Sand can accumulate enough to connect with the breakwater and form a “tombolo”. Sand may build up behind breakwaters and artificial reefs to form “salients” (bell-shaped sand deposits). The natural characteristics of the sites have to be considered since they may have an important impact on the shoreline. Rock size, face slopes, crest elevation and crest width, toe protections and aprons should be designed properly to build groynes, breakwaters and reefs. Artificial reefs mimic some functions of natural reefs, such as protecting, regenerating, or enhancing populations of living marine resources. Similarly to breakwaters, artificial reefs reduce wave energy and protect the beach from erosion. They are usually less intrusive and (depending on orientation) can have less impact on longshore processes. They are usually constructed offshore (often parallel to the shore). When this issue is poorly considered in the design and implementation phases, these structures can cause sediment starvation downdrift.Īrtificial reefs (or reef breakwaters) are rubble mound breakwaters of typically single-sized stones with a crest at or below sea level. As for groynes, disadvantages of breakwaters are primarily related to interference with longshore sediment transport. A breakwater can be built at the shoreline or offshore (detached or reef breakwater). A breakwater typically comprises various stone layers and is typically armoured with large armour stone or concrete armour units. It reduces incoming wave energy, provides a sheltered beach area and shelters vessels from waves and currents. This depends on the type of wooden structures in general screens are less effective.Ī breakwater is a coastal structure (usually a rock and rubble mound structure) parallel or close to the coast. Rock groynes can be more effective in this case, as wooden groynes tend to reflect energy rather than absorb it. Groynes can also be used in estuaries to decrease tidal flow velocities at the shoreline. They can also cause sediment transportation away from the coastal system, especially during storm events. Rip marine currents adjacent the groynes can present a hazard to the bathers. Another adverse effect (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. This is because groynes do not add sediment to the shoreface but instead distribute the available material differently. 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. Their effectiveness depends on their extension into the river or sea. Groynes trap sediments from longshore drift so that the coast behind the sand layer is protected from erosion. Timber or gabions may be used for temporary structures. They are more durable and absorb more wave energy due to their permeable nature. Wooden groynes, steel groynes, rubble-mound and sand-filled bag groynes, or groynes made of concrete elements can also be found. Rock is often used as construction material. A groyne field or system is a series of groynes acting together to protect a beach. Groynes, breakwaters, and artificial reefs are grey adaptation measures commonly implemented to counteract erosion and wave action in coastal areas (or in some case in rivers).Ī 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.
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