Waves and tides

Erosion, sediment transport and deposition shape coastal landscapes through their balance. A stretch of coast may lose sediment in a storm, then regain it when conditions calm down. How it develops in the longer term depends on that balance over time.

Wind supplies the energy for most coastal waves. Waves generally become larger and more energetic when winds are stronger, blow for longer or travel across a greater fetch—the distance wind travels across open water. This exposure helps explain the difference between an open coast and a sheltered bay.

Water moving up a beach after a wave breaks is called swash; its return towards the sea is backwash. Constructive waves tend to build beaches because their swash is stronger than their backwash. With destructive waves, backwash tends to be stronger, removing beach sediment. These are contrasting tendencies, not permanent labels attached to particular beaches.

Waves slow down as they enter shallow water. The bending of wave fronts as their speed changes is called wave refraction. It can concentrate wave energy around headlands while spreading it across bays, encouraging erosion on exposed headlands and deposition in sheltered bays.

Driven mainly by the gravitational attraction of the Moon and Sun, tides are regular changes in sea level. The difference between high- and low-tide levels is the tidal range. A large range spreads wave activity across a wider vertical zone. On a gently sloping coast, it also spreads that activity over a wider intertidal area. Tidal currents transport sediment too, particularly around estuaries and narrow inlets.

Wave erosion and sediment transport

The force of water against rock causes erosion known as hydraulic action. When waves enter joints, they can compress trapped air. Repeated changes in pressure weaken the rock and detach fragments.

Abrasion occurs when material carried against a surface wears it away. Waves throw sand, pebbles and larger fragments against cliffs, then drag them across rock surfaces. The sediment being transported acts as a tool of erosion.

Sediment supply and the coastal setting

The material available to coastal processes is the sediment supply. It comes from eroding cliffs, rivers, offshore deposits and shells. Sediment can accumulate where inputs exceed losses; beaches may narrow where losses exceed inputs. A wide beach can absorb wave energy before waves reach a cliff. If the beach is depleted, the cliff is left more exposed.

Lithology refers to the physical characteristics of rock. Resistant rock usually erodes more slowly than weak or poorly consolidated material. Waves can exploit weaknesses along joints, faults and bedding planes, so rock structure matters alongside resistance.

Vegetation traps sediment, while roots bind loose material. This can reduce erosion and help depositional features persist. Intense wave attack, though, may overwhelm that protection.

Subaerial processes

Rather than acting directly through marine action, subaerial processes operate on exposed land. Weathering weakens cliff material. Salt crystallization, for example, can force grains apart; repeated wetting and drying can weaken clay-rich rock.

Material moving downslope under gravity undergoes mass movement. Rainfall can saturate a cliff, making it heavier and less stable and causing slumping or landslides. Weakened blocks may detach as rockfalls. Waves can undercut the cliff and remove fallen debris, allowing renewed attack. Cliff retreat often results from marine erosion working together with processes operating from above.

Cliffs and wave cut platforms

A cliff is a steep coastal rock face. From the base of a retreating cliff, a gently sloping rock surface extends seawards: this is a wave cut platform.

Hydraulic action and abrasion exploit weaknesses near the cliff foot, cutting a wave cut notch. As undercutting continues, the rock above loses support and eventually collapses. Waves then remove or redistribute the debris before attacking the new cliff base. Each cycle moves the cliff inland, leaving a platform in front of it.

Stacks

A stack is an isolated, steep-sided mass of rock standing offshore. It commonly develops as erosion cuts through a headland.

Beaches

A beach is an accumulation of loose sediment along the shore, usually sand, shingle or pebbles. For a beach to form, sediment must be available and deposition must balance or exceed removal.

Constructive waves carry material up the beach. They can build a ridge, or berm, near the upper limit of ordinary swash. During storms, powerful waves may strip away this material, transferring it offshore or along the coast. A beach is a changing sediment store, not a fixed landform.

Grain size affects its shape. Water infiltrates coarse shingle readily, weakening surface backwash, so shingle beaches are generally steeper. Fine sand beaches are generally gentler. Sediment tends to accumulate in sheltered locations; exposed locations are more vulnerable to its removal.

Spits

A spit is a narrow depositional ridge attached to land at one end, with the other extending into the sea or across an inlet.

Littoral drift carries sediment along the shore. At a change in coastline direction, that sediment can continue beyond the land into more sheltered water. Deposition builds a ridge, which lengthens as material arrives. Growth depends on a continuing sediment supply. Strong currents through an inlet may stop the spit from extending fully across it.

Why coastlines advance or retreat

An advancing coastline moves seawards; a retreating coastline moves landwards. Deposition builds land outwards, whereas erosion removes it. Even if sediment supply stays the same, changes in sea level relative to the land can move the coastline.

Relative sea level is the height of the sea surface compared with the adjacent land. When it rises, low-lying land can flood and the shoreline can shift inland. A fall can expose former seabed, shifting the shoreline seawards. For the same vertical change, the shoreline generally moves further horizontally on a gently sloping coast than on a steep one.

Eustatic and isostatic processes

Eustatic change is a change in sea level that affects the oceans globally. Melting land-based ice adds water to the oceans. Warming seawater expands. Both processes can raise sea level, while the growth of land-based ice and cooling of seawater can contribute to a fall.

Isostatic change is vertical movement of the Earth's crust in response to changes in loading. A large ice sheet pushes down the crust beneath it. As the ice melts, unloading allows the land to rise through isostatic rebound. This rise can continue long after the ice has disappeared.

Both processes shape the local outcome. If land rises faster than the sea, relative sea level falls. If sea level rises faster than the land, relative sea level rises. Coastlines can therefore experience different changes even while global sea level is rising.

Relict cliffs and raised beaches

A relict cliff is a former coastal cliff that normal wave action no longer reaches. A raised beach is a former beach left above the present shoreline after a relative sea-level fall.

Fjords

A fjord is a long, narrow, deep marine inlet occupying a glacially eroded valley, usually with steep sides.

Through abrasion and the removal of rock blocks, a valley glacier erodes a deep trough. Once the glacier retreats, a relative rise in sea level floods the valley. Glacial erosion, rather than wave erosion, gives the resulting fjord its depth and steep sides. Many fjords have a shallower threshold near their mouth, where glacial erosion was weaker or deposited material remains.

From coastal sediment to windblown sand

A sand dune is a mound or ridge of sand accumulated by wind. For coastal dunes to form, suitable sand must be available on an exposed area where it can dry. Winds must be able to carry it inland, and there must be places where it can collect.

Waves and littoral drift bring sand to the beach and redistribute it. A wide upper beach supplies sand above ordinary wave action. Low tide may expose more sand, but wind has a harder time moving wet grains because moisture holds them together.

Wind transports dry sand by saltation, in which grains bounce close to the ground, and by surface creep, in which grains roll or slide. Onshore winds carry this material towards the back of the beach. Even a coast with abundant sand may develop few dunes if the sand has little chance to dry or move inland.

Trapping sand and building dunes

Wind slows locally around driftwood, small surface irregularities and pioneer vegetation. Sand collects around these obstacles to form small embryo dunes. With further deposition, they can grow into a foredune ridge behind the beach.

Vegetation helps sand accumulate. Stems and leaves slow the wind; roots bind the sand. Dune grasses such as marram tolerate burial and can grow through newly deposited sand. As plants grow, they trap more sand, allowing further plant growth. This is a reinforcing feedback between vegetation and dune development.

Development inland

Near the beach, young dunes face salt spray and shifting sand, with little moisture or nutrients available. Only tolerant plants establish easily. Farther inland, greater shelter and stability allow a wider range of vegetation to grow.

Organic matter builds up as plants die and decay. As soil develops, it retains more water and nutrients. This supports denser vegetation, which further reduces sand movement. Older dunes are therefore often more stable than newly formed ones, though the exact sequence depends on local climate, sediment and exposure.

Erosion and renewed movement

Dunes remain connected to coastal processes. Storm waves can cut into the seaward dune face, transferring sand back to the beach or offshore. If sediment remains available, later beach recovery and onshore winds may rebuild the dune.

Where vegetation cover breaks, wind can remove exposed sand and enlarge a hollow called a blowout. Sand may then move inland and bury existing vegetation. The balance keeps changing as dunes develop. Coastal processes supply or remove sand; wind redistributes it, while vegetation traps and stabilizes it.