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About 30 million people live on low-lying coral islands and atolls where they depend on coral reefs for protection against waves. Healthy coral reefs have rough surfaces and complex structures that slow incoming waves. Coral reef health, however, is threatened by the effects of climate change, including ocean acidification caused by increasing carbon dioxide in the atmosphere and ocean, coral bleaching caused by warming ocean temperatures, and smothering and light reduction caused by sediment stirred up by large waves that will become more common as sea-level rise outstrips coral reef growth.

At present, some low-lying tropical islands are flooded by wave events a few times per decade. It is expected that this rate of flooding will increase as sea level rises and coral reefs decay, because the remaining dead corals are generally smoother in structure and do less to dissipate wave energy. Loss of coral cover not only causes increased shoreline erosion but also affects the sparse drinking-water resources on these islands by allowing saltwater to wash inland. Deterioration of freshwater resources may eventually make some islands uninhabitable. 

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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 new paper, titled "The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines," gives guidance on a local reef's sensitivity to change. It was published in August 2015 in Geophysical Research Letters, a journal of the American Geophysical Union.

To gain insight into effects of changing conditions on coral reefs, the study authors used Xbeach, an open-source wave model. The computer model was first validated with field measurements obtained on the Kwajalein Atoll in the Republic of the Marshall Islands in the Pacific Ocean, and was then used to investigate how changes in certain reef properties would affect water levels, waves, and wave-driven runup (the maximum height above sea level reached by the up-rushing wave). Reef roughness, steepness, width, and the water depth above the reef platform are all important factors for coastal managers to consider when planning mitigating measures.

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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 greatest potential for island flooding. Wave runup increases for higher water levels (expected with sea-level rise), higher waves, and lower bed roughness (as coral degrades and becomes smoother), which are all expected effects of climate change. The authors conclude that rising sea levels and climate change will have a significant negative impact on the ability of coral reefs to mitigate the effects of coastal hazards in the future.

The complete citation is:

Quataert, E., Storlazzi, C., van Rooijen, A., Cheriton, O., and van Dongeren, A., 2015, The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines, Geophysical Research Letters, 42, doi: 10.1002/2015GL064861

Learn more about the USGS Coral Reef Project. Learn more about Deltares, an independent institute for applied research in the field of water and subsurface.