不同破碎波对沙质海床作用的实验研究

破碎波对近海海岸地形以及海岸建筑物影响强烈,通过物理模型实验对孤立波、规则波作用下破碎带的床面形态以及孔隙水压力进行分析。破碎波冲击海床,破碎处床面上形成沙坝和沙坑,与规则波相比,孤立波破碎时对床面的冲刷更加剧烈,床面形成的沙坝和沙坑尺度更大,且土体内孔隙水压力幅值也较大。同时研究了波面变化对孔隙水压力的影响,发现波面变化历时曲线与孔隙水压力历时曲线相似,与孔隙水压力梯度历时曲线更为相似,说明波面变化更能反映海床内部孔隙水压力梯度的变化。通过探讨波浪与海床之间相互耦合作用,发现破碎带地形变化使得波浪出现不同破碎类型,分析得出卷破波比崩破波作用下孔隙水压力幅值大。     

波浪作用下斜坡上单桩周围海床孔隙水压力响应特性试验研究

单桩基础周围斜坡海床中的波致孔隙水压力响应与纯斜坡海床存在较大差异。为了解不同波高、波周期条件下,单桩基础周围波浪传播变形及其对斜坡海床孔压振荡响应的影响,在波浪水槽末端铺设了长6 m、坡度1∶16的斜坡砂床进行试验。通过改变桩身位置和波浪参数,测量斜坡段各处波面形态,采集单桩周围孔隙水压力,分析了桩身位置及波浪参数对斜坡海床孔压响应的影响。结果表明：相同入射波条件下,随距坡脚水平距离增加,波高、近底流速和桩周孔隙水压力幅值都随之增大;桩周孔隙水压力幅值分布规律为：桩前孔压幅值明显大于桩侧与桩后孔压幅值。当Keulegan-Carpenter数大于6时,随着波高和波周期增大,马蹄涡产生的负压区使得桩侧海床孔隙水压力与纯斜坡海床孔隙水压力差值迅速增加。     

Simulating offshore sand waves

Sand waves form a prominent regular pattern in the offshore seabeds of sandy shallow seas and pose a threat to a range of offshore activities. A two-dimensional vertical (2DV) flow and morphological simulation model describing the behaviour of these sand waves has been developed. The simulation model contains the 2DV shallow water equations, with a free water surface and a general bed load formula. The water movement is coupled to the sediment transport equation with a seabed evolution equation. The domain is non-periodic in both directions. The spatial discretisation is performed by a spectral method based on Chebyshev polynomials. A fully implicit method is chosen for the discretisation in time. Firstly, we validate the simulation model mathematically by reproducing the results obtained using a linear stability analysis for infinitely small sand waves. Hereby, we investigate a steady current situation induced by a wind stress applied at the sea surface. The bed forms we find have wavelengths in the order of hundreds of metres when the resistance at the seabed is relatively large. The results show that it is possible to model the initial evolution of sand waves with a numerical simulation model. This paper forms the necessary first step to investigate the intermediate term behaviour of sand waves.     

Experimental investigation on the wave-induced pore pressure around shallowly embedded pipelines

A series of regular wave experiments have been done in a large-scale wave flume to investigate the wave-induced pore pressure around the submarine shallowly embedded pipelines.The model pipelines are buried in three kinds of soils,including gravel,sand and silt with different burial depth.The input waves change with height and period.The results show that the amplitudes of wave-induced pore pressure increase as the wave period increase,and decay from the surface to the bottom of seabed.Higher pore pressures are recorded at the pipeline top and the lower pore pressures at the bottom,especially in the sand seabed.The normalized pressure around pipeline decreases as the relative water depth,burial depth or scattering parameters increase.For the silt seabed,the wavelet transform has been successfully used to analyze the signals of wave-induced pore pressure,and the oscillatory and residual pore pressure can be extracted by wavelet analysis.Higher oscillatory pressures are recorded at the bottom and the lower pressures at the top of the pipeline.However,higher residual pressures are recorded at the top and the lower pressures at the bottom of the pipeline.     

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.     

Spatial and temporal variation process of seabed dynamic response induced by the internal solitary wave

Internal solitary wave(ISW) is often accompanied by huge energy transport, which will change the pore water pressure in the seabed. Based on the two-dimensional Biot consolidation theory, the excess pore water pressure in seabed was simulated, and the spatiotemporal distribution characteristics of excess pore water pressure was studied. As the parameters of both ISW and seabed can affect the excess pore water pressure, the distribution of pore water pressure showed both dissipation and phase lag...     

The numerical study of wave-induced pore water pressure response in highly permeable seabed

The coupling numerical model of wave interaction with porous medium is used to study waveinduced pore water pressure in high permeability seabed.In the model,the wave field solver is based on the two dimensional Reynolds-averaged Navier-Stokes(RANS) equations with a k-ε closure,and Forchheimer equations are adopted for flow within the porous media.By introducing a Velocity-Pressure Correction equation for the wave flow and porous flow,a highly efficient coupling between the two flows is implemented.The numerical tests are conducted to study the effects of seabed thickness,porosity,particle size and intrinsic permeability coefficient on regular wave and solitary wave-induced pore water pressure response.The results indicate that,as compared with regular wave-induced,solitary wave-induced pore water pressure has larger values and stronger action on seabed with different parameters.The results also clearly show the flow characteristics of pore water flow within seabed and water wave flow on seabed.The maximum pore water flow velocities within seabed under solitary wave action are higher than those under regular wave action.     

大亚湾大鹏澳菱角石沿岸泥沙动态

本文根据大亚湾大鹏澳菱角石沿岸的波浪、潮流和泥沙资料,探讨了菱角石沿岸泥沙来源、泥沙活动水深和破波带,估算了破波带沿岸输沙率和非破波带底沙单宽输沙率,说明了沿岸泥沙运移以破波带沿岸输沙为主,验证了赵子丹提出的估算沿岸输沙率的关系曲线的可靠性。     

Surf zone characterization for integration with remote sensing data

A numerical model that solves the unsteady, incompressible, Reynolds averaged, Navier–Stokes equations has been utilized to simulate 57 cases of monochromatic, breaking waves over a sloping bed. The Volume of Fluid technique is used to track the complex, discontinuous free surface and the Renormalized Group turbulence model is used for closure. The model is validated by comparing predictions with Particle Image Velocimetry data and other empirical results. The model results are used to determine a relationship between the incipient wave breaking height and the maximum orbital velocity as well as a relationship between surf zone width and breaker type. Such expressions may be useful for remote sensing methods like Synthetic Aperture Radar to derive breaker height and classification from image data.     

Coupled analysis for response and instability of sloping seabed under wave action

Wave-induced instability of seabed may cause damage to coastal and offshore structures. This issue has been investigated mostly for mildly sloping ($<5^{°}$) seabed considering uncoupled or one-way coupled response of wave and seabed interaction. However, some of the marine structures are founded on seabed with steeper slopes. In this study, the wave-induced response and instability of sloping seabed are evaluated using a coupled finite element model. The interaction between fluid and porous seabed accounting for the effect of fluid motion on the seabed response, and conversely the effect of seabed response on the fluid motion (but not on the surface wave profile) is considered. The results indicate that the system response (fluid pressure, stresses, etc.) and the extent of instantaneously liquefied zone within the sloping seabed with significant steepness are lesser than those for horizontal seabed. Moreover, for typical sediment and wave characteristics, for the flat seabed, the response obtained from fully coupled analysis is not significantly different from those obtained by uncoupled analysis. For the sloping bed, such difference is slightly greater as compared to that for the flat bed.     

随机波作用下埋管海床动态响应及液化研究

基于广义Biot动力理论和Longuet-Higgins线性叠加模型,构建波浪-海床-管线动态响应的有限元计算模型,求解随机波作用下,多层砂质海床中管线周围土体孔隙水压力和竖向有效应力的分布。采用基于超静孔隙水压力的液化判断准则,得出液化区的最大深度及横向范围,从而判断海床土体液化情况。考虑海洋波浪的随机性,将海床视为多孔介质,海床动态响应计算模型采用u-p模式,孔隙水压力和位移视为场变量。并考虑孔隙水的可压缩性、海床弹性变形、土体速度、土体加速度以及流体速度的影响,忽略孔隙流体惯性作用。参数研究表明:土体渗透系数、饱和度以及有效波高等参数对海床土体孔隙水压力、竖向有效应力和液化区域分布有显著影响。     

Penetration depth of the dynamic response of seabed induced by internal solitary waves

We use flume experiments and numerical modeling to examine the penetration depth of internal solitary waves (ISWs) on partially saturated porous sandy silt and clayey silt seabed. The results of the experiment and model showed that the instantaneous excess pore water pressure in both the sandy silt and clayey silt seabed followed the same trend of decreasing with the seabed depth. In general, the excess pore water pressure generated by the sandy silt was bigger than that by clayey silt at the same depth. The ISW-induced excess pore water pressure greatly influenced the surface seabed and showed a linear relationship. The penetration depth was approximately one order of magnitude smaller than the half-wavelength of the ISWs, which might be larger than the penetration depth induced by surface waves. Our study results are helpful for understanding the damage that ISWs inflict upon the seabed and for informing future field experiments designed to directly measure the interaction between ISWs and seabed sediments.     

内孤立波浅化破碎过程斜坡沉积物孔压响应特征实验分析

观测资料显示内孤立波沿斜坡浅化过程对海底沉积物的作用犹如一台水中吸尘器，在破碎转换阶段达到最强，甚至会触发一系列地质活动，引发地质灾害。为界定此过程中沉积物的动力响应特征和影响因素，在大型重力式分层流水槽中模拟不同振幅内孤立波和不同类型沉积物斜坡连续作用过程，利用孔隙水压力采集系统实时记录孔隙水压力变化，对比分析不同水动力、坡度、沉积物类型情况下沉积物中超孔压变化特征。分析结果表明，内孤立波破碎过程，破波位置海床表层波压力和不同深度超孔隙水压力都存在相似的"U"型负压力变化过程；破碎波经过位置沉积物表现为和表面波压力正相关的孔压响应特征。破碎点沉积物中超孔压幅值随深度减小，约在6%波长深度位置减少到坡面压力的50%。超孔压幅值和内孤立波振幅、沉积物类型和斜坡度密切相关，坡度由0.071变化到0.160时，波压力幅值可增大至1.6倍。内孤立波振幅变化不影响不同类型海床土动力响应规律，只与超孔隙水压力值大小有关，内孤立波对海床的动力作用可认为弹性作用。     

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.     

An integrated three-dimensional model of wave-induced pore pressure and effective stresses in a porous seabed: II. Breaking waves

The evaluation of the wave-induced liquefaction potential is particularly important for coastal engineers involved in the design of marine structures. Most previous investigations of the wave-induced liquefaction have been limited to two-dimensional non-breaking waves. In this paper, the integrated three-dimensional poro-elastic model for the wave-seabed interaction proposed by [Zhang, H., Jeng, D.-S., 2005. An integrated three-dimensional model of wave-induced pore pressure and effective stresses in a porous seabed: I. A sloping seabed. Ocean Engineering 32(5/6), 701–729.] is further extended to simulate the seabed liquefaction potential with breaking wave loading. Based on the parametric study, we conclude: (1) the liquefaction depth due to breaking waves is smaller than that of due to non-breaking waves; (2) the degree of saturation significantly affects the wave-induced liquefaction depth, and no liquefaction occurs in full saturated seabed, and (3) soil permeability does not only significantly affect the pore pressure, but also the shear stresses distribution.     

考虑土骨架加速度效应的海床动力反应及其影响因素分析

由Biot二维广义动力固结理论的形式基本控制方程出发,忽略孔隙流体的加速度,提出了饱和海床动力反应的时域有限元数值解法.联立静力平衡条件和Biot固结方程的退化法所得到的数值解可视为其特例.在比较算例中,退化法得到的超静孔压和有效应力幅值沿海床深度的分布与解析解一致.一般情况下,土骨架的加速度对海床的有效应力和超静孔压影响很小,控制方程可以退化为Biot理论.成层海床上部的粗砂层不会使超静孔压幅值在海床表面下较浅的深度内迅速衰减,难以改变海床的瞬时循环液化深度.     

极端波浪条件下黏土质斜坡海床稳定性解析

海洋资源开发引起海底软黏土的结构性破坏,导致土体强度弱化,在百年一遇的极端波浪作用时极易发生斜坡海床的局部失稳甚至大范围海底滑坡,给海洋工程建设和正常运营带来严重影响。目前,主要采用极限平衡法评价这类海底斜坡,但该法只能给出近似解。基于极限分析上限方法,推导了极端波浪诱发的波压力对斜坡海床的做功功率,建立了外力功与内能耗散率平衡方程;利用最优化方法,结合数值积分和强度折减技术,求解了不同时刻的斜坡海床稳定性系数,并针对扰动后的斜坡海床开展了有限元解的对比验证。在此基础上,深入探讨了不同波浪参数(波长、波高和水深)和坡长小于一个波长等极端条件下的海底斜坡稳定性。     

Experimental Study on the Bed Shear Stress Under Breaking Waves

The object of present study is to investigate the bed shear stress on a slope under regular breaking waves by a novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor. The sensors were calibrated before application, and then a wave flume experiment was conducted to study the bed shear stress for the case of regular waves spilling and plunging on a 1:15 smooth PVC slope. The experiment shows that the sensor is feasible for the measurement of the bed shear stress under breaking waves. For regular incident waves, the bed shear stress is mainly periodic in both outside and inside the breaking point. The fluctuations of the bed shear stress increase significantly after waves breaking due to the turbulence and vortexes generated by breaking waves. For plunging breaker, the extreme value of the mean maximum bed shear stress appears after the plunging point, and the more violent the wave breaks, the more dramatic increase of the maximum bed shear stress will occur. For spilling breaker, the increase of the maximum bed shear stress along the slope is gradual compared with the plunging breaker. At last, an empirical equation about the relationship between the maximum bed shear stress and the surf similarity parameter is given, which can be used to estimate the maximum bed shear stress under breaking waves in practice.     

A probabilistic analysis of random wave-induced liquefaction

Seabed instability caused by soil liquefaction due to build-up of excess pore pressure within the sedimentary seabed represents a serious threat to coastal structures. Models of varying sophistication exist for predicting the liquefaction process but most previous calculations are limited to regular waves while the real waves are random. In this study, a numerical study of liquefaction potential of a sand bed under narrow-band random waves is carried out employing ensemble modelling techniques. The aim of the work is to investigate the effect of random waves on excess pore pressure build-up and liquefaction processes and study the probability distribution of the maximum liquefaction depth. The computational results using a 1D liquefaction model indicate that the random wave-induced liquefaction can be much deeper than that of the corresponding regular waves with the largest individual waves in the random wave time series playing a dominant role in determining the maximum liquefaction depth. It is also found that the time for the maximum liquefaction depth to be reached can vary considerably from one random wave series to another, which suggests that in random waves notable densification may occur within the same timeframe as that for liquefaction.     

Experimental study of the breaking limit of multi-directional random waves passing over an impermeable submerged breakwater

This study investigates experimentally the breaking wave height of multi-directional random waves passing over an impermeable submerged breakwater. Experiments have been conducted in a three-dimensional wave basin equipped with a multi-directional random wave generator. A special type of wave gauge has been newly devised to record the water surface elevations in the breaker zone as accurately as possible. The records are analyzed to estimate the location and limit of wave breaking. Comparisons have also been made with the results of regular waves. The influence of the incident wave conditions on the breaking wave height normalized by the breakwater dimensions has been investigated. Empirical formulae have been presented to estimate the breaking limit of multi-directional random waves based on the experimental records. The formulae have been tested and found to work well not only for multi-directional random waves, but for regular waves as well.