Shore protection structures which extend from the beach backshore into the surf zone, perpendicular to the shoreline. A groin is intended to build up an eroded beach by trapping littoral drift or to retard the erosion of a stretch of beach. Often mis-identified as jetties.
Ohio's Department of Natural Resources has a web page with extensive explanations of groins and groin fields (groups of groins along a stretch of coast). The Ohio DNR page includes pictures and diagrams showing accretion and erosion resulting from groin construction, along with descriptions of materials typically used for groins, and description of some issues that can be caused by the use of groins to arrest littoral drift of sand along the shoreline.
For an extensive discussion of groins and other shoreline structures, including how they can affect the beach environment, please see the larger article on Shoreline Structures.