Flood hazard mapping using HEC-RAS and QGIS for Meja dam

Abstract

The study was conducted on the Meja dam which is located at Mirzapur Uttar Pradesh. The dam failures can cause immense damage and loss of life and property. Complete hydraulic simulation and analysis for a hypothetical dam break of Meja dam were performed using United States Army Corps of Engineers (USACE), Hydrologic Engineering Center‘s River Analysis System (HEC-RAS) computer model with river geometry derived from DEM. Public domain HEC-RAS programme for the use of combined flood routing and estimation of flood levels. The flood inundation map was generated by HEC-RAS and imported into QGIS to delineate the areas flooded under the assumed dam break scenario. The details of water surface elevations, depth of flood, flood arrival time and velocity of flood wave at different locations of downstream gives an idea about the extent of flooding. The average breach width and breach development time of dam are calculated by using the geometric information of the Meja dam through the regression equation of the Froehlich (2008). The average breach width and breach development time calculated by Froehlich (2008) are 194 m and 4.20 hr for overtopping and 141m and 3.81 hr for the piping mode of failure respectively. The bottom breach elevation of the dam which used as input for the HEC-RAS model is 155 m for both modes of failure. The maximum discharge flows out from the breached dam is 4836.42 m3/s (CMS) noted at 5:00 hours after a dam breached due to overtopping and 4169.04 m3/s (CMS) at 5:15 hours after a dam breached due to piping mode of failure. The maximum volume of water accumulated is 128900.8 and 127502.3 due to overtopping and piping failure respectively at end of simulation time. The simulation results showed that in the event of Meja dam failure, some areas including residential, agricultural and industrial areas were listed as having a very high risk of flooding due to the significant difference in water surface level and ground elevation value. Emergency evacuation will take place within 41 minutes of breach of the dam.