Local Erosion at Vertical Piles by Waves and Currents

Abstract

Although the study of local scour at cylindrical piers has been the subject of many studies, few resulted in clear design recommendations; furthermore, the results of these studies exhibit often wide differences pending on the type and range of parameters considered. There are very few studies available dealing with erosion under wave action. The paper analyses briefly the mechanics of local scour in the case of unidirectional flow both for velocities below and above critical velocities for sediment transport; it deals then with the mechanics of scour due to action of oscillatory flow and oscillatory flow superimposed on a unidirectional current. This is followed by a brief dimensional analysis of the problem. The major part of the paper is devoted to the description and analysis of experiments carried out in two flumes with different pile diameters, varying velocities and flow depths, different sediment sizes and wave amplitudes and lengths. The experimental results are presented in design graphs enabling a quick estimate of the scour depth as a function of flow depth H, pile diameter D and ratio of critical to flow velocity v _c/v for unidirectional flow. The added influence of waves can be estimated (within the experimental range) from graphs incorporating the influence of wave length and amplitude. The study results show that the maximum scour is in the region of 1.7D for v _c/v ≦ 1 with smaller values for v _c/v > 1. In the range 1 <( v _c/ v) < 2 the depth exerts little influence for values D/H > 0.6 and D/H < 0.15 with some influence in the intermediate range. If waves are superimposed on currents; the rate of scour formation increases substantially with the maximum scour increasing with wave amplitudes and wave lengths, but not exceeding the 1.7D value found for unidirectional flow alone.