Residual Sediment Transport in Tidally Energetic Estuarine Channels With Lateral Bathymetric Variation

Abstract

The residual sediment transport in tidally energetic estuarine channels is investigated by means of idealized cross-sectional modeling. The lateral bathymetric variation follows a Gaussian profile, assuming longitudinal uniformity. The total along-channel residual sediment flux is decomposed into contributions from an advective flux and a tidal pumping flux. Two important mechanisms are found to modify the tidal covariance between sediment concentration and current velocity, thereby contributing to the tidal pumping of sediment. First, longitudinal and lateral straining of salinity leads to tidal asymmetries in stratification and thus sediment resuspension. Second, lateral circulations directly redistribute suspended sediments within the cross section, which are then differentially transported by the along-channel tidal currents. A general relationship between the phasing of the lateral circulations and the resulting lateraladvection-driven tidal pumping is proposed. Reduced-physics experiments with lateral sediment advection turned off provide the first evidence that lateral-advection-driven tidal pumping plays a leading role in sediment transport for tidally energetic estuaries with nonnegligible lateral depth gradients. Additionally, a temporal decomposition breaks down the cross-sectionally averaged tidal pumping flux into individual contributions from different tidal phases (early tide, peak tide, and late tide), providing a new perspective on tidal asymmetry in sediment resuspension and settling. The direction and strength of tidal pumping (both stratification-driven and lateral-advection-driven) are shown to depend on lateral bathymetry, sediment grain size, and longitudinal buoyancy gradient forcing. Plain Language Summary Sediment plays a critical role in estuarine physical and biological processes, such as shaping the topographic characteristics of wetlands, regulating phytoplankton growth by light attenuation, and affecting the transport and fate of aqueous pollutants. In this study, we use idealized numerical simulations to investigate the physics governing the direction and magnitude of sediment transport in tidally energetic estuarine channels. The model is driven by salinity-induced density difference inside the water body, which connects saltwater in the coastal ocean and freshwater in the upstream watershed. Two mechanisms are found to drive the net along-channel transport of sediment at timescales longer than a tidal cycle: the flood-ebb asymmetry in vertical density stratification that suppresses sediment resuspension and the across-channel water circulation that redistributes the suspended sediment within the estuary's cross section. The insights gained are expected to expand our knowledge of sediment transport pathways in different estuarine systems, which can aid us in making management decisions regarding estuarine morphodynamics and ecosystem.

The Breede River estuary (Cape Province, South Africa): A historical perspective on hydrology, geomorphology, and sedimentology

“…Detailed physical investigations of this phenomenon have shown that either flood-dominance or ebb-dominance along estuary channels can be explained by the relationship between tidal flow asymmetry (local shear stress differences) and the cross-sectional channel geometry (morphology). Comprehensive treatments of this phenomenon can, for example, be found in Dronkers (1986Dronkers ( , 2005, Friedrichs and Aubrey 1988;Friedrichs (1995), Wang et al (1999), Prandle (2009), and Zhou and Stacey (2020).…”