Tidal asymmetry and variability of bed shear stress and sediment bed flux at a site in San Francisco Bay, USA

By: , and 
Edited by: J.C. Winterwerp and C. Kranenburg



The relationship between sediment bed flux and bed shear stress during a pair of field experiments in a partially stratified estuary is examined in this paper. Time series of flow velocity, vertical density profiles, and suspended sediment concentration were measured continuously throughout the water column and intensely within 1 meter of the bed. These time series were analyzed to determine bed shear stress, vertical turbulent sediment flux, and mass of sediment suspended in the water column. Resuspension, as inferred from near-bed measurements of vertical turbulent sediment flux, was flood dominant, in accordance with the flood-dominant bed shear stress. Bathymetry-induced residual flow, gravitational circulation, and ebb tide salinity stratification contributed to the flood dominance. In addition to this flow-induced asymmetry, the erodibility of the sediment appears to increase during the first 2 hours of flood tide. Tidal asymmetry in bed shear stress and erodibility help explain an estuarine turbidity maximum that is present during flood tide but absent during ebb tide. Because horizontal advection was insignificant during most of the observation periods, the change in bed mass can be estimated from changes in the total suspended sediment mass. The square wave shape of the bed mass time series indicates that suspended sediment rapidly deposited in an unconsolidated or concentrated benthic suspension layer at slack tides and instantly resuspended when the shear stress became sufficiently large during a subsequent tide. The variability of bed mass associated with the spring/neap cycle (about 60 mg/cm2) is similar to that associated with the semidiurnal tidal cycle.

Publication type Book chapter
Publication Subtype Book Chapter
Title Tidal asymmetry and variability of bed shear stress and sediment bed flux at a site in San Francisco Bay, USA
DOI 10.1016/S1568-2692(02)80010-9
Volume 5
Year Published 2002
Language English
Publisher Elsevier
Contributing office(s) California Water Science Center, San Francisco Bay-Delta, Pacific Regional Director's Office
Description 15 p.
Larger Work Type Book
Larger Work Title Fine sediment dynamics in the marine environment
First page 93
Last page 107
Country United States
State California
Other Geospatial San Francisco Bay
Online Only (Y/N) N
Additional Online Files (Y/N) N
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