Modelling the formation of coral cays on platform reefs

Abstract

In tropical seas, many platform reefs are found with nearly elliptical or circular shapes submerged just below mean sea level (MSL). Coral cays/ islands form on top of these reefs, possibly as a result of wave refraction, wave-generated currents and sediment transport. Due to climate change and human interventions these diverse, but vulnerable ecosystems are at risk of disappearing. It is therefore of interest to understand how coral cays form. In a previous study, a ray tracing model was used to simulate the process of waves refracting into energy convergence zones (ECZ), where sediment may accumulate and cays may form. In this thesis the advanced numerical wave, flow and sediment transport model XBeach, is employed to examine the effect of additional processes on the formation of coral cays, such as wave breaking, bottom friction and advection of sediment by wave-generated flow. Model simulations are performed for cases of low and high waves and of low and high tidal levels. In the case of low waves, the numerical model reproduces the ECZs as found in previous work. However, on elliptical platforms a secondary low energy convergence zone is not reproduced. The absence of this secondary ECZ on the elliptical platform is likely, due to the absence of diffraction in the model. Low waves together with the wave-generated flow are not able to mobilise the coarse sediment. When the wave height is large, most of the energy dissipates close to the reef-edge, such that only a fraction of the remaining energy flux convergences. However in contrast to the case of low waves sediment is transported to the ECZ. The driving mechanism behind the wave-generated flow that induces sediment transport is the divergence in radiation stress by wave dissipation balanced by the wave-induced water level gradient and bed friction. During high tidal levels the wave-induced set-up is small, but significant during MSL and low tidal levels. For high tidal levels and at MSL the spatial erosion and sedimentation patterns are relative similar, but the intensity is larger during high tide. The conditions in the surf zone during low tidal levels are complex and need further study.