Cohesive sediment transport modeling in estuarine shallows: the role of bed erodibility variation
Rachel M Allen1,2, Jessica R Lacy2, Andrew William Stevens3, David B Senn4, Rusty Holleman5 and Mark T Stacey6, (1)University of California Berkeley, Berkeley, CA, United States, (2)USGS Pacific Coastal and Marine Science Center Santa Cruz, Santa Cruz, CA, United States, (3)USGS Pacific Coastal and Marine Science Center Santa Cruz, Santa Cruz, United States, (4)San Francisco Estuary Institute, Richmond, CA, United States, (5)University of California Davis, Civil and Environmental Engineering, Davis, United States, (6)University of California Berkeley, Department of Civil and Environmental Engineering, Berkeley, CA, United States
Abstract:
In estuarine environments such as San Francisco Bay, hydrodynamic and sediment transport models are commonly applied to predict phytoplankton blooms controlled by light availability, and whether fringing marshes can withstand sea-level rise. While long-term observations of suspended sediment concentrations (SSC) exist along San Francisco Bay’s deep central channel, fewer studies of sediment within its shallow shoals have been conducted, yet regions where the water depth is 5 m MLLW or less account for about 2/3 of the bay’s area. Traditionally, predicting SSC in such regions requires that cohesive sediment transport models use constant erodibility parameters, such as the erosion rate parameter, and the critical shear stress. However, field observations in the shallows have shown that the erosion rate parameter can vary in space and time: it increased 5-fold during the stormy winter season and was higher near a tributary outfall. Modelers have hypothesized that temporal variation in this parameter is needed to make accurate predictions of SSC in this region.
This work implements observed values of the erosion rate parameter in a 2-way coupled flow (Delft3D4) and wave (SWAN) numerical model applied in northern San Francisco Bay, and performs a sensitivity analysis to investigate the importance of bed erodibility fluctuations on storm-event and seasonal timescales. Flow, bed stress, and suspended sediment metrics were compared to timeseries observations from 2 sites in the shallows of northern San Francisco Bay. Preliminary model results capture the hydrodynamics and wave events well, and show the spring-neap oscillation in SSC as well as its steep increase in response to wind-wave events. Understanding how this parameter impacts SSC and transport will enable sediment models to make better predictions, leading to a better understanding of phytoplankton dynamics and blooms in the estuary.
