A beach that was wide in August can be reduced to cobbles by February. In most cases the sand has not gone anywhere permanent, and the same water that removed it will bring it back.
Wave shape decides direction of transport
Waves move sand both up the beach and back down it, and which direction dominates depends on wave steepness. Long, low summer swells push material shoreward and build the beach up.
Winter storms produce short, steep waves that arrive in rapid succession. Water from one wave is still draining seaward when the next breaks, and the backwash carries sand offshore.
The result is a seasonal exchange rather than a loss. Material sits in a bar just offshore through winter and migrates back onto the face during calmer months.
The offshore bar is the missing beach
Sand removed in storms does not disperse across the ocean. It accumulates in a submerged ridge parallel to the shore, usually within sight of the low water line.
That bar performs a function while it sits there. Waves break on it further out, dissipating energy before reaching the shore and protecting the dune behind.
Swimmers encounter it as a shallow patch some way out with deeper water between it and the beach. That trough is also where returning water concentrates.
Beach profiles flatten and steepen
A summer beach is steep and narrow in profile, with a pronounced berm near the top. Winter strips the berm and leaves a flatter, wider surface of coarser material.
Coarse material is left behind because it is heavier and moves less readily. A beach that looks like shingle in winter is showing the fraction the waves could not shift.
The profile change is why beach furniture and access steps get damaged. Structures set into a summer profile end up undercut when the sand level drops.
Longshore drift complicates the picture
Waves rarely arrive exactly parallel to the shore, so sand also travels sideways along the coast. This drift is persistent and moves material in one dominant direction over years.
Groynes, harbour walls and jetties interrupt that flow. Sand accumulates on one side and the beach on the other side starves, which is genuine loss rather than seasonal exchange.
Distinguishing the two matters for coastal management. A beach that thins every winter is behaving normally, while one that fails to recover each summer is losing supply.
Replenishment works with the cycle
Where beaches are nourished with imported sand, the timing is chosen around this cycle. Placing material before the calm season lets natural transport distribute it up the face.
Grain size is matched to the native sand as closely as possible. Finer imported material washes out quickly, while material that is too coarse changes how the beach feels underfoot.
Nourishment is therefore maintenance rather than a fix. The transport processes continue, and a nourished beach needs topping up on a cycle measured in years.