River Severn Flooding & Flood Risk

The River Severn is the longest river in Britain and the one that floods more towns, more often, than any other. It gathers rain across a vast upland catchment in mid Wales and carries it 354 kilometres through Shrewsbury, Ironbridge, Bridgnorth, Bewdley, Worcester, Upton, Tewkesbury and Gloucester before it widens into its estuary. Flooding from the River Severn is a fact of life for those riverside communities, and for anyone building, buying or insuring a property near the channel it is the single question that decides a scheme. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Severn floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

In July 2007 the Severn overwhelmed the Mythe water treatment works near Tewkesbury and left around 350,000 people without mains water for up to seventeen days, while soldiers fought to keep the floodwater out of the Walham switching station that powers half a million homes. No river in England puts more at stake when it rises.

• Britain’s longest river, and its most flood-prone. From its source on Plynlimon to the Bristol Channel the Severn drains roughly 11,000 square kilometres, and it carries the most water of any river in England and Wales.
• A Welsh mountain catchment feeding English towns. Heavy rain on the Cambrian Mountains arrives downstream as a large, slow flood wave, so a wet spell in Wales can put Shrewsbury, Worcester and Gloucester under water days later.
• A designated Main River for its whole English length. The Environment Agency regulates the channel, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it; in Wales, Natural Resources Wales is the equivalent authority.
• Live gauges the length of the river. More than twenty Environment Agency gauges, from Welsh Bridge in Shrewsbury to Gloucester, appear on the map below with their latest levels updating in real time.

Check flood risk along the River Severn

The map below draws the River Severn in coral, from the Welsh headwaters to the estuary, with the Environment Agency’s live river gauges positioned along it — click any gauge to see its latest level, its typical range and its record peak. Enter a postcode near the river to zoom in and screen that location against the Environment Agency’s flood data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Why does the River Severn flood?

The Severn floods because of the shape and size of the land it drains. It rises high on Plynlimon in the Cambrian Mountains of mid Wales, at around 610 metres, in some of the wettest, steepest, least absorbent country in Britain. Rain that falls on those moorland slopes runs off quickly into a river that then has more than three hundred kilometres of falling ground to travel before it reaches the sea. By the time the whole catchment of roughly 11,000 square kilometres has fed its water into the main channel, the Severn is carrying the largest flow of any river in England and Wales — on average about 107 cubic metres a second where it passes Apperley in Gloucestershire, and vastly more in flood.

What makes the river dangerous is the combination of a flashy upland head and a slow lowland body. The Welsh tributaries respond within hours to heavy rain, but the wide, flat floodplain through Shropshire, Worcestershire and Gloucestershire drains slowly, so a flood peak can take days to pass and the water sits on the land long after the rain has stopped. Add the Vyrnwy, the Tern, the Teme and the Warwickshire Avon — the Avon being the largest tributary of all, joining at Tewkesbury — and a single wet week across the whole basin can stack peak upon peak down the length of the river.

Two large reservoirs, Llyn Clywedog above Llanidloes and Lake Vyrnwy on the Vyrnwy tributary, were built partly to hold back Welsh water and take the top off the biggest floods, and they still do useful work. They cannot, however, stop the river flooding: they control only a fraction of the catchment, and the rain that falls below them is beyond their reach. The gauged record makes the scale plain.

• A wet, steep, quick-shedding head. The Welsh headwaters receive some of the heaviest rainfall in the catchment and shed it fast, so the upper Severn rises within hours of a storm.
• A slow, wide lower body. Through the English lowlands the floodplain is broad and flat, so flood peaks pass slowly and water lingers for days — the reason Severn floods are so prolonged.
• Big tributaries that arrive together. The Vyrnwy, Tern, Teme and Warwickshire Avon can each be in flood at once, and the Avon’s meeting with the Severn at Tewkesbury is one of the most flood-prone points in England.
• A river that is only partly controlled. Llyn Clywedog and Lake Vyrnwy attenuate the largest Welsh floods but govern only part of the catchment, so the Severn still floods hard below them.

A history of flooding on the River Severn

Few rivers in Britain have a flood record as long or as well documented as the Severn’s, and the towns along it keep the evidence on their walls: brass plaques and painted marks record levels reaching back to floods in 1770, 1795 and beyond. The modern story starts with March 1947, when snowmelt and rain together drove the highest gauged flow ever measured at Bewdley, 637 cubic metres a second, and put much of Worcestershire and the Severn valley under water. For half a century the 1947 flood was the one every later event was measured against.

The autumn of 2000 changed that. Weeks of rain sent the Severn to levels not seen in living memory: Shrewsbury’s Frankwell district flooded three times in six weeks, the Welsh Bridge gauge reached 5.25 metres, and Bewdley recorded 5.56 metres — still the highest level ever measured there. The 2000 floods triggered a wave of new defences along the river and a lasting shift in how the catchment is managed.

Then came summer 2007, the most costly of them all. After the wettest May-to-July on record, the Severn and the Warwickshire Avon burst out together across Gloucestershire. Tewkesbury was cut off entirely, floodwater entered the Abbey for the first time in 247 years, and the Mythe water treatment works were inundated — leaving around 350,000 people without mains water for up to seventeen days. The Walham electricity switching station, which supplies half a million homes, was saved only by an emergency barrier thrown up by the military and the fire service. It was a national wake-up call and the trigger for the Pitt Review.

The pattern has continued. Storm Dennis in February 2020 fell in the wettest February on record: the demountable barriers at Ironbridge were pushed back and overtopped and the Wharfage was evacuated, Worcester set a new level record of 5.79 metres at Barbourne, and around 1,600 homes flooded across the catchment. The Severn at Bewdley stayed in flood for weeks — the longest sustained high flow in almost a century of record. Storm Franklin followed in February 2022, driving the highest flow ever gauged at Buildwas, near Ironbridge, at 664 cubic metres a second. For a site anywhere near the river, this is the history an assessment has to reckon with.

Which areas does the River Severn put at risk?

Flood risk on the Severn is spread the length of the river, but it concentrates on the historic riverside cores of the towns strung along it, and the Environment Agency runs a dense network of flood warning and flood alert areas to match. In its Catchment Flood Management Plan the Agency estimates that about 32,600 people in the catchment are at high risk of flooding from rivers and the sea, with a further 67,400 at medium risk. The table below traces the main settlements at risk from the Welsh source to the tidal estuary; each of the English towns is covered in more detail on our local flood risk pages, linked below.

If your site sits within one of these areas, the Environment Agency’s flood map for planning is likely to place it in Flood Zone 2 or 3, which triggers the need for a site-specific flood risk assessment. You can screen any address against the live Environment Agency data using the map above, and the towns along the river are covered in more depth on our local pages for Shrewsbury, Ironbridge, Bridgnorth, Bewdley, Telford, Worcestershire, Upton-upon-Severn, Tewkesbury, Gloucester and Gloucestershire.

Is the River Severn a Main River — and what that means for your site

Yes. The Environment Agency has designated the River Severn a Main River along its English length, which means the Agency, rather than the local council, regulates the channel and its immediate margins. That matters if you are developing nearby. Works within 8 metres of the top of the bank generally need a Flood Risk Activity Permit from the Environment Agency before they start, and a site in the river’s floodplain will normally need a flood risk assessment to support a planning application. The many smaller brooks and ditches that feed the Severn are usually ordinary watercourses instead, regulated by the county council as Lead Local Flood Authority and by the riparian owners whose land they cross. In Wales, the upper Severn is regulated by Natural Resources Wales, and a Welsh planning application follows a different route: a Flood Consequence Assessment under TAN15 rather than an English flood risk assessment.

Below Gloucester the river becomes tidal. The Severn Estuary has the second-highest tidal range in the world, and the incoming tide funnels into the narrowing channel as the Severn bore, a wave that runs upstream past Minsterworth and Newnham. For sites on the lower river, tidal flooding — and the way a high tide can hold back a river already in flood — becomes as important as the fluvial risk, and a fluvial assessment alone is not enough.

• The 8 metre byelaw margin. Building, excavation or structures near the Severn usually require a Flood Risk Activity Permit, sought alongside — not instead of — planning permission.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, or a larger site in Flood Zone 1, needs a flood risk assessment for planning; on the lower river that assessment has to cover tidal risk as well.
• England and Wales differ. The English Severn answers to the Environment Agency and the NPPF; the Welsh Severn answers to Natural Resources Wales and TAN15. We confirm which regime applies to your site.

Flood risk and drainage services for sites near the River Severn

A development near the River Severn can call for several different reports, and it helps to be clear that they answer different questions. 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 address separate things: whether the site is safe from flooding, whether the development should be located here, and where the surface water goes. Unda prepares each of them for sites across the Severn catchment.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s expectations for development in the river’s flood zones. See our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders through our flood risk survey for property purchase and flood risk assessment for insurance — often decisive on a river as flood-prone as the Severn.
• Drainage strategies and SuDS. Surface-water designs that keep runoff within the discharge limits set by the Lead Local Flood Authority — see our sustainable drainage strategy service, with preliminary drainage design at the early stage and the detailed scheme to discharge a drainage condition later.
• Sequential and exception tests. The locational justification the NPPF requires for higher-risk sites, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where a high water table on the floodplain gravels comes into play.

Where the national flood maps are not enough — a common situation for sites right beside a river this size — we can bring in hydraulic flood modelling to derive accurate flood levels, extents and depths, and a flood consequence assessment where a Welsh site needs one. Everything we do is framed by the NPPF and its planning practice guidance, including the climate change allowances that matter so much on a river with the Severn’s flood history.

Managing flood risk across the Severn catchment

Responsibility for flooding on the Severn is shared across several bodies, and it is worth knowing who does what. The river sits within the Severn River Basin District, and its flood risk is governed by the Environment Agency’s Catchment Flood Management Plan and the Severn Flood Risk Management Plan, prepared jointly with Natural Resources Wales because the catchment straddles the border. On the ground, the Environment Agency regulates the Main River, publishes the flood map for planning and issues flood warnings; the county councils — Shropshire, Worcestershire and Gloucestershire — act as Lead Local Flood Authorities for surface water, groundwater and ordinary watercourses, each with its own strategic flood risk assessment; and the district and borough councils decide planning applications along the river.

Since the floods of 2000 and 2007, the response has become steadily more strategic. Bewdley, Shrewsbury, Upton and Ironbridge all gained flood defences, and the £10 million Beales Corner scheme in Bewdley was completed in 2025. The River Severn Partnership, launched in 2019 and led by Shropshire Council and the Environment Agency, is now taking a catchment-scale view: its emerging Severn Valley Water Management Scheme, developed with Natural Resources Wales and Powys and Shropshire councils, aims to reduce flood risk to nearly 3,000 homes and businesses by combining upstream land management, nature-based storage and targeted engineering. The direction of travel is away from defending individual towns and towards managing water across the whole basin.

• Live river levels. Check the river’s gauges on the Environment Agency service — Severn at Bewdley and Worcester (Barbourne) — the National River Flow Archive’s Bewdley station record, or our own live river levels map.
• Screen a location. Try our free flood risk map, and for drainage the drainage calculators, the BRE 365 infiltration calculator and the ground conditions checker.
• Flood warnings. Sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on the River Severn

Unda understand how a large upland-fed river like the Severn behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know how a wet week on Plynlimon becomes a flood peak at Worcester days later, why the water lingers on the floodplain, and how the tidal reach below Gloucester complicates the picture — and we know what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning on a river with this record. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents, home buyers and businesses with sites the length of the valley.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping and modelled levels, the river’s gauged record, and the local planning policy that applies to the site. If you are working on a site near the River Severn, talk to one of our experienced flood risk consultants about what your application will need. You can read more about how the river’s communities have adapted over the past quarter-century in our look back at the 2000 Severn floods, 25 years on, and about the wider service in our flood risk assessment and Shropshire pages.

River Severn flood risk: your questions answered