Laboratory experiments and dual‐domain modeling of infiltration dynamics in partially saturated fractured porous media

Abstract

Many studies have shown that fracture networks can provide rapid pathways for water infiltration and time‐dependent recharge in the vadose zone of consolidated fractured rock formations. To investigate the control of fracture networks and fracture–matrix interaction on preferential flow and infiltration dynamics, we constructed a simple network system consisting of m × 2 sandstone blocks placed in between two glass plates. Water was injected from a point source directly into the fracture system with a constant flow rate of 1.5 g min−1. We used a dual‐porosity non‐equilibrium model to model the discharge dynamics and the internal fracture–matrix mass exchange and observed strong deviations from the laboratory results when the original parameterization was used. This specifically concerns the matrix–fracture volume ratio κ and the fracture flow velocity v that require modifications in terms of their definition and underlying process description. Although the original model assumes a perfectly coupled fracture and matrix domain, in the experiments the discrete nature of the fracture network led to a much stronger dominance of the rapid flow domain and hence to a reduction of κ. The newly introduced parameter κ* includes additional effects and processes related to the time‐dependent evolution and smaller size of the fracture–matrix interface. Furthermore, experiments of varying total vertical system size reveal convergence toward a unique parameter set and the existence of a representative elementary volume (REV) for the chosen setup. Though it performs less well for very small systems below REV scale, the unique parameter set describes discharge dynamics in sufficiently large systems with high accuracy.

Partially Saturated Fracture‐Matrix Infiltration in Experiments and Theory

“…Given the above assumptions about the geometrical constraints and knowledge about the critical fracture flow condition and matrix diffusivity, computationally efficient semi-analytical models could describe infiltration in more complex fracture networks. Note that other sophisticated (semi-)analytical models for fracture flow systems in impermeable media were developed (Yang et al, 2019) and studied in terms of their complex process spectrum (Kordilla et al, 2017(Kordilla et al, , 2021Noffz et al, 2019;Rüdiger et al, 2022;Shigorina et al, 2021). It remains challenging to unify various observations and provide suitable solutions for a wide range of field settings at larger scales.…”

“…The short fracture setup blocks are from the sandstone prepared for the experiments published in our previous study (Rüdiger et al., 2022), where we investigated fracture‐matrix flow in m‐by‐2 blocks fracture networks of varying numbers of consecutive X‐type intersections. By choosing the same block dimensions and analyzing a 1‐by‐2 blocks single fracture‐matrix system, we extended the previous work and studied the fracture flow propagation process in greater detail.…”

“…We obtain D m ,2 = 3.82e −6 m 2 s −1 . Parameters τ m and L x were taken from our previous study employing the same geomaterial (Rüdiger et al., 2022).…”

“…In our previous study (Rüdiger et al., 2022), we examined discharge dynamics in a m‐by‐2 matrix block system, employing m = 2, 4, 6, 8, 10, 12. The ensembled fracture system represented single fractures of different lengths and consecutive, horizontal fracture intersections.…”

Water Level in Observation Wells Simulated From Fracture and Matrix Water Heads Outputted by Dual‐Continuum Hydrogeological Models: POWeR‐FADS

“…This dual‐continuum concept was first introduced by Barenblatt et al. (1960) and Warren and Root (1963), and has since then been implemented by many authors (e.g., Al‐Shaalan et al., 2003; Choi et al., 1997; Gerke & van Genuchten, 1993a; Robineau et al., 2018; Rüdiger et al., 2022). Dual‐continuum approaches differ according to their level of complexity, with a distinction between dual‐porosity models and dual‐porosity/dual‐permeability models, also simply called dual‐permeability models (e.g., Berkowitz, 2002; Neuman, 2005).…”