Effect of hole shape and porosity in horizontal screens on vertical drop flow

A vertical drop is an effective energy dissipator through water–air interaction. This study experimentally investigates the impact of horizontal screens with varying hole shapes and porosity on the hydraulic performance of vertical drops. A total of 63 runs were conducted using three shapes (circular, square, rectangular) and two porosities (50% and 60%), tested under nine flow rates. Results show that screens reduced downstream energy by about 60%, converting flow from supercritical (Froude 3.6–4.3) to subcritical (0.59–0.78). Downstream velocities decreased by ∼68%, mitigating scour. Energy dissipation was attributed to increased friction along hole perimeters. Square holes achieved the highest reduction due to enhanced turbulence from sharp corners, while rectangular holes were least effective due to their elongated shape. Porosity had minor effects on energy loss but significantly influenced wetted and mixing lengths. The 60% square hole screen minimized both lengths, achieving up to 38% and 57% reductions compared with unscreened flow.