Flood Risk Assessment in Dorset

A flood risk assessment in Dorset has to start with the chalk. Across much of the county, winter rain soaks into the ground and comes back up weeks later, so a site well away from any river can still flood. Add the Stour, Frome and Piddle floodplains and a coast from Lyme Regis to the harbours, and most sites face more than one source. We prepare flood risk assessments in Dorset for planning, house purchases, lenders and insurers, alongside sequential and exception tests and surface water drainage strategies.

That includes Weymouth and Portland, Wareham, Swanage and the rest of Purbeck, Lyme Regis, Charmouth and Beaminster in the west, Wool, Bere Regis and Puddletown on the Frome and Piddle, Wimborne Minster, Sturminster Marshall, Ferndown, West Moors and Verwood in the east, and Shaftesbury and Sturminster Newton in the north, across the DT, BH and Dorset SP postcode districts. Bournemouth, Poole, Christchurch, Dorchester, Martinstown, Blandford Forum, Bridport and Sherborne each have a page of their own.

In February 2026 the Environment Agency still had several groundwater flood warnings in force for Dorset, weeks after the January storms, and said significant groundwater flooding was probable in parts of the county.

Tell us the address and what you plan to build or buy: call 01293 214444 or email enquiries@unda.co.uk.

Why groundwater comes first in a flood risk assessment in Dorset

Chalk underlies a broad band of the county from Cranborne Chase through the Dorset Downs to the hills above Dorchester, and it fills and drains slowly. Once it is full, water breaks out in dry valleys, cellars and low floors, and it can stay up for weeks. A river flood map will not show it, so we check groundwater on every Dorset site as a source in its own right.

• A warning service of its own. In February 2019 the Environment Agency launched groundwater flood warnings for Dorset, Salisbury Plain and the Wiltshire South Downs, covering about 10,000 residents in more than 100 vulnerable communities. Residents already signed up for flood warnings have to opt in to receive them.
• The winterbournes. Streams such as the South Winterborne and the Winterborne that joins the Stour at Sturminster Marshall flow in their upper reaches only when the water table is high, and the 2023 SFRA lists the villages from Winterborne Zelston to Winterborne Kingston, and from Winterborne Whitechurch up to Winterborne Houghton, as places at high risk.
• Mapped by the council. Dorset Council's Level 1 SFRA includes groundwater emergence mapping and tells applicants to use it as a prompt for a site-specific assessment.

Our explainers on groundwater flooding and on the communities affected by it explain why chalk behaves this way. River and tide gauges across the county feed our live river levels map, which shows the Stour, Frome and Avon as they rise. The Environment Agency's announcement of the groundwater service explains how to opt in.

Check a Dorset postcode

Type a postcode to see river and sea flood zones, surface water, reservoir extents and the nearest warning area for anywhere in the county; our flood risk map does the same for any English address. The coral line follows the ceremonial county: the Dorset Council area together with Bournemouth, Christchurch and Poole.

The screening uses national datasets and cannot show groundwater emergence. For that, the council's SFRA maps on Dorset Explorer carry the groundwater layer, historic flooding records and the climate change outlines.

Where Dorset's river flood zones fall, town by town

Five river systems carry most of the county's fluvial risk: the Stour with the Allen, Lydden and Iwerne; the Frome and its chalk tributaries; the Piddle; the Moors River and Uddens Water in the east; and the short west Dorset rivers, the Lim, Char, Brit and Asker. Dorset Council's 2023 SFRA summarises each catchment in turn; for the towns below, it places the risk as follows:

The SFRA also maps reservoir flood routes, for example down the Allen from Crichel Lake and down the Yeo through Sherborne. In the far west, small headwaters near Mosterton belong to the River Parrett, which does its flooding further downstream in Somerset.

Dorset Council or BCP Council: who reads a flood risk assessment in Dorset?

Two unitary councils have handled planning and local flood risk across the county since April 2019, when the old county council and its districts were replaced. Which one reads your report depends only on where the site is.

In August 2026 the Government invited the two councils, with Somerset and Wiltshire, to express an interest in a Wessex mayoral strategic authority. Nothing has been agreed, and the leaders have said it would not merge any councils, so planning and drainage decisions stay where the table puts them. BCP's own flood risk evidence is on its strategic flood risk assessment page.

What the Dorset SFRA and local plans ask of a site

National policy F4 in chapter 18 of the August 2026 NPPF asks for an assessment in Flood Zones 2 and 3, on sites of a hectare or more, and on Flood Zone 1 land at risk from another source. Policies F5 and F6 set the sequential and exception tests, which our guide to national flood policy explains. Dorset adds its own evidence and plan policies:

• The 2023 SFRA. The Dorset Council Level 1 SFRA, prepared by JBA Consulting and finalised in February 2023, covers rivers, the sea, surface water, groundwater and reservoirs, with climate change modelling for the 1 in 100 year flood around several key settlements.
• Updated flood zones. The council notes that Environment Agency flood zones have changed since the SFRA maps were drawn, and it may ask a site-specific assessment to redefine Flood Zones 3a and 3b from the latest mapping.
• Purbeck. The Purbeck Local Plan 2018 to 2034, adopted on 18 July 2024, carries Policy E4 on assessing flood risk, E5 on sustainable drainage and E6 on coastal change management areas.
• East Dorset. The Christchurch and East Dorset Core Strategy of 2014 still governs Wimborne, Ferndown, West Moors and Verwood, with Policy ME6 on flood management, mitigation and defence.
• West and North Dorset. The West Dorset, Weymouth and Portland Local Plan of 2015 and the North Dorset Local Plan of 2016 remain in force for Weymouth, Portland, Lyme Regis, Shaftesbury, Sturminster Newton and their villages.
• The new plan. Dorset Council restarted its plan under the new plan-making system: the scoping consultation closed on 26 October 2026, and the council's own timetable expects adoption in 2029.

Until the new plan is adopted, an assessment answers the policy of whichever older plan covers the site, and the council's local plan updates page tracks each stage. Our explainer on strategic flood risk assessments covers what an SFRA can and cannot tell you.

Drainage strategy in Dorset: infiltration on the chalk

A drainage strategy in Dorset starts with whether the ground will take the water. Chalk and the heathland sands in the east can drain well, which makes soakaways the first choice in the drainage hierarchy, but a high winter water table can rule them out. Dorset Council is consulted as lead local flood authority on the surface water of every major application, and it offers chargeable pre-application advice through its surface water planning advice service.

• Prove the soakaway. Infiltration rates come from BRE 365 testing on site, at the depth the soakaway will sit.
• Find the winter high. Where groundwater may be close, readings through the wet season show how much clearance a soakaway has; our note on groundwater monitoring for planning explains why starting late can cost most of a year.
• National standards. Since August 2026 policy F8 expects schemes to follow the 2025 National Standards for sustainable drainage, alongside the council's local advice.
• Ordinary watercourses. Works in a ditch or stream that is not a main river need the council's consent as lead local flood authority, separate from planning.

Our ground conditions and soakaway checker and infiltration rate calculator give a first view of a site before any testing. The council's local flood risk management strategy sets out how it handles surface water, groundwater and ordinary watercourses.

From Lyme Regis to Highcliffe: Dorset's coast and its shoreline plans

Two shoreline management plans set the long-term policy for every stretch of the Dorset coast, and our guide to shoreline management plans explains what hold the line, managed realignment and no active intervention mean for a site. Around the larger towns, more detailed strategies sit beneath them.

• West of Swanage. The Durlston Head to Rame Head plan covers Purbeck's southern cliffs, Weymouth, Portland, Chesil and Lyme Regis, and continues into Devon.
• Swanage to Hurst Spit. The Poole and Christchurch Bays plan runs from Durlston Head near Swanage to Hurst Spit near Milford-on-Sea, including both harbours.
• Weymouth. In October 2020 Dorset Council's cabinet approved the Weymouth Harbour and Esplanade strategy, a phased programme of replacing and raising walls round the harbour and seafront needing more than £115 million over 100 years.
• Swanage. Central Swanage, from Shore Road to Lower High Street, relies on temporary defences the Environment Agency puts up each winter; a permanent scheme is at business case stage with no funding confirmed.
• Wareham and Poole Harbour. The Environment Agency's Poole and Wareham strategy, adopted in February 2015, covers the harbour, Wareham and the bays from Poole to Durlston.

On the coast we prepare flood risk assessments that test tide, surge and wave overtopping with sea level rise, and due diligence for buyers, lenders and businesses. Where the council asks for an erosion or coastal change study as well, that is a separate report we do not prepare.

Flooding in Dorset: the record behind today's rules

Flooding in Dorset has come from rain, rivers and the ground itself, and the dates below are why both councils ask for evidence on every source:

1. 18 July 1955. A thunderstorm over the hills west of Dorchester produced one of the heaviest single days of rain ever measured in the UK, centred on Martinstown.
2. Winter 2013 to 2014. On Christmas Day 2013 the Stour reached the highest level on record at the Environment Agency's Iford Bridge gauge, on the lower river near Christchurch.
3. February 2019. The Environment Agency began issuing groundwater flood warnings for Dorset, the first time the county's chalk communities could be warned of water rising from below.
4. August 2020. Dorset Council investigated flooding in Dorchester as lead local flood authority and published the findings.
5. January and February 2026. After Storm Chandra at the end of January, groundwater warnings stayed in force across Dorset into late February.

Several of these floods came from rain and rising groundwater instead of a river overtopping, which is why a Dorset assessment looks hard at slopes, dry valleys and winter water levels as well as flood zones. Live warnings for the whole county are on the Environment Agency's check for flooding page.

Reports for Dorset sites, and the question each one answers

A flood risk assessment asks whether a site is safe, a sequential or exception test asks whether it should be built there at all, and a drainage strategy asks where the rain goes. Below, each one is paired with the Dorset situation that most often calls for it.

• Every source together. A plot in a chalk valley can face a stream, rising groundwater and runoff off the downs at once, and our flood risk assessment weighs each one and how they combine.
• For the application. Whether Dorset Council or BCP Council decides the scheme, a flood risk assessment for planning gives the planning officer and the Environment Agency the evidence on flood zones, groundwater and safe access.
• Before exchange. Searches flag cottages by the Frome at Wareham and homes near the Esplanade at Weymouth, and a flood risk survey for property purchase explains what the risk means for a particular house before a buyer commits.
• Testing other sites. New homes on Flood Zone 2 or 3 land at Wool, Sturminster Marshall or Wimborne Minster normally need a sequential and exception test comparing reasonably available sites elsewhere.
• The whole drainage picture. A drainage strategy sets out how foul and surface water leave a site, from a barn conversion in the Blackmore Vale to an urban extension.
• Rain on the site. Where infiltration depends on the chalk and its winter water table, a surface water drainage strategy shows the lead local flood authority the test results, storage and discharge route.
• After permission. If the decision notice from either Dorset council carries a pre-commencement drainage condition, discharge of drainage conditions provides what the officer needs before building begins.
• Water from below. With more than 100 Dorset and Wiltshire communities on the groundwater warning service, a groundwater flood risk assessment looks at how high the water table rises under a particular site.
• Homes in the warning areas. After the 2026 groundwater warnings, a flood risk assessment for insurance gives a broker or underwriter evidence about the individual building.
• Harbour and seafront property. Weymouth's own strategy warns that unmanaged sea flooding could blight the town's property market, and flood risk due diligence sets out tidal risk and shoreline policy for a buyer, lender or developer from Lyme Regis to the harbours.
• Keeping trade going. Businesses on Swanage's seafront and round Weymouth Harbour depend on defences that are temporary or ageing, and a business continuity flood plan prepares a firm for the next surge.
• Levels on the big rivers. Sites on the Stour, Frome or Piddle floodplains can need flood levels the national maps do not provide, which flood modelling supplies.
• Getting out in time. Where a site depends on a warning, as central Swanage does on the agency's winter defences, a flood warning and evacuation plan sets out when and how people leave.

Eight Dorset places with pages of their own, and the counties next door

These pages go further into each town's rivers, defences and council:

• The BCP towns. Bournemouth on the Bourne Stream and the bay, Poole round its harbour, and Christchurch where the Stour and Hampshire Avon meet.
• The county town and its valley. Dorchester on the Frome, and Martinstown on the South Winterborne.
• The Stour at Blandford. Blandford Forum, with Blandford St Mary across the river.
• West Dorset. Bridport and West Bay on the Brit and Asker, and Sherborne on the Yeo.
• Over the borders. Devon lies west of the Axe, Somerset to the north-west, Wiltshire beyond Cranborne Chase and Hampshire across the Avon. Our areas page lists every county and town we cover.

Flood contacts for Dorset, Bournemouth, Christchurch and Poole

If floodwater puts anyone at risk, ring 999; for a power cut, ring 105. Floodline on 0345 988 1188 gives warnings and advice at any hour, and you can sign up for flood warnings, including the separate groundwater alerts, by phone, text or email.

South West Water, trading as Bournemouth Water, supplies drinking water in parts of the east of the county; check the supplier for an address before reporting a burst main. The council's emergency contacts page lists the power and road numbers.

Flood risk in Dorset: the questions people ask us

For a quote, or to talk a Dorset site through first, contact one of our experienced flood risk consultants on 01293 214444 or at enquiries@unda.co.uk. We quote a fixed fee, our consultants are chartered, Unda Consulting Limited is a CIWEM Business Partner, and a senior consultant signs off every report.