Flood Risk Assessment in Crawley

Unda is a flood risk and drainage consultancy based in County Oak, Crawley, preparing clear, technically robust reports for residential and commercial sites across the town and the wider Upper Mole catchment. A flood risk assessment in Crawley shows Crawley Borough Council, 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. We prepare flood risk assessments in Crawley for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS) — guiding developers, planning consultants, architects, landowners, home buyers and businesses through to a decision.

Crawley is a surface-water town. Built largely after 1947 as a post-war New Town, its neighbourhoods sit at the head of the River Mole and shed rainfall fast through engineered channels and culverts towards the Gatwick Stream, Ifield Brook and their fellow tributaries. We assess every source that matters on a given site — surface water first, then the Mole and its tributaries, groundwater, and the sewers — and deal directly with Crawley Borough Council and West Sussex County Council as Lead Local Flood Authority on your behalf. 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.

In Crawley the flooding that catches schemes out is rarely the river — it is surface water running off the New Town’s clay faster than the drains can carry it, on sites that map as Flood Zone 1.

Flood risk assessment in Crawley at a glance

• Crawley sits at the head of the Upper Mole. The River Mole and its tributaries — the Gatwick Stream, Ifield Brook, Crawters Brook and Tilgate Brook — gather within a confined corridor north of the town before passing Gatwick, one of the fastest-responding catchments in the South East.
• Surface water is the dominant risk. The New Town's impermeable clay soils and dense drainage network mean intense rain runs off almost completely, overwhelming culverts and gullies long before the rivers themselves spill.
• Two flood events shape every assessment here. The September 1968 storm closed Gatwick Airport and was the worst Mole flooding in around 100 years; the December 2000 floods put around 100 Crawley homes and parts of Gatwick under water.
• The Upper Mole Flood Alleviation Scheme protects over 1,400 properties. Reservoirs at Tilgate, Clay's Lake, Worth Farm and Ifield now hold back the peak — but they manage river flood risk, not the surface water that floods Crawley streets.
• 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 Flood Zone 1 site in Crawley.

Check the flood risk anywhere in Crawley

Crawley is outlined in coral below. Screen any postcode in the borough against Environment Agency data for rivers and sea, surface water, reservoirs 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 Crawley: surface water, the Mole and its tributaries

No single map captures flood risk in Crawley. The town drains through a tight network of brooks that meet north of the centre and pass Gatwick as the River Mole, so a credible flood risk assessment in Crawley has to identify which source actually drives the risk on your site — surface water, a named tributary, or groundwater rising through the clay — because the mitigation is different for each.

The dominant story is surface water. Crawley's neighbourhoods were laid out for rapid, engineered drainage, and on the town's largely impermeable Weald Clay and Hastings Beds the runoff coefficient is close to total in a heavy storm. Runoff converges on the watercourses along roads, footpaths and landscaped corridors, and where a culvert or highway drain is undersized the exceedance flow simply travels overland. The Gatwick Stream, Ifield Brook, Crawters Brook and Tilgate Brook all feed the Mole, which is a main river by the time it reaches the airport.

Because flood zones only map rivers and the sea, a Crawley site can sit firmly in Flood Zone 1 and still be at real risk from surface water, or from groundwater where the clay gives way to more permeable beds. You can get an early read on any address with Unda's free Flood Risk Map and Live River Levels Map for the Mole and Gatwick Stream gauges.

A history of flooding in Crawley

Dated local events are the clearest test of whether risk has really been understood on a site. Crawley has two that every assessment in the town should have in view — a river flood that closed an international airport, and an urban surface-water flood that overwhelmed a modern estate — because between them they show how differently water behaves across the catchment.

• September 1968 — the Great Flood. Thunderstorms stalled over the Gatwick area on 14–15 September 1968, dropping more than 100mm in 24 hours in a roughly 1-in-200-year storm. The Mole burst its banks in the worst flooding the area had seen in around 100 years and closed Gatwick Airport for several hours; across Surrey's rivers some 14,000 properties were affected.
• December 2000 — Crawley and Gatwick. During the wettest autumn for nearly 300 years, more than 150mm of rain fell on the Upper Mole in 72 hours. Around 100 Crawley homes flooded along with parts of Gatwick; Maidenbower, and Haslett and Hazelwick Avenues in Three Bridges, were among the worst hit, and flows in the Gatwick Stream exceeded the 1-in-75-year level. Unda has written about the 2000 Crawley and Gatwick floods in detail, and on the Great Flood of 1968.
• The response: the Upper Mole Flood Alleviation Scheme. Begun in 2010 with £11m from the Environment Agency, £4m from Gatwick Airport and £100,000 from Crawley, the scheme raised the Tilgate Lake dam, rebuilt Clay's Lake, and built the Worth Farm and Ifield storage areas, reducing river flood risk to more than 1,400 properties across Crawley, Horley and Gatwick.

The defences hold back the rivers, but they do not stop surface water flooding Crawley's streets — which is why a site-specific assessment of how runoff behaves on your plot still matters.

These events shape how the Environment Agency and West Sussex County Council read new applications today. A scheme near the Mole or the Gatwick Stream is expected to address the river and lifetime climate change; a site in the older built-up neighbourhoods has to show surface water can be managed without repeating 2000. Getting this right at the first attempt matters more than ever, because the May 2026 Flood Map for Planning update added surface-water climate-change data that pulls some previously unflagged sites into a mapped risk area, and UKCP18 projections suggest short-duration rainfall could rise by up to 40% by mid-century.

Planning and drainage policy in Crawley

Two authorities govern flood risk and drainage for a Crawley site. Crawley Borough Council is the local planning authority that determines applications, and West Sussex County Council is the Lead Local Flood Authority, leading on surface water, groundwater and ordinary watercourses and acting as the statutory consultee on surface water drainage for major development.

• Crawley Borough Local Plan 2023–2040. Adopted on 16 October 2024, Policy EP1 (Development and Flood Risk) sets the borough's risk-based, sequential approach, with Policy EP2 covering flood risk for householder development.
• The 2023 Strategic Flood Risk Assessment. The borough's SFRA, prepared with the Upper Mole catchment in view, is the local evidence base a reviewer reaches for first — including for critical drainage areas in Flood Zone 1.
• Surface water drainage and SuDS. Major development must manage runoff with sustainable drainage approved by the council, following the surface-water hierarchy and the 2025 national standards for SuDS.
• Infiltration evidence is expected. Assumed soakaway rates without BRE 365 testing rarely pass on Crawley's Weald Clay, where infiltration is often poor — check the ground first with Unda's geology and ground conditions map.
• Watercourse works need consent. Building near, over or altering an ordinary watercourse such as the Gatwick Stream or Ifield Brook needs the LLFA's consent, separate from planning permission.

Why sites in Crawley need a flood risk assessment

Development in Crawley tends to fall into a few recognisable patterns, and each raises its own flood risk and drainage questions. Knowing which one applies to your site is the quickest route to a proportionate assessment and a smooth application.

• Town-centre and brownfield regeneration. Higher-density redevelopment around the centre and the railway corridor frequently sits on or near Crawters Brook and the culverted network, where surface water and drainage capacity drive the design.
• Manor Royal and Gatwick-related commercial schemes. Large roofs and hardstanding at Manor Royal and County Oak generate substantial runoff towards the Mole and Gatwick Stream, so attenuation and discharge controls are central to consent.
• New-town infill and garden land. Building within the established neighbourhoods increases impermeable area on already fast-draining clay, which the council expects to be offset with sustainable drainage.
• Prior-approval conversions. Changing offices or commercial buildings to homes raises the vulnerability of the use, so even a low-lying Flood Zone 1 building can need raised floor levels and a safe escape route to be approved.
• Surface-water Flood Zone 1 sites. Many Crawley plots screen as Flood Zone 1 for rivers yet carry mapped surface-water risk on the site or its access — the situation the 2025 PPG update brought firmly into scope.

How Unda helps with flood risk in Crawley

Unda are flood risk and drainage experts based in Crawley, at Oak Cottage in County Oak, between the town and Gatwick — the same Upper Mole catchment we assess for clients. That local knowledge is matched by the technical depth to be the accurate translation layer between your scheme and the Crawley Borough Council, West Sussex County Council, Environment Agency and NPPF requirements it has to satisfy. A Senior Flood Risk Consultant oversees and signs off every report, and we stay involved past the report to answer consultee objections and keep the application moving.

One recent regional example shows the pattern on exactly the kind of surface-water site that is common across Crawley. A developer was seeking prior approval to convert a Class E commercial building to homes on a site that screened as Flood Zone 1, where the real constraint was surface water. Our flood risk assessment confirmed a maximum modelled surface-water depth of 0.3m, and we worked with the design team to raise finished floor levels with at least 600mm of freeboard and to introduce a floodable walkway giving a safe route to flood-free ground. The scheme was revised so occupants had internal access to a higher floor, and prior approval was secured.

We have resolved similar constraints on comparable sites — from a surface-water-affected residential infill, where the safe escape route was the deciding issue, to a property purchase where careful interpretation of surface-water mapping let the buyer secure insurance on normal terms.

Flood risk assessments for every situation in Crawley

Whatever the reason you need flood risk or drainage advice in Crawley, 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 — see FRA for planning.
• Flood risk survey for property purchase. A property-specific survey for buyers and their advisers when a conveyancing search flags risk — our homebuyer flood risk survey.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover or premiums are in question — an insurance flood risk assessment.
• Flood risk assessment for BREEAM. An assessment structured around BREEAM Pol 03 for project teams pursuing certification — FRA for BREEAM.
• Sequential and exception tests. The locational case for development in or near a flood risk area — sequential and exception test reports.
• Surface water drainage strategy (SuDS). How runoff is managed to meet the surface-water hierarchy and the council's expectations — a drainage strategy and SuDS design, with discharge of drainage conditions and preliminary drainage design where needed.
• Groundwater and flood modelling. A groundwater flood risk assessment where the clay gives way to permeable beds, and hydraulic flood modelling where national mapping is not enough — both feeding the wider assessment.

You can get an early read on a site for free using Unda's Flood Risk Map, the Live River Levels Map, the geology and ground conditions checker and the drainage calculators — then talk to our flood risk consultants when you need a report. Crawley sits at the head of the Mole, which drains north into the Thames basin; for the wider county picture, see our flood risk assessment in West Sussex page.

Flood risk management and emergency contacts in Crawley

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

Flood risk assessment in Crawley: common questions