Glacial lake outburst floods are a growing threat to hydropower plants

The breach of the South Lhonak lake in Sikkim that caused the deaths of at least 35 people and the complete destruction of the state’s largest hydropower plant has put the spotlight on the growing risks of glacial lake outburst floods (GLOFs) on large dams in the Himalayas.

India has 5,334 large dams, hundreds of which are in the Himalayan ranges. In February this year, the government said another 30 large hydroelectric projects (HEPs) of over 25 megawatts each were in the pipeline in Himalayan states. Himalayan rivers have been a rich source of hydroelectric power for India and have fuelled the country’s ambition to harness this energy to meet its renewable energy goals. But the onset of climate change, compounded by high seismic activity, is likely to make these projects more vulnerable, particularly if their rivers are connected to glacial lakes.

Global warming is causing glaciers to lose their mass rapidly and leading to the formation of glacial lakes. These lakes, which have the potential to hold huge amounts of water, carry the risk of bursting due to a number of reasons, like earthquakes, avalanches, large pieces of ice falling into the lake, or erosion of the walls holding the lake. For perspective, the South Lhonak lake in Sikkim was deep enough to submerge a five storey building with ease, said researcher Ashim Sattar.

GLOFs pose a risk to human life and settlements that exist further downstream. The 2013 cloudburst in Uttarakhand’s Kedarnath, that caused the Chorabari glacier to breach, killed over 6,000 people, most of them pilgrims. In 2021, a deluge triggered by an avalanche, also in Uttarakhand, in Chamoli district, swept away the 520 MW Tapovan Vishnugad hydropower plant. The breach of Sikkim’s South Lhonak lake not only destroyed the 1500 MW Teesta III HEP, but damaged two others further downstream.

As per the National Electricity Plan 2022, India will require an additional 17 gigawatts (GW) of hydro capacity addition between 2022 to 2031 to sustain 500 GW of renewables in the grid – a goal Prime Minister Narendra Modi articulated at the COP26 in Glasgow, in 2021.

Many activists are calling for a ban on building large hydropower plants in the Himalayas, in light of the destruction such plants can unleash when met with natural disasters. While the Central Water Commission is in charge of monitoring 477 large glacial lakes in the country, a parliamentary panel report from earlier this year noted that no government agency had any information on areas that are likely to be impacted by GLOFs. A 2016 paper by Swiss and German researchers found that around 66 percent of hydropower plants in Himalayan states fell in GLOF tracks, demonstrating the scale of the risk.

Dam safety and GLOFs

For existing hydropower plants, safety measures built into a dam can determine whether it will survive a deluge or not. Dams that don’t have the capacity to handle flood levels that come with GLOFs are likely to compound impacts, as was the case with the breach at South Lhonak lake.

The Ministry of Environment, Forests and Climate Change has made it mandatory for large hydropower projects to conduct a “dam break” or “dam fail” analysis in order to obtain environmental clearance. The analysis is a simulation of the factors that would lead the dam to break and is required as a component of the project’s Environmental Management Plan (EMPs), to mitigate impacts in case such a situation arises.

A 2021 paper from the National Institute of Public Finance and Policy (NIPFP) noted that though this is a mandatory requirement, it wasn’t clear how many projects complied with this requirement. The paper looked at the Environmental Impact Assessments of six large hydro projects and found that “there is no standardisation in the parameters and protocols used for dam failure analysis. In some cases, the dam failure analysis is not part of the documents available on the website.” The Environmental Impact Assessment includes the EMP.

The NIPFP paper also notes that existing design standards for dams are based on heights and storage capacities, and “are not based on the risk created by the dam.” Though dam fail analyses are mandatory, “there is no requirement to perform a consequence analysis, i.e., a quantification of the probable loss of lives and livelihoods, and damage to property in the event the dam fails,” the paper says.

Scientists say modeling for impacts in the event of a GLOF should become a more common practice, especially if the hydropower plant is in the vicinity of at-risk glacial lakes. “We have done a study that provides the flood paths from over 300 lakes in the Himalayas. How many lakes are impacting the position of the power plant, how big the lakes are and what the worstcase scenario should be looked at whenever a hydropower plant is planned,” said Saket Dubey, Assistant Professor of the School of Infrastructure at the Indian Institute of Technology, Indore.

Typically, very large dams design their spillway capacities by looking at Probable Maximum Flood levels, or water levels at the time of the worst possible storm. The spillway is one of the most important safety features of a dam, because it controls the release of water from the reservoir, which can make space for floodwaters. The 2016 paper that estimated
regional exposure to glacial lake outburst floods, found, however, that such measures that calculate extreme flood magnitudes “largely overlooks GLOFs.”

The same paper found that 177 hydropower plants out of 259 assessed across Himachal Pradesh, Sikkim, Uttarakhand, Bhutan and Nepal were located along potential GLOF tracks. “Up to one third of these HPP could experience GLOF discharges well above local design floods, as hydropower development continues to seek higher sites closer to glacial lakes. We compute that this systematic push of HPP into headwaters effectively doubles the uncertainty about GLOF peak discharge in these locations,” says the study.

The Dam Safety Act of 2021 has provisions to improve monitoring of dams, including the integration of dam break analyses in the dam’s safety mechanisms.

Himanshu Thakkar, coordinator of South Asia Network on Dams, Rivers and People, says the provisions in the Act are not enough. “The Dam Safety Act does not require the information it monitors to be publicly available. Dam safety is first and foremost about transparency. The Committees formed under the Act don’t have much space for independent experts.”

Factoring in risks

Experts agree that the most urgent and pressing task is to first monitor glacial lakes and gather data about their expansion in order to ascertain the risks they pose downstream. A recent presentation by officials with the Central Water Commission of glacier lakes in the Chenab basin shows how the hazards posed by GLOFs can transform over time. The presentation looks at 11 hydropower projects in the basin and shows how, even though distance from the lake remains the same, as its size increases the time taken for the flood to reach the lake reduces, the volume of water it carries increases, and it the velocity with which it comes down can increase.

Dubey from IIT Indore’s own 2020 study looks at the possible impacts of GLOFs on settlements and infrastructure from 329 glacial lakes and classified them according to hazard-risks. Sikkim had the maximum number of very high hazard lakes (20) followed by the union territories of Ladakh and Jammu and Kashmir (4). The states of Himachal Pradesh and Arunachal Pradesh had two very high hazard lakes each, while Uttarakhand did not have a very high hazard lake, the study found. The hazard here refers to the likelihood of the lake breaching.

Using simulated models, the IIT Indore study found that GLOFs from 67 lakes could inundate at least one hydropower plant and 15 could inundate two. Nine lakes – a majority located in Jammu and Kashmir and Ladakh – were found capable of inundating 1000 buildings. Even though Uttarakhand did not have any “very high hazard” lakes, the impacts of a breach – should one occur – were among the highest, after Jammu and Kashmir and Sikkim, inundating over 200 buildings.

“Even if a lake is low hazard, it can still have high impacts in the event of a breach. Sometimes the events leading to a breach can’t be predicted, which is why it’s good to know what the possible impacts are,” said Dubey, adding, “If dams are planned to withstand these events, they could even help alleviate some of the destruction downstream.”

Read more: Sikkim floods: No early warning system and insufficient dam safety

Banner image: Tehri dam on the Bhagirati river in Uttarakhand. Photo by Lingaraj, G. J/Wikimedia Commons.