Flood Risk Assessment in Thames Ditton

A flood risk assessment in Thames Ditton starts with one river gauge, on Thames Ditton Island. The Thames there reached 6.70 metres on 10 February 2014, the highest level it has recorded, and the Environment Agency expects some property flooding once the river passes 6.35 metres. We prepare flood risk assessments in Thames Ditton for planning applications, property purchase, mortgages and insurance, together with sequential and exception tests and surface water drainage strategies.

Here, Thames Ditton means the Thames Ditton ward as the Office for National Statistics draws it: the village centre and Giggs Hill, the riverside roads between Summer Road and the High Street, Ditton Field, and Thames Ditton Island with its smaller neighbour, Boyle Farm Island. Long Ditton and Hinchley Wood lie outside that line, but they drain to the same River Rythe and share a settlement area in the council's flood study, so this page covers them too. Elmbridge Borough Council decides planning applications, Surrey County Council is the Lead Local Flood Authority, and our page on flood risk assessment in Surrey gives the county-wide view.

Since Thames Water fitted a non-return valve to the street drains of Alexandra Road, Queens Road and River Bank, the river no longer bubbles up through the gullies as a warning. Once the Thames tops its bank at about 6.6 metres, water reaches the road with little notice.

For a first view on a particular KT7 or Hinchley Wood plot, ring one of our consultants on 01293 214444 or email enquiries@unda.co.uk.

Thames Ditton water levels, from a normal winter to the 2014 record

Flood levels on the Thames Ditton riverside are read against the Environment Agency's River Thames level at Thames Ditton Island (station 7246). The thresholds below come from that gauge page and from a note on the local warning process, written mainly by the Agency for the Thames Ditton and Weston Green Residents' Association in May 2020. Levels are in metres above Ordnance Datum (m AOD) and leave out any allowance for climate change.

The tide plays a part even this far upstream. Teddington Weir, about 5 km downstream, is the normal tidal limit, yet the council's Level 1 Strategic Flood Risk Assessment notes that a high tide can push its influence back as far as Molesey Weir. When a high tide holds floodwater back at Teddington, the residents' association says the river at Thames Ditton can gain a short rise of up to 30 cm, which makes any flood worse. Our live river levels map shows the island gauge beside the stations upstream, and our explainer on annual chance and return periods explains what a 1 in 30 or 1 in 100 figure means for one year.

When Thames Ditton flooded: 2003, 2014, 2016, 2020 and Storm Henk

Surrey County Council describes Thames Ditton as extremely low-lying beside the Thames, with the land rising gradually to the south, and the flood records held by the council and the Environment Agency bear that out by naming only the riverside roads and the island. The dates below come from the residents' association, the council's flood study and Surrey's Section 19 investigations.

1. 2003. Parts of Thames Ditton flooded or came close three times in one year, the residents' association records. The Strategic Flood Risk Assessment lists January 2003 among the larger Lower Thames floods.
2. Winter 2013/14. The Thames peaked at 6.70 m on 10 February 2014. Nothing since has matched it. The association's note says that level flooded much of Alexandra Road and River Bank and some homes nearby. Surrey's Section 19 report for Elmbridge (October 2015) lists "Riverbank (Thames Ditton)" among the roads it closed, and a Thames Ditton Island flood forum was set up afterwards. Surrey's 2024 annex notes that island homes were flooded inside that winter.
3. June 2016. Hinchley Wood flooded from the Rythe's side streams, not the Thames. Around 60 mm of rain fell over 22 and 23 June on a 4 km² catchment centred on Hinchley Wood, watercourses in the Hengest Avenue and Clay Lane area overtopped, and 10 properties flooded inside, according to Surrey's investigation as summarised in the council's flood study.
4. December 2019 and February 2020. The river reached 6.35 m and then 6.27 m. Alexandra Road stayed clear because the association had arranged for Thames Water to fit the non-return valve to its street drain.
5. January 2024, Storm Henk. The Thames peaked between Datchet and Teddington on 10 January. Surrey's Section 19 annex for Thames Ditton found the river over the banks of the island and of the south bank by Aragon Avenue and Queens Road. Every property affected was within 20 m of the bank: a riverside pub lost its lower decking and cellar to the water, a boathouse's garages flooded, and island homes flooded outside but not inside this time.

The Storm Henk figures need care. The annex records 1 property flooded internally and 5 externally, while the summary table in the main Thames catchment report gives 0 and 5 for Thames Ditton. Either way, 2024 stayed well short of February 2014. A riverside assessment therefore tests a site against the 2014 peak and the modelled design levels above it, not against the most recent flood.

Screening a KT7 postcode on the Thames Ditton map

The Thames Ditton map below outlines the Thames Ditton ward in coral, from Ditton Field and the island down to the railway and Giggs Hill Green. Enter a postcode to see the Environment Agency's river, surface water and reservoir data and the planning flood zones for that spot, or open our national flood risk map to compare it with elsewhere. Long Ditton lies just beyond the eastern edge of the line and Hinchley Wood beyond the southern edge.

Ditton Field, River Bank and the island: where Elmbridge draws Zone 3b

Elmbridge maps its own functional floodplain, Flood Zone 3b, from the land the Thames or the Rythe would cover in a 1 in 30 year flood (1 in 75 on the Lower Mole). Across the study's settlement area for Thames Ditton, Long Ditton, Hinchley Wood and Weston Green, 8.7 km² in all, 68% is in Flood Zone 1, 24% in Zone 2, 3% in Zone 3a and 5% in Zone 3b. Most of that 3b land lies in two places: the riverside here, and a strip beside the Rythe south of Hinchley Wood.

• Open floodplain. Ditton Field is undeveloped functional floodplain, which the Strategic Flood Risk Assessment says should stay free of building so it can keep storing water.
• Built-up riverside roads. Summer Road, Aragon Avenue, Queens Road, Alexandra Road, River Bank, Riversdale Road, Thames Side and Ferry Road lie inside the 3b outline. Houses that keep water out are not themselves 3b, but the gardens and roads around them are treated as flow routes and storage that will flood often.
• In the river. Thames Ditton Island is listed in the same way, and Boyle Farm Island shares its Environment Agency warning area.

For those roads the study is strict. Rebuilding must not move a site into a more vulnerable use, so an office or shop does not become flats, and it must leave flood risk lower than before: no extra dwellings, a footprint no bigger than now, higher floor levels, less runoff and clear flow routes. New basements and basement conversions are ruled out. Even a modest extension is expected not to take storage or block water, using raised floors, permeable surfaces and, where the plot allows, level-for-level compensation. Our explainer on Flood Zone 3b sets out the national rules that sit behind this.

Why no barrier protects Thames Ditton yet

Thames Ditton has no permanent flood wall. Its temporary barrier plan is not in use either. The council's flood study (AECOM, updated 2024) says the plan remains under review and cannot be deployed until that review is finished. Environment Agency modelling found no harmful effect from putting the barrier up elsewhere along the river, and the review was moving to its next stage.

The route the Agency was studying ran from Ditton Field along the footpath between Aragon Avenue and Queens Road, then along River Bank, with a separate length on Riversdale Road, according to information it shared through the residents' association in October 2022. The association also reported Agency funding for a study of a permanent barrier, after its own survey of bank heights. Until something is built and its standard of protection is known, a flood risk assessment here has to assume the river can reach its full modelled level.

Help from upstream is limited. The Agency's 2020 note concluded that the main River Thames Scheme would have a very small effect at Thames Ditton and would not change the risk. Our article on delays to the River Thames Scheme covers where that project stands.

With the valve in the drains and no barrier, the warning service is what gives residents time to move cars and lift belongings. Three Environment Agency warning areas cover the ward: River Thames at Thames Ditton, which takes in Ditton Field and Ditton Reach; River Thames at Thames Ditton Island, which includes Boyle Farm Island; and River Rythe between Oxshott and Thames Ditton. The association suggests that mainland residents sign up for the island warnings as well, because they trigger at a lower level.

The Rythe, the Ember and the ground beneath Long Ditton and Hinchley Wood

The Thames sets the flood levels on the riverside. A site a few streets back can face a different source entirely. The council's flood study, Surrey's investigations and Thames Water's records point to six more.

Long Ditton and Hinchley Wood stand higher than the village, rising to as much as 50 m AOD at Surbiton Golf Course, so their questions are more often about surface water, the Rythe and its side streams than the Thames. The study adds that modelling of the Lower Mole leaves out some small tributaries, so a site beside one may need its own model to fix the flood level.

Planning rules behind a flood risk assessment in Thames Ditton

Elmbridge still decides applications against its Core Strategy of July 2011. The Local Plan 2037 was withdrawn on 26 February 2025, and although work on a new Local Plan has begun, nothing in it carries weight yet. From 1 April 2027 Elmbridge becomes part of East Surrey Council; our note on local government reorganisation explains what that may mean for flood risk and planning.

The householder guide asks for more than the national standing advice does. It wants floors no lower than the existing house, measures that let water in but limit the damage where it could be deeper than 0.6 m, and drainage for the extra runoff, and it warns that a thin assessment may simply be refused, with no chance to add information while the application is decided. Our guide to extensions in a flood zone sets this against the Environment Agency's standing advice.

Drainage is constrained too. The flood study expects soakaways to be heavily constrained in the centre of the area, where shallow gravels sit on London Clay, and only case-by-case infiltration elsewhere once the depth to the water table is known. Surrey also comments on smaller schemes inside its 1% surface water extent or beside a watercourse (non-major guidance), and that can include plots along the Rythe. Our ground conditions and soakaway map is a useful first look at the geology.

Which Thames Ditton projects need a flood risk assessment?

Most proposals here are house extensions, rebuilds and conversions on established plots, with the occasional riverside business or larger site. What each one needs depends on its flood zone and on whether it falls in the riverside 3b roads.

For a buyer the question is different: whether the house has flooded, how close it sits to the 6.35 m and 7.88 m lines, and what that means for a mortgage and insurance. Homes built since 2009 cannot be insured through Flood Re, so a newer house on the riverside can be harder to cover.

Reports our consultants write for Thames Ditton, Long Ditton and Hinchley Wood

Homeowners, buyers, landlords, architects, developers and riverside businesses commission these reports from us. Each row answers a question that Thames Ditton's rivers, ground or policies raise.

• Every source together. A flood risk assessment for a site here weighs the Thames, the Rythe, the Ember, surface water, groundwater in the gravels and the sewers at once.
• Applications to Elmbridge. Extensions, rebuilds and conversions need a flood risk assessment for planning that meets CS26, the householder guide and the 3b rules.
• Before you exchange. A riverside house that flooded in 2014 calls for a flood risk survey for property purchase set against the island gauge levels.
• New homes in Zones 2 and 3. A replacement house or flats near the river needs a sequential and exception test alongside the assessment.
• Runoff and reuse. Surrey asks every drainage scheme to start with rainwater reuse, and a drainage strategy shows how a Thames Ditton site does it.
• Where the water goes. On gravels over clay, with soakaways limited, a surface water drainage strategy shows how runoff meets the 1 in 2 year rate.
• After permission. Where Elmbridge attaches a drainage condition to a permission, we prepare what the discharge of drainage conditions needs, from calculations to the verification report.
• The river's real level. Where the tide, the island gauge and the Rythe interact, flood modelling fixes a design level that national mapping cannot.
• Getting out in time. With the Thames Ditton barrier plan on hold and the island cut off early in a Thames flood, a flood warning and evacuation plan sets out when to leave and by which route.
• Water under the floor. Kempton Park Gravel near the Thames calls for a groundwater flood risk assessment for any cellar or lowered floor.
• Cover and premiums. With 276 properties likely to take water over the threshold in a 1 in 100 flood, owners use a flood risk assessment for insurance to put their case to underwriters.
• Keeping a business open. Storm Henk flooded a riverside pub's cellar and a boathouse, and a business continuity flood risk assessment plans for the next time.
• Foul flows at high river. Because sewage can back up when the Thames is high, a foul drainage strategy checks where a new connection will discharge.

Who to call when the Thames or the Rythe floods

Ring 999 if the Thames or the Rythe puts anyone in danger. Report a power cut on 105. For everything else, the bodies below cover Thames Ditton, Long Ditton and Hinchley Wood.

Two boundaries are close. Homes on the far side of the Rythe at Ferry Road are in Kingston upon Thames, which has its own planning requirements. Hampton Court Palace, across the Thames, is in Richmond upon Thames. Upstream, our pages on Molesey and Esher cover the Mole and the Ember, and the full list of places is on our areas page.

Thames Ditton flood risk: questions from owners and applicants

Before any report leaves us for a Thames Ditton, Long Ditton or Hinchley Wood site, a senior consultant checks and signs it. Unda Consulting Limited is a CIWEM Business Partner, and the consultants who write these reports are chartered. Call 01293 214444 or write to enquiries@unda.co.uk.