Rock Avalanche Dams on the Trans Himalayan Upper Indus Streams: A Survey of Late Quaternary Events and Hazard-Related Characteristics

Abstract

Out of more than 320 late Quaternary rock avalanches identified in the Upper Indus Basin, some 161 formed cross-valley barriers impounding one or more rivers have been investigated. At least 228 lakes were associated with them. Only a few small lakes exist today, but many former lakes exceeded 20 km long, a few, 90 km. More than half the dams had an initial, effective barrier height of over 25 m, 33 exceeded 100 m, and two 1,000 m. Today, impoundments exist in every conceivable stage of infilling and degradation, and 117 barriers are still not fully breached. Given the number and diversity of events, and different states in which they are found, a comparative analysis is undertaken. A revised classification of landslide dams is proposed to take account of the large fraction of the events that involved complex emplacement of the rock avalanche. In addition, 26 cases are identified as hybrid dams, the barriers composed of two or more materials due to the interaction of the rock avalanche with substrates, or large uptake of moisture. Most of the newly discovered dams impounded lakes for many years, some for centuries, and the evidence suggests relatively gradual or phased breaching. Some aspects of breach histories can be reconstructed from field evidence, notably former, abandoned spillways. Dam stability is discussed in terms of the combined effects of barrier size, shape, composition, sudden emplacement and consolidation. The hybrid dams raise some special problems of the response of different materials and relations between them. The incidence of large rock slope failures relates to the region’s late Quaternary history, especially slopes over-steepened by ice action and debutressed with deglaciation. Landslide barriers have been decisive in post-glacial landform development along the Upper Indus streams, and continue to exercise control over sediment delivery and routing of floods from other causes. This newly emerging picture of landslide damming suggests an urgent need for contemporary risk assessment. In areas of potential inundation or outburst floods, there are more people, wealth and infrastructure than ever before. Most rock avalanche deposits have settlements on or near them. Much of the critical arable land is on valley fill sediments associated with landslide dams. Some implied risk profiles are reviewed, and the evidence needed to confirm or refute them.