Stability analysis of water movement in unsaturated porous materials: 1. Theoretical considerations

The occurrence of wetting front instability during water movement in unsaturated porous materials under certain conditions is discussed and a brief review presented of relevant stability studies. The principles of hydrodynamic stability analysis are summarized and then used in developing a linear perturbation equation for vertical water movement with nonsharp fronts. A numerical approach for determining the stability parameters involving the eigenvalues of a matrix solution is outlined, and the numerical results are checked against an analytical solution for a profile exhibiting a uniform distribution of capillary potential.