What causes a longshore drift?

As a coastal geomorphologist, I specialize in the study of the processes that shape the coastline. One of the key mechanisms that contribute to the dynamic nature of coastal environments is longshore drift. Longshore drift is the movement of sediments, such as sand and pebbles, along the shoreline in a generally parallel direction to the coast. This process is driven by the interaction between waves and the coastline. Here's how it typically works: 1. Wave Approach: Prevailing winds influence the direction from which waves approach the shore. When waves come in at an angle to the coast, they create a zigzag pattern as they interact with the shoreline. 2. Swash and Backwash: As waves break and move up the beach (swash), they carry sediment with them. The angle at which the waves approach the shore often determines the angle at which the swash moves sediment along the coast. 3. Backwash Return: The backwash, or the wave's return movement down the beach, is less effective at carrying sediment seaward because it is often dissipated by the beach's slope. This results in a net movement of sediment along the coast, parallel to the shoreline. 4. Tidal Influence: In some cases, the tidal currents can also contribute to the longshore drift by reworking the sediments that have been moved by the swash. 5. Coastal Features: The presence of headlands (protruding pieces of land) and bays can influence the pattern of longshore drift by causing sediment to accumulate in certain areas, creating features like spits and tombolos. Longshore drift is a significant process in the coastal sediment budget, playing a crucial role in the formation and evolution of beaches, dunes, and other coastal landforms.

The transport of sand and pebbles along the coast is called longshore drift. The prevailing wind (the direction the wind ususally blows from) causes waves to approach the coast at an angle. The swash carries the sand and pebbles up the beach at the same angle (usually 45o).