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due to interconnected fractures while blocks are impervious. In double (dual)
porosity medium, both fractures and matrix blocks contribute to groundwater
flow but fractures are the main contributors. In a situation, when fractures
are filled with clay or silty material, the fracture permeability is considerably
reduced and such a medium is termed as heterogeneous.
In fractured rocks, the concept of porous medium can be applicable where
the fractures are interconnected and the density of conductive fractures is
high. When these conditions are not met, alternate conceptualizations must
be used.
Some of the models describing flow in a fractured medium are:
1. Double porosity model.
2. Equivalent porous medium (EPM) model.
3. Discrete fracture network model.
It may be mentioned that various models require knowledge about
characteristic features of individual fractures and thereby account for the
heterogeneity. Therefore, careful mapping of fractures by surface and
subsurface measurements is important. The level of details depends on the
purpose for which the model is being developed. A greater accuracy is
required for modelling solute transport as the heterogeneity of the fracture
system greatly influences the travel time and solute concentration. It is also
important to estimate the hydraulic properties of different fracture sets. Studies
indicate that fractures parallel to the maximum compressive stress tend to be
open, whereas those perpendicular to this direction tend to be closed (Anon,
1996).
The choice of modelling strategy depends on information desired. The
choice of simulation model should depend on conceptual model . If necessary
hydrogeological data are lacking, sophisticated numerical models can not be
justified. In case sufficient data is not available, simple epm model can be
used for flow but not for transport.
Double Porosity Model
Double porosity model is also known as dual porosity model . The behaviour
of a fractured aquifer for regional groundwater investigations can be
represented by a double-porosity model. The concept of double porosity
model has been explained in detail by Streltsova-Adams (1978) and Gringarten
(1982). The model assumes two regions—the porous block and the fracture,
which have different hydraulic and hydromechanic characteristics. Three
types of distributions of matrix block are considered—horizontal slabs (strata-
type), spherical blocks, and cubes (Fig. 3). The fissured medium with
horizontal fractures has been considered to be analogous to alternate aquifer-
aquitard system (Boulton and Streltsova, 1978). The block consists of fine
pores which are separated from fractures. The blocks supply fluid to the
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