Hydrodynamics and sediment flux of hoa in an Indian Ocean atoll

摘要

Detailed hydrodynamic and sediment flux measurements are reported from 11 hoa (shallow cross-reef channels) in the Cocos (Keeling) Islands, Indian Ocean. All hoa exhibit unidirectional oceanside reef to lagoon fluxes. These currents are driven by tides, wave set-up and incident waves, and the presence of islands that generate alongshore gradients so that currents flow toward the topographic low points of hoa entrances in the reef rim. Two functional groups of hoa are identified that extend Chevalier's typology of hoa and possess different process signatures. Sediment-limited hoa (SLH) are efficient conduits of sediment transport. They are located close to the reef rim where incident wave energy propagates across the reef crest into the hoa and is able to entrain sediment. Transport-limited hoa (TLH) are characterized by low rates of sediment transport because incident wave energy is negligible as they are located much further from the reef edge than SLH. In the absence of an entrainment mechanism the oceanside reef flat is laden with sediment. Storm energy increases transport efficiency through hoa. However, the net sedimentation response differs between the two hoa types. Proximity to the reef edge means that frequent storms flush sediment from SLH increasing hoa depth and enhancing the potential for erosion of the conglomerate boundary. In contrast, the greater reef width fronting TLH means that sediment transport from reef flats to the lagoon is only activated during infrequent storms. The contrasting process signatures account for morphological differences between sediment-limited hoa (lagoonward decrease in depth and increase in width), and transport-limited hoa (lagoonward increase in depth). The number and type of functional hoa are found to have significant implications for the rate of lagoon sedimentation in enclosed atolls. In general, an increase in frequency of functional hoa and increased proportion of SLH will promote more rapid rates of lagoon infill.