Turbulence, Bottom Boundary Conditions, and Sediment Transport in a Shallow Water Habitatby Deanna M. Sereno,
Mark T. Stacey,
Abstract: Field experiments were designed to explore the effects of vegetation on the hydrodynamics and sediment transport in Franks Tract, a shallow water habitat in the Sacramento-San Joaquin Delta. Within this system, the mean currents are primarily driven by freshwater flow and diurnal tides, with some influence from wind. The dominant vegetation, Egeria densa, is buoyant and moves with the flow, yielding challenges for the experimental design and data analysis. Two types of physical observations were made: (1) Synopticsurveys of the flow field; and (2) high-resolution time series at two fixed locations within the tidal lake. The synoptic data was collected with a boat-mounted acoustic Doppler current profiler (ADCP). The fixed, insitu instrumentation consisted of an upward-looking ADCP to capture the water column dynamics at one station, with a nearby second station focused on the bottom boundary layer. The instrument grouping for boundary layer analysis included four acoustic Doppler velocimeters (ADV), two optical backscatter sensors (OBS), and two conductivity and temperature (CT) probes. Preliminary results indicate the mean circulation is dominated by tidal forcing relative to freshwater flow and wind. Further analysis of the turbulent characteristics and near-bed turbulent structure allow us to estimate the bottom stress from several methods. Finally, determination of local sediment suspension and deposition is estimated from the mass flux. Although the experimental design allowed placement of each ADV point measurement very close to an OBS conical measurement volume, the difference in measurement volumes will yield an inaccurate mass flux. Therefore, a second estimate of suspended sediment concentrations (SSC) is made using the backscatter intensity from the ADV acoustic signal. Direct comparison of SSC measured by the OBS and estimated from the ADV is complicated due to their slightly different location, different reactions to presence of vegetation, and different particle size classification recognition for acoustic and optical pulses.
Subject Headings: Sediment transport | Water supply systems | Tides | Shallow water | Aquatic habitats | Boundary conditions | Turbulence | Doppler systems | Suspended sediment | Boundary layers | Vegetation | Volume measurement | Acoustic techniques | North America | Connecticut | United States
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