NHPC Ltd., Faridabad, India
Online published on 12 January, 2016.
NHPC, A Government of India Enterprise is operating 18 hydropower Projects, which are located in heavily sediment prone Himalayas, having small and medium size of reservoirs. One of the major advantage of hydro projects out of a numerous others is the capability to provide the peaking power i.e. generate at a time when the requirement is at its peak while reducing the output at other times. To ensure the same, some diurnal storage (live capacity) requires to be provided in the reservoir. Sediment spells doom for this diurnal storage as it occupies the precious little space meant for peaking purposes. These hydropower projects are being successfully managed since last 20–25 years by developing site specific sediment management techniques. More often than not, these techniques are required to be customised for different projects depending on their reservoir size, valley shapes as well as their unique design features. In the present case study two run-off the river hydropower projects on river Ravi in Himachal Pradesh namely Chamera-I and Chamera-II projects having medium and small reservoirs have been discussed. The Catchment areas of Chamera-I and Chamera-II projects are 4725 sq km and 2596 sq km respectively. The Chamera-I reservoir extends to about 15 km along the river Ravi and about 12 km along river Siul, while Chamera-II has a reservoir stretch of about 3 km along Ravi.
To have a regular check on the balance reservoir capacities the reservoir hydrographic survey is being carried out every year generally in the post-monsoon period. The live capacities being so different in magnitude in these projects, call for different techniques for sediment management. The Chamera-I reservoir is a relatively big reservoir and would require several days for depletion and refilling the reservoir. Therefore sedimentation management is done by lowering the reservoir level from FRL to a level close to MDDL during flood season (Jun to Sep), for routing the sediment to the downstream and operating under-sluices regularly. This sediment management technique is termed drawdown sluicing or simply “sluicing” with low level outlets.
On the other hand Chamera-II reservoir is a small reservoir and sediment management is done by carrying out reservoir drawdown flushing operation (free flow flushing) once in every month of monsoon for removal of deposited sediment from the reservoir. The principle of draw down flushing entails lowering the reservoir to river level and allow the flow in the near pristine conditions.
The Chamera-I reservoir having original live capacity of 115 MCM has been maintained in satisfactory working condition. Even after 20 years of commissioning of the project, the live capacity has been maintained at 91.4 MCM (Jul-2014 survey). The reservoir of Chamera-II power station having original live capacity of 1.66 MCM has been maintained in satisfactory working condition. After 10 years of commissioning of project the live capacity has been maintained at 1.41 MCM (Jun-2014 survey). Hence the drawdown sluicing and reservoir flushing are both effective in managing the sedimentation and to maintain the useful life of reservoirs in Himalayan region, the choice of either methods would depend on the reservoir size.
Hydropower, Sediment, Reservoir, Sluicing, Flushing
