One of the most effective ways to dissipate the destructive energy of water downstream of a flow control structure is the interference of water with air (such as a vertical drop). In this study, the hydraulic parameters of a vertical drop equipped with dual horizontal screens and with the subcritical upstream flow were investigated. Experiments were performed for two drop heights, two porosities, three relative distances between the screens, and a relative critical depth that was varied from 0.077 to 0.242. The results revealed that the relative distance between the screens does not affect the relative depth and downstream residual energy. On the other hand, increasing the relative length of the drop caused a decrease in the relative depth. Also, for a vertical drop equipped with a type 1 settling basin, the relative downstream depth decreased by more than 48%. The normal residual downstream energy and the relative total length of the drop are also decreased. The results also reveal that the use of dual horizontal screens transformed the regime of flow from super- to subcritical downstream of the drop.
