The modelling of debris flow initiation in slopes is addressed in this paper. First, possible factors governing debris flow initiation are established. Then, a coupled hydro-mechanical model for deformable porous media with two pore fluids that is used to assess the problem of the debris flow initiation in slopes is briefly outlined. Various ways to identify failure and to approach the transition of the failed mass into a debris flow are discussed in the framework of small strain theory and elasto-plastic behaviour. A parametric study was carried out to evaluate the relative importance of the most commonly cited parameters that are assumed to influence debris flow initiation. It was found that the slope angle is of minor importance in the development of slope instability under loading due to internal water supply. Transient behaviour was found to be decisive, and some critical combinations of water supply over time yielded situations that were likely to encourage the onset of debris flow. The significant role of permeability as a function of the degree of saturation in relation to the water supply is demonstrated. The proposed three-phase model is shown to be an adequate and promising way to address debris flow initiation.