mass flux and undertow in a surf zone abstract the mass of water carried shoreward by the breaking waves in a surf zone will in a two dimensional situation be compensated by a seaward return flow the undertow it is shown that the undertow is driven by the local difference between radiation stress and the set up pressure gradient which only balance each other in average over the depth turbulent shear stresses are required to maintain a steady situation comparison with measurements confirms the theoretical results

a simple model for interannual sandbar behavior time and length scales of beach variability have been quantified using 16 years of beach surveys sampled at the army corps of engineers field research facility located on the u s atlantic coast between 50 and 90 of the bathymetric variability at this site was explained by alongshore uniform response over the approximately 1 km alongshore span of the surveys although the incident wave height variance was dominated by frequencies at or higher than 1 cycle yr more than 80 of the bathymetric variance at all cross shore locations was explained by frequencies 1 cycle yr interannual cycles consisting of sandbar formation migration and decay contributed to the low frequency variability the observed behavior can be explained by a simple heuristic model the model assumes that bars migrate toward a wave height dependent equilibrium position this position was shown to coincide with the wave breakpoint additionally the rate of bar response is taken to be variable and was empirically determined to be proportional to the wave height cubed the net effect of a variable response rate is to shift the expected long term mean sandbar position offshore toward the equilibrium position associated with the largest waves the model explained up to 80 of the observed bar position time series variance and up to 70 of the variance of bar crest velocity time series which were extracted from three different sandbars characteristic bar response times related to the inverse of the response rate were found to be long relative to the characteristic timescale of the forcing 1 year in our case as a result transient response i e bar position far from equilibrium tended to persist for many cycles of the forcing transient bar behavior appears in the observations when bars formed near the shoreline or when outer bars decayed and inner bars faced a changed wave climate while the present model is able to explain the evolution of these transients it does not contain a mechanism for their introduction

offshore stereo measurements of gravity waves stereo video techniques are effective for estimating the space time wave dynamics over an area of the ocean indeed a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes to prove this we consider an application of the wave acquisition stereo system wass for the analysis of offshore video measurements of gravity waves in the northern adriatic sea in particular we deployed wass at the oceanographic platform acqua alta off the venice coast italy three experimental studies were performed and the overlapping field of view of the acquired stereo images covered an area of approximately 1100 m2 analysis of the wass measurements show that the sea surface can be accurately estimated in space and time together yielding associated directional spectra and wave statistics that agree well with theoretical models from the observed wavenumber frequency spectrum one can also predict the vertical profile of the current flow underneath the wave surface finally future improvements of wass and applications are discussed

modeling the alongshore current on barred beaches mean alongshore currents observed on two barred beaches are compared with predictions based on the one dimensional time and depth averaged alongshore momentum balance between forcing by breaking waves wind and 10 100 km scale alongshore surface slopes bottom stress and lateral mixing the observations span 500 hours at egmond netherlands and 1000 hours at duck north carolina and include a wide range of conditions with maximum mean currents of 1 4 m s including rollers in the wave forcing results in improved predictions of the observed alongshore current structure by shifting the predicted velocity maxima shoreward and increasing the velocity in the bar trough compared with model predictions without rollers for these data wave forcing balances the bottom stress within the surfzone with the other terms of secondary importance the good agreement between observations and predictions implies that the one dimensional assumption holds for the range of conditions examined despite the presence of small alongshore bathymetric nonuniformities with stronger bathymetric variations the model skill deteriorates particularly in the bar trough consistent with earlier modeling and laboratory studies

cbathy a robust algorithm for estimating nearshore bathymetry 1 a three part algorithm is described and tested to provide robust bathymetry maps based solely on long time series observations of surface wave motions the first phase consists of frequency dependent characterization of the wave field in which dominant frequencies are estimated by fourier transform while corresponding wave numbers are derived from spatial gradients in cross spectral phase over analysis tiles that can be small allowing high spatial resolution coherent spatial structures at each frequency are extracted by frequency dependent empirical orthogonal function eof in phase two depths are found that best fit weighted sets of frequency wave number pairs these are subsequently smoothed in time in phase 3 using a kalman filter that fills gaps in coverage and objectively averages new estimates of variable quality with prior estimates objective confidence intervals are returned tests at duck nc using 16 surveys collected over 2 years showed a bias and root mean square rms error of 0 19 and 0 51 m respectively but were largest near the offshore limits of analysis roughly 500 m from the camera and near the steep shoreline where analysis tiles mix information from waves swash and static dry sand performance was excellent for small waves but degraded somewhat with increasing wave height sand bars and their small scale alongshore variability were well resolved a single ground truth survey from a dissipative low sloping beach agate beach or showed similar errors over a region that extended several kilometers from the camera and reached depths of 14 m vector wave number estimates can also be incorporated into data assimilation models of nearshore dynamics

remote sensing of surf zone waves using stereo imaging abstract the measurement of water wave characteristics such as wavelength and wave height in the surf zone is important for monitoring prediction of erosion and numerical model calibration traditional methods of measuring wave heights have either been limited to a small number of points or have required contact with the water an experimental study of the remote sensing of water wave elevations through the application of stereo photogrammetry is presented this method uses two spatially offset cameras with overlapping fields of view to determine water surface elevation this remote sensing approach provides data with excellent spatial coverage and spatial and temporal resolution additionally the hardware needs are minimal and the system is quickly deployed calibrated and operational in the present study a phased approach was taken with medium scale domain 50 m 2 laboratory experiments being followed by a large scale domain 10 3 10 4 m 2 field test of the method in the laboratory reconstructed surface elevations were validated using a pressure sensor and demonstrated excellent agreement in the field measured wave heights and periods were found to agree well with available buoy data

defining and quantifying microscale wave breaking with infrared imagery breaking without air entrainment of very short wind forced waves or microscale wave breaking is undoubtedly widespread over the oceans and may prove to be a significant mechanism for enhancing the transfer of heat and gas across the air sea interface however quantifying the effects of microscale wave breaking has been difficult because the phenomenon lacks the visible manifestation of whitecapping in this brief report we present limited but promising laboratory measurements which show that microscale wave breaking associated with evolving wind waves disturbs the thermal boundary layer at the air water interface producing signatures that can be detected with infrared imagery simultaneous video and infrared observations show that the infrared signature itself may serve as a practical means of defining and characterizing the microscale breaking process the infrared imagery is used to quantify microscale breaking waves in terms of the frequency of occurrence and the areal coverage which is substantial under the moderate wind speed conditions investigated the results imply that bursting phenomena observed beneath laboratory wind waves are likely produced by microscale breaking waves but that not all microscale breaking waves produce bursts oceanic measurements show the ability to quantify microscale wave breaking in the field our results demonstrate that infrared techniques can provide the information necessary to quantify the breaking process for inclusion in models of air sea heat and gas fluxes as well as unprecedented details on the origin and evolution of microscale wave breaking

a laboratory study of longshore currents over barred and non barred beaches abstract a detailed description is given of the results of laboratory experiments on wave driven longshore currents on both barred and non barred beaches the objective is to examine the cross shore distribution of the longshore current velocities for purely wave driven currents with emphasis on the position of maximum current velocity with respect to areas where wave energy is dissipated unidirectional obliquely incident waves both regular and random were used the measurements were performed in a large wave basin with a pump recirculation system to create spatially homogeneous longshore currents the experiments yielded information on wave transformation set up of the mean water level and the cross shore distribution of wave driven longshore current velocity a number of cases are presented and compared with each other the measurements show that in the case of purely wave driven longshore currents the maximum current velocities occur close to areas where wave breaking is most intense the effect of mixing bottom friction and wave rollers on the longshore current velocity profile are examined in more detail with help of numerical modelling existing model equations based on the assumption of alongshore uniformity are used the results for the mean longshore current profile on a barred beach are in close agreement with the measurements

thermal infrared multipath reflection from breaking waves observed at large incidence angles the infrared signature of breaking waves at large incidence angles was investigated using laboratory experiments and a radiometric model infrared imagery of the water surface at incidence angles greater than 70 shows multipath reflections for both spilling and plunging waves generated using a programmable wave maker for the spilling breakers the multipath signature was initially distinct from the breaking wave front roller signature but then merged to create a single large bright distributed target for the plunging breakers the roller and multipath signatures overlapped from the inception of breaking the radiance of the multipath reflection was higher than the surrounding water for simulated cold sky conditions and lower for a simulated warm sky a specular double reflection model successfully predicted the presence of multipath reflection but the magnitude was sensitive to small uncertainties in geometry wave slope and input temperatures the results show that multipath reflection from breaking waves is characteristic of large incidence angle infrared measurements and increases the area and magnitude of the infrared signature of breaking waves compared to the background

estimating wave energy dissipation in the surf zone using thermal infrared imagery thermal infrared ir imagery is used to quantify the high spatial and temporal variability of dissipation due to wave breaking in the surf zone the foam produced in an actively breaking crest or wave roller has a distinct signature in ir imagery a retrieval algorithm is developed to detect breaking waves and extract wave roller length using measurements taken during the surf zone optics 2010 experiment at duck nc the remotely derived roller length and an in situ estimate of wave slope are used to estimate dissipation due to wave breaking by means of the wave resolving model by duncan 1981 the wave energy dissipation rate estimates show a pattern of increased breaking during low tide over a sand bar consistent with in situ turbulent kinetic energy dissipation rate estimates from fixed and drifting instruments over the bar when integrated over the surf zone width these dissipation rate estimates account for 40 69 of the incoming wave energy flux the duncan 1981 estimates agree with those from a dissipation parameterization by janssen and battjes 2007 a wave energy dissipation model commonly applied within nearshore circulation models

video observations and model predictions of depth induced wave dissipation time averaged video observations of the nearshore zone show the process of wave breaking as one or more white alongshore bands of high intensity corresponding to the preferential location of breaking wave dissipation on one or more sandbars across a known depth profile similar bands of dissipation can be predicted with existing wave transformation models based on the wave energy balance this opens up possibilities for estimating bathymetry from video observations by processing observed intensities into a model predicted proxy of wave dissipation and the inverse modeling of this dissipation proxy into depth the objectives of this paper are 1 to present a technique for processing time averaged cross shore image intensity profiles into profiles that solely contain the contribution due to wave breaking and 2 to empirically determine which modeled wave dissipation proxy relates best to the cross shore shape of the obtained breaking induced intensity the processing technique involves the removal of background illumination and noise and most importantly a correction for white foam that after being generated in the breaking process remains floating at the water surface this foam thus contributes to high image intensities but is not predicted by wave transformation models based on video and bathymetric data collected at the double barred beach at egmond aan zee the netherlands and a standard wave transformation model containing the wave and roller energy balances we find that the modeled cross shore distribution of the dissipation of the energy of the surface roller the white aerated mass of water at the breaking wave front matches the observed cross shore shape of breaking induced intensity well other hypothesized dissipation proxies result in a larger 10 m seaward bias in the locations of maximum breaking induced intensity and model predicted dissipation or do not predict the cross shore change in the area below the breaking induced intensity bands correctly

a note on wave energy dissipation over steep beaches abstract this paper revisits the derivation of the parametric surf zone model proposed by baldock et al baldock t e holmes p bunker s van weert p 1998 cross shore hydrodynamics within an unsaturated surf zone coast eng 34 173 196 we show that a consistent use of the proposed rayleigh distribution for surf zone wave heights results in modification of the expressions for the bulk dissipation rate and enhanced dissipation levels on steep beaches and over saturated surf zone conditions as a consequence the modification proposed herein renders the model robust even on steep beaches where it could otherwise develop a shoreline singularity

measurements of short water waves using stereo matched image sequences abstract image analysis techniques are used for retrieving water surface elevation fields spatially and temporally from ccd images and ccd image sequences the technique proposed herein utilizes binocular stereogrammetry to recover topographic information from a sequence of synchronous overlapping video images the method used differs from the traditional stereo photogrammetric analysis of a single stereo pair because the use of video allows for a continuous sequence of stereo images to be digitally sampled and analyzed for data acquisition two synchronized progressive scan cameras were used a partially supervised 3 d stereo system called wass wave acquisition stereo system is shown here it is used to reconstruct the 3 d shape of water surface waves acquired at frame rate with small computational time needed the stereo method is presented including the derivation of a relationship relating the geometry of the stereo rig and the expected errors finally the 3 d calculated scattered points give the complete spatio temporal distribution of the water surface elevations the measurable length scales depend on the pixel resolution the triangulation accuracy and the acquisition frame rate limitations in the stereo measurements are also discussed two experiments to test and to demonstrate the system took place one on the venice lagoon north of the city of venice in september 2004 and the second on the coast of california at san diego in december 2005 for the second experiment qualitative and quantitative intercomparisons of the stereo matching and in situ sensor measurements are presented all the measurements of water surface waves indicate that the proposed approach is both accurate and applicable for measuring water surface waves moreover shape estimates are accurately and extremely dense both in space and time and the remote location of the instruments alleviates some difficulties associated with in situ instrumentation

spectral evolution of shoaling and breaking waves on a barred beach field observations and numerical model predictions are used to investigate the effects of nonlinear interactions reflection and dissipation on the evolution of surface gravity waves propagating across a barred beach nonlinear interactions resulted in a doubling of the number of wave crests when moderately energetic about 0 8 m significant wave height narrowband swell propagated without breaking across an 80 m wide nearly flat 2 m depth section of beach between a small offshore sand bar and a steep slope 0 1 beach face where the waves finally broke these nonlinear energy transfers are accurately predicted by a model based on the nondissipative unidirectional i e reflection is neglected boussinesq equations for a lower energy wave height about 0 4 m bimodal wave field high frequency seas dissipated in the surf zone but lower frequency swell partially reflected from the steep beach face resulting in significant cross shore modulation of swell energy the combined effects of reflection from the beach face and dissipation across the sand bar and near the shoreline are described well by a bore propagation model based on the nondispersive nonlinear shallow water equations boussinesq model predictions on the flat section where dissipation is weak are improved by decomposing the wave field into seaward and shoreward propagating components in more energetic wave heights greater than 1 m conditions reflection is negligible and the region of significant dissipation can extend well seaward of the sand bar differences between observed decreases in spectral levels and boussinesq model predictions of nonlinear energy transfers are used to infer the spectrum of breaking wave induced dissipation between adjacent measurement locations the inferred dissipation rates typically increase with increasing frequency and are comparable in magnitude to the nonlinear energy transfer rates

surf zone longshore currents and random waves field data and models abstract analytic and numerical models for longshore currents generated by obliquely incident random waves am compared with field observations five days of observations were selected during which the waves were narrow banded in both frequency and direction in keeping with model assumptions the extensive measurements included radiation stress and wave directional spectra in 9 m depth and a closely spaced array of current and pressure sensors on a line perpendicular to shore the longshore current models are based on balancing the gradient of the radiation stress with the alongshore bed shear and reynold s stresses assuming stationary wave conditions and straight and parallel bottom contours the spatial variation of wave height required to determine the gradient of the radiation stress is modeled using linear random wave theory given hrms in 9 m depth the model predicts hrms at shoreward locations with an average error of less than 9 using a nonlinear bottom shear stress formulation and the meas

scaling surf zone turbulence 1 turbulence in the surf zone the shallow region adjacent to the shoreline has a key role in beach erosion fertilization dispersal and larval settlement of marine invertebrates and microbial contamination dilution in beach waters breaking wave generated the dominant source surf zone turbulence is understood poorly a new surf zone turbulent dissipation rate scaling is derived that collapses new field surf zone observations with relatively high skill compared to other scalings the vertically uniform length scale is 1 6 the water depth and 15 of the wave energy flux gradient is dissipated below the mean surface field and laboratory surf zone turbulence observations are shown to be consistent using the scaling the non dimensional surf zone diffusivity and suspended sediment profile can be applied to sediment transport and a range of biological processes including microbial pathogen contamination of beach waters

what determines the position and intensity of the south atlantic anticyclone in austral winter an agcm study abstract the south atlantic anticyclone is a major feature of the austral winter climatology an atmospheric general circulation model agcm is used to study the dynamics of the south atlantic anticyclone by means of control simulations and experiments to investigate sensitivity to prescribed orography sea surface temperatures and soil wetness the south atlantic anticyclone in the first control simulation is unrealistically zonally elongated and centered too far west errors typical of coupled ocean atmosphere gcms results of the sensitivity experiments suggest that these deficiencies are associated with another family of systematic model errors the overprediction of convection over the tropical land surfaces particularly over eastern tropical africa and india and the concurrent large scale westward shift in the divergence center at upper levels and the convergence center at lower levels the results also confirm the important role of south american and african orography in localizing the south atl

surf zone dynamics simulated by a boussinesq type model part i model description and cross shore motion of regular waves abstract this is the first of three papers on the modelling of various types of surf zone phenomena in this first paper part i the model is presented and its basic features are studied for the case of regular waves the model is based on two dimensional equations of the boussinesq type and it features improved linear dispersion characteristics possibility of wave breaking and a moving boundary at the shoreline the moving shoreline is treated numerically by replacing the solid beach by a permeable beach characterized by an extremely small porosity run up of nonbreaking waves is verified against the analytical solution for nonlinear shallow water waves the inclusion of wave breaking is based on the surface roller concept for spilling breakers using a geometrical determination of the instantaneous roller thickness at each point and modelling the effect of wave breaking by an additional convective momentum term this is a function of the local wave celerity which is determined interactively the model is applied to cross shore motions of regular waves including various types of breaking on plane sloping beaches and over submerged bars model results comprise time series of surface elevations and the spatial variation of phase averaged quantities such as the wave height the crest and trough elevations the mean water level and the depth averaged undertow comparisons with physical experiments are presented the phaseaveraged balance of the individual terms in the momentum and energy equation is determined by time integration and quantities such as the cross sectional roller area the radiation stress the energy flux and the energy dissipation are studied and discussed with reference to conventional phase averaged wave models the companion papers present cross shore motions of breaking irregular waves swash oscillations and surf beats part ii and nearshore circulations induced by breaking of unidirectional and multidirectional waves part iii

remote sensing of wave roller lengths in the laboratory 1 the optical intensity signals from surf zone waves in a laboratory flume are analyzed using several different phase averaging techniques and a methodology is developed for estimating wave roller lengths and local wave dissipation the intensity signals i e phase averaged intensity profiles of individual breaking waves are compared with the wave profiles measured by in situ wave gauges and the optical signal of the wave roller is shown to ramp up from the toe of the wave roller on the front face of the wave to a maximum intensity at the wave crest the remote sensing observations capture the growth equilibrium and decay phases of the roller as it propagates over a fixed bed arranged in a bar trough morphology next for the regular wave conditions considered here the local maxima of the phase averaged intensities are shown to better indicate the initial onset of wave breaking and the occurrence of wave breaking in the bar trough as compared to the more commonly used time averaged mean intensity in addition the phase averaged profiles are used to measure the size of the roller and these measurements are compared to previous observations of smaller scale rollers in equilibrium the observed roller lengths are shown to agree with predictions from a wave roller model and to provide a new physical link between the remotely sensed signal and roller dissipation finally as an example application of these new data a simple wave height inversion model is presented that allows an estimation of surf zone wave heights from the remotely sensed roller lengths