Flood Risk Assessment in Sherborne

Sherborne's old town sits on a ridge of Jurassic limestone above the Yeo valley, but it is not the river that has been catching this Dorset town out. In October 2023, a downpour sent surface water pouring down Ottery Lane and into the Old Yarn Mill Business Centre on Cheap Street, the third flood in eighteen months for one business there, against one in the previous twenty-five years. A flood risk assessment in Sherborne has to explain that shift: why a hilltop market town is flooding more often, from a source the standard river maps barely register. Unda prepares flood risk assessments in Sherborne for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies (SuDS), working from Environment Agency data, Dorset Council's local flood evidence and the site conditions that decide what actually happens on the ground.

Contact one of our experienced flood risk consultants on 01293 214444 or email enquiries@unda.co.uk to discuss a flood risk assessment, sequential test or drainage strategy for a site in Sherborne.

A flood-level marker inside Sherborne Abbey still records a flood from May 1709, proof that this town's relationship with water long predates its modern surface-water problem, and a reminder that a site's flood history rarely starts with the last named storm.

• Dominant risk: surface water. Recent flooding in Sherborne, including the October 2023 Old Yarn Mill flood, has been flash surface-water flooding rather than the River Yeo overtopping.
• River Yeo corridor. Flood Zones 2 and 3 follow the Yeo through the town, with Ludborne Road and South Street named by the Environment Agency as areas where property flooding becomes possible.
• Groundwater-prone geology. Sherborne sits on Inferior Oolite limestone over Yeovil Sands, permeable Jurassic rock that promotes springs and recharge, relevant wherever a scheme relies on soakaways.
• Lead Local Flood Authority: Dorset Council. Dorset Council is both the local planning authority and the LLFA for Sherborne, following the 2019 unitary reorganisation.
• Live development pressure. A large urban extension on Sherborne Castle Estate land is being promoted through the emerging Dorset Local Plan, alongside allocations already adopted for Barton Farm and the former Gasworks site.
• Heritage constraints. Sherborne's conservation areas, Sherborne Abbey and the Grade I registered Sherborne Castle grounds sit close to the Yeo floodplain, adding a layer of design sensitivity to any flood mitigation.

Local flood risk and hydrology in Sherborne

Sherborne's flood risk comes from three distinct directions, and a proportionate flood risk assessment in Sherborne needs to weigh all three rather than defaulting to whichever has the most familiar flood map. The River Yeo is the obvious candidate: it rises to the west of the town and flows east through Sherborne Lake, in the grounds of Sherborne Castle, before continuing towards Yeovil. The Environment Agency's flood warning area for this stretch runs from New Road in Sherborne down to Yeovil, and names Ludborne Road and South Street as the locations where river flooding starts to affect property. The monitoring gauge at Sherborne Lake shows how narrow the margin is: the normal range is 0.02m to 0.20m, but the station's highest level on record (0.39m, reached on 8 July 2012) sits only just below the 0.43m threshold at which property flooding at Ludborne Road and South Street becomes possible.

The second, quieter watercourse is the Coombe, a small stream that historically ran along the western edge of Sherborne Abbey and Sherborne School, once powering a watermill, before its catchment was substantially built over and culverted as the town grew. A separate, larger catchment above the town drains into Sherborne Lake and is monitored by the Environment Agency before it joins the Yeo. Between them, the Yeo and Coombe systems mean that watercourse behaviour changes markedly across the town, and a site a few hundred metres from the river can still carry a real flood pathway through an old, buried stream line.

The third and, on current evidence, the most active source is surface water. The October 2023 flooding at the Old Yarn Mill Business Centre and around Cheap Street and Horsecastles was pluvial: rain overwhelmed drainage and ran downhill through the town rather than a river breaking its banks. It isn't an isolated incident either. Dorset's own former West Dorset Local Plan records that delivery of the Barton Farm housing allocation has been complicated specifically by the need to manage surface-water run-off. Cold Harbour and Bristol Road are a further recognised local pinch-point where heavy rain can send surface water down the hill quickly. Groundwater sits underneath all of this: Sherborne's Inferior Oolite and Yeovil Sands geology is permeable enough to support numerous local springs, which matters directly for any development proposing soakaways rather than piped discharge, even where the site shows no river or surface-water flag at all.

Historical flooding in Sherborne

Sherborne's flood record stretches back further than most Dorset market towns can point to. A prolonged, very heavy hailstorm in May 1709 blocked the Coombe stream where it ran beside Sherborne Abbey and flooded the building itself; a flood-level marker recording the event is still visible inside the Abbey today. Significant flooding in the town in 1979 prompted the construction of a flood defence scheme close to Sherborne railway station, which remains part of the town's protection. More recently, the pattern has shifted towards shorter, sharper events: the River Yeo's highest recorded level at the Sherborne Lake gauge came during the wet summer of July 2012, and winter 2013/14 brought the wider Wessex floods that left more than 1,000 properties across Dorset and Wiltshire with groundwater flooding, a regional event rather than one confined to Sherborne itself. The clearest recent signal, though, is the run of surface-water incidents around Cheap Street and Ottery Lane, culminating in the October 2023 flood at the Old Yarn Mill Business Centre.

Check the flood risk at your Sherborne postcode

The interactive map below draws on live Environment Agency data. Enter any postcode in or around Sherborne to see the modelled flood risk from rivers and sea, surface water and reservoirs, the flood zone designation, and any active flood warnings for that location. It is a screening tool, not a substitute for a site-specific flood risk assessment, but it is a fast way to see whether a site needs one.

Planning and drainage policy in Sherborne

Any development in Sherborne within Flood Zone 2 or 3, or covering a hectare or more anywhere in the town, needs a site-specific flood risk assessment before Dorset Council can determine the planning application, in line with the national flood risk checking service and the Flood and Water Management Act 2010 duties Dorset Council holds. Dorset Council is both the local planning authority and the Lead Local Flood Authority (LLFA) for Sherborne, a role it has held since the 2019 reorganisation that replaced the former district and county councils with a single unitary authority. Sherborne itself is covered by the adopted West Dorset, Weymouth and Portland Local Plan (2015), whose Chapter 16 sets out named site policies for the town: Barton Farm (a mixed-use urban extension whose delivery the plan itself flags as complicated by surface-water run-off), the Newland Car Park town-centre expansion sites, the Sherborne Hotel gateway site, and the former Gasworks site on Gas House Hill, whose developable land is explicitly restricted to ground outside the floodplain even though its access crosses it.

Dorset Council's Level 1 Strategic Flood Risk Assessment (February 2023) sets the county-wide evidence base that a flood risk assessment in Sherborne has to sit alongside, mapping flood zones and surface-water risk and informing where sequential and exception test requirements apply. A single Dorset-wide Local Plan is now in preparation to replace the legacy district plans, targeted for adoption in 2027; its draft options consultation already includes a proposed large-scale urban extension on Sherborne Castle Estate land to the west of the town, which will bring fresh flood and drainage evidence requirements as it progresses. Until that plan is adopted, the 2015 West Dorset plan remains the operative policy for Sherborne.

• Flood Zone 2/3, any size. A site-specific flood risk assessment is required, addressing river, surface-water and groundwater sources together.
• Flood Zone 1, 1 hectare or more. Still requires a site-specific FRA, regardless of how low the modelled river risk appears.
• Surface-water drainage strategy. Triggered by 10 or more dwellings, sites of 1 hectare or more, or smaller sites in a recognised surface-water risk area such as those around Ottery Lane and Cold Harbour.
• Sequential Test. Applies to more vulnerable uses proposed in Flood Zone 2 or 3, requiring the applicant to show no reasonably available lower-risk alternative site exists.
• West Dorset, Weymouth and Portland Local Plan, Chapter 16. The operative site allocation policies for Sherborne (SHER 1–4) until the new Dorset-wide plan is adopted.

Development context: what's being built in Sherborne

Sherborne's development pipeline sits close to some of its most sensitive ground, which is exactly why flood risk keeps surfacing in the town's planning history. The historic core carries multiple designated conservation areas, the earliest dating from 1970, and Sherborne Abbey, Sherborne Old Castle and the Grade I registered park and garden at Sherborne (New) Castle all sit near the Yeo floodplain, a heritage backdrop that adds design constraints to any flood mitigation a scheme proposes. The adopted Barton Farm allocation, a mixed-use urban extension for several hundred homes plus employment land, has already been slowed by the surface-water run-off issue the Local Plan itself identifies; the former Gasworks site on Gas House Hill is restricted to development outside the floodplain, with its access crossing land that partly sits within it. Beyond these adopted sites, the most significant live story is the proposed large-scale extension on Sherborne Castle Estate land to the west of the town, currently being promoted through the emerging Dorset Local Plan as part of a longer-term masterplan for the area, with access proposed from the A30.

• Historic core, tight land supply. Conservation area designations and heritage assets constrain where new development and any associated flood mitigation can go.
• Barton Farm urban extension. Adopted allocation whose delivery the Local Plan itself ties to resolving surface-water run-off.
• Former Gasworks, Gas House Hill. Housing and employment site restricted to land outside the floodplain; access crosses flood-affected ground.
• West Sherborne. A proposed urban extension on Sherborne Castle Estate land, being promoted through the emerging county-wide Dorset Local Plan.
• Town-centre expansion. Newland Car Park and Sherborne Hotel sites bring retail and gateway development close to the historic core.

How Unda helps with flood risk assessment in Sherborne

Unda has prepared flood risk assessments across Dorset, working with Dorset Council as the local planning authority and the Environment Agency as statutory consultee, alongside the Dorset Strategic Flood Risk Assessment as the policy framework a Sherborne site sits within. A flood risk assessment in Sherborne needs to hold three sources in view at once: the River Yeo corridor, the buried and partly culverted Coombe, and the surface-water pathways that have caused the town's most recent flooding. It also needs to set that evidence against the specific Local Plan policy a site falls under. We prepare assessments that address the source of risk that actually applies to a site, rather than defaulting to whichever flood map is easiest to pull.

Flood risk assessments for every situation in Sherborne

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 prepare flood risk assessments in Sherborne for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies.

• Planning flood risk assessments. Site-specific FRAs for residential, commercial and mixed-use applications in Sherborne, prepared to satisfy Dorset Council and the Environment Agency.
• Property-purchase flood risk assessments. An independent, site-specific assessment for buyers and lenders where a search or valuation has flagged risk, going beyond the national screening data.
• Insurance flood risk assessments. Technical evidence to support a broker renegotiation where premiums have risen or cover has been restricted.
• BREEAM flood risk assessments. Structured around BREEAM's Pol 03 flood and surface water management credit, for commercial and public-sector schemes in Sherborne.
• Sequential and exception tests. Standalone reports demonstrating that a proposed site in Flood Zone 2 or 3 is the most appropriate available location.
• Surface-water drainage strategies (SuDS). Calculated designs to manage runoff for the sites around Sherborne where surface water, not the river, is the live constraint, through to discharge of drainage conditions and preliminary drainage design.
• Groundwater assessment. A dedicated look at the Inferior Oolite and Yeovil Sands geology beneath Sherborne, relevant wherever infiltration drainage is proposed.

Free screening tools can give a quick first read before a full assessment is commissioned: the Flood Risk Map, the Live River Levels Map for checking the Yeo in real time, the Geology and Ground Conditions map for a first read on soakaway feasibility, the Drainage Calculators, and the BRE 365 Infiltration Rate Calculator for sites considering infiltration-based SuDS.

Local flood risk management and emergency contacts

Sherborne sits within Dorset Council's area for local flood risk purposes and the Environment Agency's Wessex area for main-river and coastal risk. Naming the right body correctly in a flood risk assessment matters: a known past error on other pages has been a live gauge link pointed at the wrong county, so the table below uses the correct Sherborne-specific sources throughout.

Frequently asked questions: flood risk assessment in Sherborne

Unda's flood risk consultants cover Sherborne and the wider Dorset area, including neighbouring towns such as Bridport on the Brit and Asker, and Blandford Forum on the River Stour, as well as the Dorset and Somerset areas either side of Sherborne's county boundary, and the full range of areas across England and Wales we work in. For background on the wider issues that shape a flood risk assessment in Sherborne, see our articles on fluvial flooding, surface-water flooding, groundwater flooding, groundwater flooding in Dorset and Wiltshire, main rivers and ordinary watercourses, riparian ownership, climate change allowances, residual flood risk, what the sequential and exception tests involve, the Yatton judgment and the planning balance, the four pillars of SuDS, BRE365 infiltration testing, what a flood risk assessment costs, whether flood risk affects house prices, flood history declarations when selling a property, why the UK can flood during a drought, and flooding on the neighbouring Somerset Levels.

To discuss a flood risk assessment in Sherborne, call 01293 214444, email enquiries@unda.co.uk, or use the Start a Quote form.