Debris flows are multiphase, gravity-driven flows consisting of randomly dispersed interacting phases. Their rheology and flow behaviour can vary along the runout path and depend on the sediment composition and percentage of solid and fluid phases. In an attempt to better comprehend the mechanisms and dynamics of these complex phenomena, existing one-phase numerical models are being complemented or replaced by new two-phase models. In the present paper, capabilities and limits of a quasi-two-phase code (TRENT2D) to simulate debris flows are discussed with reference to the Pellaud catchment basin (Norwestern Italy). Since TRENT2D is based on Takahashi’s theory (1978), it restricts the angles of failing slopes to less than φ/2, that is most commonly exceeded at site. As a consequence, the flow dynamics in the Pellaud upper slope was simulated with a one-phase code (RASH3D) and its outputs were used as input in the TRENT2D code, as soon as the Takahashi’s conditions are respected.