What caused the flood in Sikkim? | Explained

Glacier Lake Outburst Floods in Himalayas pose a major risk to hydropower projects and people living downstream. Early warning systems, better coordination and greater awareness are needed to mitigate the risk.

The story so far: More than 25 people have been confirmed dead, and many more are missing from a massive flood that resulted from a glacier-lake outburst in Sikkim in the early hours of October 4. The voluminous outflow has destroyed the Chungthang dam, which is critical to the Teesta 3 hydropower project, and rendered several hydropower projects along the river dysfunctional.

Technically called a Glacier Lake Outburst Flood (GLOF), these are instances of large lakes formed from the melting of glaciers, suddenly breaking free of their moraine — natural dams that are formed from rock, sediment and other debris.

The South Lhonak glacier, located in north Sikkim, is reportedly one of the fastest retreating glaciers. The glacier receded nearly 2 km in 46 years from 1962 to 2008. It further retreated by ~400 m from 2008 to 2019. There are an estimated 7,500 glaciers in the Himalayas and GLOFs have been associated with major disasters through the years.

A report in the journal Nature counts the 1926 Jammu and Kashmir deluge, the 1981 Kinnaur valley floods in Himachal Pradesh and the 2013 Kedarnath outburst in Uttarakhand as examples of GLOF related disasters. Among the Himalayan States in India, Sikkim has about 80 glaciers more than any other State. Over the years climatologists have warned that they could be responsible for lake outbursts.

There is uncertainty on this point. Satellite images from the National Remote Sensing Centre, Indian Space Research Organisation suggest around 105 hectares (about 1 square km) of the glacier-fed lake had been drained out. This was on comparing images taken on September 28, when the lake spanned 167 ha and on October 4, the night of the disaster, when it shrunk to 62 ha. On September 17, it was 162 ha. As satellite images don’t reveal the depth of the lake, calculating the volume of water in the lake is challenging without physically visiting the place, according to Kalachand Sain, Director, DST-Wadia Institute of Himalayan Geology. However, water-monitoring apparatus maintained by the Central Water Commission (CWC) reports that water levels surged nearly 60 feet above the maximum levels at Sangkalang at 1:30 pm and gushed at nearly 55 kmph. This is, at the very least, thousands of cubic metres of water in a short time and being midnight, gave very little lead time for people downstream to react.

The National Disaster Management Agency reports that “...the primary reason for the sudden surge appears to be a likely combination of excess rainfall and a GLOF event. The lake is at a height of 5,200 metres with a towering ice-capped feature at about 6,800 metres to the north of and in close proximity to the lake.” There is speculation that heavy rainfall might have tipped the moraine to collapse and trigger the flood but meteorological records don’t reveal any evidence of such heavy rain. “There was heavy rain in south Sikkim but nothing remarkable in the northern region,” M. Mohapatra, Director General, India Meteorological Department (IMD) told The Hindu. “But on the other hand, we don’t really know. At such a height there is no monitoring of rainfall.”

There is also a suggestion that a series of earthquakes in Nepal on October 3, in the afternoon (whose tremors jolted several in the Delhi National Capital Region) might have played a role. “There were two tremors in the afternoon and aftershocks until at about 9 pm on the 3rd. The distance (until Lhonak lake) is about 700 km and so it’s theoretically possible that this could be a trigger,” said Mr. Sain. “But without a deeper study it is all speculation.”