Chamoli disaster was not caused by a glacial lake outburst, flood was triggered by massive rockslide

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A recently released report suggests that Chamoli flood was not a GLOF. ( A view of Nainital, Uttarakhand, Image courtesy: Harikishore Singh/ Unsplash)
A recently released report suggests that Chamoli flood was not a GLOF. ( A view of Nainital, Uttarakhand, Image courtesy: Harikishore Singh/ Unsplash)

–By Mohd Imran Khan

Patna: Nearly a month after disaster in Chamoli in Uttarakhand state of India, a team of experts from a regional knowledge institution in their latest report claimed that the massive flash flood was not caused by a glacial lake outburst flood (GLOF) as there were no significant glacial lakes in the area.The flood was triggered by a massive rockslide. This is contrary to common views expressed by several scientists and experts soon after disaster in Chamoli.

“The Chamoli flood was not caused by a GLOF as there were no significant glacial lakes in the area.The flood was triggered by a massive rockslide just below Ronti peak, of ~22 mio m3 of rock mixed with ice and snow.The energy of the fall melted the ice creating the source of flood. This remobilized the debris and ice on the valley floor deposited by previous events, pushed the stream water and created an excessive flood wave.A couple of days prior to this, a strong western disturbance resulted in heavy precipitation in the area, which increased the flood magnitude downstream.Comprehensive monitoring of mountain environments is recommended.

Infrastructure in the flood path, particularly hydropower projects, exacerbated the impact of the flood. Infrastructure development in fragile mountain environments should consider a sustainability framework, including environmental sustainability”said a report prepared by seven scientists of Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD).

The report questioned some experts’ view of “glacier burst” or GLOF as the reason for the flood, possibly triggered by glacier collapse and raised many valid points related to increasing man-made activities.

“The rockslide-triggered flash flood in Chamoli is one of many possible hazards in the Hindu Kush Himalaya (HKH)mountains. Mountain hazards like glacial lake outburst floods, torrential floods, debris flows, landslides, and avalanches, especially caused by the coupling of avalanches, glacier movement, snow melt, and extreme precipitation are common in this region. While this event cannot be directly attributed to climate change, it is well known that climate change can lead to an increase in the frequency and severity of mountain hazards”report said.

According to the report, the HKH is a multi-hazard environment. Often these hazards are of a cascading nature with multiple hazards interconnected with a primary hazard trigger and a chain of secondary and tertiary hazards. Human interference in the mountain environment is rapidly increasing. Mountain settlements are increasing in size and land use patterns are changing. Infrastructure such as roads and hydropower projects are rapidly penetrating mountain landscapes. The interplay between natural hazards with human settlements and infrastructure is an important aspect, which can significantly escalate the impacts of events like the Chamoli flood. Disaster risk management therefore needs to incorporate a multi-hazard risk assessment approach.

In the aftermath of recent disaster events, the role of infrastructure, especially hydropower and its interplay with natural hazards has emerged as a topic of strong debate. These events have raised the question: Is hydropower a boon or bane? With the need to green the energy sector and the challenges with solar and wind energy, hydropower seemed to be a viable option. However, hydropower development faces multiple challenges. Apart from financial and technical challenges, it faces strong environmental and social challenges. On the environmental front, hydropower development impacts environmental flows, water quality, and the health of aquatic and terrestrial ecosystems.

At the same time the physical environment poses many challenges to hydropower development and sustainability. Climate change related flow variations, extreme events, erosion and sedimentation, and GLOF/LDOFs, are some of the environmental challenges to hydropower. A comprehensive sustainability framework considering financial, environmental and social sustainability can help make hydropower a viable energy option.

Vaidya et al. (2021) argue that for the sustainability of hydropower in the HKH region, environmental threats need to be minimized by mitigating risk through both structural (e.g. erosion protection work) and non-structural measures (e.g. operating rules). Besides this, mitigating the risk of climate change and flow variability is of paramount importance for future energy security for which a better understanding of future climate projections and water availability is needed. That understanding can be reflected in the design and location consideration of future hydropower projects in the region.

Dr Arun Bhakta Shrestha, who heads the transboundary river basins and cryosphere programme at the ICIMOD and one of the member of the experts team behind the report said”Based on available imagery and relying on published data, we are able to make approximate calculations of the mass movements that have taken place. We examined pre- and post-event imagery and found that a crack had formed prior to the event at the site where the rock detachment followed by a rockslide happened .This failure eventually propagated along a 550 m wide crest starting at an elevation of 5500 masl reaching down to nearly 4500 masl. Analysis of elevation models pre- and post-event suggests that the scarp left by the rockslide is 150 m deep, 100 m on average and consists largely of rock and relatively little ice. It is 39° steep, 1060 m long and has an area of ~350,000 m2. This results in an approximate volume of 22 mio m3, which corresponds with the DEM differencing that puts it at 25 mio m3.

Relying on modelled glacier thickness (21 to 25 m for the three glacierettes in the inventory, which corresponds to typical heights of such hanging glaciers; Farinotti et al. 2019), we can estimate the fraction of rock to be 85% and ice 15% and calculate a total mass of ~52 * 109 kg. With a straight slide line of 1.6 km (5500 to 3900 masl), this results in total potential energy of 8.24 * 1014 J. This energy is converted to kinetic energy during the fall and dissipated as enough heat to melt 2.7 * 106 m3 of ice (with 335 kJ per kg of ice necessary at 0 °C). Considering that not all the mass was converted into energy during the fall, this number is likely a lot lower (Huggel et al. 2005). As Huggel et al. (2005) argue and has been conclusively shown in experiments (Arakawa 1999) and for a large co-seismic event (Eberhart-Phillips et al. 2003), fluidization can also happen simply from a very large impact on present ice, which possibly happened in this case.”

In a reference to what has triggered the rockslide ,the report said precedent weather conditions a strong western disturbance passed across Kashmir and northwest India from 4 to 6 February 2021. It was fully charged with convective instability that may have contributed to the heavy precipitation. This unfortunate event occurred on 7 February. Numerical simulation of some of the attributes have been carried out which depict strong evidence of heavy precipitation contributing to high flows downstream.

The analysis of wind pattern and geopotential height contours at 500hPa level indicate that the trough of an active westerly wave was passing over Kashmir and northwestern latitudes of India with a strong vorticity and convergence combination at the leading edge of the westerly. The trough of this western disturbance showed great potential of convective instability as severe Convective Available Potential Energy (CAPE) conditions were found on the rear end of the low pressure area. The numerical simulation on 4 February is presented in Figure 7 which shows heavy precipitation over that region. The western disturbance travelled with relatively slower speed and its stagnancy produced concentrated precipitation.

At the same headwall, a large ice avalanche was previously released somewhere between 19 September and 9 October 2016, which deposited ~1.5 * 107 m3 of ice and more bedrock in the valley below (Figure 9). The resulting destabilization of the rock due to the lack of ice cover (glacial debuttressing, stress-release fracturing), and increased exposure to solar radiation and hence an increased freeze thaw cycle, in combination with a large snowfall event preceding the event of 7 February 2021 and rapid melt water production, may have favoured the fracturing of rock.

This can however not explain the depth of the fracture (~150 m), which must have evolved over a longer period of time. Fracture zones at the runout of the rockslide visible before the event suggest that such detachments have happened at the same location previously. Permafrost thaw and frost cracking has been used to explain increased rockfall activity in the Alps (Deline et al. 2015; Gruber and Haeberli 2007); however, that generally only applies for the first ~10 m of bedrock.

Report further said that as highlighted by the IPCC 2019 report, the mountainous regions are exposed to many cryosphere-related hazards. The frequency, magnitude and areas of these hazards are projected to change as the cryosphere continues to decline. The escalation of cascading hazards to a cascading disaster is a common phenomenon observed in the Hindu Kush Himalaya region (Cutter 2018; Vaidya et al. 2019). One of the prominent recent examples is the Uttarakhand flood of 2013, which started with heavy rainfall and caused a chain of events including landslides, flash floods, and the Chorabari lake outburst and debris flow, which killed more than 6,000 people and damaged roads, bridges, and buildings (Allen et al. 2016; Ray et al. 2016).

Similar hazard events or in combination with other geophysical processes can damage several hydropower stations, which can be further exacerbated with future floods in the context of global climate change (Nie et al. 2021). Hock et al (2019) also suggest that snow avalanches involving wet snow even in winter will occur more frequently in the mountainous regions. Nie et al. (2021) reported 105 existing hydropower projects (HP) (≥ 40 MW) with an installed capacity of 37 GW, 61 projects (≥40 MW) currently under construction (39 GW) and 890 projects (≥10 MW) in various stages of planning (242 GW) in the Karakoram-Himalaya region. Most of the existing hydropower projects were built in the past two to three decades, mainly starting from the downstream sections.

Now these projects are gradually moving upstream where the exposure to mountain hazards is high, the chances of multiple hazards happening in combination and occurring more frequently, and cascading effects can create compounding impacts on the system. Hydropower projects are particularly at risk because of the proximity of their infrastructure (such as diversion dams/reservoirs) to the river network where water-related hazards occur. Many hydropower projects have been damaged by events like the Chamoli flood. For example, the Dig Tsho 1985 GLOF event in the Everest region, the 2015 earthquake, the 2014 Jure landslides, the 2013 Uttarakhand flood, and the 2016 Bhote Koshi GLOF (Nepal) damaged hydropower plants in Nepal and India (Vaidya et al. 2021). Conversely, hydropower infrastructure also impacts the local environment, causing changes in natural flow regimes and environmental flows, alteration of aquatic ecosystems, and deterioration of water quality, among others.

In their recommendations, ICIMOD’s experts suggested that It is necessary to carry out quantitative studies on the status of mountains, understand their formation mechanism, and monitor dynamic processes in order to have advance knowledge of impending hazard events and improve preparedness. These should be done through ground based research, analysis of geospatial information, and modelling. All these need sustained investments from national agencies including establishment of environmental monitoring, analysis and information systems. Collaborative efforts between institutions within the region and with international institutions can help in building robust systems and capacity within the region.

(Mohd Imran Khan is a senior journalist based in Patna)

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