Showing papers on "Wave height published in 1968"

Wave ‘set-down’ and set-Up

Abstract: ‘Set-down’ and set-up, the negative and positive changes in mean water level due to the presence of a train of surface waves, was measured in a wave channel. Well outside the break point the experimental results are in good agreement with the theoretical relationship determined by Longuet-Higgins and Stewart. Near the break point, where the wave height is greater than predicted by first-order wave theory, the measured ‘set-down’ was consistently less than theory would predict from the deep water wave height. Inside the break point the bore height was found to be a linear function of the mean water depth. In this region, the gradient of the set-up, , was related to the beach slope tan β and the mean ratio of wave height to water depth by the equation .

Experimental determination of run‐up of undular and fully developed bores

Abstract: The run-up of undular surge and bore is determined experimentally for four slopes each with three different bottom roughnesses. Results indicate that dimensionless run-up curves of the height of run-up versus the height of the initial wave are approximately linear for the undular surge, F ≤ 1.35, and the fully developed bore, F ≥ 1.55, separated by a nonlinear transition region. The run-up is strongly affected by both slope and bottom roughness. Empirical prediction equations are given in the form h/y2 = ƒ1 (sin α, ƒ) + ƒ2 (sin α, ƒ) y2/y1, where h is run-up height above undisturbed water level, α is the slope angle, ƒ is a dimensionless friction coefficient, y2 is the height of the wave measured from the channel bottom, and y1 is the undisturbed water depth. Experimental data on run-up and changes in structure and celerity of the wave front during progression up slope disagree in several fundamental aspects with now published relevant theory based on the nonlinear long-wave equations. In particular, the prediction equation for run-up in the form u02/2g, independent of beach slope, is shown not to hold true, and the theoretical conclusion that the bore height η collapses to zero at the intersection of undisturbed water level with slope is also shown not to hold true for the conditions under which the present experiments were made. The predicted relationship between piston velocity V and wave height y2 is verified experimentally, and the predicted relationship between wave celerity C and y2 fits the experimental data provided that one takes into account the effects of wall and bottom roughness and the wide fluctuations in the wave front in the fully developed bore phase.

Beaches Produced by Waves of Low Phase Difference

Abstract: A theoretical and experimental study of the action of swell waves on model beaches shows that the beach slope and the distance from the shore to the breakers correlate with the breaker height, wave period and grain diameter. The experiments were carried out using quartz sands and wave periods in the range of 0.8 sec to 1.34 sec. Wave heights were used up to a maximum breaker height of 6 cm. The waves employed were of low phase difference, i.e., the ratio of time of uprush to wave period being less than 0.7. When this ratio is exceeded, the wave period ceases to be important. Using a continuity approach it is shown that the energy lost in breaking, friction and turbulence and in moving material, amounts to about 25% of the incident wave energy when the beach angle is highest, to about 50% when the beach is at its lowest level within the range considered.

Surface‐Coupled Losses in Surface Sound Channels

Abstract: Evidence is presented that for a large body of surface sound channel propagation data, collected for ionic absorption, there is a residual attenuation coefficient that may be due to the combined effects of scattering by sea waves as well as bubble‐layer absorption and scattering. The bubble‐layer properties are wakelike and are dominant whenever they are present. The bubble layer losses are proportional to the square root of the product of acoustic frequency and mean wave height.

Sea-Clutter Measurements on Four Frequencies

Abstract: : The initial processing and analysis of radar backscatter data, recorded off the coast of San Juan, Puerto Rico, in July 1965, have been completed. Radar returns were collected nearly simultaneously on four frequencies: 428 MHz, 1228 MHz, 4455 MHz, 8910 MHz, for both linear and cross polarizations. The data were processed to obtain the normalized radar cross section, sigma of the sea surface as a function of various parameters. In particular, the behavior of the median value of the cross section sigma was investigated as a function of wind direction, incident angle, polarization, radar wavelength, and gross surface conditions of wind velocity and wave height. Results are given for the upwind direction and the linear polarization (vertical and horizontal). The polarization ratio is shown to decrease with increasing sea roughness and to be a function of radar wavelength. As surface roughness increases, sigma becomes independent of wavelength for vertical polarization but maintains an inverse wavelength dependence for horizontal polarization. The results given provide estimates of sigma as a function of the above parameters over an angular range of 4 to 90 degrees (vertical incidence) and from calm sea conditions to moderately rough conditions characterized by 5- to 7-foot wave heights and 10- to 20-knot winds.

Wave measurements by a pressure type wave gauge

Abstract: The paper deals with comparative measurements of irregular model waves by a pressure type wave gauge and a continuous wire wave gauge. For the depth and wave conditions used in the study it is concluded that the height of individual waves obtained by a pressure type gauge and using the first order wave theory may be in considerable error, while wave height distributions and wave spectra are fairly good estimated.

Ship waves in shoaling waters

Abstract: Models of two types of vessels were towed at various speeds in water of uniform depth. On one side of the sailing line the ship waves were allowed to move over beaches of various slopes. The wave heights were measured and compared at two points equidistant from the sailing line— patterns were determined by photographs.

Ship waves in navigable waterways

Abstract: Model tests were conducted at the University of California, Berkeley, on six ship models to determine the heights of the waves produced by ships. Measurements were made at various distances from the sailing line with the models moving at various speeds in various water depths. The results of the tests are presented in graphical form, as prototype values.

A New Method for Obtaining the Directional Spectrum of Ocean Surface Gravity Waves

Abstract: The measurement of wave height and direction spectra is a classical problem; it has been attempted in a variety of ways with various degrees of success In this paper, the Oceanics Division (formerly National Marine Consultants) of Interstate Electronics Corporation extends these previous efforts, utilizing applicable background work to obtain the desired electronic wave height and direction system. The system basically provides the time history of amplitude and wave slope to allow the two-dimensional wave spectrum to be uniquely computed. For simplicity of thought, the surface of the sea is most easily considered as sinusoidal. However, the statistical and spectral mathematical tools used allow the more random and confused surface characteristics to be taken into account. Efficient processing, recording, reduction, and evaluation of the data thus provide the oceanographer with the desired spectral coefficients related to power density and directional spectra. Typical installations are discussed showing applications to pier or pole mounting as well as deep-moored buoy installations.

The effect of wave crests on wave forces

Abstract: This paper deals with wave forces from short crested waves as compared with two dimensional waves. The investigation comprised measurements of wind wave forces on fixed pontoons in a model, and a comparison of wave crest characteristics of wind waves in model and prototype. The model tests showed that the wave crests have a marked influence on the wave forces. The investigation of model and prototype wave data showed that the crest characteristics of the model waves compared well with that of prototype sea.

Trends of Extreme Wave Bending Moment According to Long-Term Predictions

Abstract: The long-term predictions of wave bending moment amidships were tried on three kinds of hull forms including cargo ship and tanker, where were assumed the similar ships having different lengths for each hull form in order to investigate the relationship between ship size and wave bending moment.In the first place, the standard deviations of wave bending moment induced on ship hulls in short-term irregular seas were evaluated as functions of average wave period and significant wave height, according to the linear superposition method by using the theoretically calculated response operators and the modified Pierson-Moskowitz type wave spectra (I. S. S. C. -1964 wave spectra). Then, the long-term probabilities of wave bending moment exceeding particular level values were calculated by availing the long-term wave data on average wave period and significant wave height in the North Atlantic.According to the results of long-term prediction, the following trends were concluded on the extreme values of wave bending moment expected to be exceeded with the probability of 10-8 in the North Atlantic : The effective wave height for the midship wave bending moment increases generally with increase of ship length, but takes on unexpectedly large values for ships of about 100 meter length and holds nearly constant for ships longer than about 300 meter.

Ambient noise studies. bimini, bahamas - 1967-1968.

Abstract: : During a 72 day period, 18 February to 2 May 1967, ambient noise and in situ environmental parameters were sampled at 12 minute intervals off the coast of Bimini, Bahamas. The resulting time series were analyzed by spectral and cross spectral methods. There was no significant correlation between noise data at frequencies of 25 and 100 Hz, with wind speed up to 28 knots or wave height up to 6 feet. At frequencies of 400 and 1600 Hz, the ambient noise levels shows a strong correlation with wind speed and wave height. Wind speed is shown to be the predominant factor and wave height is secondary in the production of higher frequency noise. Wave height is found to be strongly correlated to wind speed and current and temperature measurements exhibit some weak correlation with wind speed for low frequency fluctuations, i.e., below three days. High frequency periodicities of 8, 6, 4.8, 4 and 3 hours were found in the 400 and 1600 Hz records. These are thought to be harmonics of the 24 hour occurrence of a large rise in ambient noise level occurring at 2100 hours, which is produced by biological sources of unknown origin. (Author)

Structural Damage from Tsunami at Hilo, Hawaii

Abstract: A team of engineers and scientists visited Hilo on the Island of Hawaii after the tsunami of 1960 to survey the damage. The principal items studied were the general wave characteristics and the structural damage caused by the waves. The portion of the survey which pertained to onshore effects is reported. Included are studies of height of wave, structural damage caused by the wave, and the pressures on structures from the force of the wave. The velocity of the wave was estimated by three different methods. Reasonable agreement was noted among the three methods. The damage caused by the tsunami is described in some detail, together with the various conditions which affected the degree of damage. Analyses of several structural elements are presented, and conclusions and recommendations are formulated.

Radar sea return as a function of wave height and wind speed for fully developed seas

Abstract: Measuring radar sea return as function of wave height and wind speed for fully developed seas