Laos News Agency said the accident happened on Monday evening (July 23) near the border with Cambodia, releasing five billion cubic metres of water – more than two million Olympic swimming pools.
The agency said there were “several human lives claimed, and several hundreds of people missing” while some 6,600 people had been made homeless as authorities scrambled to evacuate villagers.
Communist Laos is traversed by a vast network of rivers and several dams are being built or planned in the impoverished and landlocked country, which exports most of its hydropower energy to neighbouring countries like Thailand.
Aerial footage posted on the Facebook page of local news outlet ABC Laos showed a vast brown inundation swamping houses and jungle alike over a huge area.
Another video showed families waiting for rescue on the rooftop of their house, with a nearby Buddhist temple partially submerged.
Nearly 24 hours after the collapse local authorities said they were struggling to gauge the extent of the disaster.
“We do not have any formal information yet about any casualties or how many are missing,” an official in Attapeu province, where much of the flooding occurred, said on condition of anonymity, adding that was no phone signal in the flooded region.
“We sent rescue teams who will help them and provide basic assistance first,” the official added.
The Thai government said it would also send rescue experts to its northern neighbour.
The $1.2 billion (B40.120bn) dam is part of a project by Vientiane-based Xe Pian Xe Namnoy Power Company, or PNPC, a joint venture formed in 2012 between a Laotian, a Thai and two South Korean companies, according to the project’s website.
Ratchaburi Electricity Generating Holding, the Thai company, said it had been told by operators that a 770-metre-long auxiliary dam used to divert river water had failed after heavy rainfall.
“The incident was caused by continuous rainstorm which caused high volume of water to flow into the project's reservoir,” it said in a statement.
One of the South Korean companies, SK Engineering and Construction, said it had sent a crisis team to Laos, according to Yonhap news agency, and was bringing in helicopters from Thailand.
South Korea’s foreign ministry said another Korean firm, Korea Western Power, was also involved.
The companies and others had sent helicopters, boats and rescue workers.
“All of our 53 nationals who were taking part in this construction evacuated in advance,” the ministry said in a statement.
Pope Francis said in a Vatican message he had learned with sadness of the loss of life, and expressed “heartfelt solidarity” with all those affected.
The 410 megawatt capacity plant was supposed to start commercial operations by 2019, according to the venture’s website.
The project consists of a series of dams over the Houay Makchanh, the Xe-Namnoy and the Xe-Pian rivers in Champasak province.
It planned to export 90% of its electricity to energy-hungry Thailand and the remainder was to be offered up on the local grid.
Under the terms of construction, PNPC said it would operate and manage the power project for 27 years after commercial operations began.
Dam projects in Laos, mainly providing power to neighbouring countries, have long been controversial with fears over environmental damage and the impact on local communities who are often displaced.
Maureen Harris, an expert on Laotian dams at the International Rivers NGO, said the flooding raised “major questions about dam standards and dam safety in Laos, including their appropriateness to deal with weather conditions and risks”.
“Many of these people have already been relocated or suffered impacts to livelihoods due to the dam construction and are now experiencing further devastating impacts – loss of homes, property and family members," she said.
Laos has been keen to turn itself into “the battery of Southeast Asia” with a series of massive hydropower projects that has sparked opposition in downstream Mekong nations like Vietnam and Cambodia, who fear it will disrupt vital ecosystems, fisheries and their own river systems. Communist authorities in Laos keep tight control on information and are often opaque about business deals and development projects. The media is state-controlled and the government vigorously pursues dissent or protesters.
The country has around 10 dams in operation, 10 to 20 under construction and dozens more in planning stages.
“Once they cast themselves as the battery of Asia, exporting electricity became one of the major revenue sources, so it’s basically selling natural resources such as water,” Toshiyuki Doi, Senior Advisor at Mekong Watch, said.