Can we simply predict maximum turbidity in tidal estuaries?

Tidal estuaries are often characterized by estuarine turbidity maxima (ETM), impacting physical, biogeochemical, and ecological dynamics along the land-sea continuum. Mechanisms driving ETM formation are now relatively well understood and ETM responses to environmental conditions are well characterized. However, a realistic description of estuarine sediment dynamics currently requires extensive in-situobservations and fully-validated numerical modeling. Such time-consuming and costly approaches limit their use for estuary stakeholders. 

The main goal of this study is to provide an estimate of the maximum suspended sediment concentration (SSC) that can be expected in tidal estuaries, based on key estuarine parameters, i.e., tidal range (a), river discharge, water depth (h), and estuary shape. Seven meso- to macro-tidal NW European estuaries have been analyzed using a combination of in-situ measurements and realistic numerical simulations (Elbe, Weser, Scheldt, Seine, Loire, Gironde, and Ems estuaries). These estuaries present contrasted estuarine forcing, in terms of tidal range (from 2 to 8 m), mean river discharge (from 100 to 900 m3/s), and estuary length (from 100 to 190 km). It results in maximum surface SSC ranging from 0.2 to 5 kg/m3.

Results show that the classical parameter space used for estuarine circulation classification, based on the freshwater Froude number and mixing number, is not well adapted to SSC classification. It is mainly due to not considering tidal asymmetry effects. However, the relative tidal range (i.e., the tidal range over channel depth ratio, a/h) is more relevant than the mixing number to characterize estuarine SSC, as it is related to both tidal energy and tidal asymmetries. Therefore, Frf and a/h provide an interesting parameter space to classify ETM, where SSC primarily increases with a/h, and secondly increases with Frf (Figure 1). Nonetheless, the seven investigated estuaries correspond to strongly stratified and partially mixed environments, and a wider investigation of different estuarine systems has to be carried out to assess the limitation of the observed trends.

How to cite
Grasso Florent, Bismuth Eliott, Burchard Hans, Defontaine Sophie, Dijkstra Yoeri, Geyer R, Kösters Frank, Lafite Robert, Reese Nina, Schuttelaars Henk, Sottolichio Aldo, Van Kessel Thijs, Vanlede Joris, Van Maren Bas, Verney Romaric, Walther Régis, Zorndt Anna (2023). Can we simply predict maximum turbidity in tidal estuaries?. AGU23. Wide. Open. Science. AGU Fall Meeting 2023. 11-15 December 2023, San Francisco, CA & Online Everywhere.

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