A beach is a shoreline where sand or other loose sediments are deposited and shaped by waves and currents. These sediments are primarily of sand or shingle, although clay and mud may also be present. The size and shape of a beach is a function of its composition, the amount of wave energy it receives, and whether it is protected from erosion by offshore rock reefs or structures such as groynes. Beaches are also influenced by coastal vegetation, including sea grasses and mangrove forests.
The sand and gravel that makes up a beach is generally coarser than that of a river or streambed. This coarser material is typically derived from the erosion of rocks offshore, but may be a consequence of slumping or erosion of headlands or from the breakdown of corals at depth. Some beaches are also fed by the deposition of finer sediments created by wave action or by tidal flow, as well as the leaching of fine particles from surrounding areas.
When viewed from the air, a beach has several distinct segments when compared with the land behind it. The segment nearest to the water and actively acted upon by waves is the beach dune, which has a crest (top) and a face that slopes down towards the water. The next segment above the dune is a terrace of sand and gravel, called a foreshore terrace, which may be above or below the water line depending on tide. The terrace is then topped by a series of ridges, called a beach berm, which are formed by the waves of a storm.
Between a beach dune and the ocean is a zone of tidal flats or salt marshes, depending on the location of the beach. This area is wet at low water (MLW) and dry at high water (MHW). This region of the beach is characterized by windblown sand deposition, wet sand abrasion from incoming waves and occasional tidal flow over a beach berm. It is also the site of embryonic dune and foredune formation.
The sand moving on a beach is usually carried north by the longshore current and south by waves breaking in front of it. In some cases, however, a large buildup of sand occurs in the surf zone, where waves break closest to the coast. This sand is then pushed by the waves northwards and southwards, as they continue to break close to the coast. This net movement of sand across a beach is referred to as longshore drift or longshore transport. Longshore drift is also responsible for the shape of a beach, and it is the cause of gradual changes in a beach’s profile over long periods. Freak waves or storm surges, on the other hand, can substantially change a beach’s shape and position within hours.