Bed Stress Asymmetry in Estuaries and Implications for Transport

Abstract Tidal asymmetry and river flow are key drivers of residual sediment transport. Here, we provide a generalized formulation, applicable to non‐cohesive transport, of how tide and river flow interactions influence tidal asymmetry of bed stress in mixed‐tide estuaries. Existing analytical relationships for bed stress and tidal asymmetry are extended to include the effects of multiple tidal constituents and residual velocity into a single, one‐dimensional framework. The framework includes asymmetry caused by the phase relationships between tidal constituents at diurnal, semidiurnal, terdiurnal, and quarterdiurnal frequencies, and is summarized by three non‐dimensional numbers that scale and classify the dynamics of bed stress asymmetry in semidiurnal, diurnal, and mixed‐tide estuaries. Comparison of these numbers to different sediment transport metrics shows that even modest residual velocities ( of the tidal current amplitude in semidiurnal systems) can reverse the asymmetry in bed stress and sediment transport that is induced by tidally asymmetric currents. Moreover, because sediment transport is a time integrated process, we show that flood dominant sediment transport can prevail in ebb dominant velocity fields (and vice versa). Particles with low thresholds for erosion/transport can also exhibit different and even opposing transport to those with higher thresholds. Application of the framework to observations in San Francisco Bay shows how bed‐stress asymmetry can be diurnal‐dominant in estuaries traditionally classified as semidiurnal. Many estuaries such as the San Francisco Bay exhibit tidal asymmetry and residual velocities large enough to alter or reverse transport patterns; thus, the framework described herein is widely applicable.