Flood Risk Assessment in Chertsey

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

Chertsey sits on the south bank of the River Thames, on one of the most flood-exposed reaches in England, where the river has no permanent raised defences and spreads across a wide, developed floodplain. The Thames is only part of the picture: the Abbey River — a Thames backwater dug as a mediaeval mill race for Chertsey Abbey — loops around the town, the Chertsey Bourne and the Addlestone Bourne meet the river at Chertsey Meads, and surface water and shallow groundwater add to the risk across the lower ground. We prepare flood risk assessments in Chertsey 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, then deal directly with the Environment Agency and Surrey County Council as Lead Local Flood Authority on your behalf.

Much of Chertsey is a low island of developed ground wrapped by the Thames, the Abbey River and the Bournes, so here flood risk turns on how water gets to a site, how deep it goes and how people get out — questions a national map cannot answer and an assessment has to.

To talk a Chertsey site through, contact one of our experienced flood risk consultants on 01293 214444, or start a quote online and we will reply within 60 minutes.

Flood risk assessment in Chertsey at a glance

Chertsey carries an outsized share of the Thames floodplain for the size of the town. The headline facts below shape almost every assessment we prepare here.

• The Thames wraps the town on an undefended reach. Large parts of central Chertsey, the riverside and Chertsey Meads sit in Flood Zones 2 and 3, on a stretch the Environment Agency treats as undefended developed floodplain.
• Three river systems meet here. The Thames, the Abbey River and the Chertsey and Addlestone Bournes all converge around the town and Chertsey Meads, so most sites face more than one source at once.
• Chertsey has flooded for generations. The Thames flooded the town in 1947, 1968, 2003 and the winter of 2013–14, and again in January 2024 when Storm Henk closed Chertsey Bridge and flooded homes off Eastworth Road.
• Surrey County Council is your Lead Local Flood Authority, leading on surface water, groundwater and ordinary watercourses, while the Environment Agency leads on the Thames. Runnymede Borough Council determines the application.
• The River Thames Scheme is being built to Chertsey. A new flood-relief channel running from Egham Hythe ends at Chertsey, but it is still in delivery, so the protection it promises cannot yet be assumed in a planning case.

Check flood risk anywhere in Chertsey

Chertsey is outlined below. Screen any postcode in and around the town 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 Chertsey: the Thames, the Abbey River and the Bournes

River flooding from the Thames dominates Chertsey. The river runs along the town's northern and eastern side, and on this reach it has no permanent raised defences, so it spreads across a wide, developed floodplain. The Abbey River loops a second arm of the Thames around the town, the Chertsey and Addlestone Bournes arrive from the south, and surface water and shallow groundwater add to the risk — so most sites here face more than one source at once.

What makes Chertsey unusual is the way the water wraps the town. The Thames flows past on the north and east; the Abbey River, a backwater dug by the monks of Chertsey Abbey as a mediaeval mill race and enlarged in the early nineteenth century, breaks away from the Thames upstream and rejoins it below the town, draining the low ground of Chertsey Meads. The Chertsey Bourne and the Addlestone Bourne flow up from the south and meet the Thames at the Meads. The result is a low island of developed ground all but ringed by watercourses. Working out which one drives the risk on a given site, and how it behaves, is the heart of a credible fluvial flood risk assessment here.

• Rivers (fluvial). The Thames and the Abbey River flood the low ground around the town centre, the riverside and Chertsey Meads, on an undefended reach where the river spreads freely in a large flood. Land in Flood Zones 2 and 3 along these corridors almost always needs an FRA.
• The Bournes. The Chertsey Bourne and the Addlestone Bourne drain the higher ground to the south and converge at Chertsey Meads to join the Thames, adding fluvial and surface-water risk to the south and east of the town.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in the low-lying parts of the town. 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.
• Ordinary watercourses and standing water. Ditches such as the Meadlake Ditch, the Moat, Burway Ditch and the lakes north of the town back up in heavy rain. These fall to Surrey County Council as Lead Local Flood Authority rather than the Environment Agency — which brings riparian responsibilities for any ditch on a site boundary.

Chertsey Meads is the textbook example of why a single map is never enough: a 41-hectare floodplain meadow on the bank of the Thames, classed as functional floodplain, where the Thames, the Abbey River and the two Bournes all interact at once. A scheme anywhere near it has to reconcile all of them. Our notes on the sources of flood risk and on critical drainage areas explain how these sources combine, and why the right starting point is the Environment Agency mapping read alongside the local Strategic Flood Risk Assessment.

A history of flooding in Chertsey

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

• March 1947 — the great Thames flood. The most severe flood of the River Thames in the twentieth century, driven by snowmelt and rain on frozen ground, put the Thames out across Chertsey Meads and the low ground around the town. It remains a benchmark for the modelled extents on this reach.
• 1968 and 2003. The Thames flooded the Chertsey area again in September 1968 and in January 2003, when high flows closed approaches to Chertsey Bridge. These events sit within a long record of recurring Thames flooding through the town.
• Winter 2013–14 — the modern benchmark. Across the wettest winter on record for the region, the Thames flooded property along the Chertsey riverside as part of an event that flooded more than 600 households across the wider borough, with major incidents declared and military support brought in.
• January 2024 — Storm Henk. The Thames came out of bank and Chertsey Bridge and Bridge Road were closed; Surrey County Council's Section 19 investigation records homes flooded internally off Eastworth Road, with sewer surcharge adding to the river water, and people evacuated from neighbouring Addlestone taken to Chertsey.

Ground that flooded in 1947 and 2014 floods again — a robust assessment in Chertsey treats the town's long flood record as evidence, not background colour.

For the detail of the most recent event, Surrey's Section 19 flood investigation for Eastworth Road, Chertsey is a primary record, and our explainers on why a property sits in a flood zone and how flood risk affects house prices put that history into context for buyers.

The River Thames Scheme and flood defences in Chertsey

Because this reach of the Thames has no continuous raised defences, the main intervention is the River Thames Scheme — a new flood-relief channel that runs from Egham Hythe and ends at Chertsey. It will change what a development next to it has to prove, but it does not remove the need to assess flood risk.

The scheme's Runnymede channel is around 4.8 km long and finishes at Chertsey: it passes the lakes north of the town, runs alongside Abbey Meads where new wetlands are being created, and returns to the Thames via the Burway Ditch under the M3, downstream of Chertsey Weir, with a new fish pass on the Abbey River. Across the Egham-to-Teddington reach the scheme is intended to better protect around 11,000 homes and 1,600 businesses and to lower flood levels in places, without increasing risk elsewhere. You can follow progress at the River Thames Scheme.

• It ends in Chertsey. The relief channel discharges back to the Thames at Chertsey, so the town sits at the downstream end of the Runnymede works rather than behind a continuous defence.
• 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, its protection cannot be assumed in a planning case.
• The maps ignore it. The Flood Map for Planning is drawn as if defences and future schemes are not there, so a site in Chertsey can sit in Flood Zone 3 today regardless of the scheme.

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: 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 Chertsey

Three authorities set the rules in Chertsey. The Environment Agency leads on the River Thames and the Abbey River. Surrey County Council is the Lead Local Flood Authority, leading on surface water, groundwater and ordinary watercourses and acting as a statutory consultee on surface water drainage for major development. Runnymede Borough Council determines the planning application.

A developer here deals with all three, alongside the role the LLFA plays and the Environment Agency's planning role. Flood risk and drainage in Chertsey are governed by Policy EE13 (Managing Flood Risk) of the Runnymede 2030 Local Plan, which applies the Sequential Test, requires flood risk assessments and SuDS, and safeguards the floodplain. Where part of a site sits as higher ground surrounded by floodplain — a "dry island", which describes much of Chertsey — the policy expects the assessment to address the onset and duration of a flood, whether occupiers can and will evacuate, and whether the site can run an effective flood evacuation procedure. That access-and-egress question is often where a Chertsey application is won or lost.

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

What your flood risk assessment in Chertsey must show

• Risk assessed from every credible source — the Thames, the Abbey River, the Bournes, 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 — the "dry island" question that decides many Chertsey applications.

Development and flood risk assessment in Chertsey

Chertsey carries real development pressure on a tightly constrained, low-lying town, 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.

• Town-centre regeneration. Schemes around Guildford Street and the town centre sit close to the Abbey River and the Thames floodplain, needing finished floor levels, safe access and egress, and often compensatory storage on tight urban plots.
• Riverside and infill housing. Plots along the Chertsey riverside, Bridge Wharf and Mead Lane regularly fall in Flood Zone 3, where the assessment turns on how the undefended Thames behaves in a climate-adjusted flood.
• Sites near Chertsey Meads. Development on the edge of the Meads has to respect functional floodplain, so the layout, levels and any storage have to be argued carefully against the Sequential Test and Policy EE13.
• Commercial and logistics sites. Large impermeable areas near the M3 and M25 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 Chertsey 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, as our guide on doing your own assessment explains.

How Unda helps with flood risk assessment in Chertsey

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, and we stay involved past the report to answer objections.

A flood risk assessment in Chertsey from Unda assesses every credible source — the Thames, the Abbey River, the Bournes, 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.

For the wider picture, Chertsey sits within our coverage of flood risk assessment in Runnymede and across Surrey, and every report is overseen by a chartered flood risk consultant. You can read more about Unda and browse our project case studies.

Flood risk assessments for every situation in Chertsey

People come to us for a flood risk assessment in Chertsey for many reasons — securing planning permission, buying a home, satisfying a mortgage lender or insurer, achieving a BREEAM rating, or designing the drainage for a new scheme. It helps to keep three things separate: 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. Groundwater and flood modelling are not separate reports: groundwater is a source every assessment considers, and modelling is a technique that feeds one.

Flood risk assessments

• Flood risk assessment for planning. The site-specific report 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 Chertsey riverside.
• 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 Chertsey, 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. For the full range, see our flood risk assessment service.

Chertsey flood resources and contacts

The most reliable Chertsey 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 Chertsey. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; the local warning area is River Thames at Chertsey, covering Chertsey Lock, Chertsey Bridge Road, Mead Lane, Bridge Wharf and nearby streets. 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 flood 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 flood risk and coastal change Planning Practice Guidance and the Flood and Water Management Act 2010.
• Live river levels. Whether the Thames is running high right now via our live river levels map, and a first-pass postcode screen on our flood risk map.

Flood risk assessment in Chertsey: frequently asked questions

Talk to an experienced flood risk consultant about your Chertsey site

We have prepared flood risk assessments and drainage strategies across Chertsey and the wider Runnymede borough, from the riverside and Chertsey Meads to the town centre, Eastworth and St Ann's. Get a free, no-obligation quote and a consultant will call you back within 60 minutes.