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 20,000 square kilometres (7,700 sq mi) of farmland was damaged. The disaster was described by an unnamed Thai government official as "the worst flooding yet in terms of the amount of water and [number of] people affected".
The World Bank has estimated 1.425 trillion baht (US$46.5 billion) in economic 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 3 meters (9.8 feet) 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.
Flooding timeline
With the monsoon season well underway in 2011, when noticeable rainfall started in May, major flooding began as Tropical Storm Nock-ten made its landfall in northern Vietnam, causing heavy precipitation in northern and northeastern Thailand and flash floods in many provinces from 31 July. Within one week thirteen persons had been reported dead, with ongoing flooding in the provinces of Chiang Mai, Lampang, Lamphun, Mae Hong Son, Nan, Phrae, and Uttaradit in the north, and Bung Kan, Nakhon Phanom, Nong Khai, Sakon Nakhon, and Udon Thani in the upper northeast. The upper-central provinces of Phichit, Phitsanulok, and Sukhothai were also flooded as the flooding spread down the overflowing Yom and Nan Rivers. Prachuap Khiri Khan on the gulf coast was also affected.
Flooding was still ongoing by late-August, as heavy rains were expected to continue for longer than usual due to the effect of a multi-year La Niña event. Flood waters reached a depth of 50 cm in downtown Nan, and became the highest recorded in 16 years in Phitsanulok Province, while large areas in the downstream provinces of Nakhon Sawan, Ang Thong, Ayutthaya, and Nakhon Nayok were affected and the death toll rose to 37 by 22 August. Both Bhumibol and Sirikit Dams increased discharge rates to compensate for the increase in incoming flow.
By 19 September almost all of the lower central provinces were affected by the flood: Uthai Thani, Chai Nat, Sing Buri, Ang Thong, Suphan Buri, Ayutthaya, Pathum Thani, and Nonthaburi, the latter two on the northern border of Bangkok. Broken floodgates resulted in water from the Chao Phraya flowing through irrigation canals and inundating large areas of paddy fields in Singburi, Ang Thong, and Ayutthaya, but lessened the strain on Bangkok as the fields served as water retention areas. Power boats were used to run against the river's flow while at anchor in an attempt to increase the river's discharge rate.
By the beginning of October, most dams were already near or over capacity and were being forced to increase their rate of discharge, potentially worsening downstream flooding. Flooding in Ayutthaya worsened and flood water entered the city itself, inundating the Ayutthaya Historical Park and forcing evacuations. Barriers protecting industrial estates failed, resulting in the flooding of dozens of major factories and a country-wide disruption of the manufacturing supply chains. In Nakhon Sawan, the sandbag barrier protecting the city was breached, resulting in rapid flooding of the city. Hundreds of patients had to be transferred out of Ayutthaya and Nakhon Sawan Regional Hospitals by boat as water levels rose over the hospital floors and power supplies and life support systems were disrupted.
Northeast
The Mekong, and its major tributaries Mae Mun and Mae Chi all experienced flooding. In Khon Kaen Province alone, floods destroyed close to 350,000 350,000 rai (56,000 ha; 220 sq mi) of land, stranding 315 families of Mai Si Wilai village in the middle of the swollen Nong Kong Kaew Lake in Chonnabot District, while in Phra Lap municipality on the outskirts of Khon Kaen city, over 700 displaced residents of Phra Kheu village called the shoulder of a provincial highway home. The provincial Department of Disaster Prevention and Mitigation, normally allocated 50 million Thai baht, was granted an extra 50 million for 2011 and had spent over 80 million by the end of October 2011.
South
Significant flooding occurred in late-2011 in Thailand's southern provinces. Nine provinces were affected by flash flooding owing to rains that occurred in November 2011. Saba Yoi, Khuan Niang, Rattaphum, and Singha Nakhon Districts were declared disaster zones after flood waters as deep as five meters covered nearly all areas. More than 159 small and key roads were impassable. Ten other districts were feared at risk of inundation and more rains were expected. Coastal areas in the south were also battered by high waves.
Flooding in and around Bangkok
As floodwaters drained southwards from Ayutthaya, Bangkok was becoming seriously threatened by mid-October. In Pathum Thani Province bordering Bangkok to the north, continuous efforts to reinforce and repair sandbag flood walls were undertaken to prevent the Chao Phraya and Rangsit Canals from overflowing into Bangkok. Several districts in eastern Bangkok which lie outside Bangkok's flood wall, as well as parts of the surrounding Nonthaburi, Pathum Thani, Chachoengsao, and Nakhon Pathom Provinces, became flooded as water was diverted from the Chao Phraya to the Nakhon Nayok River and outlying canals.
As flood barriers in Pathum Thani failed, even more industrial estates as well as suburban residential areas became flooded. Parts of the Phahonyothin Highway leading out of Bangkok became inaccessible, causing severe traffic jams on alternative routes. Disruption of a barrier protecting the Khlong Prapa water supply canal early on 20 October allowed floodwaters to enter the canal and rapidly flow down to Sam Sen in central Bangkok, overflowing and flooding several areas along the banks. Although the breach was controlled, residents panicked and illegally parked cars on flyovers and parts of the elevated expressway.
The entire Rangsit campus of Thammasat University in Pathum Thani, north of Bangkok, serving as the largest evacuation centre, was flooded with two meters of water. This led to the school's final exams to be postponed three times.
As the situation continued, several floodgates controlling the Rangsit/Raphiphat canal system became overwhelmed and were forced to release water towards more residential areas. Residents of several districts of Bangkok, especially those bordering Pathum Thani, were told to prepare for flooding. 15 October, saw the beginning of what would be the total inundation of Muang Ake and Rangsit University. The Rangsit gymnasium served as the main means of exit from the town for those who did not have ability to leave on boat or four-wheel drive. An army truck transferred many to the Mo Chit bus station.
The Chao Phraya River watershed drains an area of 157,924 square kilometres (60,975 sq mi). This entire area drains towards Bangkok and eventually empties into the sea. The Chao Phraya River itself, and pumping stations around Bangkok drain approximately 420,000,000 square metres (4.5×109 sq ft) per day. However, the releases from the dams upstream of Bangkok coupled with additional rainfall, led to estimates that 16,000,000,000 cubic metres (5.7×1011 cu ft) of flood waters must be drained. The flow headed towards Bangkok equated to 16 cubic kilometres (3.8 cu mi). Thailand's Royal Irrigation Department predicted, provided there is no new rain, that it would take 30–45 days for this quantity of water to reach the sea.
Until water flows out to sea, it must go somewhere. This meant flooding on unprotected low-lying land. The amount of land inundated, and the height of peak flood waters was difficult to project for three reasons:
- The exact amount of flood water flowing towards Bangkok was an estimate.
- Flood waters flowing into the Bangkok area didn't arrive all at the same time.
- The expanse of land area that flood waters can occupy before reaching the sea is variable depending on how well barriers hold.