Physical Processes on the coast

• Hydraulic action — the FORCE of breaking waves slamming into cliffs; air trapped in cracks is compressed, weakening and shattering the rock.

• Solution (corrosion) — slightly acidic seawater CHEMICALLY DISSOLVES soluble rocks such as limestone and chalk.

Marine erosion:

• Hydraulic action — force of waves on cliffs.
• Abrasion (corrasion) — sand and pebbles scrape rock surfaces.
• Attrition — pebbles in transport collide and become smaller/rounder.

Weathering:

• Mechanical — freeze-thaw, salt-crystal growth, wetting and drying.
• Chemical — acid rain reacting with rock; solution of limestone.

Longshore drift is the movement of sediment ALONG a coast.

Waves approach the beach at an ANGLE set by the prevailing wind. The swash carries material diagonally UP the beach at this angle; the backwash returns it STRAIGHT DOWN under gravity (perpendicular to shoreline). Repeating this pattern produces a zig-zag transport of sediment along the coast in the direction of the prevailing wind.

Why it varies along a coast. Headlands and bays change wave approach angles locally (waves bend / refract around headlands → concentrate energy on headlands). Hard engineering structures (groynes, breakwaters) trap sediment and BLOCK longshore drift, building up beach on one side and starving the other side of sediment. Local wind variation, embayments and offshore bathymetry also affect the angle and energy of approaching waves.

Mechanical (physical) weathering breaks rock apart without chemical change. On coasts: freeze-thaw (water in cracks freezes, expands ~9%, shatters rock — common on UK chalk cliffs), salt-crystal growth (sea spray evaporates, salt crystals grow and split rock), and wetting and drying (alternate expansion and contraction of clay-rich rocks).

Chemical weathering changes the chemistry of the rock. Sea water and slightly acidic rainwater react with limestone and chalk (CARBONATION), dissolving the rock. Iron-rich rocks may rust (oxidation).

Biological weathering involves living organisms. Tree and shrub ROOTS widen cracks in cliffs; SEAWEED and BARNACLES on rocks promote chemical reactions; burrowing organisms (sea urchins, sponges) bore into rock.

Together, these three types of weathering weaken the cliff face IN PLACE; gravity then removes the loose material through mass movement (sliding, slumping).

Coastal cliffs are shaped by THREE families of processes working together: marine (wave-driven), weathering (in-place breakdown) and mass movement (gravity-driven downslope transport).

Marine processes attack the BASE of the cliff. Hydraulic action (force of waves slamming into the cliff) and abrasion (sand and pebbles scraping the rock face) cut a wave-cut notch at the high-water mark. Continued undercutting weakens the cliff base. Solution dissolves soluble rock such as limestone or chalk.

Subaerial weathering attacks the cliff FACE from above. Mechanical weathering (freeze-thaw, salt-crystal growth from spray, wetting and drying) breaks up surface rock. Chemical weathering (acid rain, dissolution of limestone) reacts with the rock surface. Biological weathering (root wedging, burrowing organisms) widens cracks. These processes weaken the cliff face IN PLACE.

Mass movement removes the weathered material. Once the cliff base is undercut and the face is weakened, gravity drives sliding, slumping (rotational failure of saturated clay), rockfalls and soil creep. Material is dumped at the cliff base, where waves remove it through longshore drift.

Named example — Holderness coast (UK). The Holderness coast is largely boulder clay — a soft, easily-weathered material. Marine processes (the long North Sea fetch produces high-energy storm waves) attack the base; subaerial weathering (wetting-drying, salt spray) prepares the surface; and slumping dominates as the rotational failure of saturated clay cliffs is the main mass-movement process. The result is one of the fastest-eroding coastlines in Europe at ~2 m/year on average, accelerating to ~10 m in major storms. Settlements (Mappleton, Skipsea) have lost roads, farmland and houses; Mappleton in particular has been protected since 1991 with hard-engineering groynes + revetments. Holderness shows all three process families working together to produce rapid cliff retreat.

Case for wave action being most important. Wave action sets the cliff RETREAT and produces all the major depositional landforms (beaches, spits, bars). Hydraulic action and abrasion attack the cliff base and undercut it; longshore drift moves sediment along the coast; constructive waves build beaches; destructive waves erode them. Storms intensify wave action dramatically and produce most coastal change in a few hours (Hurricane Sandy 2012, Cyclone Sidr 2007). On HARD-ROCK coasts (Cornwall granite, Scottish basalt) where weathering is slow, wave action is overwhelmingly dominant.

Case against (other processes matter too). On SOFT-CLAY coasts like Holderness (UK), subaerial processes — especially slumping after heavy rainfall — drive most of the cliff retreat. On CHALK coasts (Sussex Seven Sisters), freeze-thaw weathering produces major rockfalls that waves then remove. Mass movement at the cliff face transports more material than waves can move at the base in some climates. SEA-LEVEL changes (climate change driving rise; or post-glacial rebound) operate over decades and can drown or expose coasts. People (sea-wall construction, groyne installation, beach nourishment) increasingly shape coasts in many places.

Counter-point. Wave action and subaerial processes WORK TOGETHER, not in competition. Subaerial weathering prepares the rock; waves remove it. Without waves, weathered material would pile up at the cliff base; without weathering, waves would have a much harder time attacking the cliff. The CLIFF RETREAT RATE depends on BOTH process families.

Judgement. Wave action is the most important process for SETTING the form of coastal landforms (cliffs retreat backward, beaches form, spits grow). But the RATE of cliff retreat depends on rock type and on subaerial process activity. On hard-rock coasts wave action dominates; on soft-clay coasts subaerial processes dominate. The statement is broadly true as a general claim but oversimplified — wave action is necessary, but not always sufficient, to explain coastal change.

Coastlines around the world look very different from each other not just because of climate or wave energy, but because the underlying ROCK TYPE controls which processes can be effective. Three contrasting cases illustrate the spectrum.

Case 1 — Hard-rock coast: Land's End, Cornwall (UK). The headland is composed of granite, an extremely resistant igneous rock. Marine processes dominate because the granite weathers very slowly. Hydraulic action and abrasion attack the base of cliffs, slowly forming wave-cut notches that take centuries to undermine the cliff. Distinctive landforms include narrow caves carved along joints, sea stacks (Longships) where the rock has been undermined enough to collapse, and a craggy, irregular coastline. Subaerial weathering does occur but is geological in pace. Mass movement is rare. Cliff retreat is millimetres to centimetres per year.

Case 2 — Chalk coast: Seven Sisters / White Cliffs of Dover, Sussex (UK). The cliffs are made of chalk, a sedimentary rock that is moderately resistant but vulnerable to FREEZE-THAW WEATHERING because it is porous. Marine processes (hydraulic action, solution) work on the base, but most cliff retreat comes from MASS MOVEMENT — rockfalls following heavy frost. The 2014 fall at the White Cliffs of Dover involved several hundred thousand tonnes. The cliffs are vertical and white because they lose material in large blocky falls rather than gradual abrasion. Subaerial processes are the limiting factor here; the rate of cliff retreat is ~0.5-1 m/year on average.

Case 3 — Soft-clay coast: Holderness, East Yorkshire (UK). The cliffs are made of boulder clay (glacial till), an extremely weak material that saturates and weakens rapidly with rain. Mass movement (especially SLUMPING — rotational failure of saturated clay) is the dominant process. Marine processes are also active (the long North Sea fetch produces high-energy waves) and constantly undercut the cliff base. Subaerial weathering (wetting and drying, salt spray) accelerates the weakening. Holderness is one of the fastest-eroding coasts in Europe at ~2 m/year on average, sometimes 10+ m in major storms. Settlements have lost roads, farmland and entire villages over historical time.

Comparison and assessment. The relative importance of the three process families systematically shifts with rock type:

• Hard-rock (Cornwall): marine processes DOMINANT, subaerial minor, mass movement rare. Retreat slow.
• Chalk (Sussex): marine processes important, freeze-thaw weathering + mass movement (rockfalls) dominant. Retreat moderate.
• Soft-clay (Holderness): mass movement (slumping) dominant, marine processes important, weathering accelerating. Retreat fast.

The same global wave climate produces these very different rates because of the underlying rock. This explains why coastal management strategies must MATCH the rock context — hard engineering may make sense on a fast-eroding clay coast like Holderness, but is rarely cost-effective on a slow-eroding granite headland.

Coastlines change continuously but at very different rates. EVERYDAY processes (normal tides, constructive waves, weak weathering) act constantly but produce small changes; STORM events (cyclones, hurricanes, North Sea storm surges) are infrequent but produce dramatic changes in hours. The question of which matters MORE in the long term depends on how the rates compound over time.

The case for storms being most important.

1) Wave energy scales as the SQUARE of wave height. A storm with 10 m waves carries ~100× the energy of normal 1 m waves. A single severe storm can deliver more wave energy in 6 hours than a year of normal conditions.

2) Most cliff retreat happens in storms. On the Holderness coast, average annual retreat is ~2 m, but most of this happens in a few major storm events; quiet years see almost no change. Hurricane Sandy (2012) eroded ~15-20 m of beach in a single night along parts of the US east coast — equivalent to ~10 years of normal erosion.

3) Storm surges reach previously safe areas. The 1953 North Sea storm surge raised water levels by ~5.6 m above normal high tide, flooding low-lying parts of the Netherlands and eastern England. Over 2,000 people died and the disaster triggered the Dutch Delta Works programme. Storm surges allow waves to attack cliff faces high above the normal wave zone.

4) Tsunamis (a special case). The 2004 Indian Ocean tsunami transferred enormous energy from a seabed earthquake to the coast, killing ~230,000 people across 14 countries and reshaping vast areas of coast in minutes. Banda Aceh lost much of its coastal infrastructure.

The case for everyday processes being equally or more important.

1) Cumulative small change adds up. Constructive waves deposit sediment day after day, building beaches over decades. A storm may erode in hours what took years to build, but the building continues afterwards.

2) Some landforms require continuous work. Beaches, spits and bars depend on the steady action of longshore drift — they require continual sediment supply, which storms disrupt but do not produce.

3) Weathering is constant background. Freeze-thaw on chalk cliffs, salt-crystal growth on sandstone, biological weathering by roots and barnacles all act continuously. The 2014 White Cliffs of Dover rockfall (~250,000 tonnes) was the climactic event but was prepared by years of frost weathering.

4) Climate change is gradually intensifying everyday processes. Sea-level rise (~3-4 mm/year) is small annually but cumulatively significant over decades. The Maldives faces existential threat from gradual rise more than from any single storm.

5) Some coasts have few storms. Tropical equatorial coasts (much of equatorial West Africa) experience few major storms. Their coastal evolution is dominated by everyday tide and weathering processes.

Comparison via integration over time.

Imagine a coast experiencing 1 m of cliff retreat per year on average. If 80% of this comes from 1 major storm every 5 years, the storm produces 4 m in a few hours and the other 4 years are quiet. The LONG-TERM rate (1 m/year) is the same as if it were spread evenly — but most of the visible change happens during storms. The everyday processes set the BACKGROUND rate; storms produce the DRAMATIC change.

Climate-change consideration. Both processes may intensify. More extreme rainfall and stronger storms are projected, but so is sea-level rise (which intensifies everyday tidal attack). The relative balance is shifting toward MORE storm importance in the 21st century.

Judgement. Both storms and everyday processes are necessary to explain long-term coastal change, but their roles are DIFFERENT:

• STORMS produce the visible, dramatic, rapid change. They are most important in HARD-ROCK coasts where retreat is otherwise minimal, and on populated SOFT-CLAY coasts (Holderness) where storm-driven cliff failure threatens infrastructure.
• EVERYDAY PROCESSES produce the cumulative, gradual change. They are most important in BUILDING landforms (beaches, spits) and on coasts with WEAK STORM ACTIVITY.

In the LONG TERM, both compound to produce coastal evolution. The statement is therefore HALF-TRUE: storms produce more change per event, but everyday processes operate continuously. Coastal management must address BOTH — hardening defences for storms (sea walls, gabions) AND maintaining sediment supply against everyday erosion (beach nourishment, groynes). The 21st century brings BOTH stronger storms AND faster sea-level rise — coastal management will face compounding pressures.