Flood risk zonation in order to determine river flood fringe (Case study: Gamasiyab river)

The frequency and severity of floods has increased globally. However, in many parts of the world, especially in developing countries, there is no accurate data for estimating the probable of flood risk. In this research, flood risk zonation has been done for delineation of flood fringe in Gamasiyab River (south of the Sahneh city, east of the Kermanshah province).The simulation of the flood has been carried out based on the HEC-RAS (one-dimensional hydrodynamic model), and the geometric data has been processed in the GIS by the HEC-GEORAS extension. Also peak discharge in diffrence return periods has been calculated using SMADA software by Type III Pearson distribution. The study area has been divided to four reaches based on river patterns and then manning's roughness coefficient has been calculated for each reach. The results show that 23.78 km2 and 42.1 km2 of the river floodplain have been inundated by 25-year and 100-year return periods respectively, so that amount of damage from the 100-year flood was higher than the 25-year flood. Also the width of inundation area in the reaches 2, 3 and 4 indicated that 25-year return period was not good criteria for flood fringe. It is necessary to study the other features of the river, including bed morphology, river depth and river pattern.