River Arun Flooding & Flood Risk

The River Arun carries flood risk that few rivers its size can match. It falls fast off the Weald clay above Horsham, then runs into one of the longest tidal reaches in southern England — the sea reaches 41 kilometres inland to Pallingham — so Pulborough, Amberley and Arundel can go under on a high spring tide as readily as on high river flow. Add a North Sea surge on top and the lower valley floods hard, as Littlehampton and Arundel have learned repeatedly. Unda prepares flood risk assessments, drainage strategies and related reports for sites the length of the Arun floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

One of the fastest-flowing rivers in England, the Arun is tidal for 25 miles (41 km) inland to Pallingham Quay — so the tide, not just the rain, decides flood risk from Arundel all the way up to Pulborough and Amberley.

• A flashy river with a long tidal tail. Wealden clay headwaters shed rain quickly, while the lower 41 km is tide-locked — a combination that floods the valley from Horsham down to the coast.
• A designated Main River. The Environment Agency regulates the Arun, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near its banks.
• Record levels set in December 2013. The Arun reached 4.78 m at Pulborough (24 December) and 3.76 m at Littlehampton (6 December, on a tidal surge) — both still the highest on record.
• Development pressure the length of the valley. From Horsham to Arundel and Littlehampton, sites in and near the floodplain almost always need a flood risk assessment for planning.

Check flood risk along the River Arun

The map below draws the Arun in coral, from its source east of Horsham to the sea at Littlehampton, with the Environment Agency’s live river gauges positioned along it — click a gauge to see its latest level, its typical range and its record peak. Enter a postcode near the river to zoom in and screen that location against the Environment Agency’s flood 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.

Which areas does the River Arun put at risk?

Flooding from the Arun is spread the length of the valley, but it changes character as the river drops off the Weald and enters its tidal reach. In the upper catchment around Horsham it is a fast fluvial river; below Pallingham it is tidal, and the towns beside it flood on a combination of river flow, high tides and, at the coast, storm surge. The Environment Agency runs a formal warning service for the communities most exposed, with named flood warning areas at Horsham, Pulborough, Amberley and Arundel sitting inside the wider Upper Arun and Lower Arun flood alert areas. The table below summarises the main settlements at risk, from source to mouth.

If your site sits within one of these areas, the Environment Agency’s flood map for planning is likely to place it in Flood Zone 2 or 3, which triggers the need for a site-specific flood risk assessment. You can screen any address against the live Environment Agency data using the map above, and the towns along the river are covered in more depth on our local pages for Horsham, Arundel and Littlehampton, and across the county on our West Sussex page.

Why does the River Arun flood?

The Arun floods for two quite different reasons at opposite ends of its course, and the sites at greatest risk are the ones caught by both. In the upper catchment the river drains the heavy clays and greensands of the Weald around Horsham. Clay sheds rain rather than soaking it up, so after prolonged or intense wet weather the headwater streams — known locally as ghylls — fill fast, the river rises sharply and it pushes a large volume of water downstream in a hurry. Hydrologists call a river that responds this way “flashy”, and the Arun’s reputation as one of the fastest-flowing rivers in England, running at 4 to 6 knots, is a direct expression of it.

The second mechanism is the tide. Below Pallingham Quay, 41 kilometres from the sea, the Arun is tidal, and the tidal range at Littlehampton reaches 5.2 metres on spring tides. On a high tide the sea holds back — and can reverse — the river’s drainage, so land water has nowhere to go. Because high water at Pulborough falls about four hours after high water at Littlehampton, a spring tide can keep the middle valley’s levels up for hours. When a large fluvial flood coming off the Weald coincides with a high spring tide, or worse a North Sea storm surge pushing water up the Channel, the lower Arun floods severely. That coincidence is what an assessment for a site anywhere in the valley has to reckon with.

The river rises as a series of ghylls in St Leonard’s Forest east of Horsham, flows west past the town to Nowhurst where the North River joins it, then turns south and is joined by its main tributary, the western River Rother, at Stopham near Pulborough. From there it cuts through the gap in the South Downs at Arundel to reach the English Channel at Littlehampton — 60 kilometres in all, the longest river lying entirely within Sussex.

• Wealden clay headwaters. The upper catchment sheds rain quickly, producing a fast-rising, flashy river above and through Horsham.
• A 41 km tidal reach. Below Pallingham the tide governs levels; a spring tide or surge can hold the whole lower valley up for hours.
• The Rother roughly doubles the flow. The western River Rother joins at Stopham, adding a large slug of Downs and greensand runoff just above Pulborough.
• Record peaks at the gauges. Pulborough (Swan Bridge) reached 4.78 m on 24 December 2013; Littlehampton reached 3.76 m on 6 December 2013 in a tidal surge.

A history of flooding on the River Arun

The Arun’s flood record is dominated by the winter of 2013/14, the wettest on record for much of Sussex, when the river flooded twice in a single month by two different mechanisms. First, on the night of 5–6 December 2013, a North Sea storm surge drove a tidal flood up the estuary: the gauge at Littlehampton reached its all-time high of 3.76 metres at around 1:30am, and the Riverside Industrial Estate on Bridge Road was inundated overnight, with businesses reporting the worst flooding they had ever seen. Barely a fortnight later, sustained rainfall pushed the fluvial river to its own record — 4.78 metres at Pulborough Swan Bridge on 24 December 2013.

The New Year brought no relief. In early January 2014 the Environment Agency issued more than a hundred flood warnings across the South East, houses in Arundel were pumped out by fire crews, and Rope Walk and River Road on either side of the Arun in Littlehampton flooded again — the second serious flood there in under a month. More recently, during Storm Ciarán in November 2023 the river was reported as the highest it had ever been at Arundel, and West Sussex County Council carried out formal flood investigations after the winter 2023/24 storm sequence. Further back, the autumn of 2000 — the wettest for nearly three hundred years — brought widespread flooding across Sussex that put the Arun valley and much of the surrounding area under water.

Those repeated tidal floods are why the lower river is now being heavily defended. The Environment Agency’s Arundel tidal walls scheme is raising and rebuilding defences to better protect around 130 properties in the town, and the Littlehampton tidal walls, built along the estuary, protect more than 2,000 homes and businesses at the river’s mouth. Defences reduce the odds; they do not remove the risk, and a site behind them still has to demonstrate it is safe.

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

Yes. The Environment Agency has designated the Arun a Main River, which makes it, rather than the local council, the regulator for the channel and its immediate margins. That distinction matters if you are developing nearby. Works within 8 metres of the top of the bank 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 ghylls, brooks and ditches that feed the Arun are ordinary watercourses instead — regulated by West Sussex County Council as Lead Local Flood Authority and by the riparian owners whose land they cross, and often subject to ordinary watercourse consent for culverting or diversion. Getting that classification right at the outset saves time and cost later.

• The 8 metre byelaw margin. Building, excavation or structures near the Arun 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. Our guide to flood zones 1, 2 and 3 explains the bands.
• The tidal reach needs both sources assessed. Sites below Pallingham face tidal as well as fluvial flooding, and an assessment that considers only one will not satisfy the Environment Agency.

Flood risk and drainage services for sites near the River Arun

A development near the Arun 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 Arun catchment and floodplain.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the river’s flood zones — see 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 manage runoff and meet the discharge limits set by the Lead Local Flood Authority — see our sustainable drainage strategy service, shaped by the county’s tighter Arun drainage checklists.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• BREEAM, groundwater and consequence assessments. A BREEAM flood risk assessment, a groundwater flood risk assessment where a high water table is in play, and a flood consequence assessment where one is required.

Where national mapping is not enough — a common situation for a site right beside a tidal Main River like the Arun — we can bring in hydraulic flood modelling to derive accurate flood levels and extents, prepare the preliminary drainage design and, later, the detailed scheme needed to discharge a drainage condition. Everything we do is framed by the NPPF and its planning practice guidance, including the current climate change allowances for peak river flow and sea level rise.

Managing flood risk on the River Arun: policy and contacts

Several bodies share responsibility for flood risk on the Arun, and it is worth knowing who does what. The river sits within the Arun and Western Streams catchment, part of the South East river basin district, and the Environment Agency’s Arun and Western Streams catchment flood management plan sets the long-term strategy for managing its risk. The Environment Agency regulates the Main River, publishes the flood map for planning and issues flood warnings; West Sussex County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses; and Horsham District and Arun District are the planning authorities that decide applications along the river, each with its own strategic flood risk assessment. One local constraint worth flagging: the internationally protected Arun Valley wetlands underpinned the Sussex North water neutrality requirement, which stalled development across the catchment for several years before its late-2025 withdrawal.

• Live river levels. Check the Arun’s gauges on the Environment Agency service — Pulborough (Swan Bridge), Arundel and Littlehampton — or use our own live river levels map.
• Screen a location. Try our free flood risk map and, for drainage design, the drainage calculators, the BRE 365 infiltration calculator and the geology and ground checker.
• Flood warnings. Sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188. Report flooding from ordinary watercourses to West Sussex County Council, and sewer flooding to Southern Water.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on the River Arun

Unda understand how the Arun behaves in flood — the flashy Wealden headwaters above Horsham, the long tidal reach where the sea governs levels as far up as Pulborough, and the surge-driven flooding that hits Arundel and Littlehampton at the coast — and what the planning system and the Environment Agency expect from the reports that follow. We know where the floodplain bites hardest along the valley, how a tidal Main River changes what an assessment has to demonstrate, and what a robust, compliance-led flood risk assessment needs to carry a development through planning. We have prepared assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the Arun floodplain.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping, the river’s gauged levels and modelled flows, tidal and surge data for the lower reaches, and the local planning policy that applies to the site. If you are working on a site near the River Arun, talk to one of our experienced flood risk consultants about what your application will need, or read more about when a flood risk assessment is required and how much one costs.

River Arun flood risk: your questions answered