Flood Risk Assessment in Gloucester

Gloucester is one of the most flood-exposed cities in England. It sits on the floodplain of the River Severn, on flat, low-lying ground threaded by tidal water and a network of brooks that back up when the river runs high. If you are developing, buying or insuring property here, a flood risk assessment in Gloucester is often the document that decides whether the project moves forward. Unda prepares flood risk assessments in Gloucester for planning, property purchase, mortgage and insurance, and BREEAM, alongside sequential and exception tests and surface-water drainage strategies.

We are experts on Gloucester's flooding rather than a generic report mill, and we know how the Environment Agency, Gloucestershire County Council as Lead Local Flood Authority and Gloucester City Council read a planning submission. Every report is evidence-led and signed off by a senior consultant.

Gloucester is one of the places the Environment Agency has formally identified as a Flood Risk Area, where the risk is significant at a national scale. On a site near the Severn or one of the city's brooks, flooding is a planning question you cannot afford to answer with a generic map.

Speak to one of our experienced flood risk consultants on +44 (0) 1293 214444 or start a quote for a free, no-obligation response within 60 minutes.

• On the Severn floodplain. Gloucester is drained by the River Severn, which is both tidal and fluvial through the city, on ground flat and low-lying enough to flood from several directions at once.
• Brooks that back up. The Twyver, Horsbere Brook, Sud Brook, Daniels Brook and Wotton Brook cross the city; high levels in the Severn can hold their flows back so they spill.
• A national Flood Risk Area. The Environment Agency's preliminary flood risk assessment identifies Gloucester as a Flood Risk Area for river, sea and reservoir flooding.
• Flood Zone 1 is not a free pass. Since the September 2025 Planning Practice Guidance update, surface water risk can trigger an assessment even on a Flood Zone 1 site.

Do you need a flood risk assessment in Gloucester?

You will usually need a flood risk assessment in Gloucester if your site is in Flood Zone 2 or 3, is one hectare or larger anywhere in the city, or is flagged for surface water on the Environment Agency's Flood Map for Planning. Given how much of Gloucester sits on the Severn floodplain, a large share of development sites trigger one. The only reliable way to know is to check the specific site against the mapping and the council's local evidence.

A clean, site-specific assessment can sometimes show that land mapped at risk is, in practice, safe once real ground levels are taken into account, and that can remove the need for the Sequential Test at application stage. The table below sets out when an assessment is typically required.

Why does Gloucester flood? The Severn and the city's brooks

Gloucester floods because it sits low on the Severn floodplain where tidal and fluvial water meet, and because the smaller watercourses that cross the city cannot discharge when the Severn is high. The result is a complex, multi-source flood risk: the same heavy-rainfall event can bring the river up, hold the brooks back and overwhelm the surface-water drains together. That interaction is exactly why a single map never tells the whole story here.

The tidal and fluvial Severn

The River Severn is a main river through Gloucester and carries both fluvial flow from its huge upstream catchment and tidal influence from the Severn Estuary, with the tidal limit reaching the city's weirs. The greatest risk near Gloucester comes from high fluvial flows, while tidal flooding dominates downstream towards Minsterworth. Approaching the estuary the floodplain spreads more than a mile wide, and once the Severn is out of bank the water can take weeks to drain, as recorded in the city's Strategic Flood Risk Assessment and the wider Severn River Basin District Flood Risk Management Plan.

The brooks that back up

Inland of the river, a set of ordinary watercourses drains the higher ground around the city. The River Twyver (fed by the Matson Brook), Horsbere Brook, Sud Brook, Daniels Brook and Wotton Brook all run towards the Severn, and when the river is high their outfalls drown out and they spill across low ground. This backing-up effect is central to flood risk in Longford, Sandhurst, Walham and the eastern suburbs, and it is the LLFA rather than the Environment Agency that leads on these ordinary watercourses.

Surface water and groundwater

Surface water is now the most widespread source of flood risk in England, and Gloucester's flat topography means rain struggles to drain away by gravity. The city's underlying geology — Severn floodplain alluvium of sand, silt and clay over peat in a deep buried channel — both holds water at the surface and raises the prospect of groundwater flooding that never appears on a river map. Any scheme relying on a soakaway has to prove the ground can actually take the water, which is where infiltration testing comes in.

Check flood risk anywhere in Gloucester

Gloucester is outlined below. Screen any postcode in the city 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.

A history of flooding in Gloucester

Gloucester's recent flood history explains why the regulators scrutinise development here so closely, and why a flood risk assessment in Gloucester has to be evidence-led rather than generic. Each major event reshaped the defences and the policy a developer now designs around.

• July 2007 — the defining event. Severn Trent's Mythe water treatment works at Tewkesbury was flooded, leaving around 140,000 properties across Gloucester, Cheltenham and Tewkesbury without mains water for up to 17 days. The Castle Meads electricity substation was switched off, cutting power to roughly 50,000 homes, while a major emergency effort narrowly saved the Walham substation that supplied around half a million people.
• A near-miss for the city. Had Walham been lost, plans were drawn up for the mass evacuation of vulnerable residents — a reminder that in Gloucester flooding threatens critical infrastructure, not just individual buildings.
• January 2024 — Storm Henk. Around 80 homes flooded in Gloucester and residents of Alney Island, between two arms of the Severn, were evacuated overnight as defences came close to overtopping. The Environment Agency recorded the third-highest water levels on Alney Island in 100 years.
• Defences earning their keep. Post-2007 investment in the Mythe works, the Walham substation, Horsbere Brook and the Westgate drainage scheme is credited with limiting the damage during Storm Henk, even as the Severn rose dangerously high four times that winter.

A pattern runs through all of it: ground that has flooded once tends to flood again. A credible assessment treats Gloucester's recorded flood history as evidence, not background colour.

Flood defences and alleviation schemes in Gloucester

Major investment followed the 2007 floods, and the schemes change what a development next to them has to prove. A new defence lowers the chance of flooding but never removes it, and planning policy still expects the residual risk to be assessed.

• Horsbere Brook Flood Alleviation Scheme. Built by the Environment Agency after 2007 and completed in 2011, this storage scheme holds back around 120,000 cubic metres of water to protect more than 350 homes in Elmbridge and Longlevens.
• Westgate drainage scheme. Improvements around the low-lying Westgate area form part of the post-2007 package credited with reducing flooding during later storms.
• City-wide watercourse works. Gloucester City Council has cleared ditches, streams and brooks and built storage ponds since 2007 to hold water during heavy rainfall, alongside natural flood management measures.
• Severn defences are partial, not total. The River Severn has defences at critical points through Gloucester, but the city's SFRA is clear they do not provide full protection — which is why defended sites remain in the flood zones on the Flood Map for Planning.

The catch for developers is the one we set out in our notes on residual flood risk and the difference between the long-term flood map and the Flood Map for Planning: the planning map is drawn as if the defences are not there, so a defended Gloucester site can still sit in Flood Zone 3. Climate change sharpens this further, which is why an assessment applies the Environment Agency's allowances over the development's whole lifetime.

Planning and drainage policy for development in Gloucester

Two kinds of authority set the rules in Gloucester. The Environment Agency leads on the River Severn and the other main rivers as a statutory consultee. Gloucestershire County Council is the Lead Local Flood Authority, leading on surface water, groundwater and ordinary watercourses, while Gloucester City Council is the local planning authority that determines applications. Your assessment and drainage strategy have to satisfy all three.

Development in the city is judged against the adopted Gloucester City Plan 2011–2031 (adopted January 2023) and the wider Joint Core Strategy for Gloucester, Cheltenham and Tewkesbury, supported by the council's Strategic Flood Risk Assessment. Because so much of Gloucester is at risk, the City Plan applies the flood-risk sequential approach across the whole city, and the county council's flood team reviews surface-water drainage on major schemes. Our explainers on the Environment Agency's role and the role of the LLFA set out how the pieces fit together.

What your flood risk assessment in Gloucester must show

• Risk assessed from every credible source — the tidal and fluvial Severn, the city's brooks, surface water and groundwater — using the Flood Map for Planning and the council's SFRA.
• An appropriate location, tested against the Sequential Test where policy requires it.
• Safe for its lifetime, typically 100 years for housing, once the Environment Agency's climate change allowances are applied.
• No increase in flood risk elsewhere, with attenuation and, where needed, compensatory storage so displaced water is not pushed onto neighbours.
• Residual risk managed, with finished floor levels, safe access and egress. Get the drainage wrong and the application stalls, as our note on LLFA drainage objections explains.

Development and flood risk assessment in Gloucester

Gloucester is growing through regeneration of its docks, canal corridor and city-centre sites, 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.

• Docks and canal-side regeneration. Schemes around Gloucester Docks, the Gloucester & Sharpness Canal and the historic quays often sit close to the Severn and the tidal floodplain, needing raised floor levels, safe access and a clear residual-risk case.
• City-centre and brownfield sites. Redevelopment on flat, low-lying ground frequently turns on surface water and the capacity of the brooks and sewers, where a drainage strategy is central to the application.
• Urban-edge housing. New homes towards Quedgeley, Hempsted, Hucclecote and Longlevens interact with the Sud, Daniels and Horsbere brooks and with critical drainage areas identified in the SFRA.
• Commercial and logistics land. Larger employment sites with extensive hardstanding make the surface-water strategy and infiltration evidence the heart of the submission.

Home buyers and businesses come to us as well. A flood-risk question can affect a purchase and house prices, and a conveyancing search is not the same thing as a planning-grade assessment — knowing which you actually need saves time and money, as our guide on doing your own assessment explains.

How Unda helps with flood risk assessment in Gloucester

We act as the accurate translation layer between your scheme and the requirements set by the Environment Agency, Gloucestershire County Council as LLFA and Gloucester City Council. We are experts on Gloucester's flooding — the Severn, the brooks and the city's flood record — not a templated report dressed up with a place name.

A flood risk assessment in Gloucester from Unda assesses every credible source, compares modelled flood levels against your proposed floor levels, sets safe access and egress, and recommends mitigation the regulators will accept. We are a CIWEM Business Partner with chartered members, rated 5.0 on Google, 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. We stay involved past the report to answer any questions the regulators raise.

Flood risk assessments and related reports for every situation in Gloucester

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 all three in Gloucester, alongside the right form of assessment for buyers, lenders and BREEAM.

• Flood risk assessment for planning. The site-specific report submitted with a planning application to show development is safe and does not increase risk elsewhere, prepared to the NPPF and its Planning Practice Guidance.
• Flood risk survey for property purchase. An independent, property-specific survey for homebuyers when a conveyancing search flags risk or a lender raises a concern.
• Flood risk assessment for insurance. Property-level evidence for brokers and underwriters where premiums rise, cover is restricted, or mapped risk looks inconsistent with the site.
• Flood risk assessment for BREEAM. Structured around BREEAM Pol 03 so a scheme earns its flood and surface-water credits from the outset.
• Sequential and exception tests. The locational justification that steers development to lower-risk land — important in a city where so much sits in Flood Zones 2 and 3.
• Drainage strategy and SuDS. Surface-water design to the four pillars of SuDS, with discharge of conditions and preliminary drainage design where needed.
• Groundwater and flood modelling. Groundwater is assessed within every FRA on Gloucester's alluvial ground; hydraulic modelling is brought in where the national mapping is not detailed enough.

You can start any site with our free Flood Risk Map, check current conditions on the Live River Levels Map, or test infiltration feasibility with our BRE 365 calculator and drainage calculators. For the bigger picture across the county, see our flood risk assessment in Gloucestershire page, or talk to our flood risk consultants directly.

Local flood risk management and emergency contacts in Gloucester

If you live or work in Gloucester, it is worth knowing who to contact about flood risk and who to call in an emergency. The table below lists the bodies responsible for managing flood risk in the city and how to reach them.

You can check live warnings and sign up for free flood alerts for the River Severn through the Environment Agency's check for flooding service and flood-warning sign-up. If you own land next to one of Gloucester's brooks, you are likely to be a riparian owner with maintenance responsibilities, and problems on an ordinary watercourse are reported to the county council as LLFA.

Frequently asked questions about flood risk assessment in Gloucester