Riverine flood hazard: The GAR 15 global flood hazard assessment uses a probabilistic approach for modelling riverine flood major river basins around the globe. The main steps in this methodology consists of: Compiling a global database of stream-flow data, merging different sources gathering more than 8000 stations over the globe. Calculating river discharge quantiles at various river sections. In another word calculating the range of possible discharges from very low to the maximum possible at series of locations along the river. The time span in the global stream-flow dataset is long enough to allow extreme value analysis. Where time series of flow discharges were too short or incomplete, they were improved with proxy data from stations located in the same “homogeneous region.” Homogeneous regions were calculated taking into account information such as climatic zones, hydrological characteristics of the catchments, and statistical parameters of the streamflow data. The calculated discharge quantiles were introduced to river sections, whose geometries were derived from topographic data (SRTM), and used with a simplified approach (based on Manning’s equation) to model water levels downstream. This procedure allowed for the determination of the reference Flood hazard maps for different return periods (6 are shown in the global study: T= 25, 50, 100, 200, 500, 1000 years). The hazard maps are developed at 1kmx1km resolution. Such maps have been validated against satellite flood footprints from different sources (DFO archive, UNOSAT flood portal) and well performed especially for the big events For smaller events (lower return periods), the GAR Flood hazard maps tend to overestimate with respect to similar maps produced locally (hazard maps where available for some countries and were used as benchmark). The main issue being that, due to the resolution, the GAR flood maps do not take into account flood defences that are normally present to preserve the value exposed to floods. This can influence strongly the results of the risk calculations and especially of the economic parameters. In order to tackle this problem some post processing of the maps has been performed, based on the assumption that flood defences tend to be higher where the exposed value is high and then suddenly drop as this value reduces. The flood hazard assessment was conducted by CIMA Foundation and UNEP-GRID. The flood maps with associated probability of occurrence, is then used by CIMNE as input to the computation of the flood risk for GAR15 as Average Annual Loss values in each country. Hazard maps for six main return periods are developed and available, and probable maximum loss calculations are underway which will be available within few months of GAR15 launch. For GAR15, the risk was calculated with the CAPRA-GIS platform which is risk modelling tool of the CAPRA suite (www.ecapra.org). More information about the flood hazard assessment can be found in the background paper (CIMA Foundation, 2015).
