The horizontal screen is one of the energy dissipater structures used on the brink of vertical drop. These structures increase the energy dissipation and the turbulence in the flow by causing air entrainment. In the present study, the effect of the diameter of the screen with constant porosity at three different diameters on the hydraulic parameters of the vertical drop was investigated. The experiments were performed in the relative critical depth range of 0.13 to 0.39. The results showed that by increasing the relative diameter of the horizontal screen, the relative wetting length and turbulence length increased, the residual energy remained constant and the pool depth decreased. Compared to the stilling basin, the horizontal screen significantly reduces turbulence length and residual energy. The results also showed that the application of horizontal screens at the brink of the drop, with and without a downstream rough bed, could be a suitable alternative for a stilling basin.
