National Geographic : 1962 Dec
KODACHROMEBY NATIONAL GEOGRAPHIC CHIEF PHOTOGRAPHERB. ANTHONY STEWART THE CORPS OF ENGINEERS of the United States Army has the never-ending job of guarding our coasts against the seas. With stone, timber, and concrete it builds sea walls and jetties. With bulldozers and hy draulic dredges it makes beaches and dunes; then it grasses them against erosion. Before it can build, it must know where and how to build. Its Beach Erosion Board has an extensive laboratory off MacArthur Boulevard in Washington, D. C.; the accom panying photographs show some of the lab's testing tanks. Until I spent a day there, I did not under stand how complex a thing the action of sea on shore could be. The sea is one of earth's most powerful forces, matched only by winds and earth quakes and running inland waters. It is whimsical-building one day, destroying its own handiwork the next. It can be bru tal, it can be exceedingly subtle, but it can never be still. It will move fine sand a certain distance in a given time. Mix in clay or pebbles, and the rate and extent of movement change. A host of other variables add their effects- sea-bottom contours, which help determine the shapes of waves; the waves themselves, their speed, size, angle of attack upon the shore; the tides; coastal currents. Even the chemical content of sea water, which affects its ability to dissolve earth materials, makes a difference. To build wisely, the Corps of Engineers must first identify every force at work on the scene, then know what it can do. Its special ists find few universal formulas. They try many different ideas and mate rials. I saw a man placing strange concrete objects with protruding arms into a tank. "Those arms cause the objects to lock to gether like children's playing jacks when waves tumble them," he explained. "They tend to stay put in a jetty or breakwater where stones of equal weight would be rolled away." The Corps has also investigated many other unusual defense methods, such as lines of flexible rafts anchored offshore to calm waves; nature's own inshore rafts of matted weed and kelp; and even underwater pipes laid near vulnerable shores to release strong hydraulic jets or clouds of air bubbles when wind and storm threaten, thus breaking up severe wave action. 883 N.G.S.