Flood Risk Assessment in West Sussex

Unda prepares clear, technically robust flood risk assessments and drainage strategies for residential and commercial sites across West Sussex. A flood risk assessment in West Sussex shows the local planning authority, and the Environment Agency where it is involved, that a development will stay safe from flooding for its whole lifetime and will not push flood risk onto anyone else. We have prepared assessments the length of the county — from the tidal Arun valley and the chalk villages north of Chichester to the surface-water corridors around Crawley and Gatwick — and we know how the local authorities here expect flood risk to be evidenced.

West Sussex is an unusually varied county for flooding. The Arun, Adur, Western Rother, Lavant and Ems drain the Weald and the Downs to a low, defended coast at Bognor Regis, Littlehampton, Shoreham and Selsey; the chalk of the South Downs brings groundwater flooding that no river map shows; and the urbanised north around Crawley and Gatwick sheds surface water fast. We assess every source that matters on a given site, then deal directly with the Environment Agency and West Sussex County Council as Lead Local Flood Authority on your behalf. We prepare flood risk assessments in West Sussex for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS) — guiding developers, planning consultants, architects, landowners, home buyers and businesses through to a decision.

A flood risk assessment, a sequential or exception test and a drainage strategy are three different reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go.

Flood risk assessment in West Sussex at a glance

• Five rivers and a defended coast. The Arun, Adur, Western Rother, Lavant and Ems drain to a low-lying shoreline at Bognor Regis, Littlehampton, Shoreham, Lancing and Selsey, so sites here can face river, tidal and coastal risk at once.
• Chalk groundwater is a West Sussex signature. The 1994 Chichester and Lavant flood — the worst in the area for 130 years — was driven by groundwater rising through the chalk, a source no national river map captures.
• Surface water dominates the north. The December 2000 Crawley and Gatwick floods, in the wettest autumn for nearly 300 years, put more than 100 Crawley homes and parts of Gatwick Airport under water.
• One Lead Local Flood Authority. West Sussex County Council leads on surface water, groundwater and ordinary watercourses, and is the statutory consultee on drainage for major development across the county.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, mapped surface water risk can trigger an assessment and a sequential test even on a Flood Zone 1 site.

Check the flood risk anywhere in West Sussex

West Sussex is outlined in coral below. Screen any postcode in the county against Environment Agency data for rivers and sea, surface water, reservoirs and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass and a useful starting point — not a substitute for a site-specific flood risk assessment.

Flood risk across West Sussex: rivers, coast, surface water and chalk groundwater

No single source defines flood risk in West Sussex. The county runs from the High Weald, across the chalk of the South Downs, to a low and heavily defended coast, and each band brings its own flooding. A credible flood risk assessment in West Sussex has to identify which source actually drives the risk on your site — river, tidal, surface water or groundwater — because the mitigation is different for each.

The river system is dominated by the Arun, which is tidal a remarkable distance inland — to Pallingham, some 40km from the sea at Littlehampton — so Arundel, Pulborough, Amberley and Houghton can flood on a high tide as readily as on a high river. The Adur floods the floor of its valley around Shoreham and Lancing, the Western Rother winds through Midhurst and Petworth to join the Arun, and the Ems affects Westbourne and Emsworth in the far west. On the coast, Selsey, Pagham, Climping, Bognor Regis, Littlehampton, Worthing, Shoreham and Lancing all sit behind sea defences, and tidal locking — where a river cannot discharge against a high tide — is a recurring theme. Inland, the chalk of the South Downs produces groundwater flooding in the villages north of Chichester, while the urbanised north around Crawley and Gatwick is dominated by surface water.

Because flood zones only map rivers and the sea, a West Sussex site can sit firmly in Flood Zone 1 and still be at real risk from surface water or chalk groundwater.

A history of flooding in West Sussex

Dated local flood events are the clearest test of whether risk has really been understood on a site. West Sussex has three that every assessment in the county should have in view — a chalk groundwater flood, an urban surface-water flood and a coastal defence failure — because between them they show how differently water behaves across the county.

• January 1994 — Chichester and the Lavant. Sustained rain on already-saturated chalk sent groundwater levels surging; the River Lavant exceeded its capacity and flooded Chichester and surrounding villages in the worst event for 130 years, closing the A27 and A259. Military "Green Goddess" pumps ran for weeks, and the damage was put at around £6 million.
• December 2000 — Crawley and Gatwick. During the wettest autumn for nearly 300 years, more than 150mm of rain fell on the Upper Mole catchment in 72 hours. The Mole and Gatwick Stream overtopped, flooding around 100 homes in Crawley and parts of Gatwick Airport. Unda has written about the 2000 Crawley and Gatwick floods in detail.
• February 2020 — Storm Ciara at Climping. The sea defences at Climping, west of Littlehampton, were severely damaged, causing overtopping and flooding of farmland — a reminder that a defended coast is not a risk-free coast.

These events shape how the Environment Agency and West Sussex County Council read new applications today. A scheme near the tidal Arun is expected to address tidal locking and lifetime sea-level rise; a site on the chalk has to derive a credible winter groundwater level rather than a single dry-season reading; and a development in the Gatwick corridor has to show surface water can be managed without repeating 2000. Getting this right at the first attempt matters more than ever, because the May 2026 Flood Map for Planning update added surface-water climate-change data that pulls some previously unflagged sites into a mapped risk area.

Planning and drainage policy in West Sussex

West Sussex County Council is the Lead Local Flood Authority for the whole county. It leads on surface water, groundwater and ordinary watercourses, and is the statutory consultee on surface water drainage for major development. Planning decisions, though, are taken by the seven local planning authorities, each with its own Local Plan flood policies and Strategic Flood Risk Assessment.

The county's Local Flood Risk Management Strategy (2025–2030) takes a catchment-based, source-to-sea approach and identifies 25 priority areas most at risk from surface water, which it works through over the life of the strategy. West Sussex County Council also publishes sustainable drainage guidance for developers, built around the surface-water hierarchy and the 2025 national standards for SuDS. On the ground, the policy you must satisfy depends on which district your site sits in.

• The Lead Local Flood Authority is county-wide. West Sussex County Council reviews surface water drainage on major applications across every district, and an unresolved drainage objection can hold up a decision for months.
• The planning authority is local. Arun, Chichester, Crawley, Horsham, Mid Sussex, and Adur & Worthing each determine applications against their own Local Plan; the South Downs National Park Authority is the planning authority for land within the park.
• Each district has its own SFRA. The Strategic Flood Risk Assessment for your district sets the local flood-risk evidence base and is the first document a reviewer reaches for — including for critical drainage areas in Flood Zone 1.
• Infiltration evidence is expected. Assumed soakaway rates without BRE 365 testing no longer pass, particularly on the clay of the Weald and the high water tables of the coastal plain.

Why sites in West Sussex need a flood risk assessment

Development in West Sussex tends to fall into a few recognisable patterns, and each raises its own flood risk and drainage questions. Knowing which one applies to your site is the quickest route to a proportionate assessment and a smooth application.

• Coastal and tidal infill. Constrained urban plots at Bognor Regis, Littlehampton, Worthing and Shoreham often sit in Flood Zone 1 for rivers yet carry tidal, surface-water or safe-access constraints that a sequential and exception test has to address.
• Prior-approval conversions. Changing offices or agricultural buildings to homes raises the vulnerability of the use, so even a low-lying Flood Zone 1 building can need raised floor levels and a safe escape route to be approved.
• Chalk-downland and groundwater sites. North of Chichester and across the Downs fringe, the binding constraint is often a winter groundwater level that defeats infiltration drainage — which is why groundwater monitoring has to be planned early.
• The Gatwick and Crawley corridor. High-density and commercial development on the urbanised Mole catchment is a surface-water and drainage problem first, where attenuation and discharge controls decide the layout.
• Rural and agricultural schemes. Conversions and new builds across the Weald frequently turn on ordinary watercourse consents, riparian responsibilities and off-mains drainage as much as on the river itself.

How Unda helps with flood risk in West Sussex

Unda are experts on West Sussex flooding and planning, not a local presence to drop in — we are the accurate translation layer between your scheme and the West Sussex County Council, Environment Agency and NPPF requirements it has to satisfy. A Senior Flood Risk Consultant oversees and signs off every report, and we stay involved past the report to answer consultee objections and keep the application moving.

One recent West Sussex example shows the pattern. A developer was seeking prior approval to convert a Class E commercial building to homes on a site that screened as Flood Zone 1, where the real constraint was surface water. Our flood risk assessment confirmed a maximum modelled surface-water depth of 0.3m, and we worked with the design team to raise finished floor levels to 8.54m AOD — at least 600mm of freeboard — and to introduce a floodable walkway giving a safe route to flood-free ground. The scheme was revised from four flats to two duplex units so occupants had internal access to a higher floor, and prior approval was secured.

We have resolved similar constraints across the county — from a surface-water-affected residential infill in a coastal West Sussex town, where the safe escape route was the deciding issue, to a property purchase where careful interpretation of surface-water mapping let the buyer secure insurance on normal terms.

Flood risk assessments for every situation in West Sussex

Whatever the reason you need flood risk or drainage advice in West Sussex, Unda covers it. Each report below answers a different question, and we will tell you honestly which ones your situation calls for.

• Flood risk assessment for planning. The core report for a planning application, demonstrating a site is safe for its lifetime and does not increase risk elsewhere — see FRA for planning.
• Flood risk survey for property purchase. A property-specific survey for buyers and their advisers when a conveyancing search flags risk — our homebuyer flood risk survey.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover or premiums are in question — an insurance flood risk assessment.
• Flood risk assessment for BREEAM. An assessment structured around BREEAM Pol 03 for project teams pursuing certification — FRA for BREEAM.
• Sequential and exception tests. The locational case for development in or near a flood risk area — sequential and exception test reports.
• Surface water drainage strategy (SuDS). How runoff is managed to meet the surface-water hierarchy and West Sussex County Council's expectations — a drainage strategy and SuDS design, with discharge of drainage conditions and preliminary drainage design where needed.
• Groundwater and flood modelling. A groundwater flood risk assessment for chalk and high-water-table sites, and hydraulic flood modelling where national mapping is not enough — both feeding the wider assessment.

You can get an early read on a site for free using Unda's Flood Risk Map, the Live River Levels Map for the Arun, Adur and Rother gauges, the geology and ground conditions checker, and the drainage calculators — then talk to our flood risk consultants when you need a report.

Flood risk management and emergency contacts in West Sussex

If you need to report flooding, prepare for it, or understand who is responsible for what in West Sussex, these are the bodies that matter. In a flood that threatens life, always call 999.

Flood risk assessment in West Sussex: common questions