Coastal processes

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

The longer the fetch, the more energy the wind transfers to the water, so a long fetch produces larger, more powerful (higher-energy) waves, increasing the potential for erosion.

Longshore drift is the net movement of sediment along a coastline in the direction of the prevailing waves.

It happens because waves approach the beach at an angle (driven by the prevailing wind), so the swash carries sediment up the beach at that same angle. The backwash then returns straight down the beach at 90° under gravity.

Because the up-beach movement is angled but the down-beach movement is perpendicular, each wave shifts the sediment a little further along the coast, producing a zig-zag transport in one net direction.

Mechanical (physical) weathering breaks rock apart without changing its chemistry. The main coastal example is freeze–thaw: water in cracks freezes, expands by ~9%, and repeated freezing and thawing prises the rock apart. Salt crystallisation (salt crystals growing in pores) also acts here.

Chemical weathering alters the minerals in the rock. The key coastal example is carbonation: rainwater and seawater are weakly acidic (carbonic acid) and dissolve calcium carbonate, so limestone and chalk are slowly dissolved.

Biological weathering is breakdown by living organisms — for example plant roots widening cracks, or molluscs/piddocks boring into and secreting acids onto rock.

Hydraulic action is the erosive force of the water itself. As waves break against a cliff, water is forced into cracks and joints, compressing the trapped air. When the wave retreats the pressure is released and the air expands explosively, and repeated cycles weaken and shatter the rock. It is most effective on well-jointed or faulted rock.

Abrasion (corrasion) occurs when waves pick up sand, shingle and pebbles and hurl them against the cliff, wearing it away like sandpaper. It is concentrated at the cliff base where a wave-cut notch develops.

Abrasion is usually the faster process because it uses a 'tool kit' of sediment to physically grind the rock, whereas hydraulic action mainly exploits existing weaknesses. However, hydraulic action can dominate where the rock is heavily jointed and there is little loose sediment to act as abrasive tools.

The resource shows a very large difference in erosion rate that is controlled by lithology (rock type). Coast X, made of soft glacial clay, retreats at about 2.0 m per year, whereas Coast Y, made of resistant granite, retreats at only about 1 mm per year — a difference of roughly 2000 times.

This is because clay is weak and unconsolidated: its particles have low cohesion and it saturates easily, so it is readily removed by hydraulic action, abrasion and slumping. It offers little resistance to wave attack.

Granite is a hard, crystalline, well-cemented igneous rock with strong interlocking crystals and few weaknesses, so it resists the same erosion processes and is worn back extremely slowly.

The pattern demonstrates that, where wave conditions are broadly similar, lithology is the dominant control on the rate of erosion — weak rock can retreat metres per year while resistant rock retreats millimetres per year. Other factors (wave energy, beach protection) modify this, but they cannot make granite erode as fast as clay.

Deposition occurs where waves lose energy and can no longer transport their sediment load, so material is dropped.

• Low-energy (constructive) waves dominate. Constructive waves are low and long-wavelength with a stronger swash than backwash, so they push sediment up the beach and leave it there rather than dragging it back — net deposition.
• Sheltered locations. In bays, estuaries and behind spits or headlands, waves are refracted and lose energy, so the reduced velocity means sediment settles out.
• An abundant sediment supply. Where rivers, cliff erosion or longshore drift deliver plentiful sediment into a low-energy zone, there is material available to be deposited — giving a positive sediment budget and accretion.
• Shallow water and tidal conditions. Friction with a shallow, gently shelving sea bed slows the waves, and slack water at high or low tide allows fine material (silt/mud) to settle, for example on mudflats and salt marshes.

In short, deposition needs low wave energy AND a supply of sediment — where inputs exceed outputs the coast builds outwards (accretion) rather than retreating.

Cliff retreat is not caused by waves alone; sub-aerial processes — weathering and mass movement acting on the cliff face from above — work together with marine erosion.

Weathering weakens the rock in place. Freeze–thaw (mechanical) prises jointed rock apart as water repeatedly freezes and expands in cracks; carbonation (chemical) dissolves limestone and chalk; and biological action (roots, boring organisms) opens up cracks. This weathering reduces the strength of the cliff face and produces loose debris, making the rock more vulnerable to removal.

Mass movement then moves the weakened material downslope. Depending on the geology and water content this includes:

• Rockfall — fragments loosened by freeze–thaw drop from steep, well-jointed cliffs.
• Slumping (rotational slip) — saturated clay rotates downslope along a curved slip plane, common on soft coasts.
• Landslides — blocks slide down a flat slip plane.

Marine and sub-aerial processes reinforce each other. Waves undercut the base of the cliff (a wave-cut notch), removing support so the weathered material above collapses by mass movement; the waves then remove the fallen debris, exposing a fresh face for weathering and undercutting to begin again. This coupling is why cliffs retreat steadily rather than reaching a stable slope.

The rate at which a coast erodes depends on the balance between the energy attacking it (wave factors) and its ability to resist (geology). This answer assesses whether wave energy is the more important control.

The case that wave energy is more important. Erosion needs energy, and high-energy waves erode faster. Wave energy rises with wind speed, storm frequency and, crucially, fetch — a long fetch facing the prevailing wind produces large, powerful waves. On soft, uniform coasts (e.g. glacial till at Holderness, England), erosion rates rise sharply during storm surges and high-energy conditions, and years with more storms show faster retreat. Here, variation in wave energy clearly controls how fast the coast is lost, and beach removal during storms (which exposes the cliff) further shows energy driving the rate.

The case that geology is more important. However, wave energy cannot make resistant rock erode quickly. Lithology sets a ceiling: granite and basalt retreat at ~1 mm per year even on exposed, high-energy coasts, whereas weak clay retreats at 1–2 m per year — a difference of thousands of times that no amount of wave energy can overturn. Geological structure also matters: well-jointed or faulted rock is exploited by hydraulic action, and a discordant coast erodes differentially into headlands and bays regardless of overall wave energy. So the same waves produce utterly different rates depending on the rock.

Judgement. The two factors are not independent, and their relative importance depends on context. On hard-rock coasts, geology dominates — the coast erodes slowly whatever the waves. On soft-rock coasts, geology is broadly constant, so variations in wave energy (storms, fetch, beach loss) become the main control on the rate. The most accurate view is therefore that geology sets the maximum possible rate of erosion, while wave energy determines how much of that potential is actually achieved — so neither is universally 'more important'; it depends on whether the coast is hard or soft.

Whether a beach grows (accretes) or shrinks (erodes) is strongly influenced by wave type, but the statement is only partly correct because sediment supply also matters.

Wave type does exert a major control. Constructive waves are low, long-wavelength waves with a swash stronger than the backwash, so they push sediment up the beach and deposit it — the beach grows and its profile builds up (often in calmer summer conditions). Destructive waves are high, short-wavelength waves with a backwash stronger than the swash, so they comb material back down the beach and remove it — the beach shrinks and steepens (often in stormy winter conditions). So a shift from constructive to destructive conditions can visibly change a beach, supporting the statement.

But sediment supply and the sediment budget also matter. Even with constructive waves, a beach cannot grow if it is starved of sediment — for example where longshore drift is interrupted by groynes or a harbour wall updrift, or where a river's sediment is dammed. This creates a negative sediment budget and the beach shrinks despite gentle waves. Conversely, an abundant supply can maintain a beach even under quite energetic conditions.

Judgement. Wave type is a major short-term control on whether a beach grows or shrinks, and is the best single answer over a season. However, over the longer term the sediment budget (the balance of inputs and outputs) is at least as important, because no wave type can build a beach without a supply of sediment. The statement is therefore broadly true but incomplete — beach change depends on wave type AND sediment supply together.