Groynes, breakwaters and artificial reefs

A groyne is a shore protection structure built perpendicular to the shoreline of the coast (or river), over the beach and into the shoreface (the area between the nearshore region and the inner continental shelf), to reduce longshore drift and trap sediments. A groyne field or system is a series of groynes acting together to protect a beach. Rock is often used as construction material, but wooden groynes, steel groynes, rubble-mound and sand-filled bag groynes, or groynes made of concrete elements can also be found. Rock groynes are generally preferred as they are more durable and absorb more wave energy due to their permeable nature. Timber or gabions may be used for temporary structures.Groynes trap sediments from longshore drift so that the coast behind the sand layer is protected from erosion. Their effectiveness depends on their extension into the river or sea. As any other systems acting on the long-shore drift, they can negatively affect the transportation and sedimentation pattern of underflow areas, causing downdrift erosion. Other adverse effects (especially for rivers) can be an increase in current velocity in the constricted flow area, with increasing bed erosion and a deepening of the bed level. Groynes can also be used in estuaries to decrease tidal flow velocities at the shoreline. Rock groynes can be more effective in this case, as wooden groynes tend to reflect energy rather that absorb it. This depends on the type of wooden structures; in general screens are less effective.A breakwater is a coastal structure (usually a rock and rubble mound structure) projecting into the sea that shelters vessels from waves and currents, prevents siltation of a navigation channel, protects a shore area or prevents thermal mixing (e.g. cooling water intakes). A breakwater typically comprises various stone layers and is typically armoured with large armour stone or concrete armour units (an exception are e.g. vertical (caisson) breakwaters). A breakwater can be built at the shoreline or offshore (detached or reef breakwater).Artificial reefs (or reef breakwaters) are rubble mound breakwaters of typically single-sized stones with a crest at or below sea level. They are usually constructed offshore (often parallel to the shore). They are usually less intrusive and (depending on orientation) can have less impact on longshore processes. Similarly to breakwaters, artificial reefs reduce wave energy and protect the beach from erosion. They can be continuous or segmented.To build groynes, breakwaters and reefs, rock size, face slopes, crest elevation and crest width and toe protections and aprons should be designed according to the natural characteristics of the sites as these factors have an important impact on the shoreline. Sand may build up behind breakwaters and artificial reefs to form salients. Sand can accumulate enough to connect with the breakwater and form a tombolo (a stretch of sand developed by wave refraction, diffraction and longshore drift forming a ‘neck’ connecting the structure to the shore). Considering the significant impact these structures have on the coastal environment, they should only be considered as part of a global adaptive management policy, taking into account the characteristics of the specific site and the potential effects on the whole coast. The construction of groynes and breakwaters could also be linked to a beach nourishment programme, and groynes and breakwaters can be used in a protected beach nourishment approach.