Hydrodynamic Loadings on Debris Accumulations at Low Froude Numbers in Straight Channel

Debris accumulation critically impacts hydraulic structures by altering approach flow, amplifying hydrodynamic forces, and inducing backwater rise. While previous research has extensively examined drag forces due to debris, the effects of debris porosity, its proximity to the channel bed, and upstream–downstream water level difference on hydrodynamic loadings are still not fully understood. To address these gaps, 336 experiments were conducted under subcritical flow conditions, involving nine debris configurations, characterized by different geometries and porosities. Drag and lift forces were measured to quantify debris–flow–structure interactions. The results show that drag and lift coefficients increase with blockage ratio and water level difference, whereas they decrease with Froude number and proximity ratio. Moreover, debris porosity and geometry have a negligible effect on drag coefficient but significantly influence lift coefficient. In the tested range of Reynolds numbers, both coefficients are not affected by the flow regime, with all other parameters being constant. Based on experimental evidence and dimensional analysis, empirical equations were derived for estimating drag and lift coefficients. To the best of the authors’ knowledge, for the first time, the proposed predictive relationships account for all the above-mentioned hydraulic and geometric variables, providing useful tools for improving the design and resilience of bridge infrastructures.