Lithological Control on Landslide in the Babai Khola Watershed, Siwaliks Zone of Nepal
[1]
Bharat Prasad Bhandari, Central Department of Environmental Science, Tribhuvan University, Kathmandu, Nepal.
[2]
Subodh Dhakal, Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal.
Sub-Himalayan zone also called Siwalik is located between the Main Boundary Thrust (MBT) at north and the Main Frontal Thrust (MFT) at south. As a consequence of lithospheric plate dynamic between the Indian and Tibetan plate, the young and fragile sedimentary rock of the Siwaliks area are highly weathered and highly deformed. The Siwaliks range is made up of geologically very young sedimentary rocks such as mudstones, shale, sandstones, siltstones and conglomerates, due to such young and weak geological condition, trend of occurring different types of landslide is higher in the Siwaliks zone. Present study was conducted in Babai Khola watershed of Mid-Western Nepal to find the landslide distribution in different geological formations and to identify the lithistratigraphic control for landslide occurrence. Landslide inventory map was prepared by using Google Earth pro, 2017 and Arc. GIS 10.3, and it was verified by several field survey. Geological map was prepared in the field by detailed study of Lithostratigraphy and geological structures. Loosening of cementing materials in conglomerate is found the major cause of debris fall; block slide and gulley erosion in the Upper Siwaliks. Red clay cementing materials of conglomerates undergoes weathering and makes the gravel loose and fragile. Highly weathered and easily erodible mudstone beds between thick sandstone beds are responsible for landslide in the Middle Siwaliks. The mudstones in the Lower Siwaliks are highly to completely weathered, which are in the form of residual soil in the surface at many places. These are less permeable but can be easily eroded by rain action due to soft nature. The eroded masses are soluble with water and moved through erosional gullies. So that, several gully erosion, erosion induced landslide, mud flow and mud slide were occurred in Lower Siwaliks. Total 867 landslides are identified in the study area. Among them 101 landslides are observed in Lower Siwaliks, 575 in Middle Siwaliks and 191 in Upper Siwaliks. Landslide density is found higher in Middle Siwaliks. Landslides are found majorly controlled by lithology in the study area.
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