At a depth equal to half of each wave's length, the lowest of the orbiting particles come in contact with the bottom and, instead of orbiting, begin to move back and forth. Steep waves are unstable. When the crest travels faster than the lagging bottom it tumbles forward and collapses.
Waves breaking onshore can take three forms.
A spilling breaker collapses slowly over a long distance, with its crest spilling forward down the front of the wave as it progresses. A plunging breaker gradually steepens as it enters shallow water, then breaks in a sudden, instantaneous crash as its top plunges forward. A surging breaker rushes or surges up a beach without first growing steep. The types of breakers are determined by a number of factors. Plunging breakers are only possible under ideal conditions.