Shorter estuarine dunes and upstream migration due to intratidal variations in stratification

Estuarine dunes are large-scale subaqueous rhythmic bed patterns, and differ from river dunes and marine sand waves in that they exist in environments characterized by several estuarine-specific processes. One of these processes is the tidally varying salinity stratification, with stratified conditions inhibiting vertical mixing. This causes an asymmetry between ebb and flood tide, leading to the strain-induced circulation. Here, we show through a morphodynamic linear stability model that reduced turbulent mixing in partially mixed environments leads to significantly shorter dunes compared to well-mixed environments. Furthermore, intratidal variations in stratification are a driver of upstream migration of dunes due to the tidal straining circulation. Our results constitute one explanation for differing dune wavelengths between two positions in the Elbe estuary. This study stresses the importance of (variations in) stratification for estuarine dunes, and hence improves our understanding of dune characteristics.