open sea
Coastal erosion is the wearing away of land by the sea through wave action, abrasion, hydraulic pressure, and chemical weathering — a natural process that is accelerating in many places due to sea-level rise and changes in storm frequency. When waves approach the shore at an angle, the swash runs up the beach at that angle while the backwash returns perpendicularly under gravity, moving sediment along the shoreline in a zig-zag path called longshore drift. Over decades this process can move millions of tonnes of sand, building spits and bars at river mouths whilst eroding headlands and cliffs elsewhere — as dramatically illustrated by the UK's Holderness coast, which loses up to 2 metres of chalk per year.
This simulation models longshore drift and cliff retreat: adjust wave approach angle, sediment grain size, wave energy, and rate of sea-level rise to watch the shoreline evolve. You can add groynes (low walls perpendicular to the shore) and sea walls to observe how hard-engineering interventions trap sediment upcurrent while starving downcurrent beaches — revealing the unintended consequences that make coastal management so challenging.
What is longshore drift and what drives it?
Longshore drift (also called littoral drift) is the net movement of sediment along a coastline driven by waves breaking at an angle to the shore. The swash — the water rushing up the beach after a wave breaks — carries sand diagonally up the slope in the direction the wave is travelling; the backwash returns straight down under gravity, displacing the sand slightly along the shore. Repeated millions of times daily, this creates a conveyor belt of sediment that can move thousands of cubic metres of material along a beach each year.
How does wave angle affect the rate of sediment transport?
The volume of sediment transported along the shore peaks at a wave approach angle of approximately 45° and is near zero when waves arrive perpendicular to the beach (0°) or parallel to it (90°). The CERC formula (Coastal Engineering Research Centre) quantifies this: Q = K × H^(5/2) × sin(2α), where Q is transport volume, H is breaking wave height, and α is the angle between the wave crest and the shoreline. At 45°, sin(90°) = 1, giving maximum transport; at 0° or 90°, sin(0°) = 0, giving no net transport.
What is a spit and how does it form?
A spit is a ridge of sand or shingle that extends from the land into the sea or across the mouth of an estuary, formed where longshore drift carries sediment past a change in coastline direction. As the coast curves, the current loses energy and deposits its load. Spurn Head in East Yorkshire is a classic UK example, extending 5.5 km into the Humber estuary and continually migrating westward at about 2 m/year. Spits with a curved end ("recurved spits") show where wave refraction around the tip has bent the depositional direction.
Groynes are low walls built perpendicular to the shoreline to intercept longshore drift and hold sand on beaches, typically protecting seaside resorts from erosion. While the upcurrent (uprift) beach widens, the groyne interrupts the sediment supply to downcurrent areas, starving beaches there of material and accelerating their erosion — a problem called "terminal groyne syndrome." Managers must treat the entire sediment cell as a system; groyne fields often need to be supplied with imported sand (beach nourishment) to compensate for this downstream deficit.
A wave-cut platform is the gently sloping, near-horizontal rock surface exposed at low tide at the base of a cliff. It forms as waves erode the cliff base through hydraulic action and abrasion, causing the cliff to retreat landward. The retreating cliff leaves behind the planed-off rock surface. Wave-cut platforms rarely extend more than a few hundred metres offshore because increasing water depth beyond the platform reduces wave energy reaching the cliff, slowing erosion. They are particularly well developed in Britain along chalk coasts such as at Beachy Head.
Higher sea levels push the wave attack zone up the beach and cliff, increasing the frequency with which waves reach cliff bases and reducing the width of beaches that normally absorb wave energy. A 1 m rise in sea level is estimated to cause a 50–200 m inland shift of the coastline in low-lying areas. Current projections under high-emission scenarios (SSP5-8.5) suggest 0.6–1.0 m of global mean sea-level rise by 2100, which could double or triple erosion rates on many currently stable coasts.
Beach nourishment (or replenishment) involves pumping or dumping sand dredged from offshore onto an eroding beach to restore its width and protective function. Bournemouth and Boscombe in Dorset have received over 10 million cubic metres of sand since the 1970s. Nourishment is effective but temporary: a typical scheme lasts 5–10 years before erosion removes the added sand. It is generally cheaper than hard engineering for tourist beaches and avoids the downcurrent starvation problem of groynes, but it requires ongoing investment and suitable offshore sand sources.
Managed retreat (also called coastal realignment) involves deliberately allowing the sea to flood low-lying land behind a current sea wall, creating intertidal salt marsh that dissipates wave energy and provides wildlife habitat. It is most viable where the land has low economic value and where a natural shoreline can be re-established. The UK's Wallasea Island in Essex is the largest managed retreat project in Europe — 115 ha of farmland was inundated in 2006 to create RSPB wetland habitat, with the new salt marsh now providing better flood protection than the old sea wall it replaced.
Coarse sediment (shingle, pebbles) is moved only by high-energy waves and settles rapidly when wave energy decreases, forming steep, well-drained beaches. Fine sand is transported by lower-energy conditions and settles slowly, creating gentle beach profiles. Very fine silt and clay remain in suspension and are deposited only in very sheltered, low-energy environments such as estuaries. The Hjulstrom curve illustrates these relationships: counterintuitively, very fine particles require higher velocities to erode than medium sand, because surface cohesion and electrostatic attraction between clay particles makes them harder to entrain.
England's chalk cliffs — including the White Cliffs of Dover and Beachy Head — erode primarily through wave quarrying at the base (hydraulic pressure forcing water into joint planes) and mass movement triggered by groundwater saturation. Chalk is too permeable to develop large surface drainage, so rainfall percolates through and builds up pore pressure along clay horizons, eventually causing large rotational slides. Beachy Head retreats at an average of 0.38 m/year but this proceeds in large discrete collapses rather than continuous gradual loss.