State-run National Hydroelectric Power Corporation (NHPC) on Monday allayed concerns associated with the dams it constructed in Arunachal Pradesh, saying that the mega hydro projects in the state are safe. This comes after growing concern among people following an incident of glacial lake outburst flood in the Teesta valley October 2003.
NHPC said in a statement, "There is a growing concern amongst the masses and intellectuals over the safety of the construction of mega dams in India and particularly in Arunachal Pradesh after the recent incident of the Glacial Lake Outburst Flood scenario in Teesta Valley in Oct 2023."
"The technical facts corroborating the Glacial Lake Outburst Flood (GLOF) scenario in Teesta Valley are entirely different than the prevailing glacial lakes and their likely impact on proposed Dams in Arunachal Pradesh constructed by NHPC," it added.
Elucidating further, it explained how a glacial lake outburst flood event on September 28, 2023 originating from South Lhonak lake, approximately 70 kilometers upstream of the Teesta III dam, could have been triggered due to various factors like an earthquake, flash flood, glacier calving, or breaches due to the detachment of moraine sidewalls or a combination of the events.
This resulted in an outburst with a discharge of around 8,000 to 10,000 cumec which carried massive boulders against the Teesta III dam. An assessment by the National Remote Sensing Centre (NRSC), Hyderabad, the water spread area of the South Lhonak Lake was originally around 167 ha on September 28, 2023, before the breaching of the lake and later reduced to 130 ha.
The reservoir capacity of the Teesta III, that is, Chungtham Dam project was about 5 MCM. It was a Concrete Faced Rockfill Dam (CFRD). Due to the fact that South Lhonak Lake is located in close vicinity of the Chungtham Dam and steep slope of the river course, it took merely about an hour to reach the flood from South Lhonak Lake to Chungtham Dam hence reaction time was much less. Also due to the 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, the statement mentioned.
Even though flooding was very high at NHPC projects due to combined effect of flood created from the glacial lake outburst coupled with the Chungtham dam breach flood; two Concrete dams, that is, Teesta V and TLDP IV and a barrage, TLDP-III of NHPC on the main Teesta river downstream of Teesta III dam are intact and did not fail during the incident.
NHPC Projects in Arunachal Pradesh
Most of the glacial lakes present in the Subansiri, Siang, and Dibang river catchments are present in the Chinese portion of these river catchments, which are far away from NHPC projects hence posing an insignificant threat of glacial lake outburst-induced flood.
The topographic, catchment characteristic, glacial lake locations in river basins in Arunachal Pradesh are different in the following aspects which indicates 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) gets attenuated to a very low flood value.
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 the average monsoon flood for the Subansiri, Siang, and Dibang basin projects of NHPC.
All the proposed dams in these basins are concrete dams, which are not vulnerable to failure.
The basin-wise NHPC project details to cater to GLOF events are enumerated below:
Subansiri Basin Projects:
Most of the glacial lakes are present in Subansiri river catchment are present in Chinese portion of the Subansiri river catchment which is far away from the Subansiri Upper Project (SUP) and Subansiri Lower Project (SLP) hence posing an insignificant threat of GLOF-induced flood.
The slope of river Subansiri from some large glacier lakes up to SUP dam site is of the order of about 1 in 70. Also, potentially dangerous lakes are located at more than 150 km upstream of the SUP dam. The reservoir capacities at Full Reservoir Level (FRL) and Maximum Water Level (MWL) are 1755 MCM and 1988 MCM respectively along with live storage of about 745 MCM which is very large to absorb the GLOF volume.
The slope of river Subansiri from vulnerable glacier lakes up to SLP dam site is of the order of about 1 in 100. Also, potentially dangerous lakes are located at about 260 km upstream of the SLP dam. The reservoir capacities at Full Reservoir Level (FRL) and Maximum Water Level (MWL) are 1365 MCM and 1474 MCM respectively along with live storage of 645 MCM which is very large to absorb the GLOF volume.
Kamla HE Project (KHEP), also known as Subansiri Middle Project, is located on river Kamla, a right bank tributary of river Subansiri. The slope of river Subansiri from some large glacier lakes up to KHEP dam site is of the order of about 1 in 50. Also potentially dangerous lakes are located at about 140 upstream of KHEP dam. The reservoir capacities at Full reservoir Level (FRL) and Maximum Water Level (MWL) are 1928 MCM and 2366 MCM respectively along with live storage of about 624 MCM, which is very large to absorb the GLOF volume.
Siang Basin Projects:
Glacial lakes present in Siang River are located in Chinese portion of the Siang river catchment. The slope of river Siang from some large glacier lakes up to Siang Upper dam site is of the order of about 1 in 150. Also potentially dangerous lakes are located at about 500 km upstream of Siang Upper dam. The reservoir capacities at Full reservoir Level (FRL: 497m) and Maximum Water Level (MWL: 500m) are 12987 MCM &13412.6 MCM respectively along with live storage of about 9200 MCM which are very large to absorb the GLOF volume.
Dibang Basin Projects:
The slope of river Dibang from some large glacier lakes, up to Dibang dam site is of the order of about 1 in 40. Also potentially dangerous lakes are located at about 140 km upstream of Dibang dam. The reservoir capacities at Full reservoir Level (FRL: 530.3m) and Maximum Water Level (MWL: 538m) are 3248 MCM and 3510 MCM respectively along with live storage of about 1290 MCM which is very large to absorb the GLOF volume.
Therefore, correlating flash flooding in Teesta basin with any future eventualities in Arunachal Pradesh based on some incomplete studies without considering catchment-specific features and project capabilities is not proper. The role of hydropower projects is crucial in sustainable green enery transition to arrest global warming, added the NHPC statement.
