Flood Risk Assessment in Chertsey

A flood risk assessment in Chertsey has to deal with three rivers at once. The Thames runs along the east of the town with no formal defences, the Abbey River loops off it to the north-east, and the Chertsey Bourne crosses the town from the west and cannot drain when the Thames is high, so it backs up into the streets around Eastworth Road and Fordwater Road.

On this page Chertsey means the town as the Office for National Statistics maps it: the centre around Guildford Street and Windsor Street, Eastworth, the riverside streets at Bridge Wharf and Mead Lane, Pyrcroft Road and Chilsey Green to the west, and Chertsey South along Green Lane and Bittams Lane, down to St Peter's Hospital. Almost all of it shares the KT16 postcode district. Runnymede Borough Council decides planning applications here and Surrey County Council is the Lead Local Flood Authority. We prepare flood risk assessments in Chertsey for planning, house purchases, mortgages and insurance, and the sequential and exception tests and surface water drainage strategies that often go with them.

Runnymede's own flood study identifies a "Chertsey dry island" over part of the town centre: higher ground in Flood Zone 1 or 2 that the Bourne and the Thames can cut off, which is why policy EE13 asks for an assessment there even outside the flood zones.

To talk through a site anywhere in Chertsey, call one of our flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

The Thames, the Abbey River, the Bourne and the sewers: where Chertsey's water comes from

River flooding leads in Chertsey, but each river behaves differently and a site can sit in reach of two of them. Surrey's flood investigations treat all three as main rivers, and the borough's Level 1 Strategic Flood Risk Assessment (January 2018) records no formal flood defences in the riverside wards and a town built on Bagshot sand with river gravels and alluvium above it. The table sets out each source and where it reaches.

When the Thames is in flood the Bourne's outfall at Hamm Court is held back, and the county's investigators describe the river rising with the Thames because of what they call the Chertsey Bridge stopper effect. Rain then has nowhere to go: the surface water outfall under Fordwater Bridge sits below the Bourne's flood level, and surplus water ends up in the foul sewers. An assessment here looks at the joint event, a high Thames and a full Bourne together, not each river on its own. Our page on the Chertsey Bourne follows it from Virginia Water to Hamm Court, and the River Thames page covers the main river.

Look up a postcode between the Bourne and the Thames

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 sites outside the town. The outline is the Chertsey built-up area; Chertsey Meads, the lock and the weir lie just outside it to the east.

Chertsey flooding in 2003, 2014 and 2024

The town's three recent floods each came from a different source, which is why one flood zone line does not describe a Chertsey site. Runnymede's Local Plan lists 1898, 1947, 1968, 2003 and 2013 to 2014 as the borough's major Thames floods; the events below come from the borough SFRA and from Surrey's Section 19 reports on the winter of 2013 to 2014 and January 2024.

1. January 2003: the Bourne. The Chertsey Bourne flooded more than 100 properties, according to the borough's SFRA, and both of its gauges in the town still hold their record levels from the night of 2 January 2003: 1.78 m at Guildford Street and 2.63 m at Free Prae Road, where the Environment Agency says property flooding is possible above 2.15 m.
2. February 2014: the Thames. After a winter with 275% of average rainfall in Runnymede, the Thames at Chertsey peaked in February at a level the county's investigation puts at a 1 in 15 to 1 in 20 year flood, higher than in 2003. A severe flood warning went out for the River Thames at Chertsey at 15:32 on 9 February to 518 properties, and about 90 properties flooded in the Chertsey area from the Thames and the Bourne.
3. Bridge Road, Chaseside Gardens and the roads that closed. On 13 February the Environment Agency set an AquaDam along Bridge Road to protect about 155 properties against a 1 in 100 flood, and a sandbag wall built with the armed forces protected 25 homes at Chaseside Gardens. In the event the river stopped short of the level at which the AquaDam would have come into use. Dockett Eddy, Almners Lane, Chaseside Gardens, St Ann's Road and Lyne Lane were closed, and a flood recovery centre opened in the town.
4. 2014 underground. Raised groundwater got into the foul sewers across the borough. Thames Water's Stepgates catchment was among the worst hit, Eastworth Road was severely affected, and residents had restricted use of toilets and drains for some time after the river fell.
5. January 2024: Storm Henk. Between 1 and 4 January, 50 to 75 mm of rain fell on a saturated catchment and the Thames peaked between Datchet and Teddington on 10 January. In Eastworth Road two houses flooded inside with sewage and up to 14 flooded outside, and most lost foul drainage for the 14 days the event lasted.

Surrey County Council's Storm Henk investigation groups 39 internal and 117 external flooding reports under a Chertsey sub-area, but its three sites lie downstream and across the river, at Hamm Court and along the Thameside streets beyond Chertsey Bridge, outside the town mapped here. River levels at Chertsey Lock and on the Bourne can be followed on our live river levels map, and the Environment Agency publishes each gauge: Chertsey Lock, the Bourne at Guildford Street and the Bourne at Free Prae Road.

What the Eastworth Road investigation means for new building

Surrey's Section 19 report on Eastworth Road is short, but it is the most useful document a Chertsey applicant can read before drawing up a drainage strategy. It found that the street floods from the Bourne behind the gardens, from road runoff, and from a combined sewer that has no spare room once the river is up.

• Roof water in the foul sewer. Houses on the affected row discharge their roofs straight into the foul system without any flow control. Thames Water's CCTV survey found no blockage or defect in the pipe, so the report identifies the roof connections as a fundamental cause.
• Conversions uphill. The report points to office-to-residential conversions planned upstream of the low point as extra pressure on a sewer it calls near capacity, and asks Thames Water to plan pumping capacity for Chertsey's growth in its next investment round.
• An outfall that drowns. The surface water drain runs under the pavement to a flap valve below Fordwater Bridge. The valve needs regular maintenance to swing freely and is submerged when the Bourne is in flood, which can restrict the outflow just when it is needed most.
• Answers on open ground. The recommendations favour taking gully and side-street water, including from Wheatash Road, into a new open ditch to the Bourne across undeveloped land, and a SuDS pod on each house to hold roof water back.

For a developer that changes what the drainage case must show. Foul and surface water should be kept apart, roof water should not reach the combined sewer, and any outfall to the Bourne needs to work while the Bourne is high. A scheme that adds flats near the Eastworth Road low point should expect Thames Water and the Lead Local Flood Authority to ask how it avoids making the 2024 problem worse. Our guides to foul, surface water and combined sewers and sewer flooding explain the background, and our article on office-to-residential conversions covers the planning side of Class MA changes of use.

The Chertsey dry island and the streets in Zone 3b

Two definitions in the SFRA shape a Chertsey application as much as the flood zone does. Functional floodplain, Flood Zone 3b, is undeveloped land inside the 1 in 20 year flood outline. A dry island is ground in Flood Zone 1 or 2 of half a hectare or more that is surrounded by land at higher risk. Chertsey has both, often a few streets apart.

• Zone 3b north of Eastworth Road. The SFRA puts significant land north of Eastworth Road in the 1 in 20 year outline, including parts of Drill Hall Road, Bramley Close, Pound Road, Free Prae Road and Flemish Gardens.
• Zone 3b to the west. In the St Ann's part of town the same outline takes in parts of Brookside, Pyrcroft Road, Frithwald Road, Twynersh Avenue and Grove Road, beside the Bourne and the Rutherwyk Road Ditch.
• What counts as 3b in a built-up street. Existing buildings that keep floodwater out are not treated as functional floodplain, but the gardens and open space around them are. Redevelopment in these areas must not raise the vulnerability of the use or the number of homes, and should reduce flood risk overall.
• The dry islands. The largest is the Chertsey dry island over part of the town centre. A smaller one lies east of Heriot Road, part of Bridge Wharf sits on another, and there is one south of the A320 near St Ann's Road.

Policy EE13 of the Runnymede 2030 Local Plan, adopted on 16 July 2020, brings the two together. It asks for a site-specific flood risk assessment for any development in Flood Zones 2 or 3, on sites over 1 hectare in Flood Zone 1, or in a dry island, except minor development in a dry island. Where a site is in a dry island, or the exception test applies, the applicant must consider when a flood would arrive and how long it would last, whether occupiers would be willing and able to leave, and whether the site can run an effective evacuation plan. On the Chertsey dry island the buildings may stay dry while every road out floods, so access and egress usually decide the case. Our article on dry island sites explains the problem, and our guide to Flood Zone 3b covers what 3b allows.

Which rules apply to a flood risk assessment in Chertsey?

Policy for a Chertsey site comes from national, borough and county documents, and three bodies check it: Runnymede Borough Council as planning authority, Surrey County Council on surface water and the Environment Agency on the Thames, the Abbey River and the Bourne.

Runnymede reviewed the 2030 Local Plan in January 2021 and is now preparing a new Local Plan, so EE13 and the 2018 SFRA remain the documents in force until it is adopted. Surrey's Local Flood Risk Management Strategy and its SuDS planning advice set out how the county handles surface water and what a drainage submission must contain, and our guide to the Lead Local Flood Authority explains the county's role. From 1 April 2027 Runnymede and Surrey County Council's services pass to the new West Surrey Council; until then both operate as normal, and our article on local government reorganisation explains what changes for applications.

Pyrcroft Road, Chertsey Bittams and the other allocated sites

The Local Plan allocates several large housing sites in and around Chertsey, mostly on its western and southern edges. Two were studied in Runnymede's Level 2 SFRA, and they came out very differently: one is split across all three flood zones, the other is almost all Zone 1.

The Pyrcroft Road study also shows the age of the river models. The Chertsey Bourne model dates from 2005, and only a limited output from the 2018 Thames model reached the SFRA, so the study asks for the Environment Agency's current Thames area climate change allowances to be applied, by the basic method in Zone 2 and the detailed method in Zone 3. It also asks for runoff limited to greenfield rates, infiltration where the gravels allow it and discharge to the unnamed main river on the site's eastern edge. Surrey's current drainage guidance now adds two firmer rules: rainwater reuse is always required, because Surrey is a water-stressed area, and most sites that add runoff are held to the 1 in 2 year rate. At Chertsey Bittams the risk is mainly surface water and a high water table, so the drainage strategy carries most of the work. Our article on climate change allowances explains which allowance applies, and our geology and ground conditions map and the guide to infiltration testing help with a first read of the ground.

The River Thames Scheme channel that would end at Chertsey

The River Thames Scheme, led by the Environment Agency and Surrey County Council, would add a flood channel in two sections, one through Runnymede between Egham Hythe and Chertsey and one through Spelthorne, together with more capacity at the Desborough Cut and the Sunbury, Molesey and Teddington weirs. Policy EE13 safeguards the land the Runnymede channel needs, and statutory consultation ran from 22 January to 4 March 2024, as Spelthorne Borough Council records.

Because the scheme is nationally significant it needs a Development Consent Order. When we last checked, in September 2026, no application had been made and no construction date had been set, as our article on why the River Thames Scheme is delayed explains. A Chertsey assessment therefore describes the town as it is now, with the climate change allowance for the lifetime of the development and without the channel. Even once it is built, the Flood Map for Planning may still show a site in Flood Zone 3, a point covered in our page on staying in a flood zone after new defences.

Reports we prepare for sites from the Abbey River to Chertsey South

We prepare reports in Chertsey for applicants, buyers, landowners, businesses and insurers. Each row below answers a different question, and on a site near the Bourne several often go in together.

• Planning applications. Runnymede needs a flood risk assessment for planning for development in Flood Zones 2 and 3, on dry islands and on sites over 1 hectare, as policy EE13 sets out.
• Every source together. A Chertsey site can face the Thames, the Bourne backing up, groundwater in the gravels and sewer flooding at once, and a flood risk assessment brings them together for one site.
• Buying a house. For homes on Bridge Wharf, Mead Lane or near the Bourne with a 2003 or 2014 flood history, a flood risk survey for property purchase checks the building itself as well as the postcode.
• Sites in Flood Zone 3. Where a site lies in Zone 3, or in a dry island ringed by it, the sequential and exception test shows whether the development should be there at all.
• Drainage for a planning application. From a single new house to Chertsey Bittams, a drainage strategy answers where the water goes when the Bourne is high and the gravels are full.
• Surface water on new schemes. Surrey asks for rainwater reuse on every scheme and a 1 in 2 year discharge limit on most, so a surface water drainage strategy shows how a Chertsey site meets both with an outfall that still works when the Bourne is high.
• After permission. Surrey recommends standard surface water conditions to planning authorities, and the discharge of drainage conditions needs the final calculations, details and maintenance plan for Runnymede to sign them off.
• Foul water on Eastworth Road. Surrey's 2024 investigation found the Eastworth Road combined sewer near capacity with roof water already in it, so a foul drainage strategy shows how a new scheme keeps foul and surface water apart.
• Groundwater in the gravels. Basements and soakaways in the town centre and near St Ann's Road meet groundwater the SFRA maps there, which a groundwater flood risk assessment measures against the floor and the drainage.
• Testing the Bourne in detail. The Chertsey Bourne model dates from 2005, so flood modelling can test a site's levels and the joint Thames and Bourne flood under current climate allowances.
• Getting out on the dry island. EE13 asks dry-island and exception-test sites to show how occupiers would leave, which a flood warning and evacuation plan sets out against the River Thames at Chertsey warning.
• Insurance. About 90 properties flooded in the Chertsey area in 2014, and a flood risk assessment for insurance gives an insurer evidence about the building rather than the postcode band.
• Trading estates by the Bourne. The Fordwater Trading Estate sits inside the Chertsey Bourne flood warning area, and a business continuity flood risk assessment plans for a business staying open through a flood.
• Land with split flood zones. Before buying or promoting land like Pyrcroft Road, split across all three flood zones, a flood risk feasibility study shows how much of it can be built on and where.

Who to call when the Bourne or the Thames rises in Chertsey

If floodwater puts anyone in danger, call 999 and ask for the fire and rescue service; for a power cut during flooding, call 105. Otherwise each body below deals with a different part of flooding in the town.

Register for free flood warnings for both Chertsey areas, and check the long-term picture for an address on the government's long-term flood risk service. Surrey publishes every Section 19 flood investigation it has carried out, including both Chertsey reports. On the far bank, Chertsey Lock, Chertsey Weir and the Thameside streets towards Laleham fall within Spelthorne, which has its own Spelthorne flood risk and drainage page.

What Chertsey applicants and buyers ask about flood risk

Each report we issue for a Chertsey site is checked and 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 town, call 01293 214444 or email enquiries@unda.co.uk. For sites beyond the town, see our pages on the borough of Runnymede, the neighbouring towns of Egham, Staines and Weybridge, the rest of Surrey and other areas across England.