River Dee Flooding & Flood Risk

The River Dee floods differently depending on where you stand on it. Above Llangollen it is a fast, rocky Welsh river that can rise and fall in hours; across the Cheshire Plain from Erbistock to Chester it turns almost flat and floods slowly, spreading over the water meadows at Bangor-on-Dee, Overton and Farndon for days at a time; below Chester Weir it is tidal, and flood risk becomes an estuarine question shared between Flintshire and the Wirral shore. Flooding on the River Dee is therefore never one story. It is at least three, and a flood risk assessment for a site on this river has to know which one applies. Unda prepares flood risk assessments, Flood Consequence Assessments and drainage strategies for sites the length of the Dee, from its source near Bala in Snowdonia to the Dee Estuary and the Irish Sea. To talk through a site near the river, call 01293 214444 or email enquiries@unda.co.uk.

On 21 January 2021, Storm Christoph pushed the River Dee to record levels at both ends of its floodplain in the same night: 16.46 metres at Bangor-on-Dee and 9.6 metres AOD at Farndon, while the gauge at Manley Hall recorded its highest flow since continuous records began in 1970, 575 cubic metres a second.

• A river that changes character three times. Flashy and steep through Snowdonia and the Vale of Llangollen, almost flat across the Cheshire Plain from Erbistock to Chester, then tidal below Chester Weir to the estuary.
• Two regulators share it. Natural Resources Wales regulates the Welsh reaches and the Environment Agency the English reaches, under a long-standing joint arrangement across the catchment.
• A uniquely managed flow. Releases from Llyn Celyn and Bala Lake maintain a guaranteed compensation flow over Chester Weir, and the lower Dee is the only river in the UK carrying a Water Protection Zone along its entire length to the weir.
• Record flooding within living memory. Storm Christoph in January 2021 set new all-time records at gauges from Bangor-on-Dee to Manley Hall, surpassing the previous highs set in 2011.

Check flood risk along the River Dee

The map below draws the River Dee in coral from its source near Bala to Chester, with the Environment Agency’s live gauges positioned along it. Click a gauge for 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 open 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 or a Flood Consequence Assessment.

Why does the River Dee flood?

The River Dee floods because it is really two rivers joined end to end. It rises at around 450 metres on the slopes of Dduallt above Llanuwchllyn, in the mountains south of Bala, and for its first stretch through Snowdonia and the Vale of Llangollen it drops quickly over a rocky bed, so heavy rainfall reaches the channel fast and the river can rise sharply within hours. Past Llangollen and Chirk the gradient all but disappears: the Dee crosses the Cheshire Plain through Erbistock, Bangor-on-Dee, Overton, Worthenbury, Holt, Farndon, Aldford and Eccleston on a course so shallow that the valley floor has flooded too often to be built on, which is why it has stayed farmland rather than town. At Chester the river meets Chester Weir, built in 1093 and still the effective head of tide; below it, the river is tidal all the way to the Dee Estuary, though in exceptional conditions a tide above about 3.5 metres can still top the weir and push flooding upstream of it.

The Dee’s catchment covers roughly 1,817 square kilometres to Chester, gathering the Tryweryn, Alwen, Clywedog and Alyn from the left bank and the Ceiriog and Wych Brook from the right. It is also the only major river in Britain whose flow is deliberately regulated for water supply rather than flood storage: the Dee Regulation Scheme, run jointly by Natural Resources Wales and the water companies that abstract from the river, releases water from Llyn Celyn on the Tryweryn and from Bala Lake to keep a guaranteed flow of at least 364 million litres a day moving over Chester Weir, safeguarding the drinking-water supply for around three million people across North Wales, Wirral and Cheshire. That is also why the lower Dee has carried a Water Protection Zone, the only one in the UK covering an entire river to a named weir, since 1999. None of this stops the Dee flooding in a genuine spate; it simply means the river’s baseline behaviour is more closely managed than almost any other in the country.

• Flashy above Llangollen, slow below it. The upland Welsh reach responds to rain in hours; the Cheshire Plain reach floods more slowly but over a much wider area for days at a time.
• A regulated baseline flow. Manley Hall is the control point for the Dee Regulation Scheme: when its flow drops toward 10 m³/s, releases from Bala and then Llyn Celyn keep the river, and Chester’s water supply, topped up.
• Tide-locked at Chester in normal conditions. Chester Weir marks the usual head of tide, though exceptional tides above roughly 3.5 metres can still overtop it.

A history of flooding on the River Dee

The joint Environment Agency and Natural Resources Wales flood risk assessment for the catchment identifies 1890, 1946, 1964 and 2000 as the most significant flood years across the Dee river basin as a whole, and the river’s gauge records add the detail. On 14 December 1964, Manley Hall recorded a daily mean flow of 521 cubic metres a second, still the highest in its record. Widespread flooding returned at the end of October and into November 2000, described nationally as one of the largest floods in England in recent history at the time; the Dee peaked at 455 cubic metres a second at Manley Hall on 30 October, and the Chester Suspension Bridge gauge recorded its own record daily mean flow, 306.6 cubic metres a second, on 7 November. A further serious flood on 6 February 2011 set what was then an all-time record level at Bangor-on-Dee, 16.35 metres, with a peak flow of 449 cubic metres a second at Manley Hall. Storms in late December 2015 pushed Manley Hall to 364.67 cubic metres a second, and Storm Dennis, in February 2020, produced a peak of 427.57 cubic metres a second at the same gauge.

All of these were overtaken by Storm Christoph on 19–21 January 2021, the most severe flood the modern gauge network has recorded on the Dee. At Bangor-on-Dee, residents were evacuated overnight from around 1:40am on 21 January as the river reached 16.46 metres, and Natural Resources Wales declared a Severe Flood Warning for the Lower Dee Valley from Llangollen to Trevalyn Meadows, lifted two days later. At Farndon, the Environment Agency confirmed the river reached 9.6 metres above Ordnance Datum, 240 millimetres higher than the previous record; properties on River Lane were evacuated, and the borough’s Section 19 flood investigation recorded 24 residential properties affected, 4 evacuated and 3 actually flooded, with a flood warning issued at 00:10 and a severe warning at 10:19 that morning. Across Cheshire West and Chester more widely, around 150 residents were evacuated during the storm as several watercourses in the borough came under pressure at once. At Manley Hall, the instantaneous peak flow reached 575.03 cubic metres a second, the highest value recorded at that gauge since continuous monitoring began in 1970.

Which areas does the River Dee put at risk?

Flood risk on the Dee runs the length of the river and changes in character as it goes: fluvial in the Welsh valley, slow and widespread across the Cheshire Plain floodplain, then tidal and estuarine from Chester to the sea. The Environment Agency and Natural Resources Wales between them maintain flood warning and flood alert areas the length of the river, each covering a specific stretch or community. The table below summarises the main settlements at risk from source to mouth.

If a site sits within one of these reaches, it is likely to fall in Flood Zone 2 or 3 on the Flood Map for Planning (England) or the equivalent Flood Map for Planning (Wales). That is what triggers the need for a site-specific flood risk assessment or Flood Consequence Assessment. Cheshire’s wider flood risk picture, including the Dee’s course past Chester, is covered on our Cheshire flood risk assessment page; you can screen any address directly against live Environment Agency data using the map above or our free flood risk map.

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

Yes. The River Dee is a designated Main River along virtually its whole length, but uniquely for a page like this, two different regulators enforce that status either side of the border: Natural Resources Wales for the Welsh reaches and the Environment Agency for the English reaches, working under a long-standing joint management arrangement across the catchment. That split matters practically. In Wales, any activity within 8 metres of the river or a flood defence on it normally needs a Flood Risk Activity Permit from Natural Resources Wales. In England, the equivalent rule requires a permit from the Environment Agency, for activity within 8 metres of the non-tidal reach, widening to 16 metres once the river becomes tidal at Chester. Getting this right at the outset, including which regulator actually has jurisdiction over a particular reach, saves real time later in a planning application.

The border also changes which planning report a site needs. A site on the English bank sits under the NPPF, and will normally need a flood risk assessment for planning, potentially alongside a sequential or exception test if the site is in a higher-risk flood zone. A site on the Welsh bank sits under TAN15 and Planning Policy Wales instead, where the equivalent report is a Flood Consequence Assessment, built around a justification test rather than the English sequential and exception test. Both frameworks ask a similar underlying question: can the site be made safe, allowing for climate change, without increasing flood risk elsewhere? But the report, the tests applied and the regulator reviewing it are genuinely different depending on which side of the Dee the site falls.

• Two regulators, one river. Natural Resources Wales enforces the 8 metre byelaw and Flood Risk Activity Permits on the Welsh reaches; the Environment Agency applies the same rule in England, at 8 metres for the non-tidal reach and 16 metres once the river is tidal.
• Different planning frameworks either side of the border. An English site normally needs a flood risk assessment for planning under the NPPF; a Welsh site needs a Flood Consequence Assessment under TAN15 and Planning Policy Wales.
• Flood zones drive the requirement. On either side of the river, a site the Flood Map for Planning places in Flood Zone 2 or 3 will normally need a site-specific assessment.

Flood risk and drainage services for sites near the River Dee

A site near the River Dee can call for several different reports, and it helps to be clear that they answer different questions. A flood risk assessment, a Flood Consequence Assessment, a sequential or exception test and a drainage strategy are distinct reports: one asks whether the site is safe from flooding, one is the Welsh equivalent of that same question, one asks whether the development should be located here at all, and one asks where the surface water goes. Unda prepares each of these for sites the length of the Dee catchment, on either side of the border.

• Flood risk assessments for planning (England). Site-specific assessments addressing the Environment Agency’s and the NPPF’s requirements for development near the Dee. See our flood risk assessment for planning service.
• Flood Consequence Assessments (Wales). The equivalent report for sites on the Welsh bank, built around TAN15 and Natural Resources Wales’ acceptability criteria. See our Flood Consequence Assessment 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.
• Drainage strategies and SuDS. Surface-water designs that manage runoff and meet the discharge limits set by the relevant Lead Local Flood Authority. See our sustainable drainage strategy service.
• Sequential and exception tests. Locational justification for English sites where policy requires it, 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 Dee’s lowland clays and gravels is in play.

National flood mapping is not always detailed enough on a meandering lowland reach like the Dee between Bangor-on-Dee and Chester. Where that is the case, we can bring in hydraulic flood modelling to derive accurate flood levels and extents, prepare a preliminary drainage design at the outline stage, or the detailed scheme needed to discharge a drainage condition. English work is framed by the NPPF and its planning practice guidance; Welsh work by TAN15. The distinction between a Main River and an ordinary watercourse, and between formal ordinary watercourse consent and a Flood Risk Activity Permit, is explained in more depth on our blog.

Managing flood risk on the River Dee: policy and contacts

The Dee is its own river basin district, not folded into a wider Welsh or North West district, which reflects how distinct its catchment and its cross-border management really are. The Environment Agency’s River Dee Catchment Flood Management Plan, published in 2010, and the current Dee River Basin District Flood Risk Management Plan for 2021–2027 set the catchment-scale strategy on the English side, with a parallel Natural Resources Wales plan covering the Welsh reaches. On the ground, Wrexham County Borough Council, Flintshire County Council, Denbighshire County Council and Gwynedd Council are the Lead Local Flood Authorities on the Welsh side of the catchment, and Cheshire West and Chester Council performs the same role in England. It has published Section 19 flood investigation reports for both the Farndon flooding and the wider Storm Christoph event, and worked with the Environment Agency and United Utilities on the response.

• Live river levels. Check the Dee’s gauges directly via the Environment Agency’s check-for-flooding service for Farndon, or use our own live river levels map.
• Sign up for warnings. Register for Environment Agency flood warnings for the English reaches, or Natural Resources Wales flood warnings for Bangor-on-Dee and the Llangollen to Trevalyn Meadows area on the Welsh reaches, and call Floodline on 0345 988 1188.
• Local flood wardens. Bangor-on-Dee has an active, community-run Flood Wardens scheme, coordinating directly with residents when Natural Resources Wales issues a warning; the Environment Agency maintains a similar community liaison contact at Farndon.
• Screen a location. Try our free flood risk map and, for drainage design, the drainage calculators, the BRE 365 infiltration calculator and the geology and ground conditions map.
• 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 Dee

Unda understand how a river with three distinct personalities behaves in flood: the flashy Welsh spate above Llangollen, the slow lowland rise across the Cheshire Plain, and the tidal risk below Chester Weir. We understand, too, what Natural Resources Wales, the Environment Agency and the planning system expect from the report that follows. We know where the Dee’s floodplain bites hardest, from Bangor-on-Dee to Farndon and on to Chester, and how a robust, evidence-led flood risk assessment or Flood Consequence Assessment has to demonstrate a site is safe. We prepare these reports for developers, planning consultants, architects, agents and homeowners with sites anywhere along the river, on either side of the border.

Every assessment rests on the same evidence the regulators themselves use: the gauged levels and modelled flows from Manley Hall and Chester, the Environment Agency and Natural Resources Wales flood maps, and the local planning policy that applies on each bank. If you are working on a site near the River Dee, talk to one of our experienced flood risk consultants about what your application will need.

River Dee flood risk: your questions answered