The main objective of this thesis is to investigate three-dimensional flow around a bridge pier and/or woody debris using numerical simulations in several different configurations. The study addresses the uncertainties associated with woody debris accumulation at bridge piers and highlights the importance of understanding its effects on bridge loading and river hydraulics. Since the accumulation process depends on numerous factors and cannot be reliably predicted, estimating its influence on structural loads and the surrounding flow conditions remains a significant challenge. The thesis presents the theoretical background of free-surface hydraulics, the hydromechanics of flow around bridge piers, and the drag force acting on bridge piers, woody debris, and the combined bridge pier–debris system. The numerical simulations were performed using the DualSPHysics software, which is based on the Smoothed Particle Hydrodynamics (SPH) method. The simulation results are compared with theoretical expectations. Most results show good agreement with the theoretical background, while the drag force obtained for the combined bridge pier–debris system is inconsistent due to softwarerelated limitations. Drag forces were also calculated using the conventional drag equation and the equivalent width approach. The free-surface profile is presented for each of the 18 simulations, and the observed flow behaviour is found to be consistent with hydraulic theory.
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