Flood Risk Assessment in Molesey

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

Molesey sits on the south bank of the River Thames opposite Hampton Court Palace, on low ground where the River Mole and the River Ember reach the Thames together. We assess every source that matters here — the tidal-limit Thames at Molesey Lock, the Mole and the Ember, the surface water that ponds on the flat ground between them, the groundwater in the river gravels, and the reservoirs ringing West Molesey — then deal directly with the Environment Agency and Surrey County Council as Lead Local Flood Authority on your behalf. We guide 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 insurance, BREEAM, or a surface-water drainage strategy.

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

Few places in Surrey concentrate flood risk like Molesey: it sits in the bowl where the Mole and the Ember meet the Thames, behind a flood alleviation scheme now past its design life, with raised reservoirs along its western edge — so river, surface water and reservoir risk all have to be read together for an assessment to stand up.

Flood risk assessment in Molesey at a glance

• Three rivers meet here. The Thames, the Mole and the Ember all converge at Molesey, so the town can take flood water from the tidal-limit Thames and from the Mole catchment in the same event.
• 1968 is the benchmark. After a weekend of thunderstorms in September 1968 the Mole valley was submerged, water reached around eight feet in parts of Molesey, and thousands across the Mole, Wey and Thames valleys were made homeless — the event that prompted today's flood defences.
• Winter 2013–14 hit again. The Thames and Mole flooding of December 2013 to February 2014 was around a 1-in-50-year event at Esher, with East and West Molesey among the worst-affected parts of Elmbridge.
• Surrey County Council is your Lead Local Flood Authority, covering surface water, groundwater and ordinary watercourses; Elmbridge Borough Council is the planning authority; the Environment Agency leads on the Thames and the Mole.
• 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 Molesey

East and West Molesey are 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 in Molesey: the Thames, the Mole, the Ember and surface water

Molesey faces more than one source of flooding at once. The Thames sets the base level along the northern edge at Molesey Lock and Weir; the Mole and the Ember bring water down from the Mole catchment to the south; and the flat, low ground between them holds surface water and sits on permeable river gravels where groundwater can rise. Most sites here have to answer to two or three of these sources together.

Just upstream of the town the Mole splits around Island Barn Reservoir: the northern branch keeps the name Mole, the southern branch is the Ember, and the two run either side of the reservoir before merging a few hundred metres above the Thames. Working out which channel drives the risk on a given site, and how the Thames backs water up behind it, is the heart of a credible flood risk assessment in Molesey.

• Rivers (fluvial and tidal-limit). The Thames along the north of Molesey and the Mole and Ember to the south place large parts of East and West Molesey in Flood Zones 2 and 3. Land in these zones almost always needs an assessment. See our note on flood zones 1, 2 and 3.
• Surface water (pluvial). Now the most widespread flood risk in England. The flat ground between the rivers drains slowly, so intense rain ponds in streets and gardens. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even in Flood Zone 1 — see surface water and planning.
• Groundwater. Molesey sits on Thames river-terrace gravels, where the water table can rise close to the surface and flood basements and below-ground levels without showing on a river map. This matters for any soakaway or basement — see groundwater flooding.
• Reservoirs. Island Barn, Knight and Bessborough are raised, embanked reservoirs along the western and southern edge of Molesey, so parts of the town fall within a reservoir-failure inundation extent — a residual, emergency-planning scenario the assessment should acknowledge.

Molesey is the textbook case for why a single map is never enough: the Thames floodplain runs along Hurst Park and the riverside roads, the Mole and Ember floodplain reaches in from the south, and the flat centre ponds with surface water that cannot drain by gravity. A scheme here often has to reconcile all three. Our overview of the sources of flood risk sets out how they interact, and many Molesey sites also fall within a critical drainage area.

A history of flooding in Molesey

These dated events are why Surrey County Council and Elmbridge Borough Council scrutinise development here so closely, and why a flood risk assessment in Molesey has to be evidence-led rather than generic. Each one shaped the defences and the local policy a developer now designs around.

• 1947 — the Thames floods. The Surrey Comet recorded the March 1947 floods as the most catastrophic river floods in at least 200 years, a benchmark still referenced in Elmbridge's Strategic Flood Risk Assessment.
• 1968 — the Mole valley flood. After a weekend of thunderstorms in September 1968 the Mole, Wey and Thames valleys were submerged, with water around eight feet deep in parts of Molesey and thousands made homeless. The river was modified downstream of Albany Bridge and new defences followed — the origin of the Lower Mole scheme.
• 2013–14 — the winter floods. On 13 December 2013 the gauge on the Mole near its confluence with the Thames recorded its highest peak in 16 years; the wider event of December 2013 to February 2014 was around a 1-in-50-year flood at Esher, and around a dozen Molesey properties flooded from the Thames in February 2014. It prompted Surrey's Section 19 flood investigation for Elmbridge.
• The pattern. East and West Molesey, Thames Ditton and the riverside have repeatedly appeared in flood-warning records and investigations — the recorded history is evidence an assessment has to engage with, not background colour.

Ground that flooded once tends to flood again. A robust assessment treats Molesey's recorded flood history as evidence of how water actually moves through the town, not as a footnote.

What protects Molesey: the Lower Mole Flood Alleviation Scheme

Most of Molesey's protection comes from the Lower Mole Flood Alleviation Scheme, built after the 1968 flood. It changes what a development next to it has to prove, because a defence reduces the chance of flooding but never removes it — and planning policy still expects the residual risk to be assessed.

• An engineered diversion channel. The scheme is built largely around a man-made channel along the course of the old Ember, sized to carry large flood flows through Molesey without flooding property, running from Hersham down to the Thames.
• It protects around 1,300 properties. The Environment Agency puts the scheme's protection at roughly 1,300 homes and businesses across the lower Mole, including Molesey, Esher and Hersham.
• Six ageing sluice gates. The scheme depends on a series of six main sluice gates, now showing their age — it has reached the end of its design life and the Environment Agency has consulted on refurbishment options to maintain the standard of protection.
• Residual risk remains. The Flood Map for Planning is drawn as if the scheme were not there, so defended parts of Molesey still sit in Flood Zone 3, and an assessment has to test what happens if a defence is overtopped or a gate fails.

You can follow the refurbishment programme on the Environment Agency's Lower Mole Flood Alleviation Scheme pages. For the planning consequences, see our notes on residual flood risk and on why a site stays in a flood zone after defences are built. Climate change sharpens this further, which is why an assessment applies the Environment Agency's allowances over the development's lifetime.

Planning policy and your Lead Local Flood Authority in Molesey

Two kinds of authority set the rules. The Environment Agency leads on the Thames and the Mole as main rivers. Surrey County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses, and a statutory consultee on surface water drainage for major development; Elmbridge Borough Council is the planning authority that decides the application.

A development in Molesey is assessed against national policy and Elmbridge's local framework together. Surrey County Council reviews the drainage strategy as Lead Local Flood Authority, while Elmbridge applies its own flood policy through the planning process. Our overview of who manages flood risk in England explains how the pieces fit together.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework: Surrey's Local Flood Risk Management Strategy, the Elmbridge Strategic Flood Risk Assessment and the borough's adopted flood policies (CS26, DM6 and DM13). Where a higher-risk site is involved, the Sequential and Exception Tests apply. On drainage, Surrey's SuDS design guidance, the 2025 national SuDS standards and the four pillars of SuDS shape what the authority will accept. Works within eight metres of the Thames or the Mole also need a flood-risk activity permit from the Environment Agency, and works on an ordinary watercourse need consent from Surrey County Council.

What your flood risk assessment in Molesey must show

• Risk assessed from every credible source — Thames, Mole, Ember, surface water, groundwater and the reservoirs — 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's climate change allowances are applied.
• No increase in flood risk elsewhere, with attenuation and, on the floodplain, compensatory storage.
• Residual risk managed, with safe access, egress and finished floor levels. Get one of these wrong and the application stalls, as our note on LLFA drainage objections shows.

Development and flood risk assessment in Molesey

Molesey is a sought-after Thames-side commuter 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 objection that holds an application up for months.

• Householder extensions and replacement dwellings. Much of Molesey's development is extensions, rebuilds and infill on existing plots. On the floodplain even a modest scheme can need an assessment showing it does not reduce flood storage or raise risk next door.
• Basements and below-ground space. Basement schemes are common on high-value Molesey plots, where rising groundwater in the river gravels is the deciding issue rather than the river itself.
• Riverside and conservation-area sites. Plots along the Thames at Hurst Park and the riverside roads sit in Flood Zones 2 and 3, needing finished floor levels, safe access and egress, and often compensatory flood storage.
• Commercial and mixed-use sites. Larger town-centre and brownfield schemes add impermeable area, which puts the surface-water drainage strategy at the centre of the application.

Climate change is not optional here. A Molesey assessment must apply the Environment Agency's allowances for higher Thames and Mole flows and heavier rainfall over the development's lifetime — generally 100 years for housing — which sets floor levels, freeboard and how much attenuation the drainage scheme needs.

Home buyers and businesses come to us too. A flood risk question can affect a purchase and house prices, and a conveyancing search is not the same thing as a planning-grade assessment — nor are the national flood maps always right at the level of a single building, as our note on whether flood maps are accurate explains. Knowing which report you actually need saves time and money, and our guide on doing your own assessment shows where the line falls.

How Unda helps with flood risk assessment in Molesey

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Surrey County Council and Elmbridge Borough Council. We are experts on how Molesey floods, not a generic report mill.

A flood risk assessment in Molesey from Unda assesses every credible source — the Thames, the Mole and Ember, surface water, groundwater and the reservoirs — 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 Molesey

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. We prepare all three, alongside the right form of assessment for buyers, lenders and BREEAM.

Flood risk assessments

• Flood risk assessment for planning. The core report, site-specific and prepared to satisfy the NPPF, the Environment Agency and Elmbridge 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 Mole 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.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, which is credit-driven rather than safety-driven, so a planning report alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for Molesey's river-gravel sites and any basement — assessed as a source within the assessment, or as a standalone hydrogeological study 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 the Yatton judgment explains.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering — calculates existing versus proposed runoff and proposes SuDS to hold discharge within Surrey's standards.
• 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.

Early in a drainage strategy it helps to know whether the ground will take water — on Molesey's gravels infiltration can work, but a high water table often rules it out. 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. You can screen a postcode on our flood risk map, and every report is overseen by a chartered flood risk consultant who stays involved past the report to answer objections. Molesey sits within Elmbridge: see our wider guides for Elmbridge and for Surrey, or browse all of the areas we cover.

Molesey flood resources and contacts

The most reliable Molesey 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 Molesey. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; sign up for flood warnings or call Floodline on 0345 988 1188. The warning areas covering the town are “River Thames at East and West Molesey” and “River Mole at Esher and East Molesey”.
• Surrey County Council flooding. Lead Local Flood Authority strategy, SuDS guidance and how to report flooding at surreycc.gov.uk.
• Elmbridge Borough Council flood risk. Local flood policy, the SFRA and emergency planning, including rest centres such as the one at Bishops Fox Way, East Molesey, at elmbridge.gov.uk.
• Lower Mole Flood Alleviation Scheme. Refurbishment news and scheme detail at lowermolefloodalleviationscheme.co.uk.
• Thames Regional Flood and Coastal Committee. Regional investment and priorities via the Thames RFCC.
• Live river levels. Whether the Thames or Mole are running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact Thames Water, the sewerage company for Molesey. For broader policy context, our notes on the Environment Agency's planning role and on flood zones 1, 2 and 3 are a good starting point.

Flood risk assessment in Molesey: frequently asked questions