Flood Risk Assessment in Malmesbury

A flood risk assessment in Malmesbury starts with the shape of the town. The old town stands on a hill that two branches of the Bristol Avon almost surround: the Sherston Avon comes in from the west, the Tetbury Avon, which the town calls the Ingleburn, from the north-west, and the two meet on the southern edge of the town. The hilltop stays dry. Westport, St John's Street, Lower High Street, Baskerville and Park Road sit close to the water, and they have flooded again and again.

We prepare flood risk assessments in Malmesbury for planning applications, house purchases, mortgages and insurance, along with sequential and exception tests and surface water drainage strategies for Wiltshire Council. Here, Malmesbury means the town's ONS built-up area: the hilltop centre, Westport to the north-west and Burton Hill to the south, all in the SN16 postcode district. The villages around the town come under our page for Wiltshire.

On 25 November 2012 the Sherston Avon reached 1.48 m at St Johns Weir, the highest level that gauge has recorded. About 40 homes and businesses flooded and firefighters rescued four people from two houses.

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

Why does Malmesbury flood from two rivers at once?

Because the two branches wrap the hill and join just below it. North of the old town they run within about 200 metres of each other, kept apart by a narrow ridge that turns the Sherston Avon south and the Tetbury Avon east. They meet on the town's southern edge and leave as the Bristol Avon, bound for Chippenham. Heavy rain on either catchment fills the meadows at the foot of the hill, and the low ground at the confluence takes water from both.

• The Sherston Avon. It arrives from the west through Westport. The Environment Agency measures it upstream at Fosseway and in the town at St Johns Weir, and both gauges set their records on 25 November 2012: 1.27 m at Fosseway, where property flooding becomes possible above 0.90 m.
• The Tetbury Avon, or Ingleburn. It comes down past Brokenborough, where the gauge reached its highest level, 1.55 m, on 20 July 2007. The Environment Agency expects property flooding to be possible there above 1.30 m.
• The leat, the Memorial Gardens and the meads. A mill leat beside Cuckingstool Mead carries water past the Memorial Gardens towards St John's Street and Lower High Street. Closing that route is the aim of the town's planned flood scheme.
• Below the confluence. The combined river passes Cow Bridge and runs on through Great Somerford, where the gauge record of 2.43 m still dates from 11 July 1968.

Set against the Environment Agency's Flood Map for Planning, about 6% of the built-up area lies in Flood Zone 3 and a further 4% in Flood Zone 2, nearly all of it on the valley floors around the hill (our measurement, 20 October 2026). The rest is Flood Zone 1, which does not settle a site by itself: a garden, a cellar or the road out can sit lower than the house.

Which streets in Malmesbury are most at risk of flooding?

The Environment Agency's three flood warning areas give the clearest answer, because each is drawn around the places it expects to flood first. A flood warning means flooding to some property is expected. Anyone living or working on these streets can sign up for warnings by phone, text or email.

Park Road shows why a home above the flood zone can still need an assessment. In 2020 up to 26 homes were proposed on the slope south-west of the road. Every plot on the layout was in Flood Zone 1, but the access and Park Road itself were in Flood Zones 2 and 3, and part of the land carried groundwater and surface water risk. Wiltshire Council's committee report of December 2021 recommended approval with an emergency route out through a cul-de-sac in White Lion Park, agreed by the council's drainage team and the Environment Agency, and with Environment Agency conditions on access and floor levels. It is a version of the dry island problem.

Screen a Malmesbury postcode, from Westport to Burton Hill

Type a postcode from the town centre, Westport or Burton Hill to check it against Environment Agency data for rivers, surface water and reservoirs, with climate projections to 2125, or open our full flood risk map for anywhere else.

Malmesbury flooding on record, from 1968 to Christmas Eve 2013

The town's own flood record goes back to 1880. The list compiled for the town's flood scheme presentation, drawing on Environment Agency flood outlines, local accounts and press reports, runs to the floods of 2012 and 2013 that started the current scheme. The entries below are the ones with recorded effects or a gauge record.

1. July 1968. About 30 houses in and around Lower High Street flooded and a flood fund was opened. Downstream, the Great Somerford gauge still holds its record from 11 July that year.
2. 19 to 22 January 1999. Remembered as the worst floods in living memory, with Baskerville and St John's Street worst hit and the bowls club and football club under water.
3. 30 October 2000. Both branches spilled out of bank after heavy rain. St John's Street took the main damage, houses also flooded around Park Road, and Holloway and Swindon Road went under where they cross the river.
4. 20 July 2007. The Tetbury Avon reached 1.55 m at Brokenborough, still the highest level on that gauge.
5. 25 November 2012. About 40 homes and businesses flooded. Waist-deep water cut off part of the High Street, roads into the town closed and four people were rescued from two homes. The mayor called it the worst flooding in the town centre for nearly 70 years.
6. 24 December 2013. Eight properties flooded, six of them in St John's Street. This time the water rose through floors and yards from downstream of St John's Bridge rather than coming over the road as in 2012.

Wiltshire Council's published list of Section 19 flood investigations has no Malmesbury report, so the town's record and the gauges remain the best evidence a report can cite. Readings at St Johns Weir, Brokenborough and Fosseway can be followed on our live river levels map.

What the St John's flood scheme will and will not change

After 2012, residents formed the Malmesbury Flood Working Group and began work with Malmesbury Town Council, Wiltshire Council and the Environment Agency on the St John's Street, Lower High Street and Baskerville area. Short-term works came first: a shallow swale across the mead, deeper openings under the footbridge, a lowered river bed under one arch of the bridge, a sluice at Cowbridge, and removal of an old gasometer that had been built out into the river.

In 2017 the Environment Agency concluded that a scheme to the national 1-in-100 standard could not be justified, because it would cost more than the damage it prevented. The community-led alternative raises the wall on the garden side of the leat by about a metre over some 70 metres and adds a demountable barrier across the Memorial Gardens, with pump sumps and a route for exceedance flows. Environment Agency model runs found it would protect to at least 1 in 30, and the design aims at about 1 in 50, enough to hold back a repeat of 2012 and 2013.

The Wessex Regional Flood and Coastal Committee approved £50,000 of local levy for extra modelling, with £550,000 provisionally set aside for development once an outline business case is approved, and the project passed to Wiltshire Council. A public meeting at the town hall on 11 July 2023 set out the plan for residents. Upstream, the Bristol Avon Rivers Trust finished natural flood management works on a farm above the town in May 2025: eight floodplain scrapes and twelve woody debris features, designed to hold an estimated 1,000 m³ in a flood (how such schemes slow the flow).

None of this changes a site's flood zone. The Flood Map for Planning shows river risk as if there were no defences, a demountable barrier only works if it goes up in time, and houses further along St John's Street and in Baskerville fall outside the scheme. A flood risk assessment in Malmesbury still has to deal with the residual risk behind the wall and with the warning time a demountable barrier depends on.

Core Policy 67 and the evidence Wiltshire Council asks for in Malmesbury

Wiltshire Council is both the planning authority and the Lead Local Flood Authority for Malmesbury, so one council decides the application and reviews its surface water drainage. The Environment Agency is consulted on anything affecting the two Avon branches, which are main rivers. Malmesbury Town Council comments on applications but does not decide them.

• The plan in force. Wiltshire Council withdrew its Local Plan Review from examination on 19 May 2026, after the inspectors offered a choice between withdrawal and a report recommending against adoption, and it now has to start a new plan. Until one is adopted, Malmesbury applications are decided against the Wiltshire Core Strategy, adopted on 20 January 2015.
• Core Policy 67, Flood Risk. Development in Flood Zones 2 and 3 must use the council's housing land availability evidence to show the sequential test is met, and the supporting text asks for clear evidence that no lower-risk site is available. Every new development must also include measures to reduce the rate of rainwater runoff and improve infiltration, unless site or environmental conditions make them unsuitable.
• Core Policy 13, the Malmesbury area. It plans for about 885 homes at Malmesbury between 2006 and 2026 and protects three employment areas: Malmesbury Business Park, the Dyson site and land north of Tetbury Hill.
• The neighbourhood plan. Prepared by Malmesbury Town Council with St Paul Malmesbury Without and Brokenborough parish councils, it was made in 2015 for the period to 2026, and a review went out to consultation in April and May 2022. Committee reports on the Park Road and Filands schemes weighed them against both plans.
• The SFRA. The council's Level 1 Strategic Flood Risk Assessment, final version November 2024, uses a 2016 defended model of the river at Malmesbury and takes the functional floodplain, Flood Zone 3b, from its 3.3% annual chance outline. Its appendices map groundwater emergence, sewer and surface water risk, and set out how the council expects the sequential test to be run.

Above the Core Strategy sits chapter 18 of the August 2026 NPPF. F4 decides whether a Malmesbury scheme needs an assessment, F5 and F6 set the sequential and exception tests, and F8 asks every development to meet the 2025 National Standards for sustainable drainage. Planning Practice Guidance revised on 17 September 2025 also applies the sequential approach to every source of flooding, surface water included (what that update changed).

West of Park Lane the land lies in the Cotswolds National Landscape, and the Core Strategy asks all development in the Malmesbury community area to conserve it.

From Park Road to Filands: which Malmesbury schemes need a flood risk assessment?

Most schemes beside the rivers need one, and some on the hill do too. An assessment goes with any application in Flood Zone 2 or 3, with any site of a hectare or more in Flood Zone 1, and with smaller Flood Zone 1 sites where the SFRA or the surface water maps show a problem. These are the usual cases in the town.

New homes in Flood Zone 2 or 3 also face the sequential test, which in Wiltshire draws on the council's housing land evidence under Core Policy 67. The exception test can follow; our explainer sets out how the two tests fit together.

Soakaways, springs and drainage strategies on Malmesbury's hill

Core Policy 67 expects every scheme to slow its runoff and soak water into the ground where it can. On Malmesbury's hill that depends on what is underneath. The hilltop has freshwater springs, the Park Road land carried groundwater risk, and a soakaway has to be proved by infiltration testing on the site rather than assumed. Wiltshire Council publishes its own soakaway guidance note and Drainage Betterment Strategy. Our ground conditions checker and BRE 365 infiltration rate calculator give a first reading before trial pits, and when infiltration testing is required explains what the council is likely to ask for.

Where soakaways fail, the water goes to a watercourse or a Wessex Water surface water sewer, never the foul sewer. The Park Road scheme was conditioned so that no surface water reached the public foul system, and Wessex Water asks for the drainage hierarchy to be followed before it accepts a connection. On larger sites the strategy has to show the runoff will be no worse than before, which the Filands report weighed against a known capacity problem: the drainage at Powell Rise has a history of flooding in bad weather.

What our consultants prepare for sites between the two Avons

A buyer on St John's Street, a household extending in Westport and a developer promoting land on the edge of town each need a different report.

• Every source at once. A flood risk assessment weighs both Avon branches together with surface water off the hill, groundwater from the springs and the sewers.
• Applications by the river. Sites in Westport, along St John's Street or off Park Road need a flood risk assessment for planning that answers Core Policy 67 and the 2024 SFRA.
• Before exchange in a warning area. A house on Baskerville, Mill Lane or St John's Street can be checked with a flood risk survey for property purchase that reads the 2012 and 2013 records for that street.
• Homes on the floodplain. New dwellings in Flood Zone 2 or 3 need the sequential and exception test, run against Wiltshire's housing land evidence.
• Runoff from the hill. Core Policy 67 asks every development to cut its runoff rate, and a drainage strategy shows the council how.
• Major schemes. Ten or more homes, or edge-of-town land like Filands, need a surface water drainage strategy for the Lead Local Flood Authority.
• After permission. Malmesbury permissions carry pre-commencement surface water conditions, as at Park Road and Filands, and the discharge of drainage conditions clears them.
• Cover after 2012. About 40 homes and businesses flooded in November 2012, and a flood risk assessment for insurance gives an insurer the picture for one property.
• Water from below. Water rose through floors in St John's Street in December 2013 and the Park Road land was flagged for groundwater, which a groundwater flood risk assessment tests on the site.
• Levels for one site. The SFRA's 2016 defended model was built for strategy and the town scheme needed extra modelling of its own, so a riverside site may need flood modelling to set floor levels.
• A way out when the road floods. Three Environment Agency warning areas cover Malmesbury, and a flood warning and evacuation plan sets out when to leave and by which dry route.
• Yards and shops by the river. Businesses flooded in 2012 and a builders' yard on Park Road took a foot of water in 1992; a business continuity flood risk assessment plans for the next event.
• Employment land. New buildings on the Dyson site, Malmesbury Business Park or land north of Tetbury Hill can use a BREEAM flood risk assessment for the Pol 03 credit.
• Edge-of-town land. Speculative sites such as Park Road and Filands came forward outside the neighbourhood plan, and a flood risk feasibility study tests the zones, access and drainage before land is bought or promoted.

Who to call when the Avon rises in Malmesbury

Call 999 if floodwater puts anyone in danger and 105 if the power goes off. For anything else, these are the organisations responsible in and around Malmesbury.

Downstream, the same river flows through Chippenham and on to Bath. Swindon, to the east, drains to the Thames instead. Other towns and counties are listed under the areas we cover.

Malmesbury flood risk questions from buyers, owners and applicants

Unda Consulting Limited is a CIWEM Business Partner, and our chartered consultants prepare flood risk assessments, sequential and exception tests and drainage strategies for sites across Malmesbury, from the riverside streets to the hilltop. Call 01293 214444 or email enquiries@unda.co.uk to start.