Transportation (Longshore Drift)

Definition of Longshore Drift

Longshore drift is the process by which sediment (such as sand, pebbles, and shingle) is transported along the coastline. This movement happens because waves usually approach the shore at an angle, not straight on. The angled waves cause sediment to move sideways along the beach rather than just back and forth.

The two main parts of wave movement involved are:

• Swash: The movement of water and sediment up the beach at an angle when a wave breaks.
• Backwash: The movement of water and sediment straight back down the beach due to gravity.

This combination of swash and backwash causes sediment to zigzag along the coast, gradually moving material in one direction. For example, if waves hit the beach at a 30° angle, sediment moves in a zigzag pattern along the shore.

Process of Longshore Drift

Longshore drift happens because waves rarely hit the coast head-on. Instead, they approach at an angle determined by the prevailing wind direction.

Here is how the process works step-by-step:

1. Waves approach the shore at an angle, carrying sediment with them in the swash.
2. The swash pushes sediment up and along the beach at this angle.
3. Gravity pulls the backwash straight down the slope of the beach, carrying sediment back perpendicular to the shoreline.
4. This results in a zigzag movement of sediment along the coast.
5. Over time, this causes a net movement of sediment sideways along the shore, known as longshore drift.

For example, if the wind blows from the southwest, waves will approach the coast from that direction, moving sediment northeast along the beach.

The amount of sediment moved depends on wave energy, sediment size, and beach slope.

For instance, on a gently sloping beach with fine sand and strong angled waves, longshore drift will be more effective.

Example: Imagine waves hitting a beach at a 30° angle to the shore. The swash carries sand up the beach at this angle, but the backwash pulls it straight back down. This zigzag movement moves sediment along the coast.

Effects on Coastal Landscapes

Longshore drift plays a key role in shaping coastal landscapes by moving sediment along the shore. This affects the formation and change of various coastal features:

• Beaches: Sediment transported by longshore drift contributes to the size and shape of beaches. Beaches can grow or shrink depending on the balance between sediment added and removed.
• Spits: When the coastline changes direction sharply (e.g., at a river mouth or bay), sediment carried by longshore drift can be deposited in the sea, forming a spit. Spits are narrow ridges of sand or shingle extending from the coast.
• Bars: If a spit grows across a bay and connects two headlands, it forms a bar, which can trap water behind it, creating a lagoon.
• Erosion and Deposition Balance: Longshore drift redistributes sediment, balancing erosion in some areas with deposition in others. This dynamic process constantly reshapes the coastline.

For more on how erosion and deposition shape coasts, see the Coastal Landscapes in the UK notes on Erosion and Deposition.

Human Impact and Management

Human activities can affect longshore drift and sediment movement along coasts. Coastal management structures are often built to control sediment flow and protect beaches or properties.

Groynes are wooden or rock barriers built at right angles to the shore. They interrupt longshore drift by trapping sediment moving along the beach.

By trapping sediment, groynes help build up the beach on one side, which can protect the coast from erosion. However, they can starve beaches further down the coast of sediment, causing them to shrink.

This interruption changes the shape and size of beaches, affecting coastal landscapes and habitats.

Other coastal management methods such as sea walls, rock armour, and beach nourishment also influence sediment movement, but groynes are the most directly related to longshore drift.