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Surveys of the subaerial beach (e.g., landward of approximately the MSL depth contour) are widely used to evaluate temporal changes in sand levels over large alongshore reaches. Here, seasonal beach face volume changes based on full bathymetry beach profiles (to ~8 m in depth) are compared with estimates based on the subaerial section of the profile. The profiles span 15 years and 75 km of Southern California shoreline, where seasonal vertical fluctuations in near-shore sand levels of a few meters are common. In years with relatively low winter wave energy, most erosion occurs above the MSL contour, and subaerial surveys capture as much as 0.8 of the total (relatively small) seasonal beach face volume change. In response to more energetic winter waves, beach face erosion increases and occurs as deep as 3 m below MSL, and subaerial surveys capture as little as 0.2 of the total beach face volume change. Patchy, erosion-resistant rock and cobble layers contribute to alongshore variation of the subaerial fraction of beach face volume change. [ABSTRACT FROM AUTHOR]

Airborne light detecting and ranging (lidar) systems can survey hundreds of kilometers of shoreline with high spatial resolution (several elevation estimates per square meter). Sequential surveys yield spatial change maps of beach and dune sand levels. However, lidar data include elevations of the exposed, subaerial beach and, seaward of the waterline, the ocean surface. Here, a simple method is developed to find the waterline and eliminate returns from the ocean surface. A vertical elevation cutoff is used, with the waterline elevation (W) above the known tide level because of the superelevation from wave setup and runup. During each lidar pass, the elevation cutoff (W) is assumed proportional (C) to the offshore significant wave height Hs. Comparison of in situ and lidar surveys on a moderately sloped, dissipative California beach yields C ≈ 0.4, which is qualitatively consistent with existing observations of runup and setup. The calibrated method rejects ocean surface data, while retaining subaerial beach points more than 70 m seaward of the mean high waterline, which is often used as a conservative default waterline. [ABSTRACT FROM AUTHOR]

Previous field observations indicate that the directional spread of swell-frequency (nominally 0.1 Hz) surface gravity waves increases during shoreward propagation across the surf zone. This directional broadening contrasts with the narrowing observed seaward of the surf zone and predicted by Snell’s law for bathymetric refraction. Field-observed broadening was predicted by a new model for refraction of swell by lower-frequency (nominally 0.01 Hz) current and elevation fluctuations. The observations and the model suggest that refraction by the cross-shore currents of energetic shear waves contributed substantially to the observed broadening. [ABSTRACT FROM AUTHOR]

Inverse models are developed that use data and dynamics to estimate optimally the breaking-wave-driven setup and alongshore current, as well as the cross-shore forcing, alongshore forcing, and drag coefficient. The inverse models accurately reproduce these quantities in a synthetic barred-beach example. The method is applied to one case example each from the Duck94 and SandyDuck field experiments. Both inverse solutions pass consistency tests developed for the inverse method and have forcing corrections similar to a roller model and significant cross-shore variation of the drag coefficient. The inverse drag coefficient is related to the wave dissipation, a bulk measure of the turbulence source, but not to the bed roughness, consistent with the hypothesis that breaking-wave-generated turbulence increases the drag coefficient. Inverse solutions from a wider range of conditions are required to establish the generality of these results. [ABSTRACT FROM AUTHOR]

Compares the existing methods for estimating the magnitude of shear wave velocity fluctuations in the presence of gravity waves. Application of iterative maximum likelihood estimator (IMLE); Information on IMLE and direct estimator estimates of shear wave variance for 3-h data segment; Indication of simulations with known mixes of shear and gravity waves.

Observes alongshore gradients in wave energy and propagation direction near a pier. Field observations of alongshore gradients; Predictions on the spectral refraction model; Conclusions.

Frequent surveys using modern remote sensing technology enable observations of subaerial beach morphology and surface sediment type (sand vs. gravel/cobble) with unprecedented spatial and temporal resolution. Here we show weekly terrestrial mobile light detection and ranging (LiDAR) observations of cusps on a mixed sediment beach, consisting of sand and gravel/cobble, spanning ∼2 km alongshore. Surface gravel/cobble coverage varied from almost complete to none (e.g., all sand), over times ranging from weekly to seasonal, and with location. Cusp morphologies also varied in space and time. Cusps with 11–20 m alongshore wavelengths (L) primarily developed in winter and were composite with gravel‐cobble horns and sandy bays. Cusps with L between 21 and 40 m developed throughout the year and were either composite or entirely sandy. Previously not reported, composite mega cusps (L between 200 and 400 m) occurred in winter. Composite cusps, with L between 11 and 40 m, were clustered in gravel/cobble‐rich mega cusp bays and absent on the sandier mega cusp horns. Plain Language Summary: Improved understanding of beach morphology is needed for coastal planning and hazard mitigation. Using remote sensing‐based survey technology, this study describes about 1 year of weekly observations of cusps, semicircular alongshore rhythmic patterns, along an ∼2 km beach consisting of sands and larger grain size sediments (gravels and cobbles). Cusp shape (e.g., horizontal and vertical undulations), alongshore rhythmic spacing patterns, and surface sediment types varied in space and time. The observation revealed previously unreported, composite mega cusps with rhythmic 200–400 m spacing and surface sediment zonations (sand versus gravel/cobble). The results highlight the potential of repeated surveys with modern remote sensing technology to explore the highly dynamic and complex behavior of mixed sediment beach morphology. Key Points: Cusps composed of sand, gravel/cobble, and mixed surface sediment types with 11–400 m alongshore wavelengths were observedCusp morphology (e.g., wavelengths and amplitudes) and surface sediment types considerably vary temporally and spatiallyFrequent surveys with high spatial resolution show highly dynamic and complex behavior of mixed sediment beach morphology [ABSTRACT FROM AUTHOR]

Three distinct phytoplankton blooms lasting 4-9 d were observed in approximately 15-m water depth near Huntington Beach, California, between June and October of 2006. Each bloom was preceded by a vertical NO3 flux event 6-10 d earlier. NO3 concentrations were estimated using a temperature proxy that was verified by comparison with the limited NO3 observations. The lower--water-column vertical NO3 flux from vertical advection was inferred from observed vertical isotherm displacement. Turbulent vertical eddy diffusivity was parameterized based on the observed background (< 0.3 cycles h-1) stratification and vertical shear in the horizontal currents. The first vertical nitrate flux event in June contained both advective and turbulent fluxes, whereas the later two events were primarily turbulent, driven by shear in the lower part of the water column. The correlation between the NO3 flux and the observed chlorophyll a (Chl a) was maximum (r² = 0.40) with an 8-d lag. A simple nitrate--phytoplankton model using a linear uptake function and driven with the NO3 flux captured the timing, magnitude, and duration of the three Chl a blooms (skill = 0.61) using optimal net growth rate parameters that were within the expected range. Vertical and horizontal advection of Chl a past the measurement site were too small to explain the observed Chl a increases during the blooms. The vertical NO3 flux was a primary control on the growth events, and estimation of both the advective (upwelled) and turbulent fluxes is necessary to best predict these episodic blooms. [ABSTRACT FROM AUTHOR]

Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10–20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height Hs was equal to 0.5 m and mean alongshore currents |υ| were moderate (<0.1 m s-1). On the second day, the obliquely incident waves were larger, with Hs equal to 1.4 m, and at some surf-zone locations |υ| was greater than 0.5 m s-1. The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along- and cross-shore directions, the asymptotic diffusivity is reached sooner within, rather than seaward of, the surf zone. Two-particle statistics indicate that relative dispersion grows like D2(t) ∼ t3/2 and that the relative diffusivity is scale-dependent as μ ∼ l2/3, with l being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [D2(t) ∼ t3 and μ ∼ l4/3]. Separations have a non-Gaussian self-similar distribution that is independent of time. The two-particle statistics are consistent with a nonconstant-coefficient diffusion equation for the separation probability density functions. The dispersion is explained by neither irrotational surface gravity waves nor shear dispersion. The observations imply the existence of a 2D eddy field with 5–50-m length scales, the source of which is speculated to be alongshore gradients in breaking-wave height associated with finite crest lengths. [ABSTRACT FROM AUTHOR]

Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5-10 m thick, and propagated alongshore at ∼50 cm s[SUB-1]. The density decreased 2-3 kg m[SUB-3]over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in ∼8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s[SUB-1], respectively). The southward alongshore current increased abruptly to ∼50 cm s[SUB-1]at the nose and continued to increase to a supercritical maximum of ∼70 cm s[SUB-1] about 1 km behind the nose. An onshore flow of between 5 and 15 cm s[SUB-1], which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore flow. The observed flow structure is qualitatively similar to theoretical predictions and laboratory measurements of buoyant gravity currents propagating along a sloping bottom. [ABSTRACT FROM AUTHOR]

Beach erosion and wave-induced flooding models are often initialized in O(10)-m depth, seaward of the surfzone, with wave conditions estimated from regional nonlinear spectral wave models [e.g., Simulating Waves Nearshore (SWAN)]. These models are computationally expensive for high-resolution, long-term regional O(100)-km hindcasts, and they limit examination of the effect of different climate scenarios on nearshore processes. Alternatively, computationally fast models with reduced linear wave physics enable long-term hindcasts at high spatial (<100 m) resolution. Linear models, that efficiently transform complete spectral details from deep water through complex offshore bathymetry, are appropriate for low-frequency swell wave energy propagation. Here, two numerically different linear methods are compared: backward ray-tracing and stationary linear SWAN simulations. The methods yield similar transformations from deep water (seaward of offshore islands in Southern California) to the nearshore, O(10)-m depth. However, SWAN is sensitive to model spatial resolution, especially in highly sheltered regions, where with typical (1–2 km) resolution SWAN estimates of nearshore energy vary by over a factor of 2 relative to ray tracing. Alongshore radiation stress estimates from SWAN and ray tracing also differ, and in some cases the climatological means have opposite signs. Increasing the SWAN resolution to 90 m, higher than usually applied to regional models, yields the nearshore transforms most similar to ray tracing. Both accurate rays and high-resolution SWAN require significant computation time; however, ray tracing is more efficient if transforms are needed at relatively few locations (compared with every grid point), or if computer memory is limited. Though presently less user friendly than SWAN, ray tracing is not affected by numerical diffusion or limited by model domain size or spatial resolution. [ABSTRACT FROM AUTHOR]

Aerial images are used to quantify the concentration of fluorescent Rhodamine water tracing (WT) dye in turbid and optically deep water. Tracer releases near the shoreline of an ocean beach and near a tidal inlet were observed with a two-band multispectral camera and a pushbroom hyperspectral imager, respectively. The aerial observations are compared with near-surface in situ measurements. The ratio of upwelling radiance near the Rhodamine WT excitation and emission peaks varies linearly with the in situ dye concentrations for concentrations <20 ppb ( r2 = 0.70 and r2 = 0.85-0.88 at the beach and inlet, respectively). The linear relationship allows for relative tracer concentration estimates without in situ calibration. The O(1 m) image pixels resolve complex flow structures on the inner shelf that transport and mix tracer. [ABSTRACT FROM AUTHOR]

A beach nourishment with approximately 1/3 fine-grained sediment (fines; particle diameter <63 μm) by mass was performed at Southern California's Border Fields State Park (BFSP). The nourishment was found to briefly (<1 day) increase concentrations of surf-zone fecal indicator bacteria (FIB) above single-sample public health standards [104 most probable number (MPN)·(100 mL)-1] but had no effect on phytoplankton. Contamination was constrained to the nourishment site: waters 300 m north or south of the nourishment were always below single-sample and geometric mean [≤35 MPN·(100 mL)-1] standards. Nourishment fines were identified as a source of the fecal indicator Enterococcus; correlations between fines and enterococci were significant (p < 0.01), and generalized linear model analysis identified fines as the single best predictor of enterococci. Microcosm experiments and field sampling suggest that the short surf-zone residence times observed for enterococci (e-folding time 4 h) resulted from both rapid, postplacement FIB inactivation and mixing/transport by waves and alongshore currents. Nourishment fines were phosphate-rich/nitrogen-poor and were not correlated with surf-zone phytoplankton concentrations, which may have been nitrogen-limited. [ABSTRACT FROM AUTHOR]

A drifter designed to measure surf zone circulation has been developed and field tested. Drifter positions accurate to within a few meters are estimated in real time at 0.1 Hz using the global positioning system (GPS) and a shore-to-drifter radio link. More accurate positions are estimated at 1 Hz from postprocessed, internally logged data. Mean alongshore currents estimated from trajectories of the 0.5-m-draft drifters in 1-2-m water depth agree well with measurements obtained with nearby, bottom-mounted, acoustic current meters. Drifters deployed near the base of a well-developed rip current often followed eddylike paths within the surf zone before being transported seaward. [ABSTRACT FROM AUTHOR]

The nonlinear dispersion of random, directionally spread surface gravity waves in shallow water is examined with Boussinesq theory and field observations. A theoretical dispersion relationship giving a directionally averaged wavenumber magnitude as a function of frequency, the local water depth, and the local wave spectrum and bispectrum is derived for waves propagating over a gently sloping beach with straight and parallel depth contours. The linear, nondispersive shallow water relation is recovered as the first-order solution, with weak frequency and amplitude dispersion appearing as second-order corrections. Wavenumbers were estimated using four arrays of pressure sensors deployed in 2-6-m depth on a gently sloping sandy beach. When wave energy is low, the observed wavenumbers agree with the linear, finitedepth dispersion relation over a wide frequency range. In high energy conditions, the observed wavenumbers deviate from the linear dispersion relation by as much as 20%-30% in the frequency range from two to three times the frequency of the primary spectral peak, but agree well with the nonlinear Boussinesq dispersion relation, confirming that the deviations from linear theory are finite amplitude effects. In high energy conditions, the predicted frequency and amplitude dispersion tend to cancel, yielding a nearly nondispersive wave field in which waves of all frequencies travel with approximately the linear shallow water wave speed, consistent with the observations. The nonlinear Boussinesq theory wavenumber predictions (based on the assumption of irrotational wave motion) are accurate even within the surf zone, suggesting that wave breaking on gently sloping beaches has little effect on the dispersion relation. [ABSTRACT FROM AUTHOR]