engleski

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

The interfacial friction and entrainment were investigated in a microtidal salt-wedge estuary. A detailed sampling campaign was conducted in the Rječina 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 λi 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 λ, which accounts for the channel bed friction and the interfacial friction. On the other hand, λi can be linked to a combination of Reynolds number Re and bulk Richardson Ri. Contrary to previous studies, we showed that in field conditions, λi may increase with Re.

Estuaries ; Stratified flow ; Salt-wedge ; Two-layer model ; Interfacial friction ; Entrainment

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