Scale Effect of Wave Force on Armor Units

This paper describes the scale effect in experiments on the stability of armor units from the point of view of the wave forces. The relationship between the drag coefficient C_D and the stability coefficient K_D of Hudson formula is theoretically derived to be that K_D ∞ C_D^-3 on the condition that the inertia force is negligible. Three kinds experiments were performed by using various sizes of Tetrapods ranging from 16 g to 6800 g according to the Froude law for scaling, 1) to measure wave force on an armor unit placed in an armor layer of a breakwater, 2) to determine the drag and inertia coefficients in wave fields, and 3) to determine the drag coefficient in a steady flow. It is found that the wave force in the small-scale experiments is relatively larger than that in the large-scale experiments. As the wave height increases, the drag force becomes predominant in comparison with the inertia force. It is concluded that the scale effect of the wave force on armor units is mainly due to the change of the relative drag force number as a function of Reynolds number.