An appraisal of the velocity-reversal hypothesis for stable pool-rifflesequences in the River Severn, England
The literature in support of the hypothesis together with published adverse criticism is reviewed. These observations are substantiated by a detailed study of the hydraulic geometry of stable pool-rifflesequences in the River Severn, close
Stage dependent variability in tractive force distribution through a riffle-poolsequence
High resolution data on spatial and temporal variability in flow hydraulics and sediment transport within riffle-poolsequences are required to improve understanding of how fluvial processes maintain these meso-scale bedforms. This paper addresses
this issue by providing velocity and boundary shear stress data over a range of discharges from a sequence of 4 pools and 3 riffles in the River Rede, Northumberland.
Differential bed sedimentology and the maintenance of riffle-poolsequences
The purpose of this paper is to clarify several aspects of the sedimentology of riffle-poolsequences, and to suggest how these both reflect and control riffle-pool stability in the presence of spatial differences in the turbulent near-bed flow
Morphology of riffle-poolsequence in the River Severn, England
Despite the occurence of riffle-poolsequences in many rivers there are few data concerning riffle-pool unit morphology. In this paper, field data from the River Severn in England are analysed using 3 robust objective methods : the zero-crossing
Modelling three-dimensional flow structures and patterns of boundary shear stress in a natural pool-rifflesequence
The aim of this paper is to analyse hydraulic patterns in a natural pool-rifflesequence with a view to explaining the maintenance of pool-riffle morphology. Recent investigations have stressed the importance of combinations of hydraulic
Phase-shifts in shear stress as an explanation for the maintenance of pool-rifflesequences
The AA. propose that, instead of being reversed, maxima and minima in shear stress are phase-shifted with respect to the pool-rifflesequence bedform profile, so that maximum shear stress occurs upstream of riffle crests at high flow, and downstream
to the surveyed bathymetry of a pool-rifflesequence in a straight reach of a gravel-bed river, in southeastern Australia.
at low flow. Such phase-shifts produce gradients of shear stress that explain riffle deposition, and pool scour, at high flow, in accord with sediment continuity. This proposal is supported by results of a one-dimensional hydraulic model applied
Pool and riffle characteristics in relation to channel gradient
The channel gradients, along three rivers in coastal northern California, discussed in this paper are higher than those often reported for channels with pool-riffle, rather sequences. However, the AA. have designated these as pool-riffle, than step
-pool, channels, because of the presence of a strong lateral flow component, and the lack of well-organized bed-steps spanning the entire channel width.
Sediment transport processes in pool-rifflesequences
This paper reviews the published data for sediment transport in pool-rifflesequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport. This conclusion
Spatial variations in surface sediment structure in riffle–poolsequences : a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)
Riffle-poolsequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high
a riffle-poolsequence to parameterize a physically-based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle
exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. Field data were collected from Bury Green Brook
Simulation of flow over pool-riffle topography: a consideration of the velocity reversal hypothesis
This paper reports simulations of the variability in hydraulic parameters for flows through pool-rifflesequences on the River Severn, England. Initial runs were scaled using field data. The conditions that may be associated with a velocity
to conclude from stage-dependant morphometric changes to riffle and pool maintaining processes. An index for the longitudinal amplitude of riffle-poolsequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly
to the observed dynamics of bed amplitudes, maintenance of riffle-poolsequence lags behind discharge peaks.
significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes. According
Characteristics of velocity profiles along riffle-poolsequences and estimates of bed shear stress
The main objectives of this study were, therefore : to determine average characteristics of velocity profiles above pools and riffles in an attempt to differentiate between grain and form-related stress, and to determine how these characteristics
An experimental test of whether bar instability contributes to the formation, periodicity and maintenance of pool-rifflesequences
Australia ; Channel geometry ; Experimentation ; Longitudinal section ; Model ; New South Wales ; Periodicity ; Step-pool channel ; Stream
Pool-rifflesequences (PRSs) are periodic river-bed morphologies with wavelengths several times the channel width. In this paper, the AA. investigate whether the bar instability forming alternate bars also contributes to PRS formation, periodicity
De nombreuses hypothèses ont été avancées pour expliquer la formation de méandres| elles supposent souvent une énergie suffisante pour commencer le sapement des berges. Des variations de la géométrie des chenaux ont été observées sur des séquences
The aim of this paper is to analyse data on transverse ribs, step-pools and riffle and poolsequences, measured by the authors on a few rivers in the Italian Alps and the northern Apennines. Open questions about the origin of these bedforms
in order to discriminate the fields of existence of the larger roughness transverse, cyclic elements of gravel bed rivers and to show that transverse ribs, step-pools and riffle and poolsequences are part of a coarse grained bedform continuum.
The purpose of this paper is to re-evaluate the riffle-poolsequence in Dry Creek near Winters, California, and to test the velocity-reversal hypothesis by hydraulic modelling using detailed channel topography surveyed at Dry Creek. The AA. also use
for these species. More than 3/4 of the spawning sites studied were located directly upstream from or within a riffle and were close to slow-flowing areas suitable for use as retreats. Only 20% of the spawning sites lay within a typical pool-rifflesequence. About