Burstow Stream Flooding & Flood Risk

Flooding from the Burstow Stream is a story about clay. The stream drains the eastern side of the Upper Mole catchment, rising on the sandy ridge near Copthorne and picking up the Weatherhill Stream and a network of clay-country ditches as it passes Smallfield, Burstow and Shipley Bridge on its way to the River Mole between Horley and Salfords. It is a small watercourse with a serious record: it set its highest levels ever measured at both of its Environment Agency gauges on Christmas Eve 2013, and it has flooded homes on Horley’s eastern edge and around Smallfield often enough that a dedicated flood alleviation scheme is now under construction. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Burstow Stream’s floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

On 24 December 2013 the Burstow Stream reached 4.465 metres at Bonehurst Bridge, its highest level on record, and it has flooded often enough since that Surrey County Council is fitting flood resilience measures to around 175 Smallfield properties, with a flood storage area to follow by early 2027.

• A designated Main River. The Environment Agency regulates the Burstow Stream, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to works near the channel. Most of the ditches feeding it are ordinary watercourses under Surrey County Council.
• Two live gauges. Bonehurst Bridge in Horley and Peeks Brook Lane near Gatwick’s eastern boundary both appear on the map below, and both recorded their highest ever levels on 24 December 2013.
• Two dedicated flood warning areas. The Environment Agency warns properties directly at East and North Horley and at Smallfield, within a wider flood alert area covering the Upper Mole and its tributaries.
• Development pressure on every side. Gatwick’s consented northern runway works reach sections of the stream, and new homes on Horley’s eastern side drain towards it, so planning applications near the Burstow Stream face close scrutiny of flood risk and drainage.

Which areas does the Burstow Stream put at risk?

The communities most exposed to flooding from the Burstow Stream are Smallfield, Burstow, Shipley Bridge and the eastern and northern edges of Horley, with the headwater villages around Copthorne affected mainly by runoff rather than river flooding. The Environment Agency runs two formal flood warning services on the system: Burstow Stream at East and North Horley, which also covers the Haroldslea Stream, Silverlea Ditch and the lower Weatherhill Stream, and Weatherhill Stream at Smallfield. Both sit inside the catchment-wide Upper River Mole, Ifield Brook, Gatwick Stream and Burstow Stream flood alert area.

Land beside the stream through these reaches falls in Flood Zone 2 or 3 on the Environment Agency’s flood map, which is what triggers the need for a site-specific flood risk assessment when development is proposed. The towns either side of the catchment are covered in more depth on our pages for Crawley, Surrey and West Sussex, and our guide to flood zones explains what the designations mean.

Check flood risk along the Burstow Stream

The map below draws the Burstow Stream in coral, from its confluence with the River Mole down to the edge of the Main River designation north of Copthorne, with the Environment Agency’s two live gauges pinned along it. Click a gauge for its latest level, its typical range and its record peak, or enter a postcode to screen that location against the Environment Agency’s data for rivers and sea, surface water, reservoirs and the flood zones used for planning. Treat it as a first look at a location, not a flood risk assessment.

Why does the Burstow Stream flood?

The Burstow Stream floods because most of its catchment sits on Weald Clay, which sheds rain instead of soaking it up. Its headwaters rise on the more permeable Tunbridge Wells Sands near Copthorne, but once the water reaches the clay vale it runs off fields, roads and roofs straight into the channel, so the stream rises quickly after heavy rain and spills onto the flat, low-lying land that surrounds it. Hydrologists describe rivers like this as flashy, and the record bears that out: the gauged peaks at Bonehurst Bridge arrive within hours of the rain that causes them.

The gauge data says the same thing. At Kinnersley Manor, the National River Flow Archive station on the River Mole just downstream of the Burstow confluence, the gauged catchment of 142 square kilometres has a baseflow index of 0.37, meaning barely a third of the river’s flow comes from stored groundwater and the rest is runoff. Average rainfall there is 845 millimetres a year, and the record daily flow was set on 24 December 2013, the same day the Burstow Stream set its own records. Urban cover in that catchment nearly doubled between 1960 and 2002 as Crawley, Gatwick and Horley grew, and treated effluent from the Horley and Burstow sewage treatment works adds a constant artificial component to the stream’s flow.

• Clay that sheds rain. Weald Clay across the middle and lower catchment produces rapid runoff, sharp peaks and standing water on the floodplain long after the river has fallen.
• Several watercourses, one system. The Copthorne Brook, Weatherhill Stream, Haroldslea Stream and Silverlea Ditch all feed the main channel, and when they rise together east Horley takes the combined flow.
• A gauged, monitored river. The Environment Agency records levels every 15 minutes at Bonehurst Bridge (open since 1984) and Peeks Brook Lane (since 1997).
• It meets a river under the same pressure. The Burstow Stream joins the River Mole between Horley and Salfords, so its peak often arrives when the Mole is already high and each holds the other back at the confluence.

A history of flooding on the Burstow Stream

The stream has flooded in every generation. In September 1968, when two days of rain put much of the South East under water, Horley was among the worst-hit towns in the region. The autumn 2000 floods, at the end of what the Environment Agency called the wettest autumn in nearly 300 years, pushed the whole Upper Mole system beyond capacity, flooding Crawley and closing Gatwick’s runways in the neighbouring sub-catchment.

The defining modern event came on 23 and 24 December 2013, when a saturated catchment took another band of heavy rain. The Burstow Stream rose through the night and set its highest recorded levels at both gauges on Christmas Eve. Beside Bonehurst Road, the roughly 80 park homes at Cambridge Lodge Park sit only metres from the channel and were among the properties affected as levels rose. Six years later, in the week before Christmas 2019, flood warnings were in force again for east and north Horley, with residents at Cambridge Lodge Park watching the stream rise six inches in half an hour and properties around Bonehurst Road and Peeks Brook Lane at risk.

Surrey County Council’s Section 19 investigation into flooding in Smallfield and Burstow, published in 2022, unpicked how the system fails. It found the Burstow Stream, as a Main River, overtopped onto Antlands Lane and flooded property there; overtopping at Broadbridge Lane was traced to poorly maintained ditches; foul water flooded gardens along Orchard Road, Plough Road and the eastern end of Wheelers Lane as Thames Water’s sewers surcharged; and a high risk of groundwater flooding exists between the M23 and Burstow and under the centre and west of Smallfield. The response is the Smallfield flood alleviation scheme: property flood resilience measures for around 175 homes, with delivery starting in summer 2025, and a flood storage area on Plough Road recreation ground expected to be complete by early 2027.

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

Yes. The Burstow Stream is a designated Main River, so the Environment Agency, rather than the local council, regulates the channel and its margins, and the Weatherhill Stream through Smallfield carries its own Environment Agency flood warning service. That designation has practical consequences for anyone building nearby. Works within 8 metres of the top of the bank generally need a Flood Risk Activity Permit before they start, and a site in the stream’s floodplain will normally need a flood risk assessment to support a planning application. The ditches and smaller brooks feeding the system are ordinary watercourses: works on those need ordinary watercourse consent from Surrey County Council, and the riparian owners whose land they cross are responsible for keeping them clear. The Section 19 report’s finding that ditch maintenance contributed to the Broadbridge Lane flooding shows how much that last duty matters here.

• The 8 metre byelaw margin. Building, excavation or new structures near the Burstow Stream usually require a Flood Risk Activity Permit from the Environment Agency, sought alongside planning permission rather than instead of it.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, or a site of a hectare or more in Flood Zone 1, needs a flood risk assessment with its planning application.
• Know which watercourse you are on. The Main River is Environment Agency territory; the feeder ditches answer to Surrey County Council and their riparian owners. We confirm the classification for every site, because the consenting route depends on it.

Gatwick, new homes and the Burstow Stream

Few small rivers sit under as much development pressure. At the catchment’s western edge, the Development Consent Order for Gatwick’s northern runway was granted by the Secretary of State on 21 September 2025, and High Court challenges to it were dismissed in 2026. The airport straddles the floodplains of the Upper Mole and its tributaries, including sections of the Burstow Stream at its eastern end near the Peeks Brook Lane gauge, and the consented works include extending a culvert on the Burstow tributary with flood compensation designed so that risk downstream does not increase. On the Surrey side, Horley’s eastern neighbourhoods have grown quickly, and after the December 2019 floods a borough councillor argued publicly that hundreds of new homes had made flooding in the town worse. Whatever view you take of that claim, the planning consequence is real: Surrey County Council and the Environment Agency examine drainage strategies and flood risk assessments for sites near the Burstow Stream closely, and schemes that ignore the stream’s behaviour get objections.

For a developer or homeowner, that scrutiny is manageable with the right reports. A surface water drainage strategy has to show runoff leaving the site no faster than it did before development, which on Weald Clay almost always means attenuation rather than soakaways. Our free geology and ground conditions checker shows why infiltration rarely works here, and our drainage calculators and BRE 365 calculator let you test the numbers early.

Who manages flood risk in the Burstow Stream catchment?

Responsibility is shared, and knowing who does what saves time. The Environment Agency regulates the Main River, operates the gauges and issues the flood warnings. Surrey County Council is the Lead Local Flood Authority for surface water, groundwater and the ordinary watercourses, and wrote the Section 19 report; West Sussex County Council holds the same role for the headwaters around Copthorne. Planning applications along the stream are decided by Reigate & Banstead Borough Council, Tandridge District Council and Mid Sussex District Council, each with its own strategic flood risk assessment. Thames Water owns the sewers that surcharged in Smallfield, and Burstow Parish Council runs a local flood action group. In the wider catchment, the Environment Agency’s Upper Mole flood alleviation scheme reduces flood risk to more than 1,400 homes and businesses across Crawley, Horley and Gatwick.

• Live river levels. Check the stream’s gauges on the Environment Agency service — Bonehurst Bridge and Peeks Brook Lane — or use our live river levels map.
• Screen a location. Our free flood risk map checks any postcode in England against the Environment Agency’s flood data.
• Flood warnings. Sign up for Environment Agency flood warnings for the Horley and Smallfield warning areas, and call Floodline on 0345 988 1188 for advice.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

Flood risk and drainage services for sites near the Burstow Stream

A development near the Burstow Stream can call for several reports, and they answer different questions. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports: the first asks whether the site is safe from flooding, the second asks whether the development should be located there at all, and the third asks where the surface water goes. They often travel together in a planning submission. Unda prepares each of them for sites across the Burstow Stream catchment.

• Flood risk assessments for planning. Site-specific assessments meeting the Environment Agency’s and the council’s requirements for development near the stream — 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 for commercial and larger schemes.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• Drainage strategies and SuDS. Surface water designs that work on heavy clay and meet the discharge limits Surrey County Council sets, through our sustainable drainage strategy service.
• Groundwater. Where the Section 19 report’s groundwater findings bite, between the M23 and Burstow and under Smallfield, a groundwater flood risk assessment establishes what the water table is really doing.

Where national mapping is not detailed enough for a site right beside a Main River, we bring in hydraulic flood modelling to derive accurate flood levels and extents, and we prepare the detailed schemes needed to discharge drainage planning conditions. Everything is framed by the NPPF and its planning practice guidance.

How Unda helps with flood risk on the Burstow Stream

Unda’s consultants know the Upper Mole catchment and the way the Burstow Stream behaves in flood: how fast it rises off the clay, where it overtops between Shipley Bridge and Horley, how the Weatherhill Stream complicates the picture at Smallfield, and what Surrey County Council, the district councils and the Environment Agency each expect a report to demonstrate. We prepare flood risk assessments and drainage strategies for developers, planning consultants, architects, agents, buyers and homeowners with sites throughout the catchment.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping and gauge records, the Section 19 findings, the councils’ strategic flood risk assessments and the planning policy that applies to the site. If you are working on a site near the Burstow Stream, talk to one of our experienced flood risk consultants about what your application will need.

Burstow Stream flood risk: your questions answered