Non-Darcian Movement of Unstable Infiltration Fronts in Coarse-Grained Soils

Understanding gravity-driven preferential flow in uniform porous materials is important as it can facilitate the movement of pollutants, pathogens and pesticides to groundwater. Previous studies have suggested that the dynamic contact angle could be used to model unstable gravity-driven flow in coarse sand. This study examines this theory in a broader context involving a range of porous media with different static contact angles. A high-resolution, high-speed camera recorded the movement of water in pores at the wetting front. Water velocity and associated dynamic contact angles were calculated using image analysis. The results show that the movement of the wetting front was non-Darcian as the advancement of the front occurred intermittently through the smallest pore in 0.01 s at velocities of 0.06 m/s, one pore at a time. The high velocity increased the (dynamic) contact angle. The increase followed the Baver–Hoffman equation for all media with the different static contact angles. It confirms that the matric potential across the wetting front was discontinuous for unstable gravity-driven flow.