Male Speaker: When a wave into shallow water it becomes slowed by friction with the sea floor and the previous waves back wash. Since waves closer to the shore line travel more slowly than waves farther out. The farther waves over take the closer waves increasing wave peaks. If the shore slope is steep, the wave crust peaks rapidly and curls over the slower bottom half and crushes down as a plunging breaker. After breaking the water rushes up the beach continuing its forward movement until gravity pulls it back and it returns down the slope as back wash.