Water Content Wave Approach Applied to Neutron Radiographs of Finger Flow

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 (mu m) and its contact length L (m(-1)) per unit cross-sectional area A (m(2)) between the film and the stationary parts of the sand-water-air system. The velocity v(W) (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 v(W) 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.