RESERVOIR SEDIMENTATION AND CAPACITY RESTORATION: A CASE STUDY OF BAJOLI HOLI AND TEHRI RESERVOIRS

Abstract

Reservoir sedimentation is a very complex and important phenomenon resulting from the obstruction of the natural river flow by construction of a dam across the river. Reservoirs located in the Himalayan region where sediment production is very high because of the young and fragile nature of the rocks, suffer a lot from this sedimentation disaster. The sedimentation in the Himalayan region is worst in RoR schemes since they have very small storage capacities. For this, it is imperative for dam owners, managers, operators or planners to predict and practice sediment management of different forms not only for RoR projects but for other type of reservoirs as well for sustainable use of these water resources schemes. In this study two reservoirs (Bajoli Holi and Tehri) both located in the Himalayan region but more importantly having different parameters and purposes were respectively modeled for sedimentation and capacity restoration (for Bajoli Holi only) through depleting flushing using HEC-RAS 5.0.5 software. In the first model (Bajoli Holi), the simulation was performed on the basis of proto data (sediment inflow and gradation), 3-D physical modeling data (Manning’s n values), hydrographic survey (river cross sections), an arbitrary flow hydrograph and observed flow hydrograph obtained from 3-D modeling site. For this project, the model was simulated for sedimentation for 8, 12, and 20 months correspondingly by maintaining MDDL and FRL during monsoon and non-monsoon season respectively following a flow and sediment hydrograph. Also seeing the less storage capacity of the project and as a customary practice the model was further simulated for depleting flushing during monsoon season only by fully opening all gates for 72, 96 and 120 hours in each of the sedimentation periods in order to assess the best flushing results for the sustainability of this reservoir. The simulation results indicated that out of the three conditions tested on the model, worst sedimentation occurred after 20 months of sedimentation, when only about 1.28 MCM, corresponding to 42 % out of about 3.03MCM was left. Whereas, 64 % (1.94 MCM) and 72 % (2.19 MCM) were left after 12 months and 8 months’ sedimentation respectively. Depleting flushing results showed that maximum capacity of about 93 % (about 2.81 MCM) was restored after 8 months’ sedimentation and 5 days depleting flushing. Unlike depleting flushing for the same period when iii only 87 % (2.63 MCM) and 67 % (2.02 MCM) capacities were respectively restored after 12 and 20 months’ operation. In the second model (Tehri), the mathematical model was completed in two different runs using two different sets of sediment data. Firstly, the simulation was performed for 25 years using sediment corresponding to actually observed loss of capacity by previously conducted hydrographic survey and actually observed discharge. From the analysis of both rivers it was seen that sediment mostly deposited in between chainage 22 km and 37 km in R. Bhagirathi and 7 km and 25 km in R. Bhilangna, due to the gradual drop in the river flow velocity from u/s towards the dam in both rivers. In 25 years the loss of capacity was found to be 149.51 MCM or 5.98 MCM per year, resulting to 0.17 % loss of capacity per year in Tehri reservoir. A 5.98 MCM loss per year means that MDDL capacity (925 MCM) is expected to be lost in 155 years. In the second run, for 25 years also, average sediment observed along with average flow actually observed was used. Which resulted in 216.88 MCM loss of capacity (8.68 MCM/yr.). Hence, a 0.25 %per year loss of capacity. This loss (8.68 MCM/yr.) translates to a projected loss in MDDL capacity (925 MCM) after 107 years.