River confluences: a review of recent field and numerical studies

Confluences are common components of all riverine systems and are characterized by converging streamlines and potential mixing of separate flows. The fluid dynamics of confluences is characterized by a highly complex structure with several common types of flow features observed. Many theoretical, field, laboratory and numerical studies tried to identify and clarify the main hydrodynamics, morphodynamics and ecological features of river confluences. Those studies spanned a wide range of flow and geometrical conditions ranging from laboratory-scale studies conducted in simplified geometries to field-scale studies of large-scale confluences. In particular, the identification of the main types of coherent structures generated within the post-confluence channel and of their dynamics has led to significant progress in understanding hydrodynamics and mixing processes at natural river confluences. Still, further research is needed to fully clarify how such structures interact among themselves and with the erodible bed and channel banks, how they redistribute momentum and affect transport of heat, suspended sediment, nutrients and what is their role in creating mixing patterns of dissolved and solid matter. In addition, the role of density differences between the incoming waters has garnered increasing attention in confluence studies, while the literature about the relationship between frequently observed species richness and confluence hydrodynamics is still limited. The present paper tries to review the literature on field and numerical studies of natural river confluences with a particular focus on the case in which density contrast affects confluence hydrodynamics and mixing and on large-scale river confluences. The paper discusses some main findings based on the field studies and eddy-resolving numerical simulations conducted by the authors. The application of bioenergetics metric at two large-size concordant bed confluences is also reported. Finally, the paper identifies some directions of future research that should lead to a better understanding of flow structure, transport processes, morphodynamics as well as of the ecological role of river confluences under different conditions.