Debris flows often cause devastating damage to property and can injure or kill residents in mountainous areas. The construction of check dams in debris-flow valleys is considered a useful strategy for mitigating the damages downstream. In this paper, a new type of spillway structure with lateral contraction was proposed to distribute debris
flows after the check dam storage filled up. Four different lateral contraction ratios of the spillway were considered in experiments that investigated debris-flow patterns, scour characteristics, and energy dissipation rates when debris flows passed through the spillway. The results indicated that lateral contraction considerably influenced the extension of debris-flow nappes. The drop length of the nappe at D0.7 ( means lateral contraction ratio) was approximately 1.4 times larger than at D0.4. The collision, friction,
and mixing forces between the debris-flow nappes and debris flows in downstream plunge pools dissipated much of the debris-flow kinetic energy, ranging from 42.03 to 78.08% at different contraction ratios. Additionally, based on a dimensionless
analysis, an empirical model was proposed to predict the maximum scour depth downriver of a check dam. It indicated that the results calculated by the model exhibited
good agreement with the experimental results.