This study analyzed the normal use of an unusual flood-releasing tunnel with a plug dissipator. Firstly, normal physical model tests based on the Froude criterion (1 : 50) were finished. Secondly, depression physical model tests based on the Froude criterion (1 : 50) and cavitation similarity criterion were finished. Thirdly, 3-dimensional numerical simulation of flow field was finished, and free surface profile was captured, which was based on RNG k-epsilon two-equation turbulence model and VOF method. The focus of this study is on the relationship between normal use and cavitation characteristics (e.g., pressure, turbulence kinetic energy, and cavitation number). The results show that lowering the reservoir water level, reduced by 20.41m at most, increases the risk of cavitation of a plug discharge tunnel, which means with the decrease of the flow cavitation number, the possibility of structural damage will increase dramatically, while reducing the outlet height can effectively raise the flow cavitation number, ensuring the safety of normal use. Under the conditions of free outflow, for the H-1/e values of 4.45, 4.00, and 3.55, the conditions in which the tunnel meets the requirements of anticavitation are h/D = 0.42, h/D = 0.39, and h/D = 0.35, respectively. In addition, the discharge capacity of the tunnel is not significantly reduced with the lowering of outlet height, implying that operation under a low water head of the plug discharge tunnel, as low as 3.55 of H-1/e in the test, is feasible. The results obtained in this study can serve as reference information in engineering design of the plug discharge tunnel.
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