OR/15/016 Previous estimate
| McKenzie, A A, Ward, R S. 2015. Estimating numbers of properties susceptible to groundwater flooding in England. British Geological Survey Internal Report, OR/15/016. |
The DEFRA report calculated the number of properties at risk from a count of properties lying within zones defined on Groundwater Emergence Maps (GEMS) that were prepared as part of the study. The GEMS maps are composites, with a geological base, using information on aquifer properties, groundwater levels and reported flood incidents to define areas where groundwater may reach the surface or impact on infrastructure. The coverage of the GEMS maps was limited to major aquifers (as defined by hydrogeological mapping – not the former Environment Agency definition of major aquifers). 6.4 million properties (from OS Address point data, which can underestimate the true number of properties in an area) were located on these major aquifer units. Of these, 1.7 million were located in the GEMS zones, the majority on Chalk.
This value was refined by removing 100,000 properties that fell within the areas at risk of flooding from rivers and sea in a flood with a 1 in 100 chance of occurring in any year, and so were considered more likely to be at risk from surface water flooding. Of the remaining 1.6 million properties 380,000 were on the Chalk in the areas of Southern England considered most at risk from groundwater flooding.
The report recognised that far fewer than 1.6 million properties were affected in the 2000/2001 event - because the actual emergence of groundwater is affected by local factors and geological heterogeneity, and also because in the topographically lower portions of the GEMS zones, drainage systems and construction methodologies will mitigate the actual impact of a flooding event.
The report also considered the impact of groundwater level rebound in areas where, because of urban water abstractions or mining, groundwater levels were historically artificially depressed. No separate estimates of the number of properties that may be affected were given. The interrelationship between river flooding and permeable superficial deposits (see below) was also not part of the DEFRA study.