Preferential flow dynamics in unsaturated fractured porous media: Relationship between fracture network topology and dual-domain model parameterization

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The main objective of this proposal is to establish a definite set of functional relationships between geometry/topology of fracture networks and the parameterization of dual-domain models in the context of unsaturated fracture flow dynamics. In order to understand the onset and dynamics of preferential flow paths in unsaturated fracture networks, both laboratory and field experiments will be carried out. The focus lies on the identification of easy to measure geometrical and topological parameters of the networks (intersection type, aperture, fracture density, orientation, matrix properties), and on the functional relationships between those parameters and "parsimonious" dual-domain models, which can be applied to model the bulk systems response to input signals. Specifically, we want to establish a functional relationship between the properties of unsaturated fracture networks and the active area fraction f(depth, time), as well as the fracture interfacial area density M employed in various flavours of dual-domain models.Laboratory experiments will be carried out to study the onset and formation of preferential flow paths within the fracture networks under well controlled conditions. Analogue (quasi-2D) fracture networks with various aperture widths will be constructed to generate unsaturated fracture flow either dominated by capillary (< 0.7 mm) or inertial forces (> 0.7 mm) and account for the diffusive exchange with a porous matrix by using geological materials. The source-responsive formation of flow regimes affects mass partitioning and channelling of flow paths within the fracture network. Furthermore, fracture geometry and topology of the connections are systematically altered to determine the impact on preferential flow formation. The topology of fracture connections acts as a measure for the connectivity and can be easily determined, both in laboratory and field experiments. The field experiment is installed in an outcrop of geological formations belonging to the Triassic Muschelkalk with well-defined fracture features (i.e., aperture width, orientation and frequency) while the size matches the scale of laboratory arrangement. The field study primarily serves as a validation for findings from the well-controlled analogue experiments and to understand to what degree planar 2D outcrop features can be used to predict flow dynamics in three-dimensional systems. Analytical work is done to express the connection between geometrical and topographical aspects of the fracture network to the built-up and efficiency of preferential flow paths. Hereby, we hope to test our hypotheses and ultimately reproduce flow through the fracture network.