Numerical study of pollutant movement in waves and wave-induced long-shore currents in surf zone

Water waves, wave-induced long-shore currents and movement of pollutants in waves and currents have been numerically studied based on the hyperbolic mild-slope equation, the shallow water equation , as well as the pollutant movement equation, and the numerical results have also been validated by experimental data. It is shown that the long-shore current velocity and wave set-up increase with the increasing incident wave amplitude and slope steepness of the shore plane ; the wave set-up increases with the in- creasing incident wave period;and the pollutant morement proceeds more quiekly with the increasing incident wave amplitude and slope steepness of the shore palane. In surf zones, the long-shore currents induced by the inclined incident waves have effectively affected the pollutant movement.     

近岸沿岸流及污染物运动的数值模拟

基于双曲型缓坡方程和近岸浅水方程对近岸波浪斜向入射破碎所生成的沿岸流及污染物在沿岸波流作用下的运动进行了数值模拟,并对数值模拟结果进行了验证分析。数值模拟结果表明,在相近工况参数下,随着入射波高的增大,沿岸流流速和平均水面升高值均明显增大;随着岸坡坡度的增加,沿岸流流速和平均水面升高值明显增大;随着入射波浪周期的增大,平均水面升高值明显增大。在沿岸缓坡区域,由斜向入射波浪破碎所产生的沿岸流对污染物的运动起着不可忽略的影响。     

波生流对海岸污染物输移的影响

通过物理模型实验对海岸波浪作用下污染物运动特性进行了分析,重点分析了质量输移流、沿岸流、沿岸流不稳定运动及破波带内旋涡运动等海岸水动力因素的影响.实验中坡度分别取为1:100和1:40,实验中采用CCD摄像机记录墨水的运动轨迹,同步测量流体质点速度以及波面升高.实验表明,在破碎带外污染物主要受波浪非线性引起的质量输移流的影响;在破碎带内主要受沿岸流的影响,同时还受沿岸流不稳定运动及大尺度旋涡运动的影响.     

Numerical study on water waves and wave-induced longshore currents in Obaky coastal water

In this paper, the water waves and wave-induced longshore currents in Obaky coastal water which is located at the Mediterranean coast of Turkey were numerically studied. The numerical model is based on the parabolic mild-slope equation for coastal water waves and the nonlinear shallow water equation for the wave-induced currents. The wave transformation under the effects of shoaling, refraction, diffraction and breaking is considered, and the wave provides radiation stresses for driving currents in the model. The numerical results for the water wave-induced longshore currents were validated by the measured data to demonstrate the efficiency of the numerical model. Then the water waves and longshore currents induced by the waves from main directions were numerically simulated and analyzed based on the numerical results. The numerical results show that the movement of the longshore currents was different while the wave propagated to a coastal zone from different directions.     

Rip currents under obliquely incident wind waves and tidal longshore currents

A field experiment on the nature of rip currents was conducted on the Dutch coast, which differs from previous rip current study sites because it is a wind-sea dominated environment with mostly obliquely incident waves and tidally-driven longshore currents. During the experiment three distinct flow patterns, obtained with GPS tracked drifter instruments, were observed: (1) a locally governed circulation cell, (2) an offshore current that is deflected shore parallel outside the surf zone and (3) a meandering longshore current. The transition from rip currents (flow patterns 1 and 2) to meandering longshore currents (flow pattern 3) occurred gradually within the tidal cycle with longshore currents prevalent at mid to high tide. Rip currents at this site appeared at depressions in the surf zone bar and typically occurred when the water level fell below NAP (equivalent to MSL), even in the presence of obliquely incident waves and tidally driven longshore currents. Hindcast simulations of the drifter experiments were performed with the numerical model XBeach and showed good agreement with field observations. The model was subsequently used to investigate the influence of tidal water level fluctuations, longshore currents and obliquely incident waves on rip currents.Rip currents were initiated when the water level dropped below a specific threshold with the magnitude of the rip current associated with the water level. The strength of the tidal current and its orientation with respect to the incident waves governed the offshore extent and orientation of the rip current. In contrast to other studies that suggest that rip currents solely occur under shore normal (or slightly oblique waves), in this study both observations and numerical model simulations indicate that rip currents can exist under large angles of wave incidence, when the rip channel is sufficiently wide and the wave height is small.     

Ephemeral sand waves in the hurricane surf zone

Airborne bathymetric LIDAR observations along the Florida panhandle after Hurricane Dennis (2005) show the first unequivocal observations of surf-zone sand wave trains.

These are found in depths of 5m along the trough of the hurricane bar, where hindcasts show strong longshore currents only during severe storms. The waves extend over tens of kilometers of coast after Dennis but are absent from the same area in four other datasets. Observed wavelength to water depth ratios are comparable to river dunes and tidal sand waves but height to depth ratios are smaller, with the largest wave heights around 0.1 times the water depth. The sand wave generation mechanism is hypothesized to be from wind-and-wave-induced longshore currents, which were hindcast to be large during Dennis, with destruction from water wave orbital velocities.     

Numerical modelling of wave-induced currents in the presence of coastal structures

The hydrodynamic aspects of the general methodology of calculation of nearshore processes by means of numerical models are described. The paper focuses on the method implemented for calculating combined wave refraction-diffraction and reflection due to coastal structures and the associated radiation stresses.Results of numerical modelling are compared with experimental data obtained by Gourlay in the case of a shore-connected breakwater with periodic waves.A good agreement is found between both methods of investigation as concerns the spatial distribution of wave height and mean water level (wave-induced set-up). A good similarity of the wave-induced eddies in the lee of the structure is observed. A less satisfactory agreement is obtained between the velocity distributions in several profiles normal to the shore, although the overall order of magnitude is the same.A critical review of several wave-breaking criteria and sensitivity tests of the numerical model lead to advocate the use of the CERC criterion, particularly with steep beach slopes.     

海岸沿岸流垂向分布实验研究

进行海岸沿岸流模型实验,利用ADV(acoustic doppler velocimeter)测量了沿岸流流速沿垂直岸线方向多个断面的垂向分布。采用Faria等的方法对实验结果进行了分析,将沿岸流沿水深分为上层和下层两部分:下层采用对数分布来表达沿岸流的垂向分布;上层考虑了波浪引起的自由表面的波动,流速分布为修正的对数分布。最后对沿岸流垂向分布特征做了分析,并与Visser和Hamilton的实验结果做了对比。     

利用被动示踪物模拟对黑潮入侵南海的数值研究

由于缺少观测数据和对黑潮水准确定义,很难识别出从太平洋入侵到南海的黑潮水团。本文基于一个经过观测验证的三维模式MITgcm,利用被动示踪物标记黑潮水,研究了入侵南海的黑潮水的时空变化。研究表明,在冬季,黑潮水入侵的范围最广,几乎占据了18°N-23°N和114°E-121°E的区域;并有一个分支进入台湾海峡;黑潮入侵的范围随深度增加逐渐减小。在夏季,黑潮水被限制在118°E以东,且没有分支进入台湾海峡;入侵的范围从海面到约205米是增大的,之后随深度增加逐渐减小。通过分析从2003年到2012年黑潮入侵的年际变化,与厄尔尼诺年和正常年相比,冬季黑潮入侵后向台湾海峡的分支在拉尼娜年是最弱的,这可能与中国大陆东南方向的风应力旋度有关。通过吕宋海峡的黑潮入侵通量（KIT）是西向的,其年平均值约为-3.86×10⁶ m³/s,大于吕宋海峡通量（LST,约-3.15×10⁶ m³/s）。250米以上的KIT约占了全深度通量的60-80%。此外,从2003年到2012年KIT与Niño 3.4指数的相关系数到达0.41,小于LST与Niño 3.4指数的相关系数0.78。     

Remote sensing of surf zone waves using stereo imaging

The measurement of water wave characteristics, such as wavelength and wave height, in the surf zone is important for monitoring, prediction of erosion, and numerical model calibration. Traditional methods of measuring wave heights have either been limited to a small number of points or have required contact with the water. An experimental study of the remote sensing of water wave elevations, through the application of stereo photogrammetry, is presented. This method uses two spatially offset cameras, with overlapping fields of view, to determine water surface elevation. This remote sensing approach provides data with excellent spatial coverage and spatial and temporal resolution. Additionally, the hardware needs are minimal and the system is quickly deployed, calibrated, and operational.     

Laboratory measurements of uniform longshore currents

Experiments on uniform longshore currents in a wave basin are described. The measurements were performed in a basin with a pumped recirculation through openings in the wave guides. Minimal return flows in the offshore region of the basin are found to be accompanied with longshore currents which are virtually uniform alongshore. Three-dimensional longshore current velocity distributions were measured with much attention to quality control. Detailed experimental results are presented for different wave fields, two beach slopes and two beach roughnesses.     

不规则波情况下沿岸流不稳定运动传播方向的研究

本文以沿岸流不稳定运动实验中流速仪采集到的不规则波的流速时间历程为基础,利用相关性分析,得到两个不同位置处对应流速时间历程的最大相关系数和延迟时间,进而分析沿岸流不稳定运动在1∶40和1∶100平直斜坡上的传播方向,并且分析了其随入射波高、周期的变化。结果表明沿岸流不稳定运动在海岸垂向环流系统的作用下,其传播方向会向海方向偏移,与岸线角度大约在0°到45°之间,且在1∶40和1∶100坡情况下,沿岸流不稳定运动的偏移角分别随入射周期和波高的增加而增加,但1∶100坡的增加幅值较小,大约为1∶40坡情况下的1/2。     

Effects of longshore shelf variations on barotropic continental shelf waves,slope currents and ocean modes

Effects of continental shelf bends, converging depth contours and changing depth profiles are discussed. Some analysis is carried out for previously unstudied cases. Separate oceanic interior and shelf flow problems are formulated for a sufficiently narrow shelf. The ocean interior ‘sees’ only an integrated shelf effect, typically increasing shelf-edge amplitudes, retarding longshore Kelvin-wave propagation and increasing natural mode periods by 0 (10%). On the local shelf, the flow matches to the ocean interior and is nondivergent. Effects on shelf waves and slope currents depend subtly on the nature of the longshore variations. Curvature and contour convergence do not per se imply scaterring or generation of shelf waves. Indeed, any depth h(ξ) where ▽² ξ(x,y) = 0 (a condition approximating longshelf uniformity in the topography's convexity) supports essentially the same shelf waves as do straight depth contours (DAVIS, 1983), and slope currents follow depth contours. Scattering results rather from breaks in analyticity of the depth profile. Hence calculations for small isolated features (necessarily highly convex or concave) may overestimate scattering, and superposition for realistic topography may lead to much self-cancellation among scattered waves. Otherwise, examples show a strong preference for scattering into adjacent mode numbers and into any shelf wave mode near to its maximum frequency. A shelf sector, where the maximum shelf wave frequency maxω is less than the frequency ω of an incident shelf wave, causes substantial scattering unless maxω and ω are very close. Adjustment of slope currents to changed conditions takes place through (and over the decay distance of) scattered shelf waves.     

沿岸流不稳定运动研究进展

沿岸流不稳定运动目前已成为一个研究热点,对于沿岸流不稳定的研究对于近岸泥沙、污染物输移以及海岸养护等都有很重要的意义。本文详细介绍了沿岸流不稳定运动的研究进展,具体包括沿岸流不稳定运动现场以及实验室实验研究进展,沿岸流线性不稳定理论,非线性不稳定理论以及沿岸流不稳定运动产生的其它理论分析。结果表明国外对于该问题的研究(主要是有关不稳定理论方面)已经取得很多成果,但国内目前很少,并且现在对沿岸流不稳定研究比较成熟的理论基础是不稳定理论,该理论都假定增长率最大的不稳定模式决定着沿岸流的波动特性。但对于多模式不稳定运动的研究还基本没有,对于沿岸流不稳定运动三维特性的研究(包括数值研究及实验研究)也基本没有报道。所以这两个方面可能是以后对该问题研究的重要方向。     

Influence of lateral mixing on longshore currents

A variety of different lateral mixing models have been employed in the equation of motion for determining wave-induced longshore currents. The equation of motion is cast into a general form which enables a comparison of the various models. Analytic solutions for longshore currents are developed for seven different mixing models. A nonplanar beach profile is employed because it has been found to be representative of many beaches and it allows for a distinction between offshore and depth scaling of eddies. The seven different mixing models include models which vary monotonically with horizontal distance offshore and models which change form at the breaker line. Numerical results indicate that the longshore current profile is rather insensitive to the form of the mixing model for nonplanar beach profiles.     

On low-frequency waves in the surf and swash

Low-frequency waves in the surf and swash zones on various beach slopes are discussed using numerical simulations. Simulated surface elevations of both primary waves and low-frequency waves across the surf zone were first compared with experimental data and good agreement found. Low-frequency wave characteristics are then discussed in terms of their physical nature and their relationship to the primary wave field on a series of sea bottom slopes. Unlike primary waves, low-frequency wave energy increases towards the shoreline. Low-frequency waves in the surf and swash are a function of incident waves and the sea bottom slope and hence the saturation level of the surf zone. Wave energy on a gently sloping beach is dominated by low-frequency waves while primary waves play a significant role on a steep beach. Low-frequency wave radiation from the surf zone on a given beach depends on primary wave frequency and beach slope. However, a very poor correlation was found between surf similarity parameter and low-frequency wave radiation.     

Boundary roughness and bedforms in the surf zone

Hydrodynamical models of the nearshore system frequently assume that a single friction coefficient is sufficient to represent flow conditions at a point in the surf zone. Furthermore, models attempting to relate bed configuration to surf zone flows have relied primarily upon the wave orbital velocity as an indicator of potential bedforms, and thus as the control on boundary roughness. The data presented here point out potential errors arising from either of these approaches. The results of a field experiment conducted at Wendake Beach, Ontario, show that at a single location in an active surf zone, the Darcy-Weisbach friction coefficient, f, varied by approximately 250% (in this case between 0.016 and 0.041).

It is also shown that existing bedform models, based upon primary wave motions alone, do not accurately predict conditions at this study site. For a relatively constant wave orbital velocity and velocity asymmetry, it is found that changes in bed roughness, as a result of bedform development, are reflected mainly in the vertical profile of the long-shore current velocity. A sequence of bedforms, from oscillatory ripples through flat bed, is inferred from the data, and found to be supported by diver observations and preserved primary sedimentary structures.     

Computation of the driving forces of wave-induced currents

For morphological studies in coastal areas the wave-driven currents are often of primary importance. As the computation of the wave-driven current field requires the computation of the driving forces from a wave propagation model, the quality of these forces is basic to the success of the morphological study. It is shown in the paper that, under the common conditions of slowly varying wave fields, the driving force per unit mass is closely proportional to the wave energy dissipation per unit area and that diffraction-related terms give insignificant contributions. This result holds good for wave fields without current refraction. Examples show that formulation of the driving forces in terms of the wave dissipation yields more trustworthy results than those obtained by numerical differentiation of the radiation stress tensor.     

Quasi-three-dimensional modelling of the morphology of longshore bars

A morphological quasi-three-dimensional (Q3D) area model for barred coasts has been developed. The model combines a two-dimensional depth integrated model for wave-driven currents with a model for undertow circulation currents. The combined model makes a simultaneous simulation of the bar-forming processes associated with the undertow and the horizontal wave-driven circulation currents, which may cause instabilities of the bar and the formation of rip channels. Situations with normal and oblique wave incidence are considered. Compared to the depth integrated approach the Q3D model produces less pronounced alongshore irregularities for obliquely incident waves. For normal incident waves the Q3D model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels. The sensitivity to variation of wave angle and beach slope is further investigated.