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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]

Decisions about recreational beach closures would be enhanced if better estimates of surfzone contaminant transport and dilution were available. In situ methods for measuring fluorescent Rhodamine WT dye tracer in the surfzone are presented, increasing the temporal and spatial resolution over previous surfzone techniques. Bubbles and sand suspended by breaking waves in the surfzone interfere with in situ optical fluorometer dye measurements, increasing the lower bound for dye detection (≈ 1 ppb) and reducing (quenching) measured dye concentrations. Simultaneous turbidity measurements are used to estimate the level of bubble and sand interference and correct dye estimates. After correction, root-mean-square dye concentration errors are estimated to be < 5% of dye concentration magnitude, thus demonstrating the viability of in situ surfzone fluorescent dye measurements. The surfzone techniques developed here may be applicable to other environments with high bubble and sand concentrations (e.g., cascading rivers and streams). [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]

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]

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]