TR(BR)-5/98-99 : Effect of anisotropy on seepage from a water body

Abstract

Most of the of the theoretical analyses of groundwater flow problems are based upon the assumption of isotropy and homogeneity of the porous medium. But field experience reveals the fact that soils are anisotropic to some extent, in general. Anisotropy can be stratification in an aquifer which might have taken place as a result of particle orientation during formative stages. Quite often, an anisotropic aquifer system is approximated to an isotropic one and solutions are attempted using flow models which generally assume a Cartesian coordinate system. However, anisotropic flow systems can lot be solved with the Cartesian coordinate domain. Approximating an anisotropic medium to an isotropic one may introduce errors in the computation of flow and/ or heads as groundwater flow through anisotropic soils is complex by nature. Besides, general rules applicable to methods for isotropic conditions like flow-net analysis are no longer valid in an anisotropic medium since the directions of flow and hydraulic gradient in such a medium need not be parallel as in the case of an isotropic aquifer. Review of literature shows that investigations on hydraulic potentials or flow in anisotropic aquifer systems are not extensive. This report summarises related aspects on the theory of anisotropic flow in porous media and reviews methodology for computing hydraulic potentials in an unconfined aquifer system. An algorithm has been devised by applying appropriate transformation techniques for anisotropic domain and analytical results. Numerical experiments have been performed using the algorithm to compute hydraulic potentials in certain hypothetical anisotropic aquifer systems. A number of cases have been studied with different coefficients of anisotropy for the aquifer as well as angles inclination of the bedding planes of the soil strata. The simulated hydraulic potentials in the anisotropic domain are depicted as equipotential lines in vertical sections.