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Bristol City Council

Strategic Flood Risk Assessment

Level 2 SFRA – Main Report

November 2009

FINAL

Halcrow Group Limited

Flooding at Eastville culvert

intake in July 1968 flood event




November 2009

FINAL

Halcrow Group Limited

Halcrow Group Limited Ash House Sowton Exeter EX2 7LB

Tel +44 (0)1392 444252 Fax +44 (0)1392 444301

www.halcrow.com




FINAL

November 2009

Contents Amendment Record This report has been issued and amended as follows:

Issue Revision Description Date Signed

1 1 Draft Level 2 Report Jul 2008 JMD

2 2 Final Level 2 Report Nov 2009 JMD

Contents

1 Introduction 1 1.1 Background 1 1.2 Outline approach 1 1.3 Purpose of the Level 2 SFRA 3 1.4 SFRA Level 2 aims and objectives 4 1.5 SFRA User Guide 5 1.6 Future SFRA updates 5

2 Areas of potential development investigated 7 2.1 Overview 7 2.2 Area A: City Centre & Floating Harbour 7 2.3 Area B: St Pauls, Baptist Mills & Eastville 8 2.4 Area C: East Avon and St Phillips Marsh 10 2.5 Area D: Ashton area 11 2.6 Area E: Avonmouth 12

3 Planning context 13 3.1 Overview 13 3.2 The SFRA in the planning context 13 3.3 Bristol's Local Development Framework 14 3.4 Planning horizons 14 3.5 Future development within Bristol 15 3.6 Sequential Test 15 3.7 Exception Test 18

4 SFRA linkage to high level plans 20 4.1 Overview 20 4.2 Overall responsibilities for flood risk management and activities 21 4.3 Draft South West Regional Spatial Strategy 22 4.4 Bristol Avon Catchment Flood Management Plan (2008) 23 4.5 Severn Estuary Shoreline Management Plan 24 4.6 Severn Estuary Flood Risk Management Strategy (FRMS) 24

5 Defining the flood risks – tidal & fluvial 25 5.1 Overview 25 5.2 SFRA flood zones 25 5.3 Flood depths and velocities 26 5.4 Flood Hazard 26 5.5 Flood risks to the five areas being considered for future development 27

6 Flood defence assets – tidal & fluvial 28 6.1 Overview 28 6.2 Flood defences – asset details, responsibilities, etc. 28 6.3 Condition of flood defence assets 30 6.4 Current policy for flood defences – as set by the EA strategically 31 6.5 Breach and blockage scenarios 31

7 Flood risks – tidal & fluvial 34 7.1 Overview 34 7.2 Area A: City Centre & Floating Harbour 34 7.3 Area B: St Pauls, Baptist Mills & Eastville 36 7.4 Area C: East Avon and St Phillips Marsh 38 7.5 Area D: Ashton area 39 7.6 Area E: Avonmouth 41 7.7 Summary of flood risk information 41

8 Flood risks – surface water & sewers 43 8.1 Overview 43 8.2 Flood risks 43 8.3 Surface Water Management Plans (SWMPs) 43 8.4 Mapping of surface water flood risk 44 8.5 Potential critical drainage areas in Bristol 45 8.6 Implications for Surface Water Management Plans (SWMP) 46 8.7 Implications for Water Cycle Strategies (WCS) 46

9 Flood risks – impounded water bodies 47 9.1 Overview 47 9.2 Flood risk from reservoirs 47 9.3 Statutory Requirement for Developers 48

10 Flood risk management policy – area specific 49 10.1 Overview 49 10.2 Local Development Framework Core Strategy 49 10.3 Site Allocations and Development Management DPD 49 10.4 Bristol Central Area Action Plan 49 10.5 Planning policy implications 49 10.6 Location specific development policies 51

10.7 Possible non-development zones (in areas of greatest risk, beyond

mitigation) 51 10.8 Policies for defended areas 51

11 Flood risk management policy – city wide 57 11.1 Overview 57 11.2 Developable zones where mitigation may be appropriate (for allocations) 57 11.3 Areas where the Council will consider ‘windfall’ applications 57 11.4 FRM funding and developer contributions 58 11.5 SUDS – appropriate locations and types 60 11.6 Appropriate flood avoidance, site layout, resistance and resilience measures62 11.7 Policy guidance on developing Flood Evacuation Plans 65

12 Strategic options for flood risk management 67 12.1 Overview 67 12.2 Complexity of the flood risk problem 67 12.3 Types of options 68 12.4 Possible strategic solutions to manage flood risk 69 12.5 Flood risk management options 71 12.6 Initial options appraisal 78 12.7 Preferred options 81

13 Advice for site specific flood risk assessments 82 13.1 Overview 82 13.2 FRA issues 82 13.3 Sequential testing using the SFRA 82

13.4 Using SFRA results to inform flood risk assessments for development at

each of the sites considered in this SFRA 84 13.5 Site specific risk, location of uses, access points, levels, safe design 87

14 Recommended further FRM studies 88 Table 14.1 – Current / planned studies in flood risk 88

15 Concluding remarks 90

Glossary 91

References 94

Appendices

Appendix A Sequential Test Template for Local Planning Authorities

Appendix B Scope of Flood Risk Management Options

Appendix C Tidal & Fluvial Flood Risk Maps – Area A

Appendix D Tidal & Fluvial Flood Risk Maps – Area B

Appendix E Tidal & Fluvial Flood Risk Maps – Area C

Appendix F Tidal & Fluvial Flood Risk Maps – Area D

Appendix G Tidal & Fluvial Flood Risk Maps – Area E

Appendix H Surface Water Flood Risk Maps – Areas A to E

Appendix I Surface Water Flood Risk Maps – risks identified by Bristol City Council

Tables Table 2.1 Developments in Area A

Table 2.2 Developments in Area B

Table 2.3 Developments in Area C

Table 2.4 Developments in Area D

Table 5.1 Flood Hazard Classification

Table 6.1 Main (raised) flood defences in Bristol

Table 7.1 Tidal & fluvial flood risk – additional flood risk detail

Table 10.1 Policy matrix for the lifetime of proposed developments

Table 10.2 Flood risk vulnerability & flood zone compatibility – Area A

Table 10.3 Flood risk vulnerability & flood zone compatibility – Area B

Table 10.4 Flood risk vulnerability & flood zone compatibility – Area C

Table 10.5 Flood risk vulnerability & flood zone compatibility – Area D

Table 10.6 Flood risk vulnerability & flood zone compatibility – Area E

Table 12.1 Possible strategic solutions to manage fluvial flood risks

Table 12.2 Non-structural approaches applicable to all flood risk areas (A to E)

Table 12.3 Structural approaches applicable to all flood risk areas (A to E)

Table 12.4 Structural approaches – Area A: City Centre & Floating Harbour

Table 12.5 Structural approaches – Area B: St Pauls Baptist Mills & Eastville

Table 12.6 Structural approaches – Area C: St East Avon & St Philips Marsh

Table 12.7 Structural approaches – Area D: Ashton area

Table 12.8 Structural approaches – Area E: Avonmouth

Table 12.9 Initial options appraisal

Table 12.10 Indicative cost of options

Table 12.11 Initial environmental appraisal of options

Table 13.1 Flood risk assessment issues specific to each area

Table 14.1 Current / planned studies in flood risk Figures Figure 1.1 Identified areas with flood risks where there are development pressures

Figure 1.2 SFRA user guide

Figure 2.1 Schematic of Bristol Floating Harbour and interaction with Avon & Frome

Figure 4.1 FCERM hierarchy

Figure 4.2 CFMP policy options

Figure 4.3 Extent of Tidal Severn SMP2 study area

Figure 5.1 Flood Zone classification

Figure 6.1 Standard of Protection

Figure 6.2 Location of raised flood defences

Figure 6.3 Photographs - major culverts in Bristol

Figure 6.1 Photographs - major culverts in Bristol

Figure 7.1 Flooding around the Eastville culvert intake in the July 1968 flood event

Figure 9.1 Layout of Barrow Reservoirs

Figure 9.2 Chew Valley Lake

Figure 10.1 Defended hazard map with climate change

Figure 11.1 Diagram of how SUDS can be used at a local scale

Figure 11.2 Examples of flood resistance and resilience measures

Figure 12.1 Indicative location of strategic solutions for Bristol and wider Avon catchment

Sustaining & Improving the Quality of People’s Lives Page 1

1 Introduction

1.1 Background

In February 2008, Bristol City Council commissioned Halcrow Group Ltd to prepare a

Level 2 Strategic Flood Risk Assessment (SFRA), with a focus on the nature and

extent of flood hazards in five areas with known flood risks and development

pressures. The areas investigated are shown in Figure 1.1.

This Level 2 SFRA is in accordance with Planning Policy Statement 25: Development

and Flood Risk (PPS25) and its accompanying practice guide. Bristol City Council

planners and the Environment Agency specialists in flood risk and development have

been consulted at all stages of the assessment. The flood risk modelling and mapping

methodologies applied are consistent with Environment Agency requirements.

The identification of five key areas for detailed assessment by the SFRA (Levels 1

and 2) does not imply any particular planning status for these locations or infer that

identified sites within them will be granted planning permission for any use. The SFRA

Level 1 and 2 reports form a part of the evidence base of supporting documents for the

emerging Bristol Core Strategy but do not have the status of Development Plan

Documents. Specific development sites referred to in this report are illustrative and will

not necessarily be allocated for development in forthcoming Development Plan

Documents.

1.2 Outline approach

This Level 2 SFRA refines and builds upon the recent Level 1 SFRA (2008), providing

more detailed information on all forms of flood risk: fluvial (rivers), tidal, surface water,

groundwater, sewer and from impounded water bodies (reservoirs), both now and in

the future given the likely impacts of climate change. During the Level 1 SFRA it was

identified that high flows may pose a risk to the urban infrastructure, and to areas

where potential development may take place.

A series of detailed hydraulic models have been developed to assess the tidal/fluvial

flood risks that had only previously been modelled by the Environment Agency using

their national generalised computer model. Where appropriate, 2-D modelling software

(TUFLOW) has been used to produce peak flood extents, depths and flow velocities

and this information has been used to produce flood hazard classifications and

animations to illustrate the rate of onset of flooding.

The fluvial and tidal flood extents for key annual exceedance probabilities (that is, the

probability that an event of a certain magnitude would be exceeded in a given year)

have been determined and mapped and the outputs used to inform appropriate flood

risk management policies for the area.


Figure 1.1 Identified areas with flood risks where there are development pressures (source: Bristol City Council)


The annual probability of flooding cases presented are:

• 5% fluvial AEP – classified as ‘Functional Floodplain’, Flood Zone 3b

• 1% fluvial AEP or 0.5% tidal AEP – Flood Zone 3a

• 1% fluvial and 0.5% tidal AEP adjusted for climate change

– Flood Zone 3a + climate change

• 0.1% AEP – Flood Zone 2)

Other flood risks are also considered, including sewers, surface water and groundwater

flood risks based on records of past flooding, and reservoirs flood risks based on

previous breach failure studies.

The refined assessment of flood risk has then been used to inform appropriate flood

risk management policies for the areas being considered for future development.

Based on this level of flood risk detail, a policy matrix is presented for the lifetime of

proposed developments.

1.3 Purpose of the Level 2 SFRA

PPS25 sets out government planning policy on development and flood risk, aiming to:

• ensure that flood risk is taken into account at all stages of the planning process.

• avoid inappropriate development in areas at risk of flooding.

• direct development away from the areas of highest risk.

• by substituting higher vulnerability land uses for lower vulnerability uses in higher

flood risk locations and locating higher vulnerability uses in areas of lower risk on

a strategic scale, or on a site basis.

• by providing adequate flood risk management infrastructure which will be

maintained for the lifetime of the development

• by mitigating the potential impacts of flooding through design and resilient

construction

Where new development is necessary in such areas, under exceptional circumstances,

the policy aims to make the development ‘safe’ through application of the Exception

Test (refer to Section 2.5) without increasing flood risk elsewhere and, where possible,

reducing flood risk overall.

The Level 1 SFRA (2008) mapped all sources of flood risk and thereby provided the

evidence base to inform a risk-based sequential approach to flood risk (the Sequential

Test). This approach helps ensure that development is located in areas of lowest

possible risk of flooding.

The Draft South West Regional Spatial Strategy (RSS) incorporating the Secretary of

State’s Proposed Changes (July 2008) sets out the requirements for housing and

employment within Bristol City Council over its 20 year period to 2026. In particular, it

set a city wide housing requirement of 36,500 houses. However, the emerging Core

Strategy for Bristol aims to provide 30,000 dwellings, consistent with the

recommendations of the Draft RSS Examination in Public Panel Report

(December 2007), with contingency arrangements if land is required to accommodate

higher levels of provision


Accordingly, there is a need for Bristol City Council to allocate significant land for

housing (as well as complementary employment and other land) and planners are

currently identifying suitable sites to meet this requirement. This Level 2 SFRA is

intended to help with this process, and to form part of the evidence base to ensure that

the most appropriate land is allocated for development.

Five flood risk areas were identified by Bristol City Council as requiring Level 2 SFRA

(Figure 1.1). A Level 2 SFRA is necessary as Bristol City Council is considering these

areas for future development.

Specifically, this Level 2 SFRA will demonstrate whether or not the flood risk to and

from any development will be ‘acceptably safe’ throughout the lifetime of the proposed

developments, taking account of climate change. For a development to be classed as

‘acceptably safe’ the site should be classed as ‘very low hazard’ as defined by Defra

(2005) R&D Technical report, FD2320/TR2.

1.4 SFRA Level 2 aims and objectives

The aim of PPS25 planning policy on development and flood risk is to ensure that flood

risk is taken into account at all stages of the planning process to avoid inappropriate

development in areas at risk of flooding, and to direct development away from areas at

highest risk.

The aim of a SFRA, therefore, is to map all forms of flood risk and use this as an

evidence base to locate new development primarily in low flood risk areas (Zone 1).

The work has been completed as part of the Level 1 and this Level 2 assessment to

guide the planning and development control process and help inform Bristol City

Council’s flood risk management policy.

The Level 1 and 2 SFRAs together form part of the evidence base for the Local

Development Framework (LDF) and inform decisions regarding land allocation and

policies. They provide:

• Flood zone, depth and velocity maps of fluvial and tidal flood risk for current and

climate change scenarios.

• Flood hazard mapping for design events

• Flood maps of other flood risks – surface water and reservoir breach

This information will be used to inform application of the Sequential Test and Exception

Test, as described in PPS25 (Annex D), to development proposals for the five areas

selected. This SFRA also allows Bristol City Council to:

• Prepare appropriate policies for the management of flood risk.

• Inform the Sustainability Appraisal to take account of flood risk when considering

options and in the preparation of strategic land use policies.

• Identify the level of detail required for site-specific Flood Risk Assessments.

• Provide information to developers on flood risk and flood management issues for

use in detailed site specific Flood Risk Assessments.


• Help inform the acceptability of flood risk in relation to emergency planning

capability and by considering the beneficial effects of flood risk management

infrastructure in generally reducing the extent and severity of flooding.

• Set out the strategic options for flood risk management

1.5 SFRA User Guide

The SFRA User Guide (Figure 1.2, next page) illustrates how the SFRA should be

used by planners, drainage engineers, emergency planners and others in relation to

planning and development control, site specific flood risk assessments and emergency

planning. The report structure is as follows:

• Chapter 2 describes the five areas of potential development investigated.

• Chapters 3 and 4 consider the planning context to this SFRA as defined by

PPS25 and including its linkage to other high level plans.

• Chapter 5 explains how tidal and fluvial flood risks are defined in terms of

strategic Flood Zones and flood hazards classified based on flood depth/velocity.

• Chapter 6 assesses the existing flood defences and their condition.

• Chapters 7, 8 and 9 presents the flood risk information for the tidal and fluvial

flood risk (Chapter 7), the surface water flood risk (Chapter 8) and flood risk

resulting from failure of upstream reservoirs (Chapter 9).

• Chapters 10 and 11 recommend policies related to flood risk following PPS25

and informed by the flood risk information presented in the previous chapters.

• Chapter 12 considers the strategic solutions to the flooding problems identified.

• Chapter 13 gives guidance on site specific flood risk assessments to be prepared

in support of planning applications.

• Chapter 14 summarises the conclusions of the SFRA.

1.6 Future SFRA updates

This SFRA report is a ‘live’ document and as new information becomes available

updates will be made to ensure that the latest information is used to guide the site

selection process for future developments. For this reason users of this SFRA are

recommended to check that they are using the latest SFRA document and maps.

Over coming years, further refinements may be undertaken (as part of the Environment

Agency’s flood risk mapping program), and any updates to Flood Zones 2 and 3a will

be reflected in the latest Environment Agency Flood Map (updated quarterly). As

such, it is recommended that Bristol City Council remain abreast of any further

refinements to these flood zones although significant changes are not anticipated.

Generally, it is recommended that the fluvial and tidal models should be reviewed every

five years, but even then only minor revisions are envisaged, e.g. possibly to

incorporate more recent data or to follow updates to climate change guidance (due to

be published by UKCIP in 2009).


Background to the SFRA, its purpose and the five areas of potential development investigated

Figure 1.2 SFRA user guide

Chapter 1: Introduction

Chapter 2: Areas of potential development investigated

Chapter 3: Planning context

Chapter 4: SFRA linkage to high level plans

Chapter 5: Defining the flood risks – tidal & fluvial

Report Section

Chapter 6: Flood defence assets – tidal & fluvial

Chapter 7: Flood risks – tidal & fluvial

Chapter 8: Flood risks – surface water & sewers

Chapter 9: Flood risks – impounded water bodies

Chapter 10: FRM policy – area specific

Chapter 11: FRM policy – city wide

Chapter 12: Strategic options for FRM

Chapter 13: Advice for site specific FRAs

Chapter 14: Recommended further FRM studies

Chapter 15: Concluding remarks

Key audience and information

Forward Planners The SFRA needs to be used to inform policies relating to flooding, managing flood risk, land use and development allocations

Forward Planners

To minimise the flood risks to future developments

Drainage Engineers

To raise awareness of flooding issues, and to resolve where possible

Development Control

To prevent inappropriate development and apply conditions as necessary

Emergency Planners

To prepare emergency plans appropriate to the flood risks

Developers

Awareness of the flood risks

Key Audiences:

Forward planners include policy makers at South Bristol City Council and at regional level.

Drainage Engineers includes functions at Bristol City Council.

Development Control & Emergency Planners includes functions at Bristol City Council and Environment Agency

Developers includes both companies and private developers

Forward Planners, Development Control, Emergency Planners & Developers Formal assessment approach to identify and mitigate flood risks for the life-time of developments

Forward Planners To enable planning policy to be made for flood risk areas, possible funding mechanisms, identify options to reduce flood risk, promote use of SUDS, etc. Development Control & Developers Key messages for development control, need for flood risk assessments, emergency planning, to raise awareness of SUDS techniques

Forward Planners, Development Control, Emergency Planners & Developers

Forward Planners Further studies to support the city planning and development aspirations going forward next year

Defines how the SFRA assesses tidal & fluvial flooding

How to use the SFRA to allocate sites for development and its planning context

Details FRM policy relevant to site allocation and future planning for the areas investigated

Initial appraisal of strategic options to reduce flood risk (tidal & fluvial)

Details the areas at risk of flooding (all forms), including climate change impacts for tidal & fluvial flooding

Considers the existing flood defences to protect against tidal and fluvial flooding

Details FRM policy on developable zones, ‘windfall’ sites, developer contributions and SUDS

Advice to inform the preparation of FRAs in advance of planning applications

FRM studies to determine optimal and sustainable solutions to reduce flood risks

Key messages from the SFRA relevant to all


2 Areas of potential development investigated

2.1 Overview

This chapter details the extent, character, key features and potential development of

the five areas of development investigated by this SFRA Level 2. These areas are:

• Area A: City Centre & Floating Harbour

• Area B: St Pauls, Baptist Mills & Eastville

• Area C: East Avon & St Phillips Marsh

• Area D: Ashton area

• Area E: Avonmouth area

2.2 Area A: City Centre & Floating Harbour

2.2.1 Extent

The area embraces the City’s

mediaeval core and most of its

commercial heart. Much of the

Floating Harbour is located

within this area, which includes

the Harbourside, Temple Quay

and Broadmead districts.

The main water bodies / rivers

in the area are the Floating

Harbour (as described below),

the Lower Avon, and the River

Frome, joining the Floating

Harbour through 4 separate

culverts, though principally at

St Augustine’s Reach. The Malago Stream flows northwards to join the Avon to the

east of Southville, near the A38.

2.2.2 Character

The area around the Floating Harbour has undergone many millions of pounds worth of

regeneration in past years. The waterside now hosts a large area of office space,

residential apartment developments and such cultural amenities as the Watershed

Media Centre as well as the major new Cabot’s Circus shopping development. The

intention has been to incorporate the needs of a modern edge of city centre

development without losing a sense of Bristol’s rich historic environment.

2.2.3 Bristol Floating Harbour

Bristol Floating Harbour is the principal water retaining structure in Bristol. The water

level is maintained within it by control of flows entering via the Feeder Canal and from

the River Frome culverts, as well as flows exiting through the Underfall Yard sluices

and Cumberland Basin lock gates.


The complex interaction of the Floating Harbour with the River Avon and Bristol Frome

is shown in schematic form below (Figure 2.1). It has a significant role in flood risk

reduction due to its available storage. The effect of large fluvial inflows can be

mitigated by pre-lowering of the existing levels by up to 200mm at low tide.

2.2.4 Potential development – Table 2.1

Within Area A, existing commitments in the form of land allocated in the Local Plan

(1997) and planning permissions, plus possible new development sites identified in

emerging work on the Core Strategy indicate a strong potential for development. Many

of them are relatively small sites, involving infilling and redevelopment; others are more

substantial.

Most of the sites have potential for residential use; some are expected to

accommodate commercial uses; some will have mixed uses. The possible

development sites emerging from Core Strategy work are all being assessed with

housing potential in mind. Some of the larger sites are listed below.

Table 2.1 Developments in Area A

Name of development Source Proposed use

Wapping Wharf Local Plan Mixed – residential (592 housing), community workspace, leisure, office

Great Western Dock – Gas Ferry Road Local Plan Mixed use (210 housing)

Redcliffe Way, Lawrence Hill, including George and Railway, Victoria Street; Elf Petrol Station, Temple Gate; Grosvenor Hotel, Victoria Street

Emerging Core Strategy

Residential (136 housing)

2.3 Area B: St Pauls, Baptist Mills & Eastville

2.3.1 Extent

The area of Bristol covered by Area B

includes Baptist Mills and parts of St

Pauls, Easton and Eastville It is a

finger of land either side of the

Frome, extending north-east from the

City Centre to the junction of

Fishponds Road and Muller Road

(M32, junction 2).

The A4032/M32 is its approximate

centreline, and it has a width of

approximately 1km, although it is

narrower around the Easton Way

area.

The main watercourse in this area is the Bristol Frome. Within this area it is joined by

Coombe Brook and Boiling Wells Stream.


Figure 2.1 Schematic of Bristol Floating Harbour showing interaction with River Avon and Bristol Frome

Key:

Open channel

Culvert

Sluice-controlled flow between

Floating Harbour and River Avon

Overland flow route

Feeder

Canal

Underfall

Yard

Sluices

Spills*

Northern Storm Water Interceptor

Cumberland Basin

(Lock Gates and

Stop Gates)

Bristol

Frome

River Avon

Frome Culvert

Ca

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e D

itc

h

Cu

lve

rt

Ca

stl

e G

ree

n

Cu

lve

rt

Fo

ss

e W

ay

Cu

lve

rt

Sto

ne

ga

tes

Cu

lve

rt

My

lne

s

Cu

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rt

Eastville

NSWI outfall to

Avon

M32 Culvert

Frenchay

Bristol Floating Harbour

Netham

Lock

Gates

Netham

Weir

To Avonmouth

CliftonBristol

Frome

River Avon

Spills*Spills*

Multiple overland flow routes

Overland

flow route

Overland

flow route

Spills Spills Spills


2.3.2 Character

The area is of mixed character crossing a number of informal and administrative areas:

• The northern extent of Easton (Greenbank) is undergoing gentrification and so

housing commands somewhat higher prices in this area.

• St Pauls is a mixed area of Georgian, Victorian and some modern development,

parts of which require improvement. A number of projects are proposed to

regenerate remaining unused/underused/derelict land and buildings.

2.3.3 Potential development – Table 2.2

The emerging Core Strategy identifies only sites for residential development in this

area. There are two developments, one in Ashley, on land adjacent to the scrap yard

on Gatton Road, with potential for 221 dwellings; and one in Lawrence Hill with

potential for 36 dwellings, at Millpond Street, adjacent to M32 junction 3.

The Local Plan identifies one small site with potential for residential development: on

the south side of Wilson Street there is potential for 46 dwellings. It also identifies

potential for light commercial development, for example office space on land at Lower

Ashley Road/Millpond Street.

Table 2.2 Developments in Area B

Name of development Source Proposed use Gatton Rd Residential

Millpond Street

Emerging Core Strategy Residential

Wilson Street Residential

Lower Ashley Road Local Plan

Light commercial

2.4 Area C: East Avon and St Phillips Marsh

2.4.1 Extent

The area extends easterly from Temple

Meads Station, encompassing the St.

Philips Marsh area between the Lower

Avon and the north side of the Feeder

Canal.

2.4.2 Character

St Phillips Marsh is a predominantly

industrial area, comprising light industrial

premises and an area of mixed retail and

leisure uses at Avon Meads. There is a

residential area to the north of the area,

north of the A4320. Area C also includes

the area surrounding Temple Meads

Station: there are a number of residential

properties in this area, although again

much of the land use is industrial.


2.4.3 Potential Development

Emerging Core Strategy work identified three large, potential residential development

sites, falling partially within Area C.

The largest is the St Phillips site of potentially up to 1,000 dwellings at Silverthorne

Lane between the Floating Harbour, the mainline railway line (London side of the

station) and the A4320 (Barrow Road). Recent reappraisal of this site indicates that its

residential capacity could be substantially lower.

Further to this, potential for approximately 700 dwellings is under consideration on a

former diesel depot on Bath Road, Totterdown, located between the mainline railway

line (South West side of the station), the Lower Avon and the A4.

The third area is a smaller site, located at the eastern end of Temple Quay House,

between Temple Meads Station and the Floating Harbour, with a potential capacity of

approximately 212 dwellings.

The Local Plan identifies only one area of potential development within Area C. This is

a residential development with the potential for 173 dwellings as part of the area known

as Temple Quay North, adjacent to Avon Street.

A number of the above sites could contain a proportion of commercial development.

Table 2.3 Developments in Area C


Silverthorne Lane Predominantly residential

Bath Road diesel depot Predominantly residential

Eastern end of Temple Quay House


Predominantly residential

Temple Quay North Local Plan Predominantly residential

2.5 Area D: Ashton area

2.5.1 Extent

This Area lies on the western edge of

Bristol. It extends from the mainline

railway line in the south, to the Lower

Avon in the north and encompasses the

Ashton Gate and Ashton Vale areas.

2.5.2 Character

This area is of mixed character, with

extensive industrial areas to the west and

residential to the east.

2.5.3 Potential development

Initial Core Strategy work identified three sites within, or partially within, the Ashton

Area that might be developed for residential purposes. More recent work has not

supported any new residential development within the Green Belt.


Consequently, one of the potential residential sites - at Silbury Road/Ashton Fields - is

no longer being considered for housing development [note: as this is a recent change,

this site is still shown on flood risk maps presented later as a residential site]. It is,

however, the subject of a current proposal for a major football stadium and related

commercial uses, which would change the flood risk characteristics of the area

significantly

Table 2.4 Development in Area D


Alderman Moores, Ashton Vale Residential

Land off Silbury Road/ Ashton Fields Leisure/employment

Former Engineers Depot, Clanage Rd, Bower Ashton


Residential

2.6 Area E: Avonmouth

2.6.1 Extent

The area is bounded by the River Avon,

the Severn Estuary, the northern

boundary of Bristol and the western

edges of the Lawrence Weston and

Shirehampton communities.

2.6.2 Character

The area has provided a focus for heavy

industry and warehousing since the

establishment of a dock at Avonmouth.

Avonmouth village still houses a

substantial residential community.

International and national environmental designations along the coast and a history of

occupation and maritime activity extending back to at least the Bronze Age make this

area unique within the City. The area is a focus for infrastructure, including the port,

motorways and rail links, power generation and sewerage plant.

2.6.3 Potential development

Avonmouth is part of a wider coastal area which is identified in the RSS Proposed

Changes (July 2008) as “providing for….port related development and a range of

employment uses at Avonmouth/ Severnside while managing flood risk”. Some areas

already have planning permission and/or have been designated as part of the Local

Plan.

The emerging Core Strategy aims to maintain its role in providing for industrial

warehousing and port uses, as well as accommodating waste and other environmental

technologies. The Core Strategy envisages no further releases of greenfield land in

Avonmouth, though there is expected to be further growth in the adjoining Severnside

area to the north, within South Gloucestershire.


3 Planning context

3.1 Overview

National planning policy relating to flooding is set out in PPS25: Development and

Flood Risk, taking account of current proposed amendments published for consultation

in August 2009. This is referred to throughout this SFRA where appropriate and forms

the main policy context. The practice guide to PPS25 explains how to implement the

policies defined in PPS25 to deliver appropriate sustainable development in the right

place while taking full account of flood risk.

The following extract from the PPS25 Practice Guide illustrates the flood risk

management hierarchy for taking flood risk (all forms) into account in the planning

process. It requires information on the nature of flood risk, the spatial distribution of

flood risk, climate change impacts; and the degree of vulnerability of different types of

development.

Extract: PPS25 Practice Guide

Specific elements of PPS25 are set out in detail in this Chapter, in particular the

Sequential Test and the Exception Test. More specifically, at the regional level the

planning policy context is set by the Draft South West Regional Spatial Strategy, and

there is linkage to other high level plans as considered in the next chapter (Chapter 4).

3.2 The SFRA in the planning context

This Level 2 SFRA will be used by Bristol City Council in the application of the

Sequential Test and the Exception Test as set out in PPS25, Annex D. The Sequential

Test steers development to areas of lowest flood risk, and if it is found necessary to

consider allocating development sites in flood risk areas (as indicated in Table D3 of

PPS25) the Exception Test must be applied.

Flood zones (as mapped in the level 1 SFRA) only show the extent of flooding and not

the variation in flood hazard. In order to apply the Exception Test it is necessary to

consider the actual flood risk to the site, in terms of the frequency, impact, speed of

onset, depth and velocity of flooding. This Level 2 SFRA provides this detailed flood

risk information, and thereby identifies lower risk areas within a flood zone, to inform

suitable site layout so that flood risk can be mitigated and developments made safe.


This Level 2 SFRA takes into account the defended conditions for each flood risk area

benefiting from defences, including the potential for flood defence failure such as

breach of the defences. This means that the actual protection provided by existing

flood defences can be considered for potential development areas. An appraisal of the

condition of defences has also been made based on available information.

3.3 Bristol's Local Development Framework

Bristol City Council is currently preparing its Local Development Framework (LDF). The

LDP comprises a series of documents which, in conjunction with the Regional Spatial

Strategy for the South West, will replace the Bristol Local Plan as the statutory

development plan for the city. The LDF will help define how the city will be developed

over the period to 2026.

LDFs comprise of a range of documents which are prepared over a number of years,

as detailed in the Local Development Scheme. Bristol City Council recently published

its Core Strategy for Public consultation (November 2009). The Core Strategy is the

primary document in the LDF. It utilises data obtain from a wide range of sources to

identify the city’s main social, physical and economic characteristics and the key

strategic issues it currently faces.

In addition to providing a baseline assessment of the City, the Core Strategy will

identify the City’s Spatial Vision and Objectives. These will shape the Council’s

development proposals for the City up to 2026. A Delivery Strategy will set out the

Spatial Strategy and Development Principles. A forthcoming Site Allocations and

Development Management DPD will build on the Core Strategy and identify sites for

development as well as providing detailed development management policies.

The SFRA forms part of the evidence base for the LDF and will inform many of the

documents which comprise the LDF. In particular, it has informed the spatial strategy

for the City by identifying those areas most at risk of flooding and has influenced

choices regarding the location and scale of growth.

3.4 Planning horizons

The minimum design life for non-residential development is taken as 60 years

(although at application stage, the LPA or applicant may need to specify an alternative

lifetime for specific developments). The design life for a residential development should

be taken as a minimum of 100 years.

The emerging Local Development Framework for Bristol is expected to run until 2026.

To correspond with this planning horizon, the impact of climate change on the risk of

fluvial and tidal flooding has been assessed for 60 and 100 years beyond 2026, i.e. in

year 2086 and year 2126 (using the assumptions of a 20% increase in peak river flows

and sea level rise, as detailed in Annex B of PPS25). This approach ensures that

Bristol City Council is planning in line with the LDF and beyond the life of the RSS.


3.5 Future development within Bristol

A number of the sites which have been considered by the City Council for possible

housing and for other forms of development are within Flood Zones 2 and 3 and may

be subject to other risks from surface water and reservoir breach. The Council has

been guided by emerging SFRA work in preparing its Strategic Housing Land

Availability Assessment and will draw on the final Level 2 report to undertake

sequential testing of potential sites to be included in its forthcoming Site Allocations

and Development Management

3.6 Sequential Test

The Sequential Test is used to direct all new development (through the site allocation

process) to locations at least risk of flooding, giving highest priority to Flood Zone 1.

Before the sites being considered in this SFRA can be allocated for development

Bristol City Council must complete the Sequential Test to determine whether these

sites are appropriate as strategic allocations given the flood risks associated with them.

The output from the Strategic Housing Land Availability Assessment (SHLAA) will be

critical evidence in this process. If these sites do not pass the Sequential Test they

should not be allocated and alternative sites should be brought forward. Where the

Sequential Test alone cannot deliver acceptable sites, the Exception Test will need to

be applied.

The Environment Agency (2009) recommends that the following approach is used by

local planning authorities to apply the Sequential Test to planning applications located

in Flood Zones 2 or 3. The same approach should also be used for the LDF site

selection process, which is undertaken at the larger city scale. A pro forma template,

based on the process below, is provided in Appendix A. There are three stages, as

follows:

• Stage 1 – Strategic application & development vulnerability

• Stage 2 – Defining the evidence base

• Stage 3 – Applying the Sequential Test


The Sequential Test can be considered adequately demonstrated if both of the following criteria are met:

• The Sequential Test has already been carried out for the site (for the same development type) at the strategic level (development plan) in line with paragraphs D5 and D6 of PPS25; and

• The development vulnerability is appropriate to the Flood Zone (see table D1 of PPS25)

Identify the geographical area of search over which the test is to be applied – this will usually be over the whole of the city but may be reduced where justified by the functional arrangements of the development (e.g. catchment area for a school or doctors surgery) or relevant objectives in the RSS of LDF. Equally, in some circumstances it may be appropriate to expand the search area beyond the city for uses that have a sub-regional, regional or national market.

Identify the source of ‘reasonably available’ alternative sites – these sites will usually be drawn from the evidence base / background documents that have been produced to inform the emerging LDF. For example, an important source of information from housing sites and employment land will be provided by the Strategic Housing Land Availability Assessment and the Employment Land Review (ELR).

Until the SHLAA is complete, or in the absence of background documents, ‘reasonably available’ sites would include any sites that are known to the Bristol City Council and that meet the functional requirements of the application in question, and where necessary, meet the LDF Policy criterion for windfall development (see below)

Stage 1 – Strategic application & development vulnerability

1.A Has the Sequential Test already been carried out for this development at the

development plan level? If yes, reference should be provided to the site allocation

and Development Plan Document (DPD) in question.

1.B Is the flood risk vulnerability classification of the proposal appropriate to the Flood

Zone in which the site is located according to Tables D1 and D3 of PPS25? The

vulnerability of the development should be clearly stated.

Finish here if the answer is ‘Yes’ to both questions 1.A. and 1.B.

Only complete Stages 2 and 3 if the answer to either questions 1.A and 1.B is ‘No’.

Stage 2 – Defining the evidence base

2.A State the geographical area over which the test is to be applied.

2.B If greater or less than the city boundary justify why the geographical area for

applying the test has been chosen.

2.C Identify the source of reasonable available sites, either:

• Background / evidence base documents (state which), or if not available

• Other sites known to Bristol City Council that meet the functional

requirements of the application


Compare the reasonably available sites identified under stage 2 with the application site. Sites should be compared in relation to flood risk; development plan status; capacity; and constraints to delivery including availability, policy restrictions, physical problems or limitations, potential impacts of the development, and future environmental conditions that would be experienced by the inhabitants of the development.

2.D State the method used for comparing the flood risk between sites, whether it is this

SFRA or an alternative (e.g. Environment Agency flood map, site specific flood risk

assessment) as new information becomes available.

Stage 3 – Applying the Sequential Test

3.A State the name and location of the reasonably available site options being

compared to the application site

3.B Indicate whether flood risk on the reasonable available options is higher or lower

than the application site. State the Flood Zone or SFRA classification for each site.

3.C State whether the reasonably available options being considered are allocated in

the Development Plan. Confirm the status of the plan.

3.D State the approximate capacity of each reasonably available site being considered.

This should be based on:

• the density policy within a Local Development Document (LDD)

• the current Strategic Housing Land Availability Assessment for the city

• past performance

Windfall sites

Windfall sites are those which have not been specifically identified as available in the Development Planning Process. They comprise previously-developed sites that have unexpectedly become available. Government policy in PPS3 para. 59 advises that LPAs should not normally rely on windfall sites to meet housing needs.

The Environment Agency recommend that the acceptability of windfall applications in flood risk areas should be considered at the strategic level through a policy setting out broad locations and quantities of windfall development that would be acceptable or not in Sequential Test terms. Evidence on this position should be provided as support to the soundness of the Core Strategy. Guidance on determining the housing potential of windfall (where justified) for broad locations can be found in paras 50-52 of Strategic Housing Land Availability Assessments, Practice Guide to PPS3.

In the absence of flood risk windfall policy, it may be possible (where data is sufficiently robust) for the LPA to apply the Sequential Test taking into account historic windfall rates and their distribution across the district relative to Flood Zones. Where historic and future trends evidence indicate that housing need in the district through windfall can be met largely/entirely by development outside high flood risk areas, this may provide grounds for factoring this into the consideration of ‘reasonably available’ alternative sites at the planning application stage.

Further detail on windfall sites is provided in Section 11.3.


Exception Test – Where necessary, the Exception Test should now be applied in the circumstances set out by table D.1 and D.3 of PPS25.

Applying the sequential approach at the site level – In addition to the formal Sequential Test, PPS25 sets out the requirements for developers to apply the sequential approach (see para. 14 and D8) to locating development within the site.

The following questions should be considered: • Can risk be avoided through substituting less vulnerable uses or by amending the site

lay-out? • Has the applicant demonstrated that less vulnerable uses for the site have been

considered and reasonably discounted? • Can layout be varied to reduce the number of people or flood risk vulnerability or

building units located in higher risk parts of the site?

3.E Detail any constraints to the delivery of identified reasonably available options; for

example, availability within a given time period or lack of appropriate infrastructure

i.e. flood defences which protect the site through its design lifetime. This part of the

test should include recommendations on how these constraints should be

overcome and when.

Sequential Test conclusion

Are there any reasonably available sites in areas with a lower probability of flooding,

which would be appropriate to the type of development or land use proposed?

Next step

3.7 Exception Test

The Exception Test should be applied by decision-makers only after the Sequential

Test has been applied and in the circumstances shown in Table D.1 of PPS25 when

‘more vulnerable’ development and ‘essential infrastructure’ cannot be located in Zones

1 or 2 and ‘highly vulnerable’ development cannot be located in Zone 1.

The flood risk information of a Level 2 SFRA facilitates the application of the Exception

Test. The test is applied when there are an insufficient number of suitably available

sites for development within zones of lower flood risk or due to possible increases in

flood risk arising from climate change.

For the Exception Test to be passed:

a) It must be demonstrated that the development provides wider sustainability

benefits to the community which outweigh flood risk, informed by a SFRA where

one has been prepared.

If the Development Plan Document has reached the ‘submission’ stage (see

Figure 4 of PPS12: Local Development Frameworks) the benefits of the

development should contribute to the Core Strategy’s Sustainability Appraisal.

b) The development should be on developable previously-developed land or, if it is

not on previously developed land, that there are no reasonable alternative sites

on developable previously-developed land.


c) A flood risk assessment must demonstrate that the development will be safe,

without increasing flood risk elsewhere, and, where possible, will reduce flood

risk overall.

The Emergency Services (Fire & Rescue) will need to be formally consulted for

their consideration on whether they will be able to rescue people from the

development for all flood events up to an annual probability of 0.1%.

Bristol City Council should also consult their Emergency Response Office to

confirm that systems will be available to assist people displaced during a major

flood event.

The PPS25 Practice Guide (Sections 4.47-4.61) provides further guidance on ensuring

that a development is safe), and as part of this advises that in some ‘exceptional cases’

developments or redevelopments might be acceptable if the building remains safe, but

safe access cannot be guaranteed during a flood (section 4.58).

Where safe access to a site cannot be guaranteed during a flood, the site should only

be considered as a last resort once Bristol City Council is convinced that the need for

development overrides the flood risk. An ‘exceptional case’ could be where the

development is on a dry island (the site is in Flood Zone 1) and can provide a safe

refuge or where a site is defended (from fluvial and/or tidal flooding) with residents

living on the first floor and above (the ground floor is only used for car parking).

Bristol City Council will need to apply the Exception Test as several potential

development sites fall within Flood Zone 3a, although this is not possible to determine

until the Sequential Test process is complete. Bristol City Council shall then

demonstrate in a transparent means that the positive contribution to the community of

development on the site is so great that they firmly outweigh the concerns about the

risk of flooding and safety.


4 SFRA linkage to high level plans

4.1 Overview

There are a number of existing and ongoing plans available for the Bristol City and

wider regional area. This chapter gives a brief review of the linkage of these studies to

the Bristol City SFRA. Figure 4.1 below shows the hierarchy of national, regional/sub-

regional and local plans published in the recent ‘Appraisal of Flood and Coastal

Erosion Risk Management: A Defra Policy Statement’.

The highest level of FCERM relevant to this SFRA is represented by the Bristol Avon

Catchment Flood Management Plan (CFMP) and Severn Estuary Shoreline

Management Plan (SMP). Below the SMP, the Severn Estuary Flood Risk

Management Strategy (FRMS) is currently being developed. These and other high

level plans are discussed below.

The linkage of the SFRA with these high level plans – the CFMP, SMP and emerging

FRMS – helps to identify the preferred strategic solutions to manage the flood risks for

Bristol City.

Figure 4.1: FCERM hierarchy (Appraisal of Flood and Coastal Erosion Risk Management, A Defra Policy statement - Figure 3.1, 2009)


4.2 Overall responsibilities for flood risk management and activities

The Department for the Environment, Food and Rural Affairs (Defra) has overall

responsibility for flood risk management in England. Their aim is to reduce flood risk

by:

• Discouraging inappropriate development in areas at risk from flooding.

• Encourage the provision of adequate and cost effective flood warning systems.

• Encourage the provision of adequate technically, environmentally and

economically sound and sustainable flood defence measures.

The Government’s Foresight Programme has recently produced a report called Future

Flooding, which warns that the risk of flooding will increase between 2 and 20 fold over

the next 75 years. The report produced by the Office of Science and Technology has a

long-term vision for the future (2030 – 2100), helping to ensure effective strategies are

developed now. Sir David King, the Chief Scientific Advisor to the Government

concluded:

“continuing with existing policies is not an option – in virtually every scenario

considered (for climate change), the risks grow to unacceptable levels. Secondly, the

risk needs to be tackled across a broad front. However, this is unlikely to be sufficient

in itself. Hard choices need to be taken – we must either invest in more sustainable

approaches to flood and coastal management or learn to live with increasing flooding”.

In response to this, Defra is leading the development of a new strategy for flood and

coastal erosion for the next 20 years. This programme, called “Making Space for

Water” will help define and set the agenda for the Government’s future strategic

approach to flood risk.

The strategic approach is being delivered through a strong and continuing commitment

to Catchment Flood Management Plans and Shoreline Management Plans within a

broader planning matrix which will include River Basin Management Plans prepared

under the Water Framework Directive and Integrated Coastal Zone Management.

The Government’s policy in flood and coastal erosion management has a key role to

contribute to mitigation and adaptation to climate change. Increases in sea level and

changing rivers flows (more floods / droughts) will impact on catchments and coastal

areas:

• It is expected that larger numbers of people could in the future be at risk from

flooding and coastal erosion, particularly from exceptional events, and if severe

events occur beyond the current design standards of flood defences across the

UK.

• To reduce these risks means investing significant sums each year to do so, and

increased flood and coastal defence activities are part of the adaption strategy to

protect the UK economy from the full effects of climate change.


The EC Water Framework Directive (WFD) (2000/60/EC) came into force in December

2000 and has set out a timetable for inclusion into the laws of Member States and then

for their implementation through river basin management plans (RBMP). It requires all

inland and coastal waters to reach a "good status" by 2015.

Article 4(3) of the WFD allows Member States to designate surface water bodies, which

have been physically altered by human activity, as artificial or heavily modified, subject

to a number of provisions. Good ecological potential is the environmental objective for

these water bodies.

The EC directive on the assessment and management of flood risk (the Floods

Directive) aims to reduce the risk to human health, the environment and economic

activity associated with floods. This directive will require the preparation of Flood Risk

Management Plans (FRMPs) that will sit alongside the River Basin Management Plans

prepared under the Water Framework Directive. The FRMPs to be prepared in the

future will build on CFMPs and SMPs.

The summer floods of 2007 and 2008 highlighted a wide range of challenges that we

face in relation to flooding. Sir Michael Pitt undertook a comprehensive review of the

lessons to be learned. He clearly identified the need for changes to primary legislation

and called for a single unifying act.

The Government's Floods and Water Bill (consultation draft published April 2009) will

take forward the outcomes of the Pitt Review. The content of the Bill which is related to

flooding is likely to include: measures in relation to surface water management,

transposition of the Floods Directive requirements, SUDS adoption and maintenance

measures, sewer micro-connections, critical infrastructure, information sharing, disaster

recovery, flood event management and potential amendments to the Civil

Contingencies Act.

4.3 Draft South West Regional Spatial Strategy

The Regional Spatial Strategy for the South West (RSS) provides a framework for the

future planning of the South West to 2026, by setting out policies for the location and

scale of development for the region. The LDFs of local authorities must be in general

conformity with RSS policy. RSS also provides a spatial context for plans, programmes

and investment of other agencies and organisations in the region.

The RSS is currently published in the form of the Secretary of State’s Proposed

Changes July 2008, with the final version delayed since June 2009 following a High

Court judgement that the published East of England Regional Spatial Strategy failed to

meet certain requirements of the EU Strategic Environmental Assessment Directive. As

a result of this judgement a new sustainability appraisal of the RSS is currently being

carried out.

The RSS includes the policy statement: “defend[ing] existing properties and, where

possible, locate[ing] new development in places with little or no risk of flooding” and

“use[ing] development to reduce the risk of flooding through location, development and

design” (Policy F1).


4.4 Bristol Avon Catchment Flood Management Plan (2008)

The Bristol Avon CFMP, currently in draft form, is a high-level strategic document

through which the Environment Agency will work with other stakeholders to identify and

agree policies for long-term flood risk management over the next 50 to 100 years.

Figure 4.2 below illustrates the recommended CFMP policy for Bristol Avon.

Figure 4.2 – CFMP policy options

The key messages of the CFMP are:

• Flood defences cannot be built to protect everything.

• Climate change will be the major cause of increased flood risk in the future.

• The floodplain is our most important asset in managing flood risk.

• Development and urban regeneration provide a crucial opportunity to manage the

flood risk.

• If current flood risk management activities continue, estimated average annual

damages are set to significantly increase due to increased tide levels and flood

flows predicted to result from climate change.

• Further action for Bristol should be taken to reduce flood risk to safeguard social,

economic and environmental welfare.

The CFMP recommends for Bristol a long term reduction in flood risk to be achieved

primarily through a number of measures to be determined in further flood risk, asset

management and integrated urban drainage studies. These studies will need to look

at the combined risk from the tidal Severn, Avon, Frome and other local tributaries and

Bristol City


urban drainage. However, there is limited reference to possible strategic solutions to

manage flood risk in the CFMP.

4.5 Severn Estuary Shoreline Management Plan

The Severn Estuary SMP (SMP1, 2000) is relevant to Bristol as it sets out the policy for

Avonmouth the city area upstream to Netham Weir, as its boundary extends to the tidal

limit of the Bristol Avon. The “next generation” of SMP (SMP2) is currently under

consultation, and should be available next year. The draft policy decision for Bristol and

Severnside is “hold the line” for all policy units and over all time scales, which is

consistent with the policy from SMP1.

Figure 4.3 Extent of Tidal Severn SMP2 study area …confirm with EA that figure can be included

4.6 Severn Estuary Flood Risk Management Strategy (FRMS)

The emerging Severn Estuary FRMS, covering the tidal extent of the Severn from

Gloucester to the downstream limits delineated by Lavernock Point (Cardiff) and

Hinkley Point (Somerset), will set out how to best develop SMP2 policies through to

strategic options. The strategy aims to:

• Define a 100 year plan of investment for flood defences.

• Prioritise other flood risk management measures such as providing advice to

utility companies to protect critical infrastructure, development control advice and

flood warning investment.

• Decide where we should create new inter-tidal wildlife habitats to compensate for

losses of habitat caused by rising sea levels.

The draft strategy is due to be published next year.

Bristol City


Figure 5.1 Flood Zone classification

5 Defining the flood risks – tidal & fluvial

5.1 Overview

The aim of the hydraulic modelling undertaken is to improve the Flood Zone

information for the five areas being considered for future development and to assess

the flood hazard posed. This chapter details the coastal and fluvial flood risks to each

of these areas.

5.2 SFRA flood zones

Detailed hydraulic modelling has been undertaken to refine the assessment of the

fluvial and tidal flood risks within Bristol City as presented in the Level 1 SFRA. The

SFRA flood zones (Figure 5.1) are defined as:

• Flood Zone 1 (Low probability) – This

zone comprises land assessed as

having a less than 1 in 1000 annual

probability of river or sea flooding in any

year (<0.1%).

• Flood Zone 2 (Medium probability) –

This zone comprises land assessed as

having between a 1 in 100 and 1 in

1000 annual probability of river flooding

(1% – 0.1%) or between a 1 in 200 and

1 in 1000 annual probability of sea

flooding (0.5% – 0.1%) in any year.

• Flood Zone 3a (High probability) – This zone comprises land assessed as having

a 1 in 100 or greater annual probability of river flooding (>1%) or a 1 in 200 or

greater annual probability of flooding from the sea (>0.5%) in any year.

• Flood Zone 3b (Functional Floodplain) – This zone comprises land where water

has to flow or be stored in times of flood (land which would flood with an annual

probability of 1 in 20 (5%) or greater in any year, or is designed to flood in an

extreme (0.1%) flood, including water conveyance routes). For the Avon, where

the 1 in 25 year (4%) flood event has been modelled previously (but not the 1 in

20 year event), this flood limit was taken to represent Flood Zone 3b. There is no

Functional Floodplain as a result of tidal flooding from the Severn due to the

presence of flood defences.

It should be noted, however, that flooding from sources including sewers, surface

water, groundwater and impounded water bodies (reservoirs), can occur in any zone.

For the Flood Zone 3 and Flood Zone 3 adjusted for climate change scenarios, both

the 1% AEP fluvial event and the 0.5% AEP tidal event were modelled and the worst

case was adopted: i.e. the event with greater depths and velocities.


The assumptions used to model the impacts of climate change are based on the

following precautionary predictions (to the end of this century) as advised by Annex B

of PPS25:

• Increase in fluvial flows by 20%

• Increase in tide levels by 1.0m

These values are applicable to a planning horizon to 2109 (over 100 years), and are

consistent with those used in the Bristol Avon CFMP.

The 0.1% AEP event was selected as an indicator of an extreme situation.

It should be noted that although in the SFRA Level 1 report, the future Flood Zone 3

extent was assumed to be equivalent to the current Flood Zone 2. In this SFRA Level 2

report, however, the future scenarios have been independently modelled.

5.3 Flood depths and velocities

Within a Flood Zone the depth and velocity of flood water can vary significantly. As a

result, the modelled depths and velocities for each flood zone have been mapped

separately to help inform the safest locations within the five areas. However, it is often

the different combinations of depths and velocities that are critical, such that:

‘six inches (0.15m) of fast flowing water can knock someone off their feet and two feet

(0.61m) of water is enough to float a car’ (Pitt Review, 2008)

The following section therefore considers the combination of depths and velocities

together with an appropriate debris factor in order to provide useful guidance of the

dangers to people likely to be caused by individual flood events.

5.4 Flood Hazard

In addition to TUFLOW model output of flood depth and velocity, flood hazard can also

be calculated. The output includes a grid of Flood Hazard derived from the flood depth

and velocity outputs and a debris factor. The methodology for these calculations is

given below. Flood Hazard is calculated using the following equation from Defra (2006)

R&D outputs: Flood Risks to People Phase Two Draft (FD2321/TR2).

Hazard = d x (v + 0.5) + DF where d = depth (m)

v = velocity (m/s)

DF = Debris Factor

A conservative DF of 1.0 for urban areas has been applied to this study, as advised by

Defra (2006). The value obtained for the Hazard is then used to assign a hazard

category. Based on the value of the Hazard for a given area, a Hazard Classification is

then assigned.

The Flood Hazard classifications are as shown in Table 5.1 and are divided into four

categories. The Environment Agency (Development control) have advised that where

the flood hazard for a site (for the lifetime of the development), is not classified as ‘low’

they will look to object to the development.


Table 5.1 Flood Hazard Classification (Source: Supplementary note on flood hazard ratings and thresholds for development and planning control purpose – Clarification of Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1, May 2008)

Flood Hazard Rating

Degree of flood hazard

Description

< 0.75 Low Caution – flood zone with shallow flowing water or deep standing water

0.75 – 1.25 Moderate Danger for some – Flood Zone with deep or fast flowing water that presents a hazard for some people (i.e. children, the elderly and the infirm)

1.25 – 2.0 Significant Danger for most – Flood Zone with deep or fast flowing water that presents a hazard for most people

> 2.0 Extreme Flood Zone with deep or fast flowing water that presents a hazard for all people.

5.5 Flood risks to the five areas being considered for future development

The following chapters detail the flood defences and flood risks to the five areas being

considered for future development (each area is described in Chapter 2). When

allocating sites for future development the current flood risks and potential impacts of

climate change on the Flood Zones should be considered.

For the purpose of this SFRA, linked 1D-2D models (using TUFLOW software) have

been developed for Areas A, B C and D using floodplain digital terrain models (DTM)

derived from aerial (LiDAR) survey data, and surveys of flood defences as part of the

National Fluvial and Coastal Defence Database (NFCDD) programme.

The models compute the ‘defended’ flood extent, depth and velocity of floodplain flows,

and the 1D in-channel component of the models determines the locations where flow

exceeds channel capacity leading to out-of-bank flow – this is not shown in the flood

maps. As the models include the existing flood defences they differ from the

Environment Agency published ‘undefended’ Flood Zones.

For Area E, outputs from the existing Avonmouth and Severnside FRA have been

used: details are available in the relevant report.


6 Flood defence assets – tidal & fluvial

6.1 Overview

Several areas of Bristol are protected from flooding by raised defences. This chapter

identifies these defences, assesses the condition of any key defences, details current

policy and any existing proposals for their maintenance and upgrade. The final section

briefly considers the potential implications of failure.

6.2 Flood defences – asset details, responsibilities, etc.

Formal defences protect much of Bristol against tidal and fluvial flood risk, though the

Standard of Protection (SoP) varies and is generally only 2% (1 in 50 years) standard.

There are sites where this standard is much lower. With climate change it is predicted

that this 2% standard will fall to say 4% (1 in 25 years). Figure 6.1 illustrates the SoP

on the Bristol Avon and Frome

Figure 6.1 – Standard of Protection (source: Bristol Avon CFMP)

According to the National Flood and Coastal Defence Database (NFCDD), almost all of

the defence assets in Bristol consist of ‘maintained channel’ or ‘culverted channel’. The

main assets are listed below in Table 6.1. Most flood defence assets lie in the city

centre, along the coastal fringe and in the Brislintgton area, to the south-west.

Responsibility for these is in the main owned by the Environment Agency, although

there are several that are owned by Bristol City Council and some private defences.

This figure excludes the tidal flood defences that run infront of Avonmouth

Avonmouth

Frome


Figure 6.2 Location of raised flood defences (refer to Table 6.1 for details)

Raised flood defence

Avonmouth tidal defence

Eastville/Easton flood defence

Ashton Vale flood defence

Temple Meads flood defence

Flowers Hill, Brislington flood defence

x M32 culvert

x Brislington Brook culvert

x

Colliters Brook & Longmoor culvert

Northern Stormwater Interceptor culvert


6.3 Condition of flood defence assets

The Standard of Protection provided by a defence can be reduced if a defence is in a

poor condition. The flood defence condition assessment given in Table 6.1 has been

undertaken using the surveyed information provided by the Environment Agency (in

their NFCDD database).

Table 6.1 Main (raised) flood defences in Bristol - as identified by Environment Agency

Location Type Condition Lowest Point

(mODN)

Residual Life

(years) Urgency

Eastville/ Easton

Sheet piled wall 2 10.73 11-20 No repairs

Eastville/ Easton

Concrete wall 2 9.82 11-20 No repairs

Eastville/ Easton

Sheet piled wall 2 10.56 11-20 No repairs

Ashton Vale Flood wall 2 n/a >20 No repairs

Flowers Hill Embankment 3 36.01 mAOD 11-20 Routine

Flowers Hill Raised embankment acting as defence

3 n/a 11-20 Routine

Temple Meads Station

Embankment 2 n/a >20 Routine


Masonry flood wall

2 n/a >20 Routine


Masonry flood wall

2 n/a >20 Routine

Avonmouth Earth and rubble embankment (coastal)

n/a 9.10mAOD n/a n/a

Avonmouth Regraded bank/revetment (tidal)

n/a n/a n/a n/a

Notes

‘Condition’ based on NFCDD condition scale where

� 1 - very good, fully serviceable

� 2- good, minor defects

� 3 – fair, some cause for concern, requires careful monitoring

� 4 – poor; structure unsound now or in the future

� 5- very poor, completely failed and derelict

‘Lowest point’ on the defence above sea level

‘Residual life’ is years until defence will need replacing or large-scale maintenance

‘Urgency’ of any repair needs identified


There also exist a number of culverts with a flood management function, for example:

• Armco Culvert taking flood waters from Colliters Brook, Ashton Vale – other

culverts in this area are on the Longmoor and Brislington Brook.

• Network of culverts protecting Central Bristol from high flows in the Bristol Frome:

- River Frome Central Bristol culvert

- Mylnes Culvert; M32 Culvert (upstream of Underfall Sluices)

- Castle Green Tunnel

- River Street Car Park

- Broad Weir Chamber culvert systems

- Northern Storm Water Interceptor (NSWI).

The Floating Harbour plays an important flood risk management function.

For further information regarding defence structures and flood risk management

responsibilities, refer to Section 7 of the Level 1 SFRA report (June 2008).

6.4 Current policy for flood defences – as set by the EA strategically

The need for defences within Bristol will increase in the future with increased fluvial

flood risks, rising sea levels and a potential increase in storm surge frequency and

magnitude.

The Environment Agency advocates a strategic approach to flood risk management on

a ‘whole catchment’ basis, and have adopted the Bristol Avon CFMP (in draft) policy to

‘take further action to reduce flood risk (now and/or in the future)’ for the Bristol City

area. Along the tidal areas, the Severn Estuary SMP policy to ‘hold the line’ is also

relevant. The implementation of these policies allows the standard of protection

currently provided to existing properties to be continued.

6.5 Breach and blockage scenarios

In order to assess the functionality and strategic importance of the flood defences

structures to Bristol City, six breach or blockage scenarios have been modelled. The

modelling of these scenarios complies fully with the guidance from Defra in the Flood

Risks to People (2006) document.

They are as follows:

• Northern Stormwater Interceptor blockage/failure for 1% AEP fluvial flow.

• Frome culvert (M32) blockage/failure for 1% AEP fluvial flow.

• Ashton Road culvert on Longmoor Brook blockage/failure for 1% AEP fluvial flow.

• St Phillips Marsh (listed as location: Temple Meads in Table 6.1) defence breach

for 0.5% AEP tidal event.

• Malago Stillhouse Lane culvert blockage/failure for 1% AEP fluvial flow.

The scenarios are discussed in more detail in the following location-specific chapters,

and the reasons for the selection of each blockage and breach scenarios is described

below.


Culvert blockage is the most common scenario modelled since culverts on the Frome,

Malago Stream and Longmoor Brook all represent ‘pinch points’ in the respective

catchments, where debris or other rubbish can collect on trash screens and result in

the diversion of flood water out of the channel rather than into the culvert.

One breach scenario was modelled at the primary raised defences in Bristol at St

Philips Marsh. No scenarios involving the Floating Harbour were modelled (e.g. failure

of the lock gates) since the main flood risk at the Harbour is from high tide from the

Avon overtopping the gates at Cumberland Basin.

i) Stormwater Interceptor (NSWI): Conveys flows to an estimated maximum of

80m3/s from the Frome (1% AEP1) into the Avon, provided the tide in the Avon

does not restrict outflow. Therefore, the role the NSWI plays in flood risk

management to Bristol is significant. Blockage of the intakes to the NSWI at

Eastville (Figure 6.3) was therefore considered an important scenario to model

during previous studies undertaken by Halcrow for the Environment Agency

(referenced below).

ii) Frome culvert: Goes under the M32 motorway and also represents a significant

‘pinch point’ in the Frome’s course to the centre of Bristol. All flows that are not

diverted down the NSWI must flow through this culvert and hence blockage of

its entrance (Figure 6.3) was regarded as a suitable scenario to model.

iii) Ashton / Longmoor Brook culvert entrance to Longmoor Brook at Ashton Road:

Critical point where culvert obstruction could exacerbate flooding. This area is

within the Area D (Figure 1.1). Hence this blockage scenario was selected.

iv) Defences at St Philips Marsh: Main flood defences in the Environment Agency’s

NFCDD database for Bristol. Hence a breach in these defences was modelled

as the key breach scenario to include for the SFRA.

v) Malago culvert: blockage of the Willway culvert at Stillhouse Lane was

simulated in this scenario. Blockage of this culvert entrance would result in

localised flooding in the area near this culvert entrance, within Area C (Figure

1.1). Blockage of the Airport Road Tunnel entrance in the Malago catchment

would be a further potential scenario to be investigated when the Malago model

is completed.

1 Information sourced from Halcrow studies during 2006 to 2008

for the Environment Agency Flood Forecasting Regional Team (Exeter)


Entrance to NSWI showing the holding

area, penstock sluices and overflow channel

Gates at entrance to M32 (Frome) culvert

under M32 fly-over

Figure 6.3 Photos illustrating blockage scenario locations on the Bristol Frome at

Eastville


7 Flood risks – tidal & fluvial

7.1 Overview

This chapter considers tidal and fluvial flood risks based on the results of the 2D

modelling, and with reference to the flood hazard classifications defined in Table 3.1.

The assumptions for fluvial and tidal conditions, including the climate change scenario,

are detailed in Chapter 5. The flood risks from other sources are considered in

Chapters 11 and 12.

As a significant flood hazard rating is identified for the majority of the areas, a

sequential approach within each growth area should be applied to avoid these high risk

areas, and the sites identified by Bristol City Council will need to be re-evaluated in

light of this flood risk evidence.

7.2 Area A: City Centre & Floating Harbour

7.2.1 Flood risks

Modelling results to illustrate the tidal and fluvial risks are presented in Appendix C for

Area A, including the Floating Harbour and parts of the Avon, Frome and Malago

Stream. A blockage scenario for Malago Stream is also presented.

The main risk areas are identified for the Wapping Wharf and Great Western Dock

developments, over a sizeable area. There is no risk to these areas during scenarios

less severe than the 0.5% AEP tide with climate change. At some less severe

scenarios, for example the current 0.5% AEP tide, access to the Great Western Dock

area may be impeded, but the flood extent is fairly minimal. Under the blockage

scenario the flood extent is localised, not affecting the proposed areas of development.

7.2.2 Flood Zone 3b – Functional Floodplain

The maps adjacent show that for the 5% AEP fluvial event (the tidal component used

the mean high water springs tide (MHWS)), flood risk both in general in Area A, and

specifically to proposed sites of development within that area is minimal. There are

almost no flood flows: the extreme west of the area and a tiny area adjacent to the

Industrial Museum are the only locations to experience any sort of flooding.

7.2.3 Flood Zone 3a

For this analysis the 0.5% AEP tidal scenario was chosen because the model results

showed it to be the more hazardous event (the fluvial component used a 5% AEP flow).

The Great Western Dock proposed development is the only potential development area

at risk from this event, principally because of the depth of water immediately adjacent

to it. There are almost no points where the water is moving at any significant speed, but

some areas of deep water (approaching 1m) mean that there are pockets of significant

risk condition.


7.2.4 Flood Zone 3a with climate change

When the 0.5% AEP tide is adjusted for climate change (the fluvial component of the

model used a 5% AEP flow) the inundated land in Area A increases considerably.

Water depths approaching 1m are prevalent, and depths of 1.5m are not uncommon.

There are only small areas where water is flowing at or above 0.5m/s but despite this

the hazard classification is significant for a large proportion of the area. This puts all of

the potential development sites, both major and smaller, within the area at some risk:

The Wapping Wharf development is particularly vulnerable as it is surrounded on all

sides by flood waters, in some places approaching 1m. The Great Western Dock

development is modelled as having an area of extreme hazard immediately to the

north.

7.2.5 Flood Zone 2

The maps facing show that in a flood event of this magnitude (0.1% fluvial scenario and

MHSW tide) two of the potential development areas are at risk from a moderate hazard

due to flood waters. This hazard results principally from the depth of the water as

velocities are negligible.

Areas of higher risk (significant) do occur in this study area, but these are not generally

in the immediate vicinity of the potential development areas. The only exception to this

is a small area of significant/ extreme hazard close to the Great Western Dock

development.

Only as a result of this event is flooding seen from the Malago stream. However it is

generally of low or moderate hazard with only a small area of significant hazard. There

are no potential sites for development in this part of Area A.

7.2.6 Blockage scenario – Malago Stream culvert blockage

This scenario considers the result of a blockage of the Willway Culvert at Stillhouse

Lane. The blockage is modelled at 100% which could relate to a large amount of debris

entering the culvert or collapse of the culvert structure itself. The scenario used 1%

AEP fluvial event and the Mean High Water Spring Tide (MHWS). The resultant depth,

velocity and hazard maps can be seen above.

The flood extent is very localised, and does not intersect with any of the proposed

areas of development. It can also be seen that the flood flows resulting from the

blockage form a predominantly slow moving body of water (mainly 0.25 m/s or slower)

with a depth of not more that 0.5m. As a result the hazard category is predominantly

low or moderate, with only a very small area modelled to have a category of significant

hazard.


7.3 Area B: St Pauls, Baptist Mills & Eastville

7.3.1 Flood risks - River Frome

Modelling results to illustrate the tidal and fluvial risks are presented in Appendix D.

The effect of large tides has been modelled as this area is tidally influenced since high

tides can reduce the outflow from the Northern Stormwater Interceptor Sewer (NSWI),

exacerbating flooding in the Eastville area. Blockage scenarios for the interceptor

sewer and M32 culvert are also presented in Appendix D.

The main areas of risk lie around the edge of the residential area of Stapleton to the

north of Eastville Park, and underneath the western edge of the M32 motorway. The

greatest risk is to the Stapleton area to the north of Area B, which is considered to be

Functional Floodplain (Flood Zone 3b); although no potential development is located in

this area. Flood Zone 3a intersects a small amount with the 200+ dwelling proposed

development at Lower Ashley Road.

For a scenario as severe as Flood Zone 2 (0.1% AEP), the principal proposed

development sites are in close proximity to areas of significant or extreme hazard,

placing at risk up to 250 proposed dwellings. The blockages of the M32 culvert and the

NSWI creates areas of risk to both areas of potential development and to the Stapleton

residential area as previously described, with the NSWI blockage scenario resulting in

the most widespread inundation.


For the 5% AEP fluvial event (with a MHWS tide) the flood risk to the proposed

development sites is minimal. The area that is flooded does not intersect with the sites,

and the only area with a significant hazard is some distance away and does not affect

access to the site.

There is, however, a significant area to the west and north of Eastville Park where flood

waters of over 1.5m occur, leading to a significant hazard. The scale of flooding for

events of this return period means that this is a location where residential and

commercial development should be avoided.

7.3.3 Flood Zone 3a

The development areas intersect Flood Zone 3a of the Frome (the 1% flow, with a

MHWS tide). For this scenario, flood water will be so slow moving as to be almost

standing water; however, it may reach a depth of 1.5m in the vicinity of the

development areas. The hazard is rated moderate to significant and in places extreme.

However, it is important to note that the modelled flood events intersect only to a small

extent with the proposed development and the flood risk adjacent to them is at most

moderate.

The scale of flooding in Eastville Park and the adjacent area in places warrants an

extreme flood hazard, predominantly due to the depth of water around Stapleton.


Figure 7.1: Flooding around the Eastville culvert intake in the July 1968 flood event


The maps above show that the proposed sites for development are quite severely

affected by the 1% AEP fluvial event adjusted for climate change (with a MHWS tide).

Depths of flood water approaching or exceeding 1m are widespread, and depths of

1.5m are not uncommon. In some places this is combined with velocities of 0.5-1 m/s

and, even where the velocities are lower, a flood hazard rating of significant is

modelled across much of the study area. As with scenario B2, the actual intersection

between the potential development areas and the flood extent is small, and access to

and from the sites may prove to be the most major issue.

Further to this, there is an area of extreme flood hazard around the edge of the

residential area at Stapleton and along the western edge of the M32, and so it is

considered that this area should remain free from development consideration.

7.3.5 Flood Zone 2

The maps above show that Flood Zone 2, as defined by the extent of the 0.1% AEP

fluvial event (with a MHWS tide), is very similar in extent to the climate change

adjusted Flood Zone 3. However, the areas of deep water (1.5m or greater) cover a

greater extent. Velocities surrounding the proposed development areas are in places

high, resulting in a potentially significant or extreme hazard rating.

7.3.6 M32 culvert blockage - 1% AEP fluvial event and the MHWS tide

This scenario models the results of the complete blockage or collapse of the Frome

culvert under the M32 motorway during a 1% AEP fluvial flow and MHWS.

For this blockage scenario areas of deep water flooding are likely to result. These are

predominantly in the north-eastern end of the area, extending towards the centre and in

close proximity to the proposed residential development at Lower Ashley Road. The

Millpond Street development is also at risk from deep flows (1.5m), although those near

the Gatton Road development have greater velocities in some places. These factors

lead to a significant hazard category, which also applies to Stapleton residential area.

7.3.7 NSWI blockage - 1% AEP fluvial event and the MHWS tide

The NSWI was built as a flood alleviation structure

to divert flows from the Bristol Frome at Eastville into

the tidal Avon downstream of Bristol. It is designed

to divert a large proportion of the flow into the Avon,

but its capacity to discharge can be reduced due to

high tide levels at its downstream end.

Blockage of the NSWI in this flood scenario would

result in widespread inundation to Area B, with

mainly significant or extreme hazard. As with the

previous scenario the greatest hazard is in the

north-east end of the area, and this extends far

enough to potentially threaten access to the

development at Gatton Road with floods with a

hazard category of extreme.


The Millpond Street potential development site is also at some risk as extremely

hazardous flows are also in its vicinity. The Stapleton residential area is at risk from an

area of significant hazard around its southerly edge.

This blockage has also been modelled to show some significant flood risk in the city

centre – principally in the vicinity of The Haymarket and The Horsefair, although this

area has not been modelled to be particularly widespread.

7.4 Area C: East Avon and St Phillips Marsh

7.4.1 Flood risks – River Avon

Modelling results to illustrate the tidal and fluvial risks, including a defence breach

scenario at St Phillips Marsh, are presented in Appendix E.

The Functional Floodplain lies adjacent to the developments at Silverthorne Lane, and

the two on Bath Road (Diesel Depot and Central Trading estate), and therefore

approximately 2000 dwellings may potentially be affected by flooding. As the flood

extents are small in terms of both extent and severity this is likely to principally concern

access to the development areas.

For more severe scenarios, including the St Phillips Marsh defences breach scenario,

the Silverthorne Lane site in particular is severely inundated while the other two sites

mentioned above do not intersect with these modelled flood extents.


Flooding occurs predominantly on the periphery of this area. For this scenario (5%

fluvial flow and MHWS tide) the flood flows are occasionally deeper than 1m in depth

but nowhere do they flow at velocities greater than 0.25 m/s. The hazard category is

significant in some small areas. The three potential residential developments that lie

partially or fully within the area intersect with these flooded areas, although only to very

small extents, and therefore the greatest risk is likely to be to access at the sites.

7.4.3 Flood Zone 3a

For this location the 0.5% AEP tide was used as opposed to the 1% AEP fluvial event,

as this gave a more severe flooding scenario (the 5% fluvial scenario was used). There

are significant areas where water depths are 1-1.5m, and areas where velocities are

over 1m/s. As a result of this, a hazard category of significant is widespread.

The areas of potential development are significantly affected by these flood extents:

Silverthorne Lane development, the most northerly of those shown above, is split in two

by the modelled flood flows, and has its southern extent almost entirely lined by water

with a significant hazard rating. The development at the Central Trading Estate, Bath

Road, while just outside Area C, is also affected by significant or extreme flood flows to

its northerly edge.

The development at the former diesel depot, Bath Road, appears to be protected from

the flood flows in this scenario by the local topography.



For this climate change scenario (1% fluvial flow and MHWS tide) the extent of the

inundation increases significantly, accompanied by increases in depth and velocity.

The hazard category of significant is widespread and extreme is not uncommon.

The effect on the potential development sites is also far greater under this scenario

than under others, with the St Phillips development at Silverthorne Lane almost entirely

covered by the flood extent, much of it with an extreme hazard classification. As in the

previous scenario, the Bath Road diesel depot development appears to be protected

from the flood extent by the local topography, but the Central Trading Estate

development is at significant risk.

7.4.5 Flood Zone 2

During a 0.1% AEP fluvial flood event (with a MHWS tide) a considerable area is likely

to be inundated, with much of the worst flooding occurring in the north-east of the area,

the north-east of St Phillips Marsh. The depth of flooding in this area potentially

exceeds 1.5m in places and is approaching 1m for much of this north eastern part.

The other main area of deep water is to the southern edge of the Silverthorne Lane

development. This development is particularly at risk from an event of this magnitude

with the flood extent modelled to cover much of the area occupied by the potential

development site.

The other potential development sites are apparently affected to a lesser extent by the

flood event: while the sites themselves appear to be unaffected, access may be

significantly impaired.

Velocities are mainly low, much of the flood extent is modelled to be moving at 0.25m/s

or slower, but despite this the depth factor leads to a widespread hazard category of

significant.

In small areas in the vicinity of Netham Weir velocities are great enough to result in an

extreme hazard classification. Again, in terms of development areas, the St Phillips

Silverthorne Lane area is the worst affected when measured by overall hazard.

7.4.6 Breach Scenario – 0.5% AEP tide with a 5% fluvial flow

This scenario is for a 5m breach of the St Phillips Marsh defence, designated in the

Environment Agency’s NFCDD as tidal defences. As the 0.5% AEP tidal event is

shown to overtop the defences in this area, this scenario is the same as scenario C2.

7.5 Area D: Ashton area

7.5.1 Flood risks – Ashton Brook

Modelling results to illustrate the tidal and fluvial risks, including a defence breach

scenario, are presented in Appendix F. This area is tidally influenced and the effect of

large tides has also been considered in the modelling scenarios.


The flood risk is entirely in Silbury Road/Ashton Fields, close to a potential

leisure/commercial development. This is the only development in the area that appears

to be at any flood risk, and even as a consequence of the most severe events,

including the blockage of the culvert on the Longmoor Brook the overlap of the flood

extent and the area of development is minimal.


For the 5% AEP fluvial event (MHWS tide) no flooding occurs in Area D. A small area

of flooding does occur adjacent to the western edge of the area, but this is also outside

of Bristol City Council’s boundary. However, it does lie very close to one of the potential

areas of residential development although the hazard category is low to medium and

the extent small.

7.5.3 Flood Zone 3a

Flood zone 3 for this location is defined by the 0.5% AEP tidal event (with a 5% AEP

fluvial flow) because this results in more severe flooding than the 1% AEP fluvial event.

The area of flooding is small, with velocities below 0.25 m/s. However, the occurrence

of depths of up to 1m means that in places the hazard is significant. Only one area of

potential development is at risk from this event; the land off Silbury Road/ Ashton

Fields. The main body of the development does not appear to be at risk, but access

may be impaired.


For this climate change scenario (0.5% AEP tide; 5% AEP fluvial flow) flooding is

minimal. Effectively, there is no change in the flood extent or water velocity. There is a

very slight increase in the area of flooding which is deeper than 0.5m, and a

consequent increase in the area of significant risk. There may be issues concerning

access to the area of potential development off Silbury Road/Ashton Fields

7.5.5 Flood Zone 2

The flood extents (0.1% AEP fluvial flow and MHWS tide) are similar to Flood Zone 3a,

with a slightly larger area affected by flood water of 1m depth or greater. Consequently

the area of significant hazard is also larger, although the difference is likely to be of low

significance. The only potential development to be threatened is that off Silbury Road /

Ashton Fields and issues concerning access may arise.

7.5.6 Breach scenario

This scenario models the effects of the blockage of the culvert on the Longmoor Brook

under the Ashton Road, conceivably by debris or collapse. Both scenarios used 1%

AEP fluvial event and the MHWS tide.

The consequences of the blockage of the culvert result in a flood extent that is

significantly more serious than the flood from the 1% AEP flow when the culvert is in

working order. Only a small proportion of Area D is flooded by this occurrence, but of

that, a significant proportion is given a significant risk category. The potential

development area off Silbury Road is again potentially threatened and issues

concerning access may arise.


7.6 Area E: Avonmouth

7.6.1 Flood risks – Severn Estuary

The flood risks that are tidally dominated are based on the results of the Avonmouth

and Severnside FRA (prepared by Capita Symonds; January, 2007) – see Appendix G.

The flood maps show the flood risk (1% AEP fluvial and 0.5% AEP coastal) in the

undefended condition, which show that the Avonmouth area occupies natural

floodplain, almost all of which is within Flood Zone 3. The defended ‘with climate

change’ scenario (not available for current flood risk in the FRA), again shows

widespread flooding; flood depth and hazard maps are also presented.

The majority of the deepest water is concentrated, as expected, around the coastal

fringe, as well as north of the M4, and much of the area between the estuary and the

M49. In these areas, water depths of around 2m are not uncommon, and depths of

between 1.0 and 1.5m are widespread.

In terms of hazard, a similar pattern is seen, with a rating of ‘danger for all’ along the

estuarine hinterland, much of the land north of the M4, and a significant part of the land

south of the freight railway line. A rating of ‘danger for most’ covers the majority of the

rest of the area described by the M4 and M49. The rest of the Avonmouth area

predominantly has either no flooding or a hazard of ‘danger for some’. Areas of hazard

more severe than this are negligible in other areas.

Further flood risk information is presented for breach scenarios – the worst case, based

on combining the flood risk resulting from six modelled breach scenarios located at

significant or vulnerable locations along the estuary at Avonmouth.

7.7 Summary of flood risk information

Additional flood risk detail for each area is provided in Table 7.1, intended to help with

interpretation of the flood risk maps (Appendices C to G). This table includes

information on flood depths and velocities and hazard classifications for FZ3b, FZ3a

and FZ3a with climate change.


Table 7.1 Tidal & fluvial flood risk – additional flood risk detail (FZ3b, FZ3a, FZ3a with climate change)

Flood risk Area A – City Centre & Floating Harbour Area B – St Pauls, Baptise Mills & Eastville

Area C – East Avon & St Phillips Marsh

Area D – Ashton area Area E – Avonmouth area

Flood Zone FZ3b 5% annual probability (1:20-year return period)

Very limited overbank flooding, with two small low-lying areas near the Nova Scotia and Millenium Square ferry landings.

Here the flood depths are <0.5m, velocity <0.25m/s and flood hazard classified as low.

No potential development sites are affected by this flood risk.

Very limited overbank flooding, with one small low-lying area on Pennywell Road near the Lower Ashley Road A4032 roundabout and another along Napier Road near the Eastgate Retail Park.

Here the flood depth is <0.5m and velocity <0.25m/s at Pennywell Road and flood depth <1.0m and velocity <0.5m/s at Napier Road.

Flood hazard is classified as ‘low’ at Pennywell Road and ‘significant’ at Napier Road.

No potential development sites are affected by this flood risk.

Overbank flooding occurs at a number of locations, with the main low-lying areas on the Albert Road and Victoria Road, the Feeder Road and Atlas Street, and the Feeder Road in the NE corner.

Here the flood depth is <1m, velocity <0.25m/s and flood hazard classified ‘low’ to ‘moderate’, with a small area of ‘significant’ hazard near the Feeder Road and Atlas Street.

This flood risk just encroaches along the southern edge of the St Phillips potential housing site.

Overbank flooding at only one location, in a greenfield area, opposite the A370.

Here the flood depth is <0.5m, velocity <0.25m/s and flood hazard classified as ‘low’ to ‘moderate’.

No potential development sites are affected, though this flood risk area is immediately adjacent to one potential residential site near the football ground.

Pending results from Avonmouth study

Flood Zone FZ3a 1% fluvial and 0.5% tidal annual probability (1:100-year fluvial and 1:200-year tidal return period)

Flood depth: 0.5-1.0m

Flood velocity: 0-0.25m/s

Flood hazard: generally ‘significant’

Flood extents for FZ3a lie closer to FZ3b (above) than FZ3a with climate change (below), and some development sites fall at least in part inside FZ3a.


Flood velocity: 0.25-1.0m/s

Flood hazard: mainly ‘significant’, some parts ‘low’ to ‘moderate’.

Flood extents for FZ3a lie between FZ3b and FZ3a with climate change.



Flood hazard: mainly ‘significant’, some parts ‘low’ to ‘moderate’.


Flood depth: 0-1.0m


Flood hazard: ‘low’ to ‘moderate’



Flood Zone FZ3a with climate change 1% fluvial and 0.5% tidal annual probability (refer to Section 5.2 for climate change assumptions)

Widespread flooding in the vicinity of the Floating Harbour, affecting the areas:

� north of Cumberland Road and to south of Hotwell Road and Anchor Road

� around Marsh Street and Baldwin Street � whole area north of Redcliffe Way to river � along Clarence Road

Flood depths vary up to 1.5m and velocities up to 0.5m/s generally and 1.0m/s in very localised areas.

Flood hazard: extensive area with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘extreme’ hazard (for all people).

The majority of the potential development sites fall at least part inside FZ3a with climate change, as follows:

� Land at Mcadam Way and Cumberland Road Harbourside � Great Western Dock & McArthurs warehouse Gasferry Rd � Cabot House Deanery Road � College Square Lower Lamb Street, West End � Theatre Royal and buildings Northy Kings Street � Wine Street/High Street � Wapping Wharf, Princes Wharf, City Docks � Redcliffe Way, Redcliffe � Public car park on Mud Dock, the Grove � Central Ambulance Station and adjoining land � Old Soapworks, Old Bread Street � Land East of Temple Quay House, the Square, Temple � St Phillips � Former Diesel Depot, Bath Road, Totterdown

Flooding extends into a number of built-up areas, including:

� A4032 and Wellington Road � Baptist Mills area � near Stapleton Road Rail station � Cottrell Road at A4469/M32 junction



Flood hazard: localised areas with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘low’ to ‘moderate’ hazard.

The majority of the potential development fall at least part inside FZ3a with climate change, as follows:

� Scrap yard, Gatton Road, Baptiste Mills

� Land between Millpond Street, M32 and Lower Ashley Road

� Wilson Street (South Side), St Pauls � Tesco Stores Ltd, Eastville

Widespread flooding affecting the St Phillips Marsh area:

� Gas Lane and Silverthorne Lane � Feeder Road and along Albert

Crescent across a large part of the industrial area

� Albert Road � Feeder Road and Avonside Road



Flood hazard: extensive area with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘extreme’ hazard (for all people).

The majority of the potential development fall at least part inside FZ3a with climate change, as follows:

� Former Diesel Depot, Bath Row, Totterdown

� St Phillips � Central trading estate, Bath Row,

Amos Vale � Land East of Temple Quay House,

The Square, Temple

Flood depth: 0-1.0m


Flood hazard: localised (greenfield) area with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘low’ to ‘moderate’ hazard.

No potential development sites are affected, though this flood risk area is immediately adjacent to one potential residential site near the football ground.



8 Flood risks – surface water & sewers

8.1 Overview

This chapter considers the surface and sewer flood risks. Surface water flooding was

regarded as the main cause of flooding in the summer of 2007 in England2, and

contributed to many flood events in the summer of 2008. As a highly urbanised area,

with large proportions of impermeable land, Bristol is at risk of surface water flooding.

Urban surface water flooding tends to occur shortly after intense rainfall and is the

result of the drainage system being unable to convey all surface water runoff; either

because the drainage system is full or the water cannot find its way into the drainage

system due to the high rate of runoff or localised issues such as culvert or road gully

blockage.

Sometimes referred to as ‘pluvial’ flooding, urban surface water flooding is distinct from

river flooding in that it occurs before runoff enters the watercourse.

8.2 Flood risks

As part of the Level 1 SFRA (March, 2009) a series of consultations were undertaken

to identify known local drainage issues resulting in surface water flooding. These

incidents have been added to the current Level 1 SFRA maps. Details of sites affected

by surface water flooding can be obtained by referring to the Level 1 GIS database.

The PPS25 Practice Guide (June 2008) requires that Level 2 SFRAs should identify

the location of critical drainage areas and identification of the need for Surface Water

Management Plans. Critical drainage areas are locations at which surface water

flooding is known to be a concern, either through prior incidents being recorded or

indicative mapping highlighting potential areas at risk.

Information provided by Bristol City Council, the Environment Agency and Wessex

Water (in November 2009) has been used to indicate areas that could be regarded as

having critical drainage issues for detailed study in the forthcoming Surface Water

Management Plan SWMP).

Developers should check for updated surface water flooding records after any

significant flooding incidents, to ensure that the best available information is used to

inform site allocations and windfall sites.

8.3 Surface Water Management Plans (SWMPs)

Intense rainfall events can occur anywhere as was highlighted by the summer 2007

floods which affected areas of Northern Ireland, north east England, the Midlands and

Wales. The occurrence of such events needs all stakeholders to work in partnership to

improve understanding and the management of flood risk in urban areas so that they

are better prepared for future events.

2 Source: The Pitt Review of the Summer 2007 Floods, Cabinet Office


The Pitt Review Recommendation 18: “Local Surface Water Management Plans, as set out in PPS25 and coordinated by local authorities, should provide the basis for managing all local flood risk.”

PPS25: "Surface Water Management Plans (SWMPs) are referred to in Planning Policy Statement 25 (PPS25) as a tool to manage surface water flood risk on a local basis by improving and optimising coordination between relevant stakeholders. SWMPs will build on SFRAs and provide the vehicle for local organisations to develop a shared understanding of local flood risk, including setting out priorities for action, maintenance needs and links into local development frameworks and emergency plans.”

Source: Defra (2009) Surface Water Management Plan guidance

A SWMP is a framework through which key local partners with responsibility for surface

water in their area work together to understand the causes of surface water flooding

and agree the most cost effective way of managing surface water flood risk. The

purpose is to make sustainable urban surface water management decisions that are

evidence based, risk based, future proofed and inclusive of stakeholder views and

preferences (Defra, 2009). The Pitt Review (2008) recommends SWMPs be adopted

where surface water flood risk is high.

Bristol is one of the 77 locations in England and Wales in which Defra has indicated

that a SWMP is required.

8.4 Mapping of surface water flood risk

The set of figures in Appendix H show areas of potential surface water flood risk,

derived from two sources: the Environment Agency indicative surface water flooding

maps and Wessex Water DG5 property flooding records. In interpreting these maps it

is important to appreciate the source and derivation of the information shown.

Areas susceptible to surface water flooding. These maps were developed for the

Environment Agency and were produced using a simplified method that excludes

underground sewerage and drainage systems, smaller over ground drainage systems

and buildings. They provide a general indication of areas which may be more likely to

suffer from surface water flooding.

The maps indicate three degrees of surface water flood risk, categorised qualitatively

as ‘more’, ‘intermediate’, and ‘less’. In the figures (Appendix H) the areas

representative of ‘more’, ‘intermediate’ and ‘less’ surface water flood risk are

reproduced.

The risk areas, shown as blue areas (‘less’ risk) and yellow areas (‘more’ risk), have

been derived through simplistic modelling techniques that ‘spread’ water over

depressions in the land surface. Therefore, they are not necessarily representative of

historic surface water flooding, but are useful when combined with DG5 records and

Bristol City Council records to focus attention on areas likely to be regarded as critical

drainage areas.


The figures (Appendix H) include the following information from Wessex Water:

• DG5: properties at risk of flooding from sewers due to hydraulic overload

• AMP4: schemes undertaken under Asset Management Programme 4, 2005-2010

• AMP5: schemes identified under Asset Management Programme 5, 2010-2015

• CSO: location of Combined Sewer Overflow

DG5 incident locations. These points, represented as red circles in the figures

(Appendix H)3, highlight properties where Wessex Water is aware of external flooding

up to 1 in 30 year (3.3% AEP) flood magnitude. As such, they are not necessarily

representative of flood risk at higher return periods. They are also indicative of historic

flooding, and therefore may not be representative of future flood risk. Nevertheless,

they do represent useful information, which, when combined with other information, can

aid the identification of critical drainage areas.

Bristol City Council also provided information where they are aware of surface water

issues presented as flood risk maps in Appendix I. Use is made of this information to

identify critical drainage area locations.

8.5 Potential critical drainage areas in Bristol

From the information gathered on surface water flood risks the following areas are

identified as having critical drainage issues. The SWMP should focus on these areas

as a priority.

8.5.1 Area A – North Redcliffe and Floating Harbour

There is a cluster of DG5 incidents, which are next to an area shown to be at ‘more’

risk from the ‘areas susceptible to surface water flooding’ modelling in the King Street

vicinity, and a further DG5 incident in the Wade Street area. Two further areas shown

to be susceptible to surface water flooding are in Castle Park and in East Street

8.5.2 Area B – St Pauls, Baptist Mills and Eastville

There are two DG5 records in this area. One of these, in Sussex Place, also coincides

with an area of ‘more’ risk from the ‘areas susceptible to surface water flooding’

modelling. The other, in the Wade Street area (see details for Area A above) is situated

at the south-west end of an area of ‘more’ surface water flood risk that runs to south of

the Frome.

Other surface water flood risk areas of note from the ‘areas susceptible to surface

water flooding’ modelling are shown to be between junctions 2 and 3 of the M32 in

Stapleton Road to the south of Eastville. This area is also shown to be an area of

surface water flood risk from the Bristol City Council Maps.

3 Please note that the key for these maps also indicates: AMP4 and AMP5 schemes and CSOs – these

are Wessex Water assets and information which are included in the base map for completeness – they

do not inform of surface water flood risk


8.5.3 Area C – Avon and St Phillips Marsh

Much of the area is “less” risk of flooding from the ‘areas susceptible to surface water

flooding’ modelling. The areas shown to be at “more” risk coincide with the Avon and

Floating Harbour Feeder Canal. There are no recorded DG5 incidents shown in this

development area. The Bristol City Council maps indicate that the Netham area,

between the start of the Feeder canal and the Avon, is a known area of surface water

flood risk.

8.5.4 Area D – Ashton Area

In this development area there is one recorded DG5 incident, situated in Clanage Road

by the railway, although this vicinity currently is developed at a low density. The

Greville Smyth Park area is also shown to be at ‘more’ risk of surface water flooding

from the ‘areas susceptible to surface water flooding’ modelling.

8.5.5 Area E – Avonmouth

In Avonmouth there are four DG5 locations in the Docks area. The ‘areas susceptible

to surface water flooding’ modelling shows a number of areas with ‘less’ surface water

flood risk, though no areas of ‘more’ surface water flood risk. One of these areas

coincides with the DG5 records of flooding.

8.6 Implications for Surface Water Management Plans (SWMP)

The combination of DG5 events, ‘areas susceptible to surface water flooding’ modelling

and Bristol City Council’s maps of known surface water flood risk areas suggests that

the Bristol SWMP should examine the following city areas:

• Low lying land either side of the Bristol Frome south of Eastville

• King Street area near the Floating Harbour and Wade Street in the city centre

• Land between the Avon and Feeder canal at Netham

• Greville Smyth Park area at Ashton Gate

• Docks area in south west of Avonmouth area

Other areas of known surface water flood risk outside the five development areas

(A to E) are also indicated in Bristol City Council’s maps (identified by yellow circles in

the maps reproduced in Appendix H). This information may be useful for focussing

future flood risk studies in these highlighted areas. These other areas include:

• Vicinity of city centre and Floating Harbour

• Southern area, near Knowle, Dundry, and Withywood

• North West area (in a band) encompassing Southmead, Henbury, Stoke Bishop

and Shirehampton

8.7 Implications for Water Cycle Strategies (WCS)

The Environment Agency encourages the use of WCS to address a range of water and

environmental planning issues, including flood risk management, water resources and

waste water planning processes, in areas where significant development is planned. A

WCS is recommended for Bristol as there is a requirement for a SWMP and it is

uncertain whether the environmental capacity of the water cycle to cope with future

proposed development is adequate.


9 Flood risks – impounded water bodies

9.1 Overview

This chapter considers the flood risks from impounded water bodies, namely, upstream

reservoirs and the risk of their overtopping or breach

9.2 Flood risk from reservoirs

The Barrow reservoirs to the south of Bristol are the closest reservoirs to the city

boundary, located either side of the A38 to the south west of Bristol, at National Grid

Reference 35401677. These three reservoirs are concrete bowl type with a water

supply purpose. The Barrow Reservoirs consist of three lakes - no. 1 with 25 acres, no.

2 with 40 acres and no. 3 with 60 acres.

Figure 9.1 Layout of Barrow Reservoirs

(source: Bristol Water www.bristolwater.co.uk)

Outside the city boundary but of interest from a flood risk management perspective is

Chew Valley Lake. This reservoir detains flood water from the River Chew, a tributary

of the Avon with its confluence at Keynsham, but its primary purpose is as a water

supply reservoir. Chew Valley Lake is located several kilometres upstream of Chew

Magna and was completed in 1956. It covers an area of 575 hectares and has a

capacity of nearly 20.5 million cubic metres.

Chew Valley Lake is owned by Bristol Water and is used to supply drinking water to

Bristol and the surrounding area. Its large capacity means that during flood events it

can act like a flood detention reservoir (although when full it will have a less significant

effect) and during the 1968 flood it is estimated that it held back approximately 90% of

the flow draining the catchment area upstream. Had this not been the case then the

flooding on the River Chew would have been significantly worse.


Figure 9.2 Chew Valley Lake

The Environment Agency role with respect to the reservoirs is to monitor to ensure

compliance with the Reservoirs Act 1975. This legislation requires that large reservoirs

are subject to annual safety checks by a (registered) Supervising Engineer and ten

year reviews by an Inspecting Engineer.

From Spring 2009 the owners of 'high consequence' reservoirs (Categories A & B) will

be required to produce reservoir flood plans, which will include inundation maps and

therefore inform the flood risk. However, ahead of this legal requirement, such

information is generally unavailable.

9.3 Statutory Requirement for Developers

Developers of any proposed development in Areas A, C or D (Figure 1.1) must contact

Bristol Water to determine whether the proposed development lies within the ‘wetted

area’ of Bristol Water’s flood risk maps relating to the Barrow reservoirs, Chew Magna

reservoir or Chew Valley Lake. The wetted area maps can be viewed at either the

offices of Bristol Water or the Local Authority (Emergency Planning) office. If the

former, a prior arranged appointment will be required. This is in order to determine

whether the development site is likely to be within the inundation zone should the

reservoir overtop or breaches of the embankment occur.


10 Flood risk management policy – area specific

10.1 Overview

Policy recommendations that result from the undertaking of this SFRA are presented

below as a series of matrices that allow the user to incorporate the flood risk

assessment into the planning system.

10.2 Local Development Framework Core Strategy

The ‘publication version’ of the Core Strategy, approved by a Full Council meeting, can

be found at this link: http://www.bristol.gov.uk/ccm/content/Environment-

Planning/Planning/planning-policy-documents/bristol-development-framework/bdf-

fsi/draft-core-strategy-publication-version.en

The role of this ‘publication version' document is to provide the opportunity for

comments, which are known as 'representations', to be made before it is submitted to

the Secretary of State. The Core Strategy does not set out site�specific proposals or

allocations; instead it looks at the broad locations for delivering new development.

Key to the implementation of the Core Strategy will be a set of future local development

documents which the Council is proposing to produce as parts of the Bristol

Development Framework, as discussed below.

10.3 Site Allocations and Development Management DPD

This is a key document for the implementation of the Core Strategy as it identifies

specific development sites. All sites considered for inclusion within the DPD will be

assessed in accordance with the findings of this Level 2 SFRA. This DPD will be

subject to consultation in 2010.

10.4 Bristol Central Area Action Plan

This is another key document that will set out a detailed vision to reinforce the unique

character and international reputation of the city centre. Work will begin on this in 2010

and this will include looking at the character of the central area and the delivery of new

homes and commercial, creative and leisure space. Consistent with the Core Strategy,

the Plan will also seek to maintain and improve the role of the harbour and waterways,

conserve the city centre’s architectural heritage, improve transport services and

revitalise areas in need of change.

10.5 Planning policy implications

The complex range of issues that result from this Level 2 SFRA have wide ranging

implications for future planning in Bristol. The emerging Local Development Framework

will require detailed policies to ensure development takes place in safe and sustainable

locations, while making the best use of the city’s scarce developable land.

Policies are likely to be too detailed for inclusion in the Core Strategy alone and the

Council will need to give consideration to preparing a Supplementary

Planning Document on the subject of flood risk. Such a document could provide clarity

on a range of issues as covered by this report and set out below.


The Core Strategy should provide the strategic policy basis for directing development

away from areas at risk of flooding and ensuring that where development is at risk, it

incorporates appropriate flood resistance and resilience measures. The Supplementary

Planning Document should provide additional detail to clarify how the LPA and

developers should deliver the Core Strategy policies.

It is advised that any site specific allocation identified in the Local Development

Framework, which wholly or partly lies within future flood risk areas identified in this

report, are scheduled for delivery after the Supplementary Planning Document on flood

risk has been published. This will allow site-specific and/or area-wide flood mitigation

measures to be assessed as part of the planning process.

The emphasis should be on early adoption of Development Management Policies on

flood risk, which amplifies the broad policies of the Core Strategy; and early

preparation of an SPD to cover the matters set out below.

Possible layout for a Supplementary Planning Document dealing with flood risk

Background & Context

• background (UK flooding), PPS25.

• current situation in Bristol, Environment Agency flood zones and current practice

• Sequential Test, available developable land in Bristol, the Strategic Housing Land

Availability Study, Employment Land Review.

• Exception Test, principles of safe development in flood zones, access, egress,

emergency services.

SFRA Flood Zones

• explanation of SFRA zones, probabilities, risk / hazard

• flood risk in relation to proposals

• surface water flooding

Flood policy

• Core Strategy policy

• location specific development policies

• policies for defended and undefended areas

• possible non-development zones (in areas of greatest risk, beyond mitigation)

• time-limited consents for commercial development

• developable zones where mitigation may be appropriate (for allocations)

• areas where the Council will or will not consider windfall applications

• developer contributions for flood defences

• SUDS (strategic and local, appropriate locations and types)

• appropriate flood avoidance, resistance and resilience measures (appropriate

locations/types), design

• other flood mitigation, e.g. flood storage areas (appropriate locations and types),

new technologies

• substitution of uses (more vulnerable for less vulnerable in high risk zones)

Advice for site specific FRAs

• sequential testing using the SFRA

• using SFRA results to inform FRAs

• site specific risk, location of uses within the site, access points, levels, safe design


10.6 Location specific development policies

In allocating sites for development Bristol City Council is required to adopt the climate

change fluvial and tidal flood zone maps for the lifetime of the proposed development,

as detailed below, in addition to any other sources of flooding (surface water,

groundwater and sewer). Location specific development policies detailed in Tables

10.2 to 10.6 are recommended for these areas.

When considering the layout of new developments information about flood depths and

velocities should be used to minimise any flood risk or ensure the level of risk is

appropriate to the type of development being proposed. For any sites situated behind

defences the defended flood zones should also be considered, together with

information about flood depths and velocities.

10.7 Possible non-development zones (in areas of greatest risk, beyond mitigation)

When development pressures means that it is necessary to consider development in

areas that are at medium or high flood risk and there are no other suitable alternative

sites for development after applying the Sequential Test the nature of the flood hazard

should be considered. This will allow a sequential approach to site allocation to be

adopted in each flood zone.

When allocating sites for development and designing safe access and exit routes, the

combinations of depth and velocity on the routes should correspond to the category of

‘very low hazard – caution’. The Environment Agency will look to object to development

where the flood hazard is at least ‘danger for some’. Residential development should

be avoided in all areas where the flood hazard is categorised as ‘danger for some’ or

greater.

Refer to the flood maps (in appendices) to identify the areas of greatest hazard.

10.8 Policies for defended areas

Key flood defences are located within the city centre, Eastville area, and Avonmouth.

All these existing defences should be maintained to a high standard, where they

currently protect development or will be relied upon to protect future development

(although reliance on defences to protect new development is not supported by PPS25

or the Environment Agency), with an allowance for climate change.

Sites protected from flooding by a flood defence may be at risk of rapid inundation.

Therefore, new development should be sited away from existing flood defences except

in exceptional circumstances, where a flood risk assessment shows how the building

and its users will be made safe for the lifetime of the development.

Any area behind a defence that is being considered for residential development should

make reference to the breach and overtopping assessments that have been made as

part of this SFRA to allow any development to be designed appropriately .


Table 10.2 Flood risk vulnerability & flood zone compatibility – Area A: City Centre & Floating Harbour

Essential Infrastructure

Water Compatible Development

Highly Vulnerable More Vulnerable Less Vulnerable Source of

Flooding

Description of flood risk

Permitted Development

Flood Zone 1 The areas at lowest risk of flooding are the area around the Tourist Information Centre and Planetarium in Millennium Square.

No restrictions on development other than managing surface water runoff

Flood Zone 2 The extent of Flood Zone 2 covers much of Area A: much of the City Centre area and the area around the SS Great Britain Dockyard are main areas. Much of the land alongside the Floating Harbour is also at risk

Essential transport infrastructure (including mass evacuation routes) which has to cross the area at risk, and strategic utility infrastructure, including electricity generating power stations and grid and primary substations.

Police stations, Ambulance stations, Fire stations, Command Centres and telecommunications installations required to be operational during flooding; emergency dispersal points; basement dwellings; caravans mobile homes and park homes intended for permanent residential use, installations requiring hazardous substances cosent.

Hospitals; residential institutions such as residential care homes, children’s homes, social services homes, prisons and hostels; buildings used for: dwelling houses, student halls of residence, drinking establishments, nightclubs and hotels; non-residential uses for health services, nurseries and educational establishments; landfill and sites used for waste management facilities for hazardous waste; sites used for holiday or short-let caravans and camping, subject to a specific warning and evacuation plan.

Flood Zone 3a The extent of Flood Zone 3a is predominantly a narrow strip following the channels of the Lower Avon and Floating Harbour.

Development should be avoided

Flood Zone 3b Negligible flood extent. Very small extent in the vicinity of the Planetarium, and near Hotwells Lock. Despite the small spatial extent, some of this has a ‘significant’ hazard rating.

Development should not be permitted

Actual Risk (3a with existing defences)

Negligible flood extent. Very small area of risk in the vicinity of the Planetarium, and near Hotwells Lock. Also some small flood extent near SS Great Britain dockyard and in the vicinity of the Prince Street Bridge. A further small area of flood risk from the Malago Stream is in the vicinity of Bedminster Station.

Development should be avoided.

Flood control infrastructure; water transmission infrastructure and pumping stations; sewage transmission infrastructure and pumping stations; sand and gravel workings; docks marinas and wharves; navigation facilities; MOD defence installations; ship building, repairing and dismantling, dockside fish processing and refrigeration and compatible activities requiring a waterside location; water-based recreation (excluding sleeping accommodation); lifeguard and coastguard stations; amenity open space, nature conservation and biodiversity; outdoor sports and recreation and essential facilities such as changing rooms; essential ancillary sleeping or residential accommodation for staff required by uses in this category subject to a specific warning and evacuation plan.



Buildings used for shops, financial, professional and other services, restaurants and cafes, hot food takeaways, offices, general industry, storage and distribution, non-residential institutions not included in ‘more vulnerable’, assembly and leisure; land and buildings used for agriculture and forestry, waste treatment (except landfill and hazardous waste facilities); minerals working and processing (except sand and gravel processing); water treatment plants; sewage treatment plants (if adequate pollution control measures are in place).

Reservoirs, canals and other artificial sources

The Bristol Floating Harbour is a significant source of flood risk – particularly in the regions around the SS Great Britain and Underfall Yard. This is a particular source of risk when tides are high and the movement of water through the Floating Harbour is restricted.

Tidal Flooding Both the Lower Avon and the Floating Harbour are affected by tidal levels. High tide restricts flow through the channels and can cause backing up and an increase in flood risk, especially if accompanied by high fluvial flows.

Surface Water Flooding

No recorded incidents in this area

Groundwater Flooding

No recorded incidents in this area – however, there may be some risk – particularly to sub-surface installations such as underground car parks.

Sewer Flooding 13 properties at risk (Wessex Water DG5 register) in the area predominantly on the northern bank of the Floating Harbour. Additionally, of the order of 20 properties at risk in the immediate vicinity of Area A. 2 recorded incidents in Area A in the FRIS dataset.


Table 10.3 Flood risk vulnerability & flood zone compatibility – Area B: St Pauls, Baptist Mills & Eastville



Highly Vulnerable More Vulnerable Less Vulnerable Source of Flooding Description of flood risk

Permitted Development Flood Zone 1 The areas at low risk of flooding are: parts of the area

east of the M32, some (but not all) of the area north of the B4051 in the Baptist Mills area, and isolated areas in the St Pauls area.


Flood Zone 2 Areas in Flood Zone 2 include much of the Eastville area adjacent to the M32, some of Baptist Mills, and large parts of the area between the A4320 and the A432 and between the A432 and the B4051.


Police stations, Ambulance stations, Fire stations, Command Centres and telecommunications installations required to be operational during flooding; emergency dispersal points; basement dwellings; caravans mobile homes and park homes intended for permanent residential use, installations requiring hazardous substances consent.


Flood Zone 3a Areas in Flood Zone 3a include the area around the A432 junction with the B4058 in Easton, and large parts, although not all of the St Pauls area. These areas are connected by a strip of Flood Zone 3a area along the M32 at Baptist Mills.


Flood Zone 3b Small isolated areas of flooding adjacent to the M32 at Eastville and Baptist Mills adjacent to M32 junction 3. The latter is of a ‘low’ hazard category, but the former in places has a hazard category of significant.



Detailed modelling shows less flooding than the Environment Agency Flood Zone 3a. Most of the actual risk occurs between the mainline railway line and the junction of the A432 and B4051, with some from the Frome to the north of this; as well as from the From south of M32 junction 3. Much of this flooding has a ‘significant’ category.







None present in this area

Tidal Flooding Not a factor in this area

Surface Water Flooding No recorded incidents in this area

Groundwater Flooding No recorded incidents in this area Sewer Flooding 1 recorded property at risk, in the vicinity of Stapleton Road Station, 1 in close proximity to Area B boundary, on the A432 near the junction with the B4051


Table 10.4 Flood risk vulnerability & flood zone compatibility – Area C: East Avon & St Phillips Marsh




Permitted Development Flood Zone 1 The areas at low risk of flooding are: Much of the extent

surrounding the A4320 and part of the central extent of St Phillips Marsh. Parts of the area adjacent to Temple Meads Station.


Flood Zone 2 Areas in Flood Zone 2 include the area east of the main line railway on the London side of Temple Meads Station; Marsh Bridge and the northerly and southerly edges of St Phillips Marsh, and much of its westerly edge.




Flood Zone 3a Areas in Flood Zone 3a include the area east of the main line railway towards on the London side of Temple Meads Station; Marsh Bridge and the northerly and southerly edges of St Phillips Marsh, and much of its westerly edge. For this area, the areas mentioned are sited similarly but are smaller in extent than those in Flood Zone 2.


Flood Zone 3b Some flooding on the periphery of St Phillips Marsh. Small areas of significant hazard predominantly due to the depth component.



Detailed modelling shows less flooding than the Environment Agency Flood Zone 3a. 0.5% tidal scenario shows more flooding than 1% fluvial scenario. Flooding from Lower Avon to lower St Phillips Marsh area, and from Floating Harbour. Worst flooding is around upstream end of the Feeder Canal. Areas of moderate and significant flood hazard.







Some flooding from Bristol Floating Harbour Feeder Canal, especially around its upstream divergence from the Avon, affecting the northern extent of St Phillips Marsh.

Tidal Flooding Tidal Flooding is a risk factor in this area (1 recorded incident in this area – in close proximity to Temple Meads Station): high tides can exacerbate flooding in this area, by reducing the rate at which flows can exit at the downstream end of the Floating Harbour, as well as from the Avon into the Severn Estuary.




No recorded incidents in this area – however, there may be some risk – particularly to sub-surface installations such as underground car parks.

Sewer Flooding 1 recorded property at risk – at the intersection between the A4320 and the Lower Avon


Table 10.5 Flood risk vulnerability & flood zone compatibility – Area D: Ashton




Permitted Development Flood Zone 1 Part of this area lies in flood zone 1 i.e. is at

low risk of flooding. This includes the Park and Ride at Ashton Vale, and the western and eastern fringes of the area north of the A369 and A3029


Flood Zone 2 Detailed modelling shows reduced flooding in the 0.1% case (Zone 2) than the Environment Agency Flood Zone 3a. Flooding from Colliters Brook to west of Silbury Road in Ashton Vale, slightly greater than in 1% case (Zone 3a) with defences. Moderate and significant flood hazard.



Hospitals; residential institutions such as residential care homes, children’s homes, social services homes, prisons and hostels; buildings used for: dwelling houses, student halls of residence, drinking establishments, nightclubs and hotels; non-residential uses for health services, nurseries and educational establishments; landfill and sites used for waste management facilities for hazardous waste; sites used for holiday or short – let caravans and camping, subject to a specific warning and evacuation plan.

Flood Zone 3a Flooding of Bower Ashton and railway; flooding western edge of Ashton Vale housing estate, near Colliters Brook. Fluvial flooding exacerbated by high tide level. Flooding of Ashton Gate area.


Flood Zone 3b Flooding of small area of land adjacent to BCC boundary, SE of Park & Ride. Low and moderate hazard.



Detailed modelling shows less flooding than the Environment Agency Flood Zone 3a. 0.5% tidal scenario shows more flooding than 1% fluvial scenario. Flooding from Colliters Brook to west of Silbury Road in Ashton Vale. Moderate and significant flood hazard predominates.







This area lies to the north east of the Barrow Gurney Reservoirs. In the event of breach of the dam retaining the northern-most reservoir (no.3) flood water could potentially affect the Ashton area by passing down the Colliters Brook. Detailed information on flood risk due to reservoir breach is not available as part of this SFRA.

Tidal Flooding Tidal Flooding is a risk factor in this area (no recorded incidents in this area – appreciation of risk from modelled scenarios): high tides can exacerbate flooding in this area, particularly from the Ashton Brook to the area adjacent to Ashton Vale


Surface Water Flooding is risk factor in this area (two recorded incidents – A3029 and Ashton Drive, and one close by – in Bower Ashton)



Sewer Flooding No recorded incidents in this area


Table 10.6 Flood risk vulnerability & flood zone compatibility – Area E: Avonmouth



Highly Vulnerable More Vulnerable Less Vulnerable Source of

Flooding

Description of flood risk

Permitted Development

Flood Zone 1 Only small isolated areas throughout the area are at low risk of flooding.


Flood Zone 2 When defences are not accounted for, almost the entirety of the Avonmouth area lies within Flood Zone 2. The majority of the risk is from tidal flooding although there is some risk from fluvial events and from the local Rhine system.




Flood Zone 3a When defences are not accounted for, almost the entirety of the Avonmouth area lies within Flood Zone 3a. The majority of the risk is from tidal flooding although there is some risk from fluvial events and the local Rhine system. This extent is only marginally smaller than Flood Zone 2.


Flood Zone 3b No Functional Floodplain assessment was carried out. Development should not be permitted


Actual risk for current scenario not displayed. For the future scenario, Flood Zone 3a covers the majority, possibly 80% of the Avonmouth Area.







Rhines: a network of small drainage channels across the Avonmouth area. Can be a source of flood risk when their capacity is exceeded as a result of tide, fluvial flows, rainfall, blockage or collapse.

Tidal Flooding High risk of tidal flooding in this area. Surface Water Flooding



No recorded incidents in this area.

Sewer Flooding Two recorded incidents in this area (DG 5 Register – Wessex Water) – both in the vicinity of Avonmouth Railway Station


11 Flood risk management policy – city wide

11.1 Overview

This chapter provides recommendations for flood risk management policies that apply

to all the five areas investigated, and covers developable zones, ‘windfall’

applications, FRM funding and developers contributions, sustainable drainage

techniques (SUDS), building design and safe access routes and flood evacuation

plans. The policies specific to an area are detailed in the previous chapter.

11.2 Developable zones where mitigation may be appropriate (for allocations)

Development should not be located in flood risk areas unless the Sequential Test,

and where necessary, the Exception Test have shown that it is necessary. Where this

is the case, a mitigation strategy to deal with the flood risk is required to ensure that

any development will be safe.

Wherever possible the construction of new defences to protect new development

should be avoided, since there is a residual risk that the defence may breach or be

overtopped. Possible strategic solutions to manage flood risks are identified within

each of the Areas (Chapter 12).

Any development that requires the construction of new defences will need to show

that other options (e.g. flood storage areas) have been considered and are not

feasible and that the defences are compatible with the long-term flood risk

management policies for Bristol as detailed in the Bristol Avon CFMP and the Severn

Estuary SMP (Chapter 3).

Bristol City Council should consider producing a flood risk management delivery

strategy, setting out how areas at risk of flooding will be adequately defended with

provision for the long term maintenance of the defences, emergency planning, etc.

This strategic study is one of the recommended further FRM studies (Chapter 14).

Opportunities may exist to reduce overall flood risk within a zone through the

redevelopment of existing uses, through innovative design, drainage or other forms of

flood mitigation. The merits of such schemes will need to be clearly demonstrated by

the applicants and supported by a flood risk assessment and drainage impact

assessment.

11.3 Areas where the Council will consider ‘windfall’ applications

‘Windfall’ sites are those sites which become available unexpectedly and therefore

have not necessarily been considered as part of the forward planning site allocation

process. Bristol City Council should consider windfall applications for sites with an

equal or lower risk of flooding as those sites that have already been allocated.

For the purpose of development control, policies may need to be included for

unallocated windfall sites that will set out broad locations and quantities of windfall

development that will be acceptable. Windfall sites should be subject to the same

consideration of flood risk as other allocated sites.


The Sequential Test should be applied to windfall sites, unless the area in which they

occur has been sequentially tested on the basis of this SFRA. Where the Sequential

Test has not been applied to the site or area, proposals will need to be dealt with on

an individual site basis and the developer will need to provide evidence to Bristol City

Council that they have adequately considered other reasonably available sites, both

allocated and unallocated.

A change of use to a higher flood risk Vulnerability Classification as set out in

Table D2 of PPS25 will generally not be subject to the Sequential Test however the

application will still be subject to the Exception Test where applicable and in all cases

a FRA will be required to demonstrate that the development is safe.

It is understood that the City Council has already published a Strategic Housing Land

Availability Assessment (SHLAA), which has taken account of Environment Agency

flood maps. The Council has also identified ‘reasonably available’ sites - taken to be

those identified in the Council's SHLAA, together with others with planning permission

or permitted subject to signing of a Planning Agreement and which have been

included within the Council's housing ‘trajectory assumptions’.

It is not expected that it will be necessary to rely on additional ‘windfall’ sites to meet

the level of housing provision proposed in the emerging Core Strategy. However,

were this to be the case, it would be necessary to assess such sites against

appropriate criteria.

The emerging Core Strategy acknowledges the need for sites not covered by the

forthcoming Site Allocations and Development Management DPD, that are at risk of

flooding, to be the subject of Sequential and Exception testing. Development

Management policies could provide more detail to define the tests for ‘windfall’

development, if necessary.

11.4 FRM funding and developer contributions

Defra has national policy responsibility for FRM and provides funding (through ‘grant

in aid’) to the Environment Agency, which also administers grant for capital projects to

local authorities and internal drainage boards. The Environment Agency is the

principal FRM operating authority, and generally supervises all matters relating to

flood defence including:

• building and maintaining defences and other management measures on

designated Main Rivers and Critical Ordinary Watercourses

• flood forecasting and warning

• improving public awareness of flood risk

The Government has previously announced that under the Environment Agency’s

new strategic overview role in England for all sources of flood risk, local authorities

will take responsibility for surface water management, including Surface Water

Management Plans.


Local authorities, working through local and regional resilience forums, lead in:

• planning for emergencies, including flooding events

• dealing with the consequences of flooding such as humanitarian assistance,

emergency housing and clear up operations

• building and maintaining defences on ordinary watercourses.

PPS25 (Appendix G) sets out the circumstances under which it may be necessary to

permit development that requires the provision of FRM including flood defence and

mitigation works. In view of the scale of the flood risks identified in Bristol, this is a key

consideration to ensure that development is ‘safe’ against flood risks over its lifetime.

PPS25 states the following considerations regarding the contributions that developers

should make:

• developers cannot normally call on public resources to provide defences and

other measures for their proposed developments where they are not already

programmed for the protection of existing development;

• where previously programmed defences and other measures have already

been provided at public expense to protect existing development, these may

also provide opportunities for new development, provided this does not itself

add to flood risk at other locations;

• for some previously developed land, public investment in land remediation and

infrastructure may include an element of flood defence and mitigation

investment as a means of bringing such land into beneficial use;

• where the two preceding considerations do not apply but where other material

considerations outweigh the risk of flooding, any necessary flood risk

management, including defences or flood alleviation works required because

of the development or which form a part of that development should normally

be fully funded by the developer;

• where such works would provide a wider benefit, the funding provided by

developers may be proportional to the benefits to them. For instance, the

development might fund the provision of the defences or other measures

which would then be vested in and maintained by the operating authority.

For Bristol it is clear that to deliver the growth proposed in the RSS and protect

existing areas at risk there is a need for significant future investment in FRM (Chapter

12), particularly in light of the climate change flood risks predicted (Chapter 7).

Developer contributions to offsite FRM solutions can be expected to only cover a

fraction of this, and the proportional approach advocated in PPS25 is considered

appropriate. On-site provision of FRM measures will be funded by developers.


11.5 SUDS – appropriate locations and types

Surface water drainage systems developed in line with the ideals of sustainable

development are collectively referred to as Sustainable Urban Drainage Systems

(SUDS). At a particular site, these systems are designed both to manage the

environmental risks resulting from urban runoff and to contribute wherever possible to

environmental enhancement.

SUDS objectives are to minimise the impacts from the development on the quantity

and quality of the runoff, and maximise amenity and biodiversity opportunities. FRA’s

must investigate the feasibility of all SUD techniques within their development

boundary. To achieve the SUDS objectives emphasis must be drawn towards

incorporating a SUD treatment train.

PPS25 requires that the volumes and peak flow rates of surface water leaving a

developed site are no greater than the rates prior to the proposed development

(PPS25 Annex F paragraph F10) for storm events up to and including those with a 1

in 100 year return period. The Environment Agency may require that the redeveloped

site runoff is to provide a degree of betterment on the existing site runoff whilst

allowing for climate change.

Any betterment rates or conditions on the redeveloped sites runoff should be agreed

during planning stages. This ensures that the effect of the proposed development on

downstream water courses and areas is minimal, even when climate change occurs.

As a result, SUDS can have a potential positive effect by reducing flood risk at all

sites. This report recommends that SUDS should be a requirement for all new

development. Space should be specifically set-aside for SUDS and used to inform the

overall site layout.

The selection of SUDS within a development is specific to the site conditions and

criteria to be met. The SUDS manual – CIRIA C697 published by the Construction

Industry Research and Information Association (CIRIA) provides best practice

guidance on the planning, design, construction, operation and maintenance of

Sustainable Drainage Systems to facilitate their effective implementation within

developments.

Source control techniques such as green roofs, soakaways, water butts and rainwater

harvesting aim to mimic the Greenfield runoff at the source. Where these can not be

provided or where larger attenuation volumes are required in addition to source

control measures, site and regional controls are to be implemented within

developments. The site and regional controls include filter strips, swales, pervious

paving, attenuation crates, infiltration basins and devices, detention basins, ponds

and wetlands.

The selection of SUDS to be implemented within the developments will be specific to

the site conditions, required attenuation volumes, permitted outflow rates and

allowable SUDS techniques. Local authorities may preclude the use of some SUDS

due to relevant maintenance and adoption issues.


Wetland Pond Dry Basin (in foreground)

Swale Permeable paving Filter drain/ strips

Attenuation tank Rainwater harvester

Green Roof1

Figure 11.1 Diagram of how SUDS can be used at a local scale

(Source: Dti 2009)

(Source: The Pitt Review, 2007. Learning

Lessons from the 2007 floods, Cabinet Office)


Figure 11.1 shows examples of SUDS techniques. As Bristol lies on predominantly

clayey soils the use of infiltration techniques will be limited. Appropriate SUDS will

need to be established through a site specific drainage assessment of local

geological and groundwater conditions, including specific site investigations to assess

the capabilities of infiltration techniques.

The key loss of benefit from not utilising infiltration is that these SUDS systems

attenuate peak flows but do not significantly reduce flood volumes. Discharging

attenuated site runoff directly to watercourses should be used instead of routing flows

through the sewer network. However the Environment Agency would expect that the

initial assumption of any drainage designer would be to include infiltration where

possible.

Large increases in impermeable areas contribute to significant increases in surface

runoff volumes and peak flows and could increase flood risk elsewhere unless

adequate SUDS techniques are implemented. It is relatively simple to avoid the

increase in peak flows by providing attenuation or detention storage that temporarily

store the required amounts of runoff within the site boundary. The use of water

recycling and permeable paving can allow transevaporation of up to 20% of the water

attenuated.

Specific attenuation for the Bristol area could comprise of:

• Swales that can be constructed alongside roads and within green areas to

transfer runoff to storage facilities, and also provide limited storage. Infiltration

swales are preferred as they keep dry between rainfall events and so avoid

becoming marshy, and allow as much infiltration as the surrounding ground

can accommodate.

• Pond / dry basin to provide the majority of the volume required to attenuate

the surface water runoff.

• Permeable/porous paving may be used within development areas, subject to

consideration of the adoption issues with the highway department, to

attenuate runoff at source as it will collect the rainfall below the surface and

discharge it after a significant delay.

In October 2008, the Government changed the General Permitted Development

Order making (inter alia) the hard surfacing of more than five square metres of

residential front gardens only permitted where a permeable surface is used (CLG and

Environment Agency, 2008). The purpose of this policy change is to slow any

increase in the loss of natural drainage storage and the incidence of surface water

flooding.

11.6 Appropriate flood avoidance, site layout, resistance and resilience measures

The best way to avoid flood risk is to locate the development outside areas of flood

risk i.e. Flood Zone 1. Where there are no suitable sites in lower flood risk areas, the

Sequential Approach should be applied within the development site to locate the most

vulnerable elements of a development in the lowest risk areas.


Site layouts should be designed so that the most vulnerable uses are restricted to

higher ground at lower risk of flooding, with more flood-compatible development

(managed public parking, open space etc.) in the highest risk areas. The acceptability

of parking use will be dependant on the depth and the ability to manage parking

during potential flood events.

Where development is considered necessary and it is not possible to minimise flood

risks to an acceptable level through the use of defence structures, flood storage areas

or other alternatives, the less desirable resort is to minimise the impact of flooding

through individual building design by raising finished floor levels and providing safe

access routes.

Other resistance and resilience measures are likely to be considered as unacceptable

on their own for new development since the hazard posed by flood waters still

remains, particularly for access, egress and the supply of utilities. Indeed, on their

own these measures are unlikely to be suitable as the only mitigation measure

implemented, but may be appropriate where land is being used for water-compatible

or change of use to less-vulnerable building types (see Table D.2 of PPS25) where

there is not an inappropriate risk to people or assets.

Further requirements to enable development may include appropriate flood warning,

raised floor level and raised ground levels that allow safe access and egress, i.e. dry

pedestrian egress should be possible above the 1% fluvial or 0.5% tidal flood level

plus climate change. Should this not be possible an egress route which has a flood

hazard rating of less than 0.75 and considered to have a low degree of flood hazard,

as identified in Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2320/TR1, shall be

provided. Emergency vehicles should be able to access the site during an extreme

event (an event with an annual probability of 0.1%).

Advice from the Local Authorities emergency planning officer and the emergency

services should be sought on whether they will be able to provide emergency

evacuation from the development during exceedance events (events in excess of a

design event, i.e. with an annual probability of between 0.5% and 0.1% for tidal

events or 1% and 0.1% for fluvial events).

Individual property protection can be divided into two main types (Figure 11.2):

• Flood resistance measures (also known as dry proofing) are those put in place

to prevent flood water entering a building. These measures may be acceptable

for a change of use. For new development elevating finished floor levels

above future flood levels would be more appropriate.

• Flood resilience measures (also known as wet proofing) accept that water will

enter the building but through careful design will minimise damage and allow

the re-occupancy of the building quickly.

As resilience measures still allow water to enter a building, these should not normally

be considered for new developments.


Figure 11.2 Examples of flood resistance and resilience measures (Source: Adapted from Scottish Executive, 2004)

Making a building flood resistant aims to prevent flood water entering the building.

This approach relies on flood barriers and the building structure. The flood barriers

are placed across doors and air vents and may include non-return valves on drains. It

is difficult to effectively block all flooding routes, e.g. where services enter the

building.

These types of flood resistant measures are most effective for short duration flooding

with simple measures estimated to be effective for several hours and more complex

measures effective for several days (Scottish Executive, 2004). In the case of the

River Avon, the size of the catchment will result in an extended flooding duration.

Making a building flood resilient involves a number of measures to make the building

able to cope with being inundated with flood water. Work may include the raising of

the services, in particular the service meters and electrical wiring above the flood

level. Some examples of flood resilience measures include:

• replacing floors with concrete;

• removing carpet and replacing with clay tiles;

• replacing open cell insulation with closed cell insulation.

Since any flood management measures only manage the risk of flooding rather than

remove it, flood resistance and flood resilience may need to be incorporated into the

design of buildings and other infrastructure behind flood defence systems. If a

defence does fail, the area behind the defence may be rapidly inundated with high

velocity flood water. As such, buildings should be structurally designed to withstand

the expected water pressures, potential debris impacts and erosion which may occur

during a flood event.


11.7 Policy guidance on developing Flood Evacuation Plans

11.7.1 Summary

In developing a Flood Evacuation Plan (FEP) to satisfy the requirements of PPS25,

developers must always remember that the plan is about actions to be taken to save

lives. It is not a plan that merely satisfies a “journey” to completing a development

application. An emergency evacuation plan template is to be posted on-line at the

Bristol City Council website.

Experiences during the 2007 floods in the UK and the subsequent report by Sir

Michael Pitt to Government recognised that it is not just the emergency services that

have roles in flooding emergencies. Those who plan developments and then those

who live and work in areas, which could be flooded, have personal roles to ensure

that they are prepared and can respond when warned that flooding is imminent.

The Pitt report in Recommendation 70 said “The Government should establish a

programme to support and encourage individuals and communities to be better

prepared and more self-reliant during emergencies, allowing the authorities to focus

on those areas and people in greatest need”. This work is being done within existing

communities but in new developments this guidance will assist that approach.

11.7.2 Guidance

To enable developers to comply with the requirements of PPS 25 and to support Pitt

recommendation 70, Bristol City Council Civil Protection Unit will create a template for

a Flood Evacuation Plan (FEP). Developers will be required to complete a FEP. In

order to obtain a “suitable for purpose” rating the FEP must include all elements of the

template. Failure to do so will mean that the plan is deemed “unsuitable for purpose”.

The FEP must be a living document capable of use in a flood emergency; owned and

understood by those with responsibilities for Health & Safety; by staff or employees

who have roles within that FEP or those who live in the development. It must show

records of training and exercising; instructions on what to do and by whom and a

system of checks to ensure that when the plan is activated, decisions on who did

what and by when are recorded, thereby ensuring the safe response to the threat or

evacuation from flooding.

Where the development application is only for housing the developer or management

company must ensure that purchasers are given copies of the FEP at the time of

purchase.

The thrust of developing FEPs is, therefore, to ensure that those who work and live in

areas prone or likely to flood are better prepared to be self-reliant in order that the

vulnerable in our society can be the focus of the responding services.


11.7.3 Matters to Consider in Preparing Flood Evacuation Plans

(advised by Bristol City Council)

Matters to be considered will vary according to the nature of the development

proposed and the potential severity of flooding. Applicants should refer to the FEP

template for details. However, relevant considerations may include:

• The availability of flood warning systems. Individual properties and wider sites

can be registered with the Environment Agency’s warning service “Floodline”.

This provides information on the current and future flood danger.

• Where is the location to which the FEP relates (map)?

• Occupants’ awareness of the likely frequency and duration of flood events.

Particular attention should be given to communicating warnings to vulnerable

people – e.g. those with restricted mobility or impaired hearing or sight.

• An FEP must identify who has responsibility for owning/managing the plan. It

must be reviewed at least annually and those who have a role within it must

be aware of its content by training/briefings as well as any exercise to test its

capability.

• Evacuation routes and plans and warnings will be of particular importance

where premises are used by transient occupants. It should be assumed that

occupiers will not have local knowledge and will need to be guided to a safe

route/location. Who issues warnings and/or instruction?

• The availability and knowledge of staff to respond to a flood warning including

preparing for evacuation, deploying flood barriers and other relevant

equipment or procedures. Who commands the response required? How is

communication delivered and to whom to ensure evacuation is carried out

safely and effectively? Co-ordination of evacuees- is every one safe?

• If critical workers are to remain on site; have risk assessments been made?

How will they communicate to and with management or emergency services?

What resources/equipment are available to sustain them and for how long?

Will management inform their families?

• The possible need for emergency services to rescue vulnerable occupants

and the feasibility of doing so. The need to liaise with developers of any

neighbouring site(s) in preparing FEPs in order to co-ordinate procedures.

• Measures to re-establish normal use, following a flood event.

• FEPs should indicate the safest routes to leave as early evacuation before

floodwaters affect those routes is one option. If routes are affected then

ensuring there is a safe location to stay within the building, rather than

evacuating, will normally be the safer option in the event of flooding, because

of the dangers of moving in flooded areas.


12 Strategic options for flood risk management

12.1 Overview

This chapter presents a wide range of strategic options for flood risk management for

Bristol, compatible with the related high level plans (Bristol Avon CFMP, Severn

Estuary SMP, and the emerging Severn Estuary FRMS – see Chapter 3). The aim of

this options appraisal is to establish a short-list for each of the five areas of potential

development (Figure 1.1) to be investigated in future more detailed studies.

For this preliminary appraisal the types of strategic options and approaches are first

considered, and options specific to Bristol and the wider Bristol Avon catchment then

outlined, so they can be understood and developed later if necessary. The appraisal

of these options considers the following:

• Environmental, economic and societal benefits

• Likely preferred solution, including combination of options to achieve the

desired flood risk mitigation.

• Deliverability (define in terms of positives and negatives) and outline cost

estimates.

12.2 Complexity of the flood risk problem

Many parts of Bristol area at risk of flooding, which arises from a number of sources

including tidal and river flooding, localised runoff and sewer flooding. This flooding

affects people, property and a variety of infrastructure.

The complexity of the flood risk problem in Bristol is due to:

• Bristol’s location affected by tidal and fluvial flooding

• Combination of flood water sources and pathways

• Constraints (physical and technical) of a city centre location

• Need to manage significant quantities of flood water to have any impact

• Lack of a simple cost effective engineering solution

Bristol is a significant flood risk location. Within the SFRA area there are 2,300

properties at tidal/fluvial flood risk, increasing to 14,000 with climate change (figures

from Bristol Avon CFMP).

The fluvial flood risk in Bristol is currently reduced in a number of ways. There are

extensive undeveloped areas of floodplain upstream on both the Avon and Frome,

which provide significant areas to store large volumes of water when the rivers are

out of banks. In addition to this floodplain attenuation, the river channels through

Bristol provide a considerable amount of conveyance.


This SFRA identifies that the predicted climate change will markedly increase the

extent of flooding. Therefore areas that are currently situated outside of Flood Zone 3

will be at risk of flooding in future years, and the locations that are currently at risk of

flooding may be susceptible to more frequent, more severe flooding in future years.

It is essential that the spatial and development control processes influence the

location, layout and design of future development. This will mitigate the potential

impact that climate change may have upon the risk of flooding to new developments.

There is a need to proactively deliver a reduction in flood risk through the planning

process – in simple terms, guiding vulnerable development away from areas that are

most at risk, and adopting sustainable design techniques.

12.3 Types of options

Flood risk management requires integrated solutions to manage flood risk,

addressing the issues of rainfall, runoff, rivers, coasts and flood inundation as well as

the human and socio-economic issues of planning, development and management.

Such an integrated approach is promoted in:

• Defra’s The Government’s Water Strategy for England – Future Water, and

strategy Making Space for Water

• DTI’s Foresight Future Flooding project

• European Directive on the Assessment and Management of Flood Risks

Structural options to manage flood risks can be summarised as:

• Increase flood storage as a strategic solution - dams, floodplain/wetland

storage, floodplain restoration, temporary channel storage.

• Increase flood conveyance (affects d/s) - canalisation, channel restoration,

dykes and embankments, by-pass and diversion channels, structure

upgrade/improvement – environmental and sustainability concerns, operation /

maintenance legacy.

• Flood defences - flood defences along river, ring dykes for key areas, special

structures such as tidal barriers, including the option to increase the standard

of protection provided by existing defences.

• Flood water transfer - bypass or diversion across river/tributary catchments -

not considered a feasible option for Bristol City except for interceptor sewers

The scope of structural and non-structural options for flood risk management are

detailed in Appendix B, and includes the structural (river engineering) options above,

mainly non-structural approaches to manage flood events and losses, and urban

(fabric) and rural management.


12.4 Possible strategic solutions to manage flood risk

There is limited reference to possible strategic solutions to manage flood risk in the

Bristol Avon CFMP (recommends a long term reduction in flood risk for Bristol), and

Severn Estuary SMP (recommends ‘hold the line’ of tidal defences).

The most significant long term increase in flood risk is likely to be from rising sea

levels, and the proposals of the Severn Estuary FRMS will be relevant to the

management of this in Bristol, as will any future power generation option involving a

Severn barrage. Any potential flood risk benefits to Bristol from a barrage depend on

its location, with two of the sites currently being assessed shown in Figure 12.1.

There are a number of possible strategic solutions for the Bristol Avon catchment as

illustrated in Figure 12.2 (next page). There is some potential for flood storage on the

River Frome and possibly other local tributaries. In the urban centre, opportunities

should be sought to incorporate strategic flood risk management works, including

strategic SUDS, into any redevelopment plans.

There may be scope to introduce a strategic storage facility on the River Frome,

possibly by developing the existing Tubbs Bottom site and/or other storage sites. Not

only do small to medium size flood storage areas contribute to flood attenuation (and

compensatory storage for flood defence schemes), especially for low to medium order

flood events, but they provide opportunity for environmental enhancement.

`

Base map source: Environment Agency website www.environment-agency.gov.uk

Potential barrage

locations – only location

downstream of Bristol

impacts on SFRA area

Flooding from rivers or

sea without defences

Figure 12.1

Potential Severn

Barrage locations


The option for flood storage can provide some flood relief, but not a stand-alone

strategic flood alleviation solution for Bristol City, nor does it appear a practicable one

for the Avon based on the emerging findings of a separate strategic study (by Atkins

for Bath and North East Somerset Council, due to be published January 2010). The

long duration and high volume flood hydrographs cannot be attenuated easily without

very large storage facilities and it is rare that this option is adopted in such areas.

There is potential to improve/extend the existing flood defences for greater protection,

infilling low spots and/or generally raising the defence to provide higher protection or

to sustain protection in response to increasing flood risk due to climate change.

In considering the optimal system configuration and operational rules involves

detailed asset management studies taking into account the ‘life’ of flood defence

assets, including river control structures and culverts, and their future maintenance,

refurbishment and replacement (not necessarily like-for-like replacement as

alternatives may provide more benefit at lower cost).

There may be ‘quick wins’ if any opportunity exists to increase flood flow conveyance

at ‘bottlenecks’ in the river system. As an example, it may be possible to further

improve the performance of the Floating Harbour using existing controls.

The alternative for major schemes, such as widen/deepen the river, enlarged control

structures, interceptor sewers, etc. require major investment and are difficult to justify

on economic grounds (typically the scheme cost far outweighs the benefits in terms of

flood damage avoidance that result).

Solutions to surface water flood risks are also needed and the planned SWMP,

focussing on the critical drainage areas identified by this SFRA (Chapter 7), will

identify these.

X

X

Bristol City SFRA area - most options fall within this area Avon catchment upstream – potential for strategic solution appears limited except land management in longer term Area of search for strategic storage potential on River Frome Avon tidal barrier

Refer to Figure 3.2 for original CFMP policy figure

Figure 12.1 Indicative location of strategic solutions for Bristol and wider Avon catchment


Non-structural options help ensure a sustainable long-term strategy:

• Heightening communities’ awareness of flood risks.

• Improvements to flood warning, including flood forecasting.

• Improvements to flood defence operation and maintenance activities.

• Tactical local measures, such as increasing flood resilience of properties.

• Influencing spatial planning and development, e.g. promoting SUDS, ensuring

“space for water” in floodplains, etc.

• Changes to land-use management to reduce storm runoff into the rivers.

• Integrated environmental measures which could enhance environmental value

whilst supporting flood risk management objectives.

• Institutional changes to optimise the effectiveness and co-ordination of flood

risk management (the recent Pitt review is one mechanism for this).

12.5 Flood risk management options

A strategic approach, which combines options for flood risk management, is

necessary to deliver the CFMP policy to reduce flood risk and unlock the

development potential in the flood risk areas identified (A to E). To reduce the risk

could mean lowering the probability of exposure to flooding and/or the magnitude of

the consequences of a flood.

A strategic approach to reduce flood risk, for existing/planned development, includes:

• A clear vision for land use is the most effective way of managing flood risk. In

some places it will be through adaptation of the urban environment to make it

more resilient to flooding. In other places locating new development in areas

of lowest risk may be the best option.

• As a number of development areas are located on the river floodplain,

increasing the channel capacity is severely constrained by development. Most

of the feasible storage, defence and interceptor sewer options are high cost,

and require additional measures to deal with surface water flooding problems.

• Reducing the consequences of flooding will be very important, particularly in

those areas where redevelopment rates are low (i.e. limited opportunity to

increase the resilience) or flood defences are not viable. How this can be

achieved involves working with communities and organisations to establish.

Based on the CFMP analysis (Figure 6.1), the Standard of Protection (SoP) provided

by the existing (tidal & fluvial) defence system within each area investigated is

estimated to be 1-2% SoP in Areas D and E, 2-4% SoP in Areas A and C, and 20-

50% in Area B. This compares with a target SoP is 2% (AEP) for city areas (highly

urbanised). Areas A, B and C fail this target.

The options set out in Table 12.1 below are drawn from a generic list of options

(Appendix B), following initial assessment of the flood risk issues in Bristol and

(limited) consultation with Bristol City Council and Environment Agency specialists.

An outline description of the options then follows, in Tables 12.2 to 12.8, with some

locations of possible strategic solutions identified in Figure 12.2 alongside the existing

flood defence system (Figure 6.2).


Raised flood defence

New raised defence option

Avonmouth tidal defence

Eastville/Easton flood defence

Ashton Vale flood defence

Temple Meads flood defence

Flowers Hill, Brislington flood defence

x M32 culvert

x Brislington Brook culvert

x

Colliters Brook & Longmoor culvert

North Stornwater Interceptor culvert

Figure 12.1 Location of possible strategic solutions

Avon tidal barrier option

New NSWI option

Improve Floating Harbour option


Table 12.1 Possible strategic solutions to manage fluvial flood risks

Ref. Area Flood risk Flood risk management options A-E All flood risk areas

in Bristol City Tidal Severn

River Avon

River Frome

Floating Harbour

Ashton Brook

Non-structural approaches - apply PPS25 flood risk hierarchy such as: substitute residential for less vulnerable development - flood forecasting/warning, public awareness - development control, e.g. avoid inappropriate development, etc. - land management as a longer term measure Structural approaches - flood resilience (retrofit existing, design in future development) - increase surface water attenuation and runoff management (incl. SUDS) - reinstate floodplain areas in public open spaces - Severn barrage - Avon tidal barrier - pumping at City Docks, Ashton Avenue and NSWI against the tide

A City Centre & Floating Harbour

River Avon

River Frome

Floating Harbour

- bypass or flood relief channel - widen/deepen Avon channel - enlarge Avon control structures - improve structure operations, e.g. Floating Harbour, Netham Weir - improve/extend existing defences at 8 identified low spots - increase flood storage, potentially on Frome at Tubbs Bottom or tributaries in /outside Council’s

boundary - increase the capacity of Frome and North Storm Water Interceptor - increase capacity of Floating Harbour - raise Avon channel defences downstream of Netham weir to the Avon Gorge

B St Pauls Baptist Mills & Eastville

River Frome

Avon (tidal)

- bypass or flood relief channel - widen/deepen Frome channel - enlarge Frome control structures - improve structure operations, e.g. North Storm Water Interceptor - improve/extend existing defences - increase flood storage, potentially on Frome at Tubbs Bottom or on Ladden Brook - new Storm Water Interceptor culvert - new channel defences on the Frome

C East Avon & St Philips Marsh

River Avon - bypass or flood relief channel - widen/deepen Avon channel - enlarge Avon control structures - improve structure operations, e.g. Floating Harbour, Netham Weir - improve/extend existing defences from Netham downstream of Temple Meads,Totterdown

D Ashton area Tidal Avon - improve/extend existing defences - widen/deepen Ashton Brook channel - upland storage

E Avonmouth Tidal Avon - Improve/extend existing tidal defences and as a minimum ‘hold the line’ (SMP policy)


Table 12.2 Non-structural approaches applicable to all flood risk areas (A to E)

Option Outline description

Development control, e.g. avoid inappropriate development, etc.

In line with PPS25 as a priority short term action to create safe and sustainable development that positively reduces flood risk:

• Encourage safeguarding of land that may be needed to implement future options to manage flood risk.

• Use the information of this SFRA to reduce flood risk, influence Local Development Framework (LDF) Documents, Regional Spatial Strategies (RSS), planning applications and emergency and evacuation plans.

• Seek commitments in land use planning documents to retain the remaining floodplain for flood risk management compatible uses.

• Adopt and apply policies to ensure that all new development in the floodplain is resistant and resilient to flooding.

Flood warning, flood awareness and emergency planning

An important element of flood risk management is to prepare for and to address the consequences of flooding. The priority actions are:

• Local Resilience Forums and multi-agency Emergency Response Plans

• Identify critical infrastructure at risk of flooding and encourage appropriate action.

• Ensure effective communication plans are in place before, during and in the recovery phase of a flooding incident.

• Increase public awareness including encouraging people to sign-up for the free Flood Line Warnings Direct service of the Environment Agency.

• Encourage communities to work together to prepare community flood plans that: - Identify the flood risks to the community and take action to reduce them - Identify vulnerable people in the community - Develop plans to assist/protect them - Identify resources in the community available to assist in an emergency - Provide key contact details in case of emergency.

• Up-to-date flood forecasting and warning systems should be in place, with regular review and update to ensure a reliable service.

Findings of this SFRA for existing and future developments should be incorporated into the flood evacuation (and incident management) plans. This should specifically identify strategic evacuation routes to enable emergency services to continue work during a flood event. The flood risk to key command centres and emergency facilities should also be assessed.

Apply PPS25 flood risk hierarchy such as substitute residential development to less vulnerable development

In Bristol there are significant development pressures, and a lack of available land outside Flood Zones 2 and 3. As a result some sites may have to be considered within Flood Zone 3.

In line with PPS25, it is recommended that where possible sites with a greater flood risk are retained for employment use, and those with lower risk are used for housing.

The mechanisms by which this should take place are the Sequential and Application tests, as well as site specific FRA’s which should show that opportunities to place lower vulnerability uses in higher risk areas, and housing development in lower risk areas have been taken wherever possible.

Land management as a longer term measure

Although there can be benefits for local flooding mitigation, the evidence to date has not demonstrated there are benefits in reducing extreme flood events at the catchment scale. The Environment Agency has pledged to support research into large catchment scale impacts of land management changes on flood risk, water quality and resource protection and will actively seek partnership projects (adapted from Environment Agency position statement on land management).


Table 12.3 Structural approaches applicable to all flood risk areas (A to E)


Flood resilience (retrofit existing, design in future development)

Where development is considered necessary and it is not possible to minimise flood risks to an acceptable level through the use of defence structures, flood storage areas or other alternatives, a less desirable resort is to minimise the impact of flooding through individual building design by raising finished floor levels and providing safe access routes.

Indeed, on their own these measures are unlikely to be suitable as the only mitigation measure implemented, but may be appropriate where land is being used for water-compatible or change of use to less-vulnerable building types (see Table D.2 of PPS25) where there is not an inappropriate risk to people or assets.

Long-term adaptation to be more flood resilient can be linked to redevelopment:

• Encourage refurbishment of existing buildings that increases resilience and resistance to flooding.

• Identify opportunities to recreate river corridors and wetland habitats in urban areas. Encourage new development and any redevelopment of these areas to acknowledge these areas in their site layouts and set development back, allowing space for water, habitat, wildlife and recreation.

• Encourage partners to assess the viability of future land swapping opportunities in those areas where there is a risk of flooding.

These priorities are likely to be most applicable in those areas where redevelopment is more likely in the foreseeable future. In some areas where major redevelopment is unlikely, there may be opportunities to reduce the consequences of flooding if there is a high level of property renovation and conversion taking place.

Increase surface water attenuation and runoff management (incl. SUDS)

Current Environment Agency standing advice requires that any development larger than one hectare must ensure that the post development runoff volumes and peak flow rates (1 in 100-year with climate change) are attenuated to the greenfield (pre-development) condition or at least to mimic the surface water flows arising from the site prior to the proposed development. As a result, SUDS can have a potential positive effect by reducing flood risk at all sites.

Reinstate floodplain areas in public open spaces

Option to flood public parks and other green spaces, classified as low vulnerability to flooding, and move (expect to a limited degree) the flood risk from more vulnerable land uses (see substitution). Possible locations include parts of the area bounded by the A4174, A4175, A4 and A431.

Severn barrage Tidal barrage proposal, principally for power generation, would benefit Bristol if sited downstream of Bristol, e.g. one alignment from Brean Down to Lavernock Point. This would harness the power of the tide and consequently reduce the tidal range, reducing the possibility of tidal flooding and tide-locking of fluvial flows out of Bristol. It may be necessary to install pumping – see option below.

Avon tidal barrier Tidal barrier proposal near the mouth of the River Avon (150m wide), to be operated when large fluvial peak flows coincide with high tides to prevent upstream tide surge with potential to cause flooding problems within the city area. It would require a major gated structure (e.g. rotating gates as employed on the Thames Barrier, or radial gates on the Hull tidal barrier) to span the full river width and permit navigation.

Pumping at City Docks, Ashton Avenue and NSWI against the tide

Pumping against the tide to increase the capacity of the NSWI and the Ashton Vale culvert so that when high fluvial flows are coincident with high tides the culverts continue to discharge and alleviate flooding problems upstream. For such an option to be effective would require a high capacity low lift pump station. Examples include the St Germans Pumping Station in Norfolk (Fens) designed to operate at 100m3/s, and the Altmouth Pumping Station on the River Alt near Formby with a capacity of 80m3/s.


Table 12.4 Structural approaches – Area A: City Centre & Floating Harbour


Enlarge Avon control structures

The principle control structures on the Avon are Netham Weir and the controls associated with operation of the Floating Harbour – these controls are considered in the option below. Major works to enlarge Netham Weir or install a bypass structure would increase flood flow conveyance and reduce the backwater effect upstream of this structure.

Improve structure operations, e.g. Floating Harbour, Netham Weir

By altering the timing of structure operations the Floating Harbour water levels can be lowered to increase flood storage at times of high fluvial flows and low tides. This relies on good, accurate timely forecasts and an effective protocol, carried out by well-trained operatives.

Improve/extend existing defences at eight identified low spots.

Option prevents overtopping at these points and protect vulnerable locations along the Floating Harbour at: 1) Junction lock, 2) Cumberland road adjacent to Underfall Yard, 3) Railway under Cumberland Road, 4) Weir to Bathhurst Basin, 5) Commercial Road adjacent to Bathhurst Basin, 6) Feeder Road/Cattle market Road junction by Temple Meads, 7) Feeder Road adjacent to St Vincents Trading Estate, Netham, 8) Netham Lock gates.

Increase flood storage, potentially on Frome at Tubbs Bottom or tributaries in /outside Council’s boundary

Potential storage in rural areas (large, open, low-lying, adjacent to river) by constructing a low height embankment dam and river control structure to attenuate flood flows upstream of Bristol. As a priority, further study should identify potential sites, which may need safeguarding from development, and assess benefits by modelling. Limited storage options for Avon.

Increase the capacity of Frome and North Storm Water Interceptor

Increased flow diverted along the NSWI results in less flow along the Frome and can therefore reduce flood flows. This relies on either low tide levels to allow the flow to discharge from the bottom end or sufficient capacity in the NSWI to contain backed-up flows

Increase capacity of Floating Harbour

Alternative operation of the Floating Harbour control structures to level its operating level in advance of flooding, and thereby maximise the storage available during the period of highest flows. Previous technical studies considered this option.

Raise Avon channel defences downstream of Netham weir to the Avon Gorge

Fluvial flood risks are high over this stretch of river. Raise/extend the formal flood defences to reduce the flood risk and mitigate climate change impacts (at least in part). Based on the tidal and fluvial flood risk maps (Appendix C), low-spots are evident along the river banks that can lead to overtopping and flooding of low-lying areas.

As an indication of the scale of new flood defences that would be required, FZ3a maps indicate flood depths generally up to 0.5m where it overtops along up to 3km of river bank. To protect against future flood risk (FZ3a with climate change), increases the scale of flood defences (to lesser extent than Area A).


Table 12.5 Structural approaches – Area B: St Pauls Baptist Mills & Eastville


Enlarge Frome control structures

Main control structure – lock gates at Eastville, which if enlarged would allow increase flood flow conveyance capacity via the NSWI and thereby reduce the flood risk to Eastville.

Improve structure operations, e.g. North Storm Water Interceptor

Increased flow diverted along the NSWI results in less flow along the Frome and can therefore reduce flood flows. This relies on either low tide levels to allow the flow to discharge from the bottom end or sufficient capacity in the NSWI to contain backed-up flows

Improve/extend existing defences, including new channel defences on the Frome

Raise/extend the formal flood defences to reduce the flood risk and mitigate climate change impacts (at least in part). Based on the tidal and fluvial flood risk maps (Appendix C), low-spots are evident at four locations along the river banks that can lead to overtopping and flooding of low-lying areas.

As an indication of the scale of new flood defences that would be required, FZ3a maps indicate flood depths generally up to 1m where it overtops along up to 1km of river bank. To protect against future flood risk (FZ3a with climate change), increases the scale of flood defences on both sides of the river (but not to the same extent as Area A).

Increase flood storage, potentially on Frome at Tubbs Bottom or on Ladden Brook

Potential storage in rural areas (large, open, low-lying, adjacent to river) by constructing a low height embankment dam and river control structure to attenuate flood flows upstream of Bristol. As a priority, further study should identify potential sites, which may need safeguarding from development, and assess benefits by modelling.

New Storm Water Interceptor culvert

Addition of a second NSWI to reduce fluvial flows into the Floating Harbour by transferring Frome flood flows into the downstream end of the Avon where, tide permitting, the flows discharge to the Severn.

Table 12.6 Structural approaches – Area C: St East Avon & St Philips Marsh


Enlarge Avon control structures

Principal control structures – Netham Weir and the controls associated with the Floating Harbour, and to enlarge these structures would remove potential constrictions to flood flows and reduce the (more immediate) upstream flood risks.

Improve structure operations, e.g. Floating Harbour, Nethan Weir

Alternative operation of the Floating Harbour control structures to level its operating level in advance of flooding, and thereby maximise the storage available during the period of highest flows.

Improve/extend existing defences from Netham downstream of Temple Meads,Totterdown

Raise/extend the formal flood defences to reduce the flood risk and mitigate climate change impacts (at least in part). Based on the tidal and fluvial flood risk maps (Appendix C), low-spots are evident along the river bank to the north that can lead to overtopping and flooding of low-lying areas.

As an indication of the scale of new flood defences that would be required, FZ3a maps indicate flood depths generally up to 1m, and as high as 1.5m, where it overtops along about 1.5km of river bank. To protect against future flood risk (FZ3a with climate change), increases considerably the scale of flood defences.

Table 12.7 Structural approaches – Area D: Ashton area


Improve/extend existing defences

Main flood risk from Colliters Brook, where properties to east (left bank) are currently protected by a natural flood bank up to an estimated 1 in 100 year flood standard (data from NFCDD). Improvements to this defence would lead to a higher current standard of protection and help mitigate against the predicted effects of climate change.

Raise/extend the formal flood defences to reduce the flood risk and mitigate


climate change impacts (at least in part). Based on the tidal and fluvial flood risk maps (Appendix C), only one low-spot is evident with flooding of a low-lying greenfield area.

Widen/deepen Ashton Brook

Potential to increase flood flow conveyance capacity, except this option may increase flooding problems downstream and for this reason is rejected.

Increase upstream flood storage

Potential storage in rural areas (large, open, low-lying, adjacent to river) by constructing a low height embankment dam and river control structure to attenuate flood flows upstream.

Table 12.8 Structural approaches – Area E: Avonmouth


Improve/extend existing tidal defences and as a minimum ‘hold the line’ (SMP policy)

Raise/extend the formal flood defences to reduce the flood risk and mitigate climate change impacts (at least in part). The existing defence system provides an estimated 1-2% standard of protection (Figure 6.1), which is just within the minimum target for a city area. The scope of this option is subject to separate appraisal.

12.6 Initial options appraisal

This appraisal provides an initial strategic overview of the identified options in terms

of flood risk reduction, engineering/technical feasibility, impacts on the social/natural

environment and deliverability. The multi-criteria appraisal set out in Table 12.9

displays the positives (��) and negatives (��), with an indicative cost range for major

scheme options in Table 12.10 and potential environmental impacts in Table 12.11.

For comparison with the costs quoted (Table 12.9), the potential benefits of flood

damage avoidance are considered based on flood damage estimates for Bristol

quoted in the Bristol Avon CFMP. This quotes an average annual flood damage of

£4.5 million for current (fluvial and tidal) flood risk to properties, increasing to £66

million with climate change (sea level rise +1m; fluvial flows +20%). These figures

exclude damages to infrastructure, emergency services, etc.

Using Treasury (Green Book) annual discount rates, the damage figures (uplifted by

30% for non-property damages) convert to a Present Value of £175 million, and £1.4

billion with climate change assuming higher annual damages (£66 million) only after

year 2025 in line with current Defra guidance.

The implication is that significant future investment to reduce the flood risk can be

justified, particularly for a climate change scenario. Even the more costly options for

Bristol, such as the Avon tidal barrage, may be justifiable longer term to counter

climate change effects.

As the flood risk issues are complex, only through detailed study can a cost effective

and sustainable strategy be formulated that sets out the priorities for flood risk

management and recommends the preferred options. Defra’s appraisal guidelines for

flood and coastal defences are relevant to this (FCDPAG series).


Table 12.9 Initial options appraisal

Ref. Area Flood risk Option Flood Risk reduction

Engineering/ technical feasibility

Impact on social environment

Impact on natural environment

Deliverability

A-E All flood risk areas in Bristol City

Tidal Severn

River Avon

River Frome

Floating Harbour

Ashton Brook

Non-structural approaches

- apply PPS25 flood risk hierarchy such as: substitute residential for less vulnerable development

- flood forecasting/warning, public awareness

- development control, e.g. avoid inappropriate development, etc.

- land management as a longer term measure

Structural approaches

- flood resilience (retrofit existing, design in future development)

- increase surface water attenuation and runoff management (incl. SUDS)

- reinstate floodplain areas in public open spaces

- Severn barrage

- Avon tidal barrier

- pumping at City Docks, Ashton Avenue and NSWI against the tide

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- widen/deepen Avon channel

- enlarge Avon control structures

- improve structure operations, e.g. Floating Harbour, Netham Weir

- improve/extend existing defences at 8 identified low spots

- increase flood storage, potentially on Frome at Tubbs Bottom or tributaries in /outside Bristol

- increase the capacity of Frome and North Storm Water Interceptor

- increase capacity of Floating Harbour

- raise Avon channel defences downstream of Netham weir to the Avon Gorge

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Avon (tidal)

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- enlarge Frome control structures

- improve structure operations, e.g. North Storm Water Interceptor

- improve/extend existing defences

- increase flood storage, potentially on Frome at Tubbs Bottom or on Ladden Brook

- new Storm Water Interceptor culvert

- new channel defences on the Frome

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- improve/extend existing defences from Netham downstream of Temple Meads,Totterdown

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Table 12.10 – Indicative cost of major scheme options

Option Scheme cost (capital) Basis for cost estimate

Severn Barrage more than £20 billion Published estimates

Avon Barrier £50-100 million at Clifton for channel 70m wide or higher near M5 as 150m wide

Ipswich tide gates scheme £50 million for 20 m wide channel

Upland storage £5-20 million Recent/planned schemes (Lower Lee - Cobbins Brook £6m, Salmons Brook £14m; Glasgow - White Cart £15m).

New NSWI £20-50 million, for 3km tunnel predominantly through clay

St Ives flood defences (2005), 450m unlined tunnel through hard rock (drill/blast), £3.2m

Increase Floating Harbour capacity

£2-20 million In line with asset works on Lower Lee

Pumping at City Docks, Ashton Avenue, NSWI against the tide

£5-50 million (excludes high operating cost if regularly employed for FRM)

St Germans Pumping Station, Norfolk (Fens); capacity 100m3/s; replacement station £40m Altmouth Pumping Station, River Alt, near Formby; capacity 80m3/s; major refurbishment £10m.

Enlarge control structures £5-25 million In line with asset works on Lower Lee

New/improved channel defences

Varies £2-£5 million depending on Area (A to D)

Recently built (Boscastle, St Ives) and under design (Newbury)

Severn tidal defences £32-280m for Area E Capita Symonds study

Table 12.11 – Potential environmental impacts of options

Option Potential environmental impact Apply PPS25 Development does not conflict with natural systems, therefore no impact Flood forecasting No interaction with natural environment Development control Positive impact as avoids development on environmentally rich or sensitive

areas Land management Positive impact as ensures farming and management practices are in

conjunction with pre-existing environmental processes as far as possible Flood resilience Neutral impact as does not impede natural processes Surface water management – SUDS

Positive impact – some systems can have water quality functionality + seeks to return systems to a closer approximation of natural state.

Reinstate floodplain areas

Positive impacts – seeks to return flooding mechanisms to closer approximation of natural state, impacts for habitat creation, sediment regimes and so on.

Severn Barrage Potentially important national role in meeting carbon emission targets, and European/global climate change goals. Would have potential for significant flood risk reduction to Bristol. DECC are currently undertaking a Strategic Environmental Appraisal (SEA) to assess environmental impact of barrage options, see: http://severntidalpowerconsultation.decc.gov.uk/

Avon Barrier Habitat loss due to reduced tidal range, sediment regimes disrupted, physical impediment to movement of fish and aquatic animals

Upland storage Potential positive impacts – opportunities for habitat creation, water quality improvements, fishery establishment. Some habitats e.g. hedgerows, grassland, may be lost

New NSWI Effects predominantly during construction Increase Floating Harbour capacity

Technical solution – no significant impacts except absence of negation of need for negative impacts resulting for other potential options

Pumping at City Docks, Ashton Avenue, NSWI against the tide

Effects predominantly relate to construction and energy consumption

Enlarge control structures

Positive impacts – adoption of sediment and creature movement regimes closer to undisturbed state

New/improved channel defences

Potential water quality considerations through prevention of flooding

Severn tidal defences Seek to maintain status quo in terms of flood risk. Some water quality considerations through prevention of flooding


12.7 Preferred options

The following provisional list of preferred options highlights those for strategic

assessment in the recommended further FRM studies (Chapter 14), based on

technical and economic feasibility, and analysis against environmental objectives.

All areas (A-E): take forward non-structural options as priority

• apply PPS25 flood risk hierarchy (Section 3.1)

• flood forecasting/warning, public awareness

• development control, e.g. avoid inappropriate development, etc.

• land management as a longer term measure

Areas A, B and C: Options that might form part of a strategic solution

• Promote through planning and development control:

- flood resilience (retrofit existing, design in future development)

- increase surface water attenuation and runoff management (incl. SUDS)

- reinstate floodplain areas in public open spaces

• Short term option, potentially low cost (requires additional measures):


• Medium to longer term options – major asset refurbishment or renewal

- increase capacity of Floating Harbour


• Capital scheme requires a robust business case:

- upstream storage, e.g. Tubbs Bottom (Frome)

- improve/extend flood defences

• Major capital scheme requires a robust business case:

- Avon tidal barrier

- new Northern Storm Water Interceptor

- pumping against the tide

Area D: In view of limited flood risks no area specific scheme is highlighted as even

with climate change (FZ3a maps) the flooding may not cause a problem to properties

– further study to confirm in the vicinity of Silbury Road. However, the Environment

Agency advise on the need for consideration of North Somerset’s Area of Search on

the Ashton Vale catchment as a whole.

Area E: Raise/extend existing tidal defences – option subject to separate study.

The planned SWMP will recommend options to reduce the surface water flood risks.


13 Advice for site specific flood risk assessments

13.1 Overview

This chapter provides advice for development control and potential developers for site

specific flood risk assessments. The following recommendations are in accordance

with PPS25 and the broad objectives of the Bristol Avon CFMP and Severn Estuary

SMP Policy Units for Bristol. To help understand requirements for flood risk

assessments, reference should be made to the flood risk information in Chapters 7-9.

This chapter provides guidance for development control, summarising the flood risk

information presented in the three preceding chapters, in Chapter 7 (tidal & fluvial),

Chapter 8 (surface water) and Chapter 9 (reservoir breach), and considers the FRA

issues associated with these flood risks for the five areas investigated.

13.2 FRA issues

This summary information is presented in Table 7.1 (next page), which considers the:

• envisaged regeneration

• flood risks in terms of flood depths, velocity and hazard (Section 7)

• other flood risks (Section 8 and 9)

• strategic and/or site specific solutions to reduce the risks (Section 12)

• developer contributions if appropriate (Section 11.4)

• need for floodplain storage compensation measures

• need for an emergency evacuation plan (Section 11.7)

13.3 Sequential testing using the SFRA

Future development within all areas will require application of the sequential approach

at the site level (sequential design) to ensure that the more vulnerable development

(e.g. residential housing) is located within an area of the site at least risk of flooding

(i.e. Flood Zone 1).

Areas at higher risk of flooding should ideally be set-aside as open space for amenity

and potential environmental enhancements to satisfy requirements of the Sequential

Test (see Table D.3. of PPS25 for other uses). For the more vulnerable use it is

necessary to ensure that the requirements of the Exception Test are satisfied. The

Exception Test will need to demonstrate that the development will provide wider

sustainability benefits and will not increase flood risk at the site or downstream.

The vulnerability from other sources of flooding should be considered as well as the

effect of the new development on surface water runoff.


Table 13.1 Flood risk assessment issues specific to each area

FRA issue Area A Area B Area C Area D Area E

Envisaged regeneration

mixed – residential, community workspace, leisure, office

residential, light commercial

predominantly residential residential expansion of employment development

Key flood risk issues

tidal & fluvial (extensive); surface water; impounded water bodies (contact Bristol Water)

tidal & fluvial (localised); surface water; impounded water bodies (contact Bristol Water)

tidal & fluvial (extensive); surface water; impounded water bodies (contact Bristol Water)

tidal & fluvial (localised in greenfield area); surface water (not affecting areas of potential development)

Tidal (extensive), potentially some fluvial, surface water (extensive)

PPS25 category (PPS25 Table D.2)

more vulnerable (residential), less vulnerable (other)

more vulnerable (residential), less vulnerable (commercial)

more vulnerable (residential)

more vulnerable (residential)

potentially less vulnerable (employment)

Typical flood depths for 1in 100 year, 1in 200, etc – tidal & fluvial flood risk

0.5-1.0m FZ3a

0.5-1.5m FZ3a with climate change

0.5-1.5m FZ3a


0.5-1.5m FZ3a


0-1.0m FZ3a

0-1.0m FZ3a with climate change

subject to separate study

Typical flood velocity – tidal & fluvial flood risk

0-0.25m/s FZ3a

0.25-1.0m/s FZ3a with climate change

0.25-1.0m/s FZ3a


0.25-1.5m/s FZ3a


0-0.25m/s FZ3a

0-0.25m/s FZ3a with climate change


Typical flood hazard category (refer to classification in Table 5.1) – in areas of tidal & fluvial flood risk, based on FZ3a with climate change

extensive area with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘extreme’ hazard (for all people).

localised areas with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘low’ to ‘moderate’ hazard.

extensive area with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘extreme’ hazard (for all people)

localised (greenfield) area with ‘significant’ hazard (danger for most – FZ with deep or fast flowing water presents a hazard for most people), including some ‘low’ to ‘moderate’ hazard.


Potential strategic solutions, excludes non structural measures as priority

- NSWI - upstream storage - enlarge Avon structures - improve operations - raise/extend defences

- NSWI - upstream storage - enlarge Avon structures - improve operations - raise/extend defences

- enlarge Avon structures - improve operations - raise/extend defences

implications of North Somerset’s Area of Search on the Ashton Vale catchment as a whole

- raise/extend defences

Alternative site specific solution if strategic unfeasible

- flood resilience - improve infrastructure for emergency access

- flood resilience - flood resilience - improve infrastructure for emergency access

as above - improve/extend defences - improve infrastructure for emergency access

Developer contribution

required possibly not required for as limited development

required not required expect required

Floodplain storage compensation

required in view of extensive tidal & fluvial flood risks (FZ3a with climate change)

as above required in view of extensive tidal & fluvial flood risks (FZ3a with climate change)

not required not required – tidal flooding

Emergency evacuation plan

required as above required not required subject to separate study


13.4 Using SFRA results to inform flood risk assessments for development at each

of the sites considered in this SFRA

This SFRA provides an assessment of flood risk at a level appropriate to inform

Bristol City Council’s planning decisions. Site specific flood risk assessments need to

be prepared for specific development sites by prospective developers. The following

reflects the minimum requirements under PPS25 for a Flood Risk Assessment

(reference should also be made to Tables D.1-D.3 in PPS25) on the basis of the

fluvial/tidal flood risk identified for the site for the lifetime of the proposed

development.

Sites in Flood Zone 1

Many of the sites being considered by Bristol City Council for future development, as

part of the 2008 Strategic Housing Land Availability Assessment, lie within Flood

Zone 1. This section details the requirements for development in Flood Zone 1. To

prevent a piecemeal approach, if large sites are split into units less than 1 Ha a Flood

Risk Assessment will still be required.

• In accordance with Table D3 of PPS25, any type of development can be located in

Low Probability Flood Zone 1.

• The vulnerability of the development from other sources of flooding should be

considered as well as the effect of the new development on surface water runoff.

• Floor levels should be situated above the 100 year (fluvial) and 200 year (tidal)

plus climate change predicted maximum level plus a minimum freeboard of

600mm.

• The proposed development should be set-back from the watercourse with a

minimum 8m wide undeveloped buffer zone, to allow appropriate access for

routine maintenance and emergency clearance.

• Where the site forms part of a dry island surrounded by ground which is now or will

be subject to classification as Flood Zone 3 consideration will need to be given to

how safe access will be achieved in accordance with FD2320.

• The potential to increase flood risk elsewhere through the addition of hard surfaces

and the effect of the new development on surface water runoff, with appropriate

mitigating action, should be incorporated in a Flood Risk Assessment (FRA) for the

site.

This should take the form of a Drainage Impact Assessment (DIA), required to

demonstrate that runoff from the site is the same as in the predevelopment case,

thereby ensuring flood risk is not increased (though wherever possible, betterment

should be achieved). This will involve the use of SUDS techniques which should

take into account the local geological and groundwater conditions. Where possible

these should be strategic SUDS. Space should also be set-aside for SUDS at the

master planning stage.


Sites in Flood Zone 2

All seven areas assessed by this study are intersected by Flood Zone 2 to a greater

or lesser extent. Where possible, as part of the Sequential Test process, alternative

sites in Flood Zone 1 should be considered in preference to those in Flood Zone 2.

This section details the requirements for development in Flood Zone 2.

• In accordance with Table D3 of PPS25, land use within Medium Probability Flood

Zone 2 should be restricted to the ‘essential infrastructure’, ‘water compatible’,

‘less vulnerable’ and ‘more vulnerable’ categories. Only if the Sequential Test

process has been carried out and passed can development occur in Flood Zone 2.

• ‘Highly vulnerable’ uses in Flood Zone 2 will have to pass the Exception Test.

• A FRA will be required, which should confirm flood extents and levels within the

site.

• The development should not increase flood risk elsewhere, and opportunity should

be taken to decrease overall flood risk.



600mm.

• Dry pedestrian access to and from the development should be possible above the

1 in 100 year (fluvial) and 200 year (tidal) plus climate change flood level.

• The development should be safe, meaning that: people (including those with

restricted mobility) should be able to remain safe inside the new development up to

a 1 in 1000 year event; and rescue and evacuation of people from a development

(including those with restricted mobility) to a place of safety is practicable up to a 1

in 1000 year event.

• If the land use of the development proposed is ‘highly vulnerable’, consideration

should be given to the incorporation of flood resistance and resilience measures



routine maintenance and emergency clearance.

• SUDS should be implemented to ensure that runoff from the site (post

development) is reduced. Space should be set-aside for SUDS at the master

planning stage.



• Residents should be made aware that they live in a flood risk area. The

Environment Agency plan to make their flood warning service ‘opt-out’ instead of

‘opt-in’, but until such time residents should be encouraged to sign up to Floodline

Warnings Direct, should a Flood Warning system exist (as indicated by the Level 1

SFRA).

• Car parking needs to be safe, especially in terms of flood warning and overnight

parking areas.


Sites in Flood Zone 3a

Flood Zone 3a encroaches on all seven areas assessed by the Level 2 SFRA and in

some cases (e.g. Area 3) the areas affected are large. Wherever possible,

development in Flood Zone 3a should be avoided, unless it can be clearly

demonstrated that the overall level of flood risk in an area will be reduced to an

acceptable level as a result of the development. This section details the requirements

for development in Flood Zone 3a.

• Only if the Sequential Test process has been carried out and passed can

development occur in Flood Zone 3a

• Land use with High Probability Flood Zone 3a should be restricted to the ‘less

vulnerable’ and ‘water compatible’ uses.

• ‘Essential Infrastructure’ and ‘More vulnerable’ uses in Flood Zone 3a will have to

pass the Exception Test.

• An FRA should be prepared for the site, which should confirm flood extents and

levels.

• Properties situated within close proximity to formal defences or water retaining

structures (reservoirs) will require a detailed breach and overtopping assessment

to ensure that the potential risk to life can be safely managed throughout the

lifetime of the development. The nature of any breach failure analysis should be

agreed with the Environment Agency.

• The development should not increase flood risk elsewhere, and opportunities

should be taken to decrease overall flood risk.

• If any part of the development falls within the floodplain, assess any compensatory

flood storage requirements as part of the FRA.



600mm.

• Dry pedestrian access to and from the development should be possible above the

1 in 100 year (fluvial) and 200 year (tidal) plus climate change flood level.

• The development should be safe, meaning that: people (including those with

restricted mobility) should be able to remain safe inside the new development up to

a 1 in 1000 year event; and rescue and evacuation of people from a development

(including those with restricted mobility) to a place of safety is practicable up to a 1

in 1000 year event.

• If the land use of the development proposed is ‘more vulnerable’ or ‘essential

infrastructure’, consideration should be given to the incorporation of flood

resistance and resilience measures.

• Basements should not be used for habitable purposes. Where basements are

permitted for commercial use, it is necessary to ensure that the basement access

points and any venting are situated 600 mm above the 1 in 100 year (fluvial) and 1

in 200 year (tidal) flood level plus climate change for the life of the development.

Near the coast an allowance for wave action should also be considered.


• An evacuation plan should be prepared in consultation with Bristol City Council’s

Emergency Planning team.

• Residents should be made aware that they live in a flood risk area. The

Environment Agency plan to make their flood warning service ‘opt-out’ instead of

‘opt-in’, but until such time residents should be encouraged to sign up to Floodline

Warnings Direct, should a Flood Warning system exist (as indicated by the Level 1

SFRA).



routine maintenance and emergency clearance, if appropriate.

• SUDS should be implemented to ensure that runoff from the site (post

development) is reduced. Space should be set-aside for SUDS at the master

planning stage.



Sites in Flood Zone 3b

Flood Zone 3b is identified in all seven areas assessed by the study. This section

should be used to understand the requirements of development in this high probability

Flood Zone.

• Development in High Probability Flood Zone 3b should be restricted to ‘water-

compatible uses’ only.

• PPS25 dictates that ‘essential infrastructure’ can be located in Flood Zone 3b if the

Exception test is passed. However, appropriate judgement should be exercised

when attempting the Exception Test for essential infrastructure in Flood Zone 3b.

Essential infrastructure includes: essential transport infrastructure (including mass

evacuation routes) which has to cross the area at risk; and strategic utility

infrastructure, including electricity generating power stations and grid and primary

substations.

• Essential transport infrastructure may be appropriate if designed in such a way that

flood flow routes and flood storage areas are not affected (e.g. designing a bridge

to cross the flood risk area). However, utility infrastructure may be less

appropriate due to the potential consequences that may occur should the utility site

become flooded (as demonstrated by the flooding of Mythe Treatment Works and

near-flooding of the power station in Gloucestershire during the summer 2007

flood events).

• ‘Essential infrastructure’ in this zone must be designed and constructed to

remain operational in times of flood and not impede water flow.

13.5 Site specific risk, location of uses, access points, levels, safe design

Site specific risks and recommendations are considered in the policy matrices in

Chapter 10, and generic recommendations regarding the use of SUDS, appropriate

flood avoidance, site layout, and resistance/resilience measures are detailed in

Chapter 11. The requirements for access points, levels and safe design are

dependent on flood risks at the site as detailed above.


14 Recommended further FRM studies

This chapter recommends further FRM studies due to known flooding problems and

the potential for severely damaging consequences arising from climate change. At

present, the flood depths, velocities and hazard ratings identified for parts of the

areas (A-E) investigated suggest offsite FRM solutions will be required to make

development ‘safe’ for its lifetime. Further studies are therefore recommended to

determine how this will be achieved for the growth areas in accordance with PPS25.

Table 14.1 outlines the current/planned FRM studies, with details of three key studies

given below (next page): a sequential testing study for the development areas, a joint

tidal/fluvial probability study to provide a more reliable estimate of flood risks and a

strategic planning study to determine an optimal solution to manage flood risk in

Bristol – referred to as the FRM delivery strategy in Chapter 11.

Table 14.1 – Current / planned studies in flood risk

Study Name Details Programmed Dates

Joint tidal/fluvial flood risk probability study*

Technical study to provide a more reliable estimate of the current and climate change flood risks, including updates to the SFRA Flood Zone maps.

2010 (provisional, subject to funding)

Bristol FRM delivery strategy* - Bristol central area flood risk assessment

Strategic study of low lying land within Bristol city centre to understand the affects of the combinations fluvial and tidal flooding and propose strategic solutions (Chapter 12).

2010/11 (provisional, subject to funding)

South Bristol (Dundry Hill) flood risk assessment

Study to understand the surface water runoff form the Dundry Hill and extent of properties at risk and propose solutions

April 2010 start to March 2011 (study completion)

Bristol surface water management plan (SWMP)

Surface water study of the Bristol city catchment with a combination of overland flows/drainage and sewers/ river to highlight the areas at risk of surface water flooding and propose strategic and localised solutions for the high risk areas.

Started to September 2010 (study completion)

Strategic Flood Zone mapping improvements

Halcrow prepared flood risk mapping of the Avon through Bristol using 2D modelling (2007) and are currently involved in mapping improvements for the Malago Stream and Brislington Brook.

Malago Stream and Brislington Brook work to complete early 2010

Flood forecasting improvements

Revision of Bristol Avon Flood Forecasting Model by Edenvale Young Associates for the Environment Agency.

Halcrow previously delivered 3 separate flood forecasting studies for Bristol – Bristol Avon Flood Forecasting Model (2007), Bristol Frome Model Improvements Study (2008) and the Bristol Floating Harbour Triggers study (2008).

These studies are all aimed at improving the accuracy and lead time of flood forecasts and warnings and, therefore, reduce flood risk by allowing those at risk the time to take action to reduce flood losses.

Current Bristol Avon Model improvement study due for completion in 2010

*refer to details below


Sequential testing study

A Sequential testing study is recommended as a next step for the SFRA to confirm

that the development areas (A to E) for Bristol are appropriate, based on flood risk

considerations. The study can also consider further the suitable types of development

types within each development area. If no suitable sites are available for allocation

following the Sequential testing, Exception testing will be necessary.

Joint tidal/fluvial flood risk probability study

An understanding of the relative rarity of flooding from various sources is fundamental

to developing strategic solutions to manage flood risk to Bristol. This recommended

study will:

• Assess the joint probability of flooding to Bristol from the tide, storm surge,

fluvial flows in the Avon and Frome, which meets one of the priority actions of

the Bristol Avon CFMP.

• Assess the critical flood levels in the city, as the next stage in the recent study

of the Floating Harbour triggers flood forecasting study (Halcrow for

Environment Agency, 2008) and to determine the various scenarios (tide, storm

surge, Avon fluvial, Frome fluvial, Floating Harbour starting level) that result in

flood level thresholds being reached and the rarity of each scenario.

• Determine the most frequent combination of the variables that gives rise to flood

thresholds being exceeded, and provide a more reliable estimate of the current

and climate change flood risks.

Bristol FRM delivery strategy study

In view of the tidal and fluvial flood risks identified, there is a clear need for a strategic

long term plan to manage this flood risk in a cost effective and sustainable way and

set investment priorities into the future (for FRM the planning horizon is typically set at

100 years). This recommended study will:

• Follow Defra’s project appraisal guidance (FCDPAG2) for a formal strategy

• Determine future investment priorities, recognising that strategic planning to

reduce flood risk calls for ‘joined-up’ action on every front from the full spectrum

of partners, from policy makers to vulnerable communities.

• Deliver a strategic plan of flood risk management measures supported by a

robust business case and with advice sought from local authorities,

environmental bodies, statutory and non-statutory organisations, and specialists

in planning, archaeology and the environment.

• Develop an innovative, sustainable and cost effective strategy, recommending

short, medium and long term solutions based on detailed modelling and

appraisal of the economic, environmental and planning issues.

• Include well targeted consultation, which is critical in establishing long-term

relationships between partners that will endure as the strategy is rolled out in

future years.

• Integrate with the ongoing Severn Estuary FRMS and planned Bristol SWMP.


15 Concluding remarks

This Level 2 SFRA follows PPS25 and its associated Practice Guidance (June 2008),

best practice and the guidance provided at all stages by the Environment Agency and

Bristol City Council planners. This Level 2 SFRA is required to assess the flood risk in

greater detail than the Level 1 SFRA, because it may be necessary to allocate

development or consider windfall development in areas of higher flood risk.

This Level 2 SFRA together with the Level 1 SFRA, provide the necessary

information with which to apply the Sequential Test and Exception Test to

development proposals for the Bristol City area (PPS25, Annex D) and inform what

could potentially be included in a Supplementary Planning Document on flood risk.

The Levels 1 and 2 SFRA together form part of the evidence base for the Local

Development Framework (LDF) and are intended to inform decisions regarding land

allocation and policies. The SFRA will also be considered an integral part of the

Sustainability Appraisal of relevant component documents of the LDF.

This SFRA investigates five geographical areas under consideration for development,

and its output includes Flood Zone, depth and velocity maps that represent for the

current flood risk and with climate change, and hazard mapping for SFRA Flood

Zones 2 and 3a.

The flood risk within the development areas (A to E) largely arises from tidal/fluvial

(Chapter 7) and surface water flooding (Chapter 8), with a further risk of reservoir

breach for some areas (A, C and D – Chapter 9). There is very limited flooding

indicated for Flood Zone FZ3b, and extensive flooding in many city areas for Flood

Zone 3a with climate change.

Flood risk assessment issues specific to each area are considered (see summary in

Table 13.1). The SFRA Flood Zone maps show that many of the allocated

development areas and areas of search for development lie within Flood Zone 3. The

Sequential Test should be applied to direct any development away from these higher

flood risk areas, but where this is not possible the Exception Test must be passed.

The best information is to be used to guide the site selection process for future

developments. For this reason, this SFRA is a living document (reports and maps) to

be updated as new information becomes available, e.g. further improvements to river

models, surface water flooding incidents or revised climate change guidance.

The recommended further FRM studies include a sequential testing study for the

development areas, a joint probability study to assess the likelihood of flooding in

Bristol and a strategic planning study to determine an optimal strategic solution to

manage flood risk in Bristol.


Glossary

Breach Hazard/Analysis

Hazard attributed to flooding caused by the constructional failure of a flood defences or other structure that is acting as a flood defence.

CIRIA The construction industry research and information association: and seeks to “address industry issues, challenges and opportunities and all with the aim of providing business and delivery improvement [as well as] working collaboratively across traditional sector boundaries provides opportunities to identify best practice, develop new approaches and to identify and enable innovation” http://www.ciria.org/service/AM/ContentManagerNet/HTMLDisplay.aspx?ContentID=14550&Section=about

CFMP Catchment Flood Management Plan. A CFMP is a high-level strategic plan

through which the Environment Agency seeks to work with other key-decision

makers within a river catchment to identify and agree long-term policies for

sustainable flood risk management.

Core Strategy The Development Plan Document which sets the long-term vision and objectives

for the area. It contains a set of strategic policies that are required to deliver the

vision including the broad approach to development.

Culvert A closed conduit used for the conveyance of surface drainage water under a roadway, railroad, canal, or other impediment

Defra Department of Environment, Food and Rural Affairs Development

DPD Development Plan Document. A DPD is a spatial planning document within the

Council’s Local Development Framework which set out policies for development

and the use of land. Together with the Regional Spatial Strategy they form the

development plan for the area. They are subject to independent examination.

Dry pedestrian egress

Routes to and from buildings that will remain dry and allow pedestrian/wheelchair

evacuation to dry land in times of flood.

DTM Digital Terrain Model.

Environment Agency

The leading public body for protecting and improving the environment in England

and Wales.

Exception Test If, following application of the Sequential Test, it is not possible (consistent with

wider sustainability objectives) to demonstrate that there are no reasonably

available sites in areas with less risk of flooding that would be appropriate to the

type of development or land use proposed, the Exception Test may apply. PPS25

sets out strict requirements for the application of the Test.

Flood Defence Natural or man-made infrastructure used to reduce the risk of flooding

Flood Risk Flood risk is a combination of two components: the chance (or probability) of a

particular flood event and the impact (or consequence) that the event would cause

if it occurred

FRA Flood Risk Assessment. Assessment of flood risk posed to a defined area (usually a new development site) as defined above.

Flood Risk Management

Flood risk management can reduce the probability of occurrence through the management of land, river systems and flood defences and reduce the impact through influencing development on flood risk areas, flood warning and emergency response.


Flood Risk Vulnerability

PPS25 provides a vulnerability classification to assess which uses of land maybe

appropriate in each flood risk zone.

Flood Warning A system maintained by the Environment Agency to enable warning messages to be sent to homeowners and businesses when floods are predicted.

Flood Defence A structure built and maintained specifically for flood defence purposes.

Flood Zones Nationally consistent delineation of ‘high’ and ‘medium’ flood risk, published on a quarterly basis by the Environment Agency.

Functional Floodplain Zone 3b

Defined as areas at risk of flooding in the 5% AEP (1 in 20 year) design event. In any one year the chance of a 5% AEP (1 in 20 year) event occurring is 5%. In areas where the 4% (but not 5%) AEP event has been modelled previously; this was taken to represent the functional floodplain as agreed between Bristol City Council and the Environment Agency.

GIS Geographic Information System. GIS is any system which stores geographical data, such as elevations, location of buildings and extent of flood outlines.

High probability Zone 3a

Defined as areas at risk of flooding in the 1% Annual Exceedance Probability (AEP) (1 in 100 year) design event for fluvial or 0.5% AEP (1 in 200 year) for tidal. In any one year the chance of a 1% AEP (1in 100 year )event occurring is 1% and for a 0.5% AEP (1 in 200 year) event occurring is 0.5%.

Informal Flood Defence

A structure that provides a flood defence function however has not been built and/or maintained for this purpose (e.g. boundary wall).

LDD Local Development Documents

LiDAR Light Detection and Ranging. LiDAR is an airborne terrain mapping technique which uses a laser to measure the distance between the aircraft and the ground.

LDF Local Development Framework. The LDF consists of a number of documents which together form the spatial strategy for development and the use of land.

Low Probability Zone 1

The area outside Zone 2. Defined as an area with less that 0.1% AEP (1 in 1000 year) chance of flooding. In any one year the chance of a 0.1% AEP (1 in 1000 year) event occurring is less than 0.1%.

LPA Local Planning Authority

‘Making Space for Water’ (Defra 2004)

The Government’s new evolving strategy to manage the risks from flooding and coastal erosion by employing an integrated portfolio of approaches, so as: a) to reduce the threat to people and their property; b) to deliver the greatest environmental, social and economic benefit, consistent with the Government's sustainable development principles, c) to secure efficient and reliable funding mechanisms that deliver the levels of investment required.

Medium probability Zone 2

Defined as an area at risk of flooding from flood events that are greater than the 0.1% AEP (1 in 1000 year), and less than the 1% AEP (1 in 100 year) fluvial or 0.5% AEP (1 in 200 year) tidal design event. The probability of flooding occurring in this area in any one year is between 1% (fluvial)/0.5% tidal) and 0.1%.

mAOD Metres Above Ordnance Datum

PPS Planning Policy Statements. The Government has updated its planning advice contained within Planning Policy Guidance Notes with the publication of new style Planning Policy Statements.

PPS25 Planning Policy Statement 25: Development and Flood Risk. PPS 25 reflects the general direction set out in ‘Making Space for Water’.

PPS25 Practice Guide

The Practice Guide explains how to implement PPS25’s commitment to deliver appropriate sustainable development in the right places while taking full account of flood risk.


Previously Developed (Brownfield) Land

Land which is or was occupied by a building (excluding those used for agriculture and forestry). It also includes land within the curtilage of the building, for example a house and its garden would be considered to be previously developed land.

Residual Risk The risk which remains after all risk avoidance, reduction and mitigation measures have been implemented.

Return Period The probability of a flood of a given magnitude occurring within any one year e.g.

a 1% AEP (1 in 100 year) event has a probability of occurring once in 100 years,

or a 1% chance in any one year. However, a 1% AEP (1 in 100 year) event could

occur twice or more within 100 years, or not at all.

RSS Regional Spatial Strategy. The RSS for Christchurch is the South West RSS, a regional planning policy providing the overarching framework for the preparation of LDFs. It provides a broad development strategy for the South West region up to 2026.

SA Sustainability Appraisal. An SA is an appraisal of plans, strategies and proposals to test them against broad sustainability objectives.

Sequential Test Informed by a SFRA, a planning authority applies the Sequential Test to demonstrate that there are no reasonably available sites in areas with less risk of flooding that would be appropriate to the type of development or land use proposed.

SFRA Strategic Flood Risk Assessment. An SFRA is used as a tool by a planning authority to assess flood risk for spatial planning, producing development briefs, setting constraints, informing sustainability appraisals and identifying locations of emergency planning measures and requirements for flood risk assessments.

SHLAA Strategic Housing Land Availability Assessment

SPD Supplementary Planning Document. An SPD provides supplementary guidance to policies and proposals contained within Development Plan Documents. They do not form part of the development plan, nor are they subject to independent examination.

SoP Standard of Protection. The return period against which a defence offers protection.

SWMP Surface Water Management Plan (described in detail in report).

SUDS Sustainable Urban Drainage Systems. SUDS are drainage systems which are designed to reduce the impact of urbanisation on the hydrology of a river system.

Sustainable Development

“Development that meets the needs of the present without compromising the ability of future generations to meet their own needs” (The World Commission on Environment and Development, 1987).

TUFLOW A 2D hydraulic modelling package.

UK Flood Hazard

A measure of hazard of a given flood event, calculated by using the following equation from Defra’s Flood Risks to People – Phase Two Document (FD2321/ TR2) (2006). Hazard is calculated as follows:

Hazard = d x (v + 0.5) + DF

where: d = depth (m); V = velocity (m/s); DF = debris factor


References

Bristol City Council, South Gloucestershire Council, Lower Severn Drainage Board (2007);

Avonmouth and Severnside Flood Risk Assessment – Summary Report

Bristol City Council (1997); Bristol Local Plan

Bristol City Council (2008); Bristol Development Framework – Core Strategy

CIRIA (1998) Remedial treatment for contaminated land (SP164)

CIRIA (2007) The SUDS manual (C697)

CLG (2006) Planning Policy Statement 3: Housing

CLG (2006) Planning Policy Statement 25: Development and Flood Risk

CLG (2008) Planning Policy Statement 25: Development and Flood Risk Practice Guide

CLG and Environment Agency, 2008. Guidance on the permeable surfacing of front gardens

(http://www.communities.gov.uk/documents/planningandbuilding/pdf/pavingfrontgardens.pdf)

Environment Agency/Defra (2005) The Flood Risk to People Phase 2, The Flood Risk to

People Methodology, R&D Technical Report FD2321/TR1

Environment Agency/Defra (2006) Flood Risks to People Phase Two, R&D Technical Report

FD2321/TR2

Defra (2009) Surface Water Management Plan Technical guidance, Living draft version 1

http://www.defra.gov.uk/environ/fcd/policy/swmp-guide.pdf

Defra/Environment Agency (2005a) Joint Probability: Dependence between extreme sea

surge, river flow and precipitation. R&D Technical Report FD2308/TR

Environment Agency (1999a) National Sea & River Defence Surveys condition assessment

manual A Guide to the Visual Condition Assessment of Sea and River Defences

Environment Agency (2000) Severn Estuary SMP1

Posford Haskoning (2003) Report on extreme tide levels (South West Region)

Environment Agency (draft - 2008); Bristol Avon Catchment Flood Management Plan

Environment Agency (2009) Demonstrating the flood risk (PPS25) Sequential Test for

Planning Applications, PPS25 FRSA (national) version 2.0 Advise issued on 27 January

2009

Scottish Executive (2004) Planning Advice Note PAN 69, Planning and Building Standards

Advice on Flooding

South West Regional Assembly (2006) The draft Regional Spatial Strategy for the South

West 2006 - 2026

Surendran, S., Gibbs, G., Wade, S., and Udale-Clarke, H. (2008) Supplementary note on

flood hazard ratings and thresholds for development and planning control purpose –

Clarification of Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1.

The Pitt Review (2007) Learning Lessons from the 2007 floods, Cabinet Office

The Pitt Review (2008) Learning Lessons from the 2007 floods, more information at

http://archive.cabinetoffice.gov.uk/pittreview/thepittreview/final_report.html