Landslide Science of the Himalaya in the Consequences of Deep-seated Slope Deformations
Ranjan Kumar DAHAL1#+, manita TIMILSINA2, Shuichi HASEGAWA3
1Central Department of Geology, Tribhuvan University, Nepal, 2Geotech Solutions International, Nepal, 3Faculty of Engineering and Design, Kagawa University, Japan

Himalaya Mountains need a comprehensive evaluation of geomorphological evolution of slope to understand engineering geological study of soil and rock in slopes. In the Himalayan region, role played by the deep-seated slope deformations (DGSDs) for the evolution of relief and hillock are fundamental issues of engineering geomorphology and they need to be pointed out in different geological and geomorphological conditions of the Himalaya.Being a geologically young and active Himalayan belt, the Nepal Himalaya is having issues of deep-seated slope deformations. Deep-seated slope deformations and their related large-scale landslides are complex phenomena taking place through a wide variety of mechanisms whose genesis and evolution are controlled by several factors, among which structure, relief, and tectonic and seismic activities have a particular importance. The deep-seated slope deformations have typical morphological elements which help to recognize them in topographical maps and satellite images along with field data. The surface extension of deformation is generally more than 1 square km with thickness of deformed mass ranges around several tens to hundreds of square km. These deformations related large-scale landslides lack a continuous slip or shear surface delimiting the deformed mass. The rate of movement of deformation is slow, happening in past geologic times with long periods of inactivity. But sometimes they might be active after a consequence of earthquakes or extreme rainfall or human interventions. The deformed slopes are often involved in many shallow-seated landslides and have less drainage density.Observations of project sites in the Himalaya have been suggesting that they are highly affected by existing deep-seated slope deformations related large-scale landslides. In Himalaya, site investigations for tunnels and roads do not follow engineering geomorphological study and they are limited to rock mass characterization along with geological and engineering geological mapping only. Engineering geomorphological evaluation of tunneling site before the project commencement can help to reduce project cost and unnecessary delay of the projects using a blaming phrase: “the Geological Issues”.  Therefore, this paper will highlight the importance of engineering geomorphological study emphasizing DGSDs in the Himalayan region with few examples.