Flood Risk Assessment in Salisbury

Four rivers meet in Salisbury — the Hampshire Avon, the River Wylye, the River Nadder and the River Bourne — and their convergence in the city centre has shaped the flood story here for centuries. Combine that with a chalk aquifer beneath Salisbury Plain that pushes groundwater up in wet winters, and you have a city where flood risk touches a very wide range of development sites and property types. Whether you are progressing a planning application, buying a home or commercial building, arranging insurance, or designing a new drainage scheme, a site-specific assessment is often a condition of moving forward.

Our flood risk consultants have extensive experience of flood risk assessment in Salisbury and across Wiltshire, including the policy and modelling context that Wiltshire Council and the Environment Agency apply. We prepare flood risk assessments in Salisbury for planning, property purchase, mortgage and insurance, and BREEAM — alongside sequential and exception tests and surface water drainage strategies. Call us on 01293 214444 or email enquiries@unda.co.uk to discuss your site.

Salisbury's position at the confluence of four chalk-fed rivers makes it one of the most flood-sensitive cities in the south of England, with around 2,000 properties at risk from a 1-in-100-year river flood — a number that rises significantly once climate change projections are applied.

Salisbury's flood picture: four rivers, chalk and a transformed city centre

Few English cities carry quite the same combination of flood pressures as Salisbury. The Hampshire Avon — here fed by three tributaries within the city itself — drains a large chalk catchment to the north and west before passing through the historic core. The result is a flood-risk profile that ranges from fast-rising fluvial events to slow-onset groundwater flooding that can persist for weeks, and surface-water runoff that overtops gullies and flows through older streets. Understanding which source is relevant to your site, and how those sources interact, is the starting point for any flood risk assessment in Salisbury. Use the EA Flood Map for Planning for an initial flood-zone check, or check your flood risk on GOV.UK.

The Hampshire Avon, Wylye, Nadder and Bourne

The Hampshire Avon is the primary source of river flooding in Salisbury. It passes through the city from north to south, carrying water from a catchment that stretches up to the Vale of Pewsey and beyond. The Wylye joins from the west near Bemerton, the Nadder flows in from Wilton, and the Bourne joins at Stratford-sub-Castle to the north of the cathedral close — all within a short reach of one another. When a significant rainfall event sits over the chalk uplands, all four can rise quickly and together.

• Properties at risk. Approximately 2,000 properties in the Salisbury area face river flood risk from a 1-in-100-year (1% annual probability) event. Of those, around 250 residential and 100 commercial properties are in the city centre.
• Climate change uplift. Climate change projections increase this to approximately 370 residential and 200 commercial properties at risk in the city centre — a significant uplift that planning policy requires all new development to account for over its lifetime.
• Last major event. The most recent significant river flooding in Salisbury was in 2014, when 20 properties suffered internal flooding. That event informed the subsequent Salisbury River Park flood-defence programme.
• Vulnerable locations. Areas historically most affected by river flooding include Southampton Road Industrial Estate, Dolphin Park Industrial Estate, the Muttons Bridge area and the lower-lying parts of Harnham. The Environment Agency Flood Map for Planning defines Flood Zone 2 and 3 extents across these areas.
• Residual risk behind defences. The Salisbury River Park scheme (see below) has improved protection for the city centre, but residual risk behind any engineered defence must still be assessed and addressed in a flood risk assessment — national planning policy requires it.

Groundwater from the Salisbury Plain chalk

The chalk of Salisbury Plain is a major aquifer. Rainwater falling on the downland percolates slowly through the rock and reappears months later as base-flow in the rivers and — crucially — as rising groundwater in valley floors and on lower-lying development sites. Unlike river flooding, groundwater rises from below, leaves no obvious warning signal and can persist long after the last rain has fallen. It also renders soakaways useless when the water table is high, which has direct consequences for surface water drainage design.

• Bourne Valley warning zone. The Environment Agency operates a dedicated groundwater flood warning zone in the Bourne Valley to the north of Salisbury, reflecting the documented risk of groundwater emergence in that chalk-floored valley.
• Winter 2025–26 events. The winter of 2025–26 produced the worst chalk groundwater flooding in Wiltshire and Dorset for over a decade. Storm Chandra (26–27 January 2026) triggered a surge in groundwater levels, and the EA launched a dedicated Dorset and Wiltshire groundwater flood service in response. Around 100 properties near Salisbury Cathedral were affected by groundwater in early 2026.
• Planning implications. Groundwater is a source of flooding that a flood risk assessment in Salisbury must assess explicitly. It sits within the FRA, usually as a dedicated groundwater section, and informs finished floor level, drainage design and the compatibility of SuDS elements with seasonal water table behaviour.
• Geology matters. Our geology and ground conditions tool maps chalk and superficial deposits across Wiltshire — useful for early screening before a site investigation.

Surface water in Salisbury

Wiltshire Council prepared a Surface Water Management Plan specifically for Salisbury, mapping the overland flow routes, pinch points and areas of accumulation that affect the city's older urban fabric. In the more intensively developed parts of Salisbury — particularly where Victorian-era culverts limit capacity and impermeable surfaces intercept all rainfall — surface water flooding can occur independently of river conditions. Permitted development and changes of use that increase hard-standing can tip marginal situations into problems; a drainage strategy that quantifies pre- and post-development runoff is often required even where river flood risk is low. Check your site with our Flood Risk Map and Live River Levels Map for an initial indication before instructing a full assessment.

What changed in 2024: the Salisbury River Park

The Salisbury River Park is one of the most significant flood-defence and environmental improvement schemes in the south of England in recent years. Jointly funded by the Environment Agency and Wiltshire Council, the £27 million project transformed the flood plain running through the heart of the city. Phase 1 was completed in spring and summer 2024.

• Properties protected. The scheme protects approximately 250 homes, 100 businesses and one school — around 350 properties in total — from river flooding events that previously caused serious damage.
• Engineering works. The Avon was widened to increase conveyance, a new flood-relief channel was cut, rock weirs were installed to manage flow velocities, and a flood wall was constructed at key locations to prevent overtopping into adjacent properties.
• Habitat and open space. Alongside the flood-defence elements, the scheme created 13 hectares of open space, 2 hectares of new habitat and planted approximately 600 trees — making it a combined flood-management and environmental enhancement project with lasting public value.
• Changed flood extents. Phase 1 altered the modelled flood extents for some sites in central Salisbury. If you are working from an older flood-risk report or an older version of the EA flood maps, it is important to verify that the assessment reflects the post-scheme position. A flood risk assessment in Salisbury prepared today must account for the current defence standard and any residual risk.

What a flood risk assessment in Salisbury has to cover

The scope of a flood risk assessment in Salisbury depends on the site, the development type and which sources are present. Most sites in and around the city require all material flood sources to be considered. The table below sets out the main sources, what typically triggers coverage, and what the assessment addresses for each.

For a first-pass view of flood risk on your site, our Flood Risk Map and drainage calculators are free to use. For a formal assessment, contact one of our experienced flood risk consultants on 01293 214444 or enquiries@unda.co.uk.

Flood risk assessments for every situation in Salisbury

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 for a Salisbury site, but each answers a different question: is the site safe, should the development be here at all, and where does the surface water go? We prepare all three, as well as flood risk assessments for property purchase, insurance and BREEAM — each shaped to its own purpose and audience.

Planning flood risk assessment in Salisbury

The most common reason for a planning flood risk assessment in Salisbury is that the site falls in Flood Zone 2 or 3, which the EA Flood Map for Planning defines across the low-lying areas near the Avon, Wylye, Nadder and Bourne. Sites of 1 hectare or more anywhere in the city can also trigger the requirement, as can sites identified as being at risk from surface water flooding or where a more vulnerable land use — such as housing on a former commercial site — is proposed.

A planning FRA in Salisbury demonstrates to Wiltshire Council and the Environment Agency that the proposed development will be safe for its lifetime, that it does not increase flood risk elsewhere, and that it incorporates appropriate resilience and resistance measures. Where the River Park scheme has changed the local flood picture, the FRA must reflect the current modelled extents and defence standard. Where groundwater is relevant, the report addresses that as an integrated section rather than a separate document.

Flood risk assessment for property purchase and mortgage in Salisbury

Environmental searches in Salisbury regularly flag flood risk — which is not surprising given the density of Flood Zone land in and around the city. A property purchase flood risk assessment goes beyond the broad-brush national datasets used in conveyancing reports to assess the actual building: its threshold levels, ground levels relative to flood extents, flow paths around the property, and the implications of groundwater for basements, extensions and drainage. This is used by buyers, lenders and solicitors to understand the actual risk before proceeding, or to provide evidence to a mortgage provider that has raised a query.

Insurance flood risk assessment in Salisbury

Salisbury's flood history — and its presence in national datasets as a moderate-to-high-risk area — means that some property owners and occupiers face elevated premiums, restricted cover or renewal difficulties. An insurance flood risk assessment provides underwriters and brokers with a site-specific, technically evidenced view of risk, taking into account the River Park defences, building threshold levels and any on-site mitigation. This can support premium negotiation, placement with specialist markets, or a Flood Re application.

BREEAM flood risk assessment in Salisbury

Commercial and public-sector developments in Salisbury that are seeking BREEAM certification require a BREEAM flood risk assessment structured around Pol 03 — a credit-driven framework that operates differently from the planning assessment. BREEAM is primarily concerned with the pre-development risk classification of the site; a planning FRA that demonstrates post-mitigation safety will not automatically satisfy the BREEAM credit criteria. We prepare BREEAM FRAs as a distinct report, coordinated where necessary with the wider planning and drainage submission.

Sequential and exception tests in Salisbury

The Sequential Test applies the NPPF's risk-based location principle: development should be directed to the lowest-probability flood area available. For Salisbury sites in Flood Zones 2 or 3, the applicant must demonstrate that there is no reasonably available alternative site in a lower-risk area within the relevant area of search. The September 2025 update to the Planning Practice Guidance extended the Sequential Test to apply to surface water and groundwater flood risk, not only fluvial risk — a change that has increased the number of Salisbury sites where a test is required.

Where the Sequential Test cannot be passed but development still needs to proceed in a higher-risk area, the Exception Test applies: wider sustainability benefits must be demonstrated, and the site must be made safe without increasing risk elsewhere. Wiltshire Council's Level 2 SFRA (February 2025) provides the policy framework and modelling data that supports both tests for sites in the Salisbury area.

Drainage strategies and SuDS in Salisbury

A drainage strategy and SuDS design is a separate report from a flood risk assessment — it addresses where the surface water goes, not whether the site is at risk from external flooding. The two often go together for Salisbury planning applications, but the drainage strategy has its own technical requirements, its own approving authority and its own design standard.

Wiltshire Council is the Lead Local Flood Authority (LLFA) for Salisbury and must approve the drainage scheme for all major developments. The sustainable drainage strategy follows the SuDS hierarchy — store and reuse, infiltrate, discharge to watercourse, connect to sewer — and must accommodate the 1-in-100-year storm event with a climate change allowance, without increasing runoff to adjacent land or the highway.

• 2025 National SuDS Standards. Mandatory SuDS are now required for new residential development of 10 or more dwellings, residential sites of 0.5 hectares or more, and non-residential sites of 1 hectare or more. For Salisbury, the 2025 National Standards represent the minimum acceptable standard, and Wiltshire Council's local drainage guidance sets additional expectations.
• Chalk and infiltration. The chalk that underlies much of the Salisbury area can — depending on depth to the water table — either facilitate or prevent infiltration drainage. On shallow-chalk sites where the seasonal water table is close to the surface (particularly in the Bourne Valley and some parts of the Avon flood plain), soakaways are not viable and drainage must route to a watercourse. A permeability assessment is required before drainage can be designed.
• Watercourse constraints. Discharge to the Avon, Wylye, Nadder or Bourne and their tributaries is subject to Land Drainage Act consents from Wiltshire Council (ordinary watercourses) and the Environment Agency (main rivers). Discharge rates must be calculated using MicroDrainage software, and greenfield runoff rates are the standard baseline.
• Pre-application advice. Wiltshire Council's LLFA team offers pre-application drainage advice for larger sites. We work alongside this process, using our drainage calculators and preliminary design to make the pre-application submission as efficient as possible.

What Wiltshire Council and the Environment Agency look for

Wiltshire Council is the decision-making authority for planning applications in Salisbury and the LLFA for drainage approvals. The Environment Agency is a statutory consultee for development in Flood Zones 2 and 3, for sites near main rivers and for development involving significant changes to drainage. Understanding what each organisation needs to see in a flood risk assessment in Salisbury significantly affects how a submission is structured.

• Wiltshire Level 1 SFRA (November 2024). The Wiltshire Level 1 Strategic Flood Risk Assessment provides the strategic overview of flood risk across Wiltshire, including Salisbury. It identifies the broad flood risk areas, the key watercourses and the policy context that applies to each. It is the starting point for any Sequential Test in the area.
• Wiltshire Level 2 SFRA (February 2025). The Level 2 SFRA focuses on Salisbury and provides detailed flood modelling, vulnerability mapping and specific policy guidance for the city. It refines the flood extents around the Rivers Avon, Wylye, Nadder and Bourne, and provides the data that site-specific FRAs and Sequential Tests must engage with.
• Sequential Test policy change (September 2025 PPG). The September 2025 update to the Planning Practice Guidance means the Sequential Test now applies to surface water and groundwater flood risk as well as fluvial and tidal risk. Salisbury sites that might previously have been assessed only against river flood zones must now also demonstrate that surface-water and groundwater risk has been considered in the locational decision.
• Environment Agency as statutory consultee. The EA will be consulted by Wiltshire Council on any application in Flood Zone 2 or 3 and on any development near a main river. A well-prepared flood risk assessment that engages directly with the EA Standing Advice and any site-specific model data generally results in faster consultation responses.
• Local Resilience Forum and emergency planning. Salisbury falls within the Wiltshire and Swindon Local Resilience Forum area. For flood emergency preparedness, the EA's flood warning sign-up service and Floodline (0345 988 1188) provide live flood alerts for Salisbury's main river warning areas. In a flood emergency, call 999 (emergency) or 105 (utility network damage).

The publication of the Wiltshire Level 2 SFRA in February 2025 and the September 2025 PPG update together represent the most significant change to the Salisbury planning-flood-risk framework in years — sites that were assessed before these documents were published should be reviewed against the current policy position before submitting.

Check flood risk anywhere in Salisbury

The interactive map below draws live data from the Environment Agency's flood datasets — rivers and sea, surface water, flood zones, climate-change projections to 2125, flood defences and live flood warnings. Enter any Salisbury postcode (or any postcode in Wiltshire) to see an indicative flood risk screening for that location. This is a free indicative tool; it is not a flood risk assessment and should not be relied on for planning, insurance, conveyancing or design decisions.

Frequently asked questions about flood risk assessments in Salisbury

Common questions we hear from developers, homeowners and agents working on sites in Salisbury. If your question isn't here, contact one of our experienced flood risk consultants on 01293 214444.

We prepare flood risk assessments across Wiltshire. See our pages for flood risk assessment in Wiltshire, flood risk assessment in Swindon and flood risk assessment in Malmesbury.