Sediment Trapping in Estuaries

Abstract

Estuarine turbidity maxima (ETMs) are generated by a large suite of hydrodynamic and sediment dynamic processes, leading to longitudinal convergence of cross-sectionally integrated and tidally averaged transport of cohesive and noncohesive suspended particulate matter (SPM). The relative importance of these processes for SPM trapping varies substantially among estuaries depending on topography, fluvial and tidal forcing, and SPM composition. The high-frequency dynamics of ETMs are constrained by interactions with the low-frequency dynamics of the bottom pool of easily erodible sediments. Here, we use a transport decomposition to present processes that lead to convergent SPM transport, and review trapping mechanisms that lead to ETMs at the landward limit of the salt intrusion, in the freshwater zone, at topographic transitions, and by lateral processes within the cross section. We use model simulations of example estuaries to demonstrate the complex concurrence of ETM formation mechanisms. We also discuss how changes in SPM trapping mechanisms, often caused by direct human interference, can lead to the generation of hyperturbid estuaries.

Correlation of Remotely Sensed Surface Reflectance With Forcing Variables in Six Different Estuaries

“…One of the main characteristics of estuaries is the estuarine maximum turbidity zone (MTZ), which is the area where fluvial and marine sediments are efficiently trapped (Burchard et al, ). MTZs are often observed in coastal plain (or drowned river valley), salt wedge, and river‐dominated estuaries (Jay et al, ).…”

“…MTZs are often observed in coastal plain (or drowned river valley), salt wedge, and river‐dominated estuaries (Jay et al, ). Previous studies on MTZ dynamics have shown that more than 65 estuaries in the world are known to have MTZs (e.g., Burchard et al, ; Jay et al, ; Uncles et al, ). An MTZ is a region of convergent SPM flux, where the SPM concentration is generally high (Allen et al, ; de Jonge et al, ; Postma, ).…”

“…Remote sensing is capable of providing a synoptic view of surface and near‐surface water conditions at a large spatial scale over the coastal area. However, the important estuarine processes occur in the subsurface layer, such as vertical mixing between freshwater and saline water (Wolanski, ), sediment resuspension, trapping, and flocculation (Burchard et al, ; Tao et al, ; Wolanski, ). Hudson, Talke, and Jay () quantitatively explored the link between physical processes and surface reflectance measured by the MODIS (Moderate Resolution Imaging Spectroradiometer).…”

Why and How Do We Study Sediment Transport? Focus on Coastal Zones and Ongoing Methods

“…Sediment flow can be positive from the shelf to the estuary, especially in the dry season (e.g., [46][47][48][49][50][51]). In the Red River, for example, some of the sediments deposited in the subaqueous delta during the rainy season return in the dry season by tidal pumping to the estuary and deltaic wetlands, where deposition is three times greater than in the rainy season [52].…”

“…Sediments are then brought back from the sea to the estuary near the bottom, even in the absence of density circulation, and settle and consolidate under the control of spring-neap tide cycles. This process, described in the 1970s [46], is still under study (see [51,142]). This key mechanism of delta geomorphodynamics is very sensitive to changes in flows, and river water regulation induced by dams is enough to move the estuarine turbidity zone and cause the silting up of navigation channels and river ports, as on the Red River [157].…”