Estimates of Bed Stresses within a Model of Chesapeake Bay

Estimates of near-bed turbulence and suspended sediment concentrations depend critically on bed shear stress, which varies with seafloor roughness, current velocities, and, often, wave properties. This project sought, in a computationally efficient manner, to improve representation of the bottom boundary layer within the Chesapeake Bay Program's models of water quality and sediment transport. Maps of sediment bed grain size distributions were compiled for the Chesapeake Bay, and used to provide input into a high resolution, one-dimensional (vertical) bottom boundary layer model that estimated both bed roughness and bed shear stresses. The one-dimensional model required as input grain size distributions, wave properties, and near-bed current velocities. It was run for a range of these model inputs and the estimated bed roughness, total bed stress, and skin friction shear stress values were used to compile a numerical look-up table. The full three-dimensional model could then quickly estimate bed shear stresses by accessing the look-up table for given values of grain size distribution, near-bed current velocity, and wave properties. Observations, especially from locations within the bay impacted by wave processes, were rare, but comparison of estimated values of shear stress to observations from three locations within Chesapeake Bay showed that the methodology provided reasonable estimates. Calculations were sensitive to the one-dimensional model's assumption for a minimum value of hydraulic roughness used in regions where clay and silt predominated. In such locations, the model assumed biogenic elements dominated seafloor roughness, but had little guidance from field observations regarding the behavior of these roughness elements under varying flow conditions.