Investigation of episodic flow from unsaturated porous media into a macropore

The recent literature contains numerous observations of episodic or intermittent flow in unsaturated flow systems under both constant flux and ponded boundary conditions. Flow systems composed of a heterogeneous porous media, as well as discrete fracture networks, have been cited as examples of systems that can exhibit episodic flow. Episodic outflow events are significant because relatively large volumes of water can move rapidly through an unsaturated system, carrying water and contaminants to depth greatly ahead of a wetting front predicted by a one-dimensional, gravity-driven diffusive infiltration model. In this study, we model the behavior of water flow through a sand column underlain by an impermeable-walled macropore. Relative permeability and capillary pressure relationships were developed that capture the complex interrelationships between the macropore and the overlying porous media that control flow out of the system. The potential for episodic flow is assessed and compared to results of conventional modeling approaches and experimental data from the literature. Model results using coupled matrix-macropore relative permeability and capillary pressure relationships capture the behavior observed in laboratory experiments remarkably well, while simulations using conventional relative permeability and capillary pressure functions fail to capture some of the observed flow dynamics. Capturing the rapid downward movement of water suggests that the matrix-macropore capillary pressure and relative permeability functions developed have the potential to improve descriptions of flow and transport processes in heterogeneous, variably saturated media.