A beach that is broad in August can be reduced to shingle and rock by February. In most cases the sand has not gone anywhere permanent, and the cycle is well understood.
Storm waves move sand offshore
Winter waves are steeper and carry more energy, and they break with enough force to lift sand into suspension and pull it seaward on the backwash.
That material does not disperse. It accumulates just offshore as one or more submerged bars running parallel to the shoreline.
The bars then cause waves to break further out, which dissipates energy before it reaches the beach and protects what remains of it.
Summer waves bring it back
Gentler, longer-period waves in calmer months push sand shoreward rather than pulling it out, gradually rebuilding the beach from the bars.
The process is slower than the winter loss, which is why a beach can be stripped in a single storm and take months to recover fully.
Beaches therefore have a summer profile that is wide and gently sloping and a winter profile that is narrow and steep, alternating year after year.
Longshore drift moves material along the coast
Waves arriving at an angle push sand along the shore in a zigzag, producing a steady transport of material in one direction over time.
Where that supply is interrupted, the beach downdrift starves while the one updrift accumulates, and the change is permanent rather than seasonal.
Groynes, harbour walls and breakwaters all interrupt drift, which is why a structure built to protect one beach often causes erosion further along.
Sediment supply is the long-term variable
Beaches are fed by cliff erosion and by rivers carrying material to the coast, and both sources have been reduced by coastal defences and by dams inland.
A beach with its supply cut can still cycle seasonally while losing volume overall, which is the pattern behind most genuine long-term loss.
Replenishment schemes import sand to compensate, and they work while they are maintained, since the underlying transport processes continue regardless.
What a visitor is actually seeing
A narrow winter beach is usually a healthy beach doing its job, and the exposed rock or shingle beneath is the platform the sand sits on.
Genuine loss is visible over years rather than seasons, showing as retreating dunes, undercut structures and vegetation collapsing onto the shore.
Local authorities and coastal groups monitor these changes closely, and their published information is far more reliable than a comparison with a photograph from a previous holiday.