Flood Inundation Mapping of the Mahakali River Segment, Kanchanpur District, Nepal, Using the HEC-RAS Model

Abstract

Floods are among the most frequent and destructive natural hazards worldwide. Nepal is highly vulnerable to water induced disasters, particularly floods and landslides during the monsoon season. The Mahakali River Basin, one of Nepal’s major river systems, experiences annual monsoon-induced floods that cause severe damage to livelihoods and settlements in the Dodhara Chandani and Bheemdatta Municipality of Kanchanpur District. This study aims to analyze rainfall trends, estimate peak flood discharges for multiple return periods, and develop flood hazard maps using HEC-RAS to support flood risk management and sustainable planning in the Mahakali River basin. Primary field observations were combined with rainfall, discharge, topographic, and DEM data obtained from the DHM, Department of Survey, USGS, and Google Earth to support flood hazard assessment and hydraulic modelling. Flood inundation was simulated using HEC-RAS 6.3.1 with SRTM 30 m DEM, design discharges for multiple return periods, and Manning's roughness coefficients to generate flood hazard maps. The Peak flood discharges for 2, 10, 50, 100, and 200 return periods were estimated using Gumbel’s distribution and the WECS/DHM method. The estimated peak discharges ranged from 6,642.45 to 19,086.75 m³/s using the WECS/DHM method and from 2,835.81 to 26,264.31 m³/s using the Gumbel distribution. Flood inundation mapping was performed using the one dimensional steady flow HEC-RAS model, which revealed maximum water depths of 13.56 m, 14.39 m, 14.79 m, 14.96 m, and 15.12 m for the respective return periods. Rainfall trend analysis of 2000 to 2023 indicated an increasing pattern at Mahendranagar Station (slope = 40.97, R² = 0.2649) and a slightly decreasing trend at Dodhara Station (slope = –5.65, R² = 0.0124) over 23 years. Land use analysis from 2013 to 2022 indicates a decline in agricultural and built-up areas due to flood-induced erosion and inundation. This study integrates rainfall trend analysis, flood frequency analysis, hydraulic modelling, and land-use change assessment to identify flood-prone areas and support flood risk management. The generated flood hazard maps provide a practical tool for municipal planning, flood zoning, and disaster risk reduction in the Mahakali River basin and other ungauged river basins.