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Lunana's glacial lakes: Luggye, Raphstreng, Thorthormi and Tarina Tsho
Glacial lake
The partial breaching of Lake Luggye Tsho in 1994 caused a catastrophic GLOF, the memory of which is still fresh in the minds of people who witnessed it.

This GLOF will in all likelihood go down in the history of floods in Bhutan as the most catastrophic event ever recorded both in terms of its magnitude and in terms of the damage it wreaked on the lives, property, and infrastructure of the people downstream.

The severity of this event prompted the Department of Geology and Mines (under the Ministry of Trade and Industry, Royal Government of Bhutan), to initiate a number of research activities on the glaciers and glacial lakes in the country.

Numerous studies were conducted on glacial lakes in Bhutan as part of joint Japan-Bhutan, India-Bhutan, and Austria-Bhutan projects from 1995-2004. These studies led to many scientific articles highlighting the risks associated with the lakes, discussing the mechanisms of lake expansion, and assessing the stability of the lakes. Previous sections cited some of these.

Glacial lake
This section presents different scenarios regarding lake expansion and draws both from earlier work by different experts and from the results of the present work. The discussions focus mainly on the lakes in the Pho Chu basin.

The first detailed work on the expansion of glacial lakes in the Bhutan Himalaya was a time series of sketches of the major glacial lakes in the Lunana region by Ageta et al. (2000). His subsequent study discussed the evolution of these lakes in detail using maps, photographs, and satellite images.

Ageta also studied and discussed the risk that possible outbursts pose on the geophysical environment in and around the lakes.

Punakha Dzong: Confluence of Mo Chhu and Pho Chhu


Luggye Tsho

Lake Luggye Tshois an end moraine-dammed lake in the Pho Chu basin of the Lunana region. As late as the 1950s, there were no indications of any lakes being associated with Luggye glacier. The first lake appeared only in 1967 (Gansser 1970) as a supraglacial lake and was measured to be 0.02 in 1968.

The depth of Luggye Lake (picture) was measured in 2000 and shown to be 142m. This glacial lake suffered an outburst event on 7th October 1994. The GLOF from Lake Luggye Tsho caused much damage to the downstream valley, including the religiously important Punakha Dzong. After the breach, the lake continued to grow towards the glacier snout and the glacier continued to retreat; in 2001 the lake area measured 1.12
The exposure of ice cliffs on the glacier snout show calving, which contributes to the expansion of the lake towards the glacier.

The outlet channel is at the same level as the lake surface and has a gentle slope. Evidenced by its bumpy topography, this terminal moraine has an ice core. Both the continuous sliding of the left lateral moraine at the outlet and the presence of an ice core contribute to the possibility of blocking of the previously breached outlet so that the lake could at some time in the future suffer another GLOF event).

Glacial lake
If the outlet of Lake Luggye Tso (picture) is blocked by landslides from the left lateral moraine it will cause the water level of the lake to rise, risking a GLOFevent with serious consequences for the Thorthormi lakes further downstream, especially since the Thorthormi glacier has already weakened the left lateral moraine (Ageta et al 2000). Austrian experts Leber and Hausler (2002) concur about the risk from Lake Luggye Tso.

In fact, of the possible scenarios that this group examined during their risk assessment of the Luggye GLOF, the blockage of the outlet by a landslide from the left lateral moraine was considered the "major risk" (Leber and Hausler 2002). This group recommended that the active sliding zone on the left lateral moraine be stabilised at the outlet to allow free flow of water from the lake. In contrast, Dorji (1996) observed no immediate GLOF risk from this lake because of its wide outlet channel. He commented that the risk of flood from this lake is not imminent as the outlet channel is wide enough to discharge any amount of water that will accumulate.


Raphstreng Tsho
Glacial lake
Lake Raphstreng Tsho(picture) lies at an altitude of 4360m. This lake appeared as a supraglacial lake in a 1958 topographic map; topographic maps from 1960 showed that the lake's area was 0.15 In 1986 it was 1.65 km long, 0.96 km wide, and 80m deep (Sharma et. al. 1986).
Nine years later, the Indo-Bhutan Expedition of 1995 measured a maximum length of 1.94 km, width of 1.13 km, and depth of 107m (Ageta et al 2000). The depth measured in 1999 was about 100m.

Some researchers believe that the lake's present dimensions represent its maximum since the upstream section has already reached the bedrock wall. However, field photographs show that the glacier snout is undergoing extensive calving and that the lake can still expand a few hundred more metres.

Prior to the 1994 flood from Lake Luggye Tsho, the left lateral moraine was 295 to 410m wide (Bhargava 1995). Toe erosion of the moraine initiated by the flood has reduced the width to 178m. This weakening of the lake barrier and the large size of the lake caused grave concern to the Government of Bhutan. An immediate investigation of the stability of the lake was undertaken in 1995. Three phases of mitigation work were carried out on this lake from 1996 to 1998 in an attempt to lower the water level by about 4m. A channel of 78.5m in length and 36m wide at the outlet was manually widened and deepened at the lake outlet. Nevertheless, the risk of a GLOF cannot be ruled out because a large volume of water is still stored in the lake and a chain effect of GLOFs from other adjacent lakes could occur. An additional threat to the stability of Lake Raphstreng Tsho (picture) comes from hydrostatic pressure exerted by the Thorthormi lakes, from which Lake Raphstreng Tsho is separated by only a moraine wall.

Source: ICIMOD, International Centre for Integrated Mountain Development 2008
Extract of Impact of Climate Change on Himalayan Glaciers and Glacial Lakes: Case Studies on GLOF and Associated
Author(s): Bajracharya, S. R.; Mool, P.; Shrestha, B. 2007
more information Thorthormi (Thortomi) and Raphstreng lakes


RAOnline Downloads
Regional Challenges and Local Impacts of Climate Change on Mountain Ecosystems
Source: ICIMOD
The Melting Himalayas
1.1 MB PDF-File PDF Download
Source: ICIMOD
Flashfloods in the Himalayas
244 KB PDF-File PDF Download
Glacial lake
Source: ICIMOD
Impact of Climate Change on Himalayan Glaciers and Glacial Lakes in Nepal and Bhutan
3.9 MB PDF-File PDF Download
Case Studies on GLOF and Associated Hazards in Nepal and Bhutan (2007)
This study, prepared in close cooperation with and supported by the United Nations Environment Programme Regional Office for Asia and the Pacific, investigates the impact of climate change on glaciers and glacial lakes in two major glacial hotspots in the Himalayas: the Dudh Koshi sub-basin in the Khumbu-Everest region in Nepal, and the Pho Chu sub-basin in Bhutan.
Source: United Nations

United Nations Disaster Management Team

Disaster Management Analysis in Bhutan
Thimphu, May 2005
Report prepared by Laurence Levaque
596 KB PDF Download
Source: United Nations Disaster Management Team, 2005
ICIMOD: The Glaciers of the Hindu Kush-Himalayan Region
Source: ICIMOD
The Glaciers of the Hindu Kush-Himalayan Region
1.1 MB PDF-File PDF Download
A summary of the science regarding glacier melt/retreat in the Himalayan, Hindu Kush, Karakoram, Pamir, and Tien Shan mountain ranges
Hindu Kush
Source: ICIMOD
The Last Straw
6.5 MB PDF-File PDF Download
Food security in the Hindu Kush Himalayas and the additional burden of climate change
Climate Change Glacial Lakes
Climate Change Impacts and Vulnerability in the Eastern Himalayas

keywords: natural resources and environment, Environmental impact , mountain environment , climatic change, Himalayas, biodiversity

1.5 MB PDF Download
Source: ICIMOD


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