Coastal Erosion and Longshore Drift

Swash, backwash and the zig-zag of a sand grain

Waves rarely arrive dead straight onto a beach - the prevailing wind and the shape of the offshore seabed usually send them in at a slant. When a wave breaks, its swash (the sheet of water that runs up the beach) travels up the slope at the same oblique angle the wave arrived at. But the backwash that drains back afterwards is pulled by gravity straight down the steepest line of the beach, perpendicular to the shoreline, not back along the diagonal it came up on. Each cycle therefore leaves a grain of sand a little further along the beach than where it started, tracing a zig-zag path down the coast.

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Longshore drift: a slow conveyor belt

Multiply that single zig-zag by the tens of thousands of waves that break on a beach every day and the effect stops being a curiosity and becomes a bulk transport system: longshore drift. It moves sand, shingle and finer sediment steadily in the direction the waves predominantly approach from, sometimes tens of thousands of tonnes a year along a single stretch of coast. Longshore drift is why many beaches are wider at one end than the other, and why sand disappears from one town's beach only to build up a mile down the coast.

Why waves bend: refraction and the attack on headlands

Wave speed in shallow water depends on depth, so as a wave front approaches an irregular coastline it does not slow down uniformly. The part of the wave nearest a shallow headland slows first while the rest of the wave, still in deeper water off an adjacent bay, keeps moving at speed - so the wave front bends, or refracts, to wrap around the headland. That bending focuses wave energy onto the headland's tip and spreads the same energy thinly across the bay on either side. The result is a self-reinforcing shape: headlands get attacked hardest and erode into cliffs, arches and stacks, while bays receive gentler waves and accumulate the eroded sand as beaches.

shallow-water wave speed:  c = sqrt(g * h)     // h = water depth, g = 9.81 m/s^2
shallower water near a headland -> smaller c -> wave front slows there
                                    and bends (refracts) toward the point

Where the sand ends up: spits, bars and tombolos

Wherever the coastline suddenly changes direction, for instance at a river mouth or the end of a headland, longshore drift keeps pushing sediment forward into open water instead of onto a beach, building a spit - a narrow finger of deposited sand and shingle projecting from the shore. If the far end of the spit curves back toward land, sheltered by the changing wave pattern, it forms a recurved spit or hook. Where drift from two directions meets, or where deposition bridges the gap to a nearby island, the result is a tombolo, a sand causeway connecting the mainland to what was once offshore.

Groynes: fighting the conveyor belt

A groyne is a low wall built out from the beach, roughly perpendicular to the shoreline, specifically to interrupt longshore drift. Sand piles up against its updrift side, building a wider, more protective beach exactly where it is placed. But that trapped sand was in transit to somewhere else along the coast, so the beach immediately downdrift of the groyne receives less sediment than before and tends to erode faster - which is why coastal defence schemes rarely install a single groyne and instead build a field of them, and why the far end of an unprotected coastline downdrift of a groyne field is often the most vulnerable stretch of all.

Frequently asked questions

What actually causes longshore drift?

Waves that approach a beach at an angle push water and sediment obliquely up the beach in the swash, but gravity pulls the backwash straight back down the steepest slope. Each wave cycle therefore nudges sand a little way along the shore in the direction the waves are travelling, and thousands of cycles a day add up to a strong net transport called longshore drift.

Why do headlands erode faster than bays?

Wave energy concentrates on headlands and spreads out in bays because of wave refraction: as a wave front approaches shore at an angle over an irregular seabed, the section nearest a shallow headland slows down first and the wave bends to wrap around the point, focusing its energy there. The same refraction spreads energy over a wider stretch of shoreline in the bay, so sand deposits and the coastline stays gentle while headlands are attacked directly.

Why do groynes sometimes make erosion worse further along the coast?

A groyne traps sand that is moving along the shore by longshore drift, building up a wider beach on its updrift side. But that sand was on its way somewhere else, and the beach immediately downdrift of the groyne is starved of the supply it used to receive, so it erodes faster than before. This is why groyne fields are usually built in a series, and why removing one groyne can trigger fast erosion downdrift of it.

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Coastal Erosion: How a Sideways Wave Reshapes a Shoreline