Fatal flood hits hydro works in the Himalayas 11 Feb 2021

Many fatalities and many more people are still unaccounted for following the devastating flood on the Alaknanda River high in the Himalayas in India that hit several hydropower construction sites and their dams and underground headings further downstream on the Dhauliganga River (Fig 1). The cause of the flood is reported as being failure of a glacier that caused a landslide as it crashed into the river causing the deluge downstream. Given the volume of sediment and boulders carried by the flood, others have said the cause might have been the bursting of a glacial lake.

Fig 1. Rivers high in the Himalayas are known to be prone to flash floods and landslides
Image credit: BBC

Four hydropower plants were affected by the disaster including the 13MW Rishiganga and the 520MW NTPC Tapovan-Vishnugad, the state-owned 444MW THDC Pipalkoti plant and the 400MW Vishnuprayag project in construction by the Jaypee Group. Dams were damaged and tunnel portals in the river valley were inundated, burying workers many of whom were rescued but many still missing as the underground headings are cleared. Rescued workers report how they held onto support features in the crown of the tunnels and climbed down mountains of rocks and rubble as the water receded.

Tapovan appears to have been the project most affected with the cofferdam ahead of main dam construction washed away and the portal of the access tunnel into the underground power station works blocked by the inundation. The status is unknown of the diversion tunnel on the river around the cofferdam and of the underground powerhouse caverns that are in excavation by main contractor HCC. It is reported that 25-35 people were working at an advancing face of a 1,500m heading, the purpose of which is unknown. A statement by the project owner NTPC reports that: “An avalanche near Tapovan has damaged a part of our under-construction hydropower project. While rescue operations are on, the situation is being monitored.”

See the moment a construction coffer dam was washed away
Image credit: BBC

TBM operations by Seli Overseas as a technical sub-contractor to HCC on the Tapovan-Vishnugad project are at different locations and have not been affected by the inundation. It is confirmed, however, that teams from the TBM works have joined the rescue and recover efforts at the river valley and are contributing to excavating the hardening mud and boulder inundation in the flooded headings.

The completed and privately owned Rishiganga power project upstream of Tapovan was the first to be hit by the avalanche and debris from this plant caused damage to other units downstream (Fig 1).

The disaster is being seen as the latest in a string of events that call for extreme caution in awarding infrastructure projects in the high mountain areas. The cofferdam at Tapovan is reported as being overrun and damaged by floods three times. The disaster of Sunday 7 February will prompt more thorough environmental review of projects in ecologically sensitive mountainous areas.

Emergency works to avert glacier activity disasters are known in other countries. In Canada a similar glacial failure event happened near Vancouver 220km north along the coast in the Bute Inlet. Fortunately there were no hydropower plants in the area. The slide as it came crashing into a lake created a greater wave downstream and damage along the shoreline. In Switzerland in 2010, TunnelTalk reported completion of an emergency drill+blast heading to drain a dangerously filling glacial lake to averted a potential disaster. Glacier releases are predicted to become more common with acknowledged global climate change, as the glaciers melt and their mountain slopes give way resulting in large slides.