OR/15/016 Results

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.

Counts of susceptible properties

The table below summarises the result of the estimate of properties that are in areas susceptible to groundwater flooding. On chalk and limestone aquifers 1 060 000 properties were identified as being in areas susceptible to Clearwater flooding. If properties that are also at risk from river and coastal flooding are eliminated from the count this is reduced to 920,000 properties.

For other aquifer types, 650 000 properties, or 580,000 when those also at risk from river and coastal flooding are eliminated, are in areas susceptible to Clearwater flooding.

4,000,000 properties, or 3 210 000 when those also at risk from river and coastal flooding are eliminated, are in areas susceptible to Permeable Superficial Deposit flooding.

Table 1 Properties in areas susceptible to groundwater flooding.

	Residential properties	Non residential properties	Total	Also floods from river and sea	Total less from and sea
Clearwater Chalk and limestone — emergent	375 000	125 000	500 000	81 000	420 000
Clearwater Chalk and Limestone — affecting infrastructure	423 000	141 000	564 000	63 000	501 000
Clearwater other aquifers	488 000	163 000	651 000	70 000	582 000
Permeable superficial deposits	2 981 000	994 000	3 975 000	768 000	3 206 000

Estimates of the number of properties affected

Groundwater flood susceptibility does not translate directly into numbers of properties affected in a flood incident. Each flood event will have its own unique hydrological characteristics, so flood magnitudes vary. In addition the impact of groundwater flooding on properties in an area is influenced by the nature of local drainage systems and by the extent of community adaptation. Natural drainage systems can, depending on topography, allow groundwater to drain away quickly, or may have limited capacity and cause surface ponding. Artificial drainage, either through ditches and culverts or sewer systems may act to artificially lower the water table. Houses and infrastructure can be raised above flood level. Because of these factors groundwater floods often have a greater impact on properties in rural areas. In urban areas a greater density of drains and sewers and more adaptation through building construction may mitigate flooding.

To assess the relationship between properties in flood susceptible areas and properties affected we examined reports of flooding received by the Environment Agency during the 2013/14 flood, which focussed strongly on the Clearwater Chalk flooding in Oxfordshire, Berkshire and along the South Downs. The ratio of properties in susceptible areas to reports of flooding is highly variable within individual affected communities, but across the counties an overall figure of between 5% and 15% of susceptible properties appear to have been impacted.

Assuming that vulnerability to hazard is similar across areas of the Chalk and Limestone aquifers where reports were not analysed, and outside South East England where the 2014 groundwater flood event was focussed, this implies 46 000 to 138 000 properties might be affected.

Estimating the number of properties that may be affected in other aquifers is hampered by lack of systematic observations. Large numbers of properties have been affected in susceptible areas, e.g. on Humberside in 2007, and in 2014 on the lower Thames, but the majority of affected properties have been in areas also at risk of flooding from river and sea as well.

Assuming that the lack of systematic observation is largely driven by the rarity of impact, we estimate that perhaps 2% to 4% of properties may be affected in other aquifers susceptible to Clearwater flooding, i.e. 11 600 to 23 200 properties. If similar vulnerability exists for areas affected by permeable superficial deposit flooding, it implies that between 64 000 to 128 000 properties may be affected. In addition there are 214 000 properties in areas of clearwater groundwater emergence and a further 770 000 on permeable superficial deposits where groundwater may exacerbate a flood from river or sea with a 1 in 1000 chance of occurring in any year.

Table 2 Estimate of properties affected.

Estimated properties affected	Total less river and sea	5%	10%	15%	Also floods from river and sea
Clearwater Chalk and Limestone	921 000	46 000	92 000	138 000	144 000
Estimated properties affected	Total less river and sea	2%	3%	4%	Also floods from river and sea
Clearwater other aquifers	582 000	11 600	17 500	23 200	70 000
Permeable Superficial deposits	3 206 000	64 100	96 200	128 200	768 000