A data set of 2890 field measurements was used to test the ability of several conventional flow resistance equations to predict mean flow velocity in gravel bed rivers when used with no calibration. ...The tests were performed using both flow depth and discharge as input since discharge may be a more reliable measure of flow conditions in shallow flows. Generally better predictions are obtained when using flow discharge as input. The results indicate that the Manning‐Strickler and the Keulegan equations show considerable disagreement with observed flow velocities for flow depths smaller than 10 times the characteristic grain diameter. Most equations show some systematic deviation for small relative flow depth. The use of new definitions for dimensionless variables in terms of nondimensional hydraulic geometry equations allows the development of a new flow resistance equation. The best overall performance is obtained by the Ferguson approach, which combines two power law flow resistance equations that are different for deep and shallow flows. To use this approach with flow discharge as input, a logarithmic matching equation in terms of the new dimensionless variables is proposed. For the domains of intermediate and large‐scale roughness, the field data indicate a considerable increase in flow resistance as compared with the domain of small‐scale roughness. The Ferguson approach is used to discuss the importance of flow resistance partitioning for bed load transport calculations at flow conditions with intermediate‐ and large‐scale roughness in natural gravel, cobble, and boulder bed streams.
Key Points
Some conventional flow resistance equations should not be used in shallow flows
Between‐site and at‐a‐site variation of flow resistance show similarities
Flow resistance partitioning can improve bedload transport predictions
This paper explores the potential of unmanned aerial system (UAS) optical aerial imagery to characterize grain roughness and size distribution in a braided, gravel-bed river (Vénéon River, French ...Alps). With this aim in view, a Wolman field campaign (19 samples) and five UAS surveys were conducted over the Vénéon braided channel during summer 2015. The UAS consisted of a small quadcopter carrying a GoPro camera. Structure-from-Motion (SfM) photogrammetry was used to extract dense and accurate three-dimensional point clouds. Roughness descriptors (roughness heights, standard deviation of elevation) were computed from the SfM point clouds and were correlated with the median grain size of the Wolman samples. A strong relationship was found between UAS-SfM-derived grain roughness and Wolman grain size. The procedure employed has potential for the rapid and continuous characterization of grain size distribution in exposed bars of gravel-bed rivers. The workflow described in this paper has been successfully used to produce spatially continuous grain size information on exposed gravel bars and to explore textural changes following flow events.
Bedload transport modelling in rivers takes into account the size and density of pebbles to estimate particle mobility, but does not formally consider particle shape. To address this issue and to ...compare the relative roles of the density and shape of particles, we performed original sediment transport experiments in an annular flume using molded artificial pebbles equipped with a radio frequency identification tracking system. The particles were designed with four distinct shapes and four different densities while having the same volume, and their speeds and distances traveled under constant hydraulic conditions were analyzed. The results show that particle shape has more influence than particle density on the resting time between particle displacement and the mean traveling distance. For all densities investigated, the particle shape systematically induced differences in travel distance that were strongly correlated (R
= 0.94) with the Sneed and Folks shape index. Such shape influences, although often mentioned, are here quantified for the first time, demonstrating why and how they can be included in bedload transport models.
The effect of the sediment supply on step-pool morphology and stability was studied through the investigation of 42 step-pool reaches (eight in the Vosges and 34 in the Alps, France). Step-pool ...geometry (step wavelength
L and height
H) was measured for each reach and the data set split into two distinct groups comprising 25 connected (to a sediment source) step-pool reaches and 17 disconnected step-pool reaches.
The geometry analysis showed differences between the two groups, with the shape factors
k
=
L/
H
r
and the
H/
H
r
ratios (
H
r
is the residual pool depth) being lower for disconnected step-pools. This was interpreted to result from progressively changing geometry over time, with disconnected step-pool geometry defining the probable asymptotic characteristic associated with this long-term process.
The measured step-pool dimensions were in good agreement with equations based on maximum flow resistance (Abrahams et al., 1995), energy of a falling jet (Comiti et al., 2005), and channel spanning deposition (Chartrand et al., 2011). However, the average shape factor for the Comiti et al. equation was
k
=
17 instead of 8, as the authors had proposed. The data set was extended to 255 values with data from the literature, which confirmed this average value for
k.
Field evidence (moss cover and stone imbrication) indicate that connected step-pools are mobile whereas disconnected step-pools are stable. This was investigated by a hydraulic analysis of threshold motion conditions for the boulder steps. Comparison with discharges measured at nearby gauging stations indicates that remobilization return periods are ~
20–50
years for connected step-pools (and even less when considering scouring effects) and greater than 50
years for disconnected step-pools. This difference in step mobility was ascribed to the fact that, with sediment supply, the channel can freely adjust its local slope, allowing a relatively efficient transfer downstream of large boulders by scouring effects.
► The effect of sediment supply on step-pools geometry and stability was investigated. ► A data set was built with 42 step-pools collected in the Vosges and the Alps. ► Data are in good agreement with morphological models of the literature. ► Connexion to a sediment source increases step-pools instability. ► The data analysis indicates relatively deeper pools in disconnected step-pools.
There is a paradox in the relationship between bedload transport rates and flow variables: laboratory and field studies have reported on how bedload transport rates depend on flow variables through a ...power law, but none of the empirical laws fitted to the data has managed to provide accurate predictions of bedload transport over a wide range of flow conditions. Inferring bedload transport's scaling behavior from data has remained a stubborn problem because the data are very noisy. It is, therefore, difficult to progress on this problem without some informed speculation about how bedload and flow interact. Ralph Bagnold proposed an original theory to resolve this problem. This paper reviews and updates Bagnold's model by separating the effects of flow resistance and efficiency (energy transfer from water to bedload) on dimensionless transport rates Φ. Both variables' contributions to transport rates can be parameterized separately for the three transport regimes that Bagnold defined (no transport, transitional, and sheet flow). We also consider two possible control variables: the dimensionless Shield stress τ∗ and a dimensionless number related to stream power. In the transitional regime, the dimensionless bedload transport rate scales as Φ∝τ∗3, whereas in the sheet-flow regime, it varies as Φ∝τ∗5/3. We end up with two Bagnold equations: one based on physical principles and involving Shields stress τ∗, flow resistance f, a density ratio, and a bed slope; the other based on non-linear regression and stream power. Compared to a large set of laboratory and field data, predictions from Bagnold's model show reasonable accuracy when the bed is plane.
•We derive and update Bagnold's model of bedload transport with a new focus on flow resistance.•We calibrate the model's parameters using independent datasets.•Three bedload transport regimes are identified.•Comparing Bagnold's model with laboratory data shows good agreement for plane beds.•We also propose an alternative formulation based on nonlinear regression.•Microstructural models' contribution to bedload theory is discussed.
Dense seismic array monitoring has recently allowed the detailed investigation of sources of ground vibrations and their spatiotemporal dynamics. In a context where traditional monitoring techniques ...for fluvial processes often do not provide data with adequate temporal and spatial resolution, the use of dense arrays could allow the identification and tracking of different sources of river-induced seismic ground vibrations (e.g. turbulence and bedload transport). Here, we study the potential of dense seismic array monitoring by investigating a high-flow event that occurred in summer 2019 along a 600-m-long braided reach of the Séveraisse River (French Alps). We use a network of 80 seismometers deployed on both river banks, and we supplement the seismic observations with flow gauging measurements and time-lapse imagery. During this event, we observe impulsive signals that are coherently detected over the array, and which we interpret as being associated with the bedload transport of clusters of coarse grains (blocks). Through phase-delay analysis we are able to locate these seismic events on the bend apex of an active branch of the reach. These results demonstrate the capability of such a method to locate bedload activity at high spatiotemporal resolution, providing crucial information for geomorphological investigations and natural risk management.
La mise en œuvre de techniques d’analyse par réseaux sismiques denses a récemment permis d'obtenir des dispositifs d'observation uniques pour les processus de surface de la Terre. Malgré leur grand potentiel, ces approches n'ont pas encore été appliquées pour l’étude du fonctionnement des rivières et de leur évolution morphologique. Ici, nous étudions le potentiel de l’instrumentation par réseau sismique dense en analysant les données d'une crue qui s’est produite en été 2019 le long d'un tronçon en tresse de 600 m de la Séveraisse (Alpes françaises). Nous avons installé un réseau de 80 sismométres sur les deux rives de la riviére, et nous complétons ces observations par des mesures de débit liquide et des images time-lapse. Au cours de cet événement, nous observons des signaux impulsionnels qui sont détectés de maniére cohérente à travers le réseau, et que nous interprétons comme étant associés à des clusters de sédiments transportés par charriage. Grâce à l'analyse de différence de phase, nous sommes en mesure de localiser les épisodes de mouvement sur une section de la riviére particuliérement active pendant la crue. Nos travaux démontrent la capacité unique des réseaux sismiques denses à localiser les phénoménes de charriage à haute résolution spatio-temporelle, en fournissant des informations cruciales pour les études géomorphologiques et la gestion des risques naturels.
For morphodynamic studies, it is important to compute not only the transported volumes of bed load, but also the size of the transported material. A few bed load equations compute fractional ...transport (i.e., both the volume and grain size distribution), but many equations compute only the bulk transport (a volume) with no consideration of the transported grain sizes. To fill this gap, a method is proposed to compute the bed load grain size distribution separately to the bed load flux. The method is called the Generalized Threshold Model (GTM), because it extends the flow competence method for threshold of motion of the largest transported grain size to the full bed surface grain size distribution. This was achieved by replacing dimensional diameters with their size indices in the standard hiding function, which offers a useful framework for computation, carried out for each indices considered in the range 1, 100. New functions are also proposed to account for partial transport. The method is very simple to implement and is sufficiently flexible to be tested in many environments. In addition to being a good complement to standard bulk bed load equations, it could also serve as a framework to assist in analyzing the physics of bed load transport in future research.
Key Points
River morphodynamics requires computation of the transported grain size
The GTM method computes the transported grain size independently of bed load transport
The competence method for threshold transport of the maximum transported grain size is extended to the whole grain size distribution
Sediment transport and storage in mountain stream channels are closely dependent on hillslope activities and bank erosion. This paper analyses bedload measured in 13 mountain streams (989 values with ...slope higher than 5%) in an attempt to investigate how much the sediment supply condition impacts bedload transport rates. First, the sediment supply condition of each stream was evaluated qualitatively using information available in the original studies. In the second step, the data set was analysed by comparing bedload transport rates considering a Shields stress ratio and by comparing transport rate efficiency with consideration of a dimensionless stream power. The two analyses indicate higher sediment transport rates for the streams identified as “connected” to an active source. The consequences for bedload transport prediction are discussed with a nonthreshold bedload equation. Even though predicting the exact sediment supply condition and the associated transport rate is not yet possible, this study suggests that defining an envelope delimiting minimum and maximum transport for a given river reach should be possible.
► The effect of sediment supply on mountain streams bedload transport was investigated. ► A field data set composed of 989 values was analysed. ► Streams connected to active sediment source have higher transport rate efficiencies. ► Measurements were compared to a bedload transport equation. ► An envelope delimiting minimum and maximum transport was proposed.
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