Gatwick Stream Flooding & Flood Risk

At 10:15 on Christmas Eve 2013, the Gatwick Stream at Gatwick Link recorded the highest flow since gauging began there in 1975: 14.66 cubic metres per second, roughly twenty times its average. That same morning, floodwater reached the North Terminal’s electrical switch rooms and Gatwick Airport cancelled 72 departures. The Gatwick Stream is a small river with a serious flood record. It drains the eastern half of Crawley, passes the airport in a culvert built in 1958, and joins the River Mole at Horley. Unda prepares flood risk assessments, drainage strategies and related reports for sites along the stream and across the upper Mole catchment. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

On 24 December 2013 the Gatwick Stream reached its record flow — 14.66 cubic metres per second at Gatwick Link — the morning floodwater shut two of the North Terminal’s six electrical switch rooms and Gatwick cancelled 72 of its 260 scheduled departures.

• Where it runs. The stream rises in Worth Forest below Clay’s Lake and flows north through Tilgate Forest, Maidenbower, Three Bridges and Tinsley Green, passing Gatwick Airport before meeting the River Mole near Horley, on the West Sussex–Surrey border.
• A flashy urban catchment. It drains 33.6 square kilometres of mostly built-up eastern Crawley, so heavy rain reaches the channel in hours and levels rise sharply.
• A designated Main River. The Environment Agency regulates the stream through Crawley and down to the Mole confluence, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Three live Environment Agency gauges. Maidenbower, Three Bridges and Gatwick Link all appear on the map below with their latest levels.
• Formal flood warnings. Two Environment Agency flood warning areas cover the stream — one for Maidenbower, Crawley and Tinsley Green, one for Gatwick Airport and south-west Horley — inside a catchment-wide flood alert area.

Check a site against the Gatwick Stream’s levels and flood zones

The map draws the Gatwick Stream in coral, with the Environment Agency’s three live gauges positioned along it — click a pin to see its latest level, its typical range and its record peak. Enter a postcode to zoom in and screen that location against Environment Agency 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.

The flood that closed an airport: the Gatwick Stream’s flood history

Flooding on the upper Mole system, the Gatwick Stream included, is a matter of record: 1947, 1960, 1968, 1974, 1990, 2000 and the winter of 2013–14 all brought significant floods to Crawley and Horley. Three of those events shaped how the stream is managed today.

September 1968. Two days of near-constant rain, the worst of it on Sunday 15 September, put Crawley and Horley among the hardest-hit towns in south-east England as the Mole and its tributaries rose faster than the New Town’s engineered channels could cope with. The 1968 flood is still the benchmark for the catchment — when the Environment Agency telephoned Gatwick at 4:15am on Christmas Eve 2013, its warning was that the Mole would burst its banks for the first time since 1967/68. Our article on the Great Flood of 1968 covers the event across the region.

Autumn 2000. The Environment Agency called it the wettest autumn in nearly 300 years. By early November the catchment was saturated, and 109 homes in Crawley flooded along with parts of Gatwick Airport. The second-highest flow ever gauged on the Gatwick Stream, 14.61 cubic metres per second, passed Gatwick Link on 6 November 2000. That autumn is the reason the Upper Mole Flood Alleviation Scheme exists, and we tell the story in full in our retrospective on the 2000 Crawley and Gatwick floods.

Christmas Eve 2013. On 24 December 2013, 68 millimetres of rain fell in 24 hours against a forecast of 49.6 millimetres. By 4:40am the River Mole and Crawters Brook had broken their banks and three airfield electrical sub-stations were under water; floodwater then entered the North Terminal’s basement through cable conduits and took out two of its six electrical switch rooms. Gatwick cancelled 72 of 260 scheduled departures and delayed 146 more, some by 13 hours. The Gatwick Stream itself reached both its record level, 2.173 metres, and its record flow that morning at Gatwick Link. Surrey County Council’s formal flood investigation found river levels at Horley that winter were the highest since Environment Agency records began there in the mid-1980s, and the independent McMillan Report, published on 26 February 2014, made 27 recommendations — noting that the switch-room flooding had not been foreseen in the airport’s own flood risk assessment.

The stream still reacts fast. In the storms of 20–22 January 2024 the upper Mole’s tributaries around Crawley and Gatwick rose rapidly on saturated ground, and the Environment Agency’s alert and warning areas for the stream have been activated repeatedly in recent winters.

Why does the Gatwick Stream flood?

The Gatwick Stream floods because it is a small, fast-responding river with a New Town built across its catchment. Rain landing on eastern Crawley’s roofs and roads reaches the channel in hours rather than days, and the channel itself runs through culverts sized for the storms of an earlier era. The National River Flow Archive has gauged the stream at Gatwick Link, just above the Mole confluence, since 1975, and its numbers describe the problem precisely.

• A catchment of 33.6 square kilometres. Small enough that a single intense storm can sit over the whole of it at once. The gauged catchment excludes the airport itself, whose runoff drains separately.
• A median annual flood thirteen times the average flow. The stream’s mean flow is 0.744 cubic metres per second; its median annual flood is 10.07. That gap between everyday flow and flood flow is the signature of a flashy urban river. Our guide to flood return periods and annual probability explains how such figures are read.
• An urbanising catchment. The urban fraction of the catchment rose from 0.174 in 2000 to 0.199 in 2015 on the national URBEXT measure, and Crawley continues to densify. More hard surface means faster, higher peaks.
• Rising rainfall. Average annual rainfall over the catchment is 898 millimetres on the 1991–2020 standard, up from 830 millimetres on the 1961–90 standard.
• A capped outlet. The 1958 culvert carrying the stream beneath the airport’s South Terminal and railway station can pass roughly a 1-in-50-year flood, around 15 cubic metres per second. Larger floods must be held back upstream — the job of the flood storage areas described below.

Two further details are worth knowing. Crawley’s sewage treatment works discharges to the stream about three kilometres above the Mole confluence; research in the late 1990s measured around 15,000 cubic metres a day, enough to make up as much as three-quarters of the Mole’s flow downstream in prolonged dry weather. And for planning purposes the stream is assessed as the water body “Tilgate Brook and Gatwick Stream at Crawley”, formally classed as heavily modified at Moderate ecological status. Heavily modified is the official recognition of what the flood record shows: a natural stream re-plumbed around a New Town and an airport.

Which areas does the Gatwick Stream put at risk?

The stream threads through some of the most intensively developed land in the Mole catchment, and the Environment Agency runs a formal warning service for the communities most exposed. A catchment-wide flood alert area — “Upper River Mole, Ifield Brook, Gatwick Stream and Burstow Stream” — covers the whole river, and within it two flood warning areas let the Environment Agency warn individual properties. The table below summarises the main places at risk from source to mouth.

Flooding at the bottom of the stream is rarely the stream’s alone. At Horley the Gatwick Stream meets a River Mole already carrying Ifield Brook and Crawters Brook, and a high Mole can hold the stream’s water back — Riverside Garden Park, on the Horley bank, is kept as undevelopable floodplain for exactly that reason. A site in one of these areas is likely to sit in Flood Zone 2 or 3, which triggers the need for a site-specific flood risk assessment. Crawley itself also carries the surface water risk that dominates the borough’s flood profile; the towns and counties along the stream are covered in more depth on our local pages for Crawley, West Sussex and Surrey.

The defences: how the Upper Mole scheme holds the stream back

After the 2000 flood, the Environment Agency built the Upper Mole Flood Alleviation Scheme, a chain of flood storage areas that intercept the stream’s peaks before they reach the airport culvert and Horley. The scheme reduces flood risk to more than 1,400 homes and businesses in Crawley and Horley, along with Gatwick Airport, which contributed £4 million of the roughly £15 million programme.

• Tilgate Lake, Crawley (2011). The dam was raised so the lake can hold back floodwater on the stream’s Tilgate arm.
• Worth Farm flood detention reservoir (2013). A dry dam beside M23 junction 10a that fills only in flood, throttling the Gatwick Stream’s peak. Tilgate and Worth Farm both helped reduce flooding to homes in the winter 2013–14 events.
• Clay’s Lake, near Balcombe (built from 2014). Dam works adding storage on the stream’s headwaters.
• Grattons Park, Pound Hill (2013). River restoration by Crawley Borough Council, re-meandering the channel so the floodplain slows flows naturally.

After Christmas 2013 the airport went further, answering the McMillan Report’s 27 recommendations with a £30 million resilience programme, and the South East Rivers Trust now runs natural flood management work across the upper Mole catchment. The Gatwick Northern Runway scheme, consented in September 2025, carries its own flood compensation commitments, including compensation areas beside the Gatwick Stream.

None of this removes the risk. The storage areas are built to defined standards, the South Terminal culvert still caps what the lower stream can pass, and the Flood Map for Planning deliberately shows the floodplain as if the defences were not there. A site behind the scheme can therefore still sit in Flood Zone 3 — a quirk we explain in our guide to residual flood risk.

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

Yes. The Gatwick Stream is a designated Main River on the Environment Agency’s statutory map through Maidenbower, Three Bridges and down to the Mole confluence at Horley, so the Environment Agency, rather than the local council, regulates the channel and its margins. Its headwater reaches in Worth Forest, above the designated limit, are ordinary watercourses overseen by the lead local flood authority and the riparian owners whose land they cross. The distinction has practical consequences for anyone building nearby.

• The 8 metre byelaw margin. Works within 8 metres of the bank of the Main River — structures, excavation, even some scaffolding — generally need a Flood Risk Activity Permit from the Environment Agency, sought alongside planning permission rather than instead of it. These permits cannot be granted retrospectively.
• On the ordinary-watercourse reaches, works affecting flow need ordinary watercourse consent from the lead local flood authority, and riparian owners carry maintenance responsibilities.
• 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. We confirm the classification and the requirements for every site.

Flood risk and drainage services for sites near the Gatwick Stream

A development near the Gatwick Stream 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 stream’s catchment.

• Flood risk assessments for planning. Site-specific assessments meeting the Environment Agency’s and councils’ requirements for development in the stream’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.
• BREEAM assessments. A BREEAM flood risk assessment structured around the Pol 03 credit.
• Sequential and exception tests. The locational case where policy requires it, through our sequential and exception test reports.
• Drainage strategies and SuDS. Surface-water designs that manage a site’s own runoff to the discharge rates the lead local flood authority expects — see our sustainable drainage strategy service.
• Groundwater and modelling within an assessment. A groundwater flood risk assessment where rising water tables are in play, and hydraulic flood modelling to derive accurate flood levels where national mapping is not enough — a common situation right beside a gauged, defended watercourse.

Where a drainage condition follows the permission, we prepare the detailed scheme to discharge a drainage condition, and everything we produce is framed by the NPPF and its planning practice guidance.

Who manages flood risk on the Gatwick Stream: policy and contacts

Responsibility splits at the county boundary. The Environment Agency regulates the Main River and issues flood warnings; West Sussex County Council is the lead local flood authority for the Crawley reaches and Surrey County Council for Horley. Strategically the stream sits within the Thames Catchment Flood Management Plan and the Thames river basin Flood Risk Management Plan for 2021–2027. Crawley Borough Council’s planning evidence base is the Gatwick Sub-Region strategic flood risk assessment of November 2023, which covers the borough and the wider upper Mole catchment; Reigate & Banstead’s SFRA covers the Horley side. An application near the stream will be measured against those documents and, where it applies, the sequential test.

• Live river levels. Check the stream’s gauges on the Environment Agency service — Maidenbower, Three Bridges and Gatwick Link — or watch every gauge in the catchment on our free live river levels map.
• Screen a location. Use the map on this page, our free flood risk map, and our drainage calculators for the surface-water side.
• Flood warnings. Sign up for the Environment Agency’s free warnings if a property sits in the Gatwick Stream at Maidenbower and Crawley or Gatwick Stream at South West Horley warning areas, and call Floodline on 0345 988 1188.
• Report flooding. To West Sussex County Council for Crawley, Surrey County Council for Horley, and Thames Water on 0800 316 9800 for sewer flooding. Mole River Watch, the catchment’s citizen-science group, monitors flows and water quality across the Mole and its tributaries.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on the Gatwick Stream

Unda understand how the Gatwick Stream behaves in flood: how quickly it rises on eastern Crawley’s hard surfaces, where the storage areas shave its peaks, what the South Terminal culvert does to the lower reaches, and what the Environment Agency, West Sussex County Council, Surrey County Council and the borough councils expect from the reports that follow. We have prepared flood risk assessments and drainage strategies for sites across the upper Mole catchment, and we deal with the regulators directly on your behalf.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping and modelled flows, the stream’s gauged record, and the planning policy that applies to the site. If you are working on a site near the Gatwick Stream, talk to one of our experienced flood risk consultants about what your application will need.

Gatwick Stream flood risk: your questions answered