Showing papers on "Wave height published in 2005"

Mid‐ocean microseisms

Abstract: [1] The Hawaii-2 Observatory (H2O) is an excellent site for studying the source regions and propagation of microseisms since it is located far from shorelines and shallow water. During Leg 200 of the Ocean Drilling Program, the officers of the JOIDES Resolution took wind and wave measurements for comparison with double-frequency (DF) microseism data collected at nearby H2O. The DF microseism band can be divided into short-period and long-period bands, SPDF and LPDF, respectively. Comparison of the ship's weather log with the seismic data in the SPDF band from about 0.20 to 0.45 Hz shows a strong correlation of seismic amplitude with wind speed and direction, implying that the energy reaching the ocean floor is generated locally by ocean gravity waves. Nearshore land seismic stations see similar SPDF spectra, also generated locally by wind seas. At H2O, SPDF microseism amplitudes lag sustained changes in wind speed and direction by several hours, with the lag increasing with wave period. This lag may be associated with the time necessary for the development of opposing seas for DF microseism generation. Correlation of swell height above H2O with the LPDF band from 0.085 to 0.20 Hz is often poor, implying that a significant portion of this energy originates at distant locations. Correlation of the H2O seismic data with NOAA buoy data, with hindcast wave height data from the North Pacific, and with seismic data from mainland and island stations, defines likely source areas of the LPDF signals. Most of the LPDF energy at H2O appears to be generated by high-amplitude storm waves impacting long stretches of coastline nearly simultaneously, and the Hawaiian Islands appear to be a significant source of LPDF energy in the North Pacific when waves arrive from particular directions. The highest levels observed at mid-ocean site H2O occur in the SPDF band when two coincident nearby storm systems develop. Deep water, mid-ocean-generated DF microseisms are not observed at continental sites, indicating high attenuation of these signals. At near-coastal seismic stations, both SPDF and LPDF microseism levels are generally dominated by local generation at nearby shorelines.

Morphological development of rip channel systems: Normal and near-normal wave incidence

Abstract: [1] The process of formation of a rip channel/crescentic bar system on a straight, sandy coast is examined. A short review of earlier studies is presented. A morphodynamic stability model is then formulated. The resulting model includes a comprehensive treatment of shoaling and surf zone hydrodynamics, including wave refraction on depth and currents and waves. The sediment transport is modeled using a total load formula. This model is used to study the formation of rip currents and channels on a straight single-barred coast. It is found that this more comprehensive treatment of the dynamics reveals the basic rip cells predicted in earlier studies for normal incidence. Also as before, cell spacings (λ) scale with shore-to-bar crest distance (Xb), while growth rates decrease. The λ increases with offshore wave height (H) up to a saturation value; increasing H also increases instability. Experiments at off-normal wave incidence (θ > 0) introduce obliquity into the evolving bed forms, as expected, and λ increases approximately linearly. the e-folding times also increase with θ. At normal incidence, λ increases weakly with wave period, but at oblique angles, λ decreases. Tests also reveal the presence of forced circulation cells nearer to the shoreline, which carve out bed forms there. The dynamics of these forced cells is illustrated and discussed along with the associated shoreline perturbation. Transverse bars are also discovered. Their dynamics are discussed. Model predictions are also compared with field observations. The relevance of the present approach to predictions of fully developed beach states is also discussed.

A Nearshore Wave Energy Atlas for Portugal

Abstract: The nearshore wave energy resource in Portugal has been assessed through the development of ONDATLAS. This is an electronic atlas, compatible with Internet access, containing comprehensive wave climate and wave energy statistics for 78 points at about 20 m water depth spaced variably ca. 5 km to 30 km, 5 points at deep water and 2 points at open ocean locations. The data were produced by a third-generation wind-wave model, complemented by an inverse-ray model that computes the directional spectra transformation from open ocean to the near-shore. Shoaling, refraction, bottom dissipation and shelter by the coastline and/or neighbouring islands are taken into account. ONDATLAS statistics comprise yearly and monthly values, variability and probability data for significant wave height, energy (mean) period, peak period and wave power, and directional histograms for wave and power direction. Joint probability distributions for various combinations of the above parameters are also available, as well as extreme values and return period for wave height and period parameters. A summary of the detailed verification of this model using long-term buoy measurements at four sites is presented. The main characteristics of ONDATLAS are described. The strong spatial variability that wave conditions exhibit at the coastal area are illustrated and a brief assessment of the nearshore resource at the Portugal mainland is presented.Copyright © 2003 by ASME

Modelling the effect of waves, weathering and beach development on shore platform development

Abstract: A mathematical model was used to study shore platform development Mechanical wave erosion was dependent on such variables as tidal range, wave height and period, breaker height and depth, breaker type, surf zone width and bottom roughness, submarine gradient, rock resistance and the elevational frequency of wave action within the intertidal zone Also included were the effects of sand and pebble accumulation, cliff height and debris mobility, and downwearing associated with tidal wetting and drying The occurrence, location and thickness of beaches often depended on initially quite minor variations in platform morphology, but owing to their abrasive or protective effect on underlying rock surfaces, they were able to produce marked differences in platform morphology Generalizations are difficult, but the model suggests that platform gradient increases with tidal range Platform width also increases with tidal range with slow downwearing but it decreases with fast downwearing Platform gradient decreases and width increases with wave energy, and decreasing rock resistance and platform roughness With low tidal range, platform gradient is generally lower and platform width greater with beaches of fine sand than with gravel, but the relationship is more variable with a high tidal range Platform width increases and platform gradient decreases with the rate of downwearing on bare surfaces, particularly in low tidal range environments, but the pattern is less clear on beach-covered platforms Platforms with large amounts of beach sediment tend to be narrower and steeper than bare platform surfaces Platform gradient increases and platform width decreases with increasing cliff height and with decreasing cliff debris mobility Copyright © 2005 John Wiley & Sons, Ltd

Wind wave statistics in Tallinn Bay

Abstract: The wind wave regime of Tallinn Bay, Gulf of Finland, is analysed with the use of a simplified method of long-term computations of wave fields based on a high-resolution nested WAM model and Kalbadagrund (1991-2000) wind data. The distributions of probabilities for wave heights, annual and seasonal mean wave heights, density of wave energy and its flux (wave power), and 1-year return wave heights as well as the wave field properties in extreme storms are computed. The mainland and surrounding islands together with numerous shallow areas shelter the bay from waves coming from the dominating strong wind directions. The average wave properties exhibit a significant seasonal and spatial variability. The highest waves occur in the vicinity of the Tallinn-Helsinki ship lane where the significant wave height exceeds 2 m each year and may reach 4 m in extreme NNW storms.

Offshore controls on nearshore rip currents

Abstract: [1] The rip current field resulting from the transformation of surface gravity waves over offshore submarine canyons is studied. Employing a wave transformation model and a wave-induced circulation model over observed bathymetry we find that wave height variations associated with undulations in the canyon contours cause rip current circulation cells with alongshore spacing of O(100m) even though the nearshore bathymetry displays no variations at these length scales. Further, the predicted rips correspond to observed rip currents during the Nearshore Canyon Experiment (NCEX). Motivated by these results we study the relationship between O(100 m) scale variations in offshore bathymetric contours and the resulting rip current field in the nearshore. To isolate the roles of possible bathymetric features, we construct a series of idealized case studies that include site characteristics found at NCEX that are conducive of rip current development, such as a curved shoreline, an offshore submarine canyon and undulations in the canyon contours. Our results show that the first two components are unable to produce the observed short-scale circulation systems, while wave refraction over undulations in the canyon walls at length scales of O(100 m) provides a sufficient disturbance to generate alongshore wave height variations that drive multiple rip currents for a variety of incident wave conditions. Rips are not generated when the wave period is short, or when the angle of incidence is large. Analysis of the alongshore momentum balances further demonstrates that the rip current locations are also strongly influenced by inertial effects. Hence, nonlinear processes are important within the rip current circulation cell and we find that nonlinear advective acceleration terms balance a large portion of the driving alongshore gradient in the mean water surface elevation in the vicinity of the rip currents with bottom friction accounting for the remainder. Away from the rips, the balance is between the wave forcing and the pressure gradient outside the surf zone and wave forcing and bottom friction inside the surf zone, as expected.

Wave Dissipation Across Intertidal Surfaces in the Wash Tidal Inlet, Eastern England

Abstract: Despite the recognition that intertidal surfaces play an important role in dissipating wave height and energy, few previous field measurements have been undertaken to directly quantify this effect. This paper presents the results from a one year data collection campaign in the macrotidal Wash tidal inlet, eastern England, during which wave measurements were taken at lower, mid and upper recording stations along three different shore-perpendicular intertidal transects. It has been found that the intertidal environment of The Wash is effective in dissipating wave height by, on average, 83% and in dissipating wave energy by, on average, 91% with respect to the incident wave conditions. Based upon the results from this study, it is recommended that flood defence management practice in The Wash moves away from focusing on the defence of a structural line towards the management of a defence zone, comprising both natural and structural elements.

Analytical evolution of tsunamis induced by near-shore earthquakes on a constant-slope ocean

Abstract: Strong near-shore earthquakes are the most frequent sources of tsunamis in many oceans of the world. In the framework of the nonlinear shallow-water theory, the initial sea-surface tsunami elevation is assumed to equal the sea-floor co-seismic displacement produced by the seismic event. This is quantified by means of the analytical formulas due to Okada (1985, 1992), dealing with seismic faults buried in an elastic medium. In this work the propagation of tsunamis is studied along two-dimensional profiles on an idealized constant-slope sea bed, an approximation that allows one to reduce the governing nonlinear equations to a linear problem by means of the classical Carrier & Greenspan (1958) approach. We introduce an analytical solution that is sufficiently general to account for initial conditions associated with paradigmatic cases of sea-bottom deformations produced by near-shore earthquakes, such as subsidence or uplift of the coastal area, and can be also used to treat more complex deformations. The main result is that the amplification of the tsunami height at the coast is found to range between approximately 1 and 2. The amplification is around 1 for tsunamis induced by earthquakes with their epicentre inland and tends to grow as the fault moves seaward. We restrict our analysis to earthquakes that dislocate the shore region. Within the class of sources that we consider, the tsunamis that are most amplified are the ones having initial profiles with a crest-trough-crest system or conversely with a trough-crest-trough system. The bottom slope is found to have no effect on tsunami run-ups and run-downs, but to influence tsunami periods and tsunami speed remarkably. Breaking analysis shows that wave breaking does not occur if the initial wave height is less than 8-9 m, and that the simplest sea-level profiles, which are associated with earthquakes with their epicentre on land, are not expected to break even if their initial height exceeds 19 m.

Wave height from planing and semi‐planing small boats

Abstract: The increasing number of small boats has raised concerns about their effects on the environment, particularly their waves. Bank erosion is one of the foremost concerns of boat waves but disruption of habitat, resuspension of bottom sediments, and damage to aquatic plants are other areas of concern. A large programme of field measurement of boat waves was conducted on Johnson Lake in Alaska to evaluate boats typically used on the Kenai River. The boat wave study compared wave characteristics of four boats under a variety of loadings, speeds, distances and motor powers. Over 400 tests were run on Johnson Lake with each test providing wave measurement at four locations. Two measures of waves and two types of tests were used in the study. MAXPOW was the wave height at the maximum power of the motor. MAXWAV was the maximum wave height produced by the boat which required runs at a range of speeds to determine the MAXWAV. While the MAXWAV data herein have considerable scatter in magnitude, the conditions at which MAXWAV occurs are consistent from boat to boat. To prevent generation of maximum wave heights, small boats should operate as far as possible either above or below length Froude number of 0.6, displacement Froude number of 1.3, or beam Froude number of 1.0. A general boat wave height equation was developed for the four boats based on boat speed, volume displaced by the boat and distance from the boat, and are applicable to semi-planing and planing boats based on MAXPOW and MAXWAV data. The predictive equation for V-hull boats was compared to independent data not used in the development and was found to be in agreement with the data. The predictive equation is limited to depth/boat length greater than 0.35. Published in 2005 John Wiley & Sons, Ltd.

Satellite Altimeters Measure Tsunami

Abstract: satellites obtained profi les of sea surface height on transects across the Indian Ocean between two and nine hours after the December 26 Sumatra earthquake. The data are received hours to days af-ter “real time,” too late to be used in detection and warning of tsunamis. We compared the sea level anomaly profi les of December 26 measured along the satellite tracks (Figure 1D-G) with the measurements on previous passes of the same satellites 10 days, 35 days, and 17 days earlier. This allowed us to remove the majority of permanent and slowly varying features of sea level, revealing transient signals. The altimeters also provide wind speed and wave height data, and these allowed us to interpret a sea-level anomaly at 16°S in the Jason-1 profi le (Figure 1D) as being due to a severe storm. The remaining sea-level anomaly signal appears to be associ-ated with the tsunami. The signal of the leading edge two hours after the earth-quake is particularly prominent, with an amplitude of 60 cm and two narrow peaks where the NOAA tsunami model forecast shows two overlapping peaks coalescing into one broad (250 km) crest. Increased sea-surface roughness at spatial scales from 150 to 15 km wave-lengths also appears inside the portion of the ocean excited by the tsunami.The fi rst model simulation results of the Indian Ocean tsunami (Figure 1A-C) were obtained from the “MOST” (Meth-od of Splitting Tsunamis) model (Titov and Synolakis, 1998) and were posted by V.V. Titov on the Internet Tsunami Bul-letin Board less than 12 hours after the earthquake. MOST is part of the tsunami forecasting and warning system under development for the Pacifi c Ocean (Titov et al., 2005) that will provide fast real-time estimates of tsunami amplitudes using preset models, real-time seismic data, and, most importantly, deep-ocean tsunami amplitude data from a network of deep-ocean pressure sensors. Other researchers also ran models and posted results. Results of MOST and other mod-el runs have been widely used worldwide by the media for early planning of relief efforts and for post-tsunami fi eld sur-veys. Unlike the Pacifi c, the Indian Ocean does not yet have a network of deep-ocean pressure sensors, and so coastal tide gauges provide the only direct mea-surement of Indian Ocean tsunami am-plitudes. The satellite altimeter data we present here are the only measurements of the amplitude of the December 26 tsunami in the deep, open ocean. At the time of the fi rst MOST model simulation, earthquake source mecha-nism models described a rupture con-fi ned to only the southernmost portion of the broad region mapped out by the aftershock pattern. However, it seemed clear that the tsunami should have been generated by displacements distributed along the entire aftershock zone. The initial conditions for the MOST model were set assuming this more spatially distributed source, with initial amplitude guesses based on preliminary estimates of the earthquake magnitude and one coastal tide-gauge measurement from Cocos Island. Because of the lack of

Influence of wave modelling on the prediction of fatigue for offshore wind turbines

Abstract: Currently it is standard practice to use Airy linear wave theory combined with Morison's formula for the calculation of fatigue loads for offshore wind turbines. However, offshore wind turbines are typically placed in relatively shallow water depths of 5–25 m where linear wave theory has limited accuracy and where ideally waves generated with the Navier–Stokes approach should be used. This article examines the differences in fatigue for some representative offshore wind turbines that are found if first-order, second-order and fully non-linear waves are used. The offshore wind turbines near Blyth are located in an area where non-linear wave effects are common. Measurements of these waves from the OWTES project are used to compare the different wave models with the real world in spectral form. Some attention is paid to whether the shape of a higher-order wave height spectrum (modified JONSWAP) corresponds to reality for other places in the North Sea, and which values for the drag and inertia coefficients should be used. Copyright © 2004 John Wiley & Sons, Ltd.

On the computation of the Benjamin-Feir Index

Abstract: Recently it has been shown theoretically, numerically and experimentally that the statistical properties (probability density function of wave amplitude and wave height)of long crested surface gravity waves depend not only on steepness but also on the Benjamin-Feir Index (BFI), which is the ratio between wave steepness and spectral bandwidth. The computation of this index requires the estimation of a number of parameters such as the spectral bandwidth and the peak frequency. For a given time series or a wave spectrum those parameters can be calculated using different methods, thus leading to different numerical values of the BFI. We analyze different approaches for computing the BFI and, based on numerical experiments with simulated spectra, we outline a unique robust methodology for its computation.

Evaluation of Two Numerical Wave Models with Inlet Physical Model

Abstract: This paper evaluates the performance of two numerical wave models, GHOST and STWAVE, with measurements made in an idealized inlet physical model. The emphasis of this paper is on the overall performance of these models in coastal inlets. Both wave models are similar in that they employ a finite-difference method to solve the wave action conservation equation for the steady-state wave spectral transformation. However, these models differ in the computation of diffraction, reflection, wave breaking, and representation of the directional spectrum transformation. The models’ performance is compared with a new set of physical model data for four different idealized inlet configurations. Wave height is measured in the physical model by a linear array of capacitance wave gauges, and wave direction is measured by a remote-sensing video-camera system. The comparison with data is presented as mean absolute relative errors of wave height and mean absolute difference of wave direction. Both wave models produced simil...

Drag coefficient, dynamic roughness and reference wind speed

Abstract: Surface waves are the roughness element of the ocean surface. The parameterization of the drag coefficient of the ocean surface is simplified by referencing to wind speed at an elevation proportional to the characteristic wavelength. The dynamic roughness is analytically related to the drag coefficient. Under the assumption of fetch limited wave growth condition, various empirical functions of the dynamic roughness can be converted to equivalent expressions for comparison. For datasets covering a wide range of the dimensionless frequency (inverse wave age), it is important to account for the variable rate of wave development at different wave ages. As a result, the dependence of the Charnock parameter on wave age is nonmonotonic. Finally, the analysis presented here suggests that the significant wave steepness is a sensitive property of the ocean surface and a single variable normalization of the dynamic roughness using a wavelength or wave height parameter actually produces more robust functions than bi-variable normalizations using wave height and wave slope.

HF radar wave measurements in the presence of ship echoes - problems and solutions

Abstract: HF radar is an important tool to remotely measure oceanographic parameters like currents and waves from shore over large areas up to 6500 km/sup 2/. Maps of these parameters can be obtained 3 times per hour. In areas of high ship traffic, there are ship echoes within the radar backscatter signal, which can appear in the frequency bands of the Doppler spectrum which are used to extract current- and wave information. While the current speed is derived from the first-order Bragg peaks, wave height and direction is calculated from the second-order bands around the first-order peaks. The second-order bands require 15 to 20 times the bandwidth of the first-order peaks and their amplitude is about 20 dB lower. Due to these facts, ship echoes can cause problems especially to wave algorithms. This paper discusses the special signatures of ship echoes and shows possibilities to identify and to track ships.

Non-parametric sea-state bias models and their relevance to sea level change studies

Abstract: The direct estimation of sea-state bias (SSB) from sea height residuals is extended with a parametric tting process and a successive smoothing of the remaining residuals. This hybrid method essentially produces a nonparametric SSB model in the form of a smooth grid in a 2dimensional space determined by signicant wave height and backscatter coefcient. The hybrid method allows a much higher resolution than parametric models, without the disadvantage of the direct method’s limited wind speed / wave height domain. The use of sea height residuals as input data allows estimation of a realistic SSB model with only a few months of data. With state-of-the-art geophysical corrections the hybrid SSB model can be constructed with a high level of accuracy. The impact of errors in the mean sea surface model and tide models appears marginal. Errors in the ionospheric correction, which has a geographical distribution similar to wind speed and wave height, tend to leak into the SSB model. The wave height and backscatter both show trends as a function of time. Wave heights appear to drop over time, but backscatter shows trends either way. This results in trends in SSB and hence sea level that may not be ireali. The different trends in SSB between TOPEX Side A and Side B, as well as between the CSR SSB model and our hybrid model, poses challenges on the estimation of sea level change to better than 0.2 mm/yr.

Energy transfer and dissipation during surf beat conditions

Abstract: New experimental data on energy transfer and energy dissipation during surf beat conditions are presented. Long wave-short wave interaction inside the surf zone leads to broadening of a monochromatic sea state and a net reduction in short wave height. The amplitude decay of bound sub- and superharmonics in the nearshore region is also investigated, and found to commence with the onset of short wave breaking. The sub- and superharmonic decay can be modelled using the local 2nd order solution for the bound harmonics. It is suggested that this mechanism occurs for steep wave conditions on mild slopes and where the short waves are not in shallow water at the breakpoint. A wave steepness criterion is given to describe the latter.

Statistical analysis of wave parameters in the north coast of the Persian Gulf

Abstract: . In this study we have analysed wind and wave time series data resulting from hourly measurements on the sea surface in Bushehr, the northern part of the Persian Gulf, from 15 July to 4 August 2000. Wind speed (U10) ranged from 0.34 to 10.38 m/s as alternating sea and land breezes. The lowest wind speed occurs at about midnight and the highest at around noon. The calculated autocorrelation of wind speed data shows that when the sea-land breeze is strong, the land-sea breeze is weak and vice versa. The significant wave height (Hs) varies between 0.10 to 1.02 m. The data of the present study reflects mostly the local waves or the sea waves. The calculated correlation between wind and wave parameters is rather weak, due to the continuous change in the wind direction. Wave height distribution follows the well-known Rayleigh distribution law. The cross correlation analyses between U10 and Hs reveal a time lag of 4h. Finally, we have shown that the time series of U10, Hs, and wave period are stationary. We have modeled these parameters by an auto regressive moving average (ARMA) and auto regressive integrated moving average (ARIMA) models. Keywords. Oceanography: physical (Air-sea interactions; Surface waves and tides; Upper ocean processes)

Wave Effects on Nutrient Release of Sediments from Lake Taihu by Flume Experiments

Abstract: The effect of wave disturbance on the nutrient release from lake sediments was simulated in flume experiments. The sediments were sampled from Lake Taihu, China, which is a large, shallow, eutrophic lake with a mean depth of 1. 9 m and an area of 2447 km2. The water flume was 30 m long, 0. 5 m wide and 0. 7 m deep. The sediments was 10 cm thick in the bottom of water flume in flume experiments. In a "little-wave" experiment, which means the wave height just a bit higher than the critical wave height for sediment resuspension, the depth of overlying water was 40 cm, the critical wave height for sediment resuspension was 8. 45 cm, and the wave height for sediment resuspension experiments was 8.77 cm. In a "strong-wave" experiment, the depth of o-verlying water was 30 cm, the critical wave height for sediment resuspension was 5. 93 cm, and the wave height for sediment resuspension experiments was 12. 31 cm and 13. 29 cm. In the "little-wave" experiments, contents of suspended solids (SS) was increased to 13. 6 mg/L, while in strong-wave experiments, the SS content was increased to 245. 2 mg/L. The equilibrium of the concentration of nutrients and SS in overlying water was lag more than 1 h than the change of wave height. Strong wave disturbance significantly increased the concentrations of ammonia nitrogen and soluble reactive phosphorus in overlying water. In both the little-wave experiment and strong-wave experiment, concentrations of dissolved oxygen in overlying water was increased and concentrations of dissolved organic carbon in overlying water decreased. The study indicated that strong wave might caused significant increase of internal loading of nutrients. However, flocculation and adsorption of resuspended sediments and the oxygenate action accompany by the wave disturbance may district the increasing of reactive nutrients in overlying water.

Abreast of the waves : open-sea sensor to measure height and direction

Abstract: Accurate and timely information on open-sea wave conditions can help in preventing large-scale maritime disasters. This article describes a new, low-cost Global Positioning System (GPS)-based sensor that measures wave height with an accuracy of several centimeters and direction with an accuracy of 5 degrees. The receiver is mounted on a buoy, and a high-pass filter is used to extract the movement of the buoy and thus minimize GPS positioning errors. The data provided by the sensor is intended to improve wave prediction models. In addition, since this GPS-based sensor transmits only analyzed ocean wave data, it reduces the volume of data and leads to lower operating and acquisition costs. The article describes the concept of the GPS-based wave sensor, algorithms that are used for filtering and extracting wave data, as well as the results of open-sea trials.

Experimental Study of the Probability Distributions of Green Water on the Bow of Floating Production Platforms

Abstract: Results of an experimental program with a model of a moored floating production storage and offloading vessel are used to study the probability distributions associated with various phenomena related with green water loading. Separate analysis of wave height and crests are performed in order to assess the presence and significance of nonlinearities. Time series of pitch motion and relative motion are analyzed to check for linearity of the response process. Probability distributions of the occurrence of water on deck and of the conditional distribution water height above deck are also studied.