Abstract. When debris flow discharges into the main river, the deposition of debris raises the river bed, occupies the path of water conveyance and damages or even destroys buildings, resulting in considerable economic loss and possibly fatalities. Mathematical models are normally employed to compute debris flow. However, most of these models employ empirical formulae and coefficients and their results are seldom reliable. On the other hand, scale model tests associated with debris flow have seldom been conducted due to the lack of corresponding similarity laws and the difficulty of achieving the grain diameter scale. Focusing on pseudo-one-phase flow, this paper discusses the laws of similarity for the confluence of debris flow and main river and conducts a case study of the debris flow that occurred on 13 August 2010, in the Wenjia Gully, China. After satisfying the roughness scale, the kinematic viscosity coefficient scale, and the momentum ratio scale, it was found that the deposition terrain in the model test is consistent with the one in the prototype.