Ten people are known to have died and thousands have been evacuated from their homes after water was released from the Durgapur Barrage on Thursday 27 July 2017. The water was released to prevent stress to the dam, following a rise in water behind the dam, caused by heavy rainfall associated with the Asian summer monsoon. However the state's Chief Minister, Mamata Banerjee, has suggested that the flood, and other similar floods following releases of water from other dams in India this week, has been caused not by unexpectedly high rainfall levels, but by an accumulation of silt behind the dam, leading to a reduced water retaining capacity, something which should be prevented by regular dredging of dam lakes.
Flooding in the town of Ghatal in West Midnapore district, West Bengal, on 27 July 2017. Hindustan Times.
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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