Scour and burial mechanics of conical frustums on a sandy bed under combined flow conditions

The scour and burial of conical frustums placed on a sandy bed under waves alone (WA) and combined flows (CF) conditions was investigated. The observations indicate that equilibrium burial depth is smaller than burial of other objects such as short cylinders laying on a sand bed under equivalent hydrodynamic conditions. Truncated cone offers less resistance to the flow field due to its more round shape when compared to a horizontally placed short cylinder characterized by sharp edges. A smaller disruption to the flow field translates to less turbulent intensity and to smaller sediment transport capacity of the flow around the object and less burial. The equilibrium burial depth shows a significantly weaker dependency on the Shields parameter than on the Keulegan-Carpenter number, contrary to the case of finite short cylinders. A new empirical predictor based on the relative strength of the wave to the wave plus current velocity, the Keulegan-Carpenter number, and the Shields parameter is proposed for estimating the equilibrium burial of truncated cones under combined flows. Both the Keulegan-Carpenter number and the Shields parameter determine the width of the scour hole around the cone. The former however, is the most dominant parameter influencing the length of the scour hole.     

3D scour below pipelines under waves and combined waves and currents

This paper presents the results of an experimental investigation on three-dimensional local scour below a rigid pipeline subjected to wave only and combined wave and current conditions. The tests were conducted in a conventional wave flume. The major emphasis of the investigation was on the scour propagation speed (free span expansion rate) along the pipeline after local scour was initiated at a controlled location. The effects of flow ratio (steady current velocity vs. combined waves/current velocity), flow incidence angle and pipeline initial embedment depth on free span expansion rate were investigated. It was observed that the scour along the pipeline propagated at a constant rate under wave only conditions. The scour propagation rate decreased with increasing embedment depth, however, increased with the increasing Keuglegan–Carpenter (KC) number. Under combined wave and current conditions, the effect of velocity ratio on scour propagation velocity along the pipeline was quantified. Empirical relationships between the scour propagation rate (V_h) and key parameters such as the KC number and embedment depth (e/D) were established based on the testing results.     

植物对孤立波作用下直立堤局部冲刷影响实验研究

基于水槽实验,研究植物对孤立波作用下直立堤局部冲刷的影响。通过改变实验入射波高、植物带的宽度和密度,分析各要素对直立堤局部冲刷的影响。实验对波高沿程变化、孤立波越堤、回落过程以及地形演变进行全程的测量和记录,并进行有无植物带保护的直立堤局部冲刷对比分析。结果表明,植物带的存在,使得堤后相对最大冲刷深度显著减小,堤前冲刷形态由单峰式"L"型变为双峰式淤积沙坝形态,且冲刷位置由原堤脚前移至植物带所在位置,冲刷范围大幅增加。减小植物带密度或者植物带宽度会使近岸侧或是离岸侧的相对最大冲刷深度和相对最大淤积高度有增大的趋势,但是当入射波高增大时,二者的影响不显著。该研究对实际工程中植物带的合理布置提出了具有参考性的建议,对减少堤防工程的冲刷和损失有一定现实意义。     

Random wave-induced scour at the trunk section of a breakwater

This paper provides a stochastic method by which the random wave-induced scour depth at the trunk section of vertical-wall and rubble-mound breakwaters can be derived. Here the formulas for regular wave-induced scour depth provided by Xie [Xie, S.L., 1981. Scouring patterns in front of vertical breakwaters and their influence on the stability of the foundations of the breakwaters. Report. Department of Civil Engineering, Delft University of Technology, Delft, The Netherlands, September, 61 pp.] for vertical-wall breakwater and Sumer and Fredsøe [Sumer, B.M., Fredsøe, J., 2000. Experimental study of 2D scour and its protection at a rubble-mound breakwater. Coast. Eng. 40, 59–87] for rubble-mound breakwater are used. These formulas are combined with describing the waves as a stationary Gaussian narrow-band random process to derive the random wave-induced scour depth. Comparisons are made between the present method and the Sumer and Fredsøe [Sumer, B.M., Fredsøe, J., 2000. Experimental study of 2D scour and its protection at a rubble-mound breakwater. Coast. Eng. 40, 59–87.] random wave scour data for rubble-mound breakwater, as well as the Hughes and Fowler [Hughes, S.A., Fowler, J.A., 1991. Wave-induced scour predictions at vertical walls. ASCE Proc. Conf. Coastal Sediments vol. 91, 1886–1899] random wave scour data and formula for vertical-wall breakwater. A tentative approach to random wave-induced scour at a vertical impermeable submerged breakwater is also suggested.     

推进波作用下海底管线周围局部冲刷试验研究

考虑行波作用下的海底管线的局部冲刷问题。采用波浪水槽模型试验的方法,研究波浪荷载引起的管线周围局部冲刷机理和冲刷形态,探讨行波作用下管线周围局部冲刷的演化规律,包括冲刷起动、水土界面沙波的形成以及平衡冲刷深度与KC数(keulegan-carpenter number)和相对埋置深度的关系。     

直立式防波堤口门区海啸局部冲刷规律实验研究

海啸作为世界上最严重的自然灾害之一,其巨大的破坏力使得近海建筑物遭受了严重的损害。基于波浪水槽实验,开展海啸波作用下不同口门距离的防波堤局部冲刷机理实验研究。实验结果表明,在海啸波越顶水流产生的水跌以及海啸波通过防波堤口门时产生的扰流共同作用下,口门中心线处产生明显的局部冲刷坑,随着口门距离的增加,防波堤局部冲刷先加剧后减弱。建立防波堤局部最大冲刷深度与海啸波波高、防波堤宽度及出水高度、口门间距的关系式,揭示了最大局部冲刷深度与海啸波波高、防波堤尺寸、口门距离的内在关系。     

A wave-flume study of scour at a pile breakwater: Solitary waves

Understanding the sediment transport and the resulting scour around coastal structures such as pile breakwaters under local extreme wave conditions is important for the foundation safety of various coastal structures. This study reports a wave-flume experiment investigating the scour induced by solitary waves at a pile breakwater, which consists of a row of closely spaced large piles. A wave blacking gate with a simple operation procedure in the experiment was designed to eliminate possible multiple reflections of the solitary wave inside the flume. An underwater laser scanner and a point probe were used in combination to provide high-resolution data of the bed profile around the pile breakwater. Effects of incident wave height and local water depth on the maximum scour depth, the maximum deposition height and the total scour and deposition volumes were examined. An existing empirical formula describing the evolution of the scour at a single pile in current or waves was extended to describe the scour at the pile breakwater under the action of multiple solitary waves, and new empirical coefficients were obtained by fitting the formula to the new experimental data to estimate the equilibrium scour depth. It appears that the maximum scour depth and the total scour volume are two reliable quantities for validation of numerical models developed for the scour around pile breakwaters under highly nonlinear wave conditions.     

Characteristics of Parameters for Local Scour Depth around Submarine Pipelines in Waves

The purpose of this study is to investigate the characteristics of parameters for local scour depth around submarine pipes due to waves. Because it is impossible to consider all the factors that influence the scour, it is reasonable to investigate the correlations between parameters and scour. The experimental works about scour were performed for the conditions of 40, 60, 90 m pipe diameter, various wave periods, and wave height in the case of horizontal bed and 1/10 slope bed, respectively. Analyzing the results of experiments, the Reynolds number, Shields parameter, Keulegan-Carpenter number, Ursell number, and Modified Ursell number were estimated. The correlations between the relative scour depth, which is the maximum equilibrium local scour depth divided by the pipe diameter, and 5 parameters were analyzed. It was shown that there was hardly any correlation with the Reynolds number. In the case of a horizontal bed, the Keulegan-Carpent number had the highest correlation, but in the case of the slope bed the correlation was greatly reduced. The modified Ursell number showed a high correlation regardless of the type of bed.     

Prediction of wave-induced scour depth under submarine pipelines using machine learning approach

The scour around submarine pipelines may influence their stability; therefore scour prediction is a very important issue in submarine pipeline design. Several investigations have been conducted to develop a relationship between wave-induced scour depth under pipelines and the governing parameters. However, existing formulas do not always yield accurate results due to the complexity of the scour phenomenon. Recently, machine learning approaches such as Artificial Neural Networks (ANNs) have been used to increase the accuracy of the scour depth prediction. Nevertheless, they are not as transparent and easy to use as conventional formulas. In this study, the wave-induced scour was studied in both clear water and live bed conditions using the M5’ model tree as a novel soft computing method. The M5’ model is more transparent and can provide understandable formulas. To develop the models, several dimensionless parameter, such as gap to diameter ratio, Keulegan-Carpenter number and Shields number were used. The results show that the M5’ models increase the accuracy of the scour prediction and that the Shields number is very important in the clear water condition. Overall, the results illustrate that the developed formulas could serve as a valuable tool for the prediction of wave-induced scour depth under both live bed and clear water conditions.     

Geometry of scour hole around, and the influence of the angle of attack on the burial of finite cylinders under combined flows

Experiments were conducted to investigate the geometry of the scour hole and flow structure around short cylinders under the action of waves alone (WA) and combined flows (CF). The study is aimed at better understanding the dynamics of isolated objects on a sandy floor under oscillatory flows as occurs in shallow water regions in coastal areas. Flow velocities within the fluid core were recorded and 3D mapping of the bottom was performed with sub-aquatic acoustic sensors. Experiments were conducted for cylinder Reynolds wave number and Keulegan-Carpenter number within the ranges 10⁴R_e1.7×10⁵ and 2KC71, respectively. The present experimental evidence shows that the geometric characteristics of the scour hole (length and width) depend primarily on the Keulegan-Carpenter number (KC) and the cylinder aspect ratio (a_r=L_c/D). The effect of variation in the angle of attack of the flow with respect to the cylinder main axis was also investigated. Initial orientations of zero and ninety degrees were found to be stable while cylinders with intermediate initial orientations tended to orientate their main axes perpendicular to the flow direction. The final angle of orientation was found to be primarily a function of the Shields parameter, θ, and the initial angle of attack, α_i.     

潮流作用下海上风电群桩基础局部冲刷与防护试验研究

海上风电基础局部冲刷会影响风电结构的稳定,对海上风电的安全运行至关重要。以四腿群桩导管架基础为研究对象,对潮流条件下基础局部冲刷开展试验研究。试验结果表明：0°水流条件下,最大冲刷深度为1.05倍桩径;45°水流条件下,最大冲刷深度为0.97倍桩径。并探索了新型蜂巢结构在群桩基础防冲刷中的应用,设计了蜂巢格室防护方案,发现在蜂巢防护情况下,两个流向作用于四腿群桩基础时最大冲刷深度仅为无防护情况下的27.3%和25.9%,表明蜂巢格室在海上风电基础的冲刷防护方面具有良好效果,在实际工程中具有较大的应用潜力。     

港珠澳大桥青州航道桥桥墩基础冲刷试验研究

桥墩基础冲刷是桥梁毁坏的重要因素,是桥梁基础设计的关键指标之一。目前国内外对于桥墩基础在复杂动力条件下冲刷深度的研究常采用物理模型试验方法,利用正态系列模型方法,在波流水槽中研究了水流、潮流和波流共同作用下青州航道桥索塔基础周围流态变化和局部冲刷特征。研究结果表明,桥墩最大冲刷深度和冲淤范围与水流流速、桥墩轴线与水流夹角和波浪等因素有关;在潮流最大流速和恒定流流速一致情况下,桥墩局部冲刷深度达到平衡后,将会达到与恒定流基本一致的最大冲深;波流共同作用下的最大冲刷深度比恒定流增加10%左右。设计桥墩形状在100年一遇水流和波浪共同作用下桥墩基础局部最大冲刷深度为13.7 m。     

长江口深水航道整治一期工程南导提丁坝群头局部冲刷试验研究

采用双向稳定水流、清水定床、局部动床的试验方法，用正态模型研究长江口深水航道整治一期工程南导堤丁坝群坝头局部冲刷问题。结果表明，航道开挖和双本坝的存在使得下游流速有所增加，影响程度落潮大于涨潮。坝头局部冲刷主要受落潮流控制，在沿深度为单一砂粉底质情况下，坝头冲刷坑深度分别达到27m和29m；考虑到原状底质下部为极难起动的粘土，实际冲刷坑底界到达粘土层后就不再刷深，此时冲刷坑深度为7．4m和4．7m，冲刷坑的平面范围有所加大。工程设计提出的护坦尺度能较好地保护坝头前沿滩地，适当缩小护坦尺度也能起到保护作用，但冲刷范围明显增大。     

Clear water scour around a finite array of cylinders

This experimental study presents clear-water scour and deposition patterns around hexagonal arrays of circular cylinders in steady flow conditions. Understanding the scour processes around such configurations could facilitate the design of several hydraulic and marine engineering structures, such as bridge piers and piles. The flow alteration caused by the examined porous obstacles depends on the solid volume fraction of the obstacles and on the angle of attack of the incoming flow, due to the limited number of cylinders constituting the array. Flume experiments with erodible bed were carried out for four array densities (solid volume fractions: 0.14, 0.20, 0.32 and 0.56) under three different orientations (regular, angled and staggered configurations). The scour/deposition characteristics were obtained by means of laser scanner and the results were compared to solid cylinders of equal circumambient diameter. Different angles of attack of the incoming flow lead to different blockage ratios, which have direct impact on the scour characteristics and deposition patterns. The arrays with the higher solid volume fraction generated scour/deposition patterns similar to solid cylinder, while in the arrays with the lower solid volume fractions, local scour around the individual small cylinders became evident. Finally, considering that the load bearing capacity of a pier basically depends on the area of its cross-section, a comparison of the maximum induced scour depth and volume by the cylinder arrays and the solid cylinder with equal solid cross-sectional area is presented, in order to introduce an alternative pier configuration that induces less scour. The results showed that the array of cylinders could generate 27% less scour volume and 22% less scour depth compared to its single solid cylinder counterpart.     

Hydrodynamic coefficients for inclined cylinders

The wave induced dynamic pressures around a circular cylinder of diameter 0.2 m due to regular waves were measured in a wave flume in a water depth of 1 m and in a wave basin in a water depth of 3 m. The experimental investigations were carried out with the cylinder inclined along and against the direction of wave propagation. The least-squares technique was employed to evaluate the coefficients of drag (C_D) and inertia (C_M) from the sectional force time histories obtained by integrating the measured circumferential pressure distribution. The variation of drag and inertia coefficients are presented as a function of Keulegan-Carpenter number (KC) for different inclinations of the cylinder. The comparison between the measured and the theoretical force derived from the evaluated hydrodynamic coefficients is found to be good.     

潜体绕流及远场声特性分析

首先采用SST（shear-stress transport）k-ω两方程湍流模型求解了潜体三维非定常粘性流场,然后基于Lighthill声类比理论,对潜体流噪声的远场特性进行了数值分析,并探讨了攻角的影响。研究结果显示,基于所研究的潜体外形,潜体在舷侧及深度等正横方向上的声辐射显著高于潜体在艏、艉等轴向上的声辐射,攻角的存在使声辐射增大。     

长江口南沙头通道限流潜堤局部冲刷原因及对策分析

长江口南北港分汊口河段新浏河沙护滩潜堤及南沙头通道潜堤工程是典型的航道整治工程,经调查发现限流潜堤冲刷坑在两侧余排外对称分布,12 m以深等深线呈不连续坑状,南侧个别区段冲刷坑已侵入排内。基于工程地形监测和地质资料分析,通过现场水深测量、旁扫声纳扫测、潜水探摸、水文观测和三维潮流数学模型计算等手段,对限流潜堤排外局部冲刷情况和发生原因进行研究。研究结果表明潜堤排外冲刷是越顶水流长期作用所致,冲刷发展变化具有代表性,冲刷最大深度与地质条件有一定相关性。潜堤南侧护底余排边缘局部余排受外力损毁,引起排内冲刷,如进一步发展会影响堤身及结构安全,需对护底损坏部位进行修复以保证整治建筑物护底功能正常发挥。     

新型旋转减冲装置对桩基局部冲刷防护试验研究

为减轻海上风电单桩基础周围局部冲刷对其结构安全的影响,提出了一种新型旋转减冲装置。在波流水槽中开展物理模型试验,改变波流条件、装置安装高度、安装距离,记录桩周冲刷发展历时,运用激光地形仪扫描冲刷坑形态,分析各工况下冲刷坑形态差异,验证装置不同安装距离、安装高度下的冲刷防护效果,提出了不同安装位置下的防护效率公式。结果表明：新型旋转减冲装置具有较好的冲刷防护效果,本试验工况下,桩周最大冲刷深度可减小44%左右。装置安装距离对冲刷防护效果影响较小,波流作用下的冲刷防护效果受装置安装高度影响显著,冲刷防护效果随装置安装高度的增加而减弱。     

Tentative engineering approach to scour around breakwaters in random waves

An approach by which the scour depth and protection layer width around the head of vertical-wall breakwaters, the scour and deposition depths as well as the protection layer widths at the round head of rubble-mound breakwaters in random waves can be derived is presented. Here the formulas for scour depth by Sumer and Fredsøe (1997) for vertical-wall breakwaters for regular waves and Fredsøe and Sumer (1997) for rubble-mound breakwaters for irregular waves are used. They are combined with describing the waves as a stationary Gaussian narrow-band random process to derive the scour and deposition depths as well as protection layer widths in random waves. Comparisons are made between the present approach and the Fredsøe and Sumer (1997) random wave scour data for rubble-mound breakwaters.     

海流作用下海底管道局部冲刷数值分析

在任意拉格朗日-欧拉参考坐标系下,采用基于雷诺平均的Navier-Stokes方程组(RANS)、流线迎风有限元方法、泥沙输运模型以及底床变形方程,对海流引起的海底管道局部冲刷进行了数值模拟。着重讨论了均匀来流流速和海底管道直径对局部冲刷发展过程及平衡冲刷深度的影响作用。数值结果表明,在冲刷的初始阶段,冲刷深度随时间迅速增加,之后缓慢逼近极限平衡深度;在管径一定的情况下,管道附近的局部平衡冲刷深度与流速大致呈线性关系;当流速超过某一临界区域后,最大平衡冲刷深度出现的位置并不在管道正下方,而是随流速的增加向管道下游方向移动;另外,管道直径也会对平衡冲刷深度产生比较明显的影响,在相同流速下,平衡冲刷深度大致随管径呈线性增大。在本文的计算范围内,海底管道的相对局部平衡冲刷深度基本随雷诺数线性增加,但流速对冲刷深度的影响作用要比管径的影响作用更为明显。