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Abstract: Dam reservoirs are constructed in order to flood control, reserve and provide water for downstream use, energy production and/or recreational purposes. Sedimentation is one of the most important operational conflicts in the world; surface run-off water erodes and carries the sediments on their route to the downstream all the time. Because sedimentation in reservoirs would reduce its useful volume, to reserve and retain the present reservoirs and to minimize the dissipation of reservoir volume because of sedimentation is very important. As a result, presentation of an appropriate method for increasing the efficiency of pressurized sediment flushing could be a significant way in increasing the useful lifetime of dams and also in surviving the pools with less water wasting. In this manner, using some hybrid method for increasing the efficiency of sediment flushing could be highlighted. At present, the efficiency of sediment flushing from outlet gates is very low. In this paper, a new method to increase the efficiency of sediment flushing is presented in which a structure namely SFM structure consisted of two parallel piles rows is installed on reservoir bed at the upstream of outlet gate. In this experimental research one to six pairs of piles with a permeability of 37.5% in two parallel rows at distances of 4, 8, 16 and 24 cm from each other were installed. The water flow will exit through the outlet gate after passing among the piles. Redirection of streamlines around the piles and also passing the flow along the corridor will cause some horseshow and wake vortices and also will cause the situation so as the sediments will rise and start to move. This will make the streamlines to interact and the proportional velocity of water to increase along the corridor and as a result, it is anticipated that more sediments will flush from the outlet gate and the flushing cavity volume will expand toward upstream. In this research it is noted that the SFM structure to be applicable and easy to construct inside a full-scale reservoir; so despite Madadi et al (2016), there is no ceiling on top of the columns because in prototype scale construction of such huge roof is not applicable. Because the maximum velocity gradient is normal to the outlet gate based on flow hydrodynamics, the arrangement of the columns is proposed to be perpendicular to the gate axis. Uniform non-cohesive sandy aggregates with a mean size of 0.67 mm were utilized as packed sediment in the reservoir. The results showed that the flushed sediment from the reservoir increases by 261% when the SFM structure with corridor width of 8 cm is utilized compared to that of the reference test (without SFM structure). Based on economic considerations and results of the present study (direct and indirect costs of piles construction) one can see that the four pairs of piles with permeability of 37.5% and row distances of 2Do (L/Do=2) is the most optimum case among the tested cases of SFM structure in increasing the efficiency of sediment flushing around and through outlet gate in reservoirs. Considering the results, the SFM structure is an applicable structure and further investigations should be performed in order to find its design charts.

Keywords: sediment flushing efficiency, SFM structure, corridor width, flushing, reservoir

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