Long shore drift is the movement of sand and shingle along the shoreline, caused by ebb and flow of waves, and by the wind. This continual movement means a beach in the summer may be sandy. In places where long shore drift is strong, it can wash sand across a bay or river mouth, depositing it to form split. To reduce the beach matter being washed away in this manner, barriers called groynes are often out into the sea.
A groyne (also called as groin in the United States) is a rigid hydraulic structure built from an ocean shore (in coastal engineering) or from a bank (in rivers) that interrupts water flow and limits the movement of sediment. In the ocean, groynes create beaches, or avoid having them washed away by longshore drift. A groyne is normally a straight structure perpendicular to the shoreline. Groynes work by blocking (part of) the littoral drift, whereby they trap/maintain sand on their upstream side. Groynes can have special shapes and they can be emerged, sloping or submerged, they can be single or in groups, the so-called groyne fields. Groynes are normally built as rubble mound structures, but they can also be constructed in other materials, such as concrete units, timber, etc.
Groynes can also be used for river management by maintaining a channel to prevent ice jamming, and more generally improve navigation and control over lateral erosion, that would form from meanders.
They are sometimes used as bridges to prevent bridge scour in rivers. But if a groyne is too large it may trap too much sediment, which can cause severe beach erosion on the down-drift side.
Positive aspects of groyne/groin:
- It traps sediments from longshore drift, forming a beach which attracts tourist. Creates a positive effect on the local economy.
- Coastal erosion on beach is significantly reduced, as the water can only reach the cliff during high tide.
- Low maintenance.
- Lowest cost, little money is spent to build it.
Negative aspects of groyne/groin:
- They prevent beaches down the coast from having sediments deposited there, which can lead to soil erosion. This could also destroy buildings and private land, which will cause property prices to plummet.
- Causes the coast to be ugly and unnatural.
- They can lead to cliff collapse.
- Does not protect coasts from storm-driven waves approaching the shore perpendicularly.
- Protection of the shore by use of one groyne only is most often inefficient. Therefore, shore protection by groynes is designed as a group comprising from a few to tens of individual structures.