Fractures are both rough and irregular but can be expressed by a simple model concept of two smooth parallel plates, where the cubic law describes the proportionality of the flux to the third power of the fracture aperture. However, in natural conditions, more so in the intermediate vadose zone, these assumptions are violated, particularly with regard to unsaturated conditions. In this paper we present a qualitative experimental study that seeks to investigate the flow from the fundamental concept of the horizontal smooth parallel plates under conditions of variable saturation into an intersection with and without a vertical fracture intersection. The study consists of flow visualization experiments conducted on a transparent replica of the smooth parallel plates. Findings from this research proved that saturated laminar flow is not likely achieved in nonhorizontal fractures. Draining of the saturated horizontal discontinuity occurs preferentially along the vertical face as non-uniform separate rivulets exiting at discrete points along the discontinuity; and wider preferential flow paths in the test with an intersecting vertical discontinuity. This occurs even if the water supply implies that it should achieve near-saturation. The results indicate that movement of water through the fractured vadose zone becomes a matter of the continuity principle, whereby water should theoretically be transported downward at significantly higher flow rates given the very low degree of water saturation. Current techniques which aim to quantify discrete fracture flow, notably at sub-saturation, are implicitly inaccurate, as is evident by this study's aims of qualifying rather than quantifying flow mechanisms.


Flow through discrete discontinuities can be expressed by a simple model concept of two smooth parallel plates, assuming the matrix as impermeable and saturated laminar flow conditions apply. From this strong simplification of natural conditions, the flux (Q) occurring through a single discrete fracture, can be determined using a basic analytical method, derived from the Navier-Stokes equation, known as the cubic law due to the proportionality of the flux to the third power of aperture as detailed by, e.g. Zimmerman and Bodvarsson (1996), and recently reviewed in terms of its applicability to unsaturated rock masses by Dippenaar and van Rooy (2016).

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