Flash Flood Susceptibility Assesment Based on Morphometric and Hydrological Parameter: A Case Study of a Seti Gandaki Watershed in Nepal

Nepal’s topography and high rainfall during the monsoon season are attributed to a large number of natural disasters. One frequent disaster in the region is flash flood events, which are responsible for damage in the region as well as downstream river valleys due to high floods. However, this region lacks proper data, and its terrain makes it difficult to study. Therefore, incorporating remote sensing products is essential to analyze these events. This study utilized freely available DEM, rainfall data from CHIRPS, Sentinel imagery, and soil data to assess the flash flood susceptibility of the Seti watershed in the central part of Nepal in Gandaki ProvinceThe watershed was divided into 39 sub-watersheds, and morphometric parameters were computed using GIS tools. Among these morphometric parameters, the most influential parameters were selected using principal component analysis. These morphometric parameters, along with hydrological parameters, were ranked for each sub-basin. The ranking values were then compounded to determine 5 classes of flash flood susceptibility. The outcome shows that sub-basins 1, 2, 3, 4, 5, 8, and 28 are highly susceptible to flash flood events. Among them, sub-basins 1, 2, 3, 4, 5, and 8 lie in the high mountain region with high relief and steep slopes. These sub-basins receive relatively low mean annual rainfall compared to other sub-basins. However, extreme events like extreme precipitation and cloud bursts in the region could increase the frequency of flash flood events in the future due to uncertain climate conditions. Therefore, it is essential to take action to prevent and minimize the impact of these events in the future through flash flood hazard, vulnerability, and risk assessments, and by developing mitigation and adaptation measures for future events.