Fine sediment deposits in gravel bed rivers: sensitivity analysis to particle properties using a 2D hydrodynamic and sediment model

Hydrodynamic

Entrainment and motion of coarse particles in a shallow water stream down a steep slope

We investigate the entrainment, deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope. This is an idealization of bed-load transport in mountain streams. Earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics. The intermittent character of particle transport at low-water discharges led us to consider it as a random process. Sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles. We develop a birth-death immigration-emigration Markov process to describe the particle exchanges between the bed and the water stream. A key feature of the model is its long autocorrelation times and wide, frequent fluctuations in the solid discharge, a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments. We present experimental data obtained using a nearly two-dimensional channel and glass beads as a substitute for sediment. Entrainment, trajectories, and deposition were monitored using a high-speed digital camera. The empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model. Experiments confirmed the existence of wide and frequent fluctuations of the solid discharge, and revealed the existence of long autocorrelation time, but theory overestimates the autocorrelation times by a factor of around three. Particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model, which performs well when a single particle is moving. For our experiments, the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity. We also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased. The more vigorous the stream was, the more continuous the solid discharge became. Moreover, although 90% of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10%, particles dissipate more and more energy when the bed slope is increased, but surprisingly, the dissipation rate is nearly independent of fluid velocity. A movie is available with the online version of the pape

Statistical description of sediment transport experiments

A longstanding problem in the study of sediment transport in gravel-bed rivers is related to the physical mechanisms governing bed resistance and particle motion. To study this problem, we investigated the motion of coarse spherical glass beads entrained by a steady shallow turbulent water flow down a steep twodimensional channel with a mobile bed. This experimental facility is the simplest representation of sediment transport on the laboratory scale, with the tremendous advantages that boundary conditions are perfectly controlled and a wealth of information can be obtained using imaging techniques. Flows were filmed from the side by a high-speed camera. Using image processing software made it possible to determine the flow characteristics such as particle trajectories, their state of motion (rest, rolling, or saltating motion), and flow depth. In accordance with earlier investigations, we observed that over short time periods, sediment transport appeared as a very intermittent process. To interpret these results, we revisited Einstein’s theory on sediment and derived the statistical properties (probability distribution and autocorrelation function) of the key variables such as the solid discharge and the number of moving particles. Analyzing the autocorrelation functions and the probability distributions of our measurements revealed the existence of long-range correlations. For instance, whereas theory predicts a Binomial distribution for the number of moving particles, experiments demonstrated that a negative binomial distribution best fit our data, which emphasized the crucial role played by wide fluctuations. These frequent wide fluctuations stemmed particle entrainment and motion being collective phenomena rather than individual processes, contrary to what is assumed in most theoretical models

Application de la méthode LSPIV pour la mesure de champs de vitesse et de débits de crue sur modèle réduit et en rivière

International audienceLSPIV technique enables the measurement of surface flow velocities using image sequence analysis. EDF and Irstea partnership made possible the development of Fudaa‑LSPIV freeware by DeltaCAD Company. Two software applications at flume and field scales are detailed: (i) bed shear stresses were calculated owing to LSPIV velocities, water depth and bathymetry for a physical model of the Old Rhine; (ii) the software was used to optimize the calculation parameters of LSPIV flood discharge measurement stations in Mediterranean rivers.La technique LSPIV (Large Scale Particle Image Velocimetry) permet de mesurer les vitesses de surface d'un écoulement par analyse de séquence d'images. Pour faciliter l'application opérationnelle de la méthode, un logiciel, Fudaa-LSPIV, a été développé par la société DeltaCAD dans le cadre d'une collaboration entre EDF et Irstea. Deux applications en laboratoire et en rivière sont présentées : (i) couplée avec des mesures de hauteur d'eau et de bathymétries, la LSPIV a permis d'estimer des paramètres de Shields sur le modèle physique à fond mobile du Vieux-Rhin ; (ii) le logiciel a été utilisé pour procéder à des analyses de sensibilité pour paramétrer ainsi au mieux les stations LSPIV de mesure de débit en crue de rivières cévenoles

Entrainment and motion of coarse particles in a shallow water stream down a steep slope

We investigate the entrainment, deposition and motion of coarse spherical particles within a turbulent shallow water stream down a steep slope. This is an idealization of bed-load transport in mountain streams. Earlier investigations have described this kind of sediment transport using empirical correlations or concepts borrowed from continuum mechanics. The intermittent character of particle transport at low-water discharges led us to consider it as a random process. Sediment transport in this regime results from the imbalance between entrainment and deposition of particles rather than from momentum balance between water and particles. We develop a birth–death immigration–emigration Markov process to describe the particle exchanges between the bed and the water stream. A key feature of the model is its long autocorrelation times and wide, frequent fluctuations in the solid discharge, a phenomenon never previously explained despite its ubiquity in both nature and laboratory experiments. We present experimental data obtained using a nearly two-dimensional channel and glass beads as a substitute for sediment. Entrainment, trajectories, and deposition were monitored using a high-speed digital camera. The empirical probability distributions of the solid discharge and deposition frequency were properly described by the theoretical model. Experiments confirmed the existence of wide and frequent fluctuations of the solid discharge, and revealed the existence of long autocorrelation time, but theory overestimates the autocorrelation times by a factor of around three. Particle velocity was weakly dependent on the fluid velocity contrary to the predictions of the theoretical model, which performs well when a single particle is moving. For our experiments, the dependence of the solid discharge on the fluid velocity is entirely controlled by the number of moving particles rather than by their velocity. We also noted significant changes in the behaviour of particle transport when the bed slope or the water discharge was increased. The more vigorous the stream was, the more continuous the solid discharge became. Moreover, although 90% of the energy supplied by gravity to the stream is dissipated by turbulence for slopes lower than 10 %, particles dissipate more and more energy when the bed slope is increased, but surprisingly, the dissipation rate is nearly independent of fluid velocity. A movie is available with the online version of the paper

Near-bed and surface flow division patterns in experimental river bifurcations

Understanding channel bifurcation mechanics is of great importance for predicting and managing multichannel river processes and avulsion in distributary river deltas. To date, research on river channel bifurcations has focused on factors determining the stability and evolution of bifurcations. It has recently been shown that, theoretically, the nonlinearity of the relation between sediment transport and flow discharge causes one of the two distributaries of a (slightly) asymmetrical bifurcation to grow and the other to shrink. The positive feedback introduced by this effect results in highly asymmetrical bifurcations. However, there is a lack of detailed insight into flow dynamics within river bifurcations, the consequent effect on bed load flux through bifurcating channels, and thus the impact on bifurcation stability over time. In this paper, three key parameters (discharge ratio, width-to-depth ratio, and bed roughness) were varied in order to examine the secondary flow field and its effect on flow partitioning, particularly near-bed and surface flow, at an experimental bifurcation. Discharge ratio was controlled by varying downstream water levels. Flow fields were quantified using both particle image velocimetry and ultrasonic Doppler velocity profiling. Results show that a bifurcation induces secondary flow cells upstream of the bifurcation. In the case of unequal discharge ratio, a strong increase in the secondary flow near the bed causes a larger volume of near-bed flow to enter the dominant channel compared to surface and depth-averaged flow. However, this effect diminishes with larger width-to-depth ratio and with increased bed roughness. The flow structure and division pattern will likely have a stabilizing effect on river channel bifurcations. The magnitude of this effect in relation to previously identified destabilizing effects is addressed by proposing an adjustment to a widely used empirical bed load nodal-point partition equation. Our finding implies that river bifurcations can be stable under a wider range of conditions than previously thought. Key Points Secondary flow in symmetrical bifurcations causes strong near-bed flow curvature A disproportional amount of near-bed flow enters the dominant downstream channel Flow curvature adds a stabilizing feedback on bifurcation evolution

Une paramétrisation de la vitesse de chute pour un mélange sablo-vaseux dans un modèle hydro-sédimentaire 1D pendant un évènement de chasse

International audienceSettling velocity is one of the key parameters to understand the deposit phenomena and to succeed in its representation in numerical models. To evaluate this parameter in-situ, a field campaign was carried out during a flushing event of the upper Arc River (Northern French Alps) in June 2011. Since most of the measurement methods are unsuitable in the complete concentration range of this flushing event (0.2 to 20 g/l), a simple method with a settling column was used. Water samples were regularly collected in the river from bridges and directly poured into settling columns. About ten measurements were done for each of the three experimental sites along the river system, to estimate the temporal and spatial dynamics of the settling velocity during the flushing event. Grain size characteristics of the suspension were determined using a laser grain sizer. Regarding these results and the measured concentration of each sample, a detailed description of the different settling velocity regimes is proposed. In the same time, concentration data measured during the same event give information on the suspended sediment concentration (SSC) propagations. Measurements of SSC were done at eleven sampling sites along a 100 km reach downstream from the last dam. Water level was also measured at several locations as well as water discharge. These measurements allow us to calibrate a 1D hydraulic model, used to estimate the suspended sediment mass balance between each measurement site. The deposition fluxes given by this method are compared to the measured values obtained in the settling column

Optimisation d'un nombre limité de points de mesure topographique pour des applications en rivières

International audienceA prime requirement for hydrological applications, such as sediment budgeting or numerical modelling, is that produced Digital Terrain Models (DTMs) accurately represent the shape of landforms, especially for river reaches where data are not homogeneous. DTM error is a function of data point measurement accuracy and density and also of the field survey strategy when limited amounts of data will be acquired. This paper aims to advance the importance of the field survey strategy for the specific, but common cases, where only limited topographic data will be available. This methodology is based on the idea that any feature can be properly described by a set of cross sections and breaklines describing both main and secondary directions of the flow. Then, a longitudinal linear interpolation can be applied to the defined homogeneous zones. This morphologically oriented (MO) method that includes data acquisition strategy and interpolation, was validated using a reference DTM derived from LiDAR measurements. An stimation of the uncertainties also is suggested based on the distance of the nearest point and the local slope using a geographically weighted regression. The proposed MO method is typically applicable to Alpine river reaches characterized by multiple channels that may always be underwater and not navigable such as an alternate bar system with secondary and transverse channels

Estimation du transport solide en suspension le long de l'Arc et de l'Isère (France) au cours d'une chasse de barrage

International audienceDam flushing events are conducted regularly on the Arc River (France) to remove sediments from the dam reservoirs. Four field campaigns carried out during dam flushing events in the upper Arc River basin (in June 2006, 2007, 2009 and 2010) in Northern French Alps are analyzed in this paper. All of these events have a similar discharge hydrograph. At twelve sampling sites along the river channel, measurements of suspended sediment concentration (SSC) were performed. A 1D numerical hydraulic model MASCARET (Goutal and Maurel, 2002) was calibrated and used to estimate the instantaneous discharge at any position along the river system. Thus, the data set was completed to estimate the suspended sediment flux at the twelve sites. To quantify errors, the classical error propagation method is used, taking into account both errors from measurement and modelling. A spectral analysis of the SCC signal at each sampling site gave a new approach to quantify SSC estimation errors that may occur during this type of event. After integration at each measurement site, a spatial distribution of the suspended load was obtained along the Arc and Isère rivers. This work enable us to identify specific locations of the river where deposition is accentuated or where the river bed is eroded. Thanks to these five flushing dam events, it appeared that the response of the river is not the same from one year to another

Modélisation numérique du transport solide en suspension au cours d'une chasse: de la dynamique du réservoir à la propagation en aval

International audienceDam flushing is often performed to remove sediments of all sizes from reservoirs while minimizing downstream impacts. In this study, the reservoir flushing operation and the downstream transport of suspended sediments are simulated based on a numerical tool. The study site is the Arc River in the French Alps, focussing on two reaches: locally on the Saint Martin la Porte (SMLP) Reservoir and a few metres upstream (reach 1), and downstream the 120 km between the SMLP reservoir and the city of Grenoble (reach 2).The chosen numerical code is COURLIS, developed at EDF RD, and which could be coupled with codes of the open source Telemac-Mascaret system. The model was applied to the 2012 flushing event of the Arc River dams, which are managed by EDF (Electricity of France). The bed erosion and the suspended sediment transport dynamics in the most downstream reservoir (SMLP) were first computed. The initial state of the deposited fine sediment were defined using measured topographic data of the reservoir. The upstream boundary condition for the reach 2 model was set from estimated discharges and suspended sediment concentrations. The downstream study reach is approximately 120 km long, from SMLP dam to the city of Grenoble along the Arc and Isere rivers. Topographic data of the river bed were used to define the downstream river reach, taking into account the vegetated alternate bars. Water discharges and suspended sediment concentrations were also measured at several locations in the rivers downstream of the flushed reservoirs. A new formula is proposed to estimate two mean concentration values in the cross section: one for the main channel and the other for the overbank section. A good agreement with field data is obtained for both the reservoir and downstream reach dynamics. Moreover, the key role of the alternate bars in the fine sediment dynamics is highlighted, quantified and discussed