Flood Risk Assessment in Boston

A flood risk assessment in Boston starts below the Grand Sluice, where the River Witham becomes the tidal Haven and runs between raised banks to the Wash. Almost the whole town sits in Flood Zone 3, and away from the river its rain leaves through drains that are either pumped or have to wait for the tide to fall. Our consultants prepare flood risk assessments in Boston for planning applications, buyers, lenders and insurers. Sequential and exception tests and surface water drainage strategies sit alongside them, for sites from the docks to the fen edge.

Boston, Lincolnshire, is taken here as the ONS built-up area: the old town either side of the Haven, Skirbeck and the docks to the south-east, Fenside and the Pilgrim Hospital to the north, and Wyberton and the Quadrant to the south-west, all in the PE21 postcode district. About 2 km² of that outline falls in Wyberton parish and a similar area in Fishtoft parish, although Fishtoft village itself lies outside it, as does Kirton to the south.

On 5 December 2013 a North Sea surge came over the Haven banks and, by the Environment Agency's count, flooded more than 800 homes across 55 streets in Boston. The barrier built in response guards against the tide, but in January 2025 the South Forty Foot Drain overtopped its walls and flooded 36 more.

Our flood risk consultants can talk through a Boston site on 01293 214444 or at enquiries@unda.co.uk.

The Haven, the drains and the warning areas through the town

The tide runs about 7 km up the Haven from the Wash, past the docks, to the Grand Sluice that was built in the 1760s to hold it back. Two large drains discharge into it at Boston: the South Forty Foot Drain at the Black Sluice, carrying water from the fen to the south-west, and the Maud Foster Drain on the east side of the town. The Environment Agency splits flood warnings in and around the town into areas that show where each source reaches:

The South Forty Foot Drain area fills a gap the Environment Agency found in January 2025, when only tidal warnings covered that stretch of the drain. Our live river levels map shows the Grand Sluice, Black Sluice and Maud Foster Sluice gauges and any warning in force, and the Environment Agency's Boston flood warnings page gives today's position.

Check a postcode from Skirbeck to Wyberton

Enter a postcode in the flood map of Boston below to screen one address against the Environment Agency's sea, river, surface water and reservoir layers; our full flood risk map covers the rest of England. The coral line follows the ONS built-up area, so the farmland of Wyberton Fen and Fishtoft beyond the houses is left outside it.

Laying the Flood Map for Planning over that outline on 21 October 2026, we found about 94% of the built-up area in Flood Zone 3 and 4% in Flood Zone 2, leaving under 3% in Flood Zone 1. The 2017 SFRA gives a similar picture for the borough as a whole, with 84% in Flood Zone 3. The flood zones ignore the Haven banks and the barrier, so in Boston the zone alone tells a planner little; the depth and hazard of water after a breach is what separates one site from the next.

Boston flooding on record, from 1978 to January 2025

The Environment Agency counts nine major tidal floods in Boston in the last 200 years. The recent record has two quite different kinds of flood in it, one from the sea and one from the fen, and the works that followed each:

1. 11 to 12 January 1978. A surge overtopped the defences along the Witham Haven. The Environment Agency's recorded outline of that flood lies wholly inside today's built-up area and covers about 0.26 km² of it.
2. 5 December 2013. The east coast surge overtopped the Haven banks and flooded more than 800 homes across 55 streets. Local reports describe about 160 people taken to a rest centre and water in Church Street, the High Street, Skirbeck Road, Station Street, Wormgate and St Botolph's Church. Downstream, the bank at Slippery Gowt gave way, leaving a 45 m gap and flooding commercial premises on the Riverside Industrial Estate.
3. Autumn 2014. Work began in October to rebuild the Slippery Gowt bank, widening it to 4 m and raising it to 7.3 m above Ordnance Datum.
4. December 2020. The main gate of the Boston Barrier was finished and became fully operational; it was first raised against a high tide on 7 November 2021.
5. 6 to 7 January 2025. Rain on saturated, frozen ground sent the South Forty Foot Drain to a record 3.192 m in the early hours of 7 January. The Environment Agency's review of the flooding found that river levels topped the walls behind Wyberton West Road by 8 to 22 cm, through joints that had settled and failed, and 36 homes in Boston flooded internally.

A surge flood comes with hours of warning and can reach a wide area at depth; the 2025 flood came up a drain with no river warning of its own, on a street in the tidal zone that the barrier does nothing for. Our explainers cover storm surges and the local flooding national warnings miss.

What the Boston Barrier changes, and what it leaves

The barrier stands across the Haven just below the Black Sluice and above the docks. It cuts the chance of a surge reaching the town centre. What a planner or a buyer needs to know about it:

• Design standard. A tidal surge with a 1 in 300 chance in any year, allowing for climate change, according to the Environment Agency and its designers.
• Who it protects. About 14,000 properties, around 700 of them commercial; the agency's 2023 figure is "over 13,000 homes and businesses".
• How it works. A rising sector gate 28 m wide can be raised in about 20 minutes when a high tide is forecast.
• What is still to come. A second phase adds gates at the entrance to the Port of Boston's wet dock, to protect 526 properties on the left bank behind the port. The Environment Agency's scheme page gives 2028 for completing the whole project, a date it says may change.
• Cost. Forecast at £184 million in February 2023, against £124 million at the outset, after a contractual dispute and changes in scope.

None of this takes a site out of Flood Zone 3 or removes the need for an assessment. The barrier deals with the tide; the South Forty Foot Drain, rain on flat streets and a breach of the raised banks are still possible, and Policy 4 asks every assessment for the breach or overtopping level that applies to the site. Our note on residual flood risk explains how a defended site is assessed.

Breach depths and floor levels in the 2017 SFRA

Because almost all of Boston is Flood Zone 3, the South East Lincolnshire SFRA of December 2017 works from hazard and depth instead. It mapped overtopping and individual breaches of the defences for the 0.5% tidal and 1% river events and the 0.1% event, today and in 2115, and graded the result as danger for all, danger for most, danger for some or low hazard. The standing advice published with it turns predicted depth into floor levels for new homes in the two highest bands:

Single-storey homes are expected to clear the 2115 depth or provide refuge and an evacuation plan, the Environment Agency objects to ground-floor bedrooms where depths are high, and electrical equipment goes above the 0.1% breach depth for 2115. The SFRA says the sequential test should use these hazard maps, and the Local Plan's own sites were chosen against the 2115 breach hazard map. Sea level rise is built into those 2115 runs, and an assessment adds the current climate change allowances; the Environment Agency's long view of the pumped fen around the town is set out in our Fens 2100+ explainer.

Policy 4 and the other rules a Boston application has to meet

Boston Borough Council decides applications under the South East Lincolnshire Local Plan 2011 to 2036, adopted on 8 March 2019 and shared with South Holland. Its five-year review in 2024 found no need to update it, so its flood policy is the one an application meets today:

• Policy 4, the sequential test. Development in Flood Zones 2 or 3, or at risk in a breach or overtopping scenario on the SFRA maps, has to show no lower-risk site is available, searched across the whole borough unless a specific need justifies a smaller area. Allocated sites, minor development and most changes of use are exempt.
• Policy 4, the assessment. A site-specific assessment must cover every source and climate change, and "identify the relevant predicted flood risk (breach/overtopping) level, and mitigation measures that demonstrate how the development will be made safe", with resistance and resilience measures, warning and evacuation arrangements and SuDS unless they are shown to be technically unfeasible.
• The Haven banks. "No development will be permitted within a 50m buffer from the toe of the raised Witham Haven Banks", a strip kept free for future bank raising.
• Ground-floor flats and caravans. Changes of use that create self-contained ground-floor homes in any danger zone are not supported without a habitable room above the flood level and an evacuation plan; permanent residential caravans and park homes are not permitted in flood risk areas.
• National policy. The August 2026 NPPF deals with flood risk in chapter 18. Its policy F4 decides when a site-specific assessment is required, F5 and F6 carry the sequential and exception tests, and under F8 every scheme is expected to meet the 2025 national standards for SuDS.

Validation and the borough-wide requirements, including the villages outside the town, are on our Boston Borough Council page. Change is on the way all the same. A replacement joint plan under the new plan-making system went to the joint committee in May 2026, and the reorganisation of Lincolnshire's councils announced in July 2026 was paused for a national review in September, so Boston Borough Council remains the planning authority for now.

The Quadrant, the western edge and windfall sites

Policy 1 names Boston, including the parts of Fishtoft and Wyberton parishes that the town has grown into, as one of the plan area's two sub-regional centres, alongside Spalding. The plan's two sustainable urban extensions at the town are the South West Quadrant at Wyberton (Policy 13), where the Quadrant development is planned at about 500 homes around Boston United's new stadium, and land south of the North Forty Foot Drain on the west side of the town (Policy 14). With so little of the town outside Flood Zone 3, each phase is settled on floor levels, refuge and drainage.

Allocated sites have passed the sequential test through the plan. A windfall site has not: it needs the sequential test across the borough under Policy 4, and with so little Flood Zone 1 land in Boston the argument then turns on whether any reasonably available site has a lower breach hazard, which is how the SFRA says the test should be run. A small scheme in a fen village shows how the evidence can be put together: its homes were approved in Flood Zone 3 after an earlier refusal once the site search was justified properly.

Pumps, board drains and soakaways around Boston

Little surface water leaves Boston by gravity alone. Most of it goes into a drainage board's channel, and from there into the Haven through a sluice that only opens on a falling tide, or through a pump. That makes the board's consent as decisive as the council's for most schemes:

• Witham Fourth District IDB. About 40,000 ha, starting in Boston and running north to Coningsby and Wainfleet; the board has its office in the town.
• Black Sluice IDB. Thirty-four pumping stations and about 500 miles of watercourses draining the fen west and south-west of the town into the South Forty Foot Drain; the pumping station at the Black Sluice, where that drain meets the Haven, stopped working in 2018.
• Lincolnshire County Council. As Lead Local Flood Authority it reviews drainage on major schemes under its county guide to development roads and sustainable drainage, which names high groundwater, flat ground and "typical IDB pump discharge rates of 1.4 l/s/Ha" as the fen's particular problems and asks for 40% on rainfall for climate change.
• Anglian Water. Owns the public sewers; any surface water connection needs its agreement, and the county council's guide rules out surface water to a foul sewer.

Soakaways rarely solve the problem here. The county council accepts total infiltration only with 1 m between the base and the highest recorded groundwater, ideally shown by twelve months of readings, which is a high bar on fen ground where the county expects groundwater to sit high. Where a soakaway is worth testing, our BRE 365 infiltration rate calculator works through the results, and our guide to internal drainage boards covers board consents, byelaws and easements.

From Wormgate to Wyberton West Road: when an assessment is needed

With nearly the whole town in Flood Zone 3, the answer is yes for almost any new building; what changes from one proposal to the next is how much the assessment has to show. Policy 4, policy F4 of the NPPF and the Environment Agency's standing advice for small extensions work out like this for the kinds of site Boston produces:

The national rules behind the table, including the one-hectare trigger in Flood Zone 1, are set out in our guide to when a flood risk assessment is needed.

Our reports for Boston sites, behind the banks and out on the fen

Most Boston sites have to answer for the tide and the drains in the same report, whether the client is buying a terrace in Skirbeck or building a phase of the Quadrant. The consultants who write them are chartered, and none goes out without a senior consultant's sign-off.

• Haven-side applications. Homes, conversions and commercial schemes in the town's Flood Zone 3 need a flood risk assessment for planning that sets floor levels from the SFRA's breach depths and answers each part of Policy 4.
• Tide, drain and rain together. A flood risk assessment in Boston weighs the tidal Haven, the South Forty Foot Drain, rain on flat streets and the pumped board network behind them as one picture.
• Before exchange. Searches flag nearly every Boston address, and a flood risk survey for property purchase sets the 2013 and 2025 records, the warning area and the floor level against one house.
• Windfall housing. Outside the allocations, new homes go through the sequential and exception test, with the search run across the borough as Policy 4 asks.
• SuDS as standard. The Local Plan expects major schemes to include SuDS, and a drainage strategy shows how runoff reaches a board drain or sewer at a rate the board will accept.
• Storage on wet ground. Where infiltration fails, a surface water drainage strategy holds runoff on site and limits the outflow with the board's pump rate in mind.
• Conditions on a consent. A drainage condition on a Boston permission is cleared through discharge of drainage conditions: the calculations, the board's agreement and a maintenance plan, set out for the council.
• Levels beside the banks. Next to the raised Haven banks or the South Forty Foot Drain walls, where the 2017 maps are too coarse for one plot, flood modelling fixes the level a floor has to clear.
• A plan tied to the warning. Policy 4 asks for warning and evacuation arrangements, and a flood warning and evacuation plan links a building to its own warning area, such as Grand Sluice to the docks.
• Homes that have flooded. After 2013 in the old town and 2025 on Wyberton West Road and Park Road, a flood risk assessment for insurance gives an underwriter evidence for one address.
• Port and estate property. Commercial land at the Port of Boston relies on the Haven banks and gates still being built at the wet dock, and a due diligence flood risk assessment sets that out for a buyer or lender, as for a cold store bought in a pumped fen.
• Keeping a business open. The 2013 surge reached premises on the Riverside Industrial Estate when the Slippery Gowt bank failed, and a business continuity flood risk assessment plans how a site keeps trading.
• Building near the port. New commercial buildings inside the barrier's protected area can claim the Pol 03 credits through a BREEAM flood risk assessment.
• Land before an offer. On land at the urban extensions or along the drains, a flood risk feasibility study tests whether the floor levels and the 50 m bank strip leave a workable layout.

Who answers for the river, the board drains and the sewers in Boston

Call 999 straight away if floodwater threatens anyone's safety, and 105 to report a power cut. Day to day, responsibility for flood risk in the town is split between these bodies:

Questions about Boston sites, the barrier and the drains

Unda Consulting Limited works on Boston sites of every kind, from the old town and the docks to Wyberton and Fenside, preparing flood risk assessments, sequential and exception tests and drainage strategies. Call 01293 214444 or email enquiries@unda.co.uk to talk one through. The rest of the county is on our Lincolnshire page, and the plan shared with Spalding is covered on our South Holland District Council page. Further round the Wash, Wisbech sits on the tidal Nene, and up the coast Skegness faces the open sea; other places are in the full list of areas.