Dynamics of sand ripples and burial/scouring of cobbles in oscillatory flow

The purpose of this paper is to present the results of a series of laboratory experiments aimed at better understanding (i) the dynamics of sand ripples formed in a wave-induced oscillatory flow and (ii) the influence of sand ripples on the behavior of solid objects (which are free to move) in steady oscillatory flow. This problem is closely related to the migration and burial/scouring of isolated cobbles/mines on a sandy floor by an oscillatory flow such as that occurs in coastal waters beyond the region of wave breaking. The aims of the study were (i) to mimic this process in laboratory experiments and (ii) to develop a physical understanding of the processes involved in the water motion and cobble and sandy surface interaction. The oscillatory flow was created in a long tank of rectangular cross-section using standing waves (first mode) of large amplitudes. Model cobbles (mostly disks) were placed on the sandy bottom and their subsequent behavior, the temporal evolution of the sandy bottom profile and the velocity field in water were observed. The main findings include (i) the demonstration that long-time changes of the bottom topography occur and this may lead to the periodic burial/unburial of cobbles and (ii) the scouring of the bedform surrounding the cobbles is relatively unimportant in the burial/unburial process. (C) 1999 Elsevier Science Ltd. All rights reserved.