Sediment transport and flow structures at river channel bifurcations
Background:
This is a NERC funded New Investigators Grant.
The division of one river channel into two (or more) smaller channels is a common occurrence within many fluvial systems. These fluvial forms are commonly called bifurcations, or diffluences, and they are fundamental forms within braided, anastomosed, and distributary systems such as alluvial fans, crevasse splays, and river deltas (Slingerland and Smith, 2004). Indeed, bifurcations give rise to channel patterns themselves, and within meandering systems they lie at the heart of the process of channel avulsion (e.g. Garcia and Nino, 1993). The past 25 years has witnessed substantial efforts to understand flow processes, mixing, sediment transport and the influence of channel geometry at sites where two rivers combine into one; namely river 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; Rhoads and Sukhodolov, 2001; Gaudet and Roy, 1994). This focus has reflected the importance of river channel junctions as key components of both dendritic drainage networks and braided river systems, including their associated importance for sediment transport and channel change (Mosley, 1976; Best, 1987; Ashmore et al., 1992). There has been recent recognition that in the braiding process the formation and control exerted by bifurcations in governing the downstream partitioning of flow could be as, if not more important than confluence zones (e.g. Pittaluga et al., 2001; Frederici and Paola, 2003). For example, the conclusions of the Flood Action Plan Project (FAP) 24 (1996), which examined the morphology of the major rivers of Bangladesh highlighted the importance of bifurcations and our current lack of knowledge on these vital fluvial forms. Indeed, all braided fluvial systems contain the fundamental confluence-bifurcation unit (e.g. Southard et al., 1984; Davoren and Mosley, 1986; Ferguson, 1993; Ashworth, 1996; Nicholas and Smith, 1999) and despite the extensive research into confluences, we have a far poorer understanding of flow and sediment dynamics in the bifurcation sites upstream and downstream of confluences. Crucially we know very little about the transition from confluence to bifurcation and particularly the flow characteristics that are inherited at, and downstream of, bifurcations (e.g. Richardson and Thorne, 2001). Additionally, the nature of the bifurcation will also control the character of flow in the downstream distributaries, and therefore will play a critical role in the movement of sediment through the confluence-bifurcation unit as a whole, governing the development and maintenance of mid-channel bars, which are central to the bifurcation itself.
Recent and ongoing work has mainly applied flume-based analysis of the morphological characteristics of both freely formed and forced (in channels with an imposed width expansion) bifurcations (e.g. Repetto et al., 2002; Frederici and Paola, 2003; Fu-Chun and Yeh, 2005). Research on numerical stability analysis (e.g. Colombini at al, 1987; Pittaluga et al., 2001) has also attempted to explain the conditions under which bifurcations form. As early as 1976, Mosley observed that flow divergence is one of the main causes of mid-channel bar growth and bifurcation formation, and this was confirmed by Ashworth et al. (2000) and Frederici and Paola (2003). However, we know very little about flow processes within the bifurcations themselves. This in turn has an influence on sediment movement through the reach and the overall development of mid-channel bars. The nature of the bed sediment movement through the reach raises questions over the upstream influence of confluence flow processes (e.g. Ashworth, 1996; Ashworth et al., 2000; Frederici and Paola, 2003), and in particular the inherited flow structures associated with the upstream confluence scour hole (e.g. Ashworth, 1996; Richardson and Thorne, 2001). We currently do not have the field data to begin to answer many of the complex questions concerned with bifurcations. Thus the prime aim of the research presented herein is to quantify and examine the three-dimensional flow and sediment dynamics upstream and downstream of natural channel bifurcations with the purpose of understanding the roles that three-dimensional flow velocity fields, secondary circulation inheritance, and the ensuing bed sediment transport plays in the development and maintenance of such river channel bifurcations.
Research Themes and Objectives:
The following objectives have been identified:
O1) Examine the partition of turbulent flow in natural bifurcating channels with particular regard to the character of the three-dimensional flow structures and the influence of topographic steering.
O2) Consider how upstream inherited flow structure influences the partitioning of flow and generation of flow structure within the bifurcation and how these flow structures are modified in downstream distributaries.
O3) Through the use of repeat multibeam sonar surveys quantify and examine the patterns bedforms around bifurcations and examine bedform migration rates, relating this to sediment transport pathways around bifurcations.
O4) Use a suitably verified and validated three-dimensional numerical model to look in detail at the flow structures present at natural bifurcations and use ‘what if’ scenario modelling to examine further O1 and O2.
O5) Reflect on the findings from O1-O4, and consider the wider implications and controls of bifurcations in the braiding process. This objective will seek to assess how the three-dimensional processes identified from O1-4 can be parameterised in two-dimensional and one-dimensional numerical analyses. This is important as bifurcations could then effectively represented in coarser scale models, having direct applications within modelling and management of such sites in natural channels.
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