Local scour in the vicinity of bridge piers under ice-covered conditions - experimental study and numerical simulation

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Local scour in the vicinity of bridge piers has been identified as one issue with potentially catastrophic results. Excessive scour can cause bridge pier damage, which can result in bridge failure and may cause loss of life. The proposed research will address some important gaps in the scientific literature, such as (a) assessing local scour processes around bridge piers under ice cover, (b) establishing associated equations describing the dependence of scour processes on flow intensity, particle grain size and boundary conditions, (c) understanding how ice cover alters the geometry of scour holes and deposition mounds, (d) investigating the impacts of the dimension/shape of bridge piers on local scour processes under ice cover, and (e) developing a numerical model for simulating local scour processes around bridge piers under ice-covered conditions. Extensive experiments will be carried out in a flume which is 80-m long, 2-m wide and 1.3-m deep at the Quesnel River Research Center of the University of Northern British Columbia. An acoustic doppler velocimeter will be used to determine the flow field around bridge piers. Styrofoam sheets will be used as simulated ice cover (smooth, rough and very rough). Analyses of the experimental data collected from clear-water experiments under both open-flow and ice-covered conditions will be focused on non-dimensional variables, such as shear Reynolds number, Shields parameter, and bridge pier dimensions. Equations will be established to determine scour depth around bridge piers under ice-covered conditions. Modeling the scour process includes modeling a strongly coupled system involving flow field, sediment transport and bed morphology. In this proposed research, the numerical model will consist of three modules: a hydrodynamic module for the simulation of the flow field around a bridge pier, a sediment module for the modeling of suspended sediment transport, and a bed variation module to account for the change in the bed topography. Numerical studies of the local scour around bridge piers will also be undertaken in the present research. RANS models will be applied to simulate the flow field around bridge piers under ice-covered conditions. One of the priorities for the simulation of flow fields around bridge piers is to solve the vortex systems around the infrastructure. To solve the vortex systems, either unsteady RANS or Large Eddy Simulation (LES) will be used to resolve the horseshoe vortex. In this study, the standard k-epsilon turbulence model will be used first in the simulation for the flow field around a bridge pier under ice cover. The RNG-based k-epsilon turbulence model will also be used in the simulation. Suspended load and bed load will be simulated separately under both open-flow and ice-covered conditions. For suspended-load transport, based on the hypothesis that the interaction between different size fractions of suspended load is negligible (Wu et al. 2000), the transport will be governed by the universal advection-diffusion equation. For bed-load transport, the formula proposed by van Rijn (1987) will be used. In the event that van Rijn’s approach does not work, other approaches should be attempted such as Ashida-Michiue’s empirical formula. Once the suspended load and bed load is determined, the bed deformation can be calculated by applying the mass balance equation to the bed layer. The proposed research will help us to better understand local scour processes around bridge piers under ice cover. Results will provide civil engineers with reliable results regarding local scour around bridge piers that can be used to modify guidelines for bridge design considering the impact of ice cover.