Quantitative evaluation on non-Darcian flow and non-Fickian transport: Visualization experiments on fracture networks

Accurate evaluation of fluid flow and solute transport in fracture networks is essential for understanding various hydrogeological processes. However, quantitative correlations between flow nonlinearity and transport heterogeneity in fracture networks with complex geometries remain unclear. In this study, four transparent fracture network models were designed with varying intersection angles, mechanical apertures, and topological structures. A series of flow tests and visualized solute transport experiments were then conducted on these models under a range of hydraulic gradients. The experimental results show that larger intersection angles and more complex topological structures intensify the flow nonlinearity and induce an earlier onset of non-Darcian flow. This simultaneously leads to reductions of 48.8 and 106.3% in Darcian and non-Darcian permeability, respectively. It is observed that alterations in the flow velocity directly affect the migration path and range of tracers, and the transport processes in fracture networks are significantly influenced by the geometric structures of fracture networks. Furthermore, temporal moment analysis reveals increased solute retention time and enhanced non-Fickian tailing effect for fracture networks with increased intersection angle, aperture heterogeneity, and structural asymmetry. Notably, increasing the Forchheimer coefficient from 2.34 × 10⁷ to 5.84 × 10⁷ will result in retention time increasing by 34.81–36.77% and transport heterogeneity increasing by 61.62–65.79%. This study sheds light on the quantitative correlation between the non-Darcian flow effects and non-Fickian transport in fracture networks, as well as the critical influence of fracture geometric characteristics on both processes.