Scour around submarine pipes due to high-amplitude transient waves

Estimation of scour dimensions below submarine pipelines is a vital step in designing offshore infrastructure. Extreme events like tsunami waves produce strong erosive forces below the underwater pipes, apt to create scour holes, jeopardizing the safety of the structure. Despite the importance of this issue, previous studies have mainly focused on steady flow cases, and the scour pattern below submarine pipes induced by high-amplitude transient waves has rarely been investigated. This paper reports the results of 40 experimental runs on transient wave-induced scour below a model pipe in a laboratory flume under a variety of initial conditions. The variables included the bed particle size and gradation,initial water depth, wave height, and slope of the bed layer. Waves were generated by a sudden release of water from a sluice gate, installed in the middle of the flume. A pressure transducer data acquisition system was used to record the wave heights at different time steps. The results indicate that, with a shallower initial depth of flow, the scour depth is relatively large. It was also found that there exists a direct correlation between the induced wave height and the size of the scour hole. It was observed that, in clear water conditions, the size of the scour hole in coarse sediments is smaller, while in live-bed conditions, larger scour holes are created in coarser sediments. It was also observed that at high wave amplitudes, the live-bed conditions are dominant, and consequently the bed elevation is altered.     

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。     

裸露穿河管道在压重块保护下的冲刷特性

水流冲刷引起河床变形的不确定性对水下穿越管道的安全运行造成威胁.为了探究裸露穿河管道在压重块保护措施下的冲刷特性,通过水槽模型试验,研究压重块长度、压重块布置间距、水流流速、河道水深等因素对管道周围河床地形和冲刷深度的影响.试验结果表明:水下穿越管道在压重块保护措施下的冲刷过程可分为冲刷坑形成、压重块迎流侧悬空、管道迎...     

Flow structure and scour around circular compound bridge piers – A review

Large amount of literature is presently available on the topic of scour around uniform piers and its control. However, relatively fewer studies have been carried out so far on the topic of flow field and scour around compound piers. The state of art on the topic of flow structure and scour around circular compound piers is presented herein. The available literature reveals that variations in foundation and pier geometry significantly affect the maximum depth of scour and its temporal variation. In case of compound piers the scour depth is highly sensitive to the change in elevation of the top surface of the foundation well. As compared to uniform piers, a reduction is observed in maximum depth of scour around compound pier when the top surface of the well is kept below the general level of river bed. These observations however are yet to be quantified. Results from an experimental study on flow field around the compound piers are also presented.     

挟沙水流床面冲刷深度试验研究

在清水冲刷粗化层极限冲刷深度研究的基础上,结合室内概化水槽试验,探讨了挟沙水流床面冲刷问题。不同来沙条件下床面冲刷深度水槽试验观测成果表明,随着上游单宽加沙率的增加,挟沙水流床面冲刷深度减小,挟沙水流相对冲刷深度与相对来沙条件呈线性变化趋势。参考爱因斯坦推移质输沙率公式,建立了一个能够综合反映来水来沙条件以及床面条件的床面冲刷深度计算公式。该公式计算值与试验实测值吻合良好。     

桥墩局部冲刷动态模拟及不同截面的冲刷特性

为了获得桥墩局部冲刷随时间的演变过程,探讨不同截面桥墩的冲刷特性,采用计算流体动力学软件FLUENT的自定义函数功能和动网格更新技术,考虑湍流涡增强效应的床面剪应力和坡度影响的临界剪应力,得到床面的输沙率,通过输沙率与床面高程的变化关系实现床面地形随时间的动态变化,并利用泥沙坍塌的调整来克服床面坡度超过泥沙休止角造成的模拟失真及数值不稳定。结果表明:局部冲刷最大深度、冲坑形态及流场结构的模拟结果与试验结果较为吻合;在最大墩宽一致的情况下,流线形桥墩的冲坑深度比圆柱形桥墩降低约45%,比尖角形桥墩降低约40%,冲坑范围也有所下降。     

Critical velocities for local scour around twin piers in tandem

Experiments of the local scour around twin piers are carried out under steady clear-water conditions, including 95 tests to observe the influence of the pier spacing and the flow velocity on the local scour characteristics of the twin piers. It is shown that the start of the transition region is synchronous with the sediment transport from the upstream scour hole to the downstream one. The equations for the critical velocities are derived to quantify the velocity range of each of four different scour regions. Finally, a prediction formula of the downstream pier scour depth in the radical-deviation region is established.     

往复流作用下海底管线冲刷模拟研究

海底管线冲刷悬空是诱发管道涡激振动破坏的一个主要原因。为更好地对管线冲刷悬空情况进行评估,本文应用水动力-泥沙冲刷耦合模型,对往复潮流作用淤泥质海床海底管线局部冲刷过程进行了模拟研究,模型采用标准k-ε紊流方程对N-S方程进行封闭,控制方程采用有限差分法进行离散,模拟结果表明,双向潮流作用下,管道局部冲刷轮廓基本呈现对称状态,其最大冲刷深度位置因水流流速的转向而有所摆动,往复流引起的冲刷深度较恒定流情况减小约20%左右。     

枢纽下游河床冲刷深度和水位降落估算方法

通过清水冲刷水槽试验,在探索非恒定输沙及床沙调整等规律的基础上,结合沙量平衡原理,建立了一种枢纽下游河床冲刷深度和水位降落的计算模式。洪峰过程可以概化为一种恒定的"当量流量",深槽冲刷以枯水边界作为脱离体,可将该模式延伸于天然河道。能够计算冲刷过程及终期河床冲刷纵剖面和不同流量下的水位降落。     

Effect of trapezoidal collars as a scour countermeasure around wing-wall abutments

Local scour around bridge abutments is a widespread problem that can result in structural failure. Collars can be used as a countermeasure to reduce the scour depth. In this study, the temporal scour development around a wing-wall abutment was investigated with and without collars. The tests were carried out under clear-water conditions for different abutment lengths, with collars of different sizes placed at the bed level. When no collar was used in the experiments, 70% of the maximum scour depth occurred in less than 2 h. However, when a collar with a width greater than the length of the abutment was used, no scour was observed for up to 200 min from the beginning of the experiments. The results show that an increase in the collar width not only led to a lag time for the onset of scouring but also reduced the maximum scour depth. Moreover, an increased collar width led to a better performance in mitigating scouring around smaller abutments. Generally, the scour depth decreased by 9%–37% with different collar widths.     

不同透水率对水力插板透水丁坝防冲促淤效果的试验研究

水力插板透水丁坝新型护岸结构的提出提高了新疆粉细沙河床上治河工程的防洪护岸效益。本文为了探讨透水率对水力插板透水丁坝防冲促淤效果的影响,利用动床模型水槽试验研究了水力插板透水丁坝缓流效果、局部冲刷及坝后最大淤积高度随透水率的变化规律,为透水率这个重要工程设计参数的合理选取提供理论依据。试验结果表明:水力插板透水丁坝坝后流速的减缓效果明显,且透水率对坝后流速的影响较小;坝头最大冲刷深度随着透水率的增大而变小;坝后最大淤积高度随透水率的增加先增加后减小;当透水率为30%左右时,水力插板透水丁坝的防冲促淤效果达到最佳。     

Numerical modeling of flow and scouring around a cofferdam

When planning river hydraulic structures, the analysis of the hydrodynamic and bed elevation change is of great importance, particularly the assessment of the scour depth around a cofferdam. In this study, the flow field variation and the corresponding scour depth was simulated using two-dimensional hydrodynamic analysis and a bed-load transport model. The hydrodynamic model used the streamlined upwind Petrov–Galerkin (SUPG) finite element scheme to solve the Reynolds-averaged turbulent flow equations. Based on the results of flow field analysis, the bed-load transport model was able to simulate the scour hole development, where it was shown that velocity gradients dramatically increase due to the existence of hydraulic structures. The applicability of the model was tested by simulating the velocity field and bed elevation changes around the cofferdam constructed during the Lock and Dam No. 26 replacement project in the Mississippi River. The model created in this study is able to estimate the maximum scour depth, determine the configuration of the cofferdam to suggest changes to reduce any local scour, and suggest if protection materials around the cofferdam are needed.     

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.     

桥墩振动对其局部冲刷的影响

为了探究桥墩振动对其局部冲刷的影响,以圆柱型桥墩为例,开展了不同泥沙底床条件下的振动桥墩局部冲刷水槽试验。结果表明,在振动载荷下,对于中值粒径分别为14.42μm、31.75μm和85.92μm的细颗粒底床,当振动强度从0增大至3.72,最大冲刷深度和最大冲刷半径均随之增大;中值粒径为14.42μm的泥沙底床最大冲刷深度增幅最大达910%,且粒径越大,增幅越小。对于中值粒径为260μm的粗颗粒底床,当振动强度从0增至2.31,最大冲刷深度降低了37.50%,而最大冲刷半径增加了38.37%。因此,桥墩的振动对其局部冲刷有着重要影响,且对细颗粒底床和粗颗粒底床的影响不同。桥墩振动导致有黏性的细颗粒泥沙发生流变从而加剧了其局部冲刷;而无黏性的粗颗粒泥沙受到振动作用而加密,其局部冲刷削弱。提出了考虑桥墩振动的局部冲刷深度计算公式,其理论计算值与实测值相对误差在±20%的数据达87.5%,能够为涉水桥梁基础埋深的设计提供更合理的计算依据。     

除险加固水闸的消能防冲措施

基于某除险加固工程水力学模型试验,通过对闸下水流流态、流速分布、特征断面垂向最大平均流速以及冲刷特性的观测,分析了设计方案条件闸下水流流态恶化及冲刷严重的原因,即下游翼墙加固改变了水流的边界条件,导致出闸水流受到边界挤压而主流集中,局部流速增大,冲刷加剧。通过不同消能防冲方案的试验比较,提出了利用分隔墩消除出闸水流集中的现象;分隔墩消除了主流受边界挤压现象,起到了改善闸下水流流态,均化闸下水流流速分布,降低河床最大流速,减轻闸下冲刷的作用,且体型结构简单,易于施工。     

Laboratory Investigation of Stilling Basin Slope Effect on Bed Scour at Downstream of Stepped Spillway: Physical Modeling of Javeh RCC Dam

Stepped spillway and stilling basin are one of the most important energy dissipation structures. Eventhough, most of energy dissipated by these structures, but in skimming flow, the upstream flow motion is nonaerated and the residual energy capable to destroyed structures during floods. In this study, effect of stilling basin slope on bed scour, downstream of Javeh dam was investigating. Experiments performed in hydraulic structures laboratory of the University of Kerman with six different discharges (5, 7, 13, 17, 25 and 30 l/s.m) and five various stilling basin slope (0.02, 0.01, 0, ?0.01 and???0.02). The parameters such as maximum scour depth (d_s), flow velocity (in three point), water depth on upstream and downstream of stepped spillway and stilling basin, the distance of the maximum scour depth to sill (L_s) and the gheometery of scour hole measured. Result shown that when stilling basin slopes was 0.02, the average of maximum relative scour depth, 47% Increased and in ?0.02, 52.2% Decreased. In addition, the distance of maximum scour depth until stilling basin increased by increasing and decreased by decreasing the stilling basin slope.

     

非恒定流清水冲刷输沙规律初步研究

通过波动水流及定常流量的清水冲刷水槽试验,对非恒定流输沙特性进行了研究。分析了洪峰过程中,涨、落水流速、比降、输沙率的变化特性及影响输沙率的主要因素;揭示了加速流和减速流的输沙滞后效应;提出了洪峰输沙量的估算方法以及与洪峰流量输沙等效的当量流量。对人造洪峰设计,河床变形分析计算以及河工模型设计等有一定参考意义。     

泄水建筑物下游冲刷坑二维数值模拟

对巨亭水电站泄水建筑物下游局部冲刷过程进行了二维数值模拟。采用RNG k-ε湍流模型封闭N-S方程、有限体积法在计算网格上离散求解、VOF方法追踪自由水面;采用C语言编写用户自定义函数,以泥沙起动切应力作为床面泥沙起动判别标准,通过分析比较水流剪切力与泥沙起动切应力大小,用以控制冲刷坑底部边界变化;采用局部重构模型和弹簧光顺模型更新计算区域和网格质量,藉以模拟和追踪局部冲刷坑边界变化。研究表明,模拟结果与实测资料符合良好。在冲刷坑的冲刷过程中,床面剪切应力随着冲刷的进程急剧减小,湍动能随着冲刷的进程减小缓慢;床面剪切应力和湍动能沿程变化均较为剧烈,床面剪切应力沿程先增大后减小,床面湍动能则沿程减小。冲刷坑的纵向平衡形态或床面泥沙起动主要决定于床面剪切应力,床面湍动能对泥沙起动也具有重要影响。