Frequency analysis, causes and impacts of flooding in the Bagmati River Basin, Nepal

The Bagmati River, which lies in central Nepal, originates in Bagdwar at an altitude of 2690m and flows south through the Kathmandu valley. The river basin covers an area of 3750 sq. km and includes eight districts of Nepal. Flooding along this river is a common occurrence during the monsoon season. In this study, we compared rainfall events with river discharge, predicted the rate of future flooding events, and assessed the probable causes and socioeconomic impacts of flooding in the Bagmati River Basin. Data used to compare rainfall and discharge were based on rain gauge stations with the highest 24-h records and also contributed directly to the run-off at the Pandhera Dobhan discharge station, the lowest elevation water discharge gauge, in order to facilitate a direct comparison between basin rainfall and river discharge. Future trends in extreme 24-h rainfall events and peak flood values were calculated for 2-, 5-, 10-, 25-, 50- and 100-year recurrence intervals. Extreme 24-h rainfall events varied between 195mm to 552mm among the different rain gauges, corresponding to a peak discharge at Pandhera Dobhan of 16523m(3)/s for the 100-year return interval. We also elaborate on the probable causes of flooding in the central basin and summarise major past flood events and their impacts.