The motion of large bottom particles (cobbles) in a wave-induced oscillatory flow

The purpose of this paper is to present the results of a series of experiments aimed at better understanding the dynamics of the motion of large bottom particles (cobbles) in a wave-induced oscillatory flow. This problem is closely related to the motion of cobbles along the bottom in an oscillatory flow such as 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 model to predict cobble movements. The oscillatory flow was created in a long tank of rectangular cross-section using standing waves of large amplitude. Objects of different shapes (spheres and disks) were placed in the flow and their subsequent motion along the tank floor was studied. The results of the observations were compared with the predictions of a theoretical model. For the range of parameters used in the experiments (shape, size, aspect ratio and density of cobbles, amplitude and frequency of the oscillatory flow and bottom friction), reasonable agreement between the measured and calculated values of the cobble displacements as a function of time was obtained.