A 2D integrated FEM model for surface water–groundwater flow of slopes under rainfall condition

Numerical modeling of water infiltration in slopes under rainfall conditions, especially under rainstorm conditions, is a fundamental problem for slope stability assessment. To obtain representative results, surface water–groundwater flow models are incorporated in the simulation. Based on finite element representation of Richards’ equation and of kinematic wave equations, an integrated 2D numerical model (IMCR2D) of the surface water–groundwater system was established. The model has a symmetrical matrix that modifies the flux boundary according to the runoff solution on the slope. IMCR2D was verified using two laboratory experiments, and it showed good agreement with numerical and experimental results. Additional numerical examples were used to study the effect of flux supply from runoff on infiltration. In comparison with SimMd (an existing method), IMCR2D displayed advantages in cases where surface runoff develops in an upper low-permeability section of the slope and flows down into a high-permeability section of the slope. To illustrate the advantages of the new method, the seepage field and stability condition of a case study in the Three Gorges Hydroelectric Reservoir were analyzed using IMCR2D and SimMd. The deformation of a landslide in part reflects its stability, and therefore, we also used displacement monitoring data to estimate the variation of stability conditions from that aspect. Comparison of the two numerical models indicated that flux supply greatly affects the seepage field, and that rainfall plays an important role in landslide stability evaluation, but only when considering flux supply from upper slope surface runoff.