As a swell approaches land, it interacts with the shape of the coastline and the underwater geography (bathymetry). This process is highly variable, depending on the coastline. As the swell approaches shallow water, it becomes larger and slows down, shortening the swell period. This is called shoaling.
Swell energy can become focused on one shallow area and break with immense power, such as on a reef (‘focusing’) or spread out over a wider area such as a bay (‘defocusing’). Tidal movements can exaggerate the effect of swell focusing over sandbanks, reefs and rock platforms. When the water depth becomes shallow enough, the crest of the swell starts moving faster than the base of the trough, resulting in breaking waves.
