Groyne

A groyne, spelled groin in American English, is a rigid coastal or river structure built out perpendicular to the shoreline to interrupt water flow and control the movement of sediment, usually constructed from wood, concrete, or stone. On a coast, a groyne traps sand moving along the shore to build up a beach on its updrift side and reduces wave energy on its downdrift side, slowing erosion there, while in rivers groynes slow bank erosion and help prevent ice jams that could otherwise block navigation. The earliest known groynes were built along the Nile in Egypt more than three thousand years ago, and in England the technique is documented as early as 1582 in connection with improvements to Dover harbour; Brighton built its first wooden groyne in 1713 and its first concrete groyne in 1867. A poorly designed groyne field can starve the beaches downdrift of sediment, a failure known as terminal groyne syndrome, so groyne length, height, and spacing must be carefully matched to local wave conditions.