Sinking Bangkok

For the more or less 10 million people living in Thailand’s capital city of Bangkok, flooding is a common and recurring phenomenon. This is partly due to the city’s geographic location at the southern end of the Chao Phraya River Basin, as well as its low-lying terrain of around 1.5 metres average elevation above mean sea level. The city normally experiences six months of rainy season every year from May to October.

However, conditions are soon expected to worsen for much of Bangkok. This is due to the ongoing sinking or land subsidence that is threatening the city. The implication is that while the sea level rises, the lowering of ground level continues, leaving the city doubly vulnerable to urban flooding.

On 12 May, the National Water Resource Commission (NWRC) approved a US$412 million plan to combat drought and floods as well as improve drainage and water quality in Thailand’s main interprovincial canal – the Klong Prem Prachakorn. 

Thai media reported that the plan will be sent to the cabinet for final approval. While rainfall in Thailand is expected to be five to 10 percent lower than usual over the next two months, the Office of National Water Resources (ONWR) is keeping a close eye on weather reports.

Other Southeast Asian megacities as Jakarta and Manila are being hit hardest. Thailand landed on the vulnerability list after the 2011 flood event, which cost the nation US$46 billion in repairs and rehabilitation. US$8 billion was spent on Bangkok alone. More than 800 deaths were recorded and 13 million people were affected by the flooding.

While adapting to its environmental and geological limitations is key to maintaining and improving Bangkok’s liveability by reducing the occurrence of urban flooding, other factors such as the issue of land subsidence can be more important. Bangkok, along with Venice and New Orleans, are three cities which are sinking 10 times faster than the rising sea level. According to some estimates, parts of Bangkok are sinking by two centimetres annually. For some parts of the city that are now only half a metre above sea level, these estimates are very worrying.

Built on soft clay

The city of Bangkok was developed on a marshy stretch of land in the Chao Phraya delta to accommodate a new capital after the fall of Ayutthaya to the Burmese Empire in the 18th century. It is built on highly compressible layers of soft clay. The sinking of the city is mostly driven by the sheer weight of the built-up urbanisation, as well as uncontrolled groundwater extraction.

Geological description 

Bangkok is situated on the tidal flat of marine clays of the Chao Phraya River delta. Hence, the ground surface of Bangkok is entirely underlain by marine clay, 15 m to 30 m in thickness, known as Bangkok clay. Unconsolidated and semi-consolidated sediment overlying the basement have a totally thickness of about 400 m to more than 1800 m. The aquifer system consisted mainly of sand and gravel separated by clays. Eight aquifers within the depth about 600 m have been identified. The water in the top most aquifer is not potable due to high salinity occurring since the deposition. The subsequent three aquifers (PD-100 m, NL-150 m, and NB-200 m) are the most used. However, the high production and newly developed wells at the greater depths are mostly served for industrial purposes. Due to the past uncontrolled over pumping of groundwater, certain aquifers and overlying clay layer are under substantial stress, leading to serious land subsidence which at its most severe amounts to 10 cm/year (1978-1981). In certain places, with combined surface loading, this has amounted to a maximum recorded settlement of 100 cm over a 21-year period (1978-1999) and groundwater level has declined to 55 m from ground surface. The increasing of groundwater abstraction reached it maximum at 2.6 MCMD in 1999. The mitigation actions have been required if they are to be reinstated and stabilized. The subsequent strict mitigations such as declaration of "Groundwater Critical Zone" covered large area totally seven provinces including Bangkok. The mitigation included reducing the permissible pumpage of registered wells, promoting public awareness in groundwater conservation, and finally implementing "No permission for new well rule" in the Bangkok metropolis. All these mitigations have been determined to control the total abstraction to meet permissible yield which has been studied at 1.25 MCDD. The strict mitigations finally return the good result. The current groundwater abstraction is under 1 MCMD. The overall subsidence rate has been improved to less than 2-3 cm/year in central Bangkok and less than 1 cm/year in overall area. Water level has been recovered to 40 m from ground surface. While, the ground water cone of depression areas are shift to the newly developed zone at the skirt of Bangkok metropolis. Subsidence is not the only problem when considering groundwater pumping, salt-water intrusion has become one of the major problems of groundwater. The rapid lowering of the water table by excessive extraction of groundwater has caused the shallow aquifers in Bangkok to become contaminated by salt water from the nearby ocean.

References:

https://urbanland.uli.org/sustainability/bangkok-the-sinking-city-faces-severe-climate-challenges/

https://theaseanpost.com/article/bangkok-sinking-fast