Groundwater (pluvial) flooding

Groundwater flooding

This is the most problematic type of flooding. It is different from surface water flooding caused directly by very high levels of rainfall. The geology of Broxbourne is predominantly London Clay and gravel, which contains layers of water-bearing sedimentary rock. 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 streams and rivers 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 or basements 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 channeled.

Groundwater flooding however can be predicated well in advance by the Environment Agency who monitor the aquifer levels throughout the year.  Groundwater flooding may be reduced by installing suitable land drainage if necessary.