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To address the wave control challenges in the surge tanks of hydropower plants with ultra-long headrace tunnels, this study proposed a surge tank configuration with three large-volume water chambers. The transient characteristics under critical operational scenarios such as load acceptance and rejection were analyzed using the one-dimensional method of characteristics (1D MOC). Additionally, the 1D MOC and three-dimensional fluid dynamics (3D CFD) coupled simulation method was applied to analyze the flow patterns in the surge chambers under controlled conditions. The results demonstrate that the well-designed multi-chamber geometry effectively confines water-level fluctuations within acceptable limits. Both water and air phases exhibit favourable flow patterns without significant air-pocket entrapment. These findings provide valuable insights for improving transient processes in long-distance hydropower systems and offer guidance for surge tank design optimization.