Large-scale erosional processes at the southern flank of the Ganesh Himal range, central Nepal

Abstract

An area of complex landslides is between the villages of Jarlang and Burang on the southeastern slope of Ankhu Khola valley, within the Ganesh Himal's southern flanks in central Nepal. Slow rotational rockslides within deeply weathered mica quartzites and schists have destabilized the head scarp composed of compact augen gneiss in Jarlang, causing rockslides from the main scarp. Successions of small rockslides (<10 6 m 3 ) have generated rather uniform deposits, whereas large, longrunout landslides (>10 6 m 3 ) have generated stratified deposits that have thin shear horizons.The recent erosional processes within the colluvial deposits, forced by rapid weathering of the deposits, include gully erosion and successive rotational and translational slides along the gully margins. The triggering event for the gullying and later gravitational slumping at Burang and Jarlang in 1954 was nine days of intense rainfall. The higher landslide activity within the Jarlang gully system is explained by reactivation along shear horizons in older landslide deposits.The yearly amount of mass wasting of both gullies divided by the area of the Ganesh Himal watershed yields an average rate of erosion of 6 mm/yr. This indicates that large-scale landsliding, and subsequent rapid erosion of the landslide deposits, could be the major denudation process within the Higher Himalayan range during interglacial time.