Long-lasting, often regionally extensive, groundwater flooding can be caused by the water table in an unconfined aquifer rising above the land surface in response to extreme rainfall. This is sometimes referred to as clearwater flooding and is this form of flooding that has caused significant damage to properties on the Chalk outcrop of southern England in recent years.
When does groundwater flooding occur?
Groundwater flooding occurs when existing high groundwater levels and high unsaturated zone moisture content combine with intense rainfall.
In the Chalk aquifer, this can cause groundwater levels to fluctuate over several tens of metres. For example, in the winter of 2000 to 2001, groundwater flooding was associated with unusually high levels of rainfall: for an eight-month period starting in September 2000, rainfall in south-east England was 183 per cent of the long-term average, equivalent to a greater than 100-year return period.
Roads in Berkshire had to be closed during the exceptionally wet winter of 2000 to 2001 due to groundwater flooding. BGS © UKRI.
Groundwater levels were already high as there had been several years of above-average recharge before 2000. Significant groundwater flows occurred in dry valleys in localities far removed from floodplains.
River flows also reflected the high rainfall as well as the contribution from groundwater discharge. Flows at most river gauging stations greatly exceeded the long-term averages for extended periods and, in some catchments, sustained groundwater flooding occurred for several months.
Contact
Please contact BGS Enquiries (enquiries@bgs.ac.uk) for further information.