Models to mitigate coastal hazards related to climate change

The effects of climate change are many and varied, and some of them will compound one another. Coral reefs are a very important part of island life; they protect tropical islands from erosion caused by the immense power of ocean waves, and they balance the salinization of drinking water resources which help to sustain life on those islands. Under pressure from climate change and from direct human activity, reefs are losing their ability to fill these crucial roles in island ecosystems. About 30 million people live on low-lying coral islands and atolls and are, therefore, dependent on the ecosystem protection coral reefs provide. At present, some of these islands experience flooding due to wave events a few times per decade. It is expected that this rate of flooding will increase due to the combined effects of sea level rise and coral reef decay. A new paper by researchers from the Dutch independent institute for applied research Deltares and the U.S. Geological Survey provides some guidance to coastal managers in assessing how climate change will affect their coral reefs’ ability to mitigate coastal erosion and over-salinization. 

To gain insight into the effects changing climate conditions are having on coral reefs, the study authors used Xbeach (an open-source wave model). First, field measurements obtained on the Kwajalein Atoll in the Marshall Islands were used to validate the computer model. The model was then used to investigate the effects changing certain reef properties have on water levels, waves and wave-driven runup. Reef roughness, steepness, width and the total water level on the reef platform are all important factors for coastal managers to consider when planning mitigating measures.

The results suggest that coasts fronted by relatively narrow reefs with steep faces and deeper, smoother reef flats are expected to experience the highest wave runup and, thus, the highest potential for island flooding. Wave runup increases for higher water levels, and climate change is expected to continue to raise sea levels. Climate change is also increasing water temperatures, acidity and other factors that are affecting corals and other sea life. Wave runup also increases with lower coral bed roughness. As coral degrades, it becomes smoother. Dead corals are generally much smoother in structure, and do far less to dissipate wave energy. Climate change is not only raising sea levels around the world already, it’s already having a significant negative impact on coral reefs, which in turn minimizes the mitigating effects these corals have on coastal hazards.

Loss of coral cover not only causes increased shoreline erosion but also affects the sparse drinking water resources on these islands; climate change may eventually make these islands uninhabitable for this reason alone. In order to prevent or mitigate these impacts, coastal managers need know to what extent their reef system may lose its protective function so that they can take action.

The research paper, “The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines,” is published as an open-access paper and available online. The new research has been accepted for publication in “Geophysical Research Letters,” a journal of the American Geophysical Union.

About the image: Coral reefs, Kwajalein Atoll, Republic of the Marshall Islands | USGA