冰盖下桥墩局部冲刷随时间变化的试验研究

在寒冷地区,河道中冰盖的存在会改变河道流速分布。与明流条件相比,冰盖条件下水流最大流速点会向河床移动,加剧桥墩周围的局部冲刷。过度的局部冲刷会导致桥梁倒塌。基于水槽清水冲刷试验,对冰盖与明流条件下圆柱型桥墩局部冲刷随时间的变化进行了研究,试验结果表明：冰盖下桥墩局部冲刷速率大于明流。平衡冲刷深度比明流条件下的约大12%,且冲刷平衡所需时间比明流条件下的要约大10%。分析了水流强度与无量纲冲刷深度的关系以及冰盖与明流条件下冲刷深度变化速率的差异,给出了冰盖下局部冲刷深度随时间变化的经验方程,研究成果可供实际工程参考。     

冰盖下桥墩局部最大冲刷深度试验研究

冬季寒冷的北方河流易形成冰盖或冰塞,冰盖的存在对桥墩附近局部冲刷产生影响。在清水冲刷条件下,试验研究了有无冰盖条件下,不同流速和水深对桥墩附近局部冲刷的影响。研究结果表明：对比明流条件,冰盖的存在导致更大的近底流速和近底流速梯度,从而桥墩局部最大冲刷深度更大;其它条件相同的情况下,随流速的增大,桥墩局部最大冲刷深度增大;随着桥墩墩径增大,桥墩局部最大冲刷深度增加;水深增加时桥墩局部最大冲刷深度相对减小。根据试验数据,给出了有冰盖条件下桥墩局部最大冲刷深度的计算公式,与国内外相关试验数据吻合较好。     

冰盖下桥墩局部冲刷深度预测

利用实验室清水条件下桥墩局部冲刷的试验数据,采用支持向量机（suppcrt vector machine,SVM）和BP(back propagation)神经网络的方法,基于量纲分析原理,对影响桥墩局部冲刷产生的相关因子进行分析。将试验数据的3/4作为预测模型的训练数据集、1/4作为预测模型的测试数据集。模型的输入因子有水流弗劳德数Fr、水深与墩径之比h/D、床沙中值粒径与墩径之比d50/D、冰盖下表面糙率与床面糙率之比ni/nb,输出因子为冲刷坑深度ds。采用相关系数（r）、均方根误差（δRMSE）、平均绝对百分比误差（δMAPE）、确定系数（R2）作为预测结果的评价指标,并将预测结果与试验结果做了比较。BP神经网络模型和SVM模型在预测明流条件下桥墩局部冲刷坑深度时,预测结果的r分别为0.89和0.88、MAPE分别为38.8%和31%;在预测冰盖条件下冲刷坑深度时,预测结果的r分别为0.78和0.73、MAPE分别为43%和46%。结果表明BP神经网络和SVM模型预测明流及冰盖条件下桥墩局部冲刷坑深度时具有较高的精度。     

钩环式护圈对冰盖下桥墩局部冲刷影响的试验研究

基于桥墩局部冲刷原理,在水平护圈防冲措施的基础上,设计了一种能改变桥墩周围水流流态的新型防冲设施—钩环式护圈。为探究钩环式护圈对圆柱形桥墩局部冲刷的防护效果,采用不同形状的钩环式护圈进行室内物理模型试验,分析了桥墩周围的冲刷特征和水力特性。试验结果表明：当钩环式护圈的高度为1 cm、角度为135°且安装在床面时,防护效果最好;与光墩相比,桥墩安装钩环式护圈后,最大冲刷深度最多可减小62.2%,桥墩底部垂向流速、垂向紊动强度均明显减小。通过多元回归分析建立了计算桥墩周围无量纲最大冲刷深度的经验方程,该方程对明流和冰盖条件下水流均适用。     

桥墩局部冲刷防护的石块起动

桥墩局部冲刷一直是影响桥梁安全的最大自然灾害,抛石防护是最普遍的冲刷防护形式之一。在总结已有冲刷机理的基础上,分析了包括墩前河床底部流速和墩侧河床底部流速的桥墩局部流速,并给出了桥墩冲刷防护石块起动的简化公式。结果表明,墩侧河床底部流速大于墩前河床底部流速,墩侧防护石块更易走失。当行近流速小于3m/s时,可采用抛石进行桥墩局部冲刷防护,抛石直径约为0.2m;对于行近流速为3～5m/s时,建议采用其它冲刷防护措施。     

非均质河床双排桥墩局部冲刷数值模拟研究

精确模拟山区河流非均匀沙质河床桥墩的局部冲刷对桥梁设计和安全运行具有重要的意义。以黑石渡大桥河床床沙特征为背景,采用Flow3D软件开展非均匀沙质河床上双排圆柱形桥墩冲刷三维数值模拟研究。为考虑河床非均匀泥沙的悬移质运动、泥沙挟带、推移质输运等过程,在数值模拟过程中,根据非均匀沙质河床的颗粒分布曲线,对所筛取的各个级配范围内的颗粒采用其对应的中值粒径来表征。模拟得到了双柱排桥墩局部流场结构、河床的冲淤变化和上下游桥墩周围冲刷坑形态。研究表明:受桥墩阻水作用影响,墩前壅水、墩后跌水现象明显。墩周冲刷坑基本贯通整个墩周区域,受上游墩保护作用影响,下游墩冲刷坑的发育深度和规模小于上游墩。将数值模拟结果与试验结果进行了对比分析,二者吻合较好。研究成果可为深入开展非均匀沙质河床桥墩局部冲刷研究提供参考。     

环翼式桥墩局部冲刷防护试验

基于桥墩局部冲刷原理,在传统防冲刷保护措施的基础上设计了一种能改变桥墩迎水面流态的新型环翼式桥墩,在不同流速、有无环翼式挡板以及不同的挡板位置下,试验研究了环翼式桥墩的局部防冲刷效果。研究结果表明,环翼式挡板可有效减小桥墩的局部冲刷,当桥墩上的挡板与河床的距离约为水深的1/3时,与无挡板的桥墩相比近底垂向流速最大可减小96%,最大冲坑深度可减小57.6%,环翼式桥墩防冲刷效果明显。     

河床中值粒径对桥墩冲刷坑演化过程的影响

桥墩的局部冲刷导致河床形态变化和桥墩基础埋深减小是桥梁水毁的主要原因。在大涡模拟(Large Eddy Simulation,LES)的基础上结合水流运动方程和泥沙运动的动理学理论系统地对桥墩基础处的水流冲刷问题进行全时段全方位的三维数值模拟。得到了桥墩基础处的湍流流场流线图及河床形态变化的高程图。重点研究了水流流速和河床颗粒中值粒径对桥墩周边局部冲刷的影响。结果表明:冲刷坑的深度随着初始流速的增大而增加,且冲刷坑形成速度加快;冲刷坑的深度随着河床颗粒中值粒径的减小而增大,但是当颗粒的中值粒径小到一定程度时,由于泥沙颗粒之间的黏聚力增大导致冲刷坑的深度反而减小。     

长江下游河段桥墩压缩冲刷预测研究

建桥引起的河床变化可分为压缩冲刷和局部冲刷。相比于局部冲刷,压缩冲刷研究较少,压缩冲刷使桥址断面产生整体性下降,不利于桥墩基础安全。在总结前人研究成果基础上,以长江下游世业洲桥位方案为例,建立了桥墩压缩冲刷预测模型。探讨了不同空间尺度下桥墩边界的处理方法; 针对长江下游河段水沙特点,从工程安全角度出发,提出了水沙过程的选取方法; 最后预测了桥位上下游河床变形和桥墩压缩冲刷深度,并与长江下游已建桥址断面冲刷深度进行比对,两者基本相当。结果表明, 文中确定的桥墩压缩冲刷是合理的,可为桥墩基础埋深提供技术依据。     

潮流作用下桥墩局部冲刷研究

为明确潮流与恒定流条件下桥墩局部冲刷深度的关系,通过长时间序列潮流作用下桥墩的局部冲刷试验,观测不同特征参数的潮流条件下桥墩局部冲刷最深点的发展趋势,分析往复流造成的冲刷坑内泥沙反复冲淤对冲刷坑发展过程的影响,涨落潮最大流速和历时决定了冲坑的发展的速率和达到最大冲深的可能性,在潮流速度值较大或历时占优的情况下,将取得与恒定流一致的局部最大冲深。     

Impact of bridge pier on the stability of ice jam

River ice jam is one of the most important issues in rivers in cold regions during winter time. With the extra solid boundary due to the ice cover, the flow condition under ice-covered conditions is completely different from that of a open channel flow. The presence of bridge piers will further change the velocity field around the bridge piers. As a consequence, the formation and the accumulation of ice jams in the vicinity of the bridge pier will be affected. On the other side, the formation of an ice jam around the piers can cause extra turbulence to reduce the stability of a river bridge. The present study focuses on the stress analysis of the ice jam in the vicinity of a bridge pier. By developing a governing equation for describing the equilibrium state of an ice jam, the stability of the ice jam around bridge piers is analyzed and determined. As seen from the field data in literature, the stability estimations of an ice jam around bridge piers determined by the present method agree well with the field observations. Therefore, the proposed approach can be used for the prediction of the formation of ice jams around bridge piers.     

适用于明渠流及冰盖流的统一泥沙颗粒起动流速公式

为得到冰盖流条件下的非黏性泥沙起动公式,揭示冰期河道断面强烈冲淤变化的机理,基于Einstein假定,推导得到了同时适用于明渠流及冰盖流的统一的非黏性泥沙颗粒起动流速公式,所得公式与已有冰下泥沙起动流速试验数据符合良好。当冰盖糙率为0时,冰盖流泥沙颗粒起动流速公式即化为明渠流泥沙颗粒起动流速公式。应用所得公式,比较了明渠流、冰盖流、冰塞条件下的起动流速及可起动最大泥沙粒径关系,冰塞条件下起动流速最小,可起动最大泥沙粒径最大。2014年冰期黄河头道拐断面发生强烈冲淤变化,是冰花集聚形成冰塞引起可起动最大泥沙粒径增大所致。所得公式及所揭示机理,将为进一步深入研究河道冰期泥沙输移规律提供重要思路和参考。     

桥墩影响下弯槽冰塞形成临界条件和冰厚变化的试验研究

天然河道中,桥墩的存在改变了局部水流特性,对冰塞形成及演变产生影响。通过模型试验,研究了桥墩存在及桥墩位置不同时对冰塞演变过程的影响,试验结果表明:桥墩的存在改变了桥墩附近的水流结构,使得桥墩附近水流输冰能力相对增大,冰塞厚度减小;相比于将桥墩安置在两弯连接直道段,将桥墩安置于弯道顶点处时,桥墩附近水流输冰能力增大;在两弯连接的直道段和弯道顶点处同时放置桥墩时,弯道平衡冰塞厚度介于单墩放置于两弯连接的直道段与单墩放置于弯道顶点处时的弯道平衡冰塞厚度之间。     

Impacts of ice cover on local scour around semi-circular bridge abutment简

The presence of ice cover in winter can significantly change the flow field around bridge abutments, which can also cause a different local scour pattern. To investigate the impacts of ice cover, results from a recent flume experiments were presented. Smooth and rough ice covers were created to investigate the impacts of ice cover roughness on the scour geometry around the semi-circular abutment. Three bed materials were used, with 50D s of 0.58 mm, 0.50 mm, 0.47 mm respectively. Scour volume and scour area were calculated. It was found that the maximum scour depth was located 75o inclined to the flume wall. Under rough ice cover, the scour area and scour depth were the largest. An empirical equation on the maximum scour depth was also developed.     

Armour Layer Analysis of Local Scour around Bridge Abutments Under Ice Cover

A set of experiments was conducted for the local scour around bridge abutments under ice cover in non‐uniform sediments. By taking into account of the grain size of the armour layer and ice cover roughness, the dimensionless maximum scour depth is analysed. With an increase in grain size of the armour layer, the dimensionless maximum scour depth decreases. With the increase in ice cover roughness, the dimensionless maximum scour depth increases correspondingly. By using dimensional analysis, a regression relationship supports this conclusion. The conclusions drawn from this study provide crucial evidence for the protection of bridge foundations in winter time. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental investigation and flow analysis of clear-water scour around pier and abutment in proximity

Local scour around bridge piers and abutments is one of the most significant causes of bridge failure. Despite a plethora of studies on scour around individual bridge piers or abutments, few studies have focused on the joint impact of a pier and an abutment in proximity to one another on scour. This study conducted laboratory experiments and flow analyses to examine the interaction of piers and abutments and their effect on clear-water scour. The experiments were conducted in a rectangular laboratory flume. They included 18 main tests (with a combination of different types of piers and abutments) and five control tests (with individual piers or abutments). Three pier types (a rectangular pier with a rounded edge, a group of three cylindrical piers, and a single cylindrical pier) and two abutment types (a wing–wall abutment and a semi-circular abutment) were used. An acoustic Doppler velocimeter was used to measure the three-dimensional flow velocity for analyses of streamline, velocity magnitude, vertical velocity, and bed shear stress. The results showed that the velocity near the pier and abutment increased by up to 80%. The maximum scour depth around the abutment increased by up to 19%. In contrast, the maximum scour depth around the pier increased significantly by up to l71%. The presence of the pier in the vicinity of the abutment led to an increase in the scour hole volume by up to 87% relative to the case with a solitary abutment. Empirical equations were also derived to accurately estimate the maximum scour depth at the pier adjacent to the abutment.     

冰期河流弯道冰水动力学行为研究

以黄河什四份子弯道为研究对象,基于2019-2020年度的凌情监测影像及现场试验数据,分析了河流弯道冰水动力学行为特征。结果表明:上宽下窄的河道形态是造成弯道卡冰的主要原因,流凌-封河阶段,弯顶节点工程对水流的顶托作用促进了上游回流区的形成;受弯道离心力作用,河冰聚集于河道凹岸一侧,并在回流区堆积形成冰桥,从而缩小了断面过冰面积,河道逐渐封冻;弯顶下游流速大且来冰量少,形成清沟,主流向河中发展;冰塞堆积于弯顶上游凹岸主河槽内,水流被挤压至凸岸非冰塞区,弯道主流易位;在稳封期,河道冰水动力特征基本不再变化,在解冻开河期,凸岸非冰塞区流速较大,主流区冰盖优先解冻且沿主流输移,回流区冰盖最后消融,河道主流逐渐恢复至畅流阶段,整体呈复归式。