Geotechnical investigations |
Slopes, dams and rising risks
Case studies from across India, Ethiopia and Iran highlight the invaluable role geotechnical investigations play in ensuring the integrity of dam projects
IN THE MOUNTAINOUS AREAS of India, landslides can cause extensive damage, loss of infrastructure, and hundreds of fatalities each year. When combined with the impact of extreme weather, the region’s fragile geological and geomorphological conditions can accelerate many landslides, with most occurring in the monsoon season from June to September when India receives 68% of its rainfall. In particular, the country’s North-Eastern Himalayan State of Sikkim experiences a large number on a yearly basis.
As Biwajit Bera recently discussed in Natural
Hazards Research, the increasing number of landslide incidents, glacial lake outburst floods, and other climate change-related hazards ‘can raise a critical question about the sustainability of big dam projects in this kind of active tectonic belt of the Himalayan area’. He gave the example of the Glacial Lake Outburst Flood which occurred at South Lhonak Lake in October 2023. It destroyed the Chungthang dam (part of the Teesta III hydropower project), washing away several structures and part of National Highway 10. Consequently, the author claims, it also affected part of the Teesta Stage V project too, as slope strength can be reduced as a result of toe erosion of the river. In addition, Bera says, the region also experienced continuous rainfall in June and July 2024 which can increase pore water pressure over the slope and the slope steepness, with less vegetation cover and high anthropogenic imprints significantly accelerating slope instability processes too. A massive landslide then occurred in August 2024 at Dipu-Dara near Singtam in Sikkim. Measuring over an area of 0.02km2
, it was 175m long and 136mwide,
bringing down a huge part of the slope over the National Hydroelectric Power Corporation’s Teesta V 510MW hydropower project. A large part of the powerhouse was destroyed, with the tailrace tunnel gate structure and gas insulated switchgear building also being affected. Teesta V is not expected to become operational again until 2026. The Teesta V dam is an 88.6m high, 176.5m long
concrete gravity structure impounding a regulating reservoir for daily power peaking. The head works divert flow into the desander and a 17km headrace tunnel which runs through fragile phyllite, schist, slate and quartzite rocks – decreasing rock strength, Bera claims. Numerous past earthquake epicentres are located between Dikchu and Signtam and its nearby areas, with the highest 5.45 magnitude earthquake recorded in the vicinity of the slide area in 2013. ‘The frequency and magnitude of landslides along the Singtam-Dikchu road are being increased due to fragile geology, cloudbursts and the execution of the Teesta Dam V power station and 17km underground head race tunnel. More research is required for hazard risk reduction, particularly in the populated Teesta River basin within the Eastern Himalayan terrain,’ Bera says. He goes on to claim that the Teesta V landslide highlights how hydropower station construction in the steep slopes of the Himalayas can accelerate the pathways of big destructive landslides in the region. He urges the Central Government of India and State Government of Sikkim to review these projects with ‘a new direction’ because the economic impact after this
34 | October 2025 |
www.waterpowermagazine.com
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