Investigation of Sedimentation Pattern of the River and Reservoir of Kamal-Saleh Dam Using Numerical Model GSTARS4.0

      Accumulation of sediments in dam reservoirs is one of the most important problems in the operation and maintenance of reservoir dams and this phenomenon reduces the useful life of dams. Reservoir sediment changes depend on factors such as the amount of sediment, sediment transport rate, sediment type, reservoir performance, reservoir characteristics and river flow. With the entry of sediment and its accumulation in the reservoir of the dam, the effective water storage capacity decreases. This in turn will reduce water storage capacity and loss of reservoir flood capacity. If sediment accumulates near the dam body, it may cause the bottom drains of the intake valves to be buried and can also impede their operation. In addition, sediment can erode the turbines and the lower valves of the reservoirs. The load on the dam body also increases. In this study, the sedimentation situation in the reservoir of Kamal Saleh Dam during the next 50 years will be investigated.

      More than 35 km of the river was considered for simulation. By entering the TIN map into Arc VIEW GIS software, information related to preparing the map for entering into Hec-Ras software was added with the help of Hec-Georas extension. This information included drawing lines related to the coasts, drawing lines related to the flood plain, drawing cross sections and drawing the central line. By entering the information into Hec-Ras software, REPORT output was taken in this software. In this output, the information is obtained as a text file. Therefore, they can be used to enter the text file of GSTARS4 software. In the next step, the required information was entered into GSTARS4 software. This information included 171 cross sections. Also information such as manning roughness, river flow and sediment, sediment model and stage-discharge were considered. To run the model and investigate sedimentation, two modes were considered for examining the water level inside the dam. One mode was the maximum water level in the dam (full dam) and the other was semi-full dam. Due to the water level inside the dam, sedimentation can change in these two cases. Finally, the software was run for 50 years of sedimentation. There are 14 models for sediment transport in GSTARS4 software among which the Acker and the Yang models had a suitable grain size diameter range to be used. The results showed that the use of two models will have similar results and the changes related to erosion, sedimentation and sedimentation front near the reservoir will give similar results for the two models. Therefore, the Yang model was used to continue the work.

       The results of model showed that for full and semi-full dam conditions, the sedimentation location will be different, so that in full sediment dam condition, it starts approximately from 10480 meters distance from dam and continues up to 7236 meters and then stops. But for the dam in the semi-full state, the sedimentation starts from 5766 meters and continues up to 2547 meters distance from dam. Therefore, sedimentation in the full dam reservoir starts about 4714 meters earlier and also ends earlier at about 4690 meters distance from the downstream. The distance that sedimentation is done in the case of full dam is 3243 meters and its maximum depth is 14.46 meters and for semi-full dam is 3220 meters and its maximum depth is 12.79 meters. Investigation of the cross sections showed that at a distance that sedimentation occurred in the case of a full dam (10.5 to 7 km), not only sedimentation did not occur for the semi-full dam, but also erosion is observed in the river. Of course, this erosion was obtained in a small width of the river (approximately 20 meters in width). On the other hand, if the Acker-White model was used to investigate sedimentation in the reservoir, erosion in this area was almost not observed in the case of a semi-full dam. Therefore, erosion in this area can be neglected for the semi-full dam and the cross section can be considered for the semi-full dam with the same initial bed. Also, due to the small width of erosion, it can be prevented by erosion control methods.

       It seems that there is not much difference in the amount of sedimentation in terms of sedimentation distance and its height, and only the sedimentation location has been moved, which in the semi-full dam reservoir, sedimentation is closer to the dam. This shows that the level of water in the dam can have a great impact on sedimentation in its reservoir. Of course, it should be noted that the dam is usually not in full state, so to determine the deposition, two states of full and semi-full dams were considered. Therefore, the deposition front can be placed between these two modes according to the operating condition.