The concepts of longshore current and longshore drift are often confused, though both describe processes that occur when waves hit a shoreline. They refer to distinctly different aspects of movement. Understanding the mechanics of each is necessary to grasp how coastlines are continuously shaped. The fundamental difference lies in what is being moved: the longshore current is the flow of water, while longshore drift is the resulting transportation of sediment.
The Mechanics of Longshore Current
The longshore current is a flow of water that moves parallel to the shoreline within the surf zone, the area where waves are actively breaking. This current is generated when incoming waves approach the coast at an oblique angle, meaning they do not arrive perpendicular to the beach face. As waves enter shallow water, the portion of the wave crest nearest to the shore slows down due to friction, causing the wave to bend or refract.
Even after refraction, waves still reach the beach at a slight angle, creating momentum that pushes water sideways along the coast. This continuous, parallel flow is the longshore current, sometimes called the littoral current. The speed of this current can be significant, occasionally reaching up to four kilometers per hour. Its velocity is influenced by the wave angle, the wave’s speed, and the overall slope of the beach.
The Mechanics of Longshore Drift
Longshore drift is the geological process of moving sediment—such as sand, gravel, and shingle—along the coast parallel to the shore. This movement is caused by the same angled wave approach that creates the longshore current, but the mechanism involves a distinct two-step motion on the beach face.
The first step is the swash, which is the rush of water from a breaking wave moving up the beach at an angle. The swash carries sediment particles up and along the beach slope, following the direction of the incoming wave. The second step is the backwash, which is the water flowing straight back down the beach slope toward the sea. Gravity dictates that the backwash moves perpendicular to the shoreline, pulling the sediment particles directly down the steepest gradient. This repeated pattern of angled swash and perpendicular backwash causes the sediment to move in a characteristic “zig-zag” or saw-tooth path, resulting in the net transport of material down the coast.
Distinguishing the Two and Their Coastal Impact
The distinction is one of cause and effect: the longshore current is the engine, and longshore drift is the resulting movement of material. The current is the parallel flow of water within the surf zone. Longshore drift is the physical transport of sediment particles, encompassing movement both in the nearshore zone by the current and on the beach face by the swash and backwash.
The cumulative effect of longshore drift is a continuous redistribution of sediment, which shapes coastal landscapes. When transport rates shift, material can be deposited to form large-scale landforms. These depositional features include spits, which are narrow ridges of sand extending from the coastline into the water, and bars, which are linear ridges that can close off a bay.
When human-made structures, such as groynes or jetties, are built perpendicular to the shore, they interrupt the natural flow of longshore drift. Sediment accumulates on the updrift side of the structure, leading to accretion. Conversely, the downdrift side experiences a reduction in sediment supply and often suffers from increased erosion. Understanding the direction and volume of longshore drift is important for coastal management, guiding the design of structures and informing beach nourishment projects.