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Article for Ocean Sciences 2014

Fine-grained sediment transport on a fringing coral reef flat as elucidated by sediment tracers and oceanographic measurements

Abstract: Poor land-use practices on many high volcanic islands have resulted in increased terrestrial sediment loading to fringing coral reefs around the world, which, in turn, has impacted coral reefs and coastal fisheries. A better understand of sediment dynamics on fringing coral reef flats is important for understanding not only ecosystem health, but also the evolution of high-island coral reef systems. In 2011, 80 kg of fine-grained synthetic coarse-grained silt with magnetic and fluorescent properties were deployed on a fringing reef flat to understand the natural controls on fine-grained terrestrial sediment resuspension and advection. These sediment tracers were tracked by sampling using an array of 24 rare-earth magnets and spatial sampling with a ultraviolet seabed sediment camera system; concurrent oceanographic and meteorologic time-series measurements were also made to understand the forcing and model the response of the tracers. The initial plume following deployment was much more concentrated, with very little – on the order of meters – lateral mixing occurred over alongshore dispersal lengths of 100s of meters. Subsequent resuspension of the seabed deposit resulted in greater lateral dispersal relative to alongshore dispersal than during the initial deployment. The timing of sediment tracer resuspension, advection, and deposition measured and modeled using data from the oceanographic instrumentation matched the patterns revealed by the magnets and the seabed camera. The data and resulting analyses suggest that the integration of traditional oceanographic measurements and sediment tracers are a useful tool to evaluate sediment transport pathways in coral reef environments.

Coastal reef systems, including both tropical coral and temperate rocky reefs, function as protective barriers that shelter much of the world’s coastline from offshore wave energy. Physical processes along these coasts tend to be quite distinct from classic open coast beaches. Hydrodynamics over reefs are generally strongly influenced by steep slopes, complex topography, and large bottom roughness, which result in complex wave transformation processes that also convert wave energy to other forms (e.g., mean flows and infragravity waves). Sediment transport is complicated by the small-scale interactions with reef canopies, as well as internal biogenic sediment production that ultimately make determining sediment budgets in these areas very challenging. While reefs reduce swell wave energy reaching coasts, their role in shaping coastal morphology through short-term processes (e.g., beach erosion) and longer-term changes (e.g., planform shoreline changes) is often unclear. This session aims to synthesize recent advances in this broad multi-disciplinary research area, including hydrodynamic, sedimentologic, and hydrogeologic processes in reef environments and their resulting effects on coastal geomorphology, hydrology, and ecosystems both at present and under future climate-change scenarios. Studies focusing on a diversity of reef types are welcome, including those focused on both coral and rocky (natural as well artificial) reefs, which may also utilize a variety of process-based field, laboratory, and numerical modelling approaches.

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