Jetty

A jetty is a structure, typically built of rock, concrete, or piling, that extends from the shore out into a body of water, its name drawn from the French jetee, meaning thrown out. Built singly, a jetty acts as a breakwater sheltering a harbor entrance from waves, or as a walkway and navigational guide for vessels; built in pairs at a river mouth or lagoon outlet, jetties narrow and concentrate the current so it scours a channel deep enough for ships and resists the sediment a coastline longshore drift would otherwise deposit across the entrance. Wing dams that pinch a river flow to deepen its channel work on the same principle inland, and jetties are distinguished in engineering practice from a pier, which exists mainly for vessels to berth alongside, and from a breakwater built purely to absorb wave energy rather than to direct flow.