Flood hazard zoning using HEC-RAS Hydraulic Model and ArcGIS (Case Study: CheshmehKileh River in Tonekabon County)

Flood is defined as “streamflow with high discharge which spreads temporary on the lands nearby the main river” and consequently there will be some risks. Due to proper conditions, most of the economic and social activities take place in these areas. Therefore, it is important to determine flood zonation due to flood discharge with different return periods in these areas. In this regard, the Cheshmehkileh River located in Tonekabon Township has vast river boundary zones and most of the economic and social activities are carried out in its neighborhood, which is susceptible to flood hazard according to flood conditions and occurred floods. The cross sections were derived using the digital elevation model (DEM) with scale of 1:1000 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. The obtained results showed that the longer the return period, the wider the surface affected by flood. Also, the highest flood area is related to agricultural land use with an area of 6.24 ha and after residential land uses with an area of 3.94 ha, forest with an area of 2.9 ha and orchard of an area of 0.8 ha were located at the following ranks.