Flood Hazard Mapping and Risk Evaluation using HEC- RAS Modelling and Geospatial Tools in Lower Karnali River of Nepal

Flooding is one of the major natural hazards in Nepal, and most of the Terai region are flood prone areas. Among them Rajapur Municipality of Bardiya district is one of the highly flood risk areas. With the help of the HEC-RAS (Hydrologic Engineering Center's River Analysis System), this research gives thorough hazard mapping and risk assessments in the downstream zone of the Karnali River Basin which is Rajapur Municipality for various return-period floods. The Karnali River was assessed throughout a ~38 km section from Chisapani to Nepal-India border. To perform hydrodynamic simulations, a time series of monthly discharge records from the Chisapani gauging station was used. Flooding conditions representing, 5-, 10-, 50-, 100-, and 200-year return periods (YRPs) were determined using Gumbel’s distribution with the highest daily average discharge of up 22,422 m3/s in 200 YRP. The area vulnerable to flooding in the study was carried out by household survey using (VRA) framework in Wards - 1, 3, 4, and 7 of the Municipality and risk maps were established using QGIS and HEC-RAS model. Flooding in agricultural land poses a high risk to food security, which directly impacts on residents’ livelihoods. Additionally, even after a five-year return period, the 2014 simulated flood (equal to a 100-YRP) had a significant impact on each ward and make them at high risk. In conclusion, this study can support in decision-making for better community settlement and the creation of flood control measures in Rajpur Municipality of the Bardiya district in the future.