Groynes, breakwaters and artificial reefs (2015)
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.
- Stakeholder participation
- Success and Limiting Factors
- Costs and Benefits
- Legal Aspects
- Implementation Time
- Life Time
If an EIA is undertaken, the EU Directive provides for the right to access information and to participate in the environmental decision-making procedures to the public concerned by the project. If a project creates a significant impact on a Natura 2000 site, the ‘appropriate assessment’ of the infrastructure project could include a public participation process, but this is not mandatory. Similarly, the Floods Directive, the Water Framework Directive and the Maritime Spatial Planning Directive establish public participation processes that may include these projects.
A range of stakeholders could be affected by the construction of groynes, breakwaters and artificial reefs: for local communities and landowners, hard defences could negatively impact their property. Hard defences can visually disrupt the landscape, affecting tourism interests, recreational users and other sectors. Waterborne activities can also be adversely affected if the installation of hard structures goes wrong.
Success and Limiting Factors
Artificial structures such as groynes, breakwaters or reefs tend to modify longshore drift, and have adverse effects on adjacent beaches by causing downdrift erosion. In general, to avoid these effects on the coastline, artificial nourishments and/or dune development are often preferable over hard structures such as groynes and breakwaters 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.
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 in estuaries to reduce flows.
Breakwaters provide safe mooring and berthing procedures for vessels in ports. They enhance workability and provide thus higher efficiency in loading and unloading vessels.
Artificial structures also have positive and negative impacts on beach areas and beach users. In general these structures may have a negative impact on landscape values and can create barriers to recreational use of the beach. However, breakwaters and groynes can also generate recreational value: for instance, favourable waves for surfers and safe harbours within breakwaters for marina berths. Detached breakwaters can create extra beach space, and submerged reefs can provide substrates for benthic species (flora and fauna), thus improving biodiversity. Submerged reefs can even become attractive for snorkelling. However, breakwaters can cause side deposition of mud, seaweed 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, but 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 structure dimensions. Costs can be highly influenced by availability of suitable rocks, transport costs to the construction sites and associated costs of beach nourishment, frequently required when building groynes.
In the Netherlands, groynes are estimated to cost about EUR 3,000 to 15,000 per running meter. Breakwaters are estimated to cost about EUR 10,000 to 50,000 per running meter. Artificial reefs are estimated cost around EUR 15,000 to 35,000 per running meter of structure (Deltares, 2014).
According to Scottish Natural Heritage, in 2000 construction costs for rock groynes ranged between GBP 10,000 and 100,000£ (12,500 – 125,000€) per structure. Construction costs of breakwaters are high – GBP 40,000 to 100000 (50,000-125,000€) – but they require low maintenance; for these two structures in particular, beach nourishment costs should be added. Artificial reefs construction costs are relatively high – GBP 20,000 to 60,000 (25,000 – 75,000€)/100m of structure.
The construction of coastal works to mitigate erosion and hard sea defences ‘capable of altering the coast’ fall into Annex II of the EIA Directive (codified as Directive 2011/92/EU): 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 a Natura 2000 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 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, and thus to a deterioration of ecological status. Any projects that do so 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 dikes 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.
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 of 1-5years.