An army of experts in a WhatsApp group is guiding rescue workers scouring a Nepali valley after a huge flash flood more than a week ago as they desperately search for anyone who may be alive inside tunnels of devastated hydropower plants. The messaging group was set up by engineers before the wall of water, mud and rock crashed down the Trishuli valley in Nepal on August 26. It has since become a valuable tool alongside helicopters and heavy machinery in the race to find survivors. The authorities hope some of the about 900 people reported missing from six hydropower plants - at least three of which were under construction - may have escaped the deluge because they worked in or fled into the tunnels. Even finding entrances to some of the tunnels, which were designed to channel water or used to access hydropower facilities, is tough now after the flood deposited 2.2 million tonnes of mud and debris down the valley.

"Can you send me Chilime layout drawings," a government official wrote on the engineers' WhatsApp group on Saturday as rescue workers searched the buried remains of the Chilime hydropower plant high in the Himalayan valley close to the Chinese border. Minutes afterwards, a member of the group chat, which was expanded to add more experts in support of the rescue effort, responded by sending drawings of the project's powerhouse, as well as details of access tunnels. That exchange and others posted on the group, which has more than 500 members, have been vital for operations. No one has yet been found alive in the tunnels at the 22 MW Chilime plant - or in tunnels at any of the power plants further down the valley. But Nepal's Independent Power Producers' Association said on Monday at least eight people could be trapped at Chilime, while an official at the Nepal Electricity Authority said 46 may be trapped at the Rasuwagadhi hydropower plant further south.

"The flood has altered the terrain around the tunnels and changed their structure, while mud, rocks, soil and other debris have accumulated inside," Nepal army spokesperson Raja Ram Basnet said. "This is not a normal closure of tunnels." At Chilime and other sites, rescue workers face immense challenges. More than 1,200 people were killed in last week's deluge caused by a glacier collapse. Thousands of people are still missing, including those who might be trapped inside tunnels. Alongside Chilime, rescue workers are searching five other hydroelectric sites in the valley that were part of an expanding network of plants powering the nation and exporting the surplus. The flood wiped out 10% of Nepal's generation capacity.

At the Rasuwagadhi hydropower plant, rescue workers reached 250 metres into one tunnel before finding it blocked, but they have not given up the search. "We are trying to reach there inside again to look for people," said Amshu Kiran Shahi, a board member of the Nepal Electricity Authority, adding that people may have been trapped in a chamber where they might have access to food and water. A manager at the Nepal Electricity Authority, speaking on condition of anonymity, said rescue workers had also asked former plant employees for advice about the tunnel networks via the WhatsApp group, and that drawings posted on the group had been immensely useful.