Mirali Mohammadi

This study presents a comprehensive assessment of the transient slope stability of the Gavoshan embankment dam, located in western Iran, under a range of rapid reservoir drawdown scenarios. The analysis was performed using a sequential seepage–stability framework, in which transient pore pressures were computed in SEEP/W and transferred to SLOPE/W for slope stability evaluation. Four drawdown rates of 0.15, 0.30, 0.60, and 1.20 m/day were modeled to represent a wide operational range, from gradual to accelerated depletion. The hydraulic response was examined through temporal variations in total head and phreatic line, while slope stability was evaluated based on the time-dependent factor of safety (FoS) for both upstream and downstream slopes. The findings show that during a rapid drawdown, reservoir lowering occurs faster than pore-pressure dissipation within the dam body. This results in temporary hydraulic disequilibrium, steep pressure gradients, and brief flow reversals in the upstream zone. The upstream slope exhibited the most critical response, with FoS decreasing from 2.04 to between 1.66 and 1.73 depending on the drawdown rate, followed by a gradual recovery as pore pressures equilibrated. In contrast, the downstream slope remained highly stable (FoS ≈ 1.97–2.08) under all conditions. A sensitivity analysis showed that clay-core permeability had a modest influence on stability; at a drawdown rate of 1.20 m/day, the FoS ranged from approximately 1.64 to 1.70, with lower permeability values producing slightly higher FoS under the investigated conditions. The findings indicate that the Gavoshan Dam maintains safe hydraulic and structural performance under both gradual and rapid drawdown, highlighting the necessity of transient coupled analyses for realistic and reliable dam safety assessment.

Grade-control structures (GCSs) are widely used in river engineering to stabilize bed elevation, regulate sediment movement, and control channel degradation. However, conventional impermeable GCSs often generate plunging jets that intensify local scour downstream, threatening structural stability and altering channel morphology. This study aims to evaluate the morphodynamic performance of gabion grade-control structures and to quantify the effects of gabion porosity and tailwater depth on downstream scour mitigation. A series of clear-water laboratory experiments was conducted in a recirculating tilting flume, comparing a solid GCS with gabion models of three porosities (n = 0.38, 0.45, and 0.50) under unit discharges ranging from 0.023 to 0.043 m²/s and different tailwater depths. Temporal scour evolution and equilibrium bed morphology were measured using high-resolution bed profiling. The results showed that gabion configurations consistently reduced maximum scour depth and scour length relative to the solid structure, with reductions of up to about 38% and 44%, respectively, under the tested free-overfall conditions. Increasing porosity improved scour mitigation, although the incremental benefit became small beyond n ≈ 0.45. Greater tailwater depth further reduced scour for both structure types, and the mitigation effect was stronger when combined with gabion porosity, indicating that downstream submergence and structural permeability act together to control scour development. Gabion cases also reached equilibrium more rapidly than the solid configuration and promoted more localized deposition near the structure toe. Statistical analysis identified the densimetric Froude number as the dominant driver of scour growth, whereas tailwater depth and porosity acted as significant mitigating variables. Empirical relationships developed for relative scour depth and length showed high goodness-of-fit within the tested laboratory domain (R² > 0.95). Overall, the findings indicate that gabionized GCSs can improve downstream scour performance compared with impermeable structures, while the proposed equations provide laboratory-based screening tools for assessing the combined influence of porosity and tailwater depth within the calibrated range.

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.

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 (T_r/h, where h is structure height) and tailwater depth (y_t/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 (d_s/h) and length (l_s/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 T_r/h ≈ 0.5 reduced scour by nearly 70% and further increases to T_r/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 d_s/h and l_s/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.