Ifield Brook Flooding & Flood Risk

Ifield Brook is the first significant tributary of the River Mole. It gathers the streams that drain the western half of Crawley, fills the sixteenth-century Ifield Mill Pond, then runs north through low meadows to meet the Mole on the town’s rural edge. For such a short watercourse it carries an unusual amount of consequence: it has flooded homes in Ifield repeatedly, it fed the storm that put Gatwick Airport under water in December 2013, and its floodplain now sits at the centre of the largest development argument in Sussex. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the brook’s catchment. To talk one through, call 01293 214444 or email enquiries@unda.co.uk.

When thousands of new homes are proposed beside a watercourse, the watercourse matters. Ifield Brook reached its record level — 2.51 metres at the Environment Agency’s Ifield gauge — in the early hours of Christmas Eve 2013, the same night Gatwick Airport flooded, and the West of Ifield proposals now before the planning system would build a new town district beside the floodplain that absorbed that water.

• A main river through a New Town. Ifield Brook and the streams that feed it through Crawley are designated Main Rivers, so the Environment Agency regulates them and the 8 metre byelaw margin and Flood Risk Activity Permits apply to works nearby.
• A record set in 2013. The Ifield gauge peaked at 2.51 metres at 4:45am on 24 December 2013 — the highest level in a record running back to 2004.
• Its own flood warning area. “Ifield Brook and the River Mole at Ifield and the River Mole at Lowfield Heath” covers homes near the brook and Gatwick Airport itself.
• A floodplain under the spotlight. Homes England’s application for around 3,000 homes west of Ifield was validated by Horsham District Council in August 2025 and is still to be decided.

Check flood risk along Ifield Brook

The map below draws Ifield Brook and its feeder streams in coral, with the Environment Agency’s live gauge at Ifield positioned on the channel — click the pin to see its latest level, its typical range and its record peak. Enter a postcode near the brook 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.

Why does Ifield Brook flood?

Ifield Brook floods because its catchment sheds rain instead of absorbing it. The brook drains around 15 square kilometres of impermeable Weald Clay, and a large share of that clay now lies under the streets, roofs and car parks of western Crawley. Rain that falls on Bewbush, Broadfield or Gossops Green has nowhere to soak in, so it runs off into a compact web of streams that all converge on Ifield Mill Pond — and the brook can go from placid to bank-full within hours of a heavy storm.

The numbers bear this out. At the Environment Agency’s gauge on the Mole just downstream of the brook’s confluence, the base flow index is 0.23 — meaning less than a quarter of the river’s flow comes from stored groundwater, with the rest arriving as runoff. A chalk stream, by comparison, would score above 0.9. Ifield Brook’s 15 square kilometres make up nearly half of the 31.8 square kilometre catchment measured at that gauge, in the part of the Mole basin that also receives its heaviest rainfall — an average of 851 millimetres a year around Crawley. The brook, in short, is the flashiest corner of a famously flashy river system.

• Weald Clay underneath. Nearly 90 per cent of the upper Mole catchment sits on low-permeability bedrock, so rainfall becomes runoff almost immediately.
• A New Town on top. Crawley’s post-war neighbourhoods drain quickly and deliberately — the Environment Agency classes Ifield Brook as a heavily modified water body, its channels straightened, culverted and impounded over the years.
• Everything meets at the mill pond. The streams draining Bewbush, Broadfield, Gossops Green and Crawley’s south-western neighbourhoods converge at Ifield Mill Pond, dammed in the 1500s to power the Ifield forge, so flood peaks concentrate there before the brook heads north.
• New development drains this way too. Kilnwood Vale, built on the Horsham side of the boundary, sends its watercourse west-to-east through Bewbush Water Gardens into the mill pond — a reminder that what happens upstream arrives in Ifield.

When has Ifield Brook flooded?

Often. The Environment Agency’s own account of the upper Mole names 1947, 1960, 1968, 1974, 1990, 2000 and the winter of 2013–14 as significant floods for Crawley and Horley, and the brook has played its part in most of them. September 1968 remains the benchmark for sheer volume: the highest daily flow ever measured on the Mole at Gatwick Airport — 17.4 cubic metres per second, roughly fifty times its average — was recorded on 15 September 1968, in the storm remembered across the South East as the Great Flood.

The autumn of 2000 showed what that means for property. In what the Environment Agency called the wettest autumn in nearly 300 years, 109 homes in Crawley flooded and Gatwick Airport was affected — the event that prompted the Upper Mole Flood Alleviation Scheme, and one we have written about in detail in Runways Underwater: the 2000 Crawley and Gatwick floods. Then came Christmas Eve 2013. After weeks of saturated ground, Ifield Brook rose to 2.51 metres at the Ifield gauge by 4:45am — still its highest recorded level — while a few kilometres downstream the Mole and Gatwick Stream flooded the airport and knocked out its North Terminal on one of the busiest travel days of the year. The brook has kept the warning service busy since: a flood warning was issued for the Mole and Ifield Brook at Ifield in September 2019, and in the wet winter of 2023–24, Storm Henk pushed the Mole at Gatwick to that year’s highest flow on 5 January 2024.

The response to those floods is written into the landscape. The Upper Mole Flood Alleviation Scheme built flood storage reservoirs at Tilgate Lake (2011), Worth Farm by the M23 (2013) and Clay’s Lake near Balcombe, together reducing the risk to more than 1,400 homes and businesses in Crawley and Horley and to Gatwick Airport. The fourth element — a flood storage reservoir at Ifield itself — was suspended when the contributions needed to build it could not be found. In its place came a package of local measures for Ifield Green: raised kerbs, altered road layouts and works to redirect flow away from properties, funded with £90,000 from the Thames Regional Flood and Coastal Committee and £100,000 apiece from West Sussex County Council and Crawley Borough Council. Worth knowing if you are assessing a site here: the reservoir that was planned for this brook was never built.

Which areas does Ifield Brook put at risk?

Flooding from Ifield Brook is concentrated along its meadows and the low ground beside its feeder streams, and the Environment Agency warns for it directly. A dedicated flood warning area — “Ifield Brook and the River Mole at Ifield and the River Mole at Lowfield Heath” — covers the properties most exposed, including Gatwick Airport, while the wider “Upper River Mole, Ifield Brook, Gatwick Stream and Burstow Stream” flood alert area names Ifield, Bewbush, Lowfield Heath, Charlwood, Crawley, Horley and Copthorne among the places where low-lying land floods first. The table below runs through the main communities from the headwaters down.

If your site sits in one of these areas, the flood map for planning is likely to place it in Flood Zone 2 or 3, or show surface water risk even where the river zones are clear — the situation our explainer on flood return periods and annual probability helps make sense of. The towns and counties around the brook are covered in more depth on our local pages for Crawley, Horsham, West Sussex and — for the Mole downstream through Horley — Surrey.

West of Ifield: development beside the brook’s floodplain

No English watercourse this small has a bigger planning question hanging over it. Homes England, the Government’s housing agency, has applied to Horsham District Council for around 3,000 homes on land west of Ifield — application DC/25/1312, validated in August 2025 — as the first phase of a vision reported at up to 10,000 homes wrapping Crawley’s western edge. The site sits between Ifield and Rusper, hard against Ifield Brook Meadows: the open floodplain that stores the brook’s floodwater today.

Flood risk is one of the questions the application is being tested on. Homes England has said the scheme would keep the existing floodplain as open space — with around half the site left unbuilt — and would look for opportunities to add flood alleviation measures, while objectors, including local campaign groups, argue that building over the catchment’s remaining permeable land risks pushing more water at Crawley faster. The application had not been decided when this page was written, in July 2026. Whatever the outcome, the direction of travel is clear: any site near the brook, on either side of the district boundary, will face detailed scrutiny of its flood risk and drainage evidence — and since the September 2025 update to national planning guidance, the sequential, risk-based approach applies to every source of flooding, surface water included.

Is Ifield Brook a main river — and what that means for your site

Yes. Ifield Brook and the feeder streams that join it through Crawley are designated Main Rivers on the Environment Agency’s Statutory Main River Map — unusual coverage for streams this size, and a mark of how seriously the network’s flood role is taken. Main River status means the Environment Agency, not the council, regulates the channel and its margins: works within 8 metres of the bank generally need a Flood Risk Activity Permit before they start, sought alongside any planning permission. Out in the rural headwaters towards Rusper, the field ditches and smaller watercourses are ordinary watercourses instead — there, works need Ordinary Watercourse Consent from West Sussex County Council, and riparian owners carry the maintenance duties. Getting that classification right at the outset saves real time later.

• The 8 metre byelaw margin. Building, excavation or structures near the brook or its designated feeder streams usually require a Flood Risk Activity Permit from the Environment Agency.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, a larger site in Flood Zone 1, or a smaller site with mapped surface water risk needs a flood risk assessment for planning.
• Two regulators, one catchment. The Environment Agency governs the main river network; West Sussex County Council, as Lead Local Flood Authority, governs the ordinary watercourses and surface water. We confirm which applies for every site.

Who manages flood risk on Ifield Brook?

Several bodies share the job. The brook sits in the Mole management catchment of the Thames River Basin District, and the Environment Agency — which built the Upper Mole Flood Alleviation Scheme — regulates the main river network, maps the floodplain and issues the warnings. West Sussex County Council is the Lead Local Flood Authority, responsible for surface water, groundwater and ordinary watercourses. Two planning authorities decide applications along the brook — Crawley Borough Council for most of the catchment and Horsham District Council for the upper reaches and the West of Ifield site — each with its own strategic flood risk assessment shaping what applicants must demonstrate.

Flood risk and drainage services for sites near Ifield Brook

A development near Ifield Brook can call for several different reports, and they answer different questions. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct documents: whether the site is safe from flooding, whether the development should be located there at all, and where the surface water goes. On Crawley’s clay, where soakaways rarely work and discharge rates are tightly limited, the drainage question is often the hardest of the three. Unda prepares each of them for sites across the brook’s catchment.

• Flood risk assessments for planning. Site-specific assessments addressing the Environment Agency’s requirements for development near the brook and its 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 hold runoff on site and meet the county council’s discharge limits — see our sustainable drainage strategy service, and our support for discharging drainage planning conditions.
• 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 winter water levels in the clay come into play.

Where the national mapping is not detailed enough — common for sites right beside a main river, and likely for anything substantial near the brook — we bring in hydraulic flood modelling to derive accurate flood levels and extents. Everything we prepare is framed by the NPPF and its planning practice guidance.

How Unda helps with flood risk on Ifield Brook

There are few watercourses our consultants know better than this one. We understand how the Upper Mole catchment behaves in flood — how quickly the brook rises off the clay, where its floodplain does its work through Ifield Brook Meadows, and what the Environment Agency, West Sussex County Council and the two planning authorities each expect from the evidence in front of them. We prepare flood risk assessments and drainage strategies for developers, planning consultants, architects, agents, buyers and homeowners with sites throughout the brook’s catchment.

Every assessment rests on the same evidence the regulators use: the flood map for planning, the brook’s gauged record, the strategic flood risk assessments for Crawley and Horsham, and the site itself. If you are working near Ifield Brook, talk to one of our experienced flood risk consultants about what your project will need.

Live levels, warnings and who to contact

• Live river levels. Check the brook on the Environment Agency service — Ifield Brook at Ifield — or see every gauge in the region on our live river levels map.
• Flood warnings. The Upper River Mole, Ifield Brook, Gatwick Stream and Burstow Stream alert area shows current alerts; sign up for Environment Agency warnings or call Floodline on 0345 988 1188.
• Screen a location. Try our free flood risk map, and for drainage on the clay, our geology and soakaway map, drainage calculators and BRE 365 infiltration calculator.
• Report flooding. Surface water and blocked ordinary watercourses go to West Sussex County Council; problems on the main river go to the Environment Agency.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

Ifield Brook flood risk: your questions answered