Experimental study on the role of second-order wave generation and active reflection compensation on overtopping and damage of rubble mound breakwaters

Prior to construction, coastal structures are often tested in wave flumes and basins using physical models to assess performance and stability. To ensure that such testing provides a reasonable representation of design conditions, it is crucial to generate realistic wave conditions that occur in the field. Wavemaker capabilities such as second-order wave generation (WG2) and active reflection compensation (ARC), however, may be costly, are not universally available at testing facilities, and can introduce additional complexity. Systematic studies specifically addressing the effect of these generation features on overtopping and damage of rubble mound breakwaters are lacking. In this work we used a 1:35 scale model of a rubble mound breakwater to assess the role of WG2 and ARC on overtopping and damage. Across the range of wave conditions tested it was found that overtopping volume and damage varied substantially when WG2 and/or ARC were used. Overtopping volume decreased when linear generation was used compared to WG2; whereas overtopping volume increased when ARC was turned off. Collectively, the results from the study emphasize the importance of careful paddle control to correctly simulate offshore wave conditions in physical model testing. Depending on the wave conditions and the experimental setup, using linear wave generation with and without active reflection compensation can result in substantial over- or underprediction of the performance and stability of coastal structures.