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The salinity distribution in estuaries is controlled by the mixing of salt and fresh water, which is driven by many factors including tidal amplitude, freshwater inflow, bathymetry, wind stress, and waves. The timing and location of the mixing are dependent on stratification and shear in the water column, which vary tidally (Simpson et al., 1990) and over the spring neap cycle (Bowen & Geyer, 2003). There have been many previous approaches to identify and quantify the mechanisms of estuarine mixing. Early studies focused on constant and parameterized values of eddy diffusivity (Hansen & Rattray, 1965; Lerczak et al. 2006). However, the eddy diffusivity is not in itself a measure of estuarine mixing but rather of the turbulence that would cause mixing in the presence of a vertical gradient. Several recent studies of estuarine mixing (Burchard & Rennau, 2008; MacCready et al., 2018; Wang & Geyer, 2018) have proposed that the destruction of salinity variance provides the appropriate measure of the mixing of salt in an estuary. This destruction of scalar variance has long been addressed in context with turbulent microstructure as the quantity χ s (Nash & Moum, 2002). Not only does χ s relate directly to the irreversible process that formally represents mixing (in context with the entropy of mixing, Gregg, 1984), but it also has particular significance in context with estuaries in that the strength of the exchange flow is directly proportional to the volume integral of χ s (Mac-Cready et al., 2018).

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The long-term objective is to determine the hydrodynamic processes controlling sediment transport and the associated morphologic response on tidal flats.