<p>Overtopping erosion is a critical mechanism of landslide dam failure, as it undermines dam stability and can lead to catastrophic downstream flooding. Existing physically based numerical models often assume homogeneous materials or represent soil gradation simplistically using mean particle size, limiting their applicability to widely graded materials typical of natural landslide dams. To address this gap, this study develops a novel particle-scale erosion model grounded in Newtonian mechanics, which explicitly incorporates the effects of particle-size distribution and granular contact geometry on erosion behavior. The model avoids empirical weighting schemes and instead captures erosion as a function of particle exposure and motion probability across grain-size intervals. Key model parameters are examined through sensitivity analysis, and the model is validated against flume experiments. The erosion model is then integrated into a physically based dam-breach simulation framework and applied to two case studies: the Tangjiashan landslide dam and the Banqiao Reservoir failure. Simulated peak outflows and breach timing closely match observed data, confirming the model’s accuracy and robustness. This study contributes a physically grounded approach to simulating landslide dam erosion and offers new insights into how soil gradation governs breaching processes.</p>