Flood zoning of Chalus basin using hydrologic model of HEC-RAS and Geographic Information System

In this study combination of GIS and HEC-RAS were used for flood zoning in the Chalus basin. For this purpose GIS data processing was done and the coordinate system was defined for the data and then the boundaries of the basin and sub-basin were determined with archydro. Using the extracted data TIN, DEM, mean slop and dip direction of rivers maps were produced and then using HEC-GEORAS, the data required for HEC-RAS software were prepared. Then the manning coefficient values, soil hydrologic groups and distribution maps of CN were extracted from land-use map and subsequently data transferred to the HEC-RAS and the values of manning in each of the cross sections were entered and boundary conditions were defined. Then HEC-RAS model was run in steady-state conditions. By overlaying different layers in GIS (such as the depth and velocity), dangerous areas during the flood were determined. Then the overlapping land-use map and velocity and depth layers, the land-use types at risk from flooding have been identified. Results presented that the floodplain area is equal to 8 and 24 percentage (Min. and Max). Most of the floodplain consists of rice land, building blocks, orchards, meadows and forests in the 2-year flood; therefore maximum damage happens for farmers and residences. By increasing the return period flood, the area of forest (that is placed within the floodplain) increases with higher ratio than building blocks, rice fields and pastures.