Lateral Baroclinic Forcing Enhances Sediment Transport from Shallows to Channel in an Estuary

• Published in: Estuaries and coasts Vol. 37; no. 5; pp. 1058 - 1077
• Main Authors: Lacy, Jessica R., Gladding, Steve, Brand, Andreas, Collignon, Audric, Stacey, Mark
• Format: Journal Article
• Language: English
• Published: New York Springer Science+Business Media 01.09.2014; Springer US; Springer; Springer Nature B.V
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Bathymetry; Biological and medical sciences; Brackish; Brackish water ecosystems; Coastal Sciences; Density stratification; Earth and Environmental Science; Ecology; Environment; Environmental Management; Estuaries; Freshwater & Marine Ecology; Fundamental and applied biological sciences. Psychology; Return flow; Sediment concentration; Sediment transport; Shoals; Summer; Suspended sediments; Synecology; Water and Health; Wind; Winter; INE, USA, California, San Francisco Bay; Baroclinic forcing; Estuarine shallows; San Francisco Bay; Estuarine hydrodynamics; Lateral circulation; Sediment transport; Estuaries; Brackish water environment; Sediments; Transport; Channel
• ISSN: 1559-2723; 1559-2731
• DOI: 10.1007/s12237-013-9748-3

We investigate the dynamics governing exchange of sediment between estuarine shallows and the channel based on field measurements at eight stations spanning the interface between the channel and the extensive eastern shoals of South San Francisco Bay. The study site is characterized by longitudinally homogeneous bathymetry and a straight channel, with friction more important than the Coriolis forcing. Data were collected for 3 weeks in the winter and 4 weeks in the late summer of 2009, to capture a range of hydrologie and meteorologic conditions. The greatest sediment transport from shallows to channel occurred during a pair of strong, late-summer wind events, with westerly winds exceeding 10 m/s for more than 24 h. A combination of wind-driven barotropic return flow and lateral baroclinic circulation caused the transport. The lateral density gradient was produced by differences in temperature and suspended sediment concentration (SSC). During the wind events, SSC-induced vertical density stratification limited turbulent mixing at slack tides in the shallows, increasing the potential for two-layer exchange. The temperatureand SSC-induced lateral density gradient was comparable in strength to salinity-induced gradients in South Bay produced by seasonal freshwater inflows, but shorter in duration. In the absence of a lateral density gradient, suspended sediment flux at the channel slope was directed towards the shallows, both in winter and during summer sea breeze conditions, indicating the importance of baroclinically driven exchange to supply of sediment from the shallows to the channel in South San Francisco Bay and systems with similar bathymetry.