Archive of SID A Study on Sediment Settling Pattern in the Reservoir of Shahid Rajaie Dam by Using Observed Data & Empirical Methods Zohreh Zargaran M.Sc. Student of Water Resources Engineering and Management, Islamic Azad University, Science & Research Branch, Tehran, Iran [email protected] Farhang Behrangi Ph.D. Candidate in Hydraulic Structures, School of Civil Engineering, College of Engineering, University of Tehran, Tehran, Iran [email protected] Leila Amiri Ph.D. Candidate in Environmental Engineering, Department of Civil and Environmental Engineering, K.N.Toosi University of Technology, Tehran, Iran [email protected] Abstract In addition to reducing the useful storage capacity, sedimentation in reservoirs makes changes to a river basin and reservoir morphology. Depending on the amount of incoming sediment, trap coefficient and the manner of settling, the useful life of the reservoirs varies. With the movement of sediments towards the body of the dam and the obstruction of the discharging gates and turbine valves, the dam operation will practically face problems. Therefore in sedimentation studies, besides the incoming sediment, its movement rate towards the body of the dam, and the sediment deposit procedure must be taken into account. This paper presents a study on Shahid Rajaee reservoir in Iran in which first, by using AutoCAD Civil 3D 2011 software, the geometrical information of the reservoir such as its volume, area and longitudinal profile are obtained from hydrographic maps. Then with observed data and empirical methods, the amounts of sediment inflow as well as its settling pattern in the reservoir are studied and the future of sedimentation procedure in this reservoir is anticipated. Keywords: Shahid Rajaie Dam, Sedimentation, longitudinal profile, Hydrographic, Trap coefficient, Civil3D www.SID.ir Archive of SID 1. Introduction By constructing a dam, a lake will be created in its upstream. Because of water retrogression, the speed of river behind the dam decreases; as a result, the coarse sediments will deposit far from the structure and the fine particles which are easily carried by the flow eventually sink to the bottom of the reservoir near to the body of the dam. Problems created by deposition of sediment are numerous and varied. Sediment deposition in the dam reservoir is reflected in the reduction of the storage capacity of the reservoir. Moreover, the deposition is followed by a rise in water level which leads to an increase in evaporation in the lake as well as the inundation of the lands and the creation of swamps. Disturbance in discharging gate and a reduction in dam efficiency in controlling floods are among other disadvantages of sediment deposition. This paper will briefly discuss the researches upon sedimentation in reservoirs. It will also state literature reviews on Shahid Rajaie Dam as well as the target location of the present article. Afterwards, by using hydrographic maps of Shahid Rajaei reservoir and the observed data, the amount of deposited sediment is found. At this stage, the most popular and current empirical relations involved in the deposition of dam reservoirs are applied and their efficiency in the reservoir of Shahid Rajayi dam is identified. These relations include those estimating the volume of sediment inflow and the ones determining the reservoir trap coefficient. The rest of this article concerns with identifying the amount of deposit settling and studying its distribution pattern. According to the fore studies on this dam, it has been observed that there is no concern about the useful life of the dam in regard to the volume of sediment inflow. In this research, nevertheless, deposit settling and its pattern are studied by means of empirical relations, hydrographic maps and Civil3D software to make certain that the sediments do not strike the dam body and the valves (before reservoir is filled). In the end, a conclusion is drawn from the results, and reservoir future in respect to sedimentation is predicted and some suggestions for future studies are presented. 2. Literature reviews So far there have been many studies on sedimentation in reservoirs by scholars. Some of them are done by Numerical simulations of the river and reservoir of the dam. These studies have been performed by the implementation of soft wares such as HEC-RAS (Behrangi et al., 2009), GSTAR (Pourbojarian et al., 2009) and (Behrangi et al., 2011), HEC-6 (Gibson et al., 2004). The common point between these numerical models is that in all of them the amount of sediment inflow, the flow characteristics along with geometry of river and reservoir are considered as input and the output will demonstrate the manner of sediment distribution and deposition in different parts of the dam. Other sedimentation studies are performed by means of equations and empirical methods. These studies have also been done in two categories of anticipation of amount of sediment inflow and the manner of settling in reservoirs. In the studies concerned with the first category, (Behrangi et al, 2014) studied the sedimentation behind Latian Dam by means of empirical equations presented by different scholars, hydrographic results and hydrology statics. In this research, the strength or weakness of different empirical equations for Latian dam was examined and eventually by considering 85 MCM of useful capacity and 28 years of statics, the useful life of this reservoir was estimated as 96 years. (Pourbojarian et al, 2009) also investigated sedimentation behind Sefidrood Dam through usual empirical relations. In the second category of studies with the help of empirical relations, the manner of sediment deposit has been studied in two major forms. The first form, which is the most common, is the empirical methods of area- decrease and area-increase. In this research, (Shabanloo et al, 2002) studied the amount of sediment inflow and it’s settling pattern in the reservoir of Dez dam. In this study on the basis of the amount of sediment inflow to the reservoir gained in 1983 and 1997, the amount and the manner of sediment deposit till 2021 were estimated. (Behrangi et al. 2009) in a study on Latian Dam reservoir also examined the accuracy of empirical methods by using actual values. In the recent study www.SID.ir Archive of SID it was determined that Lara’s area- decrease method has been best suited for estimating the void capacity of the reservoir while in finding sediment deposits elevation near to the body of dam and close to the gates, area-increase method is the best. Also using area- increase and area- decrease method, Hasirchian et al. (2010) have also anticipated the manner of sedimentation settling in Shahid Rajaei Dam in future. The other form of relevant studies which are related is involved with longitudinal profile of the reservoir, formation of sedimentary delta and its movement towards the body of dam. In this area, Behrangi et al. (2008) have probed the formed delta in Latian Dam and demonstrated that of useful life of Latian reservoir comes to an end before the reservoir is filled with sediment and by Lavrak branch sedimentary delta conflux to the structure. 3. Location of the present research The present study is a research on sedimentation in Shahid Rajaei reservoir. In sedimentation studies, in addition to the amount of sediment inflow and sediment deposit in reservoir, the manner of settling distribution in different elevations and also the pace of sediment deposit progress towards the body of the dam are of great importance. (Ghaderi et al. 2010) by studying the abrasion amount of the reservoir basin of Shahid Rajaei dam indicated that an average of annually 0.25 MCM sediment enters the reservoir. Meanwhile, we can see this quantity is confirmed in reports prepared by the exploiting organization. Hence, considering the 162.5 MCM capacity of the reservoir, there shouldn’t be any concern about its filling with sediments. On the other hand, the manner in which the sediments fill the reservoir should be examined to prevent early obstruction before the reservoir is filled with deposit sediment. There are two ways achieving this goal. The first is area-increase and area-decrease method through which the volume of deposited sediments in various levels of the reservoir can be anticipated. This was done by (Hasrchian el al. 2010). In this study area-decrease proved better results and the sediment amount at different levels of the reservoir in future is anticipated. They predicted that the reservoir will have been filled with sediment by 2133. The significant point in all the studies made on Shahid Rajaei Dam reservoir is the lack of studies on the pace of the deposited sediment towards the body of the dam. So in present study this has been examined with empirical relations, the form of longitudinal profile, hydrographic maps and Civil3D software. In this study some empirical relations predicting the amount of sediment inflow and deposit sediment were also surveyed for Shahid Rajae Dam and the only ones which had more accuracy for the future of the region are presented. 4. Study area Shahid Rajaie dam is situated in a place named Tange-soleiman which is located in 40 kilometers from the south east of the city of Sari in Mazandaran province. It is constructed on Tajan River in the area to provide valuable irrigation water for the fertile agricultural lands in the downstream plains of the dam and to produce hydro-electricity, flood control and tourist attraction in the beautiful forests of Northern Iran. The dam is a double curvature concrete arch dam with 430 m crest length, 27 m width of dam in foundation and 7 m in crest. The height above foundation is 138 m and 116 m from the riverbed. The spillway manages 800 cubic meters of water per second and the area of its lake at normal water level is 158 cubic meters.