The etymology and historic usage of such terms as anabranch', anastamose' and braided' within river science are reviewed. Despite several decades of modern research to define river channel typologi ...

Multichannel rivers : their definition and classification

There is a paucity of data and insight in the mechanisms of, and controls on flow separation and recirculation at natural sharply-curved river bends. Herein we report on successful laboratory experiments that elucidate flow structure in one constant-width bend and a second bend with an outer-bank widening. The experiments were performed with both a flat immobile gravel bed and mobile sand bed with dominant bedload sediment transport. In the constant-width bend with immobile bed, a zone of mainly horizontal flow separation (vertical rotational axis) formed at the inner bank that did not contain detectable flow recirculation, and an outer-bank cell of secondary flow with streamwise oriented rotational axis. Surprisingly, the bend with widening at the outer bank and immobile bed did not lead to a transverse expansion of the flow. Rather, flow in the outer-bank widening weakly recirculated around a vertical axis and hardly interacted with the inner part of the bend, which behaved as a constant-width bend. In the mobile bed experiment, downstream of the bend apex a pronounced depositional bar developed at the inside of the bend and pronounced scour occurred at the outside. Moreover the deformed bed promoted flow separation over the bar, including return currents. In the constant-width bend, the topographic steering impeded the generation of an outer-bank cell of secondary flow. In the bend with outer-bank widening, the topographic steering induced an outward expansion of the flow, whereby the major part of the discharge was conveyed in the central part of the widening section. Flow in the outer-bank widening was highly three dimensional and included return currents near the bottom. In conclusion, the experiments elucidated three distinct processes of flow separation common in sharp bends: flow separation at the inner bank, an outer-bank cell of secondary flow, and flow separation and recirculation in an outer-bank widening.

/pdf/flow-separation-at-the-inner-convex-and-outer-concave-banks-5cgkhcxn6x.pdf

Flow separation at the inner (convex) and outer (concave) banks of constant‐width and widening open‐channel bends

Bifurcation instability and chute cutoff development in meandering gravel-bed rivers

Bifurcating rivers shape natural landscapes by distributing water and sediments on fluvial plains and in deltas. Symmetrical bifurcations were often found to be unstable so that one branch downstream of the bifurcation enlarged while the other dwindled. A unified theory able to predict bifurcation stability in both gravel bed and sand bed rivers is still lacking. Here we develop a new theory for the stability of bifurcations for the entire range of gravel bed to sand bed rivers. The theory indicates opposite behavior of gravel bed and sand bed rivers: we predict that symmetrical bifurcations are inherently stable for intermediate Shields stresses but are inherently unstable for the low and high Shields stresses found in the majority of rivers on Earth. In the latter conditions asymmetrical bifurcations are stable. These predictions are corroborated by observations and have ramifications for many environmental problems in fluviodeltaic settings.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GL065175

A unified framework for stability of channel bifurcations in gravel and sand fluvial systems

Cohesive floodplain sediment and vegetation are both thought to cause meandering river patterns. Our aims are to compare the isolated and combined effects of mud and vegetation on river planform and morphodynamics in the setting of intermediate-sized valley rivers. We use a numerical model for century-scale simulation of flow, sediment transport and morphology coupled with riparian vegetation settlement, growth and mortality as functions of species traits on which flow resistance depends. Mud fluxes were predicted by excess shear stress relations in combination with the active layer formulation. We found that valley-flooding water levels increase with vegetation density, causing a higher braiding intensity rather than meandering tendency. The shear stress during floods carves channels through the muddy floodplain surface. Higher mud concentration, on the other hand, increases floodplain aggradation, reduces the overbank flow frequency and ultimately causes formation of a single-thread channel. Vegetation causes mud to deposit closer to the river channel as a levee, showing that mud sedimentation and vegetation settling mutually enhance floodplain formation. However, mud and vegetation counteract in two ways. First, vegetation enhances floodplain accretion, which ultimately increases plant desiccation for high mud concentrations. Second, vegetation increases the tendency of periodic chute cutoffs in valleys. The chute cutoffs locally reset the landscape and create new windows of opportunity for the vegetation. Surprisingly, in systems with a high mud concentration this causes hysteretic loops of vegetation cover and delayed mud deposition. Ramifications for the interpretation of Palaeozoic fluvial facies are that even rootless vegetation, capturing cohesive mud closer to the river channel to form thicker floodplain on the point bar, can enhance the tendency to meander and, under high mud supply, form stable channels. However, meandering is more unlikely in narrower valley rivers with higher vegetation density.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/esp.4437

Living landscapes : Muddy and vegetated floodplain effects on fluvial pattern in an incised river

Anastomosing rivers have multiple interconnected channels that enclose flood basins. Various theories potentially explain this pattern, including an increased discharge conveyance and sediment transport capacity of multiple channels, deltaic branching, avulsion forced by base-level rise, or a tendency to avulse due to upstream sediment overloading. The former two imply a stable anabranching channel pattern, whereas the latter two imply disequilibrium and evolution towards a single-channel pattern in the absence of avulsion. Our objective is to test these hypotheses on morphodynamic scenario modelling and data of a well-documented case study: the upper Columbia River. Proportions of channel and floodplain sediments along the river valley were derived from surface mapping. Initial and boundary conditions for the modelling were derived from field data. A 1D network model was built based on gradually varied flow equations, sediment transport prediction, mass conservation, transverse slope and spiral meander flow effects at the bifurcations. The number of channels and crevasse splays decreases in a downstream direction. Also, measured sediment transport is higher at the upstream boundary than downstream. These observations concur with bed sediment overloading from upstream, which can have caused channel aggradation above the surrounding floodplain and subsequent avulsion. The modelling also indicates that avulsion was likely caused by upstream overloading. In the model, multi-channel systems inevitably evolve towards single-channel systems within centuries. The reasons are that symmetric channel bifurcations are inherently unstable, while confluenced channels have relatively less friction than two parallel channels, so that more discharge is conveyed through the path with more confluences and less friction. Furthermore, the present longitudinal profile curvature of the valley could only be reproduced in the model by temporary overfeeding.

Evaluating competing hypotheses for the origin and dynamics of river anastomosis

Anastomosing rivers have multiple interconnected channels that enclose floodbasins. Various theories have been proposed to explain this pattern, including an increased discharge conveyance and sediment transport capacity of multiple channels, or, alternatively, a tendency to avulse due to upstream sediment overloading. The former implies an equilibrium pattern whereas the latter implies a temporary increase of the number of channels. Our objective was to test these hypotheses on a well-documented case: the upper Columbia River. We combined a geological approach with physics-based morphological modelling. Three geological sections across the entire valley were constructed based on hand auger data, covering a depth up to 8.6 m corresponding to the last 4000 years, corroborated by 14C dating. The sections were integrated with surface mapping to derive proportions of channel and floodplain sediment volumes and sedimentation rates along the river. Existing sediment transport measurements at a location upstream and downstream of the study area were re-analysed and time-integrated for comparison. A network model was built based on gradually varied flow equations, sediment transport prediction, mass conservation and detailed transverse slope and spiral meander flow effects at the bifurcations. The geological sections show a clear downstream trend of decreasing number of channels and decreasing bed sediment deposition in channels and crevasses, indicating bed sediment overloading from upstream and subsequent avulsions. The measured suspended sediment is much larger than the amount captured in the floodplains, indicating that this is not limiting the aggradation in the valley. Extensive crevasse splays in the upstream section and an increased bed elevation and gradient demonstrate a tendency to avulse due to overloading of bed sediment. The measured bedload transport indeed indicates bed material overloading. The 14C dating confirms that long-term average floodplain sedimentation rates decrease significantly in downstream direction. The overloading led to an anastomosing river pattern by in-channel aggradation and avulsions, followed by slow channel fill so that multiple channels remain open for a long time. Although multiple channels convey flood discharge, only one channel transports the majority of the sediment so that morphodynamically this is a single-channel system. The 1D network model indicates that the multi-channel system evolves towards a single-channel system within centuries because symmetric channel bifurcations are inherently unstable whilst confluenced channels attract the flow due to relative decreased friction. Moreover, the long profile of the valley could only be reproduced by overfeeding the model with a sediment pulse four times larger than the transport capacity for a century and then reduce the feed to capacity in agreement with the measurements. We conclude that this anastomosing river pattern is the result of a temporary pulse of sediment and is not an equilibrium feature.

E. Lavooi

Papers

Evaluating competing hypotheses for the origin and dynamics of river anastomosis

Anastomosing Rivers are Disequilibrium Patterns