A field study of interfacial friction and entrainment in a microtidal salt-wedge estuary

Abstract

The interfacial friction and entrainment were investigated in a microtidal salt-wedge estuary. A detailed sampling campaign was conducted in the Rjecina River estuary in Croatia from January 2014 to June 2015. The observed vertical profiles of salinity s and temperature T confirmed the presence of a highly stratified estuary, represented by an upper layer of freshwater separated from a lower salt-wedge by a sharp density interface. The entrainment rate E across the interface was estimated by a two-layer box-model, based on the observed freshwater flow rate Q and layer-averaged salinity. Interfacial friction factor $$\lambda _i$$ was estimated by fitting the results of a numerical model to the observed interface depths. For this purpose we applied a numerical two-layer shallow water model extended to account for irregular non-prismatic cross sections of the channel. We found that in microtidal conditions, the strength of the stratification is reduced with increasing Q. Furthermore, we found that as Q increases, so does the shear velocity, the interfacial friction factor and the vertical mixing across the interface. More detail analysis showed that E may be parametrized by bulk non-dimensional parameters, in particular, a combination of bulk Richardson number Ri and average friction factor $$\lambda$$ , which accounts for the channel bed friction and the interfacial friction. On the other hand, $$\lambda _i$$ can be linked to a combination of Reynolds number Re and bulk Richardson Ri. Contrary to previous studies, we showed that in field conditions, $$\lambda _i$$ may increase with Re.