An analytical solution for wave-induced seabed response in a multi-layered poro-elastic seabed

In this study, a new analytical solution for the wave-induced seabed response in a multi-layered poro-elastic seabed is developed. The seabed is treated as a multi-layered porous medium and characterized by Biot’s theory. The displacements of the solid skeleton and the pore pressure are expressed in terms of two scalar potentials and one vector. Then, the Biot’s dynamic equation can be solved using Fourier transformation and reducing to Helmholtz equations. To obtain the general solutions for the multi-layered poro-elastic seabed in the frequency-wave-number domain, the transmission and reflection matrices (TRM) method is used to form the equivalent stiffness. Using the boundary conditions and continuous conditions, the frequency-wave-number domain solutions are obtained. Finally, the time-space domain solutions for the multi-layered poro-elastic seabed are obtained by means of the inverse Fourier transformation with respect to the horizontal coordinate. Based on the new solution, a parametric study is carried out to examine the effects of soil characteristics (number of layers, permeability and shear modulus) and wave characteristics (water depth and wave steepness) on seabed responses. The results indicate that the seabed response is affected significantly by permeability, shear modulus and relative water depth.     

EPSONDE: an instrument to measure turbulence in the deep ocean

EPSONDE is a tethered free-fall profiling system used to obtain temperature microstructure and velocity turbulence data to a depth of at least 1500 m. EPSONDE, which carries a variety of slow and fast sensors, is deployed on a loose kevlar multiconductor cable by a specialized wire-handling system. Data are transmitted from this underwater unit (1792 samples per second) to a shipboard system which includes a dedicated microcomputer for data logging and online data processing. The performance of this system is demonstrated by discussing a study of turbulent mixing processes in a lens of Mediterranean water (a MEDDY) found at a depth of 1000 m in the Canary basin. These studies indicate that turbulent kinetic energy dissipation may be an important mechanism in determining the decay and lifetime of a MEDDY     

Wave-induced seabed response in a cross-anisotropic seabed in front of a breakwater: An analytical solution

To simplify the complicated mathematical process, most previous investigations for the water waves-seabed interaction problem have assumed a porous seabed with isotropic soil behavior, even though strong evidence of anisotropic soil behavior has been reported in soil-mechanics literature. This paper proposes an analytical solution of the short-crested wave-induced soil response in a cross-anisotropic seabed. As shown in the numerical results presented, the wave-induced seabed response, including pore pressure, effective stresses and soil displacements, is affected significantly by the cross-anisotropic elastic constants. A parametric study is performed to clarify the relative differences in pore pressure between isotropic and cross-anisotropic solutions.     

基于原位观测的胶州湾沉积物侵蚀再悬浮过程研究

海洋动力作用下,河口海岸地区海床通常处于动态变化之中。作为地质环境的控制因素,海床沉积物侵蚀再悬浮过程的研究具有重要意义。为阐明胶州湾海域水动力条件对海床侵蚀再悬浮的作用,本文利用海底原位观测三脚架进行了现场观测。观测结果显示：通常条件下,潮流导致的最大海床剪应力可达0.35 N/m²,高于波浪引起的剪应力。涨潮期间,海床发生侵蚀;退潮期间,海床发生淤积。风速达到5 m/s时,波浪引起的剪应力近似等于流致剪应力。风速达到7 m/s时,有效波高为26 cm,波浪对海床侵蚀再悬浮过程起主要作用;此时也会导致海水浊度显著上升,高于通常条件下的2-8倍。分析表明：通常条件下,周期性海流影响海床侵蚀再悬浮过程;而大幅度沉积物再悬浮过程由偶发的波浪事件控制。针对胶州湾沉积物动力学机制的深入研究仍待进一步开展。     

An experimental study on non-linear progressive wave-induced dynamic stresses in seabed

In this study, the dynamic stresses within the seabed induced by non-linear progressive waves were explored through a series of hydraulic model tests on a movable bed within a wave flume. By comparing Stokes’ 2nd-order wave theory with the theory of wave-induced dynamic stresses within the seabed as proposed by Yamamoto et al. [1978. On the response of a poro-elastic bed to water waves. Journal of Fluid Mechanics 87 (1), 193–206.] and Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807], the experimental results show that the pressure on the seabed surface, the pore water pressure within the seabed as well as the vertical and the horizontal stresses are all smaller than their theoretical values. If we were to obtain the characteristics of seabed soil, the analytical solution of Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807] might agree to the simulation of the wave-induced effective stresses and shear stress in the sandy seabed. A different phase shift exists among all the three soil stresses. Their influences on the three dynamic stresses within seabed soil are important for seabed stability, and can be used in the verification of numerical models. In the whole, the non-linear progressive waves and the naturally deposited seabed are found to have a strong interaction, and the behavior of the induced dynamic stresses within the seabed is very complicated, and should be investigated integrally.     

An improved methodology to measure flag performance for the shipping industry

The subject of measuring the performance of registries has been a topic of policy discussions in recent years at the regional level due to the recasting of the European Union (EU) port state control (PSC) directive which introduces incentives for flags which perform better. Since the current method used in the EU region entails some shortcomings, it has therefore been the subject of substantial scrutiny. Furthermore, the International Maritime Organization (IMO) developed a set of performance indicators which however lacks the ability to measure compliance as set out in one of its strategic directions towards fostering global compliance. This article develops a methodology to measure flag state performance which can be applied on the regional or global level and to other areas of legislative interest (e.g. recognized organizations, Document of Compliance Companies). The proposed methodology overcomes some of the shortcomings of the present method and presents a more refined, less biased approach of measuring performance. To demonstrate its usefulness, it is applied to a sample of 207,821 observations for a 3-year time frame and compared to the current method.     

An instrumentation system to measure near-bottom conditions on the continental shelf

In recent years significant interest has emerged regarding bottom currents and sediment dispersal over continental shelves and shallow marine waters. Although many papers have been written on sediment dispersal mechanisms, they include relatively few long-term observations of bottom currents and/or sediment transport. Lack of observational data is related to the hostile nature of the environment, and the difficulty associated with placing and retrieving instruments on the floor of the continental shelf during some seasons and environmental conditions.This paper describes an instrumentation system designed for use on the floor of the continental shelf. It can remain submerged for periods of one month continuously recording water speed and direction 1 m from the sea bed, differential pressure, and bed nature by means of half-hourly photographs. Four of these systems are presently in use in arrays across the continental shelf of Washington.     

Anomalous distribution of suspended matter and some chemical compositions in seawater near the seabed: Transport processes

Vertical fine distributions of suspended matter and some chemical compositions in seawater were measured in the layer near the seabed. Distributions of organic suspended matter were almost uniform throughout the layer, but others were anomalous. However, with respect to their peak heights and distribution patterns, the tendency of profiles was the same on the whole. We suggested that these anomalous distributions were primarily caused by the destruction of the large flocculent particle on the seabed and by upward diffusion of disintegrated particles. The similarity of profiles between chemical compositions of dissolved substances and suspended matter of non-dissolved substances, was explained by assuming that pore water in the large particle was simultaneously transported with disintegrated particles after the destruction. An equation for the vertical distribution of suspended matter near the seabed was derived, provided that the rate of destruction of large flocculent particle was in proportion to the current velocity on the seabed. The equation represented the existence of the anomalous distribution, which was continuously changing its pattern. Measurements of vertical profiles of suspended matter showed almost the same tendency with the theory. From the characteristics of the theoretical equation, it was expected that the eddy diffusivity near the seabed was 1050 cm²/sec.     

随机波作用下海床动态响应分析

真实的海洋波浪是随机的,而前人对海床的动态响应分析大都是选用线性波或者Stokes波理论,对海床的模拟大都采用Biot拟静力模型,忽略了流体速度及土体位移加速度的影响。联合使用Longuet-Higgins随机波模型(采用Jonswap谱)以及动力u-p形式的海床响应计算模型,使用COMSOL Multiphysics多场耦合软件的PDE模块输入方程进行有限元计算,得到随机波作用下整体海床动态响应结果。将随机波结果与一阶Stokes波和椭圆余弦波结果进行对比,并对渗透系数和饱和度进行参数分析,研究表明渗透系数和饱和度对于随机波作用下海床动态响应影响显著。     

On the wave damping due to a permeable seabed

Laboratory experiments were performed to study the wave damping induced by a porous bed. During the propagation of waves over a porous medium the wave characteristics change: a significant wave height attenuation of about 20–30% is observed and, in almost all cases, an increase in wavelength. The wave decay is found to depend on the wave characteristics like the wave height, the wavelength and the wave shape. We have also studied the influence of the geometric properties of the porous bed (i.e. thickness and length) on the wave dissipation. It is found that the attenuation of the wave height increases with the permeable bed thickness and that there is a maximum wave dissipation for a length of the porous seabed equal to 2.0–2.5 times the wavelength. A comparison is also made of our findings with available literature results. A parametric study of the wave damping has been performed by varying the values of the resistance coefficients derived by both literature and experiments. Literature analytical models have been applied by using the resistance coefficients that better describe our flow conditions. All models in use underpredict the observed wave attenuation for any sensible values of the resistance coefficients.     

深水锚在海床中复杂动力行为的分析模型

深水锚是深水系泊系统的核心组成,随着新概念和新技术在深水系泊领域的不断涌现和发展,深水锚在海床中的动力行为变得更为复杂,这给既有的分析方法带来极大的挑战。将深水锚的复杂动力行为视为拔出、旋转、嵌入三种典型行为的组合,分别给出明确定义并建立力学模型。提出最小力准则以确定锚的拔出、嵌入方向和旋转中心,该准则同时可作为拔出、旋转、嵌入行为是否发生的判定条件,从而建立起可以描述深水锚在海床中复杂动力行为的分析模型。为检验所提出的分析模型,将其应用于预测拖曳锚在海床中的嵌入运动轨迹,并与已有分析方法和离心机试验结果进行对比,验证了该分析模型的有效性。     

砂质海床上海底管线的斜向波浪力试验研究

近岸海底管线在波浪作用下的受力情况关系着海底管线的安全运营。本文利用波浪港池物理模型试验,研究了斜向规则波作用下海底管线断面和管线沿程波浪力特性,分析了波浪入射方向与管线轴线夹角α及波浪要素对海底管线所受波浪力的影响,探讨了管线冲刷与所受波浪力之间的关系。试验结果表明:管线水平力在α=60°时达到最大,而上升力在α=45°时达到最大,并且从力的大小来看水平力比上升力大。随着角度增大,管线压力沿程变化幅度先增大再减小,α在60°时管线上游端压力最大;在45°时管线压力沿程变化呈波浪型,从上游往下游增减交替变化。随着冲刷的发展,管线下方受力最大值逐渐增大,管线前后方压力差值也逐渐增大。     

An ultrasonic device to measure surface displacements of water and floating bodies

Non-intrusive measuring instruments are desirable tools to investigate the response of vessels or marine structures subjected to water waves, whether in a laboratory or in the field. An ultrasonic sensing device is designed and built to measure surface displacements of a floating body. An ultrasonic sensor can sense any surface that lies in front of it, whether it is a solid or a liquid surface. In this study, a specialized electronic circuit board is designed to measure a wide range of surface displacements. The proposed system not only can be used to measure heave, surge, and pitch of a floating body continuously but also it can act as a water waveform recording instrument. The proposed ultrasonic measuring device is far more accurate than the current standard instruments (e.g. potentiometers) based on the simple fact that the measured surface is not disturbed. Furthermore, this device is of low manufacturing cost and has a practical size that renders it suitable for a broad range of applications. Hence, it can be used in a laboratory setup as well as in conducting field tests.     

A method to measure the isotopic (C) composition of dissolved organic carbon using a high temperature combustion instrument

The stable isotopes of dissolved organic carbon (DOC) are a powerful tool for distinguishing sources and inputs of organic matter in aquatic systems. While several methods exist to perform these analyses, no labs routinely utilize a high temperature combustion (HTC) instrument. Advantages of HTC instruments include rapid analysis, small sample volumes and minimal sample preparation, making them the favored devices for most routine oceanic DOC concentration measurements. We developed a stable carbon DOC method based around an HTC system. This method has the benefit of a simple setup, requiring neither vacuum nor high pressures. The main drawback of the method is a significant blank, requiring careful accounting of all blank sources for accurate isotopic and concentration values. We present here a series of experiments to determine the magnitude, source and isotopic composition of the HTC blank. Over time, the blank is very stable at  20 ng of carbon with a δ¹³C of − 18.1‰ vs. VPDB. The similarity of the isotopic composition of the blank and seawater samples makes corrections relatively minor. The precision of the method was determined by oxidizing organic standards with a wide isotopic and concentration range (− 9‰ to − 39‰; 18 μM to 124 μM). Analysis of seawater samples demonstrates the accuracy for low concentration, high salinity samples. The overall error on the measurement is approximately ± 0.8‰.     

Unfluidized soil responses of a silty seabed to monochromatic waves

A flume experimental study on unfluidized responses of a silty bed (d₅₀=0.05 mm) to monochromatic water waves had shown that pore pressure variations were generally poro-elastic in the bulk body and displayed two other characteristic features not found in previous laboratory sand tests. They were an immediately fluidized thin surface layer induced by wave stresses inside the seabed's boundary layer and a porous skeleton with internally suspended sediments due to channeled flow motions. The analyses verified that on soils beneath the measurement points, both features resulted in relatively small-step pore pressure build-ups, while the former played a primary role. Besides, laboratory observations confirmed that there were some near-bed sediment suspensions during wave actions resulting in a flat bed form over a silty bed compared to small-scaled ripples over a sandy bed with no clearly identified suspended sediments. These characteristic silt responses suggest that sediment transport is critically associated with the internal soil responses and some field-observed sediment suspensions near above sandy beaches can further be approached in the laboratory by utilizing fine-grained soils.     

Stability and liquefaction analysis of porous seabed subjected to cnoidal wave

Cnoidal wave theory is appropriate to periodic wave progressing in water whose depth is less than 1/10 wavelength. However, the cnoidal wave theory has not been widely applied in practical engineering because the formula for wave profile involves Jacobian elliptic function. In this paper, a cnoidal wave-seabed system is modeled and discussed in detail. The seabed is treated as porous medium and characterized by Biot's partly dynamic equations (u–p model). A simple and useful calculating technique for Jacobian elliptic function is presented. Upon specification of water depth, wave height and wave period, Taylor's expression and precise integration method are used to estimate Jacobian elliptic function and cnoidal wave pressure. Based on the numerical results, the effects of cnoidal wave and seabed characteristics, such as water depth, wave height, wave period, permeability, elastic modulus, and degree of saturation, on the cnoidal wave-induced excess pore pressure and liquefaction phenomenon are studied.     

An experimental study of seabed responses around a marine pipeline under wave and current conditions

Experiments on three types of soil (d₅₀=0.287, 0.057 and 0.034 mm) with pipeline(D=4 cm) either half buried or resting on the seabed under regular wave or combined with current actions were conducted in a large wave flume to investigate characteristics of soil responses. The pore pressures were measured through the soil depth and across the pipeline. When pipeline is present the measured pore pressures in sandy soil nearby the pipeline deviate considerably from that predicted by the poro-elasticity theory. The buried pipeline seems to provide a degree of resistance to soil liquefaction in the two finer soil seabeds. In the silt bed, a negative power relationship was found between maximum values of excess pore pressure p_max and test intervals under the same wave conditions due to soil densification and dissipation of the pore pressure. In the case of wave combined with current, pore pressures in sandy soil show slightly decrease with time, whereas in silt soil, the current causes an increase in the excess pore pressure build-up, especially at the deeper depth. Comparing liquefaction depth with scour depth underneath the pipeline indicates that the occurrence of liquefaction is accompanied with larger scour depth under the same pipeline-bed configuration.     

波浪作用下粉土海床中的孔压响应试验研究

海床在波浪作用下是否稳定对海底工程的安全至关重要,海床的稳定性与土体中的孔压响应密切相关。水槽模拟试验表明:在波浪的作用下,黄河三角洲粉土海床中将产生振荡孔隙水压力和累积孔隙水压力。振荡孔隙水压力大小与土层深度、波高和粘粒含量有关,其振幅(能量)在土层中随深度的增加呈指数衰减,且粘粒含量越高衰减越快;加载波高越大,能量衰减越快。而累积孔压响应模式表现为在波浪作用最初的一段时间内,孔隙水压力快速上升,然后逐渐减小而趋于稳定,其大小和速率也与波高、粘粒含量、土层埋深有关,粘粒含量越高,孔压累积速度越低。     

探索海床液化对沉积物再悬浮贡献的波浪水槽实验

通常认为沉积物的再悬浮主要来源于过剩剪切应力对海床表面的逐层侵蚀。虽然许多研究已经注意到波致海床液化在其中的重要性,然而,至今鲜有成果对其进行可靠的定量评估。本文即尝试通过一系列大型波浪水槽实验,初步对其进行量化评估。实验结果表明：在相对波高 (波高水深比) 为 4/20 和6/20的情况下, 黄河三角洲粉质沉积物的液化分别可以贡献52.5% 和 66.8%的再悬浮沉积物, 液化贡献与相对水深呈现正相关;进一步综合前人研究结果对比分析, 构建了用于定量描述液化贡献与相对水深关系的参数化方程。液化主要通过两种机制影响再悬浮过程：(1) 液化后黏聚力的减弱与渗流托举力,导致沉积物抗侵蚀性衰减 (2) 有部分细颗粒沉积物通过液化海床内部的渗流 “泵送” 输运到海床表面。     

1985年以来黄河三角洲东营港工程变化与海床冲淤演变响应关系

河口三角洲区域地质环境脆弱,对环境变化响应敏感,三角洲港口冲淤演变受工程结构影响较大。黄河三角洲的东营港区域以粉砂质海岸为主,区域内泥沙运移活跃,此类区域港口建设的关键问题在于工程结构导致的海床冲淤变化。通过东营港建港以来实测水深数据构建水下地形数字高程模型(DEM),并结合水动力数值模拟,探讨了东营港冲淤演变过程和工程影响。结果表明,由于波浪和潮流导致的海底地形变化,东营港近岸海域的冲淤演变形势已从单一侵蚀转变到近岸侵蚀、离岸淤积的新情势;工程结构影响局地潮流流速和流向,口门处出现高速横流,最大流速可达0.7 m/s;高流速导致北防波堤的堤头位置出现直径约1 km的冲刷坑;工程结构的遮蔽区有促淤效应,遮蔽区大小与潮流流向、工程结构-岸线夹角有关,但在波浪、余流的作用下,2007—2015年工程结构遮蔽区依旧存在0.5 m以上的侵蚀。持续的侵蚀作用使海域防波堤和海堤的不稳定性加剧,迫切需要加强检测与防护。