Modelling the impact of a time-varying wave angle on the nonlinear evolution of sand bars in the surf zone

Sandy beaches are often characterized by the presence of sand bars, whose characteristics (growth, migration speed, etc.) strongly depend on offshore wave conditions, such as wave height and angle of wave incidence. This study addresses the impact of a sinusoidally time-varying wave angle of incidence with different time-means on the saturation height, migration speed and longshore spacing of sand bars. Model results show that shore-transverse sand bars (so-called TBR bars) eventually develop under a time-varying wave angle. Depending on the time-mean, amplitude and period of the varying angle of wave incidence, the mean heights and mean migration speeds of the bars can be larger or smaller than their corresponding values in the case of time-invariant angles. Bars might not even form when the wave angle varies around a too large oblique mean value, whereas bars exist in the case of a time-invariant wave angle. The oscillations in both bar height and migration speed are large if the period of the time-varying wave angle is close to the adjustment timescale of the system and if large differences in the local growth and migration rates of the bars occur during one oscillation period. The oscillations in bar height are a combination of harmonics with the principal period and half the period of the time-varying wave angle, whereas those of migration speed contain only the principal period. Bars that are subject to time-varying wave angles have larger longshore crest-to-crest spacings than those which form under fixed wave angles. Physical explanations for these findings are given. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Characteristic of the vertical seismic waves associated with the 1995 Hyogo‐ken Nanbu (Kobe), Japan earthquake estimated from the failure of the Daikai Underground Station

The dynamic behaviour of underground structures built by cut‐and‐cover methods is discussed. A simple model analysis shows that a column supporting the overburden at midspan (central column) can resonate upon incidence of an elastic wave of a specific frequency. The analytical results indicate that not only the size and material properties of the column, but also the static load acting on the column (overburden) is a decisive factor that influences the resonant frequency. Based on the results obtained by the analysis, the mechanism of the failure at the Daikai Underground Station in Kobe caused by the 1995 Hyogo‐ken Nanbu, Japan, earthquake is investigated. It is shown that the wave‐induced damage to underground structures can concentrate on the sections with specific overburden, and from the induced damage, it is possible to estimate the frequency characteristics of the associated seismic waves. Copyright © 2000 John Wiley & Sons, Ltd.     

Advanced characterization of marine dissolved organic matter by combining reversed-phase liquid chromatography and FT-ICR-MS

Marine dissolved organic matter (DOM) was separated by reversed-phase (RP) liquid chromatography method and analyzed with fluorescence/absorption detection and Fourier transform ion cyclotron mass spectrometry (FT-ICR-MS). The three key characteristics of the RP method are: (a) The C18-RP column chosen provides enhanced separation when the aqueous phase is 100% buffer-free water, and it does not degrade over time; (b) the water eluent adjusted to pH 7 significantly improves the resolution of water soluble compounds; (c) the initial flow maintained at low levels improves the separation of polar compounds. In samples, containing “fresh” DOM, specific peaks were detected, which were absent in “old” DOM samples. The combination with size exclusion chromatography (SEC) also demonstrated the relation between polarity and molecular size of DOM. FT-ICR-MS was applied to evaluate the quality of separation on a molecular scale demonstrating that physico-chemical characteristics of DOM can be related to molecular formulas. Sample extracts were separated into 4 preparative fractions, and a large suite of the identified molecular formulas only occurred in specific fractions. This is an important basis for the application of further analytical techniques in order to perform a more target-oriented analysis aiming at the determination of source and process biomarkers for DOM.     

Three-dimensional modeling of tidal circulation and mixing over the Java Sea

A combination of a three-dimensional hydrodynamic model and in-situ measurements provides the structures of barotropic tides, tidal circulation and their relationship with turbulent mixing in the Java Sea, which allow us to understand the impact of the tides on material distribution. The model retains high horizontal and vertical resolutions and is forced by the boundary conditions taken from a global model. The measurements are composed of the sea level at coastal stations and currents at moorings embedded in Seawatch buoys, in addition to hydrographic data. The simulated tidal elevations are in good agreement with the data for the K₁ and M₂ constituents. The K₁ tide clearly shows the lowest mode resonance in the Java Sea with intensification around the nodal point in the central region. The M₂ tide is secondary and propagates westward from the eastern open boundary, along with a counterclockwise amphidromic point in the western part. The K₁ tide produces a major component of tidal energy, which flows westward and dissipates through the node region near the Karimata Strait. Meanwhile, the M₂ tide dissipates in the entire Java Sea. However, the residual currents are mainly induced by the M₂ tide, which flows westward following the M₂ tidal wave propagation. The tidal mixing is mainly caused by K₁ tide which peaks at the central region and is consistent with the uniform temperature and salinity along the vertical dimension. This mixing is expected to play an important role in the vertical exchange of nutrients and control of biological productivity.     

石英示踪物源研究进展

综述了石英的性质和它作为有效指示剂来示踪物源的理论基础,进而详细介绍了使用扫描电镜、质谱、电子自旋共振和阴极发光等仪器对石英矿物的表面微结构、氧同位素比值、形成年龄和阴极发光颜色等特征进行研究来追踪物源的方法以及这些方法的可行性。还根据近年来研究者应用石英矿物来示踪物源的研究进展提出了今后利用石英研究物源需要注意的4个问题。     

Construction of invariant tori for the spin-orbit problem in the mercury-sun system

The stability of spin-orbit resonances, namely commensurabilities between the periods of rotation and revolution of an oblate satellite orbiting around a primary body, is investigated using perturbation theory. We reduce the system to a model described by a one-dimensional, time-dependent Hamiltonian function. By means of KAM theory we rigorously construct bidimensional invariant surfaces, which separate the three dimensional phase space. In particular with a suitable choice of the rotation numbers of the invariant tori we are able to trap the periodic orbit associated with a given resonance in a finite region of the phase space. This technique is applied to the Mercury-Sun system. A connection with the probability of capture in a resonance is also provided.     

Observations of volcanic tremor during the January–February 2005 eruption of Mt. Veniaminof,Alaska

Mt. Veniaminof, Alaska Peninsula, is a stratovolcano with a summit ice-filled caldera containing a small intracaldera cone and active vent. From January 2 to February 21, 2005, Mt. Veniaminof erupted. The eruption was characterized by numerous small ash emissions (VEI 0 to 1) and accompanied by low-frequency earthquake activity and volcanic tremor. We have performed spectral analyses of the seismic signals in order to characterize them and to constrain their source. Continuous tremor has durations of minutes to hours with dominant energy in the band 0.5–4.0 Hz, and spectra characterized by narrow peaks either irregularly (non-harmonic tremor) or regularly spaced (harmonic tremor). The spectra of non-harmonic tremor resemble those of low-frequency events recorded simultaneously with surface ash explosions, suggesting that the source mechanisms might be similar or related. We propose that non-harmonic tremor at Mt. Veniaminof results from the coalescence of gas bubbles while low-frequency events are related to the disruption of large gas pockets within the conduit. Harmonic tremor, characterized by regular and quasi-sinusoidal waveforms, has duration of hours. Spectra containing up to five harmonics suggest the presence of a resonating source volume that vibrates in a longitudinal acoustic mode. An interesting feature of harmonic tremor is that frequency is observed to change over time; spectral lines move towards higher or lower values while the harmonic nature of the spectra is maintained. Factors controlling the variable characteristics of harmonic tremor include changes in acoustic velocity at the source and variations of the effective size of the resonator.     

Comparison of a 3‐D DEM simulation with MRI data

This paper presents a comparison of a granular material studied experimentally and numerically. Simple shear tests were performed inside the magnetic core of magnetic resonance imaging (MRI) equipment. Spherical pharmaceutical pills were used as the granular material, with each pill's centre location determined by MRI. These centre locations in the initial assembly were then used as the initial configuration in the numerical simulation using the discrete element method. The contact properties between pharmaceutical pills used in the numerical simulation were obtained experimentally. The numerical predication was compared with experimental data at both macroscopic and microscopic levels. Good agreement was found at both levels. Copyright © 2001 John Wiley & Sons, Ltd.     

沉积物中石英E'心的热力学特性及其地质应用意义

对 8个取自南海珠江口盆地XJ33-2 -1井的样品进行了热力学实验 ,使用电子自旋共振 (ESR)技术获取了样品中石英E′心天然状况下的ESR信号强度I1对经热活化技术处理后石英E′心的ESR信号强度I2 的比值。实验结果表明 :每个样品的活化温度不完全相同 ,这与母岩的特性有关 ;I1/I2 值与古地温呈正相关关系 ;I1/I2 值与地层年龄有很强的相关性 ,相关系数高达0 .99。说明I1/I2 值具有古地温指示和地质计时的实际意义。