NAUI Master Scuba Diver 38 The Diving Environment times its height and may break again. You may use the presence of waves breaking offshore as an indicator for the location of reefs or sandbars. A frequent characteristic of waves is the variability in their height. They generally approach shore in sets of smaller waves followed by another group of larger waves, each of which is usually higher than its predecessor. These sets of waves result from the arrival of two trains of swells from two different sources. When the crests of two sets of swells coincide, they can reinforce each other and produce waves higher than those of either of the swell trains individually. When the crests of one set coincide with the troughs in the other set, a cancellation effect produces smaller waves. A definite cycle or pattern of the sets often results. By studying breaking waves, you can determine the surf beat, or frequency of the sets, and time your entry or exit to coincide with the lull period of minimum wave height, or even temporary absence of waves, in mild conditions. Types of Surf The width of the surf zone and the violence of breaking waves are influenced significantly by the slope of the beach, which is actually created in many cases by prevailing wave action (figure 2-7). On a gradually sloping beach, a moderately large swell will form spilling breakers. These waves break far from shore and continue to break all the way to the beach. The long surf zone allows waves to release their energy gradually. The water is usually turbid in the surf zone due to the sediment that is disturbed over a wide area. Plunging breakers release their energy quickly. This type of surf forms from large swells over a moderately steep bottom. As the swell moves toward shore, the waves steepen quickly and break suddenly. These waves break with tremendous force, and they are the most hazardous type. Large ones (i.e, over 0.9 m 3 ft) can easily knock over a standing diver. The crest of a plunging breaker curls over, forming a “tube” or large air pocket. A spectacular crash of water results and foam is thrown into the air as the air pocket expands after being compressed by the weight of the water in the wave. Visibility is usually better beyond plunging breakers than in an Spilling Breakers Moderate to large swells Shallow sloping area–gradual beach Tube waves Moderately steep beach slope Plunging Breakers Collapsing Breakers FIGURE 2-7. TYPES OF SURF Long Surf Zone Medium swells Steep beach slope area of spilling breakers because these waves expend their energy in a very narrow area. Collapsing breakers form from swells of medium height over a very steep bottom. As the waves break over their lower half, very little splash or foaming occurs. Thus, the wave breaks rather uneventfully. Small swells
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