Coral Reefs Protect Coastal Communities from Wave Energy Impacts

Future climate change is expected to increase the rate and severity of oceanic disturbances. Among the damages caused by disturbances such as tsunamis and storms, they create high-energy waves that damage coastal communities and ecosystems. Coral reefs and other coastal ecosystems such as sea grasses and mangroves have been recognized for their protection against the devastating effects of strong waves. Although structures such as seawalls have been built to protect coastal communities, their cost and short lifespan reduce their effectiveness as long-term solutions. In this study, Villanoy et al. (2011) seek to measure the wave energy dissipation provided by coral reefs while factoring future climate change. Using a model simulation of storm generated waves on a Philippine reef, the team of researchers measured changes in wave energy caused by varying monsoonal wind forcing and storm conditions. The results suggest that extensive reef system in the area helped dissipate wave energy, thus reducing wave run-up on land. Furthermore, a significant reduction in wave energy was observed when accounting for stronger wind and higher sea level, as well as under a non-climate change scenario. This study demonstrates that it is imperative to manage coral reef ecosystems sustainably and promote localized water quality management in order to mitigate the adverse effects on coastal communities.— Cecilia Ledesma
Villanoy, Cesar, David, Laura, Cabrera, Olivia, Atrigenio, Michael, Siringan, Fernando, Aliño, Porfirio, Villaluz, Maya. (2011) Coral reef ecosystems protect shore from high-energy waves under climate change scenarios. Climate Change doi: 10.1007/s10584-012-0399-3.

            Cesar Villanoy collaberated with colleagues at the Marine Science Institute to study the effect of coral reefs on wave propagation. The research focused on Bagacay and Rizal, two barangays located in the Municality of Sorsogon, Phillipines. The computer simulation software DELFT3D- WAVE was used to simulate wave propagation along that area and under different climate change scenarios; factoring in high resolution coastline and bathymetry from digitized maps, the model computes aspects of wind energy such as wave propagation, wave generation by wind, wind field, and water level. Bathymetry data was accessed and digitized from available navigational charts and topographic maps. Additionally, historical wind data was gathered using the QuickSCAT satellite. By measuring changes in wave energy dissipation rates and wave bottom orbital velocities, the simulations are able to determine changes in wave characteristics that are influenced by local wind and storm conditions, changes in relative seal levels, and presence of reefs. The wave orbital velocity near the bottom is used as a measurement of the energy available at the bottom for sediment transport and reworking.
            Results from the wave model simulations clearly demonstrate the importance of the coral reef areas in dissipating wave energy. While the highest dissipation takes place along the reef edges, parts of the coastline not fringed by coral reefs experienced lower energy dissipation. As a result, higher energy dissipation significant reduced the height of incoming waves across the reef edge, and lower energy dissipation areas were impacted by relatively bigger waves. Bagacay and Rizal are oriented along the southeast and northeast in the Pacific Ocean, respectively. The difference in orientation of the two areas caused different patterns in wave bottom orbital velocities. As a result, Bagacay is shown to be more sheltered than Rizal during northeast monsoon. In addition, increasing the wave height from 2 m to 4 m to simulate extreme events also increased the bottom orbital velocity. For Rizal, high bottom orbital velocity found near the town center of Gubat was consistent with observations of eroded beach and exposed tree roots. Meanwhile, significantly lower bottom orbital velocity near Rizal Beach coincided with a relatively stable beach, implying that an increase in wave height by 1-2m will have enormous consequences to wave energy reaching the coastline of Bagacay and Rizal.
             When simulating sea level rise along the edges of the reefs, the wave dissipating effects of the reefs were decreased and a higher proportion of wave energy was able to propagate across the reef and onto the coast. Assuming an increase in wave bottom orbital velocity did not mitigate the adverse effects caused by an increase in sea level. The conclusions from this study suggest that coral reefs contribute significantly to the protection of coastal communities from wave impacts. However, future increases in sea levels will likely hinder the ability of reef ecosystems to serve as adequate wave barriers. The authors stress the need to continue and enhance efforts to manage and protect these coastal ecosystems through sustainable use and preservation of biodiversity and ecosystem functions.

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