Many well-known beaches are not entirely natural features. They are maintained, periodically rebuilt with sand brought in by dredger or truck because the supply that once replaced losses has been cut off.
Beaches are conveyor belts, not deposits
Sand does not sit still. Waves arriving at an angle move it steadily along the shore, so a beach is a moving volume with material entering at one end and leaving at the other.
A stable beach is one where arrivals and departures balance. The sand present today is not the sand that was there a decade ago.
Supply comes from rivers delivering sediment, from cliffs eroding and from offshore deposits moving inshore, all of which operate slowly and continuously.
Human structures interrupt the supply
Dams trap river sediment in reservoirs, so material that would have reached the coast never arrives. The effect appears at the shore decades after the dam is built.
Seawalls and revetments stop cliffs eroding, which protects property while removing a sediment source the downdrift beaches depended on.
Harbour breakwaters and groynes interrupt the alongshore movement, building sand on one side and starving the beaches on the other.
Storms remove sand quickly and return it slowly
A single severe storm can strip a beach and carry the sand into an offshore bar within hours, leaving the visible beach dramatically narrowed.
In an unaltered system, gentler swell in the following months gradually pushes much of that material back onshore, and the profile recovers.
Where supply has been cut and sea level is rising, recovery is incomplete, so each storm leaves the beach a little narrower than before.
Replenishment is a maintenance programme
Nourishment projects dredge compatible sand from offshore borrow areas or import it by truck, then place and shape it to a designed profile.
The design assumes losses. Placed sand is intentionally over-supplied so the beach can lose material for several years before the next campaign.
Grain size determines how long the work lasts, since sand finer than the native material washes away quickly and coarser material sits at a different slope.
What it looks like from a towel
A recently nourished beach often differs in colour and coarseness from what regulars remember, and shell fragments and occasional debris come with the dredged material.
A steeper drop at the water's edge is common for the first season while waves rework the placed sand into a natural profile.
Works are usually scheduled outside peak season and outside turtle nesting periods, so signage and closed sections in shoulder months are typically nourishment rather than damage.