Flood Risk Assessment in Runnymede

Our team at Unda use years of experience to prepare comprehensive flood risk assessments and drainage strategies for commercial and residential sites across Runnymede. A flood risk assessment in Runnymede shows your local planning authority that a development will stay safe from flooding for its whole lifetime, without pushing flood risk onto anyone else.

Runnymede sits on one of the most flood-exposed stretches of the River Thames in England, where the river runs past Egham, Egham Hythe and Chertsey and through the low ground around Thorpe before it reaches Teddington. We prepare flood risk assessments in Runnymede for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS). On every one we read the sources that actually matter here — the Thames, the Bourne system, the surface water that overwhelms the towns and the groundwater that rises through the valley gravels — then deal directly with the Environment Agency and Surrey County Council as Lead Local Flood Authority on your behalf.

The Thames between Egham and Teddington is the largest area of developed floodplain in England with no permanent raised defences, so across much of Runnymede flood risk is set by the river itself — and an assessment that fails to read the Thames, the Bourne and the groundwater together will not stand up to the Environment Agency.

To talk it through, call one of our experienced flood risk consultants on +44 (0) 1293 214444, or start a quote online and we will reply within 60 minutes.

Flood risk assessment in Runnymede at a glance

Runnymede is a small borough carrying an outsized share of the Thames floodplain. The headline facts below shape almost every assessment we prepare here.

• The Thames runs through the borough. Large parts of Egham, Egham Hythe, Chertsey and Thorpe sit in Flood Zones 2 and 3, on a reach the Environment Agency treats as undefended developed floodplain.
• Over 600 households and nearly 30 commercial premises flooded internally across Runnymede in the winter of 2013–14, when Surrey declared two major incidents and brought in military support.
• Surrey County Council is the Lead Local Flood Authority, leading on surface water, groundwater and ordinary watercourses, while the Environment Agency leads on the Thames and the Bourne. We work with both.
• The River Thames Scheme is being built around the borough. A flood relief channel of around 4.8 km through Runnymede, from Egham Hythe to Chertsey, is designed to help protect roughly 11,000 homes and 1,600 businesses along the Egham–Teddington reach.
• Policy EE13 of the Runnymede 2030 Local Plan governs flood risk and drainage, applying the sequential test, requiring FRAs and SuDS, and safeguarding the floodplain.

Check flood risk anywhere in Runnymede

Runnymede is outlined below. Screen any postcode in the borough against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass, not a substitute for a site-specific assessment.

Flood risk in Runnymede: the Thames, the Bourne and surface water

River flooding from the Thames dominates Runnymede. The river forms much of the borough's northern and eastern edge, and on this reach it has no permanent raised defences, so it spreads across a wide, developed floodplain. Surface water threatens the built-up parts of Egham, Chertsey and Addlestone, and groundwater rises through the valley gravels in places — so most sites here face more than one source at once.

The Environment Agency designates the main rivers in the borough as the River Thames, the Chertsey and Addlestone Bourne, the River Wey, the Meadlake Ditch, the Moat, the Hurst Ditch and the Rive Ditch, with the Abbey River adding to the risk through Chertsey and the River Bourne feeding Virginia Water Lake. Understanding which watercourse drives the risk on a given site, and how it behaves, is the heart of a credible fluvial flood risk assessment.

• Rivers (fluvial). The Thames and the Bourne system flood the valley floors across Egham, Chertsey, Thorpe and Addlestone. Land in Flood Zones 2 and 3 along these corridors almost always needs an FRA, and the reach is undefended, so the river spreads freely in a large flood.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in the low-lying parts of Egham, Chertsey and Addlestone. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even on a Flood Zone 1 site.
• Groundwater. River terrace gravels under the Thames floodplain hold shallow groundwater that rises in wet winters and floods low ground and basements without ever showing on a river map — critical for any soakaway or below-ground level. See groundwater flooding.
• Standing water and ordinary watercourses. The former gravel-extraction lakes around Thorpe, and ditches such as the Meadlake and Hurst Ditch, back up in heavy rain. These fall to Surrey County Council as Lead Local Flood Authority rather than the Environment Agency — see the role the LLFA plays.

Thorpe is the textbook case for why a single map is never enough: the area sits on former gravel workings ringed by Manor, Fleet, Abbey and St Ann's lakes, with parts of the functional floodplain, high groundwater and Thames risk all in play at once. A scheme there has to reconcile all of them. Our notes on the sources of flood risk and on surface water and planning set out how these sources interact, and you can screen the picture for any address on the Surrey flood area page and the borough map above.

A history of flooding in Runnymede

These dated events are why the authorities here scrutinise development so closely, and why a flood risk assessment in Runnymede has to be evidence-led rather than generic. Each one reshaped local policy and the flood risk a developer now has to design around.

• 1947, 1968 and 2003 — the great Thames floods. The Thames flooded the Egham, Staines and Chertsey area severely in March 1947, the worst Thames flood of the twentieth century, and again in 1968 and January 2003. These events define the modelled extents along the reach today.
• Winter 2013–14 — the modern benchmark. Between late December 2013 and February 2014, with the peak around 9–14 February, over 600 households and nearly 30 commercial premises in Runnymede were flooded internally. Surrey received roughly two and a half times its average winter rainfall, declared major incidents on 24 December 2013 and 9 February 2014, and brought in military personnel; seven severe flood warnings, meaning danger to life, were issued on the Thames.
• January 2024 — Storm Henk. The Thames came out of bank again in early January 2024, closing Chertsey Bridge Road, Windsor Road in Egham, Coldharbour Lane at Thorpe and Chertsey Lane, with the A308 between Egham and Windsor shut. Runnymede Borough Council opened flood-recovery grants for affected households.

A pattern runs through all of it: ground that flooded in 1947 and 2014 tends to flood again. A robust assessment treats this recorded history as evidence, not background colour.

For the detail of how Surrey investigated the 2013–14 event, the council's Section 19 flood investigation report for Runnymede is the primary record, and our explainers on why a property sits in a flood zone and how flood risk affects house prices put a local history into context for buyers.

The River Thames Scheme and flood defences in Runnymede

Because this reach of the Thames has no continuous raised defences, the main intervention is the River Thames Scheme — new flood-relief channels alongside weir and capacity works between Egham and Teddington. It changes what a development next to it has to prove, but it does not remove the need to assess flood risk.

• A 4.8 km relief channel through Runnymede. The scheme's Runnymede section runs roughly 4.8 km from Egham Hythe to Chertsey, paired with a shorter channel in neighbouring Spelthorne, to carry water around the most exposed communities.
• Designed to protect thousands of properties. Across the Egham-to-Teddington reach the scheme is intended to better protect around 11,000 homes and 1,600 businesses, lowering flood levels by up to about 0.9 m in places without increasing risk elsewhere.
• Still in delivery, and delayed. The scheme has taken many years and significant cost to develop, with construction indicatively expected from around winter 2026 and operation later this decade. Until it is built and proven, the protection it promises cannot be assumed in a planning case.

The catch for developers is the one we set out in residual flood risk and why a site stays in a flood zone after defences are built: the Flood Map for Planning is drawn as if defences and future schemes are not there, so a site in Egham or Chertsey can sit in Flood Zone 3 today regardless of the River Thames Scheme. Relying on a future scheme that has faced repeated delays transfers risk onto the development, which is why an assessment applies the Environment Agency's climate change allowances over the whole lifetime. The same lesson applies upstream, where the Jubilee River shows how a relief channel redistributes rather than removes floodwater.

Planning policy and your Lead Local Flood Authority in Runnymede

Two kinds of authority set the rules. The Environment Agency leads on the River Thames and the other main rivers. Surrey County Council is the Lead Local Flood Authority (LLFA), leading on surface water, groundwater and ordinary watercourses, and is a statutory consultee on surface water drainage for major development. Runnymede Borough Council determines the application.

A developer here deals with all three, alongside the role the LLFA plays and the Environment Agency's planning role. Our overview of who manages flood risk in England explains how the pieces fit together.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework: the Surrey LLFA drainage guidance, Runnymede's Strategic Flood Risk Assessment and the flood policy in the Runnymede 2030 Local Plan. Where a higher-risk site is involved, the Sequential and Exception Tests apply, and the right flood zone and vulnerability classification have to be argued. On drainage, the 2025 national SuDS standards and the four pillars of SuDS shape what the LLFA will accept, and a strategy that follows the discharge hierarchy from the outset is the surest way to avoid an objection. You can confirm the published flood zones on the Environment Agency Flood Map for Planning.

What your flood risk assessment in Runnymede must show

• Risk assessed from every credible source — the Thames, the Bourne, surface water, groundwater and any ordinary watercourse on the boundary — using the Flood Map for Planning and local data.
• An appropriate location, tested against the Sequential Test where policy requires it.
• Safe for its lifetime, once the Environment Agency climate change allowances are applied, generally over 100 years for housing.
• No increase in flood risk elsewhere, with attenuation and, where needed, compensatory storage in the floodplain.
• Residual risk managed, with safe access, egress and finished floor levels. Get one of these wrong and the application stalls, as our note on LLFA drainage objections shows.

Development and flood risk assessment in Runnymede

Runnymede carries strong development pressure on a tightly constrained, low-lying borough, so the kind of site you bring forward shapes the assessment you need. The right scope is the difference between a clean approval and a drainage or Environment Agency objection that holds an application up for months.

• Strategic allocations. Longcross Garden Village, on the former DERA site, is a large multi-phase scheme with parts in Flood Zones 2 and 3a, where surface-water drainage and floodplain constraints shape the masterplan and demand detailed assessment and sequential testing.
• Town-centre regeneration. Schemes in Addlestone and Egham frequently sit on or near the Bourne and Thames floodplains, needing finished floor levels, safe access and egress, and often compensatory flood storage.
• Riverside and infill housing. Plots in Egham Hythe, Chertsey and Thorpe regularly fall in Flood Zone 3, where the assessment turns on how the undefended Thames behaves in a climate-adjusted flood.
• Commercial and logistics sites. Large impermeable areas near the M25 and M3 make the surface-water drainage strategy central to the application, with infiltration often ruled out by clay and high groundwater.

Climate change is not optional here. A Runnymede assessment must apply the Environment Agency's allowances for higher Thames flows and rainfall over the development's lifetime — generally 100 years for housing — which sets floor levels, freeboard and how much attenuation a drainage scheme needs.

Home buyers and businesses come to us too. A flood risk question can affect a purchase, and a conveyancing search is not the same thing as a planning-grade assessment. Knowing which you actually need saves time and money, and our guide on doing your own assessment explains where the line falls, while critical drainage areas shows how a Flood Zone 1 site can still trigger one.

How Unda helps with flood risk assessment in Runnymede

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Surrey County Council as Lead Local Flood Authority and national planning policy. We are experts on this stretch of the Thames, not a generic report mill.

A flood risk assessment in Runnymede from Unda assesses every credible source — the Thames, the Bourne, surface water, groundwater and any ordinary watercourse on the boundary — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report. Where the council needs it, a surface-water drainage strategy sits alongside the assessment as a separate report so the two cases stay consistent and an objection has nowhere to land.

Flood risk assessments and related reports for every situation in Runnymede

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 answer different questions: is the site safe, should the development be here, and where does the surface water go. We prepare all three, alongside the right form of assessment for buyers, lenders and BREEAM.

Flood risk assessments

• Flood risk assessment for planning. The core report, site-specific and prepared to satisfy the NPPF, the Environment Agency and Runnymede Borough Council. Groundwater and, where needed, flood modelling are assessed within it rather than as separate reports.
• Homebuyer and property-purchase survey. For conveyancing, where a search has flagged Thames or surface-water risk and you need the actual building assessed, not a broad map.
• Flood risk assessment for insurance. Property-specific evidence for brokers and underwriters where premiums have risen or cover has been restricted, common on the Egham and Chertsey floodplain.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning FRA alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for the Thames-valley gravels and any basement — assessed as a source within the FRA, or as a standalone hydrogeological assessment on complex sites.

Sequential and Exception Tests

The Sequential and Exception Tests are a separate, locational report about whether a development should be in a higher-risk location at all. The Sequential Test defines an area of search, identifies reasonably available lower-risk sites and justifies the proposed site; the Exception Test is the second gate where development still needs to proceed. A clean site-specific assessment showing the development is not actually within an area at risk can remove the need at application stage — a live policy area, as our note on what the Yatton judgment means for planning explains.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering — calculates existing versus proposed runoff and proposes SuDS to hold discharge within the rates Surrey will accept.
• Preliminary drainage design. The early, outline read that flags infiltration issues before they affect layout and land value.
• Discharge of drainage conditions. The detailed stage to discharge a pre-commencement or pre-occupation drainage condition.
• Flood modelling. 1D and 2D hydraulic modelling — a technique that feeds the assessment, Sequential Test or drainage design where national mapping is not enough, which is common on the Thames here.

Early in a drainage strategy it helps to know whether the ground will take water — and across much of low-lying Runnymede, clay and high groundwater mean it often will not. Our free geology and ground conditions map and BRE 365 infiltration rate calculator give an indicative read before a trial pit, and the drainage calculators help size the scheme. You can screen a postcode on our flood risk map, and every report is overseen by a chartered flood risk consultant who stays involved past the report to answer objections. For the full range, see our flood risk assessment service.

Runnymede flood resources and contacts

The most reliable Runnymede flood information comes from the lead authorities. For an immediate threat to life during a flood, always call 999; for a power cut during flooding, call 105.

• Check for flooding in Runnymede. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; sign up for flood warnings or call Floodline on 0345 988 1188.
• Surrey County Council flood risk. The Lead Local Flood Authority's drainage guidance and reporting at Surrey County Council.
• Runnymede Borough Council. Local flooding, planning policy and the Local Plan at runnymede.gov.uk.
• The River Thames Scheme. Progress, channel design and consultation at the River Thames Scheme.
• Standing advice and the law. The Environment Agency flood risk standing advice and the Flood and Water Management Act 2010.
• Live river levels. Whether the Thames or the Bourne are running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact Thames Water, which provides sewerage across the borough. For broader policy context, our notes on the Environment Agency's planning role and on flood zones 1, 2 and 3 are a good starting point, and the nearby Guildford flood alleviation scheme shows how Surrey is managing flood risk elsewhere in the county.

Flood risk assessment in Runnymede: frequently asked questions