Case history and hazard analysis of two lake-damming landslides in the Himalayas

Abstract

In investigating the hazard and case history of natural dams in the Himalayas, two sites of landslides and their former dammed lakes in the hinterland were visited between May 1994 and October 1995 and analysed from the geomorphological, geotechnical, geohydrological, tectonic, sedimentary and climatic points of view. One of the examples studied, the landslide in the valley of Birahi Ganga (Northern India), is one of the most impressive examples of recent hazards in alpinotype high mountain regions. This study was complemented by a study of the Ghatta Khola landslide (Western Nepal). In both cases, lithotectonic and climatic conditions led to the destabilisation and failure of carbonate bedrock. The occurrence of lakes, dammed over long periods behind the barriers, is of great importance, because after the sometimes fatal landslide event itself, one is confronted and has to cope with a secondary natural hazard, the possibility of a major flood due to the failure of the dam. That is why the preparatory causal factors of the origin of the two lakes (by damming up the river due to the landslide) and their stepwise disappearance (by secondary landslides within the barrier and sedimentation into the basin) were additionally focused upon. It is shown that due to very special circumstances (availability of sediments, heavy rainfall) in the Himalayas there is a progressive decrease in the potential hazard to the landscape and to human beings lower down the main valleys over a period of only a few decades.