Coastal Sediment Transport and Marine Deposition

Coastal Sediment Transport Processes

The sea transports the load or sediment it obtains through erosion in the same ways that a river does. There are four primary methods of transportation:

• Suspension: Fine sediment is carried within the water, making it look muddy or murky.
• Solution: Dissolved material is carried along in solution, making it invisible to the naked eye.
• Traction: Larger pebbles and cobbles are rolled along the sea bed by the force of the water.
• Saltation: Small pebbles are moved when one pebble hits another, causing it to bounce in a chain reaction.

Understanding Longshore Drift

As well as being moved up and down a beach, sediment can also be moved along it if the incoming waves are driven by onshore winds at an oblique angle to the coast. This movement of sediment along a beach is called longshore drift.

On coasts where longshore drift occurs mainly in one direction, beach sediment is transported further down the coast. If an obstruction prevents its replacement from further up the coast, the beach will be depleted. This causes two major problems for local authorities:

• Smaller beaches are less attractive to tourists, causing a loss of income.
• It removes the natural protection from erosion that the beach provides for cliffs.

Coastal Management and Groynes

To counter this, some local authorities install barriers called groynes at right angles to the beach to trap sediment and reduce longshore drift. Material may also be moved along some coasts in the offshore zone by longshore currents, which act like rivers of water moving through the sea along the coast.

Marine Deposition and Beach Formation

Marine deposition occurs when the sea loses energy and drops the sediment it is carrying. This process follows a specific pattern:

• When the strong swash of a constructive wave moves up a beach, it carries sand or shingle with it.
• The largest material is deposited at the upper limit reached by the swash.
• The backwash then carries smaller material back down the beach. However, it progressively loses water and energy because the beach is porous; water passes through spaces between individual particles.
• The flow of the backwash is weakened until it can only carry the lightest material. Consequently, it deposits shingle and sand particles of smaller sizes as it flows back toward the sea.

How Waves Sort Beach Material

The material on a beach is sorted by wave deposition: the largest shingle is deposited at the top of the beach, while the finest sand is deposited near the sea. The smallest mud particles settle in the low-energy environment offshore.

When a storm occurs during the highest tides, large shingle is tossed above the usual high-tide level to form a ridge at the top of the beach.