Sikkim’s Lhonak lake still at risk of GLOF, floods: Experts

 Experts suggest glacier should be monitor to prevent another glacier lake outburst flood (GLOF) downstream of South Lhonak lake in Sikkim.

Experts warn about the risk of another glacier lake outburst flood (GLOF) downstream of South Lhonak lake in Sikkim. (REUTERS)

Jayashree Nandi, HT, Oct 13, 2023 : New Delhi: The risk of another glacier lake outburst flood (GLOF) downstream of South Lhonak lake in Sikkim is still real, glaciologists and experts warned, recommending that the glacier should be monitored effectively.

According to glaciologists at the Divecha Centre for Climate Change, Indian Institute of Science (IISc), a large chunk of ice may have fallen from the glacier into the lake creating waves that toppled the moraine dam leading to a GLOF and causing severe flash floods downstream on October 4 that killed around 91 with many still missing, but there is still a lot of water left in the lake.

“The risk has not been eliminated which is obvious from satellite images. There is a slight reduction in the ice area but almost half of the glacier has not deglaciated yet. There is a lot of water and ice in the South Lhonak lake which makes it susceptible,” said Anil Kulkarni, glaciologist and distinguished scientist, IISc.

“Since most of the glacier is still intact, it will continue to retreat at the rate of 20 to 25 metres per year as per our analysis. Retreat of the glacier will lead to further expansion of the lake due to meltwater. We don’t know when further breach can happen but people need to prepare. We have already suggested mitigation options to the Sikkim government,” added Kulkarni.

According to research by IISc’s team of glaciologists, the volume of the South Lhonak lake (in 2015) was estimated as 60 ± 10.8 million cu.m with the projection that with prolonged retreat of the glacier, the lake was likely to expand to a maximum volume of 90 ± 16.2 million cu.m.

IISc’s recommended solutions include engineering interventions such as lake water siphoning and controlled widening of the discharge channel. Long-term strategies include identifying locations vulnerable to GLOF using computer simulation, monitoring lake expansion, determining structural strength of the moraine dam, maintaining safe water level in the lake through siphoning, and determining future glacier lake sites using remote sensing techniques.

“Among short term solutions, the government can also consider construction of check dams which can reduce the overall impact of GLOF,” said Kulkarni adding that “the mountain regions are already seeing a lot of changes and glaciers are vulnerable due to climate change. We need a very specific strategy for these regions like having zonation maps. We already know where these lakes are. The ‘red’ zones downstream need a special strategy like not allowing constructions, large projects etc. Locating large constructions such as dams is very risky here.”. Floods associated with the GLOF washed away part of the Teesta 3 dam – damaged parts of Teesta 5, wrecked bridges and drowned roads.

“In addition to new landslide at #SouthLhonaklake, I think there is substantially more to come down. The glacier retreated ~170-250m between March 2023 and October, which likely played a destabilizing role on the moraine (and probably exacerbated by being very wet),” tweeted Dan Shugar, geomorphologist, Associate Professor, University of Calgary on October 11.

“To my eye, there appears to be lots of sediment extending ~500m up the glacier which is at the same angle as the failed material, and with a similar slip face at the top. If the glacier retreats another few hundred meters in the next year or two, this material may come down,” he added, posting satellite images of the lake.

HT reported on October 8 that glaciologists warned earlier this year that there could be a GLOF at the Gephang Gath lake in Himachal Pradesh. There are 337 potential glacial lake sites in HP and Uttarakhand according to IISc.