Longshore drift is a geological process responsible for shaping the world’s coastlines. It is defined as the transportation of sediment, such as sand, shingle, and pebbles, parallel to the shoreline. This movement is driven by wave action and the resulting currents near the coast. The process ensures a constant redistribution of loose material, connecting areas of erosion with zones of deposition. This system of sediment movement is also referred to as littoral drift.
The Physics of Sediment Transport
The mechanism of longshore drift relies on waves approaching the beach at an oblique angle. This angled approach is often governed by the direction of the prevailing wind, which pushes the surface water and generates the incoming waves.
When a wave breaks, the surge of water that rushes up the beach face, known as the swash, carries sediment along with it at the same oblique angle as the incoming wave. Once the wave energy is spent, the water returns down the beach slope due to gravity, a movement called the backwash. This backwash flows straight down the steepest gradient, which is perpendicular to the shoreline.
This two-step sequence—swash moving material up and sideways, and backwash pulling it straight down—creates a distinct zigzag pattern of movement for each individual particle. The cumulative effect of thousands of waves repeating this action results in a net displacement of sediment in a consistent direction along the entire length of the beach.
Coastal Features Shaped by Movement
When longshore drift encounters a significant change in the shape of the coastline, or when the energy of the waves decreases, the transported sediment is deposited, creating new landforms.
A common result is the formation of a spit, which is a narrow ridge of sand or shingle extending out into the sea from the mainland, typically forming where the coastline abruptly turns inland. The sediment supply from the drift continues to deposit material into the open water, building the spit outward.
If a spit grows across the mouth of an entire bay, connecting two headlands, it forms a bar, which effectively seals off the bay from the ocean. This process traps a body of water behind the newly formed barrier, often creating a lagoon or a tidal marsh. A tombolo is a specific form of bar that connects a mainland to an offshore island, forming a sediment bridge where the waves and currents have converged.
Human Intervention and Coastal Management
The natural flow of longshore drift often conflicts with human development, leading to various attempts to control or mitigate the sediment movement. These interventions are primarily focused on protecting infrastructure and maintaining beach size for recreational purposes.
One of the most common hard engineering solutions is the construction of groynes, which are low walls or barriers built perpendicular to the shore. Groynes are designed to interrupt the drift, trapping sediment on the updrift side to widen the beach and provide better protection against erosion. However, this often leads to sediment starvation and increased erosion on the downdrift side.
Other structures, like sea walls and revetments, are built parallel to the shore to absorb or reflect wave energy, but they also disrupt the natural sediment balance. By reducing the available sediment, these structures can increase the rate of erosion on the beach face itself. A softer approach is beach nourishment, which involves dredging sand from offshore areas and artificially placing it back onto an eroding beach to replenish the lost material.