Malpa Rockfalls of 18 August 1998 in the Northeastern Kumaun Himalaya

Abstract

This paper presents the case history of Malpa rockfall of 18 August 1998, which has claimed a toll of 250 lives, including 60 pilgrims in transit camp, under the shadow of Malpa peak, 40 km north of Dharchula. The slope failure which caused the rockfall was aggravated by high angle of hillslope (78°), conducive conditions of bedrock, particularly enlarged joint space and proximity to the Main Central Thrust (MCT), which has been affected by neotectonic movements in the recent past. High frequency of discontinuities have severely deteriorated the stability of hillslopes and their safety factor 'F' has been reduced to 0.114. The rockfall vector shows a close parallelism with the geometrically determined wedge failure responsible for rockfalls.

The rockfall occurred in the metaquartzites of Pandukeshwar Formation of Vaikrata Group. Freezing and thawing due to snow and swelling action of rainwater had built up shearing stresses to a level that exceeded the shearing resistance of the slope, forming weathered metaquartzites along with interbedded sericite quartzite and a thin veneer of regolith along most vulnerable and weakest surface. Massive rockfall took place following incessant rains and cloud burst which widened the enlarged joints, particularly the joint pattern in the N45°W/80° SE direction including NW-SE trending bedrock. A number of trees and dead bodies were washed away due to the bursting of the debris dam across Malipa Gad and Kali river affecting nearly two sq. km area at Malpa.