Stability evaluation of slopes around rim of resevoir of diversion dam of vishnugad–pipalkoti hydroelectric project, Garhwal Himalaya, India

The Vishnugad–Pipalkoti Hydroelectric Project, a run-of-the river (ROR) scheme is located on Alaknanda River, a major tributary of river Ganga, in Chamoli District in the state of Uttarakhand. The project constitutes a 65m high diversion dam near village Helong (79°29’30” E and 30°30’50” N), a 13.4 km long Power tunnel (PT) and an underground power house to the south of village Hat (79°24’56” E and 30°25’31”N) to produce 444 MW of electric power. The reservoir area of Vishnugad–Pipalkoti project spread to an extent of 2.5km and is essentially constituted of quartzite rocks in and around the dam site and extent for a distance of 1.5km, where MCT separates the gneissic rocks on north. During drawdown conditions of the reservoir between MRL and DSL, the reservoir slopes may be subjected to alternate dry and water charged conditions, which may lead to reduction in the shear strength of slope forming materials. A detailed Engineering Geological evaluation of reservoir rim region has been carried out to assess the nature of instability of reservoir slopes during draw-down condition.