A beach is a landform along the water’s edge that may be made of any number of materials – primarily sand, but also gravel, cobbles, shells and rock. It is a dynamic landscape that gains material during periods of accretion and loses it during periods of erosion. Because beaches are always changing, they can appear very different in just one day.
The term beach is usually used to refer to the shoreline, but it can be extended to the area between a tidal creek and a barrier island or spit, or even the entire coastal zone. This area is dry at low tide and submerged at high tide, and it includes the beach face, front dune, back dune, tidal flats, and (parts of) salt marshes. It may be characterised by berms.
A wide beach is often described as a “bar”, and it may have the appearance of a series of parallel ridges, or an arch with a central depression. Alternatively, it may have a flat, bowl-like shape. In either case, the surface of a bar is very rough, with little or no vegetation, but the bottom of the tidal creek and the adjacent ocean floor are typically smooth, which makes it easy for the sea to flow through.
Beaches in tropical regions are usually warm and sandy, but there are exceptions – the beaches of the Arctic and Antarctic are extremely cold and rocky. The sand and other material that make up a beach are the products of weathering and erosion, which can be both natural and human-induced.
Longshore drift (or erosion by waves) is the major force that shapes a beach. Sediment is pushed by the sea on to the beach, either directly from the continental shelf or from sand bars and tidal deltas. It is then moved around by wave and wind action, and may be deposited further up the coast or lost to the sea in the form of sandbars or groynes.
The sand on a beach is typically coarse-grained, although it can be fine-grained or course. The grain size determines whether the beach gains or loses sand. Beaches built of coarse sand tend to gain sand by a process known as “backwash”, whereas fine-grained beaches, such as those on the Hawaiian islands, lose sand by a process called “upwash”.
Larger features arising from longshore drift include spits, tombolos and baymouth barriers. These sandbars form across the entrances of inlets, coves and rias. In some cases, they may silt up and become salt marshes. In addition, spits may be connected to the mainland by a lagoon. This lagoon is a temporary body of water that fills behind the spit or tombolo, and it can eventually silt up into a salt marsh. Similarly, groynes can fill up with sediment and prevent sand from leaving the beach. A beach may be artificially fed with sand to help counter erosion and maintain its width. This is called beach nourishment.