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

The B agmati R iver, which lies in central N epal, originates in B agdwar at an altitude of 2690 m and flows south through the K athmandu valley. The river basin covers an area of 3750 sq. km and includes eight districts of N epal. 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 B agmati R iver B asin. 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 P andhera D obhan 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 195 mm to 552 mm among the different rain gauges, corresponding to a peak discharge at P andhera D obhan of 16 523 m 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.