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coast protection

coast protection, methods used to protect coastal lands from erosion. Beaches can exist only where a delicate dynamic equilibrium exists between the amount of sand supplied to the beach and the inevitable losses caused by wave erosion. Various activities of man have upset this equilibrium, decidedly increasing the rate of erosion of the shorelines. For example, the plethora of dams constructed across major drainage systems has served to entrap sediment that would normally reach the coastal zone, imperiling the existence of beaches by cutting off their natural sand supply. Mining of beach sand has removed millions of tons of sand from coasts and drastically upset the balance between natural supply and losses. Historically, people have considered coast protection a local problem and have attacked the problem by building structures to inhibit the transportation of sand from a local area. However, it has been learned that building structures to solve a local erosion problem may extend and intensify the erosion problem along nearby beaches, requiring the construction of structures along an entire coast. For example, many structures block littoral drift, which is a movement of sand parallel to the coast, both on the beach and offshore, caused by waves. The blockage results in a depletion of sand downcurrent from the structure. Several different kinds of structures are built. Sea walls are constructed at the edge of the shore facing the ocean waves. Designed to protect only the beach areas behind them, they cause an increased loss of sediment in front of and beneath them. Breakwaters are long piers built offshore parallel to the shoreline; they are designed to provide calm anchorages in an area behind them called a wave shadow. At the breakwater off Santa Monica, Calif., the wave shadow impeded the littoral drift, producing a deposition of sand behind the breakwater and extensive erosion of the beach downcurrent. Groins are lines of rock or pilings constructed perpendicular to the shoreline. They act as a partial barrier to littoral drift, trapping sand on the updrift side and causing erosion on the downdrift side. Jetties are often built at river mouths and harbor entrances, projecting out into the ocean to direct and confine littoral currents and to prevent silting of the harbor entrance. Jetties cause the same problems of downdrift erosion as groins. In some instances it has been necessary to pump the sand trapped by the structure to adjacent beaches downdrift. Efforts have also been made to prevent erosion using the natural materials at hand. Artificial dunes have been built by bulldozing sand back from the beach or by placing snow fences to trap windblown sand. Since beaches themselves are effective in dissipating wave energy, one remedy to the lack of a sand supply is to pump sand directly onto the beach from interior or offshore zones. Unlike other human-made structures, artificial beaches do not harm the shore downdrift.

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