Longshore drift

Longshore drift (also known as littoral drift) is a geological process that describes the transportation of sediments (clay, silt, sand and shingle) along a coast parallel to the shoreline, which is dependent on oblique incoming wind direction. This process occurs due to the combination of beach drift and the ocean currents or longshore currents.

Process

Longshore drift begins when waves approach the shore at an angle. This causes a part of the wave's energy to move the water and sediment down the beach in a zigzag pattern, a process known as beach drift. The remaining energy of the wave causes the water to flow parallel to the shore, creating a current known as a longshore current. The combination of beach drift and the longshore current causes sediment to be transported down the beach, a process known as longshore drift.

Factors Influencing Longshore Drift

Several factors influence longshore drift, including the intensity and direction of the wind, the size and type of sediment, and the gradient and shape of the beach. The wind provides the energy for the waves, and its direction determines the direction of the longshore drift. Larger sediments such as pebbles and cobbles are transported at a slower rate than smaller sediments like sand and silt. The gradient and shape of the beach also play a role in longshore drift. Steeper beaches tend to have stronger backwash, which can pull sediment back into the ocean, reducing the effectiveness of longshore drift.

Impact

Longshore drift has a significant impact on the formation of various coastal features, including spits, bars, tombolos, and barrier islands. It also plays a crucial role in the formation and movement of beach cusps and sand dunes. However, longshore drift can also lead to problems such as beach erosion and the silting up of river mouths, which can have significant environmental and economic impacts.

Human Intervention

Human intervention often attempts to manage or halt the process of longshore drift through the construction of groynes, seawalls, and breakwaters. These structures can prevent or slow down longshore drift, protecting areas of the coast from erosion. However, they can also cause problems further down the coast, where the interruption of the natural flow of sediment can lead to increased erosion.

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  Longshore drift diagram

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  Provincetown Spit, Cape Cod

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  Groynes at Swanage Bay

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  Maumee Bay State Park aerial view