Showing papers on "Wave height published in 2003"

Boussinesq modeling of longshore currents

Abstract: [1] A time domain Boussinesq model for nearshore hydrodynamics is improved to obtain the conservation of vertical vorticity correct to second order and extended for use on an open coast using longshore periodic boundary conditions. The model is utilized to simulate surface waves and longshore currents under laboratory and field conditions. Satisfactory agreement is found between numerical results and measurements, including root mean square wave height, mean water level, and longshore current. One striking result of the simulations is the prediction of the strong longshore current in the trough shoreward of the bar as observed during the Duck Experiment on Low-frequency and Incident-band Longshore and Across-shore Hydrodynamics field campaign. The model results give insight into the spatial and temporal variability of wave-driven longshore currents and the associated vertical vorticity field under the phase-resolving, random wave forcing with wave/current interaction. Numerical experiments are carried out to examine the response of the modeled longshore currents to the randomness of surface waves and the cross-shore distributions of bed shear stress coefficient. We find that both regular and irregular waves lead to very similar mean longshore currents, while the input of monochromatic, unidirectional waves results in much more energetic shear waves than does the input of random waves. The model results favor Whitford and Thornton's [1996] finding that the bed shear stress coefficient for the area offshore the bar is larger than that in the trough, as better agreement with the field data for both regular and irregular waves is found if such coefficients are used in the Boussinesq model.

Assessment of the reliability of wave observations from voluntary observing ships: Insights from the validation of a global wind wave climatology based on voluntary observing ship data

Abstract: This paper describes development and validation of a global climatology of basic wave parameters based on the voluntary observing ship (VOS) data from the Comprehensive Ocean-Atmosphere Data Set collection. Climatology covers the period 1958–1997 and presents heights and periods for the wind sea, swell, and significant wave height (SWH) over the global ocean on 2° × 2° spatial resolution. Significant wave height has been derived from separate sea and swell estimates by taking square root of the sum of squares for the seas and swells propagating approximately in the same direction and assuming SWH to be equal to the higher of the two components in all other cases. Special algorithms of corrections were applied to minimize some biases, inherent in visual wave data. Particularly, we corrected overestimation of small seas, corrected underestimation of periods, and analyzed separation between sea and swell. Validation included estimation of random observational errors, observation of sampling errors, and comparison with the alternative wave data. Estimates of random observational errors show that for the majority of locations, observational uncertainties are within 20% of mean values, which allows us to discuss quantitatively the produced climatology. Biases associated with inadequate sampling were quantified using the data from high-resolution WAM hindcast for the period 1979–1993. The highest sampling biases are observed in the South Ocean, where wave height may be underestimated by 1–1.5 m because of poor sampling, primarily associated with a fair weather bias of ship routing and observation. Comparison to the other VOS-based products shows in general higher SWH in our climatology, especially in the midlatitudes. However, comparison with the altimeter data shows that even for well-sampled regions, high waves are still underestimated in VOS, suggesting a ubiquitous fair weather bias. Further ways of improving VOS-based wave climatologies and possible applications are discussed.

A GIS-based step-wise procedure for assessing physical exposure in fragmented archipelagos

Abstract: Wind and waves are major forces affecting the geomorphology and biota in coastal areas. We present a generally applicable method for measuring and calculating fetch length, fetch direction and wave exposure. Fetch length and direction, measured by geographic information system-based methods, are used along with wind direction and wind speed data to estimate wave height and period by applying forecasting curves. The apparent power of waves approaching the shore, used as a proxy for wave exposure, is then calculated by a linear wave model. We demonstrate our method by calculating fetch lengths and wave exposure indices for five areas with varying exposure levels and types of meteorological conditions in the Finnish Archipelago Sea, situated in the northern Baltic Sea. This method is a rapid and accurate means of estimating exposure, and is especially applicable in areas with geomorphologically varying and complicated shorelines. We expect that our method will be useful in several fields, such as basic biogeographical and biodiversity research, as well as coastal land-use planning and management.

Predicting wave exposure in the rocky intertidal zone: Do bigger waves always lead to larger forces?

Abstract: Hydrodynamic forces from breaking waves are among the most important sources of mortality in the rocky intertidal zone. Information about the forces imposed by breaking waves is therefore critical if we are to interpret the mechanical design and physiological performance of wave-swept organisms in an ecologically and evolutionarily relevant context. Wave theory and engineering experiments predict that the process of wave breaking sets a limit on the maximum force to which organisms can be subjected. Unfortunately, the magnitude of this limit has not been determined on rocky shores. To this end, at a moderately exposed shore in central California, we measured the maximum hydrodynamic forces imposed on organism-sized benthic objects and related these forces to nearshore significant wave heights. At 146 of 221 microsites, there was a significant and substantial positive correlation between force and wave height, and at 130 of these microsites, force increased nonlinearly toward a statistically defined limit. The magnitude of this limit varied among sites, from 19 to 730 newtons (N). At 37 other sites, there was no significant correlation between surf zone force and wave height, indicating that increased wave height did not translate into increased force at these sites either. At only 16 sites did force increase in proportion to wave height without an apparent upper bound. These results suggest that for most microsites there is indeed a limiting wave height beyond which force is independent of wave height. The magnitude of the limit varies substantially among microsites, and an index of local topography was found to predict little of this variation. Thus, caution must be exercised in any attempt to relate observed variations in ocean ‘‘waviness’’ to the corresponding rates of microsite disturbance in intertidal communities. Rocky intertidal invertebrates and algae live in a world of extreme environmental severity, and the risk of damage or dislodgment from wave-generated forces is thought to be among the most important determinants of survival in this habitat (e.g., Dayton 1971; Levin and Paine 1974; Koehl 1979; Paine 1979; Paine and Levin 1981; Sousa 1984; Denny 1987, 1988; Carrington 1990, 2002; Bertness et al. 1991; Hunt and Scheibling 1996; Blanchette 1997). Quantifying the hydrodynamic forces acting on organisms, and how they vary in space and in time, is therefore key to understanding the evolutionary and ecological consequences of morphological design and the subsequent effects of wave-driven forces on the dynamics of intertidal ecosystems (Denny 1988; Koehl 1996; Denny and Wethey 2001; Carrington 2002).

Revisiting Wilson’s Formulas for Simplified Wind-Wave Prediction

Abstract: A procedure for predicting the significant height and period of wind waves generated in a uniform fetch in deep water is presented in the closed-form expressions based on Wilson’s formulas, together with relevant design diagrams. Although Wilson’s formulas are relatively unknown in the United States, they are quite reliable. The relationship between the nondimensional minimum duration and the nondimensional fetch length is empirically expressed in a power law, which enables the analyst to judge whether the wave growth is limited by the wind duration or the fetch length. The correlation between the significant wave height and period also indicates the period approximately proportional to the 2/3 power of the wave height.

Wave attenuation by vegetation

Abstract: Measurements have been carried out at the Paulinaschor, a salt marsh in the Westerschelde, to obtain information of the effect of vegetation on wave attenuation. The data have been analyzed. It appears that wave height is strongly reduced by the vegetation, especially for low water depths. Further analysis of the data has been done, to achieve wave energy dissipation. An attempt has been made to formulate a theoretical approach, which is suitable for calculating wave energy dissipation due to vegetation on the basis of certain vegetation characteristics such as stem diameter, plant height and plant density. This theory has been tested by a comparison between the theoretical dissipations and the – so called – observed dissipations. This resulted in quite satisfying correlations; correlation coefficients of about 0.6 – 0.8 were calculated. By means of this analysis is a friction coefficient determined, describing the friction exerted by the vegetation. This coefficient depends on the various vegetation characteristics as mentioned before, but also at a second friction factor, that is more plant specific. Subsequently, the wave model SWAN has been suited for modelling waves over vegetation areas. The Collins friction factor is used for calibration. Values for this factor turned out to be 2 orders of magnitude bigger than the default value, for bare bottoms. A further study on this Collins coefficient showed that this coefficient is, except for a constant factor, the same as the friction coefficient that was calculated on the basis of the various characteristics. Using these calculated friction coefficients, converted to Collins coefficients, the SWAN model has been validated. The model results showed a good agreement with reality. Only the wave attenuation at the edge of the salt marsh did not correspond very well with the observed attenuation. A possible explanation could be that vegetation is modelled in SWAN through an enlarged bottom friction, in stead of 3D obstacles. Also due to the fact that the development of the orbital velocity in the vegetation is not known exactly, deviances between model outcome and observed attenuation may occur.

Wave hindcast for the New Zealand region: Deep‐water wave climate

Abstract: The wave evolution model WAM (WAve Model) has been implemented for the New Zealand region and used to simulate the generation and propagation of deep‐water waves over a 20‐year period (1979–98). The model extends to include the relevant generation areas of the south‐west Pacific and Southern Oceans. Input winds for the model were derived from the European Centre for Medium‐Range Weather Forecasts (ECMWF). The resulting synthetic wave climatology will provide a valuable tool for researchers and coastal planners, as it will help fill gaps in the available wave information for New Zealand waters. In this paper the hindcasts are described, and comparisons are made with wave height data from the altimeters flown on the TOPEX/ Poseidon and ERS1 and ERS2 satellite missions. Long‐term mean significant wave heights from the hindcast were generally 0.3–0.5 m lower than values from “buoy‐equivalent” altimeter data throughout the comparison region (150°E‐170°W, 60°S‐20°S). Hindcast distributions of significa...

On the generation of secondary microseisms observed in northern and central Europe

Abstract: [1] Microseism recordings from four European broadband stations and from three seismic arrays in Scotland, Norway, and Germany are compared with model wave data of the oceanic wave field in the North Atlantic and local ocean wave data from the Norwegian coast at 60°N, both measured during February–March 2000. Two approaches have been tested to locate generation areas of microseismic energy: a new amplitude correlation technique and beam backprojection from the three seismic arrays. Both techniques reveal that the main generation areas are located in specific regions off the coast of Southwest Norway and North Scotland. Seismic stations distant from these generation areas record a superposition of seismic energy from different source regions. Those close to a specific source region also show a high correlation with it. Both techniques give upper limits for the extent of the generation area of the strongest storm on 6/7 March at the southwest Norwegian coast of about 500 km. By using marine X-band radar measurements of the two-dimensional wave height spectrum, we estimate that the relative change of the extension of the generation area off the coast of southwest Norway during several storms is less than a factor of 3. This indicates that the size of the generation area is controlled by static features as coastline or bathymetry, and not by the extent of the storms. Microseism energy appears to be mainly controlled by the wave height in distinct and identifiable generation regions, so that the wave climate in these regions can be studied using historical records of microseisms.

A procedure for estimating wind waves and storm-surge climate scenarios in a regional basin: the Adriatic Sea case

Abstract: This study attempts to estimate the effect of CO 2 doubling on the frequency and inten-sity of high wind waves and storm-surge events in the Adriatic Sea. The meteorological forcings werederived from two 30-yr-long time-slice experiments that simulated the global atmospheric circulationin the present and the doubled-CO 2 climate scenarios. These time-slice experiments were carried outby the Danish Meteorological Institute using the ECHAM-4 model at T106 resolution. Unfortunately,the resolution of the T106 wind is inadequate for simulation of the wave field and the storm surge inthe Adriatic Sea, and it results in a gross underevaluation of extreme events. In this study, regionalsurface wind fields have been derived from T106 sea-level-pressure fields by statistical downscaling.Downscaled wind fields have been used to force a wave and an ocean model during the two 30-yr-long simulations. The downscaled wind fields produce a large improvement with respect to the T106fields, but a systematic underestimation with respect to the observed wave height and surge levelsremains present. This shortcoming of the analysis might prevent identification of very intense events.Consequently, extreme-value analysis of the results of the present climate simulation produces val-ues lower than observed, and obviously the same systematic bias is expected in the evaluation of thefuture climate. Some caution is therefore necessary in the interpretation of the results of this study.Nonetheless, the comparison between the present and future climate simulations shows no substan-tial change in the extreme surge level and a decrease in the extreme wave height.KEY WORDS: Regional climate scenarios · Storm surges · Wave height · Extremes · CO

Directional wave measurements from three wave sensors during the FETCH experiment

Abstract: [1] During the Flux, Etat de la mer et Teledection en Condition de fetcH variable (FETCH) experiment, directional wave measurements were made by an airborne radar RESSAC and by two moored buoys, an Air–Sea Interaction Spar (ASIS) and a Directional Waverider. In order to define the performance and compatibility of these wave sensors with different measuring principles, a comparison of the directional measurements in a variety of meteorological conditions during the experiment is presented in this paper. It was found that within the limits of their operational ranges, the sensors agreed on the one-dimensional spectrum and the basic parameters derived from it—significant wave height and peak frequency. The sensors reported the directional features of the wave field and the mean direction consistently, but in some cases the two buoys disagreed on the directional width of the spectrum. Most of these cases were associated with a single swell-dominated event.

Slamming response of a large high-speed wave-piercer catamaran

Abstract: Commercial and military requirements for high-speed sea transportation has led to the evolution of large, fast, lightweight vessels. Knowledge of the effect of sea loads on the structure of such vessels is required in order to carry out structural design optimization. Wet deck slam events, which occur when the vessel's motion causes an impact between the cross-deck structure and the water surface when operating in large waves, are of particular importance for high-speed catamarans. Extensive full-scale hull stress, motion, and wave measurements have been conducted on an 86-m Incat high-speed catamaran ferry during a delivery voyage. A definition of a slam event, for this vessel, is proposed and used to identify slam events from the data records. The character and effects of these slamming events are investigated with respect to a number of factors, including structural loading, wave height and length, vessel speed and heading angle,relative vertical velocity, and frequency ofoccurrence. The results of the full-scaledata analysis are presented. Particular attention is paid to thewhipping response of the structure, with the principal structural response frequencies being identified through spectral analysis.

The characteristics of wind and wave fields modelled with different resolutions

Abstract: A number of meteorological situations are studied using the operational models at the European Centre for Medium-Range Weather Forecasts, and a series of short-term forecast experiments is made for four different resolutions of the meteorological model. A comparison with the measurements and between the corresponding results allows an evaluation of the relative benefits of increased resolution. The focus is on the surface wind speeds and on the wave heights. Good quality results are found over the oceans, especially in areas with strong spatial gradients. Consistent with the already good quality of the present operational results, the improvements with increased resolution are small, but significant. In enclosed basins, as in the case of the Mediterranean Sea, it is found that both the surface wind speed and wave height are strongly underestimated. The improvements with increased resolution are greater than over the oceans, but still not sufficient to give agreement with measured data. Copyright © 2003 Royal Meteorological Society

Vertical structure of surface gravity waves propagating over a sloping seabed: Theory and field measurements

Abstract: [1] Theoretical predictions of the vertical structure of wave motion over a sloping seabed are compared with field observations close to the bed in the nearshore zone. Of particular interest is the effect of the local slope on the magnitude and phase of the vertical velocity. Field measurements of near-bed velocity profiles on a 2� bed slope were obtained using a coherent Doppler profiler. The surface elevation was measured by a colocated, upward looking, acoustic sounder. Results are presented from two intervals of different wave energy levels during a storm event: for wave height/water depth ratios smaller than 0.3 and for Ursell numbers smaller than 0.6. The local comparisons of magnitude and phase between the vertical velocity and surface elevation measurements are in good agreement with linear theory for a sloping bed, but differ greatly from that for a horizontal bottom, especially in the lower water column. The sloping bottom, however, has little effect on the horizontal velocity. Linear theory appears to adequately describe the transfer function between the surface elevation and the near-bed velocities, not only at the peak frequencies but also at their harmonics. However, in relatively shallow water the local transformations of free and forced waves at the harmonic frequencies are indistinguishable in the lower water column. Therefore, given surface elevation measurements at a particular location (which reflect the integrated effects of nonlinearities associated with wave shoaling), the vertical structure of the third moments of velocity fields estimated from linear theory is in reasonable agreement with the observations. Both theory and observations show that the skewness and asymmetry of the vertical velocity are subject to significant bottom slope effects, whereas those of horizontal velocity are not. INDEX TERMS: 4211 Oceanography: General: Benthic boundary layers; 4203 Oceanography: General: Analytical modeling; 4219 Oceanography: General: Continental shelf processes; 4546 Oceanography: Physical: Nearshore processes; 4560 Oceanography: Physical: Surface waves and tides (1255); KEYWORDS: shoaling waves, bottom slope, vertical structure, skewness, asymmetry, boundary layers Citation: Zou, Q., A. E. Hay, and A. J. Bowen, Vertical structure of surface gravity waves propagating over a sloping seabed: Theory and field measurements, J. Geophys. Res., 108(C8), 3265, doi:10.1029/2002JC001432, 2003.

Tidal modulation of storm waves on a macrotidal flat in the Yellow Sea

Abstract: On the open shores of the Yellow Sea along Korea's Baeksu coast, a severe erosional phase in a tidal flat has been attributed to hydrodynamic processes in winter To investigate the hydrodynamic behavior of winter storms on the intertidal flat, in situ measurements of currents and waves were obtained with an instrumented system deployed at a low tidal zone over a neap–spring tidal period in February 1999 The measurements included two storm events under different tidal conditions, ie the first storm occurred during a neap tide and the second weaker storm during a spring tide Near-bottom velocities recorded during storm conditions at an elevation of 15 cm show that currents were dominantly directed alongshore with a maximum speed of 055 m/s and that the wave orbital velocity and significant wave height reached up to 1 m/s and 3 m, respectively In particular, it was observed that the magnitude of wave-related parameters was greater during the spring tide (the weaker storm) than during the neap tide, regardless of the intensity of the storms Analysis of the relationship between wave height and water depth, and temporal variations in wave velocity spectra and skewness, show that the effect of storm conditions on waves was relevant to wave-breaking processes The occurrence of harmonics in the broad-banded spectra of wave velocity demonstrates redistribution of wave energy due to the breaking of waves Wave height (Hs) was linearly related to water depth (h) fluctuating with tides, yielding a correlation coefficient of 07 (=Hs/h), which was close to the breaking limit This tidal modulation of storm waves associated with wave-breaking processes is possibly responsible for the observed discrepancy between the intensity of storms and their hydrodynamic consequences on the intertidal flat

A Spectral Approach for Determining Altimeter Wind Speed Model Functions

Abstract: We propose a new analytical algorithm for the estimation of wind speeds from altimeter data using the mean square slope of the ocean surface, which is obtained by integration of a widely accepted wind-wave spectrum including the gravity-capillary wave range. It indicates that the normalized radar cross section depends not only on the wind speed but also on the wave age. The wave state effect on the altimeter radar return becomes remarkable with increasing wind speed and cannot be neglected at high wind speeds. A relationship between wave age and nondimensional wave height based on buoy observational data is applied to compute the wave age using the significant wave height of ocean waves, which could be simultaneously obtained from altimeter data. Comparison with actual data shows that this new algorithm produces more reliable wind speeds than do empirical algorithms.

Longshore sand transport

Abstract: Various reliable data sets from sites (USA and Netherlands) have been selected to analyse the longshore transport process and to establish the relationship between wave height, wave incidence angle and longshore transport, yielding a relatively simple transport formula. The data set was too small to detect any influence of particle size, wave period and profile shape. These aspects were studied by using the results of a detailed process-based model, which were parameterized and implemented in the simplified formula. The main overall result is a general expression for longshore transport of sand and gravel, including the effects of profile slope and tidal velocity. INTRODUCTION The computation of a reliable estimate of longshore sand transport remains of considerable practical importance in coastal engineering applications such as the derivation of sediment budgets for coastal areas with and without structures (breakwaters, groynes) and long-term beach stability with and without beach nourishments or coarse-grained beach protections. Most research on longshore transport has concentrated on sand sized sediment, but research on longshore transport along gravel and shingle beaches has also been performed to deal with the erosion problems along these types of beaches, which are quite common along midand high-latitude (formerly glaciated) parts of the world. The most widely used formula for longshore transport (LST) is commonly known as the CERC equation (Shore Protection Manual, US Army Corps of Engineers, 1984). This method is based on the principle that the longshore transport rate (LST, incl. bed load and suspended load) is proportional to longshore wave power P per unit length of beach; LST=K P, with K=calibration coeffcient. The CERC formula has been calibrated using field data from sand beaches. The effects of particle diameter and bed slope have been studied systematically by Kamphuis (1991), resulting in a more refined equation for longshore sediment transport. This latter equation was found to give the best agreement between computed and measured transport rates (Schoonees and Theron, 1996). Both equations (CERC and KAMPHUIS) have been used in the present study. Furthermore, a detailed process-based model (CROSMOR2000) has been used in this study to compute the longshore sand transport distribution along the cross-shore bed profile. First, this model 1 Senior engineer, Delft Hydraulics, PO Box 177, 2600 MH Delft, Netherlands; leo.vanrijn@wldelft.nl.

Initiation of sediment movement for a wave - current coexistent system

Abstract: Bed shear stress and friction coefficient have been studied. Based on the critical condition for the initiation of sediment movement by means of a lot of test data for a wave - current coexistent system, formulas of sediment initial movement for both laminar and turbulent have been set up, and wave height and water depth for initiation of sediment movement are given as well.

Cross-Shore Mud Transport and Beach Deformation Model

Abstract: The present study aims to simulate the various features of wave-mud interaction on fine-grained shore profiles including wave height attenuation, wave-induced mud mass transport, gravity-driven flow of fluid mud and the reconfiguration of profile shape. A two-dimensional mud beach deformation model is presented considering the transport of fluid mud under continued wave action and downward gravity force. The wave height transformation is computed from the energy flux conservation law combining the effects of mud bed, shoaling and wave breaking. The rheological constitutive equations of visco-elastic-plastic model (Shibayama et al., 1990) are selected for numerical simulation. Wave flume experiments are carried out and the results are utilized for the verification of numerical model. The results of the numerical model are also compared with the laboratory data of Nakano (1994). It is concluded that the model is capable to predict the observed phenomena.

Electromagnetic bias estimation using in situ and satellite data: 1. RMS wave slope

Abstract: [1] The electromagnetic (EM) bias is the largest source of error in TOPEX/Poseidon and Jason-1 satellite altimeter sea surface height (SSH) estimates. Current operational EM bias models are based on empirical relationships between the bias, wind speed, and significant wave height. These models are limited in their accuracy because wind speed and wave height do not capture enough information about the sea state to uniquely specify the bias. In order to improve EM bias estimation, we have studied the correlation between the EM bias and RMS long wave slope using data from tower-based experiments in the Gulf of Mexico and Bass Straight, Australia. Models based on significant wave height and RMS slope are more accurate than models based on wave height and wind speed by at least 50% in RMS error between predicted and ground truth bias values. Furthermore, models which incorporate wave slope exhibit reduced regional variation between the two widely separated experiment locations.