Showing papers on "Wave height published in 1993"

On the Dependence of Sea Surface Roughness on Wave Development

Abstract: The aerodynamic roughness of the sea surface, z0, is investigated using data from Lake Ontario, from the North Sea near the Dutch coast, and from an exposed site in the Atlantic Ocean off the coast of Nova Scotia. Scaling z0 by rms wave height gives consistent results for all three datasets, except where wave heights in the Atlantic Ocean are dominated by swell. The normalized roughness depends strongly on wave age: younger waves (traveling slower than the wind) are rougher than mature waves. Alternatively, the roughness may be normalized using the friction velocity, u*, of the wind stress. Again, young waves are rougher than mature waves. This contradicts some recent deductions in the literature, but the contradiction arises from attempts to describe z0 in laboratory tanks and in the field with a single simple parameterization. Here, it is demonstrated that laboratory waves are inappropriate for direct comparison with field data, being much smoother than their field equivalents. In the open ocea...

Convolution Method for Time-Dependent Beach-Profile Response

Abstract: A simple analytical solution is presented for approximating the time-dependent beach-profile response to severe storms. This solution is in the form of a convolution integral involving a time-varying erosion-forcing function and an exponential erosion-response function. The erosion-forcing function reflects changes in the nearshore water level and breaking wave height. In this paper, an idealized storm-surge hydrograph is considered from which an analytic solution is obtained for beach and dune erosion associated with severe storms such as hurricanes or northeasters. It is shown that for a given initial beach geometry and sediment size, the peak water level and the incipient breaking wave height determine the maximum erosion potential that would be achieved if the beach were allowed to respond to equilibrium. Because of the assumed exponential erosion rate, beach response obtained from the convolution method is found to lag the erosion forcing in time, and is damped relative to the maximum erosion potential such that only a fraction of the equilibrium response actually occurs.

A connection between mean wave height and atmospheric pressure gradient in the North Atlantic

Abstract: Comparison of long-term trends in mean wave height in the North Atlantic with values of annual mean atmospheric pressure gradient in the North Atlantic measured between the Iceland Low and the Azores High suggests, for the period of overlap between the data set of Bacon and Carter and of Sahsamanoglou (early 1950s to 1980), a correlation between these two measures. This paper, using monthly mean values of significant wave height (HS) derived from measurements at two sites, one oceanic (Ocean Weather Station Lima) and one coastal (Seven Stones Light Vessel), covering the period 1962–1988, investigates correlation between (firstly) annual mean values and (secondly) monthly mean values of HS and of pressure gradient. The annual mean values of pressure gradient are shown to be linked to the long-term changes in wave climate described by Bacon and Carter. The large annual cycles in the monthly mean values of both variates are found to be in phase; furthermore, correlation between the two variates independent of the annual cycle is found. Finally, the correlation between the annual means is used to generate a hindcast mean wave climate for Seven Stones Light Vessel (and also, tentatively, for the whole north-east Atlantic) from 1873 to the present, which suggests that present conditions are as high as have been observed since 1873.

Transport of Fine Sands by Currents and Waves. II

Abstract: The transport process of fine sand with a median diameter of about 100 μm in combined current and wave conditions in a laboratory basin was studied. Irregular waves with a single-topped spectrum and peak period of 2.5 s were generated. The significant wave height was varied from 0.07 m to 0.14 m. The water depth was about 0.4 m in all tests. The depth-mean current velocities were varied from 0.1 m/s to 0.3 m/s. The angles between the wave and current directions were 60°, 90°, and 120°. Instantaneous fluid velocities were measured by an acoustical probe and an electromagnetic probe. The acoustical probe was also used to measure the instantaneous sand concentrations. Time-averaged and bed-averaged concentrations were measured by a pump sampler. Current-related and wave-related transport rates were determined by decomposing, multiplying, and time-averaging instantaneous signals. The current-related transport rate in the main current direction was found to be largest when the waves were propagating normal to the current. High-frequency wave-related transport rates were found to be directed onshore because of wave asymmetry effect. Low-frequency wave-related transport rates were found to be directed offshore because of the bound long waves.

Tsunamis : forces on a vertical wall caused by long waves, bores, and surges on a dry bed

Abstract: The major objective of this study has been to investigate experimentally the forces and overturning moments produced by tsunamis on vertical walls. The experimental results are compared with several analytical and numerical models. Several types of waves were used in a horizontal tank including solitary waves, undular bores, turbulent bores, and surges on a dry bed. Bores produced from breaking solitary waves in a tilting wave tank were also investigated. Various measurements were made, including the incident wave celerity, the wave profile, the runup, force, overturning moment, and pressure time histories. The impact process of the bores in the tilting wave tank were recorded with high-speed movies. The wave profiles in the horizontal tank were defined using a laser induced-fluorescence system (LIF) which allows the free surface on a two-dimensional plane in the center of the wave tank to be recorded. This method was developed to measure accurately the surface elevation profile of turbulent high-speed flows which is difficult to measure reliably either with conventional flow visualization techniques or intrusive devices such as wave gages. The LIF method was also used to determine the runup on the wall. Strong vertical accelerations were shown to occur during the reflection of bores and steep solitary waves at a vertical wall. These reduced the force on the wall relative to a hydrostatic force computed from the maximum runup height on the wall. The accelerations also cause the maximum force to occur before and after the maximum runup for steep solitary waves and bores, respectively. For these cases, the maximum measured force and overturning moment were always less than computed from the maximum measured runup on the wall using hydrostatic considerations. The maximum force due to surges on a dry bed was also less than the hydrostatic force calculated from the maximum runup height on the wall. For all the dry bed cases studied, the maximum runup height on the wall was between 1.46 and 1.62 times the velocity head computed from the celerity of the incident surge. For the entire range of wave conditions of this study, the maximum relative runup occurred for a bore with a relative wave height of 1.23, and produced a runup equal to 3.8 times the velocity head computed from the wave celerity. The maximum measured water surface slopes along the front of long waves, bores, and dry bed surges were computed from the measured wave profiles. At the transition from undular bores to turbulent bores, there was a discontinuity in the maximum water surface slope where the slope increased by a factor of 2.5 to three for turbulent bores. This discontinuity corresponded with a rapid increase in the measured runup, force, and moment on the wall. The properly normalized force on a vertical wall due to the impingement of a bore on a mildly sloping beach is shown to be equivalent to the force produced by a bore of constant volume on a horizontal bed. This implies the results from the horizontal wave tank experiments can be used to estimate the loads expected from bores propagating on mild beaches with slopes ranging up to 0.02m/m. Two numerical models were compared with the experimental results. A boundary integral element model, which solves the potential flow problem subject to the full nonlinear free surface boundary conditions, predicted the loads imposed on the wall due to steep solitary waves quite well. A finite difference model of the Navier-Stokes equations was also used to simulate the reflection of solitary waves and mild turbulent bores at a vertical wall. This finite difference model predicted the solitary wave loads quite well; however, it over-predicted the steepness of the incident bore profiles and produced a force-time history with a high amplitude and short-duration peak, which was not observed in the measurements. Except for this sharp peak, the agreement of the finite difference model with the experimental results was quite reasonable.

A nonlinear model for the fluidization of marine mud by waves

Abstract: Rheological properties of marine mud are incorporated into a seawave-seabed interaction model. The model assumes that the viscoelastic properties of the mud are dependent on the wave-induced strains inside the stratified bed, and it solves for the depth of fluidization in the bed as a function of the imposed wave height. Below the fluidized layer the mud is assumed to behave elastically with a slow depth stratification. In the fluidized layer the behavior is assumed to be viscoelastic with a complex kinematic viscosity coefficient which is also a function of the imposed wave height.

A field experiment on the mechanics of irregular gravity waves

Abstract: In random wind-generated wave motion on the sea surface, extreme wave events have been shown theoretically to occur within groups with a well-defined configuration and time history that can be specified in terms of the space-time autocovariances of the surface displacement. The predictions of the theory have been tested in a field experiment in the Straits of Messina in which an array of nine wave gauges and nine pressure transducers supported by vertical piles provided space-time information on waves generated over a fetch of approximately 10 km. It was confirmed that the general configuration of the extreme wave groups measured was consistent with the theoretical predictions in terms of the measured space-time autocovariance. During the development stage of a group, as the height of the central (outstanding) wave grows to a maximum, the width of the wave front reduces to a minimum. As an individual wave passes through the group, its wavelength decreases as the wave height increases towards the apex, after which the wavelength increases again as the wave moves towards the front of the group and abates.

Longshore current and wave height modulation at tidal frequency inside the surf zone

Abstract: Data were acquired continuously during the 19-day DELILAH nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies. It is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations, and since radiation stress is a function of the wave height, longshore currents are forced at the tidal frequency inside the surf zone. The measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions, indicating that the tide is a dominant mechanism associated with longshore current variability. Simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by Thornton and Guza (1986) are used to explain observations. The measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model, verifying the hypothesis.

A dynamic model for wave-induced light fluctuations in a kelp forest

Abstract: We formulated a dynamic model with linear wave theory to predict the effects of wave height, wave length, and water depth on modulation of the local surface cover in a Macrocystis pyrifera canopy. This model was incorporated into a general model for attenuation of photosynthetically active radiation (PAR) in a A4. pyrifera forest with particular emphasis on light reaching understory algae. The model predicts that the period of fluctuations of PAR on the bottom matches that of the dominant wave period and that the amplitude of irradiance peaks is increased by positive local surface areal stain during the trough of a wave. Field measurements of instantaneous irradiance and surface elevation and simultaneous video recording of the surface canopy allowed investigation of these predictions. Cross-correlation analysis on irradiance and surface elevation measurements showed a significant negative correlation between surface elevation and PAR reaching the understory. This correlation cannot be attributed to the decrease in lightpath length alone, but can be explained by the change in M. pyrifera surface cover. Measured changes in instantaneous fractional canopy cover exceeded theoretically predicted values by a factor of as much as 3. Consequently, the intensities of light flecks exceeded predictions. A combination of mechanisms, as well as assumptions of the model, may explain the deviation between the magnitudes of predicted and measured fluctuations in canopy cover and light. The dominant period of ocean swells is typically in the range of 5-20 s, and light flashes with these periods have been shown to effect significant gains in light utilization efficiency by certain algae.

Global validation of the wave model WAM over a one-year period using Geosat wave height data

Abstract: The high quality of wave fields simulated by the third-generation wave model WAM has already been demonstrated in various validation studies using in situ measurements as well as data from satellites as reference. However, owing to limitations of the reference data sets, the previous studies concentrated on relatively small regions or short time periods only, for which adequate measurements were available. In this paper the first global verification of the WAM model over a full 1-year period is presented. The significant wave heights hindcast for 1988 by the WAM model as implemented at the European Centre for Medium Range Weather Forecasts are compared with measurements obtained by the Geosat radar altimeter. The wave heights from WAM and Geosat show good agreement in general. However, significant regional and seasonal differences are found. The underestimation of WAM wave heights in the southern hemisphere, which was already known from a validation study for the Seasat period, shows significant seasonal variations. The hindcast wave heights are underestimated by about 20% in large parts of the southern hemisphere and the tropical region during May-;September. For the rest of the time, the agreement with Geosat data is fairly good. Together with the fact that also the rms variability of wave heights in the tropical region is clearly underestimated by WAM, this can possibly be attributed to simplifications like the neglect of atmospheric stratification effects when converting wind speeds to the wind stress fields driving WAM. Furthermore, the intercomparison indicates that low wave heights below ∼ 1.5 m are generally overestimated by WAM. As it is planned to use altimeter wave heights for updating wave models in future data assimilation systems, it is quite important to have efficient quality control criteria for these data. We show that some additional Geosat parameters, e.g., the off-nadir angle of the altimeter, can be useful quality parameters. The difference between the Geosat and WAM wave heights shows a clear dependence on the additional parameters in some cases, which must be related to quality problems of the Geosat data. Some new criteria for the rejection of incorrect Geosat data points are obtained.

Toe structure stability of rubble mound breakwaters

Abstract: In this report an analysis of data from a series of tests is presented on the stability of the toe structure of rubble mound breakwaters. The existing knowledge on toe stability and the influence of all governing parameters in the existing knowledge is reviewed. The tests which were performed for the present research are treated extensively. The main governing parameters, which determine toe stability in rubble mound breakwaters, are: Significant wave height : Hs Nominal stone diameter : Dn50 Stone mass density : ρ s Depth above the toe : ht Damage level : Nod Parameters investigated which appeared to have no significant influence are: the fictitious wave steepness sop and the width of the toe structure bt. The choice of the governing parameters, the definition of damage levels for the design of the toe structure and the way they are made dimensionless are motivated. The results from the present tests are compared with existing design formula. With the selected parameters two formulas have been derived from the available test data. One describes toe stability using the shallow water significant wave height Hs the other uses the shallow water 2% wave height H2%. The formulas are compared with existing test results and their range of application is given.

Measurement and modelling of tropical cyclone waves in the Great Barrier Reef

Abstract: Recorded wave data from four wave measuring instruments located at various points within a section of the Great Barrier Reef during the passage of a tropical cyclone are presented. A spectral wave prediction model is used as an aid to the interpretation of the data. The tropical cyclone generated significant wave heights of approximately 10 m seaward of the reef complex. The many scattered reefs, however, act to reduce this value to approximately 6 m landward of the reef complex. Individual reefs appear to act as complete barriers to waves at typical oceanic periods, even at high tide. In a scattered reef complex such as the Great Barrier Reff, wave energy can penetrate the inter-reef gaps. Although such inter-reef gaps may appear large in comparison to the sizes of individual reefs, wave attenuation is still significant.

A two-dimensional surf zone model based on the boussinesq equations

Abstract: A simple approach to wave breaking using the concept of surface rollers is introduced in a two-dimensional Boussinesq model. A surface roller represents a passive bulk of water riding on the front of a breaking wave and the vertical redistribution of momemtum associated with the formation and change of surface rollers leads to additional terms in the Boussinesq equations. A simple geometrical method is used for the determination of the shape and location of these rollers at each time step in the simulation. This automatically results in a time-varying break point position in the case of irregular waves. Furthermore, breaking may well cease for example when waves reach a trough inshore of a bar. Comparison between one-dimensional simulations and experiments shows good agreement for the variation of wave height and mean water surface as well as surface elevation time series throughout the surf zone for both regular and irregular waves. Simulations in two horizontal dimensions are still at the initial stage. A sample simulation is shown.

Comparison of Loads Predicted Using “Newwave” and Other Wave Models with Measurements on the Tern Structure

Abstract: Global wave loads measured on the Tern platform during a very severe storm have been compared with predictions made on the basis of three different models for wave kinematics. The first is the “Newwave” theory, a broadbanded, probabilistic-based model for the extreme waves of a random seastate. The second model involves complex time-domain simulation of random directional seas. Both these theories are used with the Morison equation and realistic force coefficients to predict global forces. The third wave model is the Stokes fifth-order theory with artificial values for the force coefficients, as used in conventional design practice.

Intercomparison of Coastal Profile Models

Abstract: The present paper briefly presents 6 different models for short term coastal profile modelling for direct incoming waves. The models have been tested against measured profile evolutions from a large wave flume. Features such as wave height distribution, cross shore current profiles and sediment transport are compared and discussed.

Distribution of surface pebbles with changes in wave energy on a sandy estuarine beach

Abstract: A 29-day field experiment in Delaware Bay, NJ identified the dispersion of surface pebbles due to cross-shore transport on a mesotidal estuarine beach composed mainly of sand. Wave heights were 0.04-0.56 m. Dissipation of wave energy on the low-tide terrace increases as water levels become lower, creating a spatial gradient in wave energy delivered to the foreshore. Analysis of surface sediment samples taken daily at 4 m intervals across the foreshore and two experiments using dyed pebbles as tracers document increasing quantities of pebbles with distance downslope on the beach, corresponding to the decreasing energy gradient. There is a second locus of high pebble concentration near the upper limit of swash at high water. Numbers of surface pebbles are at a minimum following high-energy events (wind speed > 8.0 m/s, wave height > 0.3 m); the most conspicuous pebble accumulations result from low-energy conditions (wind speed < 3.0 m/s, wave height < 0.2 m). Pebbles move up and down the foreshore within the beach step. They accumulate just above the low-tide terrace on the falling tide when wave energy diminishes at low water under both storm and nonstorm conditions. Finer particles are moved onshore from this location by low-energy post-storm accretional waves and offshore by outflow from the beach water table, leaving pebbles as a surface lag. A fraction of these pebbles is moved up the beach by the swash of low-energy waves or within the beach step in subsequent tidal cycles. There is insu ficient sand in the backwash of low-energy waves to bury pebbles; they project into the flow of the swash and have low pivoting angles, increasing the probability of entrainment and movement over the sand particles to the upper limit of swash.

Wave height spectrum from sun glint patterns: An inverse problem

Abstract: The problem of retrieving spatial information of sea surface heights from aerial images is considered. In this paper, some analytical and numerical results that relate the autocorrelation of the surface heights and those of the sun glint patterns are derived. Examples of these results are presented showing the nonlinear technique which can be applied to obtain the power spectrum of a real sea surface using aerial photographs. As a first step, two kinds of roughness spectra are presented: two roughness spectra which describe single-scale surfaces, and the Pierson-Moskowitz power spectrum which describe multiscale surfaces for a fully developed sea. Both simulations are presented in one dimension. However, a single-scale bidimensional surface is analyzed considering a Gaussian roughness spectrum. Wave height spectra are obtained from the surface height autocorrelation via a Fourier transform. The results of the model compared with the theory and the data are in quite good agreement. Under favorable conditions, it is possible to invert the relation numerically and estimate the wave height spectrum from the sun glint data.

Wave-Induced Oscillations in Harbours with Dissipating Quays

Abstract: A method based on a boundary element method was described for predicting wave height distributions in a harbour of arbitrary shape and variable water depth. Effects of partial reflection along harb...

Joint Distribution of Large Wave Heights and Associated Periods

Abstract: The joint probability density of zero upcrossing wave heights and periods is investigated over the range of large wave heights. The density of interest is expressed as the product of the marginal density of large wave heights with the conditional density of associated periods. Existing theoretical approximations to these are first reviewed briefly and then compared with empirical results simulated from a generalized form of the Pierson-Moskowitz spectrum. Comparisons indicate that the marginal probability density of large wave heights is described accurately by an asymptotic expression, as was previously suggested in the literature by the writer. The conditional density of associated wave periods is approximately Gaussian, as predicted by the Longuet-Higgins approximation in 1975. However, the conditional mean period (standard deviation) tends to be larger (smaller) than that implied by the earlier theory in a manner dependent on the spectrum bandwidth. The theoretical expression describing the joint probability density is thus modified to exhibit these properties, and is then checked with simulated and actual wave data, obtaining fairly good agreement.

Wave Set-up and Wave-generated Currents on Coral Reefs

Abstract: Coral reefs are a unique environment and wave action is one of the factors determining that environment. Reef-tops exposed at low tide are essentially horizontal platforms and waves propagating over such reefs at high tide are usually shallow water ones. Both set-up and wave-induced flow are generated by waves breaking at or near the reef-edge. Laboratory experiments using both two and three dimensional reef models show that the set-up and flow both increase with increasing off reef wave height. The depth over the reef is also important with set-up being greatest at small depths and flow greatest at larger depths. At small submergence both set-up and flow are controlled by reef topography.

Preliminary analysis of the stability of rubblemound breakwater crown walls

Abstract: A series of two-dimensional hydraulic model tests was carried out to investigate the stability of rubblemound breakwater crown walls. The effect of seven design parameters on the minimum mass required for a crown wall to remain stable was studied: wave height, wave period, crown wall height, water level, front slope of the breakwater, position of the crown wall and length of stabilizing legs. Observations regarding the type of wave interaction, degree of overtopping, superstructure movement and overall hydraulic stability were studied. The coefficient of friction at the crown wall/breakwater interface was also measured. The crown wall superstructure was located on the crest of a conventional multi-layer breakwater and was subjected to both regular and irregular wave attack. Preliminary analysis of this data set is presented which shows trends established for each of the seven design parameters.

Characteristics of a solitary wave breaking caused by a submerged obstacle

Abstract: This study aims to make quantitatively clear the breaking wave characteristics of a solitary wave incident to a submerged obstacle by performing intensive numerical simulations using a boundary integral method. The incident critical wave height, the break point and the breaker height are formulated through regression analyses based on the simulated results and are shown to be determined uniquly by using the regression equations.

Effect of viscosity on dynamic response of a liquid storage tank

Abstract: An exploratory study on the effect of viscosity on the dynamic response of a liquid stomp tank is presented. The tank is assumed to be rigid. Both harmonic and earthquake excitations are used in the study. The finite clement method is employed to attack the problem. The response functions examined include the sloshing wave height and the impulsive and convective components of the hydrodynamic pressure. The study shows that the viscosity affects the sloshing wave height and the associated convective component of the pressure response, and it has a negligible effect on the impulsive component. A simple practical approach is proposed to take this effect into account.

Groups of ocean waves: linear theory, approximations to linear theory, and observations

Abstract: Statistics of wave groups observed for a wide range of wave heights, power-spectral shapes, and water depths are compared to the statistics predicted by both direct numerical simulation and analytic approximation of linear wave theory. Comparisons to numerical simulations show that the observed groups are not inconsistent with linear, Gaussian and wave fields with the same spectra as the observations. Differences between ocean observations and linear theory are owing to statistical fluctuations in group statistics estimated with the 2.3-h-long data records. One linear analytic model accounts for correlations between two successive waves, and slightly underpredicts the average number of sequential large waves for wave fields with very narrow power spectra. A newly developed extension to Rice better accounts for multiwave correlations, but has only marginally improved accuracy for these data because wave fields with very narrow power spectra rarely occurred. Both approximations overpredict the group lengths for very broad and/or multipeaked power spectra, but are still useful because the errors are small with commonly occurring spectral shapes. Direct numerical simulations, with computational expense between that of the spectral-Kimura and extended-Rice, approximations, yield the best predictions of the observed wave group statistics given a power spectrum, and also provide estimates of the statistical fluctuations of group properties about predicted mean values.

A new nearshore directional wave gage

Abstract: A field directional wave gage has been developed for requirements in coastal, port, and harbor engineering. Uses include planning and design studies, long term wave climatology, and post construction monitoring. The gage provides high quality directional wave spectra measurements from a compact, easily installed instrument. Stand alone installation with long term internal data recording is provided. Real time data access may be simultaneously provided by connecting a cable. The gage is completely bottom mounted with no surface components. Optimized for shallow and intermediate water depths required by coastal engineers, the gage may be reliably deployed in water depths too shallow for buoys or acoustic instruments. The sensors are three high resolution pressure transducers that permit the mounting frame to be resistant to fishing activities. Reliable data storage and long term deployment are achieved by performing preliminary data analysis within the instrument. Intermediate analysis results are recorded on reliable non-volatile solid state memory. The recorded data may be post processed to obtain simple estimates of mean water level, wave height, period, and direction. If desired, the Fourier series coefficients of the directional spectrum may be computed from the intermediate data. Optionally, modern high resolution methods may be used.

Sand transport under grouping waves

Abstract: Laboratory experiments as well as numerical modeling were conducted for sand transport under non-breaking grouping waves. Experiments showed that the direction of net transport of fine sand was onshore under grouping waves although it was offshore under monochromatic waves with equivalent wave height. It was also revealed that the long wave bounded to wave group plays an important role in particular for suspended load. A numerical model was developed on the basis of the second-order Stokes wave theory and one-dimensional diffusion equation of sand concentration. The validity of the model was confirmed with experimental data.