‘Mega hydel projects in Arunachal Pradesh are safe’

A Correspondent

DIBRUGARH: There is a growing concern among the masses and intellectuals over the safety of construction of mega dams in India and particularly in Arunachal Pradesh after the recent incident of the Glacial Lake Outburst Flood (GLOF) scenario in Teesta Valley in October 2023.

The technical facts corroborating the Glacial Lake Outburst Flood (GLOF) scenario in Teesta Valley are entirely different then the prevailing glacial lakes and their likely impact on proposed dams in Arunachal Pradesh constructed by NHPC. The whole concept can be understood from subsequent paragraphs.

There are several glacial lakes in Teesta Basin, upstream of Teesta III dam (Chungtham Dam), which have been studied periodically due to their potential impact on dams resulting from Glacial Lake Outburst Floods (GLOFs).

In 2015, the Central Water Commission (CWC) conducted a study titled “Status of Glacial Lakes & Water Bodies in the Himalayan Region of Indian River Basins,” in collaboration with the National Remote Sensing Centre (NRSC), ISRO.  A GLOF event originating from South Lhonak lake (located approximately 70 km upstream of Teesta III Dam) could have been triggered by various factors, such as an earthquake, flash flood, glacier calving, or breaches due to the detachment of moraine sidewalls or a combination of these events.

 This event resulted in a GLOF with a discharge of about 8000-10000 cumec, carrying massive boulders against the Teesta III dam. As per the assessment by National remote Sensing Centre (NRSC), Hyderabad, the water spread area of the South Lhonak lake was originally around 167 ha on September 28, 2023 i.e. before breaching of lake and reduced to 130 Ha after the event. The slope of river between south Lhonak Lake to Chungtham Dam is about 1 in 20. The reservoir capacity of Teesta –III (Chungtham Dam) project was about 5 MCM.

The Teesta –III dam was a Concrete Faced Rockfill Dam (CFRD). Due to fact that South Lhonak lake is located in close vicinity of Teesta –III dam (Chungtham Dam) and steep slope of river course it took merely about 1 hour to reach the flood (GLOF) from South Lhonak lake to Chungtham Dam hence reaction time was very less. Also due to very low reservoir capacity of about 5 MCM in comparison to the flood volume of 40-50 MCM generated due to lake breach, the flood volume could not be absorbed into Chungthang reservoir.

Despite the fact that flooding was very high at NHPC projects due to combined effect of flood created from GLOF coupled with Chungtham dam breach flood; 2 Concrete dams (Teesta-V & TLDP-IV) and 1 Barrage (TLDP-III) of NHPC on main Teesta river downstream of Teesta –III dam (Chungtham dam) are intact and did not fail during above mentioned October-2023 GLOF event.

Most of the glacial lakes present in Subansiri, Siang and Dibang river catchments are present in Chinese portion of these river catchments, which are far away from NHPC projects hence posing insignificant threat of GLOF induced flood.

The topographic, catchment characteristic, glacial lake locations in river basins in Arunachal Pradesh are different in following aspect which indicated there are no chance of GLOF induced flood disaster for projects in Subansiri, Siang and Dibang Basins as it was in Teesta Basin GLOF case.

The glacial lakes are located far away from NHPC dams as a result of this Glacial Lake Outburst Flood (GLOF) get attenuated to a very low flood values.

The slope of rivers are mild hence flow velocities of flood water is lower, also leads to higher travel time and more time to take action to mitigate the effect of flood. Due to presence of huge reservoir storage volume the Glacial Lake Outburst Flood (GLOF) volume will be completely absorbed into reservoirs hence no threat to dam or to the downstream population.

As per existing studies the peaks of GLOF at dam sites are even much lower than average monsoon flood for Subansiri, Siang and Dibang basin projects of NHPC. Teesta –III dam was a Concrete Faced Rockfill Dam (CFRD). Due to fact that South Lhonak  lake is located in  close vicinity of Teesta –III dam (Chungtham Dam) and steep slope of river course it took merely about 1 hour to reach the flood (GLOF) from South Lhonak lake to Chungtham Dam hence reaction time was very less. Also due to very low reservoir capacity of about 5 MCM in comparison to the flood volume of 40-50 MCM generated due to lake breach, the flood volume could not be absorbed into Chungthang reservoir.

Despite the fact that flooding was very high at NHPC projects due to combined effect of flood created from GLOF coupled with Chungtham dam breach flood; 2 Concrete dams (Teesta-V & TLDP-IV) and 1 Barrage (TLDP-III) of NHPC on main Teesta river downstream of Teesta –III dam (Chungtham dam) are intact and did not fail during above mentioned October-2023 GLOF event.

Most of the glacial lakes present in Subansiri, Siang and Dibang river catchments are present in Chinese portion of these river catchments, which are far away from NHPC projects hence posing insignificant threat of GLOF induced flood.

The topographic, catchment characteristic, glacial lake locations in river basins in Arunachal Pradesh are different in following aspect which indicated there are no chance of GLOF induced flood disaster for projects in Subansiri, Siang and Dibang Basins as it was in Teesta Basin GLOF case.

The glacial lakes are located far away from NHPC dams as a result of this Glacial Lake Outburst Flood (GLOF) get attenuated to a very low flood values.

The slope of rivers are mild hence flow velocities of flood water is lower, also leads to higher travel time and more time to take action to mitigate the effect of flood. Due to presence of huge reservoir storage volume the Glacial Lake Outburst Flood (GLOF) volume will be completely absorbed into reservoirs hence no threat to dam or to the downstream population. As per existing studies the peaks of GLOF at dam sites are even much lower than average monsoon flood for Subansiri, Siang and Dibang basin projects of NHPC.

Also Read: Arunachal: Concerns Raised Over Safety of Mega Dams in Arunachal Pradesh, North East Human Rights Urges Halt

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