Flow Structures, Shear-Generated Turbulence, and Fluid Mixing at River Confluences: The Influence of Channel Scale?

SZUPIANY, RICARDO N.; PARSONS, DANIELS; AMSLER, MARIO L.; BEST, JAMES

San Francisco, California

Encuentro; AGU Fall Meeting; 2009

American Geophysical Union

Channel confluences are key morphodynamic nodes within both dendritic channel networks and braided rivers, with past studies having investigated the flow structure and morphodynamics of small natural channels or laboratory models that often possessed relatively low width/depth (W/D) ratios (typically ~ 5 to 20). This paper reports on field-based investigations on the flow structure and fluid mixing at confluent channels many orders of magnitude larger, wherein W/D ratios are typically much higher (~ 100 and above). The results highlight major issues regarding scale variance in processes operating at river confluences, which underlie the significant control of the cross-sectional distribution of downstream flow velocity on flow structure, turbulence-generation and fluid mixing at larger confluences. The results suggest that this factor may become progressively more significant with channel scale and W/D ratio, particularly when simple discharge (or momentum) ratios between the incoming flows are used to explain the flow dynamics. For example, secondary flow cells, often proposed to occupy a large part of the channel width in small river channel confluences, are only identified in relatively small portions of the channel width at these larger spatial scales. Such a restriction seems related to the generative mechanisms of secondary flows at these higher W/D ratios, which are likely to be dominated by turbulence generated along the mixing layer between the two flows and topographic influences that limit the spatial extent of these effects. This paper concludes by highlighting the