Flow–Sediment Interaction and Local Scour Formation Downstream of a Weir: Physical Modeling Approach

The structural integrity of hydraulic structures is frequently weakened by local scour processes downstream of weirs. This study investigates the relationship between hydraulic parameters and erosion patterns to improve the predictability of bed deformation. The research methodology integrates detailed field measurements from the Radomka River in Piaseczno with laboratory experiments using a 1:30 physical scale model of the existing weir. Bed shear stress demonstrated the strongest correlation with maximum scour depth (r ≈ 0.93; RMSE ≈ 0.0032), as it directly represents the tangential force acting on sediment particles at the bed surface, which controls their entrainment, transport capacity, and ultimately the intensity of local scour development, whereas near-bed velocity showed weak and non-significant dependence (r ≈ 0.26; ρs ≈ −0.11). This weak dependence reflects the dominance of turbulence-induced velocity fluctuations and localized vortical structures in the near-bed region, which obscure the relationship between mean velocity and sediment mobilization. The relationships between mean velocity, Froude number, and scour depth were moderate (r ≈ 0.63–0.73) and showed nonlinear characteristics, confirmed by HSIC values up to 9.1 × 10−3, due to the complex interaction between flow structures and evolving bed morphology. This nonlinearity results from the interaction between turbulent flow structures, jet-induced vortices, and the dynamically evolving bed morphology, combined with the threshold-controlled and nonlinear response of sediment transport to hydraulic forcing. Among all tested parameters, bed shear stress ranked as the dominant predictor of scour depth, outperforming velocity-based indicators. These findings imply that including bed shear stress parameters significantly improves hydraulic structure safety assessments. This study based on 11 experimental runs concludes that a combined field and laboratory approach provides a robust framework for river engineering. Finally, an improved understanding of erosion mechanisms, as presented in this work, enhances the prediction of local scour development and supports the design of more resilient hydraulic infrastructure.