Showing papers on "Wave height published in 1986"

Surf Zone Longshore Currents and Random Waves: Field Data and Models

Abstract: Analytic and numerical models for longshore currents generated by obliquely incident random waves am compared with field observations. Five days of observations were selected during which the waves were narrow banded in both frequency and direction, in keeping with model assumptions. The extensive measurements included radiation stress and wave directional spectra in 9 m depth, and a closely spaced array of current and pressure sensors on a line perpendicular to shore. The longshore current models are based on balancing the gradient of the radiation stress with the alongshore bed shear and Reynold's stresses, assuming stationary wave conditions and straight and parallel bottom contours. The spatial variation of wave height, required to determine the gradient of the radiation stress, is modeled using linear random wave theory. Given Hrms in 9 m depth, the model predicts Hrms at shoreward locations with an average error of less than 9%. Using a nonlinear bottom shear stress formulation and the meas...

Numerical Simulation of Synthetic Aperture Radar Image Spectra for Ocean Waves

Abstract: A numerical model for predicting the synthetic aperture radar (SAR) image of a moving ocean surface is described, and results are presented for two SIR-B data sets collected off the coast of Chile. Wave height spectra measured by the NASA radar ocean wave spectrometer (ROWS) were used as inputs to this model, and results are compared with actual SIR-B image spectra from orbits 91 and 106. Additional parametric variations are presented to illustrate the effects of nonlinearities in the imaging process.

Generation mechanism of internal waves by tidal flow over a sill

Abstract: The generation mechanism of internal waves by relatively strong tidal flow over a sill is clarified analytically. Special attention is directed to the role of the tidal advection effect, which is examined by use of characteristics. An internal wave which propagates upstream is gradually formed over the sill through the interference among infinitesimal amplitude internal waves (elementary waves) emanated from the sill at each instant of time. In the accelerating (decelerating) stage of tidal flow, the effective amplification of an internal wave takes place as the Froude number exceeds (falls below) unity, because during this period the internal wave slowly travels downstream (upstream), crossing over the sill, where elementary waves are efficiently superimposed. When the strength of the tidal advection effect is appropriate, the internal wave formed in the accelerating stage (Ac wave) and that formed in the decelerating stage (Dc wave) overlap, so that the resultant wave height becomes very large. Since the relative position of Ac and Dc waves varies depending on the strength of the tidal advection effect, the resultant internal wave form is strongly affected by it. When the tidal advection effect is strong, the Ac wave is carried too far downstream to interfere with the Dc wave, and hence the resultant wave form consists of two crests and two troughs. When the tidal advection effect is moderate, the optimum interference between Ac and Dc waves occurs, and the resultant wave form consists of one crest and one trough, with its horizontal scale approaching that of the sill. When the tidal advection effect is weak, the internal wave height becomes very small, since elementary waves propagate almost freely in the tidal flow without coming close together, and the resultant wave form approaches a sinusoidal wave of tidal frequency.

Mathematical Model for Littoral Drift

Abstract: The longshore sediment transport along the coast is investigated by use of detailed sediment transport models in the surf zone. Combined with a detailed description of the wave height and longshore current distribution in the surf-zone, the littoral drift along the coast is calculated and presented in dimensionless diagrams for coasts with constant slope. The paper further analyzes the transverse distribution of the long-shore sediment transport on a coast with bars. The last case is compared with field measurements.

Wave Climate and the Wave Power Resource

Abstract: Wave climate statistics are reviewed from the wave power engineer’s point of view It is emphasised that there is no fixed answer to the question “What do we need to know about the wave power resource ?” but rather a feedback loop between our knowledge and the development of any particular type of wave power device

Modeling Waves in Surfzones and Around Islands

Abstract: A semi-empirical model for surf zone wave height decay is adapted to the parabolic equation method in order to include the effect of depth-limited wave breaking in combined refraction/diffraction calculations. Several exam­ ples for plane beaches are presented in order to show correspondence between the empirical model, its numerical formulation, and previous laboratory data. The model is then applied to the study of wave breaking and diffraction around offshore islands of various planforms. An analytic method for constant depth, which can be used to extend computed solutions to the farfield downwave of the island, is provided, and several simple examples treating the island as a finite width breakwater are examined.

Transformation of random breaking waves on surf beat

Abstract: Based on a previous study by the authors of regular breaking waves in the surf zone, a model for random wave transformation across the nearshore region is developed. The results of a laboratory investigation of the effect of a steady opposing current on the wave decay process are presented and a proposed governing equation verified. Surf beat effects on wave transformation are then included in the model by representing the long wave as a temporally and spatiallyvarying current and mean water level. The concept of an equivalent water depth, which contains the effect of the current, is introduced and then included in a stochastic form in the random wave model. Surf beat is found to noticeably increase the decay of the root mean square wave height, especially in the inner surf where the beat is strongest. Comparison of the models to two field data sets show very good agreement for Hotta and Mizuguchi (1980), but rather poor for Thornton and Guza (1983). Possible explanations for the unexpected behavior of the second data set, pertaining to filtering, are discussed. Finally, a possible explanation for the dependence of random wave decay on deepwater steepness, noted by Battjes and Stive (1985), is presented.

The Fleet Numerical Oceanography Center Global Spectral Ocean Wave Model

Abstract: The Spectral Ocean Wave Model (SOWM) has been an operational product at Fleet Numerical Oceanography Center since the mid 1970s; the Global Spectral Ocean Wave Model (GSOWM) was developed to replace it. An operational test of GSOWM, using buoy, ocean-weather-station, and ship-reported wave-height data for verification, was conducted during the winter of 1984/85 by several components of the Naval Oceanography Command. This test indicated that GSOWM was superior to SOWM and that both models exhibited root-mean-square significant-wave-height errors on the order of 1 m. Wave-height errors deduced from the ship observations were comparable to those calculated from the buoy data. The GSOWM scatter index, determined from the buoy and ocean-weather-station data and defined as the, standard deviation of the model-predicted wave-height error divided by the mean observed wave height, averaged 0.34. As a result of the study reported here, GSOWM replaced SOWM as the U.S. Navy's operational wave model in June ...

The role of the gravity-wave dispersion relation in HF radar measurements of the sea surface

Abstract: Recent experimental and theoretical findings raise interesting questions about the applicability of the normal gravity-wave dispersion relation at wave frequencies that exceed the spectral peak frequency. The use of the dispersion relation in analysis of HF radar Doppler sea echo is examined in this paper. Drawing on the results of perturbation theory for wave-wave nonlinear interactions, we show that this relation, so essential to echo interpretation in terms of current and wave information, can be employed with no degradation in accuracy for current measurement when the dominant wave frequency is considerably less (by as much as 10) than the radar Bragg resonance frequency. This finding is supported by comparisons of currents measured by HF radar with "surface truth;" the first-order echo must only be identifiable in order to be used accurately. Wave-height directional spectral information can be extracted from the second-order echo at a given radar frequency up to the point (in wave height) where the perturbation solution employed in the inversion process fails; then a lower radar frequency must be used. On the other hand, most conventional wave measuring instruments should not use the dispersion relation for interpretation of data well beyond the spectral peak, because they do not observe wave height as a function of both space and time independently, as does HF radar.

Ship Wave Prediction for Port and Channel Design

Abstract: Several laboratory and field investigations of ship-generated waves have been conducted during the past twenty-five years. Typically, for a given ship, reported data consists of the height of the highest wave in a wave record generated by the passing ship and its associated wave period, for the particular test run speed and distance from the sailing line. There is a need to synthesize these data and, from this, to develop ship wave height prediction methods for port and channel design. This is done herein. Also, methods are given to predict the period and crest orientation of the highest wave. Refs.

Wave Group Statistics

Abstract: Because of the dispersive property of surface waves in deep water, whitecaps are intermittent, and their lifetime is controlled partly by the lengths of wave groups, or of runs of successive high waves. This paper discusses and unifies two different approaches to the problem of wave grouping: (a) treating the sea state as a gaussian process, with group properties given by the wave envelope function, and (b) treating the sequence of wave-heights as a one-step Markov process. The latter is here related, with examples, to the spectral density function. E(σ), and it is shown that the spectral width parameter V plays an important role in both (a) and (b). It is pointed out, however, that any group analysis implicitly requires a prefiltering of the data, and an appropriate band-pass filter is determined. The Markov predictions that the distribution of high runs is a negative exponential, and that the total group length is the difference between two exponentials, are both confirmed by comparison with numerical data.

Wave Power Climate of Portugal

Abstract: The wave power available at the portuguese coast is evaluated by using three chronological series of wave records representing the northern and southern parts of the West coast and the South coast of Portugal. Analytical models for the annual mean spectra and probability distributions for wave power are derived. Statistics for extreme wave heights are also included.

Random breaking waves horizontal seabed

Abstract: According to wave theories the depth limited wave height over a horizontal seabed has a wave height to water depth ratio (H/d) of about 0.8. Flume experiments with monochromatic waves over a horizontal seabed have failed to produce H/d ratios greater than 0.55. However designers still tend to use H/d 0.8 for their design waves. Experiments have been carried out using random wave trains in the flume over a horizontal seabed. These experiments have shown that the limiting H/d ratio of 0.55 applies equally well to random waves.

Steep and breaking deep water waves

Abstract: Many naval applications require a further basic understanding of surface wave hydrodynamics up to and beyond the onset of breaking, and of the energy and momentum transfer processes which take place during and after breaking. In the present paper is described a technique for the computation of steep asymmetric wave motions up to the incipient of onset stages of breaking. Comparisons are made with related computational schemes and then the method is used for a study of limiting wave conditions and the onset of breaking. The computations are compared with recent measurements made at NRL and elsewhere of the limiting heights of deep water waves and the losses in potential energy density and momentum flux which accompany the breaking process. An estimate of the limiting wave height at breaking in deep water from the computational results is in good agreement with experiments described here. The potential energy and momentum flux loss rates of plunging breakers are about twice those of spilling breakers in deep water.

The interaction of waves and currents over a longshore bar

Abstract: A two-dimensional model for waves and steady currents in the surf zone is developed. It is based on a depth integrated and time averaged version of the equations for the conservation of mass, momentum, and wave energy. A numerical solution is described based on a fourth order Runge-Kutta method. The solution yields the variation of wave height, set-up, and current in the surf zone, taking into account the mass flux in the waves. In its general form any wave theory can be used for the wave properties. Specific results are given using the description for surf zone waves suggested by Svendsen (1984a), and in this form the model is used for the wave motion with a current on a beach with a longshore bar. Results for wave height and set-up are compared with measurements by Hansen & Svendsen (1986).

Experimental Study on the Validity Range of Various Wave Theories

Abstract: In the design of coastal structures and the study of nearshore dynamics, rational predictions of the wave kinematics are very important but difficult procedures. Although a large number of nonlinear wave theories have been proposed and used for computing the wave kinematics, there are no theories applicable from the deep water to very shallow water. It is, therefore, very important for coastal researchers and engineers to know which of theories describe well a wave field specified by the wave characteristics and water depth, and to select a particular wave theory for a problem of interest. Many intensive efforts have been made to examine the validity as well as the applicability of various wave theories. However, there are still no well-accepted guidlines for the application range of the wave theories. The validity evaluation of a particular wave theory has been basically made by means of the following two versions: the analytical (mathematical) validity and the experimental (physical) validity. The analytical validity study has been conducted by various researchers (Dean, 1970; Komar, 1976; Horikawa et al., 1977; Swart, 1978) and revealed the degree of mathematical satisification to the governing equations and boundary conditions for each wave theory. The analytical validity study probably tends to show the relative applicability for various wave theories. It does not ensure that the theory describe well laboratory or field phenomena. Based on the analytical validity of various wave theories by Horikawa et al., Isobe (1985) proposed application ranges for the finite amplitude wave theories in terms of the relative water depth and relative wave height. The experimental validity refers to how well the prediction of various wave theories agrees with actual measurements (Dean & Dalrymple, 1984). As the wave shoals, wave form becomes more asymmetrical, especially under high wave conditions of interest to design. Such nonlinearity influences greatly the wave kinematics and it makes difficult to predict readily the wave kinematics by several theories. From a practical viewpoint, it is , therefore, requested to establish the application ranges of available wave theories for shoaling waves.

Observations and Measurements of Air Flow over Water Waves

Abstract: Flow separation has been found to occur within the turbulent boundary layer over short gravity waves (~ 10-cm wavelength) under low-wind conditions (~ 1 m/s) when the waves are not breaking (wave height/wavelength < 0.06).

Observations of wind-driven surface gravity waves offshore from the Great Barrier Reef

Abstract: A 3 month long record of pressure from a nearsurface-resident instrument deployed at 18° S at the Queensland shelf break, offshore from the Great Barrier Reef, has been analysed. No discrete wind-sea and swell spectra can be identified, nor is there evidence for frequency-dependent wave dispersion. The wave spectral width and significant wave height are strongly related to local meteorological events, and the response appears to be both fetch limited and wind duration dependent. A near-gale sea state on the Beaufort scale is experienced during strong trade winds. It is suggested that waves make an important contribution to the physical oceanography, the geomorphology and the ecology of coral reefs near the shelf break of the Great Barrier Reef.

Wave Groupiness in Long‐Traveled Swell

Abstract: Grouping of high waves in a long‐traveled swell recorded at a shallow water location is examined. The swell, with most of the energy concentrated within a narrow band of frequencies (0.040–0.075 Hz), is assumed to have originated in the southern Indian Ocean. The longest run of high waves identified contained 6 waves for H>Hs and 16 waves for H>H. The observed probabilities of runlengths are in agreement with the theory of Kimura. Very high correlation between successive wave heights (up to 0.79) and significant correlation up to 5 waves apart, which is higher than all earlier reports, are observed. It is found that the correlation coefficient between successive wave heights rHH (1) is a better parameter to define the length of runs of wave heights than the spectral peakedness parameter, Qp. The high values of runlengths observed raise doubts regarding the concept of a decrease of runlengths in shallow water, which has lately been confirmed by Thompson and Seelig.

Southern Ocean Waves and Winds Derived from SEASAT Altimeter Measurements

Abstract: SEASAT altimeter measurements of significant wave height in the Southern Ocean during August and September 1978 have been examined. Significant wave heights of greater than 10 m were observed. Background levels of 2–4 m indicate generally rougher seas than occur in the North Atlantic during a comparable season. The altimeter data set is examined statistically and the spectrum of the worst case situations is determined parametrically based upon a JONSWAP form of the spectrum. Results of a statistical analysis of the altimeter data are presented together with an evaluation of the probable spectral characteristics based upon a parametric derivation of the spectrum.

Offshore sand waves

Abstract: A theoretical model for the equilibrium dimensions of offshore sand waves is presented. The model is an adaptation of the dune model by Fredsoe (1982) to the marine environment, making use of the physical analogies between offshore sand waves and river dunes. The predicted sand wave heights and lengths conform well with the observations of sand wave formation reported in the literature. One of the results from the model is that for a given wave climate sand waves will only be formed under a certain range of mean current velocities. This range becomes narrower for increasing wave height or decreasing water depth and grain size.

Interaction Between Surface Waves and Muddy Bottom Sediments

Abstract: Surface wave-induced bottom pressure fluctuations produce shear stresses in soft muddy bottom sediments that cause the sediments to undergo oscillatory motion. This motion can be described as a “mud-wave” and causes surface wave properties to vary from those that occur over a rigid bottom. Theoretical studies have attempted to describe this interaction using a variety of soil models, i.e., viscous fluid, elastic solid, viscoelastic material and nonlinear viscoelastic. Although the experimental basis for evaluating the validity of these assumptions is incomplete, it appears that a nonlinear viscoelastic soil model is required to describe the observed behavior. An example of the interaction of hurricane waves and soils found offshore of the Mississippi Delta is considered in detail. The soil is described using a model which is nonlinear in relating shear strain to shear stress and damping ratio. The surface wave-mud wave interaction for hurricane waves is significant and causes wave heights of 70 ft (21.3 m) and 80 ft (24.4 m) in deep water to decrease to values of from 10 ft (3.0 m) to 25 ft (7.6 m) at a water depth of 50 ft (15 m). Soil response during this wave-mud interaction is greatest at water depths of between 150 ft (45.7 m) and 250 ft (76.2 m). Maximum soil movements of 1.5 ft (.46 m) are predicted to occur under hurricane waves. As a means for making rough calculations of the wave-mud interaction a simplified technique for making engineering predictions is presented. The technique is based upon a nonlinear stress-strain and damping-strain soil model and predicts surface wave attenuation, soil shear stress and shear strain profiles.

A Numerical Model of Nearshore Currents Based on a Finite Amplitude Wave Theory

Abstract: A numerical model of wave-Induced nearshore currents taking into account the finite amplitude effect is developed, with a cnoidal wave theory used for the estimation of wave characteristics. The model is applied to the computation of wave transformation and nearshore currents on uniformly sloping beaches and on two-dimensional model topographies. The comparison with the results obtained by a linear model shows that wave nonlinearity has a strong influence on wave transformation in shoaling water and in the surf zone and on the strength of nearshore circulation, but that it does not have much effect on the longshore current profile. Moreover, the validity of the present model is supported by the quantitative agreement with the experiment for wave height variations, and the qualitative correspondence with the experiment for mean water level variation and longshore currents and the observation for nearshore currents.