Residence time in aquatic canopies in wave-dominated flows
School of Science
The large-scale ecological and environmental impact of coastal canopies is tightly limited by the exchange of water across their boundaries. In coastal environments, where the flow is typically wave-dominated, vertical mixing is believed to be the dominant process controlling residence time (Tres ). Recent experiments of wave-driven flows over rough boundaries, however, have revealed the generation of a strong onshore mean current (up to 50% of the orbital velocity far above the canopy) near the canopy top. It is therefore imperative to understand that these two processes, i.e. horizontal advection and vertical mixing, can control residence time in coastal canopies. Through consideration of a Peclet number (the ratio of diffusive to advective time scales), this study presents a framework for quantitative prediction of residence time in these environments. Results reveal that Pe depends heavily on wave and canopy properties and may vary significantly in real coastal canopies. Quantitative predictions for residence time in the limit of Pe << 1 (mixing-dominated exchange) and Pe >>1 (advection-dominated exchange) are presented. For Pe ~ O ( 1 ), characterization of each process will be necessary in describing residence time in these systems.
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