Flood Risk Assessment in Egham

Egham sits on the south bank of the River Thames, on what the Environment Agency calls the largest area of undefended, developed floodplain in England. That single fact shapes almost every site in the town. A flood risk assessment in Egham has to read the Thames alongside the Bourne and the ditches and former gravel-pit lakes that thread through the low ground, the surface water that overwhelms drains in heavy rain, and the groundwater that rises through the valley gravels — then show that a development will stay safe for its whole lifetime without pushing flood risk onto neighbouring homes, roads or land.

We prepare comprehensive flood risk assessments and surface-water drainage strategies for residential and commercial sites across Egham and the wider Runnymede area, dealing directly with the Environment Agency and with Surrey County Council as Lead Local Flood Authority on your behalf. We work with architects, planning consultants, developers, landlords, homebuyers and businesses, whether the report is for planning, a property purchase, a mortgage or insurance, BREEAM, or a surface-water drainage strategy. Call 01293 214444 or email enquiries@unda.co.uk to talk an Egham site through.

Egham flooded badly in February 2014, when around 250 of the 368 homes flooded across Runnymede that month were in the town — which is why the Environment Agency and Surrey County Council look so hard at development here.

Flood risk assessment in Egham at a glance

A few facts set the scene for anyone bringing a site forward in the town.

• The Thames runs along Egham's northern edge. The stretch from Egham down to Teddington is the largest area of undefended, developed floodplain in England, and Egham Hythe, Pooley Green and Thorpe Lea sit on low ground right beside it.
• Around 250 Egham homes flooded in February 2014. The town took the worst of a borough-wide event in which 368 homes were flooded, with water coming straight off the Thames into Egham Hythe and Pooley Green.
• Surrey County Council is the Lead Local Flood Authority. It leads on surface water, groundwater and ordinary watercourses, while the Environment Agency leads on the Thames as a main river. Most schemes here answer to both.
• The River Thames Scheme is planned, not built. The flood-relief channel through Runnymede has had around £104 million spent on it without construction starting, so present-day flood risk — not promised future protection — is what an assessment has to work with.

Why Egham floods: the Thames, the Bourne, surface water and groundwater

River flooding shapes Egham more than almost anywhere else on the non-tidal Thames. The town lies on the floodplain on the outside of the river's loop around Runnymede and Staines, and because this reach is largely undefended, a high flow on the Thames spreads directly across low-lying ground rather than being held back by walls or embankments. Most Egham sites, though, face more than one source at once, and reading them together is the heart of a credible assessment.

The Thames is the headline, but it is not the whole story. The Bourne and the ordinary watercourses and ditches that drain the lower ground — Meadlake Ditch among them — back up when the river is high and cannot discharge. Surface water, now the most widespread source of flood risk in England, overwhelms drains in the built-up town centre and along the older residential streets. Groundwater rises through the gravels beneath the floodplain as the river climbs. Our notes on the sources of flood risk and on surface water and planning explain how these interact on a single plot.

Egham Hythe and Pooley Green flooded directly from the Thames in 2014, and the water that pooled there could not drain away because the Bourne and Meadlake Ditch were themselves backed up by the high river. That combination — river, blocked drainage and rising groundwater at once — is what an Egham assessment has to anticipate.

The ground underfoot drives much of this. Egham and its riverside neighbourhoods sit on river terrace gravels and alluvium over London Clay, while the higher ground around Englefield Green and Royal Holloway rises onto the Bagshot Sands. The gravels can take infiltration drainage in places, but the floodplain water table swings by as much as two metres between summer and winter and can emerge from below once the Thames is in flood, so a soakaway that drains freely in August may be under water in February. A quick check of the mapped ground conditions is a sensible first move, followed by infiltration testing before any drainage design is fixed. Because a single national map never captures this interplay, an Egham assessment usually reads the Flood Map for Planning against measured ground levels and, where the river drives the risk, against hydraulic modelling rather than mapping alone.

A history of flooding in Egham

These dated events are why the authorities scrutinise development in Egham so closely, and why a flood risk assessment in Egham has to be evidence-led rather than generic. The Thames has a long memory here.

• 1894 and 1947 — the benchmark Thames floods. The great floods of 1894 and 1947 inundated the riverside settlements of northern Surrey, including Staines and Egham, and still inform how the Environment Agency models this reach today.
• 2003 — the modern wake-up call. The January 2003 Thames flood was the most serious in a generation along this stretch, and set the recent baseline against which 2014 was measured.
• February 2014 — the defining recent event. The Thames peaked around 12–13 February 2014. About 250 homes flooded in Egham, part of 368 across Runnymede, with severe flood warnings on the borough's reaches not lifted until 17 February. Water came directly off the river into Egham Hythe and Pooley Green and drained into Meadlake Ditch, which backed up and flooded the roads west of Chertsey Lane.
• Winter 2013–14 — the Bourne too. A flood alert for the Chertsey Bourne ran from 23 December 2013 to 9 March 2014, a reminder that the smaller watercourses, not just the Thames, hold water across the whole of a wet winter.

Surrey County Council's Section 19 investigation into the 2014 floods documents what happened street by street across the borough; it is a useful primary record for anyone assessing a site near the river. You can read the Runnymede Section 19 report in full.

The River Thames Scheme and what it means for development in Egham

Any honest assessment of flood risk in Egham has to be clear about the River Thames Scheme: it is the long-promised flood defence for this reach, and it is not yet built. The scheme proposes a new flood-relief channel in two sections — one through Runnymede, running from Egham Hythe to Chertsey, and one through Spelthorne — to reduce flood risk to around 11,000 homes and 1,600 businesses and to lower flood levels by up to 0.9 metres in places.

The catch is delivery. By early 2026 around £104 million had been spent on development work without construction starting, and the scheme still needed a Development Consent Order before funding could be confirmed. Our analysis of the River Thames Scheme delays sets out why this matters for planning: where a decision leans on protection that has not been built, the risk transfers to the developer and the community if delivery slips. For now, an Egham assessment has to demonstrate safety against present-day flood risk, with the scheme treated as a future benefit rather than a current defence.

The same lesson applies to the engineered channels already on this stretch. The way the nearby Jubilee River redistributes Thames floodwater shows how flood-relief infrastructure changes where water goes rather than removing it — which is exactly why residual flood risk has to be assessed on defended and to-be-defended sites alike.

Check flood risk anywhere in Egham

Runnymede, the borough Egham sits within, is outlined below. Screen any Egham postcode 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 zones and planning policy for a flood risk assessment in Egham

Two kinds of authority set the rules in Egham. The Environment Agency leads on the River Thames as a main river. Surrey County Council, as Lead Local Flood Authority, leads 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 against its adopted Local Plan.

Your assessment and any drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework. The Runnymede 2030 Local Plan, adopted in July 2020, steers vulnerable development away from areas at risk of flooding, requires development in flood-risk areas to be supported by a flood risk assessment that manages risk over the lifetime of the scheme, and requires sustainable drainage on new development. The borough's flood risk assessment applicant guide and its Strategic Flood Risk Assessment, including a Level 2 study for the Egham Gateway regeneration site, set out what is expected locally.

What your flood risk assessment in Egham must show

Whatever the site, the assessment has to demonstrate the same things to the Environment Agency and Surrey County Council, scaled to the proposal. Where a higher-risk site is involved, the Sequential and Exception Tests also apply, and on drainage the four pillars of SuDS shape what Surrey will accept.

• Every source of flooding. The Thames, the Bourne and ordinary watercourses, surface water, groundwater and any reservoir or lake risk — assessed together, not in isolation.
• Lifetime safety with climate change. Flood levels for the relevant design event plus the Environment Agency's climate change allowance, over the development's lifetime — generally 100 years for housing.
• Finished floor levels and safe access. Floor levels set above the design flood level with freeboard, and a safe route in and out during a flood.
• No increase in risk elsewhere. Where floor levels or ground are raised on the floodplain, compensatory flood storage so displaced water is not pushed onto neighbours.
• A drainage strategy that holds. Surface water managed to greenfield rates where possible, following the SuDS hierarchy and Surrey's standards.

Development and flood risk assessment in Egham

Egham is a university town, and the kind of site you bring forward shapes the assessment you need. Much of the recent pressure comes from Royal Holloway, University of London, whose growing student numbers have driven a wave of conversions and new accommodation across Egham and Englefield Green. Getting the scope right is the difference between a clean approval and a drainage or flood objection that holds an application up for months.

• HMO conversions. Converting a family house to a small shared house for students is common across Egham and Englefield Green. Even where the change of use is permitted development, adding bedrooms, hardstanding and parking increases the people at risk and the runoff from the plot, so a flood risk or drainage position is often still needed — especially on the low ground near the river.
• Larger HMOs needing permission. A house in multiple occupation for six or more unrelated occupants needs planning permission in Runnymede, which brings flood risk and surface water drainage squarely into the assessment. Putting more people into a building on the floodplain raises the bar on safe access, escape and warning.
• Purpose-built student accommodation. New student blocks concentrate a vulnerable population on constrained town-centre and brownfield sites. These schemes need a robust assessment of every flood source, careful floor levels, and a drainage strategy that works on tight plots with little room for above-ground SuDS.
• Egham Gateway and town-centre regeneration. Brownfield regeneration in and around the town centre, including the Egham Gateway area, sits close to the Bourne and the surface water network and has its own Level 2 SFRA evidence to answer to.
• Riverside and back-land plots. Infill and replacement dwellings in Egham Hythe, Pooley Green and Thorpe frequently sit in Flood Zones 2 and 3, needing raised floor levels, safe access and egress, and often compensatory flood storage.

Adding student bedrooms to a house on the Egham floodplain is not a paperwork formality: it puts more people in harm's way, so the Environment Agency and Surrey County Council expect the flood risk, the escape route and the extra runoff to be addressed properly, whatever the planning route.

How Unda helps with flood risk assessment in Egham

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Surrey County Council and national planning policy. We are experts on Egham's flooding, not a generic report mill, and we know how the Thames, the Bourne and the town's surface water behave together.

A flood risk assessment in Egham from Unda reads every credible source — the Thames, the Bourne and any ordinary watercourse on the boundary, surface water, groundwater and lake risk — 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 Egham

People need a flood risk assessment or drainage strategy in Egham for a range of reasons, and we prepare each one to the standard its audience expects. A flood risk assessment, a sequential or exception test and a drainage strategy are three different 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.

• 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.
• Flood risk survey for property purchase. For buyers and their solicitors when a search flags risk on an Egham home — common given the town's flood history — going beyond the national datasets to assess the actual building.
• Flood risk assessment for insurance. Independent, property-specific evidence where cover or premiums on an Egham property have become a problem after a flood event or a high-risk flag.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 for schemes pursuing certification, including student accommodation and commercial buildings.
• Sequential and Exception Tests. The locational justification when development is proposed in Egham's higher-risk floodplain areas.
• Surface-water drainage strategy and SuDS. Managing runoff to Surrey's standards, with discharge of drainage conditions and preliminary drainage design as the scheme develops.

Our flood risk consultants can advise which of these an Egham site needs, and our free tools — the flood risk map, the live river levels map for the Thames, the drainage calculators and the BRE 365 infiltration rate calculator — are a useful first look before you commission anything. You can read more about Unda and how we work.

We also prepare flood risk assessments and drainage strategies across the neighbouring Thames-side areas — Staines, Chertsey, Old Windsor and Datchet — and across the wider Runnymede borough and the rest of Surrey, so we understand how this reach of the river behaves from one bank to the next.

Egham flood resources and contacts

The most reliable flood information for Egham 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 Egham. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; the relevant warning area is River Thames at Staines and Egham.
• Sign up for flood warnings. Register for free Environment Agency warnings at gov.uk/sign-up-for-flood-warnings or call Floodline on 0345 988 1188.
• Surrey County Council flooding. Lead Local Flood Authority strategy, SuDS guidance and reporting at Surrey County Council.
• Runnymede Borough Council. Local Plan flood policy, the FRA applicant guide and the River Thames Scheme at runnymede.gov.uk.
• The River Thames Scheme. Progress on the flood-relief channel for this reach at riverthamesscheme.org.uk.

Flood risk assessment in Egham: frequently asked questions