Abstract
Catastrophic floods triggered by a dam-break pose significant hazards to infrastructure due to transient flows and concentrated structural loads. These hazards are intensified where in-channel structures or variable topography redirect momentum and magnify impacts on downstream infrastructures. This study aimed to numerically predict the dam-break transient flow characteristics around a bed depression, as a specific topographic condition, in the downstream channel. The numerical model was optimized and validated against experimental results reported in the literature. Subsequently, the effects of bed depression dimensions and location along the channel on transient flow were examined in twelve cases. The validation results demonstrated that the model accurately reproduced the evolution of the free surface, flow velocity, formation of the impact jet, and impact loads. The bed depression has increased plunge-pool dissipation and reduced the peak force to 16.6–18.6 N. Accordingly, the supercritical flow was characterized by a maximum Froude number (Fr) of approximately 5.4 around this depression. Increasing the depression distance attenuated the wave front and decreased the post-depression flow depth from 0.16 m to 0.03 m within approximately 2 seconds. Overall, the framework captured sharp interfaces and transient regime shifts, enabling the prediction of jetting, nappe stabilization and impact-load envelopes. The study has implications for evaluating in-channel structures and the effects of channel topography on rapid flood hazard screening and emergency planning.