[1] Alternative general forms are considered for equations to predict mean velocity over the full range of relative submergence experienced in gravel-and boulder-bed streams. A partial unification is suggested for some previous semiempirical models and physical concepts. Two new equations are proposed: a nondimensional hydraulic geometry equation with different parameters for deep and shallow flows, and a variable-power resistance equation that is asymptotic to roughness-layer formulations for shallow flows and to the Manning-Strickler approximation of the logarithmic friction law for deep flows. Predictions by existing and new equations using D 84 as roughness scale are compared to a compilation of measured velocities in natural streams at relative submergences from 0.1 to over 30. The variable-power equation performs as well as the best existing approach, which is a logarithmic law with roughness multiplier. For predicting how a known or assumed discharge is partitioned between depth and velocity, a nondimensional hydraulic geometry approach outperforms equations using relative submergence. Factor-of-two prediction errors occur with all approaches because of sensitivity to operational definitions of depth, velocity, and slope, the inadequacy of using a single grain-size length scale, and the complexity of flow physics in steep shallow streams.

Entrainment of mixed size gravel bed material was studied in nine reaches of three high‐power streams in Scotland and Norway. Paired measurements of at‐a‐point shear stress (estimated from velocity profiles) and bed load transport (by hand‐held sampler) were made. They extended to very high values (400 N m−2, 3.5 kg m−1 s−1). Analyses of maximum bed load diameter, mean bed load diameter, transport rates of individual size fractions, and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size, despite strong relative size effects. Precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates. Size‐selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant.

[1] In highly curved river bends, flow may separate at the inner bank to create a recirculation eddy with weak upstream flow. Very little is known about how recirculation eddies connect with the downstream flow or how the latter is affected by their presence. We investigate these questions using three-dimensional time-averaged computational fluid dynamics models of two natural bends with inner-bank separation. Test measurements of velocity in one bend agree well with the simulation. Common points in the two simulations are that (1) an outer-bank helix is only present in the upstream part of the bend, (2) maximum near-bank velocities are highest here rather than beyond the apex as in most bends, (3) reverse flow extends farther across the channel at the surface than the bed, and (4) flow within the recirculation eddy has a three-dimensional structure, linked with that in the outer-bank free stream. Substantial differences in detail between the two bends appear to be due to differences in upstream planform, manifested through the lateral distribution of inflow velocity.

River bifurcations are critical but poorly understood elements of many geomorphological systems. They are integral elements of alluvial fans, braided rivers, fluvial lowland plains, and deltas and control the partitioning of water and sediment through these systems. Bifurcations are commonly unstable but their lifespan varies greatly. In braided rivers bars and channels migrate, split and merge at annual or shorter timescales, thereby creating and abandoning bifurcations. This behaviour has been studied mainly by geomorphologists and fluid dynamicists. Bifurcations also exist during avulsion, the process of a river changing course on a floodplain or in a delta, which may take 102–103 years and has been studied mainly by sedimentologists. This review synthesizes our current understanding of bifurcations and brings together insights from different research communities and different environmental settings. We consider the causes and initiation of bifurcations and avulsion, the physical mechanisms controlling bifurcation and avulsion evolution, mathematical and numerical modelling of these processes, and the possibility of stable bifurcations. We end the review with some open questions. Copyright © 2012 John Wiley & Sons, Ltd.

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