November 7, 2016
Thulagi Glacier, Nepal Retreat and GLOF Potential
Posted by Mauri Pelto
Thulagi Glacier change in Landsat images from 1991 and 2016. Red arrow is 1991 terminus, yellow arrow 2016 terminus and purple arrow increasingly exposed bedrock rib amidst icefall.
Written With Prajjwal Panday: @prajjwalpanday
Thulagi Glacier terminates in a lake referred to both as Thulagi and Dona Lake. ICIMOD (2011) has identified this as a potential threat for a glacier lake outburst flood (GLOF) and has conducted extensive fieldwork there. Thulagi Lake is southwest of Mt..Manaslu in western Nepal at an altitude of 4,044 masl. Here we report on the identified threat and use Landsat imagery to identify changes in the glacier. Thulagi Lake has attracted much attention because two hydropower projects have been developed downstream on the Marsyangdi river basin, Marsyangdi Hydropower Project (69MW) and the Middle Marsyangdi Hydropower Project (70MW). Thulagi Lake began to form about 50 years ago and ICIMOD present field investigations showed that from 1995 to 2009, the length of Thulagi Lake had increased from 1.97 to 2.54 km, due to retreat and the lake area increased from 0.76 to 0.94 sq.km. ICIMOD (2011) did a bathymetric survey of Thulagi Lake using an inflatable boat. The volume was calculated to be 35.3 million cu.m in 2009 an increase from 31.75 million cu.m in 1995. The small increase despite significant area increase was because of a surface elevation lowering rate from 2003-2009 of 0.3 to 0.5 m/yr. They found the moraine walls were sinking, but more slowly at a rate of about 0.1 m/yr: The glacier experienced substantial retreat of 1.65 km from 1958 to 1995.
From 1991 to 2016 the glacier has retreated 750 m a rate of 30 m/year. The debris cover extends from the terminus 4.25 km upglacier to 4500 m. The low slope indicates the lake will continue to expand and the rate retreat should remain high. The bedrock rib is in the icefall that extends from 5600 m to 4600 m, purple arrow. The rock rib at 5000 m in the midst of the icefall is more exposed in Landsat images from 2012 to present than from 1988-2001. This suggests some thinning. All images indicate snowcover is persistent above 5800 m. The glacier terminus continues to calve into the lake as seen in the 2012 Google Earth Image, the 40 m high ice front calves only small icebergs that ICIMOD did not deem sufficiently large to trigger a GLOF event by the surge waves. They also noted that temporary blockage of the lake outlet by river ice, snow barriers, or lake ice debris, appears unlike.
Khanal et al (2015) examined the total value at risk under the modeled GLOF scenario of US $406.73 million for Thulagi. The estimated maximum flow was 4736 m3 /second for Thulagi. The majority of this potential damage was to the two hydropower projects. They noted 125 buildings and 100 acres of irrigated land at risk. A group of Nepali and US scientists carried out stability assessment of Thulagi Lake and its moraine after the April 2015 7.8 magnitude earthquake (USAID, 2015). They noted that the main moraine complex at the end of the lake is relatively stable (black arrow), while the end moraine is less stable (purple arrow). The earthquake caused some slumping of the outlet at the terminal moraine and some deterioration of this moraine. Overall the hazard due to the declining water level would offset some or all of this moraine deterioration in terms of overall risk of a GLOF. Although local people are aware of the deteriorating nature of the terminal moraine at Thulagi, community discussions revealed less concern regarding the possibility of an outburst flood (USAID 2015). However, there is a demand for risk reduction activities such as installation of early warning systems, lowering of lake levels, and development of community-based disaster response plans. There is a general consensus for a science-base community driven approach to address and find solutions for these types of lakes where communities and stakeholders participate starting from research to action.
The retreat of Thulagi Glacier is similar but less rapid than many Himalayan glaciers terminating in lakes; Thong Wuk, West Barun, Lumding and Lhonak
Middle Marsyangdi Hydropower Station and reservoir in Google Earth
Marsyangdi Hydropower Station and reservoir in Google Earth
Thulagi Lake outlet. Black arrow points to main moraine complex. Purple arrow to the less stable terminal moraine.
Thulagi Lake in 2012 Google Earth image. Yellow arrow is recently calved ice and purple arrow indicates bedrock within icefall.