Incidence and public health burden of sunburn among beachgoers in the United States.

The beach environment creates many barriers to effective sun protection, putting beachgoers at risk for sunburn, a well-established risk factor for skin cancer. Our objective was to estimate incidence of sunburn among beachgoers and evaluate the relationship between sunburn incidence and sun-protective behaviors. A secondary analysis, of prospective cohorts at 12 locations within the U.S. from 2003 to 2009 (n = 75,614), were pooled to evaluate sunburn incidence 10-12 days after the beach visit. Behavioral and environmental conditions were cross-tabulated with sunburn incidence. Multivariable logistic regression was used to estimate the association between new sunburn and sun-protective behaviors. Overall, 13.1% of beachgoers reported sunburn. Those aged 13-18 years (16.5%), whites (16.0%), and those at beach locations along the Eastern Seaboard (16.1%), had the highest incidence of sunburn. For those spending ≥5 h in the sun, the use of multiple types of sun protection reduced odds of sunburn by 55% relative to those who used no sun protection (Odds Ratio = 0.45 (95% Confidence Interval:0.27-0.77)) after adjusting for skin type, age, and race. Acute health effects of sunburn tend to be mild and self-limiting, but potential long-term health consequences are more serious and costly. Efforts to encourage and support proper sun-protective behaviors, and increase access to shade, protective clothing, and sunscreen, can help prevent sunburn and reduce skin cancer risk among beachgoers

The statistical distribution of swash maxima on natural beaches

Cartwright and Longuet-Higgins (1956) describe the statistical distribution of maxima that would result from the linear superposition of random, Gaussian waves. The distribution function depends solely upon the relative width of the power spectrum and root-mean-square value of the process time series. Runup field data from three experiments are presented to determine the extent to which the distribution of swash maxima can be approximated using the Cartwright and Longuet-Higgins probability density function. The model is found to be satisfactory for describing various distribution statistics including the average maxima, the proportion of negative maxima, and the elevation at which one third of the swash maxima are exceeded. However, systematic discrepancies that scale as a function of time series skewness are observed in the statistics describing the upper tail of the distributions. Although we conclude that the linear model is incapable of delineating these apparent nonlinearities in the swash time series, the extent of the deviation can be estimated empirically for the purpose of constraining nonlinear models and nearshore engineering design

Laboratory investigation of dune erosion using stereo video

Simple parameterizations of dune erosion are necessary for forecasting erosion potential prior to an oncoming storm. Dune erosion may be parameterized in terms of the elevation of the total water level (composed of surge, tide, and wave runup) above the dune base and period of exposure of the dune to waves. In this work, we test several versions of this model using observations from a large wave tank experiment designed to model a storm hydrograph, and we develop a new method for acquiring the appropriate data with confidence intervals using stereo video techniques. The stereo method results in observations of dune morphology at higher spatial and temporal resolutions than traditional survey methods allow. Resolution of the stereo technique was 0.1 m in the horizontal and 0.04 m in the vertical, and errors in stereo observations were on the order of 0.02 to 0.08 m (1 to 2 pixels) when compared with surveys. A new method was developed to estimate confidence intervals on stereo observations. When the unchanging dune top was repeatedly sampled, the new confidence intervals encompassed 2 standard deviations of scatter about the mean dune surface 98% of the time. Observations from the stereo method were used to quantify wave runup and dune erosion. We tested a variety of runup statistics based on a Gaussian distribution of swash properties, and found that the most predictive statistic for dune erosion was the 16% exceedance elevation above the dune base, lower than the often used 2% exceedance value. We found that the parameterization of runup was sensitive to the definition of beach slope and that the most accurate beach slope for predicting runup was through the region of the beach profile defined by the mean water level plus one standard deviation of swash. The dune base retreated along a relatively constant trajectory that was a half of the initial beach slope. Finally, a simple model for dune erosion was tested and found to reproduce 64% of the observed variance in dune erosion rate given known forcing at the dune and 49% of the observed variance in dune erosion rate given parameterized forcing. Integrating the simple model over time, 93% of the observed dune retreat distance was reproduced given offshore forcing.Keywords: Remote sensing, Stereo imaging, Wave runup, Dune erosio

Wavenumber-frequency structure of infragravity swash motions

Alongshore-separated time series of natural swash motions were obtained over a range of environmental conditions using a video technique. Although the frequency spectra and normalized wavenumber spectra for these motions were particularly bland, wavenumber-frequency spectra of these data showed clear partitions of infragravity band energy levels associated with various wave types. For the frequencies 0.025 < ƒ ≤ 0.05 Hz, 45 ± 13% of the shoreline variance was, on average, associated with high-mode (n ≥ 2) edge waves and/or leaky waves, while approximately half that amount was associated with low-mode edge waves. Gravity wave motions (comprising both edge and leaky modes) were typically dominant in a lower-frequency band (0.001 < ƒ ≤ 0.025 Hz). A substantial portion of the variance in this band (21 ± 10%, with a maximum of 38%), however, was identified as a nondispersive waveform with wavenumbers well outside of the wavenumber-frequency bounds for gravity waves. Surprisingly, this nongravity swash variance showed no significant dependence on mean alongshore current strength or mean alongshore current shear as measured in the surf zone trough separating the shoreline from an offshore bar. In addition, the celerities of these swash zone nondispersive waves were found to differ in magnitude, and in one instance, sign, from celerities of similarly structured waves measured farther offshore in the surf zone. These unexpected observations with respect to low-frequency, nongravity swash energy imply a strong decorrelation between trough and shoreline fluid motions

Setup and swash on a natural beach

Wave setup and swash statistics were calculated from 154 runup time series steep beach under incident waves varying from 0.4 to 4.0 m significant wave height. incident wave height, setup, swash height, and total runup (the sum of setup and were found to vary linearly with the surf zone similarity parameter ξ₀ = β(H₀/L₀)¯½ slope appeared the appropriate value for the calculation of ξ₀, although the setup influence of an offshore bar at low tide. For low Irribaren numbers the swash frequency band becomes saturated, while for high Irribaren numbers, no such seen. Thus the infragravity band appears to become dominant in the swash below these data, that value is approximately 1.75, although there is considerable scatter associated with that estimate

Spintessence! New Models for Dark Matter and Dark Energy

We investigate a class of models for dark matter and/or negative-pressure, dynamical dark energy consisting of ``spintessence,'' a complex scalar field $\phi$ spinning in a U(1)-symmetric potential $V(\phi)=V(|\phi|)$. As the Universe expands, the field spirals slowly toward the origin. The internal angular momentum plays an important role in the cosmic evolution and fluctuation dynamics. We outline the constraints on a cosmic spintessence field, describing the properties of the potential necessary to sustain a viable dark energy model, making connections with quintessence and self-interacting and fuzzy cold dark matter. Possible implications for the coincidence problem, baryogenesis, and cosmological birefringence, and generalizations of spintessence to models with higher global symmetry and models in which the symmetry is not exact are also discussed.Comment: 4 pages. To appear in Phys. Lett.

Infragravity waves over a natural barred profile

Measurements of cross-shore flow were made across the surf zone during a storm as a nearshore bar became better developed and migrated offshore. Measured infragravity band spectra were compared to synthetic spectra calculated numerically over the natural barred profile assuming a white run-up spectrum of leaky mode or high-mode edge waves. As in earlier studies, the spectra compared closely; however, for some frequencies the energy of the measured spectrum exceeded the energy of the synthetic spectrum, suggesting that the run-up spectrum was not white but had dominating frequencies. Utilizing cross-shore flow data and synthetic spectra from a number of cross-shore locations, an equivalent run-up spectrum was calculated for each day. On the first day of the storm, the equivalent run-up spectrum indicated a dominant wave that had a node in velocity reasonably close to the bar crest. Later during the storm, when the bar had migrated farther offshore, there was no evidence for a dominant motion having a velocity node at the bar crest. The structure of the equivalent run-up spectrum compared well with spectra of direct measurements of run-up obtained several hundred meters away. We have no clear evidence in support of the theory that infragravity waves might form or force the offshore migration of a bar. To confirm this finding, longer records obtained synoptically over a developing bar are required. The dominant wave observed early in the storm was consistent with Symond and Bowen’s (1984) theoretical prediction of resonant amplification of discrete frequencies over a barred profile

Turbulence observations of the nearshore wave bottom boundary layer

Field observations of turbulence and sediment suspension in the nearshore wave bottom boundary layer obtained during the Duck94 field experiment on the North Caroline coast showed the generation of near-bed turbulence to be highly intermittent. The intermittent nature of the flow was examined by applying homogeneous isotropic turbulence laws over small windows of data. The time-varying estimates of turbulent kinetic energy, shear stress, dissipation, and concentration were averaged over the phase of the free stream wave. The observations show that the magnitude of the turbulent kinetic energy is largest under the wave crest and decreases over the decelerating flow phase until reversal to offshore flow. The turbulent kinetic energy observations under the crests compare favorably to those of three laboratory studies. The sediment suspension observations were shown to have an intermittent structure and to be biased toward onshore decelerating phase of the flow. The shear stress was estimated with the orbital velocities by assuming an eddy viscosity model and was shown to be highly sensitive to the free stream velocity and less sensitive to the free acceleration. The intermittent signal of the turbulent kinetic energy and the suspension was not evident in the shear stress estimates, providing further support that eddy viscosity models are not appropriate for predicting the suspension of sediment. Finally, dissipation rate estimates increased with proximity to the bed but were several orders of magnitude lower than those found in the active breaking region

Field observations of the wave bottom boundary layer

This paper presents a comprehensive set of velocity and suspended sediment observations in the nearshore wave bottom boundary layer, collected during the Duck94 field experiment on the Outer Banks of the North Carolina coast. Cross-shore velocity measurements in the wave bottom boundary layer were made using five hot film anemometers, nominally spaced from 1 to 5 cm above the bed in 2 m of water depth. The time-varying location of the seabed was estimated to roughly 1 cm with a stacked set of bed-penetrating fiber-optic backscatter sensors. The instrument array was intermittently located in the surf zone on the crest of a bar. The location of the bottom varied several centimeters over a 34 min data run. Even over 4 min segments of quasi-steady statistics, occasional large waves caused short erosion and redeposition events, complicating the definition of bottom location and causing the root-mean-square velocity statistics to be nonzero below the mean bed location. This leads to obvious difficulties in comparisons with two, one-dimensional time-dependent, eddy viscosity wave bottom boundary layer models. For example, bed shears based on rms amplitude decay were lower than predicted. The observations show some evidence for a velocity overshoot region within the wave bottom boundary layer. The observations were compared with two linear eddy viscosity models. Larger estimates of a constant eddy viscosity and smaller than predicted phase leads are indicative of more rapid mixing of momentum than predicted by the models. The phase and amplitude frequency response estimated with frequency domain empirical orthogonal functions shows a nonlinear response of the wave bottom boundary layer over the incident band. These observations are among the first coherent looks at the wave bottom boundary layer under conditions of significant sediment response. They highlight the added complexity of the dynamics in natural environments

Wave variance partitioning in the trough of a barred beach

The wave-induced velocity field in the nearshore is composed of contributions from incident wind waves (f > 0.05 Hz), surface infragravity waves (f σ²/gβ), where ƒ is the frequency, σ = 2πf, k is the radial alongshore wavenumber (2π/L, L being the alongshore wavelength), β is the beach slope, and g is the acceleration due to gravity. Using an alongshore array of current meters located in the trough of a nearshore bar (mean depth ≈ 1.5 m), we investigate the bulk statistical behaviors of these wave bands over a wide range of incident wave conditions. The behavior of each contributing wave type is parameterized in terms of commonly measured or easily predicted variables describing the beach profile, wind waves, and current field. Over the 10-day period, the mean contributions (to the total variance) of the incident, infragravity, and shear wave bands were 71.5%, 14.3% and 13.6% for the alongshore component of flow (mean rms oscillations of 44,20, and 19 cm s¯¹, respectively), and 81.9%, 10.9%, and 6.6% for the cross-shore component (mean rrns oscillations of 92, 32, and 25 cm s¯¹, respectively). However, the values varied considerably. The contribution to the alongshore (cross-shore) component of flow ranged from 44.8- 88.4% (58.5-95.8%) for the incident band, to 6.2-26.6% (2.5-32.4%) for the infragravity band, and 3.4- 33.1 % (0.6-14.3%) for the shear wave band. Incident wave oscillations were limited by depth-dependent saturation over the adjacent bar crest and varied only with the tide. The infragravity wave rms oscillations on this barred beach are best parameterized by the offshore wave height, consistent with previous studies on planar beaches. Comparison with data from four other beaches of widely differing geometries shows the shoreline infragravity amplitude to be a near-constant ratio of the offshore wave height. The magnitude of the ratio is found to be dependent on the Iribarren number, ξ₀ = β(H/L₀)¯1/2. Shear waves are, as previous observation and theory suggest (Oltman-Shay et al., 1989; Bowen and Holman, 1989), significantly correlated with a prediction of the seaward facing shear of the longshore current