Flood Risk Assessment in Brighton & Hove

Our team at Unda use years of experience to prepare clear, robust flood risk assessments and drainage strategies for residential and commercial sites across Brighton & Hove. A flood risk assessment in Brighton & Hove shows your local planning authority and the Environment Agency that a development will stay safe from flooding for its whole lifetime, without increasing flood risk anywhere else.

Brighton & Hove is a city with almost no rivers, wrapped between the chalk of the South Downs and the sea. That shapes everything about flooding here: rain runs fast down steep streets to cause surface-water flooding, water rises through the chalk to flood low ground in the dry valleys, and the tide presses against the seafront and the harbour to the west. We assess every source that matters in the city — the surface water that overwhelmed drains across the city on 20 June 2023, the groundwater that has flooded Patcham and Withdean after wet winters, and the tidal risk along the front from Saltdean and the Marina round to Hove and Shoreham Harbour — then deal directly with Brighton & Hove City Council as lead local flood authority and the Environment Agency on your behalf. We prepare flood risk assessments in Brighton & Hove for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies — guiding architects, planning consultants, developers, landowners, home buyers and businesses through to a decision.

If you need a quote or want to talk a site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Brighton & Hove at a glance

• A city without rivers. Brighton & Hove has no main river running through it, so the flood risk comes from surface water, groundwater and the sea rather than an overtopping watercourse — an unusual profile that catches out generic assessments.
• One of England’s worst surface-water clusters. The city is among the ten places in England most susceptible to surface-water flooding, as rain sheds off the South Downs and runs down steep streets faster than the drains can take it.
• Chalk drives groundwater flooding. After a wet winter the chalk aquifer saturates and water surfaces in the dry valleys at Patcham, Withdean and beyond — the Environment Agency issues a dedicated groundwater flood alert for Patcham.
• Its own unitary flood authority. Brighton & Hove City Council is the lead local flood authority for the city, separate from East Sussex and West Sussex county councils next door, and it sets its own flood and drainage policy.
• The coast is held by its beaches. Shingle beaches and timber groynes are the city’s primary defence against tidal flooding and erosion, backed by the long Brighton Marina to River Adur coastal scheme.

Check flood risk anywhere in Brighton & Hove

Brighton & Hove is outlined below. Screen any postcode in the city against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass, not a substitute for a site-specific assessment.

Where does flood risk come from in Brighton & Hove?

Brighton & Hove is one of the few places where the first job of a flood risk assessment is to look past the rivers, because there are almost none. The city sits on chalk, between the South Downs and the sea, and its flooding comes from three sources that often combine on the same site: surface water, groundwater and the tide. Knowing which one governs a particular plot is what decides the data, modelling and mitigation an assessment needs.

The pattern is set by the land itself. Rain falling on the Downs runs quickly down the steep chalk valleys and city streets; water that soaks into the chalk re-emerges lower down when the aquifer is full; and the sea works against a low-lying seafront and the harbour at the western edge. Most planning sites in the city have to address more than one of these, which is why a single national map is never enough. Our overview of the sources of flood risk sets out how they interact.

Surface water (pluvial) flooding

Surface water is the defining flood risk in Brighton & Hove and now the most widespread source of flooding in England. The city’s own Strategic Flood Risk Assessment records it as the most frequent cause of flooding here, and the city is ranked among the ten worst surface-water clusters in the country. The mechanism is simple and brutal: heavy rain sheds off the South Downs and runs down steep streets, following the highway network until it ponds at the bottom of a valley faster than the drains and sewers can clear it. The flood event of 20 June 2023 is the recent example — intense rainfall produced flooding reported at multiple commercial and residential locations across the city, all of it surface water following the sloping ground. Since the 2025 Planning Practice Guidance update, surface water flooding can trigger a flood risk assessment even on a Flood Zone 1 site, so it now features on far more Brighton & Hove applications than it once did.

Groundwater flooding

The chalk that makes up the South Downs is a major aquifer, and it makes groundwater flooding a genuine constraint in the city. After a wet winter the water table climbs until it surfaces in the low-lying dry valleys, flooding ground that no river map would ever flag. Patcham is the best-known example, with a history of serious groundwater flooding around Old London Road, and the Environment Agency runs a dedicated groundwater flood alert area for it; Withdean, around Peacock Lane, has flooded the same way. Groundwater appears on no national flood map, can persist for weeks, and undermines the infiltration assumptions behind soakaways — which is why the council’s flood-risk policy asks developers to show that groundwater flow paths are preserved. Where groundwater is in question, it is assessed as a distinct source within the flood risk assessment, often after a season of monitoring to derive a credible design water table.

Tidal and coastal flooding

The sea is the third source, and it governs the front. The tidal and coastal flood risk runs the length of the city, from Saltdean and Rottingdean in the east, past Brighton Marina and the central seafront, through Hove and Aldrington to Shoreham Harbour and the River Adur in the west. Brighton & Hove has no estuary or sea wall holding back a wide floodplain in the way a tidal town does; instead its shingle beaches and timber groynes are the primary protection against tidal flooding and erosion, replenished by shingle and backed by the long-term Brighton Marina to River Adur coastal scheme. A site on or just behind the front needs an assessment that looks honestly at present-day tidal levels, allows for more than a metre of sea-level rise over the next century, and considers the residual risk if a defence is overtopped.

The Wellesbourne — Brighton’s lost river

The city does have one historic watercourse, and it explains a great deal about how Brighton floods. The Wellesbourne is a winterbourne — an intermittent stream that flowed only in winter and after heavy rain — rising near Patcham and running south down the valley now followed by the A23 London Road and the main railway line, to reach the sea at Pool Valley. It was culverted beneath the streets from 1792–93 and last flowed above ground in 1876, when London Road was flooded for three miles. It has not disappeared: heavy rain in 1960 and again in 2000 brought flooding along London Road and Mill Road in Patcham that gave the impression of the river running again. The lesson for any development along that corridor is that the old valley still drains the way it always did, whatever the surface now looks like.

A history of flooding in Brighton & Hove

Dated flood events are the clearest test of how a site really behaves, and they are why Brighton & Hove City Council scrutinises drainage and groundwater so closely. A flood risk assessment in Brighton & Hove has to treat this record as evidence, not background colour.

• 1876 — the Wellesbourne’s last run. The lost river flowed above ground for the last time, flooding London Road for three miles before it was finally taken underground for good. Strong flows had also been recorded in 1795, 1806, 1811, 1827–28 and 1852.
• 2000–2001 — the largest recorded event. The autumn and winter of 2000–2001 brought the city’s biggest recorded flooding, when extreme rainfall saturated the chalk and surface and groundwater flooded homes, with Patcham and the London Road valley among the worst hit.
• 2013–2014 — one of the wettest winters on record. Prolonged rain pushed groundwater levels up again and flooded properties in Patcham, with at least one resident displaced for months; a severe summer storm in 2014 brought further widespread flooding across the city.
• 20 June 2023 — a surface-water cloudburst. Heavy rainfall caused flooding reported at multiple locations across the city, with the fire service attending many more; the council’s Section 19 investigation confirmed it as surface water following the steep highway network.

A pattern runs through all of it: the low valleys at Patcham, London Road and the city centre flood again and again, because the water still follows the lines it always has.

Planning and drainage policy in Brighton & Hove

A flood risk assessment in Brighton & Hove has to satisfy national policy — the National Planning Policy Framework and its Planning Practice Guidance — and the local authority that applies it. Knowing who reviews what, and which local standards bite, is half the battle in getting a clean decision.

The city has its own framework, separate from the surrounding counties. Brighton & Hove City Council is a unitary authority, so it is the lead local flood authority as well as the planning authority and the highway authority. The Environment Agency leads on the sea and on any main river, while the council manages surface water, groundwater and ordinary watercourses, investigates significant floods under Section 19, and consents works to ordinary watercourses. Our overview of who manages flood risk in England explains how the pieces fit together.

• The lead local flood authority is Brighton & Hove City Council. It manages surface water, groundwater and ordinary watercourses across the city and is a statutory consultee on surface water drainage for major development — a role that matters even on a site sitting entirely in Flood Zone 1.
• The Strategic Flood Risk Assessment sets the local evidence. The council’s Level 1 and Level 2 SFRA, with its 2021 addendum, maps surface water, groundwater and tidal risk and climate allowances across the city.
• City Plan policies CP11 and DM43 govern flood risk and drainage. Policy CP11 requires development to show it is not at risk from groundwater or other sources and that groundwater flow paths are preserved; policy DM43 and the council’s Sustainable Drainage SPD require SuDS on new development.
• The seafront has its own policy. City Plan policy DM39 governs development on the seafront, where tidal risk, sea-level rise and the design of coastal defences all come into play.
• The South Downs National Park is a separate planning authority. The Downs to the north of the city are determined by the National Park Authority, an extra layer worth identifying early on any urban-fringe site.

Your assessment and drainage strategy must answer to all of this: the NPPF and its Planning Practice Guidance, the council’s SFRA and Local Flood Risk Management Strategy, and the City Plan flood and drainage policies. Where a higher-risk site is involved, the Sequential and Exception Tests apply and the right vulnerability classification has to be argued. On drainage, the four pillars of SuDS and the discharge hierarchy shape what the council will accept, and a site within a critical drainage area faces stricter requirements on attenuation and runoff.

What your flood risk assessment in Brighton & Hove must show

• Risk assessed from every credible source — surface water, groundwater and the sea — using the Flood Map for Planning and the council’s local data.
• An appropriate location, tested against the Sequential Test where policy requires it.
• Safe for its lifetime, once the Environment Agency’s climate change allowances are applied — generally 100 years for housing.
• No increase in flood risk elsewhere, with groundwater flow paths preserved and runoff attenuated to agreed rates.
• Residual risk managed, with safe access, egress and finished floor levels. Get one of these wrong and the application stalls, as our note on LLFA drainage objections shows.

What kinds of development need a flood risk assessment in Brighton & Hove?

Brighton & Hove is a dense city squeezed between the sea and the National Park, so most growth comes through infill, regeneration and the redevelopment of constrained sites — and the kind of site you bring forward shapes the assessment you need. The right scope is the difference between a clean approval and a drainage or groundwater objection that holds an application up for months.

• Seafront and harbour regeneration. Redevelopment along the front and around Shoreham Harbour and the Marina places homes and commercial space on low-lying, tidally influenced land, where City Plan policy DM39 and residual tidal risk shape the scheme.
• High-density city infill. Apartment schemes and conversions on tight central plots frequently sit in the historic Wellesbourne valley, where surface water and the surface-water sewer network drive the drainage strategy.
• Urban-fringe and valley sites. Development at Patcham, Withdean, Moulsecoomb, Bevendean and Coldean often turns on groundwater and steep-catchment surface water rather than a river, and on the right drainage information for planning.
• A change to a more vulnerable use. Converting commercial floorspace to homes raises the vulnerability classification and usually prompts an assessment, even on otherwise low-risk land.

Climate change is not optional here. A Brighton & Hove assessment must apply the Environment Agency’s allowances for heavier rainfall and rising sea levels over the development’s lifetime, which sets floor levels, freeboard and how much attenuation a drainage scheme needs.

Home buyers and businesses come to us too. A flood risk question can affect a purchase and house prices, and a conveyancing search is not the same thing as a planning-grade assessment. Knowing which you actually need saves time and money, and our guide on why expertise matters explains where the line falls.

How Unda helps with flood risk in Brighton & Hove

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Brighton & Hove City Council and national planning policy. We are experts on the city’s flooding, not a generic report mill.

A flood risk assessment in Brighton & Hove from Unda assesses every credible source — surface water off the Downs, groundwater rising through the chalk, and the tide along the front — compares modelled levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, and a senior flood risk consultant signs off every report. Where the council needs it, a surface-water drainage strategy sits alongside the assessment so the two cases stay consistent and an objection has nowhere to land. We stay involved past the report to answer queries, because a difficult groundwater or surface-water site is often won in the follow-up, not the first submission.

Flood risk assessments for every situation in Brighton & Hove

People come to us for a flood risk assessment in Brighton & Hove for many different reasons, and each is a distinct piece of work. A flood risk assessment, a sequential or exception test and a drainage strategy are three separate reports: the first asks whether the site is safe, the second whether the development should be there, and the third where the surface water goes. They often travel together in a planning submission, but they answer different questions.

Flood risk assessments

• Flood risk assessment for planning. The site-specific report submitted with an application to satisfy the NPPF, the Environment Agency and the city council. Groundwater and, where needed, flood modelling are assessed within it rather than as separate reports.
• Flood risk survey for property purchase. An independent assessment of the actual risk to a home when a conveyancing search flags surface water, groundwater or coastal flooding.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover or premiums are in question.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 to support flood and surface-water management credits, which a planning FRA alone rarely satisfies.
• Groundwater flood risk. A defensible design water table for chalk-valley sites at Patcham, Withdean and beyond, and for any basement — assessed as a source within the FRA, or as a standalone hydrogeological assessment on complex sites.

Sequential and exception tests

The Sequential and Exception Tests are a separate, locational report about whether a development should be in a higher-risk location at all. The Sequential Test defines an area of search, identifies reasonably available lower-risk sites and justifies the proposed site; the Exception Test is the second gate where development still needs to proceed. A clean site-specific assessment showing the development is not actually within an area at risk can remove the need at application stage — a live policy area, as our note on what the Yatton judgment means for planning explains.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering that calculates existing versus proposed runoff and proposes SuDS to hold discharge within the council’s standards and the discharge hierarchy.
• Preliminary drainage design. The early, outline read that flags infiltration and groundwater issues before they affect layout and land value.
• Discharge of drainage conditions. The detailed stage to discharge a pre-commencement or pre-occupation drainage condition.
• Flood modelling. 1D and 2D hydraulic modelling — a technique that feeds the assessment or drainage design where national mapping is not enough.

Not sure which you need? Our NPPF guidance sets out the requirements, or speak to one of our flood risk consultants. Our consultants work across the wider region — see our pages for East Sussex, West Sussex and Sussex as a whole.

Free flood risk tools for Brighton & Hove

Before you commission a report, our free tools give you an early read on a site anywhere in Brighton & Hove. They use the same open Environment Agency and British Geological Survey data the regulators use, and they are an indicative screening, not a substitute for a site-specific assessment.

• Flood Risk Map. Screen any postcode for rivers and sea, surface water, reservoirs and flood zones, with a climate outlook to 2125.
• Live River Levels Map. Check the latest Environment Agency gauge readings on watercourses near the city, including the Adur to the west.
• Geology and Ground Conditions Map. See the mapped chalk and superficial geology beneath a site for an early view of whether a soakaway is likely to work.
• Drainage Calculators and the BRE 365 Infiltration Rate Calculator. Size and check drainage and convert soakage test results into a design infiltration rate.

Local flood risk management and emergency contacts

Several bodies manage flood risk in Brighton & Hove, and it helps to know who does what — whether you are planning a development, reporting a flooded road or preparing for a storm. In a flood emergency that threatens life, always call 999; for a power cut during flooding, call 105.

• Check for flooding in Brighton & Hove. Environment Agency live warnings, river and sea levels and maps at check-for-flooding.service.gov.uk; sign up for flood warnings, including the Patcham groundwater alert, or call Floodline on 0345 988 1188.
• Brighton & Hove City Council flood team. Report flooding, find developer guidance and read flood investigation reports via the council’s flood and drainage policies.
• Coastal protection. The council’s coastal protection pages cover the beaches, groynes and the Brighton Marina to River Adur scheme.
• Check flood zones for planning. The Environment Agency’s Flood Map for Planning shows the flood zones that frame a planning application.
• Live river levels. Whether watercourses near the city are running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact Southern Water, which runs the surface-water and foul sewer network across the city. For broader policy context, our notes on the Environment Agency’s planning role and on flood zones 1, 2 and 3 are a good starting point.

Flood risk assessment in Brighton & Hove: frequently asked questions