Influence of density stratification and macrophytes on nutrient dynamics and resuspension in a shallow lagoon
Final Report Abstract
The project aimed to investigate the driving forces of sediment resuspension and mixing in shallow estuarine systems. We chose the South San Francisco Bay as an exemplary study site. The currents of the Bay are driven by tides and surface waves are mainly generated by north westerly winds. The winds are generated by the solar heating of the air masses in the Central Valley east of the bay and follow a diurnal pattern with maximum wind velocities around 10 m s-1 in the afternoon. Two stations were deployed along a line perpendicular to a 14 m deep channel, 1000 m (Sh station) and 2000 m (Be station) from the middle of the channel. Station depths were 2.59 and 2.19 m below MLLW, respectively. Two field campaigns were conducted to study the temporal variability in spring (24.02.2009 to 16.03.2009) and fall (09.09.2009 to 07.10.2009). We used acoustic Doppler velocimeters (ADVs) for the simultaneous determination of current velocities, and turbulence. Calibrating of the acoustic backscatter of the ADVs provided also sediment concentration and turbulent fluxes. Waves enhanced the stress at the sediment surface by up to 1/3 in addition to wave induced current shear stress, resulting in sediment peak concentrations of more than 100 g m-3. In contrast, concentrations peaked around 30 g m-3 under comparable conditions without wind waves. Data analysis using a 1-dimensional vertical mixing and settling model suggested the presence of two relevant particle groups at our field site. The first group consisted of fast settling sediment floes, which were resuspended and settled out over the semidiurnal cycle. The second group had slower settling velocities and showed a stronger seasonal variation. During spring, the concentration of fine particles was constant, while the concentrations in fall followed the spring-neap cycle with peak concentrations of ≈ 20 g m-3 during periods of high tidal energy. Particle size distributions recorded in situ suggest that floes are most likely formed by sediment bed erosion processes and not by coagulation in the water column. Sediment bed erosion was sufficiently well described as a linear function of observed sediment fluxes in 0.36 m elevation. Bed erosion was approximately 2-3 times as high as the observed fluxes. Our study highlighted the importance of current-wind interaction for sediment resuspension in shallow field sites. Since the wave induced bottom shear depends on the tidal elevation, the phasing between winds and tides has a critical influence on the overall sediment transport. The advantage of using ADVs for the characterization of sediment resuspension dynamics is its non invasiveness. In future studies, it can be used to test the transferability of the results of sediment bed erosion studies conducted in situ or in the laboratory using erosion chambers and carousels.
Publications
-
2010. Wind-enhanced resuspension in the shallow waters of South San Francisco Bay - Investigating mechanism and potential implications for sediment transport, Journal of Geophysical Research-Oceans 115, C11024
Brand, A., J. Lacy, K. Hsu, D. Hoover, S. Gladding, M. Stacey
-
Model based interpretation of sediment concentration and vertical flux measurements in the shoals of South San Francisco Bay. 6th Biennial Bay-Delta Science Conference, 27-29. September 2010, Sacramento, USA
Brand, A., S. Gladding, J. R. Lacy and M. T. Stacey
-
Wind-induced formation of turbidity gradients along the shoal channel transition in south San Francisco Bay and potential implications for sediment transport. 2010 Ocean Sciences Meeting 23 February 2010 in Portland, USA
Brand, A., J. R. Lacy, K. Hsu, D. Hoover, S. M. Gladding, M. T. Stacey
