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Publication# Morphology and morphodynamics of braided rivers: an experimental investigation

Résumé

Braided rivers form some of the most fascinating fluvial patterns found on Earth. They are identifiable by their unique morphology of complex networks of intertwined channels that spread across wide floodplains. Detailed knowledge of their dynamics is needed to define proper river management strategies that can address both human needs (e.g. protection against floods, bank migration, etc.) and natural needs (e.g. the preservation of fauna and flora, river restoration, etc.).

Recently, the study of braided rivers has undergone significant progress. Developments in the areas of laboratory experiments, monitoring techniques and field surveys, in addition to new paradigms in the field of geosciences and mathematical modelling have greatly improved our understanding of braided rivers. However, many questions remain unanswered. Is it possible to predict the long-term evolution of a braided river under steady flow conditions? More fundamentally, where do the braided pattern emerge from? Does it grow out of an intrinsic flow instability? And, if this is the case, which one? The present work aims to fill two specific gaps in the current state of knowledge: the dynamics of braided river networks and the development of a morphodynamic model that uses a non-equilibrium bedload formula that can predict bedforms that ultimately produce braiding.

This thesis studied the dynamics of the braided networks experimentally. Two laboratory-scale experiments were performed from which we extracted and investigated the braided network's temporal evolution. A set of variables describing the network was determined -namely the number of nodes, the number of links and the network's total link length. These variables were shown to relate to the flow conditions. Moreover, the evolution of the braided network was described by identifying similar network configurations as modes. The modes' evolution was well captured by their probability. Using a Markov process, we were ultimately able to reproduce the probability of occurrence of those modes.

A morphodynamic model based on the shallow-water equations and a stochastic-based bedload transport formula was developed. Applying linear stability theory, we were able to obtain marginal stability curves that predicted the development of bedforms. Two types of bedforms were identified: two-dimensional bedforms (antidunes and dunes) and three-dimensional bedforms (bars). The results agreed well with the literature. The present work was the first morphodynamical model to predict the development of both dunes and bars within the same framework using shallow-water equations. Moreover, we were able to show, albeit qualitatively, the influence of particle diffusion-present in the bedload transport equation-in the development of bedforms.

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Sediment transport

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained.

Rivière

vignette|redresse|Après le torrent se forme la rivière (Hautes-Pyrénées).
vignette|Phénomène de surcreusement du lit majeur, pouvant participer à un phénomène d'aridification, le niveau piézométrique

Méthode expérimentale

Les méthodes expérimentales scientifiques consistent à tester la validité d'une hypothèse, en reproduisant un phénomène (souvent en laboratoire) et en faisant varier un paramètre. Le paramètre que l

Christophe Ancey, Joris Heyman, Hongbo Ma, François Mettra

The phenomena of bed forms exist widely in the natural rivers and are still not fully understood. The detailed sediment dynamics near the bed is essential for this problem. However, the fluid dynamics near the bed, which drives the sediment motion, is not clear. In this talk, we focus on the fluid dynamics of supercritical flow over a sinusoidal wavy bed, especially around the wall region. This setup mimics anti-dunes morphology i.e. bedforms that are commonly found in steep mountain streams. In this case, the flow depth and the bedform amplitude have the same order of magnitude with the amplitude of the bedform. To study the detailed fluid flow, a 3-Dimensional numerical simulation of Navier-Stokes equations is performed. Two different models, Reynolds Average Simulation (RAS) and Large Eddy Simulation (LES), are used for the turbulence closure. The two models are validated with experiments carried out on a wavy bed. Particular attention is paid to the fluid shear stress on the wavy bed and the bedform equivalent roughness. LES shows more abilities for this problem. In future, various wavelength and amplitude of the sinus wave will be implemented so that new shear stress formulas and parameterization for the anti-dune roughness in shallow water equations will be proposed.

2013The study of the sediment transport in open-channel as well as in river flow is of great importance for fluvial hydraulics. While the transport of sedimentary particles at the bed, as the bed-load, has been the subject of much research, less attention has been paid to the transport of sediments in suspension. This thesis is a contribution to our knowledge of the transport of sediments in suspension. The most important theory on the vertical distribution of the mean sediment concentration in suspension flows is the diffusion-convection theory, given by the Rouse equation. This equation is rather simple and contains few parameters. About one of these parameters, the β-value, there is little known, despite its importance and physical meaning. One of the main aims of this thesis is to experimentally measure the β-values for different suspension flow configurations. The major objective was to investigate suspension flows over moveable beds without bed forms and this in capacity condition. Since flows are at times not in capacity, i.e. the flows are not saturated with sediments, an additional study (see Appendix C) was also performed. Furthermore, two special runs were done (see Appendix D), where the bed was covered with artificial bed forms. In order to evaluate the β-values, the vertical profiles of the mean velocity and its fluctuations as well as the mean concentration and its fluctuations had to be measured. This could only be done by using the new ultrasonic instrument (Acoustic Particle Flux Profiler, APFP), developed and assembled at the Laboratoire de Recherches Hydrauliques (LRH). Suspension flows were investigated focusing on the possible modification of the clear-water turbulence by suspended sediments. The concentration was carefully measured and interpreted by the Rouse equation. In particular, the ratio of the sediment, εs(y) , and momentum, εm(y) , diffusion coefficients, being the β(y) -value, was for the first time, directly measured. The strongest effect of particles on the flow was noticed on the vertical component of the turbulence intensity, , which was considerably suppressed, when compared to clear-water flow. The longitudinal velocity, u(y) , its turbulence intensity component, , and the Reynolds stress, – ρu'v' , were only slightly affected. By using the APFP instrument, the instantaneous sediment concentrations, cs, were, for the first time, directly measured. The calibration of the APFP instrument was achieved by measuring the mean concentration profiles, cs, with the suction method. The largest measured fluctuating concentrations, , and sediment flux, profiles were observed close to the bed, where the mean concentration is also very large. The sediment, εs, and the momentum, εm, diffusion coefficients, which represent the "ability" of sediment and fluid particles to be diffused in the flow by turbulence, were computed from the APFP measurements. For suspension flows over plane-bed, the sediment diffusion-coefficient profiles are always smaller than the momentum ones, εs < εm. This indicates that sediment particles undergo less diffusion than fluid particles; consequently the β-values are less than unity, β < 1. Thus, the usual assumption of εs = εm, that leads to β = 1, is not justified for the present measurements. The Rouse equation gives a better agreement to the concentration distributions measured with the suction method, by using the experimental obtained β-values, , rather than by using β = 1. For practical use, the experimental values and the ones obtained by a best-fit (least-squares method) of the Rouse equation to some concentration distributions taken from the literature are summarized in a plot. It seems that the β-values increase with either scaling parameters, vss/u*, and (vss/u*) · (Cs/csa). The effects of suspended particles on the clear-water turbulence were also investigated in non-capacity suspension flows having increasing concentrations. The measurements show that, the suppression (damping) of the vertical component of the turbulence intensity – caused by suspended particles – increases with the depth-averaged concentration. The tendency of the - values to decrease approaching the capacity condition, i.e. increasing the concentration, was also found. Suspension flows over bed forms were also investigated; in this study special attention was paid to the evolution of the flow structure behind the bed-form crest as well as the effect of bed forms on the β-values. The bed-form crest seems to generate a high-turbulence region with consequent peaks in both the longitudinal and vertical turbulence-intensity profiles as well as in the Reynolds-stress profiles. This high-turbulence region enhances the sediment diffusion coefficient but suppresses the momentum one. As a consequence, we found that for suspension flows over beds with bed forms, εs > εm, leading to β > 1. The most important result of this thesis is a recommendation that the Rouse equation with an improved β-value – itself to be estimated from the experimental plots obtained in this study – can be used to establish the dimensionless concentration profile of suspension flows. For beds without bed forms the β-values are β < l, while for beds with bed forms the β-values are β > 1. The correlation between the velocity fluctuations, associated with coherent structures (burst cycle), and the concentration was also investigated. From this study it becomes evident that the ejection event, being the most important phase of a burst cycle, is responsible of the erosion of sediment on the bed, behaving as an "injector" of sediments into the main flow.