At least thirteen people have died after a flash flood swept through villages and farmland in Lakhimpur District, Assam, on Sunday 9 July 2017, with a further sixty people having died elsewhere in the state. The flooding destroyed 183 homes in 50 villages, and damaged over 7500 more in a further 462 villages. In addition the flooding, which largely subsided after about two days, has covered about 220 square kilometres of agricultural land with a thick layer of sand, destroying crops and potentially harming agriculture in the region for years to come.
Flooding in Lakhimpur District, Assam, on 10 July 2017. Aajtak India Today.
The flooding was caused by a release of water from the Ranganadi Hydroelectric Project in Arunachal Pradesh, undertaken to prevent stress to the dam due to high water levels following heavy rains associated with the Indian summer monsoon. This caused flooding upriver of the dam, which could potentially have led to the dam overloading, with catastrophic consequences for a wide area. The release was intended to prevent such a catastrophe, but caused the river to burst its banks about 40 km downriver of the hydroelectric project, resulting in the flooding in Assam.
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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