Master of Science (M.Sc.)
Virginia Institute of Marine Science
There is currently a project underway to restore many of the man-made salt ponds along the shores of South San Francisco Bay (SSFB) back to tidal marsh, potentially reestablishing these areas as sinks for SSFB sediments. While there have been recent studies examining the evolution of newly restored marsh areas in SSFB, there have been no recent projects focusing on the expected response of valuable tidal flat environments adjacent to the restored marshes. To help fill this void, this project seeks to characterize SSFB tidal flat morphodynamics, both spatially and temporally, through examination of historic morphologic variability and change along with variations in external forcings.
Spatial and temporal trends in profiles of SSFB tidal flats are examined using bathymetric and LIDAR data collected between the 1890s and 2005. Eigenfunction analysis reveals a dominant mode of morphologic variability related to the degree of convexity or concavity in cross shore profile – classically indicative of tidally dominant, sediment rich, or wave dominant, sediment poor conditions, respectively.
Two opposing areas of equilibrium shape – north/south of a constriction in estuary width located at the Dumbarton Bridge – are highlighted by the first mode of variability in the Eigenfunction analysis, accounting for 90% of the overall spatial variation in tidal flat shape. Additionally, the eigenfunction scores which quantify the spatial pattern of increasing/decreasing convexity in the inner/outer estuary are correlated to spatial variability in fetch length, sediment grain size, recent erosion/deposition, and tidal height. Results for spatial variation found herein are generally consistent with theoretical predictions of tidal flat morphologic response to waves, tides, and sediment supply.
Trends for morphologic change between 1890 and 2005 in twelve geographically diverse regions within SSFB are compared to temporal trends in sediment discharge, mean sea level, diurnal tidal range, and Pacific Decadal Oscillation Index (as a proxy for storminess). Overall, convex vs. concave profiles were favored in the inner vs. outer estuary throughout the entire historical period. Furthermore, tidal flat morphology of the outer estuary displayed a steady increase in concavity with time. The trend of increasing concavity in the outer-estuary flats was consistent with temporal changes in hindcasted sediment discharge from the Central Valley. Although consistently convex, tidal flats located in the inner portions of SSFB exhibited greater complexity in their degree of convexity through time, and temporal changes could not easily be correlated to a given external physical forcing, suggesting a possible role for more localized variations in sediment supply.
A set of criteria for establishing dependence between morphology and external factors was created, using results of a stepwise multiple regression. Using this criteria, trends sediment supply from the Central Valley were found to have a consistency with temporal trends in outerestuary tidal flat shape. Inner-estuary flat shape change was found to be consistent with local patterns in rainfall (as a proxy for local sediment discharge) in the innermost regions, and with recent deposition or erosion in all other regions.
© The Author
Bearman, Joshua Alexander, "Factors Controlling Tidal Flat Morphology in South San Francisco Bay between the 1890s and 2005" (2008). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617872.