Sustainable Reservoir Management- A Case Study of Tarbela Reservoir

Abstract

Rivers contain a large amount of sediments and when they approach a hydraulic structure, the sediments in the water settle down and gradually deplete the reservoir capacity. This study was carried out to understand the process of sedimentation in the reservoir-river system, leading to the formation of the delta, by using the model HEC-RAS. The data input to the model was divided into three categories; Geometrical, Hydrological and that related to sediments. The observations of river-reservoir bathymetry collected from the annual field surveys served as the basis of calibration and validation of the model results. Model was calibrated and validated for three (2000-2003) and eight (2000-2008) years, respectively. Two statistical parameters were used to predict the simulation efficiency of HEC-RAS model. Model calibration and validation results were compared with actual bed profiles of 2003 and 2008, respectively. The statistical parameters showed good agreement between observed and simulated bed profiles. Manning’s roughness coefficient was optimized for different values ranging from 0.02 to 0.05, and found out that 0.03 was the most suitable one on the basis of Nash Sutcliff Coefficient, Coefficient of Determination and RMSE. Different transport functions and fall velocity methods were applied to calibrate the model and found that Ackers and White as transport function and Van Rijn as fall velocity method predicted results close to the 2003 observed bed profile. Tarbela dam is playing a key role in the food security of the country by ensuring adequate water supply all the year round to the agriculture sector-the backbone of the economy. In addition, it is generating cheap hydropower which helps to offset the effect of the costly thermal generated electricity on the overall tariff for the consumer. Apart from the gradual reservoir sedimentation depleting the live storage capacity, the immediate concern to dam functioning is posed by the advancement of the sediment delta towards the main dam. It is thought that the delta could choke the power tunnels inlet structures triggered by any seismic activity thus forcing the supply cut-off from the dam. These apprehensions to dam safety have caused the WAPDA, the dam managing authority to commission several studies to ponder ways and means to slow down the delta advancement rate. vii The studies have revealed that the delta advancement has been the maximum during the drought years when the dam was drawn down to satisfy the agriculture demand. This has suggested controlling the minimum reservoir level as one of the ways to slow down the advance of the delta. Three different dam operation scenarios were developed in this research by increasing the minimum reservoir level by 1m, 2m and 3m every year starting from 417m, and in the fourth scenario, the minimum reservoir level was kept at 417m for the next eight years (2008-2016). By increasing minimum reservoir level every year predicted a slowdown in delta advancement rate. The rate was found to be 153, 63 and 57 meter per year for first, second and third scenario, respectively. The fourth scenario had the fastest delta advancement rate of 191 meter per year which was the worst one as the pivot point of delta would reach the dam face in less time. Overall the results of this study showed that 1D HEC-RAS model can predict sediment dynamics within a reservoir for longer duration of time with less computation cost and less data requirement as compared to 2D and 3D models.