A beach is a coastal landform along the shore of an ocean, sea, lake or river consisting of loose particles such as sand, gravel, shingle, pebbles and sometimes bits of mollusc shells or coral. The particles are derived either organically (from biologic sources such as mollusc shells or marine organisms) or inorganically (such as rock fragments, igneous rocks and sedimentary rock) in origin. In addition to sand, beaches may contain mud, silt and clay. The size of the particles forming the beach is an important factor in its shape and structure. Beaches with larger particles tend to be flatter than those with smaller particles.
The process of beach formation involves erosion, which is the wearing away of rocks by flowing water and is accelerated by wave action. Erosion of a beach is influenced by many factors, including wave height and frequency, the slope of the coastline, the type and composition of the bedrock, and the characteristics of the waves, such as their wavelength, velocity and direction.
Beach erosion is the main reason that beaches change from one day to the next, as new materials are added and others removed. This happens most dramatically during low tide when sandbars form, or disappear, as the tide rises. Beaches are also constantly changing at the water’s edge, where a feedback process exists between the waves breaking over bar systems and the accumulation of sediment below breaking waves.
Because of their pleasant temperature and soft sand, beaches have long been popular for recreation and play an important role in society, attracting visitors who swim, surf, play sports, relax, sunbathe and collect shells. Beaches often have lifeguard posts, changing rooms and showers to provide safety and convenience for visitors. They may also have food vendors, ice cream stands, bars, shacks and hotels. Some are protected by barriers such as dunes or groyne fields and have facilities to accommodate recreational boating, such as piers, marinas and harbors.
Beaches are often characterized by a flat to gently sloping frontal terrace surface inclined towards the water, with a higher ridge or berm at the head of the beach and lower terraces at the foot. Beach swash, controlled by gravity and driven by waves, moves sand downstream (a process called backwash) and beach troughs carry it offshore. Over time, this movement results in the formation of an outer bar offshore at the point where energetic incident waves start to break on the beach and a nearshore bar at the location where the waves meet with less energy. The continuous interaction between waves and sediment transport by the swash and trough is known as longshore drift.