Showing papers on "Wave height published in 1998"

Ocean Waves: The Stochastic Approach

Abstract: This book describes the stochastic method for ocean wave analysis. This method provides a route to predicting the characteristics of random ocean waves - information vital for the design and safe operation of ships and ocean structures. Assuming a basic knowledge of probability theory, the book begins with a chapter describing the essential elements of wind-generated random seas from the stochastic point of view. The following three chapters introduce spectral analysis techniques, probabilistic predictions of wave amplitudes, wave height and periodicity. A further four chapters discuss sea severity, extreme sea state, the directional wave energy spreading in random seas and special wave events such as wave breaking and group phenomena. Finally the stochastic properties of non-Gaussian waves are presented. Useful appendices and an extensive reference list are included. Examples of practical applications of the theories presented can be found throughout the text. This book will be suitable as a text for graduate students of naval, ocean and coastal engineering. It will also serve as a useful reference for research scientists and engineers working in this field.

Observations of swash under highly dissipative conditions

Abstract: Video measurements of swash were made at the low-sloping beach of the multiple bar system at Terschelling, Netherlands. The majority of the measurements were conducted under highly dissipative conditions with Iribarren numbers ξ0 (the ratio of beach slope to the square root of offshore wave steepness) less than 0.2. Infragravity (0.004–0.05 Hz) waves dominated the swash with an average ratio of infragravity and total swash height Rig/R of 0.85. Using linear regression we investigated the dependence of swash parameters on environmental conditions such as short-wave height, period, and local beach slope. On average, Rig was about 30% of the offshore wave height H0; the slope in the linear H0 dependence of Rig amounted to only 0.18, considerably smaller than that observed on steeper beaches. The data set shows evidence for saturation of the higher infragravity frequencies for ξ0 less than, roughly, 0.27. In our opinion, this saturation caused the constant of proportionality in the linear relationship between Rig/H0 and ξ0 to be significantly larger than that observed under higher Iribarren number regimes. The saturated tails of the swash spectra had an approximate f−3 roll-off (where f is frequency), whereas, in general, the nonsaturated parts were white. This lack of significant peaks casts doubt on the causality between infragravity waves and nearshore bars.

A statistical comparison of wind speed, wave height, and wave period derived from satellite altimeters and ocean buoys in the Gulf of Mexico region

Abstract: The capability of spaceborne altimeters to provide precise measurement of significant wave height and wind speed has been demonstrated repeatedly. It is shown in this paper that in addition to the significant wave height and wind speed, the wave period can be calculated from the semiempirical functions established from earlier wave research. The calculated characteristic wave period using the altimeter-derived wind speed and wave height are found to be in excellent agreement with the peak wave period and average wave period from the ocean buoy measurements in the Gulf of Mexico. Also, with the long time series of collocated data set, it is possible to compare altimeter output of wind and wave parameters with ocean buoy measurements taking into consideration the spatial lags between the buoy locations and the altimeter footprints, and the temporal lags between the two sensor systems. It is found that when the spatial lags are less than 10 km, the RMS difference of the significant wave height is approximately 0.1 m, which is the digitization resolution of the output from both altimeter and ocean buoy. For the wind speed, the RMS difference approaches 1.2 m/s in the Gulf of Mexico using the empirical algorithms. The wind speed agreement is significantly improved to 0.8 m/s when the tilting effect on the altimeter cross section is accounted for. In contrast to the spatial lags, temporal lags of up to 1 hour do not appear to produce significant difference in the statistics of comparison based on this study.

Fifteen years of global wave hindcasts using winds from the European Centre for Medium‐Range Weather Forecasts reanalysis: Validating the reanalyzed winds and assessing the wave climate

Abstract: The ERA (European Centre for Medium-Range Weather Forecasts Reanalysis) project resulted in a homogeneous data set describing the atmosphere over a time span of 15 years, from 1979 to 1993. To validate (part of) these data against independent observations we use the ERA surface winds to drive the WAM wave model. The modeled significant wave heights are then compared with observations. From this comparison the quality of the forcing winds is assessed. The patterns of computed wave heights agree well with observed patterns, and they are of the right magnitude. This confirms the realistic nature of the ERA winds. If one looks in detail, it appears that the significant wave heights resulting from the model are systematically lower than the observed ones in areas of high winds and waves and higher in areas of low winds and waves. It is argued that underestimation at high winds speeds is most likely a resolution effect, as wind and thus wave peaks are missed by finite resolution in space and time, while overestimation at low wind speeds most likely results from internal WAM errors. It is concluded that the monthly mean ERA winds are slightly (less than 5%) too low in areas of high winds, while from this study it is not possible to draw a decisive conclusion on the quality of ERA winds at low wind speeds. At the same time, the hindcast data form a 15-year climatology of global waves. This climatology is analyzed in terms of annual cycle and trends. The largest trends in significant wave height occur in the North Atlantic with an increase of more than 12 cm/yr in January, and south of Africa where the increasing trend exceeds 7 cm/yr in July. These trends, however, are only marginally significant. Furthermore, they exhibit a large month-to-month variability, so that on a seasonal basis the trends are significant only in small parts of the ocean. In conclusion, we are unable to confirm a significant change in wave height during the ERA period.

Tide Effects on Wave Attenuation and Wave Set-up on a Caribbean Coral Reef

Abstract: The effects of tides on wave attenuation and wave set-up were investigated at Great Pond Bay, a Caribbean reef located in St Croix, US Virgin Islands Measurements of wave pressure fluctuations were made at three stations across the reef profile Total wave set-up was measured between the forereef and the reef crest or backreef lagoon Wave spectra indicate significant filtering of energy at the peak frequencies as waves traveled across the reef The energy dissipation calculations imply an average energy reduction of 62% between the forereef and reef crest Mean energy reduction between the forereef and lagoon was 90% Energy dissipation between the forereef and reef crest increased 15% between high and low tide and 6% between forereef and lagoon Tidal reduction of water depth at the reef crest intensified wave breaking and this condition increased energy dissipation Measurements of wave set-up ranged from 0·8 to 1·5 cm Calculations of wave set-up using Tait's 1972 model showed good agreement with observations

Louisiana barrier islands and their importance in wetland protection : Forecasting shoreline change and subsequent response of wave climate

Abstract: The role that barrier islands play in mitigating the wave climate in lower energy, bay or lagoonal environments has not yet been addressed in detail. With the exception of one study in which a shallow water wave prediction model (HISWA) (LIST et. aI., 1992) was applied to idealized barrier-bay configurations, the critical linkages among barriers, wave energy transmission into bays, regenerated local waves, and subsequent wave climate have not been made. In Louisiana, barrier disintegration is rapid over the short-term (10 2 years) and the mere potential for impacts of barrier loss on the bay wave climate is highly significant. Because of a paucity in scientific data which could be utilized to address this issue, there remains a significant debate as to the value of barrier islands in mitigating wave climate in the bays and along fringing marshes. In this paper we present historical shoreline change data which are used to predict the rapid disintegration of a section of barrier island coast along central Louisiana (Isles Dernieres) and resultant forecasted increase s in wave energy in the adjacent bays. The methods associated with shoreline, bathymetric and wave energy forecasting are briefly presented as an example of a larger, ongoing project regarding the feasibility of large-scale barrier island restoration in Louisiana. A brief overview of the magnitude and causal mechanisms associated with wetland loss are provided in addition to the implications associate d with barrier island loss and subsequent detrimental impacts on fringing marshes. The example data set presented here indicates that the role of Louisiana's barrier islands comprising the Isles Dernieres in mitigating the wave climate in their adjacent bays and fringing marshes appears critical. Considering only fairweather conditions, the data indicate that the bays adjacent to the Isles Dernieres could experience an increase in wave height of 700% if the barrier chain is reduced to shoals. Although large-scale barrier island restoration will greatly reduce wave energy in Louisiana's bays and along fringing marshes, additional devices capable of absorbing wave energy around portions of the fringing marshes will likely require construction. This may occur in areas where the fetch permits regeneration of incident waves that have propagated across the Louisiana shelf, or locally genera ted higher frequency waves.

Statistical analysis and intercomparison of WAM model data with global ERS‐1 SAR wave mode spectral retrievals over 3 years

Abstract: Ocean wave spectra were retrieved from a set of ERS-I synthetic aperture radar (SAR) wave mode (SWM) spectra between January 1993 and December 1995. An assessment is given of the SWM data quality and the retrieval performance as well as the operational feasibility of the retrieval algorithm. Sensitivity studies are performed to demonstrate the weak residual dependence of the retrieval on the first-guess input spectrum. The mean spectral parameters of the SWM retrievals are compared with spectral parameters from collocated wave model (WAM) spectra. The time series of SWM-retrieved and WAM-derived monthly mean significant wave heights Hs in various ocean basins show good overall agreement but with a small systematic underestimation of Hs by the WAM. A decomposition of the wave spectra into wind sea and swell reveals an average 10% overprediction of the wind sea by the WAM while swell is underpredicted by 20–30%. The positive wind-sea bias exhibits no clear wave height dependence, while the negative swell bias decreases with swell wave height. This could be due to a too strong damping in the WAM at low frequencies. Detailed regional investigations point to the existence of smaller-scale phenomena, which may not be adequately reproduced by the WAM at the present resolution of the wind forcing. Finally, an intercomparison is made of the observed and modeled azimuthal cutoff length scales, and global distributions are investigated. Ratios of the observed azimuthal cutoff wavenumber to the mean azimuthal wavenumber component indicate that about 75% of the swell can be directly resolved by the SAR, while about 70% of the wind sea lies at least partially beyond the cutoff.

North Atlantic Wind Waves and Wind Stress Fields from Voluntary Observing Ship Data

Abstract: On the basis of the collection of individual marine observations available from the Comprehensive Ocean–Atmosphere Data Set, major parameters of the sea state were evaluated. Climatological fields of wind waves and swell height and period, as well as significant wave height and resultant period are obtained for the North Atlantic Ocean for the period from 1964 to 1993. Validation of the results against instrumental records from National Data Buoy Center buoys and ocean weather station measurements indicate relatively good agreement for wave height and systematic biases in the visually estimated periods that were corrected. Wave age, which is important for wind stress estimates, was evaluated form wave and wind observations. The climatology of wave age indicates younger waves in winter in the North Atlantic midlatitudes and Tropics. Wave age estimates were applied to the calculations of the wind stress using parameterizations from field experiments. Differences between wave-age-based and traditional estimates are not negligible in wintertime in midlatitudes and Tropics where wave-induced stress contributes from 5% to 15% to the total stress estimates. Importance of the obtained effects for ocean circulation and climate variability is discussed.

Deformation and decay of different classes of breakers

Abstract: Deformation and dissipation of different classes of breakers ranging from a spilling breaker to a strong plunging breaker are studied. Experiments are carried out to investigate spatial changes of the wave profile and wave height attenuation of solitary waves after breaking over a reef. Moreover, the KdV-type equation in which diffusion and bottom friction are considered is numerically solved as an inverse problem to estimate the value of the eddy diffusion coefficient κ* for different classes of breakers. The results reveal that postbreaking wave deformation, wave decay, and the transmission coefficient are closely related to the breaker type. Further, it is shown that the breaker type effect on these phenomena can be evaluated quantitatively.

Experiments on Nonlinear Wave Groups in Intermediate Water Depth

Abstract: Evolution of nonlinear wave groups of various initial envelope shapes is studied experimentally in a wave tank. Experiments are performed for different values of the water depth in the tank. The experimental results are compared with the calculations based on the cubic Schrodinger equation. Qualitatively different results are obtained for water depth values corresponding to different signs of the nonlinear term coefficient in the model equation. In relatively shallow water, the demodulation of wave groups is observed, while in a deeper water the maximum wave height increases along the tank due to the focusing, as expected.

Frequency of effective wave activity and the recession of coastal bluffs: Calvert Cliffs, Maryland

Abstract: The Calvert Cliffs, Chesapeake Bay, Maryland, USA, erode by direct wave undercutting or by freeze/thaw erosion accompanied by wave removal of slope debris. Directly undercut slopes recede more rapidly, with long-term rates exceeding 1.0 m/yr; freeze/thaw slopes recede at rates approaching 0.5 m/yr. The frequency of wave height and water level at the shoreline is estimated for eleven sites based on a 37-year wind record, estimates of storm surge, offshore wave geometry, nearshore wave transformation, and breaking wave type. Locations experiencing the largest slope recession are not uniformly those with the largest cumulative wave energy; the resistance to erosion of the slope toe must also be accounted for. An index of relative wave strength is defined as the ratio of wave pressure T and the cohesive strength S of the slope material. For the Calvert Cliffs, a minimum relative wave strength for initiating erosion of intact material is 0.05 < T/S < 0.1. A cumulative duration of ≥ 50 hours per year for T/S ≥ 0.1 distinguishes undercut and nonundercut slopes and recession rates greater or lesser than 0.5 m/yr. The relative wave strength index may be used to identify sites at risk of increased erosion. At one site with a small historical erosion rate, the loss of a protective beach and associated decrease in toe elevation caused a positive shift in the frequency of large T/S. Direct wave undercutting and increased slope recession may be anticipated at this site, as indicated by the development of an undercut notch during the course of the study.

Sediment-Wave Parametric Characterization of Beaches

Abstract: Sediment-wave parameters, notably the parameter Ω = H b /W s T (where H b , is wave breaker height, W s sediment fall velocity, and T wave period), have been used in the literature to identify thresholds between various beach morphodynamic states ranging from reflective, through intermediate, to dissipative. Such parameters may be useful as elementary descriptors of beaches, especially in microtidal swell wave settings with mature sediment suites, and when used in conjunction with conceptual beach state models elaborated in recent years. Although the problems of temporal wave height variability and large tide ranges have been addressed in the parametric characterization of various beach types, these factors, together with sediment variability, may result in beach morphodynamic systems that cannot be meaningfully characterized by sediment-wave parameters. Morphodynamic parameters such as the Iribarren Number ξ b (ξ b = tan b / (H b /L o ) 0.5 , where tan b represents beach slope, H b , is wave breaker height, and L o deepwater wavelength), and the surf-scaling parameter ϵ (ϵ = a b w 2 /(gtan 2 b), where a b , is runup amplitude on the beach, w wave radian frequency, and g the gravitational constant), are based on beach slope, and avoid the problem of the choice of representative beach sediment parameters. The intertidal beach slope is also a better index of characterization of spatial and temporal changes in the reflective-to-dissipative beach morphodynamic continuum, especially in settings with large tide ranges.

A Comparison of Algorithms for Extracting Significant Wave Height from HF Radar Ocean Backscatter Spectra

Abstract: A comparison is made between three different but related algorithms for the extraction of rms wave heights from high frequency ocean backscatter radar spectra. All three methods are based on the ratio of second- to first-order energies as developed by Barrick, and each was scaled so that the mean values of the radar analysis results and the corresponding wave buoy data were zero. The rms difference between the radar wave heights and those from the buoy was taken as a measure of fit, and the recommended algorithm had an rms difference value of 7 cm. Barrick’s algorithm (after scaling), which uses a weighted second-order energy integral, performed marginally better than the others. The condition requiring wind directions other than close to orthogonal to the radar beam is retained in the recommended algorithm but is not evaluated because of sparsity of data. The algorithm for extraction of rms wave heights is validated against the buoy data over rms wave height ranges from about 0.2 to 0.7 m.

Energy Dissipation Model for Regular and Irregular Breaking Waves

Abstract: Based on a large amount of published laboratory results, reliable models are developed for computing the average rate of energy dissipation in regular and irregular breaking waves. The average energy dissipation rate is assumed to be proportional to the difference between the local mean energy density and stable energy density. Wave height transformation is computed from the energy flux conservation law based on the linear wave theory. The models are examined and verified extensively for a variety of wave and bottom conditions, including small and large scale laboratory and field experiments. Reasonable good agreements are obtained between the measured and computed wave heights and root mean square wave heights.

Extreme Waves and Coastal Erosion in the Pacific Northwest

Abstract: The extreme wave climate of a Pacific Northwest littoral cell has been characterized by examining four long-term databases of measured wave parameters. Winter waves in the region average 3.0 m in significant wave height and 12 s in period, while summer waves average 1.5 m and 8 s, respectively. There is a distinct seasonality in monthly mean wave direction, with winter storm waves arriving from the southwest and the milder summer waves arriving from the northwest. Projections of the 50-year design wave height vary greatly, depending on the data source, ranging from 8.5 m to 12.2 m. A probabilistic model, combining these long-term measurements of waves with measurements of tides and local beach morphology, has been applied to a sub-region of the littoral cell. The model quantifies the susceptibility of coastal properties to erosion by predicting the frequency with which wave runup impacts either sea cliffs or sand dunes. The coastal erosion model is based on wave runup measurements obtained on high-energy dissipative beaches typical of the Pacific Northwest. Statistics of wave runup maxima are well correlated with simple relationships between commonly measured wave and beach parameters. Measurements of extreme tides have been shown to be greater than predicted due to a variety of factors, particularly the occurrence of El Nino events. The 1997/98 El Nino has produced some of the highest water levels on record, increasing the potential for erosion within the region.

Intercomparison of the North Atlantic wave climatology from voluntary observing ships, satellite data and modelling

Abstract: Three different sources of the wave data - visual observations from the voluntary observing ships, wave hindcast from the WAM model driven by European Reanalysis Project winds, and the altimeter measurements from GEOSAT, TOPEX/POSEIDON and ERS-1 are used for the intercomparison of the North Atlantic wave fields for the period 1979–1993 Climatological spatial patterns of significant wave height seen in all three products are consistent, although the actual quantitative values indicate both positive and negative biases of about 01 to 08 m Sea and swell heights are intercompared separately for the voluntary observing ship and WAM model data Best agreement between the visually observed data, the model hindcast and the altimeter measurements is obtained in the North Atlantic mid latitudes However, long-term wave height trends in the merchant ship and the WAM model data are quite different The nature of the differences in these estimates is discussed

Gps signal scattering from sea surface: comparison between experimental data and theoretical model *

Abstract: NASA Langley Research Center, Hampton, VAABSTRACTGlobal Positioning System (GPS) signals reflected from the ocean surface havepotential use for various remote sensing purposes Some possibilities are measurements ofsurface roughness characteristics from which wave height, wind speed and direction couldbe determined For this paper, GPS reflected signal measurements taken from aircraft areused to explore the possibility of determining wind speed To interpret the GPS data, atheoretical model has been developed which describes the power of the reflected GPSsignals for different time delays as a function of geometrical and environmentalparameters The results indicate a good agreement between the measured and the modelednormalized signal power waveforms even during rapidly changing surface wind conditions The inferred wind speed, obtained by comparing actual and modeled waveforms, showsgood agreement (within 2 m/s) with data obtained from a nearby buoy1 INTRODUCTIONThe concept of bistatic radar sensing of the ocean surface using two satellites has been demonstratedby Rubashkin et al, (1993) using a transmitter in low Earth orbit and a receiver in geosynchronous orbitMartin-Neira (1993) first proposed and described a bistatic ocean altimetry system utilizing the signal of theGlobal Positioning System (GPS) The reception of surface reflected GPS signals from aircraft was firstreported by Auber et al, (1994) Recently, a number of applications have been proposed which use GPSsignals reflected from the ocean surface (Katzberg and Garrison, 1996) One of the proposals is to use thereflected power to infer information on wind speed and wave height The viability of the proposedapplications depends upon the ability to acquire and code track the reflected signal for an extended periodof time over a variety of sea states These difficulties were overcome by development of the first GPS delay-mapping receiver (Garrison et al, 1998) Qualitative assessment of characteristics of the signal using such areceiver has been presented by Garrison et al, (1997) The dependence of a received signal structure ontransmitter-receiver configuration and surface wind conditions has been studied theoretically by Zavorotnyand Voronovich (1998a) The modeling of wind speed effect on the received signal at aircraft altitudes ispresented in subsequent sections along with experimental data collected by the above mentioned delay-mapping receiver Quantitative comparison of data with the model provides us with evidence that thistechnique can be used for inferring ocean surface winds and wave heights

Fully reprocessed ERS‐1 altimeter data from 1992 to 1995: Feasibility of the detection of long term sea level change

Abstract: Global mean sea level observations are necessary to answer the urgent questions about climate changes and their impact on socio-economy. At GeoForschungsZentrum/Geman Processing and Archiving Facility ERS altimeter data is used to systematically generate geophysical products such as sea surface topography, high-resolution geoid and short- and long-period sea surface height models. On the basis of this experience, fully reprocessed ERS-1 altimeter data is used to generated a time series of monthly sea surface height models from April 1992 to April 1995. The reprocessing consists of improved satellite ephemerides, merging of Grenoble tidal model, and application of range corrections due to timing errors. With the new data set the TOPEX/POSEIDON prelaunch accuracy requirements are fulfilled. The 3-year time series is taken to estimate the rate of change of global mean sea level. A careful treatment of seasonal effects is considered. A masking of continents, sea ice, and suspect sea surface heights is chosen that is common for all sea surface height models. The obtained rate of change is compared to external results from tide gauge records and TOPEX/POSEIDON data. The relation of sea level changes and sea surface temperature variations is examined by means of global monthly sea surface temperature maps. Both global wind speed and wave height maps are investigated and correlated with sea surface heights and sea surface temperatures in order to find other indicators of climate variations. The obtained rate of changes of the various global maps is compared to an atmospheric CO2 anomaly record, which is highly correlated to El Nino events. The relatively short period of 3 years, however, does not allow definite conclusions with respect to possible long-term climate changes.

Wind wave tank measurements of wave damping and radar cross sections in the presence of monomolecular surface films

Abstract: Measurements of the damping of small gravity and gravity-capillary water surface waves covered with monomolecular organic films of different viscoelastic properties were performed in the wind wave tank facility of the University of Hamburg. The wind speed dependence of the radar cross sections for X and Ka band was measured with upwind looking microwave antennas. It is shown that Marangoni damping theory, which describes the damping of water surface waves by viscoelastic surface films, is not the only damping mechanism in wind wave tank experiments where the wind sea is not fully developed. The other source terms of the action balance equation, i.e., the energy input into the water waves from the wind, the nonlinear wave-wave interaction, and the dissipation by wave breaking, are affected differently by the various substances. It is hypothesized that this difference is caused by the different viscoelastic properties of the substances, i.e., by the different intermolecular interactions of the film molecules. A slight dip in the wind dependence of the radar cross section at Ka band at wind speeds of 8-9 m/s was measured, which corresponds to comparable reductions of the mean squared wave height and wave slope. Polarization ratios (i.e., the ratios of the radar backscatter at vertical and horizontal polarization) higher than those predicted by simple Bragg scattering theory for X band at low wind speeds and different incidence angles are explained within a (three-scale) composite-surface model. At higher wind speeds, where the polarization ratio decreases rapidly, breaking by wedges and spilling breakers is hypothesized to become more dominant. The dependence of the polarization ratio on the coverage of the water surface with a slick is explained qualitatively by means of the composite-surface model. Finally, it is stated that wind wave tank measurements in the presence of monomolecular surface films are useful for the verification of theories concerning radar backscattering, wave damping, and wind-wave and wave-wave interactions.

Tracking storm‐generated waves in the northeast Pacific Ocean with ERS‐1 synthetic aperture radar imagery and buoys

Abstract: This paper examines the capability of synthetic aperture radar imagery from ERS-1 and buoys to track the wave field emanating from an intense storm over a several-day period. The first part of the study is a validation component that compares SAR-derived wave length and direction with buoy data from two locations over 10 different dates in late 1991 and National Oceanic and Atmospheric Administration (NOAA) wave model (WAM) wave direction results. When the SAR is linear (8 out of 10 cases), mean wavelength is within 5% of the buoy measurements and mean wave direction is within 1° of direction derived from the wave model (albeit with a large standard deviation of 27°), indicating close agreement. The wave field generated from the intense storm in late December 1991 was measured by three separate ERS-1 SAR passes over a 3-day period. A simple kinematic model was used for waves propagating from a storm. Comparing the model results with both SAR and buoy data indicates that SAR-derived peak wavelength and direction measurements can be reliably used to predict arrival times and propagation direction over a several-day period and considerable distances. The measurements can also be used to derive estimated wave generation source regions about the storm as well. Such measurements are useful for comparing with wave model results, which perform less accurately for direction than wave height for example, and for predicting hazardous conditions for ship navigation and coastal regions.

Resonant interaction between an atmospheric gravity wave and shallow water wave along Florida's west coast

Abstract: On 25 March 1995, a large solitary wave, seemingly from nowhere, washed ashore along the normally tranquil Gulf Coast of Florida from Tampa Bay to south of Naples. On this Saturday morning, many be...

Enhancements in the determination of ocean surface wave height from grazing incidence microwave backscatter

Abstract: M.D. Henschel et al. (1995) described a procedure for estimating ocean surface wave height from the shadowing statistics of a grazing incidence marine radar image of the sea surface. Their model, based on the geometrical optics approach of L.B. Wetzel (1990), related the normalized lengths of the regions of illumination ("islands") and shadow ("troughs") to the significant wave height through a parameter they called the "threshold probability of illumination" (P/sub 0/). The present authors have carried out several field data collection exercises in the past few years in which a very large number of grazing incidence microwave images of the sea surface were recorded. They have used these images to calibrate and to validate the M.D. Henschel et al. model. Preliminary results, have shown that the model performed well for wave heights close to those for which it was calibrated, but significantly underestimated larger wave heights. Analysis of this failure mode, and of much more data from high sea state situations, has shown that the normalized island lengths do not decrease as quickly as the model predicts they should. The appearance of these larger islands implies that the P/sub 0/ is not a constant, but decreases with increasing wave height. A significant correlation also exists between the calculated P/sub 0/ and the mean non-shadowed NRCS of the images in the upwave direction. Recalculation of the significant wave height, based on a varying P/sub 0/ estimated from the above-mentioned NRCS value, produces a much better estimation of the measured significant wave height over a wider range of wave heights than did the original estimates based on a fixed P/sub 0/.

Estado morfodinâmico praial no instante da observação: uma alternativa de identificação

Abstract: Determinations of the morphodynamic state of beach and surf zone were made on four beaches in Rio de Janeiro state, with two beaches located northward and two westward of Cabo Frio. Twenty four monthly measurements of beach and surf zone profiles associated to measurements of wave height and period, sediment settling velocity as well as swash climate were made during low spring tide. The beaches were classified by their modal morphodynamic state, a differentiation due to different wave climate and sediment characteristics. With exception for the reflective state, low correlation between the predicted and observed Morphodynamic State were obtained using Dean's parameter (Ω). This is a consequence of the low correlation between the grain size (and settling velocity) of the beach face sediments and the topographic gradient, when applied to a single beach. A new approach for the identification of the Morphodynamic State is proposed by inferring the beach and nearshore morphology through the comparison of the wave height and period in the outer breaker zone to the height and semi-period of wave in the swash zone.

Wave Reflection from Beaches: A Predictive Model

Abstract: A model to predict reflection of random waves on a beach, including dissipation due to breaking, is presented. The evolution of the local reflection coefficient and the incident and total wave height is computed. Two initial conditions are needed; the incident H rms and the reflection coefficient, R, far offshore. The expected value of the wave breaking dissipation is estimated following BATTJES, and JANSSEN, (1978). The expected value of the reflected energy flux per unit area of beach profile is obtained hypothesizing that (1) wave reflection may be considered a linear process dependent only on the local profile geometry and wave period and (2) only the non-breaking waves contribute to the reflected flux of energy (BAQUERIZO, 1995). These results are compared with wave parameters measured during the SUPERTANK Project and with the reflection coefficient estimated by three different methods. Measured and computed data agree well for barred and non-barred beach profiles. It is shown that the local reflection coefficient evolves along the beach profile. An overall beach reflection coefficient should be defined only offshore of the depth where waves start to break and where the contribution to the wave reflection process is negligible.