Damage to the diagonal bracing of a steel pipe Sabo dam due to overflow boulders, or debris flow has been reported at the 5th Sabo dam on the Omu River after the dam reaches full sand deposition. The cause of damaged steel members remains uncertain. This study examines damage factors of the dams by overflow boulders with Distinct Element Method. The analysis parameters are based on datum from disaster investigations. A flow velocity distribution model is constructed from preliminary experiments. Following the disaster, boulder size, velocity, and water depth were determined through field surveys to evaluate potential damage to steel members of the dams. Findings reveal that the initial damage was caused by 5.0 m boulder collisions, followed by 1.0―2.0 m boulders, which continued to cause significant damage. This phenomenon led to simultaneous downstream flow of accumulated boulders at the dam’s top, resulting in damage to both diagonal and horizontal bracing of the dam.