Coherent interference of ocean swell over submarine canyons

Ocean swell can be a significant coastal hazard, potentially causing coastal disasters with costly damage to infrastructure and tragic loss of lives. We have seen observed this in the coastal region of Toyama Bay in the Sea of Japan, which has been devastated by severe winter swell events (the so-called YoriMawari-nami; YM-wave). An extreme swell event was recorded in February 2008 in which the wave height at a site in the bay reached 9+ m, which corresponds to 16 times the local climatological average. Substantial efforts by Japanese coastal engineering community have been expended to reproduce YM-wave events with phase-averaged wave models, but, to our knowledge, with no success. During an archetype YM-wave event, we found that the Noto peninsula filtered the incoming wave field such that in the bay incident waves were quasi-monochromatic. The swell then refracted over a submarine canyon resulting in a bi-modal directional distribution of wave energy. This initiated conditions favorable for coherent interference violating the assumptions of the phase-averaged approach. Coherent interference is the key mechanism for understanding the formation of the large amplitudes and high space-time variability. Our results demonstrated the significance of the coherent interference for wave statistics over coastal bathymetry and only through the phase-resolving model were we able to adequately reproduce the observation.