Study of flow through rockfill in channel = Etude des écoulements dans une mèche en canal

We present the results of an experimental, theoretical, and also numerical study of free surface buried flows that we have undertaken in the laboratory LSTE of INAT. This work was conducted as part of collaboration with the engineering international office MECATER which included the construction of a new channel with large section in INAT, this channel was designed to study the losses in porous media with high porosity for which Darcy's law is no longer applicable. The experiments consisted in determining the evolution of the free surface for different geometrical characteristics of riprap and different flow rates. These tests were used to verify different laws of head loss proposed in the literature, by performing numerical simulations of the evolution of the water surface for each experiment. The analysis of our results shows that the Stephenson’s formula is relevant to calculate the non-linear head loss for flows with high Reynolds number of pore. Furthermore, the simulations have highlighted the important role of porosity. These results should be confirmed by further tests to assess in particular the influence of the slope and to analyze the behavior of the flow at the inlet and outlet of the riprap

Evolution of flow velocities in a rectangular channel with homogeneous bed roughness.

The flow velocity above large scale roughness is investigated for low and steep slope between 0 and 4 %. The experiments were conducted in the laboratory of Fluid Mechanics Institute of Toulouse -IMFT. For the velocity measurement, the channel is equipped with a fast camera and a lightening system. The originality of the study lies in the application of a particle tracking technique (Particle Tracking Velocimetry). It is a non-intrusive measurement technique to measure instantaneous velocity in two-dimensional fields in stationary and unsteady flows. Analysis of the results is performed by processing images taken by the camera using a developed detection algorithm. A logarithmic distribution has been found for The velocity profiles which are influenced by roughness and slope. The obtained results show a depression of the maximum speed below the free surface. This behavior indicates a delay of the flow in the vicinity of the free surface and this is a direct consequence of the presence of secondary flows in these areas

Analysis of Saint Venant Model Closure Laws from 3D Model Simulations

In this work, in a first step, the effects of secondary motions on the transverse distribution of the depth average velocity in free surface flows above non uniform bottom roughness is illustrated by simulations based on an anisotropic algebraic Reynolds stress model (3D). These 3D-simulations were applied to determine the wall friction and the dispersion terms present in the depth average momentum equation. In a second step, closure laws of these terms were tested to define a 2D-Saint Venant model which is solved to calculate the transverse profile of the depth averaged velocity. This process could allow analyze of scale change problems

Study of free surface flows in rectangular channel over rough beds.

This paper presents the results of an experimental and umerical study of fully developed flow in a straight rectangular open channel over rough beds. Conical ribs were placed on the flume bottom to simulate different bed roughness conditions. Acoustic Doppler Velocimetry (ADV) measurements were made to obtain the velocity components profiles as well as the Reynolds stress profiles, at various locations. The experimental results are validated by simulations using an algebraic stress model. These investigations could be useful for researches in the field of sediment transport, bank protection, etc

Solution analytique pour le calcul de la ligne d'eau dans des écoulements à surface libre à travers des enrochements

Dans cette étude est développée une solution analytique pour le calcul de la ligne d'eau dans des écoulements à surface libre à travers des enrochements. Cette solution est basée sur la résolution de l'équation de la ligne d'eau, en utilisant la relation de Stephenson pour les pertes de charge. L'objectif est de déterminer les pertes de charge pour différentes configurations d'écoulement afin de vérifier la formule de Stephenson, et de voir sa capacité à reproduire la ligne d'eau. Cette solution analytique a été appliquée à une première série de nos essais réalisés dans un nouveau canal rectangulaire, construit à l'INAT en collaboration avec Mecater, et des experts de l'INP Toulouse. Mecater s'intéressait à ces écoulements, dans le cadre de projet avec la Nouvelle Calédonie, concernant la protection des stériles miniers contre les écoulements d'eau, dans les hautes montagnes. Ces applications nous ont permis d'avoir une analyse préliminaire. Les valeurs des hauteurs d'eau à l'entrée de la mèche étant des données obtenues pour chaque essai, et les premiers calculs à l'aide de cette solution analytique ont montré que le calcul de la courbe de remous (avec un calage du coefficient d'angularité Kt), a donné des résultats acceptables

Asymptotic Solutions of Algebraic Reynolds Stress Model Applied to Rough Bottom Open Channel Flow

We interpret experimental results on the structure of an open channel flow with a strong transverse variation of the bottom roughness. Knowing the wall parameters, we analyze the behavior of Reynolds stress components by using asymptotic solutions of an algebraic stress model developed in the wall and free surface regions. This analysis allowed us to emphasize effects of secondary flows on the production of turbulence near the wall, and the capability of this model to predict the normal components of the Reynolds tensor in the wall and free surface regions when the turbulent shear stresses are well predicted

Flow over flexible vegetated bed : evaluation of analytical models.

The development of vegetation in the river bed and in the banks can affect the hydrodynamic conditions and the flow behavior of a watercourse. This can increase the risk of flooding and sediment transport. Therefore, it is important to develop analytical approaches to predict the resistance caused by vegetation and model its effect on the flow. This is the objective of this work which investigates the ability of different analytical models to predict the vertical velocity profile as well as the resistance induced by flexible submerged vegetation in open channels. Then it is possible to select the appropriate model that will be applied in the real case of rivers. The model validation is determined after a comparison between the data measured in the different experiments carried out and those from literature. For dense vegetation, the role of the Reynolds number is emphasized in particular with a model using the Darcy-Brinkman equation in the canopy. With a simple permeability, this model is relevant to estimate friction. However, for larger Reynolds number, models based on the fully turbulent flow assumption provide better results

Modélisation des écoulements dans une mèche en pierres en canal

Dans cette étude, on s'intéresse aux écoulements dans un milieu poreux constitué de blocs de différentes dimensions. L'objectif de l'étude expérimentale, est de déterminer les pertes de charge pour différentes configurations l'écoulement, afin de vérifier la formule de Stephenson. Celle ci permet d'exprimer la perte de charge en fonction de la pente du canal, du débit, de la section du canal et des caractéristiques géométriques des pierres. Des essais ont été réalisés dans un nouveau canal rectangulaire construit à l'INAT, ayant une longueur 10 m, une largeur de 0.8 m et une hauteur de 0.6 m (Soualmia et al., 2012). Différentes conditions hydrodynamiques ont été réalisées pour des débits, et des tailles du milieu poreux variables. Les résultats de simulations des différents essais (à deux pentes différentes), ont été comparés aux résultats expérimentaux, une concordance correcte a été notée. Une étude de sensibilité à la forme de ligne d'eau montre qu'une expression de la perte de charge en α.V² permet de reproduire correctement la ligne d'eau mesurée. Parmi les termes composant le paramètre α de Stephenson, la porosité est le terme le plus sensible. Ceci est le travail préliminaire à d'autres expérimentations et améliorations en perspectives

Modeling the Impact of Riparian Vegetation on Flow Structure and Bed Sediment Distribution in Rivers

The effect of instream vegetation growth has largely been ignored by hydrological and geomorphological research in river environments, which focused instead on the function of riparian vegetation as a regulator of bank stability or as a buffer for dissolved and particulate matter entering the channel from the hillside. However, in many lowland streams, instream vegetation can be very intensive, resulting in high biomass levels during the growing season. Instream plants have a significant influence on the dynamics of flow, sediment, and nutrients. Plant growth can cause increased frictional resistance to flow and can have a short-to medium-term effects on the geomorphology of the channel. Additionally, plant development influences the velocity of river flow, affects sedimentation dynamics and increases flood risk. To achieve a balance between flooding and ecological management of rivers in the presence of vegetation, a reliable method is required to predict the resistance of channels. In the current study, a two-dimensional hydrodynamic and morphodynamic model is developed and applied using a new scaling expression of shear stress based on vegetation characteristics. These first attempts at field simulations showed qualitatively acceptable results and demonstrated the effectiveness of the model in predicting hydraulic parameters in the presence of vegetation. This model is useful in predicting the effect of vegetation on stream flow and river morphology, as well as in managing flood hazards and stream ecology

Modeling the Impact of Riparian Vegetation on Flow Structure and Bed Sediment Distribution in Rivers

The effect of instream vegetation growth has largely been ignored by hydrological and geomorphological research in river environments, which focused instead on the function of riparian vegetation as a regulator of bank stability or as a buffer for dissolved and particulate matter entering the channel from the hillside. However, in many lowland streams, instream vegetation can be very intensive, resulting in high biomass levels during the growing season. Instream plants have a significant influence on the dynamics of flow, sediment, and nutrients. Plant growth can cause increased frictional resistance to flow and can have a short-to medium-term effects on the geomorphology of the channel. Additionally, plant development influences the velocity of river flow, affects sedimentation dynamics and increases flood risk. To achieve a balance between flooding and ecological management of rivers in the presence of vegetation, a reliable method is required to predict the resistance of channels. In the current study, a two-dimensional hydrodynamic and morphodynamic model is developed and applied using a new scaling expression of shear stress based on vegetation characteristics. These first attempts at field simulations showed qualitatively acceptable results and demonstrated the effectiveness of the model in predicting hydraulic parameters in the presence of vegetation. This model is useful in predicting the effect of vegetation on stream flow and river morphology, as well as in managing flood hazards and stream ecology