Main processes of coastal erosion

Key erosive processes

• Hydraulic action - This happens when waves force air into cracks and fractures in the cliff face, compressing it under high pressure. As the wave retreats, the sudden release of pressure causes rock fragments to break off.
• Abrasion - Waves carry rocks and sediment that act like sandpaper, scraping and grinding against cliffs and other surfaces. This repeated impact breaks off pieces and smooths the rock over time.
• Attrition - Rock particles suspended in the water collide with each other due to wave motion. These collisions chip away at the fragments, gradually reducing them to smaller, smoother, and more rounded pieces.
• Corrosion - Also known as solution, this process involves the dissolution of soluble rocks, such as limestone or chalk, by seawater.

Formation of cliffs and wave-cut platforms

Cliffs are steep, often vertical rock faces formed along the coast where the sea actively erodes the land. Their development is influenced by the rock's composition (lithology) and the intensity of wave action. Over time, ongoing erosion causes cliffs to retreat inland, creating distinctive features like wave-cut platforms.

Stages in the formation of cliffs and wave-cut platforms

1. Weathering and wave erosion, such as hydraulic action, create a notch at the high water mark. This usually forms along weaknesses like exposed bedding planes (horizontal layers in the rock) or fractures.
2. Continued erosion, including abrasion, enlarges the notch into a cave as waves deepen and widen the opening.
3. The rock above the cave becomes unstable without support underneath, leading to its collapse.
4. A new notch develops in the retreated cliff, and the cycle repeats, gradually eroding the coastline backwards and leaving behind a wave-cut platform.

Wave-cut platforms are flat, horizontal expanses of rock exposed at low tide. These platforms mark the former position of the cliff base and can include layers of both resistant and less resistant rock.

Negative feedback in wave-cut platform development

As a wave-cut platform grows larger, it influences the erosion process through negative feedback. Incoming waves lose energy due to friction as they travel over the extended platform, reducing their ability to erode the cliff face. This slows down further retreat. However, if the platform itself erodes or shrinks over time, wave energy increases again, restarting more intense cliff erosion.

Wave refraction and the formation of headland features

Wave refraction occurs when waves approach an irregular coastline, such as a headland (a protruding area of land into the sea). As waves enter shallower water near the headland, friction slows them down, while waves in deeper water further out continue at full speed. This causes the waves to bend or curve around the headland, concentrating energy on its sides.

This refraction exposes both flanks of the headland to erosion, leading to the creation of distinctive landforms.

Sequence of landform development on headlands

1. Erosion exploits cracks or joints in the rock, forming small caves on opposite sides of a narrow headland.
2. If these caves enlarge and meet, they create an arch.
3. Sea spray can erode weaknesses in the arch's roof, forming a blowhole — a small opening through the rock.
4. Continued erosion of the arch and blowhole causes the roof to collapse, leaving an isolated stack.
5. The stack, now fully exposed, undergoes further weathering from wind and rain, which reduces its size. Waves may also cut a notch at the high tide mark, undercutting it until the stack topples, leaving a low-lying stump visible only at low tide.

Factors influencing rates of coastal erosion and cliff recession

Key factors affecting erosion rates

• Wave type - Destructive waves, with high energy and a strong backwash, erode more effectively through processes like hydraulic action and abrasion. They also generate more sea spray, promoting corrosion higher up the cliff.
• Wave size - Larger waves carry more energy, exerting greater force on cliffs and rocks. This results in quicker erosion compared to smaller, calmer waves.
• Lithology - Soluble or soft rocks are more vulnerable to erosion. Rocks with many fractures or defined strata are vulnerable to hydraulic action, as waves can penetrate into the cracks.

Role of tides in cliff recession

• At high tide, waves reach higher up the cliff face than at low tide, meaning they have the potential to erode a larger area of rock.
• At low tide, waves may not reach the cliff face at all.