Showing papers on "Wave height published in 1990"

Distribution of Large Wave Heights

Abstract: The statistical distribution of zero-crossing wave heights is considered within the context of a previous theory proposed by the writer some years ago. The underlying model, definitions, and assumptions are reexamined systematically to develop asymptotic approximations to the probability density, exceedance probability, and statistical moments of wave heights larger than the mean wave height. The asymptotic results have closed forms, and thus are easier to use in practical applications than the original theory, which requires numerical integration. Comparisons to empirical data are given to show that the present asymptotic theory produces the observed statistics of large wave heights faithfully to within 1%. Further, comparisons with other relevant theories also reveal that if one remains true to the theoretical definitions, then the present theory is the most accurate in predicting the exceedance distribution of large wave heights. Finally, the asymptotic theory is coupled with the statistics of wave periods to derive a theoretical expression for the joint distribution of large wave heights and associated periods. The predictive utility of this last result remains to be explored.

Effects of sea maturity on satellite altimeter measurements

Abstract: For equilibrium and near-equilibrium sea states, the wave slope variance is a function of wind speed U and of the sea maturity. The influence of both factors on the altimeter measurements of wind speed, wave height, and radar cross section is studied experimentally on the basis of 1 year's worth of Geosat altimeter observations colocated with in situ wind and wave measurements by 20 NOAA buoys. Errors and biases in altimeter wind speed and wave height measurements are investigted. A geophysically significant error trend correlated with the sea maturity is found in wind-speed measurements. This trend is explained by examining the effect of the generalized wind fetch on the curves of the observed dependence. It is concluded that unambiguous measurements of wind speed by altimeter, in a wide range of sea states, are impossible without accounting for the actual degree of wave development.

Wave loads on offshore structures

Abstract: The author discusses how knowledge of wave-induced loads is essential in both the design and operation of offshore structures. In hostile areas like the North Sea, the significant wave height (mean of the highest one third of the waves present in a sea) can be larger than 2 m 60% of the time. The most probable largest wave height in 100 years can be more than 30 m. The mean wave period can be from 15 to 20 s in extreme weather situations, and it is seldom below 4 s. Environmental load due to current and wind are also important, and in some cases the interactive effect between waves and current is significant. Current velocities of 1-2 m s{sup {minus} 1} and extreme wind velocities of 40-45 m s{sup {minus}1} must be used in designing offshore structures in the North Sea.

CODAR wave measurements from a North Sea semisubmersible

Abstract: CODAR, a high-frequency (HF) compact radar system, was operated continuously over several weeks aboard the semisubmersible oil platform Treasure Saga for the purpose of wave-height directional measurement and comparison. During North Sea winter storm conditions, the system operated at two different frequencies, depending on the sea state. Wave data are extracted from the second-order backscatter Doppler spectrum produced by nonlinearities in the hydrodynamic wave/wave and electromagnetic wave/scatter interactions. Because the floating oil rig itself moves in response to long waves, a technique has been developed and successfully demonstrated to eliminate to second order the resulting phase-modulation contamination of the echo, using separate accelerometer measurement of the platform's lateral motions. CODAR wave height, mean direction, and period are compared with data from a Norwegian directional wave buoy; in storm seas with wave heights that exceeded 9 m, the two height measurements agreed to within 20 cm RMS, and the mean direction to better than 15 degrees RMS. >

A study of differences between environmental reports by ships in the voluntary observing program and measurements from NOAA buoys

Abstract: Four years of weather observations from ships in the World Meteorological Organization Voluntary Observing Program were compared with common parameters measured simultaneously by NOAA moored buoys when ships were within 100 km of buoy stations. Common parameters were wind speed, wind direction, atmospheric pressure, surface air and sea temperatures, wave height, and wave period. From a data base containing as many as 63,354 pairs of ship-buoy reports (in the case of wind speed), the analysis provided an overall set of difference statistics as well as separate sets of difference statistics for each of the following data categories to reveal possible correlations: year, season, geographical region, buoy location, and separation between ship and buoy to a distance of 100 km. The results showed little correlation with any of these subcategories. Ship-reported wind speeds were on average 2–4 kn higher than buoy-reported winds, with standard deviations of 7–8 kn (1 kn = 0.51 m/s). Mean differences in wind direction of about 7° and standard deviations of direction differences of 45° were typical. Mean differences and standard deviations of differences for the remaining parameters were atmospheric pressure (−0.5 mbar, 4.2 mbar), air temperature (1.1°C, 4.3°C), sea temperature (0.1°C, 3.5°C), wave height (0.1 m, 2.2 m), and wave period (−0.2 s, 2.4 s). Factors responsible for these differences are noted, particularly for surface winds. This analysis represents the first known quantitative comparison of simultaneous observations made by ships and buoys for such a large data base.

Seasonal changes in the global wave climate measured by the Geosat altimeter

Abstract: Wave height measurements made during the year November 1986 to November 1987 by the radar altimeter on the US Geosat satellite are used to map the seasonal variations in the global wave climate. The largest significant wave heights are found in winter in the Southern Ocean with only a slight reduction during the southern summer. In the North Atlantic and North Pacific the significant wave heights are lower and there is a larger variation between summer and winter.

Observations of wave-induced set-up on a natural beach

Abstract: A new technique has been used to make field measurements of wave-induced set-up. The technique is reliable and accurate, and the results presented show the variation of mean set-up at the shoreline with incident wave height. The amount of set-up is well predicted by a linear function of incident wave height and is found to be in good agreement with theoretical predictions based on the observed decay of wave height across the surf zone. The maximum set-up observed was about 40 cm.

Some experiments to investigate the assimilation of SEASAT altimeter wave height data into a global wave model

Abstract: A scheme for assimilating SEASAT altimeter wave heights into a global wave model, with the additional use of the collocated altimeter wind speeds, is reported. Control experiments are described in which the wave model was run with two sets of winds, one with SEASAT scatterometer wind data included in the NWP analysis and the other without, both for a period from 0000 GMT 15 September 1978 to 0000 GMT 20 September 1978. the results of forcing the wave model with these wind fields are compared with the altimeter wave heights. the differences are discussed in order to quantify errors in both the wave model and the altimetric data. Three experiments in which altimeter wave heights were assimilated are then described. the results of these are compared with the wave fields from the model runs without wave data assimilation. the assimilation experiments show that a significant improvement in model wave height is possible and that the impact of one day of assimilation is retained for at least five days.

Spatial Variation in Sinusoidal Wave Energy on a Crescentic Nearshore Bar; Application to the Cap-Ferret Coast, France

Abstract: Simple sinusoidal wave refraction on a crescentic bar off the Aquitaine coast has been studied. In the case of parallel waves to the coast, this bar, composed of a cyclical repetition of troughs separated by ridges, induces a synchronous variation in wave height and energy flux along the shore. Energy flux is lower in the troughs than on the ridges. The longshore component of energy flux changes more rapidly than the total flux. The theoretical distribution of this energy is analyzed in comparison with the water depth changing in accordance with the tide. The results show a variation in the longshore component and thus a displacement of circulation cells and rip-currents during the course of the tide. In the case of oblique waves to the coast, the cyclic variation of energy flux decreases and the longshore component increases. The rip-currents are unwedged by comparison with the previous case. These results are compared with aerial photographs. A second crescentic bar and tidal channels complicate the refraction process.

Extreme Waves in laboratory Generated Irregular Wave Trains

Abstract: Large waves in experimental model scale random wave trains are investigated. Statistical distributions of crest heights, troughs and peak-to-peak wave heights are compared to the Rayleigh model and to linear numerical wave data. Wave heights are seen to agree well with the Rayleigh distribution. The largest individual waves from the experiment show asymmetric behaviour with higher crests and more shallow troughs than predicted by the linear theory. This is seen both from Weibull distribution plots and from wave profiles of individual large waves in the time domain. Particle velocities in extreme waves, estimated by linear transformations of wave elevation measurements, are particularly asymmetric and peaked. Proper design rules for extreme waves should be further developed. More knowledge on full scale waves must form a central part of this development.

Viscous Damping of Cnoidal Waves Over Fluid‐Mud Seabed

Abstract: The heights of water waves propagating over fluid-mud bottom can be significantly reduced due to the viscous energy dissipation occurring in the mud bottom. In some cases, exceptionally high rates of attenuation are possible whereby waves are almost completely damped within several wavelengths. This phenomenon has been observed both in field investigation and in laboratory experiment action. In this paper, the viscous damping of cnoidal waves progressing over fluid-mud seabeds is investigated, in which fluid mud is assumed to be a viscous fluid. The theoretical model adopted here is a two-layer viscous fluid model modified with three boundary layers at the water-mud interface and at the rigid bottom beneath mud layer. Viscosities of both water and fluid mud are taken into consideration. For a nonlinear shallow wave progressing over a viscous mud bed, the first-order analytical solutions are derived for the velocity distributions in boundary layers and for the attenuation rate of wave heights with distance. The attenuation coefficients are larger than those predicted on the basis of linear shallow wave theory and unlike the case of linear shallow waves, they are not independent of wave height. The viscous damping of solitary and sinusoidal waves, which are two extreme cases of cnoidal wave, are also discussed according to the present solution.

Microwave Techniques for Measuring Directional Wave Spectra

Abstract: A variety of instruments have been developed during the past thirty years for measuring ocean wave spectra. The simplest of these devices is designed to measure, either directly or indirectly, the surface elevation at a given location in space as a function of time. The observed temporal fluctuations of the surface elevation are described statistically in terms of the frequency spectrum. Using the dispersion relation for surface gravity waves, each frequency component may also be associated with a unique wavelength, assuming that the apparent frequency of the waves is not influenced by currents or by the motion of the sensor through the water. However, no information on the direction of propagation of the waves is obtainable from a point measurement of this type.

Offshore Structural System Reliability: Wave-Load Modeling, System Behavior, and Analysis

Abstract: Author(s): De, Rabi S. | Abstract: The application of system reliability methodology for offshore structural problems is investigated. The emphasis is on wave-load modeling and its implications with respect to system reliability analysis of fixed offshore structures. In addition, probabilistic and deterministic measures of "system effects" are proposed.The wave loading in system reliability analysis is usually modeled as a fixed spatial pattern of nodal forces scaled by a random intensity factor. In this work, the change of spatial pattern of the mean nodal wave forces with increasing wave height is accounted for by using the so-called "fragility approach" to systems analysis. The change of the relative importance of different member-failure sequences with wave loads corresponding to different wave heights is studied for a fixed offshore jacket in 140 feet of water. Results from simplified "fixed-pattern" analyses are calibrated against the "fragility" analysis.The nodal wave forces are implicitly assumed to be fully correlated in both the "fixed pattern" and the "fragility" analysis. In response, a framework is developed that allows modeling of a general correlation structure of nodal wave forces. Using this framework the effects of less than perfect spatial correlation among nodal wave forces on the system reliability of a fixed offshore structure are investigated.The reliability of near-ideal parallel structural systems is studied in order to understand and quantify the probabilistic and deterministic "system factors" influencing the overload capacity and redundancy of realistic statically indeterminate structures. Efficient use of reduced space Monte Carlo simulation techniques in system reliability analysis is demonstrated. Application of these findings in accelerated system reliability assessment of a fixed offshore jacket structure under wave-loading is demonstratedIn view of the importance of the load variability in system reliability assessment, a new model for short-term extreme (storm) sea-states for static structural reliability problems is presented. The model is based on a "multivariate" random variable characterization of the observed irregular process suitable for use in efficient reliability computation, e.g., FORM/SORM, and in general purpose methods such as Monte Carlo simulation. Although the model is restricted to at least semi-narrow banded time histories, it does not make any a priori assumption regarding the Gaussianity of the time series. Hence it is attractive for characterizing the skewed wave elevation processes observed in shallow water and/or in extreme (storm) sea-states. The application of the proposed model is demonstrated by analyzing a 34.13 min. long wave-elevation record collected during hurricane Camille. Calculation of response statistics such as the probability distribution of extreme base shear of a pile, the spatial correlation of sets of drag forces at different locations, etc., is demonstrated.

Long term changes in wind and wave climate on the north sea

Abstract: This paper presents the first results of a study on the wind and wave climate along the Dutch coast of the North Sea. The analysis of time series of wind force, wave height, wind direction, etc, identifies the occurrence of long term changes in wind and (wind)wave climate. In the Dutch Coastal Defence Study, the results of these climate analyses were used to develop the so called "unfavourable hydro-meteorological scenario" with which an evaluation was made of the extreme impact of the shore line retreat of the Dutch coast. The wind and wave climate study is part of a project that examines the relation between changes in the regional wind and wave climate and the long term and large scale morphological development of the Dutch coast.

A Numerical Model for Calculating Wave Height Distribution in a Harbor of Arbitrary Shape

Abstract: A numerical model based on the mild-slope-equation is proposed for calculating the height distribution of irregular waves in a harbor of an arbitrary shape and variable depth. The Green function technique is utilized for the boundary condition at a harbor entrance on approximating the waves scattered out of the harbor. To diminish the error involved in the approximation, the diffraction of incident waves at the harbor entrance is taken into account. This allows to confine the calculation region within the harbor and hence to reduce the number of grid points. Partial reflection is realized at a fixed boundary by considering an energy-dissipative region in front of a structure. By this formulation an iteration procedure becomes unnecessary because the incident wave angle does not appear in the formulas.The result of calculation shows a good agreement with experimental results and the diffraction diagram for irregular waves around a semi-infinite breakwater. Sample applications suggest that the harbo...

Microwave radar measurements of ocean wave propagation —Initial results

Abstract: A microwave radar has been developed to measure the surface orbital velocities of ocean waves, and hence estimate the wave height spectra. The Doppler radar exploits the frequency modulated continuous wave (FMCW) technique to provide good spatial resolution (much less than the ocean wavelengths of interest) making it suitable for ocean wave propagation studies. The authors present initial data demonstrating the radar's capabilities to deduce near-shore ocean wave properties. The spatial and temporal variations of the orbital velocities reveal the wave propagation, and the deduced wave height spectra show wave growth as they approach the shore.

A comparison between measured wave properties and simple wave hindcasting models in shallow water

Abstract: Significant wave heights and wave periods obtained from field measurements in Lake Balaton, Hungary, were compared with two versions of the shallow water wave hindcasting model published by the U.S. Army Corps of Engineers. The version presented in CERC [3, 4] was found to give very good hindcasts of wave height but fall -20% low on wave period. The model presented in CERC [5] was 15-20% above the earlier version at long fetches and approximately equivalent at shorter fetches.

Beach morphodynamic systems of the central Netherlands coast, Den Helder to Hoek van Holland

Abstract: The 120 km long central Netherlands coast consists of an essentially continuous sandy Holocene regressive/transgressive barrier system facing the southern North Sea. The beach and surf zone is composed of predominantly quartz sands which are coarsest at the shoreline (D50 = 286 Um) and fine seaward. Overall shoreface gradients vary between a low of 0.01 in the central region steepening to 0.015 towards dan Helder in the north and Hoek van Holland in the south. Tides are micro-tidal ranging from 1.4 to 1.7 m. The wave climate is a fetch limited strom wave environment generated by onshore winds in the North Sea together with occasional swell. Waves average 1.4 m in height with a period of 5.4 sec. They peak during the wmter storms with a January mean Ho = 1.86 m and storm waves to 3 - 4 m. During summer they decrease to a mean of - 1.0m. The interaction of the wave climate with the sandy shoreface has produced a 2 to 3 bar surf zone. Based on aerial photographs, the inner (bar 1) is modally a ridge and runnel/low tide terrace, bar 2 varies between transverse bar and rip and rhythmic bar and beach, while the outer bar 3 where present is rhythmic bar and beach to longshore bar and trough. All bars are characterised by rhythmic topography and rips which increase in spring from a mean of 500 m (bar 1) to 600 m (bar 2) and 900 m (bar 3). Groyne fields occupy 43 km of the coast and with a mean spring of 200 m induce an increase in rip occurrence and decrease in rip spacing. The morphodynamics of the beach-bar system can be explained in temporal and spatial terms by examining the impact of the wave climate on the shoreface. Temporal variation is controlled both by seasonal variation in wave height and storm frequency and by inter-storm beach recovery. It is proposed that the spatial variation in bar number (2 or 3), bar spacing and rip spacing is related to infragravity standing and edge waves generated by wave groupiness, acting across the two slope regimes (0.01 and 0.015) which produce standing wave lengths which correlate with actual bar spacing and edge wave lengths which correlate reasonably with rip spacings. Both require however field verification. The hierarchy of bar types is empirically explained by decreasing breaker wave heights across the 300-600 m wide surf zone. Finally a beach model is proposed for the coast consisting of six stages, a fully dissipative end member expected to occur during severe storm surges, two intermediate modal states consisting of the bar types mentioned above, and lower energy intermediate and a reflective member which are unlikely to occur in this wave climate.

Resampling Approach to Extreme Wave‐Height Analysis

Abstract: A bootstrap resampling approach is developed for extremal analysis of measured and hindcast ocean‐wave data for use in design, construction, operations, and maintenance of coastal structures. The resampling method produces estimates for the extrapolation bias associated with competing extreme‐probability models. A goodness‐of‐fit statistic for the extremal type 1 model is also developed. The bias estimates and goodness‐of‐fit statistic are used in selecting an appropriate probability model for a given record of extreme significant wave heights. The method is shown to provide information about model selection that is not available from present extremal analysis methodology, and may help prevent significant un‐derprediction of extremes that can occur with conventional approaches. Examples using both hindcast and measured wave data are presented to demonstrate application of the methods developed in this paper.

Stochastic Description of Offshore Environment

Abstract: This paper presents a state-of-the-art review of the stochastic description of the ocean environment. Non-linearity consideration in random seas, wave breaking, wave groups, and stochastic properties of offshore currents are discussed presenting phenomena observed in the real world.