Experimental study on bed-load sediment transport under irregular wave and current combined flow

Using an irregularly oscillating tray and flume,a series of experiments are completed to evaluate bed-load sediment transport rate under irregular wave-current coexistent fild.Testing conditions include three interaction angles 0°,45°,90° and two kinds of median sizes (0.38 and 1.10 mm).The results of transport rate show that the net sediment transport rate can be expressed approximately as the function of the maximum bottom shear stress of waves,mean shear stress of current and the grain size.     

波流联合作用下的海底沙波移动对海底底床稳定性影响的研究进展

海底沙波常见于潮流作用较强的内陆架海域,是一种脊线与潮流流向垂直的微地貌形态,一般在40～100 cm/s的流速形成。沙波既可大面积自成体系,也可分布在沙脊的坡面上,呈不对称形态。泥沙的粒度、沉积物供给以及水深[1]等因素对海底沙波的发育都有影响。     

Threshold condition of sand particles under codirectional combined wave and current flow

Flume experiments were carried out to determine the threshold of movement of sand on rippled surface and flat beds under the codirectional combined flows due to waves and currents. The results indicate that Shields's curve can be used to determine the threshold condition under combined flows, provided that the maximum bed shear stress is used.     

A Boussinesq model for simulating wave and current interaction

A new formulation of a pair of Boussinesq equations for three-dimensional nonlinear dispersive shallow-water waves is presented. This set of model equations permits spatial and temporal variations of the bottom topography and the presence of uniform currents. The newly derived equations are used to simulate the propagation of cnoidal waves and their interactions with a uniform current in a wave channel. The modified Euler's predictor-corrector algorithm for time advancing and a central difference representation for the space derivatives are applied to the computation of the basic equations. A set of open boundary conditions is developed to effectively transmit the cnoidal waves out of the computational domain. It is found that, as expected, the wave length decreases with an opposing current and increases with a following current. The wave height increases in magnitude with an opposing current and decreases with a following current. The Mach reflection due to oblique cnoidal waves propagating into an open channel with an opposing current is also investigated. Due to the opposing current, the wave patterns are compressed into smaller saddle-like regions in comparison with the Mach reflection without current effect.     

A combined numerical model of three-dimensional tidal motion and sediment transport in tidal current and its application

INTRODUCTIONTheregularpatternsofmovementanddistributionofcurrentandsedimentinwaterbodyinestuaryandatcoastareconcernedbythepublicatalltime,butonlyafewresearchesonthenumericalsimulationofthethree-dimensionalnonlinearmovementoftideandwave,inwhichLeendertse's(Leendertseetal.,1973)finitedifferencemethod(FDM)istheleadingone,havebeendoneuptonow.TheLeendertseFDMisnotverygoodinitsapplication.Themainreasonisthatinthemodethesquaregridsinaplanecomputationaldomainareadopted.Thisgridsystemcannotfitt…     

A model for wave growth in fetch-limited conditions

In this paper, it is held that the universal relationships of wave growth in fetch-limited conditions , i. e.,(f|~) p=A(x|~)-Band (m|~)0= C(x|~) Dshould satisfy the Toba 3/2 power law and the wave energy balance equation. In the ideal generation situation, theoretically it can be derived that the ideal fetch-limited wave growth relationship should have D=3B and D B =1, (i.e., B = 0.25, D = 0.75 ) and A3C=2. 1×l(T4C~(1/2)_d , where Cd is the drag coefficient. The 3/2 power law, the wave energy balance equation and the decrease of wave steepness with increasing fetch have became three requirements which should be satisfied by fetch-limited wave growth algorithms. A semi-empirical and semi-theoretical model for fetch-limited wave growth is presented. In the application to the slanting wind situation an un(?)ersal relationship of dimensionless wave energy vs dimensionless peak frequency is presented and the comparisons show that the model is in good agreement with observations.     

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.     

考虑波浪辐射应力的潮流泥沙数学模型

根据海岸河口地区潮流、波浪、泥沙的运动特点及其相互作用机制,考虑了波浪场产生的"波浪辐射应力"和"波流挟沙力"动力要素对潮流场和泥沙运动的影响,在泥沙扩散方程中考虑了紊动水流作用,采用有限元加权集中质量法,建立了考虑波浪作用的潮流泥沙数学模型,并利用锦州港港区的潮流及泥沙资料对模型进行了验证。验证结果表明,潮位吻合良好,流速、流向、含沙量分布基本相似。在此基础上,模拟了工程建设前后,港区附近潮流场和泥沙淤积的变化,分析了泥沙骤淤现象。模拟结果表明,该数学模型能够适用于海岸河口地区潮流、泥沙的模拟,为波浪作用下潮流场和泥沙淤积的预测提供一种新的模拟方法,具有良好的应用前景。     

Mean bed friction of combined wave/current flow

Bijker's model for evaluating the wave-period-mean bed friction of combined wave/current flow has been modified to account for the current velocity reduction as well as the wave boundary layer near the sea bed. A method of estimating the bed frictional dissipation rate of the combined flow is also presented. The results obtained by the present approach are compared with several sets of experimental data, and confirm its accuracy and usefulness.     

Bed friction and dissipation in a combined current and wave motion

Two simple two-layer eddy viscosity models, which facilitate analytical solutions, are presented in order to describe the velocity field and associated shear stress in a combined current wave motion. The models, which have the same eddy viscosity in the current boundary layer, but different eddy viscosities in the wave boundary layer, cover together the whole rough turbulent regime. Straightforward definitions are made for the wave friction factor and the current friction factor for the combined motion, which are in accordance with the results for pure waves and pure currents. In this way one avoids the fictitious reference velocities and elliptic integrals which e.g. Grant and Madsen (1978, 1979) experienced. The two friction factors turn out to be functions of four dimensionless parameters. A detailed calculation procedure is presented. Comparison with laboratory experiments yields promising results. A new relation connecting dissipation and bed shear stress is also developed.     

Physical model study of wave and current conditions at 17th Street Canal breach due to Hurricane Katrina

A 1:50 scale physical model was constructed for the 17th Street Canal region, New Orleans, on the southern coast of Lake Pontchartrain, as part of the Interagency Performance Evaluation Task Force (IPET) study of Hurricane Katrina. The purpose of the 1350 m² physical model that represented about 3.4 km² of the local area was to aid in defining wave and water velocity conditions in the 17th Street Canal during the time period leading up to the breaching of the floodwall within the Canal. In the immediate period following this disaster, there were many hypothesis of failure put forth in the media. Some of these hypothesis indicated wave action may have been the underlying cause of the failure of the 17th Street Canal floodwall. Some performed numerical work with inappropriate boundary conditions, which indicated strong wave-generated currents may have caused erosion along the floodwalls. This physical model study indicated a number of wave-attenuating processes occurring as waves approached the location of the breach. Wave height reduction resulted due to: (1) refraction of wave energy over the shallower submerged land areas surrounding the harbor away from the canal; (2) reflection of energy off vertical walls in the region between the entrance to the canal near the Coast Guard Harbor and the bridge; and (3) interaction of the wave with the Hammond Highway bridge, including reflection and transmission loss. Wave heights near the lakeside of the bridge were 0.3-0.9 m in height, reduced from 1.8 to 2.7 m wave heights in the open lake. Waves on the south side of the bridge, near the breach, were further reduced to heights below 0.3 m. These results supported the conclusion that waves were not a significant factor for the 17th Street Canal floodwall failure. Other IPET investigations determined floodwall failure was of a geotechnical nature due to the high surge water level. The physical model also provided calibration information for numerical wave models. The effects of debris on flow and waves after the breach was formed were also investigated.     

A coupled-mode model for water wave scattering by horizontal, non-homogeneous current in general bottom topography

A coupled-mode model is developed for treating the wave–current–seabed interaction problem, with application to wave scattering by non-homogeneous, steady current over general bottom topography. The vertical distribution of the scattered wave potential is represented by a series of local vertical modes containing the propagating mode and all evanescent modes, plus additional terms accounting for the satisfaction of the free-surface and bottom boundary conditions. Using the above representation, in conjunction with unconstrained variational principle, an improved coupled system of differential equations on the horizontal plane, with respect to the modal amplitudes, is derived. In the case of small-amplitude waves, a linearised version of the above coupled-mode system is obtained, generalizing previous results by Athanassoulis and Belibassakis [J Fluid Mech 1999;389:275–301] for the propagation of small-amplitude water waves over variable bathymetry regions. Keeping only the propagating mode in the vertical expansion of the wave potential, the present system reduces to an one-equation model, that is shown to be compatible with mild-slope model concerning wave–current interaction over slowly varying topography, and in the case of no current it exactly reduces to the modified mild-slope equation. The present coupled-mode system is discretized on the horizontal plane by using second-order finite differences and numerically solved by iterations. Results are presented for various representative test cases demonstrating the usefulness of the model, as well as the importance of the first evanescent modes and the additional sloping-bottom mode when the bottom slope is not negligible. The analytical structure of the present model facilitates its extension to fully non-linear waves, and to wave scattering by currents with more general structure.     

A Bingham-plastic model for fluid mud transport under waves and currents

Simplified equations of fluid mud motion, which is described as Bingham-Plastic model under waves and currents, are presented by order analysis. The simplified equations are non-linear ordinary differential equations which are solved by hybrid numerical-analytical technique. As the computational cost is very low, the effects of wave current parameters and fluid mud properties on the transportation velocity of the fluid mud are studied systematically. It is found that the fluid mud can move toward one direction even if the shear stress acting on the fluid mud bed is much smaller than the fluid mud yield stress under the condition of wave and current coexistence. Experiments of the fluid mud motion under current with fluctuation water surface are carried out. The fluid mud transportation velocity predicted by the presented mathematical model can roughly match that measured in experiments.     

非对称歪斜波引起的片流输沙数值研究

本文用了一个可考虑相位差作用和波浪边界层非对称性的瞬态理论模型和一个两相紊流模型共同研究非对称歪斜波引起的片流输沙现象。为了解速度偏度和加速度偏度对输沙通量和输沙率的贡献,两相流模型为理论模型提供了必要的相位超前、瞬时侵蚀深度和边界层的发展过程。理论模型研究显示了由速度偏度和加速度偏度引起的向岸阶段和离岸阶段的泥沙运动非对称性,解释了净输沙的产生原因。在以往的非对称歪斜波片流输沙研究中,净输沙的产生主要被归结于相位差作用。本文的研究则表明了非对称的边界层发展所产生的净流量和动床面效应在净输沙产生过程中的比相位差作用更为重要。     

大亚湾长周期波在缓变流作用下传播计算模型的研究

以存存缓变背景流的二维缓坡方程为控制方程建立一个计算模型来研究大亚湾长周期波的传播规律.按入射波源形式不同、周期不l司和有无背景潮流设计了几个数学试验,分别通过这儿个试验研究长周期波传播规律.从模拟结果发现,在大哑湾水域,仪在开边界附近不同源形式的入射波对波浪传播的影响有所变化;总的来说,从入射点到湾内波高削弱较快,但是对于不同周期的入射波,模拟所得波高分布变化较大;对于长周期的入射波,背景流也对波浪传播有一定影响.     

波流共同作用下珊瑚礁冠层附近平均流分布及阻力特性试验

本文通过物理模型试验研究了波流共同作用下珊瑚礁冠层附近平均流的分布特征以及阻力特性,分析了典型波浪工况下无潮流、正向潮流和反向潮流分别作用下平均流速、摩阻流速、阻力系数的沿礁变化规律。结果表明：无潮流时礁前斜坡及外礁坪上存在海底回流且在礁缘附近回流最强,在礁坪上冠层附近平均流表现为向岸流,且该流沿礁向海岸方向持续增大。相较于无潮流时,正向潮流作用下冠层内外均为向岸流,在礁坪上冠层内外的向岸流显著增大;反向潮流作用下冠层内外均为离岸流且在礁缘处达到最大,该离岸流在礁坪上逐渐减小然后趋于稳定。无潮流时礁坪上摩阻流速呈小幅波动;相较于无潮流时,正向潮流、反向潮流影响下礁坪上摩阻流速显著增大,其中正向潮流影响下增幅更大;无潮流时礁坪上水力粗糙度沿礁减小,正向潮流和反向潮流影响下水力粗糙度普遍有幅度不等的增加。三种工况下礁坪上的阻力系数均沿礁整体呈下降趋势,相较于无潮流时,正向潮流和反向潮流影响下礁坪上的阻力系数显著增大,且正向潮流作用时增幅更大。     

Underwater spreading and surface drifting of oil spilled from a submarine pipeline under the combined action of wave and current

Tremendous economic loss and environmental damages are caused by oil-spilling accidents in sea. Accurate prediction of the underwater spreading and surface drifting of oil spills is important for the emergency response. In the present study, numerical investigation on the underwater spread and surface drift of oil spilled from a submarine pipeline under the combined action of wave and current was carried out to examine the effects of physical ocean environment, leaking flux and spilled oil density and viscosity. Reynolds-Averaged-Navier-Stokes (RANS) equations, realizable k-ε turbulence model and volume of fluid (VOF) model are employed to describe the multiphase flow, and velocity-boundary wave-making technique combined with the sponge layer damping absorber technique realizes the numerical wave flume. Oil spill experiments were conducted to validate the numerical model. The calculation results indicate that compared with the environmental conditions of still water, only current and only wave, a larger scope of underwater spreading and relatively slower rising rate and relatively faster drifting rate of oil droplets are observed under the combined action of wave and current. The leaking flux affects the floating time and dispersion concentration, while the ocean environment affects the horizontal migration and surface drifting. Under the specific conditions of present work, oil density has obvious effect on the underwater spread but limited effect on the surface drifting, while oil viscosity has little effect on both the two processes.     

Stability analysis on a porous seabed under wave and current loadings

The stability of a porous seabed under wave and current loadings is particularly important for engineers to design marine structures such as submarine pipelines, breakwaters, and offshore platform foundations. Most previous investigations of dynamic response of marine structures and seabed have only considered the influence of wave loading, but the important influence of current is ignored. Even if the influence of current is considered, the interaction mechanism of both loadings has not been clearly elaborated. Based on the Biot’s dynamic theory and combined two-dimensional nonlinear progressive wave and uniform current theory, the interaction mechanism of wave and current loadings and the influence of current on wave characteristic are analyzed by numerical computations. The influence of current velocity, different permeability, and stratification in seabed on the effective stresses and pore pressures of seabed is discussed in detail. Further, the stability of seabed is evaluated through the liquefaction analysis of seabed, which will provide important reference frames to improve the design and construction of marine structures.