Wave-induced seabed response in a cross-anisotropic seabed in front of a breakwater: An analytical solution

To simplify the complicated mathematical process, most previous investigations for the water waves-seabed interaction problem have assumed a porous seabed with isotropic soil behavior, even though strong evidence of anisotropic soil behavior has been reported in soil-mechanics literature. This paper proposes an analytical solution of the short-crested wave-induced soil response in a cross-anisotropic seabed. As shown in the numerical results presented, the wave-induced seabed response, including pore pressure, effective stresses and soil displacements, is affected significantly by the cross-anisotropic elastic constants. A parametric study is performed to clarify the relative differences in pore pressure between isotropic and cross-anisotropic solutions.     

Reconstruction of the parameters of plane-layered elastic media by generalized ray theory

This paper seeks to reconstruct the parameters of elastic layered media such as P-wave velocity, S-wave velocity, density and thickness from the multioffset seismic reflection data. Since the data are highly non-linear to the low-wavenumber components, the non-linear waveform inversion method, with the aid of generalized ray theory, is proposed to solve this problem in space–time domain. As opposed to the layer-stripping method, the present method attempts to invert all layer parameters simultaneously, thus reducing the cumulative errors resulting from the upper layers. The parameters are inverted by minimizing the weighted square error between the observed data and the calculated data of the layered model, the optimization of which is based on the quasi-Newton method. In synthetic tests, we find that the inverted results are good when the variation of parameters between layers is not too large. The modified method for large variation of parameters is first to fix those of deeper layers and neglect the signals reflected from them, then recover some other parameters simultaneously until those of upper layers attain a stable value, and finally, invert all parameters simultaneously. The results so obtained show a significant improvement. This method was tested to be stable in the presence of noise in seismograms.     

Wave-induced response of seabed: Various formulations and their applicability

In this study, a set of generalized analytical solutions are developed for the wave-induced response of a saturated porous seabed under plane strain condition. When considering the water waves originating in deep water and travelling towards the shore, their velocities, lengths and heights vary. Depending on the characteristics of the wave and the properties of the seabed, different formulations (fully dynamic, partly dynamic, quasi-static) for the wave-induced response of the seabed are possible. The solutions for the response with these formulations are established in terms of non-dimensional parameters. The results are presented in terms of pore pressure, shear stress and vertical effective stress distributions within the seabed. For typical values of wave period and seabed permeability, the regions of applicability of the three formulations are identified and plotted in parametric spaces. With given wave and seabed characteristics, these regions provide quick identification of the appropriate formulation for an adequate evaluation of the wave-induced seabed response.     

Lead in the southern East China Sea

At present, in most oceans the lead (Pb) biogeochemical cycling has been disturbed by anthropogenic Pb through atmospheric input. The Pb concentrations in the upper water positively correlate with atmospheric input fluxes of Pb. The North Pacific is affected greatly by atmospheric substances via long-range transport from eastern Asia, especially from Mainland China. Mainland China may export considerable amounts of pollutants into the seas via rivers and the atmosphere owing to its recent fast growth in industry and economy. The East China Sea lies in an important geographical position--a transit between Mainland China and the western North Pacific. However, no data are available for seawater concentrations of Pb, a representative element with anthropogenic origin. In this work seawater samples from both 5 and 30-50 m water layers of 15 stations occupied over a cyclonic eddy in the southern East China Sea were analyzed for particulate Pb (PPb) and dissolved Pb (DPb). The Mean concentration of DPb (approximately 128 ng/l) in the southern East China Sea upper waters (< or = 50 m) is approximately several times higher than those in the Pacific; the high DPb concentrations in the southern East China Sea waters correspond to much higher atmospheric supplies of Pb to the East China Sea. Thus, this study partly fills the 'data gap' of the marginal seas. Also, it indicates that the East China Sea may be considerably contaminated by deposited polluted aerosols. Spatial distributions of DPb in the surface water show a tendency of increasing concentrations with distance offshore, that depends on the magnitudes of atmospheric Pb inputs and on particle scavenging processes. In contrast to DPb, spatial distributions of PPb basically display an 'omega'-like picture and a tendency of decreasing concentrations with distance offshore. These are related to riverine and scavenging sources and to the drive by the eddy. Additionally, the residence times of DPb in the surface water were estimated to be about 2 years, agreeing well with the reported data.     

Wave kinematics of partial reflection from a vertical wall

A third-order perturbation approximation for the partial reflection from a vertical wall is presented in this paper. The wave parameters are expressed in terms of the amplitude of incident waves. The reflection coefficient is defined as the ratio of the height of reflected waves to incident waves. The numerical results demonstrate the significant influences of reflected coefficient on the wave profile and wave frequency bifurcation. For example, the critical angle of wave frequency bifurcation with partial reflection is about 7.5 degrees, not 21 degrees as reported previously for fully reflection.     

Wave dispersion equation in a porous seabed

The wave dispersion equation has played a very important role in the development of ocean surface wave theories. The evaluation of the length of a water wave is an essential example of solving the dispersion relation. Conventional ocean wave theories have been based on an assumption of a rigid impermeable seabed. Thus, the conventional wave dispersion equation can only be used in the case of a wave propagating over a rigid impermeable seabed. For waves propagating over a porous seabed (such as a sandy bed), the conventional dispersion relation is no longer valid because of the absence of the characteristics of the porous seabed. The objective of this study is to establish a new wave dispersion equation for waves propagating over a porous seabed. Based on the new relation, the effects of a porous seabed on wave characteristics (such as the wavelength and wave profile) are discussed in detail.     

Higher plant n-alkane average chain length as an indicator of petrogenic hydrocarbon contamination in marine sediments

The n-alkane average chain length (ACL) is the weight-averaged number of carbon atoms of the higher plant C₂₅–C₃₃ n-alkanes. The abundance of individual n-alkanes from higher plant sources generally increases with increasing carbon number in coastal marine sediments around Taiwan, but this trend is reversed for petrogenic hydrocarbons. The ACL would potentially be lowered if petrogenic hydrocarbons were added to sediments containing biogenic hydrocarbons alone. To test this idea, a marine environment off southwestern Taiwan known to contain both biogenic and petrogenic hydrocarbons and two nearby rivers were selected for investigating possible difference in ACL values between their sediments. The average CPI of C₂₅–C₃₃ n-alkanes was 4.08 ± 2.04 (range 1.90–8.96, n = 15) for the river sediments and 1.70 ± 0.16 (range 1.43–1.97, n = 15) for the marine sediments. The ACL of C₂₅–C₃₃ n-alkanes for river sediments ranged from 29.2 to 30.5 (average 29.9 ± 0.4), and for marine sediments from 28.4 to 29.3 (average 28.9 ± 0.3). The ACL difference between marine and river sediments was significant (Student's t test at 99% confidence) although it appeared small. It is suggested that the ACL can be an additional indicator for detection of petrogenic hydrocarbons in coastal marine sediments.     

Experimental study of vortex-induced vibrations of a pipeline near an erodible sandy seabed

Based on similarity analyses, a series of experiments have been conducted with a newly established hydro-elastic facility to investigate the transverse vortex-induced vibrations (VIVs) of a submarine pipeline near an erodible sandy seabed under the influence of ocean currents. Typical characteristics of coupling processes between pipe vibration and soil scour in the currents have been summarized for Case I: pipe is laid above seabed and Case II: pipe is partially embedded in seabed on the basis of the experimental observations. Pipe vibration and the corresponding local scour are usually two coupled physical processes leading to an equilibrium state. The influence of initial gap-to-diameter ratio (e₀/D) on the interaction between pipe vibration and local scour has been studied. Experimental results show that the critical values of V_r for the initiation of VIVs of the pipe near an erodible sand bed get bigger with decreasing initial gap-to-diameter ratio within the examined range of e₀/D (−0.25<e₀/D<0.75). The comparison of the pipe vibrations near an erodible soil with those near a rigid boundary and under wall-free conditions indicates that the vibration amplitudes of the pipe near an erodible sand bed are close to the curve fit under wall-free conditions; nevertheless, for the same stability parameter, the maximum amplitudes for the VIV coupled with local scour increase with the increase of initial gap-to-diameter ratio.     

An experimental study on the flow characteristics of a standing wave: application of FLDV measurements

The determination of the characteristics of a standing wave boundary layer, such as water particle velocities and shear stresses, is a vital issue in the prediction of sediment transport rate. Thus, an accurate measure of boundary layer characteristics is a key factor in the study of the wave boundary layer. In this study, a fiber-optic laser Doppler velocimeter (FLDV) is applied to directly measure the velocity profile in the standing wave boundary layer. As the experimental data presented in this paper, the antinode points of standing waves are found to move in the temporal domain. The previous second-order solution for a standing wave boundary layer is insufficient for the prediction of larger Ursell numbers.