Flood Risk Assessment in Cambridge

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

Cambridge is a low-lying city built in and around the floodplain of the River Cam. The river runs through the heart of the city, spreading across the riverside commons in high flows, while a network of urban chalk streams — Hobson’s Brook, Cherry Hinton Brook, Coldham’s Brook and Bin Brook — threads through the suburbs and drains to the Cam. We assess every source that matters here: the River Cam itself, the surface water that overwhelms drains when summer storms hit the built-up city, the groundwater that rises through the chalk and gravels, and the ordinary watercourses on so many site boundaries. We then deal directly with the Environment Agency and Cambridgeshire County Council as the 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, mortgage or insurance, BREEAM, or a surface-water drainage strategy.

The River Cam floodplain runs straight through the middle of Cambridge — across Coe Fen, Sheep’s Green, Midsummer Common and Stourbridge Common — so the open spaces that make the city are also its working flood storage, and a great many development sites sit close to water that has to go somewhere.

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

Flood risk assessment in Cambridge at a glance

• The Cam runs through the centre. The River Cam and its floodplain pass straight through the city, with land along the river ranging from roughly 6 to 17 metres above sea level — low ground that floods readily.
• Chalk streams thread the suburbs. Hobson’s Brook, Cherry Hinton Brook, Coldham’s Brook and Bin Brook carry surface water through the built-up city and frequently sit on a development’s boundary.
• Surface water is the city’s sharpest risk. Cambridge’s drains can be overwhelmed by intense summer storms — the city centre flooded in this way in July 2021.
• Two authorities, one city. Cambridge City Council is the planning authority through Greater Cambridge Shared Planning, while Cambridgeshire County Council is the Lead Local Flood Authority. We work with both, and with the Environment Agency on the Cam.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, surface water or a critical drainage area can trigger an assessment even on a Flood Zone 1 site.

Check flood risk anywhere in Cambridge

Cambridge is outlined below. Screen any postcode in the city 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 Cambridge: the Cam, the chalk streams and surface water

Three things drive flooding in Cambridge: the River Cam, the urban chalk streams that feed it, and the surface water that builds up on the city’s low, increasingly hard ground. Most sites here face more than one of them at once, which is why a single national map never settles the question.

The Cam enters the city from the south at Trumpington, splits and braids through Sheep’s Green and Coe Fen, then runs north past the colleges, Jesus Green and Midsummer Common towards Chesterton and Fen Ditton. Its floodplain is the chain of riverside commons that define the city, and those commons do real work in a flood by holding water back. Reading which watercourse drives the risk on a given site, and how it behaves, is the heart of a credible flood risk assessment.

• The River Cam (fluvial). The Cam and its floodplain flood the low ground through the centre and along the Chesterton and Fen Ditton reaches. Land in Flood Zones 2 and 3 along the river almost always needs an assessment. See our explainer on fluvial flooding.
• Surface water (pluvial). Now the most widespread flood risk in England, and the one that catches Cambridge hardest. Intense rain overwhelms drains across the built-up city, as it did in July 2021. Since the 2025 PPG update, mapped surface water can trigger an assessment even on a Flood Zone 1 site. See pluvial flooding.
• The chalk streams. Hobson’s Brook, Cherry Hinton Brook, Coldham’s Brook and Bin Brook are small but real watercourses running through Trumpington, Cherry Hinton, Coldham’s Common and Newnham. They back up in heavy rain and are usually ordinary watercourses managed by the Lead Local Flood Authority.
• Groundwater. The chalk to the south-east and the river gravels can hold a water table close to the surface, flooding low ground and basements without ever showing on a river map — and breaking soakaway assumptions. See groundwater flood risk.
• Sewers and ordinary watercourses. The surface-water sewer network and the brooks back up together in a downpour. Works that affect a brook or ditch usually need ordinary watercourse consent from the county council.

The riverside commons are the textbook example of why a single map is never enough in Cambridge. A site set back from the river can still flood from a chalk stream on its boundary, from surface water ponding on hard ground, or from a winter water table near the surface. A credible assessment reconciles all of it. The riparian duties that come with a boundary watercourse are real too — in 2024 a Cambridgeshire homeowner was held personally responsible for clearing the watercourse along her boundary, as our note on riparian ownership explains.

A history of flooding in Cambridge

These events are why the authorities here scrutinise development so closely, and why a flood risk assessment in Cambridge has to be evidence-led rather than generic. Ground that has flooded once tends to flood again.

• 1947 — the great Fen flood. Snowmelt on frozen ground and heavy rain sent the Cam and the wider Ouse system into the worst flood of the century. South of the city the weir at Byron’s Pool, Trumpington, was rebuilt afterwards as part of managing the upper Cam.
• July 2021 — flash flooding in the city. An intense summer storm dropped a large amount of rain in a short time, overwhelming the urban drainage network and flooding parts of the city centre and nearby development — the clearest recent illustration of Cambridge’s surface-water risk.
• Winter storms 2024. The wet winter of 2024 brought high river levels on the Cam and the Great Ouse upstream, with flood warnings across the county and saturated ground that shed rain straight into the drains.
• Recurrent chalk-stream and surface-water flooding. Cherry Hinton Brook, Coldham’s Brook and the Trumpington watercourses have a long local record of surface-water and ditch flooding that rarely makes the national news but shapes what the Lead Local Flood Authority expects on nearby sites.

Planning policy and your Lead Local Flood Authority in Cambridge

Two kinds of authority set the rules in Cambridge. The Environment Agency leads on the main river — the Cam. A Lead Local Flood Authority leads on surface water, groundwater and ordinary watercourses, and in Cambridge that is Cambridgeshire County Council. Planning decisions are made by Cambridge City Council through Greater Cambridge Shared Planning, the joint service it runs with South Cambridgeshire.

A developer here deals with the city council on planning, the county council on surface water and drainage, and the Environment Agency where the Cam is involved. Our overviews of the role the LLFA plays as statutory consultee on surface water drainage, and of the Environment Agency’s role in planning, explain 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: the Greater Cambridge Strategic Flood Risk Assessment, the adopted and emerging Greater Cambridge Local Plan, and Cambridgeshire County Council’s local flood-risk guidance. Where a higher-risk site is involved, the Sequential and Exception Tests apply. On drainage, the 2025 national SuDS standards and the four pillars of SuDS shape what the authority will accept, and because much of the city sits on clay, a scheme often has to attenuate runoff and discharge at a restricted rate rather than relying on infiltration.

What your flood risk assessment in Cambridge must show

• Risk assessed from every credible source — the Cam, surface water, the chalk streams and groundwater — 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 climate change allowances are applied — generally 100 years for housing.
• No increase in flood risk elsewhere, with attenuation and, where needed, 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 Cambridge

Cambridge is one of the fastest-growing cities in the country, 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.

• North East Cambridge. The North East Cambridge Area Action Plan will deliver thousands of homes and workspaces on low ground near the Cam, once the Cambridge waste-water treatment plant relocates — an area-wide drainage and flood-risk approach is built into the policy.
• Cambridge Biomedical Campus and life sciences. High-density laboratory and hospital development on the south of the city manages surface water through attenuation basins and swales discharging at tightly restricted rates, working alongside the Hobson’s Conduit Trust where the historic watercourse is affected.
• City-centre and college sites. Dense development close to the Cam and the commons turns on finished floor levels, safe access and egress, and careful surface-water design on constrained plots.
• Suburban and brownfield sites. Infill across Trumpington, Cherry Hinton and the station area brings new impermeable surfaces over clay, where the surface-water drainage strategy is central to the application and the preliminary drainage design often shapes the layout.

Climate change is not optional here. A Cambridge 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.

On much of the city, Gault clay and a high water table mean a soakaway simply will not work, so a drainage strategy leans on attenuation and a restricted discharge to a watercourse or sewer. Our free geology and ground conditions map and BRE 365 infiltration rate calculator give an indicative read before a trial pit, and our notes on infiltration testing and permeable paving explain what the ground will and will not take. Home buyers and businesses come to us too: a flood question can affect a purchase and house prices, and a conveyancing search is not the same thing as a planning-grade assessment — our guide on whether you can do your own assessment explains where the line falls.

How Unda helps with flood risk assessment in Cambridge

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Cambridgeshire County Council and national planning policy. We are experts on the city’s flooding, not a generic report mill.

A flood risk assessment in Cambridge from Unda assesses every credible source — the River Cam, surface water, the chalk streams, groundwater and any ordinary watercourse on the boundary — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We have prepared assessments and drainage strategies across Cambridge and the wider county, 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 Cambridge

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 Cambridge City 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 river or surface-water 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 assessment alone rarely satisfies it.
• Groundwater flood risk. A defensible design water table for chalk, gravel and basement sites — 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 what the Yatton judgment means for planning explains.

Drainage strategy and SuDS

• Sustainable drainage strategy (SuDS). Surface-water engineering — calculates existing versus proposed runoff and proposes SuDS to hold discharge within the local limits.
• 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 at all: our drainage calculators help size the scheme, you can screen any 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. For the county-wide picture beyond the city, see our flood risk assessment in Cambridgeshire page.

Cambridge flood resources and contacts

The most reliable Cambridge 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 Cambridge. 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.
• River Cam at Cambridge. The Environment Agency’s River Cam at Cambridge flood warning area covers the low ground and commons along the river through the city.
• Cambridgeshire County Council flood team. Strategy, developer guidance and flood reporting at Cambridgeshire County Council.
• Cambridge City Council and Greater Cambridge planning. Planning policy, the Local Plan and the Strategic Flood Risk Assessment at Greater Cambridge Shared Planning.
• Live river levels. Whether the Cam is running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact Anglian Water, which provides sewerage across the city. For broader policy context, our notes on the Environment Agency’s planning role, on residual flood risk and on why the long-term flood risk map cannot replace the Flood Map for Planning are a good starting point.

Flood risk assessment in Cambridge: frequently asked questions