Coastal cities are susceptible to the impacts of waves, flooding, storm surge, and sea-level rise. In response to these threats, coastal jurisdictions have invested in engineered shoreline defenses such as breakwaters and sea walls that are costly to implement and maintain. Thus, there is an increasing recognition that nature-based defenses provided by healthy ecosystems like coral reefs can be an effective and cost-efficient alternative to mitigate the impacts of climatic hazards while simultaneously restoring ecosystem services. Unfortunately, coral reefs have experienced degradation worldwide, lowering their potential for wave-energy dissipation. As coastal vulnerability increases with the loss of natural barriers, it is imperative to design and test novel resilience solutions. Our study quantifies the benefits of hybrid artificial reefs for wave mitigation in a wave-tank simulator using periodic waves of three heights (0.10, 0.16, and 0.24 m) at two water levels (0.55 and 0.65 m) defined considering the Froude similarity with a prototype reef structure in South Florida. Experiments showed that an artificial trapezoidal reef model reduces wave height (> 35%) and wave energy (up to 63%) under realistic wave conditions. Moreover, adding coral skeletons of Acropora cervicornis to simulate reef restoration onto the model mitigates up to an additional 10% of wave height and 14% of wave energy through increased friction, supporting the use of hybrid approaches that integrate both gray and green infrastructure to enhance coastal resilience. Exploring wave-tank simulations provides a better understanding of wave effects before implementing larger and more costly projects in the field.
Nearly 40% of the global population lives within 100 km of a shoreline, putting billions of people and infrastructure at risk from coastal hazards such as waves, storm surge, shoreline erosion, and flooding (Small 2003; Agardy and Alder 2005; Storlazzi et al. 2019). As global climate change escalates, the threats to coastlines intensify, with extreme storm events anticipated to increase in severity with rising ocean temperatures driving sea-level rise and leading to increased breaching and urban flooding (Emanuel 2005). The impacts of Hurricanes Harvey, Irma, and Maria in 2017 included billions of dollars in damages and the loss of thousands of lives in the U.S. and the Caribbean. These events highlighted the urgent need to develop and test effective and cost-efficient methods to reduce hurricane impacts on coastal communities. Protecting coastal populations and economies from such impacts has become a scientific, social, cultural, and political focus in the U.S. around the world (Storlazzi et al. 2019). The coastline of South Florida is one of the most susceptible in the U.S. to the impacts of climatic hazards due to its large population, low-lying topography, high value of built infrastructure, and location within the tropical storm belt of the Caribbean (Beck et al. 2018). For example, the City of Miami Beach alone is projected to lose approximately 100,000 cubic yards of beach sand to wave-related erosion every year (Ousley et al. 2014). The costs of beach renourishment to compensate for this loss can reach hundreds of millions of dollars.
These concerns have led coastal jurisdictions to invest in shoreline defenses through gray infrastructure projects such as building breakwaters, sea walls, raising streets, and deploying pumps to mitigate the impacts of waves, storm surge, and flooding. This type of gray infrastructure is costly to implement and maintain. However, there is an increasing recognition that green, nature-based defenses provided by coastal ecosystems like coral reefs, mangroves, and seagrasses can be an effective and cost-efficient alternative to gray infrastructure in mitigating the impacts of climatic hazards while simultaneously restoring ecosystem services and ecological function (Guannel et al. 2016). For example, coral reefs, in addition to being hotspots of biodiversity, attractions for tourism and recreation, and essential fish habitat (Moberg and Folke 1999), protect the coastline as natural submerged barriers, reducing wave height and dissipating energy (Narayan et al. 2016). On a global scale, healthy coral reefs have been estimated to dissipate, on average, 97% of wave energy (Ferrario et al. 2014). Coral reefs found closest to areas of high human population have the highest predicted shoreline defense scores, providing significant flood-protection savings for people and property. Indeed, the loss of 1 m in reef height and relief may double the global built capital at risk from flooding during storm events (Beck et al. 2018).
Unfortunately, coral reefs have experienced significant losses over the past few decades and the average cover of corals on Florida reefs now rarely exceeds 10% (NOAA 2018). As coral abundance declines, there is a general trend toward flatter reefs with less relief, lower frictional components, and therefore lower rates of wave energy dissipation (Sheppard et al. 2005; Alvarez-Filip et al. 2009). This is a source of concern for coastal communities in Florida and elsewhere because an increase of just 5% of wave energy reaching the shoreline can result in an increase of 0.3 m of flood depth (van Zanten et al. 2014). In response to this decline and the importance of reefs to the local economy, the field of active coral reef restoration has expanded exponentially in the past 10 years. Stony corals are now propagated in both in-water and ex situ (land-based) coral nurseries and 10,000s of nursery-grown corals are being planted on Florida reefs each year, restoring both coral cover and reef topography (Lirman and Schopmeyer 2016). While the immediate target of coral reef restoration is to recover depleted coral populations and associated fisheries resources, the restoration of reef topography to increase community resilience to climatic hazards is becoming an attractive possibility (Reguero et al. 2018). However, in order to consider coral reefs as part of the coastal infrastructure protecting the shore, their direct effects on wave action must first be quantified.
In this study, we conducted experiments using the University of Miami's Surge-Structure-Atmosphere-Interaction (SUSTAIN) wind-wave tank. We measured the wave dissipation produced by a submerged artificial reef structure, both with and without coral skeletons of Acropora cervicornis, a branching coral species commonly grown in Florida's coral nurseries. These experiments allowed us to quantify the energy dissipation of hybrid artificial reefs that combine gray (cement-based) and green (nature-based) infrastructure. In particular, we measured the incremental benefit in energy dissipation offered by adding corals to cement-based substructures. This information can be used to inform the design and deployment of efficient reef restoration plots as well as artificial reef structures that merge both gray and green infrastructure for the purpose of wave mitigation and coastal protection.
Materials and procedures
Wave-energy dissipation by hybrid artificial reef structures and their relationship to wave parameters was investigated in the SUSTAIN facility at the University of Miami. SUSTAIN is a wind/wave tank capable of generating winds up to Category 5 strength on the Saffir-Simpson scale, combined with waves at a specified spectrum. The SUSTAIN flume measures 23 m L × 6 m W × 2 m H, allowing the physical testing of coral reef models at relevant scales under controlled wind-wave-surge conditions. Waves generated using a 12-paddle system can be manipulated in frequency, wavelength, and amplitude with variations of water level. In this study, the reduced-scale physical model of an artificial coral reef was tested considering periodic waves of three heights (0.10, 0.16, and 0.24 m) and two water levels (0.55 and 0.65 m).
The wave-energy dissipation caused by coral reefs is attributed to a combination of wave breaking and friction (Beck et al. 2018) and depends on environmental parameters, such as hydrodynamics and depth, and physical parameters such as the reef profile, size, and configuration (Spieler et al. 2001). Although the hydrodynamic effects of coral reefs have been well-characterized by engineering models (Hoeke et al. 2011; Quataert et al. 2015; Reguero et al. 2018), there have been few experimental studies at relevant scales. While Gourlay (1996a,b) studied the setup and wave-generated flow on an idealized two-dimensional horizontal coral reef, the frictional effects induced by the reef were omitted. Here, to separate the effects of wave-breaking and coral friction, physical testing was conducted on two model configurations: (1) breakwater only and (2) breakwater with corals. The models (breakwater and corals) were constructed on a 1 : 5 scale, representative of a realistic reef profile and staghorn colony height along the shoreline of Miami Beach, Florida. Testing the breakwater structure in the absence of corals captured the effect of wave-breaking due to wave shoaling as the water depth decreased over the structure under negligible friction. The trapezoidal breakwater was made out of wood with a 1 : 1 forereef slope, a height (bottom of the flume to the reef flat) of 0.30 m, a horizontal reef flat of 0.30 m length, and a 1:1.73 backreef slope. In the second configuration, the breakwater structure was populated with staghorn coral skeletons to capture the combined effects of wave-breaking and coral friction on a model restored reef.
|Wave frequency (Hz)
|Wave height (m)
|d = 0.55 m
|d = 0.65 m
|d = 0.55 m
|d = 0.65 m
The maximum relative wave-height reduction observed for the artificial coral reef model was approximately 39% of the incident wave height recorded for condition C (wave frequency of 0.7 Hz, height of 0.24 m, water depth of 0.55 m) (Fig. 2a). The relative wave-height reductions by the breakwater and the corals at this condition were ~ 29% and ~ 10%, respectively. Corals enhance the wave-reducing capacity of the breakwater through friction (even at depths > 5× the coral height), with their contribution depending on the wave conditions and water depth. However, in most cases it is the breakwater substructure that contributes most to the wave-reducing capacity of the reef, typically through wave breaking. The wave-height reduction attributed to the corals varies from approximately 3–10% of the incident wave height. The maximum relative wave-energy reduction observed for the artificial coral reef model was ~ 64% of the incident wave energy recorded for condition C (wave frequency of 0.7 Hz, height of 0.24 m, water depth of 0.55 m) (Fig. 2b). The relative wave-energy reductions contributed by the breakwater and the corals at this condition were ~ 50% and ~ 14%, respectively. Overall, the wave-energy reduction attributed to the corals varied from 5% to 14% of the incident wave energy. When estimated as a proportion of the total energy reduction caused by the hybrid artificial reef, corals contributed 15–36% of the total reduction of energy (Table 2). Their average contribution was higher in shallow water conditions (d = 0.5 m). Given that the wave-energy reduction mechanism remains the same between the two configurations of the reef model (no wave breaking was induced by the presence of corals on top of the breakwater model), the contribution of corals was attributed to friction.
|d = 0.55 m
|d = 0.65 m
Structurally complex coral reefs provide a much-needed line of defense against the impacts of waves and storms (Storlazzi et al. 2019). The scenarios tested in the wave simulator showed clearly that submerged structures such as reefs and artificial breakwaters can play a key role in coastal protection by reducing both wave height (> 35%) and wave energy (up to 63%). These findings thus reflect the conservative benefits of reefs at intermediate depths as their ratios of water depth-to-wavelength are between 0.05 and 0.5. It is projected that, in shallow water conditions (ratio of water depth-to-wavelength < 0.05), the protective effects of the coral reefs will be significantly higher due to an increased interaction of the waves with the reef, in agreement with previous numerical modeling studies reporting higher (> 90% wave reduction) values (Ferrario et al. 2014). It should be noted that while scaling is required for properly defining the wave conditions between a real-world prototype structure and the artificial coral reef model, relative wave-height reduction and wave-energy dissipation results can be directly applied without scaling as they represent nondimensional parameters.
The wave-energy reduction provided by the artificial reef model varied based on its geometrical characteristics and the hydrodynamic conditions tested, with the reduction being proportional to the structure height as well as the incident wave amplitude and wavelength. The main dynamical constraint on the wave-energy moving over the structure is that the waves passing over the corals “feel” the structure. This can happen in two ways, through the decrease in the depth over the structure and through frictional dissipation due to wave orbital velocity shear above the rough surface of the structure. The depth effect is typically scaled by the ratio of water depth over wavelength (d/L), and if d/L < 0.5, the wave is strongly affected by the bottom. The presence of the bottom leads to an increase in wave height known as shoaling and this increases with decreasing depth until the wave breaks. Wave breaking leads to significant wave height reduction and transfers energy to turbulence and wave run-up. The addition of corals to the structure is typically treated only through enhanced frictional dissipation which acts through the shear of the near-bed wave orbital velocity. This velocity depends on the penetration of the wave in a similar manner as the depth effect discussed above, but it also depends on the wave height. A key aspect of the frictional dissipation that is different from shoaling is that it is expressed per unit length (i.e., a longer distance of travel will lead to greater reductions in total wave height).
As coastal vulnerability increases with the loss of existing natural barriers, expanding coastal populations, and predicted increases in the frequency and intensity of extreme weather events (Crossett et al. 2004; Emanuel 2005; Storlazzi et al. 2019), it is imperative to design and test novel solutions to enhance coastal resilience. Here, we show the synergistic benefits of combining structures that incorporate gray (cement-based) and green (living, nature-based) components to minimize costs and enhance additional services unique to natural ecosystems, such as fisheries habitat and ecotourism. Moreover, unlike gray structures, natural ecosystems have the capacity to self-build and self-repair, allowing them to continue to accrete and grow after deployment, and have the potential to keep pace with projected sea-level rise. Nature-based approaches are also typically more cost-effective. For example, Ferrario et al. (2014) showed that the median project cost for built artificial structures used for shoreline protection was $19,791 m−2 compared to $1290 m−2 for ecological reef restoration projects.
In this study, we tested the enhanced benefits of adding corals onto artificial reefs and showed that dense coral aggregations made up of branching staghorn corals can mitigate up to an additional 10% of wave height and 14% of wave energy when compared to a submerged artificial reef without corals through increased friction. These experimental findings validate previous studies that report that > 10% of wave energy can be dissipated through coral friction (van Zanten et al. 2014). Reef restoration alone is limited to hardbottom habitat where reefs develop, but these locations may not always be useful to protect coastlines due to depth and distance to shore. By adding nursery-grown corals onto a cement-based substructure can create hard-bottom substrate and expand the range of habitats in which coral restoration could be employed, provided environmental conditions are suitable for coral growth and survivorship. Consequently, deployment of such structures in shallow, nearshore nonreef environments could be exploited for restoration for the purpose of coastline protection.
Wave-tank simulations as presented here open the door to further explore the effects of both ecological restoration and hybrid green-gray approaches on the dissipation of wave energy. Initial testing in a wave tank provides a better understanding of wave effects before implementing larger and more costly projects in the field. This is especially important for the construction and deployment of artificial coral reefs or reef restoration projects that presently lack design guidelines for the purpose of coastal protection, making their implementation challenging. To date, the only example of a structure incorporating active coral restoration deployed for coastal resilience is an artificial reef project implemented on the Caribbean island of Grenada (Reguero et al. 2018). This artificial reef, created by a combination of limestone boulders and metal mesh, has been shown to be effective in reducing wave energy and protecting the shoreline from beach erosion. As the need for such projects grows with coastal development and the increase in the frequency and magnitude of impacts of natural coastal hazards such as hurricanes, the design and implementation of hybrid projects that merge gray and green infrastructure for coastal protection will be a restoration priority.
Comments and recommendations
Integrating our experimental results into large-scale hydrodynamic models (Roelvink et al. 2009; Quataert et al. 2015) can guide the implementation of new artificial coral reefs and active coral reef restoration efforts for shoreline protection. In large-scale hydrodynamic models, reef wave-energy dissipation is typically described using linear wave theory (Beck et al. 2018) with global values assigned for key parameters such as wave height, wave period, and coral friction (Sheppard et al. 2005). Therefore, physical testing of coral reef models with coral populations that mimic natural structures found at the region of interest and wave-surge conditions defined based on local buoy data can inform the local-scale design and deployment of artificial reef structures and efficient reef restoration plots to maximize benefits and minimize costs.
Active coral reef restoration through the propagation and outplanting of corals onto denuded reefs has expanded in scale significantly over the past decade. With this expansion, practitioners have worked to design restoration programs that target the recovery of key ecosystem services such as the recovery of habitat, fisheries stock, and provide tourism and recreational opportunities (Lirman and Schopmeyer 2016). Because of the complex challenges presented in assessing in situ wave effects on a restored reef, coral restoration is seldom implemented with the explicit goal of providing coastal protection services, presenting a substantial gap in restoration science that remains to be explored. Quantifying these potential benefits could lend support for reef restoration along vulnerable shorelines, unlock new sources of funding, and promote the use of restoration in future adaptive management decisions, while simultaneously providing other ecosystem cobenefits and aiding in the recovery of threatened coral populations.
We would like to thank S. Chao, J. Ramaprasad, M. Beck, B. Reguero, M. Rebozo, and J. Amendolara for their contributions to this project. This project was funded by the University of Miami's U-LINK Program, the City of Miami Beach, and the National Fish and Wildlife Foundation. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the opinions or policies of the U.S. Government or the National Fish and Wildlife Foundation and its funding sources. Mention of trade names or commercial products does not constitute their endorsement by the U.S. Government or the National Fish and Wildlife Foundation or its funding sources.
Conflict of Interest