Showing papers on "Wave height published in 1999"

Seasonal variability of the global ocean wind and wave climate

Abstract: A data set spanning a period of 10 years and obtained from a combination of satellite remote sensing and model predictions is used to construct a global climatology of ocean wind and wave conditions. Results are presented for: significant wave height, peak and mean wave period and wave direction as well as wind speed and direction. The results are presented in terms of mean monthly statistics. The processed data set provides global resolution of 2°. The climatology clearly shows the zonal variation in both wind speed and wave height, with extreme conditions occurring at high latitudes. The important role played by the intense wave generation systems of the Southern Ocean is clear. Swell generated from storms in the Southern Ocean penetrates throughout the Indian, South Pacific and South Atlantic Oceans. During the Southern Hemisphere winter, this swell even penetrates into the North Pacific. The results confirm visual observations that the Southern Ocean is consistently the roughest ocean on earth. It is shown, however, that this is mainly caused by consistent high wind speeds, rather than the extended westerly fetch which exists. The west coasts of most continents have noticeably rougher wave climates than their respective east coasts, as a result of the longer generation fetches which exist on the west coasts. Copyright © 1999 Royal Meteorological Society.

Hydrodynamics induced by wind waves in a vegetation field

Abstract: The wave-induced kinematics and dynamics of a submerged or emerged vegetation field is analyzed. Using potential flow and an eigenfunction expansion, the problem is solved considering regular as well as irregular incident waves. The model takes into account the vegetation motion and solves for the complete wave system on the vegetation field and in its vicinity. The model is validated against experimental laboratory data obtained by other authors, showing a much better agreement than previous theoretical models. In this paper the model is used to evaluate wave height evolution (damping), vegetation and fluid motion, and forces and moments on the vegetation. Furthermore, the inclusion of irregular waves provides force and moment distributions on the vegetation field depending on the wave climate statistics.

Boussinesq modeling of a rip current system

Abstract: In this study, we use a time domain numerical model based on the fully nonlinear extended Boussinesq equations [Wei et al., 1995] to investigate surface wave transformation and breaking-induced nearshore circulation. The energy dissipation due to wave breaking is modeled by introducing an eddy viscosity term into the momentum equations, with the viscosity strongly localized on the front face of the breaking waves. Wave run-up on the beach is simulated using a moving shoreline technique. We employ quasi fourth-order finite difference schemes to solve the governing equations. Satisfactory agreement is found between the numerical results and the laboratory measurements of Haller et al. [1997], including wave height, mean water level, and longshore and cross-shore velocity components. The model results reveal the temporal and spatial variability of the wave-induced nearshore circulation, and the instability of the rip current in agreement with the physical experiment. Insights into the vorticity associated with the rip current and wave diffraction by underlying vortices are obtained.

Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific

Abstract: Knowing the wave climate along the California coast is vital from the perspectives of climatological change and planning shore protection measures. Buoy data indicate that the wave climate is very similar along much of the California coast. We show that elements of the wave climate can be accurately reconstructed using near-coastal inland broadband seismometer data. Such reconstructions are possible because swell approaching the coast generates pressure fluctuations that are locally transformed into seismic waves at the seafloor that propagate inland and are detectable by land-based seismometers. Buoy and seismometer data show that most of the microseism energy recorded inland near the coast is generated from wave events at nearby coastal locations. A site-specific, empirically derived seismic-to-wave transfer function is demonstrated to be applicable to seismic data from the same location for any year. These results suggest that ocean wave heights estimated from near-coastal broadband seismometer data are sufficiently reliable for monitoring the coastal wave height when buoy data are unavailable, provided that adequate simultaneous nearby buoy measurements are available to calibrate the seismometer data. The methodology presented here provides an important tool that allows the investigation of potential wave climate changes from reconstructions using archived seismic data collected since the 1930s.

Sea Spectra Revisited

Abstract: This paper comprises a compendium of updated spectrum formulations, Rayleigh factors, and associated wave height and period relationships-prepared for easy understanding and application.

Measuring waves and currents with an upward-looking ADCP

Abstract: Routine monitoring of waves and currents in the nearshore region is of great interest both scientifically and to the general public because of their role in coastline erosion and their impact on recreational activities Historically, the technology for measuring these quantities has been distinct, requiring separate instrumentation for each In this contribution the authors show that it is possible in shallow water to estimate both wave height and direction from a conventional bottom-mounted, upward-looking acoustic Doppler current profiler Height and direction spectra compare well with a co-located array of pressure gages

Random Wave Measurements in Front of Reflective Structures

Abstract: The changes in the wave spectrum and significant wave height near a reflective structure are investigated. Significant wave-height measurements with a one wave gauge show a standing wave pattern near the structure, extending offshore to about two times the spectral-peak wavelength for a JONSWAP incidental wave spectrum. The measured spectra also reveal a pattern of nodal and antinodal frequencies, even to much larger distances from the structure. These findings are in agreement with results from linear wave theory. Multigauge techniques can be used to discriminate between incident and reflected waves. Despite the standing wave pattern these techniques can be used to determine the incident wave height accurately up close to the structure—the distance being limited by the extend of the evanescent wave modes attached to the structure.

System and method for measuring wave directional spectrum and wave height

Abstract: A system and method for measuring the directional spectrum of one or more waves in a fluid medium using a multi-beam sonar system. In an exemplary embodiment, range cells located within a plurality of acoustic beams are sampled to provide current velocity data. Optionally, wave surface height and pressure data is obtained as well. This velocity, wave height, and pressure data is Fourier-transformed by one or more signal processors within the system, and a surface height spectrum produced. A cross-spectral coefficient matrix at each observed frequency is also generated from this data. A sensitivity vector specifically related to the ADCP's transducer array geometry is used in conjunction with maximum likelihood method (MLM), iterative maximum likelihood method (IMLM), or other similar methods to solve a the wave equation at each frequency and produce a frequency-specific wave directional spectrum. Ultimately, the frequency-specific spectra are combined to construct a complete two-dimensional wave directional spectrum. The system is also capable of measuring current profile as a function of depth in conjunction with wave direction and wave height.

Longshore Sediment Transport on Mediterranean Coast of Israel

Abstract: A modified version of the CERC formula, which relates longshore sediment drift to deep water wave height and direction, has been used to define the equivalent wave height. The directional distribution of these wave heights and corresponding sediment transport rates and their analytical approximations are found using statistical analysis of high quality directional wave data measured simultaneously at two sites, Ashdod and Haifa. The directional distributions enable one to determine the directional shift between the average wave directions at both sites and to find the necessary corrections of wave directions at any location along the coast. The results emphasize the importance of accounting for wave directional shift in sediment transport calculations even when the locations are close. A detailed computation of directional shift for different ranges of wave heights is performed using correlation analysis of data sets. The corrections of wave directions at various locations along the coast are found by interpolation, and an excellent agreement between sediment fluxes at each location has been obtained when using the LITPACK package for littoral transport simulations with each data set.

Wind stress and surface waves observed on Lake Washington

Abstract: New results from turbulent flux measurements made over Lake Washington include the following: 1) The only direct measure of the vertical transport of the horizontal momentum, heat, and matter in the surface boundary layer is the so-called eddy correlation method. However, even if the measurement errors are negligible, the results obtained from point observations may show large scatter due to lack of stationarity and horizontal homogeneity in the turbulent field and to the sampling variability. Scatter may be greatly reduced by spatial averaging. In this study, such an effect is achieved by determining the surface roughness length, hence the neutral drag coefficient, from the measured wave height spectrum, which reflects the atmospheric input integrated over the fetch. Applicability and usefulness of the approach for general field measurements and remote sensing is discussed. 2) The evolution of the wave field observed on Lake Washington agrees in peak frequency and the slope of the equilibrium ra...

Field observations of wave setup

Abstract: Wave setup is assumed to be a balance between the cross-shore convergence of the onshore flux of momentum (wave radiation stress Sxx) in the surfzone and a cross-shore pressure gradient. Oceanic observations between the 2- and 8-m isobaths near Duck, North Carolina, provide a test of the wave setup balance without assuming that wave height in the surfzone is proportional to water depth. Analysis of data from a cross-shore array of 11 pressure gauges and 10 sonar altimeters deployed during the fall of 1994 indicates the wave setup balance holds to at least the accuracy of the pressure measurements (a few centimeters). The correlation between the two terms in the setup balance is 0.93, and the linear regression slope is 1.05±0.19. Accurate estimates of the cross-shore pressure gradient require density measurements to adjust pressure measurements taken at different depths to the same level. The assumption that pressure and bathymetry are linear between the 2- and 8-m isobaths (or the more common assumption that the height of normally incident, shallow water waves is proportional to the water depth) introduces errors of up to 6 cm for the conditions considered here. Given this assumption, 3.5 years of data from pressure gauges in 2 and 8 m of water indicate that the wave setup balance is valid for a wide range of conditions (correlation 0.71 and regression slope 0.98±0.08).

The Global and Regional Climate System

Abstract: The relationships between planetary and regional or local scales of the climate system are discussed.

Damage Progression on Rubble-Mound Breakwaters

Abstract: : This report addresses depth-limited breaking-wave damage on rubble-mound breakwaters. Few generalized studies have been conducted on this topic; so no engineering methods exist for determining deterioration rates of breakwaters exposed to sequences of storms. A new experiment is discussed measuring incipient motion on both stone and sphere armor layers. An incipient motion criterion is derived for the dominant mode of motion: vertical lift under the steep breaking-wave face. Previous breakwater damage experiments and measurement techniques are thoroughly reviewed. A new experiment is described consisting of seven relatively long-duration breakwater damage test series. The test series were conducted in a flume using irregular waves. Wave height, wave period, water depth, storm duration, storm sequencing, and stone gradation were varied systematically. The experiment yielded relationships for both temporal and spatial damage development Maximum eroded depth, maximum eroded length, and minimum remaining cover depth are introduced to describe the damaged profile. The mean and standard deviation of these profile parameters are shown to be a function of mean eroded area. An equation is also provided to predict the standard deviation of eroded area as a function of mean damage. Relations for predicting temporal variations of mean eroded area with wave height and period varying with time in steps are shown to describe damage reasonably well.

Breaker bar formation and migration

Abstract: The formulation of Isobe and Horikawa (1982) is modified and adapted for use in a probabilistic cross-shore transport model (CROSMOR). Computed changes of wave height, peak near-bed orbital velocity and depth-averaged return flow are compared to laboratory measurements. A comparison is made between computed and measured bedprofile changes for a large scale laboratory experiment.

Influence of Steep Seabed Slopes on Breaking Waves for Structure Design

Abstract: Prediction of limiting wave heights in conditions of depth-induced breaking is subject to considerable uncertainties, yet the (local) wave height is probably the most important input variable in design of coastal, harbour or shoreline structures subject to wave action. This paper presents selected results from laboratory experiments to measure depthlimited wave breaking over steep bed slopes (1:50, 30, 20, and 1:10) in fully random wave conditions. Experimental measurements are compared with predictions for H,ms, Hs and Hmax under shoaling and breaking. The effects of shoaling and breaking on the wave height distributions are explored. An alternative empirical method to predict Hlfl and Hmax is suggested.

Evaluation of criteria for Predicting erosion and accretion on an estuarine sand beach, Delaware Bay, New Jersey

Abstract: Predicting erosion and accretion of sand beaches in estuaries is important to managing shoreline development and identifying potential relationships between biological productivity and beach change. Wave, sediment and profile data, gathered over twenty-nine days on an estuarine sand beach in Delaware Bay, New Jersey, were used to evaluate the performance of four criteria that predict beach erosion and accretion due to wave-induced cross-shore sediment movement (Dean 1973; Sunamura and Horikawa 1974; Hattori and Kawamata 1980; Kraus et al. 1991). Each criterion defines a relation, between a wave and sediment parameter, and includes a coefficient that discriminates beach erosion and accretion events. Relations, based on small-scale laboratory and field data, were evaluated for predicting erosion or accretion at the study site. Significant wave heights at the study site, monitored near high water, ranged from 0.08 to 0.52 m with periods of 2.4 to 12.8 s. Median grain sizes of sediments on the beach foreshore, gathered at low water, ranged from 0.33 to 0.73 mm. All four criteria showed a clustering of erosion and accretion events. Relations derived from small-scale laboratory data were better predictors of erosion on the profile at the field site than those derived from field data gathered on exposed ocean environments. The planar profile and dominance of incident waves of low height and short period are similar to laboratory conditions characterized by initial planar beach slopes and monochromatic waves. Decreasing the value of the empirical coefficient to account for the differences in the magnitude of wave energy and grain size increases the performance of the criteria tested to predict erosion of the profile.

Energy dissipation model for irregular breaking waves

Abstract: A simple model is presented to compute the average rate of energy dissipation in irregular breaking waves. The average rate of energy dissipation rate is assumed to be proportional to the difference between the local mean energy flux and stable energy flux. The local fraction of breaking waves is determine from the derivation of Battjes and Janssen (1978). Root mean square wave height deformation is computed from the energy flux conservation. The model is validated using root mean square wave height data from small and large scale laboratory and field experiments. Total 144 wave height profiles are used in the calibration and verification of the model. Reasonable good agreement is obtained between the measured and computed root mean square wave heights. The root mean square relative error of the model is 10.2 %.

Nonlinear features in wave-resolving microwave radar observations of ocean waves

Abstract: Transformation of sea-surface Doppler microwave backscatter observations from the space-time domain to the wavenumber-frequency domain separates linear wave energy from nonlinear effects. Here observations and modeling are used to investigate the sources of these nonlinearities. Wave breaking and electromagnetic shadowing are examined with emphasis on their relative effects both inside and outside the region of the wavenumber-frequency spectrum associated with the linear dispersion equation. Shadowing significantly reduces the variance levels within the linear spectral region. In addition, shadowing is less directly related to changes in variance outside this region, i.e., that region associated with nonlinearity in the wave field. Wave breaking has less of an effect on the variance within the linear region than shadowing. However, the modeled wave breaking does have a greater tendency to increase variance levels at frequencies less than that of the linear wave field, for any given wavenumber. Aliasing and emphasis of crest backscatter are also explored to explain features seen in some wavenumber-frequency intensity images. Two-dimensional data allow the linear wave spectrum to be separated from nonlinear effects. This results in improved wave height spectrum estimation.

Nonlinear time-averaged model in surf and swash zones

Abstract: A time-averaged model is developed to predict the cross-shore variations of the mean and standard deviation of the free surface elevation from outside the surf zone to the lower swash zone on beaches. This new model includes nonlinear correction terms in the cross-shore radiation stress and energy flux that become important in very shallow water. Empirical formulas are proposed for the skewness and kurtosis as well as the ratio of the root-mean-square wave height to the mean water depth which increases rapidly near the still water shoreline. The developed model is shown to be in agreement with three irregular wave tests on a 1:16 smooth impermeable slope and two tests of quasi-equilibrium terraced and barred beaches. The model can predict the observed large increase of wave setup near the still water shoreline. The developed model and empirical formulas will need to be verified using additional experiments.

The Budget of Turbulent Kinetic Energy in the Marine Atmospheric Surface Layer

Abstract: In the spring and autumn of 1994 a surface layer turbulence and air-sea interaction experiment (RASEX, the Riso Air-Sea Experiment) was conducted at a sea mast in the Baltic Sea, 2 km off-shore of Lolland, Denmark. This experiment brought researchers and instrumentation from the Riso Danish National Laboratory, the Woods Hole Oceanographic Institution, and NOAA’s Environmental Technology Laboratory to a sea mast facility constructed by Riso. The 48 m mast was equipped with multiple levels of mean and turbulence instrumentation for measuring the atmospheric surface layer and surface wave field, including three levels of fast response turbulent pressure sensors with dynamic noise reducing pressure probes. The pressure instrumentation together with collocated sonic anemometers allowed for the measurement of the pressure transport term in the budget of Turbulent Kinetic Energy (TKE). Sonic and cup anemometer data also provided the shear production, buoyant production, and turbulent transport terms, and the turbulent dissipation rate was computed from the inertial subrange of velocity spectra. Together with the wave height measurements, this provided not only measurement of all of the terms in the TKE budget, but also allowed for an analysis of possible wave effects on the TKE budget over the sea.

Breaking Wave Forces on Tripod Concrete Structure on Shoal

Abstract: An experimental study has been carried out on the forces from breaking waves on a tripod structure located on a shoal. The main purpose was to investigate both horizontal and vertical forces as well as overturning moments due to waves acting on the tripod. In addition, horizontal water particle velocities and crest elevations directly over the shoal have been measured. Nondimensional relations between the maximum depth average particle velocities over the shoal, the wave height, wave period, water depth, and wave crest elevation have been obtained. Further nondimensional relations between the wave crest elevation, wave height, wave period, and water depth have also been obtained. Finally, a simple wave force calculation method was developed that may be used to obtain wave forces on similar structures on other shoals that do not deviate significantly from the conditions used during the present tests.

Effects of Liquid Viscosity on Characteristics of Waves in Gas-Liquid Two-Phase Flow (Characteristics of Huge Waves and Disturbance Waves)

Abstract: In order to clarify the effects of liquid viscosity on the interfacial structure of the flow occurring in the transition region from plug flow to annular flow regimes, time-spatial measurements of liquid holdup were carried out using liquids with kinematic viscosity of 1×10-6, 10×10-6 and 20×10-6m2 / s. Based on these measurements, time-spatial maps of wave behavior were produced and the probability distributions of wave width, wave velocity and wave height were also determined. Close inspection of these data revealed that there appeared two types of waves, namely huge waves and disturbance waves, even in cases where the liquids with the kinematic viscosity of 10×10-6 and 20×10-6m2 / s were used. Moreover, the differences between these waves became greater with an increase in the kinematic viscosity of liquid. Furthermore, it was also clarified that the region, where the gas-liquid interface was covered with ripple-like waves, became wider with an increase in the kinematic viscosity of liquid.

Wave Breaking on an Opposing Current

Abstract: PROBLEM: Waves propagating into a tidal inlet will shoal and break because of changes in water depth, focusing by shoals, and interactions with an opposing (ebb) current. On an ebb current, waves steepen because their height increases and length decreases. The wave steepening can intensify wave breaking, causing a navigation hazard and inducing a wave-driven current and sediment transport. Most wave-breaking criteria are based on a maximum ratio of wave height to water depth. But, in regions where waves break because of steepening on an ebb current, the height-to-depth criterion may significantly underestimate wave breaking and overestimate wave height. CETN IV-9 (Smith 1997) describes the process of wave-current interaction in one dimension and gives the equations for calculating wave-height transformation on an ebb (opposing) or flood (following) current. Extending the discussion in CETN IV-9, the present CETN provides guidance on estimating wave breaking that may occur as waves transform on an opposing current. Before the method for calculating breaking is described, the governing equation, wave-action conservation, is reviewed (see also CETN IV-9), and wave shoaling on a current is discussed. CONSERVATION OF WAVE ACTION: Wave height in the presence of a current is governed by the conservation of wave action (Jonsson 1990, and others). The one-dimensional conservation of wave action equation is given by: