Morphology, flow structure, and suspended bed sediment transport at two large braid-bar confluences

SZUPIANY, RICARDO NICOLÁS; AMSLER, MARIO LUIS; PARSONS, DANIEL; BEST, JAMES

WATER RESOURCES RESEARCH

AMER GEOPHYSICAL UNION

Año: 2009 vol. 45 p. 1 - 19

0043-1397

Abstract: [1] Midchannel bars and their associated confluences are key morphodynamic nodes  within braided rivers, with past studies having investigated the morphodynamics of smallnatural channels or laboratory models with relatively low width/depth (W/D) ratios,  typically at <10. This paper investigates the morphology, suspended bed sediment  distribution, and flow structure at two large braid bar confluences in the Río Paraná  (Argentina), wherein W/D ratios are much higher (approaching 100) than in smaller  channels. The results highlight the significant control of the cross-sectional distribution of  downstream flow velocity on confluence flow, suspended bed sediment concentration, and  morphodynamics and indicate that this factor may become progressively more significant  with increasing channel scale and W/D ratio, particularly when simple discharge (or  momentum) ratios between the incoming flows are used to explain the flow dynamics.  Additionally, 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  highlights the importance of these findings with respect to the flow and sediment  dynamics of large channel confluences.