Groyne

A groyne (in the U.S. groin) is a rigid hydraulic structure built perpendicularly from an ocean shore (in coastal engineering) or a river bank, interrupting water flow and limiting the movement of sediment.

Groynes are generally straight but could be of various plan view shapes, permeable or impermeable, built from various materials such as wood, sand, stone rubble, or gabion, etc.

[2] The newly discovered groynes are located in what are now Egypt (Aswan), but mainly in Sudan, in areas of ancient Nubia, some of them built by the Egyptian overlords and some possibly the work of local Nubians.

[3][4][5] In 1713 the first wooden groyne to protect Brighton's seafront and coastline was built, which had been heavily damaged in the Great storm of 1703, and again in 1705.

[7] A groyne gradually creates and maintains a wide area of beach on its updrift side by trapping the sediments suspended in the ocean current.

If a groyne is correctly designed, then the amount of material it can hold will be limited, and excess sediment will be free to move on through the system.

A poorly designed groyne (too long and not suited to the unique features of the coast) can also accelerate the erosion of the downdrift beach, which receives little or no sand from longshore drift.

They maintain a channel to prevent ice jamming, and more generally improve navigation and control over lateral erosion, that would form from meanders.

Groynes can be permeable, allowing the water to flow through at reduced velocities, or impermeable, blocking and deflecting the current.