A beach is a narrow strip of land made of sand or gravel that borders a body of water, such as an ocean or lake. Beaches are constantly changing, as waves and tides deposit new materials and take away others. This process is called erosion.
Sand is the dominant material in beaches. Other materials found in beaches include rocks, shells, seaweed and marine organisms. A beach can be rocky, cliffy or covered with dunes. Beaches are also frequently shaped by humans, who make use of them for recreation and to protect their homes from coastal erosion.
The sand and other sediment that form a beach are transported from the interior of the earth by wind, water and other forces. These materials are deposited in a shoreline area known as a tidal flat or salt marsh, which is the intertidal zone.
Tidal flows of water and sand are also carried by longshore currents. These flows are influenced by the shape of the coastline and the size of the waves. The movement of these particles changes the character of underwater flora and fauna in coastal shallows, for example by engulfing reed beds.
A beach can be free or constructed. The latter are characterized by wave-cut cliffs on one or both sides of the beach. These cliffs are often formed of limestone, but may be composed of other rock types. Beaches that are free of cliffs are usually composed of sand, and are known as barrier beaches.
When waves approach a beach, they slow down and the period between their wave crests decreases. This behavior is referred to as shoaling, and it causes the waves to build up sand. This sand can be added to existing beach berms, or it may build up an outer bar offshore.
Once an outer bar has been built up, energetic incident waves can reach the beach more easily and the sand is once again moved offshore. The outer bar then decays and a new beach is created.
Coastal erosion can be accelerated by human activities, such as urban development and agricultural clearing on the land that fringes a beach. These activities change surface wind patterns, which can lead to more erosion of the beach and adjacent tidal flats.
People have developed ways to combat coastal erosion and improve the quality of a beach. For example, residents of Coney Island in Brooklyn, NY have invested in seawalls, which prevent sand and other beach materials from blowing away during storms. Beach nourishment is a common practice in which sand is pumped onto a beach to supplement its natural supply. Beaches nourished this way often appear quite natural and many people are unaware that they are artificial. For example, a visitor to Waikiki Beach in Honolulu might not know that this famous beach is largely manmade.