FLOOD MODELING OF SOAN RIVER USING HEC-RAS

Abstract

Flood is considered as the most devastating and widespread natural hazard that occurs due to heavy rainfall. This research is aimed at flood modeling of River Soan, whose catchment area receives heavy rainfall especially in the monsoon season. Terrain data plays a vital role in flood modeling process therefore DEM was compared with field surveyed cross sections to select the suitable terrain data. Comparison of results showed almost similar elevation profiles with an average relative difference of 21 ft. per terrain point of each cross section which was due to cut and fill done in the area. This assessment lead to the use of DEM for flood modeling. Temporal Landsat satellite images were taken to analyze changes in the floodplain after 1997. The analysis showed that the floodplain of the river has been restricted and narrowed down due to urbanization. Flood modeling was done based on a design discharge of 100 year flood which was estimated using a statistical distribution and its outcome was 118,130 cusecs. One model was prepared for the ground conditions that existed in 1997. Model results were compared with existing flood extent map prepared in 1997 which matched 87%. It was calibrated using different Manning's n values and finally the output matched 94%. Second model was prepared by adding embankments and a bridge to the successfully calibrated model to represent 2011 ground conditions whose results showed that the constructed protective structures can withstand 100 years flood with some areas still under risk. Therefore, new embankments should be constructed at suggested locations. The new model was run at different discharges and it was identified that the embankment will breach in River Garden area at 135,000 cusecs discharge and its raising by about one foot can make it withstand a flood of this magnitude