Before embarking on a structural reef restoration project, several considerations should be made to determine whether these methods are appropriate. Restoration projects involving hard structures may be more expensive and riskier than coral gardening and transplantation projects. For instance, if not designed and executed properly, structures may break apart or become dislodged during storms. However, the use of structures can provide benefits not attainable by coral gardening methods alone, such as speeding up natural recovery processes on a damaged reef or enhancing services likes fisheries production and coastal protection. Planning should include working with local partners, including:
- Local municipal or governmental agencies to obtain the necessary permits and environmental impact assessments
- Professionals such as coastal engineers to help in the design and planning process and with constructing artificial structures
- Local communities to reduce potential impacts to the aesthetics of the area, which may be important for the tourism industry
While some of these measures might sound daunting, many of these considerations have been well modeled and developed by the coastal engineering sector. The design and assessment of low crested submerged breakwaters (i.e., reefs) is a practice developed over many decades. Coral reef scientists and managers should work together with engineers to deliver better ecological benefits from these projects.
Coral Reefs as Breakwaters
Coral reefs are effective breakwaters that dissipate wave energy by breaking waves at the seaward edge. As waves move over reefs, the surface of coral reefs causes friction that slows waves down and makes them break or crash. Several factors significantly influence the effectiveness of coral reefs to act as breakwaters, ref including:
- Water depth: reef crests are responsible for dissipating 97% of wave energy on a reef. ref Thus, even small decreases in the height of the reef crest allows higher wave energies to reach shorelines. ref
- Reef morphology: including the shape and slope of the outer fore reef, dimensions of the reef crest and reef flat (length, width), and lagoonal coral heads, patch reefs, and or other formations.
- Reef width: including the reef crest and associated reef flat. Wider reefs dissipate more wave energy. ref Thus, even narrow reef flats can dissipate much wave energy.
- Reef rugosity: or the roughness of the reef surface, creates friction and drag as waves move over a reef that cause waves to break and the energy to be dissipated. Larger coral formations (>30 cm) create more friction and drag than sand or reef pavement. ref For instance, drag over a reef flat may be 10x greater than a sandy area, so corals on the reef crest and across the reef flat are important for dissipating wave energy. ref
Characteristics of Structures
Placement on the reef, materials used, and design are critical factors that may impact the effectiveness of structures for coral restoration. To properly design the size and placement of structures, practitioners should work with project partners to obtain detailed assessments of the existing bathymetry and dynamics of water currents around the coral reef. Natural factors may also dictate where structures are placed, such as the geomorphology of available reef habitat or areas where coral recruitment most likely occurs.
Materials used to create structures will affect the durability, stability, and overall long-term performance of the structures. Different materials are likely to attract or promote the settlement of different marine organisms. Projects should aim to build structures from natural products that support the recruitment of reef-building corals, such as limestone from coral skeletons, coral rubble or sand, or biologically-friendly, man made materials like pH-neutral concrete. ref
Natural reefs have a variety of formations and morphologies that create interstitial spaces, nooks, and crannies. These formations increase reef rugosity that reduces wave energy and promotes biological diversity by providing habitat to smaller species. The design and shape of structures should attempt to mimic natural reef formations. One way to achieve this naturally is to outplant coral fragments directly onto artificial structures.
Below are several companies that create and sell or use artificial structures for coral restoration or coastal defense projects: ref
- Reef Balls (pH-neutral concrete)
- EcoReefs (ceramic)
- BioRock (steel with electro-deposition)
- Mars Symbioscience (rebar in epoxy and coral sand)
Although these structures are increasingly being used in a variety of coral restoration projects, few studies have been done to test their use and effectiveness in promoting coastal defense. However, The Nature Conservancy is currently working in Grenada on a pilot project that tests submerged artificial structures in a shallow, high-energy coral reef environment and will assess the ability of structures to colonize coral reef organisms and buffer wave energy.
- Artificial structure projects require detailed planning and should incorporate professional expertise in their design and construction.
- Transplanting live coral fragments back to the breakwater structure after construction may speed up the colonization process and promote ecological restoration.
- Materials should include natural products that increase interstitial spaces to promote colonization of corals and provide habitat for reef organisms. Structures created to mimic the natural profile, shape, and materials of coral reefs may better promote ecological restoration.
- Large coral formations growing on the reef surface create the greatest friction and are the most important for wave attenuation, so it is important to protect existing corals and mimic natural corals in structural restoration projects.
- Reefs should not be restored in “novel” areas. If reefs did not previously occur in an area, they likely will not survive there now. Poorly conceived projects fail to meet goals, can create hazards, and make it harder to execute well designed projects later.
- Active restoration must be combined with holistic management efforts (e.g., including maintaining water quality, controlling overfishing, and habitat protection) for long-term restoration success of degraded reefs.
- Cultural values and local input should be incorporated into restoration design principles. Lessons from mangrove restoration efforts show how local observations of habitat loss and flooding risk were more important than scientific/economic data in driving policy changes to support their protection/restoration.
- Reef restoration for coastal protection is likely to be more cost-effective in areas with high coastal population/high-valued infrastructure along with coastline value.