WAMSI 2 - Dredging Node - 3.1.2 - Review of Transport Processes

A common feature of many coastal systems is the presence of large and complex bottom roughness (or canopies) on the seafloor that are formed by a wide range of different marine communities, including seagrasses, coral reef organisms and mangroves. These canopies impose substantial drag forces on the coastal flows generated by waves and currents, resulting in substantial modifications to the mean and turbulent flow structure adjacent to the seafloor. As a consequence, the transport of both natural and dredging-derived sediments (which are closely coupled to the hydrodynamics), including rates of sediment deposition and suspension, can be dramatically altered in these environments. Presently, mechanistic models of sediment transport in the presence of submerged canopies are severely lacking, with only a limited number of mostly qualitative studies providing any insight into these dynamics. New observations of sediment transport within environments such as coral reefs and seagrass meadows are thus critically needed to 1) provide the missing quantitative insight needed to better understand these processes 2) incorporate these dynamics into new predictive sediment transport formulations applicable to these environments, and 3) finally embed these dynamics in process-based numerical models that can eventually be applied by coastal engineers to enable predictions. Without this fundamental information, it is, for example, presently impossible to predict the fate and impact of sediment dredging plumes on these often sensitive environments with any degree of confidence. In this review we summarize the current state of knowledge and gaps in the various components required to predict sediment transport in coral reef and vegetated canopies, including: • The hydrodynamic interactions of currents and waves with submerged canopies, including the influence on bed stresses (Section 2) • The traditional approaches and models used to predict nearbed sediment transport in the coastal ocean (Section 3) • Existing observations of sediment transport within aquatic vegetation and over coral reefs (Section 4) • Measurement techniques and challenges for quantifying and monitoring nearbed sediment fluxes (Section 5), and • Prospects for upscaling these dynamics with numerical models to improve predictions of the transport and fate of natural and dredging-derived sediments in these environments (Section 6)