Evolutional Characteristics of Debris Flow in the Siwalik Hills of Nepal

Abstract

As a youngest Mountain of the world, Siwalik region of Nepal is facing several mass movements like landslide and debris flow in monsoon period every year. Debris flow is very common in the steep slope of weak and fragile sedimentary rocks. This paper used remote sensing data and GIS to evaluate evolutional characteristics of debris flow hazard in the Siwalik hill of Babai watershed based on the geological, topographical and hydrological factors. All together 101 debris flow polygons were made by using Google Earth and by field verification. Digital Elevation Model (DEM) was used to analyze debris flow distribution and topographical features. Lithostratigraphy was studied to evaluate geological characters and rainfall data was used to evaluate hydrological character. Average slope for debris flow evolution in Lower, Middle and Upper Siwalik is 34 &#176 , 50 &#176 and 30 &#176 respectively. The average 24 hours rainfall to occur debris flow is found 160.67 mm. The area and length of debris flow channel are significantly different in three geological formations. The temporal distribution of debris flow from 2001 to 2018 shows that the trend of debris flow generation is higher in Middle Siwalik rocks with slope greater than 30 &#176 . Three major sources of debris flows were evaluated namely slide induced debris flow, fall induced debris flow and erosion induced debris flow where the slide induced debris flow is predominant. This study can be better source to understand the general mechanism of debris flow generation to the policy makers for reducing the future impact of debris flow in the overall Siwalik zone of Nepal.