River Derwent Flooding & Flood Risk

The River Derwent carries flood risk the length of Derbyshire, and it does it fast. Rain that falls on the peat moors of the Dark Peak reaches the valley towns as a flood peak within hours, so Hathersage, Matlock, Belper, Duffield and Derby all sit downstream of a river that rises quickly and hits hard. The Derwent is the county’s longest river, and for most of its course the Environment Agency regulates it as a Main River. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Derwent floodplain, from the Peak District dales to the edge of the Trent. Reports for sites along the River Derwent come from Unda Consulting Limited, a CIWEM Business Partner. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

During Storm Babet, the River Derwent at Derby reached 3.58 metres on 21 October 2023, the highest level in a record that began in 1935, and across that single storm around 1,260 properties were reported flooded in the East Midlands.

• A flashy upland river. The Derwent drains roughly 1,194 square kilometres of gritstone moor and limestone dale, and its peaty Dark Peak headwaters shed rain quickly, so levels rise sharply after heavy or prolonged rainfall.
• A designated Main River. The Environment Agency regulates the Derwent, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Six live Environment Agency gauges. Mytham Bridge, Yorkshire Bridge, Chatsworth, Matlock, Derby City and Church Wilne all appear on the map below, with their latest levels updating in real time.
• Development pressure the length of the valley. From Matlock and Belper down to Derby, sites in and beside the floodplain almost always need a flood risk assessment for planning.

Why does the River Derwent flood?

The River Derwent floods because of where it starts and how fast it falls. It rises at Swains Greave, high on the moors between Bleaklow and Howden in the Dark Peak, at around 590 metres, and the ground up there is blanket peat over millstone grit, a surface that sheds water instead of soaking it up. When rain sets in over the moors, runoff reaches the channel quickly and the river fills, so a wet night on Howden Moor becomes a flood peak in Matlock, Belper and Derby only hours later. Hydrologists call a river that responds this fast “flashy”, and the upper Derwent is a textbook case.

The shape of the valley then concentrates the water. Through the Peak District the Derwent runs in a narrow, steep-sided trough with little room to spread, so at towns like Matlock the river has nowhere to go but up and out across the valley floor and the A6. Downstream it gathers the Noe at Bamford, the Wye near Rowsley, the Amber at Ambergate and the Ecclesbourne at Duffield, each tributary adding its own peak, before it reaches the wider floodplain around Derby and joins the Trent at Derwent Mouth, south-east of the city.

The three great reservoirs in the upper valley, Howden, Derwent and Ladybower, are often assumed to hold the floods back, but that is not what they are for. Built by the Derwent Valley Water Board from 1901 onward to supply Derby, Leicester, Nottingham and Sheffield, they are water-supply reservoirs first, and when they are already full a storm passes straight over them. Ladybower’s twin bellmouth spillways send that overflow on down the river, so a site in the valley below cannot treat the reservoirs as flood defences.

• Peat and grit that shed rain. The Dark Peak headwaters produce rapid runoff and a fast-rising river after heavy rain.
• A tight valley with little floodplain. Through the Peak District the Derwent is squeezed into a narrow trough, so it floods out across valley-floor towns rather than spreading harmlessly.
• Tributaries stack the peak. The Noe, Wye, Amber and Ecclesbourne each add their own flow as the river runs south.
• The reservoirs are for water supply, not flood storage. Once Howden, Derwent and Ladybower are full, storm flow spills over and continues downstream.

Check flood risk along the River Derwent

The map below draws the River Derwent in coral, with the Environment Agency’s live river gauges positioned along it. Click a 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.

A history of flooding on the River Derwent

The Derwent has a long flood record, and the modern gauge readings put numbers on it. Serious floods came through the valley in 2000 and again in the summer of 2007, the events that fed into the national Pitt Review of flooding, and the Derby gauge set a fresh record in November 2019. None of those matched Storm Babet.

Between 19 and 21 October 2023, Storm Babet dropped days of rain onto an already wet catchment, and the Derwent responded the way its upland regime dictates. At Derby the river reached 3.58 metres on the morning of 21 October, breaking the November 2019 record on a gauge that has run since 1935, and the Environment Agency issued a Severe Flood Warning, its most serious category, meaning danger to life, for the Derwent at Chaddesden in the north-east of the city. Upstream, the towns and villages along the river flooded in turn as the peak moved down: Baslow, Calver, Grindleford and Hathersage in the Peak District, Chatsworth in its parkland, and Darley Dale below Matlock. Across the wider Storm Babet event the Environment Agency reported around 1,260 properties flooded, with Derbyshire among the counties that declared a major incident.

That sequence is what a flood risk assessment for a Derwent-side site has to reckon with: not a single design flood in isolation, but a river that has repeatedly shown how quickly and how high it can rise.

Which places does the River Derwent put at risk?

Flooding from the River Derwent falls on the valley-floor communities strung along it, and the Environment Agency runs a formal flood warning service for the reaches most exposed. Warning areas cover the river at Belper, at Darley Abbey and at Allestree in Derby, and a catchment-scale flood alert area, “Lower Derwent in Derbyshire”, sits over the lower river as a whole. The table below sets out the main settlements at risk, from the Peak District down to the Trent.

If your site sits in 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 Derby, Matlock and Derbyshire. Downstream, the Derwent’s flow feeds the River Trent, which carries its own flood risk through the East Midlands.

Is the River Derwent a Main River, and what that means for your site

Yes. The Environment Agency has designated the River Derwent a Main River along effectively its whole course, which means the Agency, rather than the county council, regulates the channel and the land immediately beside it. That matters if you are building 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, sought alongside planning permission rather than instead of it, and a site in the floodplain will normally need a flood risk assessment to support its application. Each smaller brook or ditch that feeds the Derwent is an ordinary watercourse instead, regulated by Derbyshire County Council or Derby City Council as Lead Local Flood Authority and by the riparian owners whose land they cross. Getting that classification right at the outset saves time and argument later.

• The 8 metre byelaw margin. Building, excavation or structures near the Derwent usually need a Flood Risk Activity Permit, sought alongside 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.
• Know which watercourse you are on. The Derwent is Environment Agency territory; its feeder brooks are ordinary watercourses under the Lead Local Flood Authority. We confirm this for every site.

What flood risk on the Derwent means for a development

A scheme on or near the Derwent floodplain usually sits in Flood Zone 2 or 3, and that shapes what a planning application has to show. Finished floor levels have to be set above the design flood level with the right climate change allowance, safe access and egress have to work in a flood, and where a building takes up floodplain the scheme often has to provide compensatory flood storage so it does not push water onto its neighbours. On the lower Derwent through Derby, where the floodplain is wide and the river slow, those questions bite hardest; in the gorge at Matlock the issue is more often the speed and depth of a fast, confined flood. A sound flood risk assessment reads the Derwent’s behaviour at your specific reach rather than relying on the national map alone.

Managing flood risk on the River Derwent: policy and contacts

Several bodies share responsibility for flooding on the Derwent. The river sits within the Humber River Basin District, draining to the Trent, and the Environment Agency regulates the Main River, maintains the Derwent hydraulic model, publishes the flood map for planning and issues flood warnings. Derbyshire County Council and Derby City Council are the Lead Local Flood Authorities for surface water, groundwater and ordinary watercourses, each with its own strategic flood risk assessment, and Derby City Council has invested in the Our City, Our River flood defences that protect much of the city centre. The Environment Agency’s Derwent Catchment Flood Management Plan sets the long-term catchment approach, and the council’s Section 19 investigation into Storm Babet documents in detail what happened along the river in October 2023.

• Live river levels. Check the Derwent’s gauges on the Environment Agency service, at Derby City, Matlock and Chatsworth, or use our own live river levels map.
• Screen a location. Try our free flood risk map and, for drainage design, the drainage calculators and BRE 365 infiltration calculator.
• Catchment data. The National River Flow Archive holds the long-run flow record for the Derwent at St Mary’s Bridge in Derby.
• 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.

Flood risk and drainage services for sites near the River Derwent

A development near the Derwent 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 deal with separate things: whether the site is safe from flooding, whether the development should be located where it is, and where the surface water goes. Unda prepares each of them for sites across the Derwent catchment.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements 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.
• Drainage strategies and SuDS. Surface-water designs that meet the discharge limits set by the Lead Local Flood Authority, through our sustainable drainage strategy service.
• Sequential and exception tests. Locational justification 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 is in play.

Where the national mapping is not enough, a common situation for a site right beside a Main River like the Derwent, we can bring in hydraulic flood modelling to derive accurate flood levels and extents, and prepare the detail needed to discharge a drainage condition. Everything we do is framed by the NPPF and its planning practice guidance.

How Unda helps with flood risk on the River Derwent

Unda understand how an upland river like the Derwent behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know how quickly this river responds to rain, where its floodplain bites hardest, the Matlock gorge, the Belper reaches and the wide lower floodplain through Derby, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a scheme through planning. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the valley.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping and Derwent model, the river’s gauged levels and record peaks, and the local planning policy that applies to the site. If you are working on a site near the River Derwent, talk to one of our experienced flood risk consultants about what your application will need.

River Derwent flood risk: your questions answered