Flood Risk Assessment in Cambridgeshire

A flood risk assessment in Cambridgeshire usually starts below sea level. More than half of the county's land lies beneath mean sea level, and it stays dry because drainage boards pump water up into the rivers that carry it to the Wash. The Great Ouse and the Nene are the two main ones, with the Cam joining the Ouse above Ely. We assess sites for planning applications, house purchases, lenders and insurers, and prepare the sequential and exception tests and surface water drainage strategies that often go with them.

This page covers the ceremonial county: Cambridgeshire County Council's five districts, from Ely, Soham and Littleport in East Cambridgeshire to Huntingdon, Godmanchester, St Ives and Ramsey in Huntingdonshire and March, Chatteris and Whittlesey in Fenland, plus the separate Peterborough City Council area. Cambourne and the new town at Northstowe sit in South Cambridgeshire. Sites carry CB and PE postcodes, and the rules that apply change at each district boundary.

Holme Fen, south of Peterborough, is the lowest point in the UK at about 2.75 m below sea level. Before the Fens were drained it was not below sea level at all; the peat has shrunk as the water was taken out of it.

To talk through a Cambridgeshire site with one of our consultants, call 01293 214444 or email enquiries@unda.co.uk.

Why does the flood map put so much of Cambridgeshire in Flood Zone 3?

Because the Environment Agency's Flood Map for Planning is drawn as if the defences and pumps were not there. Cambridgeshire County Council's flood strategy states that the map returns the Fens to the periodic flooding they had before they were drained in the 17th century. Enter a postcode below for the flood zones, surface water and reservoir layers at that point, or open our full flood risk map for anywhere in England.

A fen site can screen as Flood Zone 3 and still have no recorded flood in a century, or sit in Flood Zone 1 and flood from a blocked ditch. The map cannot tell the two apart. What decides the risk is the standard the banks and pumps are held to, what happens if one of them fails, and how far below the river the floor would be. Our notes on residual flood risk and what the zones mean explain how that is assessed.

Rivers, washes and pumped fen: where Cambridgeshire's water goes

Two river systems drain the county. The Great Ouse comes in from Bedfordshire past St Neots, Huntingdon and St Ives, takes the Cam below Cambridge and leaves through Ely and Littleport into Norfolk. The Nene runs through Peterborough to the tidal channel at Wisbech, and between the two lies the Middle Level, a grid of artificial drains around March, Chatteris, Ramsey and Whittlesey whose water has to be pumped out before it can reach the sea. The county spans two Environment Agency areas, East Anglia and Lincolnshire and Northamptonshire.

The main rivers have their own pages: the River Great Ouse and the River Nene. Gauges on both, and on the Cam, feed our live river levels map. Off the main rivers, ditches and drains are ordinary watercourses, run by the county council or one of the drainage boards. Works in them need consent, and our guide to internal drainage boards explains how a district's byelaws apply to a site.

Which council decides, and which flood policy applies?

Applications are decided by the district or city council, and Cambridgeshire County Council, as Lead Local Flood Authority for all five districts, is a statutory consultee on surface water for every major scheme. Peterborough is different: the city council is a unitary authority, so it decides the application and reviews the drainage itself. Each local plan carries its own flood policy. They differ on floor levels, discharge rates and when an assessment is needed at all.

Our council pages go further on Cambridgeshire County Council's requirements and on Fenland District Council. A Cambridge city site has its own page, linked below, because Policy 32 and the city's groundwater set it apart from the rest of the county.

Cambridgeshire County Council's surface water rules, figure by figure

The county's Surface Water Planning Guidance was reissued in February 2026 and is reviewed every year. It applies the 2025 national standards for sustainable drainage alongside the Cambridgeshire Flood and Water Supplementary Planning Document, which all five districts have adopted. Its checklists set out what the county expects at outline, full, reserved matters and condition stage. Six of its requirements shape most fen schemes.

• Runoff rates. Greenfield sites discharge at the greenfield rate, or at QBAR where a simple flow control is used. Brownfield sites must cut their runoff, aiming for greenfield or 3 l/s/ha, with any relaxation capped at five times the greenfield rate.
• Volume. The 1 in 100 year, 6-hour runoff volume should not exceed the greenfield volume; any extra is held as long-term storage and released at less than 2 l/s/ha.
• Infiltration. A tested rate of at least 1 × 10⁻⁶ m/s, by BRE 365, and the base of a soakaway at least 1.2 m above groundwater. Soakaways deeper than 2 m, or borehole soakaways, count no higher than a sewer connection.
• Pumps. The county calls pumping surface water an unsustainable method. It wants as much water as possible to leave by gravity, which on flat fen land often means shallower, wider storage.
• Climate change and creep. The upper end allowance for whichever of six management catchments the site sits in, and a 10% allowance for urban creep unless the scheme is flats.
• Third-party outfalls. A discharge into a drainage board's drain at a non-standard rate needs the board's written agreement before the county will accept the strategy.

The county aims to reply to consultations within 21 days and offers pre-application advice on any scale of scheme, including a free 15-minute call. Our note on why drainage strategies draw an objection covers the usual reasons a county consultee says no.

The Ouse Washes, the barrier banks and the risk behind them

Much of the county's protection depends on a handful of structures built centuries ago and kept up since. The Ouse Washes, laid out by Vermuyden in 1653 between the Old and New Bedford Rivers, are registered as a reservoir under the Reservoirs Act 1975. That makes a breach of their banks a reservoir risk as well as a river one. It shows on the Environment Agency's reservoir flood map.

• The washes themselves. About 30 km long and up to 0.8 km wide, covering roughly 1,900 hectares and storing up to 90 million cubic metres. Safety works on the banks ran from 2017, timed around breeding and wintering birds, and the Environment Agency gives the protected area as more than 2,000 properties and about 67,000 hectares of farmland.
• The Middle Level Barrier Bank. The east bank of the washes must be safe for a 0.1% annual probability flood, and for 0.01% where communities sit next to it, according to the Environment Agency's Section 10 works page.
• Town defences. After the 1998 floods the Environment Agency built defences at Godmanchester, including a 1.5 km line through back gardens, to protect more than 560 homes and businesses. They did not stop 12 properties flooding there from other routes in December 2020.
• Fens 2100+. The Environment Agency's long-term programme for the Fens. Our account of what its evidence says covers the ageing pumps and banks and what they mean for a site-specific assessment.

Defences lower the chance of flooding. They do not remove it, and a site behind any of these still has to show it is safe if the defence is overtopped or fails, with climate change added over its lifetime. Baseline work for the Future Fens project, quoted in the county's flood strategy, put the cost of keeping today's protection in place at about £1.8 billion over 100 years, so an assessment works with the standard the banks have now. Our explainer on reservoir flooding sets out how a breach is assessed.

Beyond the Fens: chalk, clay and summer storms

Off the fen, the ground decides what floods. The county's Flood and Water SPD describes the north and centre as largely clay-like soils that will not take water, with chalk and greensand in the south. British Geological Survey mapping, quoted in the county's flood strategy, puts about 26% of Cambridgeshire at high or very high risk of groundwater flooding, much of it in the sand and gravel of the river floodplains.

• Linton, 20 July 2021. A convective storm dropped 60 mm in under three hours at the Ashdon gauge and about 99 mm by radar, an event the county's investigation puts at between 1 in 211 and 1 in 659 years. Seventeen properties flooded internally, after earlier floods in the village in 2014, 2017 and 2019.
• Alconbury, December 2020. Alconbury Brook drains 202 km² of impermeable Oxford Clay and is one of the faster-responding tributaries of the Great Ouse. It flooded 46 homes internally in Alconbury and 32 in Alconbury Weston, with 95 properties affected in all, after earlier floods in 1998, 2012 and 2016.
• Groundwater. The county asks for groundwater monitoring wherever trial pits or boreholes find water within 1.2 m of the surface, and will not accept a soakaway whose base sits closer than that. Our guides to groundwater monitoring for planning and BRE 365 testing explain the programme.

Neither source shows on the river flood zones. A Flood Zone 1 site in the south of the county or around Cambourne can still need a careful look at surface water routes and the water table, and the ground conditions checker gives a first indication of whether infiltration is worth testing.

Floods recorded across Cambridgeshire since 1947

The county council investigates any flood in which five or more properties are flooded in one event, and it has published 40 of those reports. With the Environment Agency's own summaries, they give a dated record that the flood zones cannot show.

1. March 1947. Snowmelt on frozen ground met a high tide and strong winds that stopped the Fens draining, and banks breached at places such as Bluntisham. Our account of the 1947 floods puts more than 40,000 acres of the South Level under water.
2. Easter 1998. The flood the Great Ouse is still measured against, and the reason for the Godmanchester defences.
3. August 2014. More than 300 homes flooded in the county, according to its flood strategy.
4. 23 December 2020. Storm Bella fell on a saturated catchment in what became the second wettest December since 1981, as the Environment Agency's Great Ouse summary records. More than 200 homes flooded across the county. In St Ives, 32 homes and 15 businesses reported water inside; in March, 20 of 67 reports were internal; in Ramsey, 11.
5. July 2021. The Linton cloudburst, with 17 properties flooded inside.
6. October 2023 and January 2024. Two spells of heavy rain flooded properties at Earith, where much of the village has no separate surface water sewer.
7. September and October 2024. About 183 mm of rain fell upstream in Bedfordshire in ten days. At Brampton, Brampton Brook backed up against a high Great Ouse, with 5 properties flooded inside and 58 reports of external flooding.

Every report is on the county's flood investigations page, and our note on Section 19 investigations explains what they can and cannot tell a buyer or a planning officer about a single site.

Does a flood risk assessment in Cambridgeshire apply to my site?

Under chapter 18 of the NPPF, an assessment is needed in Flood Zones 2 and 3, on sites of a hectare or more in Flood Zone 1, and wherever the flood map for planning or the district SFRA shows a risk from another source. Our summary of the NPPF and its guidance sets that out. Cambridgeshire's districts and drainage boards add requirements of their own. Some catch very small schemes.

A site at risk only from surface water may not need the sequential test if the assessment shows the development will be safe; our note on the surface water exemption explains how that works under policy F5.

Reports we prepare for sites from the Ouse valley to the Nene

The right report depends on the source of the water and who has to accept the answer: a case officer, the county's drainage team, a drainage board, a lender or an insurer. Every report is checked and signed off by a senior consultant; our consultants are chartered and Unda is a CIWEM Business Partner. Each of these is available anywhere in the county:

• Every source in one report. River, tide, pumped drains, surface water and groundwater can all reach the same fen plot, and a flood risk assessment weighs them together.
• Planning applications. From a Huntingdon extension to a Fenland housing site, a flood risk assessment for planning answers the district's flood policy and chapter 18 of the NPPF.
• Buying a home. A search can flag a house on pumped fen as Flood Zone 3 even with no flood history at all; a flood risk survey for property purchase explains what the flag means before exchange.
• Testing the location. Where most of a district is Flood Zone 3, the area of search decides the outcome, and a sequential and exception test report justifies it.
• Drainage for the application. Fenland asks for a drainage strategy with every relevant application, and the county reviews one on every major scheme.
• Runoff and storage. QBAR or greenfield rates, 1.2 m above groundwater and gravity before pumps: a surface water drainage strategy shows the county's figures are met.
• After permission. The county publishes its own checklist for condition stage, and our discharge of drainage conditions service works to it.
• Insurance. Homes in the St Ives, Alconbury and Ramsey flood records above can meet higher premiums or limited cover, and a flood risk assessment for insurance gives an insurer the property's own evidence.
• Commercial purchases. Warehouses and food stores on the pumped fen around Chatteris and March depend on banks and pumps the buyer does not control, which due diligence sets out before completion.
• Keeping a business open. Fifteen St Ives businesses reported water inside in December 2020; a business continuity assessment looks at access, stock and recovery.
• Commercial buildings. Cambridge's Policy 28 sets BREEAM levels for new non-residential space, and a BREEAM flood risk assessment addresses the Pol 03 credits.
• Foul water. Earith has no separate surface water sewer in much of the village, and Fenland's plan warns against private foul systems run by residents; a foul drainage strategy deals with both.
• Groundwater. On the chalk of the south, on floodplain gravels, or wherever pits find water within 1.2 m, a groundwater assessment fixes the level the county will test against.
• Levels where none exist. Where a fen flood outline comes from an early national model that gives an extent but no level, as at the Chatteris site below, flood modelling produces the level a floor must clear.
• Getting out safely. Homes behind the Ouse Washes banks or in a pumped district may need a flood warning and evacuation plan showing how residents leave if a bank or pump fails.
• Land and allocations. Northstowe, Waterbeach and the growth planned along the Oxford to Cambridge corridor all turn on drainage, and a flood risk feasibility study tests land before it is bought or promoted.

Two of our published jobs were on fen land. A food business bought a cold store in a pumped fen at Chatteris once its reliance on the Middle Level and a drainage board had been set out, and a small Fenland village scheme was approved after a refusal once the flood risk assessment and the sequential test were written as separate reports.

Town and neighbouring pages for Cambridgeshire

Five places in the county have pages of their own, each with its rivers, flood record and council's rules in more detail:

• Cambridge. The Cambridge page covers the Cam and its commons, the city's chalk and gravel groundwater and Policy 32.
• St Neots. Our St Neots page deals with the Great Ouse where it enters the county, and Eaton Socon across the river.
• St Ives. The St Ives page follows the Great Ouse through the town and its December 2020 flood.
• Wisbech. The Wisbech page looks at the tidal Nene and the town's defences.
• Peterborough. A separate unitary council with its own policies, on the Peterborough page.
• Over the county boundary. Norfolk and King's Lynn downstream on the Great Ouse, Bedfordshire upstream, Lincolnshire across the North Level and Suffolk to the east. Every other place is on our areas list.

Reporting flooding in Cambridgeshire and getting warnings

Where floodwater threatens anyone's life, dial 999; a power cut is reported on 105. For everything else, who to contact depends on where the water came from and which council area the site is in.

The county's Flood Risk Management Strategy 2021 to 2027, with a 2025 review of its action plan, sets out who manages each kind of flooding and the county's priorities.

Questions we are asked about sites across Cambridgeshire

Our consultants assess flood risk, run sequential and exception tests and prepare drainage strategies for sites anywhere in Cambridgeshire, from Littleport to Linton. To discuss one, call 01293 214444 or email enquiries@unda.co.uk.