When a wave approaches shore, the base of the wave encounters the bottom—the front of the wave slows down and the back overtakes the front. This forces the water into a peak where the top (crest) curves forward. This peak will eventually fall forward in a tumbling rush of foam and water called a breaker. Waves break on or near shore, they also crash over reefs or offshore sandbars if water depths are shallow.
Wave steepness is the ratio of height to wavelength. When wave steepness exceeds a ratio of 1:7, breakers form.
Example: If a moving wave has a height of one foot and a length from crest to crest of 8 feet, then the ratio is 1:8 and this wave is not going to break. However, if the height is 1 foot and the length decreases to 6 feet, then the ratio is 1:6, then the wave has now become steep enough that the crest topples over and the wave breaks.
Slope of the seabed/beach creates different kinds of breakers
There are three types of breaking waves: spilling breakers, plunging breakers, and surging breakers. Breakers may be one or a combination of these types.
Gentle slopes produce spilling breakers. Spilling breakers begin far from shore and take a relatively longer time to reach the beach. The breaking crest slides down the front of the wave in a flurry of foam as the wave moves shoreward. Spilling breakers give surfers a long slow ride.
Moderate slopes produce plunging breakers. Plunging breakers build up rapidly into a steeply leaning crest. The crest curls further forward of the rest of the wave before crashing down in the surf zone. Plunging breakers are dangerous because the crash into shallow water.
Steep slopes produce surging breakers. Surging breakers occur where waves slam directly on the shoreline. With no gentle slope the waves surge onto a steep beach, producing no tumbling surf. Surging breakers also create huge splashes on a rocky cliff shoreline.
|Figure 10.10. Spilling breakers at Torrey Pines Beach, CA.||Figure 10.11. Plunging breaker (threatens a boat).||Figure 10.12. Surging breaker on a narrow Hawaii beach.||Figure 10.13. Surging wave crashing on seacliffs|
A wave train is a group of waves of equal or similar wavelengths traveling in the same direction. Individual waves move from the back to the front of a wave train, gradually building up, peaking, then declining as it moves to the front of the wave train (Figure 10.14). The result is that individual waves within a wave train are moving about twice as the wave train itself. Surfers watching advancing waves may notice that the first waves to arrive decline in intensity as they arrive as the following waves build higher. After the highest crest passes, the trailing waves decline in intensity as the wave train passes.