Flood Risk Assessment in West Sussex

A flood risk assessment in West Sussex has to start by asking which way the water leaves the site. South of the Weald, the Arun, the Western Rother, the Adur and the Lavant cross the South Downs to a flat coastal plain whose outfalls close at every high tide. North of it, Crawley, Gatwick and East Grinstead drain away from the sea, into the Thames basin. We prepare flood risk assessments across the county for planning, property purchase, mortgage and insurance, alongside sequential and exception tests and surface water drainage strategies.

West Sussex County Council's area takes in Adur, Arun, Chichester, Crawley, Horsham, Mid Sussex and Worthing, with the part of the South Downs National Park that lies inside them. Eight planning authorities decide applications here and the county council reviews the drainage, so the same scheme can meet different rules at Bognor Regis, Shoreham-by-Sea or Burgess Hill. The town pages, linked further down, go into each place in more detail.

Between 10 and 12 June 2012 the Bognor Regis gauge recorded 114 mm of rain, its highest 24-hour total since records there began in 1974. With the rifes and sea outfalls tide-locked, about 780 properties were affected across West Sussex, 367 of them around Bognor Regis.

To talk a West Sussex site through with one of our consultants, call 01293 214444 or email enquiries@unda.co.uk.

Which way does rain leave a West Sussex site?

Most of the county drains south. The Arun and its tributary the Western Rother, the Adur, the Lavant and the Ems run off the Weald and the chalk to the Channel, and between them a coastal plain is drained by short channels called rifes. The north-east corner around Crawley, Gatwick and East Grinstead drains north instead, to the Mole and the Medway. That split decides the river, the Environment Agency area and the funding committee a site deals with.

Each of the main rivers has its own page: the River Arun, the Western Rother, the River Adur, the River Mole and the Gatwick Stream. The gauges on all of them feed our live river levels map, which shows current levels and any warnings in force.

Screening a West Sussex postcode against the flood zones

Enter a postcode below for the Environment Agency's flood zones, surface water and reservoir layers at that point, or open our full flood risk map for anywhere in England. Anyone looking for a West Sussex flood map will find this the quickest first check. It is only a screen: it cannot see a floor level, a rife that backs up on a high tide, or the winter groundwater level that decides whether a soakaway will work.

A site in Flood Zone 1 is outside the river and sea flood zones, nothing more, and surface water, groundwater and tide-locked outfalls are not part of the zones at all. Our guide to flood zones in England explains what each zone means.

Seven councils decide, and the county reviews the drainage

Planning applications in West Sussex are decided by the district or borough council, or by the South Downs National Park Authority inside the park. West Sussex County Council is the Lead Local Flood Authority for all seven districts and is consulted on surface water for every major scheme. Adur and Worthing share one planning service but keep separate local plans, so a report can be read against eight sets of policy, each backed by its own Strategic Flood Risk Assessment.

Two districts check even small schemes themselves. Arun's drainage engineers review minor and major applications against checklists the council published in October 2025. Mid Sussex has its own flood risk and drainage team that does the same and treats high and medium surface water risk as the equivalent of Flood Zone 3. Our council pages go deeper on Adur and Worthing, Arun, Crawley, Horsham and Mid Sussex; for Chichester District, the Chichester page covers the plan and its SFRA.

West Sussex County Council's August 2026 drainage standard

On major development, the county reviews surface water drainage in West Sussex against the 2025 national standards for sustainable drainage, and in August 2026 it published guidance on how it reads them. Its list for an outline application includes evidence that many councils leave to a planning condition: winter groundwater results and a decision on whether infiltration will work at all.

• Greenfield discharge. Runoff leaving the site is held to the greenfield rate, even where Southern Water or another sewerage company would accept more.
• The first 5 mm. Everyday rainfall up to 5 mm must be caught and kept on site, and interception storage is still required where the ground will not take water.
• Groundwater before soakaways. Infiltration rates come from BRE 365 testing, three tests per pit with the lowest rate used, and groundwater monitoring must find the highest level; the base of any infiltration feature sits at least 1 m above it.
• Emptied within a day. Storage must drain the 3.33% annual chance storm plus climate change within 24 hours, so it has room for the next one.
• Exceedance mapped. A plan for the 1% annual chance storm plus climate change shows no flooding off site, none to buildings or critical infrastructure, and a safe way in and out.
• No space is no excuse. The county says it will not accept high density housing or lack of space as reasons to leave out source control SuDS.

Before submitting, a developer can buy the county's pre-application advice on major schemes: at October 2026 a written flood risk report from £208 plus VAT, or a meeting and written confirmation from £894.40 plus VAT, depending on size. The county does not normally comment on minor schemes. Our note on why drainage strategies are rejected covers the commonest grounds for an objection on a major one.

Bognor Regis, the rifes and a coast that locks shut at high tide

Bognor Regis has no large river, and its flood risk comes from three other directions: the sea, heavy rain on a flat coastal plain, and the rifes that carry that rain to outfalls which cannot discharge while the tide is up. Arun District Council's March 2026 SFRA groups Bognor Regis with Aldwick, Bersted, Felpham, Middleton-on-Sea and Pagham, and puts Bognor Regis at significant tidal and coastal risk where the Aldingbourne Rife reaches the sea. Flat land is the problem. When the tide is high, rain has nowhere to go.

• June 2012. Around the town, 140 properties were affected in central Bognor Regis and Bersted, 152 in Middleton and Elmer, 43 in Felpham and 32 in west Bognor Regis. The county's report on the storm found the plain tide-locked twice a day, so the rifes stayed high for days after the rain stopped.
• November 2023. Storm Ciarán fell on ground soaked by the wettest October on record at the Bognor gauge. The county's Section 19 investigation counted about eight homes flooded inside in Bognor Regis and three in Shripney, from rivers, sewers and surface water.
• Two warning areas. The Environment Agency's Bognor warning covers the coast through Felpham, South Bersted, North Bersted and Shripney, and a separate one covers the Aldingbourne and Lidsey rifes at Felpham, including Links Avenue and the A259.
• Arun's own rules. Arun's Local Plan asks for winter groundwater monitoring and winter percolation tests on drainage schemes, and its engineers apply the district's SuDS checklists to every application they review. Inland sites in the Lidsey treatment works catchment need a Lidsey drainage impact assessment.

Bognor Regis has a page of its own, which sets out the town in more detail. Pagham and Littlehampton, either side of it, have theirs, and the district's requirements are set out on our Arun District Council page.

Chalk groundwater from the Lavant valley to the Manhood Peninsula

Groundwater floods where the chalk of the South Downs meets lower ground. After a wet winter the chalk fills, springs break out in the valleys north of Chichester and the Lavant runs high for weeks, while on the coastal plain the water table can sit close to the surface. It is the source most often missed on a Flood Zone 1 site, and the one the county's guidance now asks applicants to measure.

• The Lavant. Over the winter of 1993 to 1994 the river flooded Chichester; the county's 2011 Preliminary Flood Risk Assessment records the source as main river and groundwater together. In 2013 and 2014 high groundwater pushed up the river and leaked into the Lavant valley's foul sewers, and tankers ran for many weeks to keep sewage out of it.
• The upper Ems. The Environment Agency runs a groundwater flood warning for Stoughton and Walderton, and a groundwater alert for Compton, West Marden and Racton.
• The coastal plain. The 2011 assessment names large parts of the Manhood Peninsula as susceptible to groundwater flooding, and Arun's 2026 SFRA maps groundwater within 0.5 m of the surface across most of the strip from Climping to Ferring, Rustington and East Preston included.
• Midhurst and Petworth. The National Park's January 2025 SFRA names Midhurst as a river and groundwater risk area and records 132 sewer flooding incidents there and 42 in Petworth.

On chalk or on the coastal plain, monitoring comes first. Arun's plan asks for up to six months of it within the winter period, and the county wants the highest level found, so a scheme that misses one winter waits for the next before its soakaways can be put forward. Our guides to groundwater monitoring for planning and BRE 365 infiltration testing explain the programme, and the ground conditions checker gives a first read of the geology under a postcode.

North of the watershed: Crawley, Gatwick and East Grinstead

The part of the county that drains to the Thames is its most built-up. Crawley lies in the upper reaches of the River Mole on Weald clay that sheds rain fast, and the borough's own plan says it is at the highest risk of surface water flooding in West Sussex. East Grinstead drains to the Medway headwaters. Neither is in the Environment Agency's Solent and South Downs area. Both fall in its Kent, South London and East Sussex area. The Thames Regional Flood and Coastal Committee has also helped pay for work on the Mole.

• Autumn 2000. The Mole and its tributaries affected 109 homes in Crawley, and our account of the 2000 Crawley and Gatwick floods follows the event through.
• Storage upstream. The Upper Mole scheme built storage at Tilgate Lake, Worth Farm and Clay's Lake to reduce risk to more than 1,400 homes and businesses in Crawley and Horley, and to the airport. The Ifield Brook and Tilgate Brook pages cover the streams it controls.
• A different sewer company. Thames Water, not Southern Water, runs Crawley's sewers, and Crawley's plan sends surface water to the ground, a watercourse or a surface water sewer, never to the foul sewer.
• Water neutrality. Crawley and Horsham sat in the Sussex North water supply zone, where from 2021 new homes had to show they would not add to water demand. Natural England withdrew that position on 31 October 2025, and Horsham District Council reports that the Environment Agency formally agreed the reduced Hardham abstraction licence on 9 October 2026. That was a water supply rule. It never concerned flooding, and our guide to water neutrality covers what remains of it.

The Crawley and East Grinstead pages go further into both towns, and Horsham, on the upper Arun just over the watershed, has its own page.

Floods recorded across West Sussex since 1968

West Sussex flooding comes from river floods in the Weald, surface water in the towns and the tide along the coast, but the three largest counts of flooded properties come from storms in June and September, not from winter river floods. Each entry below comes from West Sussex County Council's own reports.

1. September 1968. After the flood of 14 and 15 September Gatwick closed for several days. On 20 September, 422 properties flooded at locations across the county, up to 50 of them in Horsham by the district's 2010 SFRA.
2. September and October 1980. Surface water flooded 488 properties in Durrington, Worthing, and 92 in North Ferring on 10 October.
3. December 1993 to January 1994. A blocked culvert flooded 98 properties in Barnham on 30 December, and the Lavant then flooded Chichester.
4. Autumn 2000. Surface water reached about 20 commercial premises in Worthing town centre in October, and in December the rivers flooded around Chichester and in the Ems and Rother valleys.
5. 25 June 2007. Flooding in Worthing led to more than 297 emergency calls and reached the hospital.
6. 10 to 12 June 2012. About 780 properties were affected, including 85 on South Terrace in Littlehampton, 36 in East Preston, 110 on the Manhood Peninsula and 28 in west Chichester and Fishbourne.
7. 1 and 2 November 2023. Storm Ciarán flooded about 11 homes inside in Bognor Regis and Shripney.
8. 8 and 9 April 2024. During Storm Kathleen the sea overtopped the defences at Medmerry Cove Seaside Park, flooding about 233 of its 268 holiday homes. In Littlehampton the River Arun overtopped its west bank at Rope Walk, reaching about 20 homes to around 500 mm and 16 businesses.

The flood zones show none of this. The 1968 to 2007 events come from the county's 2011 Preliminary Flood Risk Assessment, the June 2012 figures from its report on that storm, and the 2023 and 2024 floods from its Section 19 investigations. Those documents, not the national map, are where a site's own history turns up.

Does a flood risk assessment in West Sussex apply to my scheme?

Under chapter 18 of the NPPF, an assessment is needed in Flood Zones 2 and 3, on sites of a hectare or more in Flood Zone 1, and on any site the Environment Agency's flood map for planning or the district SFRA shows at risk from another source, surface water included. Several West Sussex councils add triggers of their own, and those catch smaller schemes than the national rules do.

A site whose only risk is surface water may not need the sequential test if the assessment shows the development will be safe; our note on the surface water exemption explains how that works under policy F5.

Our reports for West Sussex sites, from Selsey Bill to the Surrey border

Which report a site needs depends on what floods it and who will read the result: a case officer, a lender or an insurer. Our consultants are chartered, Unda is a CIWEM Business Partner, and every report is signed off by a senior consultant. We prepare each of these anywhere in the county:

• Every source in one report. Rivers, tides, rifes, chalk groundwater and Crawley's clay can all reach the same plot, and a flood risk assessment weighs them together.
• Planning applications. From a Worthing change of use to a Horsham allocation, a flood risk assessment for planning answers the district's flood policy and chapter 18 of the NPPF.
• Buying a home. Homes in the Pulborough, Felpham and Shoreham warning areas can be flagged by a search or a lender, and a flood risk survey for property purchase explains what the flag means before exchange.
• Showing there is no safer site. Where a scheme in Flood Zone 2 or 3 must show no lower-risk site is available, a sequential and exception test report sets the area of search and tests the alternatives.
• Drainage for planning. Each district and the county read drainage evidence differently, and a drainage strategy is written to the standard that applies to the site.
• Major schemes. A surface water drainage strategy shows greenfield discharge, the first 5 mm kept on site and the winter groundwater level the county asks for.
• After approval. Where consent came with a drainage condition, discharge of drainage conditions means answering the county's requirements or Arun's verification checklist.
• Insurance. With over 65,000 properties in the county's surface water risk areas, an owner facing a higher premium can use a flood risk assessment for insurance.
• Commercial schemes. At Manor Royal in Crawley, where Crawter's Brook runs through the business district, a BREEAM flood risk assessment supports the Pol 03 credits.
• Holiday parks and coastal land. After 233 holiday homes flooded at Medmerry Cove in April 2024, buyers and lenders on the Manhood Peninsula have reason to commission flood risk due diligence.
• Businesses on the water. Sixteen businesses on Rope Walk in Littlehampton flooded in April 2024, and a business continuity flood risk assessment plans for the next tide.
• Foul water. Where groundwater gets into the sewers, as it did around Bognor Regis in 2023, or Arun asks about the Lidsey works, a foul drainage strategy shows where the waste water will go.
• Chalk and the coastal plain. On the chalk north of Chichester or the high water table of the Manhood Peninsula, a groundwater flood risk assessment sets the winter level that a soakaway and a floor level both have to clear.
• Tidal rivers. On the Arun below Pallingham or behind the Shoreham walls, flood modelling gives site levels that national mapping cannot.
• Safe access. Worthing's DM20 and Arun's W DM2 both expect a flood warning and evacuation plan where getting in and out depends on warnings.
• Land and allocations. Before committing to land such as Arun's West Bank allocation at Littlehampton, planned for at least 1,000 homes, a flood risk feasibility study tests what can be built where.
• National policy. Our NPPF flood risk guidance sets out chapter 18 and the tests a West Sussex case officer will apply.

Two recent West Sussex jobs show how a Flood Zone 1 site can still turn on flood risk: a prior approval conversion where surface water forced a redesign, and a single-storey infill in a coastal town that needed an assessment, a drainage strategy and a sequential test together.

Town and river pages within West Sussex

For a site in one of these places, the town page goes further into its streets, its flood record and its council's policies:

• The coast. Selsey behind the Medmerry defences, Pagham beside its harbour, Littlehampton at the mouth of the Arun and Worthing on its long seafront.
• The Arun and the Lavant. Arundel on the tidal Arun below the Downs, and Chichester on the Lavant.
• The Weald and Mid Sussex. Horsham on the upper Arun, Crawley on the Mole, and Haywards Heath, Hassocks and East Grinstead in Mid Sussex.
• Smaller streams. Parsons Brook on the western Adur and the River Lox on the Surrey border.
• Beyond the county. Sussex as a whole, East Sussex, Brighton and Hove, Hampshire and Surrey, or every place on our areas list.

Reporting and warnings: the West Sussex flood contacts

If anyone's life is in danger, call 999, and for a power cut call 105. For anything else, the right body depends on where the water is coming from.

The county's own documents, including the Local Flood Risk Management Strategy 2025 to 2030, set out the county's priorities for the next five years.

Questions we are asked about West Sussex sites

Our consultants assess flood risk, run sequential and exception tests and write drainage strategies for sites anywhere in West Sussex, from Bosham to East Grinstead. To discuss one, call 01293 214444 or email enquiries@unda.co.uk.