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From the local to the regional scale. What is the effect of missing vertical heterogeneity from fully 3-D to 2-D depth averaged dispersion models?
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DESIGN OF ANCHORED SLABS IN SPILLWAY STILLING BASINS
No full textJOURNAL OF HYDRAULIC ENGINEERING A.S.C.E
Performance Assessment of Water Distribution Systems Subject to Leakage and Temporal Variability of Water Demand
Full text linkA water distribution system (WDS) is designed and managed to provide a reliable water supply, that is, to properly respond towater user demand, particularly in critical operating conditions such as in times of peak demand. Therefore, the assessment of the influence ofwater demand characteristics is an essential requirement in the context of WDS reliability. In this paper the impact of the pattern of hourlydemand on WDS performance is analyzed for a system subject to aging processes and temporary pipe unavailability and affected by waterlosses with different leakage levels. The hydraulic deficit that can occur when the pressure falls below the minimum service value is used as aperformance index, and its relevance is analyzed without and with preventive maintenance. The case of the synthetic Anytown network isanalyzed, but the procedure has general validity and can be applied to any real WDS. Defined in a prescribed temporal horizon the pipereplacement prioritization without preventive maintenance, the effects of pipe substitutions are analysed as a function of different schedulingtimes to quantify the reduction of the hydraulic deficit. The results show the capability of the proposed approach to define a pipe replacementprioritization and the related scheduling time, in view of the relevance that these aspects could have in any economic analysis developed todefine a proper maintenance strategy
Chemically induced flow in contaminated unsaturated soil
No full textLaboratory experiments and numerical modeling were conducted to assess the effects of glycerol concentration gradients on water flow through an unsaturated loamy sand (Yellow bush sand, Australia) at 46% glycerol mass fraction and 12.5% volumetric water content. When the contaminated and uncontaminated experimental soil sections were placed into contact, two opposite mechanisms were found to drive the flow: (i) the matric potential gradient affected by concentration-dependent surface tension, viscosity, and density repelled contaminated water; and (ii) the osmotic potential gradient had an opposite effect. Experiments demonstrated that the tested sand did not exhibit the complete semipermeable characteristic necessary to induce pure osmotic flow. Rather, physicochemical effects caused by dissolved glycerol were more relevant for water flow than those induced by the osmotic potential. We confirmed our experimental finding by numerical modeling that explicitly accounted for these effects compared with our experiments and earlier experiments conducted with 7% butanol–water mixtures as benchmarks. Our results were finally confirmed by a thermodynamic interpretation of matric and osmotic potentials. Experiments suggest that a rapid transient condition occurred on the first day and that equilibrium was recovered only after >60 d. The results support the hypothesis that chemically induced water flow in the vadose zone is contributed mainly by matric and only secondarily by osmotic effects, which have displaced 3 to 10% water during the initial transient condition. Predictive tools of contaminant hydrology in unsaturated soil may have to account for the osmotic effects in particular applications that involve soil with high osmotic efficiency
Numerical dispersion of solute transport in an integrated surface–subsurface hydrological model
No full textIntegrated surface–subsurface hydrological models (ISSHMs) are increasingly being used for the assessment of contaminant transport in the environment, in addition to their more common use in water flow applications. However, the subsurface solute transport solvers in these models are prone to numerical dispersion errors. Numerical dispersion is a well-known issue in groundwater modeling, but its impacts on the results of ISSHM simulations are still poorly understood. In this study, the CATchment HYdrology (CATHY) model is used to assess the potential impacts of numerical dispersion on the simulation of coupled surface–subsurface solute transport. We first simulate the subsurface transport of a nonreactive tracer in two soil column test cases (1D and 3D) with known analytical solutions. The subsurface solute transport solver in CATHY adopts a computationally efficient time-splitting technique whereby the advection component of the governing equation is solved on elements and the hydrodynamic dispersion component is solved on nodes. Comparison between simulation results and analytical solutions with different mesh discretizations and different values for the hydrodynamic dispersion coefficients allows for accurate quantification of the numerical dispersion error and yields insights into the parameters and other factors that control it. It is shown that, taken alone, the advection and dispersion solvers are very robust, but their combination can result in significant numerical dispersion, stemming from the exchange of concentration information from elements to nodes and vice versa in the time-splitting procedure. The tests also show that these errors can be kept under control by ensuring that the grid Péclet number is in the range 0.5-1.0 or smaller. We then apply CATHY in a third test case involving two synthetic hillslopes (concave and convex) in fully coupled surface–subsurface mode, in order to examine the impact of this subsurface numerical dispersion on simulated streamflow hydrographs, in particular with reference to pre-event water contributions to runoff. Here as well the results show that the effect of numerical dispersion can be controlled by keeping the grid Péclet number sufficiently small. This work provides a new set of benchmark test cases for integrated surface–subsurface hydrological models, extending to solute transport the flow-only suite of benchmarks recently published in two intercomparison studies
Effective dispersion in conditioned transmissivity fields
No full textWe analyze the impact of conditioning to measurements of hydraulic transmissivity on the transport of a conservative solute. The effects of conditioning on solute transport are widely discussed in the literature, but most of the published works focuses on the reduction of the uncertainty in the prediction of the plume dispersion. In this study both ensemble and effective plume moments are considered for an instantaneous release of a solute through a linear source normal to the mean flow direction, by taking into account different sizes of the source. The analysis, involving a steady and spatially inhomogeneous velocity field, is developed by using the stochastic finite element method. Results show that conditioning reduces the ensemble moment in comparison with the unconditioned case, whereas the effective dispersion may increase because of the contribution of the spatial moments related to the lack of stationarity in the flow field. As the number of conditioning points increases, this effect increases and it is significant in both the longitudinal and transverse directions. Furthermore, we conclude that the moment derived from data collected in the field can be assessed by the conditioned second-order spatial moment only with a dense grid of measured data, and it is manifest for larger initial lengths of the plume. Nevertheless, it seems very likely that the actual dispersion of the plume may be underestimated in practical applications
EVALUATION OF THE DISPERSION PROCESSES IN CONDITIONEDTRANSMISSIVITY FIELD
No full textConditioning on available measures makes the mathematical description of the hydraulic transmissivity field as inhomogeneous even if the original one isn't. Clear examples about multigaussian conditioning shows as, starting from a field that is stationary in the strict sense, the conditioned random function is no more statistically homogeneous being conditioned mean, covariance and variance depending on the spatial position. Obviously velocity fields deduced from the latter are inhomogeneous too, as shown in papers dealing with nonlocal formalism and non-Darcian flux. Among various aspects of conditioning widely discussed in the literature, we focus the attention on the effects related to the effective dispersion8 in conditioned log transmissivity fields. This topic has been tackled in the past in a hydraulic transmissivity field of finite correlation length and in self-similar porous formations with a large scale cutoff. While the former describes the impact of an uniform recharge on the conditioning process, in the latter it is shown as a single measurement reduces the difference between ensemble and effective solute moments, depending the effectiveness in reducing uncertainty on the position of the measurement respect the plume mean trajectory. Respect to these papers, we stress here the role of the flow field inhomogeneity to analyze its impact on the effective dispersion prevision in conditioned transmissivity fields. This goal is reached by use of the stochastic finite element method (SFEM) to handle nonstationarity stemming from the transmissivity conditioning in a finite 2-D domain, and by taking into account the mutual relevance of the initial plume finite size and of the inhomogeneity of the flow field in the nonergodic dispersion process. From the analysis of the developed numerical examples, it is shown as the effective dispersion may lead to a reductive forecast of the real plume spreading when, as in conditioned hydraulic transmissivity fields, the spatial stationarity of the flow field is not obeyed
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