Effect of Reef Degradation on Sediment Flushing through Reef Gaps: Whitehouse, Jamaica

In tropical environments, coral reefs and sea grasses not only offer a habitat for a diverse range of plant and animal species, but are also capable of attenuating waves, steering currents, protecting the sediment bed, and producing sediment. They therefore play an important role in determining the morphological evolution of the physical environment and offer significant coastal protection during extreme events such as hurricanes. Reefs, especially those in the Caribbean, are severely threatened by, inter alia, bleaching, disease, pollutants, ocean acidification, over-fishing, and sea-level rise. The destruction of these ecosystems directly alters the nature and functioning of the physical environment, which can have several undesirable impacts, such as increased risk of beach erosion and coastal flooding. Reef degradation (reduction in crest freeboard and structural complexity) reduces the ability of reefs to attenuate waves, making back-barrier lagoons more energetic. On the other hand, reduced wave breaking over the reef crest can result in lower setup in the lagoon and lower mean currents. This will have interesting effects on potential sediment export through reef gaps, as higher bed shear stresses (especially in areas where sea grass coverage is reduced) can increase sediment concentrations, but lower mean currents potentially reduce the overall sediment flux. Sediment exported through reef gaps has little chance of returning to the lagoon, and therefore is a major contribution to erosion when the nearshore system becomes destabilized by rapid changes in the biological environment. This work investigates the effect of reef degradation on sediment export through reef gaps using idealised schematic simulations of the Whitehouse lagoon in Jamaica. XBeach is used for the investigations, in which the reef crest freeboard, bed friction, gap configuration and vegetation (sea grass) distribution is varied to determine effects on mean currents and sediment flux. The results will indicate how the long-term stability of lagoon basins (and by extension, shorelines) will be affected by sediment loss through reef gaps. Broader impacts on coastal resilience, especially in developing countries, can help shape policy, for example in estimating the cost/benefit of environmental rehabilitation projects.