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International audience; Steady uniform and non-uniform flows through an array of emergent and slightly submerged square cylinders are experimentally investigated with a specific focus on transverse seiche waves induced by vortex shedding. The study is first and foremost aimed at assessing the effect of streamwise flow non-uniformity on seiche waves. Its secondary purpose is to investigate the change in seiche magnitude, when initially emerged cylinders become slightly submerged. Thirdly and lastly, the effect of seiche waves on mean velocities and velocity fluctuations is quantified. The lock-in process between waves and vortex shedding is unaltered by flow non-uniformity and by a change from cylinder emergence to submergence. For non-uniform flows, this results in the coexistence of two differently oscillating transverse waves close to each other. Relative wave amplitude is found to be mainly influenced by relative submergence in the case of submerged cylinders, and by Froude number and oscillation mode in the case of emergent cylinders. Finally, seiche waves modify the streamwise mean velocity, when cylinders are emergent.

This PhD thesis investigates the effect of a longitudinal change in floodplain land use on an overflooding river flow. We consider a transition between a meadow and a woodland and vice versa. This change in land use is associated with a change in hydraulic roughness, between a bed roughness (highly submerged meadow) and emergent macro-roughnesses (trees), respectively modelled by a plastic artificial grass and an array of emergent cylinders. The flows are experimentally investigated in an 18 m x 3 m laboratory flume. In a first step, we investigate the flow through a cylinder array in a single channel, focusing on the effect of bed roughness on the cylinder wakes and on the seiche phenomenon (strong free surface oscillations). In a second step, we study the development towards flow uniformity of compound channel flows with a uniform hydraulic roughness on the floodplains. The asymmetrical growth of the compound channel mixing layer, the self-similarity property and the three-dimensional organisation of the turbulent coherent structures associated with the mixing layer are analysed. In a third step, we investigate the longitudinal change in roughness in compound channel configuration, which effects on mixing layer and on coherent structures are discussed. We also assess the contributions to lateral transfers of momentum between main channel and floodplain by turbulent diffusion, by mass exchange and by secondary currents; Ce travail de thèse s'intéresse à l'effet d'une variation longitudinale de l'occupation du sol de la plaine d'inondation sur l'écoulement d'une rivière en débordement. Nous traitons le cas d'une transition entre une zone de prairie et une zone de forêt, et vice versa. Cette variation d'occupation du sol est associée à une transition de rugosité hydraulique entre une rugosité de fond (prairie fortement immergée) et des macro-rugosités émergées (arbres), modélisées respectivement par une moquette plastifiée et par un champ de cylindres. Ces écoulements sont étudiés en laboratoire dans un canal de dimension 18 m x 3 m. Dans un premier temps, nous considérons l'écoulement à travers un champ de cylindres émergents en lit simple, en étudiant l'effet du fond sur le sillage des cylindres et le phénomène de seiche (fortes oscillations de la surface libre). Dans un deuxième temps, nous nous penchons sur le développement vers l'uniformité d'un écoulement en lit composé de rugosité uniforme. La croissance asymétrique de la couche de mélange du lit composé, la propriété d'autosimilarité ainsi que l'organisation tridimensionnelle des structures turbulentes cohérentes associées à la couche de mélange sont analysées. Le troisième temps fait l'objet de la transition longitudinale de rugosité en lit composé, dont l'effet sur la couche de mélange et sur les structures cohérentes est discuté. Nous évaluons également les différentes contributions au transfert latéral de quantité de mouvement entre lit mineur et plaine d'inondation par diffusion turbulente, par échange de masse et par les courants secondaires

This paper investigates energy losses in compound channel under non-uniform flow conditions. Using the first law of thermodynamics, the concepts of energy loss and head loss are first distinguished. They are found to be different within one sub-section (main channel or floodplain). Experimental measurements of the head within the main channel and the floodplain are then analyzed for geometries with constant or variable channel width. Results show that head loss differs from one sub-section to another: the classical 1D hypothesis of unique head loss gradient appears to be erroneous. Using a model that couple 1D momentum equations, called "Independent Sub-sections Method (ISM)", head losses are resolved. The relative weights of head losses related to bed friction, turbulent exchanges and mass transfers between sub-sections are estimated. It is shown that water level and the discharge distribution across the channel are influenced by turbulent exchanges for (a) developing flows in straight channels, but only when the flow tends to uniformity; (b) flows in skewed floodplains and symmetrical converging floodplains for small relative flow depth; (c) flows in symmetrical diverging floodplains for small and medium relative depth. Flow parameters are influenced by the momentum flux due to mass exchanges in all non-prismatic geometries for small and medium relative depth, while this flux is negligible for developing flows in straight geometry. The role of an explicit modeling of mass conservation between sub-sections is eventually investigated. (C) 2009 Elsevier Ltd. All rights reserved.

This paper investigates 1D-modelling of non-uniform flows in compound channels. The issue is how to accurately predict both flow depth and mean velocity in the floodplain. A new model, called Independent Sub-sections Method (ISM), is presented here. Unlike classical 1D models that solve a dynamic equation on the total cross-section, the ISM estimates the water surface profile within each sub-section. This enables the water level and the sub-section mean velocities to be simultaneously calculated, without priority to any variable. In opposition to the DCM, corrected DCM or the EDM, the ISM assumes independent evolution of the discharge in each sub-section of the compound channel. Indeed, this method does not assume equal head loss gradients in all sub-sections, and it does not impose the downstream discharge distribution. The ISM consists in a set of three coupled 1D momentum equations (written within main channel, left-hand and right-hand floodplains) and a mass conservation equation on the total cross-section. Mass and momentum exchanges at the interfaces between sub-sections are explicitly accounted for. This method is validated against experimental data for: developing flows in straight compound channel, flows in skewed compound channel, in a symmetric converging or diverging compound channel, and in an asymmetrical compound channel with an abrupt floodplain contraction. For the 46 runs, the ISM predicts flow depth and mean velocity in the floodplain with a maximum relative error of 8% and 19% respectively. The ISM also appears to be a useful theoretical tool to improve our understanding of physical processes governing compound channel flows.

International audience; The aim of this paper is to evaluate the relative weights of three sources of energy loss for non-uniform flows in compound channel: (1) the bed friction; (2) the momentum transfer due to turbulent exchange between the main channel and the floodplains; and (3) the momentum transfer due to mass exchange between the subsections. Energy losses are resolved using a quasi-1D model, namely the Independent Subsections Method (ISM). The ISM computes the water level and the subsection-averaged velocities in the main channel and in the floodplains, simultaneously. This method was tested and validated against experimental measurements in three straight geometries and in eight non-prismatic geometries (skewed floodplains, diverging and converging compound channels). Using the ISM simulations of 46 flows, energy losses due to turbulent exchanges (St), to bed friction (Sf), and to mass exchanges (Sm) are estimated. The relative weights of the dissipation terms at the interfaces between the main channel and the floodplains (St and Sm) and of the mass conservation terms (denoted Ma) are then compared. The results show to what extent the mass conservation and the momentum transfer control the flow depth and the discharge in the floodplain.

International audience; The aim of this paper is to evaluate the relative weights of three sources of energy loss for non-uniform flows in compound channel: (1) the bed friction; (2) the momentum transfer due to turbulent exchange between the main channel and the floodplains; and (3) the momentum transfer due to mass exchange between the subsections. Energy losses are resolved using a quasi-1D model, namely the Independent Subsections Method (ISM). The ISM computes the water level and the subsection-averaged velocities in the main channel and in the floodplains, simultaneously. This method was tested and validated against experimental measurements in three straight geometries and in eight non-prismatic geometries (skewed floodplains, diverging and converging compound channels). Using the ISM simulations of 46 flows, energy losses due to turbulent exchanges (St), to bed friction (Sf), and to mass exchanges (Sm) are estimated. The relative weights of the dissipation terms at the interfaces between the main channel and the floodplains (St and Sm) and of the mass conservation terms (denoted Ma) are then compared. The results show to what extent the mass conservation and the momentum transfer control the flow depth and the discharge in the floodplain.

Flooding rivers usually present transition reaches where the floodplain width can significantly vary. The present work focuses on both physical and numerical modeling of over bank flows in such configurations. A particular attention is paid to flows in the flood plain. These flows are characterized by turbulent exchanges due to the velocity gradient between flows in the main channel and the floodplain, and by severe mass transfer and associated momentum exchange between the subsections. New experiments are carried out in non-prismatic compound channel flumes : flows in abrupt contraction of the flood plain, enlarging flood plains, flow in the vicinity of groynes. In addition to conventional 1D and 2D simulations, a new 1D modeling is presented : it is called Independent Subsections Method and separates the dynamic equations in each bed. The simulation of both water depth and discharge rate in the floodplain are in good agreement with experimental data, for eight different geometries.eight tested geometries.; La modélisation des inondations se heurte à plusieurs difficultés dès lors qu'interviennent des débordements de l'écoulement du lit mineur dans les lits majeurs contigus. Les écoulements « en lit composé » sont alors caractérisés par une interaction turbulente entre l'écoulement du lit mineur et celui du lit majeur, mais aussi par des transferts de masse entre lits lorsque la largeur du lit majeur varie. La thèse s'intéresse aux modélisations expérimentale et numérique de ces écoulements non-uniformes. De nouvelles configurations sont explorées : convergence brusque ou divergence linéaire de la plaine d'inondation, épi de type « remblai routier » dans le lit majeur. En plus de simulations numériques 1D et 2D conventionnelles, une nouvelle modélisation est présentée : nommée « Méthode 1D par lit », elle individualise la dynamique de l'écoulement dans chacun des lits et donne des résultats satisfaisants en terme de niveau d'eau et de débit dans le lit majeur, dans huit géométries différentes.

Flooding rivers usually present transition reaches where the floodplain width can significantly vary. The present work focuses on both physical and numerical modeling of over bank flows in such configurations. A particular attention is paid to flows in the flood plain. These flows are characterized by turbulent exchanges due to the velocity gradient between flows in the main channel and the floodplain, and by severe mass transfer and associated momentum exchange between the subsections. New experiments are carried out in non-prismatic compound channel flumes : flows in abrupt contraction of the flood plain, enlarging flood plains, flow in the vicinity of groynes. In addition to conventional 1D and 2D simulations, a new 1D modeling is presented : it is called Independent Subsections Method and separates the dynamic equations in each bed. The simulation of both water depth and discharge rate in the floodplain are in good agreement with experimental data, for eight different geometries.eight tested geometries.; La modélisation des inondations se heurte à plusieurs difficultés dès lors qu'interviennent des débordements de l'écoulement du lit mineur dans les lits majeurs contigus. Les écoulements « en lit composé » sont alors caractérisés par une interaction turbulente entre l'écoulement du lit mineur et celui du lit majeur, mais aussi par des transferts de masse entre lits lorsque la largeur du lit majeur varie. La thèse s'intéresse aux modélisations expérimentale et numérique de ces écoulements non-uniformes. De nouvelles configurations sont explorées : convergence brusque ou divergence linéaire de la plaine d'inondation, épi de type « remblai routier » dans le lit majeur. En plus de simulations numériques 1D et 2D conventionnelles, une nouvelle modélisation est présentée : nommée « Méthode 1D par lit », elle individualise la dynamique de l'écoulement dans chacun des lits et donne des résultats satisfaisants en terme de niveau d'eau et de débit dans le lit majeur, dans huit géométries différentes.