Hydrodynamic Aspects of Confluence of Rivers with Different Water Densities

Abstract

Confluences of rivers are characterized by highly complex intrabasin processes. The arising hydrodynamic aspects of river confluences have received a lot of attention in recent years. For modeling purposes, it is generally assumed that the water densities of the rivers under consideration are close, and the density effects associated with their difference are neglected. With this approximation, we have previously studied the confluence of the Vishera and Kama rivers. However, in some cases, the temperatures or salinity of the waters of merging rivers can differ significantly, and thereby the hydrodynamics of mixing change considerably. In this paper, the specific features of river confluences with and without taking into account density effects are examined and compared using both real and simplified (model) channel geometries. The performed calculations show that, at the density Froude number ~1 which is observed in the rivers taken as an example at the difference in salt concentration of ~0.3 g/L, the mixing characteristics of these flows significantly change when they merge. The denser waters of the Vishera River begin to move under the less mineralized waters of the Kama River. As the Froude number exceeds the critical value, a fundamental rearrangement of coherent transverse structures occurs. These effects are more pronounced for the model channel configuration, since taking into account the bottom heterogeneity significantly enhances vertical mixing. Earlier, the phenomenon in which the denser flow moves under the less dense flow was discovered by the authors for the confluence of the Chusovaya and Sylva rivers located in the backwater from the Kama hydroelectric power station.