The dam break flood is one of the potential causes of catastrophic events in cascade hydropower hub groups. Investigating the movement patterns of dam break flooding among reservoir groups under different dam break speeds is crucial for flood prevention and emergency response. In this study, the evolution characteristics of dam break floods were investigated in a cascading reservoir system, focusing on different break speeds of the upstream dam. The results indicate that the dam break speed determines the concavity or convexity of the water level curve changes in the upstream reservoir. Accordingly, dam breaks are classified into three modes: instant dam break, fast dam break, and slow dam break. An approximate critical speed has been identified to differentiate between the fast dam break and slow dam break. Further investigation into the evolution patterns of dam break floods in downstream reservoirs under different break modes was conducted. Correspondingly, the flood peak discharge and peak arrival time of the dam break floods vary differently with break speed under different break modes. Finally, a theoretical analysis for the flood peak discharge at the dam site during gradual dam break at a certain speed was established, which is able to predict the over-dam flood peak discharge in fast and slow dam break modes. This study is based on a combination of laboratory flume experiments and three-dimensional numerical simulations. This study has theoretical significance for the reinforcement of public infrastructure safety and the prevention of natural disasters.

Large stilling basins founded on heterogeneous weak-rock foundations may experience nonuniform deformation due to spatial variations in foundation stiffness. Although pile reinforcement is widely used for deformation control, its effectiveness may vary with local structure–foundation conditions. Overall and local deformation may also respond differently to pile parameters. This study conducted three-dimensional FLAC3D analyses of the Kala Hydropower Station stilling basin under the check-flood condition. Two representative transverse sections with different weak-rock distributions, pile embedment conditions, and structural configurations were compared under unreinforced and pile-reinforced conditions. The maximum resultant displacement (U_max) and maximum vertical joint offset (ΔU_joint) were used to characterize overall and local differential deformation, respectively. Under the 2.5 m × 2.5 m pile layout, U_max and ΔU_joint decreased by only 3.1% and 3.2% in Section 14–14, compared with 33.7% and 58.9% in Section 16–16. The two deformation measures also responded differently to pile spacing and pile length, and denser layouts or longer piles did not consistently reduce both overall and local deformation. Varying the thickness of competent rock overlying the weak rock produced only small changes in deformation, whereas increasing the weak-rock deformation modulus reduced both deformation measures, particularly in Section 16–16. These findings show that the effectiveness of pile reinforcement varies among structural locations with different foundation and embedment conditions. Pile layouts should therefore be evaluated for individual structural locations using both overall displacement and joint differential movement.