Waves that we see breaking on the shoreline actually start many miles or kilometers out at sea as a result of an interaction between wind and water. The flow of air over the water results in some friction between the two that transfers some of the energy in the wind to the water. This starts the water moving creating what is called swell on the open sea. If you’ve ever been seasick on a boat, the effect is the result of the boat moving up and down, and side to side on the swells that undulate across the sea surface.
If you aren’t too seasick to observe, you will see the swell moving past the boat, but while the rolling form of the water moves, there is actually virtually no movement of the water itself. The water does move, but only in a circle with a diameter of the distance from the top of the swell to the bottom of the trough in front of the peak. Swell is actually a movement of the energy imparted to the ocean from the wind blowing across it. The size of swell is determined by how fast the wind blows, how long it blows, and over how large an area it blows.
This energy, in the form of swell, can travel vast distances across the ocean with very little loss to friction. For example, the south swell that reaches south-facing beaches along the coast of southern California in the summer months are often produced by storms along the coast of Antarctica! It isn’t generally appreciated that if you sailed a boat due south from any south-facing beach in southern California the first land that you would encounter would be Antarctica. It is a simple feat for swell generated in the Southern Ocean to make its way all the to southern California.
While moving across the ocean surface in deep water, swell encounters virtually no resistance to movement. But as the swell moves into shallow water, things begin to change. A fact SCUBA divers know well is that while swell seems to be an aspect of the ocean’s surface, in fact, it also causes water beneath the surface to move in a circular motion as well. Contact with the bottom of the ocean causes these lower layers of rotating water to drag along the bottom making the overall motion more elliptical. Divers are familiar with surge, which is the more horizontal movement of the swell as it contacts the bottom.
As the bottom becomes shallower, even the uppermost movement of the swell begins to drag on the bottom causing the topmost portion of the swell to move faster than the bottom which has begun to drag along the seafloor. This causes the top of the swell to peak up and pitch forward, becoming a wave that will soon crash on the beach.
Depending on the shape of the seafloor, swell will begin to feel the bottom at different places along the shoreline. In a bay like that shown in today’s photo, swell first contacts the seafloor near the points, but not in the middle. As a result, the swell and resulting wave will move faster in the middle than near the points, causing the swell and wave to curve. You can see the breaking wave in the photo describing an arc that nearly perfectly matches the shape of the beach. This is wave refraction in action. Watching the shapes of waves approaching any beach can give you clues about the shape of the seafloor beneath the surface.
This photo was taken at Leo Carrillo State Beach in California, USA with a Canon EF28-135 mm f/3.5-5.6 IS USM lens at focal length 70 mm on a Canon EOS 5D Mk III. The exposure was set to 1/45 sec at f/11 and ISO 200.