A longshore current is a water movement that travels parallel to the coastline, existing within the surf zone where waves are actively breaking. This current acts as a natural conveyor belt for nearshore water. It extends from the line of breaking waves to the shoreline and is a constant factor on most beaches exposed to incoming surf.
How Longshore Currents Form
The formation of a longshore current is tied to the angle at which waves approach the shore. Most waves approach the beach at an oblique angle, rather than perfectly parallel. When a wave breaks, it pushes water toward the shore. Since the wave energy is directed at an angle, the water volume has a component of motion that runs along the length of the beach.
This angled push creates a buildup of water along the coast, which must flow to equalize the pressure. The displaced water flows sideways, parallel to the shoreline, within the shallow surf zone. Current speed is regulated by the size of the waves and the angle at which they break, sometimes reaching up to four kilometers per hour.
The water movement can be visualized as a continuous “zig-zag” pattern on the beach face. Water rushes up the beach face at an angle (the swash) but then returns straight back down the slope, perpendicular to the shoreline (the backwash). Each successive wave repeats this angled push, resulting in a net movement of water that travels along the coast.
The Movement of Sand and Sediment
The primary function of the longshore current is the transport of sediment, a process known as littoral drift or longshore transport. This process moves large amounts of material along coasts worldwide.
As waves break, the turbulence lifts sand and other sediment off the seabed, making it available for transport. The longshore current then carries this suspended material parallel to the shore. The angled swash pushes sand grains up the beach, and the straight backwash pulls them down, causing each particle to move in a sawtooth pattern that results in a net displacement down the beach.
This constant redistribution shapes the coastline by causing erosion in one area and deposition in another. The overall direction of the littoral drift influences the formation of coastal features like spits and barrier islands.
Longshore Currents vs. Rip Currents
Longshore currents are often confused with rip currents, but they are distinct phenomena with different flow directions. A longshore current flows parallel to the shoreline. These currents can be strong enough to sweep swimmers down the beach, but they generally do not pull people out to sea.
Rip currents, in contrast, flow perpendicular to the shore, moving straight out to the open ocean. They form when water pushed onto the shore accumulates and finds a narrow path to flow back seaward, often through a break in a sandbar. These offshore-directed currents are a significant safety hazard for swimmers, as they can rapidly pull a person away from the beach. Longshore currents can sometimes feed water toward a rip current where coastal structures force the parallel flow to turn seaward.