High-altitude glacial lake hazard assessment and mitigation: a Himalayan perspective

Abstract

Abstract Glaciers throughout the Himalayas have been receding rapidly over the last few years. In the areas vacated by the ice snouts lakes have formed behind large moraine dams. As with their Andean counterparts, these lakes can pose significant threats to downstream towns, roads and power schemes. For a developing country like Nepal, for example, the prospective loss of economically vital infrastructure and food-producing land can be devastating, in addition to the human misery. The Tsho Rolpa glacier lake has formed by the retreat of Trakarding Glacier at the head of the Rolwaling Valley in northern Nepal. The lake is now over 3 km long and contains an estimated volume of 80 x 10 6 m 3 . The snout of Trakarding Glacier terminates in the lake which, at its western end, is dammed by an ice-cored moraine. It is feared that the lake could be the source of a major glacier lake outburst flood (known locally as a ‘GLOF’) which could destroy the nearby villages of Na and Beding, as well as trekking routes, vital bridges, etc. , further downstream. At Khimti, 80 km downstream from Tsho Rolpa, a new hydroelectric power scheme is being built and could potentially be at risk. It is known that the local glaciers in the Rolwaling Valley have been the source of many GLOFs in historical as well as contemporary times. The last GLOF destroyed several houses and valuable farm land at Beding in July 1991. Following an initial GLOF-risk assessment in September 1994, a trial siphon was installed at Tsho Rolpa in May 1995. This is the first time that such measures have been used in the Nepalese Himalayas. Since then further remediation measures have been proposed and are awaiting funding. The nature of the glacial hazards at Tsho Rolpa and the engineering mitigation measures being proposed are described. Furthermore, it is concluded that an integrated national policy on glacial hazards should be developed urgently.