[ AN Mohammed ]
There is growing concern among the masses and intellectuals over the safety of construction of mega dams in India and particularly in Arunachal Pradesh after the incident of the glacial lake outburst flood (GLOF) scenario in the Teesta valley in October 2023.
The technical facts corroborating the GLOF scenario in the Teesta valley are entirely different from the prevailing glacial lakes and their likely impact on proposed dams in Arunachal Pradesh constructed by the NHPC. The whole concept can be understood from subsequent paragraphs.
GLOF scenario in Teesta basin (3-4 October 2023)
There are several glacial lakes in the Teesta basin, upstream of the Teesta III dam (Chungtham dam), which have been studied periodically due to their potential impact on dams resulting from 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 kms upstream of Teesta III dam) could have been triggered by various factors, such as an earthquake, flashflood, 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 8,000-10,000 cumec, carrying massive boulders against the Teesta III dam.
As per the assessment by the National Remote Sensing Centre (NRSC), Hyderabad, the water spread area of the south Lhonak Lake was originally around 167 ha on 28 September, 2023, ie, before breaching of lake and reduced to 130 ha after the event. The slope of the river between south Lhonak Lake and the 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 the fact that South Lhonak Lake is located in close vicinity of the Teesta III dam (Chungtham dam) and the steep slope of the river’s course it took merely about one hour to reach the flood (GLOF) from South Lhonak Lake to Chungtham dam; hence, the 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, two concrete dams (Teesta V & TLDP IV) and one 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 0ctober 2023 GLOF event.
NHPC projects in Arunachal
Most of the glacial lakes present in Subansiri, Siang and Dibang river catchments are present in the Chinese portion of these river catchments, which are far away from the NHPC projects, hence posing insignificant threat of GLOF-induced flood.
The topographic, catchment characteristics, and glacial lake locations in the river basins in Arunachal are different in following the aspects which that indicated there are no chance of GLOF-induced flood disaster for projects in Subansiri, Siang and Dibang basins as it was in the Teesta basin GLOF case.
The glacial lakes are located far away from the NHPC dams, and, as a result of this, GLOF get attenuated to very low flood values.
The slopes of the rivers are mild; hence the flow velocities of floodwater are lower and lead to higher travel time and more time to take action to mitigate the effect of flood.
Due to the presence of huge reservoir storage volume, the 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 much lower than average monsoon flood for Subansiri, Siang and Dibang basin projects of the NHPC.
All the proposed dams in these basins are concrete dams, which are not vulnerable to failure.
Basin-wise NHPC projects
- Subansiri basin projects:
Most of the glacial lakes present in the Subansiri river catchment are present in the Chinese portion of the Subansiri river catchment, which is far away from the Subansiri Upper Project (SUP) and the Subansiri Lower Project (SLP), hence posing insignificant threat of GLOF-induced flood.
The slope of the Subansiri river 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 kms upstream of SUP dam.
The reservoir capacities at full reservoir level (FRL) and maximum water level (MWL) are 1,755 MCM and 1,988 MCM, respectively, along with live storage of about 745 MCM, which is very large to absorb the GLOF volume.
The slope of the Subansiri river 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 kms upstream of SLP dam. The reservoir capacities at FRL and MWL are 1,365 MCM and 1,474 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 kms upstream of KHEP dam. The reservoir capacities at FRL and MWL are 1,928 MCM and 2,366 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 kms upstream of Siang Upper dam. The reservoir capacities at FRL: 497m and MWL: 500m are 12,987 MCM and 13,412.6 MCM, respectively, along with live storage of about 9,200 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 kms upstream of Dibang dam. The reservoir capacities at FRL: 530.3m and MWL: 538m are 3,248 MCM and 3,510 MCM, respectively, along with live storage of about 1,290 MCM, which is very large to absorb the GLOF volume.
Therefore, correlating flash-flooding in Teesta basin with any future eventualities in Arunachal, 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. (The contributor is Consultant, NHPC Ltd)
