River Mole Flooding & Flood Risk

The River Mole is one of the flashiest rivers feeding the Thames. Much of its catchment sits on heavy Weald and London Clay, and its headwaters gather runoff from the hard surfaces of Gatwick Airport and Crawley, so the river can turn from a quiet channel into a bank-topping flood within hours of sustained rain. It runs around 80 kilometres from Rusper in West Sussex to the Thames at East Molesey, putting Horley, Reigate, Dorking, Leatherhead, Cobham, Esher and Molesey at risk along the way. Unda prepares flood risk assessments, drainage strategies and related reports for sites the length of the Mole's floodplain. To talk to us, call 01293 214444 or email enquiries@unda.co.uk.

On Christmas Eve 2013 the River Mole reached its highest recorded level at Leatherhead — 3.73 metres — flooding homes in Fetcham and disrupting Gatwick Airport, part of a winter that caused around 270 internal property floods across Mole Valley; the river's worst flood on record remains September 1968, still the benchmark event behind the defences that protect the Lower Mole today.

• A flashy, clay-fed river. Weald Clay and London Clay across much of the catchment, plus the impermeable surfaces of Gatwick Airport and Crawley in the headwaters, mean the Mole responds quickly to rain and peaks sharply.
• A designated Main River for most of its length. The Environment Agency regulates the Mole, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• A dense live gauge network. Nine Environment Agency gauges from Gatwick down to Esher are shown on the map below, each updating in real time.
• Development pressure the length of the valley. From Crawley and Horley down to the Elmbridge towns, sites in or near the floodplain will almost always need a flood risk assessment.

Check flood risk along the River Mole

The map below draws the River Mole 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 flood zones. It is an indicative screening tool, not a flood risk assessment.

Why does the River Mole flood so fast?

The River Mole floods because of what lies beneath its catchment and what has been built on top of it. Much of the upper catchment sits on Weald Clay and London Clay, both low-permeability rock that sheds rainfall rather than absorbing it, and that natural tendency is sharpened by the hard, engineered surfaces of Gatwick Airport and Crawley new town in the headwaters. Together they give the Mole a genuinely “flashy” character: levels can climb several metres within hours of sustained rain, then fall away almost as quickly once it passes.

The river rises at Baldhorns Copse, just south of Rusper in West Sussex, and runs for around 80 kilometres to meet the Thames at East Molesey, opposite Hampton Court, draining a catchment of around 500 square kilometres — a little under 5% of the whole Thames catchment above Teddington. Along the way it gathers the Gatwick Stream and Ifield Brook near its source, the Earlswood Brook at Sidlow (draining the urban area of Reigate and Redhill), and further downstream the Tanners Brook and Deanoak Brook near Brockham and the Rye through Ashtead. Between Dorking and Leatherhead the river cuts through the chalk of the North Downs in the Mole Gap beneath Box Hill, a stretch long noted for its swallow holes, where the bed can run dry in low flow even though the channel still carries the full flow in flood. Near its mouth, the engineered River Ember channel and the Lower Mole Flood Alleviation Scheme's flood-relief works take some of the pressure off the final reach into the Thames.

• A clay catchment that sheds rain. Weald Clay and London Clay across much of the catchment produce rapid runoff, so the river rises fast and peaks sharply after prolonged or intense rainfall.
• An urbanised upper catchment. Runoff from Gatwick Airport, Crawley and Horley reaches the Mole quickly through the Gatwick Stream and Ifield Brook, adding to peak flows before the river has travelled far.
• A dense tributary network. The Earlswood Brook, Tanners Brook, Deanoak Brook and the Rye all feed the main channel, concentrating runoff from a wide catchment into one valley.
• A tide-influenced lower reach. Near East Molesey the Mole can be held up by high water on the Thames, which complicates simple flood thresholds on the final stretch.

A history of flooding on the River Mole

The Mole's flood record stretches back decades, and two events define it. The September 1968 flood remains the benchmark: after more than 100 millimetres of rain fell over two days, the lower Mole is reconstructed to have discharged over 240 cubic metres per second, roughly a 1-in-200-year flow. Cobham Viaduct was washed away on 15 September 1968 and the original road bridge at Box Hill was destroyed; downstream, East Molesey and Thames Ditton flooded so badly that the river was subsequently re-engineered below Albany Bridge and new flood defences built — work that grew into today's Lower Mole Flood Alleviation Scheme. Local accounts put the toll in Molesey alone at around 6,000 properties, though that figure comes from local history rather than a national statistical return.

The river also flooded badly much closer to its source. In early November 2000, during what the Environment Agency later called the wettest autumn in nearly 300 years, more than 150 millimetres of rain fell across the Upper Mole catchment in 72 hours. The Gatwick Stream and Mole exceeded their pre-development 1-in-100-year flow, flooding Maidenbower in Crawley, closing roads in Three Bridges for almost a day, and overtopping the Mole's southern bank at Gatwick Airport itself, where floodwater reached perimeter roads and service tunnels.

The most severe flood since 1968 came on Christmas Eve 2013. Exceptional rainfall over 23–24 December sent the Mole to its highest recorded level at Leatherhead, 3.73 metres, with Kinnersley Manor and Dorking also recording their highest levels on the same day. Surrey County Council's Section 19 flood investigation recorded around 270 incidents of internal property flooding across Mole Valley that winter — caused by rainfall 275% of the winter average — concentrated in Brockham, Fetcham, Leatherhead and North Holmwood; 181 properties in the district received council tax relief as a result. Around 40 homes flooded in the Cannon Grove area of Fetcham, Brockham Lane was closed as the river rose over the bridge, and guests at the Burford Bridge Hotel near Dorking were advised to shelter in place rather than evacuate. The Mole ran high again during Storm Dennis in February 2020, one of the wettest Februaries on record across southern England.

Which areas does the River Mole put at risk?

Flooding from the River Mole is concentrated on the low ground of the valley floor, and the Environment Agency runs a formal warning service for the communities most exposed. A catchment-wide flood alert area — covering the River Mole and its tributaries from Kinnersley Manor to South Hersham — spans Sidlow, Leigh, Betchworth, Brockham, Pixham, Dorking, Mickleham, Leatherhead, Fetcham, Stoke D'Abernon and Cobham, and within it more precise flood warning areas let the Environment Agency warn individual communities. 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 Crawley, Dorking, Leatherhead, Cobham, Esher and Molesey, plus Walton-on-Thames further along the Thames.

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

Yes — the Environment Agency has designated the River Mole a Main River for most of its length, which means the Agency, rather than the local council, regulates the channel and its immediate margins. That distinction has direct, practical consequences 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. Many of the Mole's smaller tributaries and headwater streams are ordinary watercourses instead, regulated by the relevant Lead Local Flood Authority and by the riparian owners whose land they cross. Because the Mole rises in West Sussex and flows through Surrey, responsibility for those ordinary watercourses is genuinely split: West Sussex County Council is the Lead Local Flood Authority around Crawley, Gatwick and Horley, and Surrey County Council takes over for the middle and lower catchment — getting this right at the outset saves time later.

• The 8 metre byelaw margin. Building, excavation or structures near the Mole 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.
• Two Lead Local Flood Authorities share the catchment. West Sussex County Council covers the upper, Sussex reaches; Surrey County Council covers the middle and lower catchment. We confirm which applies for every site.

Flood risk and drainage services for sites near the River Mole

A development near the River Mole 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 Mole valley.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the Mole’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 on a fast-responding, clay-influenced catchment 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 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 near the chalk of the Mole Gap is in play.

Where national mapping is not enough — a common situation for sites right beside a Main River with a history like the Mole's — 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.

Managing flood risk on the River Mole: policy, live levels and contacts

Several bodies share responsibility for flood risk on the River Mole, and its catchment carries some of the most established flood defence infrastructure in the South East. The Mole sits within the Thames River Basin District, and the Environment Agency's Thames Catchment Flood Management Plan splits it into a Middle Mole area, where around 210 properties currently carry a 1% annual chance of river flooding, and a Lower Mole area protected by the Lower Mole Flood Alleviation Scheme, built in response to the 1968 flood and providing defence to roughly a 1-in-200-year standard — a scheme now the subject of an Environment Agency consultation on its long-term refurbishment. Further upstream, the Upper Mole Flood Alleviation Scheme, built after the 2000 floods around Tilgate Lake, Clay's Lake and a new attenuation basin at Worth Farm, reduces risk to homes and businesses across Crawley, Horley and Gatwick, and natural flood management works on the Pipp Brook above Dorking add further attenuation upstream.

• Live river levels. Check the Mole’s gauges on the Environment Agency service — Leatherhead, Dorking and Kinnersley Manor — or use our own live river levels map.
• Flood warnings. The River Mole flood alert area and the Leatherhead & Fetcham warning area can be followed directly; sign up for Environment Agency flood warnings and call Floodline on 0345 988 1188.
• Lead Local Flood Authorities. West Sussex County Council for the upper catchment; Surrey County Council for the middle and lower catchment.
• Screen a location. Try our free flood risk map and, for drainage and ground conditions, our geology and soakaway map, drainage calculators and BRE 365 infiltration calculator.
• 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 Mole

Unda understand how a fast-responding, clay-influenced river like the Mole behaves in flood, and what the planning system and the Environment Agency expect from the reports that follow. We know how quickly this river answers heavy rain, where its floodplain bites hardest along the Leatherhead, Cobham and Esher reaches, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning. Our own offices sit in the Upper Mole catchment, in the same stretch of Crawley and Gatwick that flooded so badly in November 2000 — a piece of local history we wrote about in detail for its 25th anniversary — and that direct familiarity with how the catchment behaves informs assessments we prepare for sites the length of the valley, from the Gatwick headwaters down to the Thames.

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 Mole, talk to one of our experienced flood risk consultants about what your application will need.

River Mole flood risk: your questions answered