Showing papers on "Wave height published in 1977"

Extraction of wave parameters from measured HF radar sea-echo Doppler spectra

Abstract: Computerized techniques for extracting rms wave height and dominant wave period from HF radar sea-echo Doppler spectra are derived. Earlier theoretical models for first- and second-order sea backscatter (derived elsewhere) are employed to obtain the simple, closed-form inversion equations giving these two radar-deduced quantities. The results are general in that no specific models for the radial or azimuthal form of the wave height directional spectrum need be assumed; the resulting formulas are only weakly dependent upon the radar/wind direction. Approximations made in the derivation are stated, and they are ultimately tested by comparing wave heights and periods extracted from the inversion model with the actual values input to the original equations for scatter. The derived relationships are then tested against some eighty hours of radar measurements taken at a variety of frequencies and buoy-measured sea conditions. Theoretically predicted and empirically determined correction factors for wave height and period are in agreement. Finally, the measurements show that when sea wave height is greater than one twentieth of the radar wavelength, wave height extraction (rms) errors are less than 23%, and wave period extraction errors less than 12%.

Influence of the amplitude of a solid wavy wall on a turbulent flow. Part 1. Non-separated flows

Abstract: Measurements of the shear-stress variation along and the velocity profiles above a solid wavy wall bounding a turbulent flow are presented for waves with height-to-length ratios of 2a/λ = 0·0312 and 0·05. These are compared with previous measurements of the wall shear stress reported by Thorsness (1975) and by Morrisroe (1970) for 2a/λ = 0·012. The investigation covered a range of conditions from those for which a linear behaviour is observed to those for which a separated flow is just being initiated.Pressure measurements indicate a linear response in that the spatial variation is described quite well by a single harmonic with a wavelength equal to that of the surface. However, the variation of τw for waves with 2a/λ = 0·0312 and 0·05 can be more rapid on the leeward side of the wave. The degree of departure from a sinusoidal variation increases with increasing wave height and fluid velocity and, from the results reported in this paper, it is suggested that nonlinear behaviour will become evident when au*/v [ges ] 27.Many aspects of the flow for all three waves are described by a solution of the linear momentum equations previously presented by Thorsness (1975) and by Thorsness & Hanratty (1977). These include the phase and amplitude of the pressure profile and the first harmonic of the shear-stress profile and the velocity field outside the viscous wall region.These results suggest that up to separation the flow is approximated quite well by linear theory. Nonlinearities affect the flow only in a region very close to the wave surface and are manifested by the appearance of higher harmonics in the variation of τw.

Field data support of three-seconds power law andgu *σ−4-spectral form for growing wind waves

Abstract: Observational data on air-sea boundary processes at the Shirahama Oceanographic Tower Station, Kyoto University, obtained in November, 1969, was analyzed and presented as an example representing the structure of growing wind-wave field. The condition was an ideal onshore wind, and the data contained continuous records of the wind speed at four heights, the wind direction, the air and water temperatures, the tides, and the growing wind waves, for more than six hours. The main results are as follows. Firstly, in both of the wind speed and the sea surface wind stress, rather conspicuous variations of about six-minute period were appreciable. Secondly, the three-seconds power law and its lemma expressed byH*=BT*3/2 andδ=2πBT*−1/2, respectively, are very well supported by the data, whereH*(≡gH/u*2) andT*(≡gT/u*) are the dimensionless significant wave height and period, respectively,δ the wave steepness,u* the friction velocity of air,g the acceleration of gravity, andB=0.062 is a universal constant. Thirdly, the spectral form for the high-frequency side of the spectral maximum is well expressed by the form ofΦ(σ)=αsgu*σ−4, whereσ is the angular frequency andΦ(σ) the spectral density. The value ofαs is determined as 0.062±0.010 from the observational data. There is a conspicuous discrepancy between the spectral shape of wind waves obtained in wind-wave tunnels and those in the sea, the former containing well-defined higher harmonics of the spectral peak, and consequently there is an apparent difference in the values ofαs also. However, it is shown that the discrepancy ofσs may be eliminated by evaluating properly the energy level of the spectral form containing higher harmonics.

Observations of wave height and wave celerity in the surf zone

Abstract: Observations of wave crest elevation, wave trough elevation, and wave celerity have been made in the surf zone on a natural beach. A series of 22 wave poles having vertical gradations of 7 cm (near shore) and 11 cm (off shore) were placed across the surf zone from outside the break point to the swash zone. Movements of 10 individual waves all having a break point within one wave pole spacing of each other have been photographed, and the data on wave height changes and wave speed changes have been analyzed. Wave celerity within the surf zone, given as a ratio M to solitary wave celerity, shows a systematic increase of wave speeds near the break point to a peak of M ≃ 1.2, thence a decrease farther shoreward to M ≃ 0.8, and finally a second increase where M > 1. Wave height decay after breaking follows the theory of turbulent dissipation recently presented by Sawaragi and Iwata (1974). The wave height to water depth ratio within the surf zone is a function of distance from the break point and ranges from 2.0 to 0.6. The results indicate that the use of linear and nonlinear nonviscous wave theories to quantify surf zone wave characteristics is misleading insofar as quantitative prediction is concerned.

Wave Action and Sediment Transport on Fringing Reef.

Abstract: : Measurements of wave processes, wave-driven currents, and sediment distribution have been made in several fringing reef systems. Wave height and wave period are typically reduced by about 50% as waves pass over the reef crest. This decrease depends primarily upon reef crest water depth, so that wave conditions in the back-reef lagoon show significant changes over a single tide cycle. Wave-driven currents tend to flow continuously onshore over the reef crest. Their velocity is greatest near low tide, when wave breaking is most intense. Current in the lagoon moat generally showed a tendency to drain the lagoon except during brief intervals near flooding tide when a weak current reversal occurred. Sediment distribution in the lagoon displays a pattern that reflects current patterns in the lagoon and wave characteristics at the lagoon shoreline. (Author)

Nonlinear wave interaction with vertical cylinder of large diameter

Abstract: A nonlinear diffraction theory for interaction of waves with a vertical cylinder of large diameter is presented. The nonlinear second-order solution is examined in comparison with a linear solution and other existing second-order solutions. The computed nonlinear wave forces are found to compare very well with the experimental results. The effect of nonlinearity on the crest height distribution around the cylinder is also studied. It is found that as the ratio of wave height to water depth decreases the nonlinear solution approaches the linear solution.

The Energy Transfer Due to Air-Input, Non-Linear Wave-Wave Interaction and White-Cap Dissipation Associated with Wind-Generated Waves.

Abstract: : Miles' theory is unsatisfactory for the prediction of ocean wave growth under wind action. Hasselmann's nonlinear wave-wave interactions theory may offer another physical mechanism for constructing a reasonable wave prediction model. An experimental program was designed to investigate this possibility in the context of the momentum and energy transfer from wind to waves, nonlinear wave-wave interactions and white-capping dissipation. The wind field (mean velocity profiles and fluctuating pressure and velocity components) and wave field (wave height) were measured simultaneously in a fixed reference frame and as a function of fetch along the Stanford wind-wave channel under the conditions of steady wind and stationary wave spectrum. All the data were obtained 5 mm above the highest point of the wind waves for five stations (3 m apart on average) and at three wind speeds (7.09, 8.01 and 8.88 m/sec). The wave height, fluctuating pressure and velocity components were measured by a capacitance wave height gauge, a crystal pressure transducer and a cross hot film probe, respectively.

Determination of ocean wave heights from synthetic aperture radar imagery

Abstract: A calculation is presented for the cross-correlation of the radar images obtained by processing the same signal data over different portions of the chirp spectrum bandwidth as a function of the center frequency spacings for these portions. This is shown to be proportional to the square of the product of the characteristic function for ocean wave heights and the pupil function describing the chirp spectrum bandwidth used in the processing. Measurements of this function for ocean wave imagery over the coast of Alaska, the North Atlantic, and Monterey Bay, California, and correlation with the significant wave heights reported from ground truth data indicate that the synthetic aperture radar instrument can be used for providing wave height information in addition to the ocean wave imagery.

Theoretical study to determine the sea state limit for the survival of oil slicks on the ocean

Abstract: : Limiting sea state condition for the globular dispersion of oil slicks on the ocean have been obtained from simple considerations. Models have been derived for determining the conditions under which breaking waves will disperse an oil slick. Two types of dispersions are considered, viz., surface dispersion and globular dispersion within the water column, with emphasis on the latter type of dispersion. The results indicate that a 3 m significant wave height sea state will tend to initiate globular dispersion and that breaking waves of large crest lengths are essential for a surface dispersion to take place. The depth of penetration of globules of oil formed under breaking waves is calculated to be relatively shallow. The large number of assumptions made and their effects on the results are discussed. Several of the unknown physical parameters of importance are identified and recommendations have been made for experimental determination of some of these parameters. (Author)

Closed-Form Expressions for Determining the Fatigue Damage of Structures Due to Ocean Waves

Abstract: Closed-form mathematical expressions are derived for the fatigue damage of structures due to ocean waves. The expressions incorporate relationships between wave height and stress range, between stress range and number of cycles to failure (i.e. a fatigue curve), and the probability distribution for the occurrence of wave heights. The expressions can be utilized to predict the fatigue damage resulting from a single sea state, from a storm, or during the service life of a structure. In addition, the fatigue life of a structural element can be determined directly from the stress range resulting from the design wave, or conversely, an "allowable stress range" can be determined for the design wave which will insure a specified fatigue life. Example applications are given for areas having wave climates similar to the North Sea and similar to the Gulf of Mexico.

Prediction of irregular wave overtopping

Abstract: : Rates of wave overtopping of structures can be predicted by means of an equation presented in the Shore Protection Manual (SPM) (eg. 7-6, U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1975; Weggel, 1976). Implicit in the use of this equation is the assumption of monochromatic waves; i.e., waves of uniform height and period. A question is given on what would the overtopping rate be fore irregular wave conditions having a significant height equal to the height of the monochromatic wave used in the SPM equation. An answer is given: the irregular wave conditions are assumed to cause the runup to have a Rayleigh distribution of the type commonly associated with wave heights. The runup with a Rayleigh distribution is used in the SPM's wave overtopping equation to predict average overtopping rates for irregular waves having a particular significant height. Example problems are given to illustrate how this technique can be used to predict overtopping rates for irregular wave conditions. A table, which can be used to calculate the irregular overtopping rate using the monochromatic rate for equivalent wave conditions is also given.

Mission design for SEASAT-A, an oceanographic satellite

Abstract: SEASAT-A is a NASA earth satellite for measuring global ocean dynamics from space. Launch is planned for May 1978. The instruments on the spacecraft will provide data on wave height and direction, surface wind speed and direction, ice fields, ocean surface topography and weather. This paper is concerned with the mission design for SEASAT-A. A primary topic here is the selection of the orbit which best satisfies the measurement objectives of the various instruments. The maintenance of this orbit under drag and other perturbations is discussed. The design of the mission profile is outlined. Finally precision orbit determination for SEASAT-A is discussed since it is required to maximize ocean topography accuracy.

Analysis of short-term variations in beach morphology (and concurrent dynamic processes) for summer and winter periods, 1971-72, Plum Island, Massachusetts

Abstract: : An analysis of the relationship between wave and meteorological variables and beach morphology was undertaken during summer and winter periods, 1971-72, on Plum Island, Massachusetts. The variables were wave period, wave height, breaker type, breaker angle, longshore current velocity, wave steepness, breaker power, windspeed and direction, barometric pressure, air and water temperature, and ground water elevation. Daily topographic maps of the intertidal zone were constructed for 12 beach profiles spaced at 60-meter intervals. Variations in beach process variables, during both the summer and winter periods, were directly related to the passage of high- and low-pressure systems and to the proximity of the system to Plum Island. With an increase in breaker power and breaker steepness, the high tide beach-face gradient increased. Differences in beach morphology within a small area appear to reflect the state of recovery of the beach profiles after a storm. Adjacent profiles at different stages of maturity are controlled by the proximity of the nearshore bar. The closer the bar is to shore, the faster the sediment is returned to the beach zone.

Problems of Imaging Ocean Waves with Synthetic Aperture Radar.

Abstract: : Progress is reported on three tasks designed to provide information to evaluate the operational use of SAR, so that ultimately, meaningful ocean shallow-water wave spectra and coastal wave height information can be supplied to the Navy in real-time. Under Task 1, a limited literature search was conducted to ascertain problems encountered in imaging sea states with SAR. The search indicated four fundamental problem areas where basic research is needed, before SAR can become a useful tool for the Navy. Under Task 2, solutions to SAR signal processing problems were explored and geometrical coorections of SAR wave imagery were considered.

Ocean Wave Statistics for 1961 North Atlantic Storm

Abstract: Various statistical quantities for a random wave system are derived by different proposed means and compared to each other. A series of ocean waves recorded during a severe storm in the North Atlantic in September 1961 is utilized for this purpose. The data includes the growth and decay periods of the storm. Such quantities as the significant and maximum wave heights, wave periods, and spectral width parameter are obtained. The analysis is carried out both in the time and frequency domain. The two methods are found to show reasonably good agreement in their results. The distributions of the individual wave heights and periods in a record and their joint distribution are investigated. The histograms for these parameters compared favorably with simple theoretical models for narrow-band spectra.

On Ripple of Annular Two-Phase Flow : 2. Characteristics of Wave and Interfacial Friction Factor

Abstract: Several empirical equations have been proposed of the interfacial friction factor in a gas-liquid tow-phase annular flow, but there exists a significant inconsistency among the results calculated by these correlations. In addition, none of them indicated good agreement with the present experimental values obtained in ripple flow regime. An experiment was made to collect the information on main parameters characterizing air-water interface, such as wave height, wave propagation velocity and wave frequency, possibly interrelated with the interfacial friction factor. Using the results, there of a new relationship between the interfacial friction factor and the parameters was introduced.

Los angeles and long beach harbors model study. report 4. model design

Abstract: This report describes the design of the Los Angeles and Long beach Harbors hydraulic model for investigation of harbor oscillation characteristics of the existing harbor and for evaluation of various proposed harbor modifications. The effects of wave refraction, diffraction, viscous friction, wave reflection, wave transmission through the harbor breakwaters, wave filters, and wave absorbers were considered. Wave generator design, automated model data acquisition and control, and model data analyses are also described. This report is the fourth in series Technical Report H-75-4 published under the general title Los Angeles and Long Beach Harbors Model Study. A model distortion ratio of 1:4 and a vertical scale ratio of 1:100 (1:400 horizontal scale ratio) were selected to minimize the model area and to provide a vertical scale ratio where accurate model wave measurements could be assured. The model reproduces approximately 253 square miles of prototype area. Model limits were selected to enclose a strong convergence zone seaward of the harbor breakwater. Adjustment of the initial generated wave-front position in the model was necessary to compensate for model distortion and the maximum model depth of -300 ft mllw. The electrohydraulic wave generator used in the model study is composed of 14 separate 15-ft units for a total wave generator length of 210 ft and is capable of (a) generating waves with a prototype period ranging from 15 to 600 sec, (b) generating a wave with small variation in period and height between units, (c) defining resonant response over a narrow period band, (d) generating a variable wave height along the wave front, and (e) generating a curved wave front.

Wave-wave interactions near the shore

Abstract: Wave-wave interaction on a beach may be regarded as a rather special example of second order, resonant interaction within a rapidly changing wave spectrum. However, the existence of trapped modes, edge waves, having a very different dispersion relation from that of the incoming waves provides the possibility of transferring energy efficiently to much lower frequencies than are normally observed in the open sea. Any detailed analysis of these interactions is, however, greatly complicated by the breaking of the incoming waves as they reach water depths of the order of their wave height. Recent field and laboratory data suggest that,although the wave breaking introduces new effects, nearshore currents and set-up for example, the forcing of the purely wave-wave interaction is not greatly altered by the breaking process. However, the increased effective viscosity of the region associated with the turbulent surf zone seems to play a significant role in suppressing resonance. Given equal forcing, edge waves whose offshore length scales are large compared to the surf zone width are therefore more likely to exist.

Rough Weather Drilling Vessel Simulation

Abstract: Selecting an optimum rig design is not easy, given the many different designs proposed for rough-weather areas, but the task has been simplified by a drilling vessel simulator that calculates down time and pinpoints operations sensitive to weather-caused interruptions. Two interacting parts simulate offshore drilling operations and weather interference. While the operational model "drills" a well, the weather model generates wave heights and periods that determine vessel motions which are checked against operating limits. The weather model simulates both wave height and period, whereas previous simulators generate only wave height.

On the hysteresis of the sea surface and its applicability to wave height predictions

Abstract: Because of the low dissipation rate of wave energy on the ocean's surface, the wave height at some location and time must be dependent upon wind fields in existence there at previous times and upon swell propagated there from other regions. To study these relationships, significant wave height (SWH) measurements from the Geos-3 radar altimeter are used in conjunction with anemometer windspeed measurements from weather ships, L, C, and R. During the passage of large cyclonic disturbances near the fixed locations of these vessels in the North Atlantic in February 1976, distinct hysteresis profiles that characterize the sea's memory during generation and dissipation conditions are observed. Examples are given that demonstrate the influences of cyclone intensity, movement, velocity, and shape on the configuration of these profiles.

Laboratory study of a stationary oblique plunging breaker for surfboard testing

Abstract: Consider a water wave which is obliquely incident on a sloping beach. An observer looking along the wave front from the beach sees the portion of the wave closest to him to be already broken while that far from him is still in deep water and unbroken. In the transition region, the waveface reaches its maximum slope and this is therefore the part most suitable for the surfboard rider, who uses the waveface much like a skier uses a mountain. Because the wave breaks when the water depth reaches a particular value, the path of the break point, and therefore of the surfboard rider, is along the sea bottom contour. His average velocity is that of the intersection of the bottom contour and the wave. He therefore moves faster than the propagation speed of the wave even if his velocity is constant.

Estimating wave energy from a wave record

Abstract: This note is concerned with the calculation of wave energy from a time series record of wave heights. Various methods are used to estimate the wave energy. For wave records that contain a number of different frequencies, it is shown that the most accurate method for calculating the wave energy is by the use of Fourier analysis.

Bed shear stress coefficient within the surf zone.

Abstract: : An analytical formulation of the bed shear stress coefficient inside the surf zone is derived using the concept of radiation stress. A truncated Rayleigh p.d.f. is used to describe the wave field inside the surf zone and provides the input to calculate the variation of wave energy and longshore current as a function of wave height, water depth and distance to shore. The wave set-up is approximated using a sinusoidal wave solution. Field measurements of longshore current and waves within the surf zone are used to calculate the bed shear stress coefficient. Frequency distributions and statistics are calculated for the bed shear stress coefficient.