Flood Risk Assessment in Blandford Forum

A flood risk assessment in Blandford Forum starts with the River Stour, which runs along the south side of the town between the Market Place and Blandford St Mary, and with the defences built along it in 1986 and 1992. Behind them sits a second source: groundwater rising through the chalk, which fills the Pimperne Brook valley on the north-east side after a long wet winter. Our check of the Environment Agency's planning map puts about 6% of the built-up area in Flood Zone 3.

We prepare flood risk assessments in Blandford Forum for planning, for buyers and lenders, and for insurers, alongside sequential and exception tests and surface water drainage strategies. Blandford here means the town's built-up area as the Office for National Statistics maps it in the DT11 postcode district: the Georgian centre on the north bank, the estates running up to the bypass, and Blandford St Mary and Lower Blandford St Mary across the river. Bryanston, Pimperne and Langton Long Blandford sit just outside that outline but share the same river, chalk and council, and we cover them too.

Defences do not take Blandford out of the flood zone. North Dorset's Local Plan says that although the town is protected by a flood defence scheme, any proposal in the areas at risk of river flooding still faces the sequential and exception tests.

To talk through a site in Blandford, call our flood risk consultants on 01293 214444 or email enquiries@unda.co.uk.

Behind the flood banks: how much of Blandford is in Flood Zone 3?

Most of Blandford is in Flood Zone 1, because the town was built on a chalk bluff above the river. Our GIS check of the Environment Agency's Flood Map for Planning on 27 October 2026 puts about 19 hectares, roughly 6% of the 2022 built-up area, in Flood Zone 3, and under 1% more in Flood Zone 2 alone. Almost all of it lies in two places.

• East of the Market Place. The largest block, about 15 hectares, covers the low ground around Langton Road and the lower end of East Street, where the Pimperne Brook comes down to meet the Stour.
• The Blandford St Mary bank. A smaller strip of about 4 hectares follows the south side of the river opposite the town centre.
• The meadows in between. Crown Meadows and Stour Meadows, the open ground between the two halves of the town, are mostly outside the built-up outline. The Blandford + Neighbourhood Plan calls the floodplain separating Blandford Forum from Blandford St Mary functional floodplain, the land that has to stay free to hold water.
• The rest of the town. The estates climbing north to the bypass sit in Flood Zone 1, where surface water and groundwater, not the river, decide what a planning application must show.

Defences are left off the Flood Map for Planning. Land behind Blandford's banks is drawn as if they were not there, so a defended plot still carries its zone, and the assessment has to deal with what happens if a bank is overtopped or a pump fails. Our guide to residual flood risk sets out how that is tested, and why a site stays in a flood zone after defences are built is covered separately.

Screening a DT11 postcode against the Stour floodplain

The coral line marks Blandford's built-up area from the Office for National Statistics (Built-Up Areas 2022), taking in Blandford St Mary south of the river. Enter a DT11 postcode to see its planning flood zone, the modelled risk from rivers and surface water, and any warning in force. The same screen works for any English postcode on our flood risk map. Neither shows groundwater, and on Blandford's chalk that gap matters.

The Stour, the Pimperne Brook and the water under the town

Two main rivers meet at Blandford. The Stour carries water from north Dorset and the edge of Wiltshire past the town, and the Pimperne Brook, a short spring-fed stream off the chalk, joins it on the east side. The table sets out each source and what the record shows.

The river gauge at Blandford is the one to watch in a wet winter. Our live river levels map shows it beside other Stour gauges, and the Environment Agency's page for the Blandford gauge gives its normal range and record.

Groundwater on the chalk: soakaways, boreholes and the Pimperne Brook

Blandford sits on chalk, which soaks up rain quickly and releases it slowly. After a dry summer that makes infiltration drainage easy. After a long wet winter the water table rises, the Pimperne Brook runs high from springs rather than rain, and low ground can flood for weeks.

• February 2026. On 11 February the Environment Agency reported that 34 of the 35 groundwater flood warnings across Dorset and Wiltshire were in force, with properties affected from Pimperne to Milborne St Andrew.
• Its own warning. The Pimperne Brook has a dedicated groundwater flood warning, and the wider Cranborne Chase groundwater alert covers Pimperne and the villages to the north-east.
• Public water supply. Wessex Water draws water from boreholes at its Black Lane treatment works, and the Local Plan notes that the eastern edge of Blandford lies within a Groundwater Source Protection Area.
• What the policy asks. North Dorset Local Plan Policy 16 asks new development to take account of groundwater flooding and to protect and manage groundwater resources.

That shapes any drainage plan. A soakaway that works in September can sit in standing water in March, so winter readings carry the most weight, as our note on groundwater monitoring for planning explains. Infiltration rates come from BRE365 testing, and British Geological Survey borehole records show where the chalk lies under drift. Our ground conditions and soakaway checker and BRE 365 calculator give an early read before testing.

Floods and high water on the Blandford record

Blandford's defences exist because of floods in living memory. These are the dated events with a published source.

1. 1966 and 1974. The Stour flooded low parts of the town, according to Dorset Life's account of the river at Blandford.
2. 1979. The river flooded the town twice in one year, and work began on a scheme to stop it happening again.
3. 1986 and 1992. The Environment Agency's Dorset Stour Catchment Flood Management Plan records embankments south and west of the town built in these two stages, with a pumping station sending the Pimperne Brook into the Stour.
4. June 2012. The same plan put six properties in the Blandford area at risk in a 1 in 100 year flood with the defences in place, rising slightly but staying below ten in future.
5. 24 December 2013. The Stour reached 2.78 m on the Blandford gauge, the highest level it has recorded, at the start of the wet winter of 2013 to 2014.
6. February 2026. Groundwater flood warnings were in force across the Dorset chalk, including the Pimperne Brook. The Environment Agency's incident update of 11 February reported affected properties from Pimperne to Milborne St Andrew.

Six properties at risk is a figure that assumes the banks and the pumps work. A planning assessment has to ask the other question too, what a flood larger than the defences were built for, or a breach, would do to the site, and that is why the zone stays on the map behind them.

Policy 16, the Blandford + plan and what Dorset Council looks for

Dorset Council decides planning applications in Blandford and is the Lead Local Flood Authority. Until its new Local Plan is adopted, the policies for the town come from the North Dorset Local Plan Part 1 (2016) and the Modified Blandford + Neighbourhood Plan, made in October 2023. National policy is chapter 18 of the August 2026 NPPF.

The North Dorset Local Plan, the Blandford + Neighbourhood Plan and the Level 1 SFRA are on the council's website, and the SFRA layers can be viewed on Dorset Explorer. For major schemes, Dorset Council's surface water planning advice sets out what the Lead Local Flood Authority expects.

From Market Place infill to the land north-east of the bypass: when a flood risk assessment in Blandford Forum is needed

Four things decide it in Blandford: the flood zone, the site area, the use proposed, and whether the scheme leans on the defences or on soakaways into the chalk. Here is how the usual proposals in the town fall out.

Routes out matter as much as floor levels here. Langton Road and the bottom of East Street lie in Flood Zone 3 and inside the warning area, so a scheme that relies on them for access needs to show a dry way out to the higher streets north of the centre, or explain how occupants stay safe indoors until the water drops.

Reports for river streets, chalk plots and Blandford's growth land

In Blandford the commissions range from a single flat behind East Street to the 400-home allocation by the bypass. Each report below answers its own question, and a planning submission on the river side of town often needs more than one.

• Applications to Dorset Council. Homes, conversions and changes of use in Flood Zones 2 and 3 behind the defences need a flood risk assessment for planning that answers Policy 16 and the Local Plan's sequential approach.
• River, brook and chalk together. A flood risk assessment for a Blandford site weighs the Stour, the Pimperne Brook, groundwater and the surface water routes, then sets floor levels against the residual risk behind the banks.
• Before exchange on a river street. Buyers on West Street, Damory Street or Nightingale Court ask what the warnings mean for one house, and a flood risk survey for property purchase answers that.
• New homes in Flood Zone 3. Paragraph 8.16 of the Local Plan keeps the sequential and exception test in play for homes east of the Market Place or on the Blandford St Mary bank, whatever the defences do.
• Where rainwater goes on chalk. A drainage strategy sets out whether a site can drain into the ground, how deep the water table sits in winter and what happens when it cannot.
• The north-east allocation. Policy B2 (x) asks for a surface water drainage strategy showing the 400 homes will add no flood risk to land around them, with the Pimperne Brook below.
• After permission. Once permission is granted, the infiltration results, winter groundwater readings and storage sums a condition calls for go to Dorset Council through the discharge of drainage conditions.
• Groundwater in the Pimperne Brook valley. Where the water table rises close to the surface, a groundwater flood risk assessment looks at basements, floor levels and soakaways against winter readings.
• Levels behind the banks. The Stour and the Pimperne Brook meet by Langton Road, and hydraulic flood modelling tests how deep water would get if a bank were overtopped or breached.
• Occupied buildings in the warning area. Flats and businesses on the streets the Environment Agency warns can use a flood warning and evacuation plan to set out who leaves, when and by which route uphill.
• Homes that have had a warning. Owners on Langton Road, Nightingale Court or Sheep Market Hill facing questions from insurers can use a flood risk assessment for insurance to show how their own property stands.
• Town-centre property deals. Before buying or lending on premises along the Market Place or the East Street burgage plots, flood risk due diligence puts the zone and defence position in front of the deal.
• Employment land on the bypass. New units at Sunrise Business Park or Shaftesbury Lane seeking BREEAM credits need a BREEAM flood risk assessment for the Pol 03 issue.

Stour in flood or water rising from the ground: who to call in Blandford

Blandford has three kinds of flooding and three bodies to match. River water from the Stour or the Pimperne Brook is the Environment Agency's, water rising from the ground or running off the roads is Dorset Council's, and anything coming up through drains and manholes goes to Wessex Water. Call 999 if anyone is in danger.

Anyone on the river streets or in the Pimperne Brook valley can sign up for flood warnings by phone, text or email, and the Environment Agency's Blandford Forum page lists warnings in force.

Blandford site questions, from Langton Road to Blandford St Mary

Our consultants are chartered, Unda Consulting Limited is a CIWEM Business Partner, and a senior consultant reviews and signs off each Blandford report before it goes to you or the council. Call 01293 214444 or email enquiries@unda.co.uk to discuss a site in Blandford Forum, Blandford St Mary or the villages round about, or read about flood risk assessment in Dorset, in Dorchester and in Christchurch, where the Stour reaches the sea.