Under large unit discharges, the upstream section of stepped spillways is highly susceptible to cavitation damage due to the lack of aeration protection. This compromises its energy dissipation efficiency and can even lead to safety incidents. To address this, a novel pre-aeration facility named as gradual contraction aerators is proposed. Physical experiments were conducted to analyze and compare the hydraulic characteristics of the spillway with and without the aerators with the unit discharge, contraction angle, and contraction ratio as variables. The results demonstrate that after installing the aerators, aeration occurs effectively at both the upper and lower surfaces of the jet. The bottom air concentration along the spillway exceeds the safe threshold of 1.5%. The maximum time-averaged pressure on the step surfaces downstream of the aerator, measured to be only 1.92 kPa, occurs at the jet impingement point. This indicates that the jets from the aerators do not compromise the structural safety of the spillway. Key parameters, including the jet height, depth and inception point of quasi-uniform aerated flow were measured under various cases. Through multivariate nonlinear regression, empirical formulas relating these parameters to the Froude number, contraction ratio and contraction angle were established.
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