Some of the flooding that affected utility sites in 2000 & 2007 was pluvial in nature. Pluvial flooding is the ponding of surface water following heavy rainfall events, whereas fluvial flooding results from rivers or streams overtopping their banks. ADAS developed and tested a screening tool to identify priority sites for pluvial Flood Risk Assessment, enabling targeting of resource that will help avoid costly and disruptive damage to infrastructure.
Utility companies have a responsibility to reduce the risk of flood damage to their infrastructure and thus the adverse consequences to the general public. For fluvial flooding, areas at risk are identified and mapped by the Environment Agency. For pluvial flooding, similar risk maps are not readily available.
Utility companies would therefore benefit from a method that allows for the early assessment of risk from pluvial flooding, enabling them to prioritise Flood Risk Assessments (FRAs) to those sites that may be most at need of management. Risk from pluvial flooding is dependent on topography, catchment characteristics such as area and surface permeability and drainage system capacity.
ADAS has developed a cost-effective screening tool that enables the identification of sites that are likely to have the highest risk of pluvial flooding and therefore should be investigated as a priority.
First a Digital Terrain Model (DTM) is used to identify ‘risky’ topographical features at the site such as sinks, channels and areas of flat land via an automated procedure. Sites that do not have any of these features are classed as low priority.
For the remaining sites, natural catchments are delineated using the DTM, and features of the catchment that are required for peak runoff estimation quantified in a GIS from other data layers (e.g. soil types). A design storm peak runoff is then estimated for each site using a bespoke tool to assign sites to three priority categories for FRA – high, medium and low.
The performance of the tool has been quantified for 25 sites that underwent pluvial FRA. None of the predicted low priority sites, half of the predicted medium priority sites, and two thirds of the predicted high priority sites were judged to be susceptible to pluvial flooding following FRA. Statistically, this showed that the screening tool was a good predictor of whether or not a site was at risk from pluvial flooding.
Where screening indicates there to be a potentially elevated risk of pluvial flooding to utility infrastructure, further investigation is required. ADAS experts will provide the site operator with a thorough understanding of the risk to a site by conducting a Flood Risk Assessment.
The main benefits of the screening tool are that:
• A large number of sites can be screened in a very short space of time and at low cost;
• Identification of sites that should be prioritised for FRA will help utility companies to direct their resource most effectively, thus help avoid costly and disruptive damage to infrastructure during future heavy rainfall events.
For more information please contact Lucy Wilson on 01902 693376 or email lucy.wilson@adas.co.uk