Artificial reefs are shore parallel rock mound structures set part way down the beach face. They may be long single structures or form a series of reefs extending for some distance alongshore. They are submerged for at least part of the tidal cycle, and are therefore less intrusive on the coastal landscape, have less impact on upper beach longshore processes and add a new intertidal habitat to sandy foreshores.
Reefs dissipate part of the incident wave energy before it reaches the dune face, protecting the upper beach from erosion and encouraging deposition. Long structures (sills) reduce wave energy over an extended frontage, resulting in a more stable upper beach and dune face. Shorter, segmented reefs protect short lengths of the shore, allowing erosion to continue elsewhere. The result is an embayed shoreline with upper beach deposits (salients) forming behind the reefs.
As with all rock structures on the shoreline the rock size, face slopes, crest elevation and crest width must be designed with care. Randomly dumped rock with a high void to solid ratio is hydraulically more efficient than placed and packed rock.
Reefs may actually increase shoreline problems if they are used in areas subject to strong nearshore tidal currents. Scour along the seaward face and around the ends of reefs should be monitored, and structure maintenance undertaken prior to failure where beach levels drop.
Reef construction may need to be accompanied by an ongoing programme of beach recycling or nourishment to ensure that sediment redistribution is not unduly damaging to unprotected frontages. Regular monitoring and management are required to establish a successful scheme. Monitoring must include adjacent shorelines as well as those immediately within the reef scheme.
Reefs are of little use within estuaries where currents, rather than waves, are the main erosive force.
Political & social feasibility
Rock structures on recreational beaches should be built with a view to minimising the potential for accidents involving beach users slipping between rocks. As the structures are separated from the shore as the tide rises, and then become submerged, they are potentially hazardous to anyone using them as a perch. Rocks below the level of Spring tides will tend to be covered with marine growth including slippery algae, again forming a public hazard. The submerged reefs will form a hazard for water sports and navigation, and must be clearly marked with appropriate beacons. Wave induced currents around the ends of reefs can be locally strong and a danger to swimmers.
But the reefs will also form a new intertidal habitat, bringing rocky shore communities to a sandy beach. The structures may well prove to be popular with beach users for example for snorkeling or diving activities.
Cost of implementation & maintenance
Costs for reef schemes depend on structure dimensions and spacings, but are genereally considered to be moderate to high and additionally need some shoreline maintenance. They can be heavily influenced by the availability of suitable rock (or other material), transport and the costs of any recycling or nourishment. Work windows are limited to low tide periods and may be influenced by stormy seas. Rock structures can be assumed to have an unlimited life with respect to economic assessments.
Even though this form of defence is intended to give only partial protection to the shoreline the impacts on shoreline processes, intertidal habitats and landscape will still be high, and may be unacceptable in environmentally sensitive areas. Erosion in the lee of the gaps may well continue for several years after construction while a new beach planshape develops. Long, sill type, reefs with no gaps may suffer from a build up of fine sediment, seaweed or other debris along the inshore side - gaps provide a flushing mechanism.
Artificial reefs can reduce wave energy and therefore slow down coastal erosion. The planning and construction of these must be undertaken with much care, because they can impact negatively the coastal system. But the rocks can also from new intertidal habitats. These would not only beneficial for flora and fauna but could also be beneficial for the tourism sector.