Longshore drift (littoral drift): coastal transport of sediment

Mechanism

The core motion that produces longshore drift is the alternation of swash and backwash. Incoming waves wash up the beach at an oblique angle (the swash), carrying sediment upright and along the shore. Gravity pulls water and sediment straight down the slope on the return flow (the backwash), which moves material seaward. Repetition produces a net zigzag movement of grains along the shore. This movement is driven by the nearshore current called the longshore current; the drift is the sediment transported by that current. The role of gravity is key in returning material downslope during backwash, while wave direction sets the lateral component.

Factors affecting rate and direction

Several variables control how fast and in what direction longshore drift operates. These include the dominant wind direction and strength, the angle and energy of incoming waves, the length of open water (fetch) over which waves form, beach slope and profile, and sediment size and shape. Coarser gravels are harder to move than fine sand, so beaches with mixed sediment show differential transport. Human constructions such as harbours and groynes also alter patterns by interrupting flow or trapping material.

Landforms and coastal effects

Longshore drift builds and reshapes a range of coastal features. When transported sediment accumulates where the shoreline changes direction or where current energy drops, it can form spits, bars, tombolos and barrier islands. Famous examples of sediment accumulation and reworking can be seen in features such as tombolos joining offshore stacks or long spits sheltering estuaries. Conversely, interruption of sediment supply can lead to downdrift erosion, retreating beaches and loss of dunes. These changes affect habitats, recreation, property and navigation.

Coastal management and engineering

Because longshore drift redistributes valuable beach material, coastal managers use various methods to control or work with it. Each option has trade-offs:

• Groynes: timber, rock or concrete barriers perpendicular to the shore that trap sand and build up the updrift beach but may starve downdrift areas.
• Breakwaters and sills: offshore or nearshore structures that reduce wave energy and encourage deposition in their lee.
• Beach nourishment: adding sand to eroded beaches to mimic natural sediment supply without changing drift patterns.
• Bypassing: mechanically moving sediment around structures or from accreting to eroding stretches to restore balance.
• Seawalls and revetments: hard defenses that protect specific landward assets but can increase erosion at the base and nearby shores.

Research, monitoring and significance

Coastal scientists study longshore drift using field markers, sediment tracing, aerial photography and modern techniques such as LiDAR and GPS surveys. Understanding the process is essential for shoreline planning, ecosystem conservation and infrastructure protection, particularly as sea level rise and changing storm patterns alter wave climates. Because longshore transport links different parts of a coast, interventions in one place often have measurable consequences elsewhere, making integrated planning important for sustainable coastal management.

For further practical or regional details see resources on coastal processes and management strategies: prevailing wind effects, sediment types, beach profiles, wave action and gravity and slope.