Beyond hazard: Combining pan-European exposure data with flash flood hazard forecasts to create impact forecasts for civil protection agencies

As part of TAMIR, a European Commission Civil Protection Preparedness project (ID. 874435), probabilistic operational pan-European flash flood impact forecasts with lead times from 0 to 120 hours have been developed by combining flash flood hazard forecasts with exposure data. Working with civil protection agencies, the aim is to develop forecasts which clearly identify the areas most at risk of serious impacts and therefore may require their intervention. Firstly, the project engaged with these agencies to identify their requirements of flash flood impact forecasts and which elements of exposure and important to them when assessing impacts. Accordingly, pan-European exposure data for population and critical infrastructure (health, education, transport, and energy generation facilities) were sourced from several open source datasets (HARCI-EU, OSM, GHS). These exposure data were (if necessary) regridded and cropped to the spatial domain, transformed to reduce skewness, and rescaled between 1 and 2 to give the datasets common units. The five exposure types were then added together and re-scaled, to produce a combined exposure layer with values ranging from 1 (low exposure) to 2 (high exposure). Flash flood hazard forecasts were created in a previous project by blending hourly ensemble precipitation nowcasts with ensemble numerical weather predictions (NWP) from the ECMWF IFS (Integrated Forecast System). These forecasts are created once per hour and have a lead time of up to 5 days. The flash flood impact forecasts were created by combining the hazard forecasts and exposure data on a two-dimensional impact matrix. Both axes of matrix are split into 3 categories (low, medium, high). For exposure, the ranges for each category were chosen based on the distribution of the data. For hazard, the low, medium, and high categories indicate where the forecast probability shows a 5%-50%, 50%-80%, and 80%+ likelihood of exceeding the 5-year return period threshold.

Once developed, the impact forecasts were applied to 6 case studies of single flash flooding events across Europe chosen by the civil protection agencies, and the results presented to them. This helped evaluate the impact forecasts and enabled end users to provide feedback for further improvement. Results indicated the impact forecasts provided considerable added value compared to the hazard forecasts, by identifying targeted areas where serious impacts were observed. In the final stages of the project, the methods and products described here will be implemented in the European Flood Awareness System (EFAS) platform as a quasi-operational experimental product, and made available to the wider scientific community in the form of a Web Map Service Time (WMS-T) layer. Overall, this presentation focuses on the creation and communication of the exposure data and subsequent impact forecasts. Additionally, it outlines the evaluation of the impact forecasts, and the benefits obtained from engaging end users throughout the process. Finally, it highlights some of the challenges of using pan-European data and a continental scale forecast system to provide impact forecasts useful at the smaller scales required by decision makers.