Flood Risk Assessment in Sussex

Unda prepares clear, technically robust flood risk assessments and drainage strategies for residential and commercial sites right across Sussex. A flood risk assessment in 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.

Sussex is two administrative counties — East Sussex and West Sussex, with the city of Brighton & Hove between them — but one shared geography: the High Weald in the north, the chalk spine of the South Downs, and a long, low and heavily defended Channel coast. That mix gives Sussex river, tidal, surface-water and groundwater flooding, often more than one source on the same site. We have prepared assessments the length of the region, and we know how the local authorities here expect flood risk to be evidenced. We prepare flood risk assessments in Sussex for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies.

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.

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

Flood risk assessment in Sussex at a glance

• Two counties, two flood authorities. East Sussex County Council and West Sussex County Council are the lead local flood authorities for their own halves of Sussex; Brighton & Hove is a separate unitary authority between them. The Environment Agency covers main rivers and the sea throughout.
• Rivers drain the whole region to the Channel. The Ouse, Uck, Cuckmere and Eastern Rother in the east, and the Arun, Adur, Western Rother, Lavant and Ems in the west, give fluvial and tidal flooding from the Weald and Downs down to the coast.
• The coast is low and defended. A continuous frontage from Selsey and Bognor Regis in the west to Eastbourne, Hastings and Rye in the east sits behind sea defences, so tidal and coastal risk runs the length of Sussex.
• Chalk groundwater is a Sussex signature. The South Downs aquifer drove the 1994 Chichester and Lavant flood in the west and saturates low ground across the Downs — a source no national river map shows.
• Two landmark floods shape policy here. The October 2000 floods at Lewes and Uckfield in the east, and the December 2000 Crawley and Gatwick floods in the west, both fell in the wettest autumn for almost 300 years.
• 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 low-probability site.

East Sussex or West Sussex? Choose your area

Sussex splits into two counties for planning and flood-risk purposes, and the detail you need — the rivers, the local authority, the strategic flood risk evidence and the development pressures — depends on which side your site sits. Pick the page that matches your location for the full local picture.

• Flood risk assessment in East Sussex. Eastbourne, Hastings, Bexhill, Lewes, Uckfield, Newhaven, Seaford, Rye and the Pevensey Levels — the Ouse, Uck, Cuckmere and Eastern Rother, a tidal coast and the 2000 Lewes and Uckfield floods. Lead local flood authority: East Sussex County Council.
• Flood risk assessment in West Sussex. Crawley, Horsham, Worthing, Chichester, Bognor Regis, Littlehampton, Shoreham and Arundel — the Arun, Adur, Western Rother and Lavant, chalk groundwater and the 2000 Crawley and Gatwick floods. Lead local flood authority: West Sussex County Council.
• Brighton & Hove and unsure cases. Brighton & Hove is a separate unitary authority on the East Sussex border. If you are not sure which county your site falls in, or it sits near the boundary, just tell us the address and we will confirm which authority and evidence apply.

Check flood risk anywhere in Sussex

East and West Sussex are outlined below. Screen any postcode in the region 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 and a useful starting point — not a substitute for a site-specific flood risk assessment.

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

No single source defines flood risk in Sussex, and it differs between the two counties. The east is shaped by the tidal coast, the Pevensey Levels and the Ouse, Uck, Cuckmere and Eastern Rother; the west by the tidal Arun, the Adur and Lavant, chalk groundwater north of Chichester and the urban surface-water catchment around Crawley and Gatwick. A credible flood risk assessment in 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.

Because flood zones only map rivers and the sea, a Sussex site can sit firmly in Flood Zone 1 and still be at real risk from surface water or chalk groundwater. The table below shows where the main risks fall — follow the county link for the full detail.

The two big dated events bracket the region: the Lewes and Uckfield floods of October 2000 in the east, and the Crawley and Gatwick floods of December 2000 in the west. Both still shape how the Environment Agency and the county councils read new applications today.

Planning and drainage policy in Sussex

A flood risk assessment in Sussex has to satisfy national policy — the National Planning Policy Framework and its Planning Practice Guidance — and the local authorities that apply it. Sussex has two lead local flood authorities and several planning authorities, so the first job on any site is knowing who reviews what.

• Two lead local flood authorities. East Sussex County Council and West Sussex County Council each lead on surface water, groundwater and ordinary watercourses, and are statutory consultees on drainage for major development. Brighton & Hove is a separate unitary LLFA.
• The Environment Agency covers main rivers and the sea. It is the statutory consultee on tidal and fluvial flood risk along the Sussex coast and the main rivers, and the body whose objection you most need to avoid.
• Planning decisions are local. Arun, Chichester, Crawley, Horsham, Mid Sussex and Adur & Worthing in the west, and Eastbourne, Hastings, Lewes, Rother and Wealden in the east, 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 Strategic Flood Risk Assessment. The SFRA for your district sets the local flood-risk evidence base, including critical drainage areas in Flood Zone 1, and underpins the Sequential Test.
• Infiltration evidence is expected. Assumed soakaway rates without BRE 365 testing no longer pass on the Wealden clay or the high water tables of the coastal plain and the chalk.

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

Flood risk assessments for every situation in Sussex

Whatever the reason you need flood risk or drainage advice in 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.
• Flood risk survey for property purchase. A property-specific survey for buyers and their advisers when a conveyancing search flags risk.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover or premiums are in question.
• Flood risk assessment for BREEAM. An assessment structured around BREEAM Pol 03 for project teams pursuing certification.
• Sequential and exception tests. The locational case for development in or near a flood risk area.
• Surface water drainage strategy (SuDS). How runoff is managed to meet the surface-water hierarchy, 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 modelling where national mapping is not enough — both feeding the wider assessment.

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 Sussex

Before you commission a report, our free tools give you an early read on a site anywhere in 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 Arun, Adur, Ouse, Cuckmere, Rother and other Sussex rivers.
• 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.

Flood risk management and emergency contacts in Sussex

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

• Lead local flood authorities. East Sussex County Council and West Sussex County Council — surface water, groundwater and ordinary watercourses, and reporting local flooding.
• Environment Agency and Floodline. Main rivers and the sea, flood warnings and live levels — check for flooding, or Floodline on 0345 988 1188 and sign up for flood warnings.
• Water and sewerage. Southern Water is responsible for public sewers and sewer flooding across Sussex.
• Regional and emergency. The Southern Regional Flood & Coastal Committee oversees regional investment; the Sussex Resilience Forum coordinates multi-agency emergency response. Call 999 for danger to life and 105 for power cuts.

Flood risk assessment in Sussex: common questions