Focusing of Swell at the Nazaré Submarine Canyon

Abstract. Steep underwater canyons near the coast can significantly modulate the propagation of long waves. Many canyons share similar dimensions of approximately 150 meters deep thalweg at their terminus, and a sloping continental shelf of about 20 meters within hundred meters of the shoreline. Swell traveling above the shelf is refracted and reflected at the canyon edge, where the phase speed increases abruptly due to the increased depth, resulting in a significantly focused wave field. Outside Nazaré, Portugal, one of the largest near-shore canyons exist, extending to within ten meters of the shoreline. Here, some of the largest breaking waves (> 20 meters) in the world are formed. We deployed arrays of wave buoys above the canyon, and stereo cameras mounted at the cliff at Nazaré overlooking the canyon edge, allowing us to measure the wave field at far greater spatial and temporal resolution than any previous experiment. The swell propagation is modeled using ray tracing and a boundary element method. In particular, the shape of a submarine canyon’s edge in the final hundreds of meters toward the coastline is crucial for how and exactly where beams are focused towards the beach. Contrary to the popular explanation that the large waves are channeled through the canyon, we find they are refracted and reflected along the canyon edge. We find that waves longer than about 7 seconds are mostly reflected at the canyon, common to many canyons around the world. At Nazaré, optimal focusing occurs for waves arriving from about 275°–315°. This is in agreement with the experience from the surfing community which considers waves arriving from about 290°–315° to yield the biggest surf.