OR/15/016 Discussion
| 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 methodologies used in the 2004 Defra report that estimated that 1.6 million properties were located in areas where groundwater flooding could occur used a relatively simple model of groundwater levels, and a relatively narrow range of geological and hydrogeological conditions. We have calculated a revised estimate from BGS groundwater flood susceptibility mapping. The DEFRA estimate was focussed on major aquifers, and in particular the Chalk, and so should be compared to the new estimate of the number of properties in areas where clearwater flood emergence on Chalk and Limestone is possible — 920 000 properties. This smaller estimate of properties in areas where groundwater emergence might occur results from the use of a more detailed model of groundwater level, and a more conservative estimate of potential rise in groundwater levels than was used in the 2004 DEFRA report. If account is taken of the difference in accuracy of an NRD property count and the earlier postcode estimates, the difference is even larger.
Flooding from groundwater on other aquifers and in permeable superficial deposits was not quantified previously. We estimate that there are 580 000 properties on other aquifers, and 3 210 000 on superficial deposits.
The estimates of the number of properties in zones where groundwater emergence is possible need to be qualified by the fact that only a small percentage of these properties may be vulnerable and directly affected by an event (although secondary effects such as sewer surcharge and flooding of roads may also be a factor). Local topography, and adaption, for instance the construction of land drainage and sewers, reduce the number of vulnerable properties. Based on the analysis of flooding observed during early 2014, and taking a mid-range value for the numbers of properties at a similar level of risk, we would estimate that around 205 700 properties are actually at risk in the event of groundwater flooding of similar severity.
The total number of properties in areas at risk from groundwater flooding is now estimated to be between 122 000 and 290 000.
Our estimate is focused on flooding with a direct impact on properties. Groundwater flooding may have secondary impacts, for instance disruption to transport, sewer surcharging and damage to agricultural productivity.
Improved estimates of both groundwater flood susceptibility and receptor vulnerability are possible, at least on the Chalk where a combination of higher resolution groundwater level mapping, an inventory of historically flooded properties and modelling of where emergent groundwater runs off the landscape would all be valuable. On other aquifers the priority should be to improve the understanding of when shallow water tables give rise to flooding events.