Saturday 2 September 2017

Fifteen families evacuated ater landslide in Himachal Pradesh.

Fifteen families have been evacuated from their homes after a landslide close to the town of Bhattakufer in Shimla District, Himachal Pradesh on Saturday 2 September 2017. The incident happened at about 1.40 pm local time, when a section of cliff-face collapsed onto the Chandigarh-Shimla National Highway close to the Dhalli Tunnel, crushing between six and eight cars and damaging several houses and a temple. Nobody was hurt in the incident, but the area is still thought to be unsafe and people have been evacuated as a precaution.

The scene of the 2 September 2017 Bhattakufer landslide. Press Trust of India.

The incident happened after about three days of continuous rain, associated with the Indian Summer Monsoon. 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 approximate location of the 2 September 2017 Bhattakufer landslide. Google Maps.

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. 

Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.

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