These aquifers are susceptible to flooding as the storage capacity is often limited, direct rainfall recharge can be relatively high and the sediments may be very permeable, creating a good hydraulic connection with adjacent river networks.
Groundwater levels are often close to the ground surface during much of the year. Intense rainfall can cause a rapid response in groundwater levels; rising river levels, as the upstream catchment responds to the rainfall, can create increased heads that drive water into the aquifer.
Natural levees and man-made structures can allow river levels to rise without breaking their banks; groundwater flooding will occur in low-lying areas beyond the banks, preceding any fluvial flooding and lengthening the overall period of flooding.
However, flooding in these systems can be relatively short-lived compared with Chalk flooding as rivers, returning to pre-flooding levels, quickly drain the highly permeable aquifer. These hydrogeological settings often coincide with urban areas and it is clear that the role of groundwater in flooding needs to be addressed as the traditional engineered methods of flood protection may be circumvented by flow through the subsurface.
The difficulty is that it can be hard to distinguish this type of groundwater flooding from fluvial or pluvial floods.
The following BGS research projects address the issue of groundwater flooding in a shallow unconsolidated sedimentary aquifer setting:
Contact David Macdonald for further information