Showing papers on "Swell published in 1997"

The Air–Sea Momentum Flux in Conditions of Wind Sea and Swell

Abstract: During the Surface Wave Dynamics Experiment, direct measurements of momentum, heat, and water vapor fluxes were obtained from a mast on the foredeck of a SWATH (small water-plane area, twin hull) ship in deep water off the state of Virginia. Directional wave spectra were obtained simultaneously from a 6- or 3-wire wave-staff array mounted at the bow of the ship. One hundred and twenty-six 17-minute runs of flux and wave data obtained with the ship steaming slowly into the wind are examined for the effects of the relative direction of the wind sea and background swell on the momentum transfer. The adequacy of the inertial dissipation method, which depends on the high-frequency turbulent fluctuations for evaluating the wind stress, is also examined for any effects of swell. The results show that the presence of counter- and cross-swells can result in drag coefficients that are much larger than the value for a pure wind sea. The eddy correlation and inertial dissipation methods for measuring wind st...

Impact of sea-breeze activity on nearshore and foreshore processes in southwestern Australia

Abstract: In coastal regions sheltered from the direct impact of swell- and storm-wave activity, locally generated wind waves, particularly those associated with strong sea-breeze activity, play a dominant role in controlling nearshore and foreshore processes. Field data collected from the Perth Metropolitan Coast (western Australia) during a typical summer sea-breeze cycle, are presented. It is demonstrated that the nearshore environment responds rapidly to an increase in wind speed (up to 12 m s −1 ) during the sea breeze, resulting in considerable changes to the nearshore hydrodynamics and morphology. Incident wave energy increased during the sea breeze and was associated with development of a wind-wave field with significant wave heights up to 0.9 m. Nearshore currents responded to this change in wave climate with the development of net offshore near-bed currents and a rapid increase in the mean longshore current from −1 to 1.0 m s −1 A 10-fold increase in suspended sediment concentration and a 100-fold increase in the longshore sand transport resulted from the effects of the sea-breeze system. Erosion of the beachface was coincident with the development of the wind-wave field. Sea breeze wave-driven water circulation also completely eroded beach cusps (wavelength 20–30 m), overwhelmed the rip current system associated with the beach cusps and suppressed the infra-gravity wave frequencies in the incident wave and swash record. The beach cusps reformed after the cessation of the sea breeze. It is demonstrated that the beachface is in a constant stage of adjustment to the incident wave energy through the diurnal sea-breeze cycle alternating between dissipative and reflective morphodynamic regimes. The results may be used to determine the impact of a medium-sized storm on the beachface. It is clear that the sea-breeze system plays a major role in controlling the nearshore and foreshore processes not only in this region, but also on other geographic locations where strong sea breezes are present.

Spectral evolution of shoaling and breaking waves on a barred beach

Abstract: Field observations and numerical model predictions are used to investigate the effects of nonlinear interactions, reflection, and dissipation on the evolution of surface gravity waves propagating across a barred beach. Nonlinear interactions resulted in a doubling of the number of wave crests when moderately energetic (about 0.8-m significant wave height), narrowband swell propagated without breaking across an 80-m-wide, nearly flat (2-m depth) section of beach between a small offshore sand bar and a steep (slope = 0.1) beach face, where the waves finally broke. These nonlinear energy transfers are accurately predicted by a model based on the nondissipative, unidirectional (i.e., reflection is neglected) Boussinesq equations. For a lower-energy (wave height about 0.4 m) bimodal wave field, high-frequency seas dissipated in the surf zone, but lower-frequency swell partially reflected from the steep beach face, resulting in significant cross-shore modulation of swell energy. The combined effects of reflection from the beach face and dissipation across the sand bar and near the shoreline are described well by a bore propagation model based on the nondispersive nonlinear shallow water equations. Boussinesq model predictions on the flat section (where dissipation is weak) are improved by decomposing the wave field into seaward and shoreward propagating components. In more energetic (wave heights greater than 1 m) conditions, reflection is negligible, and the region of significant dissipation can extend well seaward of the sand bar. Differences between observed decreases in spectral levels and Boussinesq model predictions of nonlinear energy transfers are used to infer the spectrum of breaking wave induced dissipation between adjacent measurement locations. The inferred dissipation rates typically increase with increasing frequency and are comparable in magnitude to the nonlinear energy transfer rates.

Resuspension and transport of fine sediments by waves

Abstract: Although waves are the primary cause of sediment resuspension in the nearshore zone, in existing theoretical models, long-scale currents induced by the mean wind are often taken to be the only agent for the diffusion and convection of resuspended sediments. We present here theoretical examples where waves play a direct role in all aspects of sediment transport. Details are given for the simple case where only waves are present; the wave-induced current and diffusivity are shown to be no less important than similar factors in the wind-driven current. Hence, in a comprehensive model, one should include not only the current forced directly by the mean wind, but also the current forced by waves which may or may not be forced by the local wind.

Modelling of Suspended Sediment Transport in Coastal Areas Under Waves and Currents

Abstract: A coupled hydrodynamic/suspended sediment transport model including waves and currents was developed. Four models, a nested grid circulation model (2-D and 3-D), a refraction-diffraction-dissipation wave propagation model, a wave-current bottom boundary layer model and a quasi-3-D suspended sediment concentration (SSC) model are integrated. The model system has been applied to Cleveland Bay, Australia to study SSC in January and March 1993 when two high SSC events (>100 mg l−1) were observed during an intensive field monitoring programme. The swell has been found to be the dominant suspension mechanism and thus an important factor of high SSC in the bay. The model was able to predict the high SSC events in January and March 1993. A net deposition rate in Cleveland Bay was estimated to be about 0·1 mm year−1, a figure consistent with geological evidence.

Wave river water attraction

Abstract: The present invention relates to a water ride comprising a channel (2) of water connected to a beach area (22), wherein a torrential flow of water can be released into the channel, to create a swell (50) that travels through the channel, and then turns into a solitary wave (51) that spills and breaks onto the beach. The water in the channel can also be made to flow in a uniform direction (11), due to the momentum and energy transferred from repeatedly releasing the torrential flow of water into the channel. The curvature and orientation of the beach area relative to the channel causes the waves breaking onto the beach to travel radially outward in an arc-shape, causing the waves to spill over and break, as they make their way around the beach.

Frequency‐wavenumber spectrum of wind‐generated gravity‐capillary waves

Abstract: Spatial-temporal information on wind-generated gravity-capillary waves is obtained using a scanning laser slope gauge in a wind wave flume. The results show that a significant portion of short wind wave energy may be contained in wave modes that propagate with the dominant waves rather than at their phase speeds predicted by the dispersion relation. Double-peaked directional spreading of the spectrum is observed in a certain range of wavenumber and wind stress. A close examination of such results suggests that nonlinear interactions may take place between short waves and higher harmonics of dominant waves or among short waves and enhance the growth of short waves in oblique directions.

Voltage sag/swell testing station

Abstract: A computer controllable testing and monitoring station forms short-term intervals of simulated alternating current power level disturbance, either undervoltage (sag) or overvoltage (swell). The station forms the short term voltage sags or swells so that their effects on sensitive equipment can be measured. The starting point of the sag or swell disturbance, as well as the time duration of the disturbance, can be accurately controlled.

Cross-Shore Variation of Wave Reflection from Beaches

Abstract: The cross-shore variation of the local reflection coefficient R is examined under the assumptions of normally incident wind waves on beaches with alongshore uniform bathymetry. The existing three-gauge method is modified to estimate R for the sloping bottom. A new method is developed to estimate the incident and reflected root-mean-square (RMS) wave heights from the measured RMS wave height and wave setup. These methods are used to analyze six runs from the large-scale SUPERTANK data set. The estimated values of R are affected little by the formation of a bar and increase shoreward with the increased percentage of breaking waves. One set of the DELILAH field data on a barred beach is also analyzed using collocated pressure and velocity measurements. This data set confirms the negligible effect of the bar on the variation of R over the bar in the absence of wave-breaking on the bar crest. These crude estimates of R suggest that the reflected wind waves may not be negligibly small in comparison to the incid...

Heat flow over Reunion hot spot track: Additional evidence for thermal rejuvenation of oceanic lithosphere

Abstract: We present 99 new heat flow determinations obtained with a regular sampling (2–3 km) along two seismic profiles across the Mascarene Ridge, Indian Ocean. This aseismic ridge is volcanic relief due to the activity of a hot spot, now responsible of the active volcanism of Reunion Island. Our sampling technique allows us to identify short-wavelength processes such as slope-driven water flows that affect heat flow on the ridge. With the exception of these perturbed sites, heat flow appears to be very uniform across the ridge and significantly higher (6–8 mW m−2) than the reference heat flow predicted for the seafloor age. Although this anomaly is low, it confirms other evidence for thermal rejuvenation of the lithosphere based on gravity and bathymetry. The set of information now available for the Mascarene Ridge seems to indicate a mixed mode of compensation with dynamical support below the front of the swell track and thermal support behind for the older part of the swell.

Long-period unstable gravity-waves and associated VHF radar echoes

Abstract: VHF atmospheric radar is used to measure the wind velocity and radar echo power related to long-period wind perturbations, including gravity waves, which are observed commonly in the lower stratosphere and tropopause region, and sometimes in the troposphere. These wind structures have been identified previously as either inertia-gravity waves, often associated with jet streams, or mountain waves. At heights of peak wind shear, imbalances are found between the echo powers of a symmetric pair of radar beams, which are expected to be equal. The largest of these power differences are found for conditions of simultaneous high wind shear and high aspect sensitivity. It is suggested that the effect might arise from tilted specular reflectors or anisotropic turbulent scatterers, a result of, for example, Kelvin-Helmholtz instabilities generated by the strong wind shears. This radar power-difference effect could offer information about the onset of saturation in long-period waves, and the formation of thin layers of turbulence.

X-band low-grazing-angle ocean backscatter obtained during LOGAN 1993

Abstract: This paper reviews X-band ocean microwave backscatter data from the LOGAN (LOw Grazing ANgle) experiment conducted on the Chesapeake Light Tower by the Naval Air Warfare Center. The data were collected under varied wind, sea, and swell conditions that provide some new insights into low-grazing-angle backscatter phenomena. Transient backscatter peaks called "sea spikes" have long been associated with deep-water breaking waves; however, they have yet to be fully reconciled with backscatter and hydrodynamic theories. New analysis techniques have been applied to the LOGAN data that take advantage of the unique characteristics of sea spikes and their dynamics. High-resolution Doppler spectra are organized relative to the space-time centroids of the sea-spike clusters and conditionally averaged by RCS strength. The mean Doppler variation of the strongest sea spikes then map the breaking-wave structure just as Doppler histories measured at moderate grazing angles map the dynamics of the dominant linear surface-wave components. While breaking waves are manifest to some degree in backscatter data at all grazing angles, a non-Bragg-scatter mechanism accentuates the crest scattering at low grazing angles. The phenomena potentially can be exploited for remote ocean sensing and imaging.

Numerical simulations of annular extrudate swell of polymer melts

Abstract: Numerical viscoelastic simulations were carried out using a K-BKZ type of separable integral constitutive equation. Both reversible and irreversible models were tried for several types of damping functions to calculate the annular extrudate behavior of high-density polyethylene (HDPE). There are two aims in this study; first, to clarify the properties of these dumping functions, and second, to investigate the influence of rheological characteristics on annular extrudate swell. In these numerical simulations, relaxation spectrum and shear viscosity were fixed, and the other characteristics were varied. The reversional response of the damping function mainly has an effect on the magnitude of the area swell even if the die is straight. The irreversible model expresses the experimental results of annular extrudate swell better than the reversible model. The accurate fitting of N1 by the damping model is important for predicting it. The magnitude of N1 predicted from the Wagner exponential model is lower than that of the PSM model, and the area swell shows the same tendency as N1. A modified PSM model that allows the N1 curve to shift can fit the magnitude of area swell. The relationship between the diameter and thickness of the extrudate depends on N2/N1, and it was estimated by simple linear elasticity of solids. The time dependent viscosity varies with the type of damping function, and it influences the time-dependent swell.

Experimental Study on Shipping Water Volume and its Load on Deck

Abstract: In order to develop a practical prediction method for shipping water volume, load and pressure due to deck wetness, a model test was carried out for a domestic tanker in regular head seas. Shipping water height, load and pressure were measured at various ship speed and wave steepness. As for the occurrence of deck wetness, N. S. M. calculation of relative water height at bow gives a good estimation if the bow top height is reduced by considering the static swell up.A practical prediction method for shipping water height, load and pressure was proposed. The inputs of this method are relative water height at bow, pitching angle and bow vertical velocity, those can be calculated by strip theory and so on. The theory of “flood waves” was applied for evaluating the shipping water height from the relative water height at the bow and so on. This theory gives a better estimation of the water height distribution on deck than the conventional model of “dam collapsing” because it can include the effect of ship forward speed. On the shipping load and pressure, not only the static component but also the time derivative of the momentum of the shipping water were considered. It was clarified that the momentum component is important to estimate the peak of shipping water load and pressure. Having compared with measured data, it was confirmed that the combination of these two methods is practical enough for predicting shipping water load and pressure.

Observed and modeled wave results from near-stationary hurricanes

Abstract: Wave conditions in hurricanes have been difficult to study because of a lack of high-quality wave data and poor descriptions of the wind field. In the 1994 and 1995 hurricane seasons, two Category 1 hurricanes (Gordon and Felix) approached the North Carolina coast and stalled for a period of about 2 days. Although the storms were minimal hurricanes they produced large swell that persisted for several days. A wave gauging network oftwo to five directional instruments in water depths ranging from 8 to 50 m operated throughout the storms. Because of their proximity to land, both storms were extensively observed by radar and aircraft so that the wind fields are well described. The data set offers an opportunity to evaluate two prediction methods to examine the wave field during these most unusual hurricanes.

Integral control data assimilation in wave predictions

Abstract: In the present study a technique for assimilating observed wave data in numerical wave predictions is developed that exploits (a) the efficiency of a limited number of integral control variables and (b) the effectiveness of variational (model-consistent) assimilation. The formal procedure is independent of the type of control variables and of the wave model. The integral control variables in this study are chosen to represent large-scale errors in the driving wind fields and uncertainties in the wave model. The assimilation technique is validated with observations of the ERS-1 satellite altimeter and two waverider buoys in two consecutive storms in the Norwegian Sea. The assimilation of the observations reduced the errors in the predicted significant wave height at the buoy locations typically from 25% to 15%. The technique is also demonstrated with an simulation of swell prediction in the Indian Ocean based on simulated buoy data and satellite data.

Monte carlo simulation fornearshore wave statistics in southern california

Abstract: To develop the nearshore wave condition in Southern California, six meteorological weather patterns including extratropical cyclones of the northern hemisphere, northwest winds in the outer coastal waters, west to northwest local sea, pre-frontal local sea, tropical storm swell and extratropical cyclones of the southern hemisphere were identified and classified. Wave characteristics in deep water, corresponding to the categorized weather patterns, were computed and then transferred to the nearshore target sites via a spectral back-refraction transformation model. A Monte Carlo simulation technique was applied to generate a synoptic atlas of the nearshore wave climate in this region.

Estimation of Directional Surface Wave Spectra from a Towed Research Catamaran

Abstract: During the High-Resolution Remote Sensing Main Experiment (1993), wave height was estimated from a moving catamaran using pitch-rate and roll-rate sensors, a three-axis accelerometer, and a capacitive wave wire. The wave spectrum in the frequency band ranging roughly from 0.08 to 0.3 Hz was verified by independent buoy measurements. To estimate the directional frequency spectrum from a wave-wire array, the Data-Adaptive Spectral Estimator is extended to include the Doppler shifting effects of a moving platform. The method is applied to data obtained from a fixed platform during the Riso Air–Sea Experiment (1994) and to data obtained from a moving platform during the Coastal Ocean Processes Experiment (1995). Both results show that the propagation direction of the peak wind waves compares well with the measured wind direction. When swells and local wind waves are not aligned, the method can resolve the difference of propagation directions. Using the fixed platform data a numerical test is conducte...

Wave Set-up on Coral Reefs: Some Practical Applications

Abstract: Wave set-up on coral reefs both increases water levels and creates hydraulic gradients which drive water circulation systems of reef-tops. These effects are significant factors in determining flooding and erosion of reef-protected coasts and flushing of reefal lagoons, particularly in the microtidal environments of many Indo-Pacific Island countries. A procedure for calculating wave set-up on reefs under given wave and tide conditions is outlined using the results of recently published research. Aspects considered include wave dissipation and transmission, reef-top water depths, reef profiles, irregular waves and surf beat, and run-up from reef-top waves. The procedure is applied to a specific site for both swell and storm waves.

Fractal laser glints from the ocean surface

Abstract: Time series of laser glint counts from the ocean surface exhibit fractal behavior. Glint-count histogram widths do not follow Gaussian statistics, and histogram shapes are approximately log normal. Fractal dimensions for the statistically self-similar glint-count time series are found from the power spectra, which have an inverse power-law form. Glint counts in one spatial dimension from a linearly scanning laser and glint counts in two spatial dimensions from a laser glint imager behave similarly. In both sets of data, spectral density peaks exist at frequencies corresponding to swell and long wind waves. This implies that the glint-count process contains information related to long-wave modulation of surface roughness.

The Role of Wave Diffraction in the Formation of St. Ninian's Ayre (Tombolo) in Shetland, Scotland

Abstract: St. Ninian's Ayre (sand tambala), on the south-west coast of Shetland, occurs in an environment of deeply plunging hard-rock cliffs which enclose the area containing the tombolo so that the maximum fetch of local wind-driven waves is no more than 2 km. As a result of the presence of the plunging cliffs, there are no adjacent beaches to supply sand to the tombolo by longshore drift. The sea around St. Ninian's Island, at the seaward end of the tambala is so deep that wave refraction in the vicinity is restricted. Thus, the various explanations usually offered for the origin of tombolos are precluded or incomplete. Satellite image and airphotos show that wave patterns in the vicinity of the tombolo result from the prevailing ocean swell from the west being diffracted round the island, on either side. As the sea shallows, the diffracted swell is increasingly modified by refraction, before breaking on either side of the tombolo. The shorelines on either side of the tombolo are nearly parallel to the crests of the diffracted and refracted swell-derived waves that these break along the length of the tambala at the same instant. Local wind driven waves have little effect. The tombolo owes its position and shape directly to the diffraction and refraction of the ocean-swell waves transporting sand from the adjacent sea floor into the area where the waves meet behind the island. Longshore drift acts only within the confines of the tombolo and matches its shape to that the crests of the waves break over it.

Two-dimensional focusing of sonic boom noise penetrating an air-water interface

Abstract: In the late 1960s and early 1970s research on supersonic aircraft determined that sonic boom noise would penetrate the surface of the ocean. It was assumed that the surface of the air-water interface was perfectly flat. Today, sonic boom noise is once again a topic of interest. The present work concentrates on two-dimensional focusing and defocusing, caused by ocean surface swell, of the penetrating sonic boom waveform. A finite difference algorithm is used to calculate tbe pressure levels underwater due to a rounded sonic boom waveform interacting with the water interface. Numerical results are consistent with the predictions of known theories involving 1) the existence of pressure disturbances underwater and 2) their penetration depth being a function of the aircraft's speed. These calculations also indicate that 1) the swell of the ocean surface focuses and defocuses the waveform with increasing effect as the ocean wave height increases and 2) the percent change (from a flat ocean interface to a wavy ocean interface) in pressure values due to the swell increases with increasing Mach number.

Wave transformation across the inner surf zone

Abstract: Sea and swell wave heights observed on transects crossing the mid and inner surf zone on three beaches (a steep concave-up beach, a gently sloped approximately planar beach, and a beach with an approximately flat terrace adjacent to a steep foreshore) were depth limited (i.e., approximately independent of the offshore wave height), consistent with previous observations. The wave evolution is well predicted by a numerical model based on the one-dimensional nonlinear shallow water equations with bore dissipation. The model is initialized with the time series of sea surface elevation and cross-shore current observed at the most offshore sensors (located about 50 to 120 m from the mean shoreline in mean water depths 0.80 to 2.10 m). The model accurately predicts the cross-shore variation of energy at both infragravity (nominally 0.004 < f < 0.05 Hz) and sea swell (here 0.05 < f ≤ 0.18 Hz) frequencies. In models of surf zone hydrodynamics, wave energy dissipation is frequently parameterized in terms of γs, the ratio of the sea swell significant wave height to the local mean water depth. The observed and predicted values of γs increase with increasing beach slope β and decreasing normalized (by a characteristic wavenumber k) water depth kh and are well correlated with β/kh, a measure of the fractional change in water depth over a wavelength. Errors in the predicted individual values of γs, are typically less than 20%. It has been suggested that infragravity motions affect waves in the sea swell band and hence γs, but this speculation is difficult to test with field observations. Numerical simulations suggest that for the range of conditions considered here, γs is insensitive to infragravity energy levels.

Development of the BASS rake acoustic current sensor : measuring velocity in the continental shelf wave bottom boundary layer

Abstract: : Surface swell over the continental shelf generates a sheet of oscillatory shear flow at the base of the water column, the continental shelf wave bottom boundary layer. The short periods of surface swell sharply limit the thickness of the wave boundary layer, confining it to a thin region below an oscillatory, but essentially irrotational, core. For a wide range of shelf conditions, the vertical extent of the wave boundary layer does not exceed 2.5 cm and is commonly less. The extreme narrowness of this boundary layer is responsible for high levels of bottom stress and turbulent dissipation. Even in relatively mild sea states, the wave induced bottom shear stress can be sufficient to initiate sediment motion. The wave bottom boundary layer plays an important role in the processes of sediment entrainment and transport on the continental margins. This thesis documents the development, testing, and field use of a new instrument, the BASS Rake, designed to measure velocity profiles in the wave boundary layer. The mechanical design supports multiple measurement levels with millimeter vertical spacing. The mechanical design is integrated with an electronic interface designed to permit flexible acquisition of a suite of horizontal and vertical velocity measurements without sacrificing the electronic characteristics necessary for high measurement accuracy. The effects of velocity averaging over the sample volume are calculated with a model of acoustic propagation in a scattering medium appropriate to the scales of a single differential travel time axis. A simpler parametric model of the averaging process is then developed and used to specify the transducer characteristics necessary to image the wave boundary layer on the continental shelf.

The influence of state‐of‐mix on the extrudate swell of a carbon black‐filled styrene–butadiene rubber compound

Abstract: The factors which govern the extrudate swell of a styrene–butadiene rubber compound filled with 30 phr of N330 carbon black at various states-of-mix were investigated. The state-of-mix is quantified by an effective filler volume fraction, based on an estimate of the amount of rubber immobilized in the carbon black agglomerates. The swell has been found to be dominated by recoverable strain and relaxation time, which are both controlled by the effective filler volume fraction. In contrast, shear rate, wall slip, and the rubber–carbon black network have not been found to have a significant effect on the extrudate swell. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:305–315, 1997

Acoustic waves in isothermal winds in the vicinity of the sonic point

Abstract: We study the propagation of acoustic waves incident on the base of a stellar wind and the back-reaction on the mean flow,inthesphericallysymmetric,isothermalcase,bothanalyt- icallyandviadirectsimulationsoftheNavier-Stokesequations. We consider successively the quasi-linear inviscid case and the nonlinear dissipative case (shocks). We show that wave reflec- tion is small everywhere even when the WKB approximation breaks down, and conjecture that the same result could hold for radial Alfven waves in a spherically symmetric wind. We show that, after a transient acceleration, outward propagating waves lead to a lower mean wind velocity than in the unper- turbed wind, so that the average velocity may become negative belowthesonicpoint,thedifferencewiththestandardresultthat Lagrangian-mean velocities are higher in presence of waves be- ingexplainedbythedriftbetweenreferenceframes.Wepropose thatnegativeaveragevelocitiesmightprovideatestforthepres- ence of compressive waves close to the sun. We conjecture that, for MHD fluctuations, the net effect of the wave pressure on the wind velocity depends on the importance of compressive components, and that this might play a role in the observed cor- relation between the mean solar wind velocity and the level of the compressive component in the wave spectrum.

Lithospheric thinning and chemical buoyancy beneath the Hawaiian Swell

Abstract: The source of the buoyancy that supports the Hawaiian Swell is not well understood. The swell may be supported by replacement of negatively buoyant lithosphere with buoyant mantle or by emplacement of buoyant material beneath lithosphere of normal thickness. These mechanisms can be distinguished by examining the quadratic relationship between geoid height and bathymetry. At the Hawaiian Swell, the curvature of the geoid height vs. swell topography relationship is negative, indicating that the swell is supported by thinned lithosphere. The magnitude of the curvature suggests that the mantle filling in the region of thinned lithosphere is both thermally and chemically buoyant.

Methods to obtain representative surface wave spectra, illustrated for two ports of north-western Australia

Abstract: Simple automatic methods of classifying surface wave spectra in terms of spectral shape are outlined and are used to examine the monthly wave climatology for the port approaches to Dampier and Port Hedland on the North West Shelf of Australia. Waves and swell at these shallow sites occur independently in three frequency bands. These correspond to low-energy long-period swell from distant sources (periods of 12 to 20 s), cyclones or energetic local storms (periods of 8 to 12 s), and local winds (periods of 4 to 8 s). Summer cyclones generate highest waves, but sea breezes are the dominant mode of wave generation. Waves and swell tend to occur episodically and independently in any month, and the peak frequency shifts during the wave generation process, so that a representative spectrum formed by simple averaging of spectra would be grossly oversmoothed. To overcome this, the monthly climatology is presented in terms of reference spectral shapes (the Ochi and Hubble 1976 North Atlantic formulations) that are most likely to occur for particular frequency bands and ranges of significant wave height. Measured spectra may then be selected as representative spectra from groupings associated with the most likely reference shapes, with further criteria such as spectral width being used to define what is typical. In some cases, the reference spectra provide good enough fits to enable them to be used directly as representative spectra.