Sediment Transport, Bed Morphology and the Sedimentology of River Channel Confluences

Abstract

ContextRiver channel confluences are sites of significant hydraulic and morphological change within fluvial networks (Richards, 1980;Rhoads, 1987;Ferguson et al., 2006; Lane, this volume, Chapter 3) and also occur within river channels where islands or bars are present. The local and downstream effects of confluences can have a profound influence on the geomorphology and ecology of river channels -see, for example, Rice et al.-as well as on strategies for effective channel management (Pinter et al., 2004). For these reasons, the morphology of river channel confluences is of major importance within a range of considerations and disciplines. For example, scour-depth predictions at channel junctions are clearly needed in the design of engineering structures, whilst the recognition of confluence scour is important in reconstructions of ancient sedimentary environments (Bristow et al.

Subsurface flow mixing in coarse, braided river deposits

“…Therefore, they suggested to simulate the internal structure of the trough fills based on geometric considerations, i.e. by several shifted half-ellipsoids representing the trough migration (see also Best and Rhoads, 2008). In this study, the trough fills are represented by truncated ellipsoids.…”

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

“…Where the channels upstream of the confluence are meandering, the junction angle could thus change over time in response to bend migration and channel cut-off. Finally, formation of a mid-channel bar in the post-confluence channel (Mosley, 1976;Best, 1988),can occur through convergence of sediment transport pathways (Best, 1988;Best and Rhoads, 2008) and declining flow velocities and turbulence intensities downstream of the zone of maximum flow acceleration (Best, 1987;Best, 1988;Sukhodolov and Rhoads, 2001;Rhoads and Sukhodolov, 2004). Such bar formation can promote bank erosion and channel widening (Mosley, 1976), potentially driving changes in confluence morphology over time although this mid-channel bar formation is somewhat dependent on the first two factors.…”

“…1) is dynamic and responds and adjusts to upstream boundary conditions of flow and sediment supply in each tributary, and thus confluences may be expected to adjust to three broad factors. Firstly, upstream boundary conditions of discharge, or momentum, ratio between the tributaries, where momentum ratio exerts a control on scour morphology (Mosley, 1976;Best, 1986;Best, 1988;Best and Rhoads, 2008) and tributary bar morphology (Best, 1988;Biron et al, 1993;Rhoads, 1996;Biron et al, 2002;Boyer et al, 2006;Best and Rhoads, 2008). There is also some evidence that inter-event fluctuations in momentum ratio can lead to changes in bar morphology (Boyer et al, 2006), and where tributaries drain different lithological or climatic areas there could be annual or seasonal variations in momentum flux.…”

“…River confluences are important nodal points in alluvial networks, often representing abrupt downstream changes in discharge, grain size and channel geometry, which in turn may exert a significant control on channel morphology, migration and avulsion (Mosley, 1976;Richards, 1980;Ashmore, 1991;Bridge, 1993;Ashmore and Gardner, 2008;Best and Rhoads, 2008). The morphology of the confluence zone also has many ramifications for understanding and managing aspects of river behaviour, such as the fact that the dynamic morphological adjustments at these sites may make managing land use and infrastructure difficult (Ettema, 2008).…”

“…Since the depth of junction scour and mobility of the confluence are determined by flow processes in the confluence hydrodynamic zone (Best and Rhoads, 2008), it can be argued that differing junction dynamics may produce a range of characteristic confluence zone sedimentology from sandy bar development to mudfilled scours. Furthermore, understanding the planform mobility of confluences, and thus the potential spatial extent of basal scour surfaces, particularly in large rivers, is key to interpreting alluvial stratigraphy and discriminating between autocyclic and allocyclic scour surfaces (Best and Ashworth, 1997;Fielding, 2008), reconstructing palaeohydraulics and channel sedimentary architecture (Bristow et al, 1993;Siegenthaler and Huggenberger, 1993;Miall and Jones, 2003;Davies and Gibling, 2011), as well as identifying potential sites for hydrocarbon exploration (Ardies et al, 2002).…”