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Effective flow and transport properties of heterogeneous unsaturated soils

Soraganvi, VS and Ababou, R and Kumar, MSM (2020) Effective flow and transport properties of heterogeneous unsaturated soils. In: Advances in Water Resources, 143 .

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Official URL: https://doi.org/10.1016/j.advwatres.2020.103655


The heterogeneity of field scale soils poses a challenge to predictive large scale flow and transport modeling. The theory of effective macroscale parameters holds good and is applicable in dealing with such problems. But the validity of the analytic stochastic solutions obtained for randomly heterogeneous soils is debatable, as the test cases under which they are validated are of limited scope due to linearization and perturbation approximations. In this study, samples of heterogeneous soils are generated using sets of spatially correlated random field parameters that are either geometrically isotropic, or else, geometrically anisotropic with either horizontal or vertical stratification (perfect or imperfect). Several combinations of ratios of correlation length and capillary dispersion lengths are considered. Numerical simulations of unsaturated flow are performed on each randomly heterogeneous soil sample. The principal components Kii(Ψ) of the macroscale effective unsaturated conductivity are then obtained as a function of the mean suction Ψ of the sample. They are compared to stochastic spectral perturbation theory, and to a probabilistic semi-empirical Power Average Model (PAM). They are also compared with arithmetic, geometric and harmonic mean conductivity-suction curves. The numerically upscaled principal conductivity curves match quite well the PAM, better than the classical means (Arithmetic, Geometric, Harmonic), and also somewhat better than the curves obtained from stochastic spectral perturbation theory. It is observed that the upscaled principal components Kii(�), obtained numerically and with the PAM along directions �i� orthogonal/parallel to perfect stratification coincide with the harmonic/arithmetic mean curves at low suctions (i.e., near saturation), but deviate from it and come closer to the geometric mean at higher suctions. The PAM appears suitable for generation of approximate upscaled conductivity curves, e.g., for obtaining the mesh-scale or block-scale conductivity curves in large scale simulation codes. Transient solute transport simulations are then performed on the detailed random velocity fields obtained from the steady state simulations of unsaturated flow in the randomly heterogeneous soil samples. Snapshots of solute concentration C(x,z,t) are taken at different times. The temporal evolution of spatial moments of concentration is analyzed in order to characterize the macroscale advection and dispersion of the unsaturated concentration plume, and in particular, its macro-dispersion coefficient (D) and dispersivity length scale (A). For the synthetic soil samples considered in this study, the macro-dispersive spreading of the solute is stronger for flow parallel to vertical stratification, compared to flow perpendicular to horizontal stratification, and also, compared to flow in statistically isotropic non-stratified soil. © 2020 Elsevier Ltd

Item Type: Journal Article
Publication: Advances in Water Resources
Publisher: Elsevier Ltd
Additional Information: Copyright for this article belongs to Elsevier Ltd
Keywords: Dispersions; Geometry; Harmonic analysis; Parallel flow; Perturbation techniques; Soil surveys; Soils; Solute transport; Stochastic models; Stochastic systems; Velocity, Dispersion coefficient; Horizontal stratification; Large scale simulations; Random velocity fields; Solute concentrations; Spatially correlated random fields; Steady-state simulations; Vertical stratification, Soil testing, anisotropy; computer simulation; conductivity; correlation; dispersion; flow velocity; geometry; numerical model; solute transport; stochasticity; stratification; unsaturated medium
Department/Centre: Division of Interdisciplinary Sciences > Interdisciplinary Centre for Energy Research
Division of Interdisciplinary Sciences > Interdisciplinary Centre for Water Research
Division of Mechanical Sciences > Civil Engineering
Date Deposited: 10 Nov 2021 14:24
Last Modified: 10 Nov 2021 14:24
URI: http://eprints.iisc.ac.in/id/eprint/65964

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