Wave Attenuating Fringing Artificial Reefs

Introduction to reefs and coastal erosion mitigation 

Among the most biologically diverse and economically valuable ecosystems, coral reefs are proven to be efficient natural breakwaters and are considered a first line of defence against coastal erosion, storms, and flooding. An estimated 200 million people benefit from reduced risk of flooding due to the presence of reefs (Beck et al. 2022; 67). Economic benefits are widely researched with one reef situated in the US providing 1.8B USD per year in flood risk management to nearby communities (Bridges et al. 2022).  Reefs reduce wave energy by up to 97% by breaking waves and providing friction as they pass over the reef; these benefits are present even in cyclone conditions, where most of the wave energy is dissipated (Beck et al. 2022; 54,55, 57).  

The ecological and social benefits of reefs are equally profound. Reefs are considered “vital” to many communities as they provide a source of “food, shelter, medicine and cultural and aesthetic value” (Beck et al. 2022; 54). Ecologically, reefs have significant impacts on nutrient and sediment transport, as well as providing habitats to a diverse range of marine species. 

Currently, reefs are experiencing rapid decline in their conditions following global sea-level and temperature rise coupled with ocean acidification (Beck et al. 2022; 54). 

Manmade Coastline Protection Structures 

Engineered structures design to mitigate the threat of erosion on coastlines have been deployed for centuries. Sea walls, groins, and revetment are examples of such technology, they have provided significant benefits to coastal communities. However, groins and seawalls can often cause damage when planning is rushed, as well as acting as a barrier to sediment migration, water flow and nutrient transport. Furthermore these structures are not designed to provide ecological benefit (Beck et al. 2022; 64). 

Introduction to Artificial reefs

Artificial reefs are another form of mitigative structure. Their nature incorporated design (NID) provides a multifunctionality which performs similarly to conventional engineering structures with many additional benefits (Geldard et al. 2022). By mimicking natural reefs, artificial reefs reduce wave energy to protect coastlines while also enhancing several ecological processes that benefit the surrounding environment. The porous material and complex design of reef modules aim to provide habitat to marine species such as coral, seagrass, fish, and crustaceans. Sediment and nutrient transport cycles are enhanced rather than reduced by their presence and coastal communities benefit from touristic, aesthetic, and recreative opportunities. Their ability to provide ecological benefit even when used for coastal defence is significant: on page 64 of their report, Beck et al. (2022) notes that “artificial structures immediately increase reef relief and topographic complexity where they are placed, providing direct and short-term wave and erosion reduction benefits”. 

Incorporating concepts into design thinking: NID, NNBF, Systems Thinking

Artificial reef design revolves around Natural and Nature Based Features (NNBF) which provide a multi-disciplinary approach to design that incorporates “collaboration, coordination and partnerships across discipline” (Bridges et al., 2022) to provide wider benefit to communities and the local natural environment. These added benefits that exist outside of erosion mitigation are referred to as ‘Co-Benefits’. Nature Inclusive Design (NID) is a similar term to NNBF which describes design intended to enhance the ecological functioning of the surrounding local environment.   

The benefit of a nature inclusive, multidisciplinary approach to design is that it provides a holistic and resilient solution able to withstand the shocks and stresses of its local environment while providing co-benefits. The ‘NNBF’ design further incorporates a systems approach to maximise the potential of existing natural features which may benefit the local coastline. Understanding how the natural environment, artificial reefs and their co-benefits are all interconnected and can bolster each other’s capability for coastline protection is integral to nature inclusive design and systems thinking (Bridges et al. 2022).

The co-Benefits of an Artificial Reef

Co-Benefits of artificial reefs have been widely documented. Bridge et al (2021) note that reefs  provide fisheries production, habitat biodiversity, recreation, and tourism revenue as well as improvements in water quality. 

Ecosystems

As previously mentioned, artificial reefs can provide multiple ecosystem services; conventional engineering structures have reduced benefits in comparison (Gerald et al., 2022). A lesser-known benefit of artificial reefs are their role in facilitating the establishment of other coastal habitats. Bridges et al, (2021: 80) describe this phenomenon with the example of seagrass beds and dunes, who’s formation relies on wave attenuation provided by the reefs. A separate study notes that by “reducing wave energy and improving water quality, shellfish reefs can provide suitable conditions for salt marshes and seagrass beds” (Bridges et al. 2022). 

Many of these benefits also mitigate erosion, forming positive feedback loops whereby the reef creates secondary effects which themselves actively prevent coastal erosion. Seagrass is a good example of this as it attenuates wave energy and traps sediments, which are both primary goals of the artificial reef modules. These features then act in synergy, reinforcing one another. 

Sediment & Organic growth 

Artificial reefs are significantly more effective due to their ability to provide foundations for a self-sustaining system that can grow over time. This means that as sea levels rise, plant growth rises along with it, removing the need for further manmade additions to the reef to maintain effective outcome. 

Reefs can do so due to calcium carbonate cycles. Reefs accrete calcium carbonate structures due to interactions of calcifying reef organisms which act as habitat engineers (Bridges et al. 2021: 80). The calcium carbonate produced by reefs also act as a source of sand nourishment to nearby beaches, adding multiple layers of protection against erosive forces. 

Socioeconomic

Alongside the socioeconomic benefits form the reduction of coastline damage during extreme weather events, other benefits include commercial and recreational fishing opportunities, aesthetics, tourism revenue, diving attractions, food, medicine, and cultural value brought by reefs.

Picture below: Two divers explore reef modules built by Subcon (an MMA company) off the gold coast.

References 

Beck, M.W. and G-M Lange (Editors). “Managing Coasts with Natural Solutions : Guidelines for Measuring and Valuing the Coastal Protection Services of Mangroves and Coral Reefs.” World Bank, https://documents.worldbank.org/en/publication/documents-reports/documentdetail/995341467995379786/managing-coasts-with-natural-solutions-guidelines-for-measuring-and-valuing-the-coastal-protection-services-of-mangroves-and-coral-reefs. 

Bridges, T. S., J. K. King, J. D. Simm, M. W. Beck, G. Collins, Q. Lodder, and R. K. Mohan, eds. 2021. Overview: International Guidelines on Natural and Nature-Based Features for Flood Risk Management. Vicksburg, MS: U.S. Army Engineer Research and Development Center

Bridges, Todd S., et al. “Coastal Natural and Nature-Based Features: International Guidelines for Flood Risk Management.” Frontiers, Frontiers, 2022, https://www.frontiersin.org/articles/10.3389/fbuil.2022.904483/full. 

Justin Geldard , Ryan Lowe , Scott Draper , George Ellwood, Adrienne Wood, Toby Roe and Matthew Allen. “Performance of Engineered Wave Attenuating Reef Structures” University of Western Australia 2022