CASE STUDY

GLOF hazards in Bhutan and the management of glacial lakes in high mountain environments John M. Reynolds, Reynolds International Ltd, UK

The potential for glacial lakes to cause devastation became re- ality in Bhutan on the 7th October 1994 when Luggye Tsho, one of the lakes in the Lunana region, burst through its left lateral moraine (see Fig. 8–10). The ensuing Glacial Lake Outburst Flood (GLOF), which contained an estimated 18 million cubic meters of water, debris and trees, swept downstream, killed 21 people, and travelled over 204 km before crossing the border into India and finally dissipating. On the 30th April 2009, Tsho- jo Glacier is thought to have been the source of an outburst that caused panic downstream at Punakha, where the fatalities occurred in the 1994 flood. No loss of life has been reported from this latest event. Over the last ten years a significant amount of work has been and is being done in Bhutan to ad- dress the issue of hazardous glacial lakes. Not only is this im- portant in order to preserve life and protect property; it is also vital if Bhutan is to develop its substantial hydro-electric power generation potential. Having identified and prioritised its haz- ardous glacial lakes, the Government of Bhutan has taken prac- tical measures to mitigate Raphstreng Tsho between 1996 and 1998 by constructing an open channel to lower the lake level by several metres. However, in 1998 the adjacent Thorthormi Glacier was found to be downwasting, breaking up in parts, and developing interconnected supra-glacial ponds extremely rapidly. It was identified as being potentially the most hazard- ous glacial lake in Bhutan.

Indeed, as predicted, the lakes have coalesced and enlarged; the ice-cored moraine dam separating them from the lower-

that, had the remedial work not been undertaken at this lake, the number of fatalities in April would have been measured in the hundreds.

The Peruvian authorities have had substantial experience in the remediation of glacial lakes, having undertaken the first works in response to the catastrophic inundation of Hua- raz in 1941, which resulted in over 5,000 fatalities (Carey, 2009). The types of engineering works undertaken since then include the construction of open channels, culverts

lying Raphstreng Tsho has degraded significantly since 2004. It is estimated that if Thorthormi lakes burst into Raphstreng Tsho and hence downstream, it could generate a GLOF with a potential volume greater than 53 million cubic meters of water, nearly three times the volume of the 1994 flood, and cause sub- stantial damage downstream and into northern India again. In recognition of the growing urgency to mitigate the situation, a $7.8 million program to lower the Thorthormi Glacier lakes was sanctioned by the UNDP and is due to commence in June 2009.

Mitigation works both at Raphstreng Tsho and Thorthormi Gla- cier represent significant logistical and technical challenges in such remote locations. The cost of the latter project is over 2.5 times more expensive than that of the Tsho Rolpa GLOF Risk Reduction project to remediate the 110 million m3 glacial lake in Rolwaling, Nepal, which was completed in July 2000. This comprised the excavation of a 100 meter long open channel with sluice gates that allowed the controlled lowering of the lake by 3.5 m. Although a major piece of engineering construc- tion, where all the heavy machinery had to be airlifted to site in parts and rebuilt, it represents only an interim remediation, with a further lowering of the lake by 11.5 m having been rec- ommended to achieve sufficient volume reduction to afford an internationally accepted Factor of Safety. Tsho Rolpa was also significant in that it was the first place in the Himalayas where siphons had been used. Siphons were installed in 1995 and ran without significant maintenance for 18 months.

within engineered dams, siphons and tunnels. Indeed, the remediation of Laguna Parón not only gave a way of control- ling the lake for the purpose of hazard management but also to provide a means of controlling water flow into the local river system to use the water reservoir as a resource. There is now considerable interest in the control of glacial lakes as part of the management of water resources, and for lo- cal electrification and larger scale hydropower generation, as well as for protection of major infrastructure and down- stream communities.

29