Impact of ENSO on River Flooding in the Karnali River Basin of Nepal

El Niño and La Niña, collectively known as the El Niño Southern Oscillation (ENSO), play an important role in river flooding. This study investigates the relationship between ENSO and associated floods in the Karnali River Basin (KRB). A specific focus is placed on a comprehensive analysis of extreme flood events in El Niño and La Niña years, which would deepen the understanding of the influence of ENSO on hydrological and hydrodynamic processes. Precipitation and discharge data from 1962 to 2020 were obtained from the Department of Hydrology and Meteorology (DHM), Government of Nepal. The hydrodynamic model (HEC-HMS & HEC-RAS) was used to simulate the extreme floods of 1983, 2000, 2014 & 2015 at the DHM hydrological station. The year of 2015 is a strong El Niño year, while 1983 and 2014 are La Niña years. In the El Niño year 2015, the annual precipitation was 1190 mm and the average annual discharge was 1130 m ³ /s. The instantaneous peak discharge for the extreme flood event was 4560 m ³ /s. However, in the same river basin, the annual precipitation during the La Niña years of 2014 and 1983 was 1413 mm and 1283 mm, respectively. The 1D and 2D HEC-RAS models were used to simulate the flood behaviors and analyze the river channel shift during the extreme flood event in August 2015. At the station in the Chisapani area, the observed peak discharge was 3354 m³/s while the model can well capture the peak flood discharge with a simulated peak of 3365 m³/s. The models further showed that the river channel shifted at 2,000 m intervals along both the river branches during the extreme flood event. This study highlights the impact of ENSO on precipitation and river flooding in Nepal.