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

Abstract

The Bagmati River, which lies in central Nepal, originates in Bagdwar at an altitude of 2690 m 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 195 mm to 552 mm among the different rain gauges, corresponding to a peak discharge at Pandhera Dobhan of 16 523 m3/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.

Distribution of rainfall intensity during the summer monsoon season over Kathmandu, Nepal

“…TIA is located at 27°41’47”N and 85°21’32”E, and sits at an elevation of 1338m amsl. 24h accumulation rainfall data were also used in similar, previous studies (Dahal et al ., ; Dhital et al ., ; Dhital and Kayastha, ). Daily rainfall was used to define the intensity as the rate of rainfall over 24h (0300–0300 utc / 0845–0845h local time).…”

Rainfall-runoff simulation of Bagmati River Basin, Nepal

“…The lower part of the river basin is heavily impacted by flooding during the main monsoon months (July and August). Both flash floods and riverine floods frequently occur in the river basin (Dhital and Kayastha, 2013). However, flash flood frequently occurs in pre-monsoon, and riverine flood frequently occurs in monsoon and post-monsoon.…”

“…These five climatological stations are also not uniformly distributed. On the other hand, landslides and debris flow frequently occur in the river basin (Dhital and Kayastha 2013), which intensifies the flash flood events more than the riverine flood. This also may be responsible for increasing the daily peak flow.…”

Soil bioengineering application for flood hazard minimization in the foothills of Siwaliks, Nepal

“…The surrounding region is called the foothills of the Siwalik, a region known as the most unstable and natural hazard prone area in Nepal because of its weak geological formation. Debris flows and landslides in the upper part and flooding in the lower of this region frequently occur during the main monsoon months of July and August (Dhital and Kayastha, 2013). Geologically, the Siwalik zone is bounded to the north by the Main Boundary Thrust and to the south by the Main Frontal Thrust.…”