بررسی آزمایشگاهی تاثیر همپوشانی صفحات مستغرق بالادستی ساعتگرد و پادساعتگرد بر توپوگرافی قوس توام با پایه پل منفرد

Submerged vanes are simple structures that can be used to control the sediment and redistribute the flow and sediment in open channels. Submerged vanes are low aspect ratio flow-training structures mounted vertically on the riverbed at an angle to the prevailing flow. The technique of submerged vanes is a new and efficient sediment management method in rivers. Odegard and et al (1991) are the creators of the idea of using vanes, they have chosen the angle of the vanes between 15 to 25 degrees and found that the vortex created by the vanes causes a change in the amount and direction of shear stress and dispersion velocity and depth,is resulting in sediment transport. When a submerged vane is installed in the river bed with a small angle to the flow direction, due to the pressure difference on either side of the vane, vortex and induced circulation is created around the vane. as a result of this vortex, at the downstream of vane, sediment are taken from the suction side of the vane and are deposited on the pressure side of the vane. So by installing a row of vanes on the riverbed, sediment can be distributed over a larger surface. In river meandering, when flow passes through a bend, reduction of flow velocity and rising hydrostatic pressure cause super elevation phenomena at outer side and reduction of water surface at inner-side of the bend. A helical motion results, causing erosion of the outer side of the bend. Installation of submerged vanes on the stream bed can reduce erosion of the outer bank significantly. Most of previous studies in this regard have been tasted in a rectangular flume cross section. In this research use of submerged vane and its influence on the bed topography changes, especially around the bridge pier is studied. Experiments were conducted in the laboratory flume with a 180 degree bend with rational curvature of 2 , height of 90 cm, width of 100 cm and a length of straight direction upstream and downstream of the bend respectively 6.5 m and 5 m, material with specific gravity of 1.5 and a cylindrical pier of 5 cm diameter, With clear water conditions u_*/u_(*c) =0.97 ,the water discharge of 70 liters per second, with a depth of 18 cm was tested. The vane was made of 10 mm thick plaxi glass. The height of the vane above the streambed level was 4.5 cm, 75% of the flow depth. and its length is 7.5 cm (1.5 D, D= pier`s diameter). Dissimilar arrays of vanes with same angles (25 degree) to main flow direction were employed. Five experiments were carried out considering various positions of the submerged vanes and one experiment was done without the presence of vanes, only with a bridge pier located at 90 degrees. The performance and efficiency of a submerged vane is related to its position. The results illustrate that the experiment with vanes in parallel, by overlapping of 50%, under angle of 25 degrees counterclockwise with the horizon in the direction of flow, has the best performance in control of pier scour protection.