Subansiri landslides: CEA sought safety audits; NHPC said not needed

A spokesperson for the NHPC said, “Since the monsoon had already set in since the last week of March 2022, the vetting by a specialised agency regarding the adequacy of the strength of the coffer wall and the site’s slope stability was not possible in view of heavy discharge in the river.”

Pointing out that the temporary coffer wall has been dismantled since and three of the five diversion tunnels have already been plugged, the spokesperson said: “Several studies were undertaken earlier… internally as well as by outside agencies… As such, no more assessment of landslide hazards and their impact was needed. The stability of adjoining mountains is being monitored by the NHPC design team and necessary stabilisation measures have been carried out as and when required.”

As recently as in March 2019, an expert committee report, based on which the National Green Tribunal (NGT) allowed the project work to resume, claimed that “the area is free from any major active landslide problem and no significant slope instability condition exists in the immediate vicinity of the dam.”

In April 2022, referring to a recent collapse of the project’s guard wall, a field inspection report by the CEA warned that “significantly higher flow predicted during monsoons would result in higher pressure” and “advised… that the adequacy of powerhouse protection wall’s strength” be “vetted by a specialised agency before the coming monsoons.”

Pointing out that continuous water flow through river diversion tunnels (DTs), which are meant to be temporary structures, might have eroded its walls, the CEA report said: “A reassessment of the impact of river diversion through DTs for such a long period on the slope stability of surrounding mountains may be carried out by NHPC.”

Since its inception, the Lower Subansiri project has suffered multiple landslides. After work resumed in 2019, amidst gnawing safety concerns, the deadline for commissioning two of the project’s eight 250-MW units has seen four extensions: from March 2021 to August 2022, then to March 2023, and again to June 2023.

Monday’s landslide, the NHPC said, did not impact any project components or structure and would not further delay in commissioning the two units by June 2023.

The timeline of damages at the project site reveals a worrying pattern:

* January 2005: Project work begins.

* August 2005: Landslide hits surge shaft. A CAG report said the NHPC “stated that the contractor did not attend to the recommendations of the geologist despite communication from time to time.”

* January 2008: Slope failure at the powerhouse, to quote a CAG report, “attributable to non-implementation of support measures suggested… by Design Division, non-availability of data on rock movements due to lack of proper calibration of the installed instruments and non-installation of survey targets and slope movement monitoring instruments.”

* December 2011: Local resistance stops project work.

* 2013-2018: Due to recurring damages, NHPC made multiple changes in the project design, settling for a surface powerhouse rather than an underground one. The changes were formally approved in March 2018.

* December 2015: NGT stays the already stalled project work.

* June 2019: The project gets final environment clearance. Work resumes.

* May 2020: Landslides hit the dam’s right bank following prolonged rain.

* June 2020: Heavy flooding damaged the diversion tunnels and the coffer dyke.

* July 2020: The guard wall of the powerhouse collapsed.

* August 2021: The guard wall collapsed again and flood waters overflowed the dam.

* April 2022: The guard wall collapsed yet again. The CEA sought independent safety audits before monsoon.

* June 2022: The roof of the head race tunnel came unstuck, killing one.

* September 2022: Torrential rain broke part of the dam tunnel. A crater-like sinkhole was created. Landslides damaged diversion tunnels. The guard wall gave away and floodwaters entered the powerhouse.

* October 2022: A major landslide hit the diversion tunnels.

* April 2023: Yet another landslide above the diversion tunnel.