Rip currents, mega-cusps, and eroding dunes

Dune erosion is shown to occur at the embayment of beach mega-cusps O(200 m alongshore) that are associated with rip currents. The beach is the narrowest at the embayment of the mega-cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune, to undercut the dune and to cause dune erosion. Field measurements of dune, beach, and rip current morphology are acquired along an 18 km shoreline in southern Monterey Bay, California. This section of the bay consists of a sandy shoreline backed by extensive dunes, rising to heights exceeding 40 m. There is a large increase in wave height going from small wave heights in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over the Monterey Bay submarine canyon results in larger wave heights. The large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale. The strongly refracted waves and narrow bay aperture result in near normal wave incidence, resulting in well-developed, persistent rip currents along the entire shoreline.

The alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95% confidence. The alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95% confidence. Therefore, it is concluded the mega-cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega-cusp.     

Transition of Tidal Waves from the East to South China Seas over the Taiwan Strait: Influence of the Abrupt Step in the Topography

Observations of tidal waves between the East and South China Seas (ECS and SCS) over the Taiwan Strait (TS) suggest that the diurnal tides simply appear as one southward-propagating wave from the ECS to the SCS through the TS. The semidiurnal tides, however, behave differently in that they appear as a southward-propagating Kelvin wave in the western TS and a nearly standing wave in the eastern TS, and then diminish rapidly over the shallow shoal in the southern TS. A smaller-domain model, with sea-level boundary conditions derived from a larger-domain tidal model, was first used to simulate tides in the TS to an overall percentage of accuracy of about 90%. Subsequent numerical experiments and theoretical analysis revealed that the southward-propagating semidiurnal tides to be impeded and then reflected as they arrive at an abrupt, deepened step in the topography of the southern TS. This reflection enhances the amplitudes of the incident semidiurnal tides and contributes to the formation of a nearly standing wave in the eastern TS. The southward-propagating diurnal tides in the TS are connected by the diurnal tides in the northern SCS when the amplitudes of the two tide systems are comparable and their phases nearly equal at the step. This revised version was published online in July 2006 with corrections to the Cover Date.     

Air-Sea Interactions under the Existence of Opposing Swell

Data of a comprehensive laboratory study on the coexistent system of wind waves and opposing swell (Mitsuyasu and Yoshida, 1989) have been reanalyzed to clarify the air-sea interaction phenomena under the coexistence of wind waves and swell. It is shown that the magnitude of the decay rate of swell due to an opposing wind is almost the same as that of the growth rate of swell caused by a following wind, as measured by Mitsuyasu and Honda (1982). The decay rate is much smaller than that obtained recently by Peirson et al. (2003), but the reason for the disagreement is not clear at present. The effect of an opposing swell on wind waves is very different from that of a following swell; wind waves are intensified by an opposing swell while they are attenuated by a following one. The phenomenon contradicts the model of Phillips and Banner (1974), but the reason for this is not clear at this time. The high-frequency spectrum of wind waves shows a small increase of the spectral density. Wind shear stress increases a little due to the effect of opposing swell. The intensification of wind waves by opposing swell and the small increase of the spectral density in a high-frequency region can be attributed to the increase of wind shear stress. Such organized phenomena lead to the conclusion that the hypothesis of local equilibrium for pure wind waves (Toba, 1972) can also be satisfied for wind waves that coexist with opposing swell. The recent finding of Hanson and Phillips (1999) can be explained by this mechanism.     

斜向不规则波入、反射波分离的实验研究

斜向入射的不规则波在海岸结构物前发生反射形成入、反射波共存的波浪场,对这种物理场的入、反射波的分离是海岸工程研究的一个重要课题.改进了一种可用于斜向入射的不规则波分离的两点法,使得测波阵列的两波高仪可布置在结构物前任何方向上.将此方法应用于三维波浪水池的物理模型试验研究,试验了不同的两波高仪的组合、不同的基床高度、不同的入射方向和不同的波浪要素等各种条件对分离结果的影响,结果表明,各种情况在满足非奇异条件下可得到较好的分离结果.该方法计算简便,可以给出较准确的反射系数和入、反射波的频谱.     

Hydrodynamic Coefficients for Grouping Piles Under the Action of Irregular Waves

-The hydrodynamic coefficients for each of two piles and three piles in both side-by-side arrangement and tandem arrangement under the action of irregular waves are experimentally investigated. These coefficients vary with the KC number, the relative pile spacing, the number of piles and the pile location, and their relationships are presented in this paper. They can be used in Morison Equation and other equations to calculate directly the in-line wave forces and the transverse forces on each pile in array.     

非线性随机海浪模型的一种新形式

以反映随机海浪非线性的波面高度分布高阶矩为参量，提出一种新形式的非线性随机海浪模型，在三阶近似下具体导出其波面高度的表达工和推导出二阶谱。本文模式为Ｌｏｎｇｕｅｔ－Ｈｇｇｉｎｓ模式的另一种新的数学表示。     

Wave-height distributions and nonlinear effects

Theoretical distributions proposed for describing the crest-to-trough heights of linear waves are reviewed briefly. To explore the effects of nonlinearities, these are generalized to second-order waves, utilizing quasi-deterministic results on the expected shape of large waves. The efficacy of Gram–Charlier models in describing the effects of third-order nonlinearities on the distributions of wave heights, crests and troughs are examined in detail. All models and a fifth-order Stokes–Rayleigh type model recently proposed are compared with linear and nonlinear waves simulated from the JONSWAP spectrum representative of long-crested extreme seas, and also with oceanic data gathered in the North Sea. Uncertainties arising from the variability of probability estimates derived from sample populations of limited size are considered. Ultimately, the comparisons show that nonlinearities do not have any discernable effect on the crest-to-trough heights of oceanic waves. Most of the linear models considered yield similar and reasonable predictions of the observed data trends. Gram–Charlier type distributions seem neither effective nor particularly useful in describing the statistics of large wave heights or crests under oceanic conditions. However, they do surprisingly well in predicting unusually large wave heights and crests observed in some 2D wave-flume experiments and 3D numerical simulations of long-crested narrow-band random waves.     

Wave Dispersion Study in the Indian Ocean-Tsunami of December 26, 2004

A numerical study which takes into account wave dispersion effects has been carried out in the Indian Ocean to reproduce the initial stage of wave propagation of the tsunami event that occurred on December 26, 2004. Three different numerical models have been used: the nonlinear shallow water (nondispersive), the nonlinear Boussinesq, and the full Navier-Stokes aided by the volume of fluid method to track the free surface. Numerical model results are compared against each other. General features of the wave propagation agreed very well in all numerical studies. However some important differences are observed in the wave patterns, i.e., the development in time of the wave front is shown to be strongly connected to the dispersion effects. Discussions and conclusions are made about the spatial and temporal distribution of the free surface reaffirming that the dispersion mechanism is important for tsunami hazard mitigation.     

The retrieval of the nonlinear random waves from the non-Gaussian characteristics of the sea surface elevation distribution

In this paper, without recourse to the nonlinear dynamical equations of the waves, the nonlinear random waves are retrieved from the non-Gaussian characteristic of the sea surface elevation distribution. The question of coincidence of the nonlinear wave profile, spectrum and its distributions of maximum (or minimum) values of the sea surface elevation with results derived from some existing nonlinear theories is expounded under the narrow-band spectrum condition. Taking the shoaling sea wave as an example, the nonlinear random wave process and its spectrum in shallow water are retrieved from both the non-Gaussian characteristics of the sea surface elevation distribution in shallow water and the normal sea waves in deep water and compared with the values actually measured. Results show that they can coincide with the actually measured values quite well, thus, this can confirm that the method proposed in this paper is feasible.     

北部湾沿岸海区设计波浪的确定

本文基于北部湾沿岸海区多年的实测资料，于不同海区主浪向和次主浪向分别取年极值波高为样本资料．采用Ｐ－Ⅲ型和短期测波资料频率分析方法，分别估算了各海区不同重现期的设计波高，并依据式（１）计算出相应的平均周期。尽管样本资料和估算方法不变，但每次估算的结果互不相同，其差异甚至很大．利用Ｋｏｌｍｏｇｏｒｏｆｆ等适合度方法检验所得的结果，能够准确地判断出沿岸各海区不同重现期的最佳设计波浪。