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
Bristol City Council
Strategic Flood Risk Assessment
Level 2 SFRA – Main Report
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
Bristol City Council
Strategic Flood Risk Assessment
Level 2 SFRA – Main Report
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
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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.
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Figure 1.1 Identified areas with flood risks where there are development pressures (source: Bristol City Council)
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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
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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.
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• 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).
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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
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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.
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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.
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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
stl
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
lve
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
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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.
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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
Name of development Source Proposed use
Silverthorne Lane Predominantly residential
Bath Road diesel depot Predominantly residential
Eastern end of Temple Quay House
Emerging Core Strategy
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.
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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
Name of development Source Proposed use
Alderman Moores, Ashton Vale Residential
Land off Silbury Road/ Ashton Fields Leisure/employment
Former Engineers Depot, Clanage Rd, Bower Ashton
Emerging Core Strategy
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.
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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.
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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.
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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
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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
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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.
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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.
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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.
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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)
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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.
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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).
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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
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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
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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.
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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.
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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.
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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
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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
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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
Temple Meads Station
Masonry flood wall
2 n/a >20 Routine
Temple Meads Station
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
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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.
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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)
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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
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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.
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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.
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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.
7.3.2 Flood Zone 3b – Functional Floodplain
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.
Sustaining & Improving the Quality of People’s Lives Page 37
Figure 7.1: Flooding around the Eastville culvert intake in the July 1968 flood event
7.3.4 Flood Zone 3a with climate change
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.
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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.
7.4.2 Flood Zone 3b – Functional Floodplain
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.
Sustaining & Improving the Quality of People’s Lives Page 39
7.4.4 Flood Zone 3a with climate change
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.
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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.
7.5.2 Flood Zone 3b – Functional Floodplain
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.
7.5.4 Flood Zone 3a with climate change
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.
Sustaining & Improving the Quality of People’s Lives Page 41
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.
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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 depth: 0.5-1.5m
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 depth: 0.5-1.5m
Flood velocity: 0.25-1.5m/s
Flood hazard: mainly ‘significant’, some parts ‘low’ to ‘moderate’.
Flood extents for FZ3a lie between FZ3b and FZ3a with climate change.
Flood depth: 0-1.0m
Flood velocity: 0-0.25m/s
Flood hazard: ‘low’ to ‘moderate’
Flood extents for FZ3a lie between FZ3b and FZ3a with climate change.
Pending results from Avonmouth study
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 depth: 0.5-1.5m
Flood velocity: 0.25-1.5m/s
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 depth: 0.5-2.0m
Flood velocity: 0.25-1.5m/s
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 velocity: 0-0.25m/s
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.
Pending results from Avonmouth study
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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
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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.
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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
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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.
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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.
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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.
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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.
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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
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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 .
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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.
Development should not be permitted
Development should be avoided
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.
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Table 10.3 Flood risk vulnerability & flood zone compatibility – Area B: St Pauls, Baptist Mills & Eastville
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 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.
No restrictions on development other than managing surface water runoff
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.
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 consent.
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 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.
Development should be avoided
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.
Development should not be permitted
Actual Risk (3a with existing defences)
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.
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.
Development should not be permitted
Development should be avoided
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
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
Sustaining & Improving the Quality of People’s Lives Page 54
Table 10.4 Flood risk vulnerability & flood zone compatibility – Area C: East Avon & St Phillips Marsh
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 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.
No restrictions on development other than managing surface water runoff
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.
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 consent.
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 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.
Development should be avoided
Flood Zone 3b Some flooding on the periphery of St Phillips Marsh. Small areas of significant hazard predominantly due to the depth component.
Development should not be permitted
Actual Risk (3a with existing defences)
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.
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.
Development should not be permitted
Development should be avoided
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
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.
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 1 recorded property at risk – at the intersection between the A4320 and the Lower Avon
Sustaining & Improving the Quality of People’s Lives Page 55
Table 10.5 Flood risk vulnerability & flood zone compatibility – Area D: Ashton
Essential Infrastructure
Water Compatible Development
Highly Vulnerable More Vulnerable Less Vulnerable Source of Flooding Description of flood risk
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
No restrictions on development other than managing surface water runoff
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.
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 consent.
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.
Development should be avoided
Flood Zone 3b Flooding of small area of land adjacent to BCC boundary, SE of Park & Ride. Low and moderate hazard.
Development should not be permitted
Actual Risk (3a with existing defences)
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.
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.
Development should not be permitted
Development should be avoided
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
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
Surface Water Flooding is risk factor in this area (two recorded incidents – A3029 and Ashton Drive, and one close by – in Bower Ashton)
Groundwater Flooding
No recorded incidents in this area
Sewer Flooding No recorded incidents in this area
Sustaining & Improving the Quality of People’s Lives Page 56
Table 10.6 Flood risk vulnerability & flood zone compatibility – Area E: Avonmouth
Essential Infrastructure
Water Compatible Development
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.
No restrictions on development other than managing surface water runoff
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.
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 consent.
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 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.
Development should be avoided
Flood Zone 3b No Functional Floodplain assessment was carried out. Development should not be permitted
Actual Risk (3a with existing defences)
Actual risk for current scenario not displayed. For the future scenario, Flood Zone 3a covers the majority, possibly 80% of the Avonmouth Area.
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.
Development should not be permitted
Development should be avoided
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
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
Groundwater 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
Sustaining & Improving the Quality of People’s Lives Page 57
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.
Sustaining & Improving the Quality of People’s Lives Page 58
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.
Sustaining & Improving the Quality of People’s Lives Page 59
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.
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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.
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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)
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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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).
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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
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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)
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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).
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Table 12.3 Structural approaches applicable to all flood risk areas (A to E)
Option Outline description
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.
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Table 12.4 Structural approaches – Area A: City Centre & Floating Harbour
Option Outline description
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).
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Table 12.5 Structural approaches – Area B: St Pauls Baptist Mills & Eastville
Option Outline description
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
Option Outline description
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
Option Outline description
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
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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
Option Outline description
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).
Sustaining & Improving the Quality of People’s Lives Page 79
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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- 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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- improve/extend existing defences
- increase flood storage, potentially on Frome at Tubbs Bottom or on Ladden Brook
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- new channel defences on the Frome
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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
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Sustaining & Improving the Quality of People’s Lives Page 80
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
Sustaining & Improving the Quality of People’s Lives Page 81
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):
- improve structure operations, e.g. Floating Harbour, Netham Weir
• Medium to longer term options – major asset refurbishment or renewal
- increase capacity of Floating Harbour
- enlarge Avon control structures
• 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.
Sustaining & Improving the Quality of People’s Lives Page 82
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.
Sustaining & Improving the Quality of People’s Lives Page 83
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 with climate change
0.5-1.5m FZ3a
0.5-2.0m FZ3a with climate change
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 with climate change
0.25-1.5m/s FZ3a
0.25-1.5m/s FZ3a with climate change
0-0.25m/s FZ3a
0-0.25m/s FZ3a with climate change
subject to separate study
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.
subject to separate study
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
Sustaining & Improving the Quality of People’s Lives Page 84
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.
Sustaining & Improving the Quality of People’s Lives Page 85
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.
• 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.
• 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
• 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.
• 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.
• 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.
• 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.
Sustaining & Improving the Quality of People’s Lives Page 86
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.
• 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.
• 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.
Sustaining & Improving the Quality of People’s Lives Page 87
• 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).
• 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, 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.
• 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.
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.
Sustaining & Improving the Quality of People’s Lives Page 88
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
Sustaining & Improving the Quality of People’s Lives Page 89
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.
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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.
Sustaining & Improving the Quality of People’s Lives Page 91
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.
Sustaining & Improving the Quality of People’s Lives Page 92
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.
Sustaining & Improving the Quality of People’s Lives Page 93
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
Sustaining & Improving the Quality of People’s Lives Page 94
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