These are 25 West Sussex priority areas most at risk of surface water flooding

Views on a new action plan to combat flood risk across West Sussex are being sought.

Wednesday, 18th August 2021, 5:05 pm
The 25 priorty areas across West Sussex most at risk of surface water flooding

A public consultation is seeking opinions on the new Local Flood Risk Management Strategy - a long-term plan on how the county council makes communities more resilient to localised flooding primarily from surface water and groundwater sources.

It is the second cycle of the strategy drawn up for the county since West Sussex County Council became the lead authority for flooding in 2010.

The document sets out how WSCC works alongside the Environment Agency, water companies and district and borough councils to reduce flood risk and what each other’s roles and responsibilities are.

It guides the authority and its partners on how to prioritise where money should be invested to minimise flood risks for local communities.

25 priority areas

The county council says the strategy will help achieve its goal of making communities in West Sussex better prepared and more resilient to extreme weather and identifies 25 priority areas most at-risk of flooding.

These were identified using the risk of flooding from surface water dataset and over the five-year period of the strategy these areas will be prioritised for flood risk reduction of investigation work.

During the five year period of this strategy, 2021 to 2026, it is anticipated that five priority areas will be considered each year. For each area, flood risk management activities will be progressed and include supporting communities to adapt and become more resilient to flood risk, collaborating with other risk management authorities and community groups, seeking opportunities to maximise benefits and funding, enhancing and developing the evidence base and developing sustainable solutions.

Alongside the entirety of the Crawley and Worthing boroughs, the other areas are:

Adur - Lancing, Shoreham & Southwick and Sompting

Arun - Angmering, East Preston, Findon, Littlehampton and Rustington

Chichester district - Chichester city, Easebourne, Linchmere and Westbourne

Horsham district - Billingshurst, Horsham town, Itchingfield, Southwater, Steyning and Washington

Mid Sussex - Burgess Hill, Hassocks, Haywards Heath, Lindfield and Worth.

The strategy does say the prioritisation does not mean funding is available or has been agreed, nor does it exclude important work from happening elsewhere if a good cost benefit or multiple benefits can be achieved.

It also notes there are many isolated properties at risk of flooding in the county that are outside of these priority areas, but the strategy has focused on these because funding for capital projects typically requires a high cost-benefit ration that is generally not met by single residential property projects.

The plan added: “The key focus for the next five years is to carry out improvements to address known local flooding problems. The priority areas, identified by surface water flood mapping and historic flood risk, are to be considered first, but any value for money project with positive benefits, irrespective of its location, will be considered.

“In times of austerity, funding capital works is going to be challenging, especially where projects are required to have some partnership contributions in order to proceed. It may be that in many areas the risk of flooding is managed through early flood warnings and local resilience measures. Local authorities will also help communities take action to help themselves and carry out their own riparian responsibilities.”

More frequent extreme weather events

The strategy comes as parts of the South East have experienced more frequent extreme weather events in recent months as a result of changes in climate, including torrential rain, thunderstorms and localised flash flooding.

Climate projections for the UK indicate that more surface water and flash flooding because of extreme rainfall could be seen in the coming years.

Deborah Urquhart, West Sussex County Council’s cabinet member for environment, said: “Extreme weather like flooding is the reality of climate change. No longer a distant problem for future generations, it is here right now on the doorsteps of our homes.

“And unfortunately, it’s going to happen more frequently. No city, town or village is immune to flooding and we all need to understand the risks where we live.

“This strategy will identify the action we need to take over the coming years to reduce the risk of flooding affecting residents

“Flooding causes immense disruption and this strategy is instrumental in our goal to avoid being overwhelmed by the impact of severe downpours.

“We want as many people as possible to share their views and tell us if there is anything they think we have missed.”

More properties to be at risk of flooding

Based on the risk of flooding from surface water mapping, West Sussex has a total of 20,857 residential properties and 2,384 non-residential properties at risk of flooding during a one in 100-year (one per cent Annual Exceedance Probability) event.

This is predicted to increase to 39,876 residential and 3,953 nonresidential properties as a result of the impact of climate change.

The strategy describes how West Sussex has a history of fluvial, coastal, surface water and groundwater flooding.

County’s wet winter in 2019/20

More recently, flooding occurred a number of times during the 2019/20 winter as repeated storm events occurred in November, December and February. Each one resulting in rainfall falling on ever more saturated ground.

On December 19 and 20, 2019 almost 50mm of rain fell in some areas of West Sussex in a 36 hour period (recorded at a rain gauge in Clapham, near Worthing) on land which had already seen already significant rainfall in the preceding month.

This resulted in the closure, in both directions, of the M23 between junctions 10 and 11 and the disruption of rail travel on the London to Brighton mainline

The River Arun burst its banks and flooded the A29 and local businesses in Pulborough, while a watercourse in Haywards Heath burst its banks flooding America Lane and Hanbury Lane.

Flooding also resulted in road closures in Apuldram, Birdham, Crawley, Henfield, Lindfield, North Mundham, Oving, Pagham and Partridge Green.

Further minor storm events occurred in January 2020, then at the beginning of February 2020, Storm Ciara swept across the UK bringing rain and strong winds, gusting up to 97mph along the south coast.

In West Sussex, the sea defences at Climping Beach were severely damaged causing significant overtopping and flooding a large area of farmland, west of Littlehampton and cutting off access to a number of properties, while further flooding occurred in Pulborough from the River Arun.

Later in the month, Storm Dennis caused widespread flooding across the county, resulting in external property flooding in Mannings Heath and the closure of the A24 southbound near the Hop Oast roundabout and roads in Bognor Regis, Bramber, Burgess Hill, Crawley, Felpham, Hassocks, Haywards Heath, Lindfield, Pagham, Pulborough, Storrington, Three Bridges and Turners Hill. During this event almost 50mm of rain was recorded at Clapham across two days.

These events are currently under investigation by risk management authorities.

The strategy discusses the different types of flood risk ranging from surface water to groundwater, fluvial, coastal and sewer and whereabouts in the county these are most likely to occur.

Surface water flooding

Flooding from surface water runoff is caused by intense short periods of rainfall or storms when the ground is already saturated. In urban areas, surface water will accumulate where it cannot infiltrate impermeable surfaces or be collected and conveyed within existing drainage systems.

Surface water flooding is also linked to issues of poor drainage, or drainage blocked by debris, and sewer flooding.

In rural areas, downland run-off due to the landscape and topography of the South Downs can also cause flooding.

Significant surface water flood risk is generally confined to the north and east of the county, away from the highly permeable South Downs. The exceptions to this are the towns of Arundel and Chichester, which have been identified as higher risk.

A high risk of surface water flooding has been identified within the urban areas of Crawley, Chichester, Shoreham, Worthing, Horsham, Haywards Heath, Burgess Hill and East Grinstead.

Groundwater flooding

Groundwater flooding is associated with unusually high groundwater levels and is primarily caused by rising water levels in permeable aquifers following prolongued rainfall or from high water levels in nearby rivers.

Flood risk from groundwater is highest in areas where the permeable chalk bedrock meets less permeable clay, resulting in springs forming along the boundary. These areas are in the proximity of the South Downs affecting the towns and surrounding areas of Chichester and Arundel in particular.

Areas along the low-lying coastal plain are also found to be at risk. The town of Shoreham is identified to be at high risk where chalk deposits are present. Interaction between tidal levels and groundwater is known to occur here.

West Sussex County Council is in the process of installing boreholes across the county to gain a better understanding of the flood risk from groundwater within the county.

Fluvial flooding

Main rivers are designated by the Environment Agency and are watercourses that carry significant flood risk. Heavy rainfall can cause rivers to overtop their banks and spill on to the adjacent floodplain.

The major river catchments in West Sussex are the River Arun, the River Adur, part of the Upper Mole catchment and a number of smaller watercourses that make up the West Sussex Rifes. The River Arun and the River Adur both flow south and discharge into the English Channel at Littlehampton and Shoreham, respectively. The River Mole flows north through Crawley and discharges into the Thames.

Fluvial flooding is shown to impact a number of areas within West Sussex: between Selsey Bill and Pagham Harbour there are numerous small rifes which present a high risk of flooding to settlements in this area, Bramber has been identified as being at particular risk form the Adur, Arundel is at risk from the River Arun, Bognor Regis and Felpham have been identified as being at risk from the Aldingbourne Rife, Barnham is at risk from the Barnham Rife and Loxwood is at risk from the River Lox.

Coastal/tidal flooding

Tidal flood risk is caused by extreme tide levels exceeding ground and/or defence levels.

Estuaries are at particular risk due to tidal locking where rivers and sea meet. The settlements of Shoreham, Arundel, Littlehampton and Sidlesham are particularly impacted by tidal locking where fluvial interactions occur.

Wave overtopping occurs when the height of the waves exceeds the height of coastal defence and water flows over the top of the defence. In West Sussex, defences have been constructed in the urban areas of Selsey, Shoreham, Worthing, Littlehampton, Bognor Regis, Lancing, Felpham and Elmer.

Meanwhile coastal flood risk is characterised as where the sea level exceeds the elevation of the land. This often occurs where waves have not built up a natural barrier such as a dune system or shingle beaches. Coastal flooding is also linked to the stability of the coastline. Where the coast is eroding, flood risk will often increase. The parishes of Climping and Pagham are particularly vulnerable to this.

Within West Sussex there are two shoreline management plans and the Environment Agency has produced seven flood risk management strategies for the county’s coastline which sets out current and future flood risk and investment plans for the coast to manage tidal flooding.

Finally, the EA has also produced two beach management plans for the West Sussex coastline which set out the approaches for intervention and monitoring to maintain the beach where it provides an integral part of the sea defences.

Flooding from sewers

Sewer flooding occurs when intense rainfall overloads the sewer system capacity and/or when sewers cannot discharge properly to watercourses due to high water levels.

Interactions with high groundwater levels can also result in a lack of capacity in sewers. Thames and Southern Water are responsible for managing sewers and flooding from sewers within West Sussex.

Areas previously known to have experienced regular sewer flooding are Worthing and Durrington, the Manhood Peninsula, Barnham, North Lancing, Littlehampton, Shoreham and Burgess Hill.

The deadline for the public to respond to the public consultation is Wednesday September 30.

To read the West Sussex Local Flood Risk Management Strategy visit www.westsussex.gov.uk/flood-risk