Cohesive and mixed sediment in the Regional Ocean Modeling System (ROMS v3.6) implemented in the Coupled Ocean–Atmosphere–Wave–Sediment Transport Modeling System (COAWST r1234)

Abstract

Abstract. We describe and demonstrate algorithms for treating cohesive and mixed sediment that have been added to the Regional Ocean Modeling System (ROMS version 3.6), as implemented in the Coupled Ocean-Atmosphere-WaveSediment Transport Modeling System (COAWST Subversion repository revision 1234). These include the following: floc dynamics (aggregation and disaggregation in the water column); changes in floc characteristics in the seabed; erosion and deposition of cohesive and mixed (combination of cohesive and non-cohesive) sediment; and biodiffusive mixing of bed sediment. These routines supplement existing noncohesive sediment modules, thereby increasing our ability to model fine-grained and mixed-sediment environments. Additionally, we describe changes to the sediment bed layering scheme that improve the fidelity of the modeled stratigraphic record. Finally, we provide examples of these modules implemented in idealized test cases and a realistic application.

Modeling Sediment Flocculation in Langmuir Turbulence

“…Increase in erosion fluxes leads to increase in both total floc mass concentration (Figures a) in the whole water column as expected. The increase in total floc mass concentration means the increase in the probability of floc collision and median floc size (Figures b), consistent with previous studies (e.g., Sherwood et al, ; Winterwerp, ). Therefore, D 50 in simulation S 2 is the largest, and D 50 in simulation S 1 is the smallest.…”

“…This suggests turbulence plays a complicated role in aggregation: it enhances the aggregation rate of some flocs while it suppresses the aggregation rate of other flocs. The value of suggests the bias in aggregation rate due to the use of horizontally averaged mass concentration and dissipation rate from the regional model without resolving three‐dimensional nonhomogeneous turbulence (represented by the denominator) (e.g., COAWST, see Sherwood et al, ), compared to that due to the use of the mass concentration and dissipation rate diagnosed through turbulence‐resolving model (represented by the numerator).…”

“…The first two types of models are relatively easy to solve, but their applicability is restricted to unimodal sediment size distribution and cannot be extended to investigate the dynamic evolution of a bimodal distribution (e.g., Benson & French, 2007). Flocculation models have been coupled with hydrodynamic models to study flocculation processes in estuarine and coastal environments (e.g., Son & Hsu, 2011;Shen et al, 2018a;Shen et al, 2018b;Sherwood et al, 2018;Winterwerp, 2002;Xu et al, 2008Xu et al, , 2010. Winterwerp (2002) implemented a single-size flocculation model (Winterwerp, 1998) in a one-dimensional vertical model to reproduce the vertical profiles of SSC observed in a turbidity maximum in the Ems estuary.…”

“…The transport of suspended sediment in the coastal ocean plays an important role not only in shaping coastal topography (e.g., Schwab et al, ) but also in a variety of problems in oceanography related to ocean color remote sensing (D’Sa et al, ), coral reef ecology (Bannister et al, ), oxygen and nutrient dynamics (Moriarty et al, ), and the sequestration of aquatic pollutants such as spilled oil (Passow et al, ). In sediment transport modeling, settling velocity is one of the key parameters (e.g., Chen et al, ; Dyer, ; Geyer et al, ; Harris & Wiberg, ; Harris et al, ; Sherwood et al, ). Sediments can be characterized as noncohesive and cohesive.…”

“…Flocculation processes, which include aggregation and breakup, increase the complexity in determining the settling velocity and other properties (e.g., size, shape, and density) of cohesive sediment (e.g., Droppo et al, 2004;Dyer, 1989;Droppo, 2006;Mehta, 2013;Strom & Keyvani, 2011;Winterwerp & Van, 2004). Flocculation processes are also important for suspended sediment concentration (SSC) in the water column available for horizontal transport (e.g., Droppo et al, 1998;Sherwood et al, 2018;Verney et al, 2011). Therefore, an accurate prediction of cohesive sediment transport requires a better understanding of flocculation processes.…”