Groynes are structures built perpendicular to the shoreline of the coast of an ocean or river. They are built from stone, wood, or steel and are used to deflect flows away from the bank, thereby preventing erosion. They are typically shaped like a hockey stick.
There are generally two orientations for groynes that you’ll observe – they’ll be orientated in either an upstream or downstream facing direction. The downstream orientated “hockey stick” design creates an area of low velocity between the groynes where sediment is deposited, while the upstream orientated design may be used to create pool habitat but requires the structure to be keyed into both the bed and bank to remain stable.