What caused the flood in Sikkim? | Explained

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.

What is a glacier lake outburst?

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.

What triggered the Sikkim GLOF event?

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.”

The inaccessibility of the terrain makes ground observations that provide conclusive proof difficult, he added.

What was the resulting damage?

The most visible consequence of the flooding was the destruction of the Chungthang dam. Though not a storage dam it directs water to power the Teesta 3 hydropower project. The executive chairman of Sikkim Urja, which runs the project, is quoted as saying the dam was “washed away in 10 mins.” Senior officials in the CWC say that the dam is significantly damaged and the quantity of silt and water present made it difficult to estimate the extent of damage.

On Thursday, Pankaj Agrawal, Secretary, Ministry of Power, convened an “emergency meeting” to ascertain damage to projects operated by the National Hydropower Corporation (NHPC). All bridges downstream to the Teesta-V hydropower station were submerged or washed out, disrupting communication. “The floodwater overtopped the dam of the Teesta V power station [510 MW]. All connecting roads to the project sites as well as parts of the residential colony have been severely damaged,” the Power Ministry said in a statement, “Presently the power station is shut and not generating any electricity.” The Teesta 3 power project is not operated by the NHPC.

One NHPC employee at the Teesta V power station lost his life. All of the organisation’s other personnel at the site were safe. Works on the under-construction Teesta VI (500 MW) of the NHPC were disrupted with water entering into the powerhouse and transformer cavern. Dams and hydropower projects in downstream West Bengal were not significantly affected but were kept shut from heavy siltation that resulted from the floodwaters.

The State government has set up 26 relief camps in the four affected districts, the Sikkim State Disaster Management Authority (SSDMA) said on Thursday, with at least 1,025 people taking shelter in the eight relief camps in Gangtok district.

The flood destroyed 11 bridges in the State, with eight bridges getting washed away in the Mangan district alone. Two bridges were destroyed in Namchi and one in Gangtok.

Water pipelines, sewage lines and 277 houses have been destroyed in the four most affected districts of Mangan, Gangtok, Pakyong and Namchi.

Are such events likely in the future?

Several studies have over the years warned of the risk from GLOF events from the Himalayas.

“Both the frequency and severity of such events are going to increase exponentially in the future. The Himalayan ecosystem is the most fragile in the world and any disruption in the way we are managing these resources will have a problematic outcome for the people of the region. Rising temperatures are leading to a wetter future and contributing to climate change led extreme events, but it is also disturbing the fragility of the Himalayan ecosystem through hydropower and other dams. GLOF is an outcome of warming of the region and this has been a major risk for the region. Once it is formed, you never know what triggers its outburst. Sikkim is an example of this,” said Anjal Prakash, IPCC author and with the Bharti Institute of Public Policy, Indian School of Business.

While the Teesta river is a source of hydropower generation for several power projects, the risk of GLOF like events requires greater care in planning and executing dam and other infrastructure projects, which account for the huge amount of water that can potentially gush through the mountains.

Early warning systems are implementable, said Dr. Saini but require a coordinated approach such as multiple agencies promptly sharing satellite images (that are trained towards the Himalayas) and a network of sensors to provide adequate warning. “Unlike an earthquake, that is relatively sudden, GLOF events can be anticipated as we can monitor changes in the size of lakes. But this requires greater coordination.”

• 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 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.”
• The most visible consequence of the flooding was the destruction of the Chungthang dam. Though not a storage dam it directs water to power the Teesta 3 hydropower project. The executive chairman of Sikkim Urja, which runs the project, is quoted as saying the dam was “washed away in 10 mins.”