Form roughness and the absence of secondary flow in a large confluence–diffluence, Rio Paraná, Argentina

Abstract

Confluence-diffluence units are key elements within many river networks, having a major impact upon the routing of flow and sediment, and hence upon channel change. Although much progress has been made in understanding river confluences, and increasing attention is being paid to bifurcations and the important role of bifurcation asymmetry, most studies have been conducted in laboratory flumes or within small rivers with width:depth (aspect) ratios less than 50. This paper presents results of a field-based study that details the bed morphology and 3D flow structure within a very large confluence-diffluence in the Río Paraná, Argentina, with a width:depth ratio of approximately 200. Flow within the confluence-diffluence is dominated largely by the bed roughness, in the form of sand dunes; coherent, channel-scale, secondary flow cells, that have been identified as important aspects of the flow field within smaller channels, and assumed to be present within large rivers, are generally absent in this reach. This finding has profound implications for flow mixing rates, sediment transport rates and pathways, and thus the interpretation of confluence-diffluence morphology and sedimentology.

Modelling Hydraulics and Sediment Transport at River Confluences

“…Thus, the magnitude of the driving component of curvature-driven circulation is likely to be very small indeed, and it is not surprising that early results from large rivers do not find helical circulation (e.g. Parsons et al, 2007). However, for the same river, Lane et al (in press) show that there are situations in which channel-scale helicallike circulation can form.…”

“…Second, the worth of hydrodynamic models in addressing some of the key unanswered questions regarding confluences remains to be established. This is notably the case for issues regarding large river junctions, where questions are emerging about the transferability of conclusions reached from small-scale field and laboratory studies (Parsons et al, this volume, Chapter 5;Parsons et al, 2007;Lane et al, in press) where numerical modelling represents a particular challenge as a result of limits to effective computation over such large spatial scales. This chapter reviews the key findings that result from hydraulics and sediment-transport modelling of confluences and uses these to present new ideas on the generalization of observations from the laboratory scale to the very large scale.…”

The Planform Mobility of River Channel Confluences: Insights from Analysis of Remotely Sensed Imagery

“…Mosley, 1976;Best, 1988;Roy and Bergeron, 1990;Best and Roy, 1991;Biron et al, 1993;Kenworthy and Rhoads, 1995;Rhoads and Kenworthy, 1995;Rhoads, 1996;Rhoads and Kenworthy, 1998;De Serres et al, 1999;Rhoads and Sukhodolov, 2001;Biron et al, 2002;Boyer et al, 2006;Leite Ribeiro et al, 2012), and it is only with recent advances in technology that the direct field investigation of large river confluences has been possible (e.g. McLelland et al, 1999;Ashworth et al, 2000;Richardson and Thorne, 2001;Parsons et al, 2005;Parsons et al, 2007;Lane et al, 2008;Parsons et al, 2008;Sambrook Smith et al, 2009). There is therefore a S.J.…”

Introduction: River Confluences, Tributaries and the Fluvial Network

“…The biological attributes of confluences have received some attention (Cellot, 1996;Kupferberg, 1996;Franks et al, 2002;Fernandes et al, 2004;Kreb and Budiono, 2005;Kiffney et al, 2006), as have the dynamics of ice jams at confluences (Prowse, 1986;Ettema et al, 1997;Shen et al, 2000;Ettema and Muste, 2001). At this scale, improved understanding informed, and was informed by, studies of confluences in braided rivers (Ashmore, 1991;Ashworth et al, 1992;Best and Ashworth, 1997), which, arguably, has laid the foundation for recent investigations of the dynamics of river bifurcations (Dargahi, 2004;Federici and Paola, 2003;Khan et al, 2000;Parsons et al, 2007).…”