As Vedran indicates, both softwares only have a small strain formulation. Search for a software with large strain option. The software FLAC meets all your requierements.
Slope stability analysis in PLAXIS software can be performed by varying the ground water level with respect to time and study the response due to change in ground water level. This depicts the water level before and after rainfall. If you want to simulate debris flow in analysis, first you need to select the region that would fall after rainfall..this can be simulated by applying displacement as boundary conditions and study the response due to change in displacements within the selected region. i suggest you to read PLAXIS manual and collect literature related to your study is available. Find attached the paper related to the work for your reference. all the best in your research..