Investigation of long waves in the tsunami frequency band on the southwestern shelf of Kamchatka

Two inexpensive cable bottom pressure stations were installed on the southwestern shelf of Kamchatka (Okhotsk Sea) in 1987 and two more in 1988 to provide longwave measurements in the tsunami frequency band, to investigate the generating mechanism of these waves, and to test the instrumentation. Microfluctuations of atmospheric pressure were recorded simultaneously. Two cable lines were torn off by ship anchors in March 1989 but others are still working in spite of highly dynamic activity on beaches and in hard ice regimes. Careful data analysis of two months of observations (September–October, 1987) showed that: (1) the atmospheric spectra were very stable and monotonic in the period range 2–50 min and corresponded to a power law of ^–2.3, (2) the direct generation of long waves by atmospheric pressure fluctuations was negligible, (3) there was high correlation between background longwave oscillations and sea state, (4) the structure of the offshore longwave field was in good agreement with theoretical estimates of standing waves for a linear slope.     

A study on reflection pattern of swells from the shoreline of peninsular India

Information on reflected surface gravity waves from the shoreline is required for understanding the coastal hydrodynamics. We have quantified the reflected swells (frequency band 0.045–0.12 Hz) from the west and east coast of India based on the spectral wave data derived from the directional waverider buoys. Reflection coefficient, ratio of the reflected and incident spectral energy, was used to quantify the reflected waves. Influence of the seasons, cyclone, relative depth, land/sea breeze, tides and tidal current on the reflected waves were examined. For the locations off the west coast of India, seasons have large impact on the reflection coefficient and were relatively less during the monsoon season due to the increase in incident wave energy. Locations off the east coast of India show almost the same reflection coefficient throughout the year and have no significant seasonal variations. The reflection coefficient off Puducherry was higher than that for other locations due to the low incident wave energy. The reflection coefficient was low during the cyclone period, but the reflected energy during cyclone was higher than that during the normal condition due to the high incident wave energy. High-energy reflected waves show large variation with tide due to the trapping and dissipation of reflected wave by bottom friction and this effect cause low reflection in deep water location than shallow water location. The reflection coefficient decreases with increase in relative depth off west coast of India.     

Spectral wave characteristics along the central coast of eastern Red Sea

The Jeddah coast lies in the central eastern Red Sea, which is characterized by the predominant northwest winds and the associated wind waves throughout the year. A detailed investigation on the spectral wave characteristics in the nearshore regions of the Jeddah coast has not been carried out yet, primarily due to the lack of data. In the present study, we have analyzed the available wave spectra measured at two nearshore locations along the Jeddah coast using wave gauges. The wave spectra were separated into wind sea and swell components using a frequency-based algorithm, and the integral wave parameters corresponding to each component were derived. Although the measurements were limited to the summer season, notable features such as the diurnal variability and the superimposition of wind seas and swells have been identified from the spectra. The superimposition is mainly due to the interaction of the young swells propagated from the northern Red Sea and the local breezes prevailing along the coast at certain periods in a diurnal cycle. Based on the wave speed calculations and the estimated time shifts between the wind and waves, the potential swell regions have been backtracked.     

北太平洋Rossby波研究进展

系统地阐述了北太平洋Rossby波在卫星观测、理论扩展及模型应用3个方面的研究成果.随着卫星观测技术的发展,明显地探测到海表面信号的Rossby波特征,且观测到波速在热带外大于自由Rossby波理论值.为解释这个波动加速现象,在自由Rossby渡理论的基础上,考虑了大气强迫、非静止基流及海底地形等因子的作用,使得Rossby波的波动理论得到了扩展.而关于风应力强迫Rossby波的线性理论模型主要有3类:一阶斜压Rossby波模型、局地Ekman抽吸模型和Svetdrup平衡模型.这些模型被广泛地应用到北太平洋海表面高度和温度/密度跃层深度等要素变化的机理研究中,得到了不同的模型在大洋的不同区域有不同的作用等结论.最后,对线性Rossby波研究存在的问题进行了初步探讨,提出了需要进一步解决的新课题.     

径向点插值法在波浪传播数值模拟中的应用

针对波浪数值模拟中基于矩形网格的数值方法在深水到浅水的网格间距选择与复杂边界处理上的缺陷,以及基于正交曲线网格和无结构网格的数值方法前处理工作复杂的问题,引入最近在计算力学中发展起来的无网格法——径向点插值法,对经典的双曲型缓坡方程进行空间离散,并在时间上采用四阶Adams-Bashforth-Moulton格式求解建立近岸波浪传播数学模型,通过椭圆形浅滩地形和环形河道的波浪传播计算验证,表明该无网格方法可较为有效地模拟近岸波浪的传播变形,且在处理复杂边界时具有较高的精度.     

海南岛南渡江河口动力沉积模式

河口是一个迅速变化的、动态的海岸巨系统。作为陆地河流和和海洋过程的重要链接,河口的动力沉积过程一直是陆海相互作用研究的核心和焦点内容。本文基于2011年8月在南渡江河口采集的大范围表层沉积物样品,利用经验正交函数分析技术（EOF）对河口的动力沉积特征进行研究。结果表明:南渡江河口海床表层沉积物主要以砂为主,沉积物总体偏粗;河口的动力沉积特征自陆向海可分为三种动力沉积模式:①波控模式,该模式主要分布在河口地区10 m以浅的近岸区域,呈与岸线平行的带状分布,其表层沉积物以粉砂质砂为主,分选较差;②径、潮流耦合作用下的沉积模式,在径流和潮流的共同控制作用下,沉积物主要表现为粒径较粗,该模式呈扇形分布,其中20 m以浅河口海床受控于径流和潮流的共同作用,20 m以深海床表现为潮流控制的沉积模式;③台风或风暴潮控制的沉积模式,即整个河口海床都表现出受控于台风或者风暴潮作用的沉积特征,沉积物主要是以粗砂为主。常态作用下,河口以径、潮流控制的沉积模式为主,波浪、径流和潮流以及潮流控制的沉积模式自陆向海的规律性分布体现出南渡江河口近岸以波浪作用为主,而离岸则受河口尤其是洪水作用形成的喷射流以及沿岸潮流的影响。此外,尽管南渡江河口在过去的成果中将其归纳为波控河口,但目前的研究发现:该河口区域沉积类型变化明显受控于不同的动力作用,河口形态以及琼州海峡的障蔽和“狭管效应”为河口沉积环境变化的主要控制因素。     

Longwave Measurements for the Coast of British Columbia and Improvements to the Tsunami Warning Capability

A few years ago the Canadian Hydrographic Service initiated a major upgrade toall tide gauges and tsunami stations on the coast of British Columbia (B.C.). Thisprogram was undertaken to address shortcomings of the earlier digital systems andwas driven by concerns about emergency response continuity in the year 2000. By1999, thirteen tide gauge stations had been installed and were operational. Three ofthese stations (Tofino, Winter Harbour, and Langara) were selected for use as tsunamiwarning stations. Several years of continuous, high quality data have now been collectedat these stations and used for analysis of long waves in the tsunami frequency band.Careful examination of these data revealed two weak tsunamis recorded by severalB.C. stations: a distant tsunami of June 23, 2001 generated by the Peru Earthquake(M_w = 8.4), and a local tsunami of October 12, 2001 induced by the Queen Charlotte Earthquake (M_w = 6.3$). Spectral characteristics of these two tsunamis are compared with the spectral characteristics of long waves generated by a strong storm (October, 2000) and of ordinary background oscillations. The topographic admittance functions (frequency responses) constructed for all stations showed that most of them (in particular, Winter Harbour, Tofino, Bamfield, Port Hardy, and Victoria) have strong resonance at periods from 2.5 to 20 min, indicating that these locations are vulnerable to relatively high-frequency tsunamis. The Winter Harbour station also has two strong resonant peaks with periods of 30 and 47 min and with amplification factors of about 7. The estimated source functions show very clear differences between long waves associated with the seismic source (typical periods 10–30 min) and those generated by a storm, which typically have shorter periods and strong energy pumping from high-frequencies due to non-linear interaction of wind waves.     

表面波有效相速度近似分析方法

分层介质中瑞利面波有多个模态,表面瞬态响应是多个模态响应的叠加。在近场,面波模态响应传播速度随传播距离而变化;在远场,其趋于模态相速度。由分层介质表面两不同点响应互谱分析(SASW)得到的有效相速度并不对应于面波基阶模态相速度,它与波场中高阶模态能量分配比例有关。有效相速度随传播距离而变化,近场体波对有效相速度影响较大。对分层介质在简谐荷载下表面质点位移响应进行了互谱分析,得到了有效相速度理论值,通过理论值与测试值匹配分析可估算分层参数,该分析方法考虑了近场及高阶面波模态对有效相速度的影响。     

波浪作用下某防沙堤的动力固结有限元分析

基于饱和土动力固结理论,采用SWANDYNE II有限元分析,程序预测一个梯形沉箱防沙堤在某设计波浪作用下的响应。采用Pastor-Zienkiwicz Mark III广义塑性模型模拟了海床土的循环应力-应变行为。通过动三轴试验,确定了主要的模型参数,分析中只考虑了波浪对结构的作用,忽略行波对海床表面的作用。动力固结有限元分析的结果定性上与常规的拟静力极限平衡分析方法一致,有限元分析定量给出了体系的位移、应力、孔隙水压力分布和结构位移随波浪持续时间的累积过程,其研究结果初步展现了动力固结有限元方法在近海和海岸岩土工程领域的广阔应用前景。     

Modelling December 2004 Indian Ocean tsunami: a coastal study

December 2004 tsunami in the Indian Ocean region has been simulated using MIKE-21 HD model. The vertical displacement of the seabed is incorporated into the numerical simulation by using time-varying bathymetry data. In the open ocean, sea surface height from altimeter observation has been used to validate the model results. To the west of the rupture zone, the crest is observed to precede the trough of the tsunami waves while to the east, trough preceded the crest. The model performance along the coastal region has been validated using de-tided sea levels from tide gauge measurements at Tuticorin, Chennai, Vishakapattanam, and Paradip ports along the east coast of India. Unique coastal characteristics of the tsunami waves, wave height, and wave celerity are reasonably simulated by the numerical model. Spectral analysis of tide gauge observations and corresponding model results has been done, and the distribution of frequency peaks from the analysis of gauge observations and the model results is observed to have a reasonable comparison. Low-frequency waves, contributed from the coastally trapped edge waves, are found to dominate both the tide gauge observations and the model results. The subsequent increase in the tsunami wave height observed at Chennai, Vishakapattanam, and Paradip has been explained on the basis of coastally trapped edge waves. From the validation studies using altimeter data and tide gauge data, it is observed that the model can be used effectively to simulate the tsunami wave height in the offshore as well as in the coastal region with satisfying performance.     

波浪作用下沙坝剖面形成过程的数值模拟

建立了波浪、底部离岸流、泥沙运动和沙滩剖面演变耦合数学模型,模拟了实验室中波浪作用下沙坝剖面的形成过程,讨论了各个物理参数对剖面上的水动力和地形变化的影响。通过参数率定,较好地复演了不规则波作用下沙滩地形由均匀斜坡向沙坝剖面的演变,波高、底部离岸流、输沙率和剖面演变的计算结果与实测数据吻合良好。采用数值实验和误差分析方法,讨论了破碎波水滚倾角、泥沙扩散系数和床面休止角等物理参数对计算结果的影响。研究表明,所建立的模型能较好地描述波浪作用下沙坝剖面形成过程中的重要物理机制。     

Storm waves in the Canadian Atlantic: A numerical simulation

The Grand Banks and the Scotian Shelf regions of the Canadian Atlantic often experience strong winds and high waves associated with the passage of intense storms during the winter months of December to March. These storm waves are identified as a major hazard to shipping, offshore exploration and other marine activities in eastern Canada.In this study, an operational spectral ocean wave model has been used to simulate sea-states associated with selected storm events in the Canadian Atlantic. The wave model operates on a grid (with grid spacing of about 111 km) which covers a major portion of the north Atlantic. A nested fine grid (with grid spacing of about 37 km) has been designed which covers the shelf regions of the Canadian Atlantic. The model employs deep-water physics over the coarse grid while over the fine grid shallow-water processes as represented by wave refraction, wave shoaling, bottom friction and wave number scaling are included. The wave model also includes, as an optional package, the third-generation source terms as represented by the nonlinear wave-wave interaction terms.For two selected storm events, the model generated sea-states are evaluated against available buoy data as well as against hand analyzed operational wave height charts over the northwest Atlantic. The evaluation shows that the various versions of the model can simulate the observed sea-states, reasonably well. The utility of the wave model in providing numerical guidance for offshore activities is briefly discussed.     

A review of some current trends in nearshore research

Studies of sediment dynamics in the nearshore zone cover a broad spectrum of disciplines and it is difficult to determine what particular trend is emerging in current research because many problems, particularly those confronting coastal engineers, are practical rather than academic and consequently demand rapid solution. Restrictive physical factors, such as heavy swell and wave turbulence in the breaker area, have undoubtedly limited in situ measurements and may partly account for the apparent poor correlation between field data and laboratory results and theoretical prediction. The development of sophisticated field instrumentation is an attempt to resolve this problem and this Review draws attention to some of the more recent techniques that have been reported in the extensive literature during the last decade or so.For the purpose of this paper, the nearshore zone is sub-divided into swash, surf/breaker and offshore zones. A summary is given of littoral transport rates, particularly with regard to the value of tracer experiments and models, and the difficulty of measuring surge-swash velocities and bottom profiles is assessed. Brief mention is made of swash percolation and groundwater flow and, bearing in mind the engineering problem of arresting the loss of beach sand to the backshore and dune system of some coastlines, the shear stress of sea breeze has also been evaluated. The problems of predicting changes in the plan shape of beaches and in the rate of mobility of shingle under wave action are discussed. Methods for measuring suspended sediment and bulk density are also described. Variations in wave refraction and wave energy relative to theoretical computation are appraised and reference is made to the use of radar in hydrodynamic surveying.Other sections of the review refer to the relationship that appears to exist between edge waves and crescentic bars; and the use of photography in studying coastal processes.     

Dérive à la surface de l'océan sous l'effet des vagues

We model the drift velocity near the ocean surface separating the motion induced by the local current, itself influenced by winds and waves, and the motion induced by the waves, which are generated by local and remote winds. Application to the drift of ‘tar balls’, following the sinking of the oil tanker Prestige-Nassau in November 2002, shows that waves contribute at least one third of the drift for pollutants floating 1 m below the surface, with a mean direction about 30° to the right of the wind-sea direction. Although not new, this result was previously obtained with specific models, whereas the formalism used here combines classical wave and circulation forecasting models. To cite this article: F. Ardhuin et al., C. R. Geoscience 336 (2004).     

Effect of varied atmospheric stability on sea surface drag in shallow seas and its impact on wind-wave growth

This work reports an efficient bulk formulation of sea surface drag that incorporates effect of dynamic stability under varied atmospheric forcing. The proposed formulation exhibits a polynomial dependence of wind speed on air–sea temperature difference based on statistical analysis. Quality checked meteorological and oceanographic data from four shallow water buoys located off Korean seas having measurements at an interval of every 1 h were used for this study. The analyses of in situ records for this region suggest stability ranging from highly stable to very unstable conditions. Importance of this proposed formulation is better reflected during unstable condition where other popular bulk formulations fail. In addition, importance and impact of such a study on wind-wave growth using the state-of-art wave model was also investigated. Finally, we advocate a new drag formulation, which accounts for varied atmospheric stability and suggest that this should be considered as an essential pre-requisite for ocean modeling studies.     

浅水区浅地层剖面测量典型问题分析

作者在本文论述浅水区域浅地层剖面测量方法的基础上，结合海上及河道采集到的实测剖面资料，对诸如直达波、多次波、侧反射、鸣震等干扰因素的声学特征，以及浅水区的地层厚度校正问题进行了系统的分析，并针对消除和消弱这些干扰的一些方法、措施做了较全面的讨论。     

Observations of wind and waves in the central Bay of Bengal during BOBMEX-99 and their effect on mixed layer depth variability due to forced mixing

Time-series wind and wave measurements were carried out onboard INS Sagardhwani in the central Bay of Bengal during BOBMEX-99. Various other marine meteorological and oceanographic measurements relevant to monsoon studies were also collected simultaneously. The observed variations of wind and waves and the associated mixed layer depth (MLD) variability based on both temperature and density criteria at 3 hourly intervals are presented in this paper as a case study. At the time-series location (13‡N, 87‡E) wind varied between 6 and 16m/s and the predominant direction was southwesterly. The significant wave height and period varied from 1.9 to 3.7m and 8 to 13 s respectively. Some of the available statistical predictive methods for the determination of MLD by forced mixing are utilized to test the extent of mechanical mixing within the top layer of water by the local wind and wave activity. The same is extended to formulate a new empirical relation for gross estimation of effective depth within which the sound energy is generally trapped during its transmission in the surface duct. The present analysis aiming for estimation of observed MLD variability (35 to 75 m) using the suggested simple empirical relation reveals that, the mixed layer variability observed during the experiment depends on both local ocean variability as well as remote forcing as reported earlier.     

Magnetic tracing of sediment dynamics of mudbanks off southwest coast of India

In this study, we utilized environmental magnetic in combination with sedimentological and hydrodynamic data to investigate the formative processes of mudbanks along southwest coast of India. We document the linkages between enrichment of silt-sized magnetic particles and formation processes of mudbanks along Alappuzha coast. A trend of increasing magnetite concentration and coarsening in magnetic grain size is observed at mudbank stations M2 and M3, while the mud-deficient station (M1) showed an opposite trend. A strong relationship between magnetic and physical grain size for all samples implies that the magnetic particle size and clastic grain size are largely adjunct. Analysis of rock magnetic and grain size data of surficial and suspended sediments from non-mudbank (M1) and two mudbank stations (M2, M3) reflect the differential sediment partitioning and transport regimes which controlled the formation of mudbanks along Alappuzha coast. Two plausible mechanisms responsible for the formation of mudbanks are identified: grain size-selective entrainment is the dominant process during pre-monsoon; weaker hydrodynamics (waves and bottom currents) favors accumulation of silt-sized (fine and coarse) magnetic and non-magnetic fractions resulting in the formation of magnetically low-enriched sediment bed of mixed grain sizes. At the onset of monsoon, wave-induced energetic bottom currents enhance the suspension of entire sediment bedload at stations M2, M3 to form fluid mud. Concurrently, mineral-density-based selective fractionation allows the settling of coarse silt-sized magnetic particles, while the fine magnetic silt-size particles accumulate forming thick fluid mud as a suspension load resulting in the formation of mudbanks. An observed increase in magnetic susceptibility and coarsening in magnetic grain size of surficial sediments at mudbank stations (M2, M3) during monsoon period supports the interpretation. Our findings are summarized in a conceptual model which can be very well applied to investigate sediment dynamics associated with mudbank formation in coastal and shelf sedimentary systems.     

Internal wave characteristics in the eastern Arabian Sea during summer monsoon

The time series BT profiles and surface winds and atmospheric pressure, collected in the deep waters off Ratnagiri and Karwar during summer monsoon were utilized to document the characteristics of internal waves (IW). Low-frequency (≤2, cycle per day (cpd)) IW off Ratnagiri are found to propagate at 83 cm/s with wavelengths of 45 km and wave heights upto 40 m. These parameters for high-frequency (>2 cpd) IW off Karwar correspond to 99 cm/s, 3 km and 23 m. The IW off Karwar appear to leave the station at 70° (±10°) (measured from the horizontal). The data sets were further analysed to address the harmonic composition of the IW and identify the possible sources for the observed IW fields. Power spectra of the IW indicated energy peaks at inertial (0·6 cpd) and tidal (1 and 2 cpd) frequencies off Ratnagiri and in the high-frequency band of 0·5–2·0 cycles per hour off Karwar. The coherence between the IW and wind/tide is found to be good at several frequencies within the IW spectrum. This feature probably suggests tides as a source for the IW of tidal frequencies and winds and tides as a joint source for the IW at the remaining frequencies.     

Titan's native ocean revealed beneath some 45 km of ice by a Schumann-like resonance

After five years of thorough analysis of data from the Huygens Probe that descended into Titan's atmosphere in January 2005, we report major findings inferred from measurements of low frequency waves and atmospheric conductivity. The data account for the observation of a Schumann-like resonance trapped within Titan's atmospheric cavity. On Earth, this phenomenon is triggered by lightning and was anticipated to be observed on Titan, as it provides a tool to reveal the presence of a ground conductive boundary to sustain the resonance of the cavity. The Huygens observations show that the major electric field component of the signal is horizontal, which is inconsistent with lightning sources. We interpret, however, the observed signal as a second spherical harmonic of Titan's cavity, triggered and sustained by strong electric currents induced in the ionosphere by Saturn's magnetospheric plasma flow. The present study describes the characteristics of such trapped modes that allow us to constrain the parameters of the cavity and to infer the presence of a conductive layer at 45 km (±15 km) below the surface. By comparison with the presence of subsurface conductive ocean in the Galilean icy satellites, we conclude that Titan should have pursued similar processes of internal dynamics. To date, this result represents the only evidence for a buried ocean in Titan.