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

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Abstract

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 \(\lambda _i\) 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.

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Acknowledgments

This work was supported by Ministry of Science, Education and Sports of the Republic of Croatia under the project number 114-0982709-2549 (Hydrology of Sensitive Water Resources in Karst) and under the project number RC.2.2.06-0001 (Research Infrastructure for Campus-based Laboratories at University of Rijeka), which was co-funded by the European Fund for Regional Development.

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  1. Faculty of Civil Engineering, University of Rijeka, Radmile Matejcic 3, 51000, Rijeka, Croatia

    Nino Krvavica, Vanja Travaš & Nevenka Ožanić

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  1. Nino Krvavica
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  2. Vanja Travaš
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  3. Nevenka Ožanić
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Correspondence to Nino Krvavica.

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Krvavica, N., Travaš, V. & Ožanić, N. A field study of interfacial friction and entrainment in a microtidal salt-wedge estuary. Environ Fluid Mech 16, 1223–1246 (2016). https://doi.org/10.1007/s10652-016-9480-1

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