Groynes are protective structures made of stone or concrete that extend from the shore into the water. They are primarily used to prevent a beach from washing away due to erosion. The first known use of groynes dates back to 1582, and they are also known by other names such as breakwater, bulwark, groin, jetty, and seawall.

In terms of their effectiveness, groynes work by interrupting water flow and limiting the movement of sediment along the coast. This can help to reduce erosion and build up the beach on the updrift side of the groyne. However, this can also lead to increased erosion on the downdrift side, as the natural flow of sediment is disrupted.