Abstract: The operation of gates in non-pressure water conveyance tunnels often induces hydraulic transients in the form of free-surface-pressurized flow (mixed flow), which can lead to pressure surges that pose a threat to the project’s safe operation. An experimental investigation was conducted to study the hydraulic transition process of mixed flow and pressureless flow in a rectangular tunnel with an arched crown. Water depth or pressure measurements were taken along the length of the model tunnel under different flow conditions, and the results were compared with calculated values obtained using the diffusion method based on the narrow slot assumption. The experiments revealed a complete free-surface-pressurized flow regime, where the closure of the downstream gate caused a sharp increase in pipeline pressure when the initial downstream water depth approached the height of the vertical wall in the arched tunnel under high flow conditions. This process is accompanied by intense air entrainment and upstream flow propagation. Under low flow conditions, a critical mixed flow regime was observed, characterized by a sudden pressure rise without aeration. The water echo resulted in a secondary pressure spike, reaching its peak value. The research findings demonstrate that the narrow slot diffusion method could accurately simulate the complete free-surface-pressurized flow regime, but generates a larger error when simulating the critical regime. The calculated results and experimental data are in good agreement for the hydraulic transition process of pressureless flow, validating the rationality of the mathematical model.

Key words: rectangular water conveyance tunnel with arch crown, free-surface-pressurized flow, narrow slot method, numerical simulation, model experiment