Showing papers on "Wave height published in 1969"

Rip currents: 1. Theoretical investigations

Abstract: The nearshore circulation of water on a plane beach produced by a wave train, normally incident on the beach, which has a longshore variation in wave height is investigated theoretically. The radiation stress arising from the excess flux of momentum due to the presence of the waves (M. S. Longuet-Higgins and R. W. Stewart, 1964) is found to provide driving terms for a steady flow pattern only inside the surf zone. A circulation pattern is thus produced by a longshore variation in the radiation stress in the surf zone. In shallow water, the radiation stress is proportional to the square of the wave height. The nearshore circulation is therefore directly related to longshore variation in breaker height, currents flowing seaward where the breaker height is low. When the inertial terms are included in the vorticity equation, an increase in the effective Reynolds number produces a narrowing, and consequently a strengthening, of the Seaward flow, which suggests an explanation for the existence of the strong, narrow currents known as rip currents.

Specification of the wind distribution in the marine boundary layer for wave forecasting

Abstract: : The operational application of sophisticated spectral ocean wave specification models to wave forecasting requires that an adequate meteorological input to these models consist of a specification of the wind distribution in the marine surface boundary layer. This study demonstrates how this requirement can be satisfied in a computerized objective format from routinely available meteorological data and prognostic fields. Wave generation theory is employed to develop a quantitative formulation for the effects of atmospheric stability upon the development of the wave spectrum. In terms of this stability dependent spectral growth formulation, the effects of stability on wave generation are found to be significant and compare well with observational studies of the dependence of wave height and white-cap production on air-sea temperature difference. A simple model of the non-neutral, baroclinic planetary boundary layer over a sea surface described in terms of an internally prescribed roughness parameter is derived. The model is shown to provide a suitable framework for the diagnosis of the marine surface boundary layer wind distribution from prognostic fields of sea level pressure, air temperature and sea surface temperature.

Breaker Travel and Choice of Design Wave Height

Abstract: Experiments on three-plane laboratory beaches show that plunging waves travel a horizontal distance of from four to eight times breaker height during the breaking process. This suggests that the potentially damaging effect of breaking waves may be spread over a significant horizontal distance. The experiments, as well as previously available data, show that breaker depth-to-height ratios for plunging waves decrease from above 1.3 to below 0.9 as beach slope increases, so that higher waves on steeper slopes may approach nearer to shore before breaking. The combined effect of breaker travel and breaker depth-to-height ratio is such that structures sited in shallow water on moderate or steep slopes can be subject to breaking wave heights significantly larger than the design heights computed according to accepted practice. The experimental results are consistent with a solitary wave description of oscillatory waves at breaking, if the breaker depths of oscillatory waves are appropriately defined, and they are consistent with the limiting heights of breaking waves measured on rubble-mound breakwaters.

Deep‐Ocean Ambient‐Noise Spectra in the Northwest Atlantic

Abstract: Ambient sea noise was measured from an omnidirectional hydrophone located in the deep‐ocean environment. The hydrophone is installed in about 2400 f and is suspended 400 ft above the ocean bottom. The results illustrate the time variability and the correlation of deep open‐ocean ambient‐noise spectra with simultaneous wind‐speed and wave‐height measurements recorded during typical winter conditions. Broad‐band ambient noise levels were automatically recorded on magnetic tape for 2 min every 2 h. The average value of ambient‐noise in 25 logit‐filter bands of frequencies ranging from 11 to 2816 Hz was measured for each 2‐min sample. Autocorrelations of ambient‐noise levels, wind speed, and wave height, as well as cross‐correlation of ambient‐noise levels with wind speed and wave heights, are presented. The ambient‐noise spectra show the superposition of two major noise sources. One noise source is suspected to be distant shipping. This noise source predominates in the frequency range between 17 and 112 Hz and shows noise maxima occurring at 20 and 60 Hz. The other noise source is wind dependent and is observed below 17 and above 112 Hz. The variation in the standard deviation as a function of frequency at low wind speeds is suspected to be the result of the superposition of the two noise sources. The correlation coefficient of wind speed or wave height to sea noise varies as a function of frequency and wind‐speed group.

Theoretics in Design of the Proposed Crescent City Harbor Tsunami Model

Abstract: : The important wave parameters to be considered for tsunami model studies are wave height and period, and wave-front orientation. The first two of these parameters can be determined by marigraphic measurements or by visual observations; however, wave-front orientation has never been accurately observed at the problem site (Crescent City, Calif.). A digital computer program was written to plot wave rays from three recent epicentral locations to Crescent City to obtain approximate tsunami-front orientations. The refraction diagrams were checked by comparing the computed and actual arrival times of the wave fronts. The actual arrival times were obtained from recording tide stations at Crescent City and Hilo, Hawaii. Initial wave-front orientations near the earthquake epicenter were either assumed or taken from the literature, A tentative frequency relation was derived and a risk-duration relation was prepared from the frequency relation. The selected values of the test-wave dimensions (height and period) and orientation determine, to a considerable extent, the model configuration necessary to ensure results sufficiently accurate for purposes of the model study. Since experience in the design and operation of long-period wave models is scant, it was deemed necessary to conduct preliminary tests utilizing a pneumatic wave generator in a two- dimensional flume. The results of these tests were compared with theoretical predictions from an idealized mathematical model and on the basis of these tests, a preliminary model design was proposed.

Upwind-downwind-crosswind sea-clutter measurements.

Abstract: : Radar returns were collected nearly simultaneously on four frequencies-428 MHz, 1228 MHz, 4455 MHz, and 8910 MHz-for both linear and cross polarizations. In the course of these measurements, sea clutter was recorded in the upwind, downwind, and crosswind directions. The ratios of the upwind-to-downwind and upwind-to-crosswind returns were investigated as functions of the incident angle, the polarization, the radar wavelength, and the gross surface parameters of wind velocity and wave height. The upwind/downwind ratio is shown to decrease with increasing angle and surface roughness. Horizontal polarization is more sensitive to wind direction than vertical polarization. The short wavelengths are more sensitive to wind direction than the long wavelengths. The upwind/crosswind ratio remains significant as the short wavelengths for the roughest sea encountered at Puerto Rico. The results provide estimates of the upwind-downwind-crosswind relations as functions of the above parameters over an angular region of 4 to 90 degrees (vertical incidence) and from calm sea conditions to moderately rough conditions characterized by 5 - to 7-ft wave heights and 10 - to 20-knot winds. (Author)

Measurement of Ocean Waves in a Satellite Photograph.

Abstract: : Apollo 6 photographs of the surface of the ocean, made within two hours of noon solar time are found to show wave structure of the sea with remarkable clarity. The wave length distribution has been measured from about 1000 waves in a single photograph of about 10,000 sq mi area. With better photographic definition wave height could probably also be measured from deformation of edges of cloud shadows. Using this method it would be possible to measure wave length and wave height distributions daily of all oceans, with a polar orbiting satellite carrying a television camera. (Author)

Wave Action in Mission Bay Harbor, California; Hydraulic Model Investigation.

Abstract: : A hydraulic model investigation of the wave-action problems in Mission Bay Harbor was conducted to develop and test several plans of improvement proposed for reducing wave heights within Quivera Basin and Glen Rick Cove to a satisfactory level. The 1:100-scale model, molded in cement mortar, reproduced the portion of the harbor requiring remedial action, and sufficient coastline and offshore bathymetry to permit accurate simulation of storm-wave attack in the area. A 60-ft-long wave machine and electrical wave height measuring and recording apparatus were utilized in model operation.

Run-up distributions of waves breaking on sloping walls

Abstract: Distributions of run-up are calculated by assigning to each individual wave in an irregular wave train a run-up value according to Hunt's formula. The use of this formula permits a normalization of the run-up in such a way that the run-up distributions are independent of slope angle, mean wave height and mean wave period, Expressions arc derived for the probability density and the distribution function of the run-up and of the wave steepness for arbitrary joint distributions .of wave height and period. Explicit results are obtained for wind waves by assuming wave height and period squared to be jointly Rayleigh distributed with arbitrary degree of correlation. Empirical data from the laboratory are discussed, These lend support to some of the main premises used and results obtained. A few field measurements of run-up distributions are presented; the Rayleigh distribution appears to fit these data.

Experimental Research on Ship Capsize

Abstract: An experimental research on ship capsize was conducted in the experiment tank with wind tunnel at the Institute of Industrial Science, University of Tokyo, using a simple cylindrical model.Critical (minimum) wind velocity for capsizing was determined for a given period and height of regular waves. Two wave periods were chosen so as to be in near synchronism with the natural rolling and heaving periods of the model ship.The experiments revealed that : 1) for a given wave height, the critical wind velocity was somewhat lower in case of the capsize in short waves than in long waves, but the difference was rather small, 2) the ship was capsized by steady wind force (without waves) when her angle of heel had surpassed over about two-thirds of the angle corresponding to the maximum stability moment, 3) the model could, not be capsized (at least in this experiment) when the mean angle of heel was kept nearly null as the result of balancing of two heeling moments due to wind and waves.The authors can not explain these results theoretically at present but a tentative hypothesis to predict the critical wind velocity or wave height is presented.Some examples of theoretical calculation of nonsymmetric, nonlinear rolling of ships are added for a future development of this research.

Some average wave lengths on short‐crested seas

Abstract: Twelve average sea spectra are truncated at 1·6 rad s−1 and fitted with tails in which energy density varies inversely with the fifth power of frequency. By use of a spread parameter measured in typical rough weather these spectra are converted into symmetrical two-dimensional wave number equivalents, which are, inter alia, numerically integrated along normals to the dominant wave direction, yielding the wave number spectra of instantaneous wave profiles along that direction. These spectra provide unbiassed estimates of average wave lengths, while the originals provide similar estimates of average periods. Zero-crossing averages are shown to be preferable to those measured from crest-to-crest. It is found that, for the estimation of average zero-crossing wave lengths, the original frequency tail can be replaced by one in which the density varies inversely with frequency raised to the power of 5·5, and that the fourth moments of the main (truncated) spectra require reduction by about a quarter, to allow for the effect of spreading. These findings also appear to be applicable in individual situations where the dominant direction is well defined. The variations of zero-crossing periods and wave lengths with wave height are studied for two locations.