Showing papers on "Wave height published in 1975"

A mechanism for rip current generation on an open coast.

Abstract: Intersecting ocean wave trains of the same frequency will create periodic longshore variations in the mean water level both inside and outside the surf zone. In the surf zone this creates a longshore variation in wave height and wave-induced circulation as described by Bowen but previously attributed to edge waves and irregular bottom topography.

Extreme wave conditions during Hurricane Camille

Abstract: Ten hours of wave data recorded from a fixed platform during hurricane Camille were analyzed to determine wave heights and periods. Wave conditions were severe with a maximum measured wave height of 23.6 m. The analysis results indicate that a Rayleigh distribution can be used to compute wave height relationships for large hurricane-generated waves.

Computation of wave heights due to refraction and friction.

Abstract: The equations for the wave orthogonals and the wave heights are presented for depth refraction of regular, long-crested, small amplitude surface gravity waves over an arbitrary bottom, using time as the independent variable. The effect of turbulent bottom friction on the wave height is included. The computer outputs are in the form of automatically plotted wave orthogonals with the wave heights written at discrete points along these paths. Systematic tests are presented for water areas with straight and parallel bottom contours. The errors inherently connected with the introduction of a grid plus a set of formulas for the differentiation and interpolation are examined for a horizontal bottom with an infinite long sinusoidal threshold. For one plane sloping bottom, information is given for the influence of the initial curvature of the wave front, bottom roughness, and initial wave height.

Aspects of Surf and Runup Processes on Mixed Sand and Gravel Beaches

Abstract: Aspects of engineering-oriented and geomorphological runup studies are discussed in relation to the fact that there is little field data against which to check theoretical and model-derived conclusions concerning swashzone flow. Also, it is suggested that there has been little consideration of the effects of variability in wave trains on beach face changes. The "phase-difference" model of Kemp (1958) is examined and shown to be useful in this regard. The results of 21 experiments in which swash (2133) and backwash (1631) velocities were measured together with other surf and runup properties are presented and discussed. The data are for Hb 0.3 to 2.44 m; Tb 7.5-10.0 secs. and characterise non-barred, high energy, mesotidal, mixed sand and gravel beaches of mild slope (5-12°) at Kaikoura, New Zealand. Breaker height is shown to be the primary control of swash length as predicted by Kemp while phase of flow exerts important effects on the temporal structure and hence the net circulation of sediments entrained by the flow. Velocities were determined with a forceplate dynamometer having an accuracy of -15 cm sec-1. Average maximum runup velocity was 168.0 cm sec-1 and the average duration was 2.98 secs. Backwash velocities averaged 140 cm sec-1 with a mean duration of 4.25 secs. Runup velocities conformed to a near-Gaussian probability distribution and 2060 % of the incident wave velocity is transmitted to runup velocity. The relationship between relative runup velocity and breaker energy is negative-exponential in form for flow between the breakers and still water level. This result is the opposite of one obtained on steep, slopes in a model tank by earlier workers. Symbols Cb average velocity of breakers and shallow water wave velocity C. velocity at point x in the runup, i.e. a distance x from the breaker E energy per unit surface area g gravity acceleration H, Hb, HR wave height, breaker wave height, runup height H, Hb, HR the same; averages k a constant L wave length I swash length M. graphic mean grain size diameter n, N

Water Waves Generated by Landslides in Reservoirs

Abstract: A hydraulic model investigation has been conducted to ascertain the magnitude of wave height and runup to be expected from potential landslides in Lake Koocanusa at Libby Dam, Montana. Although this investigation is of a particular site, it furnishes valuable information and insight into general problems of waves resulting from landslides that might be pertinent to other areas. The model study results show the wave heights and runup to be expected at various pool elevations as well as the effect and corrective measures proposed to reduce wave heights to an acceptable level. This paper presents the Libby prototype problem, model concepts, some experimental data, and conclusions pertinent to the prediction of landslide generated water waves.

The Theory of the Rayleigh Distribution and Some of Its Applications

Abstract: The first part of the paper deals with the mathematical properties of the Rayleigh distribution, including but not limited to the density function, the moment generating function, the maximum likelihood estimator, confidence intervals, and the bivariate Rayleigh distribution. The second part deals with applications of the Rayleigh distribution to the analysis of the response of marine vehicles to wave excitation. These applications are illustrated with regard to ocean waves, short-and long- term bending moment responses of a ship, and the "target problem" applied to the relationship between wind speed, wave height, and stress.

Acoustic miniprobing for ocean microstructure and bubbles

Abstract: Analysis of sound phase and amplitude fluctuations over a 1-m range has provided in situ statistics that can be used to infer the statistics of the ocean microstructure. A particularly fruitful use of such a simple acoustic miniprobe is the study of the sound phase fluctuations near the sea surface. With the aid of simultaneous statistics of temperature and wave height the observed sound phase fluctuations have been used to calculate bubble statistics at sea. At sound frequencies less than about 25 kHz the sound speed dispersion and its fluctuations reveal the bubble volume fraction and its standard deviation, respectively. At higher sound frequencies a large resonant bubble population can be identified by a cross correlation of sound phase and ocean wave height. In this case the predominant part of the sound phase modulation is caused by the changing bubble radius due to the fluctuating wave height. The spectrum of the sound phase modulation then mimics the ocean wind wave spectrum, and its change with depth can be used to infer the change of number of resonant bubbles with depth.

The theory of wave propagation in water of gradually varying depth and the prediction of breaker type and height

Abstract: As ocean waves shoal and approach the shoreline, they peak and finally break, usually as either plunging or spilling breaker types. This study applies the theory of wave propagation in water of gradually varying depth, as developed by Biesel (1952), to determine the dependence of breaker type on the beach slope tangent s and on the deep water wave steepness H∞/L∞, the ratio of the deep water wave height H∞ to the deep water wave length L∞. By representing the fluid motion at the surface in Lagrangian coordinates, a graph (Figure 7) is developed for the breaker type on the basis of 21 combinations of s and H∞/L∞. A comparison with laboratory wave tank data on breaker types shows good agreement with this graph based entirely on theory. The steepening of the shoaling wave profile on the shoreward wave face leads to the occurrence of a vertical surface. The first appearance of such a vertical surface provides a natural breaking criterion compatible with experimental usage. On the basis of this breaking criterion the dimensionless ratios Hb/H∞ and Hb/hb, where Hb is the breaker height and hb the depth at breaking, are theoretically evaluated. The well-known dependence of Hb/H∞ on H∞/L∞ is also correctly demonstrated by the theoretical approach.

Wave Height Distribution around Vertical Cylinder

Abstract: In the present study, wave heights and dynamic pressures at the still-water level on half of a vertical cylinder surface are measured Since the height is measured only between 0° and 180°, the symmetry of the distribution cannot be shown However, Galvin and Hallermeier established in their study that symmetry existed in all cases tested The purpose of this study is to show further that the height distribution follows crest distribution and dynamic pressure distribution which, in turn, suggests that even the pressure transducers around the structure may be used to establish direction of the wave

The radar cross section of the sea at 1.95 MHz: Comparison of in‐situ and radar determinations

Abstract: The rms wave height of seven sec ocean waves determined by remote measurements using high frequency radar is compared with that determined by direct in-situ measurements using an accelerometer buoy. The radar-measured values vary as the buoy-measured values for various sea and wind conditions, but the radar values are systematically low with respect to the buoy values by about 2.8 db. The radar experiment is more straightforward than conventional measurements because the same receiving antenna is used to sample both the illuminating and echo fields. Radar-measured values for 〈σ0〉 for seven-sec ocean waves under homogeneous sea conditions did not exceed −29.7 db.

Wave height measuring device

Abstract: The invention relates to a wave measuring instrument for use on a dynamically positioned or tethered floating vessel whereby to continuously monitor wave height, wave period and vertical heave of the vessel as the vessel responds to waves, swell and currents. The instrument comprises a measuring device connected to the vessel hull in a substantially submerged position whereby to continuously record the distance between the instrument and the ocean floor as the vessel heaves. A second measuring means automatically gauges and records the normally varying distance between the instrument and the water's surface, which distance results from the usual presence of wave action whether the latter be gradually rolling or relatively turbulent. The respective measurements thus obtained are thereafter integrated and compensated for to provide a relatively accurate record of the actual wave height between trough and crest, the wave period, and the vertical movement or heave of the vessel.

Optimum Filtering and Smoothing of Buoy Wave Data

Abstract: The problem of estimating ocean wave heights given accelerometer and pressure measurements from a spar buoy is described. A Kalman optimum filter and a smoother are used to combine these measurements to estimate four state variables: wave height and its time integral, and buoy vertical displacement and velocity. The time history of the ocean waves is presented as a second-order system driven by white noise. The buoy dynamics in the vertical are represented as a second-order system driven by the waves with additional white noise. Numerical computations were carried out for the state vector of order four using five minutes of digitally recorded data from an actual spar buoy. One of the state variables, buoy vertical displacement, is compared directly with the accelerometer output as doubly integrated by another numerical technique and found to be slightly more stable. Inspection of the measurement residuals from the filter and smoother suggest that the model or parameters chosen are not in final form. this paper the state vector approach and optimum esti- ation theory are used to estimate the height of ocean waves. The impetus for this problem came from difficulties encountered in analyzing wave and motion data from Flordia State University's 100-ft buoy TRITON.) This buoy was primarily intended as a platform for meteorological in- struments, but as a side project it was equipped with resistan- ce wire gages. Early attempts at numerical double integration of the vertical accelerometer output diverged rapidly. While studying the buoy motion problem the author in- vestigated a work 2 which applied the Kalman filter to the study of ocean current meter dynamics, but was not able to pursue the problem further at that time. The work described here was begun two years later using data from a smaller spar buoy, Lockheed's Measurement and Comparison System (MCS).3 The particular problem considered was to compute a wave spectrum or time history from digitized records of relative wave height and apparent buoy vertical acceleration. Since the vertical motion of the buoy was a significant fraction of the wave height over much of the frequency range, it was not possible to combine the low frequency part from the ac- celerometer and the high frequency part from the wave gage. Furthermore it would be difficult to combine spectral com- ponents at each frequency because of unknown phase relations. Thus, it was necessary first to work in the time domain and construct a history of the ocean surface, which requires numerical double integration of the vertical ac- celerometer output. At least three factors make this numerical integration difficult: unknown initial conditions, non- verticality of the accelerometer, and the finite quantization step in the digitization. An ordinary numerical quadrature leads to rapid divergence. In the work reported here the problem was solved by the Kalman filter, which has been successfully applied to such areas as inertial navigation and orbit estimation. The results of its application to a single set of data from the MCS3 buoy are given below and possible future extensions of the method are outlined. Because the Kalman filter is a time domain technique, no spectral calculations are presented. The remain- der of this paper contains additional background on measurements from buoys and the Kalman filter.

Electronic wave analysis

Abstract: Method and apparatus, suitable for remote deployment, for measuring ocean water level and utilizing high-efficiency operational amplifiers of the Field Effect Transistor type to calculate and retain the maximum water level, mean tide level, average root mean square wave height, maximum wave height and number of waves for transmission, upon demand, to a local information center.

Waves of maximum height on uniform currents.

Abstract: Offshore design often involves the use of maximum wave height for a given water depth, yet the presence of a current is usually neglected. A constant current, having the same magnitude over the water depth, can be simply included in design by relating the wave propagating on a current to an equivalent wave propagating in still water. By relating the maximum height values for still water to those on a current, a breaking index curve is generated for waves on uniform currents. The effect of the current on the wave crest displacement is also examined. Finally, a curve is presented to show the magnitude of the maximum opposing currents for which wave groups can propagate upstream. Two examples are provided to illustrate the use of the results.

Maximum heights of ocean waves

Abstract: This paper includes laboratory and field measurements of maximum wave amplitudes of irregular waves. Few measurements of maximum irregular wave heights have been available previously, and the results in this paper will aid in evaluating limiting wave proportions. The data in section 2 of this paper cover observations of breaking waves in a laboratory irregular wave system and results for the largest waves in field observations for severe weather conditions in the North Atlantic. The data are compared with theoretical predictions based on the assumption that wave breaking is the predominant factor limiting wave height. These results may be used to estimate maximum wave conditions for design purposes when field data for maximum wave conditions are not available. Differences in wave spectra for equilibrium wave conditions are investigated in section 3, and it is concluded that the proportion of breaking waves in an irregular wave record explains the variations observed.

Floating Breakwaters-A Wave Tank Study

Abstract: As examples of the direct application of a floating breakwater, the author cities the desired protection of certain offshore operations, such as pipeline laying and tanker loading and unloading. This paper describes a laboratory study undertaken to establish which parameters affecting a breakwaters' performance are most important. Results show that wave reduction was not greatly affected by wave height. With regard to the breakwater itself, a square cross-section gives slightly better wave reduction than a triangular, circular, or trapeziodal one. Permeable and impermeable coverings give essentially the same results. Concentrating the ballast low in the profile and using two-thirds to full submergence yielded best results. As indicated by the author, probably one of the most serious and least understood problems is that of the mooring of breakwaters. The result of this study indicates that mooring line forces can be reduced by using horizontal lines and by the proper choice of attachment point.

Dual frequency scatterometer measurement of ocean wave height

Abstract: A technique for remotely measuring wave height averaged over an area of the sea surface was developed and verified with a series of aircraft flight experiments. The measurement concept involves the cross correlation of the amplitude fluctuations of two monochromatic reflected signals with variable frequency separation. The signal reflected by the randomly distributed specular points on the surface is observed in the backscatter direction at nadir incidence angle. The measured correlation coefficient is equal to the square of the magnitude of the characteristic function of the specular point height from which RMS wave height can be determined. The flight scatterometer operates at 13.9 GHz and 13.9 - delta f GHz with a maximum delta f of 40 MHz. Measurements were conducted for low and moderate sea states at altitudes of 2, 5, and 10 thousand feet. The experimental results agree with the predicted decorrelation with frequency separation and with off-nadir incidence angle.

Wave Climate and Design Waves at Moffat Beach - a Specific Analysis with Some General Implications

Abstract: An analysis of wave recordings made at Moffat Beach, Queensland, with an OSPOS wave recorder is presented. Long term wave climate information is obtained by using the recorded data to calibrate visual observations of sea and swell state made at Cape Moreton. Comparison of several representations of the long term wave height distribution indicates that estimates of design waves with a given return period vary appreciably. Wave data obtained with Wave Eider buoys off the Gold Coast is found to give wave height data and climate variations generally similar to the OSPOS data. The large differences in wave period found lead to a questioning of the suitability of the average zero crossing wave period T as a representative wave period in situations where sea and swell occur simultaneously on wave records.

Structural evaluation of fixed offshore platforms

Abstract: In fixed offshore structures, damage incurred below the waterline is often difficult to detect, but significant enough to be the source of a subsequent massive failure. A technique is described that can be used to detect subsurface structural failure by detecting changes in the natural frequencies of the structure. One tower aas extensively studied; the dynamic measurement and analysis techniques are described. A parallel computer model of this tower was used to simulate the effect of removal of structural members on natural frequency. The parameters which determine the level of minimum detectable damage are discussed. Experimental data correlating wind and wave height spectra to observed structure response is presented. Statistical Energy Analysis is introduced as a method for predicting the dynamic response of offshore towers to random waves. The method is superior to the classical random vibration approach, in that it does not require the calculation of the wave force spectrum from the wave height spectrum, thus eliminating the calculations and assumptions common to the frequently used Morrison wave force equation. SEA is also applicable to a broad range of fixed and floating structures.

A laboratory study of wind-wave-current interaction. part ii

Abstract: : Two series of experiments were performed to study the interactions between both the wind-induced drift layer and the surface wave spectrum and a spatially varying subsurface current field. In the first, measurements were made of the response of the drift layer and surface wave spectrum to an appreciable, but unknown, current gradient. This was accomplished by forcing the current from a 90 cm-deep diffuser section onto a 68.7 cm-deep flat beach. The flow, thereby, experienced a current gradient which diminished to, and remained essentially at, zero a short distance downstream of the beach leading edge. Relaxation times inferred from the data are on the order of 1 to 3 minutes. For the second test series, the beach was set at an angle of 2.65 deg to produce strain rates. Here again, the results indicate that the response of the drift layer lags behind that of the current, as strained by the beach. The measurements of surface wave spectra taken for each of the conditions tested are presently being analyzed in detail; however, some preliminary results are presented.