Showing papers on "Wave height published in 1984"

Application of a simple numerical wave prediction model to Lake Erie

Abstract: A parametric dynamical wave prediction model has been adapted and tested against semianalytic empirical results for steady conditions in a circular basin and extensive fiel measurements of wave height, period, and direction. The adapted numerical model accurately predicts the directional spreading of waves for uniform steady wind that Donelan (1980) had predicted analytically for fetch-limited waves. When the model was applied to the central basin of Lake Erie and the results compared to observations of wave height and period (at two points in the lake) and direction (at one point), results for wave height and direction estimates were excellent compared to measurements at a research tower off the southern shore, but computed wave heights were lower than observed at a weather buoy in the western part. The model somewhat underestimated wave periods at both places. Thus, with locally measured wind data as input, the model estimates wave height and direction well and wave period acceptably.

Longshore structure of infragravity wave motions

Abstract: Synchronous runup records were collected from 14 locations spaced irregularly over a 7 km stretch of a low-slope beach. The significant runup height, herein defined as the significant vertical excursion of water level at the shoreline, was typically 2 m, 60% of the incident significant wave height at the breakpoint. The runup spectra were dominated by the energy at low frequencies, with 99.9% of the variance in motions with periods longer than 20 s and 83% for periods longer than 50 s. A peak between 0.005 and 0.010 Hz was present in all spectra. Analysis of this band showed the motion to be a standing edge wave with wavelength between 5 and 10 km and significant edge wave height at the antinode of 1.3 m, accounting for half of the local significant runup height. The edge wave period was approximately 140 s and the mode number in the range 3–7. An apparently linear sand bar was present 175 m offshore, roughly the same location as the first node of cross-shore velocity for the edge wave. While the edge wave could not have been the cause of the bar (a standing edge wave does not create a linear bar), the opposite may be true, with the bar potentially providing a topographic resonance. As the local head-lands extend only a small fraction of the offshore distance of the edge wave, they are unlikely to be significant reflectors. The single rip current observed occurred at the edge wave node.

An operational coupled hybrid wave prediction model

Abstract: GONO is a numerical wave prediction model used for the preparation of forecasts as well as hind-casts. It is a hybrid model: the wind sea is described in a parametric way, but swell is treated in a spectral manner. For the wind sea there are two prognostic parameters: the zero-moment wave height and the mean direction. Pure wind-sea spectra are assumed to have a quasi-universal shape: above the spectral peak, ƒ−5 behavior is assumed; below the peak, a linear frequency dependence is taken. The directional dependence is of the cos2 θ type. Empirical relations are used to derive the full set of wind-sea parameters from the prognostic variables and the wind vector. The equations for the prognostic variables are solved on a discrete grid with the help of a simple finite-difference scheme. For the accurate propagation of swell, possibly over large distances, a ray technique is used. The full two-dimensional spectrum is reconstructed for selected grid points for which the results of the ray technique and the wind-sea calculations are combined. The model is a shallow water model because bottom dissipation effects are taken into account, but effects of refraction are disregarded. Depending on wind speed, these effects may be important in areas where the depth is less than about 100 m. The model has not been applied in regions with depths less than 15 m, therefore extreme shallow water effects are not considered. The behavior of the model was studied in quite some detail during the recent Sea Wave Modeling Project (SWAMP) in a few idealized situations. Knowledge of the model behavior in more realistic situations stems from its routine operational application. Runs are made four times a day on a grid covering the North Sea and the Norwegian Sea, and the results are monitored continuously. Presently, we have a data base containing about four years of observations and model predictions. From this data base we discuss a few interesting storms, and we present a statistical analysis of all of the available material. As part of this analysis we consider the effect of the quality of the input winds on the model performance.

Characteristics of L-band multipath fading due to sea surface reflection

Abstract: For low G/T ship-based earth stations in future maritime satellite communications, the effect of multipath fading due to sea surface reflection will be very important at low elevation angles. However, a practical model of the L -band multipath fading for fade prediction has not been available so far. A practical fading model is presented based on the shadowed Kirchhoff approximation theory for the coherent and incoherent components of the scattered power as a function of sea surface conditions. Using this model, theoretical fading depths are presented as a function of elevation angle, wave height, and antenna gain. The results indicate that intense fading occurs for wave heights greater than 50 cm, and the dependence on wave height is small under these conditions at L -band frequencies. Theoretical results presented agree well with experimental results obtained by field experiments.

The distribution of measured and simulated wave heights as a function of spectral shape

Abstract: First-order theory predicts that zero-crossing wave heights should have a Rayleigh distribution, but measured waves are slightly smaller than the theoretical prediction. Suggested explanations for this discrepancy have included the nonlinearity of the wave surface, limitations on height as a result of breaking, and the effect of spectral width. In a recent paper, Tayfun (1981 b) showed that the shape of the spectrum influences the height distribution because the wave envelope has different amplitudes at the time of the crest and trough. We have compared the distribution developed by Tayfun to simulated waves with different spectral shapes as well as to observations and found excellent agreement. The theoretical, simulated, and measured wave height distributions agree to within 1% in height. This result reinforces the conclusion that linear Gaussian simulations can reproduce many features of ocean waves.

Comparison of a Two-Dimensional Wave Prediction Model with Synoptic Measurements in Lake Michigan

Abstract: We compare results from a simple parametric, dynamical, deep-water wave prediction model with two sets of measured wave height maps of Lake Michigan. The measurements were made with an airborne laser altimeter under two distinctly different wind fields during November 1977. The results show that the model predicted almost all of the synoptic features. Both the magnitude and the general pattern of the predicted wave-height contours compared well with the measurements. The model also predicts the direction for wave propagation in conjunction with the wave height map, which is useful for practical ship routing and can be significantly different form the prevailing wind direction.

A Case Study of Wave Current Bathymetry Interactions at the Columbia River Entrance

Abstract: A unique set of field observations has documented intense wave–current–bathymetry interactions in a tidal inlet, providing a severe test of existing wave theory. Hourly wave spectral estimates were acquired over ten complete tidal cycles at locations offshore of, and on, the Columbia River Bar. This site has long been recognized as an extreme hazard to navigation due to the interaction of incident waves with a strong tidal jet typified by ebb currents in excess of 2 m s−1. During the five-day observational period, offshore significant wave height ranged from 1 to 5 m; in contrast, the Bar wave height oscillated at tidal periods, sometimes doubling during a single six-hour flood-to-ebb cycle. In one case, the Bar wave height was 7.0 m with an offshore wave height of 2.9 m. A one-dimensional model, based on linear wave action–density conservation and incorporating shoaling, refraction and radiation stress, fits the data remarkably well; 85% of the observed variance in Bar swell height and 57% of th...

Experiments and Theory of Wave‐Soil Interactions

Abstract: Wave damping and motion of clay beds are measured in a wave tank for various soil and wave conditions. The propagator matrix theory for continuously layered plasto‐elastic beds is used to model the wave‐soil interactions. In most cases, the theory agrees well with the experimental results. Wave damping and bed motion increase nonlinearly with wave height. Wave damping mechanism of clay beds is the Coulomb friction between grains. Model‐prototype scaling examples are made for application of the model data to design situations.

Aerodynamic drag: Measured at a nearshore platform on lake of Geneva

Abstract: A fixed platform was constructed 100 m from the shoreline in 3 m water depth in the Lake of Geneva; it was instrumented with velocity, temperature and wave height sensors. The 145 data (10 minutes averages) were analysed to obtain the drag coefficient for wind speeds of 7 < u to < 17 m/s and for weak stratifications. The (1)Theoretical Considerations, needed for the data interpretation are presented first.

Development of ship wave design information

Abstract: During the last three decades several field and laboratory investigations have been conducted in which the waves generated by a wide variety of vessels have been measured. There is a need to synthesize the data published from these studies and to develop general ship wave prediction methods for designers. To complete this task some additional ship wave data must be collected. This paper initiates the effort to develop these prediction methods. A summary and critique of available data are given. Then, the appropriate portion of these data is employed to develop a ship wave height predictor model that gives the maximum ship wave height as a function of ship speed and displacement, water depth, and distance from the sailing line. This is an interim model that is quite applicable but can be improved pending additional data. Finally, planned future efforts to further develop design wave prediction methods are discussed.

Estimate of Breaking Wave Height Behind Structures

Abstract: A useful and computationally simple method is given for estimating the wave breaker height and angle behind structures on a uniformly sloping bottom. The breaking wave height behind a structure is assumed to be proportional to the product of refraction, shoaling and diffraction coefficients with the incident wave height at the tip of the obstacle. The solution is obtained by iterating over breaking depth and starting angle at the tip of the structure, together with a breaking condition. The diffraction coefficient can correspond to either monochromatic or directionally random waves. A simplified procedure for calculating the diffraction coefficient of random directional waves is given in an appendix. The method has been verified with limited field and laboratory data. Possible refinements as well as limitations are discussed.

Shallow Water Wave Height Parameters

Abstract: Experimental evidence shows that two commonly-used estimates of significant wave height behave differently during the shoaling process. Further, the estimates behave differently for monochromatic and irregular wave conditions. Although the mechanics that force these differences are not fully understood, it is important to recognize that the differences exist. It is also vital to identify which wave height parameters are of interest in a study and to assure that they are treated in a consistent manner.

Self-adjusting breakwater for artificial harbors

Abstract: An artificial breakwater which dynamically changes shape and thus absorbs both surface and undersurface wave energy with varying wave height. The breakwater elements are flexible and extensible walled bags of which a major part is to be submerged below the surface of the sea. At least one anchor is fixed by a cable to the bottom of the bag. The bag is filled mostly with water and partly by air, whereby at least part of the air filled portion extends above the surface of the sea. The flexibility and extensibility of the bag is such that in the presence of waves its vertical dimension increases or reduces, thereby impeding transmission of wave energy at and below the surface.

Modeling Wave Transformation in the Surf Zone.

Abstract: : By drawing a macroscopic analogy between an idealized surf zone and a hydraulic jump, an expression for the spatial change in energy flux due to breaking is developed. Analytical solutions for wave height decay due to shoaling and breaking on a flat shelf and a plane beach are presented and the results compared with laboratory data from Horikawa and Kuo (1966). The agreement is good. Setdown/setup in mean water level, bottom friction losses, and bottom profiles of arbitrary shape are introduced next and the equations transformed for numerical solution. The model is calibrated and verified with laboratory data with good results for the wave decay, but not so favorable results for setup. A test run on a prototype scale profile containing three bar and trough systems demonstrates the model's ability to describe the shoaling, breaking, and wave reformation process. Bottom friction is found to play a negligible role in wave decay in the surf zone when compared with shoaling and breaking.

Characteristics of a breaking wind-wave field in the light of the individual wind-wave concept

Abstract: The relation between the intensity of breaking of individual wind-wave crests and parameters of wave size and wave form (e. g., height, period, steepness and skewness) is examined, and the process of change of these parameters is studied in a wind-wave tank (reference wind speed 15 m sec−1, fetch 16 m). Distributions of the wave form parameters are different for breaking and nonbreaking waves. Fully breaking waves seem to hold the relationH ∞T 2, whereH is the individual wave height andT is the period. The condition of breaking is not simply determined by the simple criterion of Stokes' limit. Wave height and steepness of a breaking wave are not always larger than those of a nonbreaking wave. This suggests the existence of an overshooting phenomenon in the breaking wave. The wave form parameters are found to change cyclically in a statistical sense during the wave propagation. The period of the cycle in the present case is estimated to be longer than four wave periods. An intermittency of wave breaking is associated with this cyclic process. Roughly speaking, two or three succeeding breaking-waves sporadically exist among a series of nonbreaking waves along the fetch.

An Analysis of Wave Data for Wave Grouping

Abstract: This paper has examined the change of wave grouping of storm waves during wave growth and decay by analyzing the field wave data observed off Gobo Coast in Wakayama Prefecture and at Lake Biwa in Shiga Prefecture (Japan) with some parameters to describe the magnitude of wave groups. For the mean lengths of runs and total runs of wave heights, the comparison of the observed results with the theoretical ones by Kimura (1980) has been done. Besides, the effect of shallow water on wave grouping has been discussed.The main results are as follows: (1) To describe the magnitude of wave groups, at least two parameters are needed; one is the magnitude of the sequence of high waves in a time series, and the other is the magnitude of the variation of wave energy or wave height around the mean value. (2) Concerning the wave data observed at Gobo, wave grouping was pronounced in the main stage of storms where three parameters of the correlation coefficient of successive wave heights, the mean length of runs an...

A coupled discrete wave model MRI-II

Abstract: An ocean wind-wave prediction model MRI-II is developed on the basis of the energy balance equation which contains five energy transfer processes, namely, the input by the wind, the non-linear transfer among the components of windsea by resonant wave-wave interactions, wave breaking, frictional dissipation and the effect of opposing winds. The non-linear energy transfer is expressed implicitly together with the wind effect by Toba’s one-parameter representation of windsea, but neither swell-swell nor swell-windsea resonant interactions are considered. Hypothetical assumptions are introduced to describe wave breaking effects. The numerical constant required in the assumptions of wave breaking is determined through trial test runs for a hindcast performed on the Northwestern Pacific Ocean. The results of the hindcast and numerical experiments of the SWAMP test cases show that MRI-II outforms our old model MRI both in describing growth stages of windsea and in application to actual wind fields.

Analysing the characteristics of wavedistribution in the south china sea onthe basis of ship reports

Abstract: Based on the data of ship reports from 1958 through 1972,this paper gives such elements as the wind direction and speed,the height,period and direction of the wave,which are calculated on the computer within a 2********o******x2********o longitudinal-latitudinal grid,and analyses the monthly and seasonal variation of wind,wave and swell,as well as their distribution.In order to gain an insight into the regional features of the wave,We divide the South China Sea into the nothern,central and southern areas and make a detailed description of the laws governing variations of the height,period and direction of both the waves and swells during the NE and SW monsoon periods as well as the frequency and distribution areas of the rough sea,and give the typical distribution of wave height and period in January and July.On the basis of a great amount of data,the author analyses and studies the basic wave characteristics in the South China Sea areas.

Breaking wave design criteria

Abstract: Breaking wave heights measured in both field and random wave laboratory experiments are examined. The dependence of breaker height and breaker depth on beach slope and deep water steepness is presented. The results are compared with the design curves of the Shore Protection Manual (SPM) and the predictions of the randan wave model by Goda (1975). The comparisons indicate that the significant breaker height, based on Goda's model, is slightly conservative for the experimental cases; but the maximum breaker heights are reasonably predicted by the model. The design procedures in the SPM are based on a monochromatic wave breaking, and appear overly conservative, particularly for low wave steepness (less than 0.01) which occur frequently on the West Coast of the United States. The use of the Rayleigh distribution to predict wave height statistics is tested with random wave data for both deep and shallow water regions.

Explicit solutions to practical wave problems

Abstract: Explicit formulae are provided for wave problems covered by linear wave theory. Two goals are pursued. The first is to provide a faster and more flexible tool than usual wave tables. The second is to provide explicit, analytical solutions to problems that have so far demanded time consuming numerical integrations. As an example we solve the problem of wave height variation due to refraction, shoaling and energy dissipation over a soft mud bottom. The obtained explicit solutions are accurate enough for practical purposes and require very little computational effort, in fact they will enable the engineer to solve many wave problems with a handheld calculator. Another advantage of analytical solutions is that they are always much more instructive than numerical results.

Waves in the Arabian Sea and the Bay of Bengal during the monsoon season

Abstract: The wave da1a recorded over the Indian seas by the Russian research vessels during the ISMEX (Indo-Soviet Monsoon Experiment) of 1973 and the Monsoon Experiment of 1977 are analysed. It is found that for given wind speeds, wave heights remain practically the same, the scattering being quite small. This is probably because during the monsoon season, the steadiness of the wind is quite high and the fetch is quite large. An empirical relationship is obtained between the wind-speed and characteristic wave height. The relationship has been verified with the data of the Russian research vessels during Monex 1979. The wave heights computed from the reported windspeeds compare quite favourably with the observed values.

A numerical model for refraction of linear and cnoidal waves

Abstract: A numerical model for refraction of linear and cnoidal waves over an arbitrary bottom is presented The model, which is based on the ray theory of refraction (11), utilizes linear theory in deep and intermediate water and cnoidal theory in shallow water The model permits one to determine nearshore wave properties using a nonlinear wave theory Nearshore wave properties such as wave height, celerity, and wave angle determined using the linear-cnoidal refraction model may vary considerably from those determined using a linear refraction model In general, cnoidal waves refract less, travel faster, and shoal higher than linear waves This result can have a considerable effect on coastal engineering design, particularly in the areas of coastal structure design and longshore transport computations

Pot-Estimation of Extreme Sea States and the Benefit of Using Wind Data

Abstract: The distribution function for the maximum peak during T years of the significant wave height is derived by means of a POT-method. Accordingly, the T-year wave peak (i.e. the level which — on the average — will be exceeded once per T years) and the sample variance of the T-year estimate can be determined. Based on a period with concurrent measurements of wave heights and wind speeds a regression analysis can be applied which allows wave peaks to be calculated during periods with only wind observations. Thus an updated estimate of the T-year event as well as the variance of the estimate can be developed, and the obtained variance reduction used as a measure of the benefit of using wind data. An example is included in order to illustrate the presented theory.

Parameterized Solution to Nonlinear Wave Problem

Abstract: A parameterized solution to the monochromatic irrotational nonlinear wave over a horizontal bottom is developed. The solution is expressed in terms of a converging truncated Fourier series. The coefficients, wave number and number of terms required are all given directly in terms of wave height, water depth and wave period. The mathematical formulation is validated both theoretically and experimentally.