Why Are There Two Walls Sticking Out of the River Mouth? To Stop It Silting

A river reaching the sea slows abruptly and drops its sediment, and waves and currents moving along the coast carry further sand across the opening, so an unprotected river mouth silts up and wanders. Two walls running seaward on either side of the channel address both problems at once. They confine the river's flow so it keeps its speed further out and continues to carry sediment away instead of dropping it, and they physically block sand travelling along the coast from crossing the entrance. The channel between them is therefore kept open by the river's own current rather than by dredging.

Several similar structures do different jobs and the names matter:

• •These extend a channel seaward and confine flow through it
• •A breakwater shelters water from waves and need not touch the shore
• •A groyne runs across a beach to trap sand moving along it
• •A pier is built to carry people or cargo over the water
• •A mole is a massive structure enclosing a harbour
• •Many real structures do more than one of these jobs

Blocking sand that was travelling along the coast has consequences on both sides and they are entirely predictable. Sand accumulates against the updrift wall, building the beach out there, which is welcomed locally. The coast on the other side receives nothing, since its supply has been intercepted, and erodes steadily, which is not welcomed at all and frequently requires its own defences or a programme of pumping sand across the entrance artificially. That transfer of a problem from one stretch of coast to another is the standard difficulty with every coastal structure, and it is why decisions of this kind are now assessed across a whole sediment cell rather than at one site.

Construction is dominated by the need to survive waves breaking directly on the structure for decades. The commonest form is a mound of graded rock, with a core of small material, layers of progressively larger stone outside it, and an armour layer of very large blocks that take the wave impact, each layer preventing the one beneath from being washed out through it. Where natural rock of sufficient size is unavailable, shaped concrete units are cast instead, in forms designed to interlock and to dissipate energy in the gaps between them. A solid vertical wall is used in deeper water and reflects waves rather than absorbing them, which causes its own problems.

The structures kill people regularly and the mechanism is specific. Water pushed against the wall by waves has to escape, and it does so as a fast current running seaward alongside the structure, which is a rip current held in place by the wall rather than wandering as it would on an open beach. Swimmers caught in it are carried out quickly and cannot swim back against it. The rocks are also slippery, gapped and constantly wet, and waves break over them without warning in conditions that look calm from the shore. Signage and rescue equipment are standard on well-managed examples and the casualty figures remain high.

Two walls running seaward confine a river's flow so it keeps its speed and carries sediment clear, and they block sand travelling along the coast from crossing the entrance, which keeps the channel open without dredging. Sand then builds up on one side and the far side erodes, since its supply has been intercepted. A fast rip current runs alongside the wall and drowns people every year.