ANALYSING HYDROLOGICAL PROCESSES AND HYDRAULIC BEHAVIOUR IN THE WEST RAPTI BASIN, NEPAL

Abstract

Extreme weather events that cause natural disasters, such as floods and landslides, have repeatedly struck Nepal. Nepal's fragile geological conditions, topographical extremes, climatic extremes, and seismic activities, combined with population growth, poverty, illiteracy, deforestation, unscientific agricultural practices, land use changes, and developmental activities such as road, irrigation, hydropower, and urbanisation, make it vulnerable to a variety of natural disasters. Flooding is a striking water-related disaster that affects most of the world. The objective of the study is to develop a flood forecasting model as well as flood hazard and inundation maps for the flood-affected region of the West Rapti River. The flood forecasting model and hazard maps serve as a decision support tool, assisting communities and stakeholders in planning and implementing initiatives to prevent potential flood hazards. Vulnerability assessments of households living in flood-prone areas can inform mitigation and resilience initiatives. The HEC-HMS is used to create the model. The model was calibrated (from 2015-2019) and validated (from 2020-2022) at each outlet of the sub-basin (i.e. gauge stations). The model's performance is evaluated for the geographic area during standardisation and validation using factors such as Nash Sutcliffe Efficiency (NSE). The acquired results were satisfactory and accepted for the modelling of runoff. This study demonstrates that the HEC-HMS hydrological model is appropriate for assessing and simulating hydrological responses in the West Rapti Basin and for the prediction of simulation of hydrological response. In this study, the HEC-RAS 2D model is used to predict flood inundation of the West Rapti River from the central mountains of Nepal. This study is focused on the evaluation of model parameters on Inundation Area, Depth, Velocity, and Travel Time. For a 500-year return period of flood, 412.05 km2 of the area got inundated in Banke & Dang District. Rapti-Sonari, Duduwa, and Nairanpur Rural Municipality were highly affected by the flood, which covered 61%,16%, and 13% of the inundated area. Duduwa, located at the lower part of the river, was the most affected municipality as it covered 73.31% of the total area inundated due to flood.