New Delhi: A study conducted by the Central Water Commission in 2015 pertaining to a potential glacial lake outburst flood-triggered disaster in Sikkim warned the state government that most of the hydropower projects on the Teesta River are highly vulnerable to such events.


A GLOF occurred in parts of Lhonak Lake, leading to a rapid rise in water levels with very high velocities downstream along the Teesta River Basin in the early hours of October 4. This resulted in severe damage in Mangan, Gangtok, Pakyong and Namchi districts. The toll due to flash floods has risen to 26, while 142 people are still missing as Army and NDRF teams continue a search for them through slushy earth and fast-flowing water in the Teesta river basin


Following the incident, Chungthang Dam was breached. Chungthang Dam is an important component of the 1,200-megawatt (MW) Teesta Stage III Hydroelectric Project.


The CWC study, titled "Glacial Lake Outburst Flood–South Lhonak System in Teesta River Basin", was taken up to analyse the impact of GLOF in vulnerable areas and the hydropower projects on the river, as reported by the news agency PTI.


CWC researchers noted that GLOFs in glacial lakes in the river basin could lead to severe flash floods downstream, impacting areas like Lachen, Chungthang, Dikchu, Singtam, Manipal, Rangpo, Bara Mungwa villages, and the entirety of hydropower projects Teesta I to VI along a 175-km stretch of the river.


In the worst-case scenario, multiple glacial lake outbursts simultaneously. The researchers said that the likelihood of such an event remains very low. The study predicted a potential 4.45-metre increase in the river's water level due to a GLOF at South Lhonak Lake, as reported by PTI.


It estimated that the lake could discharge water at a rate of 6,210 cubic metres per second, which could reach Chungthang and the Teesta III projects within two hours, as reported by PTI.


CWC Study Urged Authorities To Inform Local Bodies About Consequences Of Outburst Flood


The study urged state authorities to inform local bodies and project authorities about the likely consequences of GLOFs and incorporate this information into land use planning and project operations, as per PTI. It emphasised the need for hydropower project authorities to consider these findings and develop mechanisms for close monitoring of the lakes in order to enable timely warnings regarding abnormal water level changes and geological health.


The study also recommended the development of Standard Operating Procedures to mitigate adverse situations. According to experts, the authorities failed to install early warning systems despite multiple warnings.


A CWC official said on condition of anonymity: "Multiple studies, including one by the commission, had cautioned the state authorities about the GLOF hazard in the upper Teesta River region. We currently have one flood forecasting station 20 km upstream of Chungthang. More stations can be installed if the state submits a proposal," as quoted by PTI. Government agencies and research studies have warned about potential GLOFs in Sikkim on several occasions in the last two decades.


When Do GLOFs Occur ?


GLOFs occur when lakes formed by melting glaciers suddenly burst open. This can happen due to various reasons, such as excessive water accumulation in the lake or triggers like earthquakes. When the lake bursts, it releases an enormous volume of water all at once, causing flash floods downstream. These floods can be extremely destructive and perilous for both people and the environment in the affected area.


According to the South Asia Network of Dams, Rivers, and People, south Lhonak Lake is a glacial-moraine-dammed lake situated in Sikkim's far northwestern region. It is classified as one of the 14 potentially dangerous lakes susceptible to GLOFs.