Abstract

Finger flow along distinct paths in homogeneous soils and other porous media is well known, but its characterization and quantification with existing approaches is challenging. Neutron radiography produces 2-d images of water content distributions that allow testing of a water content wave approach to finger flow. Neutron radiographs were taken every 25 s during finger flow in a sand box that was 200 mm wide, 400 mm deep, and 5 mm thick. The sand was sieved to 0.2- to 0.5-mm diameter. The area of finger flow was 30 mm wide. We approached water content variations during infiltration and redistribution in the finger flow area with a water content wave model that assumes gravity as the driving force and viscosity to oppose it. The two model parameters are thickness of the water film F (μm) and its contact length L (m−1) per unit cross-sectional area A (m2) between the film and the stationary parts of the sand–water–air system. The velocity vW (m s−1) of the wetting front is an exclusive function of F, while mobile water content and volume flux density are functions of L and F. We observed constant vW across the entire depth of investigation, which indicates constant F despite the considerable spatial variations in bulk density and porosity. The contact length L expresses the surface area per unit volume of the medium onto which momentum dissipates. The value of L compared well with the estimated surface area per volume of the smallest sand fraction during unhampered flow; however, it increased drastically when flow was influenced by the capillary fringe at the bottom of the sand box.

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