BANGKOK — Rescuers scrambled Wednesday to help more than 3,000 people, many stranded in trees and on rooftops, after a failed dam flooded villages and farmland in southern Laos, officials said.
At least 26 people were killed and dozens more remained missing two days after the disaster, officials said. More than 6,600 people were driven from their homes by the flooding, they said, including more than 2,800 people who have been rescued.
“A second step for us will be to recover and identify the deceased, but for now we hurry to find those who are still alive in the area,” the governor of Sanamxay District, Bounhom Phommasane, told The Vientiane Times.
The auxiliary dam, part of the billion-dollar Xe-Pian Xe-Namnoy hydroelectric project, failed Monday evening, sending more than 170 billion cubic feet of water rushing downstream.
Video posted by the Thai News Agency showed vast quantities of water cascading over what appeared to be the diminished structure of the dam, known as Saddle Dam D.
The official Lao News Agency reported that the dam had collapsed. The main builder of the hydropower project, SK Engineering & Construction of South Korea, said it would investigate whether the dam collapsed or overflowed because of heavy rains.
International Rivers, an advocacy group that has opposed the rapid growth of hydropower dams in Laos, said in a statement posted online that the auxiliary dam collapsed as flooding from heavy monsoon rains caused it to overflow on Monday night.
The group, which seeks to protect rivers around the world, said the disaster showed that many dams are not designed to handle extreme weather events, such as the rains on Monday.
“Unpredictable and extreme weather events are becoming more frequent due to climate change, posing grave safety concerns to millions who live downstream of dams,” the group said.
People living below the dam had only a few hours’ warning to evacuate before it failed, the group said.
Seven villages in the Sanamxay district of Attapeu Province were flooded and more than 6,000 people were displaced by the dam’s failure, officials said.
“Communities were not given sufficient advanced warning to ensure their safety and that of their families,” the International Rivers statement said. “This event raises major questions about dam standards and dam safety in Laos, including their appropriateness to deal with weather conditions and risks.”
The Xe-Pian Xe-Namnoy is one of 70 hydropower plants that are planned, underway or have been built in Laos, most of them owned and operated by private companies, the advocacy group said.
The project consists of major dams on three tributaries of the giant Mekong River as well as several smaller auxiliary dams, or saddle dams, including the one that failed.
South Korea and Thailand were mobilizing emergency assistance. Companies from both countries are involved in building and financing the project, which was supposed to provide 90 percent of its electricity to Thailand once it began operating.
President Moon Jae-in of South Korea instructed his government on Wednesday to dispatch a rescue and emergency relief team to Laos.
“The investigation is still underway to find out the causes of the dam incident, but our government should waste no time in actively participating in the rescue and relief operations at the scene because our own businesses are involved in the construction,” Mr. Moon said, according to his office.
The South Korean government convened a vice ministers’ meeting on Wednesday to follow up on Mr. Moon’s order.
Repeated phone calls to the SK E&C spokesman’s office at the company’s headquarters in Seoul went unanswered Wednesday.
Korea Western Power Co., which has a contract to operate the power plant when it is completed, said its officials and workers from SK E&C in Laos had joined the rescue and relief efforts. SK E&C deployed one helicopter and 11 boats, and Korea Western Power sent two boats and its local medical staff.
More workers were on their way to join the efforts in Laos, said Chung Tae-heung, a spokesman for Korea Western Power.
Mr. Chung said there were two main dams and five supplementary dams in the Laos project. One of the five supplementary dams overflowed, damaging the top of the dam and causing the inundation downstream, he said.