Research commissioned by Hampshire County Council and Winchester City Council has revealed a disparity between the predicted and actual cost of prolonged groundwater flooding. Actual costs can be up to three times higher than predicted costs under current models. The research has important implications for authorities and organisations involved in flood prevention planning.
The study was conducted by Middlesex University’s expert Flood Hazard Research Centre, to build on investigations carried out by the Environment Agency following the floods of 2000-2001. In the guidance for cost-benefit analysis of flood alleviation proposals, there had been no special consideration of long-duration flooding. Long duration is what characterizes groundwater flooding and contrasts it with flash-flooding from heavy rainfall.
Groundwater is the dominant cause of flooding in the chalk landscape of mid Hampshire, and this landscape exists elsewhere in southern and eastern England. Following prolonged rainfall throughout preceding months, groundwater levels in the chalk aquifers tend to rise steadily. The risk of flooding is greatest towards the end of winter, when groundwater can rise so high that it emerges above ground level and can flow for weeks or even months at a time, along valley floors that are normally dry.
Damage to premises flooded by groundwater during the widespread flooding of 2000-2001 highlighted a disparity between the financial losses predicted using existing cost-benefit assessment guidance, and the actual financial losses suffered. This led to a belief that the existing guidance might not adequately account for long-duration flooding such as that caused by groundwater.
Although the Department for Environment, Food and Rural Affairs expressed some reservations about the report’s conclusions, it has acknowledged that very long-duration flooding is likely to lead to higher building repair and other costs than those presented in standard damage tables.
As a result of this research, those considering proposals to alleviate groundwater or other long-duration flooding may wish to consider whether they can justify use of higher damage levels in appraisals to support their bids for funding.
You can download the full report in PDF format:
Report on flood losses associated with groundwater flooding 785kb