Flood Risk Assessment in East Sussex

At Unda we use years of experience to prepare clear, robust flood risk assessments and drainage strategies for residential and commercial sites across East Sussex. A flood risk assessment in East Sussex 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.

East Sussex is shaped by water on two sides — a long, low-lying coast from Camber and Rye round to Eastbourne and Seaford, and the river valleys of the Ouse, the Uck, the Cuckmere and the Eastern Rother draining the High Weald and the South Downs to the sea. We assess every source that matters here: tidal and coastal flooding held back by the sea defences, the rivers that overtopped at Lewes and Uckfield in 2000, the surface water that overwhelms drains in Hastings and Eastbourne, and the groundwater that rises through chalk and Wealden clay. We then deal directly with East Sussex County Council as lead local flood authority and the Environment Agency on your behalf. We prepare flood risk assessments in East Sussex 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.

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

Flood risk assessment in East Sussex at a glance

• Coast and rivers both drive the risk. A 90-km coastline from Camber to Seaford sits alongside four river valleys — the Ouse, Uck, Cuckmere and Eastern Rother — so tidal, fluvial and surface-water flooding all feature, often on the same site.
• The Pevensey Levels are below the tide. Around 50 km² of low-lying land between Eastbourne and Bexhill is kept dry only by the sea defences at Pevensey Bay; without them the Levels would return to tidal marsh.
• October 2000 reshaped the county. The River Ouse and River Uck overwhelmed Lewes and Uckfield in hours, flooding more than 600 homes and 300 businesses with damage of about £118 million.
• East Sussex County Council is the lead local flood authority. It manages surface water, groundwater and ordinary watercourses across the five districts; Brighton & Hove next door is a separate authority.
• Infiltration drainage is hard here. Much of the county has poor-draining clay and high winter groundwater, so soakaways usually have to be evidenced with monitoring and testing rather than assumed.

Check flood risk anywhere in East Sussex

East Sussex is outlined below. Screen any postcode in the county 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 East Sussex?

East Sussex faces more sources of flooding than almost any other county in the South East. The coast and the Pevensey Levels bring tidal risk; the Ouse, Uck, Cuckmere and Eastern Rother bring river flooding; steep urban catchments at Hastings and Eastbourne bring surface water; and the chalk of the South Downs and the clays of the Weald bring groundwater. Most planning sites here have to address more than one. Knowing which source governs a particular site is the first job of any flood risk assessment, because it decides what data, modelling and mitigation the assessment needs.

Tidal and coastal flooding

The coast is the defining flood risk in East Sussex. Eastbourne, Hastings, Bexhill, Seaford, Newhaven and Rye all sit at or near sea level, and the tidal flood risk reaches inland up the lower Ouse to Lewes and up the Eastern Rother to Rye. The Pevensey Levels, roughly 50 square kilometres between Eastbourne and Bexhill, lie below the level of a high spring tide and stay dry only because of the sea defences at Pevensey Bay, where a shingle bank and more than 150 timber groynes hold the line at a crest of about 6 metres above Ordnance Datum. Those defences are being renewed under the Pevensey Bay to Eastbourne Coastal Management Scheme, and with sea levels projected to rise by more than a metre over the next century, a site in a defended tidal area needs an assessment that looks honestly at the residual risk if a defence is overtopped or breached.

River (fluvial) flooding

Four main rivers drain the county to the sea, and each has a flood history. The River Ouse rises in the High Weald and runs through Lewes to the sea at Newhaven, with broad floodplains at Barcombe, Hamsey, Southease and Piddinghoe. The River Uck joins it above Lewes and gave Uckfield its dramatic flooding in 2000. The Cuckmere winds through Alfriston and the Cuckmere valley, and the Eastern Rother drains the north and east of the county through Robertsbridge, Bodiam and Etchingham to Rye and the Rother Levels. Sites in these valleys are typically in Flood Zone 2 or 3 on the Flood Map for Planning, and often need hydraulic flood modelling to establish accurate flood levels.

Surface water flooding

Surface water is now the most widespread flood risk in England, and East Sussex is no exception. The steep catchments behind Hastings, Eastbourne and the coastal towns shed heavy rain quickly, and the historic drainage struggles to cope; storm Ciarán in November 2023 flooded roads and properties across Hastings, Bexhill and Newhaven. 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 East Sussex applications than it once did.

Groundwater flooding

The geology of East Sussex makes groundwater flooding a genuine constraint. The chalk of the South Downs forms a major aquifer that can saturate and flood low-lying ground after a wet winter, while in the High Weald the Hastings Beds and Tunbridge Wells Sand throw out spring lines where permeable sand meets the underlying clay. Groundwater appears on no national flood map, can persist for weeks, and undermines the infiltration assumptions behind soakaways — which is why much of the county needs a season of monitoring before a soakaway can be relied on. Where groundwater is in question, it is assessed as a distinct source within the flood risk assessment.

A history of flooding in East Sussex

Dated flood events are the clearest test of how a site really behaves, and East Sussex has some of the most studied in the country. The autumn of 2000 was the wettest in more than 270 years, and it produced the floods that still define flood policy in the county.

• October 2000 — Lewes and Uckfield. After 156.4 mm of rain fell at Plumpton in 48 hours, the River Uck tore through Uckfield town centre from about 5am at depths up to 1.9 metres, and the River Ouse overtopped Lewes’ defences behind Cliffe Bridge by early afternoon. More than 600 homes and 300 businesses flooded, around 180 people were put up in emergency accommodation, and the damage was put at roughly £118 million.
• The defences that followed. The 2000 floods led directly to the Lewes flood defence scheme and a generation of investment along the Ouse — a reminder that a defended site is not a risk-free one, and that residual risk has to be assessed.
• November 2023 — Storm Ciarán. Intense rain and coastal conditions flooded roads and properties across Hastings, Bexhill, Newhaven and Eastbourne, disrupting rail and closing roads, a recent reminder of combined surface-water and coastal risk.

You can read our full account of the 2000 event and its legacy in 25 years on: the legacy of the Lewes floods.

Planning and drainage policy in East Sussex

A flood risk assessment in East Sussex has to satisfy national policy, set by the National Planning Policy Framework and its Planning Practice Guidance, and the local authorities that apply it. Knowing who reviews what, and which local standards apply, is half the battle in getting a clean decision.

• The lead local flood authority is East Sussex County Council. It manages flood risk from surface water, groundwater and ordinary watercourses, carries out Section 19 investigations after major floods, keeps the asset register, and decides ordinary watercourse consents. Brighton & Hove, which adjoins the county, is a separate unitary authority with its own flood team.
• The Environment Agency covers main rivers and the sea. It is the statutory consultee on tidal and fluvial flood risk along the Ouse, Cuckmere, Eastern Rother and the coast, and the body whose objection you most need to avoid.
• Strategic Flood Risk Assessments set the local evidence. The Eastbourne and South Wealden SFRA, the Wealden, Hastings and Rother SFRAs, and the South Downs National Park SFRA each map flood zones, surface water and climate allowances for their areas, and underpin the Sequential Test on windfall sites.
• Local drainage standards are specific. East Sussex applies the seven national SuDS standards and asks for a minimum 5 metre undeveloped buffer from the top of bank of an ordinary watercourse, two treatment stages upstream of a soakaway, and at least 10 metres between a soakaway base and the maximum winter groundwater level.
• The South Downs National Park is its own planning authority. Much of Lewes district and southern Wealden falls within the Park, which determines applications there — an extra layer worth identifying early.

Get the flood risk and drainage evidence right first time and an East Sussex application moves; get it wrong and it stalls behind an Environment Agency or lead local flood authority objection.

What kinds of development need a flood risk assessment in East Sussex?

The development pattern in East Sussex puts flood risk at the centre of a lot of applications. Coastal regeneration in Hastings, Eastbourne, Bexhill and around Newhaven port places new homes and commercial space on defended, low-lying frontage. Town-centre infill at Lewes and Uckfield sits on river floodplain. Rural, agricultural and equestrian schemes across the Weald and the Levels involve ordinary watercourses, ditches and high groundwater. Across all of them, the same triggers decide whether an assessment is required.

• Flood Zone 2 or 3. Almost any development in a medium or high probability flood zone needs a site-specific flood risk assessment for planning.
• Sites of one hectare or more in Flood Zone 1. Larger low-risk sites still need an assessment.
• Surface water risk on the Flood Map for Planning. Since the 2025 PPG update this can trigger an assessment on otherwise low-risk land.
• Major development needing drainage. Ten or more dwellings, sites over a hectare, or 1,000 m² of floorspace will need a surface-water drainage strategy.
• A change to a more vulnerable use. Converting commercial floorspace or agricultural buildings to homes raises the vulnerability classification and usually prompts an assessment.

How Unda helps with flood risk in East Sussex

Our flood risk consultants know East Sussex well. We have prepared flood risk assessments and drainage strategies across the county, on the coast, in the river valleys and on the Levels, and we act as the accurate translation layer between your scheme and the Environment Agency, East Sussex County Council and the district planning teams. We stay involved past the report to answer objections, because a difficult tidal or surface-water site is often won in the follow-up, not the first submission.

You can read this and other examples in our tidal Flood Zone 3 case study and a property-purchase case study where a precautionary search result was clarified so a sale could proceed. Every report is overseen and signed off by a senior flood risk consultant, and we aim to reply to every quote request within 60 minutes.

Flood risk assessments for every situation in East Sussex

People come to us for a flood risk assessment in East Sussex 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 assessment for planning. The site-specific report submitted with an application to satisfy the NPPF and the Environment Agency.
• Flood risk survey for property purchase. An independent assessment of the actual risk to a home when a conveyancing search flags 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.
• Sequential and exception tests. The locational justification that steers development to lower-risk land.
• Surface-water drainage strategy (SuDS). How runoff is managed and kept within agreed discharge rates, with discharge of drainage conditions and preliminary drainage design as the later stages.
• Groundwater assessment and flood modelling. A groundwater section sits within the flood risk assessment; hydraulic modelling derives accurate flood levels 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.

Free flood risk tools for East Sussex

Before you commission a report, our free tools give you an early read on a site anywhere in East Sussex. 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 the Ouse, Cuckmere, Rother and other rivers near you.
• Geology and Ground Conditions Map. See the mapped 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 to a design infiltration rate.

Local flood risk management and emergency contacts

Several bodies manage flood risk in East Sussex, 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.

You can sign up for free Environment Agency flood warnings for the Ouse, Cuckmere, Eastern Rother, Pevensey Levels and the coast through the government’s check-for-flooding service, and report flooding from roads, ditches and surface water to East Sussex County Council as lead local flood authority.

Flood risk assessment in East Sussex: frequently asked questions