Showing papers on "Wave height published in 1989"

Structure of the turbulent flow field under breaking waves in the surf zone

Abstract: The structure of turbulence and its role in the breaking wave dynamics within the surf zone have been investigated through laboratory experiments using several flow visualization techniques and a fibre-optic LDV system. The results indicate that there exists a characteristic structure of large-scale eddies referred to here as ‘horizontal eddies’ and ‘obliquely descending eddies’, which has a significant role in the generation of Reynolds stress and thus affects the deformation of the mean flow field. The experiments also reveal that these eddies caused by the wave breaking bring a large amount of vorticity (with non-zero average) into otherwise almost irrotational velocity fields, resulting in the generation of vorticity-related mean flow fields as well as turbulence (vorticity-containing velocity fluctuation). This means that the breaking waves in the surf zone can be regarded as pseudowaves which consist of irrotational velocity components as ‘wave motion’ and appreciable amounts of rotational mean velocity components as ‘eddying motion’ (with non-zero mean vorticity) together with turbulence. It is found that the generation of the mean rotational velocity component due to wave breaking causes considerable increase in mass and momentum transport, as compared with ordinary non-breaking waves, and thus a decrease in wave height.

Random wave runup height on gentle slope

Abstract: An extensive series of labroatory tests were conducted and these tests led to the development of a formula to predict runup elevation of random waves on gentle, smooth and impermeable slopes, as a function of surf similarity parameter. On gentle slopes, a bore advancing into the shoreline cannot run up when the back‐rush of a preceding bore is large or when it is overtaken and captured by a subsequent large bore. No correspondence between individual running‐up bores and runup crests can be seen; the number of runup crests is reduces compared to the number of running‐up bores. This paper also proposed a formula for the ratio of the number of runup crests to that of incident waves; the formula can be used to estimate the mean repetition period of runup crests. The formulas proposed here are applicable to slopes, tan θ, ranging from 1/30 to 1/5 and to deep water significant wave steepness ranging from 0.007 to 0.07.

Wave transformation over coral reefs

Abstract: Ocean wave attenuation on coral reefs is discussed using data obtained from a preliminary field experiment and from the Seasat altimeter. Marked attenuation of the waves is observed, the rate being consistent with existing theories of bottom friction and wave breaking decay. In addition, there is a significant broadening of the spectrum during propagation across reefs. Three-dimensional effects, such as refraction and defraction, can also lead to substantial wave height reduction for significant distances adjacent to coral reefs. As a result, a matrix of such reefs provides significantly more wave attenuation than may initially be expected.

A new equation for calculating wave loads on offshore structures

Abstract: This paper derives an equation for the potential-flow wave loading on a lattice-type offshore structure moving partially immersed in waves. It is for the limiting case of small lattice-member diameter, and deals entirely in member-centreline fluid properties, so that it can be applied computationally by a simple ‘stick model’ computer program. This field is currently served by a simple two-term semiempirical formula ‘Morison's equation’: the new equation is effectively a replacement for the Morison inertial term, allowing the Morison drag term (or some refinement of it) to describe exclusively the effects of vorticity, which can in principle be calculated to greater accuracy when isolated in this way.The new equation calculates the potential-flow wave load accurate to second order in wave height, which is a great improvement on ‘Morison's equation’: such results can currently only be sought by very much more complicated and computationally intensive methods, of currently uncertain repeatability. Moreover the third-order error is localized at the free-surface intersection, so the equation remains attractive for fully nonlinear problems involving intermittent immersion of lattice members, which are currently beyond even the most sophisticated of these computationally intensive methods. It is shown that the primary reason for this large contrast in computational efficiency is that the loads are derived from energy considerations rather than direct integration of surface pressures, which requires a lower level of flow detail for a given level of load-calculation accuracy.These improvements must of course be seen against the current levels of uncertainty over the calculation of vorticity-induced loads, which in many applications completely dwarf inaccuracies in potential-flow load calculation. The conditions are accordingly established under which the improvements are comparable to the total wave load predicted by the Morison drag and inertia terms in combination. They are that the lattice member diameter is greater than its length/10, or the relative fluid motion/5, or the structure's motion radius/20, or the wavelength/30: if any one of these conditions is satisfied, the new equation is worthwhile even when used in combination with simple vorticity-induced load calculations from a Morison drag term.

Hindcasts and data assimilation studies with the WAM model during the Seasat period

Abstract: In the next decade a wealth of ocean surface data will become available through the launch of satellites such as ERS 1. We discuss the problem of how to make optimal use of this data. We have investigated the benefits of having a coupled wind-wave analysis over the oceans for quality assessment of satellite data, for monitoring the performance of the algorithms (e.g., for the scatterometer), and for producing consistent wind and wave analyses. Using a reliable wave prediction scheme, the WAM model, we show how to cross validate altimeter wave height with scatterometer data and how to construct an analyzed wave model spectrum from the altimeter wave height. The analyzed wave spectrum may be cross validated with the synthetic aperture radar image spectrum. Implications for improvement of, for example, the Seasat scatterometer algorithm are pointed out.

Groupiness Factor and Wave Height Distribution

Abstract: This paper examines wave height distributions in connection with wave grouping. The relationship between the groupiness factor, GF, calculated from Smoothed Instantaneous Wave Energy History SIWEH ...

Wave height estimation in stratified gas‐liquid flows

Abstract: Andritsos and Hanratty (1978b) have shown that the increase in interfacial drag caused by waves in stratified gas-liquid flows is related to the wave steepness. Recent analyses of finite amplitude Kelvin-Helmholtz waves are used to develop a correlation for the ratio of the wave height to wavelength.

Wind Wave Transformation

Abstract: Wave-induced velocities are the primary driving force for littoral sand transport. For this reason, a major component of the NSTS program was to measure wave associated velocity and elevation fluctuations. A description of the shoaling wave transformation is a necessary ingredient in the development of any sediment transport model.

Sea height wave form in equatorial waves and its interpretation

Abstract: In a previous paper a variation in sea slope of 1.6±0.6 cm per degree of latitude between 2°N and 4°N in Seasat altimeter data was associated with the passage of two temperature crests between 113°W and 130°W in the equatorial Pacific. This work has been extended to include variations in sea height between 1.5°S and 9°N with arbitrary phase in the wave. A mean wave pattern was found with maximum amplitude of 5 cm near 6°N. When the analysis was repeated for a range of wave periods, two preferred values are selected by minimum residuals. A period of 38.6±2.2 days corresponds with the passage of two observed temperature crests. A period of 22.2±0.9 days includes both these two crests and a possible intermediate crest of smaller amplitude. Fluctuating velocities in geostrophic equilibrium with the mean wave height are combined with a mean zonal flow to derive streamlines in a coordinate system moving westward with the phase velocity of the mean wave. One streamline shows the trochoidal form of the temperature front at about the appropriate latitude. Velocity field dependent estimates of the latitude of the crest and the angle of the crest are made. Flow immediately north of this streamline exhibits clockwise gyres near each trough. I suggest that this is the basic pattern of the observed form and motions in the waves. Also, selected streamlines are used to compute idealized drifter trajectories and compared with observations.

Physical Modelling of Littoral Processes

Abstract: Physical modelling of littoral processes involves the correct construction of the model, including the wavemaker system and the model basin, and the correct interpretation of the results in terms of the scaling This paper reviews these two aspects for wave tank and wave basin studies

Wave forces on pile in surface zone

Abstract: The wave forces in the surface zone for a single pile have been measured with regular waves in a laboratory. Reynolds numbers of up to 6×104 were used and the KC numbers were in the range 10–55. The wave force was found to be zero at a height ucr2/2g above the wave crest elevation and at a maximum approximately ucr2/2g below the wave crest elevation. The results have been analyzed in view of the Morison equation, and CD and CM coefficients have been obtained. Due to the fact that the measurements have been carried out with Reynolds numbers less than 6×104, it is recommended for design purposes to use the trend of the CD and CM values versus location on the pile as found from the laboratory tests in conjunction with CD and CM values as found for the appropriate flow regimes in full scale.

Joint Probability of Extreme Waves and Currents on Norwegian Shelf

Abstract: Simultaneous wave and current measurements on the Norwegian Shelf were used to estimate the appropriate current to associate with extreme waves in platform design. The primary data consisted of seven years of measurements at Tromsoflaket (71°30′N, 19 °E, 230 m depth), including 38 storms with peak significant wave heights above 7 m. The maximum wave height and maximum current occurred simultaneously in only one of the 38 storms. The wave and current timeseries data were used to calculate the time series of drag load on a simple structure and statistics of wave height and drag load were developed. The data showed that the in‐line current that must be added to extreme waves in order to preserve the extreme loads approaches 25 cm/s asymptotically as storm severity increases. Sensitivity studies suggest that this limit is largely insensitive to reasonable variations in current profile, wave kinematics theory, structure geometry, tidal current removal, length of data base, or site location. These results show ...

A parametric wave prediction model based on time delay concept

Abstract: Parametric wind-sea relationships for prediction of significant wave height (Hs) and zero-upcrossing period (Tz) for slight to moderate sea states have been presented in this paper based on the analysis of time series wind and wave data collected off west coast of India in March 1986. Wave measurements are made by deploying Data well wave rider buoy from an oceanographic research vessel Gaveshani. Observations revealed a series of growing and decaying phases of sea state with wind speeds ranging from 0 to 11.5 m/s .The characteristic feature of the proposed parametric model is the introduction of 'Time-delay' concept in place of wind duration limit. A time-lag of 6 hr is noticed between wind speed and wave height and same has been incorporated in this model which enables forecasting seas at 6-hourly synoptic time intervals. Model comparison is made and the predicted values of Hs and Tz closely agree with the recorded wave observations. Results show that the present method yields significant wave height prediction with an r.m.s. difference of 0.12 m for the observed Hs ranging from 0. 6 to 2.3m.

Modeling Cross-Shore Transport

Abstract: Seymour and King (1982) evaluated a number of existing models for cross-shore transport by testing their skill in predicting the observed beach excursions at Torrey Pines. The results were discouraging, in that none of the models showed a useful skill level. As discussed in Chapter 12, the Torrey Pines data set may not have provided a realistic test for these models. Therefore, the other three data sets described in Chapter 12 (Scripps Beach, Santa Barbara and Virginia Beach) were employed to evaluate models for cross-shore transport.

Sea Wave Simulation

Abstract: Sea wave simulation is a vast topic and growing all the time. What with time limited to two 90-minute lectures, one cannot go too deep. And yet, since this is a course, we must go as far as the actual formulas needed for the simulation. This entails the limitation of the number of subtopics. Many more methods both for computer and physical simulation could be mentioned; three-dimensional simulation could be much more extensively considered; but then, there wouldn’t be time to give the details, the actual procedures. So, directional problems, for instance, will be only briefly considered; for computer simulation, methods based on random amplitudes and random frequencies will be left out; for physical simulation, the generation of waves by wind blowers will not be considered.

Models for Surf Zone Dynamics

Abstract: Longshore current models have been tested using NSTS measurements at Leadbetter Beach, Santa Barbara. The NSTS provided data on nearshore currents for a variety of wave conditions including narrow banded (in frequency and direction) swell waves of small and moderate height and wide banded waves during local storms. The theoretical models include a two-dimensional, finite element, monochromatic wave driven model (Wu et al., 1985) and one-dimensional (i. e., no longshore variations) random wave driven model (Thornton and Guza, 1986). These models assume the waves are narrow banded in frequency and direction and use a single frequency and direction to describe the input wave field. In keeping with the narrow band wave assumption, data from four days with narrow band swell (3–6 February) were used for comparisons. Empirical orthogonal eigenfunctions were used to find longshore current patterns during all directional wave conditions (Guza et al., 1986). This chapter summarizes these papers and presents additional comparisons to a simple one-dimensional monochromatic wave model (Longuet-Higgins, 1970).

The modulation of the radar cross section of the ocean surface by an azimuthally traveling long gravity wave

Abstract: A new technique is used to calculate the modulation of the radar cross section of the ocean surface by a long gravity wave having a single narrow-band spectral peak. Spectral analysis of the measured wave height time series is used to determine the position on the dominant wave illuminated during each radar sample. When we average the samples of the signal power received from the same phase position (but during different cycles) on the long wave, the phase-interference (Rayleigh) fading is greatly reduced. The result is a measurement of the average radar cross section of the ocean surface as a function of the position on the long wave. The technique was applied to experimental runs where the radar beam was parallel to the crest of the dominant wave (cross-wave look direction). Significant modulation was observed at both C and X bands, although there was no tilt modulation. A scenario where hydrodynamic and aerodynamic effects give rise to the modulation is discussed. “Sea spikes” occurred primarily near the front face of the long wave, with some also appearing immediately after the crest has passed, supporting the theory that they are often associated with the breaking of ripple waves riding near the top of the long wave.

Shallow foreshore wave height statistics

Abstract: Wave height distributions on shallow foreshores deviate from those in deep water due to the effects of the restricted depth-to-height ratio and of wave breaking. Laboratory data of wave heights on shallow foreshores of different slopes have been analysed to determine these effects and to derive generalised empirical parameterisation. A model distribution is proposed consisting of a Rayleigh distribution or a Weibull distribution with exponent equal to 2, for the lower heights and a Weibull with a higher exponent for the higher wave heights. The parameters of this distribution have been estimated form the data and expressed in terms of local wave energy, depth and bottom slope, yielding a predictive model that is to be significantly more accurate that existing expressions.

Quasi- determinism of sea wave groups

Abstract: Provided we know that at a point x0,y0within an irregular gravity wave field, like those generated by wind on sea, at a time instant t0there is a wave with a height H great with respect to the mean, we can predict that that wave, with high probability, has been formed because of the transit of a well defined (deterministic) group, like that in Fig. 1. In mathematical terms: if the ratio between the known wave height H and the mean wave height tends to infinity, the probability that the true wave group is equal to the deterministic wave group plus a lower order random noise approaches 1 [1]. The effect of the random noise is the object of this paper. In particular, the effect on the mean heights and periods of the waves forming the group is estimated within errors of an order smaller than (H/ √m0)−1 (m0being the variance of the free surface elevation of the irregular wave field), The knowledge of the infinitesimal differences between the true wave group and the deterministic wave group, for H/ √m0→∞,proves to be useful for assessing the differences in the case that H is realistically great for a sea state. To that end data from numerical simulations of irregular gravity wave fields are used too. The conclusion is that, for a realistically great H, the deterministic wave group closely reflects the essential features of the true wave group.

Hindcasting of high wave conditions during typhoon 8712

Abstract: High wave conditions in the East China Sea during Typhoon 8712 (Dinah) are hindcasted with the use of two kinds of shallow water wave prediction models based on the radiative transfer equation. The models give reasonable agreement with the observed data. It is demonstrated that huge waves brought about by Typhoon 8712 greatly altered the spatial distribution of maximum significant wave height generated by typhoons for the past 50 years in the East China Sea, and that the return period for maximum significant wave height which occurred off Nagasaki during a typhoon is over 100 years. It is also deduced from the computation of wave transformation that the violent action of huge waves exceeding the design wave height of the breakwater was responsible for the destruction of the breakwater in the Shin-Nagasaki Fishing Port.

Test and evaluation of surf forecasting model

Abstract: : A model forecasting wave height and longshore current distribution inside the surf zone is applied to an extensive set of laboratory and field data for testing and modification. The models are tested on planar beaches as well as barred beaches for a variety of wave conditions. The wave transformation model is based on solving the energy flux equation using a bore dissipation mechanism and describing the random wave heights with the Rayleigh distribution. The two model parameters, B and gamma, where B describes the amount of foam of a breaking wave and gamma is the proportionality constant which relates the rms wave height to the water depth, are combined into a single parameter BG. BG is shown to be a function of deep water surf similarity parameter. Applied to the present data sets, the rms error of the measured wave height and the model predicted wave height was usually less than 9% and ranged from 1.5% to 15.7% with a mean of 5.3% and a standard deviation of 3.1% for the whole 74 data sets. The wave transformation model is highly robust in describing the wave height distribution in the surf zone. The longshore current model is based on solving the steady state, alongshore momentum balance for straight and parallel contours using the radiation stress concept. The model requires specifying the bed shear stress and turbulent mixing coefficients. Applied to the present data sets the rms error between the measured and modeled longshore current values ranged from 4.5% to 55.5% with a mean of 24.6%. Turbulent mixing is not required for planar beaches, but is required for barred beaches to describe the longshore currents inside the surf zone. Theses.

Improved Algorithm for Stream Function Wave Theory

Abstract: The theoretical prediction of water particle kinematics and pressure field under a progressive periodic wave in the laboratory requires the use of a suitable wave theory. An improved asymmetric stream function algorithm is proposed to represent measured periodic wave data. Numerical simulation for the periodic wave profile, in a least square error approximation to the free surface boundary condition errors, converges to the measured wave height and the mean water level. This algorithm essentially combines most computational features of Dean's irregular stream function program and Dalrymple's regular symmetric stream function program. The present algorithm is more accurate than the symmetric and asymmetric wave theories in the simulation of measured periodic wave profiles.

Measuring method for wave height

Abstract: PURPOSE:To measure a wave height with high reliability, by measuring the wave height with time by means of an ultrasonic wave which is transmitted in three directions by an ultrasonic echo sounder transducer held at a prescribed position under the surface of water. CONSTITUTION:An ultrasonic sonar 3 is provided with three sets of ultrasonic echo sounder transducers which transmit ultrasonic waves for measuring a wave height in three directions different from one another, respectively. A wave W advances along an advance line 9 perpendicular to the edge line 8 thereof and passes points C, A and B in this order. The height of the passing wave is detected with time by ultrasonic beams 7, 5 and 6 and a waveform is sensed therefrom. In order to avoid the mutual interference of the three ultrasonic beams 5, 6 and 7, a time difference is provided among the time points of transmission of the beams. By this method, the wave height is measured with high reliability.

Wave Height Changes and Mass Transport on Tague Reef, a Caribbean Reef, North Coast of St. Croix, U.S. Virgin Islands.

Abstract: This study ot wave and current transform ations across a modern corai reef w as conducted on Tague Reef, St. Croix ( U.S.V.I.), during late March and early April 1987. The project objective w as to improve our understanding of the magnitudes and frequencies a s well a s the spatial and tem poral variations of waves and currents a s they interact with a modern reef. Data were collected in 4 experiments over a 2-week period. During the observation period atm ospheric and s e a sta te conditions were typical of Caribbean-Atlantic trade wind dominated areas. Results verify that infragravity w aves of 27.7 min period and diurnal tides modulated high frequency wave and current p rocesses. P hase lags associated with th ese long-term motions cause s e a level differences across the reef crest that influence m ass transport of water and suspended sedim ent. Spectral analyses of wave data indicate that spectra shape w as conserved betw een the forereef and reef crest, and spectral broadening at the crest w as induced by redistribution and dissipation (average of 65%). Wave propagation into the backreef cau sed further spectral broadening and increased energy dissipation (average of 78%). W ater depth reduction during falling tides increased wave energy dissipation by 15% and 20% from forereef to crest and from forereef to backreef, respectively. A steady wave energy model, including frictional and turbulent dissipation, gives reasonable estim ates of wave heights across the reef. This model reveals that, in contrast to sandy beaches, bottom friction on coral reefs is com parable to turbulent energy dissipation. Currents at the reef crest occur on three important time scales: (a) short period (seconds), associated with wave activity, (b) infragravity fluctuations, and (c) diurnal variations of uncertain, but probably tidal origin. Vertical sh ear of these currents w as found to be negligible. The reef crest m ean and instan taneous flow fields suggest that seaw ater, and perhaps sedim ents and nutrients, are continuously transported lagoonw ards. In contrast, wave related instantaneous currents over the forereef indicate off-reef transport. A tim e-dependent, vertically integrated circulation model with surface slope and bottom friction explains the observed current patterns. Under low trade wind wave conditions the volume flux at the reef crest is 0.024 m3s _1, which can flush the lagoon in 37 hours.

Wave making pool

Abstract: PURPOSE: To easily attain desired wave height by providing a wave generating section, having a prescribed upward grade and a wave riding section having a small upward grade on a bottom section, and separately and independently controlling a plurality of wave making devices arranged on the deepest section side at the desired cycle and speed. CONSTITUTION: A wave making device 4 having a wave generating plate 3 is arranged in an end section width direction of the wave generating section 2 of a pool. A wave riding section 34 having the upward grade of about 1/30 is provided in a fan shape next to the wave generating section 2 having the upward grade of about 1/10, and the waves driven to a beach section 35 are bypassed to a bottom section 33 by a communicating pipe 37. Drive control programs of various waves are stored in the database of a microcomputer, the AC servo motor of the wave generating device 4 is driven, the waves formed by the wave generating plate 3 are made the maximum wave height at a water breaking point 58, and the wave height is held to the vicinity of the beach section 35 as the grade is decreased thereafter. Thus the waves suitable for surfing can be easily obtained.