Flood Risk Assessment in Wiltshire

Our team at Unda use years of experience to prepare comprehensive flood risk assessments and drainage strategies for commercial and residential sites across Wiltshire. A flood risk assessment in Wiltshire shows your local planning authority that a development will stay safe from flooding for its whole lifetime, without pushing flood risk onto anyone else.

Wiltshire is drained by two separate Avon systems and the upper River Kennet, so the flood picture changes completely from one side of the county to the other. We read every source that matters at a given site — the rivers and their tributaries, the surface water that overwhelms drains in the towns, and the groundwater that rises slowly through the chalk — then deal directly with the Environment Agency and Wiltshire Council on your behalf. We guide architects, planning consultants, developers, landowners, home buyers and businesses through to a decision, whether the report is needed for planning, a property purchase, a mortgage or insurance, BREEAM, or a surface-water drainage strategy.

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

Wiltshire's chalk is the catch. On Salisbury Plain and the Marlborough Downs the water table can climb several metres over a wet winter and rise up through the ground weeks after the rain has stopped, so a site that screens as low risk on the national map can still need a careful, source-by-source assessment to satisfy Wiltshire Council.

Flood risk assessment in Wiltshire at a glance

• Two Avon catchments split the county. The Bristol Avon drains the north and west through Malmesbury, Chippenham, Melksham, Bradford-on-Avon and the Trowbridge area; the Hampshire Avon drains the south, gathering the Wylye, Nadder, Bourne and Ebble at Salisbury.
• Chalk groundwater is a defining local risk. In winter 2025–26 the Dorset and Wiltshire chalk catchments saw their worst groundwater flooding in over a decade, prompting the Environment Agency to launch a dedicated groundwater flood warning service for the area.
• Wiltshire Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses across the county; Swindon Borough Council is the separate authority for the Swindon area.
• Flood Zone 1 is not a free pass. Since the September 2025 Planning Practice Guidance update, surface water and groundwater risk can trigger an assessment even on a Flood Zone 1 site.

Check flood risk anywhere in Wiltshire

Wiltshire Council's area is outlined below. Screen any postcode in the county 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.

Flood risk in Wiltshire: two Avons, the Kennet and surface water

Wiltshire has no coast and no tidal risk, but it carries a wide spread of river, surface water and groundwater flooding, and the balance between them shifts across the county. The north and west drain to the Bristol Avon, which rises near Sherston and runs through Malmesbury, Chippenham, Melksham and Bradford-on-Avon before crossing into Bath. The south drains to the Hampshire Avon, a 96-kilometre chalk river that gathers the Wylye, the Nadder, the Bourne and the Ebble around Salisbury. The east of the county feeds the upper River Kennet, a chalk stream rising on the Marlborough Downs and flowing through Marlborough towards the Thames.

The way each river behaves matters for an assessment. The chalk streams are normally slow to respond, fed by springs and groundwater, and some of the upper reaches are winterbournes that only flow after the water table rises. But the Nadder and parts of the upper Hampshire Avon are fed from clay catchments and can rise and fall quickly after heavy rain, which is why Salisbury — where four rivers meet — can see levels climb fast even though it sits on chalk. In the towns, surface water is now the source that catches most schemes out: short, intense storms overwhelm drains and gulleys, and Wiltshire Council has prepared Surface Water Management Plans for Trowbridge, Chippenham and Salisbury precisely because these built-up catchments flood from runoff as well as from the rivers.

Cutting across all of this is the Kennet and Avon Canal, which runs through Devizes, down the Caen Hill flight and on through Bradford-on-Avon. A raised, embanked canal above housing or a development site introduces a residual breach risk that a thorough flood risk assessment should consider where it is relevant.

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. National maps are built at a strategic scale and can miss what happens at a single plot, which is exactly why a clean map result still needs reading against the local detail (why flood maps are not always accurate).

Groundwater flooding on Wiltshire's chalk

Groundwater is the source most likely to be underestimated in Wiltshire. Much of the county sits on the chalk of Salisbury Plain and the Marlborough Downs, where the water table can swing several metres between a dry summer and a wet late winter. After prolonged rain the water rises up through the ground rather than spilling from a river, emerges in valley bottoms and seeps into basements and low-lying land, and it is slow to leave — it can persist for weeks or months after the rain has stopped.

The winter of 2025–26 was a sharp reminder. Sustained rainfall through January and February gave the Dorset and Wiltshire chalk catchments their worst groundwater flooding in over a decade, with levels in places as high as the 2013–14 floods, and the Environment Agency launched a dedicated Dorset and Wiltshire groundwater flood warning service in response. Groundwater monitoring now covers the Avon valley from Amesbury to the north of Salisbury and the Wylye valley.

• It appears on no national flood map. Groundwater is governed by geology, not by a nearby river, so a Flood Zone 1 result tells you nothing about it.
• Dry-season readings mislead. A borehole reading taken in summer can sit well below the true winter peak, so the timing of any monitoring matters.
• It breaks soakaway assumptions. A high or seasonally high water table can stop infiltration drainage working, which is why infiltration testing and groundwater separation are scrutinised closely here.

Groundwater is assessed within every flood risk assessment as one of the sources of flooding, and on complex chalk sites it can justify a season of monitoring or a standalone groundwater flood risk assessment. Our explainer on what groundwater flooding is and why it matters for planning sets out how we approach it.

A history of flooding in Wiltshire

Dated local events are part of the evidence base, and Wiltshire has plenty on record. They show the same pattern an assessment has to plan for: river flooding in the valley towns, surface water in the built-up catchments, and long, slow groundwater flooding on the chalk.

• Winter 2013–14. Prolonged rainfall flooded a wide area of the county; the villages of Aldbourne and Great Bedwyn were badly affected and the A360 near Tilshead was a particular concern, with groundwater keeping water levels high for weeks.
• Winter 2023–24. The wettest winter in the Avon catchment since records began in 1871 saturated the ground and pushed river levels up repeatedly across the county.
• November 2024. The Bristol Avon ran very high, peaking at the Great Somerford gauge and flooding low-lying property around Reybridge.
• Winter 2025–26. The chalk groundwater flooding described above — the worst in over a decade — with a dedicated Environment Agency warning service stood up for Dorset and Wiltshire.

The longer-term direction of travel is for sharper extremes at both ends, 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 defence still has to be addressed.

Planning and drainage policy for a flood risk assessment in Wiltshire

For most of the county, Wiltshire Council is the Lead Local Flood Authority, responsible for surface water, groundwater and ordinary watercourses and a statutory consultee on surface water drainage for major development. Swindon Borough Council is the separate Lead Local Flood Authority for the Swindon area. The Environment Agency remains responsible for the main rivers — the two Avons, the Wylye, the Nadder and the Kennet among them — and is the statutory consultee on development affecting them.

The evidence base a developer has to satisfy is published and specific. Wiltshire Council's Strategic Flood Risk Assessment sits in two levels — a Level 1 SFRA, updated in 2024, and a Level 2 SFRA published in February 2025 — and feeds the sequential test and the site allocations in the emerging Wiltshire Local Plan, alongside the Council's Local Flood Risk Management Strategy and its online SFRA data explorer.

• Check the SFRA before the layout is fixed. The Level 1 and Level 2 SFRA, the Local Plan and the validation list set out what Wiltshire Council expects, and a critical drainage area or mapped surface water risk can require an assessment even in Flood Zone 1.
• 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 Wiltshire?

Wiltshire's planning pipeline is concentrated on the larger towns — Trowbridge, Chippenham and Salisbury are the main growth locations — but applications come from across the county, from farm diversification and barn conversions on Salisbury Plain to riverside redevelopment and town-centre schemes. The trigger for an assessment is the same wherever the site is.

• Sites in Flood Zone 2 or 3. Any more vulnerable use, and most development, in the medium or high river flood zones 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.
• Surface water and groundwater flags. A mapped surface water risk, a location on the chalk, or a scheme relying on infiltration drainage can all require an assessment on an otherwise low-risk site.
• 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 across Wiltshire

We are experts on flood risk in Wiltshire 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 Wiltshire from Unda assesses every credible source — the rivers and their tributaries, surface water, groundwater, sewers 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.

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, and where mapping is not enough we provide detailed flood modelling to define flood levels and flow routes precisely.

Flood risk assessments for every situation in Wiltshire

People need a flood risk assessment or a drainage strategy in Wiltshire 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 — 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.
• 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 on chalk sites and flood modelling where national mapping is not enough — both feeding the wider assessment.

For drainage design you can also use our free geology and ground conditions checker, drainage calculators and BRE 365 infiltration rate calculator for a fast first pass before trial pits.

Wiltshire flood resources and contacts

The most reliable Wiltshire 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.

One example of the lead authorities at work is the Salisbury River Park scheme — a roughly £27 million Environment Agency and Wiltshire Council project that has reduced River Avon flood risk to more than 350 homes and businesses in central Salisbury while creating new riverside habitat and open space. A defended scheme 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 Wiltshire: frequently asked questions