Flood Risk Assessment in Martinstown

A flood risk assessment in Martinstown is mostly about groundwater. The village lies along the South Winterborne, a chalk stream that runs only when the water table is high, and when the chalk fills after a wet winter the water comes up through the valley floor, the roads and the drains. In July 2012, 20 properties in Martinstown and neighbouring Winterbourne Monkton flooded, and Dorset County Council's investigation named groundwater as the main source.

For sites in Martinstown we prepare flood risk assessments to support a planning application, a purchase, a mortgage or an insurance renewal, along with sequential and exception tests and surface water drainage strategies. The village's official name is Winterborne St Martin (sometimes spelt Winterbourne St Martin), the parish it sits in, and it falls in the DT2 postcode district about four miles south-west of Dorchester. Upstream, Winterbourne Steepleton and Winterbourne Abbas share the same stream, groundwater warning and council, and we cover them too.

Wessex Water says that once groundwater at the Ashton Farm borehole, south-east of the village, climbs above about 70 m above Ordnance Datum, Martinstown's sewers start to take in groundwater. It reached that level in winter 2024-25, and the company uses tankers when needed to keep the system working.

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

Screening a DT2 9 postcode along the village street

The coral line shows Martinstown's built-up area from the Office for National Statistics (Built-Up Areas 2022), listed there under the parish name Winterborne St Martin. It follows the village along the B3159 and the stream, and leaves out the farms and downland around it. Type a postcode to see its planning flood zone, modelled risk from rivers and surface water, and any warnings in force, or use our flood risk map for anywhere in England. Neither shows groundwater, which is the risk that matters most here.

The planning flood zones still cover a good part of the village. Our own overlay of the Environment Agency's Flood Map for Planning on this outline, run on 27 October 2026, puts about 16% of the built-up area in Flood Zone 3 and about 19% in Flood Zones 2 or 3, all of it along the line of the South Winterborne through the middle of the village.

When the chalk fills: groundwater in the South Winterborne valley

Groundwater flooding in the valley builds slowly and lasts. Chalk soaks up winter rain until the water table reaches the valley floor, the winterbourne starts to flow, and low ground, cellars and roads begin to flood, sometimes weeks after the rain. The Environment Agency's South Winterbourne Valley groundwater warning covers Kingston Russell, Winterbourne Abbas, Winterbourne Steepleton, Martinstown, Winterbourne Monkton, Winterbourne Herringston and Winterbourne Came.

Readings from the Ashton Farm borehole are public, and our live river levels map shows the Environment Agency's gauges and warnings in the area. The Flood Map for Planning does not show groundwater at all, which is why our explainer on communities affected by groundwater flooding puts the Dorset chalk valleys among the places where a site check has to go beyond the national map.

July 2012 and what the valley investigation found

After a wet spring, heavy rain on 6 to 8 July 2012 flooded homes the length of the valley. Dorset County Council commissioned an investigation, published in July 2013 as the South Winterbourne Flood Investigation, which looked at 42 flooded properties along the stream.

• Where. Twenty properties flooded in Martinstown and Winterbourne Monkton, 14 in Winterbourne Abbas and eight in Winterbourne Steepleton, where some flooded again in November and December 2012.
• How much rain. Friar Waddon recorded 111.2 mm in about 36 hours and Eggardon Hill 114.8 mm in 38.5 hours, after April to July rainfall of 459.8 mm, about 89% above average. The report put the event at a 1.3% to 2.2% annual chance.
• Which source. Groundwater was the primary source, with the stream, overland flow off farmland and surcharging drains adding to it. Wessex Water's operations were found to be a negligible factor.
• Who looks after the stream. The South Winterborne is an ordinary watercourse, not a main river, so the council as Lead Local Flood Authority holds the flood duty and riparian owners maintain the channel.
• What it recommended. A community flood action group, property-level protection such as raised floors and threshold defences, better maintenance of the channel and culverts, and raising or reviewing the low bridges in Martinstown.

Our own check of the Environment Agency's main river map on 27 October 2026 still shows no main river through Martinstown. Works in or beside the stream therefore need consent from Dorset Council rather than the Environment Agency, which our guide to riparian rights and ownership explains for owners of a stream-side plot.

18 July 1955: the wettest rain day measured in the UK

Martinstown holds the Met Office record for the highest rainfall in a single rain day anywhere in the UK: 279 mm on 18 July 1955, measured in the 24 hours from 09:00. Heavy thundery rain began over the downs around the village at about 14:30 that afternoon and carried on until about 05:00 the next morning.

In the village the road from Winterbourne Steepleton became a river more than four metres wide, the two sides of the street were cut off from each other, and water came up through cottage floors, according to an account in Dorset Life. Further down the valley, Radipole Lake overflowed into Weymouth, and Weymouth Museum records that a boy, Robin Crump, drowned at Watery Lane in Upwey and a man was killed by lightning at Sutton Poyntz. The Met Office record still stands, and storms of that kind are why surface water runoff from the downs is part of any assessment here, alongside groundwater.

Sewers, septic tanks and the Martinstown pumping station

In Martinstown, drainage problems usually start below ground. Wessex Water's infiltration reduction plan for Martinstown says that when groundwater rises above about 70 m at Ashton Farm, the network is at risk of being swamped by water getting into pipes and manholes, and that it reached that point in winter 2024-25 after September 2024 rainfall of more than 250% of the monthly average.

• Tankering. Wessex Water uses tankers when it must, to stop restrictions on toilet use and sewer flooding while groundwater is high, with an emergency discharge plan as a last resort.
• The pumping station. The plan lists the Martinstown sewage pumping station and a local water recycling centre, and shows flows in the network rising and falling with the groundwater.
• Work planned. CCTV surveys of public sewers, checks on private gullies and drains, more in-sewer monitors, sealing of sewers and manholes where it pays, and in the long term a scheme for network capacity.
• Private systems. Any home or plot off the main sewer relies on a septic tank or treatment plant with a drainage field, which stops working when the water table reaches it.

For a new home or conversion, that means the foul drainage needs as much thought as the flood risk: whether the sewer can take more flow in a wet winter, and if not, where a treatment plant can discharge. Our guides to septic tank rules and BS 6297 testing for drainage fields explain the tests involved.

Building in a village without a development boundary

Dorset Council decides planning applications in Martinstown and is the Lead Local Flood Authority that comments on drainage. Its adopted plan for the village is the West Dorset, Weymouth & Portland Local Plan of October 2015, read with the council's Level 1 Strategic Flood Risk Assessment.

England's national policy sits in chapter 18 of the August 2026 NPPF, where policy F4 triggers a site-specific assessment, F5 and F6 hold the sequential and exception tests, and F8 asks drainage to meet the 2025 National Standards for sustainable drainage. Dorset Council's local flood risk pages carry its strategy and flood reporting.

What a Martinstown scheme is likely to need

In a village this size most proposals are small, but small schemes here can still need flood evidence because the risk comes from below as well as from the stream. The table sets out what Dorset Council is likely to ask for.

Access is the other question. In 1955 the road through the village was cut, and the 2013 investigation looked at traffic pushing bow waves along the flooded B3159, so an assessment shows how people get in and out, or stay safe inside, while the water is up. Our article on Class Q barn conversions covers the flood rules for converted farm buildings.

Reports for chalk cottages, converted barns and plots by the stream

People in Martinstown commission flood and drainage work to build, buy, insure or lend. The reports answer different questions: whether the site is safe, whether it should be built on at all, and where the water goes.

• Applications to Dorset Council. Extensions, conversions and new homes in the stream's flood zone, or anywhere groundwater could reach them, need a flood risk assessment for planning that answers Policy ENV5.
• Groundwater, the stream and runoff together. A flood risk assessment for a Martinstown site weighs the winter water table, the South Winterborne and runoff off the downs, then sets floor levels and a dry way out.
• Buying a cottage in the village. Buyers ask whether a house flooded in 2012 and whether the groundwater warning affects it, and a flood risk survey for property purchase answers that for the one property.
• New homes in Flood Zone 3. Along the stream the council will look for a sequential and exception test showing that no lower-risk site is reasonably available and that the home can be made safe.
• Rain on chalk. A drainage strategy for a Martinstown plot starts from the depth to groundwater in winter, because a soakaway in the valley floor may sit in water for weeks.
• Keeping runoff on site. A surface water drainage strategy shows how a new roof or drive drains without adding to the stream or the lanes.
• After permission. Drainage conditions on a Martinstown consent are cleared through the discharge of drainage conditions, with the winter groundwater readings the council asks for.
• Rising water tables. With the 2013 investigation finding that groundwater was the main source, a groundwater flood risk assessment uses the Ashton Farm record and site monitoring to set floor levels and test basements or soakaways.
• Sewers that take in groundwater. Where Wessex Water's network is under strain in wet winters, or a plot needs a treatment plant, a foul drainage strategy shows how foul flows will be dealt with.
• Homes flooded in Martinstown in 2012. Owners in Martinstown whose homes flooded then, or who now face higher premiums or exclusions, can use a flood risk assessment for insurance to give insurers evidence about the building itself.
• Cut off by the water. For homes inside the South Winterbourne Valley warning area, a flood warning and evacuation plan sets out what occupants do when the groundwater warning is issued and the B3159 floods.

Groundwater up, drains slow: who to contact in Martinstown

Several bodies share the work. Dorset Council deals with the stream, groundwater and surface water, the Environment Agency issues the groundwater warnings, and Wessex Water runs the sewers and the pumping station. If anyone is in danger, call 999.

Households can sign up for the groundwater warning by phone, text or email. The 2013 investigation recommended a community flood action group for the valley, so residents may find a local group already sharing maintenance of the stream and its culverts.

Martinstown flood risk: questions we are asked

Each report for a Martinstown site is reviewed and signed by a senior consultant, the consultants who write them are chartered, and Unda Consulting Limited is a CIWEM Business Partner. Call 01293 214444 or email enquiries@unda.co.uk about a site in Martinstown, Winterbourne Steepleton or elsewhere in the valley, or read more about flood risk assessment in Dorset, in Dorchester and in Bridport.