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

桥墩附近的局部冲刷已被确定为一个具有潜在灾难性后果的问题。过度冲刷会导致桥墩损坏，从而导致桥梁失效并可能造成生命损失。 拟议的研究将解决科学文献中的一些重要空白，例如（a）评估冰盖下桥墩周围的局部冲刷过程，（B）建立描述冲刷过程对水流强度、颗粒粒度和边界条件的依赖关系的相关方程，（c）了解冰盖如何改变冲刷坑和沉积土丘的几何形状，（d）研究桥墩的大小/形状对冰层覆盖下局部冲刷过程的影响，以及（e）发展一个数值模型，以模拟在冰层覆盖情况下桥墩周围的局部冲刷过程。 将在北方不列颠哥伦比亚省大学Quesnel河研究中心的一个长80米、宽2米、深1.3米的水槽中进行广泛的实验。将使用声学多普勒流速计来确定桥墩周围的流场。泡沫聚苯乙烯板将用作模拟冰盖（光滑、粗糙和非常粗糙）。从清水实验中收集的实验数据的分析，在开放流和冰覆盖的条件下，将集中在无量纲变量，如剪切雷诺数，希尔兹参数，和桥墩尺寸。将建立方程来确定在覆冰条件下桥墩周围的冲刷深度。 冲刷过程的模拟包括模拟一个涉及流场、泥沙输移和床面形态的强耦合系统。在这项拟议的研究中，数值模型将包括三个模块：一个水动力模块的模拟桥墩周围的流场，泥沙模块的悬浮泥沙输运建模，和床变化模块占床地形的变化。 本文还将对桥墩局部冲刷进行数值模拟研究。RANS模型将用于模拟复冰条件下桥墩周围的流场。桥墩周围流场模拟的重点之一是解决基础设施周围的涡系。为了求解涡系，将使用非定常RANS或大涡模拟（LES）来求解马蹄涡。本文首次采用标准k-ε湍流模型对冰盖下桥墩周围的流场进行数值模拟。基于RNG的k-ε湍流模型也将用于模拟。 悬移质和推移质将分别在敞流和覆冰条件下进行模拟。对于悬浮物的输运，基于悬浮物的不同粒径部分之间的相互作用可以忽略不计的假设（Wu等人，2000年），输运将由通用对流扩散方程控制。对于推移质输移，将采用货车Rijn（1987）提出的公式。如果货车Rijn的方法不起作用，则应尝试其他方法，如芦田-ue的经验公式。当悬移质和推移质确定后，就可以通过对床层应用质量平衡方程来计算床层变形。 研究结果将有助于我们更好地了解冰盖下桥墩局部冲刷过程。研究结果将为土木工程师提供可靠的桥墩周围局部冲刷结果，可用于修改桥梁设计指南，考虑冰盖的影响。

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.