NUMERICAL INVESTIGATION OF STEADY LIQUID WATER-FLOW IN A VARIABLY SATURATED FRACTURE NETWORK

Numerical simulation was used to study steady liquid water movement in a 5-m by 5-m vertical section containing a hypothetical fracture netw ork under conditions of variable saturation. The fracture network was assumed to be embedded within an impermeable rock matrix. Three variat ions of a network were considered. The ''mixed'' network consisted of two fracture sets, a subvertical set containing five 125 mum average a perture fractures and a subhorizontal set containing four 25 mum avera ge aperture fractures. The other two networks had identical fracture o rientation and contained either all 125 mum or all 25 mum average aper ture fractures. The TOUGH simulator was used to calculate the total st eady liquid water flux through the network, the flux through individua l fracture segments, and the pressure head at each fracture segment. A unit hydraulic gradient was imposed on the network by applying fixed pressure head boundaries (ranging from -0.25 to 0.0 m of water) of equ al value to the top and bottom. Saturation and permeability versus pre ssure head relations for the two sets of fractures were determined wit h the VSFRAC model, which assumed that aperture was variable within an individual fracture. Results showed that the spatial distributions of pressure head and flux within the network, as well as the location of the dominant pathways, depended strongly on the prescribed boundary p ressure head. For the mixed network, both pressure head and flux tende d to become more spatially uniform when the boundary pressure head app roached the pressure head at which the permeability thickness products of the large- and small-aperture fractures are equal (the crossover p ressure head). These results imply that for systems similar to the one considered here, interpretation of actual measurements of pressure he ad and flux may be quite complex, and that representation of variably saturated fracture networks as an equivalent continuum may be more val id for some ranges in pressure head than for others. Equivalent permea bility as a function of pressure head was calculated for the fracture network, illustrating how information collected on individual fracture s may be used to estimate the flow properties of rock at larger scales .