A beach is the accumulation of loose sediment situated along the boundary between a body of water and a landmass. It represents a temporary deposit of material constantly being moved, shaped, and sorted by natural forces. The formation of a beach is a complex, ongoing geological and oceanographic process that requires a continuous supply of material and the energy to transport and deposit it. This process involves the breakdown of larger rocks into fine grains, the movement of these grains along the coast, and their final arrangement into the recognizable coastal landform.
Origin of Beach Sediment
The material that forms a beach, most commonly sand, originates from three primary sources. The most widespread source is terrigenous input, which is sediment derived from the land and transported by rivers from inland mountains to the sea. Coastal cliffs and bluffs also contribute significantly as they are eroded by wave action, supplying locally sourced material that may be coarser, such as gravel or cobbles. Another source is biogenic material, which includes the skeletal remains of marine organisms, such as shell fragments and pieces of coral, composed primarily of calcium carbonate. In geologically active regions, volcanic input creates unique beaches composed of dark, heavy minerals, and the color and texture of a beach are a direct reflection of the parent rock material.
Forces Shaping Coastal Landforms
Once sediment enters the marine environment, waves become the primary driver for transport and deposition. Waves are categorized by their effect on the beach profile, differentiating between constructive and destructive types. Constructive waves are low-energy, possessing a strong swash (the rush of water up the beach) but a weak backwash, which moves material up the beach face and leads to accretion. Destructive waves, conversely, are high-energy and feature a powerful backwash, which pulls sediment away from the shore and deposits it in offshore bars. Longshore drift transports sediment parallel to the coast because waves typically approach the shore at a slight angle, causing a net zig-zag movement of sand along the shoreline.
Anatomy and Profile of a Beach
The continuous action of these forces results in a specific physical structure that defines the beach profile. The beach is broadly divided into three main zones: the backshore, the foreshore, and the nearshore. The backshore is the landward area that remains dry under normal conditions, often containing berms built up by constructive wave action. The foreshore is the intertidal zone that is alternately covered and exposed by the tides, while the nearshore is the submerged area extending out to where the waves begin to break. The slope of the beach is closely linked to the dominant wave energy and the size of the sediment; high-energy, destructive waves create steeper profiles of coarser material, while low-energy, constructive waves result in a gentle, flatter slope of fine sand.
Constant Change and Coastal Dynamics
Beaches are not static landforms but are in a state of continuous adjustment to the surrounding environmental forces. A clear example of this is the seasonal change in the beach profile, which shifts in response to prevailing wave conditions. During calmer summer months, low-energy waves build up the beach, creating a wide, gently sloping ‘summer’ profile characterized by a prominent berm. As winter approaches, the increase in storm frequency and intensity introduces high-energy, destructive waves that erode the berm and flatten the beach profile. The removed sediment is temporarily stored offshore in submerged sandbars, forming a narrower, steeper ‘winter’ profile. Major events like hurricanes and severe storms can rapidly reshape a beach in a matter of hours, and over longer timescales, changes in sea level also influence this dynamic.