Formation of the density currents in the zone of conﬂuence of two rivers T. Lyubimova a,⇑ , A. Lepikhin b , V. Konovalov a , Ya. Parshakova a , A. Tiunov b a Computational Fluid Dynamics Laboratory, Institute of Continuous Media Mechanics UB RAS, Perm, Russia b Surface Hydrology Laboratory, Mining Institute UB RAS, Perm, Russia article info Article history: Received 1 May 2013 Received in revised form 4 October 2013 Accepted 26 October 2013 Available online 14 November 2013 This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of Ellen Wohl, Associate Editor Keywords: River conﬂuence Density currents Vertical stratiﬁcation Mineralization Numerical simulation In situ measurements summary The peculiarities of the formation of density currents in the zone of conﬂuence of two rivers with strongly different hydrochemical regimes are studied numerically and experimentally. The three-dimensional numerical simulation shows that the water of the river of higher mineralization and density ﬂows under the water of the river of lower mineralization and density and vice versa. And besides, such overlapping of the water streams is observed both upstream and downstream of the conﬂuence of two rivers. The results of numerical simulation are supported by the data of expedition observations and in situ measure- ments. A similar phenomenon, namely, a ﬂow of two overlapped oppositely directed water streams was previously discovered in the mouth zone of the rivers ﬂowing in the sea. Our study reveals the existence of a new type of the hydrological systems, in which such a phenomenon occurs. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Numerical simulation of the substance transfer processes plays a decisive role in projecting and designing of water systems for public water supplies. The peculiar features of natural water- courses (rivers, channels) are their length, small radius of curva- ture and relative shallowness. These are the advantages that can essentially simplify the mathematical description of the examined processes through the use of two-dimensional models (Graf and Altinakar, 1996), (Petrescu and Sumbasacu, 2010), (Duan and Nan- da, 2006). However, such an approach proves itself only in the case when impurities have no effect on the character of the ﬂuid motion in a water basin. In the general case, the parameters of the hydrodynamic system depend on the impurity concentration. This dependence has a profound effect on the hydrodynamic character- istics of natural pools and requires the application of three- dimensional models (Lyubimova et al., 2010). Due to the presence of heavy impurities the process of water mixing at the point of river conﬂuence is retarded. This effect is described in work (Weigold and Baborowski, 2009), which is concerned with study- ing the inﬂuence of the tributary streams of the Elba river on the quality of its water. In the case when impurities have no effect on the hydrody- namic characteristics of the system, water mixing at the conﬂuence of rivers occurs in a shear layer, which is characterized by high ki- netic turbulent energy (Rhoads and Sukhodolov, 2008). As a rule, the boundary between conﬂuent ﬂows is close to the place of their conﬂuence and can be determined based on the differences in the characteristics of these ﬂows, for example, in temperature, conduc- tivity (Gaudet and Roy, 1995; Rhoads and Kenworthy, 1995; Rho- ads and Sukhodolov, 2001), also in concentration of impurities (Kenworthy and Rhoads, 1995). In some cases the interface can ex- tend down stream for a considerable distance from the conﬂuence (MacKay, 1970). In the present work, the speciﬁc features of the conﬂuent stream mixing are investigated in relation to the ﬂow velocity in the presence of heavy impurities. A three dimensional numerical simulation of the behavior of the hydrodynamic system in the region of meeting of two rivers has been carried out. In the literature, one can ﬁnd a great number of papers dealing with three-dimensional modeling of the channel and river dynamics (see, for example, Khosronejad et al., 2007; Weiming et al., 2000; Baranya et al., 2010; Rhoads and Sukhodolov, 2001; Biron et al., 2004). In the region of river conﬂuence the intensity of horizontal water mixing can increase in the case when the width of the trib- utary stream is less than that of the main channel (Biron et al., 2004), it can also be affected by the curvature and morphology of the channel bottom. In our study, a numerical simulation of a ﬂow in the channels of invariable depth is performed in the presence of 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.10.041 ⇑ Corresponding author. Tel.: +7 3422396646; fax: +7 3422378487. E-mail address: lubimova@psu.ru (T. Lyubimova). Journal of Hydrology 508 (2014) 328–342 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol