Hampshire Avon Catchment Flood Management Plan · 4 Environment Agency Hampshire Avon Catchment...

managingflood risk

Summary Report June 2012

Hampshire Avon Catchment Flood Management Plan

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June 2012

Environment Agency Hampshire Avon Catchment Flood Management Plan 1

Introduction

I am pleased to introduce our summary of the Hampshire Avon

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

overview of the flood risk in the Hampshire Avon catchment and

sets out our preferred plan for sustainable flood risk management

over the next 50 to 100 years.

The Hampshire Avon CFMP is one of 77 CFMPs forEngland and Wales. Through the CFMPs, we haveassessed inland flood risk across all of England andWales for the first time. The CFMP considers all types ofinland flooding, from rivers, ground water, surfacewater and tidal flooding, but not flooding directly fromthe sea (coastal flooding), which is covered byShoreline Management Plans (SMPs). Our coverage ofsurface and ground water is however limited due to alack of available 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 Hampshire Avon catchment has a history of floodrisk. Over the last 30 years numerous engineeringschemes have been implemented to reduce flood riskin the catchment. At present 5,450 properties are atrisk in the catchment in a 1% event. This is expected toincrease to over 6,800 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: Dorset CountyCouncil, Salisbury District Council, ChristchurchBorough Council, West Wiltshire District Council, KennetDistrict Council, New Forest District Council, and NaturalEngland 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 Hampshire 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 River Bourne sub-area 12

2 Upper Avon and Wylye sub-area 13

3 Warminster sub-area 14

4 River Nadder sub-area 15

5 Salisbury sub-area 16

6 Lower Avon sub-area 17

7 New Forest Streams sub-area 18

8 Christchurch sub-area 19

Map of CFMP policies 20

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 catchment of the HampshireAvon is located in the south ofEngland.

The Hampshire Avon rises in the Valeof Pewsey to the north of Salisbury.The watercourses here receivesignificant flows from the chalkaquifers underlying Salisbury Plain,and then flow in a southerlydirection towards ChristchurchHarbour and Christchurch Bay on thesouth coast. Map 1 shows thelocation and extent of the River AvonCFMP area.

At Salisbury, the Avon is joined bytwo of its major tributaries - the RiverBourne and the River Nadder(including the River Wylye), and ashort distance downstream by theRiver Ebble.

The downstream limit of the CFMParea meets with the upstreamboundary of the Poole andChristchurch Bay Shoreline

Catchment overview

Management Plan (SMP) boundary atChristchurch. The Poole andChristchurch Bay SMP deals withcoastal flood management, while theCFMP considers the risk from tidalflooding.

The overall catchment area is about1,750 square kilometres, and has apopulation of around 230,000. Onlytwo per cent of the catchment isurbanised. As well as Salisbury andChristchurch, its main urban areasinclude Warminster.

The Hampshire Avon catchment ischaracterised by open chalkdownland with steep scarp slopes,sheltered valleys, chalk hills, ridgesand limestone plateaux. Thesesignificant variations in thetopography have a strong influenceon the rivers’ response to rainfall.

The upper Avon catchment is typifiedby the undulating, chalk downlandsof Salisbury Plain, which are cut by

steep combes and river valleys. Thelower catchment is characterised byrolling farmland and the New Forest.The main watercourses have widefloodplains and flow throughfarmland, woodland, scrub and openheathland.

Within the Hampshire Avoncatchment there are a number ofsites designated for theirenvironmental importance includingpart of the New Forest National Parkand the World Heritage Site ofStonehenge. Importantenvironmental sites in the catchmentinclude two Areas of OutstandingNatural Beauty, two EnvironmentallySensitive Areas, nine Special Areasof Conservation (SAC), six NationalNature Reserves, 71 Sites of SpecialScientific Interest (SSSIs) and 1,061Scheduled Monuments.


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

© Crown Copyright. Environment Agency 100026380.

Legend

Hampshire AvonCFMP

Urban areas

Main rivers

Railway

Motorway

Shaftesbury

Bruton

Westbury Tidworth

0 4 8 12 16Kilometres

Frome

Pewsey

Mere

Devizes

Warminster

Amesbury

Salisbury

Downton

Fordingbridge

Verwood

Ringwood

BlandfordForum

Christchurch

N

➜The River Avon spills on tothe floodplain close toSalisbury Cathedral


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.

The assessment was based on theuse of existing river models, ourFlood Zone maps and historicalrecords.

This catchment has a long history offlooding. The most significant eventin recent years occurred in Salisbury,Downton, Fordingbridge and

Ringwood in December 2000 when132 properties were affected by riverflooding after a period of heavyrainfall on an elevated water table.

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

• river flooding from the River Avonat Downton, Fordingbridge andRingwood, from the Avon andNadder at Salisbury, from theNadder and Wylye at Wilton andthe Bourne at Tidworth;

• tidal flooding at Christchurch;

• surface water drainage flooding,which has occurred in Warminsterand Enford.

Groundwater flooding has occurredin Netherhampton, and groundwaterhas added to river floodingdownstream.

Overview of the current flood risk

At present there are around 10,700people and 5,400 commercial andresidential properties at risk in thewhole catchment from a 1% annualprobability river flood, taking intoaccount current flood defences. Thismeans that 1% of the totalpopulation living in the catchmentare currently at risk from flooding.

It is difficult to assess the currentimpact of flooding to environmentalfeatures. Most designated sites atrisk would not actually be damagedby the inundation, althoughprolonged deep flooding can have anegative impact on the Avon ValleySite of Special Scientific Interest. 42Scheduled Monuments are at risk offlooding. The actual risk of damagefrom flooding is limited.

What is at risk?

Table 2. Critical infrastructure at risk:

20 electricity substations, 4 water treatment works, 2 care homes, 18 main roads, 5 mainline railways, 1 fire station, 3 police stations, and 3 schools

Number of properties at risk Locations

>1,000 Salisbury, Christchurch

500 to 1,000 None

100 to 500 Warminster

50 to 100 None

25 to 50 Ringwood, Fordingbridge, Downton, Shrewton, Shipton, Bellinger


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


Legend

Properties with a 1%chance of flooding

55 - 100

101 - 500

501 - 1,000

1,001 - 2,000

2,001 - 3,000

Hampshire Avon CFMPMain rivers

Warminster

Salisbury

Christchurch0 4 8 12 16Kilometres

N

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


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, andprolonged wet periods that can leadto groundwater flooding.

Over the last 25 years, engineeringschemes have been implemented toreduce flood risk in the catchment,including at Tisbury, Downton,Fordingbridge, Ringwood andChristchurch. Various structureshave also been constructed aroundSalisbury and Wilton.

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, including pumpingstations;

• maintaining river channels; • maintenance of road drainage

and sewers;

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.

• working with local authorities toinfluence the location, layout anddesign of new and redevelopedproperty and ensuring that onlyappropriate development isallowed on the floodplain throughthe application of Planning PolicyStatement 25 (PPS25).

Around 40% of the people andproperties that are at risk within thecatchment from a 1% annualprobability river flood, are located inSalisbury. A further 22% are locatedin Christchurch.

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?

➜Misty conditions at Picket Post in theNew Forest looking across to the AvonValley. This is an area whereopportunities to manage land in adifferent way could lead to a reductionin flood risk downstream


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 Hampshire Avoncatchment, climate change will havethe greatest impact on flood risk. Thefollowing future scenario for climatechange was used in the CFMP:

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

• a total sea level rise of 500 mm bythe year 2100. This will increasethe probability of tidal flooding onthe lower reaches at Christchurch.

Using river models we estimate thatby 2100, around 13,250 people and6,800 properties across thecatchment may be at risk from a 1%annual probability flood. Flood riskfrom rivers increases mainly inSalisbury, but significant increasesalso occur in the towns of Downton,Fordingbridge and Ringwood.

The sensitivity testing undertakenshowed that the effects on flood riskof land use and land managementchange are likely to be relativelysmall, and from urban developmentvery limited, at a catchment-widescale. There are potential effects at alocal scale to be managed. Thegreatest effect on future flood risk isclimate change.

0

500

1000

1500

2000

2500

3000

3500

Salisbury Christchurch Warminster

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

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 worktogether 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.


Approaches in each sub-area

We have divided the Hampshire Avon catchment intoeight distinct sub-areas which have similar physicalcharacteristics, sources of flooding and level of risk. Wehave 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. Hampshire Avon sub-areas

Bath

Mere

Wool

Wells

Calne

Frome

Bruton

Pewsey

Merley

Wilton

Wareham

Verwood

Devizes

Downton

n

Melksham

Ringwood

Tidworth

Pimperne

Westbury

AlderburyWincanton Salisbury

Bransgore

Sherborne

terWest Moors

Gillingham

Warminster

Highcliffe

Dorchester

Shaftesbury

Sandleheath

Christchurch

Broad Chalke

Fordingbridge

Shepton Mallet

Blandford Forum

Wimborne Minster

Lytchett Matravers

Sturminster Newton

Three Legged Cross

Bournemouth and Poole

Radstock

Ferndown

Ashley Heath



N

LegendHampshire Avon CFMP

Sub-area

River Bourne (Policy 4)

Upper Avon and Wylye (Policy 6)

Warminster (Policy 4)

River Nadder (Policy 6)

Salisbury (Policy 5)

Lower Avon (Policy 4)

New Forest Streams (Policy 2)

Christchurch (Policy 5)

1

2

3

4

5

6

7

8

9

Future direction for flood risk management

➜ Flooding in The Borough, Downton in December 2000


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


River Bourne

Sub-area 1

The issues in this sub-area

Flood risk is concentrated in fourlarge town/villages: Tidworth,Collingbourne Ducis, ShiptonBellinger and Newton Tony. Otherrisk areas include CollingbourneKingston, Porton and theWinterbournes.

Significant flooding was recorded inTidworth and a number of villagesalong the River Bourne, during 2000and 2003. Much of this floodingwas attributed either directly orindirectly to record groundwaterlevels, although in certain areas (forexample Shipton Bellinger) floodingwas experienced from three sourcessimultaneously (groundwater, riverand highway drainage).

The current number of properties inthe 1% annual probability event is245. This is expected to increase to255 in the future 1% annualprobability event.

Our key partners are:

Wiltshire Unitary Authority

Test Valley District Council

Wessex Water

Natural England

The main future driver of flood riskwithin the sub-area is considered tobe climate change, represented asan assumed increase in peak flow ofup to 20%. However, it isacknowledged that the impact ofclimate change on permeable chalkcatchments is uncertain.

The vision and preferred policy

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

This policy will allow presentactions to control flood risk to becontinued (for example floodwarning) and expanded, and forfuture change in flood risk to bemonitored such that appropriatefurther actions can be carried out.Any structural works may beconcentrated in the higher riskurban areas, but an improvedunderstanding of the floodmechanisms, resulting risks andclimate change implications willalso allow a better response acrossthe sub-area in relation to people,properties and other assets.

Proposed actions to implement thepreferred policy

• Work with the at-riskcommunities of CollingbourneKingston, Collingbourne Ducis,Shipton Bellinger, Newton Tony,Porton and the Winterbournes toprovide guidance and advice onreducing risk.

• Improve our understanding ofgroundwater flooding mechanics,to enhance our groundwaterflood information service.

• Identify hydraulically criticalstructures and other pinch pointsand produce a programme ofimprovements to maintaincurrent standards.


Upper Avon and Wylye


Flood flows on the Upper Avon, Tilland Wylye are dominated bybaseflow from the upstream chalkaquifers. As such the frequency offlooding is less than on the lowercatchment tributaries, but whenflooding does occur it generallyresults from excessive seasonalrainfall through the autumn andwinter and therefore tends to beprolonged. Significant flooding hasbeen recorded in the catchmentduring 1995 and 2000.

There is in the order of 400properties currently at flood risk,these being located in Pewsey,Amesbury and Durrington, and alsoat Codford St Mary, Shrewton and



Natural England

National Farmers Union

Land managers

Salisbury District Flood SteeringGroup

Wessex Water

Netheravon. The number ofproperties at risk is expected toincrease to 475 in the future 1%annual probability event.

The main future driver of flood riskwithin the sub-area is considered tobe climate change, represented asan assumed increase in peak flow ofup to 20%. However, it isacknowledged that the impact ofclimate change on permeable chalkcatchments is extremely uncertain.


Policy option 6 - we will take actionwith others to store water or mangerun-off in locations that provideoverall flood risk reduction orenvironmental benefits.

The chosen policy providesimprovements downstream inSalisbury and Christchurch. Theimplementation of the policy willallow present actions to controlflood risk to be continued (forexample flood warning) and forfuture changes in flood risk to bemonitored such that the need forfurther actions can be reviewed.

Policy 6 provides vital benefits suchas helping to improve the conditionof the Sites of Special ScientificInterest and Special Area ofConservation through increased /

managed water levels (for examplein ditches to support fen and wetgrassland). In the long-term, there isthe potential for significant localimprovements, for example anincrease in extent of wetlandhabitats (possible local andnational Biodiversity Action Planhabitats / species) adjacent todesignated sites through controlledinundation, river restoration workand restoration of floodplainmeadows. Such works would alsobe beneficial for wetland birdbreeding populations.


• Identify areas where wetlandstorage areas may be created togive benefit to flood riskmanagement downstream.

• Identify sites of criticalinfrastructure and transportroutes and work to prevent thesesites suffering flooding in thefuture.

• Develop Action Plans for the at-risk villages including Codford StMary, Shrewton and NortonBavant, including all sources offlooding.

Sub-area 2


Warminster

Sub-area 3


This sub-area covers the urban areaof Warminster on the edge of theRiver Wylye floodplain and includesthe tributaries The Were andCannimore Stream where they flowthrough the town.

Based on records of floodingincidents we estimate that at least55 properties are at flood risk in a1% annual probability event. Theserecords indicate that floodingoccurs from watercourses,groundwater and surface run-off.The number of properties at risk isexpected to increase to 80 in thefuture 1% annual probability event.

The main future driver of flood riskwithin Warminster is considered tobe climate change, represented asan assumed increase in peak flowsto the main rivers of up to 20%.However, it is acknowledged thatthe impact of climate change inurban catchments might also leadto more frequent and intensestorms, with the potential toincrease the risk of surface waterflooding.



Wessex Water

The flood risk in Warminster is verylikely to be extremely sensitive toincreasing peak flood flows fromclimate change (and potentiallyfuture development) with probablesevere impacts on major roadtransport links and vitalinfrastructure in the town centrewhere the main bottlenecks to riverflows are located.


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

This policy supports the need toreduce risk in the long term,particularly where they are sensitiveto future change and likely toincorporate vulnerable sites andvital infrastructure. Theimplementation of the policy willallow present actions to manageflood risk to be continued (forexample flood warning) andexpanded, and for future changes inflood risk to be monitored andreviewed such that appropriatefurther actions can be carried out.


Promote an integrated urbandrainage study including futuredevelopment proposals, andimplement improvements.



This sub-area covers the floodplainof the River Nadder from the sourcenear the Coombe, to Burcombe atthe downstream end, approximately3km upstream of the confluencewith the River Wylye. The sub-area islargely rural in nature, but containsthe small town of Tisbury and a fewsmall villages which straddle thefloodplain. We estimate thatapproximately 30 properties are atflood risk in a 1% annual probabilityevent. The number of properties atrisk is expected to increase to 45 inthe future 1% annual probabilityevent.

Flood hydrographs show thecatchment is more responsive torainfall than the other catchmentsupstream of Salisbury. This is largelydue to the geology in the upperreaches of the catchment beingpredominantly impermeable clays.As such flooding here is likely to be



Test Valley District Council

Wessex Water

Natural England

more frequent, in the upper reachesat least, it may be more difficult toprovide flood warnings withsufficient lead times. Significantflooding was recorded in Tisbury in2000 and further widespreadflooding in late 2002. There is noevidence of groundwater floodingcausing significant problems.However, surface water floodingshould be recognised as a potentialsource of flood risk, particularly inTisbury and Burcombe.

Potentially there is flood risk tosignificant major road infrastructure,for example, the A30 road, as well asthe Salisbury to Yeovil railway line.


Policy Option 6 - we will take actionwith others to store water or managerun-off in locations that provideoverall flood risk reduction orenvironmental benefits.

Strategic flood storage in thecatchment could offer significantbenefits towards managing futureflood risk in policy units furtherdownstream (in particular Salisburyand Lower Avon). It should be notedthat in order to gain downstreambenefits, peak flows on the Nadderneed to be stored and then releasedafter the peak from the Avon andWylye has passed downstream.


Investigate feasibility of creatingflood storage on the upper Nadderto the benefit of flood riskdownstream in Salisbury and thelower Avon towns, and carry outfeasibility studies forimplementation.

River Nadder

Sub-area 4


Salisbury

Sub-area 5


This sub-area includes the historiccity of Salisbury and the town ofWilton to the west. It covers thefloodplains of the lower reaches ofthe Wylye, Nadder and Bourne, aswell as the Lower Avon. Whilst thereare some open river corridorsthrough the area, some concentratedareas of development also lie withinthe floodplain, particularly aroundthe cathedral.

A number of properties were affectedby flooding during autumn/winter2000 and again in December2002/January 2003. Some of theproperties affected are recorded tohave been flooded by groundwater,but our analysis has shown theseproperties would also be at riskduring a significant flood event onthe river. At present, we estimatethat up to 2,350 properties are at riskin a 1% annual probability floodevent. The number of properties atrisk is expected to increase to 3,330in the future 1% annual probabilityflood event.



Wessex Water

The existing mechanism of floodingis complicated and is influenced bythe fact that Salisbury sees four mainrivers coming together, leading to anumber of possibilities with regardto the phasing and combination ofeach of the peaks. Also, permeabletertiary/alluvial deposits andanecdotal evidence suggests thatfloodwater can pass onto and acrossthe floodplain without firstovertopping the river banks (forexample in the cathedral area).


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

This policy will allow presentactions to manage flood risk to becontinued (for example floodwarning) and for existing and futurerisks to be reduced through theimplementation of a combination oflocal and strategic flood riskmanagement responses.Sustainable options will beevaluated


• Carry out further studies toprovide complete understandingof risk to Salisbury and Wilton.Model asset operations. Useoutcomes from studies and urbandrainage pilots to evaluatepossible improvements,including developing floodwarning, and asset operationprocedures.

• Identify and survey infrastructureat risk and take measures toincrease flood resilience.

• Use results of studies toencourage appropriate futuredevelopment.


Lower Avon


This sub-area covers the floodplainof the Lower Avon along with threetributaries; the River Ebble, theAshford Water and the SweatfordWater. The area is a mix of urbanand rural, and contains a number oftowns and villages, includingDownton, Fordingbridge, Ringwood,Coombe-Bisset, Britford,Bodenham, Charlton-All-Saints,Breamore, Woodgreen, and Burton.

A number of properties wereaffected by flooding in autumn2000 and January 2003. Floodingoccurred in Downton andFordingbridge in 1995, 1999, 2000and to a lesser extent, 2003. Someof this flooding is directlyattributable to groundwater, andfluvial flooding is also widelyinfluenced by groundwater levels inupstream sub-areas, with floodevents often being prolonged


Christchurch District Council

East Dorset District Council

New Forest District Council

Wessex Water

events. Schemes were put in placein Downton (2004), Fordingbridge(2006) and Ringwood (2006), whichprotect 190 of the most at riskproperties in total.

In some areas, if fluvial flood risk isreduced through defences thengroundwater flooding issues canworsen as a result.

We estimate that up to 1,200properties would be at risk in a 1%annual probability event, affectingin the order of 2,450 people,despite recent flood defenceschemes being included in ouranalysis. The number of propertiesat risk is expected to increase to1,440 in the future 1% annualprobability flood event.

The main future driver of flood riskwithin this sub-area is climatechange.


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

The implementation of this policywill allow present actions to manageflood risk to be continued (forexample flood warning) and

expanded, and for future change inflood risk to be monitored such thatappropriate further actions can becarried out. Any structural works maybe concentrated in the higher riskurban areas, but an improvedunderstanding of the floodmechanisms, resulting risks andclimate change implications will alsoallow a better response from allparties concerned.


• Investigate flood risk inBreamore, Ringwood, Woodgreenand Rockbourne and urbandrainage flood risk in Britford andimplement appropriate flood riskmitigation measures wherefeasible.

• Identify and survey infrastructureat risk and take measures toincrease flood resilience.

Sub-area 6


New Forest Streams

Sub-area 7




New Forest National Park

Natural England

National Farmers Union

Farming and Wildlife Advisory Group

Land managers


This sub-area covers the floodplainof eight different streams which flowinto the River Avon. More than halfof the area is within the New ForestNational Park.

The New Forest streams drainrelatively rural catchments, withsemi-natural vegetation. Lowerdown in the catchments there arefields and managed landscapes,many of these used for grazing. Thehighest rates of run-off are likely tooccur when the soil in the forestbecomes saturated, and so floodingis most likely during the wintermonths.

The most extensive, recent episodeof flooding recorded was inDecember 2000/January 2001,affecting at least 10 properties. Weestimate that around 100 properties

are at risk in a 1% annualprobability flood event. The numberof properties at risk is expected toincrease to 120 in the future 1%annual probability flood event

Potentially there is flood risk tosome significant road infrastructure(for example A338 and B3347) andto formal and informal amenityareas of the New Forest NationalPark.


Policy Option 2 - We can generallyreduce existing flood riskmanagement actions.

The implementation of this policywill allow any present actions tomanage flood risk to be reviewedand reduced where appropriate andacceptable. However, in areas whereflood risk is more concentrated, wewill monitor future change.

Indications are that river restorationworks elsewhere in the New ForestNational Park may have reducedpeak flows by up to 5% and delayedthe time to peak by up toapproximately seven hours. If similarwork was promoted in this sub-area,it might provide some minorbenefits downstream, thoughchanges in timing could beimportant.


• Extend hydrometric monitoring toimprove flood warning for theNew Forest Streams, and useawareness campaigns to increasethe uptake of the flood warningservice.

• Where river restoration isplanned, investigate options formaximising potential forreductions in downstream floodrisk, as part of restoring naturalfloodplains.


Christchurch




Wessex Water


The Christchurch sub-area presentsa densely populated and veryconcentrated area of risk. Manyproperties along the River Avon,and lower extents of the Mude andBure Brook, are considered to be atrisk from a combination of bothtidal and fluvial flooding. Historicflooding appears to be limited tothe months of December, Januaryand February, coinciding with thehighest probability of high flows onthe Avon (and Stour) and tidalsurges entering the harbour.

A significant risk within Christchurchis the reliance on raised defencesand it has been assumed that assea levels continue to rise over thenext 100 years, together withincreasing flood flows, then anyovertopping of defences could bevery severe and have a majorimpact on over 1,000 properties.

The number of properties at risk inthe 1% probability event is 1,080.The number of properties isexpected to increase to 1,120 in thefuture 1% annual probability floodevent.


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

The implementation of this policywill ensure that flood risk is reducedand flood risk managementresponse can be continued, whilerecognising that there is a need todo more in order to investigatewhether existing defences can bemaintained at an appropriate level,with residual losses mitigated oncedefences are overtopped and inparticular that vulnerable sites andcritical infrastructure are protected.


• Investigate flood risk inChristchurch and develop astrategy to reduce risk. Ideallythis study would look at thecombined risk from the Avon,Stour, the harbour tributaries andthe sea and urban drainage.

• Investigate impact of sea levelrise and increased risk fromstorm surge to ChristchurchHarbour.

• Identify and survey infrastructureat risk and take measures toincrease flood resilience,including undertaking awarenesscampaigns.

• Use results of studies toencourage appropriate futuredevelopment.

Sub-area 8


Map of CFMP policies

Map of the policies in the Hampshire Avon catchment

Bath

Mere

Wells

Frome

Yeovil

BrutonStreet

Pewsey

Wilton

Verwood

Devizes

Downton

t

Amesbury

Ringwood

Tidworth

Bridport

Westbury

Salisbury

Gillingham

Warminster

Shaftesbury

Christchurch

Broad Chalke

Fordingbridge

Wimborne Minster

Sturminster Newton



1

23

4 5

67

8

Legend

Main rivers

Urban areas

Preferred approach

Policy 1

Policy 2

Policy 3

Policy 4

Policy 5

Policy 6

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2 Upper Avon and Wylye

3 Warminster

4 River Nadder

5 Salisbury

6 Lower Avon

7 New Forest Streams

8 Christchurch

The sub-areas

GESW0612BWPR-E-E

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