EASTBOURNE AREA SURFACE WATER MANAGEMENT ......area. It provides a preliminary understanding of...

EASTBOURNE AREA

SURFACE WATER MANAGEMENT PLAN

Quality Management

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Quality Management DOCUMENT INFORMATION

Title: Surface Water Management Plan for Eastbourne Area

Owner: East Sussex County Council

Version: 1.3

Status: Final

Project Number: WBESWM File Name: SWMP_Halcrow_Report_for_Eastbourne.doc

REVISION HISTORY

Summary of Changes Completed by Date of Issue Version

Draft Report Oliver Evans / Chris Downs 0.1

Final Report Oliver Evans / Chris Downs 31/01/12 1.3

AUTHORS

Name Organisation and Role

Chris Downs Halcrow Project Manager

Oliver Evans Halcrow Deputy Project Manager

REPORT DELIVERY APPROVALS

Name Title Signature Date

Chris Downs Project Manager, Halcrow

Elliot Gill Technical Director, Halcrow

Imran Bukhari Project Director, Halcrow

Nick Claxton Head of Flood Risk Management, ESCC

PARTNER APPROVAL

Name Title Signature Date

Peter Padget Engineering, Eastbourne Borough Council

Graham Kean Engineering, Wealden District Council

John Challoner County Sewerage Engineer, Southern Water

Peter Amies Environment Agency

DISTRIBUTION

Name Organisation and Role

Nick Claxton Head of Flood Risk Management, East Sussex County Council

Peter Padget Engineering, Eastbourne Borough Council

Graham Kean Engineering, Wealden District Council

John Challoner County Sewerage Engineer, Southern Water

Peter Amies Environment Agency

RELATED DOCUMENTS

Doc Ref Document Title Author Date of Issue Version

PFRA Preliminary Flood Risk Assessment for East Sussex

Oliver Evans / Chris Downs

June 2011 1

Quality Management

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Halcrow Group Limited has prepared this report in accordance with the instructions of East Sussex County Council, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk.

Executive Summary

iv

Executive Summary

According to the Environment Agency, East Sussex County Council (ESCC)

was ranked 35th in the National Assessment of Flooding in England with

approximately 8,000 properties at risk of flooding. This national assessment

led to funding and prompted the study to verify the national view. The Lead

Local Flood Authority (LLFA) ESCC is responsible for managing surface water

flood risk within the County and SWMPs are an excellent tool in assist with

this. ESCC and partners agreed that a SWMP for the Eastbourne Area was

required because of the historic flooding and the predicted future flood risk

identified on the Environment Agency’s national mapping. ESCC therefore

commissioned this SWMP to gain an understanding of surface water flood risk

in Eastbourne (including Wannock and Polegate) and provide an Action Plan

to help reduce it.

The approach taken to deliver the Eastbourne Area Surface Water

Management Plan (SWMP) was based on that detailed in the related technical

guidance prepared by Defra (see Section 9). This standard approach was

enhanced and adapted using the skills and local knowledge of the partners

plus the experience from Halcrow of working across the UK on similar work.

The SWMP Action Plan and accompanying study report has been developed

by ESCC as a strategic tool, which would facilitate the local and coordinated

management of surface water flood risk within the Eastbourne catchment

area. It provides a preliminary understanding of surface water flood risk as

well as identifying potential preferred options to mitigate or reduce flood risk.

This tool will assist East Sussex County Council, Eastbourne Borough

Council, Wealden District Council and Southern Water & Environment Agency

in making subsequent sustainable surface water management decisions,

which are evidence, risk based, future proofed, joint funded where appropriate

and founded upon partnership working.

This report builds on previous studies undertaken by ESCC and has been

delivered using a tiered four phase approach as suggested in the guidance;

SWMP Document Status January 2011

This document has been produced to explain the preparation and present the

findings of the Eastbourne Area Surface Water Management Plan (SWMP) as

well as the associated Action Plan. The Action Plan will remain a live document

allowing the LLFA & RMAs to continue to monitor action delivery, develop

existing actions and add new ones, in collaboration with SWMP partners and

other relevant stakeholders.

Executive Summary

v

i.e. Phase 1 – Preparation; Phase 2 – Risk Assessment; Phase 3 – Options;

and Phase 4 – Implementation.

Phase 1: Preparation

The first phase focused on preparing and scoping the requirements of the

study. ESCC, as the Lead Local Flood Authority (LLFA), had already

established a flood risk partnership with Eastbourne Borough Council,

Wealden District Council, the Environment Agency and Southern Water to

facilitate the delivery of the SWMP. Through meetings with the partners, the

specific roles, of each, was clarified, the data sharing approach developed and

the issue of how the partnership would engage with stakeholders was agreed

in principle. The early setting-up of the partnership enabled prompt

agreement on the approach to the SWMP and the definition of the objectives.

Partners held various data, which were brought together in a GIS environment

to be shared immediately. The maps produced from this GIS dataset gave an

improved understanding of the flood risk, allowing general approaches to be

considered and an Action Plan to be developed, which will help manage and

reduce surface water flood risk going forward. This, in turn allowed an

informed judgement to be made on the level of assessment required for the

Risk Assessment Phase (Phase 2). .

Phase 2: Risk Assessment

The risk assessment phase did comprise of two distinct stages. The first

stage was to identify the intermediate hotspots, which were potential flood risk

areas determined by using the already plotted data, and review them with the

partners to confirm the most critical to consider further. The second stage,

took these high priority intermediate areas and modelled their flood risk.

The first stage identified intermediate hotspots using surface water flood risk

mapping provided by the Environment Agency along with other available data.

This was illustrated in the form of a “storyboard” (see Appendix B), using

tabulated data, text and flood risk mapping. This simple summary

presentation aided the capture of known local flood risk. It was then used, in

addition to the Environment Agency’s surface water modelling results and

historic flooding, to score and rank each hotspot. This process resulted in

highlighting thirty six. These hotspots in total contained approximately 6,000

properties predicted to be at risk of surface water flooding during a 1 in 200

year event, however since this is only predicted flood risk it is thought this

number maybe lower in reality.

Executive Summary

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Figure ES1 – Figure of Hotspots

It was agreed with the partners that the top four ranked intermediate hotspots

represented the key flood risk areas and it would be these taken forward to the

second, detailed risk assessment stage. The four high priority locations

identified were:

Mill Stream Gardens, Willingdon

Langney

Firle Road and Arndale Shopping Centre in Eastbourne Town Centre

Bourne Stream / Motcombe Park east of Eastbourne Town Centre

Executive Summary

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These four key hotspots carried forward to detailed risk modelling were seen

as the most likely to result in improved flood risk understanding and the

identification of potential flood risk reducing options from detailed modelling.

In the second stage computer modelling was used to analyse the impact of

heavy rainfall events across South Wealden and the Eastbourne Area by

assessing flow paths, flood depths, flow velocities and catchment response.

Halcrow used an existing Southern Water drainage-system computer model

and improved it to simulate flood pathways to gain an understanding of the

surface water flooding, which would be critical for option selection in Phase 3.

Figure ES2 – Extract of Detailed Flood Map

Phase 3: Options

Supported by the understanding developed in Phase 2, an option assessment

process was undertaken for each of the four top ranked hotspots areas to

reduce the associated surface water flood risk in each of them. For each

hotspot, outline solutions were developed, modelled and tested to understand

the degree of flood protection they provided.

Initially, large engineering solutions were considered to reduce flooding in the

hotspot areas. These were based on traditional fluvial flood defence

approaches such as large flood storage areas. Eastbourne was well served by

potential open areas upstream and existing storage areas downstream of the

main built-up areas. However, constraints in the topography upstream, the

Executive Summary

viii

lack of space in the developed areas and limitations on conveyance to make

use of the downstream storage areas, made these more traditional measures

impractical, and they were not progressed further.

Therefore, the computer models of the drainage systems and flood pathways

were used to assess more locally based options. These options made use of

limited modifications to the local drainage system and associated

infrastructure and were developed to an initial feasibility level. Several

possible options for further development were identified, one of which is likely

to result in the removal of properties from the Southern Water’s register of

sewer-flooded property. Further details on the options can be found in Section

6 of this report.

It was anticipated that schemes offering more than 1:25yr standard of

protection (SoP) would not be economically viable for the key hotspots and,

for the purposes of this high-level appraisal, the design SoP for all scheme

options was limited to 1:25yr (except where noted below). It was assumed that

if lower SoP scheme options were seen to deliver economic advantage, a

higher SoP could be examined at a later appraisal stage.

The outcome of the option appraisal (i.e. preferred scheme options for the four

key hotspots giving a 1:25yr standard of protection) is summarised in the

flowing table.

Table 1 – Preferred Engineering Solutions

Hotspot Preferred Engineering Solution Total

Damages

(£k)

Cost of

Scheme

(£k)

Benefit /

Cost Ratio

A –

Willingdon

Kerb raising of the western kerb on

Millstream Gardens with a Super Gully

located outside the shops on

Farmlands Avenue and a high-level

overflow along the existing footpath

490 - 770 48 10.2 – 16.0

B – Langney No detailed modelling or optioneering undertaken due to complexity of flooding

mechanisms (see Section 3.1.3)

C –

Eastbourne

Town Centre

Divert the 1050mm dia surface water

sewer at Bedfordwell Rd to Eastbourne

Park Lakes. 96 - 256 251 0.4 – 1.0

D – Bourne

Stream

Surface water storage in Motcombe

Park with highway flooding removed

from Star Road by capturing overland

flow on Upperton Road.

30 - 80 33 0.9 – 2.4

Executive Summary

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In addition to specific actions to manage issues identified in the intermediate

hotspots and their Critical Drainage Areas, there are a number of generic

actions. These generic actions should be implemented catchment-wide to

ensure the long-term, sustainable management of surface water flooding.

They will reduce the risk of the associated flooding and reducing its impact

when it occurs next. Further details of the preferred specific and generic

actions for intermediate hotspots and their Critical Drainage Areas are outlined

in Chapters 7 and 8 of this report.

To conclude the preferred proposed solution for each detailed hotspot for the

1 in 25 year rainfall event are as detailed in Table 1 above.

Phase 4: Implementation and Review

During this phase the list of actions resulting from the risk assessment and

optioneering work were brought together in the Action Plan comprising of 44

actions formalised under the following types of action:-

Engagement

Investigation

Structural Flood Reduction Measures

Non Structural Flood Reduction Measures

Data

Procedural

This Plan identified the actions required to progress the implementation of the

preferred potential options identified for the hotspots and their Critical

Drainage Areas within the SWMP boundary as set out in Phase 3. Actions

include Southern Water to progress the preferred proposed solution for

surface water storage in Motcombe Park and Eastbourne Borough Council to

lead on the investigation of the ownership and potential maintenance of the

Bourne Stream. Also included In addition to this, the action plan identified

actions that will assist ESCC in delivering its responsibilities as LLFA under

the Flood and Water Management Act. The final version of the Action Plan

was developed with key inputs from the partners. Their input helped with the

refinement of actions and setting the criteria for delivering them, which

included detailing those responsible, the timings and potential costs. An

example of this was Southern Water providing details of their OFWAT funding

programme, so this could be reflected in the cost section of the Action Plan for

the options. During this stage intermediate and short duration actions (typically

between one to three months long) were undertaken to aid the development of

the Plan.

Executive Summary

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Recommendations

It is recommended to implement the Action Plan detailed in Appendix F of this

SWMP report, and to ensure both the Action Plan and the Strategic

Environmental Assessment are periodically updated according to the latest

guidance and findings of publicised reports as they become available.

The SWMP Partnership should continue beyond the completion of the SWMP

in order to discuss the implementation of the proposed actions. The partners

should hold regular monitoring meeting to consider how the actions are

progressing and what needs to be done to drive the delivery of the Action Plan

forward.

Successful implementation of the Action Plan is likely to require additional

resources and funding from the partners.

.

Glossary

xi

Glossary

Term Definition

Aquifer A source of groundwater comprising water bearing rock, sand or gravel capable of yielding significant quantities of water.

AMP Asset Management Plan

Asset Management Plan

A plan for managing water and sewerage company (WaSC) infrastructure and other assets in order to deliver an agreed standard of service.

AStSWF Areas Susceptible to Surface Water Flooding

BGS British Geological Survey

Catchment Flood Management Plan

A high-level planning strategy through which the Environment Agency works with their key decision makers within a river catchment to identify and agree policies to secure the long-term sustainable management of flood risk.

CFMP Catchment Flood Management Plan

CIRIA Construction Industry Research and Information Association

Civil Contingencies Act

This Act delivers a single framework for civil protection in the UK. As part of the Act, Local Resilience Forums must put into place emergency plans for a range of circumstances including flooding.

CLG Government Department for Communities and Local Government

Climate Change Long term variations in global temperature and weather patterns caused by natural and human actions.

Culvert A channel or pipe that carries water at or below the level of the ground.

Defra Department for Environment, Food and Rural Affairs

DG5 Register A water-company held register of properties which have experienced sewer flooding due to hydraulic overload and which are 'at risk' of sewer flooding more frequently than once in 20 years.

DTM Digital Terrain Model

EA Environment Agency

FCERM National Strategy for Flood and Coastal Erosion Risk Management

FMfSW Flood Map for Surface Water. Environment Agency’s second generation surface water flood risk mapping dataset.

Flood Defence Infrastructure used to protect an area against floods such as floodwalls and embankments; they are designed to a specific standard of protection (design standard).

Flood Risk Area An area determined as having a significant risk of flooding in accordance with guidance published by Defra and WAG.

Flood Risk Regulations

Transposition of the EU Floods Directive into UK law. The EU Floods Directive is a piece of European Community (EC) legislation to specifically address flood risk by prescribing a common framework for its measurement and management.

Floods and Water Management Act

Part of the UK Government's response to Sir Michael Pitt's Report on the Summer 2007 floods, the aim of which is to clarify the legislative framework for managing surface and groundwater flood risk in England.

Fluvial Flooding Flooding resulting from water levels exceeding the bank level of a main river

Glossary

xii

Term Definition

FRR Flood Risk Regulations

IDB Internal Drainage Board

Indicative Flood Risk Areas

Areas determined by the Environment Agency as indicatively having a significant flood risk, based on guidance published by Defra and WAG and the use of certain national datasets. These indicative areas are intended to provide a starting point for the determination of Flood Risk Areas by LLFAs.

IUD Integrated Urban Drainage

LDF Local Development Framework

Lead Local Flood Authority

Local Authority as defined in the FWMA responsible for taking the lead on local flood risk management

LiDAR Light Detection and Ranging – digital topographic data captured by plane flight

LLFA Lead Local Flood Authority

Local Resilience Forum

A multi-agency forum, bringing together all the organisations that have a duty to cooperate under the Civil Contingencies Act, and those involved in responding to emergencies. They prepare emergency plans in a co-ordinated manner.

LPA Local Planning Authority

LRF Local Resilience Forum

Main River A watercourse shown as such on the Main River Map, and for which the Environment Agency have duties and powers

NRD National Receptor Dataset – a collection of risk receptors produced by the Environment Agency

Ordinary Watercourse

All watercourses that are not designated Main River, and which are the responsibility of Local Authorities or, where they exist, IDBs

Partner A person or organisation with responsibility for the decision or actions that need to be taken.

PFRA Preliminary Flood Risk Assessment

Pitt Review Comprehensive independent review of the 2007 summer floods by Sir Michael Pitt, which provided recommendations to improve flood risk management in England.

Pluvial Flooding Flooding generated from a rainfall event and from water flowing over the surface of the ground; often occurs when the soil is saturated and natural drainage channels or artificial drainage systems have insufficient capacity to cope with additional flow.

PPS25 Planning Policy Statement 25: Development and Flood Risk

PA Policy Area

Policy Area One or more Critical Drainage Areas linked together to provide a planning policy tool for the end users. Primarily defined on a hydrological basis, but can also accommodate geological concerns where these significantly influence the implementation of SuDS

Receptor In flood risk management, a receptor is defined as anything that is affected by flooding such as people, property, transport links and habitats.

Glossary

xiii

Term Definition

Resilience Measures

Measures designed to reduce the impact of water that enters property and businesses; could include measures such as raising electrical appliances.

Resistance Measures

Measures designed to keep flood water out of properties and businesses; could include flood guards for example.

Risk In flood risk management, risk is defined as a product of the probability or likelihood of a flood occurring, and the consequence of the flood.

Risk Management Authority

As defined by the Floods and Water Management Act;

(a) the Environment Agency,

(b) a lead local flood authority,

(c) a district council for an area for which there is no unitary authority,

(d) an internal drainage board,

(e) a water company, and

(f) a highway authority.

RMA Risk Management Authority

Sewer Flood Mitigations

Flooding Local Improvement Projects. Non-return valves and pump devices installed to prevent sewage ‘back-surging’ into basements in times of heavy rainfall and allow the property’s sewage to flow properly into the sewer network.

Sewer Flooding Flooding caused by a blockage or overloading in an urban sewerage system.

SFRA Strategic Flood Risk Assessment

Significant Harmful Consequences

Memorable past floods or otherwise registered on a national scale (such as the summer 2007 event) even if only occurring over a relatively small area

SMP Shoreline Management Plan

Stakeholder A person or organisation affected by the problem or solution, or interested in the problem or solution. They can be individuals or organisations, includes the public and communities.

SuDS Sustainable Drainage Systems

Sustainable Drainage Systems

Methods of management practices and control structures that are designed to drain surface water in a more sustainable manner than some conventional techniques.

Surface Water Rainwater (including snow and other precipitation) which is on the surface of the ground (whether or not it is moving), and has not entered a watercourse, drainage system or public sewer.

SWMP Surface Water Management Plan

WAG Welsh Assembly Government

WaSC Water and Sewerage Company

Contents

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Table of Contents


15

Quality Management ..................................................................................................................... ii

DOCUMENT INFORMATION ................................................................................................. ii

REVISION HISTORY .............................................................................................................. ii

AUTHORS .............................................................................................................................. ii

REPORT DELIVERY APPROVALS ........................................................................................ ii

PARTNER APPROVAL .......................................................................................................... ii

DISTRIBUTION ...................................................................................................................... ii

RELATED DOCUMENTS ....................................................................................................... ii

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

Glossary ....................................................................................................................................... xi

1.0 Introduction ........................................................................................................................ 20

1.1 What is a Surface Water Management Plan? ............................................................. 20

1.2 Background ................................................................................................................ 22

1.3 Objectives ................................................................................................................... 23

1.4 Study Area .................................................................................................................. 24

1.5 Links with Other Plans and Policies ............................................................................ 25

1.5.1 Environment Agency FCERM National Strategy ................................................ 27

1.5.2 Local Strategies ................................................................................................. 28

1.5.3 Preliminary Flood Risk Assessment (PFRA) ...................................................... 28

1.5.4 Strategic Environmental Assessment ................................................................ 29

1.5.5 Strategic Flood Risk Assessments (SFRA) ........................................................ 29

1.5.6 Catchment Flood Management Plan (CFMP) .................................................... 29

1.5.7 Planning Documents .......................................................................................... 30

1.6 Roles & Responsibilities of Partners ........................................................................... 34

1.6.1 East Sussex County Council .............................................................................. 34

1.6.2 The Borough and District Councils ..................................................................... 35

1.6.3 Environment Agency .......................................................................................... 37

1.6.4 Southern Water ................................................................................................. 38

1.7 Powers and Responsibilities of Businesses and Local Households ............................ 38

1.7.1 Utility and Infrastructure Providers ..................................................................... 38

1.7.2 Property Owners and Residents ........................................................................ 38

1.7.3 Riparian Ownership ........................................................................................... 39

1.8 Responsibilities of Parish Councils and Communities ................................................. 39

1.8.1 Parish Council Emergency Self-Help Plans ....................................................... 40

1.9 LLFA ........................................................................................................................... 40

2.0 Phase 1: Preparation ......................................................................................................... 42

2.1 Introduction ................................................................................................................. 42

2.2 Partnership ................................................................................................................. 42


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2.3 Data Collection ........................................................................................................... 43

2.4 Data Review ............................................................................................................... 44

2.4.1 Topography ....................................................................................................... 44

2.4.2 Geology ............................................................................................................. 46

2.4.3 Watercourses .................................................................................................... 47

2.4.4 Sewerage Network & Ownership ....................................................................... 50

2.4.5 Highways Drainage Data ................................................................................... 51

3.0 Phase 2a: Intermediate Risk Assessment ........................................................................ 52

3.1 Introduction ................................................................................................................. 52

3.1.1 Surface Water Flooding from Surface Runoff..................................................... 53

3.1.2 Surface Water Flooding from Surcharged Manholes and Gullies ....................... 54

3.1.3 Flood Risk from Groundwater ............................................................................ 55

3.1.4 Flood Risk from Ordinary Watercourses ............................................................ 58

3.1.5 Flood Risk Interactions with Surface Water ....................................................... 59

3.1.6 Effect of High Fluvial Levels on Surface Water Flooding .................................... 59

3.1.7 Effect of High Tidal Levels on Surface Water Flooding ...................................... 60

3.1.8 Residual Flood risk ............................................................................................ 61

3.1.9 Flooding of Basements ...................................................................................... 61

3.1.10 Land Use ......................................................................................................... 61

3.1.11 Significant Infrastructure .................................................................................. 61

3.1.12 Historic Flooding Incidents ............................................................................... 62

3.1.13 Proposed Future Development ........................................................................ 64

3.1.14 Surface Water Related Schemes ..................................................................... 66

3.2 Source, Pathways and Receptors ............................................................................... 67

3.3 Introduction to Hotspots and Maps ............................................................................. 68

3.3.1 Critical Drainage Areas of hotspots .................................................................... 69

3.4 Selection of Hotspots and Prioritisation ....................................................................... 71

4.0 Phase 2b: Detailed Risk Assessment ............................................................................... 73

4.1 Modelling & Detailed Assessment ............................................................................... 73

4.1.1 Introduction ........................................................................................................ 73

4.1.2 Model Updates .................................................................................................. 74

4.1.3 1D updates ........................................................................................................ 74

4.1.4 Broad scale 2D mesh generation ....................................................................... 74

4.1.5 Rainfall profiles and application to model ........................................................... 75

4.1.6 Model sensitivity and model validation ............................................................... 75

5.0 Phase 2c: Communicate Risk ........................................................................................... 77

5.1 Mapping Outputs ........................................................................................................ 77

5.1.1 Eastbourne Area Flood Depth Map .................................................................... 77

5.1.2 Detailed Hotspot Flood Depth Maps .................................................................. 79


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5.1.3 Detailed Hotspot Flood Hazard Maps ................................................................ 79

5.2 Summary of Flooding Mechanisms for Detailed Hotspots ........................................... 82

5.2.1 Hotspot A – Mill Stream Gardens, Willingdon .................................................... 82

5.2.2 Hotspot B – Langney ......................................................................................... 82

5.2.3 Hotspot C – Firle Road & Arndale Shopping Centre, Eastbourne ...................... 82

5.2.4 Hotspot D – Bourne Stream & Star Inn, Eastbourne .......................................... 82

6.0 Phase 3: Options ............................................................................................................... 84

6.1 Objectives ................................................................................................................... 84

6.2 Measures .................................................................................................................... 85

6.3 Arndale Shopping Centre ............................................................................................ 85

6.4 Eastbourne Lakes ....................................................................................................... 86

6.5 Preferred Options ....................................................................................................... 87

6.6 Options Prioritisation ................................................................................................... 88

6.7 Strategic Environmental Assessment .......................................................................... 88

7.0 Phase 4: Implementation and Review .............................................................................. 90

7.1 Action Plan ................................................................................................................. 90

7.2 Implementation Programme ........................................................................................ 93

7.3 Action Prioritisation ..................................................................................................... 93

7.4 Ongoing Monitoring .................................................................................................... 93

8.0 Conclusion ......................................................................................................................... 95

8.1 Review Objectives ...................................................................................................... 95

8.2 Recommendations ...................................................................................................... 96

8.3 Ownership of the SWMP Action Plan .......................................................................... 96

9.0 References ......................................................................................................................... 97


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Appendices

Appendix A – Data Review ...................................................................................................................... A

Data Review ................................................................................................................................. A1

Historic Flooding Data ................................................................................................................. A2

Planning Document Review ......................................................................................................... A3

Appendix B – Hotspot Area Selection ................................................................................................... B

Intermediate Hotspots Storyboard ............................................................................................... B1

Hotspot Prioritisation .................................................................................................................... B2

Appendix C – Risk Assessment: Technical Details ............................................................................. C

Environment Agency Model Review ............................................................................................ C1

Groundwater Flood Risk Technical Note ..................................................................................... C2

Appendix D – Maps .................................................................................................................................. D

Appendix E – Option Assessment Details............................................................................................. E

Appendix F – Action Plan ........................................................................................................................ F

Action Plan ................................................................................................................................... F1

Action Plan Programme ............................................................................................................... F2

Appendix G – Planner, Emergency Planner & Public Briefing Note .................................................. G

Appendix H –Environmental Assessments ........................................................................................... H

Strategic Environmental Assessment .......................................................................................... H1

Habitats Regulations Assessment Screening ............................................................................. H2


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Figures

Figure ES1 – Figure of Hotspots

Figure ES2 - Extract of Detailed Flood Map

Figure 1-1 - SWMP wheel

Figure 1-2 – Study Area

Figure 1-3 - Delivery of local FCERM

Figure 1-4 - Eastbourne Key Diagram reproduced from the EBC Core Strategy

Figure 2-1 – Phase 1: Preparation Phase of the SWMP Process

Figure 2-2 – Flood Risk Partnership

Figure 2-3 (Map 1 in Appendix D) – LiDAR Data

Figure 2-4 (Map 3 in Appendix D) – Aquifers

Figure 2-5 (Map 4 in Appendix D) - Watercourses

Figure 2-6 (Map 5 in Appendix D) – Lakes

Figure 2-7 (Map 6 in Appendix D) - Southern Water Sewerage Network

Figure 3-1 – Phase 2a: Risk Assessment Phase of SWMP Process

Figure 3-2 – Photo of Surface Runoff Flooding

Figure 3-3 – Photos showing flooding from manholes

Figure 3-4 – Photos showing groundwater flooding

Figure 3-5 (Map 8 in Appendix D) – Groundwater Flood Risk in Low-lying Areas

Figure 3-6 (Map 9 in Appendix D) – Groundwater Flood Risk

Figure 3-7 (Map 10 in Appendix D) – Fluvial Flood Risk

Figure 3-8 (Map 11 in Appendix D) – Historic Surface Water Flooding Incidents

Figure 3-9 (Map 12 in Appendix D) – EBC Core Strategy Area against SWMP Hotspot Areas

Figure 3-10 (Map 13 in Appendix D) – Hotspots in the Eastbourne Area

Figure 3-11 (Map 14 in Appendix D) – Critical Drainage Areas in the Eastbourne Area

Figure 4-1 – Phase 2b: Detailed Risk Assessment Phase of the SWMP Process

Figure 4-2 – InfoWorks 2D Mesh

Figure 5-1 – Phase 2c: Communicate Risk Phase of the SWMP Process

Figure 5-2 – Extract of the Eastbourne Area Flood Depth Map

Figure 5-3 – Extract of Flood Depth Map (Maps 16 & 18 in Appendix D)

Figure 5-4 – Extract of Flood Hazard Map (Maps 17 & 19 in Appendix D)

Figure 6-1 – Phase 3: Options Phase of the SWMP Process

Figure 7-1 – Phase 4: Implementation Phase of the SWMP Process

Many of the figures in the main report are included full size (A3 generally) in Appendix D


20

1.0 Introduction

1.1 What is a Surface Water Management Plan?

A Surface Water Management Plan (SWMP) is a plan which outlines the

preferred surface water management strategy in a given location. In this

context, surface water flooding describes flooding from sewers, drains,

groundwater, and runoff from land, small watercourses and ditches that occurs

as a result of heavy rainfall.

This SWMP study has been undertaken by Halcrow on behalf of East Sussex

County Council (ESCC) in partnership with key local partners who are

responsible for surface water management and drainage in the area. The key

local partners are: Eastbourne Borough Council (EBC), Wealden District

Council (WDC), Southern Water and the Environment Agency. The SWMP

Partners have worked together to understand the causes and effects of

surface water flooding and agree the most cost-effective package of measures

for managing surface water flood risk for the long term.

This document will act as the basis for a long-term action plan to manage

surface water and will influence further investigation, future capital investment,

maintenance, public engagement and understanding, spatial planning and

emergency planning.

A Surface Water Management Plan (SWMP) focuses on reducing flood risk for

both existing and future development. This integrated approach allows the

planning of effective surface water management measures, which will

ultimately contribute towards the development of vibrant future sustainable

communities. A SWMP attempts to look at how surface water management

will be delivered over the next 100 years. The SWMP is subject to a review

every five years, so it can reflect local changes and take account of national

developments in the management of surface water flooding.

In this context, the SWMP Defra guidance document (see Section 9 -

References) provides a definition of surface water flooding as:

“flooding from sewers, drains, groundwater, and runoff from land, small watercourses and ditches that occurs as a result of heavy rainfall.”

In more detail, this includes:

Surface water, or pluvial, flooding where runoff from high intensity

rainfall 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.


21

Flooding from groundwater where groundwater is defined as all sub-

surface water in direct contact with the ground or subsoil.

Sewer flooding which occurs when the capacity of underground

systems is exceeded due to heavy rainfall, resulting in flooding inside

and outside 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. (Sewer

flooding in ‘dry weather’ resulting from blockage, collapse or pump

failure is excluded.)

Flooding from open-channel and culverted watercourses which receive

most of their flow from the urban area and perform an urban drainage

function.

Overland flows from the urban/rural fringe entering the urban area.

Overland flows resulting from groundwater sources.

The approach to the SWMP is defined in the Defra guidance document (see

Section 9 - References). The four main phases (e.g. Preparation, Risk

Assessment, Options, and Implementation and Review), as well as the

subsets of these and the key tasks, are identified in the SWMP wheel (Figure

1-1).

Figure 1-1


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1.2 Background

Surface water flood risk is poorly understood with relatively few records of

flooding or understanding of the key assets which impact on surface water

flow. Responsibility for managing flood risk is historically unclear, being split

between the local planning authorities, the County Council, the Environment

Agency and water companies. The problem of surface water flood risk

management is given greater urgency because it is expected that heavy

storms will increase in frequency, groundwater levels will show greater

fluctuation and tide-locking will become a greater risk as a result of climate

change.

The extensive, nationwide flooding during summer 2007 led to the Pitt Review

being undertaken (see Section 9 – References). These floods were a

reminder that intense rainfall events can occur anywhere and that

stakeholders need to work in partnership if better understanding of urban flood

risk and preparation for future flood events is to be achieved. Sir Michael Pitt

recommended that local authorities should collate and map the main flood risk

management and drainage assets (over and underground), including a record

of their ownership and condition (Recommendation 16). He also

recommended that SWMPs be adopted, particularly where surface water flood

risk is high (reflected in Recommendation 18).

Informed by the ‘Integrated Urban Drainage (IUD) Pilots’ and the flooding

events which occurred in summer 2007, Defra set out its intention to use

SWMPs as the primary vehicle to manage surface water flood risk in England.

This intention was published in the Future Water Strategy (see Section 9 –

References) and included a specific surface water drainage consultation at the

same time (see Section 9 – References). The SWMP concept is recognised

and promoted within Planning Policy Statement 25 (PPS25) (see Section 9 –

References).

In response to the Pitt Review recommendations and government requests to

upper tier authorities to develop strategic level partnerships with key

stakeholders to develop and integrate flood risk management within their

areas, ESCC established the East Sussex Flood Partnership (ESFP) in

January 2010. The ESFP membership is representative of the Risk

Management Authorities (RMAs) (as defined in the Flood and Water

Management Act) in East Sussex and comprises:

the County Council in its role as Highway, Emergency Planning and

Lead Local Flood Authority;

the Districts and Borough Councils;


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the Environment Agency;

Romney Marsh area Internal Drainage Board; and

Southern Water.

The Flood and Water Management Act 2010 received Royal Assent on 08

April 2010. The Act implements those recommendations made by Sir Michael

Pitt which require primary legislation. The Act takes forward proposals from

the Pitt Review that unitary and county local authorities will lead new local

flood risk management activities as the Lead Local Flood Authority (LLFA).

The LLFA’s responsibility in relation to a SWMP is to lead in its production and

ensure that it is periodically reviewed and updated. The Act also included the

recommendation for local authorities to establish and maintain a record of

assets. The Act proposes that a good SWMP will inform the LFRM (Local

Flood Risk Management) strategy.

East Sussex County Council (ESCC) consulted with its SWMP Partners and

they agreed that a SWMP for Eastbourne area was appropriate and would

result in an understanding of the mechanisms of surface water flooding with

the potential to identify mitigation measures.

The SWMP Action Plan and accompanying study report is seen by ESCC as a

basic tool, which will facilitate the initial strategic and coordinated

management of surface water flood risk within the Eastbourne catchment area

to be undertaken by the RMAs. It will provide a preliminary understanding of

surface water flood risk as well as potential options to mitigate or reduce it.

This tool will assist the main stakeholders in making subsequent sustainable

surface water management decisions that are evidence-based, risk-based,

future-proofed and inclusive of stakeholder participation. This is to become a

‘live’ document with actions being undertaken in the following 5 years before

the next full review of the SWMP.

1.3 Objectives

The objectives of the SWMP are to:

Map all current and potential surface water flood risk areas from any

source and engage the community and stakeholders to share

knowledge

Establish and consolidate partnerships between key drainage

stakeholders to facilitate the sharing and exchange of data, skills,

resources and learning, and closer coordination to utilise cross-

boundary working opportunities


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Determine current and future surface water flooding impacts to help

establish the priorities and compare various mitigation measures

Develop a strategy to inform the provision of drainage for new

development and minimise its impact on the receiving drainage

system.

Develop an implementation plan to show how partners and

stakeholders will work together to finance and implement the preferred

strategy

Develop a framework to periodically review the plan and monitor the

effectiveness of chosen solutions

Identify effective and affordable measures to mitigate surface water

flood risk which achieves the maximum possible benefit at the least

possible cost irrespective of asset owner.

Deliver outputs to enable a real change on the ground rather than just

reports and models, whereby partners and stakeholders take

ownership of their flood risk and commit to delivery and maintenance

of the recommended measures and actions;

1.4 Study Area

The indicative study area for the SWMP has been agreed by the Partners.

The study boundary has purposely included all urban areas fringing

Eastbourne (including Wannock & Polegate) which is all of EBC and part of

WDC.

This boundary however, is indicative, so other local drainage assets or

features outside the study area have been included where appropriate. An

example of this is the Pevensey Outfalls to the east of the study area, which is

the major outfall for the study catchment.


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Figure 1-2 – Study Area

1.5 Links with Other Plans and Policies

Key studies relevant to the Eastbourne Area SWMP are the Eastbourne

Borough, southern Wealden District and Cuckmere Catchment Strategic Flood

Risk Assessment (SFRA) and the Catchment Flood Management Plan

(CFMP) which provide information on areas at risk from all sources of flooding

and can help inform the SWMP on flood risk management decisions. Also the

Shoreline Management Plan for Eastbourne provides a large-scale

assessment of risk to people and development from coastal processes and

identifies the proposed approach to shoreline management (i.e. no works,

maintain, improve).

Specific information from all of these studies currently available for the

Eastbourne area have been fed into the SWMP to ensure the wider picture

has been considered for this study.


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The FWMA must also be considered in the context of the EU Floods Directive,

which was transposed into law by the Flood Risk Regulations 2009 (the

Regulations) on 10 December 2009. The Regulations require three main types

of assessment/plan:

Preliminary Flood Risk Assessments (maps and reports for Sea,

Main River and Reservoirs flooding) to be completed by LLFAs and

the Environment Agency by the 22 December 2011. Flood Risk

Areas, at potentially significant risk of flooding, will also be identified.

Maps and management plans will be developed on the basis of these

flood risk areas.

Flood Hazard Maps and Flood Risk Maps. The Environment

Agency and LLFAs are required to produce Hazard and Risk maps

for Sea, Main River and Reservoir flooding as well as ‘other’ relevant

sources by 22 December 2013.

Flood Risk Management Plans. The Environment Agency and

LLFAs are required to produce Flood Risk Management Plans for

Sea, Main River and Reservoir flooding as well as ‘other’ relevant

sources by 22 December 2015.


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Figure 1-3 illustrates how this SWMP fits into the delivery of local Flood and

Coastal Erosion Risk Management (FCERM), and where the responsibilities

for this lie.

Figure 1-3 - Delivery of local FCERM

1.5.1 Environment Agency FCERM National Strategy

The Flood and Water Management Act 2010 requires the Environment

Agency to develop a national flood and coastal erosion risk management

strategy for England as part of its strategic overview role for flood and coastal

erosion risk management (FCERM) in England. The Environment Agency has

Environment Agency (National Strategy)

Produce a National Strategy for FCERM as part of full strategic

overview role for all FCERM (Main river, ordinary watercourse,

sea water, surface runoff, groundwater, coastal erosion and flood

risk from reservoirs). Support lead local authorities and others in

FCERM by providing information and guidance on fulfilling their

roles.

Defra

Flood and Coastal Erosion Risk Policy

Overview

Planning

Delivery

PFRAs SWMPs CFMPs SMPs

Lead Local Flood Authorities – Local Strategies

surface water, groundwater, ordinary

watercourses

EA – Main River and

Coastal

LLFAs - surface water

and groundwater

Water companies, reservoir owners, highways

authorities

Third Party assets (i.e. riparian owners)

SEAs

SFRAs


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developed this new strategy jointly with Defra to ensure it reflects current

Government policy.

The strategy sets out what needs to be done by all those involved in flood and

coastal erosion risk management to reduce the likelihood of flooding and

coastal erosion, and to manage its consequences. It provides a statutory

framework within which communities, the public sector and other

organisations will work together to manage flood and coastal erosion risks.

The strategy is central to the implementation of the Flood and Water

Management Act 2010, and provides a framework for managing all sources of

flood and coastal erosion risk in a co-ordinated way.

The strategy builds upon existing approaches to FCERM. It further promotes

the use of a wider range of measures to manage risk in a co-ordinated way

that balances the needs of communities, the economy, and the environment.

The aim of the strategy is to give a framework for FCERM that is not

prescriptive but instead encourages innovation and proportionate approaches,

and enables LLFAs to take the necessary local actions.

1.5.2 Local Strategies

The Flood and Water Management Act 2010 (FWMA) requires LLFAs to

produce Local Flood Risk Management Strategies (LFRMS). The Flood Risk

Regulations 2009 also require a Flood Risk Management Plan (FRMP) of

Flood Risk Areas by December 2015. The Action Plan from the SWMP will

feed into the LFRMS to assist in setting the strategic objectives for the whole

County.

1.5.3 Preliminary Flood Risk Assessment (PFRA)

This study focused on surface water flood risk for the whole of East Sussex

County. An assessment on the EA’s NRD (National Receptor Dataset) and

the EA’s FMfSW (Flood Map for Surface Water) dataset undertaken by the

EA, indicated parts of the Eastbourne area were highlighted as being an ‘area

above the flood risk threshold’. This meant a significant enough proportion of

receptors were deemed at risk of surface water flooding. The results of this

assessment concluded there is surface water flood risk within Eastbourne and

South Wealden, but it is not significant on a national or continental scale.

The assessment drew attention to the significant rainfall event in August 2006

which resulted in highway flooding of Terminus Road in the town centre, and

identified that low-lying areas of Eastbourne will be at higher flood risk in the

future due to tide-locking of the sea outfalls.


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1.5.4 Strategic Environmental Assessment

The Strategic Environmental Assessment (SEA) Directive (2001/42/EC) is

implemented in the UK by ‘The Environmental Assessment of Plans and

Programmes Regulations 2004 (Statutory Instrument No.1633)’. Its objective

is ‘to provide for a high level of protection of the environment and to contribute

to the integration of environmental considerations into the preparation and

adoption of plans and programmes with a view to promoting sustainable

development’.

The flood risk management plans required under the Flood Risk Regulations

2009 fall under the scope of the SEA Directive.

As part of the preparation of the SWMP a Strategic Environmental

Assessment was undertaken to consider the environmental effects of the

proposed options. This also included an appropriate assessment in terms of

the Habitats Directive, which was seen as necessary due to the proximity of

the Pevensey Levels. The Levels are located directly downstream from the

study area and are designated as a SSSI and RAMSAR site.

1.5.5 Strategic Flood Risk Assessments (SFRA)

A SFRA provides information on areas at risk from all sources of flooding. The

SFRA should form the basis for flood risk management decisions, and

provides the basis from which to apply the Sequential Test and Exception Test

(as defined in PPS25) in the development allocation and development

management process. The SFRA Level 1 historical data has been reused to

inform this SWMP, however, the Level 2 has not been used since it focuses

purely on tidal flood modelling and no relevant information was identified for

this SWMP.

1.5.6 Catchment Flood Management Plan (CFMP)

The CFMP is a strategic planning tool through which the Environment Agency

works with other key decision-makers within a river catchment to identify and

agree policies for sustainable flood risk management. The study area is

covered in the Cuckmere and Sussex Havens CFMP – Sub Catchment 6 -

Polegate/Eastbourne and Willingdon/Pevensey

This CFPMP states that the Polegate area is likely to see significant urban

development over the next 10-15 years. Together with the effects of climate

change, this development is likely to increase the level of flood risk above

existing levels. The plan also states that surface water and groundwater

flooding are a problem in Willingdon; this is caused in part by runoff from the

South Downs and by long lengths of culverted watercourses, which are of

insufficient size and in poor condition.


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The EA’s preferred policy for this sub-catchment is that, although current flood

risk is low, it is expected to increase in the future, and therefore there may be

a need to take further actions to keep pace with climate change.

Their key messages are that existing flood risk in this sub-catchment is from a

combination of fluvial, surface water and urban drainage problems due mainly

to constricted culverts and channel blockages. The current drainage system is

at full capacity. Any increase in runoff is likely to lead to more frequent and

deeper flooding. The consequences of increased flooding (due to climate

change, rises in sea level or changes in land use) would be all the more acute

in low-lying areas where extra effort will be necessary to manage this risk.

1.5.7 Planning Documents

In order to understand the context, and the potential interaction between

various sectors such as the built environment and transport infrastructure, we

conducted a review of selected national policies, and local planning policies

(mostly under production) to inform the ESCC SWMP action plan. The policy

documents reviewed were:

Flood and Water Management Act 2010 (amended 2011)

Eastbourne Borough Council Core Strategy (ongoing)

Eastbourne Town Centre Area Action Plan (ongoing, at Submission Stage)

Eastbourne County Council Local Transport Plan-3 (2011-2026)

Eastbourne Park Area Action Plan Supplementary Planning Document (ongoing)

Wealden Borough Council Core Strategy (ongoing)

A detailed review of various planning documents relating to Eastbourne

Borough Council, Wealden District Council and the East Sussex County

Council is contained in Appendix A. The key messages from each policy

document relevant to the present study are shown in Table 1-1 below.

Policy/ Plan Key pointers/ Message for the SWMP (in italics)

Eastbourne

Borough Council

Core Strategy –

Eastbourne Plan

The document lists policies on various development aspects. Of

interest will be the proposals, such as new/ intensification of

development in the Neighbourhood areas- as most of these areas are

contained within either an Intermediate or Detailed surface water

flooding hotspot identified through this ECC SWMP study modelling

exercise.

Map 1 combines the proposed EBC Core Strategy Areas for Change

and Development, with our modelled hotspot areas at risk for surface


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water led flooding. This map should be used to interpret potential Core

Strategy Key areas at risk of flooding. The identified risk may be

exacerbated if the Core Strategy Areas do not sufficiently incorporate

surface water runoff attenuation methods. For example the Detailed

Hotspot areas within the town centre have been identified as Key

Areas of change, and for residential and retail land-use intensification.

Refer to Appendix A for the proposed Neighbourhood Core Strategy

Areas, and whether these areas lie within the hotspots.

This exercise attempts to highlight the critical need for a combined

approach between the planners and the surface water management

team, at an early stage of an area development. The suggested

planning approach goes beyond S106 contributions, and the need to

incorporate effective methods within key local development

documents.

Eastbourne Town

Centre AAP

The AAP sets out vision and series of objectives to improve the retail

offering; the public realm; accessibility; maintain the cultural heritage;

and develop the area for mixed use. The document identifies 7

character areas within the town centre for potential development, and

identifies 5 Development Opportunity sites, most of which also fall

within the SWMP hotspots. The five sites are:

i) Land at the junction of Terminus Road and Ashford Road.

ii) Land adjoining the railway station and the Enterprise Centre

iii) Land between Upperton Road and Southfields Road

iv) Land at the south-eastern end of the Arndale Centre

v) Land at the former Coop on Terminus Road

It is worth noting that most of the Development Sites proposed in the

area are located in the Surface Water Flooding Hotspots, but even as

a generic point, the AAP document does not address any risk relating

to surface water runoff and this aspect is not included as part of the

‘key components for development’ section of the AAP document.

Eastbourne Park

AAP SPD

The document, under production, presents a vision for the wet

floodplain area up to the year 2026. To be adopted as a

Supplementary Planning Document, that will guide future development

in the area from a planning perspective, it emphasises the need to

maintain and/ or enhance the ecological function of the Park, as well

as expand the flood water storage capacity at Shinewater, West

Langney, Broadwater and Southbourne areas of the Park.

ESCC LTP3 The plan, a mandatory obligation for the County Council, outlines the

Vision, Objectives and Strategy for maintaining and enhancing the


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transport infrastructure and the transport services within the County.

The Sustainability appraisal of the Plan recommends the following

which relate to surface water runoff from road surfaces:

· Consider options to alleviate highways runoff that may pollute local

water bodies

· Need for the SWMP to take transport infrastructure related surface

water runoff into consideration

The LTP document stresses the need to consider transport

infrastructure as an integral component in surface water management,

in addition to the built-up area from buildings. It is important to include

car park surfaces and similar public space areas relating to transport

infrastructure (e.g., bus stands). Joint working between the surface

water management team and the LTP team is recommended.

Wealden District

Core Strategy

Similar to the Eastbourne Borough Core Strategy, this document

discusses development proposals and the policies that will direct

growth in the next 15 years.

Only part of this District area is included in the Eastbourne Area

SWMP Study area boundary, nevertheless it is worth noting that

substantial residential development is planned for the area- in various

phases, both as intensification and as urban extensions. The impact of

these built areas on the risk mapping may be worth considering for the

SWMP study.

Table 1-1 – Key messages from planning documents

Figure 1-4 overleaf is reproduced from the Eastbourne Core Strategy

document to indicate spatial proposals.


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Figure 1-4 - Eastbourne Key Diagram reproduced from the EBC Core

Strategy


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1.6 Roles & Responsibilities of Partners

All Risk Management Authorities must cooperate in the exercise of their flood

and coastal erosion risk management functions.

Each Risk Management Authority has its own distinctive roles and

responsibilities. We all recognise the benefits of working together to achieve

our common interests and to comply with the relevant acts.

1.6.1 East Sussex County Council

East Sussex County Council is the Lead Local Flood Authority for East Sussex

and has a strategic role in overseeing the management of local flood risk

associated with surface water runoff, ordinary watercourses such as streams,

ditches and groundwater.

It is also under a duty to ensure that the relevant duties under the Flood and

Water Management Act are discharged, this includes (subject to

commencement):

Meeting the requirements of the flood risk regulations,

Developing and applying a local flood risk management strategy

Reporting on local flood incidents where appropriate

Maintaining a register of assets likely to have a significant effect on flood risk

The regulation of SuDS applications via the Drainage Approval Body (anticipated October 2012)

Under the Land Drainage Act (1991) the County Council will have powers of

consenting and enforcement in respect of minor watercourses (as of 6 April

2012)

The County Council has a significant drainage role in the exercise of its duties

as a Highway Authority. Under the Highways Act (1980), the County Council is

under a duty to ensure that highway drainage systems are clear and that

blockages on the highway are cleared where reasonably practicable.

ESCC plays a leading role in emergency planning under the Civil

Contingencies Act. When a serious or wide-spread flooding incident occurs in

Sussex, ESCC’s Emergency Planning Team together with other emergency

responders from the Sussex Resilience Forum (SRF) undertakes a range of

actions to support affected communities and help limit the impacts of the flood.

In order to do this effectively and in a co-ordinated way the team produces and

contributes to multi-agency plans for responding to flooding events, including

the SRF Multi-Agency Flood Plan covering all locations in Sussex, and site-


35

specific plans for Uckfield, Lewes, Seaford & Newhaven, Bulverhythe, Rye

Bay, and Pevensey Bay, Normans Bay and Eastbourne.

1.6.2 The Borough and District Councils

The Borough and District Councils are community leaders and posses a

detailed understanding of the nature of flood risk that communities face in their

jurisdiction. They are also identified as having responsibilities under the

following:

Flood and Water Management Act 2010;

Land Drainage Act 1991;

The Planning Acts ;

Coast Protection Act (1949); and

Emergency planning.

i. Responsibilities under the Flood and Water Management Act

Though the Flood and Water Management Act 2010 provides the LLFA with

powers to undertake works for the management of flood risk from surface

runoff and groundwater, powers to undertake works on ordinary

watercourses remain with either district or unitary authorities, or internal

drainage boards, but all works must be consistent with the Local Strategy.

These powers are discretionary and the Councils’ policy is generally to

exercise them where property is at risk of flooding. The District and Borough

Councils may also give guidance and assistance on flooding issues to

residents. Under planning legislation, Borough and District Councils operate

their development planning and control functions, having due regard to

PPS25. As well as the statutory powers mentioned above, some

Boroughs/Districts have kept records of flood events and carried out both

maintenance and new works within their district. The extent of these activities

varies considerably between Councils, due to the variance of technical

resource and capacity of each Council.

Specifically, District and Borough Councils also have the following new

responsibilities:

Power to designate structures and features that affect flooding or coastal erosion;

Duty to act consistently with local and national strategies;


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ii. Responsibilities under the Land Drainage Act

From the 6th of April 2012, District and Borough Councils have no

responsibilities (except when it is a landowner) under the Land Drainage Act.

They do however have a number of permissive powers, which they can

employ in support of the Lead Local Flood Authority. These include:

Permissive powers to maintain or improve existing works or to construct new works in order to help prevent, mitigate or remedy flood damage; and

Advise the LLFA on land drainage consent applications.

iii. Responsibilities as a Planning Authority

District and Borough Councils’ planning function affects Flood Risk

Management in three key ways:

Considering flooding concerns in developing local plans

Working with the SuDS Approval Body in ensuring that planning applications and drainage applications are complementary

Considering flood risk assessments submitted in support of applications on which the Environment Agency do not require to be consulted

iv. Responsibilities as a coastal erosion risk management authority

Broadly, the Coast Protection Act 1949 provides for coast protection

authorities with general powers to execute coast protection work and provides

authorities with powers to authorise others to undertake such work.

The largely permissive coast protection powers have to be considered

alongside a range of pre-existing, and continuing, common law and local

statutory/non-statutory duties including:

Planning shoreline management activities with input from the Environment Agency.

Delivery of coastal erosion risk management activities including maintenance and repair work.

Working alongside the Environment Agency to develop and maintain coastal flood and erosion risk information.

Maintain a register of assets and other features that help to manage coastal risks.

Implement, manage, maintain and monitor shoreline management plans to understand and manage coastal flood and erosion risks.


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Assist communities in planning for the future and taking appropriate steps to adapt to changing flood and coastal erosion risks.

v. Responsibilities as an Emergency Planning authority

District and Borough Councils are ‘Category 1’ responders to emergencies

and members of the East Sussex Resilience Forum. This means that they

have duties to:

undertake risk assessments;

manage business continuity;

carry out emergency planning;

share information and cooperate with other responders; and

warn and advise the public during times of emergency.

Furthermore, in the event of an emergency, district and borough councils have

additional roles and responsibilities regarding coordinating, supporting and

managing emergency support during and after the flooding event.

1.6.3 Environment Agency

The Environment Agency has a specific role in providing strategic overview on

flood risk matters and advice and guidance to the Lead Local Flood

Authorities. The EA is a non departmental public body of Defra and is under a

duty to prepare the national flood risk strategy and it retains responsibility for

managing coastal and main river flooding. It is also the competent body

responsible for implementing the water framework directive in England and

Wales by the development of river basin management plans.

The National Strategy identifies the following strategic actions for the

Environment Agency:

Use strategic plans like the Catchment Flood Management Plan and the Shoreline Management Plan to set the direction for Flood Risk Management;

Support the creation of Flood Risk Regulations by collating and reviewing the assessments, plans and maps that Lead local flood authorities produce;

Providing the data, information and tools to inform government policy and aid risk management authorities in delivering their responsibilities;

Support collaboration, knowledge-building and sharing of good practice including provision of capacity-building schemes such as trainee schemes and officer training;


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Manage the Regional Flood and Coastal Committees (RFCCs) and support their decisions in allocating funding for flood defence and flood resilience schemes;

Report and monitor on flood and coastal erosion risk management;

Provide grants to risk management authorities to support the implementation of their incidental flooding or environmental powers;

The Environment Agency maintains a number of FRM assets (tidal, main

fluvial and ordinary watercourses) and acts as the Internal Drainage Board for

three Internal Drainage Districts in the County: the Lower Ouse, the Cuckmere

and the Pevensey levels.

1.6.4 Southern Water

Southern Water is statutory water supply and sewerage undertaker and

supplies freshwater and collects and treats wastewater in East Sussex. As

sewerage undertaker it is responsible for public combined and surface water

sewers which are essential to effective flood risk management in the county. It

is a public company regulated by the Environment Agency and, financially by

OFWAT.

1.7 Powers and Responsibilities of Businesses and Local Households

1.7.1 Utility and Infrastructure Providers

Utility and infrastructure providers such as Network Rail, energy companies

and telecommunication companies are not risk management authorities.

However, they have a crucial role to play in flood risk management as their

assets can be important consideration in planning for flooding. Moreover they

may have assets such as culverts which it is important to share with flood risk

management authorities. They already maintain plans for the future

development and maintenance of the services they provide and it is important

that they factor in flood risk management issues into this planning process.

This will ensure that their assets and systems are resilient to flood and coastal

risks and that the required level of service can be maintained in the event of

an incident. Utility and infrastructure providers may wish to invest time and

resources into developing and delivering the local flood risk management

strategy, to realise the significant benefits for them and their customers that

follow from flood risks being effectively managed.

1.7.2 Property Owners and Residents

It is the responsibility of householders and businesses to look after their

property, including protecting it from flooding. While in some circumstances

other organisations or property owners may be liable due to neglect of their

own responsibilities, there will be many occasions when flooding occurs

despite all parties meeting their responsibilities. Consequently it is important


39

that householders whose homes are at risk of flooding, take steps to ensure

that their house is protected.

These steps include:

Check whether their household is at risk from flooding from the river,

coast or local flood sources;

Ensure that preparations have been made in the event of a flood;

Take measures to ensure that their house is protected from flooding,

either through permanent measures such as sealants in the wall or

temporary measures such as floodsax or flood guards;

Take measures to make sure the house is resilient to flooding so that if

it does occur it does not cause too much damage;

Where possible take out flood insurance.

1.7.3 Riparian Ownership

Landowners, householders or businesses whose property is adjacent to a

river or stream or ditch are likely to be riparian owners with responsibilities.

Riparian owners have a right to protect their property from flooding and

erosion but in most cases will need to discuss the method of doing this with

the Environment Agency. They also have responsibility for maintaining the bed

and banks of the watercourse and ensuring there is no obstruction, diversion

or pollution to the flow of the watercourse. Full details can be found in the EA

document ‘Living on the edge’.

1.8 Responsibilities of Parish Councils and Communities

Flooding events can affect whole communities with households which do not

suffer from internal flooding still potentially being trapped as roads are blocked

or having to help support and provide shelter to their neighbours who have

suffered from flooding.

District and County Councillors have a key role in helping the Parish Councils

and communities understand their role and ensuring affected communities are

properly represented in discussions about local activities.

Communities know better than anyone the level of flood risk that they face and

can make important contributions to helping manage the levels of flood risk.

Parish councils can be the best way of letting residents know that they should

be aware of flood risk and to check with East Sussex County Council and the

http://publications.environment-agency.gov.uk/dispay.php?name=GEHO0407BMFL-E-E


40

Environment Agency as to the extent of the risk. Finding out through the

community has the advantage of being provided the information in context so

as to know the extent to which residents should be concerned.

1.8.1 Parish Council Emergency Self-Help Plans

If a parish is at risk from flooding it is advisable to create an Emergency Plan

which details who can be contacted to lead and assist in an emergency, what

equipment is available and where can be used as emergency accommodation.

1.9 LLFA

As the designated LLFA, East Sussex County Council is responsible for

leading local flood risk management across the County. Given this is a new

duty, much of the local knowledge and technical expertise necessary for the

Council to fulfil its duties as LLFA currently lies with partner organisations. It is

therefore crucial that they continue the good partnership building processes

they have already started to ensure effective and consistent management of

local flood risk throughout the County.

Aside from forging partnerships, coordinating and leading on local flood

management, there are a number of other responsibilities and powers that

have arisen for LLFAs from the Flood & Water Management Act and the Flood

Risk Regulations. This report provides information and evidence to assist the

LLFA in undertaking their responsibilities, and exercising their powers,

although it should be noted that at the time of writing some enabling sections

of the Act had yet to commence. The key legislative responsibilities and

powers invested in LLFAs are:

Investigating flood incidents – LLFAs have a duty to investigate and

record details of local flood events within their area which exceed the

threshold set by the LLFA.

Asset Register – LLFAs also have a duty to maintain a register of

structures or features which are considered to have an effect on local flood

risk, including details on ownership and condition as a minimum. The

register must be readily available for inspection. The Secretary of State

will be able to make regulations about the content of the register and

records, and these details will be made available to the public.

Drainage Approval Body (yet to be commenced) – The Flood and

Water Management Act establishes an Approval Body at County or Unitary

authority level (in this case East Sussex County Council) to ensure

national standards of sustainable drainage are enforced. Developers will

be required to gain approval for their proposed drainage systems before

they can begin construction. The SuDS Approving Body will then be


41

responsible for adopting and maintaining SuDS which serve more than

one property (other than on public roads which are the responsibility of the

Highways Agency (trunk roads) and the Highway Authority (local

network)).

Local Strategy for Flood Risk Management – LLFAs are required to

develop, maintain, apply and monitor a local strategy for flood risk

management in their areas. The local strategy will build upon information

such as national risk assessments and will use consistent risk based

approaches across different local authority areas and catchments.

Designation powers – LLFAs, as well as the Environment Agency, have

powers to designate structures and features that affect flooding or coastal

erosion in order to safeguard assets that are relied upon for flood or

coastal erosion risk management. This includes assignment of features on

private land with a duty to inform the land owner and having enforcement

powers for the unauthorised alteration of privately owned designated

features.

Works powers – LLFAs have powers to undertake works to manage local

flood risk, consistent with the local flood risk management strategy for the

area.

Transfer of powers – In April 2012 there will be a transfer of powers 23

(regulatory role for the consenting regime of ordinary watercourses) and

25 (enforcement powers to be removed from the district and boroughs to

the county council).

The Flood and Water Management Act 2010 (FWMA) presents a number of

challenges for policy makers and the flood and coastal risk management

authorities identified to co-ordinate and deliver local flood risk management

(surface water, groundwater and flooding from ordinary watercourses). ‘Upper

Tier’ local authorities (Lead Local Flood Authorities – LLFA) have been

empowered to manage local flood risk through new responsibilities for flooding

from surface and groundwater.

The FWMA reinforces the need to manage flooding holistically and in a

sustainable manner. This has grown from the key principles within Making

Space for Water (Defra, 2005) and was further reinforced by the summer 2007

floods and the Pitt Review (Cabinet Office, 2008). It implements several key

recommendations of Sir Michael Pitt’s Review of the summer 2007 floods,

while also protecting water supplies to consumers and protecting community

groups from excessive charges for surface water drainage.


42

2.0 Phase 1: Preparation

Figure 2-1 – Phase 1: Preparation Phase of the SWMP Process

2.1 Introduction

This Phase focuses on establishing a partnership for the SWMP, data

collection and determining appropriate objectives for this study. The Partners

were asked to consider the need for an Eastbourne Area SWMP.

2.2 Partnership

For the purposes of the SWMP study a partnership was established

comprising Eastbourne Borough Council (EBC), Wealden District Council

(WDC), the Environment Agency and Southern Water. Figure 2-2 shows the

organisations forming the SWMP partnership.

East Sussex County Council (ESCC) consulted with its SWMP Partners and

they agreed that a SWMP for the Eastbourne area was appropriate and would


43

result in an understanding of the mechanisms of surface water flooding with

the potential to identify mitigation measures.

Figure 2-2 – Flood Risk Partnership

2.3 Data Collection

A list of potential data sources was issued to the Partners. With their input this

list has been developed to reflect what useful data was available for the

SWMP. As data was received, it was logged into an Incoming Data Register,

with date of receipt, contact name and licence information details. A quality

scoring of the data was determined in line with the SWMP Defra guidance

document (see Section 9 - References) as follows:

1. No known deficiencies – not possible to improve in the near future.

2. Known deficiencies – best replaced as soon as new data is available.

3. Assumed – based on experience and judgement.

4. Grossly assumed – an educated guess.

The register of data received for the SWMP Scoping Study is presented in

Appendix A, with key datasets summarised in Table 2-1 overleaf:


44

Data Source

Eastbourne historic flooding incidents and hotspot areas EBC

Wealden land drainage database and hotspot areas WDC

Environment Agency Geostore data – main rivers, drainage paths, historic flood map, national receptors database, indicative surface water mapping

ESCC

Eastbourne Park Compensatory Flood Storage Scheme Report EBC

LiDAR EA

SO753 Polegate Willingdon 2007 model EA

Southern Water Sewer System GIS, Maps, and historical incidents Southern Water

EBC- Eastbourne Park and Flooding Report EBC

Southern Water sewer model & recorded flooding locations Southern Water

Table 2-1: Summary of key datasets received

There seems to be no other significantly important datasets which are

outstanding from the SWMP data collection process.

2.4 Data Review

2.4.1 Topography

LiDAR (Light Detection And Ranging) data of 2m resolution was downloaded

from the Environment Agency Geomatics website and covered the entire

study area. Both the ‘raw’ unfiltered LiDAR and filtered LiDAR (where, for

example, buildings and trees have been stripped from the raw LiDAR) were

obtained. The filtered LiDAR was merged to create the initial digital terrain

model (DTM) shown in Figure 2-3 and this gives an indication of the ‘bare

earth’ topography.


45

Figure 2-3 (Map 1 in Appendix D) – LiDAR Data

The LiDAR data shows that a large proportion of the study area is low lying

with ground levels of between 1 to 3m AOD. This corresponds mainly to the

fields and parklands along the downstream reaches of the main rivers through

the study area. However, the eastern, coastal part of Eastbourne town is also

quite low-lying, with ground levels of between 2 to 5m AOD, although there

are occasional ‘islands’ of high ground such as at Roselands, which is

between 8 to 9m AOD. Around the west and north of the study area, the land

starts to rise, reaching approximately 35m AOD at Stone Cross to the north,

and over 100m AOD along the South Downs hills to the west. At Warren Hill

ground levels reach 157m AOD, at Downside they reach between 170 to

200m AOD, and at Willingdon 194m AOD.


46

2.4.2 Geology

An overview of the study area geology was obtained from the British

Geological Survey Solid and Drift Geology maps. The maps were available in

hardcopy and as GIS layers provided by the British Geological Survey.

The maps show that the SWMP study area overlies a complex geology. The

bedrock geology is a series of rock layers, with the chalk hills of the South

Downs, Gault (mudstone, sandstone and limestone) through much of the

centre of the study area, then a band of Lower Greensand, and finally Weald

Clay underlying the north east, apart from a little area which protrudes over

the Tunbridge Sands belt to the north. Overlying deposits of Head (probably

old river deposits) occur in the valleys of the South Downs hills, while the low-

lying area in the centre of the SWMP study area is filled with Alluvium, apart

from some outcroppings of Gault and Lower Greensand. Along the shorefront,

there is a narrow stretch of Upper Greensand in front of the south part of

Eastbourne town, whereas, to the north, in front of the Alluvium, is a wider

band of Storm Gravel Beach Deposits.

Map 2 in Appendix D shows the bedrock and superficial geology of the

SWMP area, based on the GIS data provided by the British Geological

Survey. The permeability of the geology is summarised in Figure 2-4 by

identifying rock types as major or minor aquifers; very low permeability

geology (such as the clays) is not marked.


47

Figure 2-4 (Map 3 in Appendix D) – Aquifers

Chalk and Greensand are generally high permeability rock which, when

overlaid with the Environment Agency’s Groundwater Vulnerability maps,

correspond closely to the location of major aquifers. The Alluvium and Storm

Gravel Beach Deposits are also permeable and are classified as a minor,

more local aquifer.

2.4.3 Watercourses

Under Section 93 of the Water Resources Act 1991, principal rivers or locally

significant watercourses in England and Wales were classified as main rivers

and are the responsibility of the Environment Agency. The Environment

Agency also administers the Pevensey Levels Internal Drainage Board (IDB)

which covers the ordinary watercourses in the Pevensey Levels area. It is

assumed that the remaining ordinary watercourses are normally the

responsibility of riparian owners. The responsibility for Critical Ordinary

Watercourses transferred from local authorities to the Environment Agency in

2003.

A GIS layer of main rivers is available from the Environment Agency Geostore

website; this has been combined with ordinary watercourses taken from OS


48

Mastermap. The principal water features of the SWMP study area are shown

in Figure 2-5.

Figure 2-5 (Map 4 in Appendix D) - Watercourses

The study area surrounds the Willingdon Levels marshes where a number of

ordinary watercourses and drainage ditches are featured. Through the town

the drainage ditches are predominantly culverted, but on reaching the central

Willingdon Levels area they become open watercourses and lakes within a

greenfield area known as Eastbourne Park.

The drainage system is gravity operated, apart from those ordinary

watercourses draining to Lottbridge pumping station. The system currently

suffers from tide-locking about 50% of the time, where water cannot discharge

into the sea due to the outfalls being below the high tide water level. During

winter, there are times when the rate of discharge is lower than the rate at

which water drains into the system from the catchment and hence the lakes

and pasturelands beside the streams act as flood storage.


49

The Eastbourne Park area is located in the heart of the Borough, and contains

several lakes that form a compensatory flood storage scheme. It was

developed to provide a mechanism to compensate for the impact on surface

water drainage of any new development in Eastbourne. The scheme was

constructed in the early 1990s to provide protection up to the 1 in 100 year

rainfall event.

The lakes comprise Broadwater, Shinewater, Southbourne and West Langney

which interact with either ordinary watercourses or EA Main River. Figure 2-6

shows the location of all lakes constructed as part of the flood storage

scheme.

Figure 2-6 (Map 5 in Appendix D) – Lakes


50

2.4.4 Sewerage Network & Ownership

Southern Water has provided details of their sewerage network system which

is shown in Figure 2-7.

Figure 2-7 (Map 6 in Appendix D) - Southern Water Sewerage Network

The majority of Eastbourne and south Wealden comprise a combined

sewerage system with some local separately sewered areas predominantly in

areas which were developed over the last 30 years.

The majority of the sewers are owned by Southern Water, however, there are

areas within the study boundary where the picture is less clear:-

a) The Bourne Stream, running along the A259 which marks out the historical

path of the Bourne Stream. The majority of the length of this watercourse

is of unclear status and this needs to be resolved by the partnership.


51

b) Pashley Road / Cherry Gardens Road

c) Private systems. Many private sewers and laterals outside of the property

curtilage which connect to an existing public sewer were transferred to

Southern Water on 1st October, 2011. Unfortunately, however, these are

largely unmapped.

d) North Polegate

2.4.5 Highways Drainage Data

Highway drain gully data was obtained from the Highway Authority (East

Sussex County Council & local network). Overlaying this with the Southern

Water sewer network map showed that most of the gullies were located in

roads with surface or combined sewers, so it is currently assumed they

connect directly into the sewer system. Highways data supplied thus far are

shown as Map 7 in Appendix D.

Highways Agency data indicated there are some key flood risk areas along the

A27, the most significant being the junction of the A27 and A2270 (Eastbourne

Road).

Phase 1 of the action plan process formed a strong working relationship

between Eastbourne Borough Council, Wealden District Council, Southern

Water and the Environment Agency. Through this partnership a

comprehensive data collection exercise was undertaken which is discussed in

detail in Section 2.3. Objectives were agreed with the partners during an

inception meeting which can be found in Section 1.3.

Phase 2a: Intermediate Risk Assessment

52

3.0 Phase 2a: Intermediate Risk Assessment

Figure 3-1 – Phase 2a: Risk Assessment Phase of SWMP Process

3.1 Introduction

This phase focuses on the source-pathway-receptor model for giving a broad

overview to surface water flood risk in Eastbourne and South Wealden.


53

3.1.1 Surface Water Flooding from Surface Runoff

(Source: BBC Website, Eastbourne flooding 2006)

Figure 3-2 – Photo of Surface Runoff Flooding

In urban areas, surface water flooding occurs when intense rainfall is unable

to soak into the ground and when road gullies and/or pipes have insufficient

capacity to allow all surface water to enter the drainage network.

In these conditions surface water builds up locally if the ground terrain is flat,

or travels following prevailing terrain gradients. Surface water flooding is likely

to occur at locations where surface water flow paths converge, at local dips in

the ground, dry valleys and/or at overland obstructions.

Runoff is the main mechanism of surface water flooding but can be

exacerbated by the lack of maintenance of drainage infrastructure.

The impact of urbanisation (i.e. the expansion of impermeable areas (roof

areas and paved areas)), has resulted in increased runoff volumes and peak

flows reaching drainage networks. This increase has, in many cases,

exceeded the available capacity of the drainage networks (which were not

necessarily designed for extreme storm events) and has severely increased

the likelihood of surface water flooding.

Current estimates in accordance with UKCIP09 (see Section 9 - References)

indicate that climate change could increase peak rainfall intensities by up to

30% by 2115. It has been found as part of this study that large scale surface

water flood reduction schemes are not economic for the Eastbourne

catchment and therefore local based schemes were progressed. These local

schemes generally have a short design life and therefore climate-change

future-proofing of them is not necessary. The design horizons of the schemes

proposed in this report are discussed in greater detail in Section 6.1.


54

3.1.2 Surface Water Flooding from Surcharged Manholes and Gullies

The photo in Figure 3-3 show a typical situation where the water pressures in

a pipe network system are high enough to remove manhole covers.

(Source: EBC, Star Inn flooding)

Figure 3-3 – Photo showing flooding from manholes

In addition to surface water flood risk from surface runoff (see Section 3.1.1),

surface water flooding can also occur as a result of surface water inundating

foul or combined sewers causing sewer flooding from manholes and gullies.

This is the result of extreme storm events where the overwhelmed sewerage

system surcharges manholes and gullies in the critical low-spots within the

catchment, as shown in Figure 3-3.

Flooding out of manholes and gullies can originate from surface water drains

and sewers, combined sewers, or foul sewers. These are defined below:

a) Surface water drains and sewers carry only surface water originating from

roofs, pavements and roads, not foul water.

b) Foul sewers do not normally carry surface water. In some circumstances,

however, surface water runoff is able to enter the foul system, which can

result in overflowing of manholes and flooding of property.

c) Combined sewers carry both foul water and surface water originating from

roofs, pavements and roads.

Figure 2-7 in Section 2.4.4 shows the distribution between surface water

drains, foul sewers and combined sewers in the Eastbourne area.

Sewer flooding appears to be the highest risk for the older networks of sewers

in the town centre, which are designated as ‘foul’. However, the absence of


55

surface water sewers in the study areas and local knowledge confirm that they

are actually acting as combined sewers and were designed as such.

Inundation as well as direct surface water connection were significant factors

in combined sewer overloading during the 2006 storm. The situation is likely to

deteriorate further under the effects of climate change, where rainfall intensity

is predicted to increase by 30% in the next 100 years.

The same effects of climate change and future development proposals are

applicable to this form of flood risk (from overflowing manholes and gullies) as

for surface water flooding from surface runoff. Sewer flooding arising from

surface water inundation can provide additional drivers (social, environmental

and economic) for the improvement of surface water systems.

3.1.3 Flood Risk from Groundwater

Figure 3-4 – Photos showing groundwater flooding

Groundwater flooding occurs when the water table rises above the ground

surface. This is normally the result of persistent rainfall over a long period.

The water table is the level at which the ground changes from saturated

(where the soil contains as much water as it can hold) to unsaturated (where

the soil has some spare capacity). Rainfall soaks into the soil and, if

sufficiently persistent, eventually fills it up so that it cannot hold any more

water; in this case, distinguishing between whether it is surface water flooding

because it cannot infiltrate or groundwater flooding because the water table

has reached the surface becomes difficult to discern.

Groundwater flooding does not necessarily occur after a local rainfall event;

rainfall falling further away may cause the groundwater to rise over a much

wider area, e.g. due to an extensive aquifer, and thus cause groundwater

flooding at other locations. Again, if the ground is already saturated and

causing groundwater flooding, then any further local rainfall will be unable to

infiltrate and result in surface water flooding.


56

High groundwater levels can also cause springs to occur, where water

travelling through permeable rocks emerges from a hillside, or where it

reaches a more impermeable rock and is forced to the surface. This is the

case on the South Downs to the west of Eastbourne.

The water table in the low lying areas of Eastbourne close to the sea is

dominated by tidal levels. Consequently, the water table around Eastbourne

centre is an average of 0.14m AOD (which is the mean sea level at this

location). Figure 3-5 gives an indication of the areas in Eastbourne where

existing ground levels are less than 0.14m AOD.

Figure 3-5 (Map 8 in Appendix D) – Groundwater Flood Risk in Low-lying

Areas


57

Figure 3-6 below shows the historic flooding locations as well as the likely

areas which will experience groundwater flooding.

Figure 3-6 (Map 9 in Appendix D) – Groundwater Flood Risk

The cross-section line on the above figure relates to the cross sections which

have been undertaken as part of the technical note on groundwater flood risk

which can be found in Appendix C.

If sea water levels rise by 1.05m over the next 100 years (in accordance with

the latest climate change guidance from UKCIP09), then the water table in

Eastbourne town centre and Eastbourne Park will rise by a similar amount.

Traditionally it had been thought that at locations where the water table is near

the surface there is a risk that ground water will enter the sewer network

through pipe joints or cracked pipes. This results in reduced capacity of the

drainage system. Southern Water has reported that this has been a common

occurrence in the Langney area of Eastbourne.


58

However, they are increasingly finding that this traditional explanation is not

the case. Instead, the overloading is caused by:-

a) Surface water flooding as a result, for example, of soakaway failure

entering their system by other means, e.g. manhole covers and sink waste

gullies.

b) Surfacing springs in the slightly higher western part of the catchment,

flowing across the surface and flowing into the highway drains and surface

water sewerage system.

Future development will not affect groundwater flood risk provided that their

foundations do not affect groundwater flow paths. Groundwater levels follow a

seasonal trend which is likely to be amplified by climate change. The lack of

data regarding groundwater flooding within the Eastbourne area means that

this potential risk needs to be investigated further (see Action Plan in

Appendix F).

3.1.4 Flood Risk from Ordinary Watercourses

Figure 2-5 in Section 2.4.3 identifies the locations of ordinary watercourses in

the Eastbourne area.

Ordinary watercourses are all watercourses that are not designated Main

River (watercourses where the Environment Agency has responsibilities and

powers) and which the LLFA will have consent over (currently proposed to be

from April 2012) if works are required to be undertaken.

There are many ordinary watercourses within the study area which have been

reported to have flooded in the past due to lack of capacity and/or due to a

lack of adequate maintenance. The upper parts of the catchment

predominately to the west of Eastbourne on the South Downs, experience

flooding from ordinary watercourses due to the high runoff rates and short lag

time during a short rainfall event. The lower part of the catchment

experiences localised flooding from drainage ditches potentially caused, or at

least worsened, by partial blockages or excessive vegetation within the

watercourse, or due to a poor maintenance regime. It is important that the

relevant authority identifies these assets in the future for prioritised

maintenance in parallel to the task of populating the risk asset registers. The

LLFA will have the power to serve notice (currently proposed to be from April

2012) on a riparian owner to maintain watercourses and ditches necessary for

the proper operation of the surface water drainage system within the

Eastbourne area.


59

In accordance with UKCIP09, climate change will increase peak flows in

watercourses by about 20% over the next 50 to 100 years. This implies the

capacity of watercourses, and drainage ditches may not be sufficient in the

future. New development could also worsen this situation unless effective

policies are put in place now.

3.1.5 Flood Risk Interactions with Surface Water

Surface water flood risk can be increased as a result of the surface water

drainage system being impacted by high groundwater, fluvial and tidal levels.

This occurs when surface water flows are unable to drain through, for

instance, the soil, via an outfall into an open channel or to sea causing

significant ponding/backing up resulting in localised flooding.

The interaction with high groundwater levels has already been described in

Section 3.1.3. The interaction with high fluvial and tidal levels is described in

Sections 3.1.6 and 3.1.7 respectively.

3.1.6 Effect of High Fluvial Levels on Surface Water Flooding

The impact of fluvial flood risk in the Eastbourne area can be seen in Figure

3-7 below which denotes the 1 in 100yr and 1 in 100yr plus climate change

flood extents (taken from Capita Symonds 2007 study (see Section 9 –

References)).

Figure 3-7 (Map 10 in Appendix D) – Fluvial Flood Risk


60

The Environment Agency records identify three key flood events, in November

1974, July 1980 and December 2009, which caused flooding along its

designated main rivers in Eastbourne Park. Only the last of these occurred

after completion of the Eastbourne Lakes scheme and the flood extents are

confined to the Park area. According to the “Eastbourne Park and Flooding

Topic Paper No. 3” of April 2002 (see Section 9 – References) no flooding of

residential or commercial property occurred up to that time.

The flood extents of the fluvial events are mainly outside any urban areas and

therefore it seems reasonable to conclude that they do not impact directly on

surface water flood risk. In addition, the peak of surface water and fluvial

events are different for the catchment, so unlikely to occur together (see a

technical note undertaken by Halcrow (see Appendix C), which identifies this

difference for the Eastbourne Lakes).

3.1.7 Effect of High Tidal Levels on Surface Water Flooding

Surface water flooding is exacerbated during tide-locking conditions (when

flap gates at surface water outfalls close to prevent sea water entering the

system) combined with rainfall events. Tide-locking results in surface water

accumulating upstream of the outfalls in the drainage system, which leads to

reduced capacity so a storm event may cause flooding out of manholes and

gullies.

The area most prone to surface water flooding from sewers due to tide-locking

is in the northeast of Eastbourne town centre (Firle Road area) where

localised flooding of the road and occasionally properties has occurred.

Flooding in Eastbourne Park due to tide-locking is minimal due to the

significant floodplain areas and lakes available to store surface water.

Surface water flooding is also exacerbated at locations where sea-water

seepage occurs beneath the existing defences, which contributes to

maintaining a high water table.

The above effects will increase with climate change due to sea level rise as

well as increased storm intensities over the next 100 years. For Hotspot A the

impact of climate change is expected to increase flood risk by between 15-

20%, whilst for Hotspots C & D flood risk will increase by between 50-80%.

The predicted increase in flood risk over the next 100 years for Hotspot C & D

is much higher than Hotspot A since it is also impacted by sea-level rise. In

reality the actual impacts of climate change is considered to be lower than the

model is predicting, since current climate change estimates are considered to

be conservative.


61

3.1.8 Residual Flood risk

Residual flood risk occurs when the infrastructure that is designed to deal with

local flood risk fails due to poor maintenance, or when a storm event has a

higher return period than that that used for the design. Residual local flood risk

can therefore be the result of drains, pipes and gully blockages due to poor

maintenance or misuse of the drainage system as well as pump failure and

pipe collapse.

The SWMP has highlighted a critical culvert (Bourne Stream) which flows

through the centre of Eastbourne before discharging to Horsey Sewer. The

maintenance of this culvert is essential to minimise the risk of a blockage or

collapse. This is discussed in further detail in Section 5.2.4 and is included in

the Action Plan (see Appendix F).

3.1.9 Flooding of Basements

Basement flooding can be significant due to the low-lying nature of the

properties with relatively high depths of flooding. This study has been unable

to identity any dataset which indicates where basements are located within the

Eastbourne area. Such a dataset needs to be created and this action is

included in the Action Plan (see Appendix F). To help with the protection of

future development, Buildings Regulations (2002) part H on drainage and

waste disposal requires anyone developing a basement with sanitation to

consult with the relevant sewer undertaker to discuss foul flood risk to the

property and to undertake preventative measures.

3.1.10 Land Use

The study area is predominantly urban with a high proportion of densely

developed areas with mixed land-uses. Eastbourne town centre, in the south-

west of the study area, is the most heavily urbanised and includes several

industrial areas. In the centre of the study area there is a large greenfield

space, which contains lakes provided for flood storage (Eastbourne Park

Lakes).

3.1.11 Significant Infrastructure

Significant infrastructure is defined as either ‘Essential Infrastructure’ or

‘Highly Vulnerable Infrastructure in accordance with PPS25. The national and

local significant infrastructure assets within the study area include:

District General Hospital, Police Station, Fire Station and Ambulance

Station

Railway Station and trunk roads

Utilities infrastructure


62

Residential Establishments (i.e. Care Homes)

Arndale Shopping Centre

3.1.12 Historic Flooding Incidents

A range of information on historic flooding incidents has been collected and

mapped for this study. The local Strategic Flood Risk Assessment (SFRA),

compiled by Scott Wilson in September 2008 (see Section 9 – References),

and covering the borough of Eastbourne and the southern part of the district of

Wealden as far north as Heathfield, was a valuable source of information. This

SFRA sources data from records held by ESCC, EBC, WDC, the Environment

Agency and Southern Water. Where the records held by these organisations

have been updated since the SFRA, the latest version has been obtained for

the present SWMP. Individual meetings have also been held with the partners

and key stakeholders to gain an overview on flooding ‘hotspot’ areas as

defined in Section 3.4.

Data was collected from all partners and other studies and is detailed in

Appendix A. All of the data is illustrated in Figure 3-8 overleaf.


63

Figure 3-8 (Map 11 in Appendix D) – Historic Surface Water Flooding

Incidents


64

Information from the SFRA relating to historical flooding incidents within the

SWMP study area is extracted and reproduced in Table 3-1 below:

Event Date Details

1836 Records of flooding on the Pevensey Levels date back to

1836, when they were reported flooded, along with much of

the low-lying land in Sussex.

July 1893 A summer storm over Eastbourne resulted in severe flooding

of the town, converting manholes into fountains.

November 1974 Flooding of several properties and roads are recorded in

Eastbourne.

December 1984 Fluvial flooding caused watercourses to overtop resulting in

the flooding of properties and roads in Eastbourne.

March 1995 Surface water flooding in Polegate, Wannock and Willingdon

resulted in 56 properties being inundated as well as flooding

of a number of roads causing disruption, particularly through

the closure of the A22.

A report into this flooding concluded that it was due to a high

intensity rainstorm over saturated catchments leading to high

volumes of runoff. The culverts and channels at points along

Wannock Mill Stream, Mill Stream Ditch and Brook Street

Stream had insufficient capacity and thus flooding ensued.

The current policy (C5) from the CFMP for the Polegate area

is to “take further action to reduce the flood risk now and into

the future”.

August 2006 Internal flooding of Arndale Shopping Centre and significant

highway flooding of Terminus Road caused by an intense

localised rainfall event overwhelming the existing drainage

system.

Table 3-1: Historical Flooding from the Eastbourne and south Wealden

SFRA.

Historic flooding data has been collected from a number of sources, and as

much as possible mapped so that a good overview of flood risk could be

gained (see Figure 3-8, above).

The main problem areas which are emerging from the review of historical

flooding data appear to be Eastbourne town centre, the stretch along the

Bourne Stream route and the Polegate/Willingdon area.

3.1.13 Proposed Future Development

Eastbourne Borough Council through its Core Strategy has made a

commitment to deliver 5, 022 new homes over the 20 years from 2006 to


65

2027. Development within the borough is constrained by the sea to the south

east, the South Downs to the west, and the Eastbourne Park flood storage

scheme in the centre. Given these constraints, there is little available land

within the Borough which can be brought forward for development. Therefore,

growth will need to be achieved through urban change and renewal. The

Strategic Housing Land Availability Assessment (SHLAA), which has informed

the Core Strategy, assesses potential sites that may be developed for

housing, considering factors such as how many units they could

accommodate and a timeframe for when these sites could become available.

However, communication with the Council indicates that given the scarcity of

available land there are unlikely to be any major development areas and the

Council are expecting to depend heavily upon windfall sites for

redevelopment.

Wealden District Council aims to deliver 700 new dwellings in the Polegate

and Willingdon settlement between 2006 and 2030. The options for delivering

these were set out in the Core Strategy which was published for consultation

in August 2009. The options cover various ways of distributing the housing

including: growth of existing settlements proportionate to their current size;

focussing the majority of growth on Uckfield, Hailsham and/or Polegate;

growth taking particular account of criteria such as AONB, housing needs and

accessibility; growth focussed on developing villages; and growth to support

reopening the Lewes to Uckfield railway line.

A valuable spatial perspective can be gained by overlaying the areas at risk in

structural and non-structural hotspots with the proposed strategic sites

identified in the Eastbourne Core Strategy. Figure 3-9 represents this overlay,

which may provide evidence to future planning actions in the Borough in

relation to future development impact on surface water flooding.


66

Figure 3-9 (Map 12 in Appendix D) – EBC Core Strategy Area against SWMP

Hotspot Areas

3.1.14 Surface Water Related Schemes

Based on the information collected and interviews held with the stakeholders,

the present and future schemes to improve flood risk within the study area are

summarised below:

Current Schemes:

Eastbourne Borough Council has recently carried out work to Princes Park

Sluice which consisted of replacing the two old, hard to operate, wind-up

sluices with a tilting weir to enable water to discharge more quickly to sea

via Crumble Outfall.

A similar scheme is proposed for the Crumble Sewer, located at a

bifurcation (i.e. flow is split, either going to Pevensey or to Crumble

Outfalls), where they are proposing to replace the old wind-up sluice with a

tilting weir to feed water to the Princess Park Sluice.


67

Weed screens are being installed behind Sainsburys store site on Decoy

Stream, so as to be compliant with the latest legislation – this will stop

weeds going into the culverts and causing blockages. The same has been

done at the Currys store site.

The EA are currently working with Halcrow on a planned closure and

refurbishment of outfalls on the Pevensey Levels.

Future Schemes:

The AMP5 (Asset Management Plan) programme for the regulatory period

2010 to 2015 includes only one scheme involving a package pump for a

DG5 property on Terminus Road.

Eastbourne Park Lake improvements (see Section 6.4)

3.2 Source, Pathways and Receptors

The source-pathway-receptor model is frequently used in assessment of flood

risk, it identifies where the water comes from as the source, the mechanism by

which the flooding reaches the receptor as the pathway, and the object which

experiences the flooding, and normally an adverse effect from it, as being the

receptor. Likely sources, pathways and receptors for flooding are listed in

Table 3-2 below.

Type of flooding Sources Pathways Receptors

Surface water Rainfall Roads

Ditches

Overland

People

Buildings

Environment

Groundwater High water table Aquifers and the

overlying soil

People

Buildings

Environment

Sewer Rainfall

Water discharged

from property

Inundation inflows

in areas of surface

water flooding.

Sewer network People

Buildings

Environment

Table 3-2: Primary sources, pathways and receptors for surface water

flooding

Table 3-3 below indicates the sources, pathways and receptors for flooding

which have a secondary impact on surface water flooding. Although these

sources do not result directly in surface water flooding, they will impact on the


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significance of surface water flooding, through tide-locking of drainage outfalls

for example.

Type of flooding Sources Pathways Receptors

Fluvial Rainfall

High groundwater

resulting in springs

or ephemeral

watercourses

Watercourses

overtopping or

breaching

defences

People

Buildings

Environment

Tidal Wind

Tidal surge

Wave or surge

tide inundating

the land

People

Buildings

Environment

Table 3-3: Secondary sources, pathways and receptors for surface water

flooding.

The source, pathway, receptor model was used as a general approach for the

formation of the storyboard which is discussed in greater detail in the following

section.

3.3 Introduction to Hotspots and Maps

The following terminology is used throughout the SWMP. The spatial scale of

these areas follows a hierarchy and is presented below from the smallest to

largest:

1. Intermediate Hotspot is a discrete area of flooding that affects

houses, businesses and/or local infrastructure. The boundary is

defined by the readily available information on surface water flood risk

(mainly the EA’s FMfSW dataset) and confirmed by partners. These

are areas where future work may be required beyond that to prepare

this SWMP, subject to funding, to identify proposed solutions to

reduce flood risk locally (item in Action Plan)

2. Detailed Hotspot is an intermediate hotspot which has been

prioritised through the ranking process (see Section 3.4) and agreed

by partners as an area with significant surface water flood risk.

Thirty six intermediate hotspots have been identified as a result of undertaking

the Intermediate Risk Assessment and four of them have been identified for

further detailed assessment (see details in Section 4). All hotspots are shown

in Figure 3-10.


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Figure 3-10 (Map 13 in Appendix D) – Hotspots in the Eastbourne Area

3.3.1 Critical Drainage Areas of hotspots

In accordance with SWMP Defra guidance document (see Section 9 -

References) Critical Drainage Areas (CDAs) are specific areas in Flood Zone

1 only, where runoff can cause problems. To determine the CDAs LiDAR was

used to determine the upstream catchment area (in EA’s Flood Zone 1) for

each hotspot, this is shown in Figure 3-11 overleaf.


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Figure 3-11 (Map 14 in Appendix D) – Critical Drainage Areas in the

Eastbourne Area

The CDAs indicate areas where flood risk is to be carefully monitored to

minimise any increase in flood risk to the hotspot areas downstream.


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3.4 Selection of Hotspots and Prioritisation

The selection of hotspots was based on interpreting readily available

information, this included:-

1. Historic Flooding Incidents – records collected by partners on

predominantly surface water flooding.

2. Environment Agency Surface Water Mapping – predicted surface

water flood risk modelled by the Environment Agency in the form of

two datasets:

a. AStSWF (Areas Susceptible to Surface Water Flooding) – first

generation dataset indicating the susceptibility to flooding for

the 1 in 200 year rainfall event.

b. FMfSW (Flood Map for Surface Water) – second generation

flood mapping indicating deep or shallow flooding for the 1 in

30 and 1 in 200 year rainfall events. This dataset is more

accurate than the first generation data since it has taken into

consideration the influence of buildings and the sewer system.

3. Environment Agency AStGWF (Areas Susceptible to Ground

Water Flooding) – indicates the likelihood of groundwater emergence

at a 1km square grid, this dataset was predominantly used for the

PFRA study.

4. Partnership Workshops – detailed information on the frequency,

extent and impact of known flooding within the Eastbourne area.

The above information was presented in a storyboard format (see Appendix

B), which was based around using the source, pathway and receptor

descriptions and was tabled with the partners for discussion. Through the

workshop additional information was collated from partners and all relevant

organisations agreed to the proposed hotspot areas.

A prioritisation method was developed by Halcrow and agreed by ESCC to

determine which of the hotspots had the most significant flood risk and

required further investigation in the Detailed Risk Assessment phase of the

SWMP (see Section 4). The prioritisation process was to classify (low,

medium or high) each hotspot on a set criteria and derive a score which could

then rank the hotspots. The variables used to score the hotspots were:-

1. Number of buildings in the predicted flood risk flood extents created by

Halcrow and the EA

2. Amount of critical infrastructure receptors


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3. Number of historic flooding incidents

4. Number of partners agreeing that the hotspot has known flooding

issues

A higher score weighting was applied to the number of historic flooding

incidents and number of agreeing partners, to ensure known actual flood risk

data would be prioritised over predicted flood risk.

The outcome of the scoring process was that intermediate hotspots 6, 12, 22

& 15 required further investigation; this was agreed by ESCC and the

partners. The intermediate hotspots were then renamed detailed hotspots A

(no.6) - Willingdon, B (no.12) - Langney, C (no.22) – Eastbourne Town Centre

& D (no.15) – Bourne Stream. The prioritisation spreadsheet can be found in

Appendix B.

Through the data collection exercise and consultation with ESCC and partners

37 intermediate hotspot areas were identified to be at risk from surface water

flooding. Hotspot prioritisation with the partners, based on flood risk, identified

4 detailed hotspots which were taken forward to the Detailed Risk

Assessment.

Phase 2b: Detailed Risk Assessment

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4.0 Phase 2b: Detailed Risk Assessment

Figure 4-1 – Phase 2b: Detailed Risk Assessment Phase of the SWMP

Process

4.1 Modelling & Detailed Assessment

During the SWMP process it became apparent that the flooding mechanisms

associated with Hotspot B (Langney) was caused by a complex combination

of surface water and groundwater flooding (see Section 3.1.3), for this reason

it was not modelled, however, recommendations are made in the Action Plan

(see Appendix F).

4.1.1 Introduction

Two Infoworks CS (Collection System) sewer models were provided for the

Eastbourne area by Southern Water. Infoworks CS is a specialist one

dimensional pipe modelling software capable of hydrological modelling of the

urban water cycle. A rainfall profile is applied as inflows to the model based on

FEH (Flood Estimation Handbook) design rainfall and the flow of water is

modelled until it leaves the pipe system.


74

The existing 1D models were built to model piped flows but did not have an

above ground component. Water was effectively “stored” above ground until

water levels subside below ground level and water is able to return to the

system. Flow routes and the exact location of the storage areas (and therefore

flooding) were therefore disregarded without a 2D mesh component included.

The following sections describe the model updates that have been carried out

within the SWMP to modify the model so that flow routes above ground level

were included, resulting in the model being effectively converted to Infoworks

2D.

4.1.2 Model Updates

A number of updates to the Infoworks model have been carried out to improve

the schematisation of above ground flow routes and improve model detail.

Only minor updates were included within the 1D pipe network in order to

improve model schematisation for modelling surface water. All pumping

station capacities and trigger levels were left as they were included in the

model supplied by Southern Water. The main updates to the model lie within

the inclusion of a 2D domain and associated improvements to modelled flow

routes.

4.1.3 1D updates

The Bourne Stream was imported into the model and the connections

between the sewers and the watercourse were checked to allow the

interactions between the sewer system, overland flows and watercourse to be

represented.

Another change to the 1D model included the addition of tidal levels to the

model outfalls. The tidal levels added to the model were the mean high tide

levels. This ensured a conservative approach assuming that the event could

coincide with a high tide, but without being too conservative by using a spring

tide level.

4.1.4 Broad scale 2D mesh generation

A 2D triangular mesh was added to the model to simulate above ground flood

routing. Meshes were created for the hotspot areas with a maximum triangle

area of 100m2. This was designed to give an indication of flood risk across the

whole hotspot area. Mesh elevations were taken from the most recent

available 2 metre resolution LiDAR data. The following characteristics were

also included in the detailed model area mesh:

A. Buildings were included within the mesh as voids which water cannot

pass through.

B. Kerbs were included as break lines with a height of 0.125m


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C. Following sensitivity tests, a mesh roughness of 0.035 (roughness for

floodplains) was applied for Hotspots A and B, this represents the flow

paths following the rivers rather than the roads. A value of 0.016

(roughness for asphalt) was applied for Hotspots C and D; this

represents the urban nature of the catchment with the majority of the

flow paths following the roads.

Figure 4-2 – InfoWorks 2D Mesh

4.1.5 Rainfall profiles and application to model

A rainfall profile was generated using FEH design rainfall. The rainfall profile is

applied to the model in two ways:

A. Through “subcatchments” based on the drainage system and property

layouts (with contributing areas within the “subcatchment” based on

the size of permeable and impermeable areas); and

B. Rainfall applied directly to the 2D mesh outside of “subcatchment”

areas with 50% of the rainfall profile applied after assessing bedrock

permeability and through validation against historical flooding locations

(discussed further in the following model sensitivity section).

4.1.6 Model sensitivity and model validation

Model sensitivity analysis was carried out to assess how each model

parameter affects model results. Sensitivity tests were carried out for:

A. Pipe Roughness

B. Percentage rainfall applied directly to mesh


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C. Allowance for gullies

A model was run with increased pipe roughness across the catchment. This

was found to have marginal impact on the extents and locations of flooding

and therefore the original Southern Water model pipe roughness values were

used for the modelling.

The model was calibrated against a GIS layer of historically flooded properties

and known flow paths. To calibrate against this, the model was tested with

different amounts of rainfall (100%, 75%, 50% and 25%) being applied directly

to the mesh. Based on the simulation results and experience from other

SWMPs, it was finally decided that 50% was a suitable percentage to apply to

the model. This produced results which matched many of the historical flood

locations and flow paths but was also a realistic representation of runoff within

the catchment.

As the Southern Water model does not include the gullies, an allowance was

made in the model to allow flow to pass more easily between the 2D surface

and manholes, thus giving a representation of the gullies. The simulation

results showed effectively no change in the depth of flooding on the surface

(+/- 0.001 metre) and therefore the gully representation was not used in the

modelling.

An instability was identified in the Hotspot A model at the downstream end of

the Wannock Mill Stream. On these links there were shown to be large

negative flows. However, through additional analysis and model testing this

did not impact the flows in the area upstream and therefore impact on the

results found, this is discussed further in the Modelling Technical Note in

Appendix C.

In the detailed modelling stage 2D modelling of Southern Water’s sewerage

systems was undertaken for only 3 detailed hotspot areas to understand and

communicate the extent of surface water flood risk and the flooding

mechanisms.

Phase 2c: Communicate Risk

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5.0 Phase 2c: Communicate Risk

Figure 5-1 – Phase 2c: Communicate Risk Phase of the SWMP Process

5.1 Mapping Outputs

The output from both the Intermediate & Detailed Risk Assessment stages are

maps indicating the surface water flood risk for the Eastbourne area, giving an

in-depth understanding of flood risk for the hotspots.

5.1.1 Eastbourne Area Flood Depth Map

A flood depth map for the Eastbourne area (Map 15 in Appendix D) uses a

combination of the EA’s second generation surface water flood maps (FMfSW)

overlaid with Halcrow’s detailed 2D sewer modelling from the detailed hotspot

areas. This provides a detailed understanding of not only surface water runoff

and flow paths across the catchment, but also flow routes from surcharging

manholes.


78

Figure 5-2 – Extract of the Eastbourne Area Flood Depth Map


79

5.1.2 Detailed Hotspot Flood Depth Maps

Detailed mapping of surface water flood depths for all three hotspots (see

Maps 16 & 18 in Appendix D) was produced, with a blue colour banding for

depths over 150mm (threshold for internal property flooding) and light yellow

to represent flood depths over 50mm. Properties which are surrounded by a

proportion of 150+mm of flood depth are considered to have flooded internally.

The flood outlines shown are deduced from the 2D triangulated mesh and

hence are angular in form.

Figure 5-3 – Extract of Flood Depth Map

5.1.3 Detailed Hotspot Flood Hazard Maps

Detailed mapping of surface water flood hazard for all three hotspots was then

undertaken (see Maps 17 & 19 in Appendix D). Flood hazard is used to give

an indication of the risk to life posed by flood water. The severity of flood

hazard depends on the water depth and velocity, along with an additional

degree of hazard from debris in the water. This reflects the danger to life from

a combination of deep and fast flowing water. For example, shallow water,

when still, does not present a major hazard, but combined with a fast flow is

potentially dangerous.


80

A method for quantifying flood hazard is described in the Supplementary Note

on Flood Hazard Ratings and Thresholds for Development Planning and

Management Purposes – Clarification of Table 13.1 of FD2320/TR2 and

Figure FD2321/TR1 (see Section 9 – References). The formula devised is:

Flood hazard = depth x (velocity+0.5) + debris factor

For most circumstances the debris factor is unknown, but in most modelling

exercises a standard is assumed; further information is detailed in the

Modelling Technical Note in Appendix C

The flood hazard matrix used for the hazard rating is shown in Table 5-1.

Table 5-1 – Flood Hazard Matrix

Depending on the flood hazard value calculated, the severity of the hazard is

indicated by the class into which the value falls, which are shown in Table 5-2.


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Table 5-2 – Flood Hazard Classification

The flood hazard outlines shown are deduced from the 2D triangulated mesh

and hence are angular in form.

Figure 5-4 – Extract of Flood Hazard Map

Flood Hazard

Rating

Hazard to People Classification

Less than 0.75 Very low hazard - Caution

0.75 to 1.25 ‘Danger for some’ – includes children, the elderly and the

infirm.

1.25 to 2.0 ‘Danger for most’ – includes the general public.

More than 2.0 ‘Danger for all’ – includes emergency services.


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5.2 Summary of Flooding Mechanisms for Detailed Hotspots

5.2.1 Hotspot A – Mill Stream Gardens, Willingdon

Sloping urban environment with dry valleys which become flooded by the

Wannock Mill Stream during high intensity rainfall events. Localised overland

flow paths between properties and along Millstream Gardens cause further

flooding with water ponding at the junction of Millstream Gardens and

Farmlands Avenue. Surface flood water may also inundate the foul sewer

network.

5.2.2 Hotspot B – Langney

Water levels in the land drainage and the surrounding EA Main River open

sewers prevent the free discharge of the surface water sewers into the

accepting watercourses. This, coupled with relatively high groundwater,

causes backing-up and egress of groundwater into the foul sewerage system

and entry of irregular surface water flows from non-formalised sources (i.e.

poorly sealed manholes) which results in a surcharged system.

5.2.3 Hotspot C – Firle Road & Arndale Shopping Centre, Eastbourne

a) Firle Road - The surface water sewer running along Firle Road

surcharges and floods during high intensity rainfall events. This flooding

mechanism is exacerbated during high tide when the outfall is tide-locked;

this problem is expected to occur more frequently with sea-level rise due to

climate change

b) Arndale Shopping Centre – Twin surcharging combined trunk sewers

running along Terminus Road prevents effective drainage of the shopping

centre and the pedestrianised precinct during high intensity rainfall events.

5.2.4 Hotspot D – Bourne Stream & Star Inn, Eastbourne

a) Bourne Stream - Failure or blockage scenarios of the Bourne Stream

were modelled at locations along the culvert, which is currently in a poor

condition (as indicated by the limited amount of upstream CCTV surveys).

The failure/blockage locations were prioritised for areas with highly

vulnerable infrastructure (i.e. Terminus Rd outside the Train Station and

Arndale Shopping Centre). The results indicated that flood risk would be

increased in the area immediately surrounding and downstream of these

blockage locations. However, since the flood waters contained within the

culvert were effectively removed from the system at the blockage

locations, additional capacity was created further downstream thus

reducing flood risk locally in these areas.


83

b) Star Inn - A change in pipe gradient along the surface water sewer results

in a reduction in capacity at a low-spot, which causes significant flooding of

several properties and the highway.

The maps in this section indicate the flood risk in terms of depth, velocity and

hazard for each detailed hotspot area. Further modelling assessment clarified

the flooding mechanisms for each hotspot as discussed in this section to

inform the optioneering phase.

Phase 3: Options

84

6.0 Phase 3: Options

Figure 6-1 – Phase 3: Options Phase of the SWMP Process

6.1 Objectives

The objective is to form robust schemes to reduce surface water flood risk for

each of the detailed hotspots.

Each proposed scheme has been designed for the 1 in 25 year rainfall event

(unless otherwise stipulated). Significant surface water flows are also likely to

lead to surface water flooding, which has the potential to impact on all

infrastructure, traditional engineering approaches, such as increasing the

diameter of public sewers or constructing separate systems, are not

economically viable or practical on a large scale. For this reason the 1 in 25

Phase 3: Options

85

year design flood alleviation standard was used to test options to ensure cost

effective measures were recommended.

The outputs of this phase is the preferred options for each detailed hotspot

with flood damage calculations, approximate scheme costs and the associated

cost/benefit ratios (see Appendix E).

6.2 Measures

For each hotspot detailed schemes were developed, modelled and tested to

understand the degree of flood protection provided. A broad overview of the

type of options considered for each area is shown below:

Hotspot A (Willingdon) - To control flood waters within Millstream

Gardens road and improve conveyance under the shop on Farmlands

Avenue back into Wannock Mill Stream.

Hotspot C (Eastbourne Town Centre) - Construct a high-level overflow

or divert the surface water sewer to prevent flooding on Firle Road due

to tide-locking.

Hotspot D (Bourne Stream) - Increase capacity of the surface water

sewer upstream of the low-point by storing flows in Motcombe Park.

Details of each option scenario run can be found in Appendix E.

6.3 Arndale Shopping Centre

The modelling results indicated the main flooding mechanism in the Arndale

Shopping Centre area is the surcharging of the twin combined sewers running

along Terminus Road. The surcharging system prevents surface water from

draining away, coupled with the resulting backward-flow of sewage in to the

centre’s drainage system, and the result is localised flooding of both Terminus

Road and partial internal flooding of the Centre.

Possible measure to reduce the flood risk is to install flap values on the local

sewers, which are experiencing surcharging from the trunk sewers. There

would also be a need to consider the installation of removable flood gates on

the two main entrances of the Centre to prevent highway flooding entering the

low-lying shopping centre.

Further investigation into possible flood mitigation measures for the shopping

centre is recommended, for example as a separate feasibility study for the

town centre. Such a study should consult all relevant stakeholders and

consider the development of the western section of Arndale Shopping Centre

Phase 3: Options

86

and the highway modifications of Terminus Road to ensure sustainable

options are identified and flood risk is not increased elsewhere.

6.4 Eastbourne Lakes

EBC raised the issue that the Eastbourne Lakes had not been identified as an

area to consider through the SWMP prioritising process. As there are no

recorded incidences of flooding or receptor to be flooded, the lakes were not

identified as a hotspot. In addition, as the lakes are located downstream of the

main urban area at risk this meant they were judged not to be critical in terms

of the surface water flooding. To confirm that this position was correct a high

level review was undertaken of the discharge to the lakes.

It was reported that the Eastbourne Park flood alleviation scheme was set up

to offer surface water flood protection for Eastbourne due to new

development. Its construction and ongoing maintenance was funded by

development contributions calculated using a formula based on the plan area

of the development site. From a review of the latest information (a Scott

Wilson Report, June 2002), the Eastbourne Park flood alleviation scheme

offers a flood storage to a 1 in 100 year standard of protection, but it will not

be able to maintain this under the effects of climate change without

improvements to the scheme. Potential measures to address this were

recommended as:

a) Construction of new lakes with improvements to the Crumbles outfall.

b) Construction of a new Crumbles outfall

From the high-level review of the discharge to the lakes using the model

developed for the SWMP, it was found that if a significant rainfall event was to

hit the Eastbourne area and the upper fluvial catchment, the lag time for the

surface water to reach the Eastbourne Lakes area would be much less than

the time for the fluvial flood waters to reach the same location (see Halcrow’s

technical note Appendix C). In addition the SWMP model showed that the

volume generated by a storm event was only 30% of that previously

calculated. Therefore, it was considered highly unlikely that discharge from a

surface water and a fluvial flood event would arrive at the lakes at the same

time and even if it did the likely surface discharge volume would be much less

than that used to design the lake scheme. The review recommended that

there is a need to do more work to confirm the actual situation and this should

look at ways of increasing the use of the lakes for surface water flood storage.

The SWMP process has raised the interest of and eased communication

between the SWMP Partners and this has led to the agreement that a study of

Phase 3: Options

87

the current operation of the lake flood alleviation scheme will be progressed.

The first part of this study is to survey the current condition of the lakes and

their control assets. The survey information will then be used to develop a

model to understand their operation. Finally to use this model to look at how

the lakes should be operated in the future, look at ways of increasing their use

for surface water flood storage, what maintenance/ improvement works are

required to achieve this and how the developer contributions should be

handled in the future going forward.

6.5 Preferred Options

The preferred option for each detailed hotspot is as follows:-

Hotspot A - Kerb raising of the western kerb on Millstream Gardens

with a Super Gully located outside the shops on Farmlands Avenue

and a high-level overflow along the existing footpath.

Hotspot C - Divert the 1050mm diameter surface water sewer at

Bedfordwell Road to Eastbourne Park Lakes.

Hotspot D - Surface water storage in Motcombe Park with highway

flooding removed from Star Road by capturing overland flow on

Upperton Road.

Details of the preferred options, showing their impact on surface water flood

risk are shown in Maps 20, 21 & 22 in Appendix D.

It should be noted that the preferred options presented here are designs in

outline only, and demonstrate a possible workable solution to reduce surface

water flood risk with certain simplifications being made. Progress to detailed

design cannot proceed without more detailed investigation and option

appraisal at each detailed hotspot.

For example Hotspot C (Firle Road, Eastbourne Town Centre) has complex

interactions between the surface water and combined sewerage networks

which are not currently fully understood. Therefore, further work on the

existing integrated urban drainage modelling of the low lying part of the

Eastbourne Catchment with climate change will be required to move it from

strategic to a detailed level. If the further work identifies sufficient drivers to

satisfy OFWAT then, as they are essentially sewerage activities, both the

detailed investigation and the scheme can be included in Southern Water’s

business plan.

Phase 3: Options

88

6.6 Options Prioritisation

The preferred options were tabled with ESCC and the partners and were

prioritised in the Action plan (see Appendix F).

6.7 Strategic Environmental Assessment

The assessment has found that the SWMP offers significant potential to

reduce flood risk that could potentially cause damage to environmental and

cultural heritage assets. Some uncertain effects and one minor negative effect

was predicted. The uncertainty related to the freshwater ecology effects of

diverting a surface water sewer to Eastbourne Lakes and changing the flows

into and capacity of Bourne Stream. Uncertain impacts are also associated

with the new proposed storage area in Motcombe Gardens, which has the

potential to affect the depth of water levels and duration of flooding

downstream. New water storage, even if infrequent, could lead to waterlogging

of soils and affect the ability of terrestrial plant species to survive, albeit on a

small scale. The one minor negative effect identified relates to the temporary

loss of amenity greenspace associated with the proposed storage area at

Motcombe Gardens. However, it is considered that this could be avoided or

mitigated at the EIA stage if these options in Hotspot D are progressed.

For Hotspot A, no important designations for biodiversity, landscape or cultural

heritage or high value agricultural land were found to be in the immediate

vicinity of these options. SWMP options are therefore predicted to have

neutral effects on these SEA objectives. However, by improving surface water

conveyance into Wannock Mill Stream, there are likely to be benefits from

reduced flood risk to infrastructure and properties.

For Hotspot C, there are some significant material assets, landscape features,

amenity areas and heritage assets in the vicinity of the SWMP options, which

would be protected by improved drainage and reduced flood risk. Therefore,

the options score major positive effects for these SEA objectives whereas

minor positive effects are predicted for other SEA objectives. There are

potentially some opportunities to enhance biodiversity through sewer

diversions and habitat enhancement/creation at Eastbourne Lakes.

For Hotspot D, the options should result in reduced flood risk for material

assets and an area of high cultural heritage value; Old Town Conservation

Area. There are uncertain effects on natural resources and biodiversity due to

the uncertain impacts associated with the new proposed storage area in

Motcombe Gardens, which has the potential to affect the depth of water levels

and duration of flooding downstream. Further development of the option will

be needed before these effects can be detailed. In addition, its potential

(positive or negative) impacts on soil and water and habitats and species are

uncertain due to the level of available detail regarding this option at this stage.

Phase 3: Options

89

There is a potential opportunity to create small-scale wetland habitat (e.g.

Biodiversity Action Plan habitat) in the new storage area. Maintenance of the

Bourne Stream should help to conserve flows from the stream into the Horsey

and Crumbles Sewers, which are both designated SNCIs. The proposed

Motcombe Park storage area currently contains a pond but the options would

entail flooding an area of amenity greenspace in addition to the pond, hence a

minor negative score is predicted for the ‘open spaces’ SEA objective.

The SEA concluded by proposing a series of high-level monitoring measures

for the topics of biodiversity, water, cultural heritage, population and human

health and landscape and visual amenity. The SEA with the SWMP can be

made available for public consultation, and if necessary be updated following

consultation comments. Further details on the SEA can be found in Appendix

H.

A combination of options have been considered in this phase with the

preferred options comprising of either; managing overland flow paths,

improving conveyance or creating storage to reduce surface water flood risk.

All reduce flood risk for the designed 1 in 25 year rainfall event as a minimum

and will require further detailed investigation before progressing to detailed

design.


90

7.0 Phase 4: Implementation and Review

Figure 7-1 – Phase 4: Implementation Phase of the SWMP Process

7.1 Action Plan

The purpose of Phase 4 of the SWMP is to clearly identify actions and

responsibilities for the ongoing management of surface water flood risk within

the Eastbourne Study area, which have been emerged from Phases 1 to 3.

An Action Plan has been prepared for ESCC (see Appendix F). The purpose

of the Action Plan is to:

Give outline details of the actions;


91

Provide an indication of the priority of the actions;

Indicate the potential cost-of and benefit-from delivery, as well as the

potential funding sources;

Identify the timescale for delivery;

Identify the partners or stakeholders responsible for implementing the

actions;

Outline the actions required to implement the preferred options identified in

Phase 3;

Outline actions required to meet some of the requirements for ESCC as

LLFA under the FWMA 2010.

Actions within the Action Plan have been grouped under six headings as

summarised in Table 7-1 below.

Action Type General Description

Engagement Relates to those activities to engage with the

partners, stakeholders and the public generally.

The aim being to share information, promote

collaborative working and to find potential funding

streams.

Investigation To investigate asset condition, ownership and

operation.

Structural Flood

Reduction Measures

Measures where physical interventions are

proposed to reduce flood risk, which ranges from

the Phase 3 options, through support of partner

schemes to SuDs projects.

Non Structural Flood

Reduction Measures

Measures based around improved management of

existing assets or informed control of development

to reduce flood risk.

Data Collecting and assessing data to improve the

understanding of local flood risk leading to a more

effective management of the risk.


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Procedural The procedures that need to be set up and

progressed to help meet the LLFA’s duties and

deliver development management measures.

Table 7-1 - Types of Action within the Eastbourne Area Action Plan

An example of actions from the plan includes Southern Water progressing the

preferred proposed solution for surface water storage in Motcombe Park

subject to the agreement in principle of EBC with details to be included in the

Action Plan.

As identified in Table 7-1, several key actions for ESCC relating to duties and

responsibilities under the FWMA are outlined in Section 1.6. It is likely that

these actions will require consideration of internal functions within the Councils

and the adoption of new systems of data collection and asset management

(e.g. Halcrow’s developed online ‘Flood Portal’ tool).

Building on the understanding of local surface water flood risk gained as a

result of the Phase 1 work and continuing with the SWMP partnership will be

essential to the continued management of surface water across the

Eastbourne area in a joined-up manner.

One of the key drainage assets identified as needing attention from this

SWMP work is the Bourne Stream. As a result of the past changes to drainage

responsibilities, the ownership and the responsibility for this asset has become

unclear. Actions have been identified to determine the responsibility, so that

the asset can have the overdue maintenance it requires to operate effectively.

The culvert may need maintenance in order to permit inspection before asset

ownership can be determined or agreed.

The role of the lakes in Eastbourne Park in alleviating surface water flooding

was considered as part of the SWMP process. They were shown to be

dominated by fluvial rather than surface water flooding. Surface water flooding

in the Eastbourne area was found to be mainly due to runoff from the lower

slopes of the South Downs or rain that fell directly on the urban catchment.

This meant that the Eastbourne Park Lakes located downstream of the

Eastbourne urban area would not directly resolve the conveyance issues

experienced in this urban area. However, the SWMP process brought together

the right partners to start an investigation into the Eastbourne Park storage

scheme. This investigation should consider the mechanism of funding them

from developer contributions, how the currently available fund should be best

used and future contribution utilised. There seems to be a disconnection of the

surface water drainage system from the lakes in some areas. This should be


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considered further as part of the proposed work on the future operation of the

lakes.

As our understanding about surface water flood risk improves and more

information is made available, it becomes increasingly important to be able to

communicate the risk effectively both within ESCC, to other stakeholders and

members of the public. To this end, there is an action related to the future

communication of flood risk and the development of a public awareness plan,

which could be incorporated into a communication plan for the delivery of the

Action Plan.

7.2 Implementation Programme

A strategic level programme has been prepared in a ‘Gantt chart’ format to

show each of the actions from the plan, their priority and timeframes (see

Appendix F). This is seen as a tool to be developed to support future planning

and implementation of the actions.

7.3 Action Prioritisation

A judgement-based approach has been adopted to prioritise the actions in the

Action Plan. This was generally based on considering what impact completing

the action would have on the reduction of flood risk and how quickly it needed

to be undertaken. Therefore, for actions that made a big reduction to flood risk

and needed to be complete within a year they were given a ‘High’ priority.

7.4 Ongoing Monitoring


in order to manage the implementation of the proposed actions, review

opportunities for operational efficiency and to review any legislative changes.

The partners should hold a monitoring meeting every quarter to consider how

the actions are progressing and what needs to be done to drive the delivery of

the Action Plan forward. It is suggested that each partner should take it in turn

to host the meeting.

The SWMP Action Plan should be officially reviewed and updated once every

six years as a minimum. However, there may be circumstances which might

trigger a review and/or an update of the Action Plan in the interim, examples of

a possible ‘trigger-event’ are:

Occurrence of a surface water flood event;

Additional data or modelling becoming available, which may alter the

understanding of risk within the study area;

Operational decisions by partners that impact on the Action Plan;


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Additional (major) development or other changes in the catchment which

may affect the surface water flood risk.

The main output from the implementation phase is the Action Plan. The

approach and form of this Action Plan was developed during this stage. A

Gantt chart was drafted as a tool to be developed to support future planning

and implementation of the actions. The SWMP Partnership should continue

beyond the completion of the SWMP in order to discuss the implementation of

the proposed actions. The partners should hold regular monitoring meeting to

consider how the actions are progressing and what needs to be done to drive

the delivery of the Action Plan forward.

Conclusion

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8.0 Conclusion

8.1 Review Objectives

To conclude, the objectives outlined in Section 1.3 of the report are presented

in Table 8-1 below with evidence to confirm the status of each option.

Objective Status Evidence

Map surface water flood

risk - engage community

& stakeholders to share

knowledge

Completed

Eastbourne Area Flood Depth Maps show surface water risk

ESCC as part of preparing the PFRA undertook public consultation to capture localised flooding information by hosting a survey on its website for 3 months, and held drop-in sessions in each local authority area

Appendix A – details of data shared

Establish partnerships of

key stakeholders -

facilitating - data, skills,

resource & learning

sharing

Completed

SWMP Partnership formed and data collected in Phase 1 of study

Determine consequences

of flooding - establish

priorities & compare

mitigation measures

Completed Communicate Flooding mechanisms of detailed hotspot areas in Phase 2 (Risk Assessment)

Consider various measures during Phase 3 (Options)

Develop strategy to

inform on development

drainage - minimising

impact on receiving

system

Completed Identified CDAs for all hotspots in Phase 2 (Risk Assessment)

Action plan item for developments to attempt to achieve greenfield runoff

Develop implementation

plan - showing how

partners work together to

finance and implement

Completed Action Plan with proposed funding streams and Gantt Chart for delivery

Develop a periodical

review framework - to

plan & monitor

effectiveness of solutions

Completed Recommendations covering ongoing Monitoring discussed in Phase 4 (Implementation & review)

Identify effective &

affordable mitigation

measures - where

possible achieves

Completed Detailed Hotspots Options tabled detailing costs for damages and proposed schemes to calculate the cost/benefit ratio

Conclusion

96

multiple benefits

Deliver outputs to enable

a real change on the

ground – not just reports

& models - partners take

ownership of their flood

risk & commit to

recommended measures

and actions.

Completed Meeting to finalise agreement on Action Plan and proposed approach for monitoring

Plan Implementation – any possible ‘Quick Wins’

Partner approval signatures of the Final SWMP

Table 8-1 – SWMP Objectives & Evidence

8.2 Recommendations

It is recommended to implement the Action Plan detailed in Appendix F of this

SWMP report. Also it is suggested to ensure both the Action Plan and the

SEA are kept up to date by reviewing against the latest guidance and findings

of publicised reports as they become available.


in order to discuss the implementation of the proposed actions. The partners

should hold regular monitoring meeting to consider how the actions are

progressing and what needs to be done to drive the delivery of the Action Plan

forward.

For the successful implementation of the Action Plan it is envisaged that

additional resources and funding will be required from within the Local

Authorities.

8.3 Ownership of the SWMP Action Plan

ESCC’s responsibility as the LLFA in relation to this SWMP is to lead in its

production and ensure that it is periodically reviewed and updated. EBC &

WDC will be jointly undertaking the lead role in the delivery of the SWMP

Action Plan.

However, it is accepted that delivery of the Action Plan will, in most cases,

depend on finance to move projects forward and all partners will be expected

to pursue opportunities in order to seek funding that will enable this to happen.

References

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9.0 References

Surface Water Management Plan Technical Guidance, Defra, March 2010

Learning Lessons from the 2007 Floods, The Pitt Review, June 2008

Future Water – the Government’s Water Strategy for England, Defra, February

2008

Planning Policy Statement 25: Development and Flood Risk, Communities

and Local Government, March 2010 (revision)

UKCP09, UK Climate Impacts Programme (UKCIP), 2009, (www.ukcip.org.uk)

Environment Agency ABD Study for Eastbourne, Capita Symonds, 2007

Eastbourne Park and Flooding Topic Paper No. 3, Eastbourne Borough

Council, April 2002

Eastbourne Borough Council and Wealden District Council Strategic Flood

Risk Assessment Level 1, Scott Wilson, September 2008

Supplementary Note on Flood Hazard Ratings and Thresholds for

Development Planning and Control Purposes – Clarification of Table 13.1 of

FD2320/TR2 and Figure FD2321/TR1. Environment Agency/HR Wallingford,

May 2008.

Living on the edge – a guide to the rights and responsibilities of riverside

occupation. Environment Agency 3rd Edition April 2007.

Appendix A – Data Review

V0.10 11/05/2017 A1

Appendix A – Data Review

Appendix B – Hotspot Area Selection

V0.10 11/05/2017 B1

Appendix B – Hotspot Area Selection

Appendix C – Risk Assessment Technical Details

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Appendix C – Risk Assessment Technical Details

Appendix D – Maps

V0.10 11/05/2017 D1

Appendix D – Maps

Appendix E – Options Assessment Details

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Appendix E – Options Assessment Details

Appendix G – Planner, Emergency Planner & Public Briefing Note

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Appendix F – Action Plan


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Appendix H – Environmental Assessments

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