Analysis of alluvial fan surface coefficients to understand the Himalayan foothill instability in the Koshi-Mahananda interfluve area, East Nepal

The foothills of the eastern Nepal Himalayas form a colony of alluvial fans. The fan surfaces are extensively used for tea plantations and their associated land use, like dense settlements and intensive crop cultivation. Dynamic surfaces of the fans are unfavorable for a stable land use pattern and hamper human life. It is necessary to identify the dynamic natures and their cause over the fan surfaces to avoid unfavorable circumstances and for preventive measures. Fan surface coefficients show that most of the alluvial fans of the study area are highly unstable. The perennial stream supplies huge sediments in a wet climatic environment, which causes an alluvial fan instability in the eastern Nepal Himalayan foothills. The studied fan areas are derived from small river basins (2nd order) that supply a relatively low amount of sediments, but the fans are highly dynamic. This study reveals that frazil surface due to huge deforestation of geological structures under high neotectonics and high stream competency due to wet climate are responsible for this instability.