Floods - baseline (1981-2010)

Floods: areas prone to flood events with 100-year return period - baseline (1981-2010). Within the Global Assessment Report on Disaster Risk Reduction 2015 (GAR15), the global flood hazard assessment uses a probabilistic approach for modelling riverine flood major river basins around the globe. 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). Source: United Nations Office for Disaster Risk Reduction (UNDRR) GAR15