Hydrological and two-dimensional hydraulic modelling of extreme flood hazard at Hile Bazar, Dordi river basin, Nepal, using HEC-HMS and HEC-RAS 2D

Abstract

Settlements in steep Himalayan valleys are exposed to fast, deep and highly erosive floods, yet settlement-scale estimates of flood intensity remain scarce in hydropower-developed basins of Nepal. This study quantifies the 100-year flood hazard at Hile Bazar, a riverside market settlement in the Dordi River Basin, Lamjung District, downstream of the 54 MW run-of-river Super Dordi Hydropower Project. Hourly ERA5 rainfall for 1979–2025 was used to drive a HEC-HMS rainfall–runoff model of the basin, from which a 47-year Annual Maximum Series (AMS) of peak discharge was extracted (mean 914.49 m³/s, CV 0.30). A Log-Pearson Type III fit (log mean 2.9427, log standard deviation 0.1284, skew 0.0477) gave design discharges of 1,281.76, 1,430.63, 1,620.18, 1,761.09 and 1,901.35 m³/s for 10-, 20-, 50-, 100- and 200-year return periods. The Q100 hydrograph was routed through an unsteady HEC-RAS 2D model of the Hile Bazar reach with LULC-based roughness and a normal-depth downstream boundary. The Q100 flood inundates about 3.05 km². Maximum depth reaches 4.8 m in the channel and about 3.0 m in the settlement, and velocity ranges from 0.2 m/s on the floodplain to 7.0 m/s in the channel, with about 6.67 m/s in the settlement. The depth–velocity (D×V) product reaches 25–30 m²/s in the channel and about 20 m²/s in Hile Bazar, placing the settlement in the Very High hazard class, and about 52% of households lie in Moderate to Very High hazard zones. The modelled extent overlaps about 70% of the area affected by the June 2021 flood, and high-velocity zones coincide with observed bank-erosion hotspots. Because the hydropower design discharge (about 9.9 m³/s) is less than 1% of Q100, flood hazard is governed by basin rainfall–runoff response and valley morphology rather than plant operation. The results support hazard zoning, setbacks from the active channel, bank protection at high-energy bends and a rainfall- and stage-based early warning system for Hile Bazar.