Flow Patterns in an Open Channel Confluence with Increasingly Dominant Tributary Inflow

Laurent Schindfessel,Stéphan Creëlle,Tom De Mulder

doi:10.3390/w7094724

Abstract

Despite the ratio of incoming discharges being recognized as a key parameter in open-channel confluence hydrodynamics, little is known about the flow patterns when the tributary provides more than 90% of the total discharge. This paper offers a systematic study of flow features when the tributary becomes increasingly dominant in a 90° confluence with a fixed concordant bed. Large-eddy simulations are used to investigate the three-dimensional complex flow patterns for three different discharge ratios. It is found that the tributary flow impinges on the opposing bank when the tributary flow becomes sufficiently dominant, causing a recirculating eddy in the upstream channel of the confluence, which induces significant changes in the incoming velocity distribution. Moreover, it results in stronger helicoidal cells in the downstream channel, along with zones of upwelling flow. In turn, the changed flow patterns also influence the mixing layer and the flow recovery. Finally, intermittent events of stronger upwelling flow are discerned. Improved understanding of flow patterns at confluences where the tributary is dominant is applicable to both engineering and earth sciences.

Highlights

Confluences of open channels are important elements in hydraulic networks of rivers and man-made canals
Recognizing the knowledge gap, this paper offers a systematic study of the evolution of the flow patterns at a open channel confluence when the tributary discharge becomes increasingly dominant
The changes in flow features at small discharge ratios will be presented in Section 4.2 for the cases with q = 0.05 and q = 0

Summary

Introduction

Confluences of open channels are important elements in hydraulic networks of rivers and man-made canals. The associated flow patterns govern the transport of solutes and sediments in the network and influence the water levels of the incoming channels. The flow features that appear in an open channel confluence can be conceptualized as follows (Figure 1 after Best [1]): At the point where the two incoming flows meet, a stagnation zone develops, i.e., a zone of reduced flow velocity. A mixing layer departs, which delineates the merging streams. At the downstream junction corner, the tributary flow may detach, causing the formation of a separation zone. To the separation zone, the merging flows passing through a narrowed cross-section are contracted, leading to increased velocities

Results

Discussion

Conclusion