Evaluating Flood Risk Associated with Probable Maximum Precipitation in the West Rapti River Basin, Nepal

Abstract Floods, driven by extreme precipitation, pose significant risks to Nepal’s Deukhuri Valley, the headquarters of Lumbini Province, located in the West Rapti River Basin. This study assesses flood risk due to Probable Maximum Precipitation (PMP) using hydro-meteorological and statistical methods to estimate PMP, Snyder’s unit hydrograph for Probable Maximum Flood (PMF), and HEC-RAS 2D modeling for flood inundation mapping. PMP values of 507 mm (hydro-meteorological) and 575 mm (statistical) were derived, with the latter used to estimate a PMF of 11,211.1 m³/s, comparable to a 10,000-year return period flood. The flood hazard map indicates 44.0% of the inundated area as extreme hazard, with 240.5 km of roads and diverse land cover types affected. Risk mapping categorizes areas into high (362.4 ha), medium (5,599.5 ha), and low (6,624.0 ha) risk zones. These findings provide critical insights for flood risk mitigation and infrastructure planning in the study area.The study expands on flood risk assessment in Nepal's Deukhuri Valley, focusing on the impact of Probable Maximum Precipitation (PMP) in the West Rapti River Basin. Using a combination of hydro-meteorological and statistical methods, the research establishes PMP values of 507 mm and 575 mm, respectively. The higher statistical value is then utilized to calculate a Probable Maximum Flood (PMF) of 11,211.1 m³/s, which is comparable to a flood event with a 10,000-year return period. This comprehensive approach provides a robust foundation for understanding the potential magnitude of extreme flooding events in the region. The study's findings have significant implications for flood risk management and urban planning in the Deukhuri Valley. The flood hazard map reveals that 44.0% of the inundated area falls under the extreme hazard category, with 240.5 km of roads at risk. Furthermore, the research categorizes the affected areas into high (362.4 ha), medium (5,599.5 ha), and low (6,624.0 ha) risk zones. This detailed risk assessment, combined with the analysis of impacted land cover types, offers valuable insights for local authorities and planners. The information can be used to develop targeted flood mitigation strategies, improve infrastructure resilience, and inform land-use policies to minimize the potential impact of extreme flooding events on the region's population and economy. Keywords: 1. Flood risk assessment, Nepal, Deukhuri Valley, Probable Maximum Precipitation (PMP), West Rapti River Basin, Hydro-meteorological methods, Statistical methods, Probable Maximum Flood (PMF), Flood hazard mapping, Extreme flooding events, Urban planning, Infrastructure resilience, Land-use policies, Flood mitigation strategies, Risk zonation