Long-term field observations of nonlinear wave deformation and energy transfer around submerged breakwaters at two coastal sites

Submerged breakwaters (SBWs) are widely used to protect coastlines, but their effect on nonlinear wave deformation and on the energy transfer between frequency bands is not well understood. Long-term field data were analyzed around SBWs at two beaches in Korea. At Site 1 , waves were measured for 1143 days on both sides of the SBW and on its crest, which is rare in field studies. At Site 2 , waves were measured for 78 days at one seaward and two landward stations. The two SBWs differ in crest depth (0.5 and 1.5 m) and face slope. For the largest waves, the gravity-wave energy behind the SBWs fell to ∼20 % of the incident energy at Site 1 and below 10 % at Site 2 , and it was lower at Site 2 for the same relative submergence. Over the crest, the bicoherence rose sharply and the energy was transferred toward higher frequencies. At Site 1, the wave asymmetry changed sign from about −0.7 on the crest to +0.3 behind it, whereas at Site 2 the waves kept steepening behind the structure. The long-wave energy was attenuated behind the structure at Site 1 but amplified by ∼1.4–2.5 times at Site 2 , even during storms. These long waves were mostly free and consistent with standing waves reflected from the shoreline. The contrast is more likely related to the structures and their settings than to the incident wave field or the relative depth over the crest.