A beach is a narrow strip of land along the edge of a body of water. It is composed of loose materials, mostly sand, pebbles, cobbles, and shells, that accumulate during periods of accretion and move away during periods of erosion. A beach may also be a rocky reef offshore or a sandbank in a lagoon. Beaches are always changing, a process that is driven by wind and waves.
The materials that form a beach are derived from the erosion of rocks offshore and the breakdown of marine vegetation onshore. These materials are then carried in suspension by currents or waves to the shore where they may be deposited and sorted by size. The sand particles are often compacted by wave action and by the presence of established flora with network root systems that slow fluid flow at the surface layer.
Sand may be moved long distances by the forces of longshore drift, a slow current that travels southward along the coast. It can be deposited on sandbanks offshore where it may erode and drain through submarine canyons or it may be blown further inland by prevailing winds, creating beaches that are hundreds of kilometers long.
Most beaches are used for recreation and play a major role in local tourism industries. They often have amenities for visitors such as lifeguards, changing rooms and showers, shacks and bars, and restaurants. They are also exposed to the natural processes of weathering and erosion, but are often modified by human activities such as building structures (including roads) on or near the shoreline and beach nourishment operations.
Many beaches are protected by dunes which slow the movement of ocean sediment. A dune system is made up of foredunes and backdunes that are separated by intertidal areas, including a zone of water that is covered during high tide. These foredunes and backdunes are stabilized by beach grass, shrubs, palm trees, and other flora with deep network roots that slow the speed of moving wind or water at the surface layer.
Some beaches are eroded rapidly, and the cause is often linked to global climate patterns. For example, beaches in the island nation of Tuvalu in the South Pacific were retreating quickly in the 1990s, but have since recovered as the weather pattern known as the El Nino-Southern Oscillation has slowed. Beaches can be contaminated by runoff, which can alter the chemical makeup of the soil and affect the habitat of plants and animals that live there. They are also affected by pollution and sewage discharges. Beach nourishment is a method of adding new materials to a beach in order to keep it from being eroded by aggressive wave and wind conditions. This is often accomplished by placing the material in a way that it will be compacted and incorporated into the existing vegetation before aggressive scouring occurs. A well-planned nourishment project is a key part of a successful coastal protection strategy.