

The River Basin Management Plan focuses on the protection, improvement and sustainable use

of the water environment. It aims to help European Union Country’s meet Water Framework

Directive objectives such as promoting sustainable use of water as a natural resource,

contributing to mitigating the effects of floods and droughts and conserve habitats and species

that depend directly on water.

The Cam and Ely Ouse catchment supports a number of nationally and internationally important

water-related sites that are of exceptional value.

3.1.6 Countywide SWMP

The Countywide SWMP report summarises the strategic level assessment of surface water

flood risk across Cambridgeshire County as per Defra SWMP guidance. The report identifies

broad areas where surface water should be considered in more detail in a site based Flood Risk

Assessment. The report aids the development of more general planning policies to help

minimise the risk of flooding by surface water, such as policies promoting SuDS.

The SWMP identified the top ten wetspots within the county; Ely was fourth but first within the

East Cambridgeshire District. The wetspots were identified by reviewing the historical incidents,

the EA Flood Maps for Surface Water and using this information to identify susceptibility of

Cambridgeshire settlements to surface water flooding through Multi-Criteria Analysis.

3.1.7 Flood Memories

The Cambridgeshire Flood Risk Management Partnership carried out a programme of collecting

flooding information focusing predominantly on small to medium localised flooding events.

Members of the public were asked to complete a questionnaire on their memories of flooding

incidents, either via a paper or online questionnaire, or via five road shows across the county.

Over 250 responses were received.

Two records were identified in Ely from this programme. Both records related to flooding

between 1-3 years ago following a heavy downpour. It is not possible to verify whether these

two records related to the same rainfall event.



3.2 Historical Flooding


Introduce the local sources of flood risk being considered for past floods and possible future

floods.

Assess past floods which had significant harmful consequences for human health, economic

activity, cultural heritage and the environment.

The following sections outline the historical flooding recorded within the Ely study area. This text

should be read in conjunction with the Flood Incident Register given in Appendix D of this

report. It is highlighted that this report is based on the information supplied by partners up to

August 2011; the occurrence of flooding is not static and thus this represents an understanding

of the situation as of this date.

It should be noted that many of the historic records do not provide a date when the flooding

occurred, and for the majority, there is no identified cause of flooding (pluvial, fluvial, sewer etc).

It is also likely that flooding within Ely is under-reported. The historical information provided with

flooding in Ely is of varying quality and it is not always possible to ascertain the consequence or

source of flooding associated with the flood record. This is possibly due to under reporting of

problems with flooding by the general public to the Local Authority / Environment Agency.

Caution has therefore been exercised within this section of the report in interpreting the

historical record. Figure 3.1 shows the location of historical flood events in Ely based upon the

above data.

Figure 3-1 Historical Flooding in Ely © Crown Copyright and database right 2012. All

rights reserved. Ordnance Survey licence number 100026380



Whilst every effort has been made to analyse the data there is a high probability that there are

deficiencies in quantity and the attribution of historical information. It is considered that the

majority of the information pertinent to the SWMP falls within the Low to Medium Confidence

categories (see Table 3-3).

DG5 Flooding Register

AWS maintains a register of confirmed internal and external sewer flooding locations due to

hydraulic overloading. The Register only contains properties and areas at risk of internal and

external flooding if they have suffered flooding from public sewers due to overloading of the

system, the register however excludes any incident event that has a storm return period above

a 1 in 22 (4.5% AEP) and is therefore classified as a severe weather event. A sewer is

overloaded when the flow from a storm is unable to pass through it due to permanent problem

(e.g. small pipe, flat gradient).

The Register does not include properties or areas flooded due to temporary operational

problems e.g. blockage, siltation, collapse, equipment failure or operational failure. The Register

does not contain properties or areas that have been subject to a flood alleviation scheme (to a

satisfactory level of protection) or if new information reveals that the property or area does not

meet the criteria to be on the register. AWS has provided its DG5 database for the study area.

It is noted that as of February 2011, there were four properties that remain active on the

register. It should be noted that previous DG5 listings may have been removed from the register

as a result of remedial work, or the implementation of system improvements. AWS are required

to undertake capacity improvements to alleviate some of the most severe sewer flooding

problems on the DG5 register during the current 5 Year Asset Management Period (2010-15)

with priority being given to more frequent internal flooding problems.

Historical Flooding Record – Highway and Pluvial Flooding

The historical record, particularly the CCC Customer Reports of Flooding (Figure 3-1), includes

a significant number of descriptive records of flooding tagged with general descriptions such as:

blocked drains, foul sewer emissions, road drainage, foul sewer backup and drains silted up.

The records clearly demonstrate that there are problems with drainage across Ely. However,

and in general, there is insufficient information to identify whether flooding is attributable to

highway, foul or storm sewer flooding. Moreover, it is frequently not possible to determine the

frequency and precise cause of the flooding. It is considered that the quality of historical

flooding information falls within the low to medium confidence categories.

3.3 Flood Incident Register

A sub task within the data assimilation stage, as part of the countywide SWMP, was the

development of a flood incident register to show all the historical surface water flooding

incidents in Cambridgeshire, which included those occurring within the Ely study area. For each

event the location of each flood incident was registered and approximate co-ordinates for the

incident was also recorded where this was readily available or could be estimated within the

available project timescale and resources. Each flooding incident was assigned a unique flood

incident reference number. The flood incident register for the Ely area is included in Appendix D.

For many incidents the exact location of flooding was not reported, for example “flooding

occurred on High Street”. Where the exact location was not known, an indicative location was

picked at a central point on the street. Where known, the house number, incident date and time

of incident was recorded. It should be noted therefore, that the flood incident register contains



approximate grid co-ordinate locations that may not be the exact location of the historical

flooding incident.

A crucial component of the incident register is recording the confidence in the source of the

information. Some flood events were well reported, with a high level of detail regarding the

source, pathway and receptor and other reports did not provide such details. The criteria in

Table 3-3 were used to assess the confidence in the flood source. It is recommended that this

practice is continued for all new flooding incidents added to the register along with more

accurate information on incident location and flood consequence.

Table 3-3 Confidence in flood report sources

Flood Source Confidence in Flood Source

Little or no evidence to support flood source in incident report Low - Source assumed

Flood source provided by residents or non technical experts with

high level of detail in the incident report

Medium - Some evidence

Flood source provided by ‘technical experts’ e.g. IDB staff or

residents with compelling evidence i.e. photos

High - Compelling evidence

3.4 Sources of Flooding

The following sections summarise the various sources of flooding along with the key data

sources.

3.4.1 Surface Water Runoff / Pluvial Flooding

Surface water runoff occurs as a result of high intensity rainfall causing water to pond or flow

over the ground surface before entering the underground drainage network or watercourse, or

when water cannot enter the network due to insufficient capacity.

Pluvial flooding is defined as flooding that result from rainfall-generated overland flow. The

historical records include a significant number of descriptive records of flooding which imply that

there are issues with pluvial flooding. The records clearly demonstrate that there are problems

with pluvial flooding but it should also be recognised that flooding will be the result of numerous

factors rather than solely rainfall intensity or duration.

The rainfall event Annual Event Probabilities assessed by the SWMP are shown in Table 3-4

below. The modelling methodology is discussed in Section 4.

Table 3-4 Annual Event Probabilities Assessed and Equivalent Return Periods

Annual Event Probability (%)

(chance of occurring in any given year(

Approximate equivalent

Return Period

0.5% 1 in 200

1% 1 in 100

2.5% 1 in 40

3.33% 1 in 30

5% 1 in 20



3.4.2 Fluvial Flooding

The watercourses in the Ely study area are shown in Figure 3-2 below. Further details on their

categorisation and those responsible for their upkeep are given in the following sections.

Figure 3-2 Watercourses in Study Area © Crown Copyright and database right 2012. All rights


Main Rivers

Under the Water Resources Act 1991, the EA has powers to maintain and improve designated

main rivers for the efficient passage of flood flow and the management of water levels for flood

defence purposes. These powers are permissive only and there is no obligation on the Agency

to carry out such works. The current maintenance regime for designated main rivers uses a risk

based approach and government funding via Defra. The ultimate responsibility for maintaining

the bed and banks of any watercourse, including its vegetation, rests with the riparian owner(s).

The EA offers a flood warning service to areas covered by main rivers and some ordinary

watercourse tributaries. The main river in the Ely study area is the Ely Ouse. Information on the

main rivers in the county area was provided by CCC and the EA.

Ordinary Watercourses

Ordinary watercourses are all rivers, streams, ditches and drains that have not been designated

as main rivers. The main responsibility for all watercourses lies with the riparian owners. Local

Authorities are responsible for any ordinary watercourses that fall within areas where they are

the land owner, or if the watercourse is awarded. Details of the ordinary watercourses were

provided by the local authorities.



Internal Drainage Board (IDB) Drains

The IDB drains in Ely are maintained by the Ely Group of IDBs. The four IDB’s covering the Ely

area are Cawdle Fen, Littleport and Downham, Middle Fen and Mere and Padnal and

Waterden. They maintain a large number of drains and assets within the Ely vicinity, including a

reservoir and several pumping stations. The locations of the Drainage Board boundaries are

shown in Figure 3-3 below.

Figure 3-3 – Ely Internal Drainage Board Boundaries © Crown Copyright and database right 2012.

All rights reserved. Ordnance Survey licence number 100026380

3.4.3 Sewers

Sewer flooding occurs when the capacity of underground systems is exceeded due to heavy

rainfall, resulting in flooding inside and outside of buildings. However, sewer flooding can be

caused by excess surface water, blockages collapses or plant failure. Water companies, in this

case Anglian Water Services Ltd (AWS), are obliged under the Water Industry Act15

to facilitate

the drainage of surface water as detailed below.

The sewerage system within Ely comprises a combined sewer system in the City centre, with

newer suburban areas to the north and south comprised of separate foul and storm water sewer

systems. Figure 3-4 shows the complete sewer network and assets as provided by Anglian

Water.



For public sewers, sewerage undertakers, in this case AWS, are obliged under the Water

Industry Act to provide, maintain and operate systems of public sewers and works for the

purpose of effectually draining their area. There is no universal level of service associated with

the sewer network. Table 3-5 details the main sewer asset types in urban areas:

Table 3-5 Public Sewerage Systems


Public foul sewer Maintained and operated by AWS, these should carry only foul sewage but,

through misconnections, often also carry surface water, and include the recent

transfer of private sewers and lateral drains that are connected to the public

sewerage system, on the 1st October 2011

16.

Public surface water

sewer

Maintained and operated by AWS. They should carry only surface water and

include the recent transfer of private sewers and lateral drains that are

connected to the public sewerage system, on the 1st October 2011

17. .

Highway drains are often connected to public surface water sewers.

Public combined sewer Public combined sewers are maintained and operated by AWS. They carry

both foul sewage and surface water, and include the recent transfer of private

sewers and lateral drains, that are connected to the public sewerage system,

on the 1st October 2011

18.

Again, highway drains are often connected to public combined sewers

Highway Drains Maintained and operated by CCC or Highways Agency. Highway drains are

often connected to public surface water and combined sewers.

Since the first edition of Sewers for Adoption in 1980, this document has become the standard

for the design and construction of sewers to adoptable standards in England and Wales.

Sewers for Adoption currently requires public surface water sewers to accommodate flows up to

a 3.33% AEP (1 in 30 year chance) design storm.

It is highlighted however that this level of service will change if ever increasing area are

connected to the sewers over time. The design standard also does not account for the capacity

of connections such as gutters, gullies, highway drains and private drains which may limit the

flow discharging to the sewer. Anglian Water Services carry out proactive maintenance on

sewers across Cambridgeshire, however it should also be noted that there are several drains

and watercourses, previously owned by the Church Commissioners, whose ownership or

adoption is still not resolved. These now act as sewers to drain the development areas.



Figure 3-4 - Ely Sewer Network © Crown Copyright and database right 2012. All rights reserved.

Ordnance Survey licence number 100026380

Ely is served by two Wastewater Treatment Works (WwTW), Ely (Old) WwTW to the east of the

city centre and Ely (New) WwTW south of the city.

Additional Storm Water Network Data

Due to the large quantity of data gaps within the storm water network data received, a number

of consultations were carried out with AWS, CCC and ECDC in order to try and complete, as far

as was possible, the storm water records for Ely. Initially, CCC’s Highways department provided

a number of plans showing highways up for adoption under Section 38 of the Highways Act.

Gaps in sewerage network data can be as a result of assets being in private ownership.

Additional manhole surveys were carried out across the Ely catchment to fill areas of data gaps.

The locations of these additional Manhole surveys are shown in Figure 3-5.



Additional Manhole Survey Study Area

Figure 3-5 – Additional Manhole Surveys Undertaken in Ely © Crown Copyright and database right

2012. All rights reserved. Ordnance Survey licence number 100026380

3.5 Potential Indicators of Surface Water Flood Risk

3.5.1 EA Areas Susceptible to Surface Water Flooding (AStSWF) Maps

The Environment Agency have produced the outputs of a simple surface water flood modelling

at a national scale. The modelling did not take into account underground sewerage and

drainage systems or smaller over ground drainage systems. No buildings were included and a

single rainfall event was applied. The model parameters used to produce the maps were:

� 0.5% Annual Exceedance Probability (AEP) in any given year (1 in 200 year)

� 240 minute storm duration

� 1km2 resolution

� No allowance for underground pipe network

� No allowance for infiltration



The AStSWF map gives three bandings indicating areas which are ‘less’, ‘intermediate’ and

‘more’ susceptible to surface water flooding. The map is not suitable for identifying individual

properties at risk of surface water flooding.

These maps were updated and republished in January 2009. Figure 3-6 below shows the

AStSWF across the Ely study area.

Figure 3-6 Areas Susceptible to Surface Water Flooding in Ely © Crown Copyright and database

right 2012. All rights reserved. Ordnance Survey licence number 100026380

3.5.2 EA Flood Maps for Surface Water (FMfSW)

Following on from the release of the Areas Susceptible to Surface Water Flooding, the EA

updated the original mapping in order to produce the Flood Maps for Surface Water (FMfSW),

which were released in October 2010. The existing maps were updated to take account of

buildings and the underground drainage system, and more storm events were analysed. It

should be noted that these maps do not take into account artificial drainage regimes. The model

parameters used to create these new maps were:

� External Publication Scale 1:25,000

� 3.33% (1 in 30 year) and 0.5% (1 in 200 year) Annual Exceedance Probability (AEP) in

any given year return periods

� 66 minute storm duration

� 5m2 resolution with country split into 5km squares

� In rural areas, rainfall was reduced to 39% to represent infiltration

� In urban areas, rainfall was reduced to 70% to represent infiltration

� Global use of Manning’s ‘n’ of 0.1 for rural and 0.03 urban areas



The new maps have two bandings of “deep” or “shallow” and are produced for both return

periods modelled. Figure 3-7 below shows the FMfSW for the 0.5% AEP return period across

the Ely study area.

Figure 3-7 0.5% AEP Return Period Flood Maps for Surface Water in Ely © Crown Copyright and

database right 2012. All rights reserved. Ordnance Survey licence number 100026380

3.5.3 British Geological Survey Groundwater Flooding Susceptibility Maps

Groundwater flood risk has been assessed by the British Geological Survey (BGS) for the whole

country via national flood hazard maps. The groundwater flooding susceptibility data shows the

degree to which areas of England, Scotland and Wales are susceptible to groundwater flooding

on the basis of geological and hydro-geological conditions.

The dataset provided does not show the likelihood of groundwater flooding occurring, i.e. it is a

hazard not risk-based dataset. The risks have been derived using set ‘rules’ in order to identify

areas “based on geological considerations, where groundwater flooding could not occur, i.e.

areas where non-aquifers are present at the ground surface” (BGS).

Areas susceptible to groundwater accumulation are passed through a second set of rules in

order to create a groundwater level surface (this was taken from groundwater contours, inferred

river levels, borehole data and other BGS datasets). The final groundwater level was then

compared to a Digital Terrain Model (DTM), and the resulting modelled depths of groundwater

level above the surface were translated into associated risk categories ‘Very High’, ‘High’,

‘Moderate’, ‘Low’ and ‘Very Low’.



BGS note that “The susceptibility data is suitable…to establish relative, but not absolute, risk of

groundwater flooding at a resolution of greater than a few hundred metres. In all cases it is

strongly recommended that the confidence data is used in conjunction with the groundwater

flooding susceptibility data”. In addition, “the susceptibility data should not be used on its own to

make planning decisions at any scale, and, in particular, should not be used to inform planning

decisions at the site scale. The susceptibility data cannot be used on its own to indicate risk of

groundwater flooding”.

At this stage of the SWMP, these maps have been used only in a limited capacity, however, it is

expected that during future stages, these maps will be used more extensively to inform the

optioneering process.

The Ely study area is characterised by a large area of Low Susceptibility to Groundwater

flooding. There is a small area of Very High Susceptibility to Groundwater Flooding in the east

of the City. However the BGS Maps were checked against the supplied historic flood records

and there were no records that correlated with areas of Very High flood susceptibility or

suggested that groundwater was the source of flooding.

3.6 Maintenance Regimes

Maintenance regimes are critical to ensuring the continued and effective functioning of assets to

manage surface water flood risk. Existing maintenance tasks/ responsibilities have been

reviewed as part of the SWMP where information is currently available and these are listed

below. The SWMP will also assist in identifying and focussing needs in terms of future

maintenance and it is recommended that all partners and stakeholders provide the relevant

information for inclusion in the final version of this report as appropriate.

Cambridgeshire County Council Highways Authority

The CCC Highways Authority has the over-riding responsibility for all highways and highway

structures throughout the council area (with the exception of motorways and some major trunk

roads, such as the A11), and operates programmes of inspection and maintenance for bridges

and gullies within the county area.

East Cambridgeshire District Council

East Cambridgeshire District Council has responsibility for 27 miles of award watercourses, on

which is operates regular four yearly de-silting programmes and annual bank sides flailing

programmes. Where watercourses cross land under the control of ECDC such as public open

space, the District Council will have an obligation to maintain these watercourses as riparian

owners.

Ely Group of Internal Drainage Boards

The Ely Group have over 120 assets, including pumping stations, inlets and culverts, all of

which are subject to annual inspection and maintenance. They operate annual flail mowing and

weed cutting programmes for all Main Drains, by machine, boat or hand.

There are also several Catchwaters that ‘catch’ highland runoff before entering a Drainage

Board’s District. For example, the Ely study area shown in Fig 3-3, is impacted by the

Kettlesworth and Clayway Catchwaters that border the Padnal and Waterden IDB where as the

Cawdle Fen IDB is bordered by the Cawdle Northern and Cawdle Southern Catchwaters. Most

of these Catchwaters are maintained by the IDBs on behalf of Environment Agency or the

Church Commissioners.



Anglian Water

Maintenance regimes are critical to ensuring the continued and effective functioning of assets.

For the surface water network, Anglian Water assumes that the self-cleansing velocity design

standard is sufficient to clear any blockages. As a result they do not have any listed expenditure

for maintenance of surface water systems.

For foul and combined systems, Anglian Water follows a proactive approach to maintenance,

however due to additional reactive inputs, maintenance costs vary between years dependent on

any reported flooding incidents.

Existing maintenance tasks/ responsibilities has been reviewed as part of the SWMP where

information is currently available and these are listed below. The SWMP will also assist in

identifying and focussing needs in terms of future maintenance and it is recommended that all

partners and stakeholders provide the relevant information for the inclusion in the final version

of this report as appropriate.

Environment Agency

The Environment Agency carries out maintenance on those rivers or streams designated as

main rivers. The Environment Agency's annual maintenance programme can be viewed by

using their website19

. Within Ely, the Environment Agency also contributes to the cost of

maintenance of the catchwaters maintained by the Internal Drainage Boards.



4 Model Development

4.1 Model Evolution

There are a number of factors influencing surface water flooding as a result of a localised heavy

rainfall event in Ely. These include:

� Highway and building surface water runoff

� The capacity of the sewer network

� The interaction between the water levels in the river and the surface water network

� Overland flow routes following relatively steep terrain in the west of Ely.

Recent advances in hydrological and hydraulic modelling techniques have allowed for a gradual

improvement in assessing sources of flooding and flood risks. Of particular note for this study,

advances in direct rainfall bare earth modelling allow representation of storms that are not

purely fluvial. This technique allows analysis of surface water runoff, infiltration, depression

storage and rainfall distribution and its effects on flooding.

This method of ‘raining’ on the model domain allows sites at risk of surface water flooding to be

identified and also illustrates the main flood pathways by which flooding occurs. In doing so, the

model represents a means of identifying areas at risk of flooding, from which multi-criteria

analysis scores and financial damages can be calculated. Once the baseline flood risk has been

identified, the model then provides a useful tool to assess the viability of potential flood

alleviation measures.

The use of 2D surface terrain modelling is designed to ensure that the flooding mechanisms are

appropriately represented by the model. This approach enables the effect of the topography on

overland flood routes to be simulated by direct application of a rainfall profile to a 2D hydraulic

model domain.

For this study, InfoWorks CS software was used. This package utilises standard GIS practices

to manage, manipulate and present input and output data. In order to model surface flows, CS

requires terrain data. This can be from any source (GPS, LiDAR, photogrammetry etc.) but the

more detailed and accurate the source of the data, the more accurate and reliable the solution

is likely to be. For this study, the terrain used by CS has been generated from 1 and 2 metre

resolution LiDAR data provided by the EA.

In order to address the specific issues relating to the Ely SWMP, a three stage modelling

strategy was developed for this study:

� Stage 1 - Hydrological analysis and development of broad scale, bare earth and river

model of Ely (see Section 4.3).

� Stage 2 – Evaluation of preliminary results and identification and prioritisation of wetspots

using the bare earth model developed in Stage 1 through local stakeholder consultation

and Historic Flood Record Analysis (see Section 5).

� Stage 3 - Detailed modelling assessment of the identified priority wet-spots within Ely.

Creation of detailed modelling including the interaction between the ground terrain, river

and sewer network (see Section 6). This includes the development and testing of

engineering options and economic analysis (See Sections 7 and 8).



4.2 Hydraulic Modelling - Common Principles

4.2.1 2D Terrain

In InfoWorks CS, the 2D model domain is represented using a triangular “mesh” that covers the

extent of the study area. This mesh is created by sampling LiDAR data, with each triangle being

set at a ground level equal to the average of the ground levels at each of its three corners. The

mesh can be made more detailed by adjusting the size of the triangles comprising the mesh.

Further definition can be added to areas within the mesh via the lowering or raising of levels as

required, for instance to lower roadways or raise areas of land within building footprints to more

accurately model the “channelling” effect of the 2-dimensional features.

Figure 4-1 illustrates an example of the 2d mesh area with more detailed triangles within the

buildings.

Figure 4-1 2D Mesh Representation © Crown Copyright and database right 2012. All rights


4.2.2 2D Roughness

A generic roughness of 0.02 has been applied across the whole of the 2D mesh area to

represent the urban areas within the catchment. Roughness polygons have also been used in

the model to represent a higher roughness in rural/open areas of land. A roughness value of

0.045 has been applied in these areas.

Sensitivity testing of the model to differences in 2D roughness was tested and the results are

discussed in Section 6.5.



4.2.3 Infiltration

Whilst the direct rainfall model explicitly simulates the channelling and pooling of surface water,

losses to the ground through infiltration are not immediately accounted for. Such a scenario – in

which no infiltration losses are represented – could be assumed to be indicative of a frozen or

highly saturated catchment response. However this is a very conservative assumption and

hence it is desirable to include a measure of infiltration losses in the model to make it more

representative.

As it is not possible to apply areas of differing infiltration across the 2D catchment in the

Infoworks CS software a single infiltration value of 11mm/hr has been applied across the whole

of the 2D study area. This value was calculated based on the differing soil types identified

during hydrological analysis as well as the amount of impermeable surfaces across the study

terrain.

The process for defining a standard infiltration to the 2D was done by

1. Defining the different infiltration rates related to the underlying geology defined in the

Cranfield University’s Soilscape (http://www.landis.org.uk/soilscapes/) website.

2. Calculating the percentage area of permeable area of differing infiltration within the Model

Extent.

3. Defining a single infiltration rate for the whole catchment based on the contributing areas of

differing infiltration within the Model Extent.

This final adjusted value derived from the method above was then deducted from each timestep

of the rainfall hyetograph to represent loss to infiltration.

Whilst ideally the varying infiltration methods across the study catchment would be modelled

such as lower infiltration values for impermeable areas, the method described above makes

best use of both the data provided and the capabilities of the modelling software used.

4.2.4 Representation of Buildings

Buildings have been represented through raising the 2D mesh 5m within the building footprint.

This allows routing of overland flow around buildings. This method forces runoff to flow around

the building, representing a more realistic routing of surface water flows - in particular, in

Cambridgeshire where ground is relatively flat causing shallow and slow velocity flooding.

4.2.5 Representation of Roads

MasterMap base data was used to extract all roads within the study area. This separate road

polygon dataset was stamped onto the underlying DTM with a 100mm drop applied. The

100mm height difference is designed to represent the kerb level. This method allows flow to run

along the lower road network before spilling over the kerb and affecting other areas. This

represents a more realistic routing of surface water flows.

4.2.6 Representation of Foul Flows

The EA National Receptor Database (NRD) was used to calculate the number of residential

properties within the urban area of Ely. An average population per property was obtained from

the National Statistics summary.



4.3 Stage 1 - Hydrological Analysis/Bare Earth Modelling

4.3.1 Bare Earth and River Model Construction

The first stage of modelling was to create the Bare Earth and River Model. This model

comprises a 2D model terrain mesh and the local river system of the Great Ouse, plus Clayway

Catchwater, Angle Drove and Hurst Drove. This would improve on the existing Environment

Agency Surface Water flood mapping in that it determined a critical storm duration specific to

the study area and a more refined ground terrain model.

For the purpose of economic assessment the Bare Earth and River Model (and results) is

referred to as the "Do Nothing" option (See Section 7.3 for further detail). The extent of the

model is shown in Figure 4-2 below.

Figure 4-2 Ely Bare Earth and River Model- Extents of Infoworks CS Domain © Crown Copyright and


The Ely Infoworks CS domain was established by drawing a polygon around the surface water

catchment. This catchment is generally contained by the A10 to the west and River Ouse to the

East. The IDB area to the west of the A10 was also included within the model as was the Hurst

Lane Reservoir.

For the initial investigation and determination of the critical storm duration, a mesh with a

maximum triangle size of 100 m2 was chosen for the 2D mesh. The resolution of this grid is

Hurst Lane

Reservoir



detailed enough to model key flow routes across the study area without having a detrimental

impact on model run times.

Table 4-1 Stage 1 Model Parameters

Model Parameters

Grid Size 100 m2

Time Step 1 second

Storm Duration 30, 60, 120, 240, 480, 960 minutes

Return Periods 0.5% AEP

The results for a 0.5% AEP 60 minute storm are shown in Figure 4-3.

Figure 4-3 Bare Earth and River Model Results for 0.5% AEP storm © Crown

Copyright and database right 2012. All rights reserved. Ordnance Survey

licence number 100026380



4.3.2 Design Rainfall

Design rainfall for a variety of return periods and storm durations was generated using Depth

Duration Frequency (DDF) rainfall catchment descriptors, derived from the FEH CD-ROM.

These catchment descriptors are input directly into Infoworks CS, which automatically creates

rainfall hyetographs that are then applied over the catchment area. Figure 4-4 shows a

hyetograph used with the Bare Earth and River Model. The hyetograph defines point rainfall and

duration and is applied over the entire extent of the model.

Figure 4-4 Hyetograph for 0.5% AEP 480 minute duration storm

4.3.3 River Levels

As there a number of Surface Water outfalls in to the River Ouse in Ely, consideration of the

water level in the River Ouse during the rainfall event was important. The Environment Agency

provided modelled flood levels from the 25 year, 50 year 100 year and 200 year flood event.

Following discussions with the client and project team, it was agreed that all rainfall events were

modelled with the 25 year river level on the Ely Ouse without undertaking detailed joint

probability assessments. The levels were obtained from the CFMP Environment Agency Model,

Model Flood Group Reference: EA05225.

This approach was considered to be simpler, realistic and sufficiently conservative given the

large difference between the longer reaction time of the River Ouse catchment compared with

the much shorter critical storm duration of 60mins in Ely. For example, 200 year fluvial event

coinciding with a 200 year rainfall event was considered to be too conservative for the purpose

of SWMP preparation and option development process.

It was also worth noting that the prioritised wetspots areas within Ely are also generally located

on higher ground outside the main influence zone of Ouse water levels but some sensitivity runs

were undertaken to assess the likely impact of higher water levels in the Ouse and are

discussed in Section 6.5.



5 Wetspot Selection & Prioritisation

5.1 Approach

The principal purpose of a strategic assessment is to identify broad locations which are

considered more or less vulnerable to surface water flooding. These are then taken through to

an intermediate assessment. This chapter describes the selection and prioritisation of key areas

within Ely in line with the strategic and intermediate risk assessment phases. This stage

includes consultation with key stakeholders and analysis of historical flood records.

5.2 Stage 2 – Identification of Potential Wetspot Areas

During stakeholder meetings and site visits, with Cambridgeshire County Council Highways and

Anglian Water in April 2011, 3 wetspots within Ely were identified. The areas identified were

predominantly located in the North West of Ely close to the A10, around Silver Street, and

around St Johns Road. Site visits and further consultations were undertaken with

Representatives of the Ely Group of Internal Drainage Boards and East Cambridgeshire District

Council to confirm these wetspots.

Whilst the consultation allowed identification of Wetspots within Ely for the purpose of mitigation

options development, the resolution of the 2D mesh used generally allowed the same level of

analysis and mapping outputs production across the whole of Ely. This also enabled the full

consideration of influence of flood risk from the wider Ely catchment within the designated

priority wetspots without creating several detailed models covering smaller areas. The wetspot

boundaries are shown in Figure 5-1 and subsequent sections discuss further details.

Figure 5-1 – Ely Wetspots Determined Following Consultation Copyright and database right 2012.




6 Detailed Assessment

6.1 Stage 3 Detailed Modelled Development

The detailed model was developed to create an integrated terrain, river and sewers.

This chapter describes development of the detailed model, which also includes the underground

drainage network, to enable a detailed assessment across the study area whilst focussing on

the flood mitigation options development within the 3 prioritised wetspots in Ely.

6.1.1 Underground Drainage Network

Anglian Water provided their current sewer network in GIS format, including foul and storm

water sewers, as well as ancillary assets such as outfalls and pumping stations. However, as

described in Section 3.4.3 further consultation and survey were undertaken to fill in the

noticeable data gaps prior to constructing the detailed model.

The Ely storm network utilises existing drains and watercourses within the study catchment. To

the east of and North east of the catchment the storm network discharges in to the River Ouse

and the Clay Way Catchwater. The Clay Way Catchwater outfalls in to the Ouse approximately

2km North of Ely. These areas include the majority of the Commercial areas of Ely.

To the West the storm water network for the predominantly residential area outfalls to the Hurst

Lane Drove, flows from which are controlled by the Hurst Lane Drove Reservoir. The extent of

the sewer network is shown in Figure 6-1.

6.1.2 Coupling of Sewer Model

Once the sewer model was fully populated, it was incorporated into the existing bare earth

model and river model, which also includes parts of local drainage (e.g. Clayway Catchwater

and Hurst Drove systems) that was surveyed by Maltby Surveys to improve the representation

of the open channel within the urban area.

For the purpose of the Economic assessment, the InfoWorks CS model (and results) that

includes the underground drainage network, is referred to as the “Do Minimum” model (See

Section 7.3 for further detail); therefore, this terminology has also been used to describe this

detailed model from here onwards.



Figure 6-1 Existing Sewer Network Ely © Crown Copyright and database right 2012. All rights


6.2 Model Results Analysis

6.2.1 Wetspot 1 - North West Ely

The results from the 0.5% AEP Do Minimum Model run are shown in Figures 6-2 and Figure 6-3

to show how flows and depths propagate in this area.



Legend

Figure 6-2 – Depth of Flooding in North West Ely 0.5% AEP Rainfall Event © Crown Copyright and


Legend

Figure 6-3 – Speed of Flooding in North West Ely 0.5% AEP Rainfall Event © Crown Copyright and


These figures show how key flow routes enter this area from the east and south east, with some

overland flow along roads, and some across the school fields of the Ely Community College.

There is deep ponding (between 1-2m) of flood water close to the A10 due to the low lying

ground and road embankment.

The EA has advised that the area to the east of the A10 bypass is designed to be an area of

public open space low-land area that can take extreme event flooding as part of an early

strategic drainage scheme.



The EA has also advised that in Wetspot 1, the development area to the south of Tennyson

Place where the cover photo was taken also had an early Strategic Drainage Scheme agreed

with the EA and the IDB downstream of the A10 in the late 1990s.

6.2.2 Wetspot 2 – St Johns Road

Figure 6-4 and 6-5 show flows and depth in the area of St John’s Road. Flow along St Johns

Road show two distinct flow paths, one from Hereward Street/Debden Green, towards Ely City

Centre, the second from outside St Johns Community Primary School towards the outskirts of

Ely. Flow towards the City Centre appear to cause flooding above 0.3m in the area of St John’s

Farm and the Junction of St John’s Road and West End.

Legend

Figure 6-4 – Depth of Flooding around St John’s Road, Ely 0.5% AEP Rainfall Event © Crown Copyright and




Legend

Figure 6-5 – Speed of Flooding around St John’s Road, Ely 0.5% AEP Rainfall Event © Crown Copyright

and database right 2012. All rights reserved. Ordnance Survey licence number 100026380

6.2.3 Wetspot 3 – Silver Street

The area of Silver Street was highlighted during a meeting with representatives of

Cambridgeshire County Council Highways department. It is likely that overland flows in this

wetspot, may contribute to flooding issues around Silver Street. Figures 6-6 and 6-7 show

maximum depth and flow in the area during the 0.5% AEP rainfall event. As with the previous

two wetspots, overland flow routes along roads have an important role in flood risk during the

0.5% AEP rainfall event. There is significant ponding in the grounds of King’s School and also

on the road outside the school as a result of these overland flow paths.



Legend

Figure 6-6 – Depth of Flooding around Silver Street, Ely 0.5% AEP Rainfall Event © Crown Copyright and


Legend

Figure 6-7 – Speed of Flooding around Silver Street, Ely 0.5% AEP Rainfall Event © Crown Copyright and


6.3 Model Verification

Unfortunately, there is currently limited information in the form of historical evidence and no

historical flow monitoring to verify the IWCS model for Ely. The available information is in the

form of anecdotal reports and limited records of historic flooding. Based on this information the

model seems to produce representative outputs for the identified wetspots. However, further

information may be available in relation to existing drainage assets such as privately owned

drains, that was not made available or detailed to the project team at the time the modelling was

undertaken. Therefore, in limited areas, the model may not include facilities that serve to

protect the City and it’s environs that are in situ at present.



6.4 Model Assumptions

A number of assumptions have been made within the Ely model. These have been summarised

below.

- Where gaps existed in the AWS foul network dataset an assumed cover level, invert level

and/or pipe diameter was required. These gaps have been filled using automated inference

tools in IWCS.

- Where the AWS Impermeable Area Survey data coverage for Ely did not show which SW or

foul network a property drained to it was assumed that the property was drained by

soakaway.

- The model does not have any representation of the highway gully system. This information

was not readily available and would have been difficult to incorporate into IWCS. Therefore

there may be some surface water risk shown in the model extents which would in fact be

drained away through the gully system to some extent for the lower return period events.

- The model may not adequately represent privately owned assets such as those currently

owned by Church Commissioners or the assets that have been sold off by the Church

Commissioners over the years but the existing drains still are not formally adopted by

Anglian Water. Therefore, the existing drainage system may differently than currently

modelled if these data gaps were filled.

6.5 Model Sensitivity

The results of sensitivity testing of the model are included in Appendix I of the report and

discussed below.

6.5.1 Blockage Analysis

Following consultation with the Project Board on the Draft Report, the Environment Agency

highlighted the Cawdle Main Drain Culvert as a location where blockages may occur due to the

number of service crossings through the culvert. Blockages in the culvert were considered at

three separate locations, at the upstream end of the Culvert (Infoworks Link Location: Angel

Drove Bal Pond Ditch.1), halfway along the culvert underneath the A142 roundabout (Infoworks

Link Location TL54792651.1) and at the downstream end of the culvert just before the outfall in

to the River Ouse (Infoworks Link Location: TL54793751) and are shown in Figure 6-8. Each

blockage was run in a separate model (i.e. the blockages did not coincide) with a 50% blocked

cross-section.

The results showed that a blockage at the location Blockage 3 had the greatest impact on the

flood extents, particularly in the Gas Lane and Station Road area of Ely. All three blockage

scenarios increased the flood risk in the area of the Tesco superstore car park and in the

industrial estate to the south of the Tesco site at Bartholemew’s Walk and St Thomas’s Place.

These results suggest that further investigation into the capacity of this culvert may be

necessary, particularly if the adjoining area and the area downstream of the existing culvert are

targeted for future development.



Figure 6-8 Locations of Blockage Locations on Angel Drove Main River Culvert



6.5.2 River Levels

The 200 Year Do Minimum Model was run using the 100 Year Modelled Flood Level in the

Ouse. This test was performed to test the sensitivity of the 1D network to the likely ‘outfall

surcharging’ effect of higher water levels in the Ouse.

There was a local impact of the using the 1 in 100 year Flood Level in the Do Minimum model.

However the overall impact on the flood outline (at depths above 0.1m) was a less than 1%

increase in flooded extent.

6.6 Model Outputs

6.6.1 Flood Depth, Velocity and Hazard Maps

Flood depth, velocity and flood hazard mapping has been produced based upon the detailed

model for 1 in 30 (3.33% AEP) and 1 in 200 year (0.5% AEP) return periods using a storm

duration of 60 minutes. The mapping is included within Appendix F and Appendix H includes

additional maps associated with the various options discussed in subsequent sections.

Flood hazard are important factors in the assessment of flood risk and evacuation of the general

public. Three categories of flood hazard have been identified in the DEFRA / Environment

Agency Documents: Flood Risk Assessment Guidance for New Development20

, (DEFRA Report

FD2320) and Flood Risks to People Methodology21

(DEFRA Report FD2321). These are

“Danger for All”, “Danger for Most” and “Danger to Some”. The equation below gives the

relationship between hazard, depth, velocity and debris:

H = (v+0.5) x d +Df Where

H = hazard

v = velocity

d = depth

Df = debris factor

Df = 0.5 for d < 0.25m

Df = 1.0 for d > 0.25m

The mapping presented in the SWMP has been based upon the following thresholds, taken

from DEFRA Report FD2320. However it should be noted that DEFRA Report FD2321 places a

different hazard rating of the transition to Category 3. The FD2320 indicates that the change

occurs at 2.0 whereas the FD2321 report indicates that this happens at 2.5. This has a

significant impact on the interpretation of the results for the SWMP which are discussed below

but it should be noted that the results are presented conservatively as set out below.

Danger to Some Category 1 H > 0.75

Danger to Most Category 2 H > 1.25

Danger to All Category 3 H > 2.00

The colouring of the flood hazard mapping is commensurate with the hazard categorisation

given in Figure 6-9. Areas coloured red are considered dangerous for all; areas in dark yellow

are dangerous to most; light yellow is dangerous to some and blue areas are inundated areas

mainly on the margins of the flood plain which are considered to hold little hazard. The time



series graphs show the depth (left axis) and hazard category (right axis) for specific control point

locations as discussed above.

Figure 6-9 Hazard Categorisation

0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.5 2.0

0

0.10

0.25

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

Danger for Some

Danger for Most

Danger for All

Velo

cit

y o

f F

loo

din

g (

m/s

)

Depth of Flooding (m)



7 Engineering Options Identification & Assessment

7.1 Measures Identification

The engineering elements evaluated in this section are based upon employing the most

appropriate techniques for the various sites. The engineering elements proposed within this

section fall into a range of categories as shown in Figure 7-1 and where possible and

economical the use of sustainable drainage systems (SuDS) and surface water reduction

strategies has been promoted over hard infrastructure alternatives such as the upgrading of

existing sewers.



Figure 7-1 Surface Water Flood Mitigation Options

The key constraints (see Figure 7-2) associated with the implementation of all of the options are

space and cost.

Figure 7-2 Engineering Options Constraints

Accordingly, the engineering options proposed within the report have been designed to be

accommodated within the urban environment.

It should be noted that the engineering options proposed are potential solutions to current

issues and priorities. During the course of the SWMP time frame, it is possible that these issues

or priorities may change and new constraints and priorities may present themselves. The

Installation of

Source Control

Measures

Upgrading

Sewer

Infrastructure

Installation of

new sewers

Underground

Attenuation

Separation of

Foul & Surface

Up-grading of

Existing Sewers

Wetland / Basin

Attenuation Peripheral

Protection

Retrofitting of

SuDS Systems

Overland Flood

Routing

Installation of

Source Control

Measures

Upgrading

Sewer

Infrastructure

Disruption during

Construction

Capital Cost of

Major

Infrastructure

Timing of

Investment

Space within the

Urban

Environment

Maintenance

Costs

Maintenance of

Green

Infrastructure

Adoption by

Operating

Authorities

Space within the

Urban

Environment



options may, therefore, be difficult to implement, and it should be borne in mind that the

engineering works for some options are proposed over a long period.

Across Ely, there are several open spaces which can be utilised for attenuation. Attenuation

has been explored at several locations with the introduction of attenuation basins, wetlands and

ponds and there has been consideration of the use of swales where possible.

After a desk based visual inspection of key flow routes in the study area, open spaces at a

number of schools and recreation areas have been investigated as potential sites for

attenuation structures. It should be noted however that other pressures such as the need to

expand and improve existing school sites may be contrary to using school open spaces in flood

mitigation works. New developments may however offer alternative opportunities for partnership

working, such as utilising green roofs in new school developments.

The street environment and conservation issues in Ely are also significant constraints in the

installation of new drainage infrastructure. Within the wetspots possible techniques including

road side rain gardens and improvements to kerb drainage systems are discussed in detail in

the following sections.

7.2 Ely Engineering Measures and Options

This section of the report considers the engineering elements and the option combinations for

the mitigation of surface water flooding in Ely. The engineering elements and option

combinations considered in this document have been developed from work undertaken during

the course of the project. The majority of engineering elements proposed are targeted at

alleviating flood risk within the 3 wetspots identified during Stage 2 of the intermediate

assessment.

The hydrological and hydraulic, flood risk and economic analyses have allowed the options to

be developed further in order to compare the proposed schemes in terms of cost and technical

suitability.

Cambridgeshire County Council, the Lead Local Flood Authority under the Floods and Water

Management Act, has powers to carry out works for the management of surface water run-off,

ordinary watercourses and groundwater.

Table 7-1 gives a summary description of the engineering elements and Figures 7-3 to 7-6

provides the location of each element. The engineering elements have been combined to form

the option combinations, which have been modelled and analysed, to evaluate their technical

suitability and economic benefits to select a preferred engineering option. The Option

Combinations is included in Section 7-4.

Table 7.1 Ely Engineering Elements

Engineering

Element

Engineering Element

Name Description

Engineering

Element

Alleviating Flood

Risk in Wetspot

ELY-A Highfield Special School – Attenuation Basins

Construction of two attenuation basins at Highfield School.

Wetspot 1

ELY-B City of Ely Community College – Attenuation Basins

Construction of four attenuation basins in the ground of Ely Community College and associated bund.

Wetspot 1

ELY-C St Audrey’s Way – Network Improvement

Construction of a speed hump to hold back water and an additional entrance

Wetspot 1



Engineering

Element

Engineering Element

Name Description

Engineering

Element

Alleviating Flood

Risk in Wetspot

and Speed Hump point to the existing sewer network at St Audrey’s Way.

ELY-D A10 Attenuation Basin

Increasing attenuation storage within the existing public open space alongside the eastern side of A10 near Columbine Road.

Wetspot 1

ELY-E Abbots Way – Attenuation Basin

Construction of two interconnected attenuation basins and associated overflow spillway at Abbots Way Recreation Ground.

Wetspot 1

ELY-F Priors Court – Network Improvement

Extension to the existing surface water network in Priors Court to provide an additional entry point to the existing sewer network and an increase in pipe size diameter of 65m of storm network in Priors Court.

Wetspot 1

ELY-G Priors Court – Swale Swale constructed to the east of Priors Court to intercept overland flows.

Wetspot 1

ELY-H Dunstan Street – Rain Gardens & Speed Humps

Construction of two speed humps at the entrance to Priors Court and the new Housing Development off Dunstan Street to hold back water and route overland flow in to two attenuation basins on Dunstan Street.

Wetspot 1

ELY-I

Columbine Road - Beanie Beany Block Curbs Kerbs & Rain Garden

Replacement of existing pavements with Beanie Beany Blocks designed to take intercept overland flow and route into existing drainage network. Construction of Rain Garden.

Wetspot 1

ELY-J West Fen Road – Rain Gardens

Construction of two rain gardens to capture overland flow on West Fen Road.

Wetspot 1

ELY-K Beresford Road – Attenuation Basins

3 Attenuation Basins constructed along Beresford Road to complement the existing drainage ditch.

Wetspot 1

ELY-L Hertford Close – Attenuation Basin

Construction of one attenuation basin on at Hertford Close.

Wetspot 2

ELY-M Longfields – Rain Gardens

Construction of two rain gardens to capture overland flow at Longfields.

Wetspot 2

ELY-N Pilgrim’s Way – Rain Gardens

Construction of two rain gardens to capture overland flow at Pilgrim’s Way.

Wetspot 2

ELY-O Witchford Road – Attenuation Basin

Construction of one attenuation basin on at Witchford Road.

Wetspot 2

ELY-P St Johns Road – Attenuation Basin

Construction of one attenuation basin and associated Bund in the grounds at St John’s Farm, Ely. This option also includes lowering a curb kerb to route flow into the Attenuation Basin.

Wetspot 2

ELY-Q Arundel – Attenuation Basin

Construction of one attenuation basin and associated Bund in the grounds at St John’s Farm, Ely. This option also includes construction of a speed hump and lowering a curb kerb to route flow into the Attenuation Basin.

Wetspot 1



Engineering

Element

Engineering Element

Name Description

Engineering

Element

Alleviating Flood

Risk in Wetspot

ELY-R Ely Community College – Channel Widening

Channel widening of a drainage ditch close to Ely Community College, this option also included construction of a speed hump to route flow into the widened ditch.

Wetspot 1

ELY-S King’s School – Attenuation Basins

A series of 3 attenuation ponds in the grounds of King’s School (this option extends on existing pond in the King’s School grounds). The ponds are linked by 2 pipes totalling.

Wetspot 3

ELY-T Ely Rain Gardens (177 Rain Gardens)

Utilisation of existing green areas alongside roads that were shown to be situated in key flow routes in the Ely Bare Earth (Do Nothing) model.

All

ELY-U Ely Community College – Channel Widening

ELY-R Optimised - Channel widening of a drainage ditch close to Ely Community College excluding the construction of a speed hump.

Wetspot 1

ELY-V Highfield Special School – Attenuation Basins

ELY-A Optimised – Depth of 1x Attenuation structure dropped by a further 0.1m

Wetspot 1

ELY-W City of Ely Community College – Attenuation Basins

ELY-B Optimised – Attenuation basin depth increased by 0.1m

Wetspot 1

ELY-X Dunstan Street – Rain Gardens & Speed Humps

ELY-H Optimised – 1x Road Attenuation structure dropped by a further 0.5m.

Wetspot 1

ELY-Y

Columbine Road - Beanie Beany Block Curbs Kerbs & Rain Garden

ELY-I – Optimised – Rain garden depth dropped by a further 0.25m

Wetspot 1

ELY-Z Beresford Road – Attenuation Basins

ELY-K Optimised – 2x Attenuation basin depth increased by 0.2m in total.

Wetspot 1

ELY-AA Hertford Close – Attenuation Basin

ELY-L Optimised – Attenuation basin depth increased by 0.2m.

Wetspot 2

ELY-AB Longfields – Rain Gardens

ELY-M Optimised – Depth dropped on 2x Rain garden by a further 0.15m

Wetspot 2

ELY-AC Pilgrim’s Way – Rain Gardens

ELY-N Optimised – Depth dropped on 2x Rain garden by a further 0.15m

Wetspot 2

ELY-AD St Johns Road – Attenuation Basin

ELY-P Optimised – Depth of attenuation basin dropped by further 0.1m

Wetspot 2

ELY-AE Arundel – Attenuation Basin

ELY-Q Optimised – Bund height increase by 0.2m.

Wetspot 1

ELY-AF King’s School – Attenuation Basins

ELY-S Optimised - Depth of 2x attenuation ponds dropped by a further 0.6m in total.

Wetspot 3

Figure 7-3 to 7-5 shows the Engineering Options Combinations modelled to reduce surface

water flood risk in the 3 Ely Wetspots.



Figure 7-3 Wetspot 1 Engineering Elements Ely A – C, Ely Q - R © Crown Copyright and database

right 2012. All rights reserved. Ordnance Survey licence number 100026380

Figure 7-4 Wetspot 1 Engineering Elements Ely-D to K © Crown Copyright and database right 2012.




Figure 7-5 Wetspot 2 Engineering Elements Ely-L to P © Crown Copyright and database right 2012.


Figure 7-6 Wetspot 3 Engineering Elements Ely S © Crown Copyright and database right 2012. All

rights reserved. Ordnance Survey licence number 100026380



7.3 Ely Engineering Option Combinations

In order to address flooding within Ely and for the purposes of the SWMP, combinations of

options have been developed. These have been tested for their effectiveness of reducing

flooding in each wetspot.

The engineering elements described in Section 7.2 have been combined into Option

Combinations which have been evaluated for the purposes of the SWMP. These combinations

reflect the predicted flooding mechanisms described above with the objective of determining

their technical suitability and whether there is an economic case for the mitigation of surface

water flooding in all 3 wetspots.

The Option Combination Elements were combined into ‘Do Something’, which includes ‘Do

Minimum’ and other Option Combinations C1, to C3, as shown in Table 7-2 overleaf.

All Option Combinations are listed below:-

� Do Nothing – The "Do Nothing" option assumes that no maintenance, clearance or other

intervention is made to interfere with the natural fluvial processes or sewer network (See

Section 4.1.3). The evaluation of the "Do Nothing" option is a technical requirement

required by the Treasury in order to enable comparisons to be made between the "Do

Minimum" and "Do Something" options.

� Do Minimum– Maintenance of the existing storm sewer, ordinary watercourse and

highway drainage including, gully cleaning, jetting, removal of debris / vegetation;

treeworks and periodic removal of deposition and sediments.

� Option Combination C1 – This comprises all the possible engineering options included

to intercept overland flows and based on the results of the Do Nothing and Do Minimum

runs.

� Option Combination C2 – This option assess the benefit of utilising existing small road

side green areas for conversion in to rain gardens across Ely.

� Option Combination C3 – This option represents an optimised version of Option

Combination C1. Optimisation was achieved via utilising and improving the most cost-

effective engineering elements and removing the least cost-effective engineering

elements from the Option Combination C1.



Table 7-2 Option Combinations – Ely

Engineering

Element

C1 C2 C3

ELY-A ��

ELY-B ��

ELY-C ��

ELY-D ��

ELY-E ��

ELY-F ��

ELY-G ��

ELY-H ��

ELY-I ��

ELY-J ��

ELY-K ��

ELY-L ��

ELY-M ��

ELY-N ��

ELY-O ��

ELY-P ��

ELY-Q ��

ELY-R ��

ELY-S ��

ELY-T ��

ELY-U ��

ELY-V ��

ELY-W ��

ELY-X ��

ELY-Y ��

ELY-Z ��

ELY-AA ��

ELY-AB ��

ELY-AC ��

ELY-AD ��

ELY-AE ��

ELY-AF ��



8 Economic Appraisal

8.1 Introduction

The engineering elements described in Section 7 have been combined to form flood risk

mitigation option combinations shown in Table 7-2. These option combinations have been

assessed in relation to whole life costs, flood damages and residual damages in accordance

with the methodology contained in the following documents.

� Flood and Coastal Erosion Project Appraisal Guidance (FCERM-AG Manual)22

.

� The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment

Techniques23

The latter document is also known as the Multi-Coloured Handbook, and is based on the Multi-

Coloured Manual (MCM) originally published in 2005. The evaluation of the residual risk of

flooding has been discussed in the relevant sections associated with the option combinations.

The residual flood risk damages relating to the “Do Something” options have been discounted

annually to enable a direct comparison with other options. A variable discount rate, beginning at

3.5%, has been adopted for the cost benefit analysis, in line with HM Treasury guidance.

A value engineering exercise was undertaken to evaluate the Option Combinations which

indicated that a significant factor in the costs of works was associated with the excavation and

disposal to landfill of materials for the formation of attenuation basins and other flood mitigation

infrastructure. Accordingly the economic analysis assumes that all excavated materials will be

re-used on site to avoid the cost of disposal of the material. This could include the formation of

embankments and other landscaping features. This avoids costs associated with disposal

including land fill tax. It also promotes the sustainable credentials of the project by reducing the

carbon footprint associated with the transportation of materials for disposal.

Whilst an optimism bias of 60% has been applied to all of the cost estimates (as per the current

guidance applicable for a strategy of this nature) there are a number of economic risks or

uncertainties associated with the development of the cost estimates. The principal economic

risks associated with the preferred option are:

� Cost associated with dealing with utilities which have not been itemised in the cost

estimates

� The cost of land negotiations and compensation for disruption

� Agreement with local residents to implement/contribute towards the costs of property

level measures

It is therefore considered that significant effort should be placed into obtaining agreements with

landowners and stakeholders to undertake the proposed works. In order to mitigate this risk it is

recommended that CCC and the Cambridgeshire Flood Risk Management Partnership enter into

discussion with all landowners and stakeholders at the earliest opportunity during the design

process to ensure their collaboration. A review of the service/utilities locations during pre-

feasibility into the scheme would help to identify their impact on scheme costs.

8.1.1 Methodology – Damages Assessment

The assessment of cost associated with flood damage of properties in Ely has been assessed

using the DEFRA and Environment Agency approved approach outlined in the Multi- Coloured

Handbook. This method for assessing damages uses depth/damage curves based on property



type, age and social class of the dwellings occupants, in order to evaluate the overall damage

avoided in a flood risk area.

For the preferred option, flood depth results for each return period were extracted for all

properties within the modelled region. With respect to the flood depth, damages result from the

physical contact of flood water with damageable property.

National Property Dataset (NPD) has been used to form the basis of the damages assessment.

In order to calculate flood damages at a property level, the following information is required for

each property includes:

� A property ‘type’

� A floor area (for non-residential property)

� A property threshold

The NPD dataset used for this study does not provide the property threshold level therefore

LiDAR inclusive of an additional 0.1m was used to determine the threshold level of each

property. By applying flood depths to each property (adjusted to account for threshold), the flood

damages were calculated using the depth-damage curves for each individual event. Annual

Average Damages (AAD) were then calculated for each scenario: ‘Do Nothing’, ‘Do Minimum’

and ‘Do Something’ preferred option.

Depending on the size or severity of each individual flood event of a given annual probability,

each flood event will cause a different amount of flood damage. The Average Annual Damage

(AAD) is the average damage per year in monetary terms that would occur at each specific

address point, within the modelled domain, from flooding over a 100 year period, assuming that

present-day conditions (in terms of frequency of extreme rainfall) are maintained.

In many years there may be no flood damage, in some years there will be minor damage

(caused by small, relatively frequent floods) and, in a few years, there may be major flood

damage (caused by large, rare flood events). Estimation of the AAD provides a basis for

comparing the effectiveness of different flood alleviation and management measures (i.e.

through measuring the reduction in AAD).

The methodology for assessing the benefits of flood alleviation combines:

� An assessment of risk, in terms of the probability or likelihood of future floods to be

averted; and

� A vulnerability assessment in terms of the damage that would be caused by those floods

and therefore the economic saving to be gained by their reduction.

Through assessment of the associated damage values and the benefits incurred through

Engineering Options, proposed schemes are compared against each other using their benefit-

cost ratio (BCR).

To identify a preferred option, a comparison was undertaken between the consequences of ‘Do

Something’ Engineering Options against the baseline option, ‘Do Minimum’. The SWMP Project

Board requested that the ‘Do Minimum’ scenario was used as the baseline for the economic

analysis as it assumes a continuance of existing maintenance of the sewer and local drainage

network. The Board felt that the ‘Do Nothing’ scenario was unrealistic as the water companies

have a statutory right to maintain public sewers.

The cost of each option combination and the relative damages incurred are combined to create

a benefit cost ratio. This ratio is used to assess the viability of the preferred option and also the

levels of effectiveness for how capital can be spent to protect against the effects of flooding. The



BCR is the ratio of benefits produced through introduction of flood alleviation options, expressed

in monetary terms, relative to its cost, identifying the greatest ‘value for money’.

The Multi-Coloured HandbookError! Bookmark not defined.23

states that;

‘Projects are only viable if the benefits exceed the costs (i.e. the ratio of benefits to costs is

greater than 1.0). Where benefits marginally exceed costs, there is often high uncertainty as to

whether an option is justified, because only a small change or error in either the benefits or

costs would tilt the balance the other way. So when comparing a ‘Do Something’ option to the

baseline option, confidence is needed that a ‘Do Something’ option is clearly preferable.’

In this regard, the decision process explored whether the best value for money is provided while

achieving the most appropriate standard of risk management defence. This is undertaken by

assessing the incremental benefit-cost ratio of each economically viable option.

Key Parameters

For reference, key parameters which have guided the economic assessment process, in line

with FCERM-AG/MCM techniques, are repeated below:

• Property Values: Properties were assigned a market value in order that present value

damages (PVd) were ‘capped’ if they exceeded a property’s market value over the

appraisal period. These ‘capping values’ were derived according to Environment

Agency best practice. Distributional impacts (DI) were considered, in order to remove

social class bias from the property value estimates. A DI factor was calculated using

Approximate Social Grade (UV50) data for the relevant Cambridgeshire authority,

available from neighbourhood.statistics.gov.uk.

• Emergency services costs: These were incorporated in the assessment by adding

5.6% to all calculated property damages. This is as stated in the Multi-coloured

Handbook and is based on data from the 2007 floods, a revision downwards from the

previous values of approximately 10%, reflecting economies of scale of providing

emergency services in urban areas during flood events.

• Temporary accommodation costs: These were incorporated in the assessment by

allowing for an average rental cost, post-flood, of £5.7k per property flooded,

determined in an Environment Agency review of the summer 2007 floods.

Section 8.2 highlights the key assumptions made in damage assessment (including limitations

and further recommendations) and specific attention should be given to these prior to using the

currently published SWMP economic assessment outputs. Section 8.3 also summarises key

exclusions from the damage assessment.

8.1.2 Economic Risks

The principal economic risks are associated with the construction of all Engineering Options

are:-

� Cost of possible diversion of utilities;

� Cost of land negotiations

� Compensation for disruption

� Buildability

� Ecolological and other environmental risks and associated costs



� Sensitivity of flood damage assessment and model limitations (e.g. actual property

threshold levels and flood levels – see section 8.2 and 6.3)

It is recommended that as part of Business Case Justification/ Project Appraisal Reporting

process and the subsequent detailed design of the flood mitigation proposals the above risks are

addressed accordingly. The project lead should approach utility companies to obtain agreements

for the relocation of services as necessary. In addition the project lead should engage with all

landowners and stakeholders at the earliest opportunity during the design process to ensure

their collaboration.

8.1.3 Cost Estimates

Each of the proposed Option elements has been costed in accordance with information on

maintenance expenditure obtained from Cambridge City Council and Cambridgeshire County

Council and SPON’s Civil Engineering and Highways Price Book24

.



8.1.4 Benefit Cost Analysis

Table 8-1 summarises the Present Value Damages associated with the ‘Do Nothing’, ‘Do

Minimum’, Option C1, C2, and C3 for each of the wetspots.

North-West Ely –

Wetspot 1

St Johns Road –

Wetspot 2

Silver Street –

Wetspot 3

Option

Combination

Present Value Damages (£)

Do Nothing 32,546,000 6,655,000 32,628,000

Do Minimum 26,690,000 5,542,000 31,840,000

Option C1 22,802,000 5,072,000 31,563,000

Option C2 22,762,000 5,069,000 31,264,000

Option C3 22,113,000 5,044,000 31,346,000

Table 8-1 Flood and Residual Flood Damages

Based upon the assessment of damages and the cost estimates given for each option

combination, the present value damages have been combined with the whole life cost estimates

within Table 8-2 to 8-5. These tables summarises the costs, benefits and residual damages

associated with each option.

Standard routine costs for maintenance of the existing drainage assets have not been included

in the appraisal since they are not funded via capital funding scheme grants and would be

commensurate for all options.



A B C D E

Costs and benefits £

Do

Nothing

Do

Minimum

Option

C1

Option

C2

Option

C3

1 PV costs from

estimate 964,543 224,500* 931,772

2 Optimism bias

adjustment

(C1*0.6) 578,725 134,700* 559,063

3 Total PV Costs

from appraisal

(PVc)

(C1+C2) 1,543,268 359,200* 1,490,835

4 PV damage (PVd *) 32,546,000 26,690,000 22,802,000 22,762,000* 22,144,000

5 PV damage

avoided

(B4-C4) 3,888,000 3,928,000* 4,546,000

6 Total PV benefits

(PVb)

(=C5) 3,888,000 3,928,000* 4,546,000

7 Net Present Value

(NPV)

(C6-C3) 2,344,731 3,568,800* 3,055,165

8 Average benefit cost

ratio

(C6/C3) 2.52 2-10* 3.05

*See Section 8.4 for further discussion on Option C2 Cost Benefit Ratio

Table 8-2 Summary of Costs and Damages for North West Ely – Wetspot 1

Based on the benefit cost ratios calculated above Option C3, the optimised engineering options

solutions is shown to be the most economically viable solution for the North-West Ely wetspot.

This highlights the importance of optimising engineering options, even at the strategic stage of

the assessment. However, the model results for Option C3 has shown a slight increase in water

levels (up to 30mm - compared to Do Minimum results) in Kent Close area where it appears to

have suffered from previous flooding based on the recent feedback received during the

finalisation of this report. The slight increase in water levels is likely to be due to the proposed

two speed humps at the entrance to Priors Court as part of Engineering Element ELY-X. Based

on the examination of model results for the Option C1, this impact can be successfully mitigated

by reintroducing the Engineering Element ELY-G (Priors Court – Swale) to the preferred option.

Alternatively, the speed humps may be simply removed but this may increase water levels again

in Priors Court area slightly reducing the effectiveness of the proposed measure ELY-X.



A B C D E


Do

Nothing

Do

Minimum

Option

C1

Option

C2

Option

C3

1 PV costs from

estimate 129,298 67,350* 58,345

2 Optimism bias

adjustment

(C1*0.6) 77,578 40,410* 35,007

3 Total PV Costs

from appraisal

(PVc)

(C1+C2) 206,876 107,760* 93,352

4 PV damage (PVd *) 6,655,000 5,542,000 5,072,000 5,069,000* 5,044,000

5 PV damage

avoided

(B4-C4) 470,000 473,000* 498,000

6 Total PV benefits

(PVb)

(=C5) 470,000 473,000* 498,000

7 Net Present Value

(NPV)

(C6-C3) 263,123 365,240* 404,647

8 Average benefit cost

ratio

(C6/C3) 2.27 2-10* 5.33

*See Section 8.4 for further discussion on Option C2 Cost Benefit Ratio

Table 8-3 Summary of Costs and Damages for St Johns Road

Based on the benefit cost ratios calculated above Option C3, the optimised engineering options

solutions is shown to be the most economically viable solution for the St Johns Road wetspot.

This highlights the importance of optimising engineering options, even at the strategic stage of

the assessment.



A B C D E


Do

Nothing

Do

Minimum

Option

C1

Option

C2

Option

C3

1 PV costs from

estimate 18,523 157,150* 48,996

2 Optimism bias

adjustment

(C1*0.6) 11,113 94,290* 29,397

3 Total PV Costs

from appraisal

(PVc)

(C1+C2) 29,636 251,440* 78,393

4 PV damage (PVd *) 32,628,000 31,840,000 31,563,000 31,264,000* 31,346,000

5 PV damage

avoided

(B4-C4) 277,000 576,000* 494,000

6 Total PV benefits

(PVb)

(=C5) 277,000 576,000* 494,000

7 Net Present Value

(NPV)

(C6-C3) 247,364 324,560* 415,607

8 Average benefit

cost ratio

(C6/C3) 9.35 2-10* 6.30

* See Section 8.4 for further discussion on Option C2 Cost Benefit Ratio

Table 8-4 Summary of Costs and Damages for Silver Street

Based on the benefit cost ratios calculated above Option C1, the pre-optimised engineering

element option, is shown to be the most economically viable solution for the Silver Street

wetspot. The Optimised Version option considered also significantly increased the flood

damages avoided and has a good benefit cost ratio and therefore further optimisation may be

possible by utilising the outputs from the Model option C1 and Model Option C3. This can be

done at the Project Appraisal and Action Plan Preparation stages.

8.1.5 Funding Sources – Flood Defence Grant in Aid

From 2012/13, the amount of Flood Defence Grant in Aid (FDGiA) available from the

Government (through Flood and Coastal Partnership Resilience Funding) to any surface water

alleviation capital scheme will directly relate to the number of households protected, the

damages avoided, and the wider benefits of a project. As part of implementing this new funding



policy, revised Outcome Measure definitions have been agreed to replace the previous suite of

measures that expired in March 2011. The Outcome Measures are detailed in the list below:

• Outcome Measure 1 – Economic Benefits

• Outcome Measure 2 – Households at risk

• Outcome Measure 2b – Households at very significant and significant risk

• Outcome Measure 2c – Deprived households at very significant and significant risk

• Outcome Measure 3 – Households at risk from coastal erosion

• Outcome Measure 3b – Households at risk from coastal erosion in 20 years

• Outcome Measure 3c – Deprived households at risk from coastal erosion in 20 years

• Outcome Measure 4a – Hectares of water dependent habitat created or improved

• Outcome Measure 4b – Hectares of intertidal habitat created

• Outcome Measure 4c – Kilometres of rivers protected under the EU Habitats/Birds

The key data from the Engineering Options that will be required for completion of the Flood

Defence Grant in Aid Calculator for each individual wetspot will be available to CCC through

SWMP outputs.

8.2 Damages Assessments - Assumptions

Assumption 1 – Property thresholds across the study catchment are 0.1m and no

flooding of properties will occur below this 0.1m threshold. Due to the number of

properties across the study area it would not be possible to estimate threshold levels for each

property. As such an assumption of a threshold level of 0.1m at all properties has been made.

Furthermore it has been assumed that no damage occurs to property when the flood level at the

property is between 0 - 0.1m (below the threshold). It is possible that flood water can still enter

properties below the threshold level via airbricks but this is not considered in this damages

assessment.

The 0.1m property threshold level is a standard assumption that a property will have a small

step on entry however this threshold may result in an overestimation in the number of properties

at risk and the level of damages. Sensitivity analysis has been undertaken to assess the impact

of raising the property threshold to 0.2m and 0.3m respectively in the economic analysis.

By increasing the threshold level to 0.2m in the economic analysis there was a 79% average

reduction in the number of flooded properties across the 3 wetspots. With a threshold level of

0.3m set there was a 90% average reduction in the number of flooded properties across the

wetspots. Wetspot 2 showed the most significant difference in flooded properties with 93% of

the properties within the wetspot being eliminated from analysis when the threshold level was

increased to 0.3m.

This sensitivity testing has highlighted the importance of establishing accurate property

threshold levels to ensure that the most appropriate and effective options can be identified.

Therefore, doorstep surveys of sample of impacted properties are strongly recommended so

that a representative assumption can be taken about property thresholds as part of further

detailed Business Case Justification/ Project Appraisal Reporting for each wetspot.



Assumption 2 – Damage to property does not occur at return periods lower than 20 year.

The lowest return period modelled was the 20 year Rainfall Event. Whilst it is possible within

the flood damages equations to interpolate flood damages for return periods below the lowest

return period modelled, these damages are not based on any modelled outputs and as such are

subject to significant uncertainty. Furthermore, since they occur more frequently within the

appraisal process, they have a disproportionate impact on present value damages. As such,

and in keeping with the approach set out in FCERM-AG, it has been assumed that no damages

occur to property within the study area at flood events lower than the 20 year return period.

Assumption 3 – Maximum depth extracted from model results is representative of flood

depth at properties. The worst case depth at each property is captured the maximum depth of

flooding at each property has been currently used in the economic analysis. However in some

cases the maximum depth extracted may have been exaggerated by anomalies in the

underlying LiDAR. In some cases this can lead to an overestimation in the level of damages

recorded at a property. Also, the use of average flood depth at the property may have a

significant impact on the calculated flood damages.

A sensitivity test has been undertaken using average flood depth at each property (available

from the 2D model results). This test suggests that total PV Damages for each wetspot for each

modelled scenario could be reduced by 91% on average.

A further sense check has been undertaken at the top ten properties that contribute most to the

damage assessment for each wetspot to ensure that the assumptions used in the economic

analysis are not skewing results at a handful of key properties.

Assumption 4 – Raw modelled outputs have been used to calculate flooding at

properties. The modelled outputs have not been post-processed to remove small isolated

patches of flooding. This may result in properties being identified as flooded when in reality the

model is ponding in an anomalous low spot in the underlying LiDAR. The Environment

Agency’s current surface water mapping was processed to remove such small flooded areas.

However, it should be noted that the EA mapping was carried out on a coarser scale ground

terrain than the detailed surface water modelling described in this report.

Sensitivity testing showed that removing such small areas from the SWMP flood outlines

removed approximately 40% of impacted properties from the assessment of flood risk for the

200 year Do Minimum event scenario.

8.3 Damages Assessment – Exclusions

The following key items were excluded from the assessment:

Risk to life: whilst all flooding poses a risk to life, it can be argued that the nature of the

widespread surface water flooding such as is assessed in this study limits maximum depths and

velocities such that overall risk to life is low. Furthermore, its calculation for a large study area

could require appraisal time that would be disproportionate to the scale of benefits expected.

Transport disruption: flooding in a populated urban area has the potential for significant

impact of transport networks, which can add to the economic impact of flooding. Although

surface water flooding is frequently associated with transport disruption, it is not practical to

assess, on the scale of this study, the sort of alternative routes and diversions required. Since

these are unlikely to result in significant benefits in comparison to property damages, it is

recommended that assessment of this is left until further appraisal stages.

Environmental benefits: no accounting has been made for the potential environmental/amenity

improvements associated with any of the proposed options.



Health and social benefits: these perceived benefits attributable to undertaking flood

prevention works and increasing health and well-being were not included. This view was taken

because it was considered unlikely that the local population would necessarily perceive any

benefit from a form of flooding which does not result in a noticeable flood pathway or a great

depth of flooding (as would be the case for river or sea flooding).

8.4 Option C2 – Rain Gardens

Option C2 focused on identifying small land parcels that could be potentially converted into

roadside rain gardens across the Ely study area through a high level assessment. These rain

gardens are currently located both within and outside the three wetspot areas. The total cost of

the rain gardens was estimated to be £2.5m. Where rain gardens fall outside wetspot

boundaries it was not possible to fully separate out which rain garden was actually providing

benefit to which wetspot. Therefore, currently there is some uncertainty in the benefits and cost

estimates given in tables 8-2 to 8-4 under each wetspot with this option. As such, the cost-

benefit ratio is not a definitive value but based on the upper and lower values of cost-benefit

ratios calculated for this option across the three wetspots.

It should be noted that this model option was run mainly to assess how small parcels of land

could be used to provide flood mitigation, should acquisition of land for larger flood relief

schemes prove difficult or prohibitive. The results indicate that further consideration to these rain

gardens may be given during the detailed design stage as part of the SWMP Action Plan for

these wetspots if necessary.



9 Summary

In order to address the specific issues relating to the Ely SWMP, a three stage modelling

strategy was developed and implemented:

� Stage 1 - Hydrological analysis and development of broad scale, bare earth and river

model of Ely (see Section 4.3).

� Stage 2 – Evaluation of preliminary results and identification and prioritisation of wetspots

using the bare earth model developed in Stage 1 through local stakeholder consultation

and Historic Flood Record Analysis (see Section 5).

� Stage 3 - Detailed modelling assessment of the identified priority wet-spots within Ely.

Creation of detailed modelling including the interaction between the ground terrain, river

and sewer network (see Section 6). This includes the development and testing of

engineering options and economic analysis (See Sections 7 and 8).

The SWMP direct rainfall bare earth modelling and review of historical data have improved the

understanding of future surface water flood risk within Ely at a strategic level.

The detailed modelling has defined the surface water flood risk across Ely. The model results

have substantially refined the extent of surface water flooding from the Environment Agency

AStSWF and FRM4SWF and been verified where possible using available historical data.

A range of potential engineering measures and options have been identified, modelled and

costed for Ely, which highlight the need and benefit of reducing the future flood risk. These

engineering options should be considered along with non engineering policy measures in order

to maximise benefits. Funding constraints and stakeholder buy-in are likely to be a key obstacle

to implement catchment wide solutions in Ely, highlighting the need for further stakeholder

consultation and prioritisation of viable measures.

Whilst the ‘Do Minimum’ Option is effective at reducing flood damages when compared with a

theoretical ‘Do Nothing’ scenario, the ‘Do Minimum’ option does not deliver any reduction in the

number of properties vulnerable to flooding and will not address increasing flood risk associated

with climate change. This is a critical factor in relation to adopting a strategy to deal with climate

change within the city.

The suitability of reliance only in the ‘Do Minimum’ option is also questionable in terms of new

duties imposed by the Flood and Water Management Act, social and environmental acceptance

and future uncertainty. This clearly highlights the need for further consideration and

implementation of a broad strategy, including the refinement of engineering associated with

Option Combinations (to optimise the benefit cost ratios), Quick Wins and Policy Initiatives.

9.1 Key Surface Water Flooding Issues

At higher return period floods where the sewer network is surcharged, water can pond to the

south of the A10 in Wetspot 1 and in the area of Silver Street in Wetspot 3. Clear overland flow

paths are visible within roads creating areas of hazard along access and egress routes.

One of these overland flow routes results in large depths of flooding at the Ely Community

College. In the Silver Street area of Ely, there are large flood depths around the King’s School.

The flooding of schools will results in significant damages using The Multi-Coloured Manual

method for assessing flood damages.

In the Priors Court area in Wetspot 1 the capacity of the sewer network at the head of the

system can also cause surface water flooding issues to a number of residential properties. It



appears that overland flows at the high return periods cannot enter the system due to

surcharging of the network downstream of the Priors Court.

In summary, detailed modelling of the Ely Wetspots (see Appendix F) identified a series of

potential issues in the study area:

� Noticeable flood depths at a number of schools, residential and commercial zones in the

study area.

� Overland flow routes along access/egress routes in the City centre, residential and

commercial zones.

� Areas of the sewer network where the existing system is overwhelmed at high return

period events causing flooding in the urban area such as St John’s Road, Silver Street

and Priors Court

� Therefore, both structural and non structural solutions to mitigate and manage the predicted

surface water flood risk are required.

9.2 Preferred Options For Further Investigation

The currently preferred options for Ely are:

1. Increased maintenance of ordinary watercourses and surface water drains across the

whole area whilst targeting on predicted high to moderate risk areas.

2. Engineering Option Combination C3 in Wetspot 1 & 2, Engineering Option Combination

C1 in Wetspot 3.

3. Investigate the current flood resilience of schools identified as being at risk from Surface

Water Flooding within Ely.

Appendix H also includes a schematic of the preferred engineering option elements as well as

the examples of residual flood risk mapping outputs associated associated with these options.

It should be noted however that in Wetspot 1 the model results for Option C3 has shown a slight

increase in water levels (up to 30mm - compared to Do Minimum results) in Kent Close area

where it appears to have suffered from previous flooding based on the recent feedback received

during the finalisation of this report. The slight increase in water levels is likely to be due to the

proposed two speed humps at the entrance to Priors Court as part of Engineering Element ELY-

X. Based on the examination of model results for the Option C1, this impact can be successfully

mitigated by reintroducing the Engineering Element ELY-G (Priors Court – Swale) to the

preferred option. Alternatively, the speed humps may be simply removed but this may increase

water levels again in Priors Court area slightly reducing the effectiveness of the proposed

measure ELY-X. Therefore, further work is recommended to develop the proposed preferred

options as part of the next stages of the SWMP.

As highlighted in Section 8, an optimisation exercise was undertaken to minimise costs

associated with engineering elements and evaluate the Option Combination that gives best

economical advantage. For example, the economic analysis assumes that all excavated

materials will be re-used on site to avoid the cost of disposal of the material.

This could include the formation of embankments and other landscaping features. This avoids

costs associated with disposal including land fill tax. It also promotes the sustainable credentials

of the project by reducing the carbon footprint associated with the transportation of materials for

disposal.



Whilst an optimism bias of 60 % has also been applied to all of the cost estimates (as per the

current guidance applicable for a strategy of this nature) there are a number of economic risks or

uncertainties associated with the development of the cost estimates. The principal economic

risks associated with all of the option combinations are:

� The availability of land to form the attenuation storage areas, swales and rain gardens

� Cost associated with dealing with utilities and environmental constrains which have not

been itemised with the cost estimates.

� The cost of land negotiations and compensation for disruption

It is therefore considered that significant effort should be placed into obtaining agreements with

landowners and stakeholders to undertake the proposed works. In order to mitigate this risk it is

recommended that CCC/East Cambridgeshire District Council and the Cambridgeshire Flood

Risk Management Partnership enter into discussion with all landowners and stakeholders at the

earliest opportunity during the design process to ensure their collaboration.

The implementation of the proposed preferred engineering option combination described in the

report will have significant beneficial impact on dealing with predicted flooding risk but it should

also be recognised that there will be additional benefits streaming from the implementation of

flood mitigation strategies. This includes:

� Beneficial impacts on bio-diversity associated with wetland options.

� Beneficial impacts on bio-diversity associated with the implementation of greener highway

source control measures which includes planting in verges.

� Improvements in the design of the urban realm through the shift from grey to green

infrastructure. This includes retro-fitting and incorporation of green infrastructure with

highways design and other areas of urban design.

� Potential benefits in integration of investment with targets associated with bio-diversity.

� Potential benefits in amenity function and connectivity across the Ely study area.

9.3 Benefits of SWMP

The modelling results, assessments and maps created during this Detailed SWMP, with

emphasis on the 3 identified wetspots, can be used as follows:

� Indication of potential development constraints and opportunities to reduce the predicted

flood risk

� Identification of which stakeholders should be consulted with regard to new development

� Highlights broad scale risk and indication as to whether a developer is required to

undertake further investigation

� Evidence as to why Developers should undertake further investigation and develop

appropriate mitigation measures

� The CCC Highways Department can see where highways flooding has occurred in the

past and during times of high rainfall focus maintenance and emergency response efforts

in these areas

� The Emergency Planning team can use historical flooding data, modelling outputs and

impacted flood receptors, to identify more vulnerable areas and prepare suitable

emergency planning measures



� Development of future planning policies and local flood risk management policies as part

of East Cambridgeshire District Council’s future Local Development Documents and

CCC’s Local Flood Risk Management Strategy. In particular, with regard to the

consideration of surface runoff from any infill development within the three prioritised

wetspots.



10 Next Steps

10.1 Surface Water Management Action Plan – Preparation, Implementation and Monitoring

The next stage of the SWMP will be the Implementation and Review Stage as illustrated above.

It will involve the review of evidence and recommendations from the current stage of this

Detailed Ely SWMP and earlier Countywide Strategic SWMP in order to prepare, implement and

monitor an appropriate Action Plan for the three wetspots.

Consideration could also be given for combining implementation of the engineering elements

across the wetspots so that certain areas/ mitigation elements may be prioritised to formulate

the preferred option or strategy where the greatest benefit-cost can be achieved through a

combined Action Plan.

This combined approach may potentially provide a greater justification for capital investment

and stakeholder support; in particular, within the short to medium term period where the impact

of current economic climate is even greater rather than trying to solve the predicted flooding

issues in isolation.

Other key considerations for detailed design to take in to account are:

� Limited extents of open land in the study area to create attenuation features require

careful planning and negotiations with the impacted land owners. Location of attenuation

structures should be carefully balanced against economic, social and environmental

needs.

� Resolution and description of features in the urban realm should be improved within the

hydraulic model for the purposes of the detailed design. For example, there is limited

representation of smaller linear features (e.g. garden walls and kerbs) which may have a

localised impact on flood routing within the urban environment.



� Data quality and coverage limitations in relation to the existing surface water sewers

system and small urban watercourses to decide where improvements can be made,

including the private and lateral drains recently taken in to Anglian Water ownership and

relevant information on the Catchwaters in the area.

� Implementation, construction and maintenance costs, if applicable, for any designs that

are progressed to the detailed stage, along with the source of any required funding.

The Action Plan will collate all the information from the earlier phases to enable the

implementation of the preferred structural and non-structural options according to an agreed

and coordinated delivery programme. In summary, it should outline:

� The preferred options

� The actions required by each partner and stakeholder

� Who should lead in developing the actions, and

� The timetable for implementation and monitoring

The Action Plan will also inform CCC’s Local Flood Risk Management Strategy as the LLFA,

under the FWMA, by providing information on where surface water flooding may occur within

the Ely study area and the rest of county allowing members of the public to prepare for flooding

from surface water and other local sources accordingly.

Changes to National Planning Policy Guidelines will facilitate the change from Local

Development Frameworks to Local Plans. It is suggested that the Action Plan should specify

the need for the identification of the wetspot areas identified in this report within the Local Plan

and incorporate details on how the changes can be derived through Community Infrastructure

Levy payments and as to how the existing Green Areas can be safeguarded from development

in the future to reduce existing flood risk.

A monitoring strategy should be incorporated within the Action Plan and Local Flood Risk

Management Strategy in order to monitor the effectiveness of the implemented options and to

keep them up to date. This SWMP will be reviewed and updated every 6 years; ideally this

should be undertaken in conjunction with related Countywide SWMP review process in order to

coordinate the process and avoid a potential piecemeal approach to surface water management

needs across the county.

The Cambridgeshire Flood Risk Management Partnership (CFRMP) Stakeholder Engagement

Plan measures effectiveness by gauging key contact from the different stakeholder groups to

gain feedback levels of understanding of messages that have been communicated. It is

proposed that the following publication of this SWMP all key contacts are consulted accordingly

to agree the way forward so that a robust Business Case can be produced to secure funds for

implementing suitable flood mitigation capital schemes alongside non-structural measures of the

Action Plan. However, the exact timescale for the preparation, implementation and monitoring of

the Action Plan is yet to be decided by the CFRMP, subject to the availability of necessary

funding through Flood and Coastal Flood Resilience Partnership Funding.

10.2 Engage with Stakeholders

One of the key objectives of the SWMP process is to engage with partners and stakeholders.

There may be opportunity to utilise and refine the CFRMP Stakeholder Engagement Strategy to

engage stakeholders on the preferred options and development of Action Plan.

As the Engineering Option proposed are taken forward to the Flood Defence Grant in Aid

application stage, further consultation with Stakeholders will be required as to maintenance of

the proposed engineering options, how the engineering options may sit within the East



Cambridgeshire District Council Local Plan and the flood risk and amenity value function of the

engineering option.



11 References

1 Cambridgeshire Flood Risk Management Partnership (2011) Cambridgeshire Surface Water Management Plan –

Strategic Assessment Report

2 Sir Michel Pitt (2008) Learning Lessons from the 2007 Floods

3 Department for Communities and Local Government (2010) Planning Policy Statement 25 Development and Flood Risk

4 Defra (2010) Surface Water Management Plan Technical Guidance

5 Making Space for Water; Taking for a new Government strategy for flood and coastal erosion risk management in

England (2005)

6 Environment Agency (2010) Preliminary Flood Risk Assessment (PFRA) Final Guidance. GEHO1210BTGH-E-E

7 http://www.communities.gov.uk/planningandbuilding/planningsystem/planningpolicy/planningpolicyframework/

8 www.ciria.org.uk

9 Defra (2010) Flood and Water Management Act Section 9 – Local Flood Risk Management Strategies

10 Environment Agency (2010) Great Ouse Catchment Flood Management Plan

11 Atkins (2005) East Cambridgeshire Level 2 Strategic Flood Risk Assessment

12 Scott Wilson (2011) East Cambridgeshire Level 1 Strategic Flood Risk Assessment

13 Mott MacDonald (1998) Ely Drainage Area Study

14 Environment Agency (2009) River Basin Management Plan, Anglian River Basin District, Main Document

15 Water Industry Act 1991

16 The Water Industry (Schemes for Adoption of Private Sewers) Regulations 2011 -

http://www.legislation.gov.uk/uksi/2011/1566/pdfs/uksi_20111566_en.pdf





19 http://www.environment-agency.gov.uk/homeandleisure/floods/109548.aspx

20 Defra and Agency (2005) Framework and Guidance for Assessing and Managing Flood Risk for New Development,

Flood Risk Assessment Guidance for New Development, FD2320 Technical Report 2

21

Defra and Agency (2006) The Flood Risks to People Methodology, Flood Risks to People Phase 2, FD2321

Technical Report 1

22

Environment Agency (2010) Flood and Coastal Erosion Project Appraisal Guidance

23 Flood Hazard Research Centre (FHRC) (2010) The Benefits of Flood and Coastal Risk Management: A Handbook of

Assessment Techniques



24 SPON (2010) Civil Engineering and Highways Price Book