Showing papers on "Wave height published in 2004"

Observation of strongly non-Gaussian statistics for random sea surface gravity waves in wave flume experiments.

Abstract: We study random surface gravity wave fields and address the formation of large-amplitude waves in a laboratory environment. Experiments are performed in one of the largest wave tank facilities in the world. We present experimental evidence that the tail of the probability density function for wave height strongly depends on the Benjamin-Feir index (BFI)---i.e., the ratio between wave steepness and spectral bandwidth. While for a small BFI the probability density functions obtained experimentally are consistent with the Rayleigh distribution, for a large BFI the Rayleigh distribution clearly underestimates the probability of large events. These results confirm experimentally the fact that large-amplitude waves in random spectra may result from the modulational instability.

Last century changes in ocean wind wave height from global visual wave data

Abstract: [1] Centennial time series of visually observed wave height were derived from the International Comprehensive Ocean-Atmosphere Data Set along the major ship routes in the World Ocean and homogenized. They demonstrate positive trends in significant wave height over the North Pacific with a maximum of 8–10 cm/decade in the northeast Pacific. In the North Atlantic and other basins significant upward changes (up to 14 cm/decade) are observed only for the last 50 years and not for centennial records. Long-term changes in wind wave height are closely associated with the North Atlantic Oscillation in the Atlantic and with North Pacific Oscillation and El-Nino–Southern Oscillation in the Pacific.

Long-Term Validation of Wave Height Measurements from Altimeters

Abstract: Since July 1991, six altimeter missions have been launched successfully, and they have provided almost continuous wave height measurements for more than 12 years. Long-term series of wave height measurements are of major interest for climatology and oceanic wave modeling. Before using such data, the measurements have to be validated, and the homogeneity of the data from various satellites has to be checked. Significant wave height measurements from ERS, TOPEX/Poseidon, GEOSAT Follow-on, Jason-1 and ENVISAT altimeters are validated using cross-altimeter and buoy comparisons. Emphasis is put on the two recent missions Jason-1 and ENVISAT. Corrections for biases and trends are proposed for the six altimeters, allowing the generation of consistent and homogeneous data. Tests of these corrections are performed over global ocean simple statistics.

On the fractal dimension of sea surface backscattered signal at low grazing angle

Abstract: Fractal analysis of sea surface backscattering signal (sea clutter in radar terminology) represents a novel technique for the study of sea surface roughness. When Kirchhoff's assumption is satisfied, the fractal dimension of the signal is linearly related to the fractal dimension of the sea surface. Moreover, such a relationship is independent of transmitted frequency, polarization, time, space, sea wave propagation direction, incident angle (within the constraint of Kirchoff's assumption) and significant wave height. Nevertheless, for a low grazing angle, the Kirchhoff approximation does not hold and the behavior of the sea clutter fractal dimension cannot be theoretically predicted. The purpose of this paper is to investigate the fractal dimension of the sea clutter at low grazing angle, in order to extend the theoretical results. Moreover, the effects of the presence of a target on the sea surface are analyzed by means of the fractal dimension. Such an analysis is performed by using live recorded clutter data. In detail, the fractal dimension's dependence on space, time, sea wave propagation direction, sea wave height, transmitted polarization and presence of targets is investigated. A discussion on the use of the sea clutter fractal dimension for sea surface monitoring is addressed.

Measurement and prediction of wave‐generated suborbital ripples

Abstract: [1] Experiments in a large-scale wave flume using regular and irregular waves with periods between 4 s and 6 s and heights between 0.2 m and 1.55 m have examined the formation of wave-generated ripples using sediment beds composed of four grain sizes (D50 = 0.349 mm, 0.329 mm, 0.220 mm, and 0.162 mm) in a water depth of approximately 4 m. Estimates of wave-generated ripple height,η, and wavelength, λ, were obtained using zero down-crossing analyses of bed profiles measured by acoustic means along a 4-m transect normal to the ripple crests. Further information pertaining to λ was obtained from bed images obtained using scanning sonar. The analyses reported here focus on the sequence bedforms that evolved in response to stepwise increases and decreases in wave height. Results show that ripples for the most part are suborbital in nature and do not conform well to empirical equations used frequently to predict η and λ values in the field. On the basis of the present data, two new equations for prediction of η and λ are obtained and their use in field situations where hydrodynamic and sedimentary conditions favor development of suborbital bedforms is recommended.

Dynamics of a reflective beach with a low tide terrace

Abstract: Many beaches are characterised by having steep upper slopes, flat low-tide terraces, and mixed or bimodal sediment distributions. This study compares the surf zone hydrodynamic and sediment transport processes of these two dynamically different regimes on the beach at Teignmouth (UK) as part of the COAST3D project. In-situ measurements were made using pressure transducers, electromagnetic current meters and optical backscatter sensors simultaneously on the steep part of the beach (slope 1/10, D 50 1.1 mm) and on the terrace (slope 1/100, D 50 0.25 mm) during a range of incident wave conditions (H b 0.25–0.75 m). The steep upper beach was characterised by: plunging breakers, high wave height/water depth ( γ ) values, large gravity band cross-shore velocity variance, a well-developed subharmonic, strong undertow, high suspended sediment concentrations, and sediment transport dominated by a strong mean offshore component. The shallow shelving terrace was characterised by spilling breakers, low γ values, weak undertow, gravity velocity variance decreasing shoreward, infragravity cross-shore velocity variance increasing shoreward, and stronger longshore currents than undertow. Mean suspended sediment concentrations on the terrace were an order of magnitude less than on the steep beach. The oscillatory cross-shore components of transport were onshore in the incident wave band but increasingly dominated by the infragravity band in progressively shallower water. Longshore sediment transport rates on the terrace were large compared to cross-shore transport rates. The measurements suggest that the dynamics of the terrace/steep beach exhibit characteristics of separate dissipative/reflective sites. However, interaction between the different regions does occur. In particular, the low tide terrace acts to reduce the wave height that is capable of reaching the steep beach, thereby protecting it to some degree from erosion.

Effects of Sand Mining on Physical Processes and Biological Communities Offshore New Jersey, U.S.A.

Abstract: Physical processes and biological data were collected and analyzed for eight sand resource areas on the New Jersey Outer Continental Shelf to address environmental concerns raised by the potential for mining sand for beach replenishment. Nearshore wave and sediment transport patterns were modeled for existing and post-dredging conditions, with borrow site sand volumes ranging from 2.1 to 8.8 × 106 m3. Wave transformation modeling indicated that minor changes will occur to wave fields under dominant directional conditions and selected sand extraction scenarios. Localized seafloor changes at borrow sites are expected to result in negligible impacts to the prevailing wave climate at the coast. At potential impact areas along the New Jersey coast, wave height changes averaged approximately ±3 to 15% when compared with wave heights for existing conditions. For all selected sand borrow sites offshore New Jersey, average variation in annual littoral transport was approximately 10% of existing values. Be...

A 2-d random wave transformation model with gradational breaker index

Abstract: Random wave transformations with breaking in shallow water of 2-D bathymetry are computed with the parabolic equation. A new system of gradational breaker index, the value of which gradually decreases as the level of wave height within a wave group is lowered, is introduced to simulate gradual shape variations of wave height distribution in the surf zone. Wave attenuation in the trough area of a barred beach is treated with a secondary gradational breaker index, which is applied for locations in water of constant or increasing depth. Its empirical coefficients are assigned values different from those in water of decreasing depth. The wave attenuation factor due to bottom friction is formulated by evaluating the rate of energy dissipation by shear stress along the sea bottom. Computation is made for directional spectral components with multiple levels of wave heights under the Rayleigh distribution, and the results are synthesized for the calculation of wave height distributions. The new computational scheme succeeds in reproducing the random wave breaking diagrams by Goda (1975), and shows good agreements with several experimental results on wave transformations over horizontal shelves, barred beaches, and an elliptical shoal. The scheme also yields wave height predictions in good agreement with several field measurements across the surf zone.

Ship Shoal as a Prospective Borrow Site for Barrier Island Restoration, Coastal South-Central Louisiana, USA: Numerical Wave Modeling and Field Measurements of Hydrodynamics and Sediment Transport

Abstract: STONE, G.W.; PEPPER, D.A.; XU, J., and ZHANG, X., 2004. Ship Shoal as a prospective borrow site for barrier island restoration, coastal south-central Louisiana, USA: numerical wave modeling and field measurements of hydrodynamics and sediment transport. Journal of Coastal Research, 20(1), 70‐89. West Palm Beach (Florida), ISSN 07490208. Ship Shoal, a transgressive sand body located at the 10 m isobath off south-central Louisiana, is deemed a potential sand source for restoration along the rapidly eroding Isles Dernieres barrier chain and possibly other sites in Louisiana. Through numerical wave modeling we evaluate the potential response of mining Ship Shoal on the wave field. During severe and strong storms, waves break seaward of the western flank of Ship Shoal. Therefore, removal of Ship Shoal (approximately 1.1 billion m3) causes a maximum increase of the significant wave height by 90%‐100% and 40%‐50% over the shoal and directly adjacent to the lee of the complex for two strong storm scenarios. During weak storms and fair weather conditions, waves do not break over Ship Shoal. The degree of increase in significant wave height due to shoal removal is considerably smaller, only 10%‐20% on the west part of the shoal. Within the context of increasing nearshore wave energy levels, removal of the shoal is not significant enough to cause increased erosion along the Isles Dernieres. Wave approach direction exerts significant control on the wave climate leeward of Ship Shoal for stronger storms, but not weak storms or fairweather. Instrumentation deployed at the shoal allowed comparison of measured wave heights with numerically derived wave heights using STWAVE. Correlation coefficients are high in virtually all comparisons indicating the capability of the model to simulate wave behavior satisfactorily at the shoal. Directional waves, currents and sediment transport were measured during winter storms associated with frontal passages using three bottom-mounted arrays deployed on the seaward and landward sides of Ship Shoal (November, 1998‐January, 1999). Episodic increases in wave height, mean and oscillatory current speed, shear velocity, and sediment transport rates, associated with recurrent cold front passages, were measured. Dissipation mechanisms included both breaking and bottom friction due to variable depths across the shoal crest and variable wave amplitudes during storms and fair-weather. Arctic surge fronts were associated with southerly storm waves, and southwesterly to westerly currents and sediment transport. Migrating cyclonic fronts generated northerly swell that transformed into southerly sea, and currents and sediment transport that were southeasterly overall. Waves were 36% higher and 9% longer on the seaward side of the shoal, whereas mean currents were 10% stronger landward, where they were directed onshore, in contrast to the offshore site, where seaward currents predominated. Sediment transport initiated by cold fronts was generally directed southeasterly to southwesterly at the offshore site, and southerly to westerly at the nearshore site. The data suggest that both cold fronts and the shoal, exert significant influences on regional hydrodynamics and sediment transport.

A freak wave in the Black Sea: Observations and simulation

Abstract: Recently, the problem of the generation of anomalous high waves of unusual form on the ocean surface has been at the center of intense theoretical and applied work. The phenomenon of anomalously high waves (often called freak waves) is related to the sudden generation of intense waves that can be two or more times as high as the significant wave height. The suddenness and large energy of these waves have resulted in numerous shipwrecks and deaths, and the history of such disasters is described, for example, in [1]. Researchers have suggested the following possible mechanisms for the generation of such waves: linear (or linear with account for nonlinear corrections) focusing of wave groups as a result of dispersion, variable conditions of wave propagation (variable bathymetry, change in direction, and trapping by currents), variable atmospheric forcing, nonlinear modulation instability, and interaction of soliton-type wave groups (see review [2]). In this paper, we give an example of a recorded wave in the Black Sea that is undoubtedly anomalous. We carried out analysis and numerical simulation of this wave, which allowed us to make a judgment about this wave’s spatial dynamics. An attempt was made to determine the mechanism of the formation of such waves. The results are compared with the numerical simulation of the evolution of the so-called New Year Wave that was performed in [3]. The New Year Wave became a popular object of investigation after being registered on an oil platform on January 1, 1995. The anomalous wave described in the present communication was characterized by specific features (in addition to those typical of previously recorded anomalous waves) that provided its more significant intensification relative to the New Year Wave. 1. Experimental data. Waves on the open part of the Black Sea are have been virtually unexplored using instrumental measurements (except for occasional onboard reports). The available coastal observations mostly represent short-term and episodic data. In 1996, a Directional Waverider Buoy (DATAWELL Co., Netherlands) was deployed in the open sea ( 44°30 ′ 40 N, 37°58 ′ 70 E; depth 85 m) near Gelendzhik [5, 6]. The buoy was intended for making direct measurements of wave parameters, including direction of propagation, preprocessing, and transfer of gathered data to a coastal receiving device. The data were transferred from the buoy to the coastal device via a radio channel every 3 h (every 1 h when the significant wave height exceeded 1.5 m). To date, the available volume and composition (dataset) of the software programmable information includes more than 15 000 wave records.

Wave characteristics off Visakhapatnam coast during a cyclone

Abstract: EVALUATION of wave characteristics during severe sea states is essential for the adequate design and constru ction of coastal structures. The significant wave height is the generally used wave height parameter and is comp arable to the statistically determined a verage of the highest onethird wave in a record. Significant wave height ( Hs) is the wave height value approximately observed and recorded by a human when making a visual observation. The maximum value of wave height in a record is called the maximum wave height (Hmax). The data available on wave characteristics off Visakhapatnam coast are visual data 1 observed from ships of opportunity and reported in Indian Daily Weather Reports (IDWR). Since the ships usually avoid the cyclone track, the wave conditions during c yclone are missed. In the present study, the chara cteristics of waves measured during the passage of a cyclone, which occur frequently along this coast during the nort heast monsoon (October to January) are presented. The wave climate of this region varies fro m the southwest monsoon (June to September) to the northeast monsoon. IDWR results show that Hs has a range of 1 –3 m during May–September, 0.5–2 m during October to December (except during the cyclone periods), and gene rally less than 1.5 m in the remaining period. The average wave period varies from 9 to 12 s for the greater part of the year. The waves predominantly approach the coast from the south during March to September and from the east during December to February. The wave direction is tra nsitional, shifting from south to east during October to November. The tides in this region are semi -diurnal. As reported in the Indian Tide Table for Visakhapatnam, the mean spring tidal range is 1.43 m and the neap tidal range is 0.54 m.

Long wave forcing on a barred beach

Abstract: We present new laboratory data on long wave forcing over a barred beach profile under random wave breaking conditions. The data include incident and radiated wave amplitudes, wave set-up, and detailed measurements of the cross-shore variation in long wave amplitude, including shoreline (swash) amplitudes. The total surf zone width was varied via changes in both wave height and the water level over the bar crest. The data obtained from the barred beach are also compared with previous data obtained from a plane beach under essentially identical short wave forcing conditions. The presence of the bar induces a frequency downshift in the spectral peak of the radiated long waves, a consequence of the increased surf zone width on the barred beach and a clear signature of long wave forcing by a time-varying breakpoint. Further comparisons of the two data sets suggest that the bar leads to resonant trapping and amplification (or suppression) of the shoreline motion at discrete long wave frequencies. Well-defined standing long wave motion occurs at discrete frequencies inside the bar and the resonant response is consistent with a simple seiche between the bar crest and shoreline, in agreement with previous numerical model studies. The long wave structure offshore of the breakpoint depends on the relative positions of the bar, shoreline and breakpoint, and is inconsistent with a numerical solution for a free standing long wave over the barred beach profile.