Simulation of the Infiltration of Fractured Rock in the Unsaturated Zone

Abstract

We study infiltration of rainwater into fractured rock and the accompanying capillary exchange processes between fractures and matrix, hereafter referred to as fracture–matrix transfer (FMT). Its influence on the velocity of the wetting front for uniform and variable aperture fractures is of prime interest because it determines the penetration depth of infiltration pulses. FMT is modelled explicitly in a discrete fracture and matrix (DFM) framework realised using a hybrid finite element–finite volume discretisation with internal boundaries. The latter separate the fracture mesh from the rock matrix mesh with the benefit that the flow that occurs within the minute fracture subvolume can be tracked with great accuracy. A local interface solver deals with the transient nonlinear aspects of FMT, including spontaneous imbibition of the rock matrix. Two- and three-dimensional heuristic test cases are used to illustrate how FMT affects infiltration. For the investigated scenario, we find that—beyond a critical fracture aperture around 5–10-mm—infiltration rate is no longer affected by FMT. Fracture aperture variations promote in-fracture-plane fingering, with counter-current flow of water (downward) and air (upward). Fracture flow interacts with FMT in a complex fashion. For systems with a small fracture porosity (≤0.01%), our results suggest that intense, hour-long rainfall events can give rise to tens-of-meter-deep infiltration, depending on fracture/matrix properties and initial saturation of the fractured rock mass.

A model for saturated–unsaturated flow with fractures acting as capillary barriers

“…Referring to the unsaturated case, both types of representations have been proven useful. For more details, see Brouyère (2006); Chen et al (2022); Kordilla et al (2012); Kuráž et al (2010); Robineau et al (2018); Spiridonov et al (2020) for continuum-type models, and for models explicitly representing the fractures see Koohbor et al (2020); Li and Li (2019); Therrien and Sudicky (1996); Tran and Matthai (2021).…”

“…(2020) for continuum‐type models, and for models explicitly representing the fractures see Koohbor et al. (2020); Li and Li (2019); Therrien and Sudicky (1996); Tran and Matthai (2021).…”

A Model for Saturated-Unsaturated Flow with Fractures Acting as Capillary Barriers

“…Referring to the unsaturated case, both types of representations have been proven useful. See, for example (Brouyère, 2006;Chen et al, 2022;Kordilla et al, 2012;Kuráž et al, 2010;Robineau et al, 2018;Spiridonov et al, 2020) for continuum-type models, and (Koohbor et al, 2020;Li & Li, 2019;Therrien & Sudicky, 1996;Tran & Matthai, 2021) for models explicitly representing the fractures.…”