Showing papers on "Swell published in 1999"

Seasonal variability of the global ocean wind and wave climate

Abstract: A data set spanning a period of 10 years and obtained from a combination of satellite remote sensing and model predictions is used to construct a global climatology of ocean wind and wave conditions. Results are presented for: significant wave height, peak and mean wave period and wave direction as well as wind speed and direction. The results are presented in terms of mean monthly statistics. The processed data set provides global resolution of 2°. The climatology clearly shows the zonal variation in both wind speed and wave height, with extreme conditions occurring at high latitudes. The important role played by the intense wave generation systems of the Southern Ocean is clear. Swell generated from storms in the Southern Ocean penetrates throughout the Indian, South Pacific and South Atlantic Oceans. During the Southern Hemisphere winter, this swell even penetrates into the North Pacific. The results confirm visual observations that the Southern Ocean is consistently the roughest ocean on earth. It is shown, however, that this is mainly caused by consistent high wind speeds, rather than the extended westerly fetch which exists. The west coasts of most continents have noticeably rougher wave climates than their respective east coasts, as a result of the longer generation fetches which exist on the west coasts. Copyright © 1999 Royal Meteorological Society.

A case study of air-sea interaction during swell conditions

Abstract: Air-sea interaction data from a situation with pronounced unidirectional swell have been analyzed. Measurements of turbulence at three levels (10, 18, and 26 m above mean sea level) together with directional wave buoy data from the site Ostergarnsholm in the Baltic Sea were used. The situation, which lasted for ∼48 hours, appeared in the aftermath of a gale. The wind direction during the swell situation turned slowly within a 90° sector. Both during the gale phase and the swell phase the over-water fetch was >150 km. The wind speed during the swell phase was typically 4 m s−1. During the swell phase a wind maximum near or below the lowest wind speed measuring level 10 m was observed. The net momentum flux was very small, resulting in CD values ∼0.7 × 10−3. Throughout the lowest 26 m, covered by the tower measurements, turbulence intensities in all three components remained high despite the low value of the kinematic momentum flux -u′w′¯ resulting in a reduction of the correlation coefficient for the longitudinal and vertical velocity from its typical value around −0.35 to between −0.2 and 0 (and with some positive values at the higher measuring levels), appearing abruptly at wave age c0/U10 equal to 1.2. Turbulence spectra of the horizontal components were shown not to scale with height above the water surface, in contrast to vertical velocity spectra for which such a variation was observed in the low-frequency range. In addition, spectral peaks in the horizontal wind spectra were found at a frequency as low as 10−3 Hz. From a comparison with results from a previous study it was concluded that this turbulence is of the “inactive” kind, being brought down from the upper parts of the boundary layer by pressure transport.

On momentum flux and velocity spectra over waves

Abstract: Data from a research tower in Lake Ontario are used to study the validity of Monin--Obukhov scaling in the marine atmospheric boundary layer under various wave conditions. It is found that over pure wind seas, the velocity spectra and cospectra follow established universal scaling laws. However, in the presence of swells outrunning weak winds, velocity spectra and cospectra no longer satisfy universal spectral shapes. Here, Monin–Obukhov similarity theory, and the classical logarithmic boundary layers, are no longer valid. It is further shown that, in the presence of such swells, the momentum flux can be significantly modified in comparison to pure wind sea values. The implications of these findings for bulk flux estimations and on the inertial dissipation method for calculating fluxes are discussed.

Wind Sea Growth and Dissipation in the Open Ocean

Abstract: Wind sea growth and dissipation in a swell-dominated, open ocean environment is investigated to explore the use of wave parameters in air–sea process modeling. Wind, wave, and whitecap observations are used from the Gulf of Alaska surface scatter and air–sea interaction experiment (Critical Sea Test-7, Phase 2), conducted 24 February through 1 March 1992. Wind sea components are extracted from buoy directional wave spectra using an inverted catchment area approach for peak isolation with both wave age criteria and an equilibrium range threshold used to classify the wind sea spectral domain. Dimensionless wind sea energy is found to scale with inverse wave age independently of swell. However, wind trend causes significant variations, such as underdeveloped seas during rising winds. These important effects are neglected in wind-forced air–sea process models. The total rate of wave energy dissipation is conveniently estimated using concepts from the Phillips equilibrium range theory. Replacing wind ...

Changes of wind waves in the North Atlantic over the last 30 years

Abstract: In order to evaluate long-term climatic changes in wind wave height, visual wave estimates available from the Comprehensive Ocean–Atmosphere Data Set (GOADS) were updated for the period from 1964 to 1993. Analysis of the accuracy of visual estimates shows that observations from merchant ships can be used for the study of climate changes in storminess. Climate changes obtained in significant wave height, computed on the basis of the voluntary observed data, are quite consistent with those shown by the instrumental records at OWS L, Seven Stones Light Vessel and NDBC buoys. The linear trends in significant wave height, as well as in the wind sea and swell heights, were computed for the entire North Atlantic. Significant wave height increases of 10–30 cm/decade over the whole of the North Atlantic, except for the western and central subtropics were found. Changes in the swell height are very consistent with those seen in significant wave height. Nevertheless, wind sea indicates strong upward tendencies only in the central mid-latitudinal North Atlantic and does not show any significant trends in the Northeast Atlantic, where instrumental records of Bacon and Carter report secular changes of about 1% a year. Wind waves of smaller occurrences show significantly negative changes in the Northeast Atlantic; that is in agreement with the wind sea periods changes. Possible mechanisms driving the swell changes with no pronounced increase of the sea height and wind velocity are discussed. Changes in the intensities of intramonthly variability in different synoptic ranges are considered as the major agent of the increasing swell. The conclusion is made that the upward swell changes are driven by the intensification of high frequency synoptic processes. Copyright © 1999 Royal Meteorological Society

Ocean wave height determined from inland seismometer data: Implications for investigating wave climate changes in the NE Pacific

Abstract: Knowing the wave climate along the California coast is vital from the perspectives of climatological change and planning shore protection measures. Buoy data indicate that the wave climate is very similar along much of the California coast. We show that elements of the wave climate can be accurately reconstructed using near-coastal inland broadband seismometer data. Such reconstructions are possible because swell approaching the coast generates pressure fluctuations that are locally transformed into seismic waves at the seafloor that propagate inland and are detectable by land-based seismometers. Buoy and seismometer data show that most of the microseism energy recorded inland near the coast is generated from wave events at nearby coastal locations. A site-specific, empirically derived seismic-to-wave transfer function is demonstrated to be applicable to seismic data from the same location for any year. These results suggest that ocean wave heights estimated from near-coastal broadband seismometer data are sufficiently reliable for monitoring the coastal wave height when buoy data are unavailable, provided that adequate simultaneous nearby buoy measurements are available to calibrate the seismometer data. The methodology presented here provides an important tool that allows the investigation of potential wave climate changes from reconstructions using archived seismic data collected since the 1930s.

Directional spreading of waves in the nearshore

Abstract: Observations of surface gravity waves shoaling between 8-m water depth and the shoreline on a barred beach indicate that breaking results in an increase in the directional spread of wave energy, in contrast to the directional narrowing with decreasing depth predicted by refraction theory (Snell's law). During low-energy wave conditions, when breaking-induced wave energy losses over the instrumented transect are small, the observed mean propagation direction and spread about the mean both decrease with decreasing depth, consistent with the expected effects of refraction. Nonlinearity causes high-frequency components of the spectrum to become directionally aligned with the dominant incident waves. During high-energy wave conditions with significant wave breaking on the sand bar, the observed mean directions still decrease with decreasing depth. However, the observed directional spreads increase sharply (nominally a factor of 2 for values integrated over the swell-sea frequency range) between the outer edge of the surf zone and the crest of the sand bar, followed by a decrease toward the shoreline. Observations on a nonbarred beach also show directional broadening, with spreads increasing monotonically from the outer edge of the surf zone to a maximum value near the shoreline. Although the mechanism is not understood, these spatial patterns of directional broadening suggest that wave breaking causes significant scattering of incident wave energy into obliquely propagating components.

Observations of the Scale and Occurrence of Breaking Surface Waves

Abstract: Breaking of surface waves was monitored with conductivity measurements at wind speeds up to 18 m s−1. This method of wave breaking detection is well defined but excludes microbreakers and breaking of very short gravity waves. Observations in both fetch limited and open ocean conditions reveal that wind speed or wave age are insufficient to characterize breaking activity. A scaling of the breaking frequency based on wind energy input is proposed. This scaling collapses the authors’ diverse datasets, consistent with energy dissipation being determined primarily by the high frequency tail of the wave spectrum. Breaking waves with significant air entrainment were observed to have wavelengths between ∼0.1 of the dominant waves and that of the largest wind waves. The median value of the period of breaking waves is approximately half the period of the dominant waves and the mean height of breaking waves is ∼0.7 times the significant wave height. Less than 10% of observed breaking events resulted in deep...

Evidence for the Effects of Swell and Unsteady Winds on Marine Wind Stress

Abstract: Over the past four decades much effort has been directed toward determining a parameterization of the sea surface drag coefficient on readily measurable quantities, such as mean wind speed and atmospheric stability. Although such a parameterization would have obvious operational advantages, the considerable scatter present between experiments, or within any one experiment, indicates that it is not easily achievable. One likely candidate for much of the scatter is the underlying wave field. Unfortunately, few campaigns over the years have included spectral measurements of the waves. Among those that have, the results are inconclusive. Here data are presented from the Surface Wave Dynamics Experiment and High Resolution Remote Sensing Program campaigns in which 3-m discus buoys were instrumented with K-Gill and sonic anemometers and complete motion packages to measure the direct (eddy correlation) stress and, concurrently, the directional ocean wave spectrum. These data are examined for the effects of swell on the drag coefficient. It is found that much of the scatter in the drag coefficient can be attributed to geophysical effects, such as the presence of swells or nonstationary conditions.

Assessment of the relative importance of major sediment transport mechanisms in the central Great Barrier Reef lagoon

Abstract: The delivery, flux and fate of terrigenous sediment entering the Great Barrier Reef lagoon has been a focus of recent studies and represents an ongoing environmental concern Wave‐induced bed stress is the most significant mechanism of sediment resuspension in the Great Barrier Reef, and field data and mathematical modelling indicates that the combined effects of short‐period wind waves, longer period swell waves, and tidal and wind‐driven currents can often exceed the critical bed stress for resuspension Suspended‐sediment concentrations at 20 m water depth indicate resuspension seldom occurs on the middle shelf under normal wave conditions Non‐cyclonic turbidity events are generally confined to the inner shelf The wave climate in the southern sector of the central Great Barrier Reef lagoon is the most erosive, and resuspension of outer shelf sediments was hindcast for recorded cyclones Wind‐driven, longshore currents are fundamental to the northward movement of sediment, and the annual northward mas

Bound waves and bragg scattering in a wind-wave tank

Abstract: We present optical and microwave measurements that show the presence of bound waves traveling at the speed of the dominant wave in a wind-wave tank. We suggest that when these bound waves are much shorter than the dominant waves, they are preferentially located on the leeward face of the dominant wave and hence have a mean tilt. We hypothesize that the turbulence associated with these bound waves suppresses freely propagating, wind-generated waves where bound waves are present so that we may divide the rough water surface into patches containing free and patches containing bound waves. This model is shown to account for the observed histograms of slope measured in the tank and, at least qualitatively, for the observed decrease in the probability of finding bound waves with increasing wind speed. Furthermore, if we add these bound, tilted waves to the free waves of the standard Bragg/composite-surface scattering model for microwave scattering from rough water surfaces, then the model can account for many otherwise unexplained features of the scattering. Principal among these features are the rapid decrease in polarization ratio and rapid increase in the first moment of the microwave Doppler spectrum with increasing wind speed when the antenna is directed upwind, features that occur to a much lesser extent when the antenna looks downwind.

Turbulent shear flow over fast-moving waves

Abstract: We divide the interaction between wind and ocean surface waves into three parameter regimes, namely slow, intermediate and fast waves, that are distinguished by the ratio c/u. (c is the wave phase speed and u. is the friction velocity in the wind). We develop here an analytical model for linear changes to the turbulent air flow caused by waves of small slope that is applicable to slow and to fast waves. The wave-induced turbulent shear stress is parameterized here with a damped mixing-length model, which tends to the mixing-length model in an inner region that lies close to the surface, and is then damped exponentially to zero in an outer region that lies above the inner region. An adjustable parameter in the damped mixing-length model controls the rate of decay of the wave-induced stress above the inner region, and shows how the results vary from a model with no damping, which corresponds to using the mixing-length model throughout the flow, to a model with full damping, which, following previous suggestions, correctly represents rapid distortion of the wave-induced turbulence in the outer region. Solutions for air flow over fast waves are obtained by analysing the displacement of streamlines over the wave; they show that fast waves are damped, thereby giving their energy up to the wind. There is a contribution to this damping from a counterpart of the non-separated sheltering mechanism that gives rise to growth of slow waves (Belcher & Hunt 1993). This sheltering contribution is smaller than a contribution from the wave-induced surface stress working against the orbital motions in the water. Solutions from the analysis for both slow and fast waves are in excellent agreement with values computed by Mastenbroek (1996) from the nonlinear equations of motion with a full second-order closure model for the turbulent stresses. Comparisons with data for slow and intermediate waves show that the results agree well with laboratory measurements over wind-ruffled paddle-generated waves, but give results that are a factor of about two smaller than measurements of purely wind-generated waves. We know of no data for fast waves with which to compare the model. The damping rates we find for fast waves lead to e-folding times for the decay of the waves that are a day or longer. Although this wind-induced damping of fast waves is small, we suggest that it might control low-frequency waves in a fully-developed sea.

Image forming device

Abstract: PROBLEM TO BE SOLVED: To eliminate the vanishing of a lateral line and to enhance printing quality by lowering the turning-on duty of a laser beam on a side affected by the other beam distribution being different from the main beam distribution on the surface of a photoreceptor SOLUTION: The laser light beam is emitted toward the photoreceptor 17 from the laser scanner unit 18 of a base 1a and the beam profile thereof in a sub-scanning direction on the surface of the photoreceptor 17 is measured It is measured whether the beam profile is provided with swell other than the main beam distribution on an upstream side or a downstream side in the sub- scanning direction or not Then, the turning duty of a white dot on the side affected by the swell is controlled so as to be lowered That means, when the swell exists on the upstream side in the sub-scanning direction, such possibility that even a black dot part is exposed by the effect of the swell of the next white dot which is exposed and the thinning of the dot is caused in the case that the dot of a line just before the white dot is the black dot, that means, the dot which is not exposed Then, the turning-on duty of the beam is lowered in the case of the exposing dot of the line just after the dot which is not exposed

Air-sea exchange : physics, chemistry and dynamics

Abstract: Preface. 1. Historical Perspective G.L. Geernaert. 2. Theory of air-sea momentum, heat and gas fluxes G.L. Geernaert. 3. The leading edge of turbulence instrumentation S. Oncley. 4. Dynamical coupling of surface waves with the atmosphere V.K. Makin, V.N. Kudryavtsev. 5. Effect of surface gravity waves on near-surface atmospheric turbulence T. Hara, et al. 6. The budget of turbulence kinetic energy in the marine atmospheric surface layer J.M. Wilczak, et al. 7. The marine atmospheric boundary layer during swell, according to recent studies in the Baltic Sea A. Smedman, et al. 8. Flux measurements at the Noordwijk Platform W. Oost. 9. Exchange measurements above the air-sea interface using an aircraft C.A. Vogel, T.L. Crawford. 10. The coastal zone L. Mahrt. 11. Footprints of atmospheric phenomena in synthetic aperture radar images of the ocean surface: a review P.D. Mourad. 12. The droplets produced by individual bubbles bursting on a sea water surface D.E. Spiel. 13. Sea spray production and influence on air-sea heat and moisture fluxes over the open ocean E.L. Andreas, J. Decosmo. 14. Modelling the interaction between the atmospheric boundary layer and evaporating sea spray droplets J. Kepert, et al. 15. Physical and chemical processes governing fluxes and flux divergence of gaseous ammonia and nitric acid in the marine atmospheric boundary layer L.L. Sorensen. 16. Engineering spectra over water J. Mann. 17. Identifying coherent structures in the marine atmospheric boundary layer H. Shirer, et al. 18. Turbulent fluxes and coherent structures in marine boundary layers: investigations by large-eddy simulation J.C. McWilliams, P.P. Sullivan. 19. Doppler sonar observations of Langmuir circulation J.A. Smith. 20. Future directions G.L. Geernaert. Index.

First results from the Hawaiian SWELL pilot experiment

Abstract: In a year-long pilot experiment to the southwest of the Hawaiian Islands, we recorded teleseismic intermediate-period Rayleigh waves on ocean-bottom “L-CHEAPO” instruments using differential pressure gauges as sensors. We analyzed over 70 events and obtained accurate phase velocity estimates at periods between 15 and 70s. The average seismic structure beneath the pilot array does not deviate significantly from a standard seismic model of 100 Myr old oceanic lithosphere. However, we find strong lateral velocity variations across the array with large negative anomalies appearing within roughly 300km of the island chain. We are able to image the edge of the Hawaiian Swell and hence demonstrate the importance of making phase velocity measurements on the ocean floor.

Numerical simulations of annular extrudate swell using various types of viscoelastic models

Abstract: In this study, numerical simulations of annular extrudate swell of high density polyethylene (HDPE) were carried out. Some important viscoelastic models, such as the Larson, the PIT and the K-BKZ model, were employed for the swell calculation through various types of dies. These numerical results were compared with the experimental ones. The numerical results of the swell behaviors were very different in each viscoelastic model, while their simple shear flow characteristics were almost the same. As a result, the uniaxial elongational viscosity for large deformation as the steady state region is not important, but the property for relatively small deformation has remarkable effects on the numerical results for the die that have a uniaxial contraction region. Both reversible and irreversible types were tried for the Larson model. It was found that there was a difference between the irreversibility assumption of the K-BKZ model and the Larson model. While there was a serious difference in the response for reversing strain among these viscoelastic models, the response was very important to predict annular extudate swell behavior.

Effect of die gap width on annular extrudates by the annular extrudate swell simulation in steady-states

Abstract: We calculated the steady-state annular extrudate swell of polymer melts through flow geometries encountered in processes used to control parison thickness. A streamline-upwinding finite element method with an under-relaxation for the rate of deformation tensor was used. The Giesekus model was employed as the constitutive equation. An operation that widens the die gap is appropriate for the control of parison thickness corresponding to the change of die gap width. However, a control process that decreases the die gap width is not useful, because the parison thickness does not correspond to the die gap width. Furthermore, thickness swells change strikingly with the Weissenberg number. It is difficult to control the parison outer diameter in the case of a converging die, because the change of the outer diameter swell becomes large with increasing Weissenberg number. In the case of a diverging die, the changing value of the outer diameter swell is smaller than that in the case of a converging die.

Spatial and temporal structures of suspension and transport over megaripples on the shore face

Abstract: Megaripples of 0.10–0.15 m height and 0.75–0.9 m wavelength occurred under swell wave activity on a macrotidal beach in southern Cornwall, England. Megaripples are relatively common features on the shore face in wave-dominated environments, although their occurrence is not well predicted by bed form models; of the four bed form models examined [Clifton, 1976; Mogridge et al., 1994; Nielsen, 1981; Wiberg and Harris, 1994], only that of Clifton [1976] predicted the formation of megaripples. The average suspended sand concentration profiles (measured using acoustic backscatter) differed considerably, in absolute magnitude and vertical gradient, depending on whether the measurements were made over the crest or trough of the bed form. The temporal structure of the suspension also varied with position relative to the megaripple. Utilizing the motion of the bed form beneath the instruments, patterns of suspension over the megaripple were calculated as a function of the phase of the wave. Sand transport was computed from the cospectra of the suspended concentration with an electromagnetic current meter at 10 cm from the seabed. Transport was dominantly at the infragravity frequencies and was consistently directed offshore, while the transport associated with the swell waves was smaller and varied with position over the megaripple; the motion of the megaripple itself was toward the shore.

Footprints of Atmospheric Phenomena in Synthetic Aperture Radar Images of the Ocean Surface: A Review

Abstract: Measurements of satellite-based, vertically polarized radar backscatter from the ocean surface translate via empirical formulae into wind speed and direction. (Early references include Jones and Schroeder (1977), Jones et al. (1982), and Schroeder et al. (1982). See Stoffelen (1998) and its references for the various “CMOD” algorithms that relate wind speed and backscatter for C-band (5.3 GHz) scatterometers.) This relationship exists because the wind roughens the water surface via the production of gravity-capillary waves (Dorman, Mollo-Christensen 1973; Kahma, Donelan 1987; Caulliez et al. 1998) which, in turn, effectively backscatter radar signals via Bragg scattering for grazing angles between 20° and 70° (Plant 1990). Gravity-capillary waves may also be generated by the crumpling of the front of wind-driven gravity waves that are near breaking. (Jessup et al. 1997) reviews the literature on gravity capillary waves and also offers infrared images of their microscale breaking.) These waves can be an additional significant source of direct C-band radar backscatter for grazing angles between 60° and 10° (Plant 1997) as well as at smaller grazing angles (Smith et al. 1996). They can also induce multiple scattering (Trizna, Carlson 1996; Trizna 1997). Small-scale bores created by microscale breaking are also a significant source of backscatter at low grazing angles (Trizna 1997). The crumpling waves are a source of radar backscatter independent of local, short-term wind conditions. So are wind-driven gravity waves and swell, which modulate the gravity-capillary wave field, thereby producing significant variations in radar backscatter (Donelan, Pierson 1987).

Seasonal Evolution of Shoreface and Beach System Morphology in a Macrotidal Environment, Dunkerque Area, Northern France

Abstract: Differential maps have been digitalized and compared from successive bathymetric and topographic surveys performed under different meteorological conditions from May 1992 to December 1994 in the shoreface-to-beach system of the 30 km long coastal area extending from Gravelines to the French-Belgian border in southernmost North Sea. The data, which concern a low-relief sandy coastal system exposed to a macrotidal regime with flood-driven eastward sediment transport, have been interpreted according to meteorological, hydrodynamical and aerodynamical characteristics. A balanced sediment budget was observed, which contradicts previous data suggesting a progressive long-term erosional trend. This result indirectly underlines the key-role on the sedimentary budget of exceptional events such as severe storms. The two types of erosion identified comprise the action of frontal waves and the combined interaction of tidal currents and wind. Five hydrodynamical cells have been recognized along the shoreface. The corresponding segmentation is attributed to the specific distribution of the wave energy along the coast due to the presence of submarine banks responsible for the deformation of the wave propagation. The morphological changes of the beach depend on the tide and swell action responsible for the construction/migration/destruction of the ridge and runnel system.

Test of sediment initial-motion theories using irregular-wave field data

Abstract: Sediment initial-motion theories predict the flow conditions that coincide with onset of sediment motion, but usually the most that can be determined from burst-sampled field data is whether or not sediment was in motion. Furthermore, initial-motion theories are usually based on regular-wave laboratory data, with little or no guidance given on how such theories are to be transferred to the real world of irregular waves. A field dataset obtained from the zone of wave shoaling beyond the surfzone is used to compare the performance of three initial-motion theories in a way that takes explicit account of wave irregularity and the limitations of burst sampling. The dataset comprises video images of the seabed and measurements of waves, currents and suspended sediment. The initial-motion theories tested were Komar & Miller’s (1975, J. Sedim. Petrol., 45, 362–367) ‘wave-orbital-speed’ theory, a ‘wave-stress’ theory, and a ‘wave-plus-current-stress’ theory. Fourteen transitions from no sediment motion to sediment motion were observed, but not all of those represented a challenging test of theory. Using the criterion that the best theory is the one that minimizes errors in classifying bursts as ‘no sediment motion’ or ‘sediment in motion’, Komar & Miller’s (1975) theory was found to perform the best when waves were characterized by significant wave height and mean spectral period, both of which can be estimated directly from pressure data alone. Komar and Miller’s theory was better at predicting the onset of extended transport events, which occurred during confused seas, rather than the transition back to moribund seabed under clean swell at the end of such events. The conclusion regarding best theory is dependent on the choice of scales used to characterize wave motion. The way forward therefore is to standardize and define terms precisely.

Prediction of potential vertical swell of expansive soils using a triaxial stress path cell

Abstract: Preconstruction treatment or the selection and design of a foundation system both rely on accurate estimates of the potential heave of the supporting expansive soil. The majority of volume change testing of expansive soils has been performed under one-dimensional loading conditions in the oedometer. However, due to differences between laboratory test constraints and field conditions, the amount of volume change measured in various oedometer testing methods may differ dramatically from heave observed in the field. This paper presents the results of an experimental investigation in which the feasibility of using a stress path triaxial cell for evaluating the vertical swell of expansive soils under multi-dimensional loading conditions was examined. Several series of triaxial swell tests were conducted in which the influence of confinement on the predicted vertical swell was evaluated. The results of these tests were compared with the volume changes observed for samples tested under identical initial conditions in the oedometer. The applicability of the triaxial testing technique was further ascertained by predicting surface heaves observed in an instrumented field test. The triaxial swell tests provided reasonable estimates of the measured field heaves in comparison to the oedometer tests which yielded rather conservative predictions.

Beach Recharge Design and Bi-Modal Wave Spectra

Abstract: Recent field and laboratory research indicates the potential importance of complex wave spectra (combining swell and wind sea) in the design of gravel beach recharge schemes. The paper discusses the research and introduces a swell wave atlas as an aid to understanding the occurrence probability of complex wave conditions around England and Wales.

Marine self potential exploration

Abstract: Recent marine self potential (SP) measurements south of Eyre Peninsula, South Australia and Rose Canyon, San Diego, California have been made In both cases, a series of horizontal electrode dipoles were towed close to the seafloor in water depths of up to 100 m to measure the electric potential gradients generated by mineralisation beneath the seafloor A proton-precession magnetometer was also towed at the surface in the Eyre Peninsula experiment Marine SP measurements show significantly lower noise levels than land measurements, due to the uniform marine environment and low contact resistance of the electrodes, so that anomalies of less than a few tens of microvolts per metre can be detected The major source of noise is from ocean swell and waves, which may be minimised by coherent stacking of signals, and by bandpass filtering South of Eyre Peninsula SP electric field anomalies of 100 µV/m and width 2 km were observed in a number of traverses perpendicular to the trend of an onshore mylonite zone Little correlation exists between the SP and magnetic data, suggesting that the SP sources are probably due to non-ferrous minerals such as graphite, and/or from the electrokinetic effect of groundwater flow through the fracture zone

Observations on waveforms of capillary and gravity-capillary waves

Abstract: Due to extreme conditions in the field, there has not been any observational report on three-dimensional waveforms of short ocean surface waves. Three-dimensional waveforms of short wind waves can be found from integrating surface gradient image data (Zhang 1996a). Ocean surface gradient images are captured by an optical surface gradient detector mounted on a raft operating in the water offshore California (Cox and Zhang 1997). Waveforms and spatial structures of short wind waves are compared with early laboratory wind wave data (Zhang 1994, 1995). Although the large-scale wind and wave conditions are quite different, the waveforms are resoundingly similar at the small scale. It is very common, among steep short wind waves, that waves in the capillary range feature sharp troughs and flat crests. The observations show that most short waves are far less steep than the limiting waveform under weak wind conditions. Waveforms that resemble capillary-gravity solitons are observed with a close match to the form theoretically predicted for potential flows (Longuet-Higgins 1989, Vanden-Broeck and Dias 1992). Capillaries are mainly found as parasitic capillaries on the forward face of short gravity waves. The maximum wavelength in a parasitic wave train is less than a centimeter. The profiles of parasitic wave trains and longitudinal variations are shown. The phenomenon of capillary blockage (Phillips 1981) on dispersive freely traveling short waves is observed in the tank but not at sea. The short waves seen at sea propagate in all directions while waves in the tank are much more unidirectional.