The effect of small density differences at large confluences

Confluences are fascinating elements in river systems. The merging of tributary rivers produces complex hydrodynamics that can play an important role in sediment transport problems, ecological studies or the routing of a pollutant. If the two merging rivers originate from different regions they can have different characteristics. One such a characteristic can be the water density. The influence of many different properties of a confluence on the hydrodynamics has already been investigated. The effect of bed discordance has been described [Biron et al., 1996], as well as that of the momentum ratio [Best & Reid, 1984]. Also a lot of work has been carried out on the mixing layer that may form (van Prooijen & Uijttewaal, 2002). The effects of density differences, on the contrary, have received little attention. Rare examples are the work by Cook & Richmond (2004) and Lyubimova et al. (2014). The aim of this research is to see how and when density differences are important in confluences with respect to other occurring flow structures. Non-dimensional flow parameters will be linked to certain types of flow. These nondimensional parameters can then be used to determine which flow processes can occur. This research shows the importance that small density differences can have on the local hydrodynamics near a confluence. We explain the processes and present suitable nondimensional parameters to describe the flow.