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Comparison of predicted with observed attenuation of pressure fluctuations shows that wave heights can be estimated with observations from a pressure sensor that is buried a known depth in fine sand. The attenuation of pressure fluctuations within the sand bed under unbroken shoaling waves, bores in the surf zone, and swash near the shoreline was measured with vertical stacks of buried pressure sensors. The attenuation increased with increasing frequency and depth below the bed surface, consistent with previous observations under nonbreaking waves in deeper water and with model predictions based on poro-elastic theory. In the limit of an infinitely deep soil skeleton that is much more compressible than the pore fluid, the predicted pressure fluctuations decrease exponentially with increasing burial depth, and the attenuation is independent of the sediment properties. For the fine-grained sand beds considered here, this exponential limit accurately predicts the observed attenuation.

A technique to simulate non-Gaussian time series with a desired ('target') power spectrum and bispectrum is applied to ocean waves. The targets were obtained from observed bottom pressure fluctuations of shoaling, nonbreaking waves in 2-9 m water depth. The variance (i.e., frequency integrated spectrum), skewness, and asymmetry (i.e., frequency integrated bispectrum) of the simulated time series compare favorably with the observations, even for highly skewed and asymmetric near-breaking waves. The mean lengths of groups of high waves from non-Gaussian simulated time series are closer to observed values than those from Gaussian simulations. The simulations suggest that quadratic phase coupling between waves (of different frequencies) in shallow water results in longer wave groups than occur with linear, uncoupled waves having the identical power spectrum.

Extensive field observations are used to characterize seiches (periods 0.5-30 min) in three small harbors with similar surface areas ([approximately]1 [km.sup.2]), water depths (5-12 m), and swell wave climates. On the continental shelf just offshore of each harbor mouth, the energy levels of waves in the infragravity frequency band 0.002-0.03 Hz (periods 0.5-10 min) vary by more than a factor of 200 in response to comparably large variations in swell energy levels. Energy levels in this swell-driven frequency band also vary (less dramatically) in response to changes in the swell frequency and with tidal stage. Motions at longer seiche periods (10-30 min) are primarily driven by meteorological and other processes (a tsunami-generated seiche is described). As has often been observed, the amplification of seiche energy within each harbor basin (relative to energy in the same frequency band outside the harbor) varies as a function of seiche frequency, and is largest at the frequency of the lowest resonant harbor mode (i.e., the Helmholtz or grave mode). At all three harbors, the average amplification of the grave mode decreases (by at least a factor of 2) with increasing seiche energy, a trend consistent with a nonlinear dissipation mechanism such as flow separation in the harbor mouth or sidewall and bottom friction.

The roles of frequency dispersion, nonlinearity, and laminar viscosity in the evolution of long waves over distances of many wavelengths in constant water depth are investigated with numerical solutions of the Boussinesq equations. Pronounced frequency doubling and trebling is predicted, and the initial evolution to a wave shape with a pitched-forward front face and peaky crests is followed by development of a steep rear face and a nearly symmetric crest/trough profile. While reducing overall energy levels, laminar viscosity acts to prolong cycling of third moments and to inhibit the onset of disordered evolution characteristic of nonlinear, inviscid systems. Preliminary laboratory results show some qualitative similarities to the numerical simulations. However, these laboratory experiments were not suitable for detailed model-data comparisons because dissipation in the flume could not be accounted for with either laminar or quadratic damping models. More carefully controlled experiments are required to assess the importance of viscosity (and the accuracy of the Boussinesq model) in the evolution of nonlinear waves over distances of many wavelengths.

Statistics of wave groups observed for a wide range of wave heights, power-spectral shapes, and water depths are compared to the statistics predicted by both direct numerical simulation and analytic approximation of linear wave theory. Comparisons to numerical simulations show that the observed groups are not inconsistent with linear, Gaussian wave fields with the same spectra as the observations. Differences between ocean observations and linear theory are owing to statistical fluctuations in group statistics estimated with the 2.3-h-long data records. One linear analytic model accounts for correlations between two successive waves, and slightly underpredicts the average number of sequential large waves for wave fields with very narrow power spectra. A newly developed extension to Rice better accounts for multiwave correlations, but has only marginally improved accuracy for these data because wave fields with very narrow power spectra rarely occurred. Both approximations overpredict the group lengths for very broad and/or multipeaked power spectra, but are still useful because the errors are small with commonly occurring spectral shapes. Direct numerical simulations, with computational expense between that of the spectral-Kimura and extended-Rice approximations, yield the best predictions of the observed wave group statistics given a power spectrum, and also provide estimates of the statistical fluctuations of group properties about predicted mean values.

Field observations of a shoaling, nonbreaking, directionally spread wave field are simulated in a laboratory basin to determine whether laboratory artifacts cause significant distortions of the shoaling process. The laboratory wave field is measured with scaled arrays of surface-elevation sensors similar to the arrays used for the field observations. However, differences in the laboratory and field beach slopes (0.33 and 0.025, respectively) do not allow precise replication of the field conditions in the laboratory. Therefore, a nonlinear wave propagation model with no adjustable parameters (previously successfully compared to a wide range of field data) is used to show that differences between the laboratory and field data sets are caused primarily by the different beach slopes. The observations demonstrate, in agreement with the model, that it is possible to compensate partially for differences in beach slope by altering the initial conditions. With such compensation, the evolution of surface-elevation power spectra, bispectra, and skewness and asymmetry are remarkably similar in the laboratory and field. Frequency-directional spectra measured just outside the surf zone also show similar nonlinear effects in both field and laboratory data. Based on this case study, the laboratory directional wave basin appears to be useful for investigating the linear and nonlinear evolution of random, two-dimensional waves on beaches.

Discusses the similarity of statistics of the nearshore velocity field in the wind-wave frequency band estimated from an acoustic Doppler, acoustic travel time and an electromagnetic current meter. Estimates of seafloor location made with the acoustic Doppler sensors and collocated sonar altimeters; Increase in deviations from linear theory in the observed relationship between pressure and velocity fluctuations; Overview of previous studies in the surf zone.