Dynamics of cobbles in the shoaling region of a surf zone

The motion of large bottom particles (cobbles/mines) was studied in the lab oratory under simulated surf conditions. A series of experiments was conduc ted in a large wave tank, 32x0.9x1.8 m, equipped with a computer-controlled wave maker and a sloping beach. As a first step, a solid impermeable beach with artificial roughness was used in the experiments. Cobbles of differen t size were placed along the floor and their evolution with time was studie d and compared with the model predictions. Onshore and offshore mean motion s of cobbles, as well as steady oscillations with zero mean displacement, w ere observed for different conditions. To explain the results of observatio ns a theoretical model was advanced. The model takes into account all main governing parameters (size and density of cobbles, bottom slope, dynamic an d static friction at the bottom, background flow characteristics, etc,). St andard para meterizations were used for a pressure accelerating term, drag, lift and other nonlinear forces. For the range of parameters used in the e xperiments, satisfactory agreement between the measured and calculated valu es of the cobble displacements as a function of time was obtained. The mode l is practically insensitive to the vertical accelerating pressure term but sensitive to the dynamic and static friction. One of the most important va riables in the model, which is known with the least accuracy, is the virtua l mass coefficient for disk-shaped cobbles moving with variable velocity al ong a solid boundary. (C) 2001 Elsevier Science Ltd. All rights reserved.