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The Australian Forum for Operational Oceanography (F00), started in 2015, established a working group to focus on wind-waves. One of the aims of this working group was to identify the [...]

During 2006-2007, the KwaZulu-Natal coast of South Africa was exposed to several large swell events ([H.sub.o] > 3 m), near the peak of the lunar nodal cycle, causing shoreline recession. The largest swell ([H.sub.s] = 8.5 m) struck the coast on the March equinox (18th-20th) and generated a strong storm-return flow. Observations made before, during and after record dramatic coastal erosion (shoreline recession of up to 40 m and substantial property damage). This swell event removed the semi-continuous nearshore bar system and 'conditioned' the coast such that lesser subsequent swell events accomplished much greater amounts of coastal erosion than expected (up to 100 m at certain erosion hotspots) because waves reached the coast without significant energy dissipation. Subsequent bar generation rebuilt the inshore bars within six months. The styles of erosion during the March '07 event and other 2007 swells were markedly different. Lesser swells are focused by headlands and result in megarip development and activation of erosion hotspots. The March '07 event still-water level was raised (equinoctial spring high tide and a storm surge of 0.33-0.45 m) to a level that rendered most headlands (and erosion hotspots) ineffective and resulted in laterally extensive erosion of soft shorelines. Results record cumulative effects of successive swell events on coastal behaviour that proved to be critical in enabling erosion to proceed at rapid rates after the coast had been initially destabilized. Unlike hurricanes and tsunamis, surges associated with swell events are relatively minor and therefore extensive erosion is linked with high lunar tides. There is circumstantial evidence that swell-induced erosion follows the broad 18.6 yr lunar nodal tidal cycle when the chances of large swells coinciding with high water levels are increased. Keywords: coastal erosion, megarip, high swell, disequilibrium coastline, storm-return flow. doi: 10.1017/S0016756810000361

In the present work, we study the capabilities of a one-side segmented wavemaker to generate multidirectional sea states. We want to reproduce typical West Africa sea conditions in a rectangular basin, that is, two spectra representing swell and wind waves with [+ or -] 45 deg main directions relative to the basin axis. For that purpose, we study two generation techniques proposed, respectively, by Dalrymple and Molin which rely on the control of sidewall reflections in order to enlarge the testing zone. Both numerical and experimental results show that the use of such methods in combination with a one-side wavemaker provides high quality wave fields as well as large extent testing area. The Molin method appears to be more efficient in terms of wave field quality within the testing area although the tuning of its setting parameters requires more care than the Dalrymple method. [DOI: 10.1115/1.2904953]

The motion response of an FPSO is sensitive to the relative intensities and directions of the wind-sea and swell components in a sea state, and the operability of the FPSO is a function of the long-term variation in these components. Estimations of the operability are therefore dependent both on how the sea state is described in terms of its constituent wind-sea and swell components, and on how the long-term variability of the sea state is captured. However, there is currently no consensus on how either the sea state or its long-term variability should be described. We investigate these issues by means of a study of the responses of a typical FPSO to the wave fields at a location offshore Namibia and a location off the west coast of New Zealand. We make use of a state-of-the-art program for splitting a directional wave spectrum into wind-sea and swell components, and we examine the effect on the motion responses of allowing the spectra to be split into many swell partitions or constraining the spectral split to a maximum of two partitions, as is often assumed in response calculations. The resulting decompositions are used to examine the effects of swell on hull motions and, hence, to identify methods for generating sea state criteria for operability. In addition, one-year metocean conditions are estimated; these are relevant for analysis of the limits on operations. Keywords: waves, swell, wind-sea, power spectral density, FPSO, motion response

The probability that a wave crest in a random sea will exceed a specified height has long been recognized as important statistics in practical work, e.g., in predicting green water load and volume on a ship. Nonlinear probability density functions for predicting green water load and volume are presented. The models are based on the parametric model of Ogawa (2003, 'Long-Term Prediction Method for the Green Water Load and Volume for an Assessment of the Load Line,' J. Marine Sci. Technol., 7, pp. 137-144) combined with transformation of a second order wave crest height model. The wave crest height model is obtained from second order wave theory for a narrow-banded sea state in combination with transformation of the Rayleigh distribution. Results from the second order models are compared with model tests of a cargo ship presented in Ogawa (2003, 'Long-Term Prediction Method for the Green Water Load and Volume for an Assessment of the Load Line,' J. Marine Sci. Technol., 7, pp. 137-144) and the Ogawa models.

Methods for separating the spectral components and describing bimodal wave spectra are evaluated with reference to wave spectra from directional wave measurements made at the Maui location off the west coast of New Zealand. Two methods involve partitioning bimodal wave spectra into wind-sea and swell components and then fitting a spectral function to each component, while the third assigns an average spectral shape based on the integrated spectral parameters. The partitioning methods involve separating the wave spectrum into two frequency bands: a low-frequency peak, the swell component, and a high-frequency peak, the wind-sea. One partitioning method uses only the frequency spectrum while the other analyzes the complete frequency-direction spectrum. Comparison of the spectral descriptions and derived parameters against the measured counterparts provides insight into the accuracy of the different approaches to describing actual bimodal sea states.

The spatial spectrum of short sea waves is locally modulated by the presence of longer waves or currents; in the remote sensing literature, this process is described by the hydrodynamic modulation transfer function (HMTF). Such modulations are important in understanding radar images of sea waves with water wavelengths longer than the radar range resolution. Existing models for the HMTF utilized in remote sensing are based on approximations derived from consideration of conservation of wave action. However, the accuracy of these approximations has been quantified only through comparison with experimental data; in such comparisons, numerous empirical models for terms such as wind forcing and breaking wave dissipation are required, which make direct evaluation of the hydrodynamic effects difficult. A method for providing direct insight into the hydrodynamic modulation of short sea waves by longer waves is described in this paper, through use of numerical nonlinear hydrodynamic codes for sea surface evolution. The codes applied are reviewed, and a Monte Carlo simulation process based on a stochastic spectrum of short waves propagating over a single deterministic long wave is described, including the data analysis techniques developed to extract a numerical HMTF from the simulated surfaces. HMTF values obtained from the simulations are compared with those from a first-order wave action solution and are found to be in reasonable agreement, although differences on the order of 10% are observed. A numerical evaluation of long wave effects on the short wave dispersion relation is also provided. Index Terms--Hydrodynamics, modulation transfer function.

Diffraction of both regular and irregular waves by a concrete gravity substructure (CGS) was investigated using experimental surface elevation data and computational results of the linear diffraction code DELFRAC. The influence of the box-shaped base that supports the four vertical columns was studied independently from the columns, using data from regular wave model tests of the Malampaya CGS. OELFRAC was shown to give accurate results for the focusing of waves over the submerged structure. Results from regular wave data analysis of model tests of the complete Sakhalin II project Lunskoye CGS were compared to the predictions by the linear diffraction code. For the wave cases tested, the first-order amplitudes were accurately predicted. Diffraction of irregular waves at the Lunskoye CGS was studied in a similar way and linear diffraction theory for random seas gave an excellent prediction of incident wave spectral diffraction, including the peaks in the diffracted spectrum near twice the peak frequency in the input spectrum. The results obtained for the Lunksoye CGS in the present study were consistent with results found in similar studies on less complex structures. An attempt to predict the extreme crest heights from the diffracted spectrum was made using a Weibull distribution, and a second-order expansion of the sea surface that captures the effects of wave steepness, water depth, and directional spreading with no other approximation than the truncation of the expansion at second order. Depth induced breaking appeared to be an important phenomenon limiting the crest heights. The crest heights in a 100-year sea state at the Lunskoye CGS were accurately predicted. [DOI: 10.1115/1.2199562]

The paper presents an analysis of structural design wave loads on an FPSO. The vertical bending moment at midship induced by rogue waves are compared with rule values. The loads induced by deterministic rogue waves were both measured in a seakeeping tank and calculated by an advanced time domain method. Two procedures are used to calculate the expected extreme vertical bending moment during the operational lifetime of the ship. The first one relies on a standard linear long term prediction method, which results from the summation of short term distribution of maxima weighted by their probability of occurrence. The short term stationary seastates are represented by energy spectra and the ship responses by linear transfer functions. The second one is a generalization of the former and it accounts for the nonlinearity of the vertical bending moment, by using nonlinear transfer functions of the bending moment sagging peaks which depend of the wave height. [DOI: 10.1115/1.2166656]

Dynamics of a large moored floating body in ocean waves involves frequency dependent added mass and radiation damping as well as the linear and nonlinear mooring line characteristics. Usually, the added mass and radiation damping matrices can be estimated either by potential theory-based calculations or by experiments. The nonlinear mooring line properties are usually quantified by experimental methods. In this paper, we attempt to use a nonlinear system identification approach, specifically the reverse multiple input-single output (R-MISO) method, to coupled surge-pitch response (two-degrees-of-freedom) of a large floating system in random ocean waves with linear and cubic nonlinear mooring line stiffnesses. The system mass matrix has both frequency independent and frequency dependent components whereas its damping matrix has only frequency dependent components. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known that can either be calculated or obtained from experiments. For numerical illustration, a floating half-spheroid is adopted. The motion as well as the loading are simulated assuming Pierson-Moskowitz (PM) spectrum and these results have been analyzed by the R-MISO method yielding frequency dependent coupled added mass and radiation damping coefficients, as well as linear and nonlinear stiffness coefficients of mooring lines satisfactorily. [DOI: 10.1115/1.2199557] Keywords: added mass, damping, hydrodynamics, mooring, nonlinear, system identification, random, reverse multiple input-single output, spectrum, wave

Synthetic aperture radar (SAR) imaging of ocean waves involves both the geometry and the kinematics of the sea surface. However, the traditional linear wave theory fails to describe steep waves, which are likely to bring about specular reflection of the radar beam, and it may overestimate the surface fluid velocity that causes the so-called velocity bunching effect. Recently, the interest for a Lagrangian description of ocean gravity waves has increased. Such an approach considers the motion of individual labeled fluid particles and the free surface elevation is derived from the surface particles positions. The first order regular solution to the Lagrangian equations of motion for an inviscid and incompressible fluid is the so-called Gerstner wave. It shows realistic features such as sharper crests and broader troughs as the wave steepness increases. This paper proposes a second order irregular solution to these equations. The general features of the first and second order waves are described, and some statistical properties of various surface parameters such as the orbital velocity, slope, and mean curvature are studied. [DOI: 10.1115/1.2199563]

The problem of communication in the sea has been considered as propagation of radio waves in a three-layered medium (air, sea, and ground). With the aid of the perturbation calculus, this paper analyzes the influence exerted onto the electromagnetic field of arrangements radiating a pure transverse electric field in the sea. The sea height varies continuously with the distance from the transmitting end due to sea waves. Knowledge of the solution for the case of uniform sea height is presumed. The problem of the transition conditions at the upper boundary of the sea is solved in the imaging space of a Hankel transformation. Its reversal produces an integral presentation of the interfering field, which was previously quite difficult to evaluate. In this study, closed-form expressions for the far-field of a vertical magnetic dipole embedded below the sea surface are obtained by using a new technique to evaluate this integral with the aid of the complex image theory. The results obtained are compared with those mentioned elsewhere. Index Terms--Far-field, radiation in the sea, rough surface, stratified media.

Observations show that convective perturbations of the tropical atmosphere are associated with substantial variations of clouds and water vapor. Recent studies suggest that these variations may play an active role in the large-scale organization of the tropical atmosphere. The present study investigates that possibility by using a two-dimensional, nonrotating model that includes a set of physical parameterizations carefully evaluated against tropical data. In the absence of cloud--radiation interactions, the model spontaneously generates fast upwind (eastward) moving planetary-scale oscillations through the wind-induced surface heat exchange mechanism. In the presence of cloud--radiative effects, the model generates slower upwind (eastward) propagating modes in addition to small-scale disturbances advected downwind (westward) by the mean flow. Enhanced cloud--radiative effects further slow down upwind propagating waves and make them more prominent in the spectrum. On the other hand, the model suggests that interactions between moisture and convection favor the prominence of moist Kelvin-like waves in tropical variability at the expense of small-scale advective disturbances. These numerical results, consistent with theoretical predictions, suggest that the interaction of water vapor and cloud variations with convection and radiation plays an active role in the large-scale organization of the tropical atmosphere.

An encouraging way to convert ocean wave energy into electricity through direct drive is by using a linear, synchronous, longitudinal-flux permanent-magnet machine (LFM), where the piston is driven by a buoy. In this paper, the speed of the piston is assumed to be constant or sinusoidal. The paper presents an automated method for optimizing the design variables of an LFM with a rectifier, using time-stepping finite-element analysis. The method yields feasible LFM designs tailored to a given ocean wave climate, assuming constant or sinusoidal piston speed. The method will help to avoid a large amount of field calculations to obtain an optimal LFM driven by a buoy. The systematic approach results in a transparent investigation, giving the engineer an easy way to determine the best design. The paper presents LFM designs for a calm site in the Baltic Sea as examples. Index Terms--Finite-element analysis, linear generator, ocean wave, optimization, permanent magnet, rectifier, variable speed.

This paper summarizes the results of an experimental and theoretical investigation of: (1) pipeline flotation in a soil (liquefied under waves); and (2) density of the liquefied soil. In the experiments, the soil was silt with [d.sub.50]=0.078 mm. Pipeline models of 2 cm diameter were used. They were buried in the soil at different depths in the range 3-15.5 cm. The total depth of the silt layer was 17.5 cm. Waves (with 17 cm wave height and 1.6 s wave period, the water depth being 42 cm) were used to liquefy the soil. The pipes with specific gravity smaller than 1.85-2.0 floated when the soil was liquefied, the critical specific gravity for pipe flotation. The lower bound of the above range corresponds to the initial pipe position near the surface of the bed, and the upper bound to that near the impermeable base. Furthermore, the pipe floated (or sank) to a depth where the pipe specific gravity was equal to the previously mentioned critical specific gravity for flotation. The density of liquefied soil was determined in two different ways in the experiments: (1) from the aforementioned flotation tests where the pipe acted as a 'hydrometer', the instrument to measure fluid density; and (2) from the force balance equation for the pipe, in the vertical direction, corresponding to the critical condition for the pipe flotation. The results from these two methods agree well. The density of liquefied soil varied with depth; it was 1.85 near the surface of the bed and 2.0 near the impermeable base. A hydrodynamic model has been developed to predict the density of liquefied soil. The model is based on the force balance (in the vertical direction) for a soil grain of the liquefied soil. The model indicates that the density of liquefied soil is influenced by the soil category; the soil 'class'; the initial soil specific gravity; the specific gravity of soil grains; and the coefficient of lateral earth pressure. CE Database subject headings: Gas pipelines; Liquefaction; Underwater pipelines; Waves.

This paper presents the results of a numerical modeling study of wave-induced liquefaction around offshore structures. The finite-element model was first validated against both analytical solutions and laboratory data and was shown to be able to reproduce the observed behavior of the pore pressure variations. The model includes a constitutive model that can predict both residual and momentary liquefaction. The model was used to study liquefaction around a buried pipeline under wave action. In the analysis, it was assumed that the soil in the trench was loose and therefore susceptible to residual liquefaction. The numerical modeling focused on some of the questions that were raised in the physical modeling studies of Sumer et al. in 2006 and Teh et al. in 2003. In particular the influence of pipe diameter, boundary conditions around the pipeline, trench depth, etc., have been assessed and the results are in general agreement with the experimental findings. CE Database subject headings: Numerical models; Liquefaction; Pipelines; Waves; Offshore structures.

This paper presents the results of an experimental study on liquefaction around a pipeline buried in a soil exposed to a progressive wave. The soil used in the experiments was silt with [d.sup.50]=45 mm. The pore-water pressure was measured in the far field and on the pipe simultaneously. The tests indicate that the buildup of pore pressure and the resulting liquefaction in the soil are influenced by the presence of the pipe. The pore pressure builds up much more rapidly at the bottom of the pipe than in the far field (at the same level as the pipe bottom). By contrast, the buildup of pore pressure at the top of the pipe is not influenced radically by the presence of the pipe. The tests further indicate that as the liquefaction initially occurs in the very top layer and develops downwards, this picture changes in the vicinity of the pipe; in the latter case, the liquefaction initially occurs at the bottom of the pipe, and develops along the perimeter of the pipe upwards. The influence of the 'no-slip' condition at the pipe surface on the end results has been investigated and found very significant. The influence of the wave height, the influence of the pipe diameter, the influence of the no-liquefaction-regime conditions, and the influence of a sinking pipe have also been investigated. CE Database subject headings: Gas pipelines; Liquefaction; Underwater pipelines; Waves.

The existing Det Norske Veritas (DNV) Recommended Practice RP E305 for pipeline on-bottom stability is mainly based on the pipe-soil interaction model reported by Wagner et al. in 1987, and the wake model reported by Lambrakos et al. in 1987, to calculate the soil resistance and the hydrodynamic forces upon pipeline, respectively. Unlike the methods in the DNV Practice, in this paper, an improved analysis method is proposed for the on-bottom stability of a submarine pipeline, which is based on the relationships between [U.sub.m]/[(gD).sup.0.5] and [W.sub.s]/([gamma][D.sup.2]) for various restraint conditions obtained by the hydrodynamic loading experiments, taking into account the coupling effects between wave, pipeline, and sandy seabed. The analysis procedure is illustrated with a detailed flow chart. A comparison is made between the submerged weights of pipeline predicted with the DNV Practice and those with the new method. The proposed analysis method may provide a helpful tool for the engineering practice of pipeline on-bottom stability design. CE Database subject headings: Submarine pipelines: Offshore pipelines: Analytical techniques: Sea floors; Wave forces.

The objective of this research, part of the EU FP5 REBASDO Program, is to examine the effects of second order wave diffraction in wave run-up around the bow of a vessel (FPSO) in random seas. In this work, the nonlinear wave scattering problem is solved by employing a quadratic boundary element method. A computer program, DIFFRACT, has been developed and recently extended to deal with unidirectional and directional bichromatic input wave systems, calculating second order wave diffraction loads and free surface elevation under regular waves and focused wave groups. The second order wave interaction with a vessel in a unidirectional focused wave group is presented in this paper. Comparison of numerical results and experimental measurements conducted at Imperial College shows excellent agreement. The second order free surface components at the bow of the ship are very significant, and cannot be neglected if one requires accurate prediction of the wave-structure interaction; otherwise a major underestimation of the wave impact on the structure could occur. [DOI: 10.1115/1.2185130] Keywords: second order wave diffraction, FPSO, focused wave group

We investigate the drift of small bodies (bodies which do not significantly disturb the fluid) in waves. We begin with the slope sliding models of Ruiner and Marchenko, and we establish that the models are different and that the correct model is due to Marchenko. We then investigate the slope sliding model when we have linear wave forcing of constant frequency (for which the differences between the two models vanish). We establish that the drift velocity, for this case, is independent of initial position, which contradicts published results in which drift velocity was shown to depend on initial position. The reason for the error in previous results was the slow decay of the transient motion. For this reason we present a numerical method to calculate the drift velocity, which avoids the problems associated with the slow decay of the transient motion. Some results for the drift velocity are given. CE Database subject headings: Wave forces; Floating bodies; Models.

Short-crested waves are three-dimensional waves that may be generated through a reflection of a two-dimensional Stokes wave on a seawall. Thus, they are more likely to be observed near ports or any consequent marine structure. Two numerical methods are used to compute three-dimensional surface gravity short-crested waves on deepwater. The first method is an asymptotic procedure and the second computes a direct numerical solution. One of the main properties is the four-wave resonance. Such resonance introduces nonuniqueness with several solution branches connected through a turning point. We show that both computational methods are reliable for nonresonant waves, but that the direct numerical method converges faster. For resonant waves, the direct method is more appropriate because all solution branches can be obtained. The asymptotic method computes only one branch of solutions for any given parameter values, and is uncertain around and past any turning point. Stability analysis of the branches shows that, although sporadic, an instability associated with harmonic resonance is more likely to appear for one branch in the vicinity of the turning point. Consequently this could amplify the unstable resonant mode. The nonuniqueness of the solution requires careful attention in every study on the impact of surface waves on marine structures. It is shown here that the wave force exerted on a seawall may change drastically from one branch of solution to another. CE Database subject headings: Sea walls; Resonance; Three-dimensional analysis; Waves forces; Water crest.

An approximate analytical assessment of long wave development due to nonlinear shoaling is obtained and verified against wave-flume experiments. The phase averaged equation for the nonlinear evolution of shoaling waves by Agnon and Sheremet serves as a starting point. Their shoaling interaction function J is significantly simplified through the mild slope assumption. Results, which may be used to assess the long wave input for engineering applications such as harbor resonance studies, are given. CE Database subject headings: Long waves; Random waves; Wave spectra; Harbors; Coastal engineering.

The nearshore wave energy resource in Portugal has been assessed through the development of ONDATLAS. This is an electronic atlas, compatible with Internet access, containing comprehensive wave climate and wave energy statistics for 78 points at about 20 m water depth spaced variably ca. 5-30 km, 5 points at deep water, and 2 points at open ocean locations. The data were produced by a third-generation wind-wave model, complemented by an inverse-ray model that computes the directional spectra transformation from open ocean to the nearshore. Shoaling, refraction, bottom dissipation, and shelter by the coastline and/or neighboring islands are taken into account. ONDATLAS statistics comprise yearly and monthly values, variability and probability data for significant wave height, energy (mean) period, peak period and wave power, and directional histograms for wave and power direction. Joint probability distributions for various combinations of the above parameters are also available, as well as extreme values and return period for wave height and period parameters. A summary of the detailed verification of this model using long-term buoy measurements at four sites is presented. The main characteristics of ONDATLAS are described. The strong spatial variability that wave conditions exhibit at the coastal area are illustrated and a brief assessment of the nearshore resource at the Portugal mainland is presented. [DOI: 10.1115/1.1951779]

Nonlinear roll damping has a profound influence on ship motions and stability in ocean waves. In this study, an experimental investigation is conducted on the nonlinear roll damping of a ship in regular and irregular waves. The random decrement method, previously used in linear roll damping prediction, is extended to nonlinear roll damping estimation in the data process. The accuracy of the nonlinear roll damping obtained by using the random decrement method is found to be dependent on the values of the threshold and segment number. [DOI: 10.1115/1.1951780]

It is well known that look cross spectra processed from synthetic aperture radar (SAR) contain valuable information on ocean waves. With the launch of the European satellite ENVISAT, SAR look cross spectra (SLCS) have become available on an operational basis. Activities therefore exist at different European weather centres to use the data for assimilation into numerical wave models. Furthermore there is scientific interest in SLCS, e.g., concerning the estimation of the phase speed of ocean waves. For the estimation of ocean wave parameters, it is important to have information about the accuracy of SLCS. In this paper, errors of estimated SLCS due to SAR image speckle, spectral estimation errors, and image pattern decorrelation associated with ocean wave motion are analyzed. A probability model is proposed for the estimated SLCS based on the respective cross-spectrum coherence. The model is used to calculate signal-to-noise ratios and confidence limits for the SLCS phase and magnitude, as well as the real and imaginary part. The coherence is factored into a component describing look decorrelation due to SAR image speckle and a second factor describing the effect of sea surface motion. It is shown that the ocean-wave-dependent decorrelation can be simulated using existing nonlinear integral transforms for the look variance spectrum and the SLCS. The decorrelation effect associated with speckle noise is related to SAR system parameters, e.g., the spatial SAR resolution. The probability model is used to investigate the optimal choice of look processing parameters like the look separation time. A statistical analysis based on a global dataset of a reprocessed dataset of European Remote Sensing 2 satellite SLCS is presented confirming the applicability of the probability model. The implications of the results for the retrieval of two-dimensional wave spectra from SLCS are summarized. Possible future applications of the model like, for example, the investigation of the turbulent air flow over waves, are discussed. Index Terms--Coherence, cross spectrum, ocean waves, synthetic aperture radar (SAR).

Byline: Kern E. Kenyon (1) Keywords: Shoaling waves; wave force; wave torque Abstract: Freely propagating surface gravity waves are observed to slow down and to stop at a beach when the bottom has a relatively gentle upward slope toward the shore and the frequency range of the waves covers the most energetic wind waves (sea and swell). Essentially no wave reflection can be seen and the measured reflected energy is very small compared to that transmitted shoreward. One consequence of this is that the flux of the wave's linear momentum decreases in the direction of wave propagation, which is equivalent to a time rate of change of the momentum. It takes a force to cause the time rate of change of the momentum. Therefore, the bottom exerts a force on the waves in order to decrease the momentum flux. By Newton's third law (action equals reaction) the waves then impart an equal but opposite force to the bottom. In shallow (but finite) water depths the wave force per unit bottom area is calculated, for normal angle of incidence to the beach, to be directly proportional to the square of the wave amplitude and to the bottom slope and inversely proportional to the mean depth; it is independent of the wave frequency. Constants of proportionality are: 1/4, the fluid density and the acceleration of gravity. Swell attenuation near coasts and some characteristics of sand movement in the near-shore region are not inconsistent with the algebraic structure of the wave force formula. Since the force has a depth variation which is significantly faster than that of the dimensions of the particle orbits in the vertical direction, the bottom induces a torque on the fluid particles that decreases the angular momentum flux of the waves. By an extension of Newton's third law, the waves also exert an equal but opposite torque on the bottom. And because the bottom force on the waves exists over a horizontal distance, it does work on the waves and decreases their energy flux. Thus, theoretically, the fluxes of energy, angular and linear momentum are not conserved for shoaling surface gravity waves. Mass flux, associated with the Stokes drift, is assumed to be conserved, and the wave frequency is constant for a steady medium. Author Affiliation: (1) 4632 North Lane, Del Mar, CA, 92014-4134, U.S.A. Article History: Registration Date: 11/07/2005 Received Date: 30/10/2003 Accepted Date: 24/03/2004

Byline: Guan-Yu Chen (1), Chung-Ching Chien (2), Ching-Ho Su (2), Hsiang-Mao Tseng (2) Keywords: Harbor resonance; infragravity wave; edge wave Abstract: This article first reviews previous numerical studies on the resonance problem of Hua-Lien Harbor. All the research leads to the conclusion that resonance can be stimulated by 2-D infragravity waves. However, a literature survey suggests that outside the harbor these plane infragravity waves are too small to excite violent water-body movement inside. On the other hand, 3-D infragravity waves trapped along the coastline, also known as edge waves, are more likely to exist outside the harbor and their effect needs to be thoroughly discussed. Based on previous measurements, the response of Hua-Lien Harbor is best simulated using edge waves of 160 and 140 second periods and their resonance mechanisms are analyzed. The former case has a longitudinal resonant mode and hence the amplitude in the inner harbor is large. The latter case has a transverse mode in the outer basin and hence only berths in the outer basin are influenced. These features are both consistent with field measurement. Therefore, it is very likely that edge waves are responsible for the resonance of Hua-Lien Harbor. Finally, based on observation after the construction of the present offshore breakwaters, a theory is proposed to explain the trapping of incident edge waves, and a measure to further reduce the resonance is discussed. Author Affiliation: (1) Institute of Physical Oceanography, National Sun Yat-Sen University, 70 Lien-hai Rd., Kaohsiung, 80424, Taiwan (2) Center of Harbor and Marine Technology, IOT, Wuchi, 435, Taiwan Article History: Registration Date: 11/07/2005 Received Date: 01/10/2003 Accepted Date: 15/03/2004

Six one-dimensional-vertical wave bottom boundary layer models are analyzed based on different methods for estimating the turbulent eddy viscosity: Laminar, linear, parabolic, k--one equation turbulence closure, k--[epsilon]--two equation turbulence closure, and k--[omega]--two equation turbulence closure. Resultant velocity profiles, bed shear stresses, and turbulent kinetic energy are compared to laboratory data of oscillatory flow over smooth and rough beds. Bed shear stress estimates for the smooth bed case were most closely predicted by the k--[epsilon] model. Normalized errors between model predictions and measurements of velocity profiles over the entire computational domain collected at 15[degrees] intervals for one-half a wave cycle show that overall the linear model was most accurate. The least accurate were the laminar and k--[epsilon] models. Normalized errors between model predictions and turbulence kinetic energy profiles showed that the k--[omega] model was most accurate. Based on these findings, when the smallest overall velocity profile prediction error is required, the processing requirements and error analysis suggest that the linear eddy viscosity model is adequate. However, if accurate estimates of bed shear stress and TKE are required then, of the models tested, the k--[omega] model should be used. CE Database subject headings: Shear stress; Boundary layer; Kinetics; Hydrologic models; Eddy viscosity.

Landslide generated impulse waves were investigated in a two-dimensional physical laboratory model based on the generalized Froude similarity. The recorded wave profiles were extremely unsteady and nonlinear. Four wave types were determined: weakly nonlinear oscillatory wave, non-linear transition wave, solitary-like wave and dissipative transient bore. Most of the generated impulse waves were located in the intermediate water depth wave regime. Nevertheless the propagation velocity of the leading wave crest closely followed the theoretical approximations for a solitary wave. Between 4 and 50% of the kinetic slide impact energy propagated outward in the impulse wave train. The applicability ranges of the classical nonlinear wave theories to landslide generated impulse waves were determined. The main wave characteristics were related to the landslide parameters driving the entire wave generation process. The slide Froude number was identified as the dominant parameter. The physical model results were compared to the giant rockslide generated impulse wave which struck the shores of the Lituya Bay, Alaska, in 1958. CE Database subject headings: Landslides; Tsunamis; Two-dimensional models; Wave generation: Alaska.

A time-dependent model for obliquely incident nonlinear waves is developed and applied to predict longshore current and sediment transport. The wave model is based on the Boussinesq equations for breaking and nonbreaking waves. Wave breaking is introduced by adopting the surface roller concept. Longshore current velocity is calculated using the time-averaged alongshore momentum equation, including the effects of the cross-shore circulation on the dispersion of momentum. The wave module provides the longshore current and the sediment transport modules with all required hydrodynamic information such as radiation stress, bottom velocity, undertow velocity, and eddy viscosity coefficient. The Dibajnia and Watanabe formula is adopted to predict sheet-flow transport; whereas, for the suspended load, an energetics approach is used. Model results are compared with experimental data as well as with the Kamphuis and Costal Engineering Research Center formulas for the total alongshore sediment transport rate. CE Database subject headings: Littoral currents; Sediment transport; Time dependence; Transport rate: Breaking waves.

Present parametenzations of air-sea momentum flux at high wind speed, including hurricane wind forcing, are based on extrapolation from field measurements in much weaker wind regimes. They predict monotonic increase of drag coefficient ([C.sub.d]) with wind speed. Under hurricane wind forcing, the present numerical experiments using a coupled ocean wave and wave boundary layer model show that [C.sub.d] at extreme wind speeds strongly depends on the wave field. Higher, longer, and more developed waves in the right-front quadrant of the storm produce higher sea drag; lower, shorter, and younger waves in the rear-left quadrant produce lower sea drag. Hurricane intensity, translation speed, as well as the asymmetry of wind forcing are major factors that determine the spatial distribution of [C.sub.d]. At high winds above 30 m [s.sup.-1], the present model predicts a significant reduction of [C.sub.d] and an overall tendency to level off and even decrease with wind speed. This tendency is consistent with recent observational, experimental, and theoretical results at very high wind speeds.

Real time wave measurements in Turkey are often interrupted because of operational difficulties encountered. Therefore, the lacking significant wave height, period and directions were simultaneously estimated from the dynamic Elman type recurrent neural networks. Their predictions were compared with the commonly applied static feed-forward multilayer neural networks and with the stochastic Auto Regressive (AR) and Exogenous Input Auto Regressive (ARX) models. Two distinct learning algorithms, the steepest descent with momentum and the conjugate gradient methods were employed to train the neural networks. It was concluded that, the recurrent neural network generally showed better performance than the feed-forward neural network in the concurrent forecasting of multiple wave parameters. Both artificial intelligence techniques demonstrated a good performance when compared to the predictions of AR and ARX models. Prediction methods are also compared using continuous artificial data generated with known properties by measuring their performance in predicting the removed segments of various lengths. The multivariate ENN model successfully predicted the removed segments of artificially generated wave data. Hence, the learning ability of artificial intelligence techniques was verified signifying the robustness and fault-failure tolerance of neural networks. CE Database subject headings: Neural networks; Wave measurement; Auto-regressive models; Turkey; Predictions.

Directional wave statistics can be used to optimize the design of offshore structures which are not symmetrical. Unfortunately there does not appear to be any consensus on how directional design criteria should be developed or used. Using simple examples, we show that commonly used methods are unconservative. In order to preserve the reliability given by designing to omnidirectional criteria, the product of the probabilities of nonexceedance from all of the direction bands must equal the omnidirectional probability. We suggest a simple method for assuring consistency between omnidirectional and directional criteria. CE Database subject headings: Ocean environments; Random waves; Offshore structures; Design criteria.

The hydrodynamic behavior of fixed and heave motion floating breakwaters is studied in the present paper, using a finite-difference, mathematical model based on the Boussinesq type equations. The flow under the floating breakwater is treated separately as confined flow. The pressure field beneath the floating structure is determined by solving implicitly the Laplace equation for the potential of the confined flow using the appropriate boundary conditions. The dynamic equation of heave motion is solved with the consequent adjustments of the continuity equation in the case of a heave motion floating breakwater. Numerical results, concerning the efficiency of fixed and heave motion floating breakwaters, are compared to experimental results satisfactorily. The ability of the numerical model to predict the pressure field beneath the floating structure and the vertical force acting on it is thoroughly examined by making comparisons of the numerical results with large-scale experimental data. The experiments were conducted in the CIEM flume of the Catalonia Univ. of Technology, Barcelona, Spain. The final goal is to study floating breakwaters efficiency in shallow and intermediate waters. CE Database subject headings: Floating structures; Breakwaters; Boussinesq equations; Mathematical models; Wave action.

Statistical information about the joint occurrence of metocean parameters is of importance for many offshore activities. For instance, in marine operations, environmental limitations may be brought about by both wind and wave conditions. Thus, knowledge of their joint occurrence is important as the persistence duration (i.e., the duration of the sea state persistence above or beneath a given level) and the seasonal dependence of wind and waves appear to be of large interest. However, such a modeling becomes difficult as the number of considered variables increases, especially when utilizing a common parameterization of some conditional distributions. This paper proposes a general methodology that aims at modeling seasonal joint distributions of n such parameters from their correlation structure and the n marginal distributions fitted by generalized gamma ones. Two methods are proposed in order to derive an approximate joint distribution from the modeled margins. The first one matches the correlation matrix only, whereas the second one, which is based on a multivariate Hermite polynomials expansion of the multinormal distribution, is able to match joint moments of order higher than two. However, more restrictive conditions are shown by the latter. An application to the simple example of the joint occurrence of significant wave height and the mean wind velocity at the 10m elevation is used to illustrate the methods. Eventually, examples of applications like simultaneous persistence of wind and wave conditions as well as seastate forecasting from statistics are given.

A Multivariate Neural Network (MNN) algorithm is proposed for the reconstruction of significant wave height time series, without any increase of the error of the MNN output with the number of modelled data. The algorithm uses a weighted error function during the learning phase, to improve the modelling of the higher significant wave height. The ability of the MNN to reconstruct sea storms is tested by applying the equivalent triangular storm model. Finally an application to the NOAA buoys moored off California shows a good performance of the MNN algorithm, both during sea storms and calm time periods.

Green Water Loading in the bow region of a Floating Production Storage and Offloading unit (FPSO) in head sea waves is studied by numerical means. A 2-D method satisfying the exact nonlinear free-surface conditions within potential-flow theory has been developed as a step towards a fully 3-D method. The flow is assumed 2-D in a plane containing the ship's centerplane. The method is partly validated by model tests. The importance of environmental conditions, 3-D flow effects, ship motions, and hull parameters are summarized. The wave steepness of the incident waves causes important nonlinear effects. The local flow at the bow is, in general, important to account for. It has become popular to use a dam-breaking model to study the propagation of water on the deck. However, the numerical studies show the importance of accounting for the coupled flow between the deck and outside the ship. When the water is propagating on the deck, a suitable distance from the bow can be found from where shallow-water equations can be used. Impact between green water on deck and a vertical deck-house side in the bow area is studied in details. A similarity solution for impact between a wedge-formed water front and a vertical rigid wall is used. Simplified solutions for an impacting fluid wedge with small and large interior angles are developed, both to support the numerical computations and to provide simpler formulas of practical use. It is demonstrated how the local design of the deck house can reduce the slamming loads. The importance of hydroelasticity during the impact is discussed by using realistic structural dimensions of a deck house. This indicates that hydroelasticity is insignificant. On the contrary, first results from an ongoing experimental investigation document blunt impacts against the deck during the initial stage of water shipping, which deserve a dedicated hydroelastic analysis. Keywords: Green Water, Impact Loading

The growth of surface gravity waves is investigated in an open sea region dominated by swell. An extensive database was collected by the Brazilian Oil Company during the South Atlantic Deep Water Program (PROCAP) in Campos Basin, off coast of Rio de Janeiro. This is the most important petrolic basin in Brazil where tens of drilling platforms are located. The data set contains over 5800 directional wave spectra measured by a heave-pitch-roll buoy in conjunction with meteorological data. A spectral approach is applied in a novel method for the partitioning of sea from swell and for the adjustment of the spectral parameters. The wave growth is investigated and the regression laws of the windsea extracted from the swell-contaminated spectra are compared to earlier works in sheltered areas. It is shown that, on the grounds of the equilibrium range theory, the high-frequency spectral level (the alpha parameter) scale with the reciprocal wave age in the same manner as in unimodal swell-free spectra.

Byline: Takuji Waseda (1), Yoshiaki Toba (2,3), Marshall P. Tulin (4) Keywords: Wind waves; local equilibrium; fetch; wind gust Abstract: An experiment was conducted in a small wind-wave facility at the Ocean Engineering Laboratory, California, to address the following question: when the wind speed changes rapidly, how quickly and in what manner do the short wind waves respond? To answer this question we have produced a very rapid change in wind speed between U .sub.low (4.6 m s.sup.-1) and U .sub.high (7.1 m s.sup.-1). Water surface elevation and air turbulence were monitored up to a fetch of 5.5 m. The cycle of increasing and decreasing wind speed was repeated 20 times to assure statistical accuracy in the measurement by taking an ensemble mean. In this way, we were able to study in detail the processes by which the young laboratory wind waves adjust to wind speed perturbations. We found that the wind-wave response occurs over two time scales determined by local equilibrium adjustment and fetch adjustment, It .sub.1/T = O(10) and It .sub.2/T = O(100), respectively, in the current tank. The steady state is characterized by a constant non-dimensional wave height (H/gT .sup.2 or equivalently, the wave steepness for linear gravity waves) depending on wind speed. This equilibrium state was found in our non-steady experiments to apply at all fetches, even during the long transition to steady state, but only after a short initial relaxation It .sub.1/T of O(10) following a sudden change in wind speed. The complete transition to the new steady state takes much longer, It .sub.2/T of O(100) at the largest fetch, during which time energy propagates over the entire fetch along the rays (dx/dt = c .sub.g) and grows under the influence of wind pumping. At the same time, frequency downshifts. Although the current study is limited in scale variations, we believe that the suggestion that the two adjustment time scales are related to local equilibrium adjustment and fetch adjustment is also applicable to the ocean. Author Affiliation: (1) Frontier Research System for Global Change and International Pacific Research Center, University of Hawaii, HI, 96822, U.S.A (2) Earth Observation Research Center, National Space Development Agency of Japan, Tokyo, 104-6023, Japan (3) Japan Marine Science and Technology Center, Yokosuka, 237-0061, Japan (4) Ocean Engineering Laboratory, University of California, Santa Barbara, CA, 93106, U.S.A Article History: Registration Date: 08/10/2004

Risk assessment and management based hurricane and earthquake criteria have been developed for design and requalification of Petroleos Mexicano platforms in the Bay of Campeche. Results from recent field tests were used to verify the loading and capacity analytical models and to characterize the biases and uncertainties in these models. American Petroleum Institute wind and wave loading and earthquake response spectra-based procedures were used to determine the loadings associated with extreme condition hurricanes and earthquakes. Assessment of the platform capacities included recognition of the time-dependent loading and nonlinear platform capacity characteristics. The platform element capacities were based on current American Petroleum Institute working stress design and load and resistance factor design procedures. Design and requalification criteria to evaluate the platform system reserve strength ratios that resulted from the application of the risk assessment and management are summarized in this paper. Working stress design and LRFD parameters are not detailed in this paper due to paper length limitations.

The interaction of monochromatic incident waves with a horizontal porous flexible membrane is investigated in the context of a 2D linear hydroelastic theory. First, a multidomain boundary element method is developed for a porous membrane based on Darcy's fine-pore model. The inner solution uses a discrete membrane dynamic model and simple-source distribution over the entire fluid boundaries. The outer solution is based on an eigenfunction expansion method. The inner solution is then matched to the outer solution at the matching boundaries. Second, the corresponding analytic diffraction and radiation solutions are obtained based on the multidomain eigenfunction expansion method to better understand the relevant physics and confirm the boundary element method results. Using the developed computer program, the wave-blocking performance of a submerged horizontal porous membrane is tested with various system parameters, membrane porosity, and wave characteristics. The wave numbers above the porous membrane are generally complex, and the imaginary parts contribute to the dissipation of wave energy. Therefore, the overall performance of the horizontal flexible membrane can be further enhanced by using proper porous material. The theory and numerical results are also verified through a series of model tests in a 2D wave tank.

For protection against storm tides, four mobile barriers, each of which consists of 20 gates hinged at the bottom axis, have been proposed to span the three inlets of the Venice lagoon. In stormy weather these gates are raised from their housing to an inclination of 50 degrees from the horizon, so as to act as a dam and to keep the water-level difference up to 2 m across the barrier. The gates were originally expected to swing in unison in response to the normally incident waves, but subsequent laboratory experiments revealed that the neighboring gates can oscillate out of phase in a variety of ways and affect the intended efficiency. In this paper we extend the linear theory of Mei, Sammarco, Chan, and Procaccini for trapped waves around vertical rectangular gates and examine the inclined gates by using the hybrid finite-element method to account for the prototype geometry of the gates, the local bathymetry, and the intended sea-level differences. Finite elements are employed only in the immediate neighborhood of the gate, while formal analytical representations are used away from it. The factors affecting the trapped wave period are studied, and the results are compared with existing laboratory experiments by Delft Hydraulics Laboratory.

The present paper describes a theoretical analysis and an associated numerical model used to assess the performance of a breakwater consisting of a perforated front wall, an impermeable back wall, and a rock-filled core. The numerical method is based on an eigenfunction expansion and utilizes a boundary condition at the perforated wall that accounts for energy dissipation. The numerical model is validated by comparison with previous numerical studies of the limiting cases of a permeable seawall and a perforated breakwater with an impermeable back wall. Relevant numerical results that are presented relate to the reflection coefficient, the wave runup, and the wave force. The effects of porosity, breakwater geometry, and relative wavelength are discussed; the choice of suitable parameters needed to model the permeability of the breakwater is described; and an example application to a practical design situation is given.