Groundwater is normally contained within rock layers (‘aquifers’) beneath the ground. Our aquifers are a key store of drinking water in the UK – holding about 20 times more water than all our surface water reservoirs. In the South East of England groundwater provides about 90% of all drinking water.
The water table goes up and down within the aquifers depending on how much groundwater is stored. During droughts, the water table is low; during wet periods the water table rises. As the water table falls and rises, spring flows from the aquifers also go up and down. After periods of persistent heavy rainfall, like early 2012, the water table may rise to levels that it does not often reach. Springs flow at very high rates, and new springs may emerge up-slope from the usual spring line. If this extra water cannot drain away, a flood happens. Unlike typical river flooding, this emergent groundwater is usually crystal clear.
This is how the most common type of groundwater flooding occurs, and it mainly occurs in valleys of the chalk downlands of the south and east of England. In July 2012 there was groundwater flooding in Dorset that disrupted main transport routes to the Olympic sailing just weeks before the start of the event. Similar disruption in 2000 cut-off Brighton from London as a main road and railway line to and from the city were inundated. The city centre of Chichester was cut-off in January 1994 because of groundwater flows from the normally gentle River Lavant.
At other locations, groundwater flooding may occur on the floodplains of larger rivers, or at the coast. As river levels rise, the surface water moves into the river banks to flow through the flood plain deposits. The water then seeps out of the ground in depressions at locations not connected with the main flood. This happened in Oxford in July 2007, for example, as a response to rising levels in the River Thames and its tributaries.
Groundwater flooding is rare, but when it happens the floodwaters can linger for months. Because of this, the cost of groundwater flooding to householders or their insurers can be around three times that caused by river flooding. Disruption caused by long-term closures of roads and railways can be very costly.