Beaches are the scene of constant renewal, with every wave that breaks on the shore adding and removing material. In extreme conditions a storm can create or destroy entire landscapes before your eyes.
The main culprit behind all the heave-ho is longshore drift.
What is longshore drift?
Longshore drift is a process where sediment-laden waves, blown by prevailing winds, hit the shore
at an oblique angle and create a ‘swash’.
Gravity then pulls the water, pebbles and sand back down the beach’s steepest gradient, generally at
right angles to the shoreline. The passage of every grain of sand describes a zig-zagging, sawtooth pattern along the beach until it reaches another current, an obstacle such as a groyne or a headland.
How are sandbars and spits formed?
Where the angle of the shore changes, the strength of the longshore current dissipates and drops the sediment, which builds up to form a sandbar or spit.
One of Britain’s best examples is at Orford Ness in Suffolk; you may be familiar with its elbow-like form from maps.
Diffraction of waves around a spit leads to hook-like sandbanks – good examples abound on North Norfolk and Solent coastlines – but where they diffract around an offshore island, tombolos form such as St Ninian’s Isle off Shetland and Asparagus Island at Kynance in Cornwall.
Longshore drift affects every beach, but not all spit-like structures are its creation. Barrier beaches such as Slapton Ley in Devon were formed offshore and then rolled into their current position by rising sea levels, stopping when the beach reached higher ground.
In Dorset, the lagoons behind Chesil Beach owe everything to timing – sea levels stopped rising
around 5,000 years ago and halted the movement of the beach on to the land.
Main image: St Ninian’s Isle © Getty Images