River Kennet Flooding & Flood Risk

The River Kennet looks like a placid chalk stream, but it floods in three very different ways. Clear, groundwater-fed water rises slowly out of the Marlborough Downs, yet the same catchment delivers flash surface-water flooding off the chalk, out-of-bank fluvial flooding when the ground is saturated, and prolonged chalk-groundwater flooding that can sit in low-lying property for weeks. From Marlborough through Hungerford, Newbury and Thatcham to the Thames at Reading, flooding from the River Kennet has repeatedly hit homes, businesses and development sites. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Kennet floodplain — to talk one through, call 01293 214444 or email enquiries@unda.co.uk.

On 20 July 2007, a rainstorm estimated at a 1-in-169-year severity sent water pouring off the hills into Thatcham and flooded around 1,100 properties in a single afternoon — one of the worst flash floods England saw that summer, and a reminder that a “gentle” chalk river is anything but.

• A chalk river with a split personality. Very high baseflow keeps the Kennet running clear and steady, yet the catchment still produces flash surface water, fluvial and groundwater flooding.
• A designated Main River. The Environment Agency regulates the Kennet, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• Five live Environment Agency gauges. Winterbourne Monkton, Marlborough, Newbury, Calcot and Blakes Lock in Reading all appear on the map below, with their latest levels updating in real time.
• Development pressure the length of the valley. From Marlborough down to Reading, sites in and near the floodplain almost always need a flood risk assessment for planning.

Check flood risk along the River Kennet

The map below draws the River Kennet in coral, 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 Kennet flood?

The River Kennet floods for reasons rooted in its chalk. For most of the year the chalk of the Marlborough and Berkshire Downs acts like a giant sponge, storing winter rain and releasing it slowly, which is why the Kennet is one of England’s classic chalk streams and runs clear and steady where a clay river would be flashy and brown. That same chalk is what makes its flooding so varied. When the aquifer fills in a wet winter, groundwater emerges through springs and seepages and can flood low ground for weeks; when heavy rain falls on already-saturated ground, or on the town surfaces and clay-capped hills, water runs off fast and the river spills its banks; and the Kennet’s more responsive tributary, the River Enborne, can add a sharp fluvial peak of its own.

The river rises from chalk springs around Avebury and Silbury Hill in the Marlborough Downs of Wiltshire and runs roughly 72 kilometres east through Marlborough, Hungerford, Newbury and Thatcham before joining the River Thames in central Reading, on the reach above Sonning Lock. In summer it can contribute up to half of the Thames’s flow. Along the way it gathers the River Og at Marlborough, the River Dun at Hungerford, and then the Lambourn, the Enborne and Foudry Brook in the Newbury and Reading reaches. To its lowest flow gauge at Theale, the Kennet drains a catchment of around 1,033 square kilometres.

• Chalk baseflow, then sudden response. The Kennet’s base flow index at Theale is about 0.88 — extremely high — but the Environment Agency records that it “can react quickly to heavy rainfall”, with a responsive contribution from the River Enborne.
• Three flood mechanisms, one catchment. Surface-water run-off off the downs, out-of-bank fluvial flooding on the main river, and slow chalk-groundwater flooding all affect the valley — often the reason a single site has to weigh more than one source.
• A record flow set in February 2014. The most downstream gauge, the Kennet at Theale, recorded its highest daily mean flow on 8 February 2014 at 48.86 cubic metres per second — against a long-term average of about 9.7.
• The Kennet & Avon Canal shares the valley. Between Reading and Newbury the canalised Kennet and the navigation run together, so canal, river and floodplain interact along the lower reaches.

A history of flooding on the River Kennet

The Kennet’s flood record shows all three of its mechanisms in turn. The defining modern event came on 20 July 2007, when a rainstorm of around 1-in-169-year severity fell on the hills north of Thatcham. The surface-water sewers, already full from rain on the town itself, could not cope, and run-off poured down into the streets: about 1,100 properties flooded, and many residents were still out of their homes 18 months later. West Berkshire Council, as Lead Local Flood Authority, investigated the event and the flooding it caused across the Kennet valley.

The winter of 2013 to 2014 brought the other two mechanisms together. Groundwater rose out of the chalk and, combined with surface water, flooded low-lying areas around Marlborough, Hungerford and Newbury, while the river itself ran to record fluvial flows — the Kennet at Theale peaked at 48.86 cubic metres per second on 8 February 2014. Those flows were the first real test of the newly completed Newbury Flood Alleviation Scheme, opened just days earlier on 7 February 2014, which was seen to perform well in protecting the town centre from river flooding. Further back, in January 2003, high water on the Kennet and the Thames flooded around 250 properties in Reading, where flood marks at nearby Mapledurham Lock record the great floods of 1894, 1947 and 2003.

Which areas does the River Kennet put at risk?

Flooding from the River Kennet is concentrated on the valley floor, and the Environment Agency runs a formal warning service for the communities most exposed. Two catchment-scale flood alert areas cover the river — one from its source down to Newbury, the other from Thatcham down to Reading — and within them sit more precise flood warning areas at Hungerford, Newbury, and the Theale-to-Reading reach, where the Environment Agency can warn individual properties. The table below summarises the main settlements at risk from source to mouth.

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 Reading, Berkshire and Wiltshire.

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

Yes. The Environment Agency has designated the River Kennet a Main River along its course, which means it, rather than the local council, is the regulator for the channel and its immediate margins. That distinction 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, ditches and mill streams feeding the Kennet are ordinary watercourses instead, regulated by the county council as Lead Local Flood Authority and by the riparian owners whose land they cross — and getting that classification right at the outset saves time later.

• The 8 metre byelaw margin. Building, excavation or structures near the Kennet 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.
• Know which watercourse you are on. The main river is Environment Agency territory; its tributaries are ordinary watercourses under the Lead Local Flood Authority. We confirm this for every site.

Flood risk and drainage services for sites near the River Kennet

A development near the River Kennet 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 Kennet 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 manage runoff and meet the discharge limits set by the Lead Local Flood Authority — see 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 rising chalk water table is in play — a live issue in the Kennet valley.

Where national mapping is not enough — a common situation for sites right beside a Main River — we can bring in hydraulic flood modelling to derive accurate flood levels and extents, and prepare the detailed scheme needed to discharge a drainage condition. Everything we do is framed by the NPPF and its planning practice guidance, including the climate change allowances that apply to peak river flows.

Managing flood risk on the River Kennet: policy and contacts

Several bodies share responsibility for flood risk on the River Kennet, and it is worth knowing who does what. The river sits within the Kennet management catchment, part of the Thames River Basin District. The Environment Agency regulates the Main River, publishes the flood map for planning and issues flood warnings; the county and unitary councils act as Lead Local Flood Authorities for surface water, groundwater and ordinary watercourses — Wiltshire Council in the upper catchment, West Berkshire Council through Hungerford, Newbury and Thatcham, and Reading Borough Council at the confluence — each with its own strategic flood risk assessment. At Newbury, the £2 million Newbury Flood Alleviation Scheme, opened in February 2014, reduces the risk of river flooding to the town to a 1-in-100 (1%) annual chance and protects more than 380 homes and around 70 commercial properties.

• Live river levels. Check the river’s gauges on the Environment Agency service — Kennet at Marlborough and Kennet at Newbury — or use our own live river levels map.
• Screen a location. Try our free flood risk map and, for drainage design, the drainage calculators, the BRE 365 infiltration calculator and the ground conditions and soakaway map.
• Flood warnings. Check the River Kennet from Thatcham down to Reading flood alert area, 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 Kennet

Unda understand how a chalk river like the Kennet behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know why this river can look benign and still flood three different ways, where its floodplain bites hardest through Thatcham, Newbury and the Theale reaches, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the valley.

Trading since 2014, we have completed more than 5,000 flood risk assessments across England and Wales. Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping, the river’s gauged levels and modelled flows, and the local planning policy that applies to the site. If you are working on a site near the River Kennet, talk to one of our experienced flood risk consultants about what your application will need.

River Kennet flood risk: your questions answered