Evaluation Of Sediment Management Strategies For Large Reservoirs

Multipurpose large dams play a key role in the development of world by providing water for irrigation, flood control and hydropower. Tarbela is one of the world's largest earth and rock fill dam. Being multipurpose dam, it provides vital role for economic stability and social development of Pakistan. Tarbela Reservoir has lost its significant capacity due to sediment deposition. The objective of the study was to evaluate different options for evacuation of deposited sediments and reducing sediment inflows to Tarbela Reservoir through sediment modeling by HEC-RAS. Sediment flushing from existing power tunnels was evaluated in first option and found not feasible due to the downstream constraints and loss of 7848 MW hydropower from Tarbela and Ghazi Barotha. New sediment bypass tunnels were proposed on right bank of the dam to overcome the constraints in second option. Sediment modeling was performed by HEC-RAS to evaluate each scenario of sediment flushing with different parameters. The sediment balance ratio and long term capacity ratio was also checked for each scenario for technical evaluation and also economic analysis was performed. Most technical viable scenario was flushing for 90 days at reservoir drawdown level of 390 m with discharge of 5000m 3 /s. However, this scenario was not economically feasible as net present value was negative, internal rate of return was 3-4 %, and benefit cost ratio was found less than one. The 3 rd option, with under construction multipurpose Diamer Basha Dam on upstream of Tarbela Reservoir, was also evaluated on HEC-RAS. Results depicted that large amount of sediments were trapped in the upstream reservoir which ultimately reduced significantly the inflow of sediments and delta movement in Tarbela Reservoir. This option is recommended because it will enhance the life of Tarbela Reservoir and it will keep on providing multiple benefits for longer time.

ESTIMATING SEDIMENT YIELD AT TARBELA DAM AND FLOOD FORECASTING THROUGH CONTINUOUS PRECIPITATION-RUNOFF MODELING OF UPPER INDUS BASIN

The live water storage of the reservoirs is decreasing by the sedimentation, which is affecting the reservoir’s capacity and cause a severe problem for the irrigation system at the downstream side. Floods occur at the downstream by the poor management at upstream due to the heavy rainfall and snow melting. For annual accumulations of sediment load and estimation of the peak flow at Tarbela reservoir near Besham Qila station having area of 170,000 km2 was selected. Estimation of the peak flow and sediment yield at the Tarbela reservoir, SWAT (distributed hydrological model) was used. The expected decrease in reservoir storage capacity was also estimated with the SWAT model. For runoff modelling, calibration was done for three years (2004-2006) and validation was also done for three years (2007-2009). Nash-Sutcliffe Efficiency (NSE) and Standard Error of Estimate existed the statistical indices to evaluate the results. Coefficient of determination (R2) was found as 0.75 for the calibration period and 0.80 for the validation. Whereas, NSE for calibration was observed 0.69 and 0.70 for the validation. Monthly mean sediment yield was about 0.13 BCM estimated at the Tarbela reservoir near Besham Qila.

Effect of Sediment Load Boundary Conditions in Predicting Sediment Delta of Tarbela Reservoir in Pakistan

Setting precise sediment load boundary conditions plays a central role in robust modeling of sedimentation in reservoirs. In the presented study, we modeled sediment transport in Tarbela Reservoir using sediment rating curves (SRC) and wavelet artificial neural networks (WA-ANNs) for setting sediment load boundary conditions in the HEC-RAS 1D numerical model. The reconstruction performance of SRC for finding the missing sediment sampling data was at R2 = 0.655 and NSE = 0.635. The same performance using WA-ANNs was at R2 = 0.771 and NSE = 0.771. As the WA-ANNs have better ability to model non-linear sediment transport behavior in the Upper Indus River, the reconstructed missing suspended sediment load data were more accurate. Therefore, using more accurately-reconstructed sediment load boundary conditions in HEC-RAS, the model was better morphodynamically calibrated with R2 = 0.980 and NSE = 0.979. Using SRC-based sediment load boundary conditions, the HEC-RAS model was calibrated with R2 = 0.959 and NSE = 0.943. Both models validated the delta movement in the Tarbela Reservoir with R2 = 0.968, NSE = 0.959 and R2 = 0.950, NSE = 0.893 using WA-ANN and SRC estimates, respectively. Unlike SRC, WA-ANN-based boundary conditions provided stable simulations in HEC-RAS. In addition, WA-ANN-predicted sediment load also suggested a decrease in supply of sediment significantly to the Tarbela Reservoir in the future due to intra-annual shifting of flows from summer to pre- and post-winter. Therefore, our future predictions also suggested the stability of the sediment delta. As the WA-ANN-based sediment load boundary conditions precisely represented the physics of sediment transport, the modeling concept could very likely be used to study bed level changes in reservoirs/rivers elsewhere in the world.

On some microearthquakes near Tarbela reservoir during three low water stands

Detailed 3D stress analyses with a simulated model of Tarbela reservoir indicate that most nearby seismogenic faults inferred from observations of preimpoundment seismicity in the region should be stabilized even when the reservoir has low water levels during dry seasons. We hypothesize that the hypocenters of some microearthquakes occurring close to the reservoir during dry seasons of 1977, 1980, and 1981 were located on those of the above faults that are destabilized by accumulation of broadly north-south horizontal compressive stresses of plate tectonic origin. An order of magnitude analysis shows that the rate of accumulation of these stresses may be sufficiently rapid to overcome the stability imparted to the seismogenic faults by the Tarbela reservoir.