Flood Risk Assessment in Cambridgeshire

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

Cambridgeshire is one of the lowest-lying counties in England. Large parts of the Fens sit at or below sea level, kept dry only by a centuries-old system of embankments, drains and pumps, while the Great Ouse, the Cam, the River Nene and the Middle Level network carry water across the county towards the Wash. We assess every source that matters here — the main rivers, the surface water that overwhelms drains in Cambridge, Peterborough and the market towns, the high groundwater that rises through fen peat and gravels, and the ordinary watercourses managed by the internal drainage boards — then deal directly with the Environment Agency and the relevant 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.

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 Cambridgeshire at a glance

• Five rivers reach the Wash through the Fens. The Great Ouse and its tributary the Cam, the Nene through Peterborough, and the Middle Level system all drain across Cambridgeshire, so many sites face river water and drainage-board water at the same time.
• Much of the county lies at or below sea level. The Fens stay dry only through pumped drainage; the Middle Level’s St Germans station is the largest land-drainage pumping station in the United Kingdom.
• Two Lead Local Flood Authorities. Cambridgeshire County Council leads on local flood risk across most of the county, and Peterborough City Council is a separate Lead Local Flood Authority for the unitary area. We work with both.
• 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 Cambridgeshire

Cambridgeshire is outlined below. Screen any postcode in the county 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 Cambridgeshire: the Great Ouse, the Nene and the Fens

River and drainage-board water dominate Cambridgeshire. The Great Ouse and its tributary the Cam drain the south and east of the county, the Nene runs through Peterborough and Wisbech to the Wash, and the Middle Level carries the heart of the Fens. Add the surface water that overwhelms drains in the towns and the high groundwater of the peat, and most sites here face more than one source at once.

The Cam gathers the south of the county and joins the Great Ouse near Ely, the largest catchment in the area. Below Earith the Great Ouse splits into the Old and New Bedford Rivers, which sandwich the Ouse Washes — a deliberate washland that floods in winter to hold back the river. Across the central Fens the Old River Nene and the Middle Level drains carry water that has to be pumped uphill into the main rivers before it can reach the sea. Understanding which watercourse drives the risk on a given site, and how it behaves, is the heart of a credible flood risk assessment.

• Rivers (fluvial). The Great Ouse, the Cam and the Nene flood the valley floors and fen edges. Land in Flood Zones 2 and 3 along these corridors almost always needs an assessment.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms drains in Cambridge, Peterborough and the market towns. Since the 2025 PPG update, surface water on the Flood Map for Planning, or a critical drainage area, can trigger an assessment even on a Flood Zone 1 site.
• Groundwater. Fen peat, river gravels and the chalk to the south-east can hold water tables close to the surface, flooding low ground and basements without ever showing on a river map — and breaking soakaway assumptions. See groundwater flood risk.
• Tidal (the Wash). The lower Nene and Great Ouse are tidal towards Wisbech and King’s Lynn, so the north of the county carries a tidal as well as a fluvial risk, assessed against the 0.5% annual probability event.
• Sewers and ordinary watercourses. Fen drains, ditches and the surface-water sewer network back up in heavy rain. These ordinary watercourses are the Lead Local Flood Authority’s and the internal drainage boards’ responsibility rather than the Environment Agency’s — see the role the LLFA plays.

The Fens are the textbook case for why a single map is never enough. A site can screen as low risk on the river map yet sit behind a drainage board’s pumped system, below the level of the very drain that is supposed to take its water, with a winter water table near the surface. A credible assessment reconciles all of it. Works that affect a fen drain or ditch usually need ordinary watercourse consent, and the riparian duties that come with a boundary watercourse are real — 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 Cambridgeshire

These dated events are why local authorities here scrutinise development so closely, and why a flood risk assessment in Cambridgeshire has to be evidence-led rather than generic. Each one shaped the defences and the drainage standards a developer now has to design around.

• 1947 — the great fen flood. In March 1947, snowmelt on frozen ground and heavy rain breached fen banks along the Great Ouse and Nene, flooding tens of thousands of acres of farmland and many fen communities. It drove the post-war investment in the Ouse Washes and the pumping network that protects the Fens today.
• December 2020 — the Great Ouse. Prolonged rainfall pushed the Great Ouse over its banks around St Neots and the surrounding villages, with homes and businesses damaged and floodwater lingering for days.
• 2020–21 — Alconbury Weston. Repeated flooding affected over 200 homes in late 2020 and early 2021, a reminder that the risk here is not only on the largest rivers.
• January 2024 — Storm Henk. The Great Ouse burst its banks at St Neots and St Ives, while saturated ground brought widespread surface-water flooding to Ely, March and Huntingdon, closing roads and affecting dozens of properties.
• October 2024 — record river levels. Heavy rainfall brought flooding to Peterborough and St Ives, with the Great Ouse reaching record levels near Wyboston, Eaton Socon and St Neots and prompting flood warnings across the county.

A pattern runs through all of it: ground that flooded once tends to flood again. A robust assessment treats this recorded history as evidence, not background colour.

Flood defences, the Ouse Washes and pumped drainage in Cambridgeshire

Cambridgeshire is one of the most heavily managed landscapes in the country, and the schemes that keep it dry change what a development next to them has to prove. A defence reduces the chance of flooding, but it never removes it, and planning policy still expects the residual risk to be assessed.

• The Ouse Washes. A washland flood-storage reservoir held between the Old and New Bedford Rivers, roughly 30 km long, that floods deliberately in winter to hold back the Great Ouse — capable of storing in the order of 90 million cubic metres — protecting the fenland behind it. It is also a Site of Special Scientific Interest, Special Protection Area and Ramsar wetland.
• A landscape that relies on pumping. The Middle Level Commissioners and more than twenty internal drainage boards run hundreds of kilometres of drains and dozens of pumping stations, including St Germans — the largest land-drainage pumping station in the United Kingdom — to keep the Fens workable.
• Defences reduce risk but never remove it. The Environment Agency is raising and strengthening the Ouse Washes embankments through its ongoing works, yet the Flood Map for Planning is drawn as if defences are absent, so a defended fen site can still sit in Flood Zone 3.

The catch for developers is the one we set out in residual flood risk: because the planning map ignores defences, a site protected by a fen bank or a pumped system still has to demonstrate it is safe if that protection is overtopped or fails. The public long-term flood risk map cannot be used in place of the Flood Map for Planning, and climate change makes this sharper still, 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 Cambridgeshire

Two kinds of authority set the rules. The Environment Agency leads on the main rivers — the Great Ouse, the Cam and the Nene. A Lead Local Flood Authority (LLFA) leads on surface water, groundwater and ordinary watercourses: Cambridgeshire County Council across most of the county, and Peterborough City Council as a separate LLFA for the unitary area. In the Fens, internal drainage boards add a third layer.

A developer here deals with whichever council covers the site, alongside the Environment Agency, and often an internal drainage board where a fen drain or ordinary watercourse is involved. Our overview of the role the LLFA plays as statutory consultee on surface water drainage, and of the Environment Agency’s role in planning, 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: the Cambridgeshire and Peterborough Flood and Water Partnership’s guidance, the Cambridgeshire Flood and Water Supplementary Planning Document, each authority’s Local Flood Risk Management Strategy and Strategic Flood Risk Assessment, and the adopted and emerging Local Plans. 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 each authority will accept, and on much of the fen a scheme has to attenuate runoff and discharge at a restricted rate to a drainage-board watercourse rather than relying on infiltration.

What your flood risk assessment in Cambridgeshire must show

• Risk assessed from every credible source, 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 Cambridgeshire

Cambridgeshire is one of the fastest-growing parts of 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.

• New settlements and urban extensions. Northstowe and Waterbeach New Town add tens of thousands of homes on fen-edge land where surface-water drainage and the Cam and Old West River shape the masterplan.
• Cambridge growth and life sciences. High-density housing and laboratory development around the city often sits close to the Cam, needing finished floor levels, safe access and egress and careful surface-water design.
• Fenland and market towns. Regeneration at Wisbech, March, Chatteris and Whittlesey turns on the Middle Level drains and the tidal Nene rather than a single main river.
• Logistics and rural schemes. Distribution and agricultural development along the A1, A14 and A47 brings large impermeable areas, where the surface-water drainage strategy is central to the application.

Climate change is not optional here. A Cambridgeshire 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 fen, peat, 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 an ordinary watercourse that a drainage board must consent. 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 Cambridgeshire

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, the relevant Cambridgeshire LLFA, the internal drainage boards and national planning policy. We are experts on the county’s flooding, not a generic report mill.

A flood risk assessment in Cambridgeshire from Unda assesses every credible source — the Great Ouse, the Cam and the Nene, surface water, the high fen groundwater, sewers 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 Cambridgeshire, 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 Cambridgeshire

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 your Cambridgeshire LPA. 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 fen 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 fen, gravel and chalk 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 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 drainage board’s 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.

Cambridgeshire flood resources and contacts

The most reliable Cambridgeshire 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 Cambridgeshire. 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.
• Cambridgeshire County Council flood team. Strategy, developer guidance and flood reporting at Cambridgeshire County Council.
• Cambridgeshire and Peterborough Flood and Water Partnership. The county-wide partnership and the Flood and Water SPD developers work to, via Cambridgeshire County Council.
• Greater Cambridge planning evidence. The Strategic Flood Risk Assessment for Cambridge and South Cambridgeshire at greatercambridgeplanning.org.
• Internal drainage boards. Ordinary watercourse consent and fen drainage via the Middle Level Commissioners and the local boards.
• Live river levels. Whether the Great Ouse, the Cam or the Nene are running high right now via our live river levels map.

For flooding from blocked or burst sewers, contact Anglian Water, which provides water and sewerage across most of the county. For broader policy context, our notes on the Environment Agency’s planning role and on critical drainage areas are a good starting point.

Flood risk assessment in Cambridgeshire: frequently asked questions