Morphodynamic effects of porosity and tailwater depth on scour mitigation by gabion grade control structures

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.