Showing papers on "Wave height published in 1991"

Evaluation of Beach Erosion and Accretion Predictors

Abstract: This paper examines the capability of simple criteria to predict whether a beach will erode or accrete by wave-induced cross-shore sand transport. Emphasis is on beach change of engineering interest such as associated with storm erosion, poststorm recovery, and seasonal wave conditions. The criteria, originally developed based on data from small and large tanks and monochromatic waves, correctly predict most erosion and accretion events in a newly compiled field data set encompassing beaches around the world. Previous studies that found such criteria unsuccessful are reviewed and found to be questionable. Correspondence between events in the field with random waves and in large wave tanks with monochromatic waves is obtained if mean wave height is used in field applications; however, any statistical wave height can be used by adjustment of one empirical coefficient in each criterion. Two dimensionless parameters (fall speed parameter and a newly introduced Froude number) used in some criteria are shown to be related to a critical wave energy dissipation needed to suspend sediment.

Numerical Model of Longshore Current for Bar and Trough Beaches

Abstract: This paper describes a numerical modeling system called NMLONG that was developed to calculate the wave height, wave direction, mean water level, and steadystate timeaveraged longshore current velo...

Laboratory Study of Wave-Breaking Over Bars and Artificial Reefs

Abstract: One‐hundred eight regular‐wave tests and 12 random‐wave tests were performed in a tank to investigate properties of waves breaking on irregular beach profiles In the tests, solid objects representing natural barred profiles, terraced profiles, and artificial reefs were installed on a concrete slope of 1/30 Wave conditions covered deep‐water steepnesses from 00085 to 009 A large data base of previous measurements of wave breaking on plane slopes was also compiled and analyzed for comparison to the barred profile measurements Significant differences were found in such properties as breaker type, breaker depth and height indices, plunge distance, and splash distance for plane and barred slopes The ratio of wave height and water depth of random waves transforming over a bar was not constant, as previously reported from field measurements, but showed a bimodal distribution with maxima of approximately unity at the bar crest and foreshore and minima of approximately 045 in the offshore and bar trough

Comparison of Spectral Refraction and Refraction-Diffraction Wave Models

Abstract: Wave energy estimated from linear, spectral wave propagation models incorporating refraction and refraction‐diffraction are compared over two bottom configurations: an analytic circular shoal and relatively smooth coastal bathymetry from San Diego, California. The agreement between the two models improves with an increase in the width of the incident directional spectrum and with a decrease in the complexity of the local bathymetry. There are, however, significant differences between the model transformations of directionally narrow spectra on both bathymerries. Pure refraction models are not quantitatively accurate in these cases. These comparisons also demonstrate the importance of directional wave spreading in transformations over even relatively simple natural bathymetry. Data from a fundamentally low‐resolution pitch‐and‐roll buoy, if used as the sole source of directional information for incident waves, can lead to significant uncertainty in wave heights estimated by the refraction‐diffraction model.

Nonlinear Methods in Offshore Engineering

Abstract: 1. Introduction. Definition of nonlinear systems. Consistent methodology. Probability distributions. 2. Environments. Wave theories. Nonlinear waves. Random waves. Wave simulation. Waves plus current. Breaking waves. Wind spectra. 3. Wave Loading on Structures. Wave force formulations. Morison equation. Modified Morison equation. Four-term Morison equation. Transverse force. Random wave force. Wave breaking force. Steady drift force. 4. Offshore Structure Response. Nonlinear motion response. Response analysis of jacket structure. Single-degree of freedom system. Moorling line analysis. Time domain solution of moored systems. Low frequency oscillation. High frequency oscillation. Damping at low and high frequency responses. Riser deflection analysis. 5. Distribution of Short-Term Wave Parameters. Wave elevation distribution. Nonlinearity of sea waves. Wave height distribution. Distribution of wide-banded wave amplitudes. Nonlinear Gaussian waves. Nonlinear non-Gaussian waves. Wave group statistics. Wave period distribution. Non-stationary wave height distribution. Wave height-period distribution. Extreme wave height-steepness distribution. Review of nonlinear wave statistics. 6. Short Term Loading Distribution. Linear systems. Linearized systems. Loading spectra. Statistics of narrow-band Morison force. Statistics of wide-band Morison force. Statistics of wave-current force. Statistics of free surface structural response. Review of nonlinear excitation statistics. 7. Short Term Response Distribution. Structural response spectrum. Statistics of nonlinearly dampled system. Statistics of drift force. Statistics of low frequency motion. Discussion of nonlinear response statistics. 8. Long-Term Response Prediction. Long-term wave height distribution. Joint distribution of H s and T z . Extrapolation of wave scatter diagram to longer duration. Long-term wave load distribution. Prediction of extreme wave loads. Return periods of extreme events and non-encounter probabilities. Bivariate short- and long-term prediction. Time and frequency domain long-term predictions. Short and long-term response prediction for nonlinear systems. Review of extreme value prediction for linear and nonlinear systems. References at the end of each chapter. Author index. Subject index.

Field measurements of wave-induced pressure over wind-sea and swell

Abstract: Results of two field experiments in the North Sea are presented. Pressure was measured at two fixed heights above the mean water level and correlated with simultaneous wave height measurements. Roughly 90 hours of data have been analysed and the results are in agreement with earlier results obtained by Snyder et al. (1981). Measurements over swell give no indication of wave decay or growth for waves travelling faster than the wind or against the wind.

Wave variance partitioning in the trough of a barred beach

Abstract: The wave-induced velocity field in the nearshore is composed of contributions from incident wind waves (f > 0.05 Hz), surface infragravity waves (f 2/g), where fis the frequency,  = 2f, }c is the radial alongshore wavenumber (2re/L, L being the alongshore wavelength), [ is the beach slope, and g is the acceleration due to gravity. Using an alongshore array of current meters located in the trough of a nearshore bar (mean depth = 1.5 m), we investigate the bulk statistical behaviors of these wave bands over a wide range of incident wave conditions. The behavior of each contributing wave type is parameterized in terms of commonly measured or easily predicted variables describing the beach profile, wind waves, and current field. Over the 10-day period, the mean contributions (to the total variance) of the incident, infragravity, and shear wave bands were 71.5%, 14.3% and 13.6% for the alongshore component of flow (mean rms oscillations of 44, 20, and 19 cm s -1, respectively), and 81.9%, 10.9%, and 6.6% for the cross-shore component (mean rms oscillations of 92, 32, and 25 cm s -1, respectively). However, the values varied considerably. The contribution to the alongshore (cross-shore) component of flow ranged from 44.888.4% (58.5-95.8%) for the incident band, to 6.2-26.6% (2.5-32.4%) for the infragravity band, and 3.433.1% (0.6-14.3%) for the shear wave band. Incident wave oscillations were limited by depth-dependent saturation over the adjacent bar crest and varied only with the fide. The infragravity wave rms oscillations on this barred beach are best parameterized by the offshore wave height, consistent with previous studies on planar beaches. Comparison with data from four other beaches of widely differing geometries shows the shoreline infragravity amplitude to be a near-constant ratio of the offshore wave height. The magnitude of the ratio is found to be dependent on the Iribarren number, 0 = [(H/Lo) -1/2. Shear waves are, as previous observation and theory suggest (Oltman-Shay et al., 1989; Bowen and Holman, 1989), significantly correlated with a prediction of the seaward facing shear of the longshore current.

Stability of mound breakwater's head and trunk

Abstract: The stability of a cubic armored breakwater head and trunk under monochromatic wave trains is experimentally studied. For the head there is a sector, roughly of 60°, where the unit stability is minimum. The stability function values (similar to Hudson's number) are 1.3‐three times higher there than those obtained from biodimensional tested sections. Head shape, at least for the cases tested, does not exert significant influence on the head stability. Trunk sections tested under long crested waves with a standing longitudinal wave height variation (here called quasi‐three‐dimensional [3D] tests) are less stable than two‐dimensional (2D) tested sections. Furthermore, trunk and head sections show a different behavior as the damage grows; the radio of initiation of damage wave height to destruction wave heights is much lower for head than for trunk sections. Head sections are more brittle than trunk sections. Additionally, exploration was carried out into standing longitudinal variation of wave height along t...

Wind wave cast in the Mediterranean Sea

Abstract: An advanced, third-generation wave model has been repeatedly applied to the Mediterranean Sea. We have studied the accuracy of the results and the factors that control it. The grid resolution, when less than 0.5°, is shown to have in general negligible effect in the open sea. The related accuracy of description of the coastal border can have a relevant effect on the wave field on the side and on the wake of the coastal details, for a distance of a few grid points. The difficulty of correctly evaluating the wind in the Mediterranean basin is dramatically shown by comparing the results obtained using wind fields with different resolution. The crucial role of the orography in shaping the wind fields and the need for higher-resolution models is established. It is found that owing to the strong dependence of wave height on wind speed, for resolution coarser than 70 km the loss of accuracy of the wind fields, even if meteorologically acceptable, makes them useless for an efficient evaluation of the wave conditions. The exception, explained by the local absence of relevant mountain ridges, is the case of a southerly wind in the eastern Mediterranean. The accuracy of wave forecasting is obtained by comparing analysis and forecast fields for a very severe storm. In connection with the preferential west to east flow of most of the Mediterranean storms, and with the scarcity of information on the Atlantic Ocean, the equality of the meteorological forecast rapidly deteriorates beyond 1 or 2 days of forecast. This leads to an even faster deterioration of the wave forecast, whose practical limits in the western Mediterranean can at present be established at 1 day.

The elevation of shore platforms : a laboratory approach to the unsolved problem

Abstract: A wave-flume experiment was conducted to investigate the influence of wave height and rock strength upon the elevation of shore platforms having a marked scarp at the seaward margin. Breaking waves were allowed to act on a steep model cliff to produce a platform. Results indicated that platform elevation increases with increasing rock strength if other factors are constant. It was suggested that a considerable difference in platform elevation in the field can arise depending on wave height and rock strength. The water depth in front of platforms proved to be important for the study of platform elevation.

Alongshore Sediment Transport Rate Distribution

Abstract: The initial analysis of a new set of experimental data on alongshore sediment transport rate distribution is presented. The distributions were generally found to be bimodal but tended to become single-peaked for lower wave heights and smaller grain sizes. The ratio of suspended load to bedload was fairly constant throughout these tests, even though grain size, wave height and wave period were varied substantially.

Influence of waves on air-water gas transfer

Abstract: Experiments on air‐water gas transfer were performed in a flume with a mechanical wave maker for the determination of the gas‐transfer coefficient with waves. Oxygen was used as the transferred gas. The experiments indicate that nonbreaking deep‐water gravity waves significantly influence air‐water gas transfer. The gas‐transfer coefficient was correlated with the wave characteristics. A renewal model was used for the analysis of the data, and the renewal rate was dependent on a wave Reynolds number. The gas‐transfer coefficient was linearly dependent upon the product of wave height and wave frequency. This is also equivalent to the gas‐transfer coefficient being directly proportional to the wave velocity at the water surface. Comparison with data reported in the literature indicates that a significant portion of the observed gas transfer in wind‐wave flume experiments can be attributed to nonbreaking waves. The presence of bubbles or bubble‐entraining breaking waves enhances the gas‐transfer coefficient ...

NMLONG: Numerical Model for Simulating the Longshore Current; Report 1, Model Development and Tests

Abstract: : This report presents the mathematical formulation and verification of a numerical model that simulates wave transformation and longshore current over a bar and trough beach profile. The model is intended for application on projects involving dredged material berms and other situations where a bar and trough topography prevents use of simple analytic or numerical solutions that are restricted to a uniformly sloping beach. The model is called NMLONG, an acronym that stands for Numerical Model of the Longshore current, and it can be conveniently run on a desk-top (personal) computer. The model incorporates all known features of the wave and longshore current system that appear in research-type engineering models run on mainframe computer systems. These features include wave and wind driving, wave breaking and reformation over multiple bar and trough profiles, and lateral mixing. The model also allows choice of linear or quadratic bottom friction and regular or random wave heights as options. The main restriction of the model is longshore uniformity of the waves and beach topography. The model was verified with several field and laboratory data sets. The wave calculation reproduced cross-shore heights of laboratory monochromatic waves on both plane-sloping and bar and trough profiles, and it also reproduced field measurements of wave height frequency of occurrence at different depths in the surf zone. The longshore current model reproduced measured currents on plane-sloping laboratory beaches and currents measured in the field or irregular bottoms. Sensitivity analyses examined linear and nonlinear bottom friction formulations, strength of lateral mixing, effect of wind on setup/setdown and the current, and wave-current interaction.

numerical simulation of shoreline change at Lorain, Ohio

Abstract: Design of detached breakwaters for shore protection is difficult because the response of the shoreline to these structures is governed by at least 14 geometric‐, wave‐, and sediment‐related variables. Numerical models of shoreline change have the potential of incorporating these variables to predict the time evolution of the beach plan shape; however, wave transmission at breakwaters has been a major process absent from such models. In this paper, a pragmatic method of calculating the breaking‐wave height and angle under combined transmission, diffraction, refraction, and shoaling is tested to predict shoreline change measured at Lorain, Ohio, the site of a three‐segment transmissive breakwater system. The model successfully simulated observed shoreline change at the site that occurred over three time periods ranging from one to five years. Model sensitivity to changes in key parameters and simulations of alternative shore‐protection designs to the Lorain project are also described.

The dependence of wind stress on wave height and wind speed

Abstract: Three near-neutral boundary layer data sets were investigated with the aim of finding a dependence of wind stress on both wind speed and significant wave height. The data set most representative of open-ocean wave height, wind speed, and momentum flux conditions, was selected and analyzed by means of the least-squares method to produce a new parameterization for the wind stress as a function of both wind speed and significant wave height. This study shows that the wind stress, and consequently the drag coefficient, decreases with increasing wave height for a fixed wind speed. The study also shows that the curvature of the wind profile decreases with increasing wave height and that the C(DN) = A + BU-bar(10) form for the drag coefficient parameterization is inadequate. A drag coefficient that applies to both smooth and rough flows is proposed. These results are more applicable for open-ocean deep-water conditions and less applicable for sheltered, closed, shallow water sites.

Waves generated by a plunger-type wavemaker

Abstract: Based on a two dimensional linear water wave theory, the boundary element method (BEM) is developed and applied to study the waves generated by a plunger-type wavemaker. The computational domain is truncated by introducing two auxiliary boundaries at three times wave length distance away from the wavemaker. Radiation condition is used along the auxiliary boundaries. The accuracy of the numerical technique is demonstrated by comparing numerical results with previously published results, experimental and other techniques. However the actual wave height is smaller in the experiments than predicted by numerical models. The discrepancy between predicted and observed wave height can be attributed to the effects of leakage of wave energy under the plunger-type wavemaker. In order to predict waves generated by plungertype wavemaker accurately, the influence of leakage, under the plunger-type wavemaker, is included in the present study. The ratios between wave amplitude and stroke are established for two different...

Ocean circulation and climate change

Abstract: Recent numerical simulations using global ocean circulation models are reviewed together with model experiments involving further important climate sub-systems with which the ocean interacts: the atmosphere, the air-sea interface and the global carbon cycle. A common feature of all ocean circulation experiments considered is the strong sensitivity of the circulation to relatively minor changes in surface forcing, particularly to the buoyancy fluxes in regions of deep water formation in high latitudes. This may explain some of the well-known deficiencies of past global ocean circulation simulations. The strong sensitivity may also have been the cause of rapid climate changes observed in paleoclimatic records and can lead further to significant natural climate variability on the time scales of a few hundred years through the stochastic forcing of the ocean by atmospheric weather variability. Gobal warming computations using two different coupled ocean-atmosphere models for the “business-as-usual” scenario of the Intergovernmental Panel on Climate Change yield a significantly stronger warming delay due to the heat uptake by the oceans in the Southern Ocean than estimated on the basis of box-diffusion models. Recent advances in surface wave modelling, illustrated by a comparison of wave height fields derived from the WAM model and the GEOSAT altimeter, hold promise for the development of an improved representation of ocean-atmosphere coupling based on an explicit description of the dynamical processes at the air-sea interface. Global carbon cycle simulations with a three dimensional carbon cycle model tuned to reproduce past variations of carbon cycle indices show a significant impact of variations in the ocean circulation on the CO 2 concentration in the atmosphere and thereby on climate. The series of experiments suggest that for the study of climate in the time scale range from 10 -1 -10 3 years, it would be highly desirable, and has indeed now become feasible, to couple existing, verified, climate sub-system models together in a comprehensive fully interactive model including the oceans, sea-ice, atmosphere, surface interface and the global carbon cycle. DOI: 10.1034/j.1600-0889.1991.t01-2-00008.x

A C-band scatterometer for remote sensing the air-sea interface

Abstract: An airborne C-band scatterometer system (C-Scat) has been developed to remotely sense ocean surface winds and improve upon the present understanding of the relationship between normalized radar cross section (NRCS) and ocean surface roughness influences such as wind speed and direction, wave height and slope, and the air-sea temperature difference. The scatterometer utilizes a unique frequency-steered microstrip array antenna that is installed beneath the fuselage of an airplane. The antenna is electronically scanned in elevation, from 20 degrees to 50 degrees off-nadir, and mechanically spins in azimuth. The system is capable of measuring ocean surface NRCS from altitudes as high as 25000 ft. The system is divided into four subsystems: the transmitter and receiver, the spinning antenna, the computer control and data acquisition subsystem, and the digital and analog interface electronics. >

Dredging Research Program: Simulation of Time Sequences of Wave Height, Period, and Direction

Abstract: : This report describes a procedure for generating simulated time sequences of wave height, period, and direction data corresponding to specific locations. The techniques uses a finite length wave record to compute a matrix of coefficient multipliers that can be used to generate arbitrarily long time sequences of simulated wave data which preserve the primary statistical properties of the finite data set. The procedure was developed for simulating the 20-year Wave Information Study (WIS) hindcast database; however, it can be applied to any appropriate data sequence. Application of the method is demonstrated through comparisons of simulated data to hindcast data corresponding to a Gulf of Mexico WIS station located offshore of the entrance to Mobile Bay, Alabama. Results show that the simulated series exhibits the primary statistical properties of the WIS data, including seasonality and wave sequencing.

Wave Forces on Wall

Abstract: Revised criteria is presented for wave shoaling after breaking on a plane beach and for resulting wave forces on a wall landward of the still-water line. Existing criteria is based on the assumption that the wave height above still water will remain constant between the point of wave breaking and the shoreline. A review of experiments carried out by several different researchers shows a marked reduction in wave height following breaking; and the assumption used in existing criteria will overestimate wave heights by a factor of four and wave forces by a factor of 16. New criteria is presented that more accurately predicts the height of a broken wave. Revised equations are given for calculating resulting wave forces.

Ocean circulation and climate change

Abstract: Recent numerical simulations using global ocean circulation models are reviewed together with model experiments involving further important climate sub-systems with which the ocean interacts: the atmosphere, the air-sea interface and the global carbon cycle. A common feature of all ocean circulation experiments considered is the strong sensitivity of the circulation to relatively minor changes in surface forcing, particularly to the buoyancy fluxes in regions of deep water formation in high latitudes. This may explain some of the well-known deficiencies of past global ocean circulation simulations. The strong sensitivity may also have been the cause of rapid climate changes observed in paleoclimatic records and can lead further to significant natural climate variability on the time scales of a few hundred years through the stochastic forcing of the ocean by atmospheric weather variability. Gobal warming computations using two different coupled ocean-atmosphere models for the “business-as-usual” scenario of the Intergovernmental Panel on Climate Change yield a significantly stronger warming delay due to the heat uptake by the oceans in the Southern Ocean than estimated on the basis of box-diffusion models. Recent advances in surface wave modelling, illustrated by a comparison of wave height fields derived from the WAM model and the GEOSAT altimeter, hold promise for the development of an improved representation of ocean-atmosphere coupling based on an explicit description of the dynamical processes at the air-sea interface. Global carbon cycle simulations with a three dimensional carbon cycle model tuned to reproduce past variations of carbon cycle indices show a significant impact of variations in the ocean circulation on the CO 2 concentration in the atmosphere and thereby on climate. The series of experiments suggest that for the study of climate in the time scale range from 10 -1 – 10 3 years, it would be highly desirable, and has indeed now become feasible, to couple existing, verified, climate sub-system models together in a comprehensive fully interactive model including the oceans, sea-ice, atmosphere, surface interface and the global carbon cycle. DOI: 10.1034/j.1600-0870.1991.00008.x

Trends in Phreatic Surface Motion in Rubble‐Mound Breakwaters

Abstract: Experimental studies were undertaken to investigate phreatic surface motion in rubble-mound breakwater models This motion was assessed as a function of a number of governing variables, including armor unit type, geometry of the armor layer, breakwater slope, and core permeability Tests were conducted in a two-dimensional wave flume using regular waves Both the maximum and average phreatic surface elevations were found to increase with increasing wave height, increasing wave period, and steeper slopes The setup within the core was also significantly higher for tests with spheres in the armor layer, due to the increased permeability normal to the slope Increasing core permeability resulted in sharp rises in the maximum and average phreatic surface elevations

Variations on Higher-Order Shoaling

Abstract: Variations on the classical integral approach to shoaling may be categorized in terms of the choice of steady-wave theory and, more importantly, the choice of dependent variables and conservation laws. One-variable models (wave-height) appear to underpredict both wave height and wave number, and assume that both undertow current and setup are zero. Models with two-variables (wave height and undertow cuurrent) and three variables (wave height, undertow current, and setup) provide almost identical predictions of wave height, wave number, and undertow current. Setup, which is assumed to be zero in one- and two-variable models, is predicted by three-variable models. Fourier 18 predictions of the shoaling evolution of wave height, wave number, undertow current, and setup of the mean water level are presented for a range of deep-water incident wave heights.

Extension of mild slope equation for waves propagating over a permeable submerged breakwater

Abstract: A wave equation is presented for predicting reflected and transmitted waves for a permeable submerged breakwater. This equation includes the mild slope equation derived by Berkhoff(1972), which is a vertically integrated refraction-diffraction equation. Therefore, the equation derived can also predict the combined effects of refraction and diffraction. Numerical calculations with a dissipation term due to breaking are performed to obtain reflection and transmission coefficients as well as wave height distributions. Through the comparisons with the experimental results, the validity of the model is confirmed.

Transformation of Random Breaking Waves and Its Empirical Numerical Modeling Considering Surf Beat

Abstract: The free surface characteristics of random waves in the shoaling and surf zones on a beach of uniform slope were examined experimentally. The effect of incident wave grouping was found for larger waves outside the surf zone; however, this effect disappeared as the fraction of breaking waves increased. The wave height distributions were not necessarily described by the Rayleigh distribution in shallow water. The assumption that the distribution of breaking wave heights is proportional to that of non-breaking wave heights was not valid in the region where the fraction of breaking waves was small. A numerical model which accounts for the effects of incident wave grouping and surf beat empirically was shown to be capable of predicting the experimental results such as the cross-shore transformations of the representative wave heights, wave height distributions and fraction of breaking waves. The mean lengths of runs and total runs, skewness, kurtosis, nondimensional crest height were observed to increase and then decrease from deep water to shallow water. The nondimensional crest width decreased and then increased in an opposite manner. For the case of deep water wave steepness smaller than 0.03, the location of the maxima and minima was where the significant wave height was maximum, and for the case of deep water wave steepness larger than about 0.04, this location was approximately at h/H 0 =1.2 for 1:30 slope. There were essentially no effects of incident wave grouping on nonlinearity and asymmetry of wave profiles in the shoaling and surf zones. The skewness and asymmetry of the wave profiles may be important in predicting the net cross-shore sediment transport in the shoaling and surf zones.

Wave group properties of coastal waves

Abstract: This paper examines the following properties of observed wave groups: (1) the change of wave grouping from offshore to shallow water region including the surf zone; (2) the relation between wave groupiness and wave height distributions; and (3) the distribution of run lengths and time series of wave heights of extreme wave groups which contain the maximum wave height in each wave record. Main results are summarized as follows: (1) wave groups become flattened as they propagate landward; (2) no clear maximum in mean length of runs is seen around the position where the significant wave height is the maximum, in contrast to the laboratory data, while the tendency of the change of GF is the same as the experimental data; (3) when wave grouping becomes pronounced, the distribution of wave heights becomes wider; (4) extreme wave groups accompany more large waves than ordinary wave groups; (5) for the case of narrow spectra, the time series of wave heights of extreme wave groups is represented fairly well by that of the wave group formed by the side-band instability, and the distribution of three high waves is also in qualitative agreement with that of envelope soliton, and (6) for wide spectra, which may occur during the peak of storms, wave heights in front and rear of the maximum wave are half as large as the maximum wave height, implying that a maximum wave tends to appear without accompanying other large waves.