Effects of Sediment‐Induced Density Gradients on the Estuarine Turbidity Maximum in the Yangtze Estuary

Abstract

Many estuaries have regions with locally elevated suspended sediment concentration (SSC), which are referred to as an Estuarine Turbidity Maximum (ETM) (Dyer, 1986;Schubel, 1968). ETMs are areas with a convergence of sediment transport, often corresponding to the landward limit of salt intrusion. With high SSCs, the ETMs lead to dynamic bed behavior and may influence tidal propagation through damping of turbulence (Burchard & Baumert, 1998;Geyer, 1993;Talke & Jay, 2020). Understanding the dynamics of ETMs is important for the management of navigation channels, freshwater resources and ecosystem services.ETMs reflect trapping of sediment in the longitudinal direction, often as a result of (at least one of) two main mechanisms: Tidal asymmetry and estuarine circulation. Tidal asymmetry is the distortion of the tidal wave resulting from non-linear interactions between the tide and channel morphology (

Feedback Effects of Sediment Suspensions on Transport Mechanisms in an Estuarine Turbidity Maximum

“…This positive feedback between sediment and salinity was identified by Zhu, vanMaren, et al. (2021): The SedDE enhances along‐channel salinity‐induced density gradients and stratification, strengthening estuarine circulation.…”

“…Typically, stronger flood dominance imports more sediment and leads to further landward migration of the ETM. Vertical SSC gradients suppress turbulence, increase vertical and longitudinal salinity‐induced density gradients and gravitational circulation, which further increases vertical SSC gradients and provides a positive feedback for trapping sediment (Dijkstra et al., 2019b; Winterwerp et al., 2009; Zhu, van Maren, et al., 2021). Reduced deposition parameterizes the effect of poorly understood near‐bed processes such as consolidation, flocculation, and hindered settling on the vertical sediment concentration gradient (van Maren et al., 2020; Winterwerp et al., 2021).…”

“…Asymmetries in sediment settling velocity and critical shear stress for erosion generate settling and scour lags (Friedrichs, 2011; Postma, 1961; van Straaten & Kuenen, 1957), which in combination with spatial or temporal hydrodynamic asymmetries contribute to estuarine sediment trapping (Chernetsky et al., 2010). Sediment‐induced density effects (SedDE) may also play a role in ETM migration through horizontal sediment‐induced density gradients (Talke et al., 2009; Zhu, van Maren, et al., 2021), vertical mixing (Lin et al., 2021; van Maren et al., 2020; Zhu, van Maren, et al., 2021) or modified bed roughness (Dijkstra et al., 2019b; Dijkstra, Schuttelaars, Schramkowski, Brouwer, 2019; Gabioux et al., 2005; Jalón‐Rojas et al., 2018; van Maren et al., 2015; Zhu, Guo, et al., 2021). These sediment‐induced density effects introduce a positive feedback between sediment trapping and tidal asymmetry: smoothening of the bed leads to tidal amplification in estuaries and therefore enhanced flood‐dominated tidal flow (Gabioux et al., 2005; Jalón‐Rojas et al., 2016, 2018; Wang et al., 2014; Zhu, Guo, et al., 2021), resulting in progressively more sediment trapping and potentially a regime shift toward hyper‐turbid conditions (Dijkstra et al., 2019b; Dijkstra, Schuttelaars, Schramkowski, Brouwer, 2019; Lin et al., 2021; Winterwerp, 2011; Winterwerp & Wang, 2013; van Maren et al., 2015).…”

Exploring the three-dimensional flow-sediment dynamics and trapping mechanisms in a curved estuary: The role of salinity and circulation

“…Compared with the water depth, the bathymetry change (up to ~0.3 m) is relatively small within the whole simulation period (14 days). Therefore, the feedback of bathymetry change on the flow and sediment dynamics is not considered (Zhu et al, 2021). After 7 days, i.e., at spring tide, erosion generally occurs in the deep channel while deposition occurs in the shoal with both the value up to 0.3 m (Figure 9A), which is consistent with the suspended sediment laterally trapped on the shoal described above.…”

“…The study of flow and sediment dynamics is the key to exploring the pattern of estuarine sediment transportation, the characteristics of the ETM, and the evolution of estuarine morphology (Jay and Dungan Smith, 1990;Geyer, 1993;Grabemann et al, 1997;Schoellhamer, 2000;Chernetsky et al, 2010;Sommerfield and Wong, 2011). The previous analysis of the estuarine dynamics was mostly limited to the longitudinal profile (Burchard and Baumert, 1998;Wai et al, 2004;Burchard et al, 2018;Xiao et al, 2018;Jaloń-Rojas et al, 2021;Zhu et al, 2021). However, in recent years, more attention has been paid to the lateral tidal current and sediment transport pattern mechanism in the estuary.…”