Spatial joint probability for flood and coastal risk management and strategic assessments:Method report - SC140002/R1

This report outlines the process and methodology of generating and selecting reasonable worst case scenarios used to inform the evidence base for the 2016 update to the National Risk Assessment of inland flooding risks. The method uses carefully collated historical river flow and rainfall data from the Heffernan and Tawn joint probability model (previously recommended in the Environment Agency research project SC060088 which investigated the spatial coherence of flood risk) to create a large number of extreme but statistically plausible events. A set of extreme events defined at river flow and sub-daily rainfall gauges across England and Wales were developed into inland flood scenarios based on the statistical simulations. The likelihood associated with each of the new scenarios can be assessed in terms of a hydrological proxy for the aggregate severity of the event, based on the average of the extreme flow or rainfall values over the network of gaugesas well as a simplified measure of exposure. Although this is a very simple metric chosen as a proxy to indicate the relative severity of each event, this approach was considered a reasonable basis to inform and support the development of the scenarios through a blend of statistical analysis and expert judgement. The hydrometeorological plausibility of each event was also considered within this process, based on qualitative analysis of the large-scale climatological and meteorological drivers for flooding in the British Isles. A number of events were selected for consideration to provide example scenarios of extreme rainfall and river flow events that would cause flooding. One example joint fluvial–coastal scenario was also selected. A narrative accompanying each scenario gives an overview of the hydrometeorological conditions under which these events may occur, accompanied by historical context comparing the scenario to past flooding events. To calculate the consequences of flooding, hazard modelling for each scenario was conducted to create a hazard footprint using 2D hydrodynamic modelling. The hazard footprints generated were then usedin impact analysis by the Health and Safety Laboratory to convert receptor data to relevant metrics for flood risk assessment. The metrics give an indication of flood severity for each of the selected scenarios. The methods described here were used to inform evidence base for flooding risks in the National Risk Assessment2016