Local Scour Reduction around Cylindrical Piers Using Permeable Collars in Clear Water

Abstract

To address the shortcomings of the solid collar, mainly the potential risk of deformation and damage due to its weight after overhanging above the river bed caused by local scour, a lighter form of a local scour reduction measure, the permeable collar, was proposed. The aim was to reduce the local scour depth around cylindrical piers, and a total of 20 tests were conducted in this study. First, the influence of the porosity of the permeable collar on the local scour reduction efficiency was studied under the condition of the constant size of the installation height, the diameter, and the thickness of the permeable collar based on indoor model experiments. Then, under the condition of optimal porosity, the influence of the installation height, the diameter, and the thickness of the permeable collar on reduction efficiency were studied in order. The results show that the porosity of the collar has a great influence on the local scour reduction. The reduction efficiency increases first and then decreases with the increase in collar porosity. When the collar porosity is 50%, the reduction effect is the best, and can reach up to 78.1%. The reduction efficiency decreases with the increase in installation height of the collar, and the reduction effect is the best when the collar is placed on the surface of the riverbed, which is close to the reduction efficiency when using a solid collar placed on the surface of the riverbed. The reduction efficiency increases with the increase in collar diameter, which increases rapidly when the diameters are less than 3.5 D (D is the diameter of bridge piers) and then slows down until tending to 100%. The reduction efficiency decreases rapidly with the increase in the collar thickness, when the thickness of the collar is 0.15 D, the reduction effect can reach up to 89.6% in this study. The results can give a perspective on the reduction of local scour depths around bridge piers.