Why Is That Island Attached to the Beach? Waves Bending Around an Obstacle

An island offshore blocks waves arriving from the open sea, so the water directly behind it is sheltered while the water to either side is not. Waves passing the ends of the island bend inwards into that sheltered zone, a process called diffraction, and they arrive from both sides, meeting behind the island and cancelling much of their energy. Sediment being carried along the coast by wave action reaches that low energy zone and stops, because the water no longer has the energy to keep it moving. Deposition therefore concentrates directly behind the island, building a bar outwards from the shore and eventually reaching the island itself.

The same process produces several outcomes depending on how far it goes:

• •A bulge in the beach behind the island, where deposition has begun but not reached it
• •A bar exposed only at low tide, which submerges twice a day
• •A permanent connection above all tides, carrying road or settlement
• •A double form, where bars run to both ends of the island enclosing a lagoon
• •A complex form where several islands are linked in sequence
• •A drowned form, visible only in the seabed, from a period of lower sea level

Several are large enough to carry towns. Chesil Beach in Dorset links the Isle of Portland to the mainland with an eighteen mile shingle ridge, behind which lies a lagoon, and the shingle is graded in size along its length in a way that has been studied for two centuries. Mont Saint Michel in Normandy sits at the end of a natural connection that is now supplemented by a causeway, and the silting caused by that causeway was serious enough to prompt an expensive project to restore the tidal flow. Monte Argentario in Italy is joined to the coast by two bars enclosing a lagoon, which is the classic double example. Numerous smaller ones connect islands used as harbours and defensive sites.

Deposition by moving water builds a family of features and distinguishing them clarifies what is happening in each case. A spit extends from a headland into open water, built by sediment carried along the coast and dropped where the shoreline turns, and it ends in the water rather than reaching anything. A bar closes across a bay from one side to the other, cutting off a lagoon behind it. A cuspate foreland is a triangular projection built where two directions of sediment movement converge. A barrier island lies parallel to the coast with a lagoon between, and is not attached at either end. Each is built by the same process of sediment moving until the energy carrying it falls, and the shape depends on the geometry that causes the fall.

The process is used deliberately and causes unintended trouble in equal measure. Offshore breakwaters built to protect a stretch of coast create exactly the sheltered zone that causes deposition, so a beach builds behind them, which is frequently the intention and which starves the coast further along. Where the deposition reaches the breakwater, the structure becomes connected to the shore and stops functioning as designed, so engineers size and position breakwaters specifically to encourage a bulge without allowing a full connection. Causeways built to an island for access interrupt water movement and cause silting on one side, which has required expensive remediation at several sites. Predicting the outcome requires modelling the local wave climate rather than applying a rule.

An island shelters the water behind it, waves bend around both ends and meet there losing energy, so sediment carried along the coast is deposited in that zone and builds a bar outwards to the island. The result ranges from a bulge in the beach to a permanent connection carrying a road. Offshore breakwaters create the same effect deliberately, and a bar reaching the structure stops it working as designed.