Groyne
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A groyne (groin in the United States) is a method of coastal defense against erosion. Groynes are structures running perpendicular to the shoreline. They go across a beach and into the sea. Groynes are usually made of wood, concrete or, most commonly, piles of large rocks. The effect of a groyne is to accumulate sand on the updrift side where longshore drift is predominantly in one direction. They are effective at causing the deposition of beach material on the one side, but there is a corresponding loss of beach material on the downdrift side, requiring that another groyne be built there.
Groynes are extremely cost-effective coastal defense measures, requiring little maintenance, and are one of the most common coastal defense structures. However, groynes are increasingly viewed as detrimental to the aesthetics of the coastline, and face strong opposition in many coastal communities.
[edit] In rivers
Groynes, are often constructed (nearly) perpendicular to the river banks, beginning at the riverbank with a root and ending at the regulation line with a head. They serve to maintain a desired channel for the purpose of preventing ice jamming, improved navigation and erosion control. Groynes have a major impact on the river morphology; generally speaking, they cause autonomous degradation of the river. The areas between the groynes are referred to as groyne fields.
