Flood Risk Assessment in Horley

A flood risk assessment in Horley, Surrey, starts from where the town sits. It lies on flat Weald Clay where the Gatwick Stream joins the River Mole, and the Burstow Stream curves round its eastern and northern edges before meeting the Mole too. We prepare flood risk assessments in Horley for planning applications, house purchases, mortgages and insurance, alongside sequential and exception tests and surface water drainage strategies, for sites from Horley Row and Langshott to Lee Street and Meath Green.

Horley is home to about 22,000 people across the RH6 7, RH6 8 and RH6 9 postcode sectors, in the south of Reigate and Banstead borough. Reigate and Banstead Borough Council decides planning applications here, and Surrey County Council is the lead local flood authority. The page also covers the places around the town that share its rivers without a page of their own: Westvale Park on the north-west edge, Salfords to the north, Hookwood across the Mole and Smallfield to the east. The rest of the county is on our Surrey page.

On 23 December 2013, at two minutes to ten at night, the Environment Agency sent a flood warning for the Burstow Stream at East and North Horley to 1,523 properties. The next day the Mole at the Horley gauge rose to 3.44 m, the highest it has reached since records began there.

Call 01293 214444 or email enquiries@unda.co.uk to talk through a Horley site with one of our flood risk consultants.

Where the Mole, the Gatwick Stream and the Burstow Stream meet Horley

Three main rivers shape flood risk in Horley, and they meet on ground so flat that the Burstow Stream spreads into a wide floodplain. The council's 2017 Strategic Flood Risk Assessment puts houses in the south of the town in Flood Zone 3 from the Mole. Which of the three affects a site depends on which side of town it is on.

Our river pages cover each one in depth: the River Mole, the Gatwick Stream, the Burstow Stream and, for Salfords, the Salfords Stream. The airport upstream lies in Crawley borough and is covered on our Crawley page.

Look up a Horley postcode, from Langshott to Lee Street

Enter an RH6 postcode in the map below to see the Environment Agency's river, surface water and reservoir data for it; for addresses anywhere else in England, use our flood risk map. The coral line is the built-up area of Horley as the Office for National Statistics draws it, from Horley Row and Langshott in the north to the streets beside the airport boundary in the south. Westvale Park, Riverside Garden Park on the Gatwick Stream, Hookwood, Salfords and Smallfield fall outside it, and the same Environment Agency data applies there.

What the Horley gauge recorded at Christmas 2013

The Environment Agency's River Mole gauge at Horley shows what happens to the west side of the town as the river rises. Its normal range is 0.19 m to 2.42 m. At 3.10 m, property on Lee Street has flooded before, and the record of 3.44 m was set on 24 December 2013, when properties at Hookwood flooded too.

Surrey County Council's flood investigation for the borough puts the average flow of the Mole at Horley at 1.4 cubic metres a second. That winter it passed 10 cubic metres a second 26 times, and at Christmas it peaked at about 60, the second-highest flow in 53 years. Two more gauges cover the other rivers: Gatwick Link on the Gatwick Stream and Bonehurst Bridge on the Burstow Stream, where the car park of the Cambridge Hotel has flooded at 3.90 m. All three, and the stations upstream, are on our live river levels map, and the Horley readings are on the Environment Agency's gauge page.

Horley's floods on record, from 1960 to the winter of 2013/14

Horley has flooded often enough for the Environment Agency to list it, with Crawley, in seven separate years since 1947. The council's flood studies pick out the years when the town itself was hit hardest.

1. June 1960 and November 1974. Both are recorded in the 2017 SFRA as floods in Horley specifically, between the region-wide events.
2. September 1968. The council's earlier 2012 SFRA calls it the worst flood in living memory, with major flooding in Horley; our account of the 1968 flood follows the storm across the South East.
3. Autumn 2000. Horley was particularly affected, the 2012 SFRA records, in what it estimated as roughly a 1 in 30 year event; the same floods put Gatwick's runways under water.
4. 23 and 24 December 2013. River, surface water and groundwater flooding together, with the Mole at its highest since Environment Agency records began in about 1985. Meath Green Lane, Lee Street, Tanyard Way and the A217 Reigate Road were closed, and drains on Balcombe Road in east Horley were overwhelmed.
5. January and February 2014. The Burstow Stream warning went out five more times, each to about 1,500 properties, and the Gatwick Stream warning four times to about 150. Surrey County Council counted around 60 incidents of internal property flooding across Reigate and Banstead that winter, and named the Mole as the main source in Horley, Salfords, Redhill and Reigate.

After 2000 the Environment Agency built the Upper Mole flood alleviation scheme. Its storage reservoirs upstream at Tilgate, Worth Farm and Clay's Lake reduce flood risk to more than 1,400 homes and businesses in Crawley and Horley. The SFRA warns that the flood zones were drawn from older models that take no account of those reservoirs. Water that still gets past a defence is residual flood risk, and an assessment in Horley deals with it either way.

Weald Clay, high groundwater and the RH6 sewers

Sites away from the rivers sit on Weald Clay, which sheds rain quickly, and the valleys carry river terrace deposits and alluvium where water lies close to the surface.

• Soakaways rarely work. The 2012 SFRA described the subsoil as generally solid Weald Clay and soakaways as unsuitable, so most Horley drainage relies on storage and a controlled outfall; our ground conditions and soakaway guide explains the testing that proves it either way.
• Groundwater susceptibility peaks around Horley. Of the three councils its study covered, the 2017 SFRA found susceptibility to groundwater flooding greatest around Horley, over an extensive area, and linked it to the alluvial clay, silt, sand and gravel. The groundwater flooding explainer sets out what that means for basements and foundations.
• Sewers in RH6 8 and RH6 9. Thames Water's register in the SFRA records 24 sewer flooding incidents in RH6 8 and 34 in RH6 9, the two highest postcode sectors in Reigate and Banstead, out of 222 in the borough.
• Surface water on named roads. The flood investigation points to Reigate Road, Meath Green Lane and Bonehurst Road on the surface water maps, and the borough's allocation for the former Chequers Hotel on Bonehurst Road asks for measures to attenuate surface water.

What Reigate and Banstead and Surrey County Council ask of a flood risk assessment in Horley

A flood risk assessment in Horley is judged against Policy CCF2 of the borough's Development Management Plan, the 2017 SFRA, Surrey County Council's drainage requirements and chapter 18 of the National Planning Policy Framework. CCF2 asks for an assessment on any site in Flood Zone 2 or 3, any site over a hectare in Flood Zone 1, and any site with critical drainage problems or other sources of flooding.

From 1 April 2027 the new East Surrey Council takes over planning from Reigate and Banstead, and the borough says the Local Plan 2044 will pass to it then. Our note on local government reorganisation covers what the change of council means for an application already in progress. Until a new plan is adopted, CCF2 and the 2017 SFRA remain the tests. Surrey County Council comments on drainage for major schemes, and on smaller ones where the site lies inside the 1 in 100 year surface water extent or beside a watercourse.

Westvale Park, the business park land and the town centre

Horley is a major growth area within the borough, in the council's own words, and flood risk has been part of planning it since the 2004 Horley flood study. The Horley Master Plan provided for 2,600 new homes, and the council's 2012 SFRA records that the old Local Plan policy for the new neighbourhoods, Hr2A, required detailed flood models for them.

• Westvale Park. The north-west neighbourhood will have 1,510 homes when complete, with a neighbourhood centre and a primary school that opened in 2020.
• Horley Strategic Business Park (HOR9). A 31 ha allocation west of Balcombe Road, beside the M23 spur, for a business park of mainly offices with at least 5 ha of open space. Policy HOR9 allows nothing but car parks and supporting infrastructure on the land in Flood Zone 2, asks for a buffer and ditch improvements, and wants early talks with Thames Water. The council's business park page lists no planning application yet.
• Town centre sites. The Development Management Plan allocates sites including the High Street car park, the library and the police station. Most of the centre stood in Flood Zone 1 in the 2012 SFRA, so surface water and sewer capacity tend to matter more there than the rivers.

Matching a Horley project to the right report

A Horley site can need up to three separate reports. The flood risk assessment deals with safety on the site and the effect on neighbours, the sequential test with whether a lower-risk site was available, and the drainage strategy with the rain that lands on roofs and paving.

Reports we prepare for sites between the Mole and the Burstow Stream

Each of these is written for the address, the watercourse beside it and the policies Reigate and Banstead and Surrey County Council apply:

• Three rivers, clay and sewers. A flood risk assessment for a Horley site looks at the Mole, the Gatwick Stream and the Burstow Stream, then at surface water, the high groundwater the SFRA expects around the town and the RH6 sewers.
• Applications in the river corridors. Homes, conversions and extensions in Flood Zone 2 or 3, and any site over 1 hectare, need a flood risk assessment for planning that meets Policy CCF2 and the 2017 SFRA.
• Before exchange on an RH6 house. A flood risk survey for property purchase looks at one building: its floor level, the nearest gauge record and whether it sits in the Burstow Stream or Gatwick Stream warning area.
• Unallocated land on the floodplain. Policy CCF2 does not permit development in Flood Zone 2 or 3 where a reasonably available lower-risk site exists, so the sequential and exception test has to show the search was made and what it found.
• Where the water goes on clay. Our drainage strategy work covers surface water and foul flows for Horley schemes, from a pair of houses off Lumley Road to a phase of Westvale Park.
• Surrey County Council's standards. A surface water drainage strategy sets out rainwater reuse, storage and an outfall that works on Weald Clay, with discharge held to the rates Surrey County Council will accept.
• Once permission is granted. The discharge of drainage conditions gathers the calculations, levels, maintenance plan and verification Reigate and Banstead and the county ask for before work starts or homes are occupied.
• Flood zones drawn before the reservoirs. The 2017 SFRA assumes the Mole's flood zones take no account of the Upper Mole storage reservoirs, so on a site near the river flood modelling can establish the flood levels the assessment relies on.
• Two warning areas over the town. Where a site depends on the Burstow Stream or Gatwick Stream warnings for a safe way out, a flood warning and evacuation plan says who acts on the warning, when, and by which road.
• Homes with a recorded flood. Property on Lee Street flooded when the Mole gauge reached 3.10 m, and Horley homes flooded inside in winter 2013/14; a flood risk assessment for insurance gives an insurer evidence about one address.
• Land before it is bought. On land like the 31 ha business park allocation, parts of whose north and east are in Flood Zone 2, a flood risk feasibility study shows early how much can be built and where storage has to go.
• Groundwater close to the surface. The SFRA finds groundwater susceptibility highest around Horley, on alluvium and river terrace deposits; a groundwater flood risk assessment checks a basement, a sunken garden or an infiltration feature against it.
• Offices on the business park. The HOR9 allocation is for a business park of mainly offices, the kind of building often certified under BREEAM; a BREEAM flood risk assessment covers the flood risk and runoff credits.

Horley's flood contacts: Surrey, the Environment Agency and Thames Water

In Horley the right number depends on the source: the river, a blocked road gully or a sewer backing up. Where life is at risk, ring 999 before anyone else.

Hookwood, across the Mole, is in Mole Valley district, so a planning application there goes to a different council; our Mole Valley page covers its policies. The 2017 SFRA lists Horley among the places with a community flood action group. For how all these roles fit together, see who manages flood risk in England.

Questions about flood risk assessments and flooding in Horley

For a house near the Burstow Stream, a plot at Westvale Park or land beside the M23 spur, call 01293 214444 or email enquiries@unda.co.uk. A chartered consultant at Unda Consulting Limited will reply with the reports that address needs. Further afield, our pages for East Grinstead and Dorking cover the Medway headwaters to the east and the Mole downstream, and the full list of areas the rest of England and Wales.