Pressure Head Profile within Growing Fingers in Initially Dry Glass Beads

Abstract

Wetting front instability is one of the mechanisms producing preferential flow in soils. In this study, we measured the pressure head within growing fingers in three-dimensional soil columns and examined the applicability of the prefingered flow theory and the similarity solution. The pressure head within a growing finger was measured during infiltration into a porous medium consisting of fine over coarse layered glass beads in a transparent, acrylic column. To ensure the measurement of pressure heads within growing fingers whose pathways cannot be known a priori, we made 10 insertion ports for every measurement position in the column. The fingertip velocity was essentially constant with time, while the infiltration rate increased. Fingers merged as they grew, which complicated the finger flow physics in three dimensions. The profiles of the pressure head were nearly linear during early finger growth. As the finger lengthened, the pressure head at all measured points decreased to an almost constant value. The prefingered flow model showed good applicability not only to induction zone formation, but also to the early finger growth when it remained saturated. Conversely, the similarity solution was not as applicable to growing fingers that were not in a steady state.