What Is Longshore Drift? Sand Walking Along the Beach

The mechanism depends on waves arriving obliquely rather than parallel to the shore, which is the normal condition since waves are generated by wind blowing from whatever direction it happens to blow. The swash, the rush of water up the beach after a wave breaks, travels in the direction the wave was moving, carrying sand and shingle with it at an angle to the shoreline. The backwash then returns under gravity, which pulls it straight down the steepest slope, meaning perpendicular to the shore. Each cycle therefore displaces a particle a small distance along the beach, and the net transport accumulates. The direction is set by the prevailing wave approach, which depends on the prevailing wind and on the fetch, meaning the distance of open water the wind has crossed, so a coast typically has a dominant drift direction that is consistent over years while reversing during storms from another quarter.

Longshore drift constructs several recognisable landforms wherever sediment moving along a coast meets a change in conditions:

• •Spits, long ridges of sand or shingle extending from a headland into open water where the coastline turns, frequently with a recurved end where waves from another direction bend the tip landward
• •Bars, which form where a spit grows entirely across a bay and seals it off, converting the bay into a lagoon
• •Tombolos, where a spit connects the mainland to an island
• •Cuspate forelands, triangular accumulations where drift from two directions converges
• •Barrier islands, offshore ridges that migrate landward over time and that front substantial lengths of some coastlines
• •Beaches themselves, which in most places are not static deposits but rivers of sediment in continuous transit, supplied from cliff erosion and from river mouths

Treating a beach as a sediment conveyor makes the consequences of interrupting it obvious. A groyne, a barrier built perpendicular to the shore, traps drifting material on its updrift side, which builds a wider beach there and starves the coast beyond it, producing accelerated erosion downdrift. The same applies to harbour breakwaters, which frequently require continuous dredging on one side and cause erosion on the other. Sea walls protect the land directly behind them and prevent the cliff behind from eroding, which sounds entirely beneficial until it is noticed that the eroding cliff was the sediment supply for the beaches downdrift, so the wall starves them and the beach in front of the wall itself narrows as waves reflect off it. That pattern, in which a defence transfers a problem rather than solving it, is well documented and is the reason coastal management is now organised around whole sediment cells, meaning lengths of coast within which sediment circulates, rather than around individual settlements.

Approaches have shifted from resisting to accommodating. Hard engineering including walls, groynes, revetments and breakwaters remains in use where the assets behind justify it, and is expensive to build, expensive to maintain and finite in life. Soft engineering, principally beach nourishment, adds sand dredged from elsewhere to maintain a beach that absorbs wave energy naturally, which works and must be repeated indefinitely. Dune restoration and saltmarsh creation provide natural absorption. Managed realignment deliberately moves defences landward and allows flooding of low-lying land, creating intertidal habitat that dissipates wave energy and reduces the length of defence required, which is cheaper in the long run and politically difficult because it means abandoning land. Shoreline management plans now classify each stretch by policy: hold the line, advance the line, managed realignment or no active intervention, and the last category means telling residents their properties will eventually be lost, which is the hardest conversation in the field.

Waves arriving at an angle push sediment up the beach diagonally while gravity returns it straight down, so each grain zigzags along the coast in a direction set by the prevailing wave approach. That transport builds spits, bars, tombolos and barrier islands and keeps beaches supplied. Groynes and breakwaters trap sediment and starve the coast beyond, and sea walls stop the cliff erosion that was feeding downdrift beaches, which is why management now works at the scale of whole sediment cells.