Bypassing of High Sediment Loads at Nathpa Jhakri Hydroelectric Project – A Case Study

The useful life span of reservoir is determined bythe rate of sedimentation, which gradually reduces the storage capacity of the reservoir. This reduction in storage capacity ultimately fails to deliver the benefits of the scheme for which it was built. In spite of various options available for positively managing sedimentation in the reservoir such as catchments conservation programmes, upstream trapping of the sediment by constructing check dams etc. many reservoirs in Indiaand those especially in Himalayan ranges are silting up at alarming rate. To harness the substantial water power potential available in Himalayan ranges, many run-of-the-river schemes are coming up. In those schemes, a small reservoir is created by constructing a dam and the available water is diverted, where large head is available for power generation. However, due to steep river slope and high-suspended sediment concentration, these reservoirs are prone to silt up rapidly. Many such reservoirs are silted up in one season ony. In the case of some reservoirs it has been seen that due to silting up of the reservoirs and in adequate detention time for settlement of suspended sediment, large quantity of suspended sediment enters in to the water conductor system and damages the turbines. Sluicing and density current venting are the two ways of passing the sediment through reservoirs. Sluicing involves providing low level large capacity sluices in the dam through which the deposited sediment can be flushed out periodically by lowering the water level so as have river flow, which will generate shear velocities along the bed. In density current venting method it is not necessary to lower water level but it is applicable in very exceptional circumstances where sediment induced density current carry fine sediment towards the dam. The number of cases, where density current venting has been successful is minimal (Rodney White). Another method of reducing the entry of sediment into the main reservoir is “bypassing of high sediment loads”. In this method, the main reservoir is filled at low or medium flows when the sediment concentrations are low and bypass high flows when the sediment concentrations are high. In this method the reach of the reservoir between the main dam and the sediment bypassing channel/tunnel acts as a settling tank and there is a substantial reduction in entry of coarse and medium sediment into the power intakes. More-or-Iess similar methods have been envisaged by Satluj Jal Vidyut Nigam Limited (SJVNL) for their prestigious 1500 MW Nathpa Jhakri Hydro Electric Project (NJHEP). CW&PRS had an opportunity to conduct hydraulic model studies to test the adequacy of proposed new diversion tunnel in directly diverting the sediment downstream of the main dam bypassing the main reservoir. Hydraulic model studies have been conducted on 1:60 geometrically similar scale model and efficacy of the proposal is presented in the paper.