<p>Predicting sediment dynamics during catastrophic high-magnitude tailings dam ruptures is critical for environmental assessment and risk mitigation, yet remains a challenge due to rheological complexities. This study performs an exploratory evaluation of the capabilities and limitations of a coupled non-Newtonian hydrodynamic and mobile-bed model (HEC-RAS 2D) in representing the geomorphic response of the 2019 Brumadinho disaster. Through a robust framework of 630 Monte Carlo simulations, a global sensitivity analysis was conducted to quantify the influence of sediment load, grain size, specific gravity, and adaptation length on model outcomes, with the best-fit runs evaluated by statistical tests (Kruskal–Wallis). Validation against field-observed bed changes and mass/volume balances achieved a consistent 90–92% accuracy across the best-fit simulations. Contrary to traditional sediment transport paradigms, results revealed that grain size had a negligible impact, whereas sediment load and specific gravity emerged as the primary drivers of the system’s response. Furthermore, the model successfully captured the significant attenuation of peak discharges within the watershed. These findings demonstrate that while hydrodynamic-sediment transport flow is a powerful tool for disaster reconstruction, the empirical nature of transport functions requires careful parameterization of sediment load to address uncertainties in hyperconcentrated flow.</p>