A numerical study is conducted for converging stepped spillways with various spillway slopes, sidewall convergence and flow discharges to understand the influences of converging sidewalls on skimming flows. Compared with the traditional uniform-width stepped spillway, the converging sidewall changes the skimming flow features by generating standing waves on the free-surface, curving the mainstream streamlines and squeezing and distorting the bottom rotations. For free-surface, mainstream and bottom rotations, the width of disturbed regions by the converging sidewall varies, with that for bottom rotations being 1.5 times that for mainstream and 3 times that for free-surface. The variation rules of disturbed region widths along the spillway are obtained, and the maximum widths increase as the convergence angle and incoming flow discharge increase, and the spillway slope decreases. Three equations are established for predicting the maximum widths of disturbed regions for free-surface, mainstream and bottom rotations of skimming flow. Since the disturbed region of skimming flow generated by the converging sidewall is characterized by strong standing waves, high flow velocity and low air concentration, appropriate engineering measurements should be taken for the disturbed region to meet the challenges to overtopping, energy dissipation and cavitation control.

On a stepped channel, three distinct flow regimes, nappe, transition, and skimming, occur successively with increasing discharge. This work proposes a criterion of the transition flow occurrence: a critical slope of the channel exists between mild and steep slopes, and for mild slopes, air pocket disappearance and nappe parallel to channel slope are defined as the lower and upper limits of transition flow, whereas for steep slopes, the two phenomena oppositely represent the upper and lower limits. The analytical approaches based on the criterion demonstrate the critical slope of 30° and predict the lower and upper limits of transition flow. The experimental data verify the existence of the critical slope and its prediction accuracy, and indicate that the prediction of the lower and upper limits of transition flow works well for both mild (30°) slopes. During stepped channel design, employing the critical slope can control the transition flow occurrence, therefore reducing the risks caused by structure vibration and excessive splashing.

(2019). Flume experiments on baffle-posts for retarding open channel flow. Journal of Hydraulic Research: Vol. 57, No. 2, pp. 280-282.