A beach is a narrow strip of land, made of sand or gravel, that borders a body of water. It may be a free beach, where waves act on it, or a sediment-covered beach that is acted upon by tides. A beach is the result of natural erosion and deposition processes, which continually change its shape, size and composition. It contains materials such as sand, pebbles and rocks, as well as marine organisms and seaweed.
Many beaches are polluted, and their condition is affected by human activities and climate. Beach pollution often includes garbage, such as plastic bags, cans and containers, as well as raw sewage and toxic chemicals. Some beaches have been closed after storms because of high levels of bacteria and other harmful substances in the water, which wash up from the ocean or from drainage pipes into the beach area.
The beach is a habitat for a variety of animals, including birds, marine mammals and reptiles. These animals are attracted to the sandy or gravelly sediments, which provide cover and food. The beach also serves as a barrier to the open sea, protecting land from waves and currents.
A beach is usually composed of sand or gravel, with a small percentage of silt and mud. The sand can be coarse or fine, and it may contain quartz grains, feldspars, micas and calcium carbonate, depending on where the beach is located. Beaches with a lot of calcium carbonate are found in the tropics, where the sun’s heat and humidity help to make the sand turn white.
Beaches are characterized by the Dean parameter, a measure of the wave energy required to move sediment down the coast. Those beaches with less energy require lower amounts of sediment, and those with more need more sediment. The type of sediment in a beach is also important; for example, sand or gravel beaches occur on coasts exposed to strong waves and currents, whereas mud or silt beaches appear in sheltered zones with weaker wave action.
In addition to beach sand, some beaches have areas of rocky or cliffy coastline, called headlands. These areas are generally defended by sea walls and other coastal defense structures. Beaches also have areas of tidal flats and salt marshes, which serve as critical fish nurseries, bird habitats and other wildlife refuges.
A large number of different types of beaches exist, each with its own distinctive features. A beach is usually described as dissipative or reflective, based on its morphological characteristics and the Dean parameter value. Dissipative beaches have a gentle coastal profile and are subjected to energetic short-crested waves, whereas reflective beaches have steep slopes and are dominated by low-energy swell waves. Beaches are further classified as breaker bars or nearshore sandbars. Breaker bars are elongated bodies of sand and gravel built in the surf zone by the action of waves. Nearshore sandbars are sand or gravel areas that extend from the beach, and gap between adjacent sandbars form rip currents, which discharge seaward into the surf zone.