Three‐Dimensional Sediment Dynamics in Well‐Mixed Estuaries: Importance of the Internally Generated Overtide, Spatial Settling Lag, and Gravitational Circulation

Abstract

To investigate the dominant sediment transport and trapping mechanisms, a semi‐analytical three‐dimensional model is developed resolving the dynamic effects of salt intrusion on sediment in well‐mixed estuaries in morphodynamic equilibrium. As a study case, a schematized estuary with a converging width and a channel‐shoal structure representative for the Delaware estuary is considered. When neglecting Coriolis effects, sediment downstream of the estuarine turbidity maximum (ETM) is imported into the estuary through the deeper channel and exported over the shoals. Within the ETM region, sediment is transported seaward through the deeper channel and transported landward over the shoals. The largest contribution to the cross‐sectionally integrated seaward residual sediment transport is attributed to the advection of tidally averaged sediment concentrations by river‐induced flow and tidal return flow. This contribution is mainly balanced by the residual landward sediment transport due to temporal correlations between the suspended sediment concentrations and velocities at the M2 tidal frequency. The M2 sediment concentration mainly results from spatial settling lag effects and asymmetric bed shear stresses due to interactions of M2 bottom velocities and the internally generated M4 tidal velocities, as well as the salinity‐induced residual currents. Residual advection of tidally averaged sediment concentrations also plays an important role in the landward sediment transport. Including Coriolis effects hardly changes the cross‐sectionally integrated sediment balance, but results in a landward (seaward) sediment transport on the right (left) side of the estuary looking seaward, consistent with observations from literature. The sediment transport/trapping mechanisms change significantly when varying the settling velocity and river discharge.

Factors Controlling Seasonal Phytoplankton Dynamics in the Delaware River Estuary: an Idealized Model Study

“…Hence, a model for this three-dimensional behavior at least needs to capture the large-scale lateral circulation, the lateral distribution of sediment, and formation of stratification in the channel. An idealized three-dimensional model like iFlow has already been developed for sediment and salinity by Kumar et al (2017) and Wei et al (2018). Like iFlow, the scaling and solution method in this model is developed for well-mixed estuaries and would require substantial changes to account for stronger stratification.…”

Unraveling Interactions between Asymmetric Tidal Turbulence, Residual Circulation, and Salinity Dynamics in Short, Periodically Weakly Stratified Estuaries

“…The estuarine dynamics considered here is strongly nonlinear due to interactions between turbulence, shear, and stratification: small-scale turbulence can strongly influence the water motion and salinity distribution; meanwhile, the shear acts to generate turbulence, which is inhibited by stable stratification and promoted by unstable stratification. To resolve the estuarine dynamics in a way that allows for a systematic decomposition of the abovementioned interactions, the 3D semi-analytical model of Wei et al (2017) is extended in this study. In Wei et al (2017), the nonlinearly coupled water motion and salt dynamics are resolved, decomposing temporal variations into a semidiurnal (M 2 ) tidal constituent and a residual (M 0 ) signal both for the water motion and salinity.…”

“…To resolve the estuarine dynamics in a way that allows for a systematic decomposition of the abovementioned interactions, the 3D semi-analytical model of Wei et al (2017) is extended in this study. In Wei et al (2017), the nonlinearly coupled water motion and salt dynamics are resolved, decomposing temporal variations into a semidiurnal (M 2 ) tidal constituent and a residual (M 0 ) signal both for the water motion and salinity. In this model, a time-independent vertical eddy viscosity A y and diffusivity K y are prescribed, thus neglecting the influence of temporal variations of A y and K y on the hydro-and salt dynamics.…”

“…In long estuaries, however, tidal return flow can generate strong seaward residual flow (Dronkers 1986). Moreover, the residual salt flux due to tidal advection of salinity (i.e., tidal pumping) dominates the landward salt transport in long estuaries (Wei et al 2017), but is close to zero in very short estuaries due to weak correlations between the tidal velocity and salinity (Schettini et al 2017). Another important distinction is linked to the along-channel salinity gradients, which are usually large in short estuaries (such as the Tamar, Tees, and Wyre estuaries in the United Kingdom) compared to long estuaries (Lewis and Uncles 2003) and can lead to stronger gravitational circulation in shorter estuaries (with large S i number).…”

Axial Wind Effects on Stratification and Longitudinal Sediment Transport in a Convergent Estuary During Wet Season

“…Thus, understanding sediment dynamics is of considerable ecological and economical importance. Sediment transport is modulated by a variety of physical processes, including tides, river discharge (Allen et al, 1980;Gong et al, 2014), winds (Chen & Sanford, 2009a), baroclinic circulation (Meade, 1969;Wei et al, 2018), Earth's rotation (Huijts et al, 2006;Schulz et al, 2017), waves (George et al, 2018;Hoefel, 2003), geometry and bathymetry (Ralston et al, 2012;Kumar et al, 2017), tidal asymmetries in mixing (Scully & Friedrichs, 2007a, 2007bSommerfield & Wong, 2011;Gong et al, 2016), and settling and erosion lags (Cheng & Wilson, 2008;Chernetsky et al, 2010). The effects of river discharge, tides, and waves on sediment dynamics have been intensively studied.…”