Abstract: In order to modify the spillway structure of a check dam and specify the geometry parameters of the spillway, the debris-flow pattern, scour depth, and energy dissipation were discussed under the conditions of different bottom surfaces and lateral contraction ratios based on physical modeling experiments. According to the buried depth design criteria for check-dam foundations and critical equilibrium for nonviscous particles under water, the rational value of the lateral contraction ratio was also discussed. The results indicated that the scour hole with deep scour depth was near the dam foundation when the spillway surface was a plate or curved surface with a large inclined angle. When a curved surface with a horizontal outlet was considered, not only was the scour depth decreased to some extent, but the distance between the plunging point and the dam toe was also enhanced. Generally, upon increasing the lateral contraction ratio, the energy dissipation rate increased due to the stronger interaction between the debris flows and erodible bed downstream if the debris-flow scales and spillway curvature were fixed. When the lateral contraction ratio was in the range 0.2-0.6, the scour depth behind the check dam satisfied the engineering design. Meanwhile, the distance between the plunging point and the dam toe was reasonable. This study can provide some reference points for the design of debris-flow mitigation in the engineering of check dams.