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Unlock This CourseLongshore drift is one of the most important coastal processes that shapes our coastlines. It's the movement of sand and sediment along a beach, creating some of the most spectacular coastal features we see today. Understanding this process is crucial for coastal management and explains why some beaches grow whilst others shrink.
Key Definitions:
Waves approach the coast at an angle due to prevailing winds. The swash carries sediment up the beach at this angle, but gravity pulls the backwash straight down. This creates a zigzag movement of sediment along the coast.
Longshore drift happens in a step-by-step process that repeats thousands of times each day. The direction and strength of this movement depends on several key factors that work together to transport millions of tonnes of sediment each year.
The process follows a predictable pattern that creates the characteristic zigzag movement of beach material:
Waves approach the coastline at an angle, typically between 10-30 degrees, determined by the prevailing wind direction.
The wave breaks and water rushes up the beach at the same angle, carrying sediment with it.
Gravity pulls the water straight back down the beach, taking some sediment with it.
Several factors determine how much sediment moves and in which direction. Understanding these helps explain why some coastlines experience rapid change whilst others remain relatively stable.
Prevailing winds determine the angle at which waves approach the coast. Stronger winds create larger waves that can move more sediment. In the UK, south-westerly winds are dominant, causing eastward drift along many coasts.
Higher energy waves can transport larger particles and more sediment. Storm waves are particularly effective at moving material, sometimes shifting entire beaches in a single event.
Beyond wind and waves, several other factors affect longshore drift rates:
Longshore drift creates some of the most distinctive coastal landforms. These features develop over hundreds or thousands of years as sediment accumulates in specific locations.
Spits are elongated ridges of sand or shingle that extend from the coastline into the sea. They form where the coastline changes direction, causing sediment to be deposited in deeper water.
Sediment builds up where the coast changes direction. The spit grows seaward as more material is deposited at its tip.
Wave refraction around the spit's end often creates a curved or hooked shape, forming a recurved spit.
Sheltered areas behind spits often develop into salt marshes as fine sediment settles in calm water.
Spurn Head is a 5.5km long spit extending into the Humber Estuary. Formed by eastward longshore drift, it demonstrates classic spit features including a curved end and salt marsh development. The spit is constantly changing shape due to ongoing sediment transport and occasional storm damage.
When spits continue growing, they can create bars that completely block bays, or tombolos that connect islands to the mainland.
Understanding longshore drift is essential for coastal management. Human activities often interfere with natural sediment transport, creating problems both upstream and downstream of intervention points.
Wooden or stone barriers built perpendicular to the beach to trap sediment. Whilst they protect one area, they can cause erosion further along the coast by reducing sediment supply.
This occurs when coastal defences stop sediment transport, causing severe erosion on the downdrift side. The sediment that would naturally replenish beaches further along the coast becomes trapped, leading to increased erosion rates.
In 1991, rock groynes were built to protect Mappleton from cliff erosion. Whilst successful in protecting the village, they caused accelerated erosion at nearby Cowden, 3km south. The groynes trapped sediment that would naturally have protected Cowden's cliffs, demonstrating the terminal groyne effect.
Scientists use various methods to measure longshore drift rates and understand sediment transport patterns. This information is crucial for coastal management decisions.
Longshore drift operates on coastlines worldwide, creating spectacular landforms and presenting management challenges in different environments.
Chesil Beach in Dorset is an 18km long barrier beach formed by longshore drift. The pebbles are graded by size, with larger stones at the western end where wave energy is higher.
Cape Cod in Massachusetts, USA, demonstrates large-scale longshore drift effects. The cape's distinctive hook shape results from sediment transport around its tip, constantly reshaping the coastline.
Climate change is affecting longshore drift patterns through changing storm frequencies, sea level rise and altered wind patterns. These changes will impact coastal landforms and management strategies in the future.