Experimental and Numerical Investigation of Scouring Process around the Piers with Lateral Boundaries Fitted by Velocity Profile under Steady State Flow condition

The piers’ shape may be considered as one of the key factors in controlling the scour process around it، as this mainly changes the stream flow pattern around the structure. Furthermore، piers with variable section in depth may also be of interest in reducing the scour around them. The aim of this study was to evaluate the effectiveness of the piers with lateral boundaries fitted by velocity profile due to steady flow، in reducing local scour، when compared to the circular and conical shaped piers. The section of the piers was circular and the widths or diameters of these piers reduce from bed to water surface with a slope fitted by velocity profile. Due to increasing flow discharge close to the water surface، the flow which has maximum velocity in this region will then pass around the piers easier than the cylindrical pier and its velocity is reduced. Therefore، in using these piers، the dynamic pressure in upper regions decreases relative to that of the cylindrical pier and there will be a weaker down flow in front of pier which will consequently cause decreasing of the scour depth. The models tested in this study were three models with logarithmic profiles fitted by (d50=0. 78، 1. 1، 1. 3 mm) and three models with conical profiles by (degrees، in which φ is the angle of lateral slope). Then، the relative scour depth for these models compared with the cylindrical reference model and the maximum scour depth decreased with the rate of 55. 9%، 65. 4%، 73. 2%، 38. 5%، 47. 2% and 52. 7%، respectively. These tests have been performed in a wide flume at hydraulic research laboratory and under clear water conditions over12 hours using relatively uniform sediments particles. For the numerical models، the maximum scour depth around the piers was simulated using Flow-3D model. It was found that the numerical model results were in reasonably good agreement with the experimental data.