Chertsey Bourne Flooding & Flood Risk

The Chertsey Bourne is a small river with a problem at its mouth. It runs roughly 13 kilometres from the outfall of Virginia Water Lake, through Thorpe Green and Chertsey, to the Thames at Hamm Court. It floods on its own account, when rain fills a sandy, groundwater-fed catchment faster than the channel can carry it away. It also floods when the Thames sits so high that the Bourne has nowhere to discharge to, and the water banks up behind the confluence and spreads back through Chertsey. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Chertsey Bourne floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

On the night of 2 January 2003 the Chertsey Bourne reached 2.626 metres at the Free Prae Road gauge in Chertsey. That is still its highest recorded level, and more than a metre above the top of the gauge’s normal range.

• A designated Main River. Runnymede Borough Council lists the Chertsey and Addlestone Bourne among the borough’s main rivers, so the Environment Agency regulates the channel and its 8 metre margin rather than the council.
• Three live Environment Agency gauges. Trumps Green at Virginia Water, and Guildford Street and Free Prae Road in Chertsey, all report levels in real time and all three appear on the map below.
• Its worst floods are shared with the Thames. In February 2014 the Environment Agency issued a flood warning for the part of Chertsey nearest the Bourne once the Thames began backing up into it.
• Sandy ground, high water table. The catchment drains Bagshot Formation sands over London Clay, with alluvium on the valley floor, so groundwater rises into the floodplain as well as spilling onto it.

Check flood risk along the Chertsey Bourne

The map below draws the Chertsey Bourne in coral, from the lake at Virginia Water down to the Thames, with the Environment Agency’s live river gauges set along it. Click a gauge to see its latest level, its typical range and its record peak. Enter a postcode near the river to zoom in and screen that location against Environment Agency data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Which areas does the Chertsey Bourne put at risk?

Flooding from the Chertsey Bourne is concentrated on a narrow ribbon of low ground either side of the channel, and the Environment Agency runs a formal warning service along the whole of it. A single flood alert area, “Chertsey Bourne including Virginia Water, Thorpe Green and Chertsey and the Moat at Thorpe”, covers the river end to end. Inside it sit three more precise flood warning areas: one at Thorpe Green, one at Chertsey, and one covering the streets closest to the bank, where the Environment Agency can warn individual properties. The table below runs from the lake to the Thames.

A site in one of these areas will very likely fall in Flood Zone 2 or 3 on the Environment Agency’s flood map for planning, and will need a site-specific flood risk assessment before an application can be determined. You can screen any address against the live Environment Agency data using the map above. The towns along the river are covered in more depth on our local pages for Chertsey, Runnymede, Egham, Staines and Weybridge.

Why does the Chertsey Bourne flood?

Two things drive flooding on the Chertsey Bourne, and they often happen together. The first is the ground the river drains. Its catchment sits on the permeable sands of the Bagshot Formation over London Clay, and its lower valley is floored with alluvium. Water moves easily through the sand and the water table lies close to the surface, so a wet winter lifts groundwater into gardens and cellars on the floodplain at the same time as the channel is filling. Surrey County Council’s investigation into the winter 2013–14 floods recorded groundwater flooding in Chertsey and Thorpe alongside the river flooding.

The second is the Thames. The Bourne joins it at Hamm Court, a couple of hundred metres below Shepperton Lock. When the Thames is high the Bourne cannot get out, and its levels rise back upstream through Chertsey. That is what happened in February 2014, when the Environment Agency issued a warning for the streets nearest the Bourne after reports came in of the Thames backing up into it. It is fluvial flooding with a second river setting the terms.

The river has little slack to absorb either. Below the lake it runs east through Wentworth and Virginia Water, is crossed three times by the M3 and once by the M25 within a few kilometres, threads Thorpe Green, then passes Chertsey between housing and the Fordwater Trading Estate. Its lowest reach is canalised, running as a deep, metal-lined channel where it meets the Addlestone Bourne near Woburn Park before the two enter the Thames as one.

• Sand over clay, with groundwater near the surface. Permeable Bagshot sands and a naturally high water table mean groundwater flooding often arrives with river flooding.
• Two Bournes, one outfall. The Chertsey Bourne and the Addlestone Bourne join in Woburn Park before entering the Thames, so a wet catchment upstream of either reaches the same constrained outfall.
• A backwater river, though not a tidal one. The Bourne is not tidal, but a high Thames acts on it much as a tide would, holding back its flow and lifting its levels through Chertsey.
• Crossings and culverts leave no room. Three M3 crossings, the M25 and a canalised lower reach all constrain the channel when levels rise. The floodplain either side does the work instead.

A history of flooding on the Chertsey Bourne

The Bourne’s gauged record is short. The Chertsey stations were installed in 1997 and Trumps Green in 1995. What they have caught still tells you most of what an assessment for a nearby site has to reckon with.

January 2003 remains the benchmark. Late on the 2nd the river peaked at 2.626 metres at Free Prae Road and, fifteen minutes earlier, at 1.783 metres at Guildford Street. Both are records that stand today.

Winter 2013–14 was longer and did more damage. A flood alert was issued for the Chertsey Bourne on 23 December 2013 and stayed in force until 9 March 2014, eleven weeks in all. Levels rose and fell with each band of rain. When the Thames reached a level higher than 2003, Surrey County Council’s Section 19 investigation records that “as reports came in of the Thames backing up the Chertsey Bourne, a flood warning was also issued on 9 February for the area of Chertsey closest to the Chertsey Bourne”. Ninety properties flooded in the Chertsey area from the Thames and the Bourne together. Roads nearest the Bourne went under, groundwater came up in Chertsey and Thorpe, and the Environment Agency has since carried out channel capacity works on the river.

Alerts have followed regularly. Storm Brendan put the Bourne into flood alert in January 2020, with water on low-lying land and roads either side of the M25 near Thorpe and Virginia Water. Flooding of low-lying land was expected around Free Prae Road and Guildford Street in January 2024, and again during Storm Bert and Storm Conall that November. Two of the river’s highest gauged levels came in summer rather than winter: 2.073 metres at Trumps Green in August 2006, and 2.165 metres at the Thorpe gauge in August 2009. Summer storms on this catchment are not a small risk.

Is the Chertsey Bourne a Main River, and what that means for your site

Yes. Runnymede Borough Council lists the Chertsey and Addlestone Bourne among the borough’s main rivers, alongside the Thames, the Wey, the Meadlake Ditch, the Moat, the Hurst Ditch and the Rive Ditch. Designation puts the Environment Agency, rather than the council, in charge of the channel and its immediate margins, and it changes what a development nearby has to do.

Works in, over, under or within 8 metres of the top of the bank will normally need a Flood Risk Activity Permit from the Environment Agency, sought alongside planning permission rather than instead of it. Outfalls, culverts, bank protection, retaining structures and temporary works all fall inside that margin. The ditches and feeder streams draining into the Bourne and into Virginia Water Lake are ordinary watercourses, consented by Surrey County Council as Lead Local Flood Authority and maintained by the riparian owners whose land they cross. Working out which of the two you are on, before a scheme is drawn, saves an expensive rethink later.

• The 8 metre byelaw margin. Building, excavation or structures near the Bourne usually need a Flood Risk Activity Permit from the Environment Agency.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3 needs a site-specific flood risk assessment for planning, and so does a site of a hectare or more in Flood Zone 1. Our guide to when you need a flood risk assessment sets out the triggers.
• Groundwater belongs in the assessment. On this catchment’s sands, a summer soakaway test can mislead. Winter groundwater levels are the ones that govern the design.
• Know which watercourse you are on. The main river is Environment Agency territory. The ditches feeding it answer to the Lead Local Flood Authority and to riparian owners.

Gauges and record levels on the Chertsey Bourne

Four Environment Agency stations have measured the Chertsey Bourne. Three still report: Trumps Green above Virginia Water, and Guildford Street and Free Prae Road in Chertsey. The fourth, at Thorpe, is closed, though its record peak of 2.165 metres on 23 August 2009 remains on the Environment Agency’s books. The Environment Agency lists the three live stations under the name “River Bourne (Chertsey)” on its check-for-flooding service.

Flood risk and drainage services for sites near the Chertsey Bourne

A development near the Chertsey Bourne can call for several different reports, and it helps to be clear that they answer different questions. 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 address separate things: whether the site is safe from flooding, whether the development should be located here, and where the surface water goes. Unda prepares each of them for sites across the Bourne’s catchment.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the river’s flood zones. See our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders through our flood risk survey for property purchase and flood risk assessment for insurance.
• Drainage strategies and SuDS. Surface-water designs that meet the discharge limits set by the Lead Local Flood Authority — see our sustainable drainage strategy service.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where a high winter water table on the sands is in play.

Where national mapping is not enough, which is common on a small river with a backwater problem, hydraulic flood modelling can derive the flood levels and extents a planning officer will accept, and a detailed scheme can discharge a drainage condition. All of it is framed by the NPPF and its planning practice guidance.

Managing flood risk on the Chertsey Bourne: policy and contacts

Responsibility for the Bourne is shared. The Environment Agency regulates the main river, publishes the flood map for planning and issues the Chertsey Bourne flood alert and its warnings. Surrey County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses, and the statutory consultee on drainage for major applications. Runnymede Borough Council decides planning applications under Policy EE13 of its 2030 Local Plan, adopted on 16 July 2020, which applies the sequential approach, requires a site-specific flood risk assessment accounting for climate change over the development’s lifetime, and expects sustainable drainage. The borough’s Strategic Flood Risk Assessment of January 2018, and the Level 2 assessment that April, form the evidence base an assessment is measured against.

The River Thames Scheme comes up often here, and the answer needs to be exact. Nothing is built. The scheme’s Runnymede channel would run from Egham Hythe to Chertsey and is still at the pre-application stage of a Development Consent Order. It is a Thames intervention. It would lower Thames levels around Chertsey, and so ease the backwater effect at the Bourne’s outfall, but it does not re-engineer the Bourne. An assessment has to take the river as it is today, which is why a site here often needs a sequential test as well as an assessment.

• Live river levels. Check the river’s gauges on the Environment Agency service — Free Prae Road, Guildford Street and Trumps Green — or use our own live river levels map.
• Screen a location. Try our free flood risk map and, for drainage on the sands, the drainage calculators, the BRE 365 infiltration calculator and the ground conditions and soakaway map.
• Flood warnings. Sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on the Chertsey Bourne

Unda understand how the Chertsey Bourne behaves in flood: how quickly a sandy, groundwater-fed catchment delivers water to the channel, how far the Thames reaches back up it when the lower river is high, and what the Environment Agency and Runnymede’s planning officers expect an assessment to demonstrate before a scheme in Flood Zone 2 or 3 can proceed. We prepare flood risk assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites between Virginia Water and Hamm Court.

Every assessment rests on the evidence the regulators use themselves: the Environment Agency’s flood mapping, the river’s gauged levels, the Runnymede Strategic Flood Risk Assessment, and the policy that applies to the site. If you are working on a site near the Chertsey Bourne, talk to one of our experienced flood risk consultants about what your application will need, or read more about how we approach a flood risk assessment.

Chertsey Bourne flood risk: your questions answered