Modélisation du comportement hydraulique des déversoirs d’orage latéraux en régime transcritique

Le comportement hydraulique des déversoirs d’orage latéraux est le plus souvent marqué par une évolution discontinue de la ligne d’eau caractérisée par le ressaut hydraulique (Torrentiel/fluvial) et, dans certains cas, par une évolution rapidement variée causée par un écoulement également transcritique mais dans le sens fluvial/Torrentiel. Concernant les modèles actuels, ils ne permettent pas de simuler le comportement en transitoire de ce type d’ouvrage. Compte tenu de cela, nous avons modélisé l’ouvrage par les équations de Barré de Saint Venant écrite sous forme conservative en régime transitoire et couplée au modèle de déversoir de Hager. Le caractère conservatif de ces équations permet de transcrire dans un seul système d’équations les écoulements graduellement et rapidement variés. Afin de résoudre ces équations, nous avons utilisé le schéma numérique UPWIND à « capture de choc » du second ordre du type TVD (Total Variation Diminishing) utilisant le solveur de Roe. Dans l’objectif de valider notre démarche, nous avons créé un pilote de déversoir d’orage sur le site d’Obernai (laboratoire d'hydraulique du lycée agricole). Nous avons fait varier : le diamètre de la conduite aval par rapport au diamètre de la conduite amont, la longueur du déversoir, les pentes des conduites amont et aval ainsi que la hauteur de crête. La comparaison des débits déversés entre les modèles physique et numérique a montré que l’erreur observée rapportée au débit amont n’excédait jamais 13 % avec une majorité des cas entre ±5%.In recent years, French and European legislation has introduced regulations about wastewater discharge into natural environments and particularly about combined sewer overflows. As a consequence, it has become essential to control the hydraulic behaviour of these structures and to estimate the pollution loads released at this level. The side weir is the regulation structure that permits the hydraulic regulation of the waste water carried by the sewer system. When the upstream flow intensity exceeds a value referred to as reference flow, the side weir directly rejects part of the waste water to the natural environment. The hydraulic behaviour of the sewer side weir was shown to involve a discontinuous evolution of the water depth, characterized by a hydraulic jump (transition from supercritical to subcritical flow) and also by a rapidly varying transcritical evolution (subcritical to supercritical).Initially, the side weir flow was determined with the use of empirical relations. Using formulae of Engels, Coleman and Smith, Balmaceda and Gonzales or Dominguez, it was possible to calculate the outflow according to the water level at the upstream and/or the downstream region of the weir. These relations were applicable only for certain flow regimes and in certain cases in which the geometry of the side weir was specified. Subsequently, a more physical approach, initiated by Ackers, was based on the assumption of constant energy along the side weir. This approach made it possible to focus not only on an assessment of the side channel flow, but also on the water profile at the crest. Unfortunately, as the study of El Kashab shows, this method falls short in certain cases because the equations are inappropriate. For example, in the case of hydraulics the constant energy approach was not applicable. Finally, the method that is currently used is based on a momentum equation, which makes it possible to establish equations for shallow water. This approach seemed the most appropriate in the case of the side weir. The numerical solution of these equations was always based on an algorithm that describes all the possible cases according to the flow regime and the hydraulic conditions in the side weir. One must know the flow regime a priori. These models don’t properly simulate the transitory behaviour of these kinds of works.In this article we propose hydraulic modelling of a sewer side weir that integrates the geometrical characteristics of the flow (height and length of the crest, variation of width along the crest), and avoids the need for a priori knowledge of flow conditions in the side weir. The model also takes into account hydraulic discontinuities (hydraulic jump, transitions from free surface to pressurised flow) and the transitory character of the flow. The numerical results were compared with measured values obtained from a test bench.For the 1D approach, the solution was found using the 1D shallow water equations written in a conservative form for a transitory situation. The conservative characters of the equations permit us to consider gradually and rapidly varied flows in a single system of equations. In order to account for the lateral overflow, we used the Hager relation, which involves the intensity and direction of the lateral velocity vector and also the influence of width variation of the side weir. The shallow water equations system couldn’t be analytically solved. As a consequence, numerous numerical methods have been developed such as characteristic methods or finite difference methods. Unfortunately, these methods are inadequate when discontinuities such as hydraulic jumps or flow regime transitions (froude number close to 1) appear. To solve these problems, numerical (shock capturing) schemes were developed, based on a finite volume formulation. Godunov was the initiator of this type of finite volume numerical scheme. Eleuterio improved the precision and the ability of these numerical schemes to converge. The result was a combination of total variation diminishing (TVD) interpolation with an appropriate Riemann solver. We used a second order TVD Upwind shock capturing numerical method associated with the Roe Riemann solver.In order to validate the numerical model, we have built a sewer side weir physical test bench at the Obernai site. The variable parameters were the downstream pipe diameter in comparison with the upstream pipe diameter, the side weir length, slopes of the upstream and downstream pipes and the height of the crest. The tested cases permitted us to sweep a slope ranging from 0.5 ‰ to 1 % for the upstream and downstream pipes.Globally, 114 configurations have been tested with the 1D numerical model. The operating curve represents a criterion for characterising the operating of the storm overflow. As long as the upstream flow does not reach the reference value, there is no overflow. As soon as the upstream flow exceeds the value, the downstream flow remains close to the reference value. Because comparisons were made in relation with the upstream flow, the criterion used for judging the modelling performance was the absolute error value in relation to the upstream flow. It is important to note that the results need to be weighted as experimental measurements have a margin of error of approximately 5%.Comparisons between numerical and experimental results permit the following conclusions:1. The distribution of the number of errors was very close to the normal Gaussian distribution curve, with a slight shift towards the positive side. This indicates that the errors were random and therefore very close to experimental values.2. In general, the maximum errors varied from -10 to 13%, with the majority of cases occurring between –5 and +5%. This shows that the performance of the tool was very useful in cases that are as complex as side weirs. However, the model has a slight tendency to overestimate the overflow rate as compared to experimental measurements

Soil and Cultivar Type Shape the Bacterial Community in the Potato Rhizosphere

The rhizospheres of five different potato cultivars (including a genetically modified cultivar) obtained from a loamy sand soil and two from a sandy peat soil, next to corresponding bulk soils, were studied with respect to their community structures and potential function. For the former analyses, we performed bacterial 16S ribosomal RNA gene-based PCR denaturing gradient gel electrophoresis (PCR-DGGE) on the basis of soil DNA; for the latter, we extracted microbial communities and subjected these to analyses in phenotype arrays (PM1, PM2, and PM4, Biolog), with a focus on the use of different carbon, sulfur and phosphorus sources. In addition, we performed bacterial PCR-DGGE on selected wells to assess the structures of these substrate-responsive communities. Effects of soil type, the rhizosphere, and cultivar on the microbial community structures were clearly observed. Soil type was the most determinative parameter shaping the functional communities, whereas the rhizosphere and cultivar type also exerted an influence. However, no genetically modified plant effect was observed. The effects were imminent based on general community analysis and also single-compound analysis. Utilization of some of the carbon and sulfur sources was specific per cultivar, and different microbial communities were found as defined by cultivar. Thus, both soil and cultivar type shaped the potato root-associated bacterial communities that were responsive to some of the substrates in phenotype arrays

Social Control in Transnational Families: Somali Women and Dignity in Johannesburg

Transnational mobility often separates families and distances individuals from the kinship and social structures by which they organized their lives prior to migration. Myriad forms of insecurity have been the impetus for Somali movement into the diaspora, with people fleeing the realities of conflict that have marked Somalia for decades while physically dividing families as individuals settle in different countries around the world. Mobility has altered the dynamics of households, families, and communities post-migration, reshaping social constructions as individuals move on without the familial support that sustained them in Somalia. While outcomes of these hardships are variable and often uneven in different settlement spaces, migration can offer new opportunities for people to pursue avenues from which they were previously excluded, such as by assuming roles and responsibilities their relatives once filled. These changes precipitate shifting identities and are challenging for women who find themselves self-reliant in the diaspora, particularly in the absence of (supportive) husbands and close kin.Drawing on ethnographic research in Johannesburg’s Somali community, this chapter explores the assumption that migration provides an opening for women to challenge subordinating gender norms. Settlement often grants women greater freedom to make choices in their lives, such as in employment and personal relationships, and yet they remain constrained by networks that limit their autonomy. Even with transnational migration and protracted separation, women are family representatives who must uphold cultural notions of respectability despite realities that position them as guardians and family providers. Women remain under the watchful eye of their extended families through expansive networks and the ease of modern communication, which facilitate a new form of social control as women’s behavior is carefully monitored and reported to relatives afar. These actualities raise questions about the degree to which transnational movement is a liberating force for women or rather a reconfiguration of social control. I argue that despite women’s changing position in their households and families, they remain limited by social control within their extended families and communities

Soil Microbial Community Changes in Wooded Mountain Pastures due to Simulated Effects of Cattle Grazing

The effect of cattle activity on pastures can be subdivided into three categories of disturbances: herbage removal, dunging and trampling. The objective of this study was to assess separately or in combination the effect of these factors on the potential activities of soil microbial communities and to compare these effects with those of soil properties and plant composition or biomass. Controlled treatments simulating the three factors were applied in a fenced area including a light gradient (sunny and shady situation): (i) repeated mowing; (ii) trampling; (iii) fertilizing with a liquid mixture of dung and urine. In the third year of the experiment, community level physiological profiles (CLPP) (Biolog Ecoplates¿) were measured for each plots. Furthermore soil chemical properties (pH, total organic carbon, total nitrogen and total phosphorus), plant species composition and plant biomass were also assessed. Despite differences in plant communities and soil properties, the metabolic potential of the microbial community in the sunny and in the shady situations were similar. Effects of treatments on microbial communities were more pronounced in the sunny than in the shady situation. In both cases, repeated mowing was the first factor retained for explaining functional variations. In contrast, fertilizing was not a significant factor. The vegetation explained a high proportion of variation of the microbial community descriptors in the sunny situation, while no significant variation appeared under shady condition. The three components of cattle activities influenced differently the soil microbial communities and this depended on the light conditions within the wooded pasture. Cattle activities may also change spatially at a fine scale and short-term and induce changes in the microbial community structure. Thus, the shifting mosaic that has been described for the vegetation of pastures may also apply for below-ground microbial communities

Soil microbial communities following bush removal in a Namibian savanna

Savanna ecosystems are subject to desertification and bush encroachment, which reduce the carrying capacity for wildlife and livestock. Bush thinning is a management approach that can, at least temporarily, restore grasslands and raise the grazing value of the land. In this study we examined the soil microbial communities under bush and grass in Namibia. We analyzed the soil through a chronosequence where bush was thinned at 9, 5, or 3 years before sampling. Soil microbial biomass, the biomass of specific taxonomic groups, and overall microbial community structure was determined by phospholipid fatty acid analysis, while the community structure of Bacteria, Archaea, and fungi was determined by multiplex terminal restriction fragment length polymorphism analysis. Soil under bush had higher pH, C, N, and microbial biomass than under grass, and the microbial community structure was also altered under bush compared to grass. A major disturbance to the ecosystem, bush thinning, resulted in an altered microbial community structure compared to control plots, but the magnitude of this perturbation gradually declined with time. Community structure was primarily driven by pH, C, and N, while vegetation type, bush thinning, and time since bush thinning were of secondary importance