Sciency Thoughts: Fourteen people now confirmed dead following landslide in Himachal Pradesh.

Friday 13 August 2021

Fourteen people now confirmed dead following landslide in Himachal Pradesh.

Fourteen people have now been confirmed dead, and another sixteen are still missing, following a landslide in Himachal Pradesh State, India, on Wednesday 11 August 2021. The event happened on National Highway Five, close to the village of Nigulsari in Kinnaur District at about 12.30 pm, and resulted in a number of vehicles, including a crowded bus, being hit by large boulders and swept into a gorge below the road, through which the Sutlej River runs. There is thought to be little hope of finding any further survivors, with vehicles buried beneath a pile of rocks and debris 3-5 m deep and 15-30 m wide.

The remains of a bus which was hit by a landslide in Himachal Pradesh, India, on 11 August 2021. VV Krishnan/Firstpost.

Mountainous areas of Himachal Pradesh (which is most of the state) are notoriously prone to landslides, particularly during the monsoon season, which lasts from July to September, when very high rainfall levels can trigger many such events. Landslides are a common problem after severe weather, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

The section of National Highway Five that was hit by a landslide in Kinnaur District, Himachal Pradesh, on 11 August 2021. CNN.

Monsoons are tropical sea breezes triggered by heating of the land during the warmer part of the year (summer). Both the land and sea are warmed by the Sun, but the land has a lower ability to absorb heat, radiating it back so that the air above landmasses becomes significantly warmer than that over the sea, causing the air above the land to rise and drawing in water from over the sea; since this has also been warmed it carries a high evaporated water content, and brings with it heavy rainfall. In the tropical dry season the situation is reversed, as the air over the land cools more rapidly with the seasons, leading to warmer air over the sea, and thus breezes moving from the shore to the sea (where air is rising more rapidly) and a drying of the climate. This situation is particularly intense in South Asia, due to the presence of the Himalayas. High mountain ranges tend to force winds hitting them upwards, which amplifies the South Asian Summer Monsoon, with higher winds leading to more upward air movement, thus drawing in further air from the sea.