Showing papers on "Swell published in 1985"
TL;DR: In this article, an alternative Stokes theory for steady waves in water of constant depth is presented where the expansion parameter is the wave steepness itself, and the first step in application requires the solution of one nonlinear equation, rather than two or three simultaneously as has been previously necessary.
Abstract: An alternative Stokes theory for steady waves in water of constant depth is presented where the expansion parameter is the wave steepness itself. The first step in application requires the solution of one nonlinear equation, rather than two or three simultaneously as has been previously necessary. In addition to the usually specified design parameters of wave height, period and water depth, it is also necessary to specify the current or mass flux to apply any steady wave theory. The reason being that the waves almost always travel on some finite current and the apparent wave period is actually a Dopplershifted period. Most previous theories have ignored this, and their application has been indefinite, if not wrong, at first order. A numerical method for testing theoretical results is proposed, which shows that two existing theories are wrong at fifth order, while the present theory and that of Chappelear are correct. Comparisons with experiments and accurate numerical results show that the present theory ...
488 citations
TL;DR: In this paper, a theory that strong reflection can be induced by the sandbars themselves, once the so-called Bragg resonance condition is met, was presented. But this theory is limited to weak reflection and fails at resonance.
Abstract: One of the possible mechanisms of forming offshore sandbars parallel to a coast is the wave-induced mass transport in the boundary layer near the sea bottom. For this mechanism to be effective, sufficient reflection must be present so that the waves are partially standing. The main part of this paper is to explain a theory that strong reflection can be induced by the sandbars themselves, once the so-called Bragg resonance condition is met. For constant mean depth and simple harmonic waves this resonance has been studied by Davies (1982), whose theory, is however, limited to weak reflection and fails at resonance. Comparison of the strong reflection theory with Heathershaw's (1982) experiments is made. Furthermore, if the incident waves are slightly detuned or slowly modulated in time, the scattering process is found to depend critically on whether the modulational frequency lies above or below a threshold frequency. The effects of mean beach slope are also studied. In addition, it is found for periodically modulated wave groups that nonlinear effects can radiate long waves over the bars far beyond the reach of the short waves themselves. Finally it is argued that the breakpoint bar of ordinary size formed by plunging breakers can provide enough reflection to initiate the first few bars, thereby setting the stage for resonant reflection for more bars.
319 citations
TL;DR: In this paper, a new spectrum model for the ocean surface was proposed and the two unknown parameters in this spectrum were determined by fitting it to radar observations, and it was shown that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the 0\deg-70\deg range.
Abstract: A new spectrum model for the ocean surface is proposed. We determine the two unknown parameters in this spectrum by fitting it to radar observations. We find that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the 0\deg-70\deg range. The spectrum model is combined with a model for swell to examine the effect of swell on the radar cross section. We find that the effect of swell is significant for low radar frequencies ( L band) and near normal incidence but can be nearly eliminated by using higher frequencies ( K_{u} band) and large angles of incidence ( \approx 50\deg ).
282 citations
TL;DR: In this article, a numerical model is presented that predicts the transformation of monochromatic waves over complex bathymetry and includes both refractive and diffractive effects, and the solution is obtained for a finite number of rectilinear grid cells that comprise the domain of interest.
Abstract: A numerical model is presented that predicts the transformation of monochromatic waves over complex bathymetry and includes both refractive and diffractive effects. Finite difference approximations are used to solve the governing equations, and the solution is obtained for a finite number of rectilinear grid cells that comprise the domain of interest. Model results are compared with data from two experimental tests, and the capability and utility of the model for real coastal applications are illustrated by application to an ocean inlet system.
106 citations
TL;DR: In this paper, the authors investigated the resuspension of bottom sediments by waves and tidal currents in three characteristic environments of middle Chesapeake Bay (shallow platform, deep platform, and main channel).
Abstract: Resuspension of bottom sediments by waves and tidal currents was investigated in three characteristic environments of middle Chesapeake Bay (shallow platform, deep platform, and main channel). In the shallow near shore platform wind waves frequently resuspended significant amounts of sediment, some of which was transported offshore. In both the shallow and adjacent deep platform regions, tidal currents were too weak (<20 cm cm/sec) to resuspend bottom sediments. In the main channel, peak current velocities were substantially stronger (40 cm/sec), but were still not competent to erode the bottom. The stability of the bottom in this area is related to the activities of the benthic organisms which are influenced by seasonal anoxia.
60 citations
TL;DR: In this paper, the effects of steady, uniform currents on random waves and associated waterparticle kinematics were investigated, and the basic equations describing the interactions between waves and currents were described.
Abstract: The effects of steady, uniform currents on random waves, and the associated waterparticle kinematics, are investigated. The basic equations describing the interactions between waves and currents ar...
52 citations
TL;DR: In this article, the solution and integral properties (wave energy, energy flux, radiation stress, etc.) of the first-order cnoidal wave theory are given as power series of the complementary nome of the theta function.
51 citations
TL;DR: In this article, the authors investigated the feasibility of constructing a system of underwater structures which would act like a lens and focus water waves prior to harnessing their energy by a horizontal, flat plate which is moored to the seabed.
Abstract: A Norwegian research group has investigated the feasibility of constructing a system of underwater structures which would act like a lens and focus water waves prior to harnessing their energy. In the present work we consider modelling one of these structures by a horizontal, flat plate which is moored to the seabed. The water is assumed to be incompressible and inviscid and two-dimensional, linear, irrotational theory is used. Solutions to the scattering and radiation potentials are obtained by the method of matched eigenfunction expansions. Comparisons are made with various approximate solutions and results are presented illustrating the effect of varying the mooring stiffness in the cables on both the responses of the plate and the far-field wave motion.
49 citations
TL;DR: In this paper, it was shown that the momentum transferred from wind to water surface goes largely into water waves when the steepness of the waves is large, and only a small amount is advected by the wind waves.
Abstract: Momentum balance in the air-sea boundary process is discussed on the basis of a recent study (Mitsuyasu and Honda, 1982). It is shown that the momentum transferred from wind to water surface goes largely into water waves when the steepness of the waves is large. For wind-generated waves, however, much of the momentum transferred from wind to waves is lost by wave breaking, and only a small amount is advected by the wind waves.
47 citations
TL;DR: A 30-MHz ground-wave ocean surface radar has been deployed inside the Great Barrier Reef where the water is sheltered from ocean swell and coincident observations of sea-wave spectra, surface currents and boundary-layer winds are used to evaluate the radar performance and to modify some of the methods of data analysis to suit these conditions.
Abstract: A 30-MHz ground-wave ocean surface radar has been deployed inside the Great Barrier Reef where the water is sheltered from ocean swell. The spatial resolution of the radar is 3 km radially and 3.5o in azimuth. In each cell a 102.4-s time series is used to determine radial surface currents, wind directions, root- mean-square wave heights and wind speeds. Coincident observations of sea-wave spectra, surface currents and boundary-layer winds are used to evaluate the radar performance and to modify some of the methods of data analysis to suit these conditions. Surface current values are observed by the radar to an accuracy of ±0.05 m s-1, wind directions to ±10o , root-mean-square wave heights to 0.15 m and wind speeds to ±3 m s-1. In some spectra, the peak in the second-order continuum caused by the non-directional sea- wave spectrum is not resolved from a second-order resonance line. This disallows the derivation of the period of the dominant sea wave on a routine basis.
45 citations
TL;DR: In this paper, a numerical model for refraction of ocean swell by currents associated with a warm core ring was developed and tested with Seasat synthetic aperture radar (SAR) data.
Abstract: A numerical model for refraction of ocean swell by currents associated with a warm core ring was developed and tested with Seasat synthetic aperture radar (SAR) data. The wave field of SAR orbit 1232 was measured using optical Fourier transforms. The wave refraction model produced rays by simultaneous, numerical integration of the Hamiltonian ray equations applied to a moving medium. Wave orthogonals were constructed from wave number vectors calculated at each incremental time step. The flow field used by the model to simulate a warm ring was a steady, circular jet, with the radial profile of tangential velocity composed of a power function joined to a Gaussian. Initial wave conditions for simulation of refraction by the SAR-imaged ring were determined from measurements outside the ring. No data were available from which to determine the current field of the SAR-imaged ring, so the current field input to the model was adjusted until the output wave field most nearly resembled the SAR observations. The relative locations of convergence and divergence of rays were as observed on the SAR image, and the relative energy density in crossed seas was correctly predicted. However, predicted patterns of wavelength variation (presuming that incident waves were uniform in wavelength) were not observed.
Journal Article•
TL;DR: Aerial photographs showing strong refraction of swell near the northern California coastline were obtained on September 8, 1982 The possibility that this refraction is due to a large horizontal current shear is discussed; a maximum shear value of the order of 10−3 s−1, with the velocity directed along a linear slick-like front, is calculated from the wave data in the photo.
Abstract: Aerial photographs showing strong refraction of swell near the northern California coastline were obtained on September 8, 1982 The possibility that this refraction is due to a large horizontal current shear is discussed; a maximum shear value of the order of 10−3 s−1, with the velocity directed along a linear slick-like front, is calculated from the wave data in the photo In situ current data obtained at 90 m depth at a nearby midshelf site on two surface moorings separated by 390 m also show occasional periods of large horizontal velocity shear On January 4, 1982, a maximum horizontal shear of order 10−3 s−1 was observed for several hours These separate observations taken together indicate that large horizontal surface velocity shears of order 10−3 s−1 occur over the northern California continental shelf, and that large surface current shear can be usefully detected and measured by high flying high resolution sensors
TL;DR: In this paper, two field experiments were carried out to determine critical conditions at the threshold of sand motion under irregular sea waves, and the experiments were performed outside the breaker zone at Blackpool Sands, Start Bay, Devon, in waters of depth 4-10 m. Despite the irregular nature of the (swell) waves and the mixture of grain sizes on the bed, agreement between the present field results and previous laboratory results is reasonable, provided that proper allowance is made for the presence of sand ripples.
Abstract: Two field experiments were carried out to determine critical conditions at the threshold of sand motion beneath irregular sea waves. The experiments were performed outside the breaker zone at Blackpool Sands, Start Bay, Devon, in waters of depth 4–10 m. From synchronous video and near-bed velocity records, critical conditions at the threshold of motion have been established in terms of the measured free-stream velocity amplitude. Despite the irregular nature of the (swell) waves and the mixture of grain sizes on the bed, agreement between the present field results and previous laboratory results is reasonable, provided that proper allowance is made for the presence of sand ripples. In order to define threshold motion conditions on a more rational basis than in terms of the free-stream velocity amplitude, calculations of the bottom stress have been made on the basis of laboratory-derived wave drag coefficients. The threshold-motion conditions thus established from the field data are in good agreement with sediment threshold values from Shields' curve derived from laboratory work, at least for cases in which it has been possible to calculate the skin-friction contribution to the total bed shear stress.
TL;DR: In this paper, the mid-ocean bathymetric swells observed around some volcanic island chains can be modeled using the thermal conduction equations of an oceanic lithosphere moving over a heat source located at the base of the plate.
Abstract: The mid-ocean bathymetric swells observed around some volcanic island chains can be modeled using the thermal conduction equations of an oceanic lithosphere moving over a heat source located at the base of the plate. Some previous studies of this problem have suggested that the heat source must lie well within the lithosphere in order to explain the observed bathymetry and surface heat flow. These conclusions are based on an isothermal boundary condition at the base of the lithosphere, but this choice requires that the heat source is maintained above this boundary and can actually lead to a flux of heat back into the mantle, particularly in the vicinity of the heat source. If the lower boundary is insulated insofar as the anomalous temperature field is concerned, flux from the heat source back into the mantle is prevented, resulting in a more rapid uplift of, and in an increased heat flow across, the seafloor. Previous solutions have considered only the steady state solutions, whereas the present model also evaluates transient solutions. For rapidly moving oceanic plates the transient effects can be neglected provided that the heat source has been active for at least 107 years and that the direction of motion and the intensity of the heat source have remained constant in this interval. Solutions, with the heat source located near the base of the conductive layer, of the conduction equations for heat flow, bathymetry, and geoid heights in the spatial domain reproduce the principal observational characteristics of the Hawaiian swell at distances away from the immediate origin of the swell, and this part of the swell can be used to estimate the thermal conduction model parameters. The model does not wholly explain the observations near the origin and the mismatch may reflect a dynamic component in the support of the swell.
TL;DR: In this article, a simplified glacier tongue, modelled as a beam interacting with water waves, is analyzed in order to understand better the possible mechanism of iceberg calving, and the analysis and results are mainly numerical, although some analytical results valid for a limited range of frequencies are also given.
TL;DR: In this article, field trials of an optical scanner for measuring directional slopes of the sea surface have been carried out at the Research Pier of the Coastal Engineering Research Center (CERC), and preliminary results include upwind-downwind and crosswind components of the mean-square slopes under various wind, swell, and atmospheric stability conditions.
Abstract: Field trials of an optical scanner for measuring directional slopes of the sea surface have been carried out at the Research Pier of the Coastal Engineering Research Center (CERC). Preliminary results include upwind-downwind and crosswind components of the mean-square slopes under various wind, swell, and atmospheric stability conditions.
TL;DR: In this paper, the authors investigated the consistency of wind wave and swell reports from ship reports and found that there is very little consistency in the reporting of wind waves and swell periods and swell directions.
Abstract: Observations of wind waves and swell from ship reports are investigated. Comparisons are made between estimates of wave parameters made from ships in southern oceans by calculating correlations as a function of ship separation, and analyzing the resulting series. It is shown that there is very little consistency in the reporting of wind wave and swell periods and swell directions. Heights fare considerably better, although it is shown that one observer still fails to account for at least 50% of the variance in the observations of another observer. Further, some comparisons of ship-reported wave heights with measurements show a high bias in the observations of at least 0.5 meters. Some weak quality control criteria fail to be met by a large number of observations. Despite the inconsistencies, the intercomparisons show that the data is representative of many of the physical characteristics of wavefields, and therefore can be useful in climatological studies.
TL;DR: In this paper, it has been suggested by model studies that swell formation may occur in a mobile layer subjected to faulting where the fault throw is less than the mobile layer thickness.
Abstract: It has been suggested by model studies that swell formation may occur in a mobile layer subjected to faulting where the fault throw is less than the mobile layer thickness. In such a case in the real earth, differential overburden loading due to deposition or erosion at a later stage will cause a mobile layer (e.g. salt rock) to flow from the region of higher to the region of lower load stress via the fault zone, resulting in swell formation on the upthrow side of the fault. Two different examples of this effect are illustrated in marine seismic sections from the North Sea. Associated with one of these examples are seismic events which appear to indicate mass movement of sediment down the fault scarp prior to the deposition of the main body of Zechstein halite at this location.
TL;DR: In this paper, a theoretical analysis of the scattering of high-frequency (HF) electromagnetic waves from rough surfaces is proposed, when applied to a model for the ocean surface, leads to new predictions of its second-order backscattered radar cross section in addition to those provided by existing theories.
Abstract: A theoretical analysis of the scattering of high-frequency (HF) electromagnetic waves from rough surfaces is proposed. The analysis, when applied to a model for the ocean surface, leads to new predictions of its second-order backscattered radar cross section in addition to those provided by existing theories. Some of the predictions of the theory have already been verified experimentally.
TL;DR: In this paper, the effects of currents on wave statistics are investigated and the influence of current-induced changes to a typical scatter diagram is shown, in the absence of wave refraction, an adverse current (one opposing the direction of wave propagation) is seen to increase the range of H s in the scatter diagram whilst reducing the spread in mean zero-upcrossing wave period, T z.
TL;DR: In this article, the Eulerian water particle velocities under paddle-generated regular waves in a closed channel are not always accurately predicted by a conventional application of Stokes' first, second or fifth order wave theory.
Abstract: The Eulerian water particle velocities under paddle-generated regular waves in a closed channel are not always accurately predicted by a conventional application of Stokes’ first, second or fifth order wave theory. Phenomena, which give rise to errors, include mass-transport, the partial clapotis formed by reflection from the spending beach and the free second harmonic wave produced by the sinusoidal motion of the paddle. Measurements taken with a laser doppler anemometer indicate that the amplitudes predicted for the second harmonics of the velocity components can be over 100% in error. Furthermore, there is a mean horizontal velocity which is often greater than the amplitude of the second harmonic and can be 20% of the amplitude of the first harmonic. The observed mean velocities are compared with predictions based on Longuet-Higgins’ conduction solution. Because the design of a wave facility influences the particle kinematics, it is concluded that local empirical data will generally be required to achieve accurate predictions.
TL;DR: In this article, a two-year series of directional wave measurement off the Eastern Mediterranean coast of Israel reveals an abundance of high storm waves, some of which have significant height in excess of 5 meters and periods as long as 15 seconds.
01 Jan 1985
TL;DR: The tohoku model as mentioned in this paper is a hybrid model that combines a single-parameter growth equation for wind wave with swell components and includes an interchange between swells and wind waves.
Abstract: The tohoku model is a hybrid model, since it combines a single-parameter growth equation for wind wave with swell components and includes an interchange between swells and wind waves.
TL;DR: In this article, the problem of shoals and sand spits at harbor and river mouths is considered and reflected waves interact with the incident component to form a short crested wave system that suspends sediment and aids its transmission downcoast.
01 Jan 1985
TL;DR: The Hybrid Parametric (hypa) surface wave prediction model for deep water was developed in Sonderforschungsbereich 94 of the University of Hamburg as mentioned in this paper, which includes, besides parametric prediction of the one-dimensional windsea spectrum, an additional prediction parameter, the mean windsea direction.
Abstract: The Hybrid Parametrical (hypa) surface wave prediction model for deep water was developed in Sonderforschungsbereich 94 of the University of Hamburg. This model is an extension of the earlier norswam model (Gunther et al., 1979a, b; Ewing et al., 1979). It includes, besides parametric prediction of the one-dimensional windsea spectrum, an additional prediction parameter, the mean windsea direction (Gunther et al., 1981). The swell propagates on prescribed characteristics for different frequency and direction bins in a way similar to that in the norswam model. In the following sections the three parts of the model (windsea, swell, and windsea—swell exchange) are described.
TL;DR: In this article, the forces exerted by non-breaking, normally-incident water waves on a sloping sea wall are investigated within the framework of linearised potential theory and the solution is in the form of an eigenfunction expansion, the coefficients of which are found by two methods.
Abstract: The forces exerted by non-breaking, normally-incident water waves on a sloping sea wall are investigated within the framework of linearised potential theory. The slope of the sea wall is assumed to be large. The solution is in the form of an eigenfunction expansion, the coefficients of which are found by two methods. The first is a perturbation scheme based on the smallness of the reciprocal of the slope and is carried out to second order in this quantity. The second is a Galerkin technique. Results are presented for the case of a planar, outward-sloping sea wall. In shallow water it is found that the normal wave force decreases as the slope of the wall increases. In deep water, the reverse is true whilst in water of intermediate depth the normal wave force is only weakly dependent upon the slope of the sea wall.
01 Jan 1985
TL;DR: In this article, the current velocity spectra were scaled with the friction velocity of the wind and the distance from the mean water surface, and the low frequency part of the spectra revealed an approximate similarity irrespective of the existence of wind waves.
Abstract: Winds, wind-generated waves and drift currents were measured in a wind-wave tank both for pure water (ordinary tap water) and for the water containing a surfactant. For the latter no wind waves were generated within a range of the wind speed in the experiment. If the current velocity spectra were scaled with the friction velocity of the wind and the distance from the mean water surface, the low frequency part of the spectra revealed an approximate similarity irrespective of the existence of wind waves.