Page:Popular Science Monthly Volume 86.djvu/567

Rh the rate of erosion at different points which could be attributed to variations in resistance of materials.

An examination of the Seabright shore indicates that the greatest damage to bulkheads resulted from the direct impact of the waves, whereas the buildings suffered most from the undermining of the ground upon which they stood. When it is remembered that ocean waves strike a vertical face with a force of from a few hundred pounds to more than six thousand pounds per square foot, their enormous destructive power may readily be appreciated. Solid blocks of granite have been shattered by wave impact upon the coast of Holland, and it is therefore not surprising that the wooden bulkheads of the Jersey coast should yield to the attack of the sea wherever they were not reinforced by parallel rows of piling with heavy stone filling, or otherwise rendered especially strong. Fig. 7 shows one of the weaker bulkheads in the early stages of destruction.

Most of the bulkheads were surmounted by a broad boardwalk which served to shed falling wave crests back into the sea, and thus protected the cliff from erosion. The force generated by masses of water falling from the great height to which they are projected when a storm wave strikes a vertical wall, may be sufficient to crush such a boardwalk, even if supported by heavy timbers. During a severe gale at Buffalo, New York, many large timbers, 12 &times; 12 inches in thickness, 12 feet long, and 10 feet between supports, were broken like match sticks by the impact of falling water which had been hurled from 75 to 125 feet into the air by breaking waves. There are several localities in the Seabright district where the demolishing of the bulkheads had been hastened in this manner.

Many of the bulkheads are protected by rows of piling set some distance out in the sea to break the force of the oncoming waves. Even where the sea attack was powerless to break these pilings or to tear them from their positions, the waves passed between the pilings and still retained sufficient force to destroy the bulkheads which presented a more continuous surface to their impact. Fig. 6 shows such a series of protecting piling, which remained largely intact while the bulkhead immediately in front of the house was battered down and the house itself destroyed. Fig. 2 shows a series of pilings surmounted by undamaged bath-houses, back of which the shore has been so badly eroded that the superjacent houses have collapsed.

As a rule, the houses were not damaged as much by direct wave impact as by the undermining of the beach upon which they stood. It is of course true that a building from under which most of the support was already removed by the sapping action of the waves, often received its "death blow" from some extra-large wave: and a building which was once tipped over into the edge of the sea as a result of being undermined was soon pounded to pieces by the waves. The Octagon Hotel