This is the most problematic type of flooding in Hampshire. It is different from surface water flooding caused directly by very high levels of rainfall. The geology of Hampshire is predominantly chalk, which contains layers of water-bearing rock, clay, or sand. These layers are called aquifers. There is a natural cycle in which the aquifers are filled with rain water in the winter and discharged into chalk streams such as the Test and Itchen in the summer months to provide a regular river flow all the year round.
When the aquifers are filled to overflowing in the winter, natural springs and winterbournes are activated (winterbournes are streams or rivers that are dry in the summer months). Exceptional periods of rain can cause groundwater flooding from springs and winterbournes which inundate roads and overwhelm drainage systems. This type of flooding can last for weeks or months. An early indication of groundwater flooding is often when property cellars start to fill with water.
Engineering solutions that can be put in place to mitigate the impact of this type of flooding are limited simply because of the huge volumes of water involved and because it is not contained or channelled.
Groundwater flooding, however can be predicated well in advance by the Environment Agency who monitor the aquifer levels throughout the year.
Any groundwater highway flooding should be reported where appropriate traffic management arrangements can be put in place.
Additional information added August 2007:
Research commissioned by Hampshire County Council and Winchester City Council has revealed a disparity between the predicted and actual cost of prolonged groundwater flooding. Actual costs can be up to three times higher than predicted costs, when using current models. The research has important implications for authorities and organisations involved in flood prevention planning.
Read more about the research