Groynes are wooden or stone fences that are built at right angles to the coast. They trap material transported by longshore drift.
The benefits of using groynes are that groynes create wider beaches which slow the waves. This gives greater protection from flooding and erosion. They're a fairly cheap defence strategy.
However, the drawbacks of using groynes are that they starve beaches further down the coast of sand, making them them narrower. Narrower beaches don't protect the coast as well, leading to greater erosion and floods.
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