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The Process of Longshore Drift

You might think the sea just moves water in and out, but it is actually a massive coastal conveyor belt for sand and rocks.
This movement is driven by the prevailing wind, which causes waves to approach the beach at an oblique angle of approximately 45°.
First, the wave breaks and the swash pushes sediment diagonally up the beach in the same direction as the incoming wind.
Then, the backwash pulls the water and sediment straight back down to the sea at a 90° angle because gravity pulls it down the steepest gradient.
This continuous cycle of diagonal swash and vertical backwash creates a zig-zag movement, transporting material bit by bit along the coast through a process called longshore drift.

Conditions for Coastal Deposition

Understanding wave energy explains why some coasts lose sand while others build massive beaches.
Deposition occurs when waves lose velocity and energy, meaning they can no longer transport their sediment load.
This loss of energy happens in shallow water due to increased friction with the seabed, in sheltered areas like bays, or where the coastline changes direction.
Constructive waves are the main agents of coastal deposition and are more frequent during the calmer summer months.
These are low-energy waves (less than 1m high, arriving at 6–9 waves per minute) that feature a strong swash and a weak backwash.
The strong swash carries sand and shingle up the beach, but the weak backwash lacks the energy to pull it back down, causing sediment to accumulate.

The Formation of Beaches

Every time you walk across a shingle beach, you might notice the stones are much harder to walk on near the back than the wet, flat sand near the water.
A beach forms in sheltered bays where wave energy is low and constructive waves push sediment ashore.
On shingle beaches, the weak backwash loses even more energy through , a process where water sinks into the gaps between the porous stones rather than flowing back to the sea.

Students often forget to explain why sediment builds up during deposition; you must explicitly state that the weak backwash lacks the energy to remove the material.

For 'Explain the process' questions on longshore drift, Edexcel expects you to link the 90° angle of the backwash directly to the force of gravity.

Always use the term 'percolation' when explaining why the backwash is particularly weak on a shingle beach compared to a sandy beach.

If asked to draw or label a diagram of a spit, ensure you include the prevailing wind direction, the zig-zag arrows of longshore drift, the recurved end, and the sheltered salt marsh behind it.

The direction from which the wind blows most frequently in a specific location.

The forward movement of water up a beach after a wave breaks.

The backward movement of water down a beach back into the sea, driven directly by gravity.

The zig-zag movement of sediments along a coast caused by waves approaching at an oblique angle and receding straight back.

The process where the sea drops the material it is carrying because it has lost the energy to transport it.

Low-energy waves with a strong swash and weak backwash that build up beaches.

A depositional landform made of accumulated sand or shingle found in sheltered coastal areas.

The process of water filtering through the gaps in porous materials like shingle or sand.

A ridge found at the back of a beach marking the high-tide mark or the limits of the swash.

An extended stretch of beach material joined to the mainland at one end and projecting out into the sea.

The landward-curving tip of a spit, often caused by a temporary secondary wind direction or wave refraction.

A low-energy, sheltered environment behind a spit where fine silts and muds are deposited.