River Adur Flooding & Flood Risk

Flooding from the River Adur affects a run of West Sussex settlements from the rural Weald down to the coast, but nowhere more so than Shoreham-by-Sea, where the river meets the tide. The Adur rises as two separate streams at Slinfold and on Ditchling Common, joins at Henfield, and runs tidal for around nine miles down to the estuary at Shoreham. Steyning, Bramber and Upper Beeding sit close to its banks in between. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the River Adur floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

The Environment Agency's own catchment plan expects the number of properties at risk from a 1% annual-probability flood in Shoreham and the Adur Estuary to rise from just 11 today to 1,757 by 2100 as tidal and river flooding combine — and in April 2024 the Adur had already burst its banks through the town.

• A river of two arms. The western Adur rises at Slinfold, the eastern Adur on Ditchling Common; they meet at Henfield, where the combined river becomes tidal and embanked for around nine miles down to the estuary at Shoreham-by-Sea.
• A designated Main River. The Environment Agency regulates the Adur along its length above Old Shoreham Bridge, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Three live Environment Agency gauges. Beeding Bridge, Sakeham Weir at Wineham and the tidal gauge at Shoreham Harbour all appear on the map below, updating in real time.
• Flood risk set to grow fastest at the coast. The Environment Agency's Adur Catchment Flood Management Plan finds the sharpest rise in flood risk anywhere on the river is at Shoreham and the Adur Estuary, driven by combined tidal and fluvial flooding under climate change.

Check flood risk along the River Adur

The map below draws the River Adur in coral along both arms — from Slinfold in the west and from near Burgess Hill in the east — down through Henfield and on to the sea at Shoreham-by-Sea, with the Environment Agency's live river gauges positioned along it — click a gauge to see its latest level, typical range and record peak. Enter a postcode near the river to zoom in and screen that location against Environment Agency data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Why does the River Adur flood?

The River Adur floods because it gathers water from two very different arms of West Sussex and then squeezes it through a narrow, tide-locked estuary. Its upper tributaries drain permeable sandstone, but the lower catchment turns to heavy, impermeable clay, so the combined river responds quickly to rainfall. Once it meets the tide at Henfield, around nine miles of embanked channel carry that flow down to Shoreham-by-Sea, where high water can hold flood levels up rather than letting them drain away.

The western Adur rises near Slinfold and flows through Coolham, Shipley and West Grinstead, gathering Hammer Pond, Lancing Brook and Knepp Mill Stream on its way towards Henfield. The eastern Adur rises separately on Ditchling Common, in East Sussex, and flows past Burgess Hill and Hassocks before reaching Shermanbury and Henfield, where the two arms meet. The Environment Agency's own description of the catchment — which covers around 500 square kilometres of the Low Weald and the northern slopes of the South Downs — calls the Adur “a typical lowland river” whose upper tributaries are dominated by sandstone, “which changes to impermeable clay in the lower stretches, making the river very reactive to rainfall.”

• A roughly 500 km² catchment. The Environment Agency's Adur Upper operational catchment drains through 19 named water bodies, including the Adur itself, Herrings Stream, Cowfold Stream, Honeybridge Stream and Woodsmill Stream.
• Sandstone giving way to clay. The upper catchment is dominated by permeable sandstone; the lower catchment turns to impermeable clay, so rainfall reaches the channel quickly rather than soaking away.
• Tidal for around nine miles. The combined river becomes tidal at Henfield and stays embanked and tide-influenced all the way down to the estuary at Shoreham-by-Sea.
• Three gauged reaches. Beeding Bridge, below Bramber and Upper Beeding; Sakeham Weir at Wineham on the eastern arm; and the tidal gauge at Shoreham Harbour all report levels in real time.

A history of flooding on the River Adur

The River Adur's worst flooding follows two different patterns: a flashy fluvial response inland when heavy rain follows a wet spell, and a tide-driven overtopping at the coast when a storm surge coincides with high water. Both have produced record-breaking levels within the last 25 years, and Shoreham-by-Sea has flooded as recently as April 2024.

Inland, the Adur's record on the eastern arm came during the exceptionally wet autumn of 2000, when saturated ground sent rivers over their banks across Sussex through October and November; the Sakeham Weir gauge at Wineham reached its highest level on record, 2.90 metres, on 12 October 2000. On the tidally-influenced middle reach, the Beeding Bridge gauge below Bramber and Upper Beeding recorded its highest level, 3.35 metres, during the coastal storm surge of 6 December 2013 — part of the same east-coast surge event that caused serious flooding elsewhere in the south of England that winter.

More recently, Shoreham suffered major flooding in April 2024 when the Adur burst its banks along the A259 Brighton Road during severe weather, flooding the road and the entrance to Sussex Yacht Club. The Environment Agency, West Sussex County Council and Adur & Worthing Councils are now building new flood defences along that stretch — a steel-piled concrete, brick and flint wall up to 2.2 metres high, with construction due to run from spring to autumn 2026. The scheme follows the earlier Shoreham Adur Tidal Walls scheme, which the Environment Agency completed to reduce tidal flood risk through the town centre.

Further upstream, the low-lying washland east of Henfield — known locally as “the brooks” — is expected to flood most winters. The Environment Agency's own catchment plan treats this as a beneficial part of the river's natural function in the upper catchment, and the Henfield Levels it floods across support wetland habitat for overwintering birds.

Which areas does the River Adur put at risk?

The River Adur puts a run of West Sussex settlements at risk from source to sea, but not in the same way: inland towns face a fast fluvial response to rainfall, while Shoreham-by-Sea faces a tide-driven risk that the Environment Agency expects to grow faster than almost anywhere else on the south coast. The table below summarises the main settlements at risk, in order downstream.

These figures draw on the Environment Agency's Adur Catchment Flood Management Plan, which found around 91 properties at risk from a 1%-probability river flood in Steyning and Upper Beeding today, rising only to 112 by 2100, against just 11 properties at risk in Shoreham and the Adur Estuary today — a figure the plan expects to reach 1,757 by 2100 once tidal and fluvial flooding are considered together under climate change. Properties on the river's upper reaches, around Slinfold, Coolham, Shipley and West Grinstead, face a much lower risk throughout — fewer than ten today or in the future. For the wider county picture, including the Adur's neighbouring rivers, see our West Sussex flood risk guide; the coastal towns either side of the estuary are covered by our Worthing flood risk guide.

Is the River Adur a Main River — and what that means for your site

Yes. Above Old Shoreham Bridge, the River Adur is the Environment Agency's responsibility as a designated Main River; below the bridge, the tidal harbour is managed instead by the Shoreham Port Authority. That distinction is not academic if you are developing nearby: works within 8 metres of the top of the bank on the Main River reach generally need a Flood Risk Activity Permit from the Environment Agency before they start, and a site in the river's floodplain will normally need a flood risk assessment to support a planning application.

The many smaller streams that feed the Adur — Herrings Stream, Cowfold Stream, Honeybridge Stream, Woodsmill Stream and the rest — are ordinary watercourses instead, regulated by West Sussex County Council as Lead Local Flood Authority and by the riparian owners whose land they cross. Getting that classification right at the outset saves time later, and we confirm it for every site we assess.

• The 8 metre byelaw margin. Building, excavation or structures near the Main River reach usually require a Flood Risk Activity Permit, sought alongside — not instead of — planning permission.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, or a larger site in Flood Zone 1, needs a flood risk assessment for planning.
• Know which watercourse you are on. The Main River is Environment Agency territory; its tributaries are ordinary watercourses under the Lead Local Flood Authority. We confirm this for every site.

Flood risk and drainage services for sites near the River Adur

A site near the River Adur can call for several different reports, and it helps to be clear that they answer different questions. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they address separate things: whether the site is safe from flooding, whether the development should be located here, and where the surface water goes. Unda prepares each of them for sites across the River Adur catchment.

• Flood risk assessments for planning. Site-specific assessments addressing the Environment Agency's requirements for development in the river's flood zones, through our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders through our flood risk survey for property purchase and flood risk assessment for insurance.
• Drainage strategies and SuDS. Surface-water designs that meet the discharge limits the Lead Local Flood Authority applies, through our sustainable drainage strategy service.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where the valley's higher water table is in play.

Where national mapping is not detailed enough for a site right beside a Main River, we can bring in hydraulic flood modelling to derive accurate flood levels and extents, and prepare the detailed scheme needed to discharge a drainage condition. Everything we do is framed by the NPPF and its planning practice guidance.

Managing flood risk on the River Adur: policy and contacts

Several bodies share responsibility for flood risk on the River Adur, and it is worth knowing who does what. The river sits within the Adur and Ouse catchment, part of the South East River Basin District. The Environment Agency regulates the Main River, publishes the flood map for planning and issues flood warnings for the river; West Sussex County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses across the catchment; and Horsham District Council and Adur District Council are the planning authorities along most of its length, each drawing on the joint Adur and Worthing Strategic Flood Risk Assessment and the Environment Agency's Adur Catchment Flood Management Plan. Downstream, the Environment Agency's Adur Tidal Walls scheme and the new Shoreham town centre flood defences set out how tidal risk through the estuary is being managed.

• Live river levels. Check the river's gauges on the Environment Agency service — Beeding Bridge, Sakeham Weir and Shoreham Harbour — or use our own live river levels map.
• Flood alerts. The River Adur East Branch and Lower Adur flood alert areas cover the river from its upper eastern arm down to Shoreham-by-Sea.
• Screen a location. Try our free flood risk map and, for drainage design, our drainage calculators and BRE 365 infiltration calculator.
• Flood warnings. Sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 to report a power cut.

How Unda helps with flood risk on the River Adur

Unda understand how a two-armed, part-tidal river like the Adur behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know how quickly the upper catchment responds to rain, how far the tide reaches back up the estuary, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning at Henfield, Steyning, Shoreham-by-Sea or anywhere in between. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the valley.

Every assessment rests on the same evidence the regulators use: the Environment Agency's flood mapping, the river's gauged levels, and the local planning policy that applies to the site. If you are working on a site near the River Adur, talk to one of our experienced flood risk consultants about what your application will need.

River Adur flood risk: your questions answered