Quantifying the Distribution and Influence of Non-Uniform Bed Properties on Spatial Patterns of Turbidity in Shallow Coastal Bays

Abstract:

Increases in turbidity in shallow coastal bays typically occur in response to resuspension of bed sediment driven by the interaction between wind-generated waves and the bay bottom. The mass and grain size of resuspended sediment at any location in a bay are strongly controlled by local bed properties. This suggests that spatial variations in bed properties, such as grain size, porosity and erodibility, should be reflected in spatial patterns of turbidity. Unfortunately, knowledge of bed properties in coastal bays is typically sparse at best. In this study we address two related questions: 1) how best to estimate the spatial distribution of bed properties in shallow coastal bays; and 2) how does the spatial distribution of bed properties affect the spatial pattern of wave-driven turbidity? We explored these questions using field observations and the Delft3D coastal hydrodynamic and sediment transport model for a system of shallow coastal bays on Virginia’s Atlantic coast. This system comprises bays of varying geometry with similar wind and tidal forcing. We found a strong relationship between water residence time and measured grain size. We leveraged the relationships between residence time and grain size fractions to map the spatial distribution of grain size throughout the system based on a recent mapping of residence times for the Virginia coastal bays. This spatially varying map of grain size was used as the initial bed conditions for 2-month-long runs of Delft3D forced with measured wind and tides. The resulting patterns of suspended sediment concentration (SSC) and turbidity differ dramatically from those obtained from simulations with an initially spatially homogeneous bed, with high wave-driven SSC near the mainland in the non-uniform case but high SSC near the inlets for the uniform case. The pre- and post-simulation bed properties are compared to each other and with measured bed properties to investigate how close the residence-time generated bed is to equilibrium bed conditions.