Originating from the southern slope of Himalaya, the Karnali River poses a high flood risk at downstream regions during the monsoon season (June to September). This paper presents comprehensive hazard mapping and risk assessments in the downstream region of the Karnali River basin for different return-period floods, with the aid of the HEC-RAS (Hydrologic Engineering Center’s River Analysis System). The assessment was conducted on a ~38 km segment of the Karnali River from Chisapani to the Nepal–India border. To perform hydrodynamic simulations, a long-term time series of instantaneous peak discharge records from the Chisapani gauging station was collected. Flooding conditions representing 2-, 5-, 10-, 50-, 100-, 200-, and 1000-year return periods (YRPs) were determined using Gumbel’s distribution. With an estimated peak discharge of up to 29,910 m3/s and the flood depths up to 23 m in the 1000-YRP, the area vulnerable to flooding in the study domain extends into regions on both the east and west banks of the Karnali River. Such flooding in agricultural land poses a high risk to food security, which directly impacts on residents’ livelihoods. Furthermore, the simulated flood in 2014 (equivalent to a 100-YRP) showed a high level of impact on physical infrastructure, affecting 51 schools, 14 health facilities, 2 bus-stops, and an airport. A total of 132 km of rural–urban roads and 22 km of highways were inundated during the flood. In summary, this study can support in future planning and decision-making for improved water resources management and development of flood control plans on the southern slope of Himalaya.
The study, conducted in western hilly areas of Nepal, inventoried and mapped over 4,222 springs from five different watersheds. The study showed that more than 50% of the spring sources were found under natural conditions, i.e., open spring whereas 15% of them were of pond type. Similarly, the other 15% spring was recorded as a concrete structure or tank while 1% was determined to be a well. Attempts were made to identify if a change in water discharge from springs relates to rainfall patterns. The inter-annual variability analysis shows a significant fluctuation suggesting variation in water discharge across spring sources. The lowest amount of yearly rainfall received in the river basin is governed by decreasing water flow from the springs in the upper and mid-hills of Nepal. Besides, the intra-annual variation (i.e., seasonal and concentrative nature of rainfall only during monsoon) leads to shortage of drinking water and other domestic purposes (e.g., cooking, cleaning) during the dry months of the year. This study, based on the estimation of discharge flow in these springs, revealed that about 70% were decreasing and, in particular, the flow over the recent ten years decreased significantly.
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