Master of Science (M.Sc.)
Virginia Institute of Marine Science
In order to understand the processes controlling the temporal variability in settling velocity (Ws) and bed erodibility (ε), in the middle reaches of the York River estuary, VA, the relationships between the hydrodynamics and particle types were investigated with a near-‐bed Acoustic Doppler Velocimeter (ADV) and the York River 3-‐D Hydrodynamic Cohesive Bed Model.
ADV observations of the flow characteristics that occurred over a strong temporal transition period indicated that Ws and ε were characterized by two distinct regimes with contrasting sediment and water column characteristics: (i) a physically-‐dominated regime (Regime 1) which was a period dominated by flocculated muds (flocs), and (ii) a biologically-‐influenced regime (Regime 2) which was a period dominated by biologically formed pellets mixed with flocs. During Regime 1, Ws averaged about 0.5 mm/s, and ε averaged about 3 kg/m2/Pa. In contrast, during Regime 2 average Ws increased to 1.5 mm/s, and average ε dropped to 1 kg/m2/Pa. The change between these two regimes and the transition in Ws and ε were linked with the arrival and departure of a seasonal density front.
Comparison between ADV observations and the results from the York River 3-‐D Hydrodynamic Cohesive Bed Model suggested that the current model version was not conducive to examining the temporal variability in settling velocity associated with the transition of the distinct sediment regimes. The existing model version estimated realistic values for current speed and concentration and resolved the daily variation associated with in current speed, bed stress, concentration, and settling velocity. However, model estimates of bed stress, current speed, settling velocity, and erodibility did not suggest the presence of two distinct sediment regimes. The model did a poor job of predicting peak bed stresses and settling velocities. Both were over estimated by a factor of 2 throughout most of the study period. Possible modifications to create a version that is able to simulate the bed stresses and sediment properties (i.e. erodibility and settling velocity) during each regime with more accuracy are: (1) define finer sediment classes in the model that are more representative of the water column and not just the seabed, (2) use a consolidation time scale of 5 days rather than 24 hours to allow more sediment to be suspended at lower bed stresses, (3) further reduce hydraulic roughness, and (4) turn on sediment induced stratification.
© The Author
Fall, Kelsey A., "Relationships among Fine Sediment Settling and Suspension, Bed Erodibility, and Particle Type in the York River Estuary, Virginia" (2012). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617918.