The effect of small density differences at large confluences

Abstract

Confluences are special, complex elements within river systems which have been subject of much research. However, little attention has been given to the effects of density differences in confluences. The aim of this research is to see when and how 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 non-dimensional parameters can then be used to determine to some degree which flow processes can occur. Using numerical simulations of a schematized confluence the effects of density differences were identified. These effects were visible on several aerial photographs of large confluences. For one specific confluence, the Rio Negro – Solimões confluence near Manaus, Brazil, a separate model was made and run. The numerical simulations show that the denser water flowed under the lighter water. In a cross-section the shape of the interface between the two waters is very similar to the shape in a lock-exchange. In downstream direction the horizontal part of the interface moves upwards and eventually reaches the surface. This limits the distance over which the two waters can move over one another If the river downstream of the confluence is insufficiently wide, another process occurs. If the dense or light water reaches the opposite bank, it will well up or down respectively. When in a confluence a velocity difference is present between the two tributary rivers, a mixing layer is likely to develop. However, density differences can cause motion perpendicular to the main direction of flow. This motion hampers the development of a mixing layer. If density differences become too large compared to the velocity difference, the mixing layer may not develop at all. We found similarities between the flow characteristics in the numerical models and those visible on aerial photographs of several large confluences. Only aerial photographs with a colour difference between the two tributaries were used. The numerical model of the Rio Negro – Solimões confluence shows similar characteristics as those in the schematized numerical model. Aerial photographs of this confluence did not show these characteristics. The reason for this is boils of heavier Solimões water at the Rio Negro side of the river disturbed a clear surface pattern. These boils do indicate however that heavy water is present under the lighter water. Oblique photographs of this confluence often showed the absence of a mixing layer and the presence of floating foam around the interface of the two waters. These two observations can be explained by the theory derived from this research. This research shows that small density differences can have a significant impact on the hydrodynamics downstream of a confluence. It proved to be possible to link non-dimensional parameters to certain types of flow. However many aspects are still unknown and more research into these is recommended.