Numerical modeling for hydrodynamics and suspended sediment transport in estuarine marshes [electronic resource]

Sediment migration within estuarine marshes is a long-term process that determines the stability of vegetated wetlands and affects estuarine ecosystems. In order to un- derstand this process, new numerical modeling techniques have been developed and applied to a salt marsh in San Francisco Bay. The salt marsh is influenced by numer- ous physical mechanisms including tides, river inflows, vegetation-induced drag, wet- ting and drying of shallow mudflats, and culverts. Therefore, three new developments have been added to the latest SUNTANS model including hybrid grid compatibility, a marsh model to parameterize the effects of drag associated with marsh vegetation and a culvert model that simulates pressurized (rigid-lid) flow. Using this model, a high-resolution, three-dimensional (3D), unstructured hybrid-grid implementation has been applied to the study domain and validated with observations of currents, salinity, water level, and suspended sediment concentrations at three locations in the marsh during wet and dry periods. The 3D model is used to compute sediment fluxes due to tides during low and high inflow events. The results indicate that the tides provide a consistent landward flux of sediment into the marsh from San Francisco Bay. During river inflow events, sediments that originate upstream in the marsh are transported back into the Bay. Large vegetation coverage prevents erosion in the shallows and stimulates sediment resuspension in the main channels that incise the vegetated mudflats. Culverts in the system limit the seaward flux of sediments, thereby increasing the potential for accretion. As introduced by Casulli (2009), subgrid bathymetry ensures accurate represen- tation of cell volume and face cross-sectional area regardless of grid resolution, and enables a drastic reduction in computational cost particularly in narrow channels that can be accurately modeled with a 1D along-channel, cross-sectionally averaged discretization. Consequently, in order to develop an efficient numerical model and enable long-term simulations for studies of morphodynamics within estuarine wet- lands, a subgrid bathymetry implementation to improve 1D, 2D and 3D simulations of hydrodynamics and sediment transport within estuarine areas is proposed. For hydrodynamics, application of the subgrid bathymetry method along with modified discretization schemes for mass and momentum conservation are proposed, while a pathway to parameterize 3D effects on momentum drag force due to bottom roughness and vegetation have been developed to benefit 1D and 2D simulations. New parame- terization methods with subgrid bathymetry that can effectively recover subgrid-scale sediment resuspension and deposition in 1D and 2D simulations have also been devel- oped. These new methods are tested with application to the same domain as the high- resolution 3D simulations, and it is shown that in many instances highly efficient 1D and 2D simulations with subgrid bathymetry can reproduce time-consuminng high- resolution 3D simulations for hydrodynamics and sediment transport. The results indicate that 1D modeling may be a good choice for long-term analysis of sediment migration especially in shallow, less stratified estuarine regions.

Advisor: Fringer Oliver B. (Oliver Bartlett) primary advisor. Thesis advisor
Fong Derek advisor. Thesis advisor
Monismith Stephen Gene advisor. Thesis advisor