The influence of hydraulics and geometrical modifications on downstream scour in inclined drop structures

This study experimentally investigates the hydraulic and geometric controls on local scour downstream of inclined drop structures (IDSs), emphasizing downstream face inclination and tailwater depth as key design variables. Twenty-seven physical model tests were conducted with systematically varied slope angles (21.8°, 26.6°, 33.7°), tailwater depths (free-flow, ×1.5, ×2), and multiple flow discharges. Results indicate that slope angle strongly governs scour morphology, steeper configurations concentrate jet momentum, increasing maximum scour depth by up to 32.4%, whereas shallower slopes produce longer scour cavities, with length increases exceeding 36%. Increased tailwater depth consistently mitigated scour, reducing depth and length by up to 16.5% and 14%, respectively, through decoupling of vortex activity from the bed. Temporal analyses showed that over 80% of equilibrium scour depth develops during the initial phase, underscoring the dominance of early-stage hydraulics. Novel multivariate regression models based on dimensionless parameters achieved prediction errors within ± 10% and demonstrated robustness in sensitivity analyses. Collectively, these findings provide quantitative benchmarks for design-oriented and performance-based optimization of IDS configurations, enabling the selection of scour-minimizing and hydraulically efficient designs that balance structural stability, hydraulic efficiency, and sediment transport management.