Tidal Variation in Cohesive Sediment Distribution and Sensitivity to Flocculation and Bed Consolidation in An Idealized, Partially Mixed Estuary

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.

Seabed Resuspension in the Chesapeake Bay: Implications for Biogeochemical Cycling and Hypoxia

“…Both observed and modeled data were averaged over May-July 2002 and2003 Overall, differences between model results and observations (Fig. 3, Supplement A) are inevitable considering the relatively coarse spatial and temporal resolution of the observational datasets, as well as the model's necessary simplification or neglect of many processes such as aggregation (Tarpley et al 2019) and seasonal succession of plankton communities (e.g., Malone and Ducklow 1990). The difference in age between decades-old observations (Harding et al 2002;Smith and Kemp 1995) and modern model estimates from 2002 to 2003 also limited the evaluation of primary production and oxygen consumption.…”

“…This implies that accounting for resuspension in observational and modeling studies is likely also important for understanding biogeochemical dynamics in other estuaries and coastal regions where turbidity affects primary production (Cloern 1987; e.g., Delaware Bay: Pennock and Sharp 1986;McSweeney et al 2016). Moreover, a better understanding of processes such as seabed consolidation and particle aggregation that affect spatial variability in resuspension and TSS would be useful for further improving model skill (Tarpley et al 2019;Moriarty et al 2014).…”

Formation of Oil-Particle-Aggregates: Numerical Model Formulation and Calibration

“…Unlike distribution-based models, size-class-based models can provide detailed information about floc populations. Recently, a size-class-based flocculation model (FLOCMOD) (Verney et al, 2011) has been implemented within the COAWST (Coupled-Ocean-Atmosphere-Wave-Sediment Transport) model (Sherwood et al, 2018) and tested in an idealized, partially mixed estuary to investigate cohesive sediment distribution over a tidal cycle (Tarpley et al, 2019). To date, however, these types of flocculation models have solely been used to characterize the fate and aggregation of sediment.…”

“…FLOCMOD also assumes a constant fractal dimension (Verney et al, 2011;Sherwood et al, 2018). For example, Tarpley et al (2019) used a fractal dimension of 2.4 for FLOCMOD in a two-dimensional sediment transport model.…”

Sensitivities of Bottom Stress Estimation to Sediment Stratification in a Tidal Coastal Bottom Boundary Layer

“…Suspended sediment concentration gradients are probably significant near the bottom in coastal waters, and hence the associated density stratification cannot be ignored [4,[13][14][15][16]. Being similar to the thermally stratified atmospheric boundary layer, many studies have also adopted an approach that modifies the Karman's constant (κ) by introducing the Richardson number, which represents the importance of stratification, and performing to recover the predicted log velocity distribution in the sediment-stratified flow [17][18][19].…”