Field-Scale Numerical Investigation of Flow and Bed Stresses at a Bridge Site: Effects of Piers and Pressure-Flow with Deck Overtopping

Detached Eddy Simulations (DES) with deformable free surface are used to analyze the three-dimensional flow fields and the bed shear stresses at a multiple-pier bridge in a river reach with natural bathymetry. The bed shear stresses and the dynamics of coherent structures are analyzed for different flow regimes (free surface and pressure flow with deck overtopping) and different scenarios, which include the removal of some of the piers and/or the bridge deck. Using data from this full-scale case study, the paper discusses how the presence of pressurized flow beneath the bridge deck and the proximity of piers affect the scour potential. In the present case study, the contraction caused by adjacent piers has a minor effect, since piers are relatively thin and far from each other. The bed shear stresses and the erosion potential are found to be more than twice larger when the flow is pressurized compared with the corresponding simulation in which the bridge deck is removed. When the flow is pressurized, the piers have small effects on the erosive capacity of flow; hence, for relatively high flow conditions, pressure scour is dominant compared to local and contraction scour. The results from DES are also compared with those obtained using the Reynolds-averaged Navier Stokes (RANS) approach. The mean flow field and mean bed shear stresses are fairly close to each other, particularly for pressurized-flow conditions at the bridge deck. Simulation results show that in regions with large-scale coherent structures, RANS computations underestimate the turbulent kinetic energy and, in turn, the fluctuations of bed shear stress that play a role in local scouring.