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Almost 300,000 homes at risk from groundwater flooding

New research from the British Geological Survey has suggested that up to 290,000 homes could be at risk from groundwater flooding.

The new report - Estimating numbers of properties susceptible to groundwater flooding in England - is based on information gathered during the flooding in 2014.

Nonetheless, the BGS has said that it is difficult to understand and model risk from groundwater flooding compared to other sources of flooding and more research is needed.

“Groundwater follows underground flow paths and the exact locations where water may emerge, and the volumes of water, depend on variations in geology,” states the report. “This may be further influenced by human activity such as underground foundations and excavations for infrastructure. Consequently, most of the maps in existence are based on an understanding of hydrogeology, and show areas where it is possible that floodwater might emerge; they do not necessarily show areas that might be flooded by groundwater.

“Groundwater will not emerge uniformly and that some places will not experience groundwater emergence. Groundwater will not flood the whole of these areas of possible emergence and, because it will always follow the easiest path, it can flow downhill to areas away from these emergence zones. Whether or not properties flood internally will depend on their construction and any mitigation measures in place, as well as groundwater levels. This means it is not appropriate simply to add up the number of properties in emergence zones and class them all as at risk of flooding.”

According to the report, up to 4.7M properties lie in areas where the hydrogeology suggests that groundwater emergence may be possible. However, using data from the 2014 flooding within the chalk aquifer the BGS has estimated that the real figure may be up to 290,000.

The BGS has said that although groundwater flooding may be locally significant, far fewer properties are at risk from groundwater flooding than from river, sea or surface water flooding. However, the report adds that underground infrastructure and basements – especially where basement conversions have been undertaken – are at a greater risk of groundwater flooding.

The BGS has said that the figures in the report are estimates based on assumptions outlined in detail in the report. It says that the estimates could be improved with more detailed modelling and mapping, which in turn would need to be supported by better, more consistent and complete collection and collation of records of observed groundwater flooding.

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