When water flows over step-pools, plunging from steps to pools, tumbling and hydraulic jumps will be produced. These three kinds of flows result in spill resistance, which generates significant energy dissipation. Since the geometry of mountain step-pools is complicated for flow structure analysis, a 3D fluid dynamic numerical model was used to analyze energy dissipation and flow resistance in regular step channels. Results show that in cases of low flow, the flow is in a nappe flow condition, and plunges from steps to pools and hydraulic jumps are produced. The total roughness
coefficient of the nappe flow can reach 0.1. For cases of high flow, water submerges the steps,resulting in a skimming flow, and the total roughness coefficient is comparatively small. Whether the flow is in a nappe or a skimming flow condition, the total roughness coefficient is higher than the channel surface-roughness coefficient set in the numerical simulations. The relationship between the
energy dissipation and the total roughness coefficient investigated in this study can be used as a reference
for hydraulic design in steep mountain streams.
Key Words : Step-pool, spill resistance, 3D fluid dynamic numerical model, nappe flow, skimming flow.