Flood Risk Assessment in Runnymede

A flood risk assessment in Runnymede has to deal with four rivers meeting in one small borough: the Thames along its eastern edge, the River Wey at its south-eastern corner, and the Chertsey and Addlestone Bournes running across it to join them. Runnymede Borough Council asks for an assessment for any development in Flood Zones 2 and 3, for sites over a hectare in Flood Zone 1, and for most schemes on the dry islands that the river leaves cut off when it floods. Our consultants prepare flood risk assessments in Runnymede for planning applications, home buyers, lenders and insurers, together with sequential and exception tests and surface water drainage strategies.

This page covers the whole Borough of Runnymede, about 78 km² of north-west Surrey between the Royal Borough of Windsor and Maidenhead, Spelthorne, Elmbridge, Woking and Surrey Heath. Its three towns are Addlestone, Egham and Chertsey, and all three have pages of their own. Around them lie Virginia Water, New Haw, Woodham, Ottershaw, Lyne, Longcross and Thorpe, and roughly 79% of the borough is Green Belt. Addlestone and New Haw addresses mostly carry KT15 postcodes and Virginia Water GU25, with KT16 and TW20 covering Chertsey, Egham and Thorpe.

Surrey County Council counted about 1,170 cases of homes and other buildings flooded inside across Runnymede in the winter of 2013 to 2014, on a stretch of the Thames the borough's Local Plan describes as the largest area of developed floodplain in England without flood defences.

For a site in Addlestone, Virginia Water, Thorpe or anywhere else in the borough, call our flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

How much of each Runnymede ward lies in the flood zones?

The answer swings from almost none to all of it. The council's Level 1 Strategic Flood Risk Assessment, published in January 2018, measured every ward against the Environment Agency's flood zones and against its own map of the functional floodplain, the land that floods in a 1 in 20 year event. The east of the borough, on the Thames gravels, holds most of the risk; the heath and higher ground to the west hold very little.

The figures use the ward boundaries the SFRA worked with, which differ from today's wards such as Addlestone South and Woodham & Rowtown, so they describe areas of the borough, not current wards. Where no detailed river model exists, as on the Ripley Springs, the Rive Ditch and the smaller tributaries of the Chertsey Bourne, the SFRA treats the whole of Flood Zone 3 as functional floodplain until an applicant's own modelling shows otherwise. Our explainer on Flood Zone 3b sets out why that line matters so much for housing.

The borough boundary on the map, from Virginia Water Lake to the River Wey

Enter a postcode in the map below to screen it against Environment Agency data for rivers, surface water and reservoirs, or open our full flood risk map for anywhere in England. The coral line follows the Runnymede borough boundary. Downstream across the Thames lie Staines and the rest of Spelthorne, Weybridge sits beyond the Wey to the south-east, and Wraysbury faces the Runnymede meadows from the Berkshire bank. The planning flood zones are drawn without defences, which on this reach makes little difference, because there are none.

Thames, Wey, two Bournes and the ditches between them

Most of Runnymede's flood risk comes from rivers, and most of that from the Thames. The SFRA lists twelve main rivers in the borough, so the Environment Agency, not the county council, is the authority for far more watercourses here than their size suggests.

• The Thames and the Abbey River. The Thames runs along the north-eastern boundary over a wide, flat floodplain with no formal defences. The M3, the M25 and the railway embankments hold water back in places, but the SFRA does not treat them as defences because they were not built as such. The Abbey River is a side channel of the Thames from Penton Hook to just below Chertsey Weir. Our page on the River Thames covers the whole catchment.
• The Chertsey Bourne system. The Chertsey Bourne leaves Virginia Water Lake and runs through Thorpe Green and Chertsey to the Thames at Hamm Court. It gathers the Moat from Thorpe Lea, the Ripley Springs from Englefield Green, the Hurst Ditch, the Meadlake Ditch through Pooley Green and the Rutherwyk Road Ditch, and when the Thames is high these ditches back up as well as filling from rain.
• The Addlestone Bourne. From Dunford Bridge on the A320 at Ottershaw it flows through Addlestone to the Wey, and the SFRA records it flooding parts of New Haw Road, Boundary Road North, Bourne Road and Palace Road. The Woburn Park Stream links it with the Chertsey Bourne.
• The River Wey and its navigation. The River Wey and the Wey Navigation form the south-eastern edge past New Haw and Wey Meadows, with the Rive Ditch draining in from Sheerwater near Nine Arches Bridge. The Basingstoke Canal runs along Woodham's southern boundary.
• Lakes and reservoirs. Virginia Water Lake, first dammed in 1753, the Obelisk Pond and Coxes Mill Pond sit inside or on the edge of the borough, and the Thames Water reservoirs lie just across the river. The SFRA finds no record of reservoir flooding, but the breach maps reach parts of Virginia Water along the Chertsey Bourne and much of Thorpe, so a site there is screened for it. Our guide to reservoir flooding explains how that residual risk is handled.

The Thames rises slowly, after days of rain, which gives time for warnings. The SFRA notes that the two Bournes can rise with much less notice. River levels across the borough, including the Environment Agency's Addlestone Bourne gauge at Addlestone, can be followed on our live river levels map. That gauge normally reads between 0.31 m and 0.90 m; its highest level on record is 1.979 m, on 31 October 2000.

North and south of the M3: why drainage differs across the borough

The motorway marks an old administrative line that still shapes where a site's rainwater can go. South of the M3, in Addlestone, Woodham and Ottershaw, urban areas have long been served by surface water sewers. North of it there are, in the SFRA's words, virtually no adopted public surface water sewers, and drainage relies on letting water soak into the ground. The SFRA traces the difference back to the former Chertsey and Egham urban district councils.

• Ground that varies. The SFRA's British Geological Survey summary finds all of Woodham compatible in some way with infiltration, much of New Haw and Virginia Water likewise, and very significant constraints across most of Thorpe, partly from old landfills where soakaways should not be used. Our geology and ground conditions map gives a first indication for a site before any trial pit.
• Groundwater on the gravels. The sands and gravels under the Thames floodplain hold a shallow water table, and the British Geological Survey data shows the greatest potential for groundwater flooding in Egham and Thorpe. During 2013 and 2014 the Environment Agency kept a groundwater flood alert in force for the Egham area from 9 January to 12 March, and a groundwater source protection zone covers part of the east of the borough.
• Surrey's drainage rules. Surrey County Council's guidance for major development applies the 2025 National Standards for sustainable drainage. Because Surrey is a water-stressed area it always expects rainwater to be collected for reuse, most sites are held to the 1 in 2 year runoff rate, and private surface water may not drain into highway drains or onto the road.
• Sewers under pressure. Thames Water is the sewerage company. In the winter of 2013 to 2014 groundwater and river water got into the foul sewers across the flooded area, and the SFRA records hundreds of properties with poor drainage or no toilet facilities for long periods, problems that outlasted the river flood.

The county council is a statutory consultee on surface water for major applications but not on groundwater, so the groundwater evidence for a Runnymede scheme has to stand on its own; our article on groundwater in planning applications covers what is usually submitted. Reuse is required whatever the ground, so a strategy normally pairs rainwater harvesting with a soakaway or an outfall to a watercourse or sewer.

Floods on Runnymede's record, from 1898 to Storm Henk

The council's Local Plan lists major Thames floods in 1898, 1947, 1968, 2003 and the winter of 2013 to 2014, and the Environment Agency holds records of river flooding in the borough in 1929, 1954, 1974, 1988, 1990 and 2000. The urban areas hit hardest have been parts of Egham Hythe and Chertsey, from the Thames and the Chertsey Bourne, and New Haw, from the Addlestone Bourne.

1. October 2000. The Addlestone Bourne at Addlestone reached the highest level on its gauge's record, 1.979 m, on 31 October.
2. 2003. The Chertsey Bourne flooded more than 100 properties, a larger impact from that river than in 2013 and 2014, according to the SFRA.
3. December 2013. The Wey was the first of the borough's rivers to respond. Surrey Fire and Rescue evacuated residents of Wey Meadows on Christmas Day, although the Wey's peak proved lower than feared.
4. January to February 2014. Thames flood warnings covered the borough from early January. On 9 February severe flood warnings were issued for every Thames reach in Runnymede, a major incident was declared and military help was requested. The Silver Command co-ordinating the response for all of Surrey ran from Addlestone Police Station, and the river peaked around 12 to 13 February.
5. The count afterwards. Surrey County Council's Section 19 report, published in October 2015, recorded about 1,170 incidents of internal property flooding at 115 sites and around 4,000 properties affected. The council's own SFRA puts the figure at 742 households flooded inside, with about 60 businesses applying for grant relief.
6. January 2024. Storm Henk brought surface water first and Thames flooding several days later. Surrey's Section 19 report for the Thames catchment groups its sites by stretch of river, not by borough. Its two Egham sites, Green Lane and the riverside off Windsor Road and Yard Mead, are both in Runnymede and had 13 properties flooded inside and 53 outside. Its Chertsey area lies mostly on the Spelthorne bank and takes in one Runnymede site, Hamm Court, where 3 flooded inside and 57 outside. The roads off Chertsey Lane, Timsway and Bundys Way among them, are filed under Staines. There is no borough total. Runnymede Borough Council offered households flooded inside a £500 recovery grant and three months without council tax.

For 2013 and 2014 the county counted incidents and the borough counted households, and the Section 19 report itself warns that agencies recorded flooding inconsistently, so a site's own history is worth more than either total. Our explainer on Section 19 investigations describes how those reports are triggered.

Policy EE13 and the 2018 SFRA: what Runnymede Borough Council expects

Applications are decided against the Runnymede 2030 Local Plan, adopted on 16 July 2020, and chiefly its policy EE13, Managing Flood Risk, read with the 2018 SFRA. Above them sits chapter 18 of the August 2026 NPPF: policy F4 on when an assessment is needed, F5 and F6 on the sequential and exception tests, and F8 on sustainable drainage. A new Local Plan is being prepared, but until it is adopted EE13 applies.

• Who needs an assessment. EE13 asks for a site-specific assessment for any development in Flood Zone 2 or 3, on sites over 1 ha in Flood Zone 1, and on dry islands. A footnote counts minor development, changes of use and replacement buildings as development for this purpose; only on a dry island is minor development exempt.
• Dry islands. Land in Flood Zone 1 or 2 surrounded by higher-risk land, mapped by the SFRA in Egham Hythe, in Chertsey and across the Green Belt around Thorpe Park. On a dry island, and wherever the exception test applies, the council wants to know when a flood would arrive, how long it would last, how willing and able occupants are to leave, and how the site would be evacuated. Our article on dry island sites covers how those cases are decided.
• No loss of flow or storage. Where a scheme adds at least one home, or 250 m² of other floorspace, it must not materially impede flood flows, reduce floodplain storage or make flooding worse on or off the site.
• Rates and volumes. Runoff may be no greater than before development, and previously developed sites must actively reduce rates and volumes, with sustainable drainage unless it is shown to be inappropriate.
• Floor levels and a dry route out. The SFRA asks for floors 300 mm above the 1 in 100 year flood level plus climate change. An escape route may be wet only if the hazard stays at Very Low along its whole length, and until new hazard mapping is issued the council takes a depth of no more than 250 mm where water moves slowly.
• Storage given back level for level. Lost floodplain storage is replaced level for level and volume for volume, on or next to the site. Voids under raised floors count as mitigation, not compensation, need openings of at least 1 m in every 5 m of wall and a maintenance plan, and are not enough on their own on an undeveloped site. Our explainers on compensatory storage and floodable voids cover both.

The SFRA expects the sequential test to be run across the whole borough unless the scheme's purpose justifies a smaller area, and the NPPF's approach to the sequential test decides whether a lower-risk site must be preferred. The council's householder guide for extensions under 250 m² in Flood Zones 2, 3a and 3b is still on its website, but it cites a 2001 policy that the 2030 plan replaced, so a submission should answer EE13 directly.

From 1 April 2027 the borough becomes part of the new West Surrey Council, which takes on both Runnymede Borough Council's planning role and Surrey County Council's role as lead local flood authority. Until then both councils carry on as now, and our article on local government reorganisation explains what applicants should watch for.

Longcross, Ottershaw and New Haw: where the 2030 plan puts new homes

Much of the borough's planned growth sits on higher ground, away from the Thames, but several allocations come with flood conditions written into the Local Plan itself. The adopted plan sets out what each site's assessment must address.

• Longcross Garden Village. Policy SD9 plans the largest single scheme, with about 1,779 homes in the Longcross area, and asks for a village-wide sustainable drainage scheme that brings runoff as close as possible to greenfield rates, with clear maintenance arrangements.
• Ottershaw. At Ottershaw East (SL12) the plan points to surface water risk in the south-east of the site and a flood flow path running across it from north-west to south-east.
• Addlestone. For Parcel B of the Veterinary Laboratories site (SL11) the assessment must consider surface water flooding on the access road. No other Addlestone allocation in the plan has that requirement.
• New Haw and Woodham. The plan expects about 123 homes and some 20,000 m² of industrial and storage floorspace here, including at Byfleet Road, one of five sites the council tested in its April 2018 Level 2 SFRA.
• Virginia Water. About 424 homes are planned for the area, including the North and South allocations (SL9 and SL10). Their site text sets no flood requirement of its own, but the SFRA notes surface water ponding beside the Chertsey Bourne and the Moat in the ward.
• Chertsey Bittams. Parts of Parcel A (SL14) lie in Flood Zones 2 and 3, so the plan asks for a sequential approach to the layout; the five Bittams parcels are covered further on our Chertsey page.

Allocation does not remove the need for an assessment. EE13 applies to allocated sites as to any other, and where the Local Plan's site text identifies a risk, the assessment is expected to deal with it.

The River Thames Scheme and Runnymede's undefended reach

The River Thames Scheme is the Environment Agency and Surrey County Council's response to a reach of the Thames with no formal defences between Egham and Teddington. In Runnymede its main element is a flood channel from Egham Hythe to Chertsey, and policy EE13 safeguards the land it needs. The policy also expects any loss of the approved flood compensation scheme at Thorpe Park during construction to be replaced.

As our article on why the River Thames Scheme is delayed explains, it had no development consent order and no construction date in September 2026. A site-specific assessment therefore works from the river as it is, and treats any future channel as a residual risk question, not a reason to lower floor levels or skip the sequential test.

Loft conversions, rebuilds and dry islands: which Runnymede proposals need an assessment?

Most do once they touch Flood Zone 2 or 3, and some do outside it. The table gives the usual position under EE13 and the SFRA; the plot's flood zone, the SFRA's functional floodplain map and its dry island map settle each case.

Extensions are covered in more depth in our guide to building an extension in a flood zone, and rebuilding in replacing a house in a flood zone.

Reports we prepare for sites from Longcross to the Thames

Applicants, home buyers, lenders, owners and businesses in the borough commission these reports. Each answers a different question, and a riverside scheme in Runnymede often needs several at once.

• Every source on one site. A flood risk assessment weighs the Thames and the Bournes against the gravels' groundwater, the ditches that back up and the reservoir breach extents.
• Planning applications. Development in Flood Zones 2 and 3, on dry islands or on larger sites needs a flood risk assessment for planning that answers policy EE13 and the 2018 SFRA.
• Buying a house. Before exchange, a flood risk survey for property purchase looks at the house itself, its floor level and how its street fared in 2014 and 2024.
• Homes in Flood Zone 2 or 3. New and replacement homes need the sequential and exception test, normally searched across the whole borough.
• Soakaways or sewers. A drainage strategy has to show where the water goes on either side of the M3, and how rainwater is reused.
• Major schemes. A surface water drainage strategy meets Surrey's 1 in 2 year runoff limit and the National Standards.
• After permission. Discharge of drainage conditions supplies the drainage detail and maintenance plan a Runnymede condition asks for.
• Watercourses with no model. Where the SFRA treats all of Flood Zone 3 as functional floodplain, along the Ripley Springs, the Rive Ditch and the Bourne tributaries, flood modelling can show whether a site really lies within it.
• Cut off by water. Dry island sites, where EE13 asks how occupants would leave, need a flood warning and evacuation plan.
• Shallow water table. In Egham, Thorpe and other places on the Thames gravels, a groundwater flood risk assessment sets the water level that floors, voids and soakaways must allow for.
• Premiums after 2014. With about 1,170 internal flooding incidents recorded in the borough in the winter of 2013 to 2014, an owner whose insurer has since raised cover costs can use a flood risk assessment for insurance to set out the building's own floor levels and flood record.
• Foul sewers that flooded. Where foul sewers were overwhelmed in 2014, a foul drainage strategy shows how a scheme's wastewater will be handled.
• Employment land. At the Longcross Enterprise Zone and the borough's strategic employment areas, a BREEAM flood risk assessment supports the Pol 03 credits.
• Buying commercial property. Industrial land off Hamm Moor Lane and at the Thorpe Industrial Estate lies partly in the flood zones, and commercial due diligence tests it before purchase.
• Keeping a business open. About 60 Runnymede businesses applied for flood grant relief after 2014, and a business continuity flood risk assessment helps an occupier plan for the next event.
• Allocated land. For land at Longcross or around Ottershaw, a feasibility study tests flood and drainage constraints before a layout is fixed.

Who answers for flooding in Runnymede

If floodwater threatens anyone's life, call 999; for a power cut during flooding, call 105. Several bodies share responsibility for everything else, and the right one depends on where the water comes from.

Free flood warnings can be set up for each warning area in the borough, from the Chertsey Bourne at Thorpe Green to the River Thames at Hamm Court and the Addlestone Bourne at Woodham. Surrey County Council publishes its completed flood investigations for every borough in the county.

Runnymede sites and flooding: questions from applicants and buyers

Every report we issue for a Runnymede site is signed off by a senior consultant; our consultants are chartered, and Unda Consulting Limited is a CIWEM Business Partner. For a site anywhere in the borough, call 01293 214444 or email enquiries@unda.co.uk. For the rest of the county, see our page on flood risk in Surrey, or browse all the areas we cover.