Abstract
Particle settling velocity and erodibility are key factors that govern the transport of sediment through coastal environments including estuaries. These are difficult to parameterize in models that represent mud, whose properties can change in response to many factors, including tidally varying suspended sediment concentration (SSC) and shear stress. Using the COAWST (Coupled Ocean-Atmosphere-Wave-Sediment Transport) model framework, we implemented bed consolidation, sediment-induced stratification, and flocculation formulations within an idealized two-dimensional domain that represented the longitudinal dimension of a micro-tidal, muddy, partially mixed estuary. Within the Estuarine Turbidity Maximum (ETM), SSC and median floc diameter varied by a factor of four over the tidal cycle. Downstream of the ETM, the median floc size and SSC were several times smaller and showed less tidal variation (~20% or less). The suspended floc distributions only reached an equilibrium size as a function of SSC and shear in the ETM at peak tidal flow. In general, flocculation increased particle size, which reduced SSC by half in the ETM through increased settling velocity. Consolidation also limited SSC by reduced resuspension, which then limited floc growth through reduced SSC by half outside of the ETM. Sediment-induced stratification had negligible effects in the parameter space examined. Efforts to lessen the computation cost of the flocculation routine by reducing the number of size classes proved difficult; floc size distribution and SSC were sensitive to specification of size classes by factors of 60% and 300%, respectively.
Highlights
Estuaries are valuable coastal ecosystems that provide habitat and nursery services to many fishery species, including finfish, crustaceans, and mollusks
The Estuarine Turbidity Maximum (ETM) is a key feature of partially mixed estuaries that occurs at the convergence of freshwater and sea water, which can trap sediment leading to a peak in suspended sediment concentration (SSC), and moves with changes in hydrodynmic conditions [6]
The flocculation model adequately represented the dynamics throughout the entire estuary, and the results demonstrated that sediment conditions in the ETM were more variable over the tidal cycle with SSC ~3–10 times greater and median diameters ~2–8.5 times larger than downstream
Summary
Estuaries are valuable coastal ecosystems that provide habitat and nursery services to many fishery species, including finfish, crustaceans, and mollusks. A more fundamental approach is to add a formulation that incorporates density changes due to SSC into the equation of state (the function that quantifies fluid density) [73,74], and use the combination of water density and SSC stratification in a turbulence closure model to determine the eddy diffusivity This requires that the hydrodynamic model have sufficiently high vertical resolution to represent the large gradients in SSC that produce the density-induced stratification found near the bed [74]. Due to computational limits, many implementations of sediment transport for muddy environments in the past have used simplified forms for flocculation and bed erodibility despite the potential reduction in the model skill and challenges in parameterizing cohesive formulations
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