Erosion of sediments beneath an oscillating cylinder

Abstract

The depth of erosion of cohesionless sediments underneath horizontal circular cylinders that are oscillated periodically in still water was of primary interest in this study. In this investigation anthracite was used for the sediments. First the formation of vortex ripples was studied by oscillating a box containing anthracite on its own in still water without the cylinder. The geometrical characteristics of the ripples obtained compared well with those of other investigations. In the experimental range studied, the low density of sediments had no explicit influence on the features of the vortex ripples. The box filled with anthracite and the cylinder were oscillated together to perform periodic motion and the depth of erosion beneath the cylinder was measured. For the diameters of cylinder tested, the height of the vortex ripples formed on both sides of the cylinder was comparable with the depth of scour underneath the cylinder. The experimental results are categorised as clear water scour and live bed scour. For both cases, the relative scour depth was weakly dependent on the Shields parameter for Keulegan–Carpenter numbers less than about 30. For Keulegan–Carpenter numbers greater than about 30, it increased with the increasing value of the Shields parameter.