Propagation of topographically trapped waves on aβ-plane

Topographically trapped (subinertia) waves that propagate along a coast lying in an arbitrary direction on aβ-plane are studied. It is found that the waves also propagate in the direction normal to the coast within an envelope due to theβ-effect. The dispersion relation is hardly affected by theβ-effect except in a long wavelength or long period range in which generalized Haurwitz waves (Takeda, 1984b) exist. In the long wavelength or long period range, two types of waves exist: topographically trapped type waves and generalized Haurwitz type waves.     

胶州湾湾口海底沙波地形地貌特征及其活动性研究

利用多波束、侧扫声纳以及单道地震资料对胶州湾湾口潮流作用下形成的典型海底沙波地貌的平面形态、剖面特征和分布特点进行了分析研究。根据实测的水文资料计算了不同潮流流速下沙波的瞬时移动速度,推测了直脊型沙波和新月型沙波的形成]化过程:区内新月型沙波在西向优势流的作用下大约以50m/a的速度向西迁移,直脊型沙波则在两端方向不一致的优势流长期作用下,发生逆时针旋转,同时在往复流的作用下以一个平衡位置左右摆动;就地貌形态而言,新月型沙波是不稳定的,直脊型沙波达到相对平衡状态。     

Generation of internal waves in a three-layer ocean

Within the framework of the linear theory of long waves taking into account the action of the Coriolis force, we solve the problem of generation of internal waves by a barotropic tide impinging on a bottom irregularity of the sea-ridge type. The cross section of the ridge is assumed to be rectangular and the stratification of the ocean is regarded as stepwise with two thermoclines (three-layer model). We study the dependences of the characteristics of generated waves on the parameters of stratification and the period of the impinging barotropic tide. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

北部湾海浪季节变化和驱动因素的数值模拟研究

基于NCEP CFSV2再分析风场驱动SWAN模型,对南海至北部湾为期1年的海浪逐时过程进行了数值模拟,利用Jason-2卫星和近岸浮标整年观测数据检验了模拟效果。在此基础上,评估了模型空间网格尺度对北部湾内波浪模拟的影响,分析了波浪的季节变化特征,辨析了局地风和南海传入浪对海湾波浪的驱动贡献。研究显示:（1）较Jason-2卫星观测值,有效波高模拟值的均方根误差和分散系数分别约为0.4 m和0.2;较北部湾湾顶近岸浮标逐时观测值,有效波高的均方根误差和分散系数分别约为0.2 m和0.4,平均波周期的均方根误差和分散系数分别约为0.6 s和0.2,平均波向的均方根误差约为30°;（2）空间网格分辨率为12'×12'的模型对北部湾20 m以深开敞海域波浪的模拟效果良好,模拟值较2'×2'模型的平均相对偏差在10%以下;（3）北部湾冬季盛行东北向波,夏季盛行偏南向浪,季风转换期盛行东南向浪,全年波浪在季风期强于季风转换期,冬季最强、冬夏转换期最弱;（4）局地风对北部湾波浪的驱动贡献自湾口向湾内增强,季风期强于季风转换期;南海传入浪的驱动贡献自湾口向湾内减弱,季风转换期强于季风期;海湾中部和北部的波浪以局地风为主控因素,海南岛南部和东部水域以传入浪的影响为主,海南岛西南水域受局地风和传入浪的共同控制。     

Joint distributions of wave height and period: A critical comparison

Three theoretical joint distributions of wave height and period are compared with data. Both crest-to-trough waves and zero-upcrossing waves are considered. The data consist of two long Waverider records (of 12 and 8 hr) and a shorter record taken from a reservoir. It is found that a distribution based on the theory of Gaussian processes fits the crest-to-trough waves well in all cases. In contrast, the two narrow band approximations only fit the data from the reservoir, which is more narrow band. The effect of high frequency cut-off on the distribution of height and period are considered.     

Effect of the Geometric Characteristics of a Basin on the Space Structure of Forced Oscillations

By the method of mathematical simulation, we study the effect of changes in the width and depth of a ring-shaped basin with parabolic profile of the bottom on the space structure of waves generated by variations of atmospheric pressure periodic as a function of time. Our investigation is carried out under the assumptions of the linear theory of long waves with regard for the action of the Coriolis force. The structures of the profiles of liquid surface are compared for the cases of free and forced waves. We also establish the dependences of the period of atmospheric disturbances on the geometry of the basin for which the number of nodal points of the free-surface profile in the case of forced oscillations coincides with the number of nodal points of the profile of liquid surface in the case of free oscillations.     

Effects of depth discontinuity on harbor oscillations

The effects of water depth discontinuity near the harbor mouth on harbor oscillations are examined. Linear long-wave equations are used as the basis of the present study. For simplicity, only the normal incident waves are considered. Assuming that the harbor mouth is small in comparison with wavelength, the method of matched asymptotic expansion is employed to obtain the ocean impedance and the harbor responses. It is found that the incident waves can be trapped over the depth discontinuity which causes large oscillations near the harbor mouth. The radiation damping also decreases because of the appearance of the depth discontinuity, which leads to large amplifications at the lowest mode.     

Dynamic response of a poro-elastic soil to the action of long water waves: Determination of the maximum liquefaction depth as an eigenvalue problem

In this work, a theoretical analysis of the dynamic response of a poro-elastic soil to the action of long water waves is conducted. For some combinations of the physical parameters of the soil and the water waves, the vertical stress tends towards zero at a certain unknown depth in the soil, as measured from the top of that medium. Under this condition, the liquefaction of the soil is imminent, at which time the excess pore pressure is essentially equal to the overburden soil pressure. Physical problems of this type have been widely studied in the specialized literature. However, most major studies have focused on solving the governing equations together with a liquefaction criterion. Here, the maximum momentary liquefaction depth induced by long water waves is considered as part of the problem, which is treated as an eigenvalue problem. To solve this problem, the governing equations are written in dimensionless form. The theoretical results show that for long waves, the horizontal displacements are smaller in magnitude than the vertical displacements, and when the wavelength or wave period increases, the maximum liquefaction also increases. Analytical solutions for the excess pore pressure and the horizontal and vertical displacements are obtained. The analytical results for the pore pressure are found to be very close to the analytical results reported in the specialized literature.     

Experimental hydrodynamic investigation of a fixed offshore Oscillating Water Column device

Oscillating Water Column (OWC) is one of the pioneer devices in harnessing wave energy; however, it is not fully commercialized perhaps due to the complicated hydrodynamic behavior. Previous studies are significantly devoted to OWC devices located in nearshore and coastal regions where incident wave energy would experience dissipation more than offshore. In this paper, a 1:15 scaled fixed offshore OWC model is tested in a large towing tank of National Iranian Marine Laboratory. Wave spectrum shape effect on the efficiency of the OWC model is addressed. Moreover, the paper investigates the effects of the geometric and hydrodynamic factors on OWC device efficiency and uncovers new points in nonlinear interaction occurring inside the chamber; i.e. sloshing. The results indicate that shape of the spectrum inside the chamber is affected by the type of incident wave spectrum, especially for long waves. Pierson–Moskowitz spectrum leaded to higher efficiency rather than JONSWAP spectrum at longer incident wave periods. According to efficiency analysis, increasing wave height may lead to air leakage from the chamber followed by vortex generation, which is a reason for decreasing the efficiency of the OWC device. Furthermore, no shift in the resonant period of the OWC model, due to wave height increase, was observed at the opening ratios equal or smaller than 1.28%. Spectral analysis of water fluctuation inside the OWC chamber illustrates two modes of sloshing. The first mode can be seen at short period waves while the second mode is visible at long period waves. The sloshing modes approximately vanish by increasing draft value.     

Swell-dominant surface waves observed by a moored buoy with a GPS wave sensor in Otsuchi Bay,a ria in Sanriku,Japan

Real-time monitoring of wind and surface waves in Otsuchi Bay, Iwate, Tohoku, Japan, commenced in October 2012, using a mooring buoy with an ultrasonic anemometer and a single-mode GPS wave sensor. Wind and wave data are distributed hourly in real time via the Internet along with a chart of their time series. We analyzed data monitored in the first 3 months in order to assess the variability and occurrence of wind and waves and to elucidate the main reasons for wave variation in Otsuchi Bay. The monitoring data revealed that surface waves in the bay were predominantly affected by swells propagated from the northeastern offshore region and that the wave height was significantly correlated with the component of wind velocity toward Otsuchi Bay in the northeastern offshore region that faces the bay mouth. The offshore wind field was expected to provide information useful for predicting coastal waves in a ria bay in Sanriku such as Otsuchi Bay. However, it should be emphasized that the horizontal distribution of the offshore wind field which has a significant effect on the surface waves in a ria bay depends heavily on the topographic shape of the bay.     

Surf zone dynamics simulated by a Boussinesq type model. Part II: surf beat and swash oscillations for wave groups and irregular waves

This is the second of three papers on the modelling of various types of surf zone phenomena. In the first paper the general model was described and it was applied to study cross-shore motion of regular waves in the surf zone. In this paper, part II, we consider the cross-shore motion of wave groups and irregular waves with emphasis on shoaling, breaking and runup as well as the generation of surf beats. These phenomena are investigated numerically by using a time-domain Boussinesq type model, which resolves the primary wave motion as well as the long waves. As compared with the classical Boussinesq equations, the equations adopted here allow for improved linear dispersion characteristics and wave breaking is modelled by using a roller concept for spilling breakers. The swash zone is included by incorporating a moving shoreline boundary condition and radiation of short and long period waves from the offshore boundary is allowed by the use of absorbing sponge layers. Mutual interaction between short waves and long waves is inherent in the model. This allows, for example, for a general exchange of energy between triads rather than a simple one-way forcing of bound waves and for a substantial modification of bore celerities in the swash zone due to the presence of long waves. The model study is based mainly on incident bichromatic wave groups considering a range of mean frequencies, group frequencies, modulation rates, sea bed slopes and surf similarity parameters. Additionally, two cases of incident irregular waves are studied. The model results presented include transformation of surface elevations during shoaling, breaking and runup and the resulting shoreline oscillations. The low frequency motion induced by the primary-wave groups is determined at the shoreline and outside the surf zone by low-pass filtering and subsequent division into incident bound and free components and reflected free components. The model results are compared with laboratory experiments from the literature and the agreement is generally found to be very good. Finally the paper includes special details from the breaker model: time and space trajectories of surface rollers revealing the breakpoint oscillation and the speed of bores; envelopes of low-pass filtered radiation stress and surface elevation; sensitivity of surf beat to group frequency, modulation rate and bottom slope is investigated. Part III of this work (Sørensen et al., 1998) presents nearshore circulations induced by the breaking of unidirectional and multi-directional waves.     

基于震级上限估计的渤海海啸危险性与共振特征研究

根据渤海区域地质断层特征和历史地震活动规律,分析得出渤海内潜在最大震级上限为8.1级,并对该海啸源可能的两组震源机制分别进行了数值模拟。模拟结果显示：渤海局部区域海啸波幅最大可达 1.5 m,最大流速可达2.8~3.0 m/s,具备造成灾害损失的风险。在该海啸源情景下,渤海海盆内易激发长期的水位自由振荡,部分区域水位振荡可持续 20 h以上,振荡波幅的大小与海啸首波波幅相当或更大。基于快速傅里叶变换方法对海啸波进行频谱分析,部分长周期频谱成分满足区域固有共振特征。因此,渤海内一旦发生海啸,不仅要关注海啸首波可能造成的灾害性影响,还要密切关注海啸首波到达后,可能产生的长时间、长周期的海啸波共振以及往复式海啸流造成的影响。     

Short-wave measurements by a fixed tower-based and a drifting buoysystem

The small-scale roughness of the sea surface acts as an important link in air-sea interaction processes. Radar and sonar waves are scattered by short surface waves providing the basis for remote sensing methods of the sea surface. At high wind speeds, breaking waves occur. Bubbles penetrate into the water and drastically increase acoustical reverberation, transmission loss and ambient noise. Thus, the development of short waves and wave breaking have to be known to apply radar remote sensing to the surface and to deduce from radar backscatter which sonar conditions prevail. To measure the wind dependence of short waves an experimental device was constructed for use from stationary platforms. It is nearly all-weather capable and can easily be handled by a crane. On the other hand, frequencies of short waves measured in a fixed position are extremely frequency shifted by currents. This limits the usefulness of tower-based measurements, e.g., the short wave modulation by wind and waves or currents can only be estimated in a rough approximation. Consequently, a buoy was developed to reduce the frequency shifts. The principle of the buoy is to drift in the local surface current and to follow the amplitudes of long waves. Therefore, short waves are measured in facets of long waves and the Doppler shifts are minimized. The wind is measured at a constant height above the long wave profile and relative to the moving facets. The paper describes the conventional measuring device and points out the necessity of the drifting buoy system. Examples of wind and wave spectra are presented and short wave modulations by long waves are depicted, too. From these measurements, new insights in short wave behaviour have to be expected     

Modulation of short wind waves by a long surface wave: a mechanism for feedback with an air flow

The paper concentrates on the evolution of a spectrum of short wind waves (SW) along the profile of a long surface wave (LW). Short wave spectral variations are considered in the relaxation approximation. The SW spectrum is modulated by the orbital velocities of long waves and by the variations of wind stress along the LW profile. The latter effect occurs due to wind flux perturbations induced by both the long wave proper and variations of the sea surface roughness induced by the SW modulations. To describe this effect, a feedback mechanism is introduces—the growth of energy of short waves results in the larger roughness of the sea surface, thereby contributing to the local wind stress, which facilitates, in turn, the growth of short waves. With moderate and strong winds being involved, this effect (aerodynamic feedback) is shown to be dominant in the short wave spectrum modulation. The mechanism becomes more efficient with intensification of the wind and decreasing of the long waves' frequency. Results of model calculations are in agreement with the known experimental data. Translated by Vladimir A. Puchkin.     

Experimental study of the transformation of bound long waves over a mild slope with ambient currents

The effect of currents on the variation of cross-shore bound long waves forced by bichromatic waves over a plane slope was investigated in the laboratory. In still water the growth rate of the shoaling bound long waves over the slope is proportional to h^– 5/2 (h is still-water depth). It was found that the opposing current makes the amplitudes of the bound long waves greater than those of still water for all cases. However, the amplitudes of bound long waves in a following current are reduced in the weakly modulated cases but are enhanced in the fully modulated case.     

Characteristics of storm waves off the Mediterranean coast of Israel

A two-year series of directional wave measurement off the Eastern Mediterranean coast of Israel reveals an abundance of high storm waves. Some of these waves have significant height in excess of 5 meters and periods as long as 15 sec.The evolution of the storm waves is described and related to the growth and paths of the storm fronts in Mid-Mediterranean. Shorter-period waves are found to always lead the arrival of longer-period swell. This characteristic is explained by a short decay distance and/or a high migration velocity of the storm front.The scatter plot of significant wave height vs period for the recorded events of each storm describes an open-loop time sequence. The difference in period between that of the peak height event and the period of a fully arisen sea of the same height is found to be indicative of the true decay distance the waves have travelled.     

The effect of currents on the generation of internal waves by a barotropic tide near an underwater ridge

In the frame of the linear theory for long waves, the paper studies long waves generated by a barotropic tide running at an arbitrary angle over a bottom ridge. The ocean is assumed to be two-layered. In the area of a ridge with a rectangular cross section, geostrophic flows are considered, coupled with the inclinations of the free surface and interface. Wave amplitudes are shown to depend on the angle of an onrunning tide and this allows us to reach a conclusion about the influence of a geostrophic current on the generation of internal waves. Translated by Vladimir A. Puchkin.     

Forced Waves in a Bounded Basin with Two Pycnoclines

The behavior of waves induced in a continuously stratified fluid with two pycnoclines by periodic variations of pressure on the free surface is studied within the framework of the linear theory by taking into account the action of the Coriolis force. The fluid fills a bounded basin of constant depth. We establish the dependences of the amplitudes and velocities of waves on the period of disturbing pressure. The process of transition of generated waves from the barotropic character of propagation to baroclinic with changes in the period of disturbing pressure is studied in detail.     

Influence of the Space Distribution of Atmospheric Pressure on the Structure of Free Wave Surface in a Basin of Variable Depth

Within the framework of the linear theory of long waves, we study forced oscillation of liquid in a ring basin of variable depth by using numerical methods. As a generator of waves, we use periodic (in time) variations of atmospheric pressure. The action of the Coriolis force is taken into account. The liquid is regarded as homogeneous and inviscid. We analyze the dependences of the structure of the free wave surface (the number and location of nodal lines) on the period and space distribution of disturbing pressures. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 11–23, March–April, 2005.     

海浪感应电磁场的理论计算

根据麦克斯韦电磁理论,在地磁场中运动的海水将产生感应电磁场。利用一个简单的数学物理模型对感应电磁场在海水内部的分布进行了计算,结果表明海浪产生的电磁场明显依赖于海浪波动的周期及浪高。在100 m的海水深度内,海浪产生的磁感应强度的大小为纳特数量级,而电场强度的大小为几个微伏每米。在同一海水深度处,磁感应强度随海水波动的周期呈现近线性变化,而电场强度的大小有一个极值,该极值随海水深度的增加向长周期方向移动。海浪产生的电磁场是影响海洋电磁探测数据精度的主要噪声之一。