The drift shadow phenomenon in an unsaturated fractured environment

The presence of subterranean holes creates a capillary barrier in an unsaturated environment. This phenomenon has been referred to as "Drift Shadow" and indicates a region that is sheltered from the downward percolating water. If the lateral hydraulic conductivity is insufficient to divert the water, fully saturated conditions are reached locally, and seepage occurs as the capillary barrier fails. Natural heterogeneities in hydrological properties can reduce the probability of seepage only if the flux is largely diverted around the drift. Previous numerical studies have been performed investigating various aspects of capillary barrier performance in engineered or naturally layered systems. Many authors examined the impact of heterogeneity on the distribution and rate of water seepage across a capillary barrier and into a drift, but the seepage exclusion problem has not been formally analyzed for fractured formations, in which the physical processes governing seepage in porous media also represent key factors. This paper analyzes the effect that a fracture network can have on the drift shadow. In a fractured environment, the effectiveness of the capillary barrier is determined by the capability of individual fractures to hold water by capillary forces and by the permeability and connectivity of the fracture network, which allow water to be diverted around the drift. The orientation of any individual fracture in relation to the opening, the discreteness and the anisotropy of the fracture network are all geometric factors affecting seepage, because they have a relevant influence on the hydraulic properties in the immediate vicinity of the drift wall. © 2008 Taylor & Francis Group, London.