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Deltares | R&D Highlights 2015
Climate change effects
on the protection of
coasts by coral reefs
Coral reefs under pressure from climate change and direct human activity may provide tropical islands with less protection from wave attack, erosion and the salinisation of the drinking water resources that help to sustain life on those islands. Deltares collaborated with the U.S. Geological Survey to assess how climate change will affect the potential mitigation of coastal hazards by coral reefs.
Inhabitants of low-lying coral islands and atolls depend on coral reefs for protection. At present, some of these islands are flooded a few times a decade due to wave events. It is expected that the frequency of flooding will increase due to sea-level rise and the decay of coral reefs because the smoother structure of the dead coral dissipates wave energy less effectively. The loss of coral cover not only causes increased shoreline erosion but also degrades the sparse drinking water resources on these islands, and this may eventually make these islands uninhabitable. In order to prevent or mitigate these impacts, coastal managers need to know the extent of the degradation of the protective function of their reefs so they can take action. The present study provides an indication of local reefs’ sensitivity to change.
We used the open-source wave model XBeach to understand the effects of changing conditions on coral reefs. The model was first validated using field measurements obtained on the Kwajalein Atoll in the Marshall Islands in the Pacific Ocean. It was then used to investigate the effects of changing reef properties on water levels, waves, and wave-driven run-up. 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 of the model runs suggest that coasts fronted by relatively narrow reefs with steep faces and deeper, smoother reef flats are expected to experience the highest wave run-up and therefore to be at more risk of flooding. Wave run-up increases with higher water levels (which are to be expected with sea-level rise), higher waves, and lower bed roughness (as coral degrades and becomes smoother), all of which are expected as a result of climate change. Rising sea levels and climate change will have a significant negative impact on the capacity of coral reefs to mitigate the effects of coastal hazards in the future.
Further reading: Quataert et al (2015). The influence of coral reefs and climate change on wave-driven flooding of tropical coastlines, Geophys. Res. Lett., 42, 6407– 6415, doi:10.1002/2015GL064861 ellen.quataert@deltares.nl T +31(0)88 335 8516 ap.vandongeren@deltares.nl T +31(0)88 335 8351 Flood Risk
Reef flat width [m] Fore reef slope
