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managingflood risk

Summary Report June 2012

Bristol Avon Catchment Flood Management Plan

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June 2012

Environment Agency Bristol Avon Catchment Flood Management Plan 1

Introduction

I am pleased to introduce our summary of the Bristol Avon

Catchment Flood Management Plan (CFMP). This CFMP gives an

overview of the flood risk in the Bristol Avon catchment and sets

out our preferred plan for sustainable flood risk management over

the next 50 to 100 years.

The Bristol Avon CFMP is one of 77 CFMPs for Englandand Wales. Through the CFMPs, we have assessedinland flood risk across all of England and Wales for thefirst time. The CFMP considers all types of inlandflooding, from rivers, ground water, surface water andtidal flooding, but not flooding directly from the sea(coastal flooding), which is covered by ShorelineManagement Plans (SMPs). Our coverage of surfaceand ground water is however limited due to a lack ofavailable information.

The role of CFMPs is to establish flood risk managementpolicies which will deliver sustainable flood riskmanagement for the long term. This is essential if weare to make the right investment decisions for thefuture and to help prepare ourselves effectively for theimpact of climate change. We will use CFMPs to help ustarget our limited resources where the risks aregreatest.

This CFMP identifies flood risk management policies toassist all key decision makers in the catchment. It wasproduced through a wide consultation and appraisalprocess, however it is only the first step towards anintegrated approach to Flood Risk Management. As weall work together to achieve our objectives, we mustmonitor and listen to each others progress, discusswhat has been achieved and consider where we mayneed to review parts of the CFMP.

The Bristol Avon catchment has a history of flood risk,and over the last 60 years numerous engineeringschemes have been implemented to reduce flood riskin the catchment. At present 7,000 properties are at riskin the catchment in a 1% event. This is likely to increaseto over 20,000 properties in the future.

We cannot reduce flood risk on our own, we willtherefore work closely with all our partners to improvethe co-ordination of flood risk activities and agree themost effective way to manage flood risk in the future.We have worked with others including: Bristol CityCouncil, Natural England, Wessex Water and theNational Farmers Union to develop this plan.

This is a summary of the main CFMP document, if youneed to see the full document an electronic version canbe obtained by emailing [email protected] or alternatively paper copies can be viewed at any ofour offices in South West Region.

Richard CresswellSouth West Regional Director

2 Environment Agency Bristol Avon Catchment Flood Management Plan

The purpose of a CFMP in managing flood risk 3

Catchment overview 4

Current and future flood risk 6

Future direction for flood risk management 10

Sub-areas

1 Bristol sub-area 12

2 Bath sub-area 13

3 Upper Avon sub-area 14

4 Lower Avon sub-area 16

5 Upper Bristol Frome sub-area 17

6 Mendip slopes and Long Ashton sub-area 18

7 Wootton Bassett and Dauntsey sub-area 19

8 Wiltshire Towns sub-area 20

9 Bradford-on-Avon and Frome sub-area 21

10 Markham Brook and Avonmouth sub-area 22

Map of CFMP policies 23

Contents


The purpose of a CFMP in managing flood riskCFMPs help us to understand thescale and extent of flooding now andin the future, and set policies formanaging flood risk within thecatchment. CFMPs should be used toinform planning and decisionmaking by key stakeholders such as:

• the Environment Agency, who willuse the plan to guide decisionson investment in further plans,projects or actions;

• Regional Assemblies and localauthorities who can use the planto inform spatial planningactivities and emergencyplanning;

• Internal Drainage Boards (IDB),water companies and otherutilities to help plan theiractivities in the wider context ofthe catchment;

• transportation planners;

• land owners, farmers and landmanagers that manage andoperate land for agriculture,conservation and amenitypurposes;

• the public and businesses toenhance their understanding offlood risk and how it will bemanaged.

Figure 1. The relationship between CFMPs, delivery plans, projects and actions

CFMPs aim to promote moresustainable approaches tomanaging flood risk. The policiesidentified in the CFMP will bedelivered through a combination ofdifferent approaches. Together withour partners, we will implementthese approaches through a rangeof delivery plans, projects andactions.

The relationship between the CFMP,delivery plans, strategies, projectsand actions is shown in Figure 1.

Policy planning• CFMPs and Shoreline Management Plans.

• Action plans define requirement for deliveryplans, projects and actions.

Note: Some plans may not be led by us – we may identify theneed and encourage their development.

Policy delivery plans (see note)• Influence spatial planning to reduce risk and

restore floodplains.

• Prepare for and manage floods (including localFlood Warning plans).

• Managing assets.

• Water level management plans.

• Land management and habitat creation.

• Surface water management plans.

Projects and actions• Make sure our spending delivers the best

possible outcomes.

• Focus on risk based targets, for example numbersof households at risk.


The Bristol Avon catchment islocated in the west of England. Itdrains parts of Gloucestershire,Wiltshire and Somerset and flowsthrough the major cities of Bristoland Bath to the Severn Estuary atAvonmouth.

Map 1 shows the location and extentof the River Avon CFMP area. Itincludes the Somerset Frome andthe Bristol Frome, plus a number ofother tributaries includingSemington Brook, the River Chewand Midford Brook. The downstreamlimit of the CFMP area overlaps withthe upstream boundary of the SevernEstuary Shoreline Management Plan(SMP).

The Severn Estuary SMP deals withcoastal flood management, while theCFMP considers tidal flood risk alongthe River Avon upstream of NethamWeir to the tidal limit at Keynsham.

Catchment overview

The overall catchment area is about2,221 square kilometres, and has apopulation of around 1,050,000. Tenper cent of the catchment isurbanised. As well as Bristol andBath, its main urban areas includeChippenham, Frome, Trowbridge,Devizes, Melksham, Malmesbury,Calne, Keynsham, Westbury,Midsomer Norton and Radstock, Yateand Chipping Sodbury, Bradford-on-Avon and Corsham.

The Bristol Avon catchment isdelineated by the Mendip Hills to thesouth, the Cotswold Hills to thenorth, the Marlborough Downs andSalisbury Plain to the east and theSevern Estuary to the west. The RiverAvon’s direction and path is dictatedby the catchment’s topography andresults in the river following acrescent shape, initially flowingsouth from the Cotswolds beforebending west through Bath andBristol.

The main geological features of thecatchment are the limestone MendipHills, the oolitic limestone Cotswoldsand the chalk downs in the east, allof which are major aquifers affectingthe hydrology of the catchment.Impermeable clays lie between thewest-sloping strata of the limestoneand the chalk, while sandstone andmudstone are exposed in the west ofthe catchment.

Within the River Avon catchmentthere are a number of sitesdesignated for their environmentalimportance including Special Areasof Conservation (SAC), SpecialProtection Areas (SPA) and Ramsarsites. Important environmental sitesin the catchment include four Areasof Outstanding Natural Beauty(AONB) including the Cotswolds andthe Mendip Hills, five SACs, 23 SPAs,98 Sites of Special Scientific Interest(SSSIs) and 299 ScheduledMonuments.


Map 1. Location and extent of the Bristol Avon CFMP area

© Crown Copyright. Environment Agency 100026380.

Legend

ThornburyTetbury

Malmesbury

Swindon

ChippingSodbury

Nailsea

Yatton

Cheddar

Wells

Glastonbury

Bruton

Bristol

Bath

Radstock

WestburyFrome

Chippenham

Calne

Devizes

Warminster

0 4 8 12 16Kilometres

Bristol Avon CFMP

Urban areas

Main rivers

Railway

Motorway

N

➜ High water levels on the Avon at Old Bridge – since replaced by Churchill Bridge – in Bath in 1960


Current and future flood risk

Flood risk has two components: thechance (probability) of a particularflood and the impact (orconsequence) that the flood wouldhave if it happened. The probabilityof a flood relates to the likelihood ofa flood of that size occurring within aone year period. It is expressed as apercentage. For example, a 1% floodhas a 1% chance or 0.01 probabilityof occurring in any one year, and a0.5% flood has a 0.5% chance or0.005 probability of occurring in anyone year. The flood risks quoted inthis report are those that takeaccount of flood defences already inplace.

This catchment has a long history offlooding, which resulted in manyflood defence schemes being built,particularly in the period 1935 to2000. Since then, high flows on theRiver Avon in 2000 and 2008 whichwould have caused widespreadflooding resulted in little damage.

Currently the main sources of floodrisk for people, property,infrastructure and the land are:

• river flooding from the River Avonand its tributaries, particularly inBristol, Bath, Malmesbury,Chippenham, Chew Magna,Frome, Melksham, Bradford-on-Avon and Midsomer Norton;

• tidal flooding from the River Avonbetween Avonmouth and Bristol,where tidal water could result intidelocking on tributaries drainingto the river;

• surface water drainage and sewerflooding, which has occurred inparts of Bristol, Bath, MidsomerNorton, Chipping Sodbury andCorsham. Several other townshave the potential to be at riskfrom surface water flooding.

Overview of the current flood risk

At present there are around 17,000people and 7,000 commercial andresidential properties at risk in thewhole catchment from a 1% annualprobability river flood. This meansthat 1.6% of the total populationliving in the catchment are currentlyat risk from flooding.

It is difficult to assess the currentimpact of flooding to environmentalfeatures. Many designated sites atrisk from flooding would not actuallybe damaged by the inundation.

43 Scheduled Monuments are at riskof flooding, but again, the actual riskof damage from flooding is limited.

What is at risk?

➜

Sluice gates built on the River Avonat Twerton as part of the Bath FloodAlleviation Scheme. They are vitalfor maintaining the river level inBath and open automatically to letflood flows through.

Table 2. Critical infrastructure at risk:

2 ambulance stations, 53 electricity sub-stations, 4 care homes

Number of properties at risk Locations

>1,000 Bristol, Bath

500 to 1,000 None

100 to 500 Chipping Sodbury and Yate, Melksham

50 to 100 Trowbridge, Calne, Chew Magna, Keynsham, Bradford-on-Avon, Malmesbury

25 to 50 Chippenham, Frome


Table 1. Locations of towns and villages with 25 or more properties at risk in a 1% annual probability river flood

Map 2. Flood risk to property in a 1% annual probability river flood, taking into account current flood defences


0 4 8 12 16Kilometres

Legend

Properties with a 1%chance of flooding

25 - 100

101 - 250

251 - 500

501 - 1,000

1,001 - 3,850

Bristol Avon CFMP

Main rivers

Malmesbury

ChippenhamCalneBristol

Keynsham

BathMelksham

Bradford-on-Avon

Trowbridge

Frome

Chipping Sodburyand Yate

Chew MagnaN


How we currently manage the risk

The catchment has a history of floodrisk, generally due to the highrainfall that can lead to extensiveflooding of the river valleys.

Over 50 years at the end of the 20thCentury, numerous engineeringschemes were implemented toreduce flood risk in the catchment,including:

• widening and deepening of riversand removal of obstructions inBath, Chippenham, Frome,Trowbridge, Melksham,Malmesbury, Calne, Radstock,Keynsham, Castle Combe andGreat Somerford. Protectionvaries from 4% annual probabilityin Malmesbury to 1% in Bath;

• building flood bypass tunnels: thebypass tunnel at MidsomerNorton which provides protectionup to a 1% annual probabilityriver flood, while at Ashton Vale inBristol, this protection is reducedto 3% due to the risk oftidelocking of the tunnel outfall.The Northern StormwaterInterceptor at Eastville divertsflood flows from the centre ofBristol directly to the River Avon;

• constructing reservoirs. The floodstorage reservoir at Iron Actonreduces flood risk downstream onthe Bristol Frome throughFrampton Cotterell to Eastville.Other reservoirs at WoottonBassett and Emerson’s Greenhave similar impacts on theHancock’s Water and Folly Brookrespectively.

These measures have all reducedflood risk.

In addition to these engineeringschemes other flood riskmanagement activities are carriedout in the catchment. These includeactivities which help to reduce theprobability of flooding and thosethat address the consequences offlooding.

Activities that reduce the probabilityof flooding include:

• maintaining and improvingexisting flood defences andstructures;

• maintaining river channels; • maintenance of road drainage

and sewers; • working with local authorities to

influence the location, layout anddesign of new and redevelopedproperty and ensuring that onlyappropriate development isallowed on the floodplain throughthe application of Planning PolicyStatement 25 (PPS25).

Activities that reduce theconsequences of flooding include:

• understanding where flooding islikely by using flood risk mapping;

• providing flood forecasting andwarning services;

• promoting awareness of floodingso that organisations,communities and individuals areaware of the risk and are preparedin case they need to take action intime of flood;

• promoting resilience andresistance measures for thoseproperties already in thefloodplain.

Around a third of the people andproperties that are at risk within thecatchment from a 1% annualprobability river flood, are located inBristol. A further 15% are located inBath.

The distribution of properties at riskfrom a 1% annual probability riverflood, is illustrated in Map 2. Table 1summarises where there is flood riskto more than 25 properties. Werecognise that there is also apotential risk from surface water andgroundwater flooding. However,further studies following on from theCFMP are needed by us and ourpartners to quantify this potentialrisk.

Where is the risk?


The impact of climate change and future flood risk

In the future, flooding will beinfluenced by climate change,changes in land use (for exampleurban development) and rural landmanagement. In the Bristol Avoncatchment, climate change will havethe greatest impact on flood risk,with urban development being afurther impact on the Bristol Frome.The following future scenario forclimate change was used in theCFMP:

• 20% increase in peak flow in allwatercourses. This will increasethe probability of large-scale floodevents;

• a total sea level rise of 1,000 mmby the year 2100. This willincrease the probability of tidalflooding on the lower reachesfrom Avonmouth to Keynsham andincrease the length of timewatercourses will be tide locked.

Using river models we estimate thatby 2100, around 50,000 people and20,000 properties across thecatchment may be at risk from a 1%annual probability flood. Flood riskfrom rivers increases mainly in theBristol and the Bristol Fromecatchment, but significant increasesalso occur in the Wiltshire towns ofChippenham, Trowbridge and Calne.

The sensitivity testing undertakenhas shown that the main drivers ofchange to flood risk in the BristolAvon catchment to be climatechange and in some locations, urbandevelopment.

Figure 2 shows the differencebetween current and future floodrisks from a 1% annual probabilityriver flood at key locations in thecatchment. Following on from theCFMP, organisations need to work

0

2000

4000

6000

8000

10000

12000

14000

16000

Bristol Bath Chipping Sodbury & Yate Chew Magna Keynsham Calne Chippenham Frome

Num

ber o

f Pro

pert

ies

at F

lood

Ris

k

Current Future

Figure 2. Current and future (2100) flood risk to property from a 1% annualprobability river flood, taking into account current flood defences

together to investigate flood riskfrom other sources (e.g. surfacewater and ground water flooding) inmore detail.

In general, it is unlikely that theimpact of flooding on environmentalsites will change significantly in thefuture.


Future direction for flood risk management

Approaches in each sub-area

We have divided the Bristol Avon catchment into ninedistinct sub-areas which have similar physicalcharacteristics, sources of flooding and level of risk. We have identified the most appropriate approach tomanaging flood risk for each of the sub-areas andallocated one of six generic flood risk managementpolicies, shown in Table 3.

To select the most appropriate policy, the plan hasconsidered how social, economic and environmentalobjectives are affected by flood risk managementactivities under each policy option.

Map 3. Bristol Avon sub-areas

Pill

Bath

M

Wells

Calne

Frome

Bruton

Pewsey

Yatton

Swindon

Tetbury

Devizes

Cheddar

Nailsea

Bristol

Patchway

Melksham

Westbury

Thornbury

Malmesbury

Warminster

Portishead

Chippenham

Shepton Mallet

Wootton Bassett

Yate

Radstock


N LegendBristol Avon CFMP

Sub-area

Bristol (Policy 5)

Bath (Policy 5)

Upper Avon (Policy 6)

Lower Avon (Policy 3)

Upper Bristol Frome (Policy 6)

Mendip Slopes and Long Ashton(Policy 4)

Wootton Bassett and Dauntsey (Policy 3)

Wiltshire Towns (Policy 4)

Bradford-on-Avon and Frome (Policy 3)

Markham Brook and Avonmouth (Policy 4)

1

2

3

4

5

6

7

8

9

10

➜ River Chew at the village of Pensford after floods of July 1968


Policy 1

Areas of little or no flood risk where we will continue to monitor and advise

This policy will tend to be applied in those areas where there are very few properties at risk of flooding. It reflects a commitment to work with the natural flood processes as far as possible.

Policy 2

Areas of low to moderate flood risk where we can generally reduce existing flood risk management actions

This policy will tend to be applied where the overall level of risk to people and property is low to moderate.It may no longer be value for money to focus on continuing current levels of maintenance of existing defencesif we can use resources to reduce risk where there are more people at higher risk. We would therefore reviewthe flood risk management actions being taken so that they are proportionate to the level of risk.

Policy 3

Areas of low to moderate flood risk where we are generally managing existing flood risk effectively

This policy will tend to be applied where the risks are currently appropriately managed and where the risk offlooding is not expected to increase significantly in the future. However, we keep our approach under review,looking for improvements and responding to new challenges or information as they emerge. We may reviewour approach to managing flood defences and other flood risk management actions, to ensure that we aremanaging efficiently and taking the best approach to managing flood risk in the longer term.

Policy 4

Areas of low, moderate or high flood risk where we are already managing the flood risk effectively but where wemay need to take further actions to keep pace with climate change

This policy will tend to be applied where the risks are currently deemed to be appropriately-managed, butwhere the risk of flooding is expected to significantly rise in the future. In this case we would need to do morein the future to contain what would otherwise be increasing risk. Taking further action to reduce risk will requirefurther appraisal to assess whether there are socially and environmentally sustainable, technically viable andeconomically justified options.

Policy 5

Areas of moderate to high flood risk where we can generally take further action to reduce flood risk

This policy will tend to be applied to those areas where the case for further action to reduce flood risk is mostcompelling, for example where there are many people at high risk, or where changes in the environment havealready increased risk. Taking further action to reduce risk will require additional appraisal to assess whetherthere are socially and environmentally sustainable, technically viable and economically justified options.

Policy 6

Areas of low to moderate flood risk where we will take action with others to store water or manage run-off inlocations that provide overall flood risk reduction or environmental benefits

This policy will tend to be applied where there may be opportunities in some locations to reduce flood risklocally or more widely in a catchment by storing water or managing run-off. The policy has been applied toan area (where the potential to apply the policy exists), but would only be implemented in specific locationswithin the area, after more detailed appraisal and consultation.

Table 3. Policy options


Bristol

Sub-area 1

The issues in this sub-area

This sub-area is a mostly urbanarea, covering the city of Bristol andits suburbs.

The defences through Bristolinclude the Northern Storm WaterInterceptor (NSWI), diverting floodflows from the Bristol Frome awayfrom the city into the tidal Avon.There are also smaller diversionchannels on the Ashton, Longmoorand Colliters Brooks and theBrislington Brook. The FloatingHarbour in the centre of the city hasa vital role in protecting the cityfrom combined tidal and fluvialflooding, effectively acting as alarge storage area.

It is estimated that approximately2,200 properties lie within thecurrent 1% annual probability floodextent. Many of these properties areprotected by defences. Within thefuture 1% annual probability floodextent the number of properties isexpected to increase to 14,000.

There are a number ofenvironmental designations at riskof flooding including the HorseshoeBend (Shirehampton), Avon Gorgeand three Scheduled Monuments.

Flooding affects a significantamount of critical infrastructure inBristol. This includes hospitals,police stations, and fire stations.Numerous roads are at risk offlooding including the M4 and M32motorways. The increase in futureflood risk will mainly be driven byclimate change.

Climate change and increasingdevelopment pressures have beenidentified as the main drivers forincrease in flood risk.

The vision and preferred policy

Policy Option 5 - We can generallytake further action to reduce floodrisk.

Taking further action to reduce theflood risk will ensure that thestandard of protection throughBristol is improved where required.This will ensure that the effects ofincreased flows as a result ofclimate change and futuredevelopment do not result in anincrease in the level of flood risk invulnerable areas.

Proposed actions to implement thepreferred policy

• We will carry out a study todetermine the combined fluvial /tidal flood risk to Bristol from thetide, the River Avon and theBristol Frome in order to reduceuncertainty relating to the level ofrisk this poses.

• This information will then beused to inform and furtherdevelop our flood riskmanagement strategy for Bristol.

• We will identify if there are otherspecific areas where tide-lockingof tributaries (for example theMalago Stream flowing into theRiver Avon from the South) arecausing flooding problems, andlook at ways of mitigating thisrisk.

• Carry out integrated urbandrainage studies to identifycurrent and future risks, andpropose mitigation.

• We will investigate the benefits ofimproved flood forecasting andflood warning using improvedmeteorological technology.

Our key partners are:

City of Bristol Unitary Authority

South Gloucestershire UnitaryAuthority

North Somerset Unitary Authority

Wessex Water

Met Office


Sub-area 2

Bath


This sub-area covers the urban areaof Bath and includes a large numberof designated sites. Bath is a WorldHeritage Site.

Approximately 1,100 properties arewithin the current 1% annualprobability flood extent. This figureincreases to an estimated 1,800properties for the future 1% annualprobability flood extent.

Although the majority of theproperties and people are at risk offlooding from the River Avon, asignificant number are at risk fromtributaries, in particular thoseflowing into the River Avon from thenorth (right bank). Bath has aformal defence scheme to protectthe city from the River Avon.

Flooding significantly affects criticalinfrastructure in Bath. Ambulancestations, health surgeries and apolice station are at risk. Transportnetworks are also at risk.


• Carry out improvements toexisting assets throughdevelopment opportunities onthose lengths identified as belowstandard, and identify an overallstrategy for the future protectionof Bath and for its existingdefences.

• We will increase awareness ofrisk and response to floodwarnings, and discourageinappropriate development.


Bath & North East Somerset UnitaryAuthority

Two Scheduled Monuments,including the Roman Baths and partof the World Heritage Site, are atrisk of flooding.

The increase in future flood risk willmainly be driven by climate change,which is predicted to result inincreases to peak river flows.


Policy Option 5 - We can generallytake further action to reduce floodrisk.

The current level of flood risk inBath is considered unacceptableand under the chosen policy thisrisk would be reduced. Futureincreases in flood risk due toclimate change could be balancedby reducing flows throughincreasing storage in the UpperAvon sub-area.

➜

Flooded streets at Southgate in Bath inDecember 1960


Wiltshire Unitary Authority

South Gloucester Unitary Authority

Cotswold District Council

Natural England

Wessex Water

National Farmers Union

Farming and Wildlife Advisory Group

Local farmers and landowners


Upper Avon

Sub-area 3


This sub-area covers thepredominantly rural area of theUpper Avon catchment and includesthe Semington Brook. The mainflood risk comes from the RiverAvon, though there is also a floodrisk associated with the tributaries.Overall, there are no particularlylarge concentrations of people orproperties at risk within the sub-area.

There are very few formal defenceswithin the sub-area and most of thewatercourses remain in a naturalstate. Malmesbury is the mainsettlement, but generally across thesub-area most properties at risk areisolated. There are a number of oldriver control structures that havebeen replaced with flood defencecontrol structures on the River Avon

and tributaries. Within the sub-area,approximately 400 properties are atrisk of flooding during the current1% annual probability flood event.These numbers increase toapproximately 600 properties forthe future 1% annual probabilityevent. The floodplain of the RiverAvon covers a wide extent andflooding of the agricultural land is asignificant factor.

An AONB, three SSSI and fourScheduled Monuments are at risk offlooding. A school, health centre,electricity sub-station, a sewagetreatment works, a water treatmentworks and a fire and ambulancestation are at risk. Railway lines androads, including the M4 motorway,are also at risk. The increase infuture flood risk will mainly bedriven by climate change, withfuture land use changes and landmanagement practices unlikely tohave a major effect on future floodrisk.


Policy Option 6 - We will take actionwith others to store water ormanage run-off in locations thatprovide overall flood risk reductionor environmental benefits.

This policy offers the potential forconsiderable environmentalbenefits. The floodplain of the RiverAvon is quite wide for much of this

sub-area, meaning that there ispotential for significant floodplainstorage. This will have the effect ofattenuating flows and retainingfloodwater, thereby reducing theflood risk to areas at riskdownstream, and locally.


Proposed actions to implement the preferred policy

• We will carry out a detailed study to determine specific areas where storage of floodwater may befeasible. This is likely to include a detailed topographic study (e.g. upstream of Malmesbury), and willpropose the implementation of feasible schemes. This work will also identify urban areas and smallersettlements within the sub-area at risk of flooding so that the risk to these would not be increased byadopting this policy.

• We will identify areas which could benefit from increased flooding, for example suitable areas in whichto develop water meadows, wet woodland and other wetland habitat, or opportunities to convert arableland to permanent pasture that may also allow flooding. We will identify opportunities to createrecreational public amenities or water resources benefits through such storage options. We will proposea schedule of schemes for suitable sites.

• We will carry out pilot studies on the Semington Brook, Brinkworth Brook and at Little Somerford intothe benefits of planting wet woodlands in floodplain.

• We will monitor the effects of upland storage on flows through Bradford-on-Avon and Bath.

➜ The confluence of the River Avon and the Brinkworth Brook at Great Somerford during flooding


Lower Avon

Sub-area 4


This sub-area covers much of therural area of the lower Bristol Avoncatchment and includes the ByBrook, River Boyd and the lowersections of the Somerset Frome andMidford Brook. This sub-area coversa large proportion of the Bristol AvonCFMP.

There are very few formal defenceswithin the sub-area with mainlyisolated properties at flood risk. Alimited flood warning service isoffered to the main areas at risk.

Approximately 200 properties arewithin the current 1% annualprobability flood extent, most ofwhich are well dispersed across thesub-area.

This figure is expected to increase toan estimated 340 properties withinthe future 1% annual probabilityflood extent.

The increased frequency of floodingwill bring a limited opportunity toincrease the area of water meadows,wetland and/or wet woodland,including around Bradford-on-Avonand Newton St Loe.

Proposed actions to implement the preferred policy

• Through the development of aSystem Asset Management Plan,study the cost-efficiency ofexisting asset maintenance inrelation to flood risks at sitessuch as Bathford, Swineford,Batheaston etc and implementany recommendedimprovements.


Bath and North East SomersetUnitary Authority


Mendip District Council

Numerous features havedesignations, including 15 Sites ofSpecial Scientific Interest, 11Scheduled Monuments, two Areas ofOutstanding Natural Beauty and theSalisbury Plain and Mells ValleySpecial Areas of Conservation.

Three electricity sub-stations andthree water treatment works are atrisk of flooding, along with railwaysand major roads, including the M4and A36.

The increase in future flood risk willbe driven mainly by climate changewhich is predicted to result inincreases to fluvial flows. Neitherland use or land managementchanges are expected to have asignificant influence on future floodrisk in the sub-area.


Policy Option 3 - We are generallymanaging existing flood riskeffectively.

Flood risk is predicted to increase inthe future through climate changebut the effective decrease in thestandard of protection that this willbring is not expected to havesignificant social or economicimplications and this policytherefore represents the bestbalance of costs and benefits,socially, economically andenvironmentally.


Sub-area 5

Upper Bristol Frome


This sub-area covers the upperBristol Frome catchment andincludes the towns of FramptonCotterell, Chipping Sodbury andYate. The main flood risk comes fromthe Bristol Frome, although a fewminor tributaries contribute to theflood risk.

The majority of properties at risk arein Chipping Sodbury and Yate, butisolated properties at FramptonCotterell and on the StockwellWatercourse, Bradley Brook and FollyBrook are also at risk. Tubbs Bottomdetention dam was constructed toreduce the risk of flooding to areasdownstream.

Around 300 properties are within thecurrent 1% annual probability floodextent. This figure is expected toincrease significantly to 1,600properties in the future. An electricitysub-station, a care home, a firestation, schools and health centresare at risk from flooding. Part of theM4 motorway and the A432 road arealso at risk.


• We will carry out a detailed studyto consider, firstly, the operationof Tubbs Bottom detentionreservoir to further benefitdownstream, and secondly, theopportunities for furtherfloodplain storage on the LaddenBrook and tributaries.

• We will discourage inappropriatedevelopment in flood risklocations, especially criticalinfrastructure.




Policy Option 6 - We will take actionwith others to store water ormanage run-off in locations thatprovide overall flood risk reductionor environmental benefits.

Increasing storage through thisapproach, including increasing theefficiency of Tubbs Bottomdetention reservoir, has thepotential to reduce the flood risk inthe urban areas of the sub-areathrough increased floodplainstorage upstream. This would bringan associated reduction in theseverity and frequency of floodingto people and properties. The floodrisk to the health centres, school,care homes and the fire station willbe reduced, as will the flood risk tothe roads within the urban areas.

➜ Construction work at Tubbs Bottom


Sub-area 6

flood risk to the schools, healthcentres, sewage treatment works orindustrial units currently at risk offlooding.


• We will review emergencycontingency planning, especiallyin the light of climate change,increase awareness of risk andresponse to flood warnings, anddiscourage inappropriatedevelopment.

• We will investigate the benefits ofimproved flood forecasting andflood warning using improvedmeteorological technology.



Bath and North East SomersetUnitary Authority


Met Office


This sub-area covers the slopes ofthe Mendips and the upper reachesof the Midford Brook, SomersetFrome and River Chew. An area tothe west of Bristol around LongAshton is also included. It is apredominantly rural area, but doescontain the towns of MidsomerNorton, Chew Magna, Keynshamand Radstock.

The flood risk mainly arises from therelatively fast response of thewatercourses due to their locationon the slopes of the Mendip Hillsand from direct surface run-off.Notable areas at risk of floodinginclude Chew Stoke, Hallatrow,Nunney, Witham Friary, Mells,Pensford, Chilcompton, ComptonDando and Vobster. Approximately580 properties (of which 470 areresidential) are at risk of floodingcurrently, rising to 790 (600residential) in the future.

Mendip Slopes andLong Ashton

Within the current 1% annualprobability flood extent there arevarious environmental designations.These include nine SSSIs, twoAONB, two SAC and 15 ScheduledMonuments.

Seven electricity sub-stations arealso at risk along with healthcentres, schools, a sewagetreatment works, railways and majorroads.

The increase in future flood risk willmainly be driven by climate changewith future changes in land use andland management practices unlikelyto have much of an effect on futureflood risk. The steeper nature of thewatercourses combined with theincreased flows predicted underclimate change may lead to a largerincrease in flood risk compared tosome of the other sub-areas in theBristol Avon CFMP.


Policy Option 4 - We are alreadymanaging the flood risk effectivelybut we may need to take furtheractions to keep pace with climatechange.

This policy will require us to domore in the future to contain whatwould otherwise be increasing risk.There should be no increase in


Sub-area 7


• Undertake integrated urbandrainage studies, in particular formain line railway and M4motorway flood risks, andimplement any recommendedimprovements.

• Through the development of aSystem Asset Management Plan,study the cost-efficiency ofexisting channel maintenance inrelation to Dauntsey Green, andimplement any recommendedimprovements.


This sub-area includes the towns ofWootton Bassett and Dauntsey,both of which are located in theupper reaches of the catchment

Whilst this sub-area ispredominantly rural, at WoottonBassett significant development ispredicted. The main flood risk is to40 residential properties atDauntsey, rising to 60 with climatechange increases. There is a flooddetention reservoir in WoottonBassett. This was primarily installedto ensure that increased run-offfrom development upstream did notincrease flooding to agriculturalland.

Wootton Bassett andDauntseyOur key partners are:


Highways Agency

Network Rail

There is one Scheduled Monumentat risk of flooding within the current1% annual probability flood extent.Also at risk are roads, including partof the M4 motorway and the railwayline from Bristol to Swindon. Aschool in Dauntsey is also at risk.

The increase in future flood risk willmainly be driven by climate change,which is predicted to result inincreases to river flows and surfacerun-off.



Although this approach may lead toa slight increase in the frequency offlooding in the future and to thelevel of disruption, this is notdeemed significant enough toincrease the level of flood riskmanagement. Policy 3 representsthe best balance of costs andbenefits, socially, economically andenvironmentally.


Wiltshire Towns

Sub-area 8


• We will develop a prioritisedprogramme of strategies formaintaining the level of risk intothe future. As well as benefitingfrom upstream storage, we wouldlook to include the possiblefuture modification of existingassets, including utilisingdevelopment opportunities forthe removal or replacement ofsluice structures at Chippenhamand Melksham. We would look atoptions to make channelimprovements and undertakebank raising in Trowbridge andWestbury. We would also look atoptions to make improvements tochannels, culverts and theirscreening, and storage inCorsham and Calne.

• Improve flood forecasting andflood warning using improvedmeteorological technology andimprove response through raisingawareness.

• Undertake integrated urbandrainage studies, in particular formain line rail flood risks, andimplement any recommendedimprovements at Corsham.

• Discourage inappropriatedevelopment in Corsham, Calne,Trowbridge and Westbury.

• Reinforce contingency planningand self-help in Corsham andCalne.


This sub-area covers the towns ofChippenham, Melksham, Corsham,Calne, Westbury and Trowbridge.The main flood risk to Chippenhamcomes from the River Avon, thoughseveral small tributaries flowthrough the town and increase therisk.

None of these towns havesignificant flood risk, and, exceptfor Corsham, are protected to areasonable level by past schemes.Development pressures affect allthe towns and climate change willincrease properties at riskdramatically. It is estimated thatapproximately 400 properties liewithin the current 1% annualprobability extent. Within the future1% annual probability flood extentthe number of properties isexpected to increase to 2,600.Major infrastructure including mainline rail, roads and an electricity substation will be at increased risk.



Met Office


Policy Option 4 - We are alreadymanaging the flood risk effectivelybut we may need to take furtheractions to keep pace with climatechange.

Under this approach, further actionwill be taken to sustain the currentlevel of flood risk into the future.The majority of the 2,600 propertiesat risk in the future would see therisk remain similar to that atpresent.


Sub-area 9





This sub-area covers the towns ofBradford-on-Avon and Frome.

Both towns would be cut in half ifmajor flooding takes place. UnlikeBradford-on-Avon, Frome alreadyhas some protection afforded by anearlier scheme. Both towns sufferfrom the impracticality of futureimprovements to channels andbridges in the town.

There are currently 75 propertieswith the current 1% annualprobability flood extent and this isexpected to rise to around 190 inthe future.


• We will reinforce contingencyplanning and self-help, increaseawareness of risk and responseto flood warnings, anddiscourage inappropriatedevelopment.

• Through the development of theSystem Asset Management Plan,study the cost-efficiency ofexisting maintenance of the twotown centre’s bridges, channelsand culverts and implement anyrecommended improvements.

Bradford-on-Avon and Frome

Three Scheduled Monuments inBradford-on-Avon are at risk offlooding. Also at risk in the town arean electricity sub-station, a firestation and a police station. TheA363 road and the railway linethrough the town are also at risk.

There are no environmentaldesignations at risk of flooding inFrome. The A361 and A362 roads,along with the railway, are also atrisk. Flooding could also affect anelectricity sub-station and twohealth centres in the town.



The overall increase in the level offlood risk is likely to be small.


Sub-area 10


• Review emergency contingencyplanning, especially in the lightof climate change

• Increase awareness of risk andresponse to flood warnings.

• Discourage inappropriatedevelopment.

• Encourage the production ofsurface water management plansfor Pill and Shirehampton

• Consider future improvements forthe Pill pumping station.


City of Bristol Unitary Authority


Port of Bristol Authority

Wessex Water


This sub-area covers both banks ofthe tidal River Avon belowShirehampton including theMarkham Brook and Chapel Pill. Thisincludes the urban areas ofShirehampton, Pill and Easton-in-Gordano and a small area ofAvonmouth including Portbury andAvonmouth Docks. Most ofAvonmouth is covered by the SevernTidal Tributaries CFMP.

The over-riding flood risk in this sub-area is from tidal flooding and bothbanks of the Avon have raised tidaldefences.

Behind the defences, the main riskof flooding is from surface waterexacerbated by tide-locking.

Around 60 properties, mainlyresidential, are at risk of floodingfrom the current 1% annualprobability flood event, thoughthese are protected from flooding bythe existing defences. The number

Markham Brook andAvonmouth

of properties at risk during the future1% event increases to an estimated120.

The Severn Estuary SpecialProtection Area, and Ramsar are notaffected by fluvial and surface waterflooding in this sub-area.

An electricity substation, a firestation and 10 sheltered houses areat risk from the current 1% annualprobability flood event. A main roadis also at risk.


Policy Option 4 - we are alreadymanaging the flood risk effectively,but we may need to take furtheractions to keep pace with climatechange.

Increased river flows, surface run-offand sea level rise as a result ofclimate change are likely to be thethree main drivers of future floodrisk within this sub-area.

Under this approach, further actionwill be taken to sustain the currentlevel of flood risk into the future.The estimated 120 properties at riskof flooding would see the riskremain similar to that at present.


Map of CFMP policies

Map of the policies in the Bristol Avon catchment

1 Bristol

2 Bath

3 Upper Avon

4 Lower Avon

5 Upper Bristol Frome

6 Mendip Slopes and Long Ashton

7 Wootton Bassett and Dauntsey

8 Wiltshire Towns

9 Bradford-on-Avon and Frome

10 Markham Brook and Avonmouth

The sub-areas

GESW0612BWPY-E-E

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bristol avon catchment flood management plan - gov.uk · 2014-03-19 · 4 environment agency...

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