Groynes, breakwaters and artificial reefs

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). 

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. 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. 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. This is because groynes do not add sediment to the shoreface but instead distribute the available material differently. 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. Rip marine currents adjacent the groynes can present a hazard to the bathers. They can also cause sediment transportation away from the coastal system, especially during storm events. 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 than 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) parallel or close to the coast. It reduces incoming wave energy, provides a sheltered beach area and shelters vessels from waves and currents. 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). As for groynes, disadvantages of breakwaters are primarily related to interference with longshore sediment transport. When this issue is poorly considered in the design and implementation phases, these structures can cause sediment starvation downdrift. 

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. Artificial reefs mimic some functions of natural reefs, such as protecting, regenerating, or enhancing populations of living marine resources. They can be continuous or segmented.  

Rock size, face slopes, crest elevation and crest width, toe protections and aprons should be designed properly to build groynes, breakwaters and reefs.  The natural characteristics of the sites have to be considered since they may have an important impact on the shoreline. Sand may build up behind breakwaters and artificial reefs to form “salients” (bell-shaped sand deposits). Sand can accumulate enough to connect with the breakwater and form a “tombolo”. This is a stretch of sand developed by wave refraction, diffraction and longshore drift which forms a ‘neck’ connecting the structure to the shore. 

Considering the significant impact these structures can have on the coastal environment, they should only be considered as part of a holistic adaptive management policy. Such a policy should take into account the local characteristics of the specific site where they are built and the potential effects on the whole coast. The construction of groynes, breakwaters and artificial reefs can also be integrated into a beach nourishment programme, complement other green measures and be part of integrated coastal zone management plans.  

IPCC categories

Structural and physical: Engineering and built environment options

Stakeholder participation

Stakeholder participation generally does not play a major role in the technical design and construction of groynes, breakwaters, and artificial reefs, unless they are part of a larger coastal defence plan. However, these measures can raise concern about possible environmental impacts on the marine ecosystem and about the visual impacts on the coastal landscape. Conflicts can arise between various groups and economic sectors: the tourism sector generally benefits from protected beaches or safer touristic moorings, while local communities can be worried about the modification of the coastal landscape, habitat and biodiversity loss and environmental impacts in general. Such conflicts call for an active involvement of all interested actors, including local authorities, coastal communities, tourism operators, research institutions and/or NGOs. 

These defensive structures might be the subject of Environmental Impact Assessments, depending on national laws and regulation. When they are implemented in sites protected under the EU Natura 2000 network, an appropriate assessment considering their potential impacts is usually required. Both processes must ensure the right to access information and formally consult the interested parties. Similarly, the EU Floods Directive, the EU Water Framework Directive and the EU Maritime Spatial Planning Directive call for public participation processes that may include this typology of projects. 

Success and Limiting Factors

In general, groynes are efficient to protect certain parts of the coast and maintain upper beach stability. They are effective on sand and shingle beaches, and in rivers and estuaries to reduce flows. They can provide co-benefits for recreation and tourism, by contributing to the widening of beaches. Breakwaters provide additional co-benefits, such as safe mooring and berthing procedures for vessels in ports. They enhance workability and provide thus higher efficiency in loading and unloading vessels. Submerged reefs, in addition to counteracting the wave strength, can provide substrates for benthic species (flora and fauna), attracting fish and thus improving biodiversity. Submerged reefs can even become attractive for snorkelling, creating new tourism opportunities. 

Groynes, breakwaters and artificial reefs can be combined with a number of other approaches, including green measures as beach nourishment and dune construction and strengthening. They have been applied around the world for many years. Consequently, a broad experience can support their correct design and construction.  

Artificial defence structures also have negative impacts that suggest a careful appraisal before their implementation. Groynes, breakwaters, or reefs tend to modify longshore drift, and have adverse effects on adjacent beaches by causing downdrift erosion. To avoid these effects on the coastline, artificial nourishments and/or dune development are often preferable over hard structures unless there are other needs, such as the safe berthing of ships. However, the extent of the blocking of longshore drift, disturbance of adjacent beaches and degradation of landscape values depends very much on the design, orientation of the structure and the main wave/sediment transport direction at the specific site. 

Artificial structures can also have negative impacts on beach areas and beach users. These structures may have a negative impact on landscape values. Breakwaters can cause side deposition of mud, can retain algae and capture litter or debris from ships, making the beach area both unpleasant and unsafe. Currents around the ends of breakwaters and reefs can be strong and dangerous for swimmers. Submerged reefs are offshore, away from beach users. If they don't function as intended, they can be a hazard for navigation and water sports, such as surfing. 

Costs and Benefits

Construction costs depend significantly on the structure dimensions. Costs can be highly influenced by the selected construction material, local availability of suitable material, amount of the needed material, transport costs to the construction sites, environmental conditions on site and associated costs of beach nourishment. The latter are frequently required to enhance grey measures’ efficacy. Cost assessment should also include the analysis of wave conditions and sea currents as well as sediment transport dynamics. The assessment should be performed by specialized staff in order to properly design the structures. 

According to the estimates reported in UNEP-DHI (2016), purchase and transport of rocks based on transport distance of about 50 km can cost about 25 USD/ton (about 21 euros per ton) while placing costs are about 40 USD/ton (about 34 euros per ton). Additional costs, though rarely considered, are related to the maintenance of these measures in the long period. 

Legal Aspects

The construction of coastal works to mitigate erosion and of hard sea defences ‘capable of altering the coast’ fall into Annex II of the EU Environmental Impact Assessment (EIA) Directive: Member States decide whether projects in Annex II should undergo an EIA procedure, either on a case-by-case basis or in terms of thresholds and criteria. However, this requirement does not affect the maintenance and reconstruction of these works. 

Any infrastructure project likely to have a significant impact on sites protected under the Natura 2000 network (Birds Directive and Habitat Directive) site must be subjected to an ‘appropriate assessment of its implications for the site’ to determine whether the project will adversely affect the integrity of the site. 

The EU Water Framework Directive calls for the Good Environmental Status of Europe’s water bodies, including coastal waters. Coastal defences could alter the hydromorphological characteristics of coastal waters, for example in terms of water flow, sediment composition and movement. This could lead to a deterioration of the ecological status. Any projects with those effects would need to meet criteria set out in Art. 4 of the Directive. The EU Floods Directive (2007/60/EC) provides a legal framework for flood actions and defence. The construction and restoration of coastal defence structures could be part of measures under flood risk management plans. The 2014 Maritime Spatial Planning Directive requires the consideration of the interactions between land and sea, along with maritime activities and adaptation to climate change. Groynes, breakwaters and breakwater artificial reefs systems could affect these land/sea interactions. 

Implementation Time

Groynes, breakwaters and artificial reefs are rather simple, small-scale interventions, which are implemented at the local scale (stretch of a coast in erosion). The simple placement of the material on-site can take short time (from weeks to months). However, the whole process of selecting the best solution, collecting and analysing data on waves, currents and sediment transportation, properly designing the infrastructure and involving the stakeholders in a participated process certainly requires more time (about 1 year). 

Life Time

When constructed, the protective function of breakwaters, groynes and artificial reefs can be maintained for many years, requiring only basic monitoring and maintenance. Breakwaters have a typical design lifetime of 30-50 years. This is the case for most rock structures. Wooden groynes have a lifetime of about 10-25 years; and groynes made of gabions have a lifetime of only 1-5years. 

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