Fracture flow is fairly well documented with the widespreadapplication of, for instance, the cubic law and assumed smooth parallel plate model.Geometrical intricacies such as aperture, roughness and infill do however significantlyinfluence the validity of the cubic law with even its application to smooth parallelsystems being contestable. Rock mechanical discontinuity surveys provide valuableinformation regarding the discontinuity geometry that can likely contribute to theevaluation of flow through individual fractures with variable properties. The hydraulicaperture is available for the transmission of flow, while normal and shear stresses alterdiscontinuity properties over time. In this, numerous advances have been made tobetter accommodate deviations of natural discontinuity geometry to that of smoothparallel plates and at partial saturation. The paper addresses these advances and detailsconditions under which the cubic law, even in local form, fails to adequately estimate thehydraulic properties. The role of roughness in open discontinuities is addressed inparticular, as contact areas and high amplitude roughness cause most extensivedeviation from the cubic law. Aperture of open fractures still governs hydraulicproperties, but inertial forces control flow in very rough fractures, in which instancesthe applicability of the cubic law should be revisited. Open questions are finally posed,assessment of which will contribute significantly to the understanding of flow throughindividual discontinuities as well as fracture networks.
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