Interfacial Mixing in Estuaries and Fjords

Abstract

Mathematical modelers of stratified flows in estuaries and fjords need to include in their equations terms representing the transfer of fluid across the density interface. Using a combination of results obtained from theory and laboratory and field measurements, it is found that two distinct types of interfacial mixing can be identified. When the interfacial transition layer is in a subcritical state, mixing takes place in either direction to a greater or lesser extent, depending on the turbulence in each layer. This type of mixing is called entrainment and is due to Kelvin‐Helmholtz instabilities. When the interfacial transition layer is in a critical or supercritical state, a two‐way transfer process called turbulent‐diffusion mixing takes place. In this condition, violent vortex motions, generated by internal wave interference, exchange equal volumes of fluid between the layers. The resulting equations are compared with the results of other researchers and found to have similar characteristics.