Page:Popular Science Monthly Volume 86.djvu/564

560 cliffs, now grass covered, may be seen at these points. But the bar now lies a short distance in front of these old cliffs, and protects them from erosion. Shrewsbury and Navesink Rivers are really bays of the ocean formed by a sinking of the land or a rising of the ocean level which permitted the seawater to flood pre-existing river valleys. They are "drowned valleys" but not "rivers" in the true sense of the term.

In order fully to appreciate the effects of storms upon the New Jersey coast, it is necessary to keep in mind some of the conditions affecting wave energy. The destructive power of a wave depends in part upon its size, and this in turn partly upon the water depth. Waves usually break and dissipate their energy when they come into water of a depth equal to the wave height. Hence, the deeper the water immediately at the shore the larger the waves which can attack it, and the greater the damage they will effect at that point. It follows from this that the rise of the tide must increase the destructive power of storm waves on the coast, not only because it brings the zone of wave activity farther in upon the shore, but also because the deepening of the water as the tide rises against the steeper upper part of the shore profile permits larger and more powerful waves to break against the shore cliffs. In all the recent storms the chief damage to the New Jersey coast occurred at the high-tide periods, and the citizens worked feverishly during low water to prepare for the violent wave attack which they knew would ensue at the next high tide.

On-shore winds increase the destructive power of the waves in a variety of ways. First of all, they raise the water level by blowing the surface of the sea along the coast faster than the water escapes seaward