Coastal Dynamics: Waves, Swash, Backwash, and Marine Erosion

Coastal Dynamics: Waves and Marine Erosion

Waves are fundamental forces shaping our coastlines. Their interaction with the shore involves two primary movements: swash and backwash.

Understanding Wave Dynamics

Swash and Backwash: The Basics

• Swash: When water rushes up the beach.
• Backwash: When water drains back down the beach.

Destructive Waves: Shaping Coastlines

Destructive waves are powerful forces that remove material from the beach. They are characterized by a weak swash and a strong backwash, pulling sand and pebbles back down the beach as the water retreats.

• Often steep, high waves that are close together.
• Crash down onto the beach.
• Come in very quickly, at a rate of 10 to 15 waves per minute.
• Also known as plunging waves.

Constructive Waves: Building Beaches

In contrast, constructive waves build up the beach. They feature a very strong swash and a weak backwash, pushing sand and pebbles up the beach and leaving them behind when the water retreats, as the backwash is not strong enough to remove them.

• Often low waves with longer periods between them.
• As they break, they spill up the beach.
• Also known as slipping waves.
• Come in at a rate of 6-8 waves per minute.

Marine Erosion Processes

Marine erosion refers to the wearing away of the Earth's surface by the sea. Several distinct processes contribute to this coastal change.

Attrition: Sediment Reduction

The effects of attrition are increased the further the distance and the longer the time period over which material is moved by the waves. Loose sediment, knocked off the cliff by hydraulic action and abrasion, is swirled around by the waves. It constantly collides with other sediment and gradually gets worn down into smaller and rounder sediment.

An angular boulder will eventually be broken down into small, round grains of sand (composed of very tough mineral quartz), which are very resistant to further breakdown. Rounded beach material of intermediate size between boulders and sand is known as shingle.

Hydraulic Action: Water Pressure Erosion

This process occurs when water is forced into cracks in the rock. This compresses the air inside. When the wave retreats, the compressed air blasts out, which can force the rock apart.

Corrasion (Abrasion): Sediment Impact

Corrasion, also known as abrasion, happens when loose rocks, called sediment, are thrown against the cliff by waves, chipping off bits of rock.

Corrosion (Solution): Chemical Dissolution

Corrosion, or solution, occurs when seawater dissolves material from the rock. This process is particularly prevalent along limestone and chalk coasts, where calcium carbonate is dissolved.