Flood Risk Assessment in Walton-on-Thames

Unda prepares comprehensive flood risk assessments and drainage strategies for residential, commercial and mixed-use sites across Walton-on-Thames. A flood risk assessment in Walton-on-Thames shows your local planning authority that a development will stay safe from flooding for its whole lifetime, and that it will not increase flood risk anywhere else.

Walton sits on the south bank of the River Thames in the borough of Elmbridge, on a reach where the river is joined a little downstream by the River Mole and the engineered River Ember at Molesey. Flooding here arrives from more than one direction: the Thames itself, the Mole and Ember, the surface water that overwhelms drains after intense rain, and the groundwater that rises through the river-terrace gravels. We assess every source that matters on your site, then deal directly with the Environment Agency and with Surrey County Council as Lead Local Flood Authority, guiding architects, planning consultants, developers, landowners, home buyers and businesses through to a decision, whether the report is needed for planning, a property purchase, a mortgage or insurer, BREEAM, or a surface-water drainage strategy.

If you need a quote or want to talk a Walton site through, contact one of our experienced flood risk consultants on 01293 214444.

Walton-on-Thames sits on the Thames just upstream of the point where the Mole and Ember join the river, so a single site here can take flood water from the Thames and the Mole at the same time, which is why an assessment has to read both systems together before the planners will accept it.

Flood risk assessment in Walton-on-Thames at a glance

• Most sites face more than one source. River flooding from the Thames, surface water in the built-up streets and groundwater in the valley gravels all occur in Walton, and riverside schemes often face two or three together.
• The 2013-14 winter floods hit the Thames valley hard. Walton, Weybridge and Molesey all flooded, and on 13 December 2013 the gauge on the River Mole near its confluence with the Thames recorded its highest level in 16 years.
• Surrey County Council is your Lead Local Flood Authority, a statutory consultee on surface water drainage for major development; Elmbridge Borough Council is the planning authority that applies the Sequential Test and its 2024 Strategic Flood Risk Assessment.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, surface water risk can trigger an assessment even on a Flood Zone 1 site.

Check flood risk anywhere in Walton-on-Thames

The Walton-on-Thames area is outlined below. Screen any 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 risk across Walton-on-Thames: the Thames, the Lower Mole and surface water

Walton-on-Thames faces flooding from three main directions. The River Thames runs along its northern edge and is the dominant risk, spilling onto the low ground around Walton Bridge and Desborough Island. The Lower Mole and the engineered River Ember add river risk where they reach the Thames at Molesey, and surface water overwhelms drains across the built-up town. Groundwater rises through the valley gravels. Most sites face more than one of these at once.

The northern edge of Walton drains straight to the Thames, where the "River Thames at Walton" flood warning area covers Desborough Island, Walton Bridge and the Elmbridge Leisure Centre. South and west of the town the Lower Mole catchment takes over: the River Mole and the man-made River Ember channel run north through Hersham towards the Thames, and their floodplains reach into Hersham, Walton and Molesey. Working out which watercourse drives the risk on a given site, and how it behaves, is the heart of a credible fluvial flood risk assessment.

• Rivers - the Thames (fluvial). The dominant risk. The Thames floods the riverside around Walton Bridge and Desborough Island, and land in Flood Zones 2 and 3 along the river almost always needs an FRA.
• Rivers - the Lower Mole and Ember (fluvial). The Mole and the engineered Ember channel carry the Lower Mole catchment north to the Thames at Molesey; their floodplains affect Hersham and the south of Walton.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in the town's streets, and 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. The river-terrace sands and gravels carry a shallow water table that floods low ground and basements without showing on a river map. Critical for any soakaway or below-ground level. See groundwater flooding.
• Reservoirs and ordinary watercourses. The Environment Agency reservoir dataset shows the northern fringe of Walton could be affected if Queen Mary Reservoir in neighbouring Spelthorne failed, and many sites sit beside a ditch or culvert classed as an ordinary watercourse, which brings riparian responsibilities.

Because these sources interact, a single national map is rarely enough. Our notes on the sources of flood risk and on critical drainage areas explain how a Flood Zone 1 site can still need assessment, and why the right starting point is the Environment Agency mapping read alongside the Elmbridge Strategic Flood Risk Assessment.

A history of flooding in Walton-on-Thames

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

• 2013-14 - the Thames winter floods. Through December 2013 and into February 2014, the wettest winter in England since records began in 1766 flooded East and West Molesey, Cobham, Thames Ditton, Weybridge and Walton. The Environment Agency issued Thames flood warnings to roughly 1,770 registered Molesey properties in February 2014, and the event has been estimated at around a 1-in-50-year annual chance at Esher.
• 13 December 2013 - the Mole peak. The gauge on the River Mole near its confluence with the Thames recorded its highest level in 16 years out of 26 years of record, the high-water mark for the recent Lower Mole floods.
• September 1968 - the Mole catchment flood. The major flood that overwhelmed the lower Mole and prompted the Lower Mole Flood Alleviation Scheme, the engineered channels and sluices that still protect Hersham, Walton and Molesey today.
• 1947 - the benchmark Thames flood. The Thames burst its banks on a scale not matched again until 2014, the event older Walton residents still measure later floods against.
• Recurring surface-water flooding. Surrey County Council's Section 19 flood investigations for Elmbridge record flooding away from the main rivers, where intense rain overwhelms the drains in the built-up parts of the borough.

Ground that has flooded once tends to flood again, so a robust assessment in Walton treats this recorded history as evidence rather than background colour.

Do you need a flood risk assessment in Walton-on-Thames?

You need a flood risk assessment for most development in Walton-on-Thames within Flood Zones 2 or 3, for sites of one hectare or more in Flood Zone 1, and increasingly for smaller Flood Zone 1 sites where the Flood Map for Planning shows surface water risk or the Elmbridge SFRA identifies a critical drainage area. If you are unsure, send us the address and we will tell you quickly whether one is required.

The detail expected scales with the application: an outline scheme needs to show the principle is sound, a full application needs design and calculations, and discharge of a condition needs full technical compliance. Our guide to whether you can do your own assessment explains where the line falls, and the difference between the public flood-risk map and the Flood Map for Planning explains why the right dataset matters.

Planning policy and your flood authorities in Walton-on-Thames

Three authorities set the rules for a Walton site. The Environment Agency leads on the main rivers, the Thames and the Mole. Surrey County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses. Elmbridge Borough Council is the planning authority that decides your application.

Surrey County Council as Lead Local Flood Authority

For major development - 10 or more dwellings, sites of 0.5 hectares or more, or 1,000 square metres or more of floor space - Surrey County Council is a statutory consultee, and its advice on surface water drainage is a material consideration the borough must take into account. An LLFA objection usually stops an application being determined until it is resolved, so getting the drainage strategy right first time is the surest way to keep a scheme moving.

• The Surrey SuDS pro-forma. The county's SuDS design guidance and Surface Water Drainage Statement pro-forma record how a scheme follows the drainage hierarchy and the national SuDS standards.
• Greenfield runoff rates. Surrey expects discharge as close to greenfield as possible and no greater than existing rates, with the 1-in-100-year-plus-climate-change rate held to no more than the existing 1-in-100 rate.
• The drainage hierarchy. Infiltration first, then a watercourse, then a surface water sewer, then a combined sewer, each step down justified by evidence such as BRE 365 infiltration testing.
• The four pillars. Schemes are expected to deliver water quantity, water quality, amenity and biodiversity together, in line with the four pillars of SuDS and the 2025 national standards.

Elmbridge Borough Council, the SFRA and the Sequential Test

Elmbridge Borough Council is the planning authority for Walton, and it holds its own evidence base. Its Level 1 Strategic Flood Risk Assessment, updated in April 2024, maps flood risk from rivers, surface water, groundwater, sewers and reservoirs across the borough. The council's Flood Risk planning guidance directs development to the areas of lowest flood risk and applies the Sequential Test, and where necessary the Exception Test, with a site-specific assessment required to demonstrate safety.

• The Elmbridge SFRA. A Strategic Flood Risk Assessment is where the borough's site-specific flood evidence is most likely to be found, and it is more current than the national datasets alone.
• The Sequential Test. Where a higher-risk site is involved, the test must show the development is in the most appropriate available location, and the right vulnerability classification has to be argued.
• National policy still applies. Your assessment must satisfy the NPPF and its Planning Practice Guidance as well as the Elmbridge framework.

What your flood risk assessment in Walton-on-Thames must show

• Risk assessed from every credible source, 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 100 years for housing.
• No increase in flood risk elsewhere, with attenuation to greenfield rates and, where needed, compensatory storage.
• Residual risk managed, with safe access, egress and finished floor levels, the area where many defended-site applications are won or lost.

Development and flood risk assessment in Walton-on-Thames

Walton is a high-demand part of the South East, and 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 objection that holds an application up for months.

• Riverside and town-centre redevelopment. Thames-side plots and town-centre regeneration often sit in Flood Zones 2 and 3, needing finished floor levels, safe access and egress, and frequently compensatory storage on tight urban sites.
• Backland and infill in a high-value suburb. Much of Walton is established residential, so released and infill plots are scrutinised closely, and surface water and ordinary watercourses often drive the drainage strategy.
• Sites behind the Lower Mole scheme. Land in Hersham and south Walton benefits from the Lower Mole Flood Alleviation Scheme, but the defences are being refurbished and the River Thames Scheme upstream is not yet built, so residual risk still has to be assessed.
• Householder extensions and basements. Conversions, extensions and basements across the town frequently turn on groundwater and infiltration feasibility, so it pays to check the geology early with our ground conditions checker.

Climate change is not optional here. A Walton assessment must apply the Environment Agency's allowances for higher river flows and rainfall over the development's lifetime, 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 house prices and insurance, and a conveyancing search is not the same thing as a planning-grade assessment. Our flood risk survey for property purchase and insurance flood risk assessment are built for exactly those situations.

How Unda helps with flood risk assessment in Walton-on-Thames

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Surrey County Council as LLFA and Elmbridge Borough Council. We are experts on this stretch of the Thames valley, not a generic report mill, and we stay involved past the report to answer objections. We are a CIWEM Business Partner with chartered members, and a senior consultant signs off every report.

That Surrey project, written up in our note on an Environment Agency objection withdrawn for a mixed-use redevelopment in Flood Zones 2 and 3, shows how a planning-compliant assessment supported by current modelling moves a riverside scheme from objection to approval, the same approach we bring to a site on the Thames at Walton.

Flood risk assessments for every situation in Walton-on-Thames

People come to us for a flood risk assessment in Walton 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 assessment for planning. The site-specific report that satisfies the NPPF and shows your development is safe for its lifetime without increasing risk elsewhere.
• Flood risk survey for property purchase. An independent, property-specific survey for buyers and their advisers, going beyond the conveyancing search to assess the actual building.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover has been restricted, declined or priced on flood risk.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 so a scheme keeps its flood and surface-water credits.
• Sequential and exception tests. The locational justification that a higher-risk site is the most appropriate available, prepared alongside the assessment.
• Groundwater flood risk. A defensible design groundwater level for the valley gravels, tested against floor levels, basements and drainage.
• Flood modelling. 1D and 2D hydraulic modelling of the Thames or the Mole where national mapping does not reflect how water behaves on the ground.
• Surface-water drainage strategy (SuDS). A scheme that manages runoff to Surrey's standards, following the drainage hierarchy and the four pillars of SuDS.
• Discharge of drainage conditions. The detailed design that clears a pre-commencement or pre-occupation drainage condition, with preliminary drainage design for outline applications.

A senior consultant signs every report; read more about Unda and our chartered flood risk consultants, or see how Walton fits the wider picture in our guide to flood risk assessment in Surrey and the other areas we cover.

Walton-on-Thames flood resources and contacts

The most reliable Walton flood information comes from the lead authorities. For an immediate threat to life during a flood, always call 999.

• Check for flooding. Environment Agency live warnings, river levels and maps, including the River Thames level at Walton gauge and flood-warning sign-up.
• Surrey County Council flooding and SuDS. The county's SuDS planning advice and its Local Flood Risk Management Strategy, plus how to report flooding from surface water or an ordinary watercourse.
• Elmbridge Borough Council. The borough's flood risk planning pages and its emergency planning and rest-centre arrangements.
• Lower Mole Flood Alleviation Scheme. Background and consultation on the Environment Agency's refurbishment of the Lower Mole scheme that protects Hersham, Walton and Molesey.
• Sewer flooding. For flooding from blocked or burst sewers, contact Thames Water as the local sewerage undertaker.
• Floodline and emergencies. Floodline on 0345 988 1188 for recorded warnings, 999 in a flood emergency and 105 for power cuts.
• Live levels. Whether the Thames or the Mole are running high right now via our live river levels map, and a first-pass postcode screen on our flood risk map.

For broader policy context, our notes on flood zones 1, 2 and 3 and residual flood risk are a good starting point, and the full flood risk assessment service sits behind every Walton report.

Flood risk assessment in Walton-on-Thames: frequently asked questions

Talk to an experienced flood risk consultant about your Walton-on-Thames site

We have prepared flood risk assessments and drainage strategies right across the Thames valley, from the riverside at Walton, Weybridge and Molesey to the Mole catchment at Hersham. Get a free, no-obligation quote and a consultant will call you back within 60 minutes.