Flood Risk Assessment in East Grinstead

A flood risk assessment in East Grinstead is mostly about rain rather than rivers. The town sits on the High Weald ridge in Mid Sussex, close to where the River Medway rises near Turners Hill, and the young Medway is not even a designated main river where it crosses the district. What floods here is surface water running off the sandstone and clay. For sites here our consultants write flood risk assessments for planning, purchase, mortgage and insurance, together with sequential and exception tests and surface water drainage strategies.

On this page East Grinstead means the built-up area the Office for National Statistics maps for the town: the High Street and town centre, Imberhorne and the western estates, Baldwins, Ashplats and Herontye, all within the RH19 postcode district. Felbridge, across the county line in Tandridge district, and the village of Ashurst Wood to the south-east are separate built-up areas outside the outline, although Ashurst Wood falls under the same Mid Sussex and West Sussex rules.

Mid Sussex District Council treats land at high or medium risk of surface water flooding as Flood Zone 3, and the district's 2024 flood study counts 771 properties in East Grinstead parish at risk in a 1-in-100 year storm.

Talk an East Grinstead site through with one of our flood risk consultants on 01293 214444 or at enquiries@unda.co.uk.

A ridge town between the Medway and the Eden

East Grinstead stands at the top of two catchments. The Upper Medway catchment runs from the river's source at Turners Hill, west of the town, to its meeting with the Eden at Penshurst, and the West Sussex flood strategy lists East Grinstead among its large urban areas. The land between the town and Crawley drains instead to the Eden. On both sides the rivers and their flood alerts belong to the Environment Agency's Kent, South London and East Sussex area.

• Not a main river here. The Medway is not a designated main river where it crosses Mid Sussex, and the Environment Agency holds no detailed flood model for that stretch, the council's 2024 Level 1 SFRA records. The streams that feed it are ordinary watercourses: West Sussex County Council regulates works on them and the riparian owner maintains them.
• An internal drainage district to the south. The only Internal Drainage Board watercourses in Mid Sussex lie south of East Grinstead and run to the Medway. Works within 8 m of one need the consent of the Upper Medway Internal Drainage Board, and a site sending surface water into its district can be charged a Surface Water Development Contribution. Our note on internal drainage boards covers how that consent works.
• Flood alerts begin beyond the town. The Environment Agency's Upper River Medway alert covers the river from Forest Row to Penshurst, and its Rivers Eden and Eden Brook alert starts at Blindley Heath. None of the agency's warning or alert areas lies inside East Grinstead.
• The nearest gauges. The agency reads the Medway at Forest Row and the Eden Brook at Lingfield, and runs a rain gauge at Weir Wood. All three appear on our live river levels map.

Water leaving the town ends up in the River Medway, whose worst floods happen far downstream. For a planning application in the town, the river evidence is usually short and the surface water evidence long.

Screen an RH19 postcode from Imberhorne to Herontye

Enter a postcode below to screen it against Environment Agency data for rivers and the sea, surface water and reservoirs, or open our full flood risk map for anywhere in England. The coral line follows the ONS built-up area for East Grinstead, so Felbridge to the north-west and Ashurst Wood to the south-east sit outside it. The district's flood study calls Mid Sussex generally an area of low flood risk, and on this ridge the surface water result tends to tell you more than the flood zone.

Surface water first, then sewers, springs and the reservoir

Surface water is the main source of flooding in East Grinstead. The 2024 Level 1 SFRA puts the town with Haywards Heath, Burgess Hill and Hassocks as the district's highest-risk areas for surface water, and the West Sussex Local Flood Risk Management Strategy 2025–2030 lists East Grinstead as a Mid Sussex priority area where the risk is mostly from surface water.

The council's screening rule is what turns those counts into planning requirements. Land at high or medium surface water risk is treated as Flood Zone 3 and land at low risk as Flood Zone 2, so a site on a mapped flow path is judged much as a riverside site would be. Our guide to surface water flooding and planning explains how that risk is mapped and modelled.

What the records show for East Grinstead, 1960 to 2014

The flood record for the town is thin, because so little of it lies by a river. The entries below come from the Environment Agency's recorded outlines, the district's SFRA, the gauges on the Medway and the Eden Brook, and the local press.

1. November 1960. The Medway outline from this month is one of only a handful of fluvial flood records the Environment Agency holds for Mid Sussex, most of them in rural areas.
2. 15 September 1968. Thunderstorms across South East England caused the Great Flood of 1968, and the worst-hit areas included Crawley, East Grinstead and Horley. The agency also holds a Medway outline from that September. Our account of the 1968 floods follows the event across the region.
3. 24 December 2013. The Medway gauge at Forest Row reached 2.26 m and the Eden Brook gauge at Lingfield 2.90 m, the highest level each has recorded. Mid Sussex holds property flooding reports for the winter of December 2013 to February 2014.
4. 9 January 2014. Firefighters were called to a block of flats in Moat Road where the basement held around eight inches of water, the Sussex Express reported the next day.
5. 2019 to 2023. Reported flood incidents across Mid Sussex numbered 11 in 2018, 58 in 2019, 64 in 2022 and 37 in 2023 (SFRA 2024, Table 9). The council keeps the records by postcode area rather than by property.

Some of the response has been local and small. The district council lists a balancing pond at Gleave Close, rebuilt to hold more water, and a storm water attenuation tank with a restricted outlet beneath the Norton House car park in Tower Close among its flood works.

Weir Wood, spring lines and the Wealden ground

Two things beside and beneath the town shape a drainage strategy here more than any river does: the reservoir to the south-east and the ground itself.

• Weir Wood Reservoir. Southern Water owns Weir Wood, west of Forest Row and just outside Mid Sussex. In the Environment Agency's dry-day failure scenario a small area at the district boundary south-east of East Grinstead floods; in the wet-day scenario the extent reaches the south-western boundary of Sunnyside. The SFRA asks for that risk to be assessed as if the reservoir lay inside the district, and our explainer on reservoir flooding sets out how an assessment handles it.
• Sandstone, siltstone and clay. The SFRA describes the bedrock from East Grinstead to Burgess Hill as Wealden Group sandstones, siltstones and clays. The former police station site lies in a Wadhurst Clay minerals safeguarding area and the Imberhorne allocation in an Ardingly stone one, which shows how quickly the ground changes across the town.
• Springs and perched water. The Site Allocations DPD tells the Imberhorne developer to keep the spring lines along the Worth Way. Water perched on a clay band inside the sands can pass for a high water table, and our note on perched groundwater explains how to tell the two apart.
• Proving a soakaway. Mid Sussex's application checklist asks for infiltration tests at submission only where infiltration is the sole outlet, but the county's standards want winter groundwater monitoring and a metre between the highest groundwater and the base of any soakaway. On a sloping RH19 plot, BRE 365 testing settles whether one can work.

Which East Grinstead proposals need a flood risk assessment?

National policy in chapter 18 of the NPPF asks for a site-specific assessment in Flood Zones 2 and 3, on sites of a hectare or more in Flood Zone 1, and where other sources put a site at risk. Mid Sussex screens surface water harder than that, and it checks the access and escape route as well as the site.

Mid Sussex runs its own Flood Risk and Drainage Team, which reviews minor applications as well as major ones, while West Sussex County Council comments as Lead Local Flood Authority on major development. Every application validated after 10 May 2022, including condition discharges, uses the Environment Agency's May 2022 climate change allowances.

Policy DP41, the emerging DPS4 and the county's drainage standards

Applications in East Grinstead are decided against the Mid Sussex District Plan 2014–2031, adopted on 28 March 2018. Its replacement, the District Plan 2021–2039, is still at examination: the inspector recommended main modifications in July 2026, and resumed hearings are set for 3 to 6 November 2026, according to the council's examination updates.

Above all of this sits chapter 18 of the NPPF, where policy F8 expects development to follow the 2025 National Standards for sustainable drainage. Until the new plan is adopted DP41 decides. If MM4 stands, the detailed numbers for an East Grinstead scheme will come from the county's standards and the council's own detailed drainage checklist rather than the plan itself: greenfield Qbar discharge, floor levels at least 150 mm above surrounding ground, and soakaways sized for the six-hour 1-in-100 year storm with climate change.

Imberhorne, the Felbridge Water and the town's allocated sites

The allocations on the edge of East Grinstead show, site by site, what the council expects on flooding and drainage. Three are adopted and a fourth is proposed in the plan at examination.

• SA20, land south and west of Imberhorne Upper School: 550 homes on 64.8 ha. A flood risk assessment for each part of the site, room kept for the natural flood flow routes crossing it (including those arriving from off site), at least 5 m from the top of bank of every watercourse, the Worth Way spring lines protected, and natural flood management and infiltration SuDS in the masterplan.
• SA19, land south of Crawley Down Road, Felbridge: 200 homes on 8.5 ha. The southern boundary meets the Felbridge Water and its flood zones. A sequential layout keeps all development out of the 1-in-100 year extent with climate change, hydraulic modelling is likely to be needed to define it, and any attenuation ponds go in the flood risk buffer.
• SA18, the former police station in College Lane: 22 homes. Surface water drainage to minimise runoff through SuDS, a slope stability assessment beside Blackwell Hollow, and occupation phased to the delivery of sewerage infrastructure.
• DPA4, land off West Hoathly Road: up to 45 homes. Proposed in the 2021–2039 submission plan and subject to the examination outcome.

Windfall plots across the town raise the same questions on a smaller scale. A plot can carry a ditch, a culvert or an old field drain that no map shows, and altering a culvert or building beside an ordinary watercourse needs ordinary watercourse consent from the county council outside the internal drainage district.

Reports for East Grinstead sites, from the High Street to the Worth Way

Three reports answer three different questions on a site in East Grinstead: is it safe, should it be built here, and where does the rain go. Our West Sussex prior approval case shows how surface water alone can reshape a Flood Zone 1 conversion.

• Every source on one plot. Surface water, a headwater stream, groundwater in the sandstone and the Weir Wood failure case can all meet on one site, and a flood risk assessment weighs them together.
• Applications to Mid Sussex. Where a site or its access touches mapped surface water risk, the council's Flood Risk and Drainage Team will expect a flood risk assessment for planning that answers DP41 and the SFRA.
• Buying in RH19. A conveyancing search that flags surface water risk on a house in Imberhorne or Herontye can be answered for that one building with a flood risk survey for property purchase.
• Homes on land treated as Flood Zone 3. Because the council equates medium and high surface water risk with Flood Zone 3, new housing there can need the sequential and exception test, unless the surface water exemption applies.
• SuDS from the start. DP41 asks for SuDS on ten or more homes and the county's standards cover every major scheme, so a drainage strategy shows the council where the water goes.
• Greenfield runoff on a slope. With discharge held to greenfield rates and attenuation kept inside the red line, a surface water drainage strategy is where infiltration results and storage volumes are settled.
• Conditions after consent. Applications submitted without full drainage details are expected to resolve them by condition, and the discharge of drainage conditions clears that before work starts.
• Springs and perched water. Spring lines by the Worth Way and water held in the Wealden sands make a groundwater flood risk assessment worth commissioning before a scheme relies on infiltration.
• Sewer capacity. Where sewerage capacity limits occupation, as policy SA18 sets out for the former police station, a foul drainage strategy shows how waste water will be handled.
• Cover and premiums. With 771 properties in the parish at risk in a 1-in-100 year storm, owners and brokers can use a flood risk assessment for insurance as evidence for one building.
• Unmodelled streams. The Environment Agency has no detailed model of the Medway in Mid Sussex and policy SA19 expects the Felbridge Water to be modelled, so flood modelling fills the gap.
• Large sites at Imberhorne. On allocated or speculative land such as SA20, a flood risk feasibility study tests layout and drainage before the masterplan is fixed.
• Birches and Charlwoods. The Birches (13.2 ha) and Charlwoods (5.7 ha) industrial estates are protected employment sites where intensification is supported, and a BREEAM flood risk assessment answers the Pol 03 credit for new buildings there.

Reporting a flood and who manages it in East Grinstead

Call 999 if flooding puts anyone in danger and 105 for a power cut. Beyond that, who to contact depends on where the water came from.

Anyone living near the Medway or the Eden Brook can sign up for flood warnings for those alert areas. Sewer flooding inside a property goes to Southern Water, and our guide to sewer flooding explains who is responsible for what.

For the county picture, see our page on flood risk assessment in West Sussex; for district policy in more depth, our Mid Sussex District Council page. Nearby towns with pages of their own are Crawley, Haywards Heath and Uckfield, and every place we cover is listed on our areas page.

Before you instruct: East Grinstead questions answered

To talk through a site in East Grinstead, call one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk. A senior consultant signs off every report, and Unda Consulting Limited is a CIWEM Business Partner.