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

Abstract

In this idealized numerical modeling study, the composition of residual sediment fluxes in energetic (e.g., weakly or periodically stratified) tidal estuaries is investigated by means of one-dimensional water column models, with some focus on the sediment availability. Scaling of the underlying dynamic equations shows dependence of the results on the Simpson number (relative strength of horizontal density gradient) and the Rouse number (relative settling velocity) as well as impacts of the Unsteadiness number (relative tidal frequency). Here, the parameter space given by the Simpson and Rouse numbers is mainly investigated. A simple analytical model based on the assumption of stationarity shows that for small Simpson and Rouse numbers sediment flux is down estuary and vice versa for large Simpson and Rouse numbers. A fully dynamic water column model coupled to a second-moment turbulence closure model allows to decompose the sediment flux profiles into contributions from the transport flux (product of subtidal velocity and sediment concentration profiles) and the fluctuation flux profiles (tidal covariance between current velocity and sediment concentration). Three different types of bottom sediment pools are distinguished to vary the sediment availability, by defining a time scale for complete sediment erosion. For short erosion times scales, the transport sediment flux may dominate, but for larger erosion time scales the fluctuation sediment flux largely dominates the tidal sediment flux. When quarter-diurnal components are added to the tidal forcing, upestuary sediment fluxes are strongly increased for stronger and shorter flood tides and vice versa. The theoretical results are compared to field observations in a tidally energetic inlet.

Stratification in the presence of an axial convergent front: Causes and implications

“…The net sediment flux estimates for the primary site are in agreement with those of Burchard et al (2013) who used a numerical model to examine the influence of the Simpson number (Si) and Rouse number (Ro) on sediment transport. They predicted that in the Conwy estuary, where Si and Ro are low (equal to 0.2 and 0.06e0.12, respectively), there would be a negative, or downestuary, flux of sediment, presumably in the presence of tidal asymmetry.…”

“…The SSC is able to be mixed vertically on the flood, resulting in an influx of sediment. This is the situation described by Burchard et al (2013) for a flood-dominant tidal system.…”

A numerical model for the entire Wadden Sea: Skill assessment and analysis of hydrodynamics

“…Also hydrographic data are sufficiently well‐reproduced with correlations coefficients generally larger than 0.7. The proper representation of hydrographic data is a precondition for the reproduction of effects of tidal straining due to horizontal density gradients, a process which is a substantial driver of sediment flux into the Wadden Sea and other tidally energetic inlets [ Scully and Friedrichs , ; Burchard et al ., ]. A direct assessment of the seasonal variability of the density gradients toward the Wadden Sea was provided by data obtained from a ship of opportunity regularly commuting between Büsum and Helgoland during the summer season.…”

Maximum sinking velocities of suspended particulate matter in a coastal transition zone

“…Residual currents of the estuarine circulation flow shorewards in near-bottom waters and offshore in surface waters (Burchard et al, 2008;Flöser et al, 2011). This probably causes a net SPM flux into the Wadden Sea (Burchard et al, 2008(Burchard et al, , 2013. The Wadden Sea is hypothesized to act as bioreactor, where organic components of SPM become remineralized and are exported in dissolved form (Postma, 1984;Ebenhöh et al, 2004;Grunwald et al, 2010) by the offshore-directed component of the estuarine circulation.…”