Reservoirs across the world are losing storage capacity due to sediment deposition. Construction of new dams to compensate for the capacity loss is not feasible since most of the ideal sites have already been developed. An alternative is to develop projects to sustain its life by sediment management. The concept of sustainable development is gaining popularity and many of the recent hydroelectric projects are designed and operated on this concept. The prime design criteria of such projects are to provide efficient sediment management technique. Hydroelectric projects are being developed as run-of-the-river schemes. Drawdown flushing is the most efficient sediment management technique in such projects and is being practiced widely for sediment management. Conditions for effective flushing are mainly natural and hence design of such projects is site specific.
Most of the hydroelectric projects in the Himalayan region are recently being developed as run-of-the-river schemes. The projects are designed by providing large size dual purpose sluice spillways with crest very near to the riverbed for passing flood and sediment during flushing/sluicing. The alignment, size and crest/invert levels of various components; viz., spillways and intakes are governed by the sediment deposition levels along the reservoir reach. Hence, estimation/prediction of sedimentation pattern in the reservoir, efficiency of flushing and distribution of sediment deposits after flushing is essential for optimizing the design of various hydraulic structures of the project. It is also required to optimize the reservoir operation schedule. Hydraulic model simulations are the generally accepted tool for such investigations, where the governing parameters are mostly site specific.
Prediction/estimation of the sediment deposition pattern in the reservoir is the prerequisite for predicting the efficiency of hydraulic flushing. Numerical model simulations can be used to predict the sediment deposition pattern in the reservoir. The long-term sedimentation pattern in narrow and elongated reservoirs of run-of-the-river projects can be predicted using one dimensional (1D) numerical model.
In the present study, simulations conducted using hydraulic model for sediment removal by drawdown flushing of the reservoir of the Tangon Limb of the run-of-the-river Etalin hydropower project in Arunachal Pradesh, India is presented. Three alternate sedimentation profiles computed using 1D numerical model simulations were studied on a 1: 70 scale geometrically similar model. To optimize the flushing schedule, simulations were carried out with flushing discharges of 200, 500 and 750 m3/s for 36 h duration. Flushing simulations were carried out with three alternative sediment deposition profiles: viz., Profile I in which the delta front of sediment deposition reaches near the intake, Profile II where the five year deposition profile joined to the spillway crest and Profile III with ten year profile joined to the spillway crest. Studies indicated that 1D numerical models can be used to predict the long term sediment deposition pattern in narrow and elongated reservoirs of run-of-the-river projects. Simulations of drawdown flushing on physical scale model indicated that in all the three cases of sedimentation profiles, the reservoir bed levels stabilize after a flushing duration of about 36 h. However, the quantity of sediment flushed increases with the flushing discharge. It was observed that about 0.83 Mm3 of the deposited sediment can be removed with the flushing discharge of 750 m3/s in case of Profile I. Similarly, 0.66 Mm3 of sediment can be flushed with the flushing discharge of 500 m3/s. It was observed that for profile II, about 0.54 Mm3 of the deposited sediment can be removed with the flushing discharge of 750 m3/s. The quantity of sediment flushed with the flushing discharge of 500 m3/s was 0.37 Mm3. For profile III, the quantity of sediment flushed was almost same as for profile II. Flushing with the discharge of 750 m3/s may be carried out for a duration of about 36 hours for effective sediment removal. If the discharge of 750 m3/s does not occur in a year, flushing may be carried out with a lower discharge of about 500 m3/s for a longer duration. Flushing with lower discharge of 200 m3/s is not effective. Since the flushing channel is developing along the right bank from dam to about 600 m length of reservoir upstream, the intake remains clear of sediment deposition for all the flushing conditions.
