Book Description

The long-term evolution of shallow intertidal zones is of particular interest in light of efforts to restore coastal marshes. Factors that affect that rate of accretion or erosion of a marsh include current speeds, wind waves, tides and sediment load. These forcings change with time and vary on different time scales, and for any particular marsh the importance of each forcing may be different. Observations indicate that shallow intertidal basins tend to be characterized by a bimodal distribution of water depths into higher marshplain and lower mudflats. For shallow intertidal marshes, wind waves and tidal currents would both affect the deposition and erosion of sediments. It is hypothesized that for a shallow intertidal marsh with significant wind effects, wind waves will cause increased turbulence that enhances resuspension of sediment, leading to lower mudflats rather than higher marshplains. In this project, an attempt is made to study the factors that affect sediment deposition and resuspension, and the variability of these factors with time. Field data was collected for two weeks at a tidal wetlands restoration site, the Sonoma Baylands, California. Two locations within the field site with different wind fetches are compared to show the impact of wind on turbidity. At each site, wind speed, current velocity, wave, and turbidity data was collected. Turbidity is used as a proxy for suspended sediment concentration. Relative wave height and wave shear stress were found to be most correlated with turbidity. At the site with shorter fetch, current shear stress was almost as important as wave shear stress, but it was not significantly correlated at the site with longer fetch and larger waves. The importance of wind waves for sediment will help develop understanding of the mechanisms and factors affecting the transition of a shallow coastal region into either a marshplain accreting to mean high-high water level, or eroding into mudflat. The results of this study will be useful in determining the importance of wind waves on sedimentation, thus helping in developing strategies for restoring or protecting tidal marshes. To further study the process of sediment transport in San Francisco Bay, a three-dimensional coupled hydrodynamic, wind wave and mud suspension model SUNTANS was developed. The model was calibrated to existing meteorological, current, salinity, wave and sediment conditions. The model was found to be in general agreement with existing patterns of salinity, wave and sediment distribution. Higher sediment concentrations were generally found in locations of higher wind, shallow depths, complicated bathymetry, and/or down wind locations, in line with the established understanding of wind-induced sediment resuspension together with current induced erosion and advective transport.