Four people have died in a landslide in the Lower Siang District of Arunachal Pradesh on Friday 29 June 2018. The incident happened at about 2.30 pm local time when a large boulder struck a minibus carrying twenty members of the Indo-Tibetan Border Police. In addition to the four fatalities, eight other passengers were injured, with two being described as being in a serious condition.
The remains of an Indo-Tibetan Border Police vehicle hit by a boulder during a landslide in Arunachal Pradesh on 29 June 2018. Press Trust of India.
The incident is the latest in a series of such events in the state associated with heavy rains caused by the summer monsoon. 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. Arunchal Pradesh has a monsoon season that begins around the end of April or beginning of May and ends around September, bringing 2-4000 mm of rain to the region each year.
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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