<p>Excavating spillways is a common measure for mitigating landslide dam risks. While proven effective, the mechanisms through which spillway morphology influences breach dynamics are not fully quantified. This study employs numerical simulations to analyze how various morphological parameters (position, slope, longitudinal profile, cross-section) influence breach processes. The results reveal that a spillway confines the overflow to a predefined channel, preventing random erosion and reducing peak discharge by 13.7 to 25.1% compared to unexcavated cases. The position has an important effect on flow divergence: a central position promotes bidirectional flow, reduces velocity by impeding water with dam materials on both sides, whereas a side position accelerates unilateral erosion. The slope regulates the flow efficiency: a downhill slope enhances drainage and reduces peak values, while an adverse slope increases water retention and risk. The profile shape modulates flow resistance: downstream expansion increases cross-sectional area, reducing velocity (continuity principle), while contraction may induce turbulence. The cross-sectional shape influences the hydraulic radius: a trapezoidal section offers an optimal balance between stability and efficiency. These findings provide mechanistic insights for optimizing spillway design in emergency scenarios.</p> Graphical abstract <p></p>