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Photo Courtesy: PIB India & PIB Uttarakhand

“The Uttarakhand incident does not appear to be a GLOF (Glacier Lake Outburst Flood). This event possibly occurred due to the detachment of ice from a glacier and can be termed as a Glacier Ice burst or Ice Avalanche.”

By RMSI

On 7th February 2021, an ice sheet broke off from the Nanda Devi glacier, triggering a cascade of events that led to widespread damage in Uttarakhand’s Chamoli district. The incident occurred in the Rishi Ganga Basin, a tributary of Dhauli Ganga. Dhauli Ganga River rises about 5,070 m and meets Alaknanda at Vishnuprayag, with a total length of about 94 km. Several glaciers exist in the upper part of the Dhauli Ganga River Basin. Figure 1 shows an aerial map depicting the Dhauli Ganga River, Rishi Ganga River, locations of the hydropower projects, and source of the event.

Cause
Though the jury is still out analyzing the cause, glaciologists and geologists identified a steeply hanging bit of the Nanda Devi glacier at an altitude of 5600 m. This is believed to have developed a crack in the glacier’s body and possibly caused detachment of a large extent of the glacier from its main body. The ice-avalanche, covering approximately 14 sq km area, swept debris from the valley, thus, pushing a large amount of rock, moraines, mud, and debris, and prompting a flood in the Rishi Ganga, Dhauli Ganga, and Alaknanda rivers. The floodwater travelled over 100 km down steep slopes through the Ganga tributaries before attaining its normal condition in the channel.

During this period (January/ February), temperatures are very low in the glaciated regions of the Himalayas. There is minimum flow in the rivers due to the absence of liquid precipitation at high elevations. All the water bodies/lakes freeze at high altitudes. Therefore, the possibility of Glacier Lake Outburst Flood (GLOF), which represents a sudden release of water from a glacier/ moraine-dammed lakes, is negligible.

Figure 1: Map Showing The Rivers, Hydropower Projects And Source Of The Event

The Uttarakhand incident does not appear to be a GLOF. This event possibly occurred due to the detachment of ice from a glacier and can be termed as ‘Glacier Ice burst’ or ‘Ice Avalanche’.

The intense flow of rock and ice can generate tremendous heat, which can melt large volumes of ice, leading to a huge water volume that flood rivers in a flash. Various government organizations such as the Wadia Institute of Himalayan Geology (WIHG), Dehradun; Indian Institute of Remote Sensing (IIRS), Dehradun; Indian Institute of Technology (IIT), Roorkee & Indore, and others dealing with remote sensing, glaciology, geology, geomorphology are investigating to identify the exact cause behind this disaster. At present, different experts are offering different hypotheses. We expect these activities to conclude after the field visits and data analysis in the days to come.

The intense flow of rock and ice can generate tremendous heat, which can melt large volumes of ice, leading to a huge water volume that flood rivers in a flash. Various government organizations such as the Wadia Institute of Himalayan Geology (WIHG), Dehradun; Indian Institute of Remote Sensing (IIRS), Dehradun; Indian Institute of Technology (IIT), Roorkee & Indore, and others dealing with remote sensing, glaciology, geology, geomorphology are investigating to identify the exact cause behind this disaster. At present, different experts are offering different hypotheses. We expect these activities to conclude after the field visits and data analysis in the days to come.

Impact
The glacial burst triggered an avalanche and a deluge in the Rishi Ganga, Dhauli Ganga, and Alaknanda rivers. It impacted the river channels and the hydropower projects located in the downstream area. Ice pieces of the glacier were observed up to Pipalkoti located on the Alaknanda River, about 50 km from the Rishi Ganga power project.

This event is reminiscent of the Kedarnath deluge of 2013, which led to widespread devastation in Uttarakhand.

The gushing water carrying the debris and boulders along with the ice mass caused significant damage in this area. This flowing ice mass swept away the 13.2 MW Rishi Ganga Hydel Project. The damage to its hydraulic structure led to the immediate release of water stored, thus, creating a situation similar to dam failure.

Further, the flash flood extensively damaged NTPC’s 520 MW Tapovan-Vishnugad Hydropower Project, which is under construction downstream on the Dhauli Ganga River.

Tapovan-Vishnugad Hydropower Project is a run-of-the-river hydroelectricity project, whereby little or no water storage is provided. Vast quantities of sediment were deposited in the Tapovan reservoir and tunnels. The height of sediment deposition in the Tapovan reservoir was about 35 m. Water and debris damaged and washed away structures and machinery.

Several project staff and labourers were trapped in the tunnels filled by the debris (Figure 2).

Figure 2: Images Showing Damage Due To Sediment Flow Into Tunnels And Reservoirs

The Rishi Ganga Power Corporation and NTPC are assessing the damage to their projects. Preliminary estimates suggest damages worth INR 1500-2000 crore.

A massive search and rescue operation is underway by the State Disaster Response Force (SDRF) of Uttarakhand along with a unit of the Indo-Tibetan Border Police (ITBP), with 30 confirmed deaths so far and over 150 missing.

The disaster also washed away five bridges of the Border Roads Organization that connected narrow roads in the hills with 13 villages, including Gahar, Bhangyun, Raini Palli, Pang Lata, Suraithota, Tolma, and Fagrasu. Out of 17 Gram Sabha areas affected by the disaster; 11 were inhabited, while the residents in the other areas had moved to lower areas due to winter.

Exposure
Although this event damaged two hydropower projects, many hydropower projects are vulnerable to natural disasters caused by floods, cloud bursts, GLOF, snow avalanches, ice avalanches, landslides, and rockslides, etc., in the Himalayan region.

NTPC’s Tapovan-Vishnugad project was under construction when the disaster occurred, while the Rishi Ganga project had resumed generating power since June 2020. Nearly 75 percent of the total cost of the Tapovan-Vishnugad project, estimated to be about INR 4000 crore, has been spent.

A list of important projects under different stages of development in Uttarakhand is given in Table 1.

Table 1: List of major hydropower projects (>25 MW) in Uttarakhand

The location of major projects is shown in Figure 3. To estimate the total cost of these projects, one can consider INR 7-8 crore per MW.

 

In October 2020, the National Disaster Management Authority (NDMA) had issued detailed guidelines on how to reduce and deal with disasters caused by what is scientifically called Glacial Lake Outburst Floods (GLOFs).

Figure 3: Map Showing Major Hydro Power Projects In Uttarakhand

Analysis
To understand the hydraulic behavior of the flood event, RMSI attempted to map the severely affected areas based on publicly available information about the event and applied flood modeling software (HECRAS-2D) to simulate possible flooding conditions as shown in Figure 4.

Figure 4: Map Showing The Extent And Depth Of Flood Along With Affected Areas

The flood model was validated at two locations near Raini village where flood depth for a short duration was observed around 12-15 m as published by some news agencies. Modelled flood depth near NTPC Tapovan-Vishnugad project was also validated where flood depth of 1.5 m to 2.0 m was reported near river bank close to the project site. The Central Water Commission (CWC) has published water level information at Joshimath gauging site for this event. The simulated flood depth at Joshimath gauging site was also validated with water surface elevation (1,388 m) observed by CWC at 11:00 AM on 7th February.

Disclaimer: This report contains analysis that is based on publicly available data. The actual extent and intensities of catastrophic events may differ from the results of this simulation analysis due to the accuracy and quality of available data. RMSI specifically disclaims all responsibilities, obligations, and liability with respect to any decisions or advice made or given as a result of the information in this report.

About the Contributor
RMSI is a leading global natural catastrophe risk management consultancy offering solutions for risks associated with natural and human-made hazards. With more than two decades of experience, its expertise lies in hydrology, hydraulics, probabilistic hydro-meteorological hazard modeling, and climate change impact assessment. RMSI has vast experience of working in varied geographies such as India, Bangladesh, Vietnam, Morocco, Yemen, Sana, Nigeria, Timor Leste, Romania, and Beirut. It can help states develop flood risk assessment and flood forecasting models and create emergency action plans for the flood-prone zones.

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