Flood Risk Assessment in Gloucestershire

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

Gloucestershire is shaped by the River Severn, the longest river in Britain, and by the River Avon, which joins it at Tewkesbury. That meeting point has flooded the county more dramatically than anywhere else. We assess every source that matters here: the Severn and Avon, the smaller rivers that run through the towns (the Chelt through Cheltenham, the Twyver through Gloucester, the Frome through Stroud, the Wye on the western edge), the surface water that overwhelms urban drains, and the groundwater that rises through the Cotswold limestone. We then deal directly with the Environment Agency and Gloucestershire County Council, as Lead Local Flood Authority, 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, 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.

Flood risk assessment in Gloucestershire at a glance

• The Severn and the Avon meet at Tewkesbury. Two of Britain's largest rivers converge in the north of the county, so a wet spell across either catchment becomes a prolonged flood at the confluence and along the Severn vale below it.
• Around 11,000 properties sit in the 1% annual river-flood band. Gloucestershire is treated as a high-risk county for flooding, driven by the Severn, the Avon and their tributaries, with surface water adding to the risk in the towns.
• 2007 is the benchmark event. Floodwater entered the Mythe water treatment works near Tewkesbury and left roughly 420,000 people without mains water; it reached Tewkesbury Abbey for the first time in 247 years.
• Flood Zone 1 is not a free pass. Since the 2025 Planning Practice Guidance update, surface water risk can trigger an assessment even on a Flood Zone 1 site, and Gloucestershire County Council reviews drainage on every major application.

Check flood risk anywhere in Gloucestershire

Gloucestershire 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.

Why Gloucestershire floods: the Severn, the Avon and surface water

River flooding dominates Gloucestershire. The River Severn runs the length of the county and is joined by the River Avon at Tewkesbury, where the combined flow produces the longest and deepest floods in the vale. Surface water now threatens the built-up parts of Gloucester, Cheltenham and Stroud, and groundwater rises through the Cotswold limestone in the east, so many sites here face more than one source at once.

The Severn gathers a vast upland catchment in Wales and the Marches before it reaches Gloucestershire, so heavy or prolonged rain far upstream arrives here as a slow, sustained flood that can sit on the floodplain for days. The Avon adds the drainage of Warwickshire and the south Midlands, and the two meet at Tewkesbury. Smaller rivers carry the risk through the towns: the Chelt through Cheltenham, the Twyver through Gloucester, the Frome through Stroud and Ryeford, the Leadon near Newent, the Isbourne at Winchcombe, and the Coln and Churn in the Cotswolds. Understanding which watercourse drives the risk on a given site, and how it behaves, is the heart of a credible flood modelling and flood risk assessment.

• Rivers and the sea (fluvial and tidal). The Severn, the Avon and their tributaries drive the headline risk, and the lower Severn is tidal. The Severn Estuary has one of the largest tidal ranges in the world, which can hold river floods back at high tide.
• Surface water (pluvial). Now the most widespread flood risk in England. Intense rain overwhelms the older drainage in Cheltenham and Gloucester, where Severn Trent Water has invested heavily to improve sewer capacity. Since the 2025 PPG update, surface water on the Flood Map for Planning can trigger an assessment even on a Flood Zone 1 site.
• Groundwater. The permeable limestone of the Cotswolds means springs, seasonal seepage and a rising water table can affect sites that screen as low risk on the national maps. Groundwater flooding appears on no standard flood map and is assessed within the flood risk assessment.

Cheltenham is the textbook case for why a single map is never enough: the Chelt runs through the town centre behind a flood alleviation scheme, while low-lying neighbourhoods flood from surface water that has nowhere to drain by gravity. A scheme there has to reconcile both. Our notes on the surface water and planning picture and on the Severn floods set out how these sources interact across the county.

A history of flooding in Gloucestershire

These dated events are why local authorities here scrutinise development so closely, and why a flood risk assessment in Gloucestershire has to be evidence-led rather than generic. Each one reshaped local policy and the flood defences a developer now has to design around.

• 1947 — the post-war floods. A severe winter followed by a rapid thaw and heavy rain pushed the Severn and Avon over their banks across the vale, in one of the worst flood years of the twentieth century in Britain.
• 2007 — the summer floods. Exceptional July rainfall sent the Severn and Avon to record levels. Tewkesbury was cut off with around three feet of water in parts of the town, and floodwater reached Tewkesbury Abbey for the first time in 247 years. The Mythe water treatment works flooded, leaving roughly 420,000 people without mains water, and the tap water for about 140,000 homes was not declared safe again until 16 days later.
• 2007 — the near-misses. A combined military and civilian effort kept floodwater out of the Walham electricity substation, which supplies around half a million people; the Castle Meads substation in Gloucester was lost and restored the next day. The event drove a national rethink of how critical infrastructure is protected.
• 2012 to 2020 — the repeat warnings. Further flooding in 2012, 2014, 2019 and 2020 affected Severn-side communities such as Sandhurst, Maisemore and Apperley, a reminder that the risk here returns in many winters, not only in landmark years.

High-risk areas across Gloucestershire

No two sites in the county carry the same risk, but several places are repeatedly flagged in flood alerts and in the Strategic Flood Risk Assessments. Knowing where a site sits in relation to them is the starting point for the right scope of assessment.

• Tewkesbury. The land around Severn Ham, at the meeting of the Severn and Avon, is among the most flood-prone in the county and frequently sits in Flood Zone 3.
• Sandhurst, Maisemore and Apperley. Low-lying parishes on the floodplain between Gloucester and Tewkesbury that are regularly flagged when Severn levels rise.
• Gloucester. Severn-side districts and the city's older drainage make parts of the city vulnerable to both river and surface water flooding.
• Cheltenham. The Chelt and surface water threaten central and low-lying neighbourhoods, managed in part by the town's flood alleviation scheme.
• Stroud and the valleys. The Frome and its tributaries respond quickly to rainfall, giving the Stroud valleys a flashy, fast-onset risk.

Gloucestershire County Council has mapped a number of rapid response catchments, where rainfall turns into flooding so quickly there may be little time to warn anyone, which is one reason a site-specific assessment matters even where the national map looks reassuring.

Planning policy and your Lead Local Flood Authority in Gloucestershire

Two kinds of authority set the rules. The Environment Agency leads on the River Severn, the River Avon and the other main rivers. A Lead Local Flood Authority (LLFA) leads on surface water, groundwater and ordinary watercourses, and in Gloucestershire that is Gloucestershire County Council, which reviews surface water drainage on every major application.

Gloucestershire County Council is the LLFA for the whole administrative county, covering Gloucester, Cheltenham, Tewkesbury, Stroud, Cotswold and the Forest of Dean. It publishes a Local Flood Risk Management Strategy and developer guidance on sustainable drainage, alongside the role the LLFA plays as statutory consultee. The neighbouring unitary authority of South Gloucestershire is a separate LLFA and sits outside the county council's area. Our overview of the Environment Agency's role in planning explains how the pieces fit together.

Your assessment and drainage strategy must satisfy national policy — the NPPF and its Planning Practice Guidance — and the local framework: the county's Local Flood Risk Management Strategy, the relevant Strategic Flood Risk Assessment and the adopted Local Plan that covers your site. Where a higher-risk site is involved, the Sequential and Exception Tests apply, and the right vulnerability classification has to be argued. On drainage, the four pillars of SuDS and the GCC standard shape what the council will accept, so runoff is typically held to greenfield rates with a climate change allowance on large sites.

What your flood risk assessment in Gloucestershire must show

• Every source of flooding assessed. Rivers and the sea, surface water, groundwater, sewers and reservoirs, rather than just the headline river.
• Safe for the development's lifetime. Generally 100 years for housing, with Environment Agency climate change allowances applied to river flows and rainfall.
• No increase in risk elsewhere. Finished floor levels, safe access and egress, and where needed compensatory flood storage and a drainage strategy.

Development and flood risk assessment in Gloucestershire

Gloucestershire takes significant growth across very different landscapes, and the kind of site you bring forward shapes the assessment you need. The right scope is the difference between a clean approval and a drainage objection that holds an application up for months.

• Severn-side and city-centre regeneration. Regeneration in Gloucester and along the Severn frequently sits in Flood Zones 2 and 3, needing finished floor levels, safe access and egress, and often flood modelling and compensatory storage.
• Edge-of-settlement housing. Strategic allocations around Cheltenham, Tewkesbury and Bishop's Cleeve turn on surface water and drainage, where a robust surface water drainage strategy keeps the application moving.
• Cotswold and rural schemes. Conversions and new builds in the Cotswolds and the Forest of Dean often turn on groundwater and the feasibility of soakaways on limestone, and on the right ground conditions work.

Whatever the site, an early read pays for itself. Our flood risk map, live river levels map and drainage calculators (including the BRE 365 infiltration calculator) give a fast first pass before we scope the full report.

How Unda helps with flood risk assessment in Gloucestershire

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Gloucestershire County Council and national planning policy. We are experts on the county's flooding, not a generic report mill.

A flood risk assessment in Gloucestershire from Unda assesses every credible source: the Severn and Avon, the town rivers, surface water, groundwater, sewers and any ordinary watercourse on the boundary. We compare modelled flood levels against your proposed floor levels, set safe access and egress, and recommend mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, with more than 5,000 flood risk assessments completed since 2014, and a senior consultant signs off every report. Where the council needs it, a surface-water drainage strategy sits alongside the assessment as a separate report so the two cases stay consistent and an objection has nowhere to land. You can read more about Unda and our flood risk consultants.

Flood risk assessments and related reports for every situation in Gloucestershire

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 each one for sites across Gloucestershire.

• Flood risk assessment for planning. The site-specific report prepared to satisfy the NPPF, the Environment Agency and your Gloucestershire LPA. Groundwater and, where needed, flood modelling are assessed within it.
• Flood risk survey for a property purchase. An independent, property-specific survey for buyers and their advisers when a conveyancing search or lender flags flood risk.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where cover has been restricted, declined or priced on flood grounds.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03, so the credits for flood and surface water management are supported from the outset.
• Sequential and Exception Tests. The locational case that the development is in the most appropriate available site, prepared alongside the flood risk assessment.
• Drainage strategy and SuDS. Surface water managed down the SuDS hierarchy to the GCC standard, with discharge of drainage conditions and preliminary drainage design where the scheme needs them.

Gloucestershire flood resources and contacts

The most reliable Gloucestershire 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.

• Check for flooding in Gloucestershire. Environment Agency live warnings, river levels and maps at check-for-flooding.service.gov.uk; sign up for flood warnings or call Floodline on 0345 988 1188.
• Gloucestershire County Council flood team. Strategy, developer guidance, flood zones in Gloucestershire and how to report flooding, as Lead Local Flood Authority.
• Rapid response catchments. Guidance on the county's fastest-responding catchments at Gloucestershire County Council.
• Severn Trent Water. The water and sewerage company for the county, and the first contact for sewer flooding.
• Local flood action groups. Community groups including the Isbourne Catchment Group, Northleach Flood Action Group and the Newent Neighbourhood Flood Association work alongside the council and the Environment Agency.

Flood risk assessment in Gloucestershire: frequently asked questions