Showing papers on "Wave height published in 1987"

Adjustment of Reflective Beaches to Waves

Abstract: A new mode of beach response to changes in wave height is identified for reflective beaches where waves break close to the shoreline. This response involves vertical adjustment of the beach face morphology, especially the beach step. It is found that profile relief varies with wave height rather than inversely to it as expected from existing models. As a result the characteristic steep reflective profile can be maintained under certain conditions, during periods of beach erosion. A field program which included surveys of 67 beach profiles on a persistently reflective beach measured vertical adjustments of the beach step and associated wave and sediment conditions. Based on these data a model is developed to describe the changes in the amplitude of the beach step related to changes in breaker height and sediment size. It was found that as breaker height increases the step height increases, while the surf zone width remains constant. Larger grain sizes and changes in grain size across the beach face are also associated with larger step heights. This step adjustment model implies that if wave height increases, under the range of conditions observed, the resultant increase in step amplitude and associated deepening of the nearshore will be sufficient to delay wave breaking, allowing a reflective profile to be maintained. Therefore the model can account for the environmental conditions which produce changes in step heights and favour the maintenance of a reflective system. The model also implies that a threshold exists between this mode and the conventionally recognised mode of beach response and therefore differentiates between transient reflective profiles, and those of less mobile reflective beach systems.

Prediction of Occurrences of Steep and High Waves in Deep Water

Abstract: Estimates for encounter probabilities of occurrence of steep and high waves in deep water for given sea states by using two different parametric models for the joint distribution of crest front steepness and wave height are presented. The parametric models are fitted to the same data set, with data from a zero‐downcross analysis of wave data obtained from measurements at sea on the Norwegian continental shelf. The probability of occurrence of waves with different crest front steepness are estimated with each parametric model for a family of JONSWAP spectra. An example is given where the probabilities of occurrence of “extreme waves,” which are considered to be critical to capsizing of smaller vessels, are estimated with one parametric model for sea states described by significant wave height and mean zero‐crossing period.

Analysis of prototype freak waves

Abstract: It is described how continuous storm wave records taken in the North Sea were used to isolate and analyze freak wave events. The analyses were performed on several levels. First, an investigation was made of whether surface profiles and horizontal orbital velocities could be explained by 2-D (long crested) wave theories up to the fifth order. Second, the phenomenon was treated as a 3-D (directional) interaction case. Velocity vectors in three levels were studied in order to reveal possible special 3-D characteristics. Low- and high-pass filters were also used as elements in the analysis of the composition of the freak waves. From the limited amount of data analyzed so far, the high frequency part of the freak waves seems to be strongly directional. To describe the character of the high frequency waves in detail, a three-dimensional second-order (higher harmonic) separation technique was used to isolate bound and free components. As a reference for the future analysis of freak waves, the wave climate producing the ones discussed was also analyzed in detail. The development over time of mean directions, directional spread and spectral density is presented.

Breakwater Gap Wave Diffraction: an Experimental and Numerical Study

Abstract: Breakwater gap configurations with gap‐to‐wavelength (B/L) ratios of 1.64, 1.41, 1.20, 1.00, 0.75, and 0.50 are investigated, both experimentally (using close‐range photogrammetry) and numerically (using finite and infinite elements). The experimental results, when compared to the finite element and available analytical results, show that: (1) The measured wave heights in the shadow zones (those regions sheltered by the breakwater arms) tend to be larger than predicted theoretically due to the combined effect of secondary waves generated at the breakwater tips and wave orthogonal spreading near the gap centerline (and subsequent wave orthogonal bunching in the shadow zones) caused by wave steepness differences along the crests; and (2) the wave heights outside the shadow zones tend to be smaller than predicted theoretically, again due to wave orthogonal spreading caused by the greater steepness of waves near the gap centerline. The results suggest that linear theory provides conservative wave height estim...

Laboratory Study of Steep and Breaking Deep Water Waves

Abstract: Measurements are presented here of the onset of breaking of deep water waves. The results of recent experiments conducted at NRL to study the growth of steep waves to breaking in a convergent chann...

Short-Term Impoundment of Longshore Sediment Transport.

Abstract: : The cross-shore distribution of longshore sediment transport is investigated through the distribution of sediment impounded against a shore-perpendicular barrier over short-term intervals in field and laboratory environments. For each field experiment, a low-profile groyne was deployed across a natural beach in less than 8 hr and profiles near the groyne were repeatedly surveyed for 8 to 20 hr thereafter. For each laboratory experiment, a low-profile barrier was installed across a pre-equilibrated fine sand model beach, and profile changes near the barrier were measured after 5 to 40 minutes of regular, obliquely incident, unidirectional wave action. Breaking wave angle and longshore current and wave height across the surf zone were also measured. The effects of crossshore transport and tidal fluctuation were addressed in the survey data, and the effectiveness of the barriers as impoundment agents is discussed. Local downdrift profile changes were found to be poor indicators of the local updrift impoundment. In general, the longshore transport profiles were found to be bimodal with peaks just landward of the breakpoint and near the shoreline. The relative significance of the longshore transport shifted from the near-breakpoint peak to the near-shoreline peak as the wave condition varied from spilling to collapsing breakers. Keywords: Beach erosion; Groins(Shore protection).

Friction velocity scaling in wind wave generation

Abstract: This note is devoted to the problem of the appropriate scaling of parameters relevant for sea waves, such as wave height, peak frequency, duration, and fetch. In the past, the growth of sea waves has often been analysed in terms of the wind velocity at a fixed height, despite the fact that many authors have stressed the importance of scaling with the friction velocity. This problem would be immaterial if the ratio between the friction velocity and the wind speed at a fixed height were a constant. There is, however, ample evidence that this ratio increases with wind speed (Smith and Banke, 1975; Smith, 1980), in agreement with dimensional considerations by Charnock (1955) on the friction height. As a result, the scaling problem is an important one. In this note we conjecture that the correct procedure is to scale wave parameters with friction velocity, and we discuss experimental evidence for the correctness of this conjecture. Comparing two independent datasets (‘JONSWAP’ and ‘KNMI’), we find some evidence supporting our ideas. Further confirmation remains desirable, however, and suggestions are made as to how this might be obtained.

Sediment transport concentrations and sediment transport in case of irregular non-breaking waves with a current

Abstract: In many coastal, engineering problems the sediment transport plays a part. A transport gradient causes accretion or erosion. Various models, such as that of Bijker, Engelund and Hansen (van de Graaff and van Overeem, 1979) and Nielsen (1985) are available to estimate the sediment transport rate if the hydraulic and environmental conditions (wave height, current velocity and direction, sediment size) are known. Since reliable data under field conditions are extremely scarce, the reliability of these models is not known, while also no understanding of the basic relations between the sediment transport, current velocity and wave height can be obtained. To extend the knowledge of the basic phenomena, a laboratory study was carried out. Fluid velocities and sediment concentrations have been measured in case of irregular non-breaking waves alone, and in combination with following or opposing currents.

Nearshore Sand Transport.

Abstract: : Sand transport as bedload on nearly flat beds in shallow water outside the breakers is the subject of this study. The appropriate variables necessary for computation of sediment transport are grouped into a few dimensionless force ratios using the techniques of dimensional analysis, forming a sediment transport model. Field experiments measuring fluid velocity and sand transport were performed seaward of the breaker region. Fluorescent sand tracer was used to measure both sediment-transport velocity and thickness. Thirty tracer experiments were performed under differing wave and sediment conditions. Transport thickness is well correlated with orbital diameter but not wave height or fluid velocity. Different powers of the fluid velocity are compared with sediment transport. The lower velocity moments perform much better than the higher moments. Use of a threshold criterion is essential in predicting whether the sand transport is onshore or offshore. Results suggest that the appropriate power of fluid velocity necessary for computing and transport may itself be a function of the flow intensity.

Steady Wave-Drift of Modeled Ice Floes

Abstract: Systematic laboratory measurements are performed to determine the steady drift of isolated, tabular ice-floe models in regular waves. A relatively simple expression for the steady wave-induced drift of two-dimensional floes of uniform thickness and density in deep water is developed. This empirical wave-drift formula indicates that the drift velocity depends primarily on the ratio of wave period to ice-floe roll period, wave steepness, and length of the floe. The highest drift velocity is produced by waves with period somewhat greater than the ice-floe roll period, with fundamentally different drift behavior evident on either side of this maximum (long-wave and short-wave drift response). The dependence on wave height is found not to be linear but increases with wavelength (from approximately \IH\N¹\u/² to \IH\N³\u/². A simplified equation for the roll period of a prismatic body of uniform density and rectangular cross section is developed and found to be of practical value. Floe curvature (in the vertical plane) is found to affect drift rates substantially.

Transformation of Significant Wave Heights

Abstract: A formula for transforming significant wave heights of wind seas between locations of differing depths is developed from the concept of the equilibrium range in the spectrum. The transformation is assumed valid for single‐peaked equilibrium wind seas which do not contain swell components. The method does not account for the effects of refraction, diffraction, or situations which may be fetch‐ or duration‐limited, therefore the method should not be applied when these effects may be dominant. The transformation is tested using 198 data pairs which seemed to fit the necessary criteria. The significant wave height at the shallower gage (8 m depth) was predicted using the measured significant wave height at the deeper gage (18 m depth). Comparisons between predicted and measured significant wave heights indicate that the wind sea transformation generally performs better than unrefracted shoaling of significant wave height using linear theory, but that underprediction occurs at longer wave periods.

Transformation of regular waves

Abstract: Results are presented from calculations for shoaling waves using a development of Dean's non-linear stream function which display better agreement with previous experimental data for parallel waves in shallow water than other currently available theories. The results are presented in a manner suitable for design applications. In addition to wave height evaluation, curves have been given to aid the prediction of other parameters of design interest, such as crest elevation and horizontal water particle velocity. It is demonstrated that the stream function series type of solution is particularly suitable for this application, because the integral wave properties required for shoaling predictions may be obtained directly from the stream function solution, without performing integrations in the body of one wave. It is also shown that the conservation of wave action flux and wave energy flux lead to identical results in terms of wave shoaling predictions.

A user's guide to the littoral environment observation retrieval system

Abstract: : The lack of available data on waves, currents, and sand movement along beaches in the United States is major problem confronting planners and designers of coastal projects Data from instruments such as wave and current gages are expensive to obtain and are rarely available at the precise location where needed The Littoral Environment Observation (LEO) program was established to help overcome some of these problems Under this program, volunteer observers are recruited to obtain daily visual observations of such observations of such coastal variables ar breaker height, wave period, direction of wave approach, wind speed and direction, longshore current velocity, and beach slope Observers obtain the data by using simple, inexpensive equipment and for some data, eg, wave height and direction, observers are asked to simply record a visual estimate The LEO program has been ongoing since 1968, and observations have been made at over 200 sites along the coasts of the United States This information collected has been placed in a computer data base and is available to the coastal community This guide describes the LEO data collection process, the parameters involved, and the use of the LEO Retrieval System The LEO Retrieval System is a computer-based analysis system that performs a variety of data retrieval, data analysis, and report processing functions Keywords: Beaches, Computer program, Handbooks, Manuals, Data management, Currents, Retrieval, Monitoring, Sand movement, Waves

SIR-B ocean-wave enhancement with fast Fourier transform techniques

Abstract: Shuttle Imaging Radar (SIR-B) imagery is Fourier filtered to remove the estimated system-transfer function, reduce speckle noise, and produce ocean scenes with a gray scale that is proportional to wave height. The SIR-B system response to speckled scenes of uniform surfaces yields an estimate of the stationary wavenumber response of the imaging radar, modeled by the 15 even terms of an eighth-order two-dimensional polynomial. Speckle can also be used to estimate the dynamic wavenumber response of the system due to surface motion during the aperture synthesis period, modeled with a single adaptive parameter describing an exponential correlation along track. A Fourier filter can then be devised to correct for the wavenumber response of the remote sensor and scene correlation, with subsequent subtraction of an estimate of the speckle noise component. A linearized velocity bunching model, combined with a surface tilt and hydrodynamic model, is incorporated in the Fourier filter to derive estimates of wave height from the radar intensities corresponding to individual picture elements.

Paleo-wave Conditions Reconstructed from Ripples in the Pleistocene Paleo-Tokyo Bay Deposits

Abstract: Paleo-wave conditions during the Shimosueyoshi Transgression (130, 000-100, 000 years B.P.) are estimated from oscillatory ripples preserved in prodelta, shoreface and tidal flat deposits of the Paleo-Tokyo Bay. Possible combinations of wave conditions and water depths that could have generated the observed ripples are determined by ripple spacing and grain size of ripple forming sediments using the method of Komar (1974) and others, which are based mainly on Airy wave theory. In addition, paleo-depths are calculated independently by the following two methods using stratigraphic thickness from the ripples to the above foreshore deposits and the height of longshore bars containing some of the ripples in shoreface deposits. Therefore, combinations of wave height and wave period under the water depth, which was estimated by the above method, can be determined.Waves with a height of lower than 2.3 m and a period of 2-8 s are obtained from the ripples in prodelta deposits. Such waves represen “storm waves” of the present Tokyo Bay. This wave condition may have been formed by relatively small waves of post-storm stage, because the ripples occur in the upper part of storm-generated sheet sand and are covered by a clay layer deposited from suspended matter in flood fluvial water into the Paleo-Tokyo Bay. Waves with lower than 2.5 m in height and 1.5 to more than 10 s in period are reconstructed from the ripples in shoreface deposits. These waves can generally represent “storm waves” of the Tokyo Bay and “fairweather waves” of the Kashima beach facing the Pacific Ocean. Waves of smaller 1 m high and less than 5.5 seconds are reconstructed from the ripples in tidal flat deposits. These small waves are approximately equal to “fairweather waves” of the Tokyo Bay. No ripples representing more big waves, such as winter waves and typhoon-generated storm waves on the present Pacific coast, are preserved in the Paleo-Tokyo Bay sediments. This may be caused by the shallow seawater-depth, less than 10 m, of the Paleo-Tokyo Bay where big wave motion during storm event may have changed most of the bottom sediments to flat bed rather than ripples.

Approximating SIR-B response characteristics and estimating wave height and wavelength for ocean imagery

Abstract: NASA Space Shuttle Challenger SIR-B ocean scenes are used to derive directional wave spectra for which speckle noise is modeled as a function of Rayleigh random phase coherence downrange and Poisson random amplitude errors inherent in the Doppler measurement of along-track position. A Fourier filter that preserves SIR-B image phase relations is used to correct the stationary and dynamic response characteristics of the remote sensor and scene correlator, as well as to subtract an estimate of the speckle noise component. A two-dimensional map of sea surface elevation is obtained after the filtered image is corrected for both random and deterministic motions.

Spot welding method

Abstract: PURPOSE:To secure a stable current path and to obtain a sound joining part by performing a pre-electrification with the current having small wave height value for specified wave height time prior to the joining of both metal plates. CONSTITUTION:The preelectrification current whose wave height time is 2mm/sec or less is passed between both electrodes 4a, 4b by a capacitor power source 11 with pressurizing the electrode 4a by the specific pressurizing force. A phosphate film 3 is removed by the electric resistance heating due to the pre-electrification only at the contact part of the projection 10 of a 1st metal plate 1a and a 2nd metal plate 1b. A stable current path is then secured with the direct contact of the mutual base metal parts of both metal plates 1a, 1b to arrange the preparation for the electrification of a rapidly instantaneous large current. When the welding current having larger wave height value than that of the precurrent and <2mm/sec wave height time is then passed, a thin welding nugget is formed to obtain a sound joining part.

Measurements of Wind Dependent Acoustic Transmission Loss in Shallow Water under Breaking Wave Conditions

Abstract: The enhancement of transmission loss induced by breaking waves in shallow water of 30 m depth, measured at a fixed 10 km-range under isothermal conditions rather depends on wind speed than on wave height. The attenuation coefficient, growing weakly at low wind speeds increases after a thresholdlike transition by the third to the fifth power of the wind speed, probably due to air bubble suspension. The transition wind speed around 10 m/s decreases with increasing frequency. The time lag between changing wind speed and attenuation is of the order of minutes. The loss differs at increasing and decreasing fetch. The close relation between wave height and attenuation at low sea states and predominating rough boundary scattering disappears at high sea states.

Nonlinearity of sagging moment of fine ships

Abstract: Rational design of ship's longitudinal strength should, be based on accurate evaluation of bending moment and shearing force. The nonlinear strip theory and model experiments reveal that the internal force by waves in rough seas differ from the ones obtained by the linear theory. Rules of classification societies are based on the long term prediction of structural responses, which is performed under the assumption of linearity in response and uses of the linear superposition method. According to the recent studies, the effect of nonlinearity becomes to be taken into consideration in the rules of classification societies.In the present paper, calculations are performed for a fine container ship and for a fullbodied bulk carrier using the nonlinear strip theory. Numerical results are obtained for regular waves of various wave height, wave length and wave encounter angle and for irregular waves of ISSC spectra by the nonlinear strip theory, and are compared with the result of O. S. M. : the linear theory. Results obtained are as follows : 1) Sagging moment increases as wave height increases by the effect of nolinearities, and the moment is about 170% of the one obtained by O. S. M. at the wave height 8 m in regular head sea for the container ship.2) On the contrary, the effect of nonlinearity in hogging moment is not so significant as in sagging moment. Therefore, long term prediction of hogging moment is obtained appropriately by the linear theory.3) Approximate prediction of long term response of bending moment is performed taking into account of the nonlinearity, and compared with the present rules.

A Numerical Model for Shoaling and Refraction of Third-Order Stokes Waves over an Irregular Bottom.

Abstract: : This numerical model solves for water wave height, angle, and number directly on a rectangular grid. Required input are the deepwater wave height, period, and direction and the bathymetry in the region of interest. The model employs a finite difference scheme. The irrotationality equation of the wave number vector is solved for the wave angle, and the conservation of energy flux equation is solved for the wave height. Iteration is required. A closed form expresssion, to third-order, for the time-averaged, vertically integrated energy flux is derived. Stokes' second definition of wave celerity is used in the derivation to reduce the number of intermediate calculations. Expressions for the wave energy and the group velocity are also derived. The model is written such that both first-order (linear) and third-order stokes wave theory model computations may be conducted. The modeling process begins at higher intermediate depth, or deep water, and waves are propagated shoreward until an Urseell number of 25 or another, user-specified, value is reached. The model is applied for the following cases: (a) comparison of small amplitude and finite amplitude wave refraction and shoaling on a plane beach, (b) refraction and shoaling over an irregular bottom configuration, and (c) comparison of the model shoaling predictions to laboratory data of Iverson (1951).

A Wind- and Wave-Driven Nearshore Current Model, The One-Dimensional Model.

Abstract: : A model to calculate the longshore component of currents resulting from winds and waves is described. The model is one-dimensional in the sense that it calculates variables at a number of grid points along a line normal to shore. The user of the model must supply the depth at each grid point along the line; a wave height, period, and mean direction at the seaward end of the line; and the wind speed and direction. The model then calculates the steady-state free surface elevation, longshore and cross-shore current, and wave height and direction at each grid point. Input parameters are described, and a sample application is provided. The model is intended for use where a quick but accurate estimate of the longshore current at an unobstructed, relatively straight shoreline is needed. For more complicated coastal configurations, a two-dimensional model is recommended. (Author)

Method and device for controlling solder jet wave height

Abstract: PURPOSE:To enable the control of a solder jet wave height in high accuracy to the set wave height and prevent a soldering defect by making the wave height cross section of the solder jet jetted from a jet nozzle in a visible image and measuring the wave height of the solder jet from the visible image. CONSTITUTION:With driving a jet pump 4 by a pump speed control mechanism 9 the molten solder 2 inside a solder tank 1 is forcibly fed, jetting from a jet nozzle 3. The wave height 7 of the solder jet 6 of the solder 2 is adjusted by controlling the revolutions of the pump 4. The solder jet wave height cross section of the solder 2 is reflected on a heat resistant transparent plate 5 and made in a visible image. The solder jet wave height 7 can correctly be measured with non-contact by an image sensor 13 from the solder jet wave height cross section reflected on the transparent plate 5. The measured data 14 thereof is fed to a data processing means 16 and inputted into CPU16 by performing A/D conversion. The CPU16 compares with the set wave height decided in advance, outputting the deviation value 20 thereof to a pump speed mechanism 9.

Tsunami flooding extension for coastal zones of Mexico

Abstract: Joint probabilities of runup height due to astronomical tides and tsunamis are calculated for five bays on the Pacific Ocean coast of Mexico. Runup heights equaled or exceeded on the average of once per 100 years, and once per 200 years, for those bays, are evaluated. Calculations are based on wave height probabilities obtained from sets of harmonic constituents and tsunami heights recorded during the last 28 years at tidal stations on each location. Extension of potential tsunami terrestrial flooding for populated coastal zones around those bays can be determined from the previous results. One case is illustrated showing endangered features in the hazardous zones, and recommendations to prevent casualties and property damages are given.