Heavy precipitation in the Chamoli region from February 4-6 and an overall rise in temperature in Uttarakhand in last four decades could have led to the rock collapse which triggered the deadly flash floods in the state last month, according to a study by the International Centre for Integrated Mountain Development.
The report by Kathmandu-based ICIMOD, which was released on Friday, found that a crack had formed prior to the incident at the site where the rock detachment led to a rockslide from the Raunti peak.
The rockslide brought along with it mounds of earth and snow which could have triggered the flash floods. With a straight slide line of 1.6 km (5500 to 3900 masl), the kinetic energy during the fall generated enough heat to melt the ice.
As the debris moved downstream in the Rishiganga river, it pushed running river water ahead of it, including water from small ponding structures along the river path.
So far 72 bodies and 30 human body parts have been recovered from different places in the avalanche-hit areas, out of which 41 bodies have been identified. As many as 132 persons are still missing.
The ICIMOD is a regional organisation that works on different aspects in the Himalayan-Hindukush region. It has eight members — India, China, Pakistan, Afghanistan, Myanmar, Bangladesh, Bhutan and Nepal.
At the same headwall of the Raunthi peak, a large ice avalanche was previously released somewhere between 19 September and 9 October 2016, which deposited ice and more bedrock in the valley below, the report states.
The resulting destabilisation 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 February 7, 2021 and rapid melt water production, may have favoured the fracturing of rock, the report states.
This can, however, not explain the depth of the fracture (150 metres), which must have evolved over a longer period of time, the report states.
The ICIMOD report further states that a strong western disturbance passed across Kashmir and northwest India from February 4-6. It was fully charged with convective instability that may have contributed to the heavy precipitation. The Uttarakhand tragedy occurred on February 7.
Numerical simulation of some of the attributes have been carried out which depict strong evidence of heavy precipitation contributing to high flows downstream, it said.
The numerical simulation on February 4 shows heavy precipitation over the region. The western disturbance travelled with relatively slower speed and its stagnancy produced concentrated precipitation, the report states.
Precipitation data derived from Global Precipitation Measurement (GPM) Mission suggests that there were continuous precipitation events from February 3-5 which resulted in approximately 58 mm of cumulative precipitation. Most of the precipitation in the high altitude areas of Chamoli would have occurred as snowfall, it states.
The temperatures are also rising in Uttarakhand, the report states, saying maximum temperature in the Chamoli area has increased at the rate of 0.032 degrees C per year between 1980 and 2018.
Furthermore, January 2021 was the warmest January on record in Uttarakhand for six decades, the report states.
While a hazard event like the flood at Chamoli cannot be directly attributed to climatic changes, the increased thaw-freeze cycle of permafrost could have partially contributed to the event, it states.
A report by the Wadia Institute of Himalayan Geology (WIHG) also stated that the collapse of a massive rock, which brought down a large amount of earth and snow, may have triggered the flash floods.
The debris also blocked the mouth of a stream in the upper reaches of the Rishiganga river. This led to formation of a glacial lake which is being monitored continuously.