Sediment yield and depostion pattern in long conical tehri reservoir

Tehri Storage Dam constructed across the mighty river Bhagirathi with a catchment area of 7511 km² is located at Tehri which is 85 km from the famous holy place Rishikesh, Uttarakhand, India. Tehri HPP (100 MW) is already commissioned and Tehri PSP (1000 MW) is in advance stage of construction. The-long and conical shaped reservoir of the Tehri Dam with a gross storage of 35.40 billion m³ and a live storage of 26.15 billion m³ spreads in an area of approx. 42 km². Sedimentation of reservoirs is a matter of great concern as it not only occurs in dead storage but also encroaches the live storage capacity thus-impairing the intended benefits from the reservoir. Depositional patterns of sediment in reservoir vary with differences in hydrologic conditions, sediment grain size, and reservoir geometry. In reservoirs with fluctuating water levels like Tehri reservoir, previously deposited sediments may be extensively eroded and reworked by stream flow, failure of exposed slopes, and wave action. Also in long and narrow reservoirs like Tehri reservoir, the bathymetric profile commonly-associated with delta deposits may be absent, but an area characterized by a rapid shift in grain size, marking the downstream limit of coarse material deposition, may still be present. Sediment deposition-is initially focused in the deepest part of each cross section, creating deposits having a near-horizontal-surface regardless of the original cross section shape. Because the upstream area of such long and narrow reservoir is shallow with little storage capacity, the longitudinal growth of the delta may initially be very rapid and sometimes sedimentation can also result in deltas becoming exposed above the reservoir pool. Distribution of both fine- and coarse-grained sediment deposits in a reservoir can be predicted by both empirical and numerical techniques. The hydrographic survey is a direct method to find out the depth of-sediment deposition, the pattern of sediment deposition and the loss of the reservoir capacity. Considering generally weak nature of Himalayan rocks and steep valley slopes in the catchment of the project, CWC has recommended bed load as 15% of the suspended sediment load for Tehri reservoir and accordingly total sediment load is assumed to be about 161.84 lac t /year for the reservoir. Periodic bathymetric survey since 2005 is being conducted at Tehri reservoir and based on the analyses of survey data collected so far, the overall reduction rate of the live storage of Tehri reservoir since the year 2005 comes to be 4.23 MCM / year. The pattern of sediment deposited at different depth of Tehri reservoir after 2013 survey, plotted on the figure given by USBR for classification of reservoirs, shows that the current pattern of-deposition lies reasonably close to type - III reservoir curve. This paper provides an insight of the methodology adopted in the study and results of the analyses of survey data for deposition pattern of sediment in long conical Tehri reservoir.