Figure 5.8: Typical diffraction pattern through a gap between two detached breakwaters in Pesaro, Italy. In the case of a long detached breakwater, \(\lambda /L \gg 1\), diffraction occurs around each breakwater head, but wave energy will not spread in the entire zone behind the breakwater. When a thin pile is standing in waves with a large wavelength, \(\lambda /L \ll 1\), wave energy spreads behind the entire pile. The extent of energy penetration in the area landward of an obstacle depends on the ratio of a characteristic lateral dimension of the obstacle, e.g., the length of a single detached breakwater \(\lambda\) to the wavelength \(L\). Further from the breakwater into the wave zone the wave height gradually approaches the incident wave height. ![]() The wave heights decrease deeper into the shadow zone. Due to the lateral transfer of wave energy into the shadow zone, the wave height along this ray is lower than the incident wave height in the case of constant depth (and thus constant celerity), the wave height is 50% of the original wave height according to linear theory and order 70% for irregular directional waves. The diffracted wave crests will form concentric circular arcs with the wave height decreasing along the crest of each wave.įigure 5.7 also shows the wave ray that separates the shadow or diffraction zone from the wave zone. The remainder of the wave front will bend around the obstacle and thus penetrate into the zone in the lee of the obstacle (shadow zone). A part of the wave front is blocked by the breakwater and is reflected seaward. Figure 5.7 shows the diffraction of an incident wave train in the case that there are no depth changes. ![]() If obstructions to the wave propagation (an offshore island, a breakwater, a headland) or abrupt changes in the bottom contours are present, there is a large (initial) variation of wave energy along a wave crest which leads to transfer of energy along the wave crests. \)įigure 5.7: Diffraction of an incident wave train.
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