Coastal processes

Fetch is the uninterrupted distance of open water over which the wind blows to generate waves.

The longer the fetch, the more energy the wind can transfer to the water, so the waves are larger and carry more energy.

A coast facing a long fetch of open ocean therefore experiences high-energy waves, while a sheltered coast with a short fetch experiences low-energy waves, even if the wind speed is the same.

Abrasion (also called corrasion) is the process by which waves pick up sand, pebbles and rock fragments and hurl them against the cliff, scraping and grinding it away like sandpaper. It therefore erodes the CLIFF.

Attrition is the process by which the rock fragments and pebbles carried by the sea collide with one another, gradually becoming smaller, smoother and more rounded. It therefore wears down the SEDIMENT, not the cliff.

The key difference is the target: abrasion uses the sediment as tools to wear the coast away, whereas attrition is the sediment wearing itself down through repeated collisions.

Wave refraction is the bending of wave crests as they approach an irregular coastline made up of headlands and bays.

The water is shallower off the headlands than in the bays, so as a wave approaches, the part of its crest over the headland slows down first through friction with the seabed, while the part over the deeper bay continues at speed. This causes the crest to bend so that it wraps around the headland.

Because the wave energy becomes focused onto the headland, erosion is concentrated there. High-energy processes such as hydraulic action and abrasion attack the headland, producing features such as caves, arches and stacks.

In the bays, by contrast, the bending spreads the wave energy out over a wider area, so the waves arrive with less energy. These weaker, more constructive waves deposit sediment, building beaches in the sheltered bays.

Over a long period, refraction therefore wears the headlands back and allows sediment to accumulate in the bays, so the coastline tends to become straighter.

Sub-aerial processes act on the cliff face above the waves and work alongside marine erosion to make a cliff retreat.

Weathering weakens the rock in place. Physical weathering such as freeze–thaw — where water in cracks freezes, expands by about 9% and shatters the rock — and salt crystal growth, where salt left by evaporating spray grows in cracks, both break up the cliff face. Chemical weathering, such as carbonation dissolving limestone, weakens it further.

Once the rock is weakened, mass movement moves the material downslope under gravity. Rockfalls drop loosened blocks from steep cliffs, while landslides and mudflows occur when the cliff becomes saturated after heavy rain and a mass of material slides or flows to the base.

Meanwhile the sea erodes and removes the debris from the cliff foot, undercutting and steepening the cliff. This removal makes the cliff unstable again, triggering further weathering and mass movement.

Cliff retreat is therefore the combined result of sub-aerial processes attacking the face from above and marine erosion removing material from the base — neither acting alone.

Coastlines are shaped by two broad sets of processes: marine processes, driven by waves (erosion by hydraulic action, abrasion, attrition and solution, together with the transport and deposition of sediment), and sub-aerial processes that act on the cliff above the waterline (weathering such as freeze–thaw, salt crystal growth, carbonation and hydrolysis, and mass movement such as rockfalls, landslides and mudflows). This essay assesses their relative importance and argues that, while marine processes are the most powerful agents on high-energy coasts and provide the energy that ultimately drives cliff retreat, sub-aerial processes are essential partners — and on weak, well-jointed rock or in certain climates they may do most of the visible work — so neither can be ranked first in isolation.

Marine processes are clearly powerful and, on many coasts, dominant. Hydraulic action and abrasion attack the base of a cliff, undercutting it to form a wave-cut notch; once the overhang collapses, the cliff retreats. Destructive, high-energy waves erode beaches and, through wave refraction, concentrate energy on headlands to carve caves, arches and stacks. Marine transport — above all longshore drift — then moves the eroded sediment along the coast, building spits and bars elsewhere, so the sea both erodes and redistributes material. The rapid retreat of the Holderness coast, where weak boulder-clay cliffs are battered by high-energy North Sea waves and the debris is swept away by strong longshore drift, shows marine processes operating at speed. Because the sea removes the debris from the cliff foot, it controls the pace at which the whole system can work — without removal, eroded material would simply pile up and protect the cliff.

However, sub-aerial processes are far more than a minor influence, and on some coasts they shape the cliff more than the waves do. Weathering weakens the rock face in place: freeze–thaw and salt crystal growth break up well-jointed rock, while carbonation and hydrolysis chemically weaken limestone and granite. This prepared, weakened material is then moved by mass movement — slumping is common where permeable rock overlies impermeable, saturated clay, and mudflows and rockfalls reshape the cliff profile from above. On a coast of weak, jointed rock in a wet or freeze–thaw climate, the cliff profile (its angle, slumped terraces and debris aprons) is largely the product of sub-aerial activity; marine erosion merely clears the base. The gentler, slumped cliffs of soft-rock coasts contrast sharply with the steep cliffs of resistant rock, a difference that reflects sub-aerial as much as marine processes.

The most accurate assessment is that the two sets of processes are interdependent and operate together, so their relative importance is context-dependent rather than fixed. Marine erosion undercuts and steepens the cliff and removes the debris, which destabilises the face and accelerates sub-aerial weathering and mass movement; the sub-aerial processes in turn deliver loosened material to the base for the sea to remove. The balance shifts with conditions: marine processes dominate on high-energy, storm-exposed, macro-tidal coasts of resistant rock, where wave attack is relentless; sub-aerial processes dominate on sheltered, low-energy coasts of weak, well-jointed rock in wet or freezing climates, where weathering and slumping outpace wave attack. Rock type, climate, tidal range and storm frequency therefore decide which set leads at any one place.

In conclusion, marine processes provide the energy and the removal mechanism that ultimately drive coastal change, and on exposed, high-energy coasts they are the more important. But sub-aerial processes are not subordinate everywhere: on weak, jointed rock and in wet or freeze–thaw climates they shape the cliff profile more than the waves do, and in all cases the two work as one system. The relative importance of marine and sub-aerial processes is therefore not fixed but depends on the rock, the climate and the wave energy of the particular coast — and any answer that simply ranks one above the other oversimplifies a coastline that is the product of both.

The rate at which a coastline erodes — how quickly cliffs retreat and beaches are cut back — depends on the energy attacking the coast and on the resistance of the material being attacked. Wave energy, set by fetch, wind speed and wind duration and concentrated by wave refraction, is one key control; but rock type and structure, sub-aerial processes, sediment supply and human activity also influence the rate. This essay assesses the importance of wave energy and argues that, although it supplies the force that does the eroding, the rate of erosion is determined just as much by the resistance of the rock and the other factors — so wave energy is necessary but not, on its own, the most important factor.

Wave energy is undeniably central. High-energy destructive waves, generated by strong winds over a long fetch, deliver hydraulic action and abrasion with great force, especially during storms when a single event can cause years' worth of retreat. Refraction concentrates this energy on headlands, and a coast facing the prevailing wind across thousands of kilometres of open ocean — such as an exposed Atlantic coast — erodes far faster than a sheltered coast with a short fetch and low-energy, constructive waves. The clear link between high wave energy and rapid retreat strongly supports the statement, and explains why storm frequency is such a good predictor of erosion rates.

Yet identical wave energy produces very different rates on different rocks, which shows that resistance matters at least as much. Weak, unconsolidated boulder clay at Holderness retreats rapidly, whereas resistant granite headlands subjected to the same Atlantic waves erode extremely slowly. Rock structure is also decisive: well-jointed, faulted or steeply dipping rock gives the sea lines of weakness to exploit, so it erodes faster than massive, unjointed rock of the same hardness. The same energy therefore yields fast erosion on weak, jointed rock and negligible erosion on strong rock — so the material, not just the energy, controls the rate.

Several further factors modify the rate. Sub-aerial weathering and mass movement weaken the cliff face and supply debris, speeding retreat where the rock and climate favour them. Sediment supply matters too: a wide beach absorbs wave energy and protects the cliff, so where longshore drift or river input keeps a beach well supplied, erosion slows — and where sediment is starved (often by human defences updrift), the cliff is exposed and erodes faster. Human activity cuts both ways: sea walls and groynes slow erosion locally but can accelerate it downdrift by interrupting the sediment cell.

The most accurate assessment is that wave energy supplies the FORCE, but the RATE of erosion is the outcome of that force acting against the coast's resistance and protection. The two cannot be separated: the highest erosion rates occur where high wave energy meets weak, jointed rock with little protecting beach, as at Holderness; the lowest occur where even high energy meets resistant rock or a wide, energy-absorbing beach. Wave energy is therefore a necessary condition for rapid erosion but not a sufficient one on its own.

In conclusion, I agree only partly with the statement. Wave energy is the most important single source of the force that erodes a coast, and where it is high, erosion can be very rapid. But the rate of erosion is determined by the interaction of that energy with rock type and structure, sub-aerial processes, sediment supply and human management. Wave energy is best seen as the driver whose effect is governed by the resistance it meets — so it is the most important factor only when the rock is weak and unprotected, and the statement overstates its role as a universal control.