| Abstract: |
The MUltiDisciplinary Benthic Exchange Dynamics (MUDBED) program is exploring how both physical and biological processes influence seasonal variations in turbidity and sediment properties in muddy estuarine environments. Suspended sediment concentrations can be influenced by hydrodynamic forces, sediment settling velocity, and bed erodibility. In turn, sediment flux convergence and divergence can modify both suspended sediment and seabed properties. In estuaries, stratification and sediment trapping vary over seasonal, storm, and tidal timescales in response to tides, freshwater discharge, and winds. In the York River, VA these processes commonly produce an ephemeral Secondary estuarine Turbidity Maxima (STM), downstream of the main Estuarine Turbidity Maximum (ETM).
A three-dimensional numerical hydrodynamic and sediment-transport model of the York River Estuary has been developed to examine feedbacks between sediment flux convergence, erodibility, and settling velocity. The model uses the Regional Ocean Modeling System (ROMS) coupled to the Community Sediment Transport Modeling System (CSTMS), and includes a bed consolidation component that represents critical shear stress for erosion as increasing with depth in the bed and with time since deposition. Specifically, the sediment bed model follows Sanford (2008) in tracking the instantaneous critical shear stress for erosion as a function of depth in the bed and time. This critical shear stress profile then is nudged toward an equilibrium value at specified rates of consolidation (for depositional cases) and swelling (for erosional cases). Multiple grain types are used in the model to represent small, medium, and large flocs.
Model results will be shown that represent conditions in the York River during times that coincide with MUDBED field experiments. The model neglects both direct biological processes and flocculation dynamics, but tidal fluctuations and variations in freshwater discharge influence model estimates of bed erodibility and effective settling velocity. For example, though the model is initialized using uniform sediment bed properties, the channel quickly becomes scoured of fine-grained material, with a concomitant reduction in predicted erodibility, similar to what is seen in the actual York River. Likewise, an STM forms within the model during wet months, when erodibility increases.
Challenges in using this approach include specification of model parameters such as (1) the rate parameter (often called M) used to estimate the erosion rate of sediment, (2) the equilibrium critical stress profile with depth in the sediment bed, (3) timescales for consolidation and swelling, and (3) settling velocity for the grain sizes used. Future applications of this type of bed consolidation model will need to consider appropriate ways to specify these parameters.
REFERENCE:
Sanford, L.P., 2008. Modeling a dynamically varying mixed sediment bed with erosion, deposition, bioturbation, consolidation, and armoring. Computers & Geosciences, 34: 1263-1283. |
