Seven people have died in flooding in northern Thailand since 10 October 2017, as the country has suffered its heaviest rains for thirty years, with 214 mm of rain falling on Bangkok in a single night this weekend (more than two thirds of the rain that would usually be expected to fall on the city in the entire of October). In order to protect the city, authorities have restricted the outflow of water from the Chao Phraya Barrage, leading to flooding in communities behind the dam; this comes after a decision to increase the outflow from the dam earlier this month that resulted in flooding in communities downstream of the dam, an operation which cannot be repeated without effecting the city, as high rain levels have prevented the floods from subsiding.
An earth barrage in the Khok Samrong district of Lop Buri Province that gave way on Tuesday 17 October 2017, causing flooding in villages downstream. Thia Visa.
Thailand has a tropical climate, with a monsoon season that usually lasts from June to October. Typically September produces the highest rainfall, with the rains trailing off in October. However, as with other countries in Southeast Asia this year, the rains have been exceptionally heavy this year, so that reservoirs and barrages have filled to capacity by the beginning of October, and the rains have persisted longer, pushing water storage systems beyond their capacity.
Flooding in the Sapphaya District of Chai Nat Province, caused by a discharge of water from the Chao Phraya Barrage earlier this month. Chudet Sihawong/The Straits 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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