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可能造成灾难性后果的问题。过度的冲刷会导致桥墩损坏，从而导致桥梁损坏，并可能造成人员伤亡。 拟议的研究将解决科学文献中的一些重要空白，如：(A)评估冰盖下桥墩周围的局部冲刷过程；(B)建立描述水流强度、颗粒尺寸和边界条件对冲刷过程的依赖关系的相关方程；(C)了解冰盖如何改变冲刷洞和堆积丘的几何形状；(D)调查桥墩的尺寸/形状对冰盖下桥墩周围局部冲刷过程的影响；以及(E)开发一个数值模型，用于模拟冰盖条件下桥墩周围的局部冲刷过程。 在北不列颠哥伦比亚省大学奎斯奈尔河研究中心，将在长80米、宽2米、深1.3米的水槽中进行广泛的实验。将使用声学多普勒测速仪来确定桥墩周围的流场。泡沫塑料将被用作模拟冰盖(光滑、粗糙和非常粗糙)。在明流和覆冰条件下，清水试验收集的实验数据的分析将集中在无量纲变量上，如剪切雷诺数、盾构参数和桥墩尺寸。将建立公式，以确定冰盖条件下桥墩周围的冲刷深度。 对冲刷过程的模拟包括对涉及流场、泥沙运动和河床形态的强耦合系统的模拟。在这项拟议的研究中，数值模型将由三个模块组成：用于模拟桥墩周围流场的水动力模块，用于模拟悬沙运动的泥沙模块，以及用于考虑河床地形变化的河床变化模块。 本文还将对桥墩周围的局部冲刷进行数值研究。采用RANS模型对结冰条件下桥墩周围的流场进行数值模拟。桥梁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问题。对于涡系的求解，将采用非定常RANS或大涡模拟(LES)来求解马蹄形涡系。在本研究中，将首次使用标准k-epsilon湍流模型对冰盖下桥墩的绕流进行数值模拟。模拟中也将使用基于RNG的k-epsilon湍流模型。 在明流和覆冰条件下，将分别模拟悬移质和推移质。对于悬移质运输，基于不同尺寸的悬移质之间的相互作用可以忽略不计的假设(Wu等人。2000年)，输运将受普遍的平流-扩散方程支配。对于推移质输运，将采用van Rijn(1987)提出的公式。如果van Rijn的方法不起作用，应该尝试其他方法，如Ashida-Michiue的经验公式。一旦确定了悬移质和推移质，就可以通过对床层应用质量平衡方程来计算床面变形。 拟议的研究将有助于我们更好地了解冰盖下桥墩周围的局部冲刷过程。研究结果将为土木工程师提供有关桥墩周围局部冲刷的可靠结果，可用于修改考虑冰盖影响的桥梁设计指南。

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.