Wave Types and Coastal Processes

• Constructive waves: These are low-energy "spilling" waves that build up the beach by depositing sediment. They have a low wave frequency (typically 6–8 waves per minute), a low wave height (under 1 metre), and a long wavelength. They feature a strong swash (water moving up the beach) and a weak backwash (water moving down the beach), creating a gentle, wide beach profile common in summer.
• Destructive waves: These are high-energy "plunging" waves that remove sediment, causing erosion. They have a high frequency (10–14 waves per minute), a high wave height (over 1 metre), and a short wavelength. They possess a weak swash and a strong backwash, leaving a steep, narrow beach profile more common in winter.

Weathering and Mass Movement

You can easily snap a dry twig, but breaking a solid cliff face seems impossible without heavy machinery. Yet, nature slowly shatters solid rock daily through sub-aerial processes (processes occurring on the cliff face).

Weathering is the disintegration of rock in situ (in its original place).

• Mechanical weathering involves physical breakdown. In freeze-thaw weathering, water enters cracks and freezes when temperatures drop below 0°C. The ice expands by 9%, exerting intense pressure. When it thaws, pressure is released. Repeated cycles shatter the rock, often forming scree (piles of angular rock fragments) at the cliff base via a rockfall.
• Chemical weathering changes the mineral composition. In carbonation, rainwater absorbs $CO_2$ to form weak carbonic acid, which reacts with calcium carbonate in limestone or chalk, dissolving it into soluble calcium bicarbonate.
• Biological weathering is the breakdown caused by living organisms, like plant roots expanding in cracks.

Mass movement is the downward movement of weathered material due to gravity, often triggered when heavy rainfall saturates the ground, increasing its weight and reducing friction.

• Rotational slumping occurs when permeable sand sits above impermeable clay. Rain saturates the sand, lubricating the slip plane (boundary). The saturated section slips down a concave (curved) plane, as seen at the Holbeck Hall Hotel in 1993.
• A landslide involves rapid movement along a straight, flat slip plane, whereas a mudflow is the rapid flow of saturated, fine-grained material on steep slopes.

Coastal Erosion Processes

Every time you watch a storm batter the coast, destructive waves are actively carving away the land.

Erosion is primarily driven by destructive waves, concentrating high energy in small areas. There are four main processes of coastal erosion:

• Hydraulic action: Waves force air into cracks in the rock. The trapped air is compressed, and when the wave retreats, the pressure is explosively released, shattering the rock. Cavitation is a high-level component of this, involving intense pressure changes and fizzing water.
• Abrasion: Waves hurl loose load (pebbles and stones) against the cliff face, creating a sandpapering effect that wears the rock away.
• Attrition: Rocks and pebbles carried by waves collide with each other, becoming smaller, smoother, and more rounded. Crucially, attrition does NOT erode the cliff itself; it only affects the sediment.
• Solution: Seawater dissolves soluble minerals within the rocks, such as chalk and limestone.

Transportation and Deposition

Why does sand always seem to pile up against wooden groynes on a beach? This happens due to the constant zigzag movement of sediment along the coast.

Sediment is transported along the coast through longshore drift. The prevailing wind causes waves to approach the beach at an oblique angle. The swash carries sediment diagonally up the beach. However, gravity always pulls the backwash straight down the beach at a 90° angle. This creates a net zigzag movement of sediment along the shoreline.

Material is moved within the water in four ways:

• Traction: Large boulders are rolled along the seabed.
• Saltation: Small pebbles are bounced along the seabed.
• Suspension: Fine material (silt/clay) is carried within the water.
• Solution: Minerals are carried invisibly in chemical form.

Deposition occurs when wave velocity decreases and energy is lost, causing sediment to "drop out." This is driven by constructive waves in shallow water (where friction is high), sheltered bays, or river mouths where competing currents reduce water speed. Strong swash carries sediment up the beach, the water soaks into the sand (percolation), and the weak backwash lacks the energy to remove it. This often leaves a berm (a ridge of material) at the top of the beach.

The Influence of Geological Structure

If you walk along the Dorset coast, you will see a jagged coastline of headlands and bays in one area, and a perfectly smooth curved cove in another.

Coastal landforms are heavily dictated by geological structure and lithology (the physical and chemical characteristics of rocks). Differential erosion occurs when rocks of varying hardness erode at different rates.

• Discordant coastlines: Rock strata (layers) run perpendicular (90°) to the coastline. At Swanage Bay in Dorset, less resistant Wealden Clay eroded rapidly to form a bay, while resistant Chalk and Portland Limestone eroded slowly, jutting out as headlands.
• Concordant coastlines: Rock layers run parallel to the shore. At Lulworth Cove, the sea eventually breached the hard outer layer of Portland Limestone. It then rapidly eroded the softer Purbeck Clay and Wealden Sands behind it, creating a circular cove backed by resistant Chalk.

How Rock Type Impacts Coastal Forms

Understanding rock types explains why some coastal towns lose metres of land every winter, while others barely change over centuries.

Soft, unconsolidated material like boulder clay is highly vulnerable to erosion and slumping. The Holderness Coast retreats by up to 2 metres per year because its soft clay is easily washed away and becomes heavy when saturated.

In contrast, resistant rocks like granite, limestone, or chalk (e.g., Flamborough Head) erode very slowly, forming high, steep cliff profiles with bare rock faces. However, even hard rocks have lines of weakness, such as faults and joints. Hydraulic action exploits these weaknesses, hollowing them out to create features like the Cave-Arch-Stack-Stump sequence seen at Old Harry Rocks.