what does groynes mean?

Answer 1

A groyne is a framework of timber, or now sometimes a low broad wall of concrete or masonry run out into the sea, for the purpose of arresting the washed-up sand and shingle and thus raising a barrier against the encroachment of the sea.

Answer 2

Usually they were low filled-in wooden fences on a beach, running straight out to sea as far as they could go. They would only be put on beaches which could get a particularly damaging combination of wind and wave action. The purpose of the groynes was to stop the sand being washed sideways, and all ending up at one end of the beach.