Groyne

Groyne (pronounced groin)

A wall or jetty built out from a riverbank or seashore to control erosion; also known as a spur or breakwater, acting as a "strong, low sea wall".  The US spelling is for all purposes: groin.  The noun plural is groynes.

1580s: Origin contested, some sources claim it’s an alteration of groin, the use based on a vague anatomical similarity.  The alternative etymology, this one linked to porcine proboscis anatomy, traces the origin to the Middle English groyn (snout) from the Old French groign & groin (muzzle, snout; promontory, jutting part) from the Late Latin grunium & grunia, derived from the Classical Latin grunnire (grunt like a pig).  It has troubled etymologists because the use of groin in the sense of "pig's snout" from circa 1300 was obsolete long before the 1580s.  All agree however the link of the Latin and French forms to the colloquial English grunter (a pig).

Groynes are fixed hydraulic structure extending either from a costal shoreline or riverbank which interrupt the natural water-flow, thereby limiting the movement of sediment.  Historically, groynes were made from timber or stone but synthetic materials are now also used.  Ocean groynes tend towards being perpendicular to the shore, extending from the upper foreshore or beach into the water where as riparian structures are often laid at more acute angles.  Groynes may be wholly or partially submerged and the space between two is called a groyne field although they’re generally built in a series, often in tandem with seawalls.

English spelling evolved under many influences and this produced anomalies and absurdities, some of which the Americans corrected and it would be nice if the rest of the English-speaking world could be convinced to adopt the more rational of their innovations.  However, like cheque & check, groin (part of a body or vault) & groyne (a breakwater) share one pronunciation yet more than one meaning so maintaining two different spellings seems useful.  It's one of those examples where the often commendable simplification of US English is not helpful.