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可能造成灾难性后果的一个问题。过度冲刷会造成桥梁桥墩损坏，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流动强度、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孔和沉积丘的几何形状，(d)研究冰层覆盖下桥墩的尺寸/形状对局部冲刷过程的影响，以及(e)建立一个模拟冰层覆盖条件下桥墩周围局部冲刷过程的数值模型。在北不列颠哥伦比亚大学奎斯奈尔河研究中心，将在长80米、宽2米、深1.3米的水槽中进行大量实验。用多普勒测速仪测定桥墩周围的流场。聚苯乙烯泡沫板将用作模拟冰盖（光滑，粗糙和非常粗糙）。从清水实验中收集的实验数据在开放流和冰覆盖条件下的分析将集中在非量纲变量上，如剪切雷诺数、盾构参数和桥墩尺寸。建立了确定冰雪条件下桥墩周围冲刷深度的方程。对冲刷过程进行建模包括对流场、泥沙输运和河床形态等强耦合系统进行建模。在本研究中，数值模型将包括三个模块：用于模拟桥墩周围流场的水动力模块，用于模拟悬沙输运的泥沙模块，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局部冲刷进行数值研究。采用RANS模型对覆盖冰条件下桥墩周围流场进行模拟。桥墩周围流场模拟的重点之一是解决基础设施周围的涡流系统。为了求解马蹄形涡，可以采用非定常RANS或大涡模拟（LES）来求解马蹄形涡。在本研究中，将首先采用标准k-epsilon湍流模型对冰雪覆盖下桥墩周围流场进行模拟。基于rng的k-epsilon湍流模型也将用于模拟。在开流和覆冰条件下，将分别模拟悬浮荷载和床荷载。对于悬载输运，基于悬载不同大小部分之间的相互作用可以忽略不计的假设（Wu et al. 2000），输运将由通用平流-扩散方程控制。对于河床输运，将使用van Rijn（1987）提出的公式。如果van Rijn的方法不起作用，则应该尝试其他方法，例如Ashida-Michiue的经验公式。一旦确定了悬浮载荷和床层载荷，就可以将质量平衡方程应用于床层，计算床层变形。这项研究将有助于我们更好地了解冰覆盖下桥墩周围的局部冲刷过程。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.