A beach that looks depleted one season and full the next is behaving normally. Sand moves offshore and back on a seasonal cycle, and permanent loss has a different cause.
Storm waves move sand offshore
Steep, energetic waves pull sand from the visible beach and deposit it in bars beneath the water a short distance out.
This is a protective response rather than a loss, because the submerged bar causes waves to break earlier and reduces the energy reaching the shore.
The beach appears narrower and steeper during this period, which is why coastal resorts look markedly different outside their main season.
Calm conditions rebuild the profile
Gentler waves push sand back towards the shore over weeks, gradually restoring the wide, shallow profile associated with summer.
The cycle repeats annually, and a beach in equilibrium ends each year roughly where it started despite large movements within it.
Judging a beach by a single visit is therefore unreliable, since the same location can present very differently a few months apart.
Longshore drift supplies the sand
Waves arriving at an angle move sand steadily along the coast, so any beach is a point on a conveyor rather than a closed system.
Sand arrives from up-current and departs down-current continuously, and the beach exists because those two flows are approximately balanced.
Rivers and eroding cliffs feed the system, which is why changes far from a beach can affect how much sand reaches it.
Structures interrupt the supply
Harbour walls, groynes and jetties block the along-shore movement, accumulating sand on one side and starving the coast on the other.
The effect is visibly asymmetric and is one of the clearest indications that a beach's loss is structural rather than seasonal.
Dams and river management reduce the sand entering the system in the first place, producing a slower decline across an entire coastline.
Replenishment buys time rather than resolving it
Pumping sand from offshore onto a depleted beach restores the profile quickly and is widely used where tourism depends on the beach existing.
The imported sand enters the same transport system and is carried away at a similar rate, so the operation must be repeated periodically.
Grain size matters to how long it lasts, since sand finer than the original is removed faster, which is one reason results vary between projects.