How the South Downs chalk aquifer determines flooding in West Sussex

Chalk is an aquifer, which means that it is able to soak up and retain water like a sponge. Water can move through the cracks in chalk, which are called fissures. Flooding is caused when prior conditions of high groundwater levels and high unsaturated zone moisture contents mix with lots of rainfall.

When it comes to the chalk on the South Downs, this is likely to cause groundwater levels to increase rapidly over several tens of metres. This results in mass flooding. Groundwater protection measures have been put in place in parts of the area since the 1920s and are extensive. These include controls on land use near groundwater boreholes; known as source protection zones, and a comprehensive aquifer protection policy. This has been based on assessments of the vulnerability of the aquifer to contamination across Sussex.

Lavant has been the subject of numerous flooding reports over many years and records go back to the 19th Century. The most recent has been be the January 2015 Upper Lavant Valley Flood Risk. Due to the underlying chalk, the River Lavant is primarily a groundwater-fed stream.

The river therefore responds to changes in groundwater levels throughout the year; it frequently runs dry during the summer months when groundwater levels within the chalk catchment are low. The flooding happens after heavy rainfall when surface water runoff cannot be absorbed in heavily saturated ground.