Author(s): Wessel M. Van Der Sande; Pieter C. Roos; Theo Gerkema; Suzanne J. M. H. Hulscher

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Keywords: Estuaries; Sand dune; Morphodynamics; Linear stability analysis; Estuarine circulation

Abstract: Sand dunes are large-scale rhythmic bed patterns which occur in estuaries. Their wavelengths are of the order of tens to hundreds of meters, and they have heights on the order of meters. Estuarine sand dunes are subject to a complex mixture of marine, riverine and estuarine-specific processes. One of such processes is the estuarine circulation, which is a tide-averaged circulation with a seaward component near the surface, and a landward component near the bed. Here, we investigate the effect of two sources of estuarine circulation (gravitational circulation and tidal straining circulation) on the dimensions and dynamics of sand dunes. To do so, we develop an idealized process-based morphodynamic model which describes the motion of water (forced by river and tide) over a mobile sandy bed on a local model domain. Here, ‘local’ means that we describe relevant physical processes on a model domain the size of one sand dune, implying that large-scale variables (e.g. salinity and stratification) are imposed diagnostically on the local domain. With this model, we then perform a linear stability analysis, which reveals whether and how small-amplitude perturbations in a flat bed will grow or decay; i.e., they show whether the environmental conditions favour dune development or not. Furthermore, the model shows the wavelength which grows fastest from a flat bed, as well as the corresponding growth rate and migration rate. In our model formulation, the gravitational circulation is induced by an external baroclinic forcing; tidal straining circulation is represented through an eddy viscosity parameterization which depends on the gradient Richardson number (and therefore on the local shear and vertical density gradient). Through this parameterization, the turbulence model captures the reduced mixing due to stratification. Model results show that both types of circulation (gravitational circulation and tidal straining circulation) are well-represented by the model. Furthermore, the model shows that sand dunes develop from a flat bed, and that they migrate upstream (i.e., against the river flow) when the estuarine circulation is strong compared to the river flow velocity.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221104

Year: 2022