The effect of tidal asymmetry and temporal settling lag on sediment trapping in tidal estuaries

Abstract

Over decades and centuries, the mean depth of estuaries changes due to sea-level rise, land subsidence, infilling, and dredging projects. These processes produce changes in relative roughness (friction) and mixing, resulting in fundamental changes in the characteristics of the horizontal (velocity) and vertical tides (sea surface elevation) and the dynamics of sediment trapping. To investigate such changes, a 2DV model is developed. The model equations consist of the widthaveraged shallow water equations and a sediment balance equation. Together with the condition of morphodynamic equilibrium, these equations are solved analytically by making a regular expansion of the various physical variables in a small parameter. Using these analytic solutions, we are able to gain insight into the fundamental physical processes resulting in sediment trapping in an estuary by studying various forcings separately. As a case study, we consider the Ems estuary. Between 1980 and 2005, successive deepening of the Ems estuary has significantly altered the tidal and sediment dynamics. The tidal range and the surface sediment concentration has increased and the position of the turbidity zone has shifted into the freshwater zone. The model is used to determine the causes of these historical changes. It is found that the increase of the tidal amplitude toward the end of the embayment is the combined effect of the deepening of the estuary and a 37% and 50% reduction in the vertical eddy viscosity and stress parameter, respectively. The physical mechanism resulting in the trapping of sediment, the number of trapping regions, and their sensitivity to grain size are explained by careful analysis of the various contributions of the residual sediment transport. It is found that sediment is trapped in the estuary by a delicate balance between the M 2 transport and the residual transport for fine sediment (w s = 0.2 mm s −1 ) and the residual, M 2 and M 4 transports for coarser sediment (w s = 2 mm s −1 ). The upstream movement of the estuarine turbidity maximum into the freshwater zone in 2005 is mainly the result of changes in tidal asymmetry. Moreover, the difference between the sediment distribution for different grain sizes in the same year can be attributed to changes in the temporal settling lag.

Longitudinal variation in lateral trapping of fine sediment in tidal estuaries: observations and a 3D exploratory model

“…Jay and Musiak (1994) and Sanford et al (2001) demonstrated that net sediment transport due to ebb-flood asymmetry in intensity of turbulence during a tidal cycle significantly affects the location of the turbidity maximum. A similar conclusion was made by Chernetsky et al (2010) with respect to net sediment transport that results from the interaction between the M 2 tide and external M 4 tide. Recent studies (Donker and de Swart 2013;Winterwerp et al 2013) stressed the role of flocculation and hindered settling in sediment trapping.…”

“…Note that in contrast to previous studies (e.g. Huijts et al (2006) and Chernetsky et al (2010)), the equation is two-dimensional. Solutions are obtained by a novel iteration procedure, where in the initial step only the term involving the lateral sediment transport is considered.…”

Residual Sediment Fluxes in Weakly-to-Periodically Stratified Estuaries and Tidal Inlets

“…Consequently, Burchard and Baumert (1998) used a limited amount of sediment in their model to study the equilibrium dynamics of an ETM. In the analytical approach of Huijts et al (2006) and Chernetsky et al (2010), the inclusion of a spatially varying erosion coefficient is essential to describe the trapping of fine sediment in the lateral and longitudinal directions, respectively. Limited sediment availability is reflected by the observation that erodibility decreases with depth within the sediment layer and can be expressed as a critical shear stress increasing with depth (Sheng and Villaret 1989;Cartwright et al 2009;Dickhudt et al 2009).…”

The hyperturbid state of the water column in estuaries and rivers: the importance of hindered settling

“…The focus on vertical processes, thereby ignoring horizontal gradients, is a reasonable approximation in many estuarine systems, since vertical exchange processes are typically dominant up to leading order (see, e.g. the scaling analysis 97 in the models of Chernetsky et al 2010;Dijkstra et al 2017).…”

“…Several studies have made this connection by using the combined knowledge of measurements and model results (Chernetsky et al 2010;De Jonge et al 2014;Van Maren et al 2015). A possible feedback mechanism that could result in such a transition was proposed by .…”