F.E. Roelvink1*, A.R. van Dongeren1 , C.D. Storlazzi2, S.G. Pearson1,3
3 Delft University of Technology, S.G.Pearson@tudelft.
Coral reefs are crucial for the protection of tropical coastal communities from wave-driven flooding and coastal erosion, while also providing other vital ecosystem services. Key to the shoreline protection service of coral reefs is the eﬃcient wave dissipation by wave breaking and friction. At the reef crest, the high-frequency incident sea-swell (SS, 5 - 25 s periods) wave height rapidly decays by wave breaking. The oscillation of the short-wave breakpoint over time generates low-frequency (LF, 25-1000 s) motions that propagate across the reef ﬂat, where friction reduces the low-frequency wave energy. However, coral reef degradation, sea-level rise and the possible storm intensification increase flooding and erosion hazards on low-lying tropical coastlines. The nature-based solution of coral restoration is proposed to satisfy the need for increased coastal resilience. The large spatial scales of coral reefs and often limited funding necessitate an eﬃcient approach in designing and restoring coral reefs for the purpose of coastal hazard risk reduction.
Here we address the effects and efficiency of coral restoration through numerical modelling of theoretical reef restorations on characteristic coral reef profiles. Using the k-means method, 30,000 reef profiles across the USA were clustered into four distinct profile types representing 70% of all surveyed reefs: (1) typical fringing reef profile; (2) convex profile; (3) straight sloping fore reef profile; and (4) three-slope profile with a steep nearshore slope, followed by a relatively horizontal shelf, and an offshore fore reef slope. To investigate the potential of coral reef restoration to reduce flooding, the XBeach model was used to compute wave transformation over the reef types and the subsequent flooding of coastlines by adding theoretical coral reef restorations, represented using increased reef height and hydrodynamic roughness at set locations on the profiles.
Restoration potential is distinctly different among varying profile shapes. The reef flat on fringing and convex reefs acts as a natural wave attenuator, reducing the flood risk reduction effect of potential restorations. The straight and three-slope profile are relatively vulnerable to coastal flooding, but also responsive to reef restoration measures. Average wave runup reductions of appr. 30% occur for the three-slope profile due to the effective reflection at restorations on the horizontal shelf. For the straight, sloping fore reef profile, runup reductions of up to 10% are observed, whereas runup reduction across the typical fringing and convex reef reaches 5-10%, much smaller than reduction across straight reefs when measured in absolute terms. For the typical fringing and convex profile, the restoration location significantly affects the reduction potential, as restorations near the breakpoint can strongly increase the setup across the reef flat. The results provide insight into which restoration locations have the greatest runup reduction potential. By allocating ever- limited funds to well-designed coral restorations, vulnerable coastal areas may receive much-needed support for combating the effects of climate change.
Figure 1 Restored Acropora's colonies at Hatamin Island, Indonesia. Source: Coral Guardian