River Thames Flooding & Flood Risk

The River Thames is the longest river in England and, in flood-risk terms, the most consequential. Above Teddington it is a slow, wide lowland river that spills across a broad floodplain from Oxford down through the Berkshire and Surrey reaches; below Teddington it becomes a tidal river running through the heart of London to the North Sea, held back by the Thames Barrier and a chain of tidal walls. Both halves put homes, businesses and major infrastructure in harm’s way, and both raise hard questions for anyone building near the water. Unda prepares flood risk assessments, drainage strategies and related reports for sites the length of the Thames floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

Behind the tidal Thames’s defences sit around 1.42 million people and roughly £321 billion of property — the single largest concentration of flood risk anywhere in the United Kingdom, and the reason the estuary has its own 100-year management plan.

• Two rivers in one. The Thames is non-tidal (fluvial) from its Cotswold source to Teddington Lock, and tidal from Teddington through London to the estuary — the flood mechanisms, and the assessments, differ between the two.
• A Main River throughout. The Environment Agency regulates the Thames along effectively its whole length, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to works near the channel.
• A long, slow, well-gauged river. At 344 kilometres and draining a non-tidal catchment of about 9,948 square kilometres, the Thames rises and falls slowly — and the gauge at Kingston holds the longest continuous flow record of any river in Britain.
• Its floods are national events. From the great flood of 1894 to the wettest winter on record in 2013–14, out-of-bank flooding on the Thames has repeatedly made headlines — and sites in its floodplain almost always need a flood risk assessment for planning.

Check flood risk along the River Thames

The map below draws the River Thames in coral, from its source in the Cotswolds to the estuary, with the Environment Agency’s live river gauges positioned along it — click a gauge to see its latest level, its typical range and its record peak. Enter a postcode near the river to zoom in and screen that location against the Environment Agency’s flood data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Which areas does the River Thames put at risk?

The Thames threatens a long ribbon of towns, and the risk changes character as you move downstream. Above Teddington the danger is fluvial — the river swells after prolonged rain and spreads slowly across its floodplain, giving days of warning but flooding wide areas of Oxford, the Berkshire towns and the Surrey reaches. Below Teddington the danger is tidal — a North Sea surge driven up the estuary on a high spring tide, which is why central London relies on the Thames Barrier. The table below sets out the main settlements at risk from source to sea, and each is covered in more depth on its own local page.

The non-tidal Thames: Oxford to Teddington

On the fluvial reaches, flooding builds over days as water works downstream from the Cotswolds. The worst-hit communities cluster where the floodplain is wide and low: Oxford and Abingdon on the upper river, Wallingford and Reading in the middle, and the notorious Berkshire and Surrey reaches around Maidenhead, Windsor, Datchet, Wraysbury, Staines-upon-Thames, Chertsey and Walton-on-Thames.

The tidal Thames: Teddington to the estuary

Below Teddington Lock the river answers to the tide. A high spring tide meeting a North Sea storm surge is the danger here, and the defence is the Thames Barrier together with the tidal walls that line the river through Richmond, Twickenham, central London and out past Greenwich, Thamesmead and Dartford to Canvey Island and Southend-on-Sea. Here the residual risk — what happens if a defence is overtopped or fails — matters as much as the everyday standard of protection.

If your site sits within any of these areas, the Environment Agency’s flood map for planning is likely to place it in Flood Zone 2 or 3, which triggers the need for a site-specific flood risk assessment. You can screen an address against the live Environment Agency data using the map above, and our guide to flood zones 1, 2 and 3 explains what each one means for a proposal.

Why does the River Thames flood?

The Thames floods for a straightforward reason: it drains a very large, gently sloping lowland catchment, and there is nowhere for a lot of water to go quickly. The river rises at Thames Head near Kemble in Gloucestershire, about 110 metres above sea level, and falls only slowly over its 344-kilometre journey to the sea. Its non-tidal catchment covers roughly 9,948 square kilometres and gathers water from around 38 tributaries between the source and Teddington — among them the Churn, Windrush, Evenlode, Cherwell, Ock, Thame, Pang, Kennet, Loddon, Wey and Mole, with the Colne, Brent, Wandle, Lea, Roding and Darent joining lower down. When a wet spell saturates that catchment, every one of those tributaries delivers at once, and the main river has to carry the combined load across a wide, flat floodplain.

Because the catchment is so large and low, the Thames is a slow-responding river rather than a flashy one. Levels take days to build and days to fall, so communities usually get meaningful warning — but when the river does go out of bank it stays out for a long time, and the flooded area is vast. Below Teddington Lock a second mechanism takes over entirely: the river is tidal, and the real threat is a North Sea surge pushed up the funnel-shaped estuary on a high spring tide, which is what the Thames Barrier exists to stop.

• A huge lowland catchment. Around 9,948 square kilometres of gently sloping land drains to the non-tidal Thames, so heavy or prolonged rain across the basin arrives at the main river together.
• Dozens of tributaries. The Cherwell, Kennet, Wey, Mole, Colne and Lea are among roughly 38 tributaries feeding the Thames — a high point on any one of them can coincide with a high main river.
• Slow to rise, slow to fall. The Thames is a slow-responding river: floods build over days and recede over days, so inundation is prolonged and widespread rather than sudden.
• Tidal below Teddington. Downstream of the lock the flood driver is the tide, not rainfall — a storm surge on a spring tide is the danger the tidal defences are designed for.

The gauge at Kingston, the lowest on the non-tidal river, has measured flow since 1883 — the longest continuous record in Britain — and gives the clearest picture of how the Thames behaves in flood.

A history of flooding on the River Thames

The Thames has one of the best-documented flood records in the country, and it reads as a warning. The benchmark remains the great flood of November 1894, when more than eight inches of rain fell across the Thames Valley over 26 days; the river rose first in its upper reaches and then downstream, peaking on 17 November when Kingston was the last town to flood. Hundreds of families abandoned their homes, and the 1894 peak is still the highest in the gauged record.

The 20th century’s worst event came in March 1947, when a sudden thaw after a bitter, snow-bound winter combined with heavy rain. Blizzards had frozen the upper catchment; when it melted, the effect was severe. The railway at Datchet became a canal between its platforms, as it did at Wraysbury, and low-lying parts of Maidenhead and Staines went under. Widespread Thames Valley flooding returned in the autumn of 2000, a wake-up call that prompted much of the modern investment in defences.

The most recent major episode was the winter of 2013–14 — the wettest on record. Persistent rain through December, January and February left the ground saturated and the river swollen for weeks. In Surrey alone, Environment Agency and council investigations recorded roughly 1,170 internal property-flooding incidents in Runnymede, with more in Spelthorne and Elmbridge; riverside properties flooded from Staines to Sunbury, Chertsey Bridge closed, and homes built since the 1947 flood took water for the first time. On the tidal river the same winter saw the Thames Barrier close 50 times in 13 weeks — and, crucially, no properties in central London flooded as a result.

The defences and schemes that hold the Thames back

No river in Britain is defended on the scale of the Thames, and understanding those defences is part of understanding the risk — because the standard of protection they offer, and the residual risk if they are ever overtopped, both feed directly into a flood risk assessment. Several major schemes shape the picture.

• The Thames Barrier. Operational since 1982 and the world’s second-largest movable flood barrier, it has closed for flood defence more than 220 times — 50 of them in the winter of 2013–14 alone. It closes when a North Sea surge and high Teddington flows together threaten to push central London levels past a set trigger, and is expected to protect London to its current standard until around 2070. Our explainer on the Thames Barrier and how it defends London covers it in detail.
• The Jubilee River. Opened in 2002, this 11.6-kilometre relief channel diverts high flows around Maidenhead, Windsor and Eton, taking water from the Thames above Boulter’s Lock and returning it downstream of Eton. It transformed flood risk for those towns, though it remains a subject of debate over downstream effects.
• The River Thames Scheme. The largest fluvial scheme in a generation, the River Thames Scheme will cut flood risk to roughly 11,000 homes and 1,600 businesses between Egham and Teddington through new flood-relief channels and capacity works. Its budget has risen to around £635 million and its timeline has slipped — the concerns we set out in our note on the River Thames Scheme delays.
• Oxford and the upper river. Upstream, the long-awaited Oxford Flood Alleviation Scheme is finally progressing — see our update on the Oxford scheme approval.
• Thames Estuary 2100 (TE2100). The Environment Agency’s long-term plan for the tidal river, Thames Estuary 2100, runs from Teddington to Sheerness and Shoeburyness, protects around 1.42 million people and £321 billion of property, and sets out how the Barrier will be improved or replaced as sea levels rise.

Defences reduce risk; they do not remove it. A development behind a Thames wall or downstream of a relief channel still has to demonstrate it is safe over its lifetime, allowing for the standard of protection, its residual risk and the relevant climate change allowances. Our note on residual flood risk explains why the “what if the defence fails” question is so important on a defended river like the Thames.

Is the River Thames a Main River — and what that means for your site

Yes. The Environment Agency has designated the Thames a Main River along effectively its whole length, which means the Agency — not the local council — regulates the channel and its immediate margins. That distinction has real consequences if you are developing nearby. Works within 8 metres of the top of the bank generally need a Flood Risk Activity Permit from the Environment Agency before they start, sought alongside planning permission rather than instead of it, and a site in the river’s floodplain will normally need a flood risk assessment to support an application. The many smaller brooks and ditches that drain to the Thames are ordinary watercourses instead, regulated by the relevant lead local flood authority and by the riparian owners whose land they cross.

• The 8 metre byelaw margin. Building, excavation or structures near the Thames usually require a Flood Risk Activity Permit, obtained alongside — not in place of — planning permission.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, or a larger site of a hectare or more in Flood Zone 1, needs a flood risk assessment for planning.
• Tidal or fluvial changes the brief. Above Teddington the assessment centres on out-of-bank river levels and floodplain storage; below it, on tidal surge levels, defence standards and residual risk. We tailor the report to the reach.
• Know which watercourse you are on. The Thames is Environment Agency territory; its tributaries and ditches are ordinary watercourses under the lead local flood authority. We confirm this for every site.

Flood risk and drainage services for sites near the River Thames

A development near the Thames can call for several different reports, and it helps to be clear that they answer different questions. 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 address separate things: whether the site is safe from flooding, whether the development should be located here, and where the surface water goes. Unda prepares each of them for sites the length of the Thames floodplain.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the river’s flood zones — see our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders through our flood risk survey for property purchase and flood risk assessment for insurance — important on a river where risk can affect house prices and mortgage lending.
• Drainage strategies and SuDS. Surface-water designs that keep runoff within the discharge limits set by the lead local flood authority — see our sustainable drainage strategy service.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where the high water table of the Thames gravels is in play.

Where national mapping is not enough — a common situation for sites right beside a Main River as large as the Thames — we can bring in hydraulic flood modelling to derive accurate flood levels and extents, prepare the detailed scheme needed to discharge a drainage condition, and design compensation and floodable voids where floodplain storage must be preserved. Everything we do is framed by the NPPF and its planning practice guidance.

Managing flood risk on the Thames: policy and contacts

Responsibility for flood risk on the Thames is shared. The river runs through the Thames River Basin District, and the Environment Agency regulates the Main River, publishes the flood map for planning, operates the Thames Barrier and issues flood warnings. Each county and borough along the river is its own lead local flood authority for surface water, groundwater and ordinary watercourses, and each planning authority decides applications against its own strategic flood risk assessment. Alongside the fluvial and tidal risk sits the separate issue of sewer capacity and pollution, the subject of both the Thames Tideway super sewer and the wider Thames Water crisis. For how the Agency itself approaches development, see our guide to the Environment Agency, flood risk and planning.

• Live river levels. Check the Thames gauges on the Environment Agency service — for example the River Thames at Kingston — or use our own live river levels map.
• Screen a location. Try our free flood risk map, and for drainage design the drainage calculators, the BRE 365 infiltration calculator and the geology and ground conditions map.
• Flood warnings. Sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on the River Thames

Unda understand how the Thames behaves in flood — how slowly it rises and how long it stays out of bank on the fluvial reaches, how the tide and a North Sea surge drive the risk below Teddington, and what the planning system and the Environment Agency expect from the reports that follow. We know where the floodplain bites hardest, from the Berkshire and Surrey reaches to the defended stretches through London, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the river.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping and modelled levels, the river’s gauged flows, the standard of protection offered by the relevant defences, and the local planning policy that applies to the site. If you are working on a site near the River Thames, talk to one of our experienced flood risk consultants about what your application will need.

River Thames flood risk: your questions answered