Flood hazard zoning using HEC-RAS in GIS environment and impact of manning roughness coefficient changes on flood zones in Semi-arid climate

Development of rural areas in rivers margins and flood plains, without understanding and attention to the dynamical and hydrological conditions and upstream areas of the rivers will cause to increase flood risk and loss of life and financial. Flood zoning has many applications, in floodplain management. For management of the three major branches of the Halilrood River, Rabor, Roodar and Javaran in Safarood watershed in Kerman Province, flood hazard zonation was considered in this research. For this purpose, Flood levels in 2 to 200 return periods in three mentioned reaches (72.13 km) using HEC-RAS and HEC-GeoRAS extension were determined in GIS environment. Initially, Rivers Plan was divided to 163 cross sections and in 75 of them, hydrological data were collected and roughness coefficient was determined by Manning method. Results showed that 90.26 percent of the total area affected by the 200-year flood, Susceptible to flooding by the flood under 25 years as well as study on flood depth in the floodplain showed that 68.26 percent of the average depth of floods with 200 years return period is belonged to floods with under 25 year return period. Survey on roughness coefficient in the return period of 200 years, revealed that with a doubling of the roughness coefficient, area under flood 7.15 percent increase which proves direct relationship between the return period and flood area. The result confirms accuracy of combination geographic information system and HEC-RAS software in flood zoning and river modeling.