Showing papers on "Wave height published in 1971"

Run-up distributions of waves breaking on slopes

Abstract: Run-up of irregular waves which break on a slope is calculated by assuming that on the average the run-up of each wave with a given height and period equals the run-up of a periodic wave train of the same height and period. General expressions are derived for the distributions of the run-up and the wave steepness as functionals of an arbitrary joint distribution of the wave height and the square of the period. Explicit results are obtained for the case when these variates are jointly Rayleigh distributed with arbitrary degree of correlation. Some of the assumptions are verified by a comparison of the analytical results with previous experimental data.

Momentum flux and wave spectra observations from an ocean tower

Abstract: Three hundred seventy-seven eddy correlation estimates of momentum flux were obtained with a ‘flux meter’ system and cross-spectrum analysis at an elevation of 7.5 meters above mean sea level at Argus Island Tower during 1964, 1967, and 1969. Argus Island, a former research tower which stands in 60 meters of water, is located 45 km southwest of Bermuda with an unrestricted fetch in most directions. The influence of the pre-existing ocean waves in the open ocean (January to May) is manifest in a decreasing drag coefficient as mean horizontal wind velocities increase up to 6.5 m sec−1. As the wind velocity increases up to 14.5 m sec−1, the differential interplay of wave background and atmospheric stability is associated with a nearly linear increase of drag coefficient under unstable conditions, and stability effects appear to be more dominant than the surface waves. However, the influence of the ocean-surface waves is clearly evident when drag coefficients are observed at neutral and stable conditions. Under these conditions the observations indicated an oscillation of the drag coefficient as function of mean horizontal wind velocities between 7 and 12 m sec−1. There is some evidence of the dependence of the drag coefficient on dimensionless wave-phase velocity and dimensionless wave height. Wave-induced peaks in vertical velocity and quadrature spectra, corresponding to ocean-wave spectral peaks, are illustrated for light winds (4–6 m sec−1) and moderate winds (9.5–10.5 m sec−1). The ratio of the standard deviation of the vertical wind velocity to the friction velocity increased linearly as a function of the ratio of wave-phase velocity of the spectral peak to the friction velocity at all observed stabilities.

General theory of radiative transfer across the random atmosphere-ocean interface

Abstract: The general problem of radiative transfer across a random air-water interface is formulated and several important cases are solved. In particular the reflected sky radiance from a rough sea with given wave height and wave slope statistics is determined by solving a novel class of stochastic integral equations. This class of equations allows for all orders of multiple scattering of radiant energy between the atmosphere and the sea, and the air-sea interface, including radiometric self-interactions of the interface. The general formulation is carried out using as a representation of the air-sea interface a general (non stationary) random function. The bases for practical numerical solutions of the problems are given in terms of the mean square heights and mean square slopes of the ocean surface. The model contains implicitly an hierarchy of successively simpler interface models for use in practice, the choice of a model being a function of available data. The extension of the present theory to cover the case of polarized radiant flux is indicated.

Measurement of Ocean Wave Heights with the Random-Signal Radar

Abstract: When a radar antenna is pointed directly down at the sea surface, the return signal is caused mainly by specular points on the surface. In this case, the sea surface can be modeled as a collection of Poisson-distributed point scatterers, each having a certain radar range and a certain vertical velocity. This model can be characterized by a deterministic function, called the scatterer density function, which is the density of specular point scatterers in the two-dimensional range-velocity space. This function can be estimated by using a high-resolution random-signal radar and provides information relating the dynamic aspects of the ocean surface to the distribution of wave heights. A special case of this problem is the estimation of average wave height, or sea state, and a specific technique to estimate this quantity is proposed and analyzed. This technique has the advantage of being a direct estimation procedure; i.e., the quantity measured is wave height, not a quantity that has been empirically related to wave height.

An exact numerical solution of the solitary wave

Abstract: The solitary wave, first studied in the field by J. Scott Russell, is an elevation of the water surface moving with essentially constant form at a celerity dependent upon the ratio a of its height to the depth of water upon which it propagates. The fore and aft portions of its profile are symmetrical, and gradually approach the undisturbed water-surface elevation at large distances from the crest. While the solitary wave can be observed upon a running stream, the essential features are retained when considering propagation in standing water. It is under these latter circumstances that the fluid motion in the wave is most nearly irrotational and most accurately subject to analysis by potential-flow techniques.

Fourier Analysis of Weather and Wave Data from Holland, Michigan, July 1970.

Abstract: : During July, 1970, weather and wave parameters were measured at two hour intervals on the beach and in the nearshore area two miles north of Holland, Michigan. Parameters measured include barometric pressure; wind speed and direction; air and water temperature; sky condition; lake and groundwater level, wave period and height; breaker depth, type, distance and angle; and longshore current velocity. The computer was used to calculate the phase and amplitude for the first 15 Fourier harmonics and to plot the observed data and cumulative curves. The curves were influenced by low pressure systems which passed north of the area of 4, 9, 15 and 19 July. Wave height and direction are related to cyclonic winds moving counterclockwise around the low pressure system. Longshore current velocity can be predicted as a constant times the derivative of the barometric pressure. (Author)

The relationship between microscales and wind-wave spectral development.

Abstract: The objective of this study was to describe the mechanics of wind wave generation and spectral development. Intermittency, high frequency microstructure in wind and wave fields, and strong nonlinear coupling involving a wide range of scales are shown to be crucial elements in the transfer of momentum to, from, and within the wave field. None of these elements are included in available theories. Measurements of wave height and of the turbulent atmospheric and subsurface boundary layers were made, from a small surface following platform and from a stable 38.5m spar buoy. The structure of moving gust patterns (cat's paws) is described and related to the generation of surface waves. Results from this and other background studies are then applied to a discussion of spectral growth during a two day period of active wave generation. Cat's paws contain 'bursts' of intense turbulent stress and buoyancy fluctuations separated by quiescent 'intervals'. There is a difference of over three orders of magnitude in fluctuation strength between these features. Rapid growth rate generation of high frequency surface waves and atmospheric turbulence occurs during the bursts. The resultant microscale components aid the growth of lower frequency instabilities by strong nonlinear coupling between scales of motion and by acting as drag or roughness elements. Evidence of strong coupling between frequency bands and of weakly resonant capillary-gravity wave interactions is presented. Thermal stratification has a strong influence on fluctuation magnitude and can delay the onset of surface wave generation. Major spectral growth is highly unsteady. Much of the momentum flux from air to sea occurs during intermittent events that are similar in nature to cat's paws, and goes directly into high frequency waves. The bursts occur predominantly over large groups of surface waves and involve strong nonlinear interactions between media and frequency bands. The long-term equilibrium balance between wind and water is disrupted by variations in surface currents. There are 'critical' wind speeds characterized by anomalous relationships between parameters of the predominantly logarithmic velocity profile.

Interaction Between Gravity Waves and Finite Turbulent Flow Fields.

Abstract: : A laboratory study of the interaction of deep water gravity waves progressing into a turbulent flow field produced by a finite width grid towed in a wide tank showed wave height attenuations of nearly 90% in the grid wake and wave height amplifications of nearly 75% in the still water outside the wake. The transverse gradient of longitudinal flow in the wake was predominantly responsible for the large wave deformations and precluded an evaluation of direct turbulence effects. A simple, analytical solution using wave refraction, defraction and superposition concepts is developed which qualitatively reproduces the measured results. (Author)

A study of wave resistance characteristics through the analysis of wave height and slope along a longitudinal track

Abstract: The system of waves generated by a ship has a direct relationship to its hull form. The purpose in analysing the wave system is therefore to determine this relationship and thereby its wave resistance characteristics. In particular such an analysis could produce ships of minimum resistance. It is possible in principle to compute from theory the wave pattern generated by a ship, but at present the only practical theory available for this purpose is that of the thin ship. This linear theory is not sufficiently accurate except possibly for submerged simple forms such as bulbs. The results indicate that the linear wave theory may, in the case of relatively fine hull forms, satisfactorily predict the major part of the wave-resistance characteristics of a hull from Longitudinal Cuts. For full hull forms it is possible that wave-breaking becomes a major component of resistance which does not show up in the form of a free wave pattern. What has been presented is mainly a detailed description of the method of analysis and the experimental techniques together with a few numerical results. It is hoped that the Longitudinal Cut method will prove successful in the future.

Wave Forecasting for West Coast of India

Abstract: The applicability of the general wave forecasting procedures, e.g., the SMB and the PNJ methods, to the West Coast of India is studied. The study consisted of the analysis of the synoptic charts to obtain the necessary wind characteristics. The computed wind characteristics were used in the general forecasting methods to yield significant wave heights. These were compared with the wave characteristics as recorded by a subsurface pressure type recorder after suitable modification to account for the attenuation of wave pressure with depth. The predicted wave heights and periods compared well with the recorded wave heights and periods and the SMB method predicted wave heights better for the case studied.

Measurements and calculations of ship resistance in waves

Abstract: : The comparison of the constant-thrust and constant-velocity methods of towing in waves for the measurement of added resistance, as reported in previous experiment, was extended to additional models in regular waves. The results indicate a reasonably good agreement between the two methods. The constant-velocity method was used to test the effect of wave height on added resistance. The results demonstrate that added resistance is proportional to the seond power of the wave height for higher waves only. For low waves the relationship could have considerable variation. There are indications that it may be possible to establish standard series of hull forms for added-resistance calculations from available data.

Stability tests on a rubble mound breakwater head in regular and irregular waves. sorvaer fishing port, norway

Abstract: Results from a two-dimensional case study carried out at RHL showed that irregular waves seemed to represent a more severe wave attack than regular waves with heights equal to the significant wave heights of the irregular wave. It was further concluded that the factors that influence the stability of a breakwater are many and complex and vary within wide ranges from project to project. The best basis for breakwater design is still model testing, preferably with irregular waves. This conclusion was also positively underlined by van Oorschot. In view of these conclusions a case study has been carried out at RHL regarding the stability of the head of the rubble mound breakwater under construction at the fishing port of Sorvaer in the northernmost county Finnmark of Norway. The sponsor of the tests was the Norwegian State Harbour Works. The study involved evaluation of the design wave height from meteorological data as well as model testing. The evaluation of the design wave height followed more or less standard procedures, the presentation of which is of less general interest. The tests involved intensive investigations of the breakwater stability in regular waves as well as in irregular waves. It is the latter tests that are considered to be of most general interest and which are reported in this paper. The following conclusions were drawn from the tests results: 1. The stability of the armour layer seems to be the same in regular and irregular waves when the significant wave heights of the irregular wavetrain correspond to the height of the regular waves. 2. The two long terms indicated a breakdown for lower waves than in the other tests. However, since only two such tests were run, no definite conclusions can be drawn on this point. 3. Regarding erosion on the foot berm this seemed in some cases to be the same in regular and irregular waves. The tests indicated that the erosion was larger with irregular waves than with regular waves when the waveheights were just above the waveheight when erosion started. At larger waveheights the erosion tended to be approximately the same for regular and irregular waves. None of the tests showed damage to the armour layer due to erosion of the foot berm.

Regular Wave Effects on Walls Made of Cylinders

Abstract: This paper presents the results of wave action on walls made of closely spaced cylindrical cells 10m to 20m in diameter with both vertical and chamfered superstructures In these tests the waves, wall dimensions, and water depth in front of the walls were subject to large-scale changes The total wave force which acts on the cylinder with vertical superstructure is approximately equal to that acting on the corresponding length of the plane vertical wall In case of chamfered superstructure, the total force is essentially decreased The wave pressures are variably distributed around the cylinder surface and depend upon wave steepness Recommendations for calculating the wave pressure against walls made of cylindrical cells with both vertical and chamfered superstructures have been worked out from the results of more than 600 experiments

Dynamic analysis of the Naval Postgraduate School ocean instrument platform

Abstract: The dynamic and static response of the proposed NPS ocean instrument platform is investigated by developing and solving linear differential equations of motion of the tower in surge, heave, and pitch. The motion is expressed as a response spectrum which is directly proportional to a wave spectrum as the exciting force. The analysis is made for various configurations of the lateral restraining cables using both a five point and nine point mooring system. For all configurations j the heave response of the tower is found to be less than one percent of wave height. The stability of the tower in pitch is found to be considerably improved after shifting 1'rom a five point mooring system zo a nine point mooring system and optimizing the location of the cable attachment points. Using this design, a significant wave height of 7-7 feet is found to produce a significant pitch of 5-^ degrees, a significant surge of .97 feet, and a 5.28 foot excursion of the lower platform on the tower. All oscillations will be superimposed on any heel angle of the tower which may exist due to steady wind forces. The angle of heel for a wind of 20 mph is evaluated to be 1.9 degrees.