Infragravity‐wave (periods of one‐half to a few minutes) energy levels observed for about 1 year in 8‐m water depth in the Pacific and in 8‐ and 13‐m depths in the Atlantic are highly correlated with energy in the swell‐frequency band (7‐ to 20‐s periods), suggesting the infragravity waves were generated locally by the swell. The amplification of infragravity‐wave energy between 13‐ and 8‐m depth (separated by 1 km in the cross shore) is about 2, indicating that the observed infragravity motions are dominated by free waves, not by group‐forced bound waves, which in theory are amplified by an order of magnitude in energy between the two locations. However, bound waves are more important for the relatively few cases with very energetic swell, when the observed amplification between 13‐ and 8‐m depth of infragravity‐wave energy was sometimes 3 times greater than expected for free waves. Bispectra are consistent with increased coupling between infragravity waves and groups of swell and sea for high‐energy incident waves.

Model predictions of bound (i.e,, nonlinearly forced by and coupled to wave groups) infragravity wave enryaecompared with about 2 years of observations in 8-to 13-rn depths at Imperial Beach. California, and r-% Barbers Pointm Hawaii. Frequency-directional spectra of free waves at sea and swell frequencies, estimated with a small array of four pressure sensors, are used to predict the bound wave spectra below 0.04 Hz. The predicted S• total bound wave energy is always less than the observed infragravity energy, and the underprediction increases c with increasing water depth and especially with decreasing swell energy. At most half, and usually much less. of the observed infragravity energy is bound. Bound wave spectra are also predicted with data from a single wave gage in 183-m depth at Point Conception. California, and the assumption of unidirectional sea and swell. Even with energetic swell, less than 10% of the total observed infragravity energy in 183-m depth is bound Free waves, either leaky or edge waves, are more energetic than bound waves at both the shallow and deep sites. The low level of infragravity energy observed in 183-m depth compared with 8-to 13-m depths, with similarly moderate sea and swell energy, suggests that leaky (and very high-mode edge) waves contribute less than 10% of the infragravity energy in 8-13 m. Most of the free infragravity energy in shallow water is refractively 98 sra andnriaý cTeea4 INTRODUC171ONFollowing the early observations by Munk [1949] and Tucker Infragravity waves are believed to be an important factnr in [19501 suggesting that the seaward propagating infragravity wave several nearshore processes. The purpose of this paper is to estiamplitude was at least as large as the amplitudes of the shoreward mate the contribution of bound waves to infragravity energy propagating bound wave, Longuet-Higgins and Stewart 11962] observed well outside the surf zone in depths of both -10 and speculated that the incoming bound wave somehow reflects from -200 m. Infragravity motions (typical periods of 25-200 s on the shoreline and radiates seaward as a free wave. Numerical Pacific coasts) coupled to incident wave (typical periods of models [Symonds et al., 1982) suggest that slow modulation of the 4-25 s) groups were first observed in roughly 15 m depth by breakpoint position at the group frequency results i, long-wave Munk [1949] and Tucker [19501, who showed suggestive correlaradiation seaward from the breakpoint, but laboratory results are tions between wave groups and low-frequency motions. In both inconclusive. Kosten~se [1984] measured the amplitudes of cases the infragravity wave heights were about 10% of the infragravity waves in shallow water induced by wave grouping in incident wave heights. a long wave channel with a plane beach at one end. The observed Weak nonlinear interactions between first-order free waves (sea and theoretical bound wave amplitudes agreed, but there were and swell) of nearly equal frequency is one possible mechanism of significant differences b...

Seiche measured within a small (0.6 by 0.6 km), shallow (12-m depth) harbor is dominated by oscillations in several narrow infragravity frequency bands between approximately 10 '3 and 10 '2 Hz. Energy levels within the harbor are amplified, relative to just outside the harbor in 8.5-m depth, by as much as a factor of 20 at the lowest (grave mode) resonant frequency (~ 10 '3 Hz) compared to amplifications of roughly 5 at higher resonant frequencies (~10 '2 Hz). At nonresonant frequencies, energy levels observed inside the harbor are lower than those outside. These amplifications are compared to predictions of a numerical model of seiche excited by linear, inviscid long waves impinging on a harbor of variable depth. The amplification of higher-frequency (~10'2-Hz) seiches is predicted within a factor of about 2. However, at the grave mode (10 '3 Hz), the observed amplification decreases with increasing swell and seiche energy levels, possibly owing to the sensitivity of this highly amplified mode to dissipation not included in the inviscid model. The energy levels of higher-frequency seiche within the harbor were predicted from the offshore sea and swell spectra by the ad hoc coupling of the linear model for the amplification of harbor modes with a nonlinear model for the generation of bound infragravity waves outside the harbor. The predictions are qualitatively accurate only when the swell is energetic and bound waves are a significant fraction of the infragravity energy outside the harbor. 18,210 OKIHmO ET AL.: EXCITATION OF SEICHE OBSERVED IN A SMALL HARBOR lO 1 lO o 10-• 0-2 _ ,'" • ß ß ,-ß ß -.,, ß ß -, ß _ ,, ß -,,,' ß iiI ß ß ß _ . ß ,,' ß -, _ . ,, ß ß 71 IllIll

ABSTRAC!: .E~tensive field observation~are used to characterize seiches (periods 0.5-30 min) in three small harbo~s wIth sImIlar surface areas (-I km), water depths (5-12 m), and swell wave climates. On the continental shelf Just off~hore of each~arbor mouth, the energy levels of waves in the infragravity frequency band 0.002-0.03 Hz (penods 0.5-10 mm) vary by more than a factor of 200 in response to comparably large variations in swell energ~levels. Energy levels in this swell-driven frequency band also vary (less dramatically) in response to. ch~ges~n the swell freque.ncy and with tidal stage. Motions at longer seiche periods (10-30 min) are pnmanly dnven by mete?rolo~lcal and .other processes (a tsunami-generated seiche is described). As has often been observed, the~mpltficatlOn 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 I.owest resonant harbor mode (Le.• the Helmholtz or grave mode). At all three harbors. the average ampli-fi~atlOn of t?e grav.e~od.e decreases .(by at least a factor of 2) with increasing seiche energy. a trend consistent w~th. a nonltnear diSSipatIOn mechamsm such as flow separation in the harbor mouth or sidewall and bottom fnctlOn.

Infragravity waves, motions with frequencies immediately below wind‐generated sea and swell, are believed to be radiated seaward from the surf zone, where they are excited by nonlinear interactions between waves in the sea‐swell frequency band. We describe tidal modulation of sea surface elevation variance at infragravity frequencies observed in 8–30 m water depth, a few kilometers from shore at several California sites. Infragravity spectral levels vary by as much as a factor of 10 between high and low tide, possibly because the surf zone width and beach face slope vary significantly with tidal level on the usually concave‐shaped beaches onshore of the observations. At a fixed sensor and with approximately constant sea‐swell energy, the observed infragravity energy is lowest at low tide.

Sustained, quantitative observations of nearshore waves and sand levels are essential for testing beach evolution models, but comprehensive datasets are relatively rare. We document beach profiles and concurrent waves monitored at three southern California beaches during 2001–2016. The beaches include offshore reefs, lagoon mouths, hard substrates, and cobble and sandy (medium-grained) sediments. The data span two energetic El Niño winters and four beach nourishments. Quarterly surveys of 165 total cross-shore transects (all sites) at 100 m alongshore spacing were made from the backbeach to 8 m depth. Monthly surveys of the subaerial beach were obtained at alongshore-oriented transects. The resulting dataset consists of (1) raw sand elevation data, (2) gridded elevations, (3) interpolated elevation maps with error estimates, (4) beach widths, subaerial and total sand volumes, (5) locations of hard substrate and beach nourishments, (6) water levels from a NOAA tide gauge (7) wave conditions from a buoy-driven regional wave model, and (8) time periods and reaches with alongshore uniform bathymetry, suitable for testing 1-dimensional beach profile change models.

No abstract

[1] Two existing models for bed form orientation are tested against measurements of megaripple crest line orientation from Scripps Beach. Optical time-averaged images of the surf zone sand bed at 10 min intervals are processed with an automatic crest line tracing algorithm. Flow measurements concurrent with images of the sand bed collected during 3-7 April and 17-23 July 1999 are used as input to the gross bed form normal transport model (bed forms align to maximize gross transport across crests) predicting steady state crest orientation and the defect model (bed forms reorient at a rate proportional to the differential velocity of crest line ends, or defects) predicting a time-dependent orientation. Generally poor agreement was found between predictions and measurements of crest line orientation. The gross bed form normal transport model predicted crest line normal orientation within 20°of measured orientation for 38% of records, with increased agreement coinciding with longitudinal and oblique bed forms. The defect model predicted crest line normal orientation within 20°of measurements for 22% of records when the ratio between defect and transport event timescales is assumed )1 and 14% of the records for the ratio (1. Although migration and mean sediment transport direction are assumed to coincide in both models, RMS deviation in bed form migration direction from the calculated 30 min mean sediment transport direction was found to be 38°. Additionally, RMS deviation of megaripple migration direction from crest line normal orientation was 39°.