Longshore drift is a geological process involving the movement of sediment along a coastline parallel to the shore. This continuous transportation of material, which includes sand, shingle, silt, and clay, is driven by wave action within the surf zone. The process is also known as littoral drift, describing how coastal landscapes are constantly reshaped by the sea’s energy. Understanding longshore drift is important because it influences shoreline evolution, affects beach stability, and is a major consideration in coastal engineering and management projects.
How Coastal Sediment Moves
The mechanism for longshore drift is a two-step sequence involving how waves interact with the beach when approaching at an angle. The sequence begins with the swash, the rush of water carrying sediment up the beach after a wave breaks. Since incoming waves arrive obliquely, the swash drives the sediment diagonally up the beach slope in the direction of the wave.
Once wave energy is dissipated, the water returns to the sea under gravity, an action known as the backwash. The backwash pulls the water and sediment straight back down the steepest gradient of the beach, making its path perpendicular to the shoreline. This repeated cycle of diagonal swash followed by perpendicular backwash creates a distinctive zig-zag pattern of sediment movement. The cumulative effect is a net transfer of material along the coast in the direction of the dominant wave approach.
The Physical Results of Drift
Longshore drift leads to the creation of significant depositional coastal landforms. When the coastline abruptly changes direction, such as at a bay or river mouth, waves lose energy and can no longer carry their full sediment load. The resulting deposition of sand and shingle forms a long, narrow ridge extending into the water known as a spit. Spits are attached to the land at one end and often develop a curved or “hooked” end due to secondary wave action or opposing currents.
If a spit continues to grow completely across a bay, it connects two headlands or the mainland to an island, forming a bar. This continuous ridge of sediment effectively seals off the bay, trapping a body of water often referred to as a lagoon. Conversely, longshore drift contributes to beach erosion where the natural supply of sediment is cut off, such as on the down-drift side of man-made structures like groynes or jetties.
What Controls the Speed of Movement
The rate and efficiency of longshore drift are governed by several variables related to wave characteristics and beach geography. A primary factor is the angle of wave approach, where waves approaching the shore closer to a 45-degree angle produce the most rapid transport. If waves arrive nearly parallel or perpendicular to the coast, the effectiveness of the zig-zag movement is reduced. The energy and velocity of the waves play a major role, as more powerful waves increase the speed of the longshore current and the capacity to move sediment.
The physical characteristics of the beach influence the speed of the drift. Steeper beach slopes generally result in a stronger backwash and more rapid movement of material. The type and size of the sediment affects how it is transported; finer sands are more easily suspended and carried by the current than heavier pebbles or shingle. The distance over which the wind blows to generate the waves, known as the fetch, determines wave strength and the overall magnitude of the drift.