The Bird in the Waterfall by Jerry Dennis and Glenn Wolff

Dramatic Changes
Jerry Dennis & Glenn Wolff

When waves enter shallow water they undergo dramatic changes. 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. The particles above them continue to revolve, but their path is elliptical rather than circular, a restriction of free movement that causes an accordion-style compression of the long and rounded crest into a short and steep-sided peak. 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. Bottom must be smooth and uniform; currents and wind must not create instability in the wave. Most of the time, on most beaches, conflicting forces slow the wave and cause it to break gradually in a spilling or surging breaker. But if allowed to break all at once, the effect can be spectacular. All the energy the wave has carried for hundreds or thousands of miles is poised to explode in a single, shattering moment. Water and air meet land in a burst of spray, with noise that can be heard a mile or more away.

You can get a fairly accurate picture of the bottom contours of a coast by watching wave behavior. Large waves that do not break until they are very near shore indicate deep water; breakers far out mean shallow water. Waves that break offshore then re-form and continue on before breaking again probably indicate a submerged sandbar or other structure. Above the sandbar the waves become steeper and break, but as they enter the deeper water in the trough between the bar and the shore (or between a succession of bars) they lengthen and become stable enough to cease breaking.

Like waves of light, water waves can be diffracted, reflected, and refracted when they encounter stationary objects or shoreline features.

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