Forecasting groundwater flooding risk in England this winter

Awareness of groundwater flooding has increased this past winter following heavy rainfall which has caused levels in aquifers to rise.

Jon Haycox, Data Manager

Rainfall in February 2020

As early as August 2019, GeoSmart Information was informing users of our groundwater flood forecast service that the recharge season had begun early and groundwater levels were high in Lincolnshire. In Barrow Upon Humber, tankers were required to remove excess waste that had overwhelmed the sewer network¹. The impact of groundwater flooding was also felt over Christmas when the railway line between Brighton and Haywards Heath was closed due to flooding of Patcham Tunnel, and the A35 and A354 roads were closed in Dorset due to flooding in areas identified as being at risk of groundwater flooding².

February 2020 was the wettest on record and it was the 5th wettest Winter³. Rainfall from Storm Ciara was heaviest over the Severn Basin in areas without aquifers vulnerable to groundwater flooding. Groundwater levels, in many places already above normal for the time of year, rose as a result of rainfall. Storm Dennis lead to heavy rainfall over southern England and groundwater flooding was predicted in Dorset, Wiltshire, Oxfordshire and Hampshire. Environment Agency warnings for groundwater flooding were issued, at one point more than 30 were in place. High groundwater levels remained through February and into March highlighting the temporal aspect of risk which the forecast service can be used to ascertain.

GeoSmart’s groundwater forecast map on 1 March 2020. Red markers indicate where groundwater levels are above a threshold or exceeding a threshold within 5 days.

Most recently, groundwater levels in South London have been of concern. The Caterham Bourne in Croydon began flowing and preparations were made for potential flooding which included building sandbag walls and installing additional pipework in rear gardens of homes to make sure the watercourse runs freely⁴. Trash screens were also being regularly cleared. River flooding can be exacerbated by groundwater and areas around Woldingham and Purleybury are at risk. There have been unconfirmed reports of flooding on Woldingham Road but the weather was dry so no significant groundwater flooding has been reported. The Environment Agency (EA) Groundwater Flood Alert for South East London was in place until the start of April.

Ten EA Groundwater Flood Alerts remain in place across England in mid-April more than a month since significant rainfall has fallen across Southern England. GeoSmart’s system forecasts groundwater levels so risks levels are lowered in realtime for areas where groundwater is falling. Groundwater levels are still high in the North Wessex Downs, Hampshire and the Kent Downs.

Groundwater forecast for Purleybury borehole on 17 February 2020. The borehole threshold is an indicator for Caterham Bourne and Wandle groundwater flooding where considerable impact was experienced in winter 2013/14

Aquifers in these areas tend to respond slowly to rainfall, rising gradually but also falling from the peak levels gradually. This highlights the spatial and temporal variations of groundwater flood risk across England and the need for real time monitoring of groundwater levels to give decision makers information for key infrastructure and underground utilities.

Please contact us to discuss how we can tailor a groundwater flood forecast service to your application to give you advanced warning of groundwater flood risks to enable smarter resilience planning and provide critical answers on when and how long flooding could occur.

1 Groundwater flooding season begins early, GeoSmart Information, 2 December 2019.

2 Groundwater Flooding: New Year Update, GeoSmart Information, 7 January 2020.

3 Record breaking rainfall, Met Office, 2 March 2020.

4 Preparing for possible flooding of the Caterham Bourne, Tandridge District Council, 6 March 2020.

Tags: flood risk, Flooding, Groundwater, groundwater risk