River Arun Flooding & Flood Risk

River Arun flooding comes from two directions. Rain on the Weald Clay above Horsham runs off fast and lifts the river quickly, and from the coast at Littlehampton the tide reaches 41 km inland to Pallingham Quay, so Pulborough, Amberley, Arundel and Littlehampton can flood on a high tide as well as on river flow. The Arun is the longest river wholly in Sussex, and we prepare flood risk assessments and drainage strategies for sites along its valley in West Sussex.

In April 2024 a storm surge on a spring tide carried the Arun over its west-bank walls at Rope Walk in Littlehampton, flooding about 20 homes and 16 businesses.

To talk through a site, call 01293 214444 or email enquiries@unda.co.uk.

• Course. About 60 km (37 miles), from small streams in St Leonard's Forest east of Horsham to the English Channel at Littlehampton.
• Catchment. 379 square kilometres above the Pallingham gauge, 84% of it on low-permeability rock, mostly Weald Clay, according to the National River Flow Archive.
• Tidal limit. Pallingham Quay, upstream of Pulborough and 41 km from the sea, so roughly two-thirds of the river's length feels the tide.
• Regulator. The Environment Agency, which manages the Arun as a Main River.

How far inland is the River Arun tidal?

All the way to Pallingham Quay, 41 km (25.5 miles) up from the harbour mouth. At Littlehampton the range on a spring tide is about 5.2 metres, against 2.7 metres on a neap. Twice a day the tide holds the river back: while it is high, water coming down from the Weald has nowhere to go, and levels at Amberley and Pulborough stay up after the rain has stopped. The current is strong too: the Arun is usually listed among the faster-flowing rivers in England, at 4 to 6 knots.

Even the flow gauge at the tidal limit feels it. The National River Flow Archive notes that its weir at Pallingham drowns on the higher tides, which is why flood flows there are hard to measure. For a site anywhere between Pallingham and the coast there are two sources of flooding to assess, the river and the sea, and the worst case is the two peaking together. Our explainers on tidal flooding and on storm surges cover how each is handled in planning.

• Littlehampton. At the mouth the sea sets the flood level more often than the river does; a surge on top of a spring tide is what comes over the walls.
• Arundel. The town's gauge reached its highest level, 3.65 m, on a tidal surge on 6 December 2013, not during the river floods that followed at Christmas.
• Amberley and Pulborough. Above the gap in the Downs the tide still controls low water, so a river flood that arrives on a high tide spreads across the wet meadows either side of the channel.

Why River Arun flooding comes from two directions

The upper Arun is a clay river. The National River Flow Archive puts its base flow index at Pallingham at 0.21, which means about four-fifths of the water reaches the channel as quick run-off off the land instead of seeping out of the ground over weeks. The archive describes the regime as very responsive and notes a growing urban area around Horsham. Heavy rain on the ghylls of St Leonard's Forest and the fields around the town reaches the river fast and sends it up sharply.

The Western Rother behaves differently. Most of its catchment is chalk and greensand, its base flow index at Hardham is 0.64, and it rises more slowly and stays high for longer. It joins the Arun near Pulborough, close to where the tide takes over, and its average flow at Hardham (4.5 cubic metres a second) is not far short of the Arun's own at Pallingham (5.6). The River Rother page follows its floods through Midhurst and Fittleworth, and the River Lox is one of the clay streams that feeds the upper Arun, at Drungewick.

The lower valley floods hardest when a long wet spell has filled both rivers and a spring tide or a surge arrives at the same time. December 2013 showed each on its own, a surge on the 6th and the river at Christmas. Littlehampton had the surge again in April 2024, two days after heavy rain.

Live gauges and record levels on the Arun

The map below draws the River Arun in coral from St Leonard's Forest to the sea, with the Environment Agency's live gauges along it. Enter a postcode to screen a location against the Agency's flood data, or open our live river levels map to compare the Arun with the Rother and the other Sussex rivers.

Each gauge reads against its own datum, so a level means something only against that gauge's own history. Five of the eight highest readings the Agency lists came in December 2013.

At Pallingham the archive's highest daily flow is 162 cubic metres a second, on 24 December 2013, about 29 times the long-term average there. Pulborough's normal range tops out at 3.71 m; the Agency warns that property can flood above 4.36 m.

Which places does the River Arun flood?

The kind of flood changes down the valley. Horsham sits on the quick-rising clay river. From Pulborough down the river is tidal, and flood water spreads across wide meadows: the Arun Valley wetlands between Pulborough and Amberley, 529 hectares protected as a Ramsar site, are floodplain that goes under in wet winters. At Arundel and Littlehampton the tide and surge decide most floods. The Environment Agency covers the valley with two alert areas, Upper Arun and Lower Arun, and runs a warning area for each community below.

Pulborough, Amberley, Ford and the villages of the upper valley are covered on our West Sussex page.

When has the River Arun flooded?

The Arun's record floods are recent. The winter of 2013 to 2014 set the highest levels at most of its gauges, flooding the valley from both ends within three weeks, and the worst tidal flood since came at Littlehampton in April 2024.

1. 5 to 6 December 2013. A tidal surge took the Arundel gauge to 3.65 m, still its highest reading, and flooded Rope Walk in Littlehampton overnight.
2. 24 and 25 December 2013. Rain on saturated clay set the highest levels on record at Tanbridge and Alfoldean above Horsham, at Pulborough Swan Bridge (4.78 m) and, a day later, at Houghton Bridge (3.69 m). The River Lox, a clay tributary, reached its own record at Drungewick on Christmas Eve.
3. January 2014. A high tide flooded Rope Walk and riverside businesses in Littlehampton again, the second time in under a month, according to a local press report.
4. 8 to 9 April 2024. Storm Pierrick's surge arrived on a spring tide two days after Storm Kathleen's rain. The Arun came over the west-bank walls at Rope Walk. Those stand at about 3.23 m above Ordnance Datum, against 3.94 m on the east bank, and about 20 homes flooded to around half a metre, along with 16 businesses. Water outside reached about 1.5 m in places, and the Littlehampton gauge peaked at 3.78 m at 00:30 on 9 April, according to West Sussex County Council's flood investigation.

Do the Arundel and Littlehampton walls take a site out of the flood zone?

No. A wall lowers the chance of water reaching a building, but the Environment Agency's flood map for planning still shows defended land on the Arun as Flood Zone 3, and an assessment for a site behind a wall has to show what happens if it is overtopped or breached. Our flood risk map gives the zone for any postcode in the valley.

• Arundel. The Environment Agency finished the Arundel Tidal Walls Scheme in December 2024, after work that began in 2019 and included a new 93 m sheet-pile wall. It protects 176 homes and many businesses on River Road, Tarrant Wharf and both banks nearby (Environment Agency), and future upkeep of the town's river walls now falls to their riparian owners.
• Littlehampton. The East Bank scheme, a £22 million project with a 450 m tidal wall, protects over 2,000 homes and businesses on the town side, according to the Environment Agency. The west bank at Rope Walk is lower and its walls vary in condition, which is where the river came over in 2024.
• The tidal river as a whole. The Agency's Lower Tidal River Arun Strategy sets how the banks from Pallingham Weir to Littlehampton and Ford are to be managed.
• What a defence leaves behind. Residual flood risk is the depth and speed of water behind a defence if it fails; on the tidal Arun it sets floor levels and escape routes for many schemes.

Is the River Arun a Main River?

Yes. The Environment Agency regulates the Arun as a Main River, with powers over the channel, its banks and its defences, and work within 8 metres of the bank may need a permit from the Agency as well as planning permission. The ghylls, ditches and small streams that feed it are mostly ordinary watercourses, where West Sussex County Council as lead local flood authority and the landowners along them carry the responsibilities, and culverting or diverting one may need ordinary watercourse consent. How the two regimes differ is covered in our explainer on Main Rivers and ordinary watercourses.

• Flood zones set the trigger. Under chapter 18 of the National Planning Policy Framework, any application in Flood Zone 2 or 3 needs a flood risk assessment, and so does a site of a hectare or more in Flood Zone 1. Our guide to flood zones 1, 2 and 3 explains the bands.
• Two sources below Pallingham. On the tidal river the assessment covers river flooding and the tide together, using the Agency's climate change allowances for peak river flow and for sea level.
• Clay and the coastal plain. Weald Clay seldom takes soakaways, and Arun District Council asks for winter groundwater monitoring and winter infiltration tests before it will accept infiltration. The BRE 365 infiltration calculator and our article on Arun's SuDS checklists cover the tests.
• Water neutrality. The Arun Valley wetlands were the reason for the water neutrality requirement on development in Horsham district and the wider Sussex North supply zone; Natural England withdrew it on 31 October 2025.

Flood risk and drainage reports for sites in the Arun valley

Three questions come up on most sites in the Arun valley, and each has its own report. A flood risk assessment asks whether a building will be safe from the river and the tide for its lifetime without pushing water onto anyone else. A sequential test asks whether the scheme could go on land at lower risk. A drainage strategy works out where rain falling on new roofs and paths will go when the outfall is tide-locked or the clay will not soak it up.

• Any site in the Arun's flood zones. A flood risk assessment starts from the Agency's mapping, the gauge records above and, below Pallingham, the tide levels at Littlehampton.
• Planning applications. A flood risk assessment for planning answers what Horsham or Arun District Council and the Environment Agency will ask about building on the Arun floodplain.
• Buying near the river. A flood risk survey for property purchase sets out the river's flood record, the zone and the defences in front of a house before exchange.
• Housing on the floodplain. Housing on the floodplain at Pulborough, Arundel or Littlehampton normally needs the sequential and exception test to show there was no reasonably available site at lower risk.
• Rain falling on the site. A drainage strategy shows how a scheme on the Arun's clay or its coastal plain will deal with rainfall without adding to the river.
• Discharge rates and storage. A surface water drainage strategy sets the outflow and the storage needed where the outfall backs up at high tide.
• After permission. Our consultants prepare the evidence for the discharge of drainage conditions that Arun and Horsham attach to permissions on the valley floor.
• Levels the national map cannot give. Flood modelling works out depths and levels where a site sits close to the tidal Arun and the mapped zone is too coarse to set a floor level.
• Getting people out. A flood warning and evacuation plan sets out how occupants leave a building on the tidal reach, using the Agency's warnings for Arundel, Ford and Rope Walk.
• Insurance. Homes at Rope Walk flooded in 2013, 2014 and 2024, and a flood risk assessment for insurance gives an underwriter the evidence for a property near the river.
• Commercial land at the mouth. Before buying riverside commercial land in Littlehampton or Ford, a due diligence flood risk assessment sets out the tidal exposure for lenders and investors.
• Keeping a business open. Sixteen businesses at Rope Walk were flooded inside in April 2024; a business continuity flood risk assessment plans for the next tide.
• Groundwater. Arun District Council asks for a groundwater flood risk assessment where its flood study shows more than a 25% chance of groundwater flooding, which matters on the low ground of the lower valley.

Who manages flood risk on the River Arun?

The Environment Agency looks after the river, the tidal walls and the warnings. West Sussex County Council deals with surface water, groundwater and the ordinary watercourses, and investigates serious floods. Planning decisions sit with Horsham District Council on the upper river, Arun District Council on the lower valley and the South Downs National Park Authority through Amberley and Houghton; our pages on Arun and Horsham set out each council's requirements.

• Catchment plans. The South East river basin district flood risk management plan and the earlier Arun and Western Streams catchment flood management plan.
• Regional committee. The Southern Regional Flood and Coastal Committee approves the Agency's flood and coastal work in Sussex.
• Emergency planning. The Sussex Resilience Forum (local resilience forum contacts); Arun District Council also runs the Arun Flood Forum with the Agency, the county council and Arundel Town Council.
• Sewers. Southern Water deals with flooding from the public sewers.
• Warnings. Sign up for flood warnings for the areas above, or call Floodline on 0345 988 1188.
• In an emergency. Call 999 if life is in danger, and 105 for a power cut.

Questions about flooding on the River Arun

For a site anywhere from Horsham to the harbour at Littlehampton, call 01293 214444 or email enquiries@unda.co.uk. Unda Consulting Limited's chartered consultants will tell you which reports the scheme or the sale needs.