Abstract: In an attempt to clarify the hydraulic transient process of mixed free-surface-pressure flow in water conveyance tunnel and to optimize the discharge of sluice gate, the calculation principle and method of hydraulic transients for mixed free-surface-pressure flow in a circular water conveyance tunnel are researched based on the slot method and diffusion method in explicit finite-difference method. A trunk canal project in a large irrigation area is taken as research background. The formulas of hydraulic elements for the transient flow of circular tunnel is given, and time-dependent curves of water depth and pressure of each tunnel section in the presence of varying flow rates are obtained. Results manifest that water pressure rises more sharply as it gets closer to the tunnel downstream section when the water level of circular tunnel is capped. Under the condition of open flow, the maximum water depth in the tunnel drops linearly with the increase of sluice discharge but is hardly affected by the closing time of check gate. According to numerical simulation, the reasonable design discharge of the sluice gate is given to provide reference for the design of similar projects.

Key words: circular tunnel, mixed free-surface-pressure flow, hydraulic transient, slot method, sluice gate, optimal sluice discharge