Simulation of stable and unstable flows in unsaturated homogeneous coarse sand

A numerical model developed to simulate stable and unstable flows in unsaturated porous media is described. Results of numerical studies carried out to simulate laboratory experiments with the assumption of stable flow demonstrate the occurrence of unstable flow for the initial conditions of both air dry and field capacity for unsaturated infiltration in sands. This indicates that the Richards flow equation based on moisture content and potential variables averaged over total cross-sectional area may not be applicable for flow under instability-prone boundary conditions. The unstable flow due to wetting front instability is modelled using the steady-state theory proposed by Hillel & Baker (1988). Simulation results for fingered flux calculated with the theory represent the experimental data reasonably well. The pore water velocity remains constant irrespective of the incident flux as long as the flux is smaller than the hydraulic conductivity value at the water entry suction of the porous media. The effect of antecedent wetness on the unstable flow behaviour is to reduce the pore water velocity drastically from air dry to field capacity conditions.