A beach is an area of sand or other granular material that gathers at the shore of a body of water. The sand comes from erosion of rocks far away and near the beach, from coral reefs, and from other natural or man-made sources. A beach can be any size and shape, but it generally has gentle slopes because waves move the sand along the waterline. Beaches can have a wide range of colors, including white, black, gray, red, and yellow. Beaches may have a variety of other features as well, such as sea grass, tidal flats, salt marshes, and sand bars.
Beaches form when the wind, ocean currents, and tidal movements transport sediment toward the coast. This sediment is usually sand, but it can also be gravel, cobble, shingle, or rock. Beach sand often has a high silica content, which makes it very hard and durable. This sand is often used to build houses and roads. It is also used in erosion control devices, such as beach nourishment, to reduce the speed of coastal erosion.
Most beaches are lined with trees, shrubs, or other plants that provide shelter from storms and wind. Beaches are often popular places to picnic, swim, surf, fish, and sunbathe. Some have piers, lifeguard stands, and other structures for people to use. Some are crowded during the summer, when tourists flock to the shore to enjoy the warmth and beauty of the beach.
Many beaches are polluted with bacteria, raw sewage, and other waste from inland sources. This pollution can make the water unsafe for swimming, and it can harm marine animals. Beaches are often closed after a strong storm or if heavy amounts of pollutants wash onto the beach.
Sandbars are submerged ridges of sand or coarse sediment built offshore from a beach by swirling turbulence in the sandy bottom. The trough excavated by the turbulence is filled in with sand from the beach, while sand suspended in the backwash and in rip currents as well as sand moving shoreward from deeper water are added to the bar.
The beach state is determined by the way that waves interact with the beach morphology (shape). Three wave transformation zones—shoaling, breaking, and swash—determine the shape of a beach. Ocean waves shoal by gently depositing sediment across the surf zone, but as they interact with the beach they transform to become breakers and finally swashes. Each of these phases determines the shape of a beach and the characteristics that it has. This process is called beach morphodynamics. The beach state is determined by the point where the solid and dashed lines on a beach-state diagram meet. Below this line the beach is reflective; above, it is dissipative. Beaches between these two states are intermediate.