Eddy-resolving CFD modelling of a river flow at a full-scale, multi-pier bridge over naturally-deformed bathymetry

Numerical simulations are conducted to evaluate the three-dimensional flow field and the bed shear stress in the vicinity of a multiple-pier bridge located in the Po river (Italy), considering the naturally-deformed bathymetry. The use of Detached Eddy Simulations (DES) allows to explicitly resolve the unsteady motion of the energetically important turbulent eddies, and the Volume of Fluid (VoF) method is used to consider the deformations of the free-surface. Simulations are conducted in different hydrodynamic regimes, including free-surface flow and pressure flow that generates in case of deck overtopping. The objective is to investigate the applicability of the DES approach and the VoF technique for simulating the flow dynamics in a full-scale river reach with irregular geometry and a man-made structure on the riverbed. The complex interplays among the river flow, the deformed bathymetry, and the bridge structure are explicitly accounted for, with a precision that far exceeds the typical level of detail achieved through standard methods used for the simulation of river flows (e.g., two-dimensional depth averaged models). In the case of free-surface flow, the deformed bathymetry, typical of natural rivers, as well as the non-zero angle of attack and the complex shape of the bridge piers, influence the flow field at the bridge site and the distributions of bed shear stresses. This aspect highlights some limitations that arise when canonical cases (i.e., piers of regular shape and angle of attack of 0 over a flat bed) are considered in place of real complex geometries. The impact of the lateral flow contraction on the flow fields and on the potential of sediment erosion is limited in the present case due to the reduced width of the piers and the large distance between them, resulting in a low blocking ratio. Transitioning to the pressure-flow regime increases the free surface elevation upstream of the bridge and induces the formation of a high-velocity orifice flow beneath the deck, with regions of high velocity extending far downstream. Recirculation regions are observed below and downstream of the deck. Compared to an equivalent free-surface case with the same discharge and stage, pressure-flow induces much higher bed shear stresses at the bridge site, entailing an increased erosion potential. In these conditions, the flow acceleration around the piers and the lateral flow contraction have a lower impact on the erosive capacity, as confirmed by a pressure-flow simulation conducted by removing the piers.