Seasonal variations of wind and waves over Taiwan waters

We here investigate the frequency and intensity of oscillations in oceanographic data within intraseasonal time scales using spectral analysis of surface wind and wave time-series data collected at off-island weather stations or moored buoys around Taiwan. Data from marine weather stations were used to trace atmospheric conditions, while we used buoy data to examine sea states. The spectra and wavelet scalogram of the wind fields revealed oscillations with a period of around 20–33 days, and the energy density of the wind field at the off-island stations was stronger than that at the data buoy stations. However, the wavelet scalogram of the wave height measured at the buoy stations was stronger than its associated wind field. This long-period oscillation is consistent with the wavelet scalogram of the wind field calculated from the off-island weather stations. About 20–33 day oscillations exist within intraseasonal variations, which are closely linked to the atmospheric environment and to wind and ocean wave fields. Oscillations with a period of 5–10 days are a pronounced feature over northeastern Taiwan waters during the winter season and can be interpreted as the wave pattern following synoptic weather systems.     

Numerical investigation of tsunami-like wave hydrodynamic characteristics and its comparison with solitary wave

Solitary waves have been commonly used as an initial condition in the experimental and numerical modelling of tsunamis for decades. However, the main component of a tsunami waves acts at completely different spatial and temporal scales than solitary waves. Thus, use of solitary waves as approximation of a tsunami wave may not yield realistic model results, especially in the coastal region where the shoaling effect restrains the development of the tsunami wave. Alternatively, N-shaped waves may be used to give a more realistic approximation of the tsunami wave profile. Based on the superposition of the sech²(*) waves, the observed tsunami wave profile could be approximated with the N-shaped wave method, and this paper presents numerical simulation results based on the tsunami-like wave generated based on the observed tsunami wave profile measured in the Tohoku tsunami. This tsunami-like wave was numerically generated with an internal wave source method based on the two-phase incompressible flow model with a Volume of Fluid (VOF) method to capture the free surface, and a finite volume scheme was used to solve all the governing equations. The model is first validated for the case of a solitary wave propagating within a straight channel, by comparing its analytical solutions to model results. Further, model comparisons between the solitary and tsunami-like wave are then made for (a) the simulation of wave run-up on shore and (b) wave transport over breakwater. Comparisons show that use of these largely different waveform shapes as inputs produces significant differences in overall wave evolution, hydrodynamic load characteristics as well as velocity and vortex fields. Further, it was found that the solitary wave uses underestimated the total energy and hence underestimated the run-up distance.     

Sediment grains moved by passing tsunami waves: Tsunami deposits in deep water

Tsunamis propagating in the open ocean have associated horizontal particle velocities that do not change with depth — yet the limiting water depth where a tsunami of given characteristics will initiate sediment motion remains unknown. Based upon linear wave theory and a parametrization of the Shields curve, equations are derived and solved, using an iterative scheme, to address the topic of grain movement by tsunami waves as a function of water depth and wave amplitude. The focus is on waves in deep water where tsunami waves behave linearly and on non-cohesive sediment grains. Furthermore, the question is addressed of which grain sizes are picked up on a sloping beach as the wave shoals. According to the results, even the Boxing Day tsunami in 2004 was incapable of moving fine sand in water deeper than 985 m in the Bay of Bengal and 335 m in the Indian and Pacific oceans. The results suggest that tectonic tsunamis of size equal to or smaller than the Boxing Day tsunami cannot initiate motion of deep-water cohesionless sediments that can be correlated on an oceanic basin-wide scale.     

波浪动力因素变化对沙质岸滩演变的影响

通过波浪水槽实验,对海平面变化造成的波浪动力因素改变引起的沙质岸滩形态响应开展机理性研究。实验采用1∶10单一沙质斜坡概化岸滩,利用3种不同实验水深模拟海平面变化,考虑椭圆余弦波、非规则波、规则波和孤立波4种类型波浪作用。实验对波浪在斜坡上的传播变形、破碎、上爬和回落过程的波高及波浪作用后的岸滩地形进行了测量。实验结果表明,椭圆余弦波、规则波和非规则波作用下,平衡岸滩呈现出滩肩形态,孤立波作用下则呈沙坝形态。海平面上升造成波浪动力增强,沙质岸滩平衡剖面形状基本保持不变向岸平移,槽谷、滩肩、沙坝位置以及岸线蚀退距离,均呈现出良好规律性。     

植被斜坡岸滩海啸波消减数值模拟研究

An explicit one-dimensional model based on the shallow water equations(SWEs) was established in this work to simulate tsunami wave propagation on a vegetated beach. This model adopted the finite-volume method(FVM)for maintaining the mass balance of these equations. The resistance force caused by vegetation was taken into account as a source term in the momentum equation. The Harten–Lax–van Leer(HLL) approximate Riemann solver was applied to evaluate the interface fluxes for tracing the wet/dry transition boundary. This proposed model was used to simulate solitary wave run-up and long-periodic wave propagation on a sloping beach. The calibration process suitably compared the calculated results with the measured data. The tsunami waves were also simulated to discuss the water depth, tsunami force, as well as the current speed in absence of and in presence of forest domain. The results indicated that forest growth at the beach reduced wave energy loss caused by tsunamis. A series of sensitivity analyses were conducted with respect to variable parameters(such as vegetation densities, wave heights, wave periods, bed resistance, and beach slopes) to identify important influences on mitigating tsunami damage on coastal forest beach.     

2017年9月8日墨西哥沿岸Mw8.2级地震海啸观测数据分析与模拟

2017年9月8日4时49分（UTC），墨西哥瓦哈卡州沿岸海域（15.21°N，93.64°W）发生M_w8.2级地震，震源深度30 km。强震在该海域引发海啸，海啸对震源附近数百千米范围内造成了严重影响。位于太平洋上的多个海啸监测网络捕捉到了海啸信号并详细记录了此次海啸的传播过程。本文选用了近场2个DART浮标和6个验潮站的水位数据，通过潮汐调和分析和滤波分离出海啸信号，对近场海啸特征值进行了统计分析，并采用小波变换分析方法进一步分析了海啸的波频特征。基于Okada弹性位错理论断层模型计算得到了强震引发的海底形变分布，并采用MOST海啸模式对本次海啸事件近场传播特征进行了模拟，模拟结果与观测吻合较好。最后，基于实测和模拟结果，详细分析了此次地震海啸的近场分布特征，发现除受海啸源的强度和几何分布特征影响外，近岸海啸波还主要受地形特征控制，在与特定地形相互作用后波幅产生放大效应，会进一步加剧海啸造成的灾害。     

Hindcasting wave spectra in finite‐depth waters

A model is described for hindcasting or forecasting waves in finite‐depth waters. The model is particularly applicable to coastal sites where the water is depth‐limited. The wave energy density spectrum is modelled in the frequency‐directional domain. For each spectral component a ray is defined along which wave energy propagates to reach the site. For sites exposed to the open ocean a background spectral wave model is required to provide input to the ray endpoints. Further growth and dissipation is then effected along the rays according to the local wind and water depth. The model was used to hindcast wave spectra over a period of 9 months for a site in the Canterbury Bight, New Zealand. The results were compared with measurements from a Waveridcr buoy at the site. The model succeeds in explaining about 40% of the variance in measured significant wave heights. However, the present application is handicapped by errors inherent in the background spectral model and in specifying the local wind.     

基于射线理论的海脊俘获波机制

虽然众多现场实测资料和数值模拟均表明海脊可以俘获海啸波并引导其传播至远场地区，带来严重的灾害，但少有研究阐述其产生机理。本文基于射线理论，推导了指数型海脊上波浪传播轨迹的理论解，并提出了震源位于海脊顶部的海啸被海脊完全俘获的条件。基于该俘获条件进一步给出了海脊对海啸俘获效率的表达式，用以评估海啸中被海脊俘获影响至远场的能量占海啸总能量的比例。     

自然海况下波浪特性的初步研究

通过分析大量的外海浮标观测资料 ,发现波龄和无因次波高之间存在非常好的相关性 ,自然海况下的波浪场满足 3/ 5指数律 ,其波龄可达到几十 ,远远超过风浪波龄的上限 1 .4,说明波浪组成波之间波 -波共振非线性相互作用是波浪内部结构的主要调节机制 ,使波高和周期之间具有很好的相关性。     

越洋海啸的数值模拟及其对我国的影响分析

简要介绍了地震海啸产生的物理机制、海啸波在大洋中的传播特性以及海啸所具有的超强破坏力可能引发的巨大灾害;概述了全球地震海啸发生的频率和太平洋区域历史海啸的时空分布;整理分析了我国沿海发生海啸的频次和空间分布。针对越洋海啸传播的特点,采用基于波浪追逐原理和自适应网格加密技术的海啸数值模型对1960智利海啸进行了数值模拟,将模拟的结果与历史记录进行了对比,验证了模型的可靠性。通过对数值模拟结果的分析,初步讨论了我国沿海地区越洋海啸的危险性,并定量阐述了越洋海啸对我国各海区的影响。     

基于谱分析的GPS浮标测量参数提取方法

介绍采用谱分析原理提取海浪信息的方法。根据GPS浮标测量得到的海浪高度数据,先通过滑动平均的方法分离海浪信息和潮流潮位信息,然后对海浪信息采用AR模型法进行功率谱估计,并对海浪信息进行连续小波变换,对AR模型法功率谱估计结果和小波变换结果进行综合研究,提取海浪特征参数———周期。最后通过一个GPS浮标试验对上述方法进行了验证。     

冰区四季通用灯浮标的二阶波浪力计算

冰区四季通用灯浮标是一种为满足北方冬季冰冻港口一年四季的助航服务需求研制的新型灯浮标,二阶波浪力对其漂浮姿态和漂移运动有较大影响。文中研究了浮标受到的二阶波浪力的数值计算方法,计算了不同流速下罐形和锥形灯浮标的二阶波浪力。研究结果显示,罐形和锥形灯浮标受到的一阶波浪力相差不大,罐形的二阶波浪力明显小于锥形,具有一定的外形优势。     

Study of the Characteristics of Wave Buoys by Comparison Measurements

A method is proposed for estimating the accuracy of measuring the wave parameters by wave buoys. The results of comparative field tests of the developed Storm and Waverider wave buoys and a string wave probe are given. It is shown that the standard deviation of the error of the Storm buoy does not exceed 0.1 m for a sea state of 5. The research has made it possible to improve the methods for obtaining the spectral and statistical characteristics of sea waves of different intensity.     

基于浮标站和ERA5 再分析资料的浙江沿海台风浪特征分析

为了分析台风影响下浙江沿海风和浪的演变特点，利用浙江省海洋浮标站监测数据和欧洲中期天气预报中心第五代全球气候大气再分析数据（European Centre for Medium-Range Weather Forecasts Reanalysis v5，ERA5），选取2010年以来严重影响浙江的7次台风个例，对台风作用下浙江沿海海面风和浪的演变特点进行分析。结果表明：在台风影响过程中，海浪波型多数呈现混合浪-风浪-混合浪的演变规律；涌浪波型的出现与台风强度及其与浮标站的距离和方位有关，也与海洋潮汐现象紧密相关。台风影响期间，浙江沿海浪高的变化受风速和风向共同作用影响。在风向不变的情况下，持续风速增大对浪高的增大有明显作用；风向的变化也会对浪高变化产生影响，向岸风和离岸风的转变会造成浪高出现剧烈变化。ERA5 再分析资料有效波高在台风浪较大时会呈现偏小的趋势，分析订正后的ERA5 有效波高发现，台风浪有效波高大值区与台风中心位置相关。研究结果可为严重影响浙江沿海的台风浪预报服务提供参考。     

New buoy observation system for tsunami and crustal deformation

We have developed a new system for real-time observation of tsunamis and crustal deformation using a seafloor pressure sensor, an array of seafloor transponders and a Precise Point Positioning (PPP ) system on a buoy. The seafloor pressure sensor and the PPP system detect tsunamis, and the pressure sensor and the transponder array measure crustal deformation. The system is designed to be capable of detecting tsunami and vertical crustal deformation of ±8 m with a resolution of less than 5 mm. A noteworthy innovation in our system is its resistance to disturbance by strong ocean currents. Seismogenic zones near Japan lie in areas of strong currents like the Kuroshio, which reaches speeds of approximately 5.5 kt (2.8 m/s) around the Nankai Trough. Our techniques include slack mooring and new acoustic transmission methods using double pulses for sending tsunami data. The slack ratio can be specified for the environment of the deployment location. We can adjust slack ratios, rope lengths, anchor weights and buoy sizes to control the ability of the buoy system to maintain freeboard. The measured pressure data is converted to time difference of a double pulse and this simple method is effective to save battery to transmit data. The time difference of the double pulse has error due to move of the buoy and fluctuation of the seawater environment. We set a wire-end station 1,000 m beneath the buoy to minimize the error. The crustal deformation data is measured by acoustic ranging between the buoy and six transponders on the seafloor. All pressure and crustal deformation data are sent to land station in real-time using iridium communication.     

NWF: Propagation of Tsunami and its Interaction with Continental Shelf and Vertical Wall

In this article, tsunamis represented as solitary waves was simulated using the fully nonlinear free surface waves based on Finite Element method developed by Sriram et al. (2006). The split up of solitary wave while it propagates over the uneven bottom topography is successfully established. Wave transmission and reflection over a vertical step introduced in the bottom topography is in good agreement with the experimental results from Seabra-Santos et al. (1987). The wave transformation over a continental shelf with different smooth slopes reveals that the solitary wave reflection increases while the continental slope varies from flat to steep. The interaction of the solitary wave with a vertical wall for different wave steepness has been analysed. The reflected shape of the profile is in good agreement with the observation made by Fenton and Rienecker (1982) and an increase in wave celerity is observed.     

Physical modelling of tsunami using a new pneumatic wave generator

The physical simulation of tsunami in the laboratory has taken a major leap forward with the construction and testing of a new wave generator, capable of recreating scaled tsunami waves. Numerical tools fail to reproduce tsunami nearshore and onshore processes well, and physical experiments in large scale hydraulic facilities worldwide have been limited to the generation of solitary waves as an (controversial) approximation for evolved forms of tsunami. The new concept in wave generation presented herein is born of collaboration between UCL's Earthquake and People Interaction Centre (EPICentre) and HR Wallingford. It allows for the first time the stable simulation of extremely long waves led either by a crest or a trough (depressed wave). This paper presents the working concepts behind the new wave generator and the first stages of testing for verifying its capacities and limitations. It is shown that the new wave generator can not only reproduce solitary waves and N-waves with large wavelengths, but also the 2004 Indian Ocean Tsunami as recorded off the coast of Thailand (“Mercator” trace).     

Down-scaled regional ocean modeling system (ROMS) for high-resolution coastal hydrodynamics in Korea

A down-scaled operational oceanographic system is developed for the coastal waters of Korea using a regional ocean modeling system(ROMS).The operational oceanographic modeling system consists of atmospheric and hydrodynamic models.The hydrodynamic model,ROMS,is coupled with wave,sediment transport,and water quality modules.The system forecasts the predicted results twice a day on a 72 h basis,including sea surface elevation,currents,temperature,salinity,storm surge height,and wave information for the coastal waters of Korea.The predicted results are exported to the web-GIS-based coastal information system for real-time dissemination to the public and validation with real-time monitoring data using visualization technologies.The ROMS is two-way coupled with a simulating waves nearshore model,SWAN,for the hydrodynamics and waves,nested with the meteorological model,WRF,for the atmospheric surface forcing,and externally nested with the eutrophication model,CE-QUAL-ICM,for the water quality.The operational model,ROMS,was calibrated with the tidal surface observed with a tide-gage and verified with current data observed by bottom-mounted ADCP or AWAC near the coastal waters of Korea.To validate the predicted results,we used real-time monitoring data derived from remote buoy system,HF-radar,and geostationary ocean color imager(GOCI).This down-scaled operational coastal forecasting system will be used as a part of the Korea operational oceanographic system(KOOS) with other operational oceanographic systems.     

浙江省海洋水质监测浮标设计与实现

作为一种高技术监测手段,海洋浮标可实现现场自动监测而不受恶劣海况的影响,近年来得到了广泛的重视和快速发展。文中介绍了浙江近岸海域海洋水质监测浮标的结构设计和自动监测实现。浮标设计为生态浮标、海滨浮标和专项浮标3种类型,由浮体、标架、供电设备、防护设备、锚系、传感器、数据采集传输、电子/电池舱、岸站接收系统等9个部分组成。浮体设计为圆饼形,浮体下方正中安装稳定锤以保证具备足够的稳定性。供电设备由太阳能电池板、大容量蓄电池组成,可保障海上30 d连续阴雨天气、恶劣海况下的不间断供电。浮标经喷防污漆、牺牲阳极保护、裹铜皮、加过滤网和人工清除等5种方法防腐、防污处理后投放入海,采用常规维护、应急维护和年度维护等3种方式保障浮标系统运行稳定。投放后可实现对标浮所在海域水质、气象、生态等参数的连续、实时、自动监测。最后对浮体直径设计和浮标投放点选址提出了建议和要求。该设计可为其它类似区域的水质监控提供良好的参考。