beach

beach, a gently sloping zone where deposits of unconsolidated sediments are subject to wave action at the shore of an ocean or lake. Most of the sediment making up a beach is supplied by rivers or by the erosion of highlands adjacent to the coast.

Beaches extend from a low waterline landward to a definite change in material or physiographic form, such as the presence of a cliff or dune complex marking a clear demarcation of the edge of a coast. The surf zone is the area between the landward limit of the waves and where the farthest seaward wave breaks. The foreshore, the active portion of the beach, is a seaward-sloping surface extending from the low tide limit of the beach to the crest of a ridge, called the berm, formed by storm waves. Water motion landward and seaward across the foreshore is called swash and backwash, respectively. The foreshore's slope angle is related to the size of the beach material and the vigor of the waves. The backshore extends landward from the berm as a broad terrace or gently landward-sloping surface, often broken by one or more beach ridges. Seaward of the surf zone is the offshore zone, which commonly contains a trough and an offshore bar where the waves begin to break before reforming and dispensing their energy on the beach. Along low sandy coasts, such as the Eastern coast of the United States, a long, narrow beach, called a barrier beach, is commonly separated from the coast by a narrow lagoon. Where a beach extends from land and terminates in open water it is called a spit or a hook.

Beaches undergo a cyclical migration of sand between the beach and the offshore zone caused by seasonal changes in the supply of sedimentary material and by the changes in intensity and direction of the approaching waves. The action of tides causes daily cycles of cut and fill. Waves approaching the shore obliquely move the sediment along the beach in a zigzag pattern called longshore transport. Since beaches are mobile deposits, they owe their existence to a constant replenishment of sand. In many coastal areas a deficiency in the supply of sand from human intervention or the natural changes in the coastal environment results in serious erosion problems. Artificial replenishment by pumping sand onto the beach from offshore or halting the moving sand from longshore drift by building breakwaters are two solutions to erosional problems.

See W. Bascom, Waves and Beaches: The Dynamics of the Ocean Surface, (1980).

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