Hampshire Avon Catchment Flood Management Plan Summary Report June 2012 Managing Flood Risk We Are the Environment Agency

Home , Alderbury, Enford, Highcliffe, Netheravon, New Forest, River Ebble

Hampshire Avon Catchment Flood Management Plan Summary Report June 2012 managing flood risk We are the Environment Agency. It’s our job to look after your environment and make it a better place – for you, and for future generations. Your environment is the air you breathe, the water you drink and the ground you walk on. Working with business, Government and society as a whole, we are making your environment cleaner and healthier. The Environment Agency. Out there, making your environment a better place.

Published by:

Environment Agency Manley House Kestrel Way Exeter EX2 7LQ Tel: 0870 8506506 Email: [email protected] www.environment-agency.gov.uk

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 for The Hampshire Avon catchment has a history of flood England and Wales. Through the CFMPs, we have risk. Over the last 30 years numerous engineering assessed inland flood risk across all of England and schemes have been implemented to reduce flood risk Wales for the first time. The CFMP considers all types of in the catchment. At present 5,450 properties are at inland flooding, from rivers, ground water, surface risk in the catchment in a 1% event. This is expected to water and tidal flooding, but not flooding directly from increase to over 6,800 properties in the future. the sea (coastal flooding), which is covered by We cannot reduce flood risk on our own, we will Shoreline Management Plans (SMPs). Our coverage of therefore work closely with all our partners to improve surface and ground water is however limited due to a the co-ordination of flood risk activities and agree the lack of available information. most effective way to manage flood risk in the future. The role of CFMPs is to establish flood risk management We have worked with others including: Dorset County policies which will deliver sustainable flood risk Council, Salisbury District Council, Christchurch management for the long term. This is essential if we Borough Council, West Wiltshire District Council, Kennet are to make the right investment decisions for the District Council, New Forest District Council, and Natural future and to help prepare ourselves effectively for the England to develop this plan. impact of climate change. We will use CFMPs to help us This is a summary of the main CFMP document, if you target our limited resources where the risks are need to see the full document an electronic version can greatest. be obtained by emailing This CFMP identifies flood risk management policies to [email protected] assist all key decision makers in the catchment. It was or alternatively paper copies can be viewed at any of produced through a wide consultation and appraisal our offices in South West Region. process, however it is only the first step towards an integrated approach to Flood Risk Management. As we all work together to achieve our objectives, we must monitor and listen to each others progress, discuss what has been achieved and consider where we may need to review parts of the CFMP. Richard Cresswell South West Regional Director

Environment Agency Hampshire Avon Catchment Flood Management Plan 1 Contents

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

2 Environment Agency Hampshire Avon Catchment Flood Management Plan The purpose of a CFMP in managing flood risk

CFMPs help us to understand the • Internal Drainage Boards (IDB), CFMPs aim to promote more scale and extent of flooding now and water companies and other sustainable approaches to in the future, and set policies for utilities to help plan their managing flood risk. The policies managing flood risk within the activities in the wider context of identified in the CFMP will be catchment. CFMPs should be used to the catchment; delivered through a combination of inform planning and decision different approaches. Together with • transportation planners; making by key stakeholders such as: our partners, we will implement • land owners, farmers and land these approaches through a range • the Environment Agency, who will managers that manage and of delivery plans, projects and use the plan to guide decisions operate land for agriculture, actions. on investment in further plans, conservation and amenity projects or actions; The relationship between the CFMP, purposes; delivery plans, strategies, projects • Regional Assemblies and local • the public and businesses to and actions is shown in Figure 1. authorities who can use the plan enhance their understanding of to inform spatial planning flood risk and how it will be activities and emergency managed. planning;

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

Policy planning • CFMPs and Shoreline Management Plans. • Action plans define requirement for delivery plans, projects and actions.

Policy delivery plans (see note) Projects and actions • Influence spatial planning to reduce risk and • Make sure our spending delivers the best restore floodplains. possible outcomes. • Prepare for and manage floods (including local • Focus on risk based targets, for example numbers Flood Warning plans). of households at risk. • Managing assets. • Water level management plans. • Land management and habitat creation. Note: Some plans may not be led by us – we may identify the • Surface water management plans. need and encourage their development.

Environment Agency Hampshire Avon Catchment Flood Management Plan 3 Catchment overview

The catchment of the Hampshire Management Plan (SMP) boundary at steep combes and river valleys. The Avon is located in the south of Christchurch. The Poole and lower catchment is characterised by England. Christchurch Bay SMP deals with rolling farmland and the New Forest. coastal flood management, while the The main watercourses have wide The Hampshire Avon rises in the Vale CFMP considers the risk from tidal floodplains and flow through of Pewsey to the north of Salisbury. flooding. farmland, woodland, scrub and open The watercourses here receive heathland. significant flows from the chalk The overall catchment area is about aquifers underlying Salisbury Plain, 1,750 square kilometres, and has a Within the Hampshire Avon and then flow in a southerly population of around 230,000. Only catchment there are a number of direction towards Christchurch two per cent of the catchment is sites designated for their Harbour and Christchurch Bay on the urbanised. As well as Salisbury and environmental importance including south coast. Map 1 shows the Christchurch, its main urban areas part of the New Forest National Park location and extent of the River Avon include Warminster. and the World Heritage Site of CFMP area. Stonehenge. Important The Hampshire Avon catchment is environmental sites in the catchment At Salisbury, the Avon is joined by characterised by open chalk include two Areas of Outstanding two of its major tributaries - the River downland with steep scarp slopes, Natural Beauty, two Environmentally Bourne and the River Nadder sheltered valleys, chalk hills, ridges Sensitive Areas, nine Special Areas (including the River Wylye), and a and limestone plateaux. These of Conservation (SAC), six National short distance downstream by the significant variations in the Nature Reserves, 71 Sites of Special River Ebble. topography have a strong influence Scientific Interest (SSSIs) and 1,061 on the rivers’ response to rainfall. The downstream limit of the CFMP Scheduled Monuments. area meets with the upstream The upper Avon catchment is typified boundary of the Poole and by the undulating, chalk downlands Christchurch Bay Shoreline of Salisbury Plain, which are cut by

4 Environment Agency Hampshire Avon Catchment Flood Management Plan Map 1. Location and extent of the Hampshire Avon CFMP area

Devizes Legend

Hampshire Avon Pewsey CFMP Urban areas Tidworth Westbury Main rivers

Warminster Railway Frome Motorway Amesbury

Bruton Mere

Salisbury

Shaftesbury Downton N Fordingbridge Blandford Verwood Forum Ringwood

0 4 8 12 16 Kilometres Christchurch © Crown Copyright. Environment Agency 100026380.

➜ The River Avon spills on to the floodplain close to Salisbury Cathedral

Environment Agency Hampshire Avon Catchment Flood Management Plan 5 Current and future flood risk

Overview of the current flood risk What is at risk?

Flood risk has two components: the Ringwood in December 2000 when At present there are around 10,700 chance (probability) of a particular 132 properties were affected by river people and 5,400 commercial and flood and the impact (or flooding after a period of heavy residential properties at risk in the consequence) that the flood would rainfall on an elevated water table. whole catchment from a 1% annual have if it happened. The probability probability river flood, taking into Currently the main sources of flood of a flood relates to the likelihood of account current flood defences. This risk for people, property, a flood of that size occurring within a means that 1% of the total infrastructure and the land are: one year period. It is expressed as a population living in the catchment percentage. For example, a 1% flood • river flooding from the River Avon are currently at risk from flooding. has a 1% chance or 0.01 probability at Downton, Fordingbridge and It is difficult to assess the current of occurring in any one year, and a Ringwood, from the Avon and impact of flooding to environmental 0.5% flood has a 0.5% chance or Nadder at Salisbury, from the features. Most designated sites at 0.005 probability of occurring in any Nadder and Wylye at Wilton and risk would not actually be damaged one year. The flood risks quoted in the Bourne at Tidworth; by the inundation, although this report are those that take prolonged deep flooding can have a account of flood defences already in • tidal flooding at Christchurch; negative impact on the Avon Valley place. • surface water drainage flooding, Site of Special Scientific Interest. 42 The assessment was based on the which has occurred in Warminster Scheduled Monuments are at risk of use of existing river models, our and Enford. flooding. The actual risk of damage Flood Zone maps and historical from flooding is limited. Groundwater flooding has occurred records. in Netherhampton, and groundwater This catchment has a long history of has added to river flooding flooding. The most significant event downstream. in recent years occurred in Salisbury, Downton, Fordingbridge and

6 Environment Agency Hampshire Avon Catchment Flood Management Plan 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 Warminster 1,001 - 2,000

2,001 - 3,000

Hampshire Avon Salisbury CFMP Main rivers

0 4 8 12 16 Kilometres Christchurch

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

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

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

Environment Agency Hampshire Avon Catchment Flood Management Plan 7 Where is the risk? How we currently manage the risk

Around 40% of the people and The catchment has a history of flood Activities that reduce the probability properties that are at risk within the risk, generally due to the high of flooding include: catchment from a 1% annual rainfall that can lead to extensive • maintaining and improving probability river flood, are located in flooding of the river valleys, and existing flood defences and Salisbury. A further 22% are located prolonged wet periods that can lead structures, including pumping in Christchurch. to groundwater flooding. stations; The distribution of properties at risk Over the last 25 years, engineering • maintaining river channels; from a 1% annual probability river schemes have been implemented to • maintenance of road drainage flood, is illustrated in Map 2. Table 1 reduce flood risk in the catchment, and sewers; summarises where there is flood risk including at Tisbury, Downton, Activities that reduce the to more than 25 properties. We Fordingbridge, Ringwood and consequences of flooding include: recognise that there is also a Christchurch. Various structures potential risk from surface water and have also been constructed around • understanding where flooding is groundwater flooding. However, Salisbury and Wilton. likely by using flood risk mapping; further studies following on from the • providing flood forecasting and These measures have all reduced CFMP are needed by us and our warning services; flood risk. partners to quantify this potential • promoting awareness of flooding risk. In addition to these engineering so that organisations, schemes, other flood risk communities and individuals are management activities are carried aware of the risk and are prepared out in the catchment. These include in case they need to take action in activities which help to reduce the time of flood; probability of flooding and those • promoting resilience and that address the consequences of resistance measures for those flooding. properties already in the floodplain. • working with local authorities to influence the location, layout and design of new and redeveloped property and ensuring that only appropriate development is allowed on the floodplain through the application of Planning Policy Statement 25 (PPS25).

➜ Misty conditions at Picket Post in the New Forest looking across to the Avon Valley. This is an area where opportunities to manage land in a different way could lead to a reduction in flood risk downstream

8 Environment Agency Hampshire Avon Catchment Flood Management Plan The impact of climate change and future flood risk

In the future, flooding will be Using river models we estimate that Figure 2 shows the difference influenced by climate change, by 2100, around 13,250 people and between current and future flood changes in land use (for example 6,800 properties across the risks from a 1% annual probability urban development) and rural land catchment may be at risk from a 1% river flood at key locations in the management. In the Hampshire Avon annual probability flood. Flood risk catchment. Following on from the catchment, climate change will have from rivers increases mainly in CFMP, organisations need to work the greatest impact on flood risk. The Salisbury, but significant increases together to investigate flood risk following future scenario for climate also occur in the towns of Downton, from other sources (e.g. surface change was used in the CFMP: Fordingbridge and Ringwood. water and ground water flooding) in more detail. • 20% increase in peak flow in all The sensitivity testing undertaken watercourses. This will increase showed that the effects on flood risk In general, it is unlikely that the the probability of large-scale flood of land use and land management impact of flooding on environmental events; change are likely to be relatively sites will change significantly in the • a total sea level rise of 500 mm by small, and from urban development future. the year 2100. This will increase very limited, at a catchment-wide the probability of tidal flooding on scale. There are potential effects at a the lower reaches at Christchurch. local scale to be managed. The greatest effect on future flood risk is climate change.

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

3500

3000

2500

2000

1500

1000

500

Number of Properties at Flood Risk at Properties Number of 0 Salisbury Christchurch Warminster

Current Future

Environment Agency Hampshire Avon Catchment Flood Management Plan 9 Future direction for flood risk management

Approaches in each sub-area

We have divided the Hampshire Avon catchment into eight distinct sub-areas which have similar physical characteristics, sources of flooding and level of risk. We have identified the most appropriate approach to managing flood risk for each of the sub-areas and allocated one of six generic flood risk management policies, shown in Table 3.

To select the most appropriate policy, the plan has considered how social, economic and environmental objectives are affected by flood risk management activities under each policy option.

➜ Flooding in The Borough, Downton in December 2000 Map 3. Hampshire Avon sub-areas

Calne Legend Bath Melksham Hampshire Avon CFMP Pewsey Devizes Sub-area

1 River Bourne (Policy 4) Radstock Westbury 2 Upper Avon and Wylye (Policy 6) Frome 3 Tidworth Warminster (Policy 4) Wells 4 River Nadder (Policy 6) Shepton Mallet Warminster 5 Salisbury (Policy 5) 6 Lower Avon (Policy 4) 7 New Forest Streams (Policy 2) Bruton Mere 8 Christchurch (Policy 5) Wilton n Wincanton Salisbury Gillingham Alderbury Broad Chalke 9 Shaftesbury Downton N Sherborne Sturminster Newton Sandleheath Fordingbridge

Pimperne Verwood Blandford Forum Three Legged Cross Ringwood Ashley Heath West Moors ter Wimborne Minster Ferndown Merley Bransgore

Lytchett Matravers Highcliffe Christchurch 0481216Dorchester Kilometres Bournemouth and Poole Wareham © Crown Copyright. Environment Agency 100026380.Wool

10 Environment Agency Hampshire Avon Catchment Flood Management Plan Table 3. Policy options 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 defences if we can use resources to reduce risk where there are more people at higher risk. We would therefore review the 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 of flooding 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 review our approach to managing flood defences and other flood risk management actions, to ensure that we are managing 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 we may 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, but where the risk of flooding is expected to significantly rise in the future. In this case we would need to do more in the future to contain what would otherwise be increasing risk. Taking further action to reduce risk will require further appraisal to assess whether there are socially and environmentally sustainable, technically viable and economically 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 most compelling, for example where there are many people at high risk, or where changes in the environment have already increased risk. Taking further action to reduce risk will require additional appraisal to assess whether there 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 in locations 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 risk locally or more widely in a catchment by storing water or managing run-off. The policy has been applied to an area (where the potential to apply the policy exists), but would only be implemented in specific locations within the area, after more detailed appraisal and consultation.

Environment Agency Hampshire Avon Catchment Flood Management Plan 11 Sub-area 1

River Bourne

Our key partners are: The main future driver of flood risk Proposed actions within the sub-area is considered to to implement the Wiltshire Unitary Authority be climate change, represented as an assumed increase in peak flow of preferred policy Test Valley District Council up to 20%. However, it is acknowledged that the impact of • Work with the at-risk Wessex Water climate change on permeable chalk communities of Collingbourne Natural England catchments is uncertain. Kingston, Collingbourne Ducis, Shipton Bellinger, Newton Tony, Porton and the Winterbournes to The vision and provide guidance and advice on The issues in this reducing risk. sub-area preferred policy • Improve our understanding of groundwater flooding mechanics, Flood risk is concentrated in four Policy Option 4 - we are already to enhance our groundwater large town/villages: Tidworth, managing the flood risk effectively flood information service. Collingbourne Ducis, Shipton but we may need to take further actions to keep pace with climate Bellinger and Newton Tony. Other • Identify hydraulically critical change. risk areas include Collingbourne structures and other pinch points Kingston, Porton and the and produce a programme of This policy will allow present Winterbournes. improvements to maintain actions to control flood risk to be current standards. Significant flooding was recorded in continued (for example flood Tidworth and a number of villages warning) and expanded, and for along the River Bourne, during 2000 future change in flood risk to be and 2003. Much of this flooding monitored such that appropriate was attributed either directly or further actions can be carried out. indirectly to record groundwater Any structural works may be levels, although in certain areas (for concentrated in the higher risk example Shipton Bellinger) flooding urban areas, but an improved was experienced from three sources understanding of the flood simultaneously (groundwater, river mechanisms, resulting risks and and highway drainage). climate change implications will also allow a better response across The current number of properties in the sub-area in relation to people, the 1% annual probability event is properties and other assets. 245. This is expected to increase to 255 in the future 1% annual probability event.

12 Environment Agency Hampshire Avon Catchment Flood Management Plan Sub-area 2

Upper Avon and Wylye

Our key partners are: Netheravon. The number of managed water levels (for example properties at risk is expected to in ditches to support fen and wet Wiltshire Unitary Authority increase to 475 in the future 1% grassland). In the long-term, there is annual probability event. the potential for significant local Natural England improvements, for example an The main future driver of flood risk increase in extent of wetland National Farmers Union within the sub-area is considered to habitats (possible local and be climate change, represented as Land managers national Biodiversity Action Plan an assumed increase in peak flow of habitats / species) adjacent to up to 20%. However, it is Salisbury District Flood Steering designated sites through controlled acknowledged that the impact of Group inundation, river restoration work climate change on permeable chalk and restoration of floodplain Wessex Water catchments is extremely uncertain. meadows. Such works would also be beneficial for wetland bird breeding populations. The issues in this The vision and sub-area preferred policy Proposed actions Flood flows on the Upper Avon, Till Policy option 6 - we will take action and Wylye are dominated by with others to store water or mange to implement the baseflow from the upstream chalk run-off in locations that provide preferred policy aquifers. As such the frequency of overall flood risk reduction or flooding is less than on the lower environmental benefits. • Identify areas where wetland catchment tributaries, but when storage areas may be created to flooding does occur it generally The chosen policy provides give benefit to flood risk results from excessive seasonal improvements downstream in management downstream. rainfall through the autumn and Salisbury and Christchurch. The • Identify sites of critical winter and therefore tends to be implementation of the policy will infrastructure and transport prolonged. Significant flooding has allow present actions to control routes and work to prevent these been recorded in the catchment flood risk to be continued (for sites suffering flooding in the during 1995 and 2000. example flood warning) and for future. future changes in flood risk to be • Develop Action Plans for the at- There is in the order of 400 monitored such that the need for risk villages including Codford St properties currently at flood risk, further actions can be reviewed. Mary, Shrewton and Norton these being located in Pewsey, Bavant, including all sources of Amesbury and Durrington, and also Policy 6 provides vital benefits such flooding. at Codford St Mary, Shrewton and as helping to improve the condition of the Sites of Special Scientific Interest and Special Area of Conservation through increased /

Environment Agency Hampshire Avon Catchment Flood Management Plan 13 Sub-area 3

Warminster

Our key partners are: The flood risk in Warminster is very Proposed actions likely to be extremely sensitive to to implement the Wiltshire Unitary Authority increasing peak flood flows from climate change (and potentially preferred policy Wessex Water future development) with probable severe impacts on major road Promote an integrated urban transport links and vital drainage study including future The issues in infrastructure in the town centre development proposals, and implement improvements. this sub-area where the main bottlenecks to river flows are located.

This sub-area covers the urban area of Warminster on the edge of the The vision and River Wylye floodplain and includes preferred policy the tributaries The Were and Cannimore Stream where they flow Policy Option 4 - we are already through the town. managing the flood risk effectively, Based on records of flooding but we may need to take further incidents we estimate that at least actions to keep pace with climate 55 properties are at flood risk in a change. 1% annual probability event. These This policy supports the need to records indicate that flooding reduce risk in the long term, occurs from watercourses, particularly where they are sensitive groundwater and surface run-off. to future change and likely to The number of properties at risk is incorporate vulnerable sites and expected to increase to 80 in the vital infrastructure. The future 1% annual probability event. implementation of the policy will The main future driver of flood risk allow present actions to manage within Warminster is considered to flood risk to be continued (for be climate change, represented as example flood warning) and an assumed increase in peak flows expanded, and for future changes in to the main rivers of up to 20%. flood risk to be monitored and However, it is acknowledged that reviewed such that appropriate the impact of climate change in further actions can be carried out. urban catchments might also lead to more frequent and intense storms, with the potential to increase the risk of surface water flooding.

14 Environment Agency Hampshire Avon Catchment Flood Management Plan Sub-area 4

River Nadder

Our key partners are: more frequent, in the upper reaches Proposed actions at least, it may be more difficult to to implement the Wiltshire Unitary Authority provide flood warnings with sufficient lead times. Significant preferred policy Test Valley District Council flooding was recorded in Tisbury in 2000 and further widespread Investigate feasibility of creating Wessex Water flooding in late 2002. There is no flood storage on the upper Nadder Natural England evidence of groundwater flooding to the benefit of flood risk causing significant problems. downstream in Salisbury and the However, surface water flooding lower Avon towns, and carry out feasibility studies for The issues in this should be recognised as a potential source of flood risk, particularly in implementation. sub-area Tisbury and Burcombe.

This sub-area covers the floodplain Potentially there is flood risk to of the River Nadder from the source significant major road infrastructure, near the Coombe, to Burcombe at for example, the A30 road, as well as the downstream end, approximately the Salisbury to Yeovil railway line. 3km upstream of the confluence with the River Wylye. The sub-area is largely rural in nature, but contains The vision and the small town of Tisbury and a few preferred policy small villages which straddle the floodplain. We estimate that Policy Option 6 - we will take action approximately 30 properties are at with others to store water or manage flood risk in a 1% annual probability run-off in locations that provide event. The number of properties at overall flood risk reduction or risk is expected to increase to 45 in environmental benefits. the future 1% annual probability event. Strategic flood storage in the catchment could offer significant Flood hydrographs show the benefits towards managing future catchment is more responsive to flood risk in policy units further rainfall than the other catchments downstream (in particular Salisbury upstream of Salisbury. This is largely and Lower Avon). It should be noted due to the geology in the upper that in order to gain downstream reaches of the catchment being benefits, peak flows on the Nadder predominantly impermeable clays. need to be stored and then released As such flooding here is likely to be after the peak from the Avon and Wylye has passed downstream.

Environment Agency Hampshire Avon Catchment Flood Management Plan 15 Sub-area 5

Salisbury

Our key partners are: The existing mechanism of flooding Proposed actions is complicated and is influenced by to implement the Wiltshire Unitary Authority the fact that Salisbury sees four main rivers coming together, leading to a preferred policy Wessex Water number of possibilities with regard to the phasing and combination of • Carry out further studies to each of the peaks. Also, permeable provide complete understanding The issues in this tertiary/alluvial deposits and of risk to Salisbury and Wilton. Model asset operations. Use sub-area anecdotal evidence suggests that floodwater can pass onto and across outcomes from studies and urban the floodplain without first drainage pilots to evaluate This sub-area includes the historic overtopping the river banks (for possible improvements, city of Salisbury and the town of example in the cathedral area). including developing flood Wilton to the west. It covers the warning, and asset operation floodplains of the lower reaches of procedures. the Wylye, Nadder and Bourne, as well as the Lower Avon. Whilst there The vision and • Identify and survey infrastructure are some open river corridors preferred policy at risk and take measures to through the area, some concentrated increase flood resilience. areas of development also lie within Policy Option 5 - we can generally • Use results of studies to the floodplain, particularly around take further action to reduce flood encourage appropriate future the cathedral. risk. development. A number of properties were affected This policy will allow present by flooding during autumn/winter actions to manage flood risk to be 2000 and again in December continued (for example flood 2002/January 2003. Some of the warning) and for existing and future properties affected are recorded to risks to be reduced through the have been flooded by groundwater, implementation of a combination of but our analysis has shown these local and strategic flood risk properties would also be at risk management responses. during a significant flood event on Sustainable options will be the river. At present, we estimate evaluated that up to 2,350 properties are at risk in a 1% annual probability flood event. The number of properties at risk is expected to increase to 3,330 in the future 1% annual probability flood event.

16 Environment Agency Hampshire Avon Catchment Flood Management Plan Sub-area 6

Lower Avon

Our key partners are: events. Schemes were put in place expanded, and for future change in in Downton (2004), Fordingbridge flood risk to be monitored such that Christchurch District Council (2006) and Ringwood (2006), which appropriate further actions can be protect 190 of the most at risk carried out. Any structural works may East Dorset District Council properties in total. be concentrated in the higher risk urban areas, but an improved New Forest District Council In some areas, if fluvial flood risk is understanding of the flood reduced through defences then Wessex Water mechanisms, resulting risks and groundwater flooding issues can climate change implications will also worsen as a result. allow a better response from all The issues in this We estimate that up to 1,200 parties concerned. properties would be at risk in a 1% sub-area annual probability event, affecting in the order of 2,450 people, Proposed actions This sub-area covers the floodplain despite recent flood defence of the Lower Avon along with three schemes being included in our to implement the tributaries; the River Ebble, the analysis. The number of properties preferred policy Ashford Water and the Sweatford at risk is expected to increase to Water. The area is a mix of urban 1,440 in the future 1% annual • Investigate flood risk in and rural, and contains a number of probability flood event. Breamore, Ringwood, Woodgreen towns and villages, including and Rockbourne and urban Downton, Fordingbridge, Ringwood, The main future driver of flood risk drainage flood risk in Britford and Coombe-Bisset, Britford, within this sub-area is climate implement appropriate flood risk Bodenham, Charlton-All-Saints, change. mitigation measures where Breamore, Woodgreen, and Burton. feasible. A number of properties were The vision and • Identify and survey infrastructure affected by flooding in autumn at risk and take measures to 2000 and January 2003. Flooding preferred policy increase flood resilience. occurred in Downton and Fordingbridge in 1995, 1999, 2000 Policy Option 4 - we are already and to a lesser extent, 2003. Some managing the flood risk effectivity, of this flooding is directly but we may need to take further attributable to groundwater, and actions to keep pace with climate fluvial flooding is also widely change. influenced by groundwater levels in The implementation of this policy upstream sub-areas, with flood will allow present actions to manage events often being prolonged flood risk to be continued (for example flood warning) and

Environment Agency Hampshire Avon Catchment Flood Management Plan 17 Sub-area 7

New Forest Streams

Our key partners are: are at risk in a 1% annual Proposed actions probability flood event. The number to implement the Christchurch District Council of properties at risk is expected to increase to 120 in the future 1% preferred policy New Forest District Council annual probability flood event • Extend hydrometric monitoring to New Forest National Park Potentially there is flood risk to improve flood warning for the some significant road infrastructure Natural England New Forest Streams, and use (for example A338 and B3347) and awareness campaigns to increase National Farmers Union to formal and informal amenity the uptake of the flood warning areas of the New Forest National service. Farming and Wildlife Advisory Group Park. • Where river restoration is Land managers planned, investigate options for The vision and maximising potential for reductions in downstream flood The issues in this preferred policy risk, as part of restoring natural floodplains. sub-area Policy Option 2 - We can generally reduce existing flood risk This sub-area covers the floodplain management actions. of eight different streams which flow into the River Avon. More than half The implementation of this policy of the area is within the New Forest will allow any present actions to National Park. manage flood risk to be reviewed and reduced where appropriate and The New Forest streams drain acceptable. However, in areas where relatively rural catchments, with flood risk is more concentrated, we semi-natural vegetation. Lower will monitor future change. down in the catchments there are fields and managed landscapes, Indications are that river restoration many of these used for grazing. The works elsewhere in the New Forest highest rates of run-off are likely to National Park may have reduced occur when the soil in the forest peak flows by up to 5% and delayed becomes saturated, and so flooding the time to peak by up to is most likely during the winter approximately seven hours. If similar months. work was promoted in this sub-area, it might provide some minor The most extensive, recent episode benefits downstream, though of flooding recorded was in changes in timing could be December 2000/January 2001, important. affecting at least 10 properties. We estimate that around 100 properties

18 Environment Agency Hampshire Avon Catchment Flood Management Plan Sub-area 8

Christchurch

Our key partners are: The vision and Proposed actions preferred policy to implement the Christchurch District Council preferred policy New Forest District Council Policy Option 5 - We can generally take further action to reduce flood • Investigate flood risk in Wessex Water risk. Christchurch and develop a The implementation of this policy strategy to reduce risk. Ideally will ensure that flood risk is reduced this study would look at the The issues in this and flood risk management combined risk from the Avon, sub-area response can be continued, while Stour, the harbour tributaries and recognising that there is a need to the sea and urban drainage. The Christchurch sub-area presents do more in order to investigate • Investigate impact of sea level a densely populated and very whether existing defences can be rise and increased risk from concentrated area of risk. Many maintained at an appropriate level, storm surge to Christchurch properties along the River Avon, with residual losses mitigated once Harbour. and lower extents of the Mude and defences are overtopped and in Bure Brook, are considered to be at particular that vulnerable sites and • Identify and survey infrastructure risk from a combination of both critical infrastructure are protected. at risk and take measures to tidal and fluvial flooding. Historic increase flood resilience, flooding appears to be limited to including undertaking awareness the months of December, January campaigns. and February, coinciding with the • Use results of studies to highest probability of high flows on encourage appropriate future the Avon (and Stour) and tidal development. surges entering the harbour.

A significant risk within Christchurch is the reliance on raised defences and it has been assumed that as sea levels continue to rise over the next 100 years, together with increasing flood flows, then any overtopping of defences could be very severe and have a major impact on over 1,000 properties.

The number of properties at risk in the 1% probability event is 1,080. The number of properties is expected to increase to 1,120 in the future 1% annual probability flood event.

Environment Agency Hampshire Avon Catchment Flood Management Plan 19 Map of CFMP policies

Map of the policies in the Hampshire Avon catchment LLegendegend

BathBathh DDevizesevizes Hampshire Avon PPewseyewsey CFMP MainMain riversrivers

WestburyWestburryy UrbanUrban areasareas FrFromeomem TidwTidworthorrtth 2 WellsWeells PreferredPreffeerred approachapproach 3 WWarminsterarminster PPolicyolicy 1 1 AmesburyAmesbury PPolicyolicy 2 SStrtreetreetee BrutonBruton PPolicyolicy 3 WWiltonillton PPolicyolicy 4 MereMere 4 5 PPolicyolicy 5 t SaliSalisburysbuurry PPolicyolicy 6 ShShaftesburyaffttesbuurryy GillinghamGGillinghhaam BrBroadoad DoDowntonwntono ChChalkealke 6 N YeovilYeoovil StSturminsterurminmininsterr 7 NNeNewtNewtone on FFoFordingbridgeordiingbridge VVerwooderwood RinRingwoodinngwood

WWimborneimmmbmborrnrne MinMinsterster

8 BridportBridport

004812164 8 12 16 KiKilometreslometres ChriChristchurchstchurch

The sub-areas

1 River Bourne 2 Upper Avon and Wylye 3 Warminster 4 River Nadder 5Salisbury 6 Lower Avon 7 New Forest Streams 8 Christchurch

20 Environment Agency Hampshire Avon Catchment Flood Management Plan Environment Agency Hampshire Avon Catchment Flood Management Plan 21 Would you like to find out more about us, or about your environment?

Then call us on 08708 506 506* (Mon-Fri 8-6) email [email protected] or visit our website www.environment-agency.gov.uk incident hotline 0800 80 70 60 (24hrs) floodline 0845 988 1188

* Approximate call costs: 8p plus 6p per minute (standard landline). Please note charges will vary across telephone providers.

Environment first: Viewing this on-screen? Please consider the environment and only print if absolutely necessary. If you're reading a paper copy, please don't forget to reuse and recycle.

GESW0612BWPR-E-E