Flood Risk Assessment in Thames Ditton

Unda prepares clear, policy-compliant flood risk assessments and drainage strategies for residential and commercial sites in Thames Ditton, on the south bank of the River Thames in the Borough of Elmbridge. A flood risk assessment in Thames Ditton shows Elmbridge Borough Council, the Environment Agency and Surrey County Council that a development will stay safe from flooding for its whole lifetime, and will not push flood risk onto neighbouring land.

Thames Ditton sits opposite Hampton Court Palace, where the River Rythe joins the Thames and the river runs past Thames Ditton Island and Boyle Farm. We assess every source that matters here — the Thames itself, the Rythe, the nearby Ember and Mole, the surface water that overwhelms drains on the area’s London Clay, and the groundwater that rises under low-lying riverside plots — then deal directly with the Environment Agency and Surrey County Council as Lead Local Flood Authority on your behalf. We prepare flood risk assessments in Thames Ditton for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies.

Thames Ditton floods from the river and from its own drains: at the Thames Ditton Island gauge a level near 6 metres AOD is critical, and in February 2014 the Thames reached about 6.7 metres AOD and water entered riverside homes — so an assessment here has to read the river, surface water and groundwater together.

Speak to one of our experienced flood risk consultants on +44 (0) 1293 214444, or email enquiries@unda.co.uk, for a free, no-obligation quote with a reply within 60 minutes.

Flood risk assessment in Thames Ditton at a glance

• On the tidal-influenced Thames. The normal tidal limit is at Teddington Weir, about 5 km downstream, but on a high tide the tidal influence reaches back to Molesey Weir and can add a ‘spike’ of up to 30 cm to river levels at Thames Ditton.
• Repeated riverside flooding. Thames Ditton has flooded or been threatened in 1968, three times in 2003, the winter of 2013–14 and the winter of 2019–20.
• Surrey County Council is the Lead Local Flood Authority, responsible for surface water, groundwater and ordinary watercourses; the Environment Agency leads on the Thames and the other main rivers; Elmbridge Borough Council is the local planning authority.
• Roughly a 10% chance each year of a flood on the scale of 2014 affecting the high-risk riverside area, according to Environment Agency modelling shared with the local flood group.

Check flood risk anywhere in and around Thames Ditton

The Borough of Elmbridge, which contains Thames Ditton, 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 — useful, but not a substitute for a site-specific assessment. For more detail, try our free Flood Risk Map, Live River Levels Map and Geology & Ground Conditions map.

Flood risk in Thames Ditton: the Thames, the Rythe and surface water

Flood risk in Thames Ditton comes mainly from the River Thames, but it is not the only source. The River Rythe enters the Thames here after draining a long clay catchment, the Ember and Mole reach the river just upstream at East Molesey, and heavy rain runs off the area’s London Clay faster than it can soak away. A sound assessment reads all of these together rather than treating the Thames in isolation.

The Thames at Thames Ditton is technically non-tidal above Teddington Weir, but it behaves tidally in a flood: when a high tide stops water passing freely over the weir at Teddington, levels back up and rise at Thames Ditton. The Environment Agency operated the Thames Barrier more than 30 times in February 2014 to manage exactly this effect. The result is a place where fluvial, tidal and surface-water risk all have to be weighed.

The local geology shapes the drainage picture as much as the river does. London Clay holds water rather than letting it drain, so soakaways often will not work and runoff has to be managed another way. That is why so many Thames Ditton schemes need a surface-water drainage strategy as well as a flood risk assessment.

A history of flooding in Thames Ditton

Dated flood events are the clearest measure of real local risk, and Thames Ditton has a long record. The riverside community has been flooded or seriously threatened several times in living memory, and the local flood group keeps detailed records of river levels and street heights along the high-risk frontage.

• 1968 — the Great Flood. The Rivers Ember and Mole burst their banks and floodwater reached from the Portsmouth Road to the Thames, damaging homes across the village and the western part of Weston Green.
• 2003 — three separate episodes. Parts of Thames Ditton were flooded or threatened three times in a single year, a reminder that high-river events are not rare here.
• Winter 2013–14. The Thames reached about 6.7 metres AOD at the Thames Ditton Island gauge — above the roughly 6 metre critical level — and water entered riverside properties along Alexandra Road and the river frontage.
• Winter 2019–20. High river levels again threatened the high-risk area; a Thames Water non-return valve installed after 2014 helped prevent street flooding in Alexandra Road and Queens Road.

Because a property can flood even when headline mapping looks reassuring, it is worth understanding why flood maps are not always accurate at the level of an individual plot — the gap a site-specific assessment is designed to close.

Planning and drainage policy for a flood risk assessment in Thames Ditton

Two layers of policy apply. National policy — the NPPF and its Planning Practice Guidance — requires development to be located and designed so it is safe for its lifetime and does not increase flood risk elsewhere. Local policy is set by Elmbridge Borough Council and Surrey County Council, and a Thames Ditton assessment has to satisfy both.

• Lead Local Flood Authority. Surrey County Council leads on surface water, groundwater and ordinary watercourses, and maintains the county Local Flood Risk Management Strategy.
• Strategic Flood Risk Assessment. The Elmbridge Level 1 SFRA (2024), prepared by AECOM, maps flood risk from all sources across the borough and guides site-specific assessments.
• Local Plan flood policy. Elmbridge Core Policy CS26 (Flooding) seeks to reduce flood risk and requires development to be located, designed and laid out to be safe, supported by the council’s Flood Risk Supplementary Planning Document.
• Surface water and SuDS. Where a scheme increases impermeable area it must follow the SuDS hierarchy; on London Clay that usually means attenuation and controlled discharge rather than infiltration.
• Sequential and exception tests. Development in Flood Zone 2 or 3 normally triggers the Sequential Test, and sometimes the Exception Test, alongside the flood risk assessment.

Local development and the flood risk it creates

Thames Ditton is a sought-after riverside village, and the kinds of development common here each raise their own flood and drainage questions. Many sites sit close to the Thames or the Rythe, on clay, in a conservation area, which is exactly where careful assessment earns its keep.

• Replacement dwellings and riverside rebuilds. Plots near Ditton Reach and the river frontage often fall in Flood Zone 2 or 3, so finished floor levels, safe access and compensatory flood storage have to be designed in.
• Basements and lower-ground extensions. Close to the river the water table is high, so basements need a groundwater assessment as part of the flood risk work.
• Householder extensions and infill. Even modest schemes can be caught by surface-water risk on the Flood Map for Planning, which now triggers assessment far more often than it used to.
• Island and conservation-area properties. Thames Ditton Island and Boyle Farm sit in the river itself, where flood resilience and access in a flood are central to any proposal.

Where flood levels are uncertain, hydraulic modelling can define them more accurately than national mapping, and a preliminary drainage design can flag issues before they affect site layout or value.

How Unda helps with a flood risk assessment in Thames Ditton

Unda are experts on flood risk in Thames Ditton and across the wider Borough of Elmbridge and Surrey. A flood risk assessment in Thames Ditton from Unda assesses every credible source — the Thames, the Rythe, the Ember and Mole, surface water and groundwater — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation that the Environment Agency and the council 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.

You can see how we approach difficult sites across our flood risk consultancy and other areas we cover.

Flood risk assessments for every situation in Thames Ditton

People need flood and drainage reports in Thames Ditton for many reasons, and Unda prepares each one. A flood risk assessment, a sequential or exception test and a drainage strategy are three different reports: the assessment asks whether the site is safe, the sequential test asks whether the development should be there, and the drainage strategy asks where the surface water goes. They often travel together in a planning submission, but each answers a different question.

• Flood risk assessment for planning. The site-specific report submitted with a planning application to satisfy the NPPF and the Environment Agency.
• Flood risk survey for property purchase. An independent homebuyer assessment for buyers, solicitors and lenders during conveyancing.
• Flood risk report for insurance. A property-specific insurance report where premiums rise, cover is restricted or a flood has occurred.
• Flood risk assessment for BREEAM. A report structured around BREEAM Pol 03 for schemes pursuing certification.
• Sequential and exception tests. The locational policy reports for development in higher-risk areas.
• Drainage strategy and SuDS. A surface-water drainage strategy, with discharge of drainage conditions and detailed design where required.

Our free drainage calculators and BRE 365 infiltration calculator give engineers a quick, defensible first pass, and our wider drainage services take a scheme from sketch to an approved design.

Local flood risk management and emergency contacts

If you need to report flooding, sign up for warnings or reach the right authority in a flood, the table below sets out who does what in Thames Ditton. The Environment Agency gauge at Thames Ditton Island gives the live river level.

• Check the live river level. The Environment Agency station at Thames Ditton Island shows the current Thames level; a reading near 6 metres AOD is the local critical threshold.
• Sign up for flood warnings. Register for Environment Agency flood warnings for Thames Ditton and Thames Ditton Island, and check the local flood risk service.
• The River Thames Scheme. The Environment Agency’s River Thames Scheme will cut flood risk between Egham and Teddington, but is expected to have little direct effect on Thames Ditton itself; separate local defences are being explored.
• Report flooding to Surrey. Use Surrey County Council to report flooding from surface water or an ordinary watercourse.

Frequently asked questions