1,720,981 research outputs found
Dinamica delle calotte glaciali: una revisione critica della SIA (shallow-ice approximation)
No full textInterpretation of numerical transport tests with single and dual porous medium approaches
No full textSINGLE AND DUAL-DOMAIN MODELS TO ASSESS THE EFFECTS OF HETEROGENEITY ON THE SOLUTE TRANSPORT IN ALLUVIAL AQUIFERS
Get PDFGroundwater contamination is a fundamental environmental concern, since aquifers are a major source of drinking water in many regions of the world. The effects of different sources of pollutants on water quality can be investigated through the modeling of water flow and solute transport in the aquifers.
An important issue to be addressed in the development of such models is the heterogeneity of the aquifers, which can occur at different spatial scales. The fine scale heterogeneity significantly affects the transport of contaminants at the scales of interest for practical applications.
The primary objective of this thesis is to implement some models to effectively describe non-reactive solute transport in heterogeneous alluvial aquifers, by considering the porous medium as either an equivalent homogeneous volume (single-domain model) or a superposition of two domains (dual-domain models). Specifically, the dual-porosity model assumes that water flows in only one of the two domains, which can exchange solute by diffusion. The dual-permeability models assume that water flows in both domains, which have different hydrodispersive parameters and can be coupled, i.e., they can exchange solute, or uncoupled.
These models are applied for the interpretation of some numerical tracer tests performed in portions of aquifers with different degrees of heterogeneity and at different scales: from a laboratory test on a decimeter-scale sand column, to numerical transport experiments on meter- and decameter-scale blocks of sediments, to an hectometer-scale field tracer test performed at the Cape Cod site.
The effective model parameters are linked to the heterogeneity pattern of the different tests and the ability of the different models to describe the effects of structured heterogeneity on the solute transport is compared.
The uncoupled dual-permeability model is shown to be the best one for alluvial aquifers characterized by the presence of preferential flow paths, which are connected bands of high-permeability sediments
Single- and Dual-domain Models of Solute Transport in Alluvial Sediments: the Effects of Heterogeneity Structure and Spatial Scale
No full textFine-scale heterogeneity of alluvial aquifers controls solute transport in groundwater at the scales relevant for practical applications: the architecture of sedimentary structures might create preferential flow paths (PFPs) or hydraulic barriers, which affect the breakthrough curves (BTCs). Objective of this paper was the assessment of the relevance of single- and dual-domain models for different heterogeneity patterns and scale lengths in alluvial sediments. Three case studies have been analysed with a classical single-domain model (SDM) and with three dual-domain models (DDMs): a dual-porosity model (DPorM) and two dual-permeability models (DPerM), which differ for the presence or the absence of solute exchange between the two domains. The first case study includes numerical tracer tests in metre-scale blocks of alluvial sediments; the second is a laboratory experiment of tracer injection in a decimetre-scale column of homogeneous sand; the third is a field tracer test performed at hectometre scale at the Cape Cod site. The relevance of the solute exchange in the DDMs is analysed with the characteristic advection and exchange times and with the Péclet and Damköhler numbers. The SDM is satisfactory for alluvial sediments with unstructured heterogeneity. The uncoupled DPerM is shown to be a better approach than the DPorM in sediments with PFPs; in this case, the coupled DPerM does not improve significantly the results of the uncoupled DPerM. A minor difference between the results of the three DDMs is observed for sediments in which the non-Fickian behaviour is not clearly determined by the presence of PFPs
Interpretation of numerical transport experiments in alluvial sediments with single-domain, dual-porosity and dual-permeability models
No full textModel calibration for ice sheets and glaciers dynamics: a general theory of inverse problems in glaciology
No full textA sensitivity analysis for a thermomechanical model of the Antarctic ice sheet and ice shelves
No full textVerso la modellazione del reticolo idrografico subglaciale antartico: modellazione dinamica della calotta
No full textModel calibration for ice sheets and glaciers dynamics: a general theory of inverse problems in glaciology
Get PDFNumerical modelling of the dynamic evolution of ice sheets and glaciers requires the solution of discrete equations which are based on physical principles (e.g. conservation of mass, linear momentum and energy) and phenomenological constitutive laws (e.g. Glen’s and Fourier’s laws). These equations must be accompanied by information on the forcing term and by initial and boundary conditions (IBCs) on ice velocity, stress and temperature; on the other hand the constitutive laws involve many physical parameters, some of which depend on the ice thermodynamical state. The proper forecast of the dynamics of ice sheets and glaciers requires a precise knowledge of several quantities which appear in the IBCs, in the forcing terms and in the phenomenological laws. As these quantities cannot be easily measured at the study scale in the field, they are often obtained through model calibration by solving an inverse problem (IP). The objective of this paper is to provide a thorough and rigorous conceptual framework for IPs in cryospheric studies and in particular: to clarify the role of experimental and monitoring data to determine the calibration targets and the values of the parameters that can be considered to be fixed; to define and characterise identifiability, a property related to the solution to the forward problem; to study well-posedness in a correct way, without confusing instability with ill-conditioning or with the properties of the method applied to compute a solution; to cast sensitivity analysis in a general framework and to differentiate between the computation of local sensitivity indicators with a one-at-a-time approach and first-order sensitivity indicators that consider the whole possible variability of the model parameters. The conceptual framework and the relevant properties are illustrated by means of a simple numerical example of isothermal ice flow, based on the shallow-ice approximation
Single and dual domain models to evaluate the effects of preferential flow paths in alluvial porous sediments
No full textTypical features of preferential flow paths were evidenced by numerical tests of convective transport of conservative solutes performed in three blocks of alluvial sediments at the scale of depositional elements. The numerical experiments are analysed with standard single-domain models (SDMs) and with dual-domain models (DDMs): the model parameters are identified by minimisation of the misfit between the "experimental" and the modelled cumulative breakthrough curves (BTCs) and between the "experimental" and the modelled temporal moments of the BTCs. The results for the SDMs show different behaviours for the three model blocks and for the different flow directions, in good agreement with their hydrostratigraphic characteristics. The results for the DDMs sometimes correspond to cases for which one of the two domains is dominant and its values of diffusivity and average velocity are close to those obtained for the SDM; in some cases the DDM performs much better than the SDM and correctly represents the effects of preferential flow paths. Finally the relevance of the DDM is analysed in the framework of multi-objective optimisation: a proper choice of the objective-functions yields Pareto sets whose geometries are different for single- and dual-domain media
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