Comparison of New Large and Small Scale Overtopping Tests for Rubble Mound Breakwaters

Many breakwaters are, due to functional requirements, designed for small wave overtopping discharges. From the EC-research projects OPTICREST and CLASH it is known that overtopping discharges determined from conventional Froude scale models of rubble mound breakwaters are smaller than measured in corresponding prototypes. The present study examines this scale effect by comparing overtopping discharges in small scale and large scale tests. The length scale ratio between the models was 5.7. The reason for using a large scale model instead of a prototype is the difficulties in accurate reproduction of prototype conditions for waves and structure in the model, as prototype conditions are not known in sufficient details. The model was designed to avoid as many model effects as possible. Despite this some differences in wave kinematics were detected, most probably due to differences in wave generation hardware and software. This demonstrates that, even under controlled conditions, it is very difficult to reproduce similar conditions. From the present test results it is concluded, despite the model effect, that a significant scale effect on small overtopping discharges exists also when wind effects are absent. The scale effect seems to be very dependent on the top geometry of the breakwater with much larger scale effects for a low crown wall than for a high one. The paper presents an explanation of this difference.