Flood Risk Assessment in Salisbury

A flood risk assessment in Salisbury has to deal with water from two directions: rivers that meet in the middle of the city, and chalk groundwater that can rise beneath it for weeks after the rain has stopped. The Avon comes in from the north, the Nadder brings the Wylye in past Harnham, and the Bourne arrives from the east through Laverstock. We prepare flood risk assessments in Salisbury for planning applications, property purchases, mortgages and insurance, together with sequential and exception tests and surface water drainage strategies.

Salisbury here means the built-up area as the Office for National Statistics draws it: the city centre and the Cathedral Close, Fisherton, Churchfields and St Paul's, Stratford-sub-Castle and Bishopdown to the north, Laverstock and Petersfinger to the east, Harnham to the south, and Bemerton and Bemerton Heath to the west, in the SP1 and SP2 postcode districts. Old Sarum, Wilton and Britford lie just outside it. Wiltshire Council decides planning applications across the whole area and is its Lead Local Flood Authority.

The highest river levels ever recorded at Harnham Bridge and on the Bourne were both set on 7 January 2024, at the end of what the Environment Agency called the wettest winter in the Avon catchment since records began in 1871.

To talk through a Salisbury site with one of our flood risk consultants, call 01293 214444 or email enquiries@unda.co.uk.

Where the Avon, Nadder and Bourne meet in Salisbury

Three rivers converge on the city. The Avon, fed largely by chalk springs, runs south through St Paul's and the city centre to the Close; the Nadder, already joined by the Wylye near Quidhampton, passes Harnham and meets the Avon near the cathedral; the Bourne joins on the eastern edge after Ford and Laverstock. The Environment Agency issues flood warnings for named streets along all three, and the table below sets out what each source reaches.

The Ebble joins the Avon further downstream, outside the city. Within Salisbury, the risk changes over short distances: a street beside the Avon in the Friary and a cul-de-sac on the slopes of Bemerton Heath can sit a few hundred metres apart and face entirely different questions.

Screening a postcode from Stratford-sub-Castle to Harnham

The tool below is a flood risk map of Salisbury: type a postcode to screen it against Environment Agency data for rivers, surface water and reservoirs, or open our full flood risk map for anywhere in England. The coral line follows the ONS built-up area, so Old Sarum and Wilton fall outside it. Bear in mind that the planning flood zones show where water would go with no defences in place, which means land behind the River Park walls still shows as Flood Zone 2 or 3. Groundwater, the second half of the Salisbury picture, is not shown at all.

What the Salisbury River Park changed, and what it left in place

The River Park, a £27 million scheme led by the Environment Agency with Wiltshire Council, Salisbury City Council and the Swindon and Wiltshire Local Enterprise Partnership, was marked as nearing completion on 13 September 2024. According to the Environment Agency, it has reduced flood risk from the Avon to more than 350 properties in the city centre. The need for it came from 2019 flood modelling, which showed a growing number of central properties at risk. The Environment Agency's account of the works lists four main changes.

• Walls and embankments. New flood embankments and flood walls up to 1.4 m high run through Fisherton Recreation Ground and along Ashley Road.
• A second channel. A new channel and wetland now cut through Fisherton, with boardwalks and planting.
• Room in the car park. A green corridor 50 m wide through the Central Car Park is left free so floodwater can pass through it.
• Weirs instead of a gate. The old radial gate has gone, replaced by fish-friendly rock weirs.

The scheme covered Ashley Road Open Space, Fisherton Recreation Ground, the Maltings and the Central Car Park. The project's own summary says other areas at risk are not covered, and Wiltshire Council has said later phases will follow as funding allows. The Level 2 SFRA of February 2025 sets out the planning position. It says areas at risk of a breach, the Maltings and Central Car Park among them, are "still likely to be at risk", and that risk levels will not be certain until updated defended model outputs have been tested. A site behind the new walls therefore still needs an assessment that deals with residual risk behind defences as well as the protected case.

Chalk groundwater under the city and up the Bourne valley

Salisbury stands on chalk, and Wiltshire's Level 1 SFRA of November 2024 lists the city with Marlborough, Pewsey and Warminster among the places at risk of groundwater emergence. The aquifer reacts slowly, so groundwater flooding tends to follow a long wet spell rather than a single storm, and it can last for several weeks. Because the Avon is fed by that same groundwater, a high water table also makes the river respond faster to the next rainfall.

On 10 February 2026 the Environment Agency reported that Salisbury city centre was experiencing high groundwater levels and that it expected flooding, with the Till and the Bourne rising slowly and feeding a high Avon. By 27 February there were 34 groundwater flood warnings across Dorset and Wiltshire, and levels in the Bourne valley were still creeping up. The record at the Clarendon borehole east of the city is older still: 77.14 m on 20 February 2014.

On a site, groundwater matters most for basements and floor slabs, and for whether a soakaway will work at all. The Level 2 SFRA asks for groundwater monitoring through the winter months, November to March, before infiltration drainage is relied on. Begin that monitoring in April and the results arrive the following spring. Our guides cover groundwater flooding and planning and groundwater monitoring timescales.

Flood years on Salisbury's rivers and streets

Salisbury's flood record is mostly one of wet winters, when rivers and groundwater rise together, with the odd summer downpour that overwhelms the drains. These are the dates the public record supports.

1. 1915. Photographs of the flooded cathedral, Fisherton Street and Water Lane appear in the River Park project's account of the city's flood history.
2. Winter 2013/14. Over 500 properties flooded across Wiltshire between December 2013 and March 2014. In Salisbury, 20 properties flooded internally in 2014, the most recent significant river flood. The Ashley Road gauge reached its highest level, 1.36 m, on 6 January 2014.
3. 25 August 2015. Heavy rain overwhelmed the highway drains under the Castle Street railway bridge, a spot the council's Section 19 report describes as a known problem area over 20 years.
4. 2019. New flood modelling showed more central properties at risk, which led to the River Park.
5. 7 January 2024. Record levels at Harnham Bridge, 1.43 m, and on the Bourne, 1.83 m, after the wettest winter in the Avon catchment since 1871.
6. September 2024. Phase 1 of the River Park was marked as nearing completion.
7. January to March 2026. Groundwater flooding across the chalk of Wiltshire and Dorset, with high groundwater in the city centre and the Bourne valley.

The Ashley Road gauge shows property flooding as possible above 1.11 m, a level it passed in 2014. Levels on the Avon, the Bourne and the Nadder at Wilton can be followed on our live river levels map, and the Environment Agency's warnings for Salisbury are free to sign up for.

Surface water, the A36 underpasses and sewer capacity

When the council's 2019 Level 1 SFRA ranked Wiltshire's settlements by surface water risk, Salisbury came first, with 1,476 properties at risk. The Surface Water Management Plan of November 2011, which covered Salisbury alongside Chippenham and Trowbridge, picked out hotspots in the city where surface water flooding is more likely. Neither replaces a look at the flow routes across a particular site.

The A36 passes through the city in a series of underpasses, at the Cattle Market, St Marks roundabout, the Avon river bridge and Castle Hill roundabout, each drained by pumps that National Highways maintains. The city's flood plan notes that these pumps are not monitored, so flooding in an underpass has to be reported to National Highways directly. Below ground, Wessex Water runs the sewers. The Core Strategy says that development upstream at Old Sarum, Hampton Park and Longhedge needs a new relief sewer "to ensure that risk from sewer flooding is resolved", along with more capacity at the Petersfinger sewage works.

Building at Churchfields, the Maltings and Southampton Road

The biggest regeneration sites in Salisbury sit on the valley floor, where river, groundwater and surface water risks overlap. The adopted Core Strategy names them, and the Level 2 SFRA and the site templates say what each must deal with.

Small schemes meet the same constraints. A change of use above a shop on Fisherton Street sits inside the city-centre warning area, and a new home on a garden plot in Laverstock may sit near the Bourne's floodplain. Both need the flood sources checked before the drawings are fixed.

Core Policy 67 and the Wiltshire SFRA evidence

Wiltshire Council is both the planning authority and the Lead Local Flood Authority for Salisbury, and the Environment Agency advises it on the main rivers. The local plan in force is the Wiltshire Core Strategy, adopted in January 2015. The council submitted a Local Plan review in November 2024 but withdrew it from examination on 19 May 2026, after the inspectors offered a choice between withdrawal and a report recommending non-adoption. Its draft policies carry no weight; the council's Local Plan library records the withdrawal, and a new plan has to begin by 31 October 2026.

• Core Policy 67, sequential test. Development proposed in Flood Zones 2 and 3 must use the council's housing land availability evidence when it applies the sequential test.
• Core Policy 67, drainage. All new development must reduce the rate of runoff and improve infiltration unless site or environmental conditions make that unsuitable.
• Level 1 SFRA. The November 2024 Level 1 SFRA, which replaced the 2019 edition, maps every source of flooding across the county and sets out a method for the sequential test.
• Level 2 SFRA. The February 2025 Level 2 SFRA tested eight draft allocations, with the Maltings and Central Car Park the only one in Salisbury, and sets out how sites can pass the Exception Test.
• Climate change. For the Avon catchment, the Level 2 SFRA applies peak river flow allowances of 38% (central) and 56% (higher central) for the 2080s.
• Floor levels and access. Finished floor levels 600 mm above the 1% annual probability flood level with climate change, and an 8 m easement either side of a main river, measured from the top of the bank.

For drainage, the council's planning information for developers includes a guide to householder and minor extension assessments, a note on surface water soakaways and a SuDS technical checklist. National policy is chapter 18 of the NPPF published in August 2026: F4 sets when an assessment is needed, F5 and F6 cover the sequential and exception tests, and F8 expects every development to follow the 2025 National Standards for sustainable drainage. Our explainers on how the NPPF treats flood risk and what an SFRA is for give the background.

Is a flood risk assessment in Salisbury needed for your scheme?

Under NPPF policy F4, a site in Flood Zone 2 or 3 needs an assessment whatever its size, and so does a site of 1 ha or more in Flood Zone 1. In Salisbury, groundwater and surface water often add to the case. The table shows how typical schemes in the city fall.

Housing on an unallocated site in Flood Zone 2 or 3 also has to pass the sequential test, with the area of search set for the scheme. Our guide to the Flood Map for Planning explains how the zones are drawn.

What our consultants prepare for Salisbury's rivers, chalk and city sites

A buyer in the Cathedral Close, a developer at Churchfields and a business on Southampton Road each need a different report, and the groundwater question can apply to all three. Our consultants are chartered, and a senior consultant signs off every report before it is issued. Unda is a CIWEM Business Partner.

• Applications near the rivers. A scheme beside the Avon, the Nadder or the Bourne needs a flood risk assessment for planning that answers Core Policy 67 and the Wiltshire SFRA.
• Rivers, chalk and drains together. A flood risk assessment weighs river flooding against the groundwater, surface water and sewer risks that are just as real in Salisbury.
• Buying in the Close or Harnham. A flood risk survey for property purchase looks at one building's floor levels and its exposure to the river and groundwater before exchange.
• Homes in the flood zones. New housing on the valley floor, from Churchfields to the Friary, usually needs a sequential and exception test.
• Runoff from every site. Core Policy 67 asks all new development to cut runoff, so even small Salisbury schemes need drainage strategy evidence of some kind.
• Soakaways or a river outfall. A surface water drainage strategy tests whether the chalk will take water after winter monitoring, or whether runoff has to be held back and released to a watercourse.
• Conditions on the permission. Where Wiltshire Council attaches a drainage condition, discharge of drainage conditions supplies the detail and maintenance plan the council needs.
• Rising water from below. Basements and deep foundations on the valley floor can need a groundwater flood risk assessment that draws on borehole records such as Clarendon's.
• Levels behind the new walls. The Level 2 SFRA says defended model outputs have yet to be tested, so a city-centre scheme may need site-specific flood modelling to set its floor levels.
• Getting out in a flood. Sites with access cut by flooding, as the Level 2 SFRA found at the Maltings, can need a flood warning and evacuation plan built around the city's warning areas.
• Cover on streets that flooded. Homeowners on streets that flooded in 2014, or that sit inside a warning area, can use a flood risk assessment for insurance to show an insurer the risk to their property.
• Shops and offices in the centre. The retail-led plans for the Maltings and the employment land at Churchfields can call for a BREEAM flood risk assessment for the Pol 03 credits.
• Buying on Southampton Road. Industrial and retail property on the Southampton Road estates and Dolphin Park lies inside the Bourne and Avon warning areas, which due diligence on a purchase or loan should measure.
• Keeping trading through a flood. Businesses in the Friary, on Fisherton Street or at Dolphin Park can plan for warnings and lost access with a business continuity flood risk assessment.
• Sewer capacity to the north. With a relief sewer and more capacity at Petersfinger needed for the northern sites, a foul drainage strategy shows how a scheme connects without adding to sewer flooding.

Floodline, Wessex Water and the flood wardens: who to contact in Salisbury

If floodwater threatens anyone in Salisbury, call 999; for a power cut, call 105. For everything else, the organisations below each look after a different part of the city's flood risk.

Salisbury sits within our Wiltshire coverage. Upstream on the Wylye, Warminster faces a similar mix of river and chalk groundwater. Downstream, the Avon flows on through Fordingbridge and Ringwood in Hampshire to reach the sea at Christchurch. Other places we cover are listed on our areas page.

Before you commission a flood risk assessment in Salisbury: common questions

Unda Consulting Limited works on sites across Salisbury, from city-centre conversions to sites on the edge of the city, preparing flood risk assessments, sequential and exception tests and drainage strategies for homes, businesses and development land. To discuss a site in Salisbury, Harnham, Bemerton or Laverstock, call 01293 214444 or email enquiries@unda.co.uk.