Rainfall, Flood Peaks and Sewage Overflows: January 20-22 storm on the River Mole

Here's a quick round-up of the recent flooding on the River Mole (20–24 January)

Between 20 and 22 January, around 40 mm of rain fell across the River Mole catchment. That total isn’t remarkable on its own — but the way the rain arrived made this flood more interesting than a simple one-off downpour. (No photos here... please see our Facebook page for observations from our members of this flood event.)

Instead of falling all at once, the rain came in several distinct bursts.

Rain came in a series of fronts wrapping round a Low. Radar image courtesy Netwx radar

The first rain, shown in the radar image above (courtesy netwx radar), arrived on the evening of Tuesday 20 January, with the heaviest rainfall from a similar front around 5 pm on Wednesday 21 January.

This stop-start pattern created a very different response across the catchment.

In the upper catchment — around Ifield, Gatwick and Crawley — tributaries reacted rapidly to each burst of rain shown by the Gatwick river gauge above. River levels rose quickly, dipped slightly, then rose again with each new pulse of rain. Because the ground was already wet, each rise tended to happen more quickly as soils and floodplains had less capacity to store water.

Further downstream, those individual peaks began to merge into one larger flood wave. Water from different tributaries arrives at different times, and as these flows combined, the smaller ups and downs are smoothed out into a single, clearer flood peak. In the chart below showing the Kinnersley Manor gauge near Sidlow Bridge note also the contrast between the steep rise in river level and the more gentle recession of the falling river - a clear sign of prolonged rain continuing after the main deluge that is represented in the flood peak.

Kinnersley Manor gauge shows a single flood peak

Several major tributaries — including the Burstow Stream and Salfords Stream — join the River Mole near Kinnersley Manor (Sidlow Bridge). By the time floodwater reaches this point, the separate peaks have effectively blended together in one large peak as shown above. As a result, between Horley and Sidlow Bridge the river begins to show just one main flood peak rather than several smaller ones.

From Sidlow, the flood wave then travels downstream as a single surge, typically reaching Brockham within about 3–6 hours. In very extreme floods the Mole can behave in surprising ways — on rare occasions Brockham has even peaked before upstream gauges, possibly exhibiting the enormous wave length of a flood peak in a prolonged rainfall event — but this event followed the more familiar pattern.

By the time the flood wave arrived in Dorking all signs of the separate rainfall episodes have gone and the peak discharge may appear to have derived from a distinct single rainfall event.

How quickly did the river respond to rain?

For this event, lag times (the time between peak rainfall and peak river flow) were very close to long-term averages for similar storms (based on 2022–2026 data):

• Horley: ~9 hours

• Dorking: ~20 hours

• Leatherhead: ~25 hours

• Cobham: ~32 hours

• Esher: ~41 hours

Serious water quality impacts

The rainfall also triggered over 1,000 hours of sewage discharges in the 4 days between 20-24 Jan into the River Mole from Thames Water sewage works and pumping stations.

Gant chart showing EDM start and stop times (some are still going!)

Some works — including Crawley, Holmwood, Leatherhead, Horley and Earlswood (Reigate) — began overflowing overnight on 20–21 January after less than 10mm of rainfall, while Dorking and Esher triggered later, around late morning on 21 January after no more than 15mm of accumulated rain. Note that several storm overflows are still discharging at the time of writing (am Saturday 24 Jan).

As a result, E. coli and bacterial levels are likely to be elevated for some time, and so contact with the river — for both people and dogs — should be limited over the coming days.

How big was this flood?

This was a moderate flood, with a return period of less than 2 years under the FEH22 model. It was clearly smaller than the floods of 4 January 2024 and 4 January 2025.

For comparison:

• January 2025: around 11 m³/s at Gatwick, 69.5 m³/s at Esher, and 3.77 m ASD at Brockham

• This event: around 10.2 m³/s at Gatwick, 62 m³/s at Esher, and 3.6 m ASD at Brockham

• January 2024: Esher peaked even higher, at 76 m³/s

The January 2025 flood also saw much higher levels on the Salfords Stream (around 2.7 m ASD), which caused flooding across the A23 at Salfords Bridge. This time levels reached about 2.3 m ASD, avoiding bridge flooding. Several low-lying roads and bridges around Leigh, Flanchford, Betchworth and Brockham did flood, but Brockham Bridge itself remained dry, despite levels coming close to the threshold.

Overall, a discharge of around 60 m³/s is fairly typical for a modest annual flood on the River Mole.

For longer-term context, it’s worth remembering that during the December 2013 floods, flows exceeded 80 m³/s at Kinnersley Manor, 100 m³/s at Leatherhead, and nearly 150 m³/s at Esher — following around 80 mm of rain in just 24 hours after an already very wet December. Further back, in the 1968 flood the Mole catchment experienced rainfall in excess of 100mm in 24 hours and the discharge at Esher was estimated at 241 m3/s.

Thank you for reading this summary. I've not included photos of the flooding.. these are available on our Facebook page at River Mole River Watch.