Tilgate Brook Flooding & Flood Risk

Tilgate Brook is a small watercourse with a serious flood record. It rises around Tilgate Lake on the southern edge of Crawley and runs north through Furnace Green to meet the Gatwick Stream, and because it drains the sandstone and clay of the town’s southern neighbourhoods through a channel that is heavily culverted and squeezed by development, it can rise quickly and spill into homes and roads after heavy rain. It was one of the watercourses behind the flooding that hit Crawley in 1968 and 2000, and it is now the subject of its own Environment Agency flood warning service. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Tilgate Brook catchment. To talk it through, call 01293 214444 or email enquiries@unda.co.uk.

More than 1,100 properties in Tilgate and Furnace Green now sit within the Environment Agency’s Tilgate Brook flood warning area — a direct legacy of the 1968 and 2000 floods and the reason a site anywhere near the brook needs its flood risk taken seriously.

• A flashy urban brook. Tilgate Brook drains the built-up southern side of Crawley, where paved catchments and culverts push rainfall into the channel quickly, so levels rise sharply after heavy or prolonged rain.
• A designated Main River. The Environment Agency regulates Tilgate Brook through Crawley, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Storage at Tilgate Lake. The dam at Tilgate Lake was raised in 2011 under the Upper Mole flood alleviation scheme to hold water back and protect homes downstream in Furnace Green and beyond.
• A live Environment Agency gauge. The Crawley gauge on the brook appears on the map below, with its latest level updating in real time.

Check flood risk along Tilgate Brook

The map below draws Tilgate Brook in coral, with the Environment Agency’s live river gauge positioned on it — click the pin to see its latest level. 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 Tilgate Brook flood?

Tilgate Brook floods because of where it runs and what has been done to its channel. It rises on the Hastings Beds of sandstone and clay on the high ground south of Crawley, around Tilgate Park and Tilgate Forest, and then flows north straight through the built-up neighbourhoods of Tilgate and Furnace Green before joining the Gatwick Stream. Along that urban stretch the brook has been straightened, reinforced and heavily culverted to squeeze it between houses, roads and the railway, and every hard surface it passes sheds rainfall into the channel rather than letting it soak away. The result is a small watercourse that fills fast, peaks sharply and gives little warning — the classic behaviour of a flashy urban brook.

The brook is one arm of the wider Upper Mole system. It shares a catchment with the Gatwick Stream, Crawters Brook, Stanford Brook and Ifield Brook, all of which drain the New Town and converge in a tight corridor north of Crawley before feeding the River Mole near Gatwick Airport. Because so many of these streams meet in such a small area, the timing of their peaks matters: when Tilgate Brook rises at the same time as its neighbours, the combined flow is what overwhelms the town’s channels.

• An urbanised, culverted channel. Significant lengths of the brook through Furnace Green and Tilgate run in culverts and straightened cuts, which reduce capacity and raise levels quickly in heavy rain.
• Sandstone and clay headwaters. The brook rises on the Hastings Beds around Tilgate Forest, where clay-rich ground sheds rain and feeds runoff into the channel.
• Part of a converging system. Tilgate Brook, the Gatwick Stream, Crawters Brook and Stanford Brook all meet within a short reach north of Crawley, so their flood peaks can combine.
• A gauged reach at Crawley. The Environment Agency records levels on the brook at Crawley, one of the network of gauges the Upper Mole scheme relies on.

A history of flooding on Tilgate Brook

Crawley’s worst floods are written into the record of the Upper Mole, and Tilgate Brook is named among the watercourses responsible. The Environment Agency lists the brook, alongside the Gatwick Stream, Crawters Brook and Ifield Brook, as a cause of the significant flooding the town suffered in 1947, 1960, 1968, 1974, 1990, 2000 and the winter of 2013–14. The September 1968 storm that devastated much of Surrey and Sussex hit Crawley hard, with Furnace Green and Three Bridges among the worst-affected areas.

The benchmark modern event came in the autumn of 2000. During the wettest autumn in nearly 300 years, more than 150 millimetres of rain fell on the Upper Mole catchment in 72 hours, the Mole and Gatwick Stream overtopped, and around 100 to 109 homes in Crawley were flooded — Gatwick Airport itself was inundated. That event triggered the Upper Mole flood alleviation scheme, and the first component the Environment Agency built was the raising of Tilgate Dam, completed in 2011, which holds floodwater back in Tilgate Lake and releases it slowly through a restricted outlet to protect Furnace Green and the neighbourhoods downstream. Together with the storage reservoir at Worth Farm, the raised dam helped reduce flooding during the winter of 2013–14.

Which areas does Tilgate Brook put at risk?

Flooding from Tilgate Brook is concentrated on Tilgate and Furnace Green, the neighbourhoods the brook runs directly through, and the Environment Agency operates a dedicated flood warning service for them. The Tilgate Brook flood warning area — covering the brook at Tilgate and Furnace Green — was launched in 2021 and gives more than 1,100 properties direct warnings, sitting within the wider Upper River Mole flood alert area. Downstream, the brook’s flow joins the Gatwick Stream, which has its own warning areas at Maidenbower and towards Horley. The table below summarises the main areas at risk from source to confluence.

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 Crawley and the wider county are covered in more depth on our local pages for Crawley, West Sussex and Sussex. The story of the town’s worst modern flood is told in our article on the 2000 Crawley and Gatwick floods, and the wider 1968 event in remembering the Great Flood of 1968.

Is Tilgate Brook a Main River — and what that means for your site

Yes. The Environment Agency has designated Tilgate Brook a Main River through Crawley, which means it, rather than the local council, is 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 brook’s floodplain will normally need a flood risk assessment to support a planning application. The smaller ditches and drains that feed the brook 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 getting that classification right at the outset saves time later. It also determines whether you need ordinary watercourse consent from the council for any works on a feeder ditch.

• The 8 metre byelaw margin. Building, excavation or structures near Tilgate Brook 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.
• Know which watercourse you are on. The brook itself is Environment Agency territory; its feeder ditches are ordinary watercourses under the Lead Local Flood Authority. We confirm this for every site.

Flood risk and drainage services for sites near Tilgate Brook

A development near Tilgate Brook 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 Tilgate Brook catchment.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the brook’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 on Crawley’s paved catchments and meet the discharge limits set by the Lead Local Flood Authority — see our sustainable drainage strategy service.
• 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 a high water table is in play.

Where national mapping is not enough — a common situation for sites right beside a Main River — we can bring in hydraulic flood modelling to derive accurate flood levels and extents, prepare the preliminary drainage design a scheme needs, and produce the detailed work required to discharge a drainage condition. Everything we do is framed by the NPPF and its planning practice guidance, and where a scheme falls in a critical drainage area we build that into the design from the start.

Managing flood risk on Tilgate Brook: policy and contacts

Several bodies share responsibility for flood risk on Tilgate Brook, and it is worth knowing who does what. The brook sits within the Mole catchment, part of the Thames River Basin District. The Environment Agency regulates the Main River, publishes the flood map for planning and runs the Tilgate Brook flood warning service; West Sussex County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses; and Crawley Borough Council is the planning authority that decides applications along the brook, with its own strategic flood risk assessment for the Gatwick sub-region and the Upper Mole catchment. The role of the Lead Local Flood Authority is set out in our guide to what an LLFA does.

• Live river levels. Check the brook’s gauge on the Environment Agency service — Tilgate Brook at Crawley — 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 ground conditions and soakaway map.
• Flood warnings. Sign up for the Environment Agency’s flood warnings covering Tilgate Brook, and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on Tilgate Brook

Unda understand how an urban brook like Tilgate Brook behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. Our office is in County Oak on the northern edge of Crawley, so we know this catchment well — how quickly the brook responds to rain, how the Tilgate Lake storage and the culverted reaches through Furnace Green change its behaviour, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites across the catchment.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping, the brook’s gauged levels and the Upper Mole modelling, and the local planning policy that applies to the site. If you are working on a site near Tilgate Brook, talk to one of our experienced flood risk consultants about what your application will need.

Tilgate Brook flood risk: your questions answered