Page:Popular Science Monthly Volume 86.djvu/565

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along the bottom as undertow. In the second place, on-shore winds, by driving the surface of the sea landward, insure a vigorous undertow seaward; this undertow carries the beach material out to deep water, thereby aiding beach destruction. Vigorous on-shore winds drive in large waves, whereas off-shore winds, no matter how violent, can not form large waves in the immediate vicinity of the shore. During the recent storms on-shore winds raised the high-tide level on the New Jersey coast from one to several feet above its normal elevation, much beach sand was sucked out to sea by the resulting undertow, and large waves were driven upon the shore with terrific violence.

Other things being equal, the greatest damage will occur where the land exposed to wave erosion is lowest. Waves may expend their energy in two ways: in eroding the land or sea-bottom, or in transporting débris. If the land is high, the waves break at the base of a high cliff which sheds much débris into the water as its base is undermined. This débris must be removed by the waves if effective erosion is to continue, as otherwise the cliff would soon be protected by the accumulated waste. Removal of the débris requires much of the waves' energy, and leaves them less competent to wear back the cliff. If the land is low, the low cliff sheds but a small amount of waste upon the shore, the waves quickly dispose of it and energetically continue their landward advance. It is true that the effect of cliff height may be more than offset by other factors, among which the form of the adjacent sea bottom is important. Of still greater importance for such a region as the one in question is the effect of artificial sea defences, such as breakwaters, bulkheads and similar devices. At Seabright and adjacent