Riprap mitigation of downstream scour at grade-control structures considering tailwater depth and layer thickness

Downstream scour at grade-control structures (GCSs) poses a serious threat to structural stability and riverbed integrity. Among various countermeasures, riprap has long been recognized as a cost-effective and readily available solution for scour protection. This research examines the role of riprap in reducing scour downstream of a vertical drop GCS, with emphasis on the influence of riprap thickness (Tr/h, where h is structure height) and tailwater depth (yt/h). Experiments were conducted under clear-water conditions to monitor both the temporal and spatial development of scour and to establish new empirical relationships for normalized maximum scour depth (ds/h) and length (ls/h). The results show that riprap substantially reduces scour dimensions and accelerates stabilization. In the absence of protection, maximum scour depth reached up to 1.2 h under high discharges. Increasing riprap thickness to Tr/h ≈ 0.5 reduced scour by nearly 70% and further increases to Tr/h ≈ 0.66 achieved reductions exceeding 89%, with scour almost eliminated at low flows. Tailwater depth provided an additional stabilizing effect, reducing jet impact velocity and vortex intensity. Doubling the tailwater depth decreased scour by 20–30%, and when combined with thick riprap layers, reductions greater than 90% were achieved in both depth and length. The proposed empirical equations demonstrated strong predictive capability for both ds/h and ls/h, yielding R² values of 0.913 and 0.902, RMSE values of 0.107 and 0.405, and MAE values of 0.082 and 0.310, respectively. The sensitivity analysis further identified riprap thickness and tailwater depth as the most influential parameters governing the protective performance. These findings confirm the riprap effectiveness as a simple, economical, and robust strategy for mitigating downstream scour, offering valuable guidance for the hydraulics and geotechnical design of grade-control systems.