The difference in soil properties determines the different breaching characteristics exhibited by landslide dams (LDs) and debris-flow dams (DFDs). In this study, two types of soil were prepared by controlling the initial water content and the mixing time of the soil to construct the LD and DFD. Based on observations of breach in dams with six different grain size distributions, the following conclusions were drawn: (1) the erosion resistance within the soil leads to a slower failure speed for DFD under the same grain size distribution and particle density. However, both types of dams exhibit a nonuniform downcutting process in the longitudinal direction, induced by uneven velocities. (2) Laterally, DFDs are characterized by the creep slide of the breach bank, distinct from the intermittent slide observed in LD. (3) For the range of conditions tested, the peak discharge of LD significantly exceeds that of DFD. Additionally, the flood curve of LD exhibits a bimodal characteristic, attributed to the slide of the bank slope and the nonuniform distribution of particles within the dam. Finally, a prediction formula for the downcutting coefficient of the breach was established and validated by past studies. This study provides a basis for predicting outburst floods of LD and DFD.