Response of Wave Dissipating Blocks Composing a Detached Breakwater to Waves with Various Periods

Abstract

We discuss the response of the wave dissipating blocks composing a detached breakwater to the waves with various periods, based on both the experimental and numerical results. First, in the hydraulic experiments, the number of the blocks fallen by the irregular waves was the largest when the still water level is moderate, namely the HWL. When the large and long wave is incident, the overflow that lasted for a long time fell many blocks at the back of the breakwater. When the irregular waves, long wave, and irregular waves are continuously incident in this order, the first irregular wave train shifted several blocks, and the subsequent long wave dropped these shifted blocks, displacing the blocks around them, whereafter the second irregular wave train dropped many of the displaced blocks. Second, in the vertically two-dimensional calculations, when the long wave struck the breakwater, a large-scale vortex was created near the seabed in front of the breakwater and remained during the pushing wave. The overflow due to the long wave generated negative wave pressure at the top behind the breakwater. Based on both the numerical and experimental results behind the breakwater, when a large positive wave pressure is generated after a negative wave pressure that is not large in absolute value, the number of fallen blocks will reduce. When large and short waves cause overtopping, and negative wave pressure frequently appears behind a breakwater, the blocks at the back of the breakwater can gradually shift, loosening their engagement.