Utilise live storage in NHPC project to minimise flood: CWC

The CWC has suggested "utilising" live storage in a hydroelectric project located downstream to moderate ensuing flood in Sikkim in the wake of an artificial lake created by blockage of Kanaka river.
Following massive landslide on August 13, the artificial lake was created 4 km upstream of confluence of Teesta and Kanaka rivers. The operational hydroelectric project of National Hydroelectric Power Corporation (NHPC), Teesta-V, is situated about 21 km downstream of the present landslide dam.
The landslide had occurred at 'So-Bhir' cliff at Dzongu.
The lake could burst any moment releasing huge volume of water into the Teesta, suddenly causing a major problem for the people of Dikchu, Makha, Singtam, Rangpo and some areas of north Bengal.

"The live storage of Teesta-V, in consultation with the project authorities concerned, may be utilised to moderate the landslide dam breach flood in the reach of river Teesta, downstream of Teesta-V HE Project," the Central Water Commission (CWC) has suggested in its report sent to the Sikkim government.
The report has been sent also to NHPC and National Disaster Management Authority (NDMA) for "suitable action" on their part, Union Water Resources Ministry said.

According to the CWC, the estimated volume for 65 m height of (the artificial) dam, based on the hydrodynamic simulation, is about 7.55 million cubic meter (MCM). The volume of the lake, for 60 m height, has been estimated about 6.1 MCM.

"For the estimated volume of 7.55 MCM...For the dam break scenario, the initial condition flood in the Kanaka river has been adopted as 100 cumec (cubic metre per second), while the same for Teesta river has been adopted as 450 cumec.
"Since Teesta-V can play a crucial role for flood moderation in case of breaching of land slide dam, the river reach for dam break simulation has been considered from land slide dam location up to the Teesta-V project," it said.
According to the estimates, the available storage at Teesta-V between minimum draw down level (MDDL) and full reservoir level (FRL) is 5.4 MCM.

"Hence, in the event of breaching of landslide dam, moderation of flood peak is possible downstream of Teesta-V through a controlled release of about 1,000 cumec from Teesta-V spillway.
The CWC though stated the estimates have "limitations"
and they are obtained using river cross sections from ASTER Digital Elevation Model (DEM) "which may differ in comparison to surveyed river cross sections".
A high rainfall in the drainage area of the rivers may influence the attenuation pattern of the flood peak, resulting in some higher discharge in comparison to estimated one, it said.
The CWC also said it is not possible to predict exactly the breach parameters of artificial lake, hence, "a major variation in (artificial) dam breach parameter may result in variation of estimated flood peak".