When a wave enters water that is shallower than half its wave length, the motion of the water near the bottom is retarded by friction. This causes the bottom of the wave to slow down. As the water becomes more shallow the wave speed decreases, the wave length becomes shorter, and the wave crest increases in height. This continues until the crest of the wave becomes too high and is moving too fast. At this point the crest of the wave becomes unstable and crashes down into the preceding wave trough; when this happens the wave is said to be breaking. The type of breaker (that is, whether spilling, plunging, or surging) is determined by the steepness of the wave in deep water and the slope of the beach. Figure 6-9 depicts the general characteristics of the three types of breakers.

SPILLING BREAKER. Spilling breakers occur with shallow beach slopes. The water at the crest of a wave may create foam as it spills down the face of the wave. Spilling breakers also occur more frequently when deepwater sea waves approach the beach. This is because the shorter wavelength of a sea wave means that the wave is steeper in the deep water and that the water spills from the crest as the waves begin to feel bottom. Because the water constantly spills from the crest in shorter wavelength (shorter period) waves, the height of spilling waves rarely increases as dramatically when the wave feels bottom, as do the longer period waves forming at the crest and expanding down the face of the breaker.

PLUNGING BREAKER. Plunging breakers occur with a moderately steep bottom. In this type of breaker, a large quantity of water at the crest of a wave curls out ahead of the wave crest, temporarily forming a tube of water on the wave face before the water plunges down the face of the wave in a violent tumbling action. Plunging breakers are characterized by a loud, explosive sound made when the air trapped in the curl

Figure 6-9.-General characteristics of spilling, plunging, and surging breakers.

is released Plunging breakers are more commonly associated with swell waves that approach the beach with much longer wavelengths. The shortening of the wavelength as the wave feels bottom causes a great mass of water to build up in the crest in a short time. Longer period swell waves may double in height when feeling bottom.

SURGING BREAKER. Surging breakers are normally seen only with a very steep beach slope. This type of breaker is often described as creating the appearance that the water level at the beach is suddenly rising and falling. The entire face of the wave usually displays churning water and produces foam, but an actual curl never develops.

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