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Flow Structures and Flow Partitioning at River Channel Bifurcations
A research project of the Department of Geography.
A joint project between Durham University (Professor S.N. Lane and Dr R.J. Hardy) and the University of Leeds (Professor J.L. Best).
There has been a strong growth in the scientific interest in the role of bifurcations (also known as diffluences) as part of the braiding process. During the 1980s and 1990s, substantial efforts were made to understand flow processes, sediment transport and channel geometrical influences with respect to tributary junctions or confluences (e.g. Ashmore et al., 1992; Best, 1987; Best and Reid, 1984; Best and Roy, 1991; Biron et al., 1993a, 1993b, 1996a, 1996b; Bradbrook et al., 1998, 2000a,b, 2001; Lane et al., 1999; 2000; Moseley, 1976; Rhoads and Kenworthy, 1995, 1998; Roy et al., 1994). This focus reflected the importance of river channel junctions as part of dendritic drainage networks, but also their common occurrence in braided rivers, including their associated importance for sediment transport and channel change (Mosley, 1976; Ashmore et al., 1992). During this period, bifurcations were relatively neglected. However, over the last 5 years, there has been progressive recognition that bifurcations exert a crucial control over the nature of the river braiding process. Indeed, there may have been a disproportionate influence on confluences in braided rivers, even though bifurcations may be more important as controls on the downstream partitioning of flow (Frederici and Paola, 2003), and given how important they appear to be in the different modes of channel changes that have been identified in braided rivers (Ashmore, 1991; Ferguson, 1993). Despite extensive research into confluences, there has been much less research into what happens in the bifurcations downstream of confluences, nor of the transition from a confluence to a bifurcation in relation to the flows that the bifurcation inherits. Similarly, the nature of the bifurcation will control the nature of the flow in the downstream distributaries and we know little about the implications of this for distributary behaviour.
The aims of this research are to investigate the interaction between bifurcation geometry, flow partitioning and flow structures in relation to channel braiding processes in order:
- to understand and to explain the characteristic range of bifurcation morphologies; and
- to validate and to inform system-scale analyses of systems that contain channel bifurcations
This individual(s) appointed to this research project will address the following:
The prime focus of the bifurcation research to date has combined: (1) flume-based analysis of the morphological characteristics of both freely formed (in initially-straight channels) and forced (in channels with an imposed width expansion) bifurcations (e.g. Frederici and Paola, 2003); with (2) numerical stability analysis (e.g. Pittaluga et al., 2001); in order to identify and to explain the conditions under which bifurcations form. This work has led to major progress in our understanding of bifurcations, but there have been very few attempts to explore the morphological characteristics using either more extensive survey of the bifurcations found in simulated braided river channels or analysis of data acquired from field braided rivers.
Bifurcation Flow Partitioning and Flow Structures
Given that the prime focus of bifurcation measurements to date has involved scaled laboratory studies, we know very little about flow processes within bifurcations. This knowledge is important as the flow structures that form within the bifurcation region will have implications for flow partitioning between the two distributaries as well as the flow structures inherited by them (Paola, 2001). On the basis of research into both river meanders and confluences, we can hypothesise that the controlling variables will be: (1) flow forcing by streamline curvature, in relation to width:depth ratio and flow inertia; (2) its interaction with bifurcation asymmetry; and (3) topographic forcing (e.g. McArdell and Faeh, 2001; ‘altimetric effects’ after Pittaluga et al., 2002), where there is divergence between flows at the surface and flows at the bed due to the depth dependence of the magnitude of topographic forcing terms and flow at the bed adjusts to topographic change more readily than that at the surface. This may be exacerbated by the effects of inertia upon water surface gradients which may reverse up onto the bar head. We now have robust models for flow processes in confluences in braided rivers, but not for river channel confluences.
Downstream Flow Structures
The nature of the bifurcation will influence both flow partitioning and flow structures within the distributaries. Evolution of the braid bar system will depend upon the way in which distributaries modify flow structures formed within the bifurcation. For instance, how is vorticity generated at the bifurcation advected downstream? How is this controlled by distributary morphology? Bridge and Gabel (1992), Paola (2001) and Pittaluga et al. (2001) noted the presence of strong curvature in braid bar distributaries and the role of bifurcations in generating that curvature has yet to be addressed (Pittaluga et al, 2001).
Bifurcations Within the Confluence-Bifurcation Unit
Given the importance that has been attached to the confluence-bifurcation unit (e.g. Southard et al., 1984; Davoren and Mosley, 1976; Ferguson, 1993; Ashworth, 1996), questions emerge over the way in which bifurcation processes and their downstream effects upon distributary flow structures are influenced by confluence processes operating upstream. This issue has been noted by a number of researchers (e.g. Ashworth, 1996; McLelland et al., 1996; Frederici and Paola, 2003). The majority of the laboratory and stability analysis experiments have focused upon a uniform upstream inflow. However, confluence research has demonstrated that it can take some time for the two confluent flows to mix (Roy and Gaudet, 1994), and this may be associated with the way in which flow out of a confluence scour-hole is forced to diverge at the bed concurrently with the core of high intensity turbulent lifting from the bed (McLelland et al., 1996). Ashworth (1996) noted that the distance between scour hole formation and the initial growth of a downstream mid channel bar is a function of the total discharge of the tributaries, the tributary discharge ratio and junction angle, sediment discharge and the width of the downstream confluent channel. Of particular interest here is the issue raised by Richardson and Thorne (2001): to what extent and in what way do bifurcation characteristics relate to the characteristics of flow inherited from upstream?
The research will:
- Identify the geometrical characteristics of bifurcations as found in a range of different braided river systems
- Use the information acquired from 1 to design and to undertake a series of laboratory experiments that identified controls upon flow partitioning and flow structure formation
- Extend the laboratory experiments in 2 to explore the effects of distributary morphology and upstream inherited flow upon flow partitioning and flow structure formation, again for idealised bifurcations
- Extend the combinations of variables identified from 1 and explored in 2 and 3 to a wider variable set using three-dimensional computational fluid dynamics
- Undertake scaled laboratory simulation and CFD simulations for a small number of idealised bifurcations identified under 1
- Situate the reductionist approach adopted to this study within broader thinking regarding the role of bifurcations in braided river behaviour