Five billion cubic metres of water, enough to fill about 2 million Olympic swimming pools, was released from the Xepian-Xe Nam Noy hydropower dam, which was under construction in southeastern Attapeu province.
The US$1-billion project was to build two dams with five supplier dams used to hold water beyond what is held by the major dams.
Pictures showed villagers stranded on the roofs of their submerged houses.
Boats have been used to try to evacuate victims in San Sai district with more than 6,600 left homeless, the state-run KPL agency reported.
The company building the dam, SK Engineering and Construction, said monsoon rains caused the structure to collapse, adding that it was working to rescue the villagers.
SK said it ordered the evacuation of 12 villages as soon as it became clear that the dam could collapse.
“We are running an emergency team and planning to help evacuate and rescue residents,” a company spokesman told the media.
Attapeu, the country’s southernmost province bordering Cambodia and Vietnam, is known for agriculture, dense forests and wood-based industries, with hydropower being one of its major exports.
The dam is a part of the impoverished, landlocked country’s attempt to become the “battery of Asia” by selling power to its neighbours from a series of environmentally ruinous dams. The authorities are planning 11 major hydropower dams along the Mekong River, and 120 tributary dams over the next 20 years.
The Xe-Pian Xe-Namnoy hydropower project involves Laotian, Thai and South Korean firms.
South Korea’s foreign ministry announced that 50 SK staff and three from Korea Western Power who were stationed at the site had been evacuated.
The 410-megawatt project was designed to generate electricity by diverting the Houay Makchanh, Xe-Namnoy and Xe-Pian rivers on the Bolaven plateau in the province of Champasak before channelling it into the Xe-Pian, which is a Mekong tributary.
Laos already exports two-thirds of its hydropower, with electricity constituting around 30 per cent of all Laotian exports.
The plan is to sell 90 per cent of the energy produced to Thailand.
Prime Minister Thongloun Sisoulith has suspended government meetings and led the cabinet to monitor rescue efforts, the state agency said.
One of the worst Laotian natural disasters occurred in 2013 when five major storms hit over three months, with an estimated 347,000 people affected by severe flooding, according to ReliefWeb.
Laos was already one of Asia’s poorest countries. Picture credit: YouTube