Severe flooding occurred during the 2011 monsoon season in Thailand. The flooding began at the end of July triggered by the landfall of Tropical Storm Nock-ten. These floods soon spread through the provinces of northern, northeastern, and central Thailand along the Mekong and Chao Phraya river basins. In October floodwaters reached the mouth of the Chao Phraya and inundated parts of the capital city of Bangkok. Flooding persisted in some areas until mid-January 2012, and resulted in a total of 815 deaths (with three missing) and 13.6 million people affected. Sixty-five of Thailand's 76 provinces were declared flood disaster zones, and over of farmland was damaged. The disaster has been described as "the worst flooding yet in terms of...water and people affected."
The World Bank has estimated 1,425 trillion baht (US$46.5 billion) in economic damages and losses due to flooding, as of 1 December 2011. Most of this was due to the manufacturing industry, as seven major industrial estates were inundated in water as much as deep during the floods. Disruptions to manufacturing supply chains affected regional automobile production and caused a global shortage of hard disk drives which lasted throughout 2012.
The World Bank's estimate for this disaster means it ranks as the world's fourth costliest disaster as of 2011 surpassed only by the 2011 Tōhoku earthquake and tsunami in Japan, Great Hanshin earthquake in 1995, and Hurricane Katrina in 2005. A 2015 study suggests increasing odds for potential flooding similar to the 2011 flood intensity to occur in the future.
Thailand's southern provinces also saw seasonal flash-flooding towards the end of the year, although these were not as destructive as the floods in the more northerly provinces.
Background
Various regions of Thailand are prone to seasonal flash-flooding due to their tropical climate. The floods often occur in the north and spread down the Chao Phraya River through the central plains, in the northeast along the Chi and Mun Rivers flowing into the Mekong, or in the coastal hillsides of the east and south. Remnants of tropical storms that strike Vietnam or the peninsular south commonly increase precipitation, resulting in further risk of flooding. Drainage control systems, including several dams, irrigation canals and flood detention basins, have been constructed, but are inadequate to prevent flood damage, especially in rural areas. Much effort, including a system of drainage tunnels begun in 2001, has been put into preventing the inundation of the capital city, which lies near the mouth of the Chao Phraya and is prone to flooding, with considerable success. Bangkok has seen only brief and minor flooding since the major flood of 1995. Other regions, however, had experienced severe flooding as recently as 2010.
Rainfall in March 2011 over the area of northern Thailand was an extraordinary 344 percent above the mean. Bhumibol Dam in particular received 242.8 mm of rain, well above the normal 25.2 mm. Since 1 January the dam had accumulated 245.9 mm, 216.0 mm or 186 percent above normal.
Bangkok's tendency to flood is particularly acute. Bangkok and adjacent provinces are only 50 cm to two meters above sea level. Sea levels are rising four mm every year. The rising sea levels push more water into the Chao Phraya River. To combat rising sea levels a barrier dam stretching from Chonburi to Hua Hin has been proposed, at a cost of 500 billion baht. The three-meter high dykes that sequester the river are subsiding, as is the rest of the city. In the past, some areas of Bangkok were sinking as much as three centimeters per year, due to excessive groundwater extraction. Groundwater extraction was halted in 1977. That step has slowed subsidence to about one centimeter per year on average. However, the sheer weight of Bangkok's ever expanding infrastructure has exacerbated the problem. Bangkok has some 700 buildings more than 20 storeys high and 4,000 buildings eight to 20 storeys high. The sheer weight of these structures displaces the spongy soil and increases subsidence. In a 2015 report, Thailand's National Reform Council warned that relocation of the capital was not out of the question.