A beach is an area of accumulated loose sediment (usually sand or pebbles) along the edge of a body of water. Though usually associated with oceans, beaches can also form on the shores of rivers, lakes and ponds. Beaches are subject to constant change in shape and composition, due to the natural processes of erosion, depositional transport, and wave and tide action. Many of these changes are relatively slow, occurring over periods of years or more, but freak wave events can substantially alter the appearance of a beach within hours.
Beaches are primarily composed of fine to medium-grained sediments. These materials are eroded from inland areas and carried to the sea by waves, currents and wind. Sediment particles are affected differently by these forces according to their size and state of compaction. Larger particles tend to be eroded more readily, while smaller particles are held in suspension longer and are carried farther afield by wind or water. Sediment that has been compacted by plant roots or other means is more resistant to erosion.
The composition of a beach depends also on the type of sediments and the environment that surrounds it. For example, sandy beaches are often characterized by rocky outcrops and sand-filled dunes, while gravel-dominated beaches may be forested by a shrubby vegetation or have mangrove forests nearby. Beaches facing energetic wave and wind conditions will be dominated by horn-shaped or knobby ridges of sand and shingle, known as berms. These ridges are created by the force of winds pushing up against a cresting wave. A beach that is protected from such strong winds by a barrier of coral reefs or a body of land may have smoother, flatter and more uniform slopes.
A major factor in beach erosion is the influx of sand from offshore areas. The longshore drift of sand, driven by wind and waves, moves the material south from a source area near the mouth of a bay or river. This movement can engulf reed beds or change the character of underwater coastal shallows by removing sand and shifting it elsewhere.
Another significant contributor to beach erosion is human intervention. The burning of vegetation on the head of a beach for farming and development causes changes in surface wind patterns that expose the sand to erosion by waves and currents. The sand is then carried down the coast and deposited at other beaches or in lagoons or deltas. This process of sand transport can also disrupt the ecological balance of a beach, destroying dunes and forming gullies and mangrove forests. In addition, the influx of sand can reduce the depth at which water enters the beach, which changes the characteristics of the shoreline. For these reasons, beaches are a sensitive and important environmental resource that requires careful management.