Flood Risk Assessment in Sussex

A flood risk assessment in Sussex starts by settling which authority, which river and which stretch of coast a site answers to. The historic county is now split between three lead local flood authorities, thirteen planning authorities and the South Downs National Park, and several of its rivers cross the county line. Our consultants assess sites across Sussex for planning applications, house purchases, lenders and insurers, and prepare sequential and exception tests and surface water drainage strategies alongside them.

This page covers East and West Sussex together with the city of Brighton and Hove: about 220 km of coast from Chichester Harbour to Rye Bay, the South Downs, the Sussex Weald and the BN, RH, TN and PO postcode areas, plus GU28 and GU29 around Petworth and Midhurst. Sussex stopped being a single ceremonial county in 1974, and its flood duties are still divided three ways. The county and town pages linked further down carry each area's own detail.

On 12 October 2000 the Ouse, a river that rises near Lower Beeding in West Sussex, flooded more than 600 homes and 300 businesses in Lewes, in East Sussex.

To talk through a site anywhere in Sussex with one of our experienced flood risk consultants, call 01293 214444 or email enquiries@unda.co.uk.

Which lead local flood authority covers a site in Sussex?

That depends on the district. West Sussex County Council is the lead local flood authority for the seven West Sussex districts, East Sussex County Council for the five East Sussex districts, and Brighton and Hove City Council for the city, where it is also the planning authority. Each comments on surface water, groundwater and ordinary watercourses for major applications, while the district, the city or the National Park decides the application. Our guide to the lead local flood authority explains the role.

Inside the National Park the planning authority changes but the flood authority does not, so a drainage strategy for a site near Midhurst still goes to West Sussex County Council and one near Alfriston to East Sussex County Council. Every district also keeps its own Strategic Flood Risk Assessment, and our guide to what an SFRA contains explains how officers use one.

Screening an address on the Sussex map

The map outlines the whole historic county: the West Sussex and East Sussex council areas and Brighton and Hove together. Enter a postcode to see Environment Agency data for rivers and the sea, surface water, reservoirs and the planning flood zones, the same layers as our flood risk map. It is a first screening; how a site-specific assessment works explains what the council will expect beyond it.

Behind the map, the Environment Agency runs its Sussex flood warnings from three of its operational areas. On 20 October 2026 its flood-monitoring service listed 165 warning and alert areas touching Sussex, and the Solent and South Downs area runs 136 of them. Another 28 belong to the Kent, South London and East Sussex area, which takes in the Rother levels around Rye, Forest Row, the Upper Medway and Eden near East Grinstead and the streams through Crawley. The Thames area runs one, for the Upper Wey.

Floodline on 0345 988 1188 and the flood warning sign-up cover every one of these areas, and our live river levels map shows the readings at the Sussex river gauges in between warnings.

Which Sussex rivers cross into another county?

Five do, and two leave Sussex altogether. The Ouse rises in West Sussex and meets the sea in East Sussex; the Medway and the Mole rise in West Sussex and head north into Kent and Surrey; the Western Rother arrives from Hampshire; and the Eastern Rother becomes the Kent border before it reaches Rye Bay. A site beside one of them can sit under one council while its river is shaped by land in another county.

The split also decides which committee funds the defences. The Mole catchment around Crawley comes under the Thames Regional Flood and Coastal Committee and the rest of Sussex under the Southern committee, and West Sussex County Council sits on both. The Environment Agency's Upper Mole scheme, with storage at Tilgate Lake and Worth Farm, was built to protect more than 1,400 homes and businesses in Crawley and Horley and Gatwick Airport after flooding in 2000 reached 109 homes in Crawley; our account of the 2000 Crawley and Gatwick floods tells that story.

The Arun, the Adur and the Cuckmere stay inside Sussex from source to sea. Their valleys have pages of their own: the Arun, the Adur, the Cuckmere and the Ouse tributary the Uck.

Three shoreline management plans along the Sussex coast

The Sussex coast runs for about 220 km, and three shoreline management plans divide it between them. Each sets a long-term policy for every frontage, such as holding the existing line or managed realignment, and an assessment for a coastal site has to fit the policy for its own stretch. The Environment Agency's shoreline planning service holds all three, and our explainer on shoreline management plans covers what the policies mean for a planning application.

• North Solent (SMP13). From Selsey Bill west along Bracklesham Bay and round Chichester Harbour, Thorney Island and Bosham into Hampshire.
• Beachy Head to Selsey Bill (SMP12). The longest Sussex frontage: Bognor Regis, Littlehampton, Worthing, Shoreham, Brighton and Hove, Newhaven and Seaford, most of it behind sea walls, groynes and shingle.
• South Foreland to Beachy Head (SMP11). Eastbourne, the Pevensey shore, Bexhill, Hastings, Pett Level and Rye Bay, then on into Kent.

Of the 165 Environment Agency warning and alert areas touching Sussex, 57 sit on the sea or a harbour. The policy for the frontage, the defence standard and the warning area together set what a coastal assessment has to address.

Chalk, clay and groundwater under the Downs and the Weald

Groundwater is the source a river map leaves out, and in Sussex it follows the chalk of the South Downs. The Environment Agency runs eight groundwater flood warning and alert areas touching the county: two for the Lavant valley villages from West Dean to Chilgrove, Stoughton and Walderton, the upper Ems valley, Finchdean and Rowlands Castle on the Hampshire border, Denton near Newhaven, and two at Patcham on the northern edge of Brighton. Our explainer on groundwater flooding covers how it behaves.

• East Sussex wards on the chalk. East Sussex County Council's flood strategy places Lewes Bridge, Devonshire in Eastbourne, and Newhaven Denton and Meeching at the highest groundwater risk in the county.
• Brighton and Hove's aquifer. The city's drainage guidance wants the base of any soakaway at least 1 m above the seasonal high water table, and notes that the Environment Agency treats outer source protection zones in the city's chalk as strictly as inner ones.
• The Weald clay. North of the Downs the ground drains poorly. On a site near Hailsham all but one soakage test fell below a workable rate and groundwater was struck 0.73 m below the surface, so storage had to replace infiltration.

East Sussex expects groundwater monitoring through a six-month winter, preferably October to March, and at least 10 m between the base of a deep-bored soakaway and the highest recorded winter groundwater. Infiltration tests to BRE 365 are the usual starting point. Our ground conditions map gives an early view of the geology under a site, and the BRE 365 infiltration rate calculator turns soakage test results into an infiltration rate.

Floods that shaped planning in Sussex, 1960 to 2024

The dates and figures below come from council flood strategies and investigation reports, the National Park's 2025 flood risk assessment and Parliament's debate on the autumn 2000 floods.

1. November 1960. The Ouse floods Lewes, one of the river floods listed in the South Downs National Park's 2025 flood risk assessment.
2. 12 October 2000. The Ouse floods more than 600 homes and 300 businesses in Lewes. Almost 100 properties flood at Robertsbridge on the Eastern Rother, Uckfield's high street goes under, and in Parliament the damage across Sussex is put at about £40 million. Our look back at the Lewes floods follows what changed.
3. 7 to 19 November 2000. Groundwater rising through the chalk at Patcham floods the ground floors of 15 houses, closes the A23 south of the A27 and disrupts the London to Brighton railway for five days.
4. 10 to 12 June 2012. 114 mm of rain falls at Bognor Regis, the highest 24-hour total there since 1974, and about 780 properties on the West Sussex coastal plain are affected while tide-locked outfalls cannot drain.
5. 13 August 2015. A short, intense storm over Eastbourne leaves 21 properties reporting flooding, the Arndale Centre among them, and East Sussex County Council opens a formal investigation.
6. 16 January 2023. Hastings town centre floods around a culverted watercourse. Southern Water had taken responsibility for the culvert in September 2022, and the county's final report came in October 2023.
7. 8 and 9 April 2024. Storm Kathleen's surge overtops defences on the West Sussex coast, flooding about 233 holiday homes at Medmerry Cove and around 20 homes on Rope Walk in Littlehampton.

The 2015, 2023 and 2024 floods each led to a Section 19 report, the record a lead local flood authority keeps of what flooded and why; our explainer on Section 19 flood investigations sets out when one is required.

What planning policy does a flood risk assessment in Sussex have to meet?

National policy is the same in every Sussex district: chapter 18 of the NPPF, with policy F4 setting when a site-specific assessment is needed, F5 and F6 the sequential test and the exception test, and F8 expecting development to follow the 2025 National Standards for sustainable drainage. Our article on planning for flood risk under the NPPF walks through them. The local layer is where Sussex differs from place to place.

• The South Downs National Park. The Local Plan adopted in July 2019 applies policy SD49 on flood risk management and SD50 on sustainable drainage inside the Park in all three council areas. Its review went out for final consultation from 12 May to 23 June 2026 and is due to be submitted for examination by the end of the year.
• Brighton and Hove. City Plan Part One policy CP11 (2016) covers flood risk and City Plan Part Two policy DM43 (2022) sustainable drainage. The city's drainage guidance, SPD16, expects brownfield schemes to cut peak runoff by at least 50% where they can, and ranks the city among England's ten Flood Risk Areas with the most people at risk.
• Shoreham Harbour. A joint plan adopted in October 2019 by Adur District Council, Brighton and Hove City Council and West Sussex County Council covers the harbour on both sides of the city boundary, with at least 1,400 homes by 2032 and policy SH6 on flood risk and sustainable drainage.
• The East Sussex districts. East Sussex County Council asks every application to show how the national standards have been met, recommends a summary page in the drainage strategy setting that out, and keeps a 5 m strip free of development from the top of a watercourse bank or the edge of a culvert.
• The West Sussex districts. West Sussex County Council restricts surface water discharge to greenfield rates under its August 2026 guidance on the national standards and runs a paid pre-application service for major schemes; our West Sussex page sets out the seven district plans.
• Water neutrality. Natural England withdrew its water neutrality position for the Sussex North supply zone on 31 October 2025, and Horsham District Council reports that the Environment Agency formally agreed the reduced Hardham abstraction licence on 9 October 2026. It is a water supply question rather than a flood one, and our article on water neutrality gives the current position.

Sussex sites in our case studies, from the Weald clay to the coast

Our published case studies include sites in both counties, each named only to the town or district.

• 130 homes near Hailsham. A probabilistic groundwater model fixed a plausible winter water table, and the attenuation storage was sized around it. East Sussex County Council withdrew its objection in early 2020 and outline consent followed in August 2021: how the groundwater model cleared the objection.
• 24 homes on an East Sussex mill site. No usable public sewer and an outfall to a ditch feeding the Pevensey Levels. Storage, flow controls and treatment shared a 3.2 l/s cap, the Pevensey and Cuckmere Water Level Management Board gave consent, and both drainage conditions were discharged.
• A new home in Worthing. Flood Zone 1 but inside a high surface water band. A stand-in storm event agreed with the council's drainage reviewer led to approval with conditions.
• A West Sussex conversion. A commercial-to-residential prior approval where modelled surface water reached 0.3 m. The layout changed from four flats to two duplex units and prior approval was secured.
• An infill plot in a West Sussex coastal town. Updated surface water mapping meant a flood risk assessment, a drainage strategy and a sequential test together for a single-storey home in Flood Zone 1.

Several of these sat outside the river and sea flood zones and still needed full flood evidence, which is why the surface water maps matter as much as the flood zones here. More of our work is on the case studies page.

What we prepare for sites between Chichester Harbour and Rye Bay

Each report answers its own question: whether a site is safe, whether it should be built there at all, and where its water goes. A senior consultant signs off every report, our consultants are chartered, and Unda Consulting Limited is a CIWEM Business Partner.

• Planning applications. A scheme in Flood Zone 2 or 3, or on a hectare or more, needs a flood risk assessment for planning, and so can a smaller Zone 1 site in Sussex where surface water or groundwater is mapped.
• River, tide and chalk together. A flood risk assessment at Lewes or Shoreham often has to weigh a tidal river, the sea and groundwater from the Downs on the same plot.
• Buying beside the Ouse or the Rother. Homes in Lewes, Robertsbridge or Rye can prompt lender questions that a flood risk survey for property purchase answers before exchange.
• Homes in Flood Zone 3. Housing on the Pevensey Levels, the Manhood Peninsula or the Arun floodplain has to pass the sequential and exception test before floor levels come into it.
• Three sets of drainage rules. West Sussex, East Sussex and Brighton and Hove each read a drainage strategy against their own guidance on the national standards.
• Clay in the Weald, chalk on the Downs. A surface water drainage strategy on the Weald clay usually depends on storage, while on the chalk it has to show the soakaway sits well clear of groundwater.
• After consent. Where a Sussex district or the National Park attaches a drainage condition, we prepare the evidence for the discharge of drainage conditions.
• Insurance where it has flooded before. After floods such as Lewes in October 2000 and Patcham the following month, a flood risk assessment for insurance gives an underwriter evidence about the building itself.
• Employment floorspace at Shoreham Harbour. The joint plan seeks 16,000 m² of employment floorspace in Adur and 7,500 m² in Brighton and Hove, and commercial buildings there can use a BREEAM flood risk assessment for the Pol 03 credit.
• Port and harbour land. Industrial land around Shoreham and Newhaven harbours faces the tide, so flood risk due diligence belongs before the deal.
• Keeping a business trading. The 2015 Eastbourne storm reached the Arndale Centre and the 2000 flood hit 300 Lewes businesses; a business continuity flood assessment helps an occupier plan for the next event.
• No sewer to connect to. On rural Sussex sites without a usable public sewer, like the East Sussex mill site above, a foul drainage strategy settles treatment and the outfall early.
• Chalk groundwater. With eight Environment Agency groundwater warning and alert areas touching Sussex, a groundwater flood risk assessment is often part of the work on a Downs site.
• Where river meets tide. The Ouse is tidal below Barcombe and the Eastern Rother below Scots Float Sluice; where national mapping gives no usable level for a site, flood modelling can.
• Tidal frontages. From Chichester Harbour to Rye Bay, 57 warning and alert areas sit on the sea or a harbour, and a flood warning and evacuation plan shows how occupants leave in time.
• Land for allocations. Before land is bought towards the 1,400 homes the Shoreham Harbour plan seeks, or for a South Downs allocation, a flood risk feasibility study tests what the site can take.
• National policy. Our NPPF guidance sets out how chapter 18 applies to a Sussex site.

Area pages for towns and districts across Sussex

For a site in one of these places, the local page goes into its own streets, flood record and council policies:

• West Sussex. The county page, with Arundel, Chichester, Crawley, East Grinstead, Haywards Heath, Horsham, Littlehampton, Pagham, Selsey and Worthing.
• East Sussex. The county page, with Bexhill, Eastbourne, Hastings, Lewes, Newhaven, Pevensey Bay, the Pevensey Levels, Uckfield and Winchelsea Beach.
• Brighton and Hove. The city page, covering the unitary authority.
• Beyond Sussex. Hampshire to the west, Surrey to the north and Kent to the east.

Every other county and town we write about is listed on our areas page.

Who handles flooding in Sussex, and who to call

Call 999 if anyone's life is at risk and 105 for a power cut. Flood response across West Sussex, East Sussex and Brighton and Hove is coordinated through one Local Resilience Forum; for everything else, the right body depends on where the water comes from.

East Sussex County Council's local flood risk management strategy for 2016 to 2026 runs to the end of this year, with its replacement due for consultation, and Brighton and Hove's new strategy was still a consultation draft in July 2025.

Questions about a flood risk assessment in Sussex

To discuss a site in East or West Sussex or in Brighton and Hove, contact one of our experienced flood risk consultants on 01293 214444 or at enquiries@unda.co.uk.