Flood Risk Assessment in Malmesbury

Malmesbury stands on a Cornbrash limestone hilltop that the Bristol Avon almost wraps entirely: the Sherston Avon closes in from the west, the Tetbury Avon comes down from the north, and the two branches meet on the town's southern edge. That near-island setting makes it one of the most flood-exposed towns in the north of Wiltshire, and it is why a flood risk assessment in Malmesbury has to weigh river, surface-water and groundwater flooding together rather than judging any one source on its own.

We prepare flood risk assessments and drainage strategies for commercial and residential sites in and around Malmesbury, showing Wiltshire Council that a development will stay safe from flooding for its whole lifetime without pushing flood risk onto anyone else. We read every source that matters at a given site, deal directly with the Environment Agency and Wiltshire Council on your behalf, and guide architects, planning consultants, developers, landowners, home buyers and businesses through to a decision. We prepare flood risk assessments in Malmesbury for planning, property purchase, mortgage and insurance, and BREEAM, together with sequential and exception tests and surface-water drainage strategies.

Two arms of the Bristol Avon meet just below the old town, so a single site can take river water from one branch, surface water off the hill and rising water from the other branch in the same storm — which is why a generic, single-source assessment rarely satisfies Wiltshire Council.

If you need a quote or want to talk a site through, contact one of our experienced flood risk consultants on 01293 214444.

Flood risk assessment in Malmesbury at a glance

A quick summary for anyone weighing up a site in the town, before the detail below.

• Two rivers close around the town. The Sherston Avon and the Tetbury Avon (known locally as the Ingleburn) wrap the hilltop and join just south of it, so Malmesbury carries flood risk on more than one front.
• Three Environment Agency flood warning areas cover the town. The Tetbury Avon at Malmesbury, the Sherston Avon at Malmesbury, and the Bristol Avon (upper) from Malmesbury to Chippenham, reflecting how many properties sit close to the water.
• Storm Bert (November 2024) peaked within 3cm of the 2012 flood at the Environment Agency's St John's gauge, the event that first triggered the town's flood investigations.
• Wiltshire Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses; the Environment Agency is responsible for the two Avon branches as main rivers.

Flood risk in Malmesbury: two rivers around a hilltop town

River flooding shapes Malmesbury more than almost any other town in the catchment. The Sherston Avon arrives from the west and the Tetbury Avon comes down from the north, wrapping the limestone hill the old town stands on before they meet just below it. Both rivers respond quickly to heavy rain, and because they close around the town from two sides, a property can face one branch, surface water running off the streets and the hill, and the other branch in the same storm.

The way each branch behaves matters for an assessment. The Tetbury Avon, known locally as the Ingleburn, can rise fast at the Brokenborough gauge upstream of the town after heavy showers. The Sherston Avon drains a broad catchment to the west. Where they join, the combined Bristol Avon runs east towards Chippenham, which is why the Environment Agency operates a separate flood warning area for the Bristol Avon (upper) from Malmesbury downstream. National maps are built at a strategic scale and can miss what happens at a single plot, so a clean map result still needs reading against the local detail (why flood maps are not always accurate).

You can get an immediate, indicative read for any address using our free flood risk map, and see how the rivers are behaving right now on our live river levels map. Both are screening tools, useful context but not a substitute for a site-specific report.

A history of flooding in Malmesbury

Dated local events are part of the evidence base, and Malmesbury has a clear recent record. They show the pattern an assessment has to plan for: both Avon branches out of bank, surface water running off the hill, and low-lying streets and meadows underwater. Each event has fed the case for the town's flood scheme and the way the regulators now review development here.

• 2012. Flooding through the town triggered the current series of investigations and proposals for reducing flood risk, and prompted the community to set up a dedicated Malmesbury flood and emergency information service.
• January 2024, Storm Henk. The Avon burst its banks after heavy rain; conditions on Park Lane closed the Buildbase yard, and the Conygre Mead nature reserve was underwater.
• November 2024, Storm Bert. Over 22 to 24 November the flood level peaked at the Environment Agency's St John's gauge within 3cm of the 2012 flood, a near-record event for the town.
• November 2025. Heavy rainfall and high river levels brought further flood warnings to the Tetbury Avon, the Sherston Avon and the Bristol Avon around Malmesbury.

The longer-term direction of travel is for sharper extremes, with rivers swinging more abruptly between flood and drought (climate whiplash). That is why a credible assessment models the design flood for the development's lifetime, usually 100 years for housing, with the appropriate climate change allowance, and why residual flood risk behind any future defence still has to be addressed.

Check flood risk anywhere in Malmesbury

Malmesbury is outlined below. Screen any postcode in and around the town against Environment Agency data for rivers and sea, surface water and the planning flood zones, with a climate outlook to 2125. It is an indicative first pass, not a substitute for a site-specific assessment.

The ground beneath Malmesbury: Cornbrash, springs and drainage

Geology decides how a site drains, and Malmesbury is not the chalk country of southern Wiltshire. The old town stands on the Cornbrash, a Jurassic limestone, with Great Oolite limestones and the Forest Marble around it and the Kellaways Beds passing up into Oxford Clay to the south-east. The hilltop carries several freshwater springs, a reminder that water moves through this ground as well as over it.

That mix matters for drainage design. Fissured limestone can let water away quickly in one spot and hardly at all a few metres away, while the clay to the south-east resists infiltration altogether. A soakaway that looks viable on a desk study can fail in the ground, so infiltration should be tested early to BRE Digest 365 with a discharge route ready if it fails (do you need infiltration testing?).

• Limestone is unpredictable for soakaways. Infiltration rates vary sharply across a single site, so assumed rates rarely pass; the Lead Local Flood Authority expects test data.
• Clay caps infiltration to the south-east. Where Oxford Clay underlies a site, an infiltration-led strategy usually has to give way to controlled discharge to a watercourse or sewer.
• Check the ground before the layout is fixed. Our free geology and ground conditions checker and BRE 365 infiltration rate calculator give a fast first pass before trial pits.

Planning and drainage policy for a flood risk assessment in Malmesbury

Wiltshire Council is the Lead Local Flood Authority for Malmesbury, responsible for surface water, groundwater and ordinary watercourses and a statutory consultee on surface water drainage for major development. The Environment Agency is responsible for the two Avon branches as main rivers and is consulted on development affecting them. The evidence a developer has to satisfy is published and specific.

• Check the SFRA before the layout is fixed. Wiltshire Council's Strategic Flood Risk Assessment — a Level 1 updated in 2024 and a Level 2 published in February 2025 — and the Council's online SFRA data explorer set out what is expected, and a critical drainage area or mapped surface water risk can require an assessment even in Flood Zone 1.
• The Malmesbury Neighbourhood Plan carries weight. The made Neighbourhood Plan forms part of the development plan for the area, so its policies count alongside the Wiltshire Local Plan when an application is judged.
• Follow the drainage hierarchy. Surface water should be managed by infiltration, then a watercourse, then a sewer as a last resort, with sustainable drainage on major development designed to the 2025 national standards and the four pillars of SuDS.
• Expect the sequential test to widen. Since the September 2025 Planning Practice Guidance update it must consider surface water and groundwater, not just rivers, and a failed sequential test is not automatically fatal (the Yatton judgment).
• Get the first submission right. From April 2026 the appeals process puts more weight on the quality of the first-time submission, so an Environment Agency or Lead Local Flood Authority objection is far cheaper to avoid than to fix.

What kind of development needs a flood risk assessment in Malmesbury?

Development around Malmesbury runs from town-centre and riverside schemes to edge-of-town housing, barn conversions and farm diversification on the surrounding land, and the town is also home to Dyson's global headquarters, so commercial schemes feature too. The trigger for an assessment is the same wherever the site is, and proximity to either Avon branch makes it more likely here than in most places.

• Sites in Flood Zone 2 or 3. Any more vulnerable use, and most development, in the medium or high river flood zones along the two Avon branches will need a site-specific assessment and may need the sequential and exception tests.
• Larger sites in Flood Zone 1. Development of one hectare or more needs an assessment even in the low-risk zone, as does any site where the SFRA flags surface water or a critical drainage area.
• Edge-of-town housing. Recent proposals at Park Road and Sherston Road drew flood objections from the town and the Environment Agency, a reminder that flood risk and the Cotswolds National Landscape both bear on what is acceptable here.
• Major development drainage. Ten or more dwellings, a site of half a hectare or more, or 1,000 square metres of floorspace will need a surface water drainage strategy for the Lead Local Flood Authority.

If you are unsure, our guide to whether you can do your own flood risk assessment sets out where standing advice ends and a competent report becomes necessary, and the NPPF guidance explains the national policy tests in full.

How our flood risk consultants help in Malmesbury

We are experts on flood risk in Malmesbury rather than a local office, and that is the point: we are the accurate translation layer between your scheme and the Environment Agency, Wiltshire Council and national planning policy. A flood risk assessment in Malmesbury from Unda assesses every credible source — both Avon branches, surface water off the hill, groundwater and any ordinary watercourse on the boundary — compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We stay involved past the report to answer objections, and a senior consultant signs off every job. Where mapping is not enough, we provide detailed flood modelling to define flood levels and flow routes precisely.

It helps to be clear about what you are commissioning. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they answer different questions: is the site safe, should the development be here, and where does the surface water go. We will tell you which you actually need before you spend anything.

You can read more about how our flood risk consultants work or about Unda.

Flood risk assessments for every situation in Malmesbury

People need a flood risk assessment or a drainage strategy in Malmesbury for a range of reasons, and we prepare each to the standard its audience expects.

• Flood risk assessment for planning. The site-specific report submitted with a planning application to satisfy the NPPF, our planning flood risk assessment service.
• Sequential and exception tests. The locational justification when a site sits in a higher-risk zone, our sequential and exception test reports.
• Surface water drainage and SuDS. How runoff is managed to meet Wiltshire Council's standards, from a drainage strategy through to discharging drainage conditions and preliminary drainage design.
• Property purchase and mortgage. An independent flood risk survey for a property purchase when a search or lender flags risk during conveyancing, common on the streets near both Avon branches.
• Insurance. A property-specific flood risk report for insurance where cover has been restricted or premiums have risen.
• BREEAM. A flood risk assessment for BREEAM structured around the Pol 03 credit.
• Groundwater and flood modelling. A groundwater assessment where springs or a high water table are a concern, and flood modelling where national mapping is not enough, both feeding the wider assessment.

For drainage design you can also use our free drainage calculators for a fast first pass before detailed work begins.

Malmesbury flood resources and contacts

The most reliable Malmesbury flood information comes from the lead authorities. For an immediate threat to life during a flood, always call 999; for a power cut during flooding, call 105.

The town's flood scheme shows the lead authorities at work. After the Environment Agency concluded there was no viable conventional scheme, a community-led project taken forward with Malmesbury Town Council, the Environment Agency and Wiltshire Council secured funding from the Regional Flood and Coastal Committee to extend the modelling and costing. The measures under investigation include a temporary barrier at the Memorial Gates, raising the wall on the north bank at Cuckingstool Mead and improving river flow downstream of St John's Bridge. A future defence like this still leaves a residual risk that an assessment has to address, because the flood maps show risk as if the defences were not there.

Flood risk assessment in Malmesbury: frequently asked questions