Near-wall velocity measurements by Particle-Shadow-Tracking

We report a new method to measure the velocity of a fluid in the vicinity of a wall. The method, that we call Particle-Shadow Tracking (PST), simply consists in seeding the fluid with a small number of fine tracer particles of density close to that of the fluid. The position of each particle and of its shadow on the wall are then tracked simultaneously, allowing one to accurately determine the distance separating tracers from the wall and therefore to extract the velocity field. We present an application of the method to the determination of the velocity profile inside a laminar density current flowing along an inclined plane

Étude écosystémique: cas du secteur 103 de la zone portuaire de Montréal

L’étude d’une situation environnementale exige un regard global sur l’objet d’étude. Il est difficile dans ce contexte, d’admettre, comme Descartes, que la réalité est trop complexe pour être comprise dans son ensemble. L’étude d’un objet environnemental ne peut se faire si l’on ne considère pas la relation de l’objet d’étude avec le monde qui l’entoure. À ce niveau, le processus réducteur de la méthode scientifique traditionnelle, s’il fournit une multitude d’éléments de réponse, ne permet p..

Le gouvernement du Québec et sa Politique de la lecture et du livre de 1998 : les objectifs et les réalisations

Au printemps 1998, le gouvernement du Québec, dirigé par Lucien Bouchard, adoptait la Politique de la lecture et du livre. Presque une décennie plus tard, il est pertinent de faire un retour sur cette politique, pour s’en rappeler l’élaboration, pour en comprendre les contours, et surtout pour en évaluer les résultats. Au fil des années, des évaluations ont été effectuées, des ajustements ont été apportés, des pans de cette politique ont été abandonnés. Des mesures mises de l’avant pour une période limitée (trois ou cinq ans) ont été annulées à la fin de ce terme, et d’autres l’ont été à la suite d’un changement de priorités d’un nouveau gouvernement. C’est l’objectif de cet article de dessiner le portrait d’ensemble de ce grand dessein politique lancé en 1998 et de comparer les objectifs qui ont été avancés avec les réalisations qui ont suivi. En définitive, on peut dresser un tableau nuancé, soulignant des réalisations éclatantes et durables comme la création de la Grande Bibliothèque et de l’Observatoire de la culture et des communications du Québec, mais retenant aussi des essais intéressants qui ont été abandonnés pour diverses raisons de politique gouvernementale.In the spring of 1998, the Government of Québec, led by Lucien Bouchard, adopted the Politique de la lecture et du livre (The Reading and Book Policy). Nearly a decade later, it is appropriate to recall how the policy was put together, to understand its components and, especially, to evaluate its impact. Over the course of time, evaluations were undertaken, adjustments made and parts of the policy abandoned. Measures, put in place for a limited period (3 or 5 years), were cancelled at the end of the prescribed period and others were abandoned as the result of a shift in priorities of a new government. The purpose of this article is to provide a broad review of the policy since 1998 and to compare the objectives with the results. A general portrait can be made that underscores some of the more remarkable and long-lasting results, such as the creation of the Grande Bibliothèque and the Observatoire de la culture et des communications du Québec, while outlining a few interesting projects that were put aside for various policy reasons.En la primavera de 1998 el Gobierno de Quebec, encabezado por Lucien Bouchard, adoptó la Política de la lectura y del libro. Casi un decenio después, es pertinente retornar a esta política con el propósito de recordar su elaboración, comprender sus límites y sobre todo evaluar sus resultados. A lo largo de los años, se ha evaluado y afinado esta política y se han abandonado algunas partes de ella. Se han anulado medidas -aplicadas por un periodo limitado (3 ó 5 años)- al final de ese tiempo; y, otras, seguido al cambio en las prioridades de un nuevo gobierno. El objetivo de este artículo es presentar el perfil de conjunto de este gran ideal político lanzado en 1998 y comparar los objetivos avanzados con los resultados posteriores. En definitiva, se puede elaborar una tabla matizada, subrayando los resultados más significativos y duraderos como la creación de la Gran Biblioteca y el Observatorio de la Cultura y las Comunicaciones de Quebec y conservando los interesantes intentos dejados de lado por diversas razones de la política gubernamental

Measuring bedload in gravel-bed mountain rivers: averaging methods and sampling strategies Measuring bedload in gravel-bed mountain rivers: averaging methods and sampling strategies

International audienceA dataset of more than 1,000 individual bedload samples coupled with hydraulic flow variables (water depth and velocity) was collected on two high mountain rivers the torrent de Saint Pierre, a proglacial gravel-bed river in the French Alps, in July 2002 and the Urumqi River, in the Chinese Tianshan mountains during summer 2005 and 2006. Analysis of the dataset leads to question the usual section averaged sampling procedure of bedload using Helley-Smith type bedload sampler. It is shown that this procedure is inadequate to catch the full range of flow conditions. Comparison between moving averages on individual datasets and section averages furthermore show that this technique can lead to significantly different rating curves with predictions differing by more than an order of magnitude. Single point sampling is shown to be much more adequate than multiple point sampling and section averaging provided the dataset is sufficiently large

Self-similar growth of a bimodal laboratory fan

International audienceUsing laboratory experiments, we investigate the growth of an alluvial fan fed with two distinct granular materials. Throughout the growth of the fan, its surface maintains a radial segregation, with the less mobile sediment concentrated near the apex. Scanning the fan surface with a laser, we find that the transition between the proximal and distal deposits coincides with a distinct slope break. A radial cross section reveals that the stratigraphy records the signal of this segregation. To interpret these observations, we conceptualize the fan as a radially symmetric structure that maintains its geometry as it grows. When combined with slope measurements, this model proves consistent with the sediment mass balance and successfully predicts the slope of the proximal–distal transition as preserved in the fan stratigraphy. While the threshold-channel theory provides an order-of-magnitude estimate of the fan slopes, driven by the relatively high sediment discharge in our experimental system, the actual observed slopes are 3–5 times higher than those predicted by this theory

Dissolution in a porous rock: effect on the concentration-discharge relationships

Abstract Various empirical or physically-based theory have been proposed to understand how the solute concentration of a stream varies with the discharge. We focus here on the influence of the dynamics of the groundwater flow on the chemical erosion rate. To do so, we couple a onedimensional aquifer model to a first order dissolution equation. If the aquifer extends far below the stream level, the theoretical discharge-concentration equation corresponds to the empirical "working model" proposed by Johnson in 1969, thus providing a physical interpretation of its parameters. Conversely, if the aquifer lays mainly above the stream level, a significantly different relationship is found. These theoretical findings are then compared to two field-data sets. From this comparison, we conclude that the dynamics of the groundwater flow could play a significant role in moderating the impact of dilution on the stream concentration at large discharges