Flood Hazard zoning in Abr watershed, Shahroud city, in order to reduce risk using the HEC-RAC model

Identifying flood prone areas is a key step in flood risk management and flood risk zonation provides a quantity measurement of flood risk, as a result of this, it is possible to send appropriate warnings in flood danger periods and facilitate rescue operations. In this study, the floods in the Abr watershed in several return periods were studied using Arc-GIS software and HEC-RAS hydraulic model. The cross sections were derived using the digital elevation model (DEM) in Arc/GIS environment and HEC-GeoRAS extension and the obtained outputs were entered into HEC-RAS hydraulic software. Manning roughness coefficients and flood discharge with five return periods of 2, 5, 10, 25, 50 and 100-year were entered into HEC-RAS model and water surface profile was computed in flow path and flood zonation for different return periods was derived using the HEC-RAS output information through geographical information system. results showed that in most sections of floods with a return period of 5, 10, 25, 50 and 100 years, it rises above the main section of the river and spreads around the river. Also, the agricultural lands along this river have been affected by the occurrence of floods with a low return period (2 years) and this will be dangerous for the residents of this village and agricultural lands.Area of flood zones for 5, 10, 25, 50 and 100-year return periods is 375, 476, 598, 759 and 1395 square kilometer respectively.