Suspended sediment transport in an estuarine tidal channel within San Francisco Bay, California

Abstract

A recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within San Francisco Bay, California. Measurements were made every 15 min over six successive flood and ebb tidal cycles. They included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed; mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed; near-bottom pressure fluctuations; vertical temperature gradient; and bottom photographs. Additionally, suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle. The instrument system was retrieved during each slack water period to exchange suspended sediment sample bags. While the instrument was deployed STD-nephelometer measurements were made thoughout the water column and water samples were collected each 1–h and bottom sediment was sampled at the deployment site. Size distributions of the suspended sediment samples, estimates of particle settling velocity ( μ s ), friction velocity ( U ∗ ), and reference concentration ( C a ) at z = 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles. Three suspended sediment particle conditions were evaluated: (1) individual particle sizes in the 4–11 Φ (62.5-0.5 μm) size range with the reference concentration C a at z = 20 cm ( C Φ ); (2) individual particle sizes in the 4–6 Φ size range, flocs representing the 7–11 Φ size range with the reference concentration C a at z = 20 cm ( C f ); and (3) individual particle sizes in the 4–6 Φ size range, flocs representing the 7–11 Φ size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress. An analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables μ s , U ∗ , and C a . In addition, computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions.