Showing papers on "Wave height published in 1970"

Effects of an artificial sea slick upon the atmosphere and the ocean

Abstract: Vertical mean wind profiles, hot-film data, and wave height data measured during the passage of an artificial sea slick are compared with similar measurements without a sea slick. The effects of the slick are modifications of profile roughness length z0, and a possible increase in mean wind speed. Power spectral density plots of wave records obtained before, during and after the slick show wave energy modifications for wavelengths up to 10 m. Coherence values computed from a cross-spectral density of the wind field and simultaneously measured wave field both during and after the slick demonstrate the importance of small waves to air-sea interaction processes. Phase angles indicate the maximum horizontal velocity occurs over the wave trough for all spectral components, some of which are above and some below the wind-wave “matched layer.” During a stick, however, the horizontal velocity maximum occurs over the wave crest for those waves remaining coherent with the wind field.

Piling-Up behind Low and Submerged Permeable Breakwaters

Abstract: An experimental study of the phenomenon of piling-up behind low and submerged breakwater is presented. Piling-up occurs in completely enclosed areas or behind two-dimensional breakwaters as a result of overtopping of the breakwater. The water that spills into the protected area is accumulated there until the mean water level inside the protected area is higher than the man sea level outside. This difference in elevation, or height of piling-up, reaches eventually a value sufficient to cause a mean outflow of water through and above the breakwater equal to the mean inflow by water overtopping the structure and spilling into protected area. For a given breakwater and mean sea level, the height of piling-up was found to be an increasing function of the height of the waves in the deep sea. Expressing values relative to this wave height, the relative height of piling-up was found to be a function of the relative depth of submergence, or of the relative height of protrusion of the breakwaters. The maximum value of piling-up for the structures tested was found to be of the order of 60% of the wave height. Values of this magnitude were found for low breakwaters with crest elevations above mean sea level of between 50% and 90% of the wave height.

Aie entrainment and energy dissipation in breakers

Abstract: Even m shallow water, only a part of wave energy is lost by turbulent viscosity and bottom friction, most of wave energy transfer takes place m the narrow zone of surf at the shore. Till to the point of breaking, the theoretical conception of an one-phase flow may be applied to the problem. From beginning of breaking, however, the effect of aeration can not be neglected. Prom a simple physical consideration, the sudden reduction of wave height and wave energy inside the surf zone can be explained by the entrainment of air bubbles into the water. Except compression and surface tension effects, most of wave energy is stored at first by the static energy of the air bubbles which are driven into the water. Using idealized assumptions for calculation (uniform concentration of air bubbles a.s.o.), it can be shown that m a plunging breaker the wave energy is dissipated on a very short way (less than on wave length), for a spilling breaker however, this way is of the order of some wave lengths.

Waves generated by a piston-type wavemaker

Abstract: When a wavemaker generates a finite number of waves, it has been found that one of the first and one of the last waves in such a burst is considerably larger than the average A mathematical model, based on the linearized governing equations, is used for the particular problem of the waves generated by a sinusoidally moving piston-type wavemaker starting from rest Theoretical results for the magnitude of the large wave relative to the average agree fairly well with experiments, however, the actual wave height is smaller in the experiments than predicted by theory It is shown, by extending the classical wavemaker theory to second order, that finite amplitude effects do not offer an explanation However, pistons rarely fit the tank dimensions exactly, and an approximate evaluation indicates that the discrepancy between predicted and observed wave heights can be attributed to the effects of leakage around the piston.

Wave refraction theory in convergence zone

Abstract: An experimental investigation utilizing nonbreaking waves and one bottom topography with parallel circular contours symmetric about the center of the basin was performed to assess the limits of applicability of linear wave refraction theory in a convergence zone. Analytical computations of reflections from underwater topographic variations and of viscous dissipation of energy at the bottom agreed with experimental measurements. It was vividly illustrated that a significant amount of energy was involved in diffraction along the wave crest, thus the necessity of developing a theory to include the stepwise computation of diffraction processes simultaneously with the computation .of wave refraction is now evident. Nonlinear transfer of energy from lower to higher frequency components was investigated by performing a harmonic analysis over one wave period. As expected, nonlinear effects increased with both wave period and wave height (for a given wave period). Only 20 percent of the energy remained in the fundamental component for the longest period largest amplitude wave tested. Higher frequency components ranged from being completely coupled to the fundamental frequency to almost completely uncoupled. It was clearly demonstrated that in some circumstances the consideration of nonlinear effects becomes extremely important.

Tsunami Refraction Diagrams by Digital Computer

Abstract: Important wave parameters to be considered for tsunami model studies are wave height, period, and orientation. The first two of these parameters can be determined by marigraphic measurements or by visual observations; however, wavefront, orientation, which is an important variable in the construction and operation of a hydraulic model, has never been accurately observed at the problem site (Crescent City, California). A digital computer program was written to plot wave rays from tsunami sources to obtain tsunami-front orientations in the Crescent City area. The refraction diagrams are checked by comparing the computed and actual arrival times of the initial 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 are either assumed or taken from the literature. The computer program has been successfully tested against a closed form solution for the refraction around a selected island shape. The technique also compensates for the distorted picture of the earth’s surface as presented on a Mercator projection of that surface.

Computation of alongshore energy and littoral transport

Abstract: This work encompasses a study of the littoral regime of a section of the coastline of the city and county of San Francisco, California The study included a complete refraction analyses of all applicable deep water wave directions and periods, the determination of breaker locations, and a computation of alongshore energy and potential littoral transport for seven stations located just offshore along the thirty foot depth contour The waves are refracted from deep water locations to the shoreline using the Stanford Wave Refraction computer program Wave breaking is assumed to take place whenever the computed wave height exceeds 0 78 times the local water depth The effect of limiting the refraction coefficient is explored since the refraction theory, in analogy with its parent theory of geometric optics, fails along caustic curves and predicts unreasonably large values for the refraction coefficient.

Energy distribution curves of developing and fully arisen seas

Abstract: The PIERON-MOSKOWITZ (PM) ocean wave spectrum, derived from analyses of many ocean wave records, contains two constants α and β for fully arisen sea (FAS) conditions. This frequency spectrum can be converted to a period base, termed an energy distribution curve (EDC) that contains the same contants. The EDC is almost triangular in shape, with the period Tmax at the apex increasing with the steady wind velocity. A unique relationship result between the ignificant wave height H⅓ and Tmax. A new dimensionless form of the PM spectrum and EDC is presented which incorporates wave age (C/U). The triangular form of the latter permits the definition of working upper (TU) and lower (TL) period limit of the many wave trains present. Using a recently derived growth relationship for wave in a developing sea, the variations of α and β in the spectral equation of the FAS are determined. Knowing the fetch of the FAS, value of α and β have been related to the ratio F/FFAS for fetch controlled conditions. Substitution of t...

Evaluating forecasts of ocean-wave spectra

Abstract: An experimental study has been made to determine the accuracy of numerical forecasts of wave spectra over 36-hour periods. The forecast wave data were evaluated by comparison with measurements obtained from ocean weather ships and the Argus Island tower during three separate storms in the North Atlantic. Significant wave heights up to 11.1 meters and wind speeds up to 30 m/sec were observed at some of the five different locations where the evaluations were made. The maximum significant wave height average for all locations was 7.3 meters. The procedure used was to inspect visually the forecast spectra and significant wave height data for two different spectral models superimposed on instrumentally observed spectra and significant wave heights and to make note of how they correlated. Also, a numerical assessment obtained from statistical analyses indicated a negligible negative bias and an rms error of ±1.72 meters in the significant wave heights for 196 samples. The results of the evaluation indicate that routine numerical wave spectra forecasting up to 30- or 36-hour periods is feasible for the North Atlantic. The greatest single factor that could aid in improving such forecasts would be more accurate forecasts of the surface wind fields over the oceans.

Littoral environment observation program in california, preliminary report february-december 1968,

Abstract: : The report describes the Littoral Environment Observation (LEO) Program, and assembles in one paper the data collected under the program from February through December 1968. LEO is a cooperative effort of the State of California and the Corps of Engineers to collect information which will increase understanding of the littoral processes and physical characteristics of the California shore. The littoral variables collected under the LEO program include the following beach characteristics: foreshore slope, width and elevation of berm, presence of cusps, and samples of the sediments. The beach samples are analyzed for mean and median diameter, standard deviation, skewness, and kurtosis. Sea variables include tide level, wave height, wave period, wave direction, type of breaker, direction and velocity of littoral currents, presence of rip currents, and water temperature. Wind velocity and direction are recorded, and panoramic photographs are obtained. The objective is to establish a bank of repetitive, systematic measurements of meteorological and oceanographic forces affecting the shoreline and the response of the shore to these forces. The data collected are being used as a base to analyze physical characteristics of the shoreline and the littoral processes affecting it. (Author)

Characteristics of Waves Broken by a Longshore Bar

Abstract: A two-dimensional model submerged offshore bar was installed in a Texas A&M Hydrodynamics Laboratory wave tank Monochromatic waves with a range of heights and periods were generated at this bar for three different depths of water over the bar For each wave, water surface time-histories were measured at points before and after the bar and spectral analyses of these measurements were performed The analysis of each wave record yielded an equivalent wave height which is proportional to the square root of the wave energy per unit surface area The ratio of the reformed to incident equivalent wave height is shown to relate to the ratio of incident wave height to water depth over the bar The predominant periods of the reformed waves are found to be the same as for the incident waves but the presence of energy at higher frequencies is also observed The cause of these higher frequency waves is discussed.

Comparison Of Nearshore Wind-waveMeasurements And Model Results In TheDevelopment Of A Coupled Wind-wave,Hydrodynamic And Sediment Transport ModelFor Estimating Coastal Zone Erosion

Abstract: This study is a part of a project to develop a connected sediment transport, windwave and 2D hydrodynamic model to estimate coastal erosion. The wind-wave model was adapted from SWAN-nearshore wave model. As a preliminary work, wind-wave model results were compared with nearshore wave height measurements from two sites in Finnish coast of the Baltic Sea. More comprehensive results of the development project will be published in near future.

Barge trains in a coastal seaway, part 1: model tests in oblique seas

Abstract: Tests of a 1/50-scale model of push-tow barge trains were carried out under various wave conditions, including wave-to-heading angle change, to obtain forces on tow connections and motions of the train. Test results are presented for lateral bending moment, longitudinal and vertical forces on tow connections and for oscillatory motions such as roll, pitch and heave, with variations in wave conditions (wave length, wave height and wave-to-heading angle), number of barges, types of connections (hinge, short and long linkage), loading conditions and speeds. Test results of forces and moments indicate that the lateral bending moment is the most critical component in the resultant force generated on connections of barge trains in waves. For example, the maximum magnitude of measured moment coefficient (moment/(beam x one barge displacement)) was approximately 0.25 at the middle connection of the three-barges and tug train in moderate waves (wave height/wave length = 1/100) at wave-to-heading angles of 120 degrees and 60 degrees. The maximum longitudinal force coefficient (force/one barge displacement) obtained in tests was approximately 0.05 in moderate head seas (wave height/wave length = 1/100). Lateral bending moment and longitudinal and vertical forces were roughly doubled when the wave height relative to the wave length was increased from 1/100 to 1/50.

Wave investigations in shallow water

Abstract: Examination of the significant heights of zero-crossing waves in the Elbe Estuary has yielded two noteworthy results: 1 In the deeper water of the estuary, the value of the quotient relating the significant and the mean wave heights is larger than on the bordering tidal flat. 2. The value of this function is dependent on the height of the waves; on the tidal flat this dependency is considerably more sensitive than in deeper water. With increasing wave height the value of significant wave height divided by mean height becomes smaller The propagation direction of waves moving onto the tidal flat is contingent upon the position of intertidal channels Such channels sharply reduce the possible propagation directions The waves nearly always move up-channel regardless of the wind direction It is possible to derive special wave period and wave height distributions representing the conditions m very shallow water.

Wave Forecasting and Hindcasting

Abstract: The purpose of this chapter is not that of providing instruction on how to make wave forecasts and/or hindcasts, but rather to illustrate the basic concepts and procedures which are employed by-specialists who regularly furnish forecasting services. The objective here is that of providing sufficient insight into the mechanics of wave forecasting and the underlying premises so that information contained in the forecast and/or hindcast can be used to maximum advantage.

Numerical Validation Of Some Wave HeightDistributions

Abstract: This paper studies, numerically, the validity of different wave height distributions and the minimum number of components needed to validate the distribution under study. Also, the effect of bandwidth of the wave spectrum is considered. Rayleigh distribution, the modified Rayleigh distribution and the BetaRayleigh distribution are considered in this study

Forces Induced by Breaking Waves

Abstract: Chapter VII dealt principally with fluid-induced forces and where the fluid motion appeared in the form of surface gravity waves, only non-breaking waves were considered. In this chapter, our concern is with the forces induced by waves breaking on structures. Generally, information is meager; however, some specific information is available in the case of plane barriers. Two cases of interest have been studied. First, we consider barriers oriented in the vertical plane and then those in the horizontal plane.

Invited PaperNumerical Simulation Of Coastal Changes

Abstract: Two-dimensional changes of beach profile are computed by the continuity equation of sediment. Wave transformation used in the continuity equation is obtained by solving the shallow water equations numerically. The application of this method for different wave height, period, and sand particle diameters shows that this approach is reasonable.