surf zone currents over irregular bathymetry drifter observations and numerical simulations 1 surf zone currents over irregular bathymetry were observed with gps tracked drifters and modeled with the depth integrated nonlinear shallow water equations trajectories of drifters released in 1 2 m depth sometimes defined rip currents and surf zone eddies features that have been difficult to resolve with fixed instruments the drifter delineated surf zone circulation evolved as the tidal level changed during each 4 6 hour deployment in one case as the tide dropped a shore normal rip current present for the first 2 hours evolved to a more shore parallel flow in a second case on a rising tide a well developed bifurcated rip current was replaced by a weak amorphous circulation rip current velocities were strongest near the surf zone edge and decayed by an order of magnitude within 2 surf zone widths from the shoreline an eddy was observed to persist over a bathymetric depression within the surf zone for at least 1 hour observed and numerically simulated drifter trajectories define similar flow features but the observed and modeled velocities differed by roughly a factor of 2 and flow evolution observed during roughly 0 5 m changes in tidal level were reproduced only if the simulated water levels varied by about 1 m the alongshore momentum balance was spatially variable with wave forcing balanced by pressure gradient friction and advective terms the cross shore momentum balance was everywhere dominated by wave forcing and pressure gradient terms of about equal magnitude in the vicinity of a simulated rip current the residual of the cross shore wave forcing and pressure gradient increased and was balanced by advection terms

physical modeling of intermediate cross shore beach morphology transients and equilibrium states laboratory experiments on cross shore beach morphodynamics are presented a lightweight sediment density rho s 1 19 g cm 3 model is used in order to fulfill a shields number and rouse number scaling this choice aims at correctly reproducing bed load transport as well as suspension dynamics terraces and barred beach profiles obtained in the experiments also present close similarities with profiles observed in the field in order to question the concept of equilibrium beach profile wave forcings conforming to a jonswap spectrum were imposed over long periods up to more than a hundred hours an average bottom evolution velocity is defined and used to determine when the profile reaches equilibrium usually beach profiles are characterized according to the wright and short 1984 classification based on the dean number w this well known classification is investigated and refined in the intermediate range that is for 1 omega 5 for w close to 1 a typical reflective profile is obtained terraces are obtained for the omega 2 5 cases for omega approximate to 3 7 the profiles exhibit two parts a mild dissipative offshore slope producing low reflection and a steeper beach face with slightly higher reflection the wave dissipation velocity skewness and acceleration skewness are computed from the free surface elevation time series the dissipation and wave nonlinearities patterns are similar for similar equilibrium beach profiles that is with the same dean number dissipation peaks coincide with bottom slope transitions as higher energy dissipation occurs with milder bottom slope sections besides the uniformity of volumetric wave energy dissipation seems to concern only a limited zone of beaches with a widely developed surf zone

analysis techniques for coherent airborne radar sounding application to west antarctic ice streams 1 analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment echoes from a general interface consist of both specularly reflected and diffusely scattered contributions specular reflection results from smooth uniform interfaces whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media this article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of whillans ice stream and ice stream c west antarctica additionally reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over ice stream c analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates comparison of low and high resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and or scattering interface combining the results from these independent analyses provides classification of the subglacial environment classified regions include smooth seawater smooth saturated sediments accreted marine ice rough bottom crevasses mixed conditions with partial subglacial water and dry frozen conditions

application modelling and validation of surfzone drifters lagrangian data of surfzone flows are scarce but important for understanding the spatial variability of currents surfzone drifters capable of collecting high frequency accurate lagrangian data in the surfzone have been developed the main issues of a robust design capable of autonomous positioning fixing the dynamic response of the drifters position fixing accuracy and field validation are addressed a simple low cost drifter design using non differential global positioning system gps position fixing is described a drogue consisting of a series of small parachutes prevents the drifter surfing in breaking waves the methodology and results of simulations of drifter response in idealised surfzone waves are presented and predict that the drifter will perform well in low to medium energy surfzones an analysis of positioning errors shows that only surfzone motions with frequencies of less than 0 05 hz can be reliably resolved without differential correction a field validation of the drifters shows that although there is significant scatter in the data the best fit correspondence of the drifter velocity and depth averaged acoustic doppler current profiler adcp measurements is good

observations of nearshore circulation alongshore uniformity 1 nearshore circulation observed for 4 months on a 200 m long stretch of natural beach during the sandyduck field experiment is shown to be alongshore uniform an alongshore momentum balance between wind and wave forcing and bottom stress cross shore integrated between the shoreline and approximately 4 m water depth holds on each of five instrumented cross shore transects skill 0 87 the corresponding five best fit drag coefficients are similar consistent with the assumption that terms in the momentum balance associated with alongshore nonuniformity are negligible in addition the alongshore nonuniformity of the circulation and bathymetry were examined at five cross shore locations except near the shoreline the circulation and bathymetry were rarely strongly alongshore nonuniform and the circulation nonuniformities were usually no larger than expected from current meter noise alone near the shoreline the bathymetry was more irregular and the circulation was often detectably nonuniform although no relationship between bathymetric and circulation nonuniformities was found the closure of the alongshore momentum balances on cross shore transects and the observed alongshore uniformity of the circulation on four of five alongshore transects demonstrates that the simplified dynamics of alongshore uniform circulation are valid during the experiment

modelling rip current flow and bather escape strategies across a transverse bar and rip channel morphology abstract rip currents are a hazard to bathers on surf beaches worldwide yet due to logistical and ethical concerns there has been minimal examination of how bathers caught in them should attempt to escape this study presents the first numerical model of simulated bathers escaping from a rip current an understanding of the underlying coastal geomorphology and hydrodynamics form a critical basis for the model with the intention that outcomes of the model should be evaluated from a physical science perspective field observations of a transverse bar rip channel were used as model inputs escape simulations were performed with moderate energy h s 1 m oblique offshore wave forcing used to generate two non stationary asymmetric flow fields dominant surfzone exits and dominant recirculation the model treats bathers as particles that move with the underlying flow with an additional swimming velocity added iterating until a safety condition based on depth and current speed is satisfied or until a maximum time limit is reached simulations compared the strategies of stay afloat and various fixed swim directions using two bather heights and four swim speeds up to 0 4 m s the overall optimal escape strategy was to swim parallel in the direction of alongshore flow time to safety was less for i recirculating flow ii taller bathers and iii greater swim speeds across all simulations 44 of floaters reached safety within 10 min while 80 of slow swimmers 0 2 m s succeeded in the same time interval this suggests that slow sustainable swimming may be a preferable escape action to floating optimal swim direction varied with start location from onshore near the shoreline parallel to shore mid rip channel and diagonally onshore outer rip channel at lower swim speeds the optimal swim direction had greater dependence on the underlying flow field as opposed to distance to safety therefore choice of direction is more complex for slower swimmers critically no strategy succeeded in all scenarios at all locations across the surfzone and small changes to input conditions water level start time and location swim direction had major impacts on outcomes future modelling efforts should examine additional surf zone rip current morphologies and wave tide inputs with a goal of determining generalised rip escape principles that may be communicated to the general public

wave beach and dune interactions in southeastern australia abstract a morphodynamic classification of surfzones beaches and dunes of the microtidal low to high energy southeast australian coast is presented the first section a waves and beaches briefly deals with the transformation of deep water wave energy as it crosses the shelf nearshore and surfzone depending on the deep water wave characteristics and shelf and inshore morphology resultant breaker wave energy may be high moderate or low and the accompanying beach surfzone morphodynamic state in fine to medium sand beaches either dissipative intermediate or reflective dissipative beaches have wide surfzones with shore parallel bar s and channel s and predominantly shore normal circulation intermediate beaches are characterised by rip circulation crescentic transverse bars and megacusps and reflective beaches by a barless surfzone and steep cusped or bermed beach each beach form has a characteristic level of beach stability zone of sediment storage and mode of beach and dune erosion landward aeolian sediment transport of swash deposited sand section b is dependent on the subaerial beach topography and the aerodynamic flow regime across that topography characteristic profile shapes are ascribed to each beach type dissipative intermediate and reflective aeolian sand transport rates are potentially highest on dissipative beaches moderate on intermediate beaches and lowest on reflective beaches these rates determine the potential size of foredunes which are correspondingly largest on dissipative beaches and smallest on reflective beaches the combination of mode and frequency of beach dune erosion rates of aeolian sand transport and foredune volume and morphology provide an explanation of the nature and morphology of landward occurring large scale dune systems dissipative beaches are frequently characterised by large scale transgressive dune sheets intermediate by a trend from large scale parabolic dune systems high wave energy to small scale blowouts low wave energy and reflective beaches by minimal dune development

along track focusing of airborne radar sounding data from west antarctica for improving basal reflection analysis and layer detection this paper presents focused synthetic aperture radar sar processing of airborne radar sounding data acquired with the high capability radar sounder system at 60 mhz the motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking the processing and reflection analyses are applied to data from kamb ice stream west antarctica the sar processor correlates the radar data with reference echoes from subsurface point targets the references are 1 d responses limited by the pulse nadir footprint or 2 d responses that include echo tails unfocused sar and incoherent integration are included for comparison echoes are accurately preserved from along track slopes up to about 0 5deg for unfocused sar 3deg for 1 d correlations and 10deg for 2 d correlations the noise clutter levels increase from unfocused sar to 1 d and 2 d correlations but additional gain compensates at the basal interface the basal echo signal to noise ratio improvement is typically about 5 db and up to 10 db for 2 d correlations in rough regions the increased noise degrades the clarity of internal layers in the 2 d correlations but detection of layers with slopes greater than 3deg is improved reflection coefficients are computed for basal water detection and the results are compared for the different processing methods there is a significant increase in the detected water from unfocused sar to 1 d correlations indicating that substantial basal water exists on moderately sloped interfaces very little additional water is detected from the 2 d correlations the results from incoherent integration are close to the focused sar results but the noise clutter levels are much greater

rip spacing and persistence on an embayed beach 1 four years of daily time exposure images from an embayed beach were examined to study the spacing persistence and location preferences of rips in a natural rip channel system a total of 5271 rip channels was observed on 782 days occurrence statistics showed no evidence of the preferred location pattern associated with standing edge waves trapped in an embayed beach the histogram of rip spacing the primary diagnostic observable for most models was well modeled by a lognormal distribution mean spacing of 178 m however spacings were highly longshore variable time mean of the standard deviation longshore mean of rip spacing was 39 so they are of questionable merit as a diagnostic variable storm driven resets to the longshore uniform condition required by most models occurred only four times per year on average making rip generation models relevant to only a small fraction of the system behavior rip spacings after the 15 observed reset events were uncorrelated with bar crest distance the lifetime of the 324 individual rip channel trajectories averaged 45 6 days rips were equally mobile in both longshore directions but the coefficient of variation of rip migration rates was large even for high migration rates the mean migration rate was well correlated to a proxy for the longshore current r2 of 0 78 thus there is no significant evidence that the formation spacing and migration of rip channels on this beach can be explained by currently existing simple models moreover the alongshore uniform initial conditions assumed by these models are rare on palm beach making the models generally inapplicable

field observations on the morphodynamic evolution of a low energy rip current system abstract a field experiment was conducted at palm beach nsw australia to investigate morphodynamic relationships operating within a rip current system during a near complete sequence of beach state evolution under decreasing energy conditions observations and measurements were obtained from nearshore surveys multiple dye releases oblique photographs and the deployment of various combinations of nine ducted impellor flow meters and five pressure sensors over a seven day period in rip and feeder channels the dataset provides a unique opportunity to assess and quantify aspects of the accretionary model of wright and short mar geol 56 1984 93 118 for intermediate beaches the morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width an increase in channel depth a reduction in the cross sectional area available for rip flow and an increase in morphological relief rip flow velocity increased during the observed evolution and was also modulated by the tide experiencing maximums at low tide and minimums at high tide morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transverse bar and rip state the existence of a dominant feeder in a twin feeder and rip system contributed to the migration and establishment of a rip head over the seaward slope of the opposite longshore bar net erosion in the rip channel was balanced by net deposition in the feeder channels and bar crests and qualitative observations suggest that this rip head bar both receives transported sediments from the rip channel and contributes sediments to the ongoing accretion of the beach system a direct relationship was found to exist between rip morphology and flow with rip velocity increasing as cross sectional rip channel area decreases and these results are incorporated in a conceptual modification of the model of wright and short the results of this study are restricted to a subset of commonly occurring beaches but show that accretionary beach state evolution on intermediate beaches is indeed characterised by an increase in rip current velocity the nature of which can be linked to morphologic control in addition the rip head bar is identified as a potentially key component of low energy rip system evolution

drifter measurements in a laboratory rip current 1 this paper investigates circulation in a laboratory rip current system using large numbers of lagrangian drifters with supplementary current meter and water level measurements these represent the most dense measurements to date of laboratory rip circulation over a large spatial area overall circulation is found to be very unsteady with strong changes in circulation apparent at many length and time scales drifter and current meter measurements are compared in the rip channel and stokes drift is found to be a significant component of drifter velocities in this region estimated generation of circulation is compared with computed bottom friction dissipation with good agreement the mean cellular circulation about a dividing streamline is found to be surprisingly small when compared with computed rates of generation of circulation finally the divergence of volume flux computed using drifters is found to be significant with depth varying wave induced mass transport depth varying eulerian velocities and velocity drifter concentration effects the likely sources of error

laboratory measurements of the vertical structure of rip currents 1 this paper describes the three dimensional variation of rip currents through the use of experimental measurements of rip currents in a directional wave basin the rip currents are unstable appearing sporadically at any given location the vertical profile of the rip current is found to vary from depth uniform inside the channel to depth varying further offshore offshore from the channel the rip current has much stronger velocities at the surface than near the bottom the instantaneous profiles twist rapidly over depth farther offshore and are fairly uniform in the channel the depth variations are shown to be sensitive to the total volume flux in the rip

laboratory experiment on rip current circulations over a moveable bed drifter measurements 1 this study describes a laboratory experiment on rip current circulations over a moveable bed rip current characteristics over eight contrasting nature like beach morphologies are investigated the seabed varied from reasonably alongshore uniform to strongly alongshore nonuniform with crescentic patterns and bar rip morphologies representative of a full morphological down state sequence the same offshore shore normal waves were generated by the wavemaker for the eight situations with the same mean water level to study the sensitivity of rip current characteristics as a function of the beach morphology only in each case a 30 to 60 min video run was used to track a large number of drifters released within the surf zone results show the presence of classic rip current patterns with counterrotating cells and a relatively narrow offshore directed jet varying from shore normal to strongly skewed wave driven circulations were strongly unstable computed standard deviations of flow intensity and direction provide high resolution information on the spatial variability of rip current instabilities highly pulsating and weakly directionally variable offshore directed flow is observed in the rip neck for well developed bar rip morphologies that turns into a weakly pulsating and highly directional variable rip current flow with decreasing beach alongshore nonuniformity proposing a definition of rip current intensity based on the rip current circulation geometry rip current intensity was found to linearly increase with increasing measure of beach alongshore nonuniformity within both the low energy and moderate energy rip current regimes to date our laboratory experiment provides the first extensive quantitative rip current information during a full down state sequence for a given wave condition

observations of nearshore circulation rip currents a sector scanning doppler sonar was deployed at the end of the scripps institution of oceanography pier aimed in toward the surf zone estimates of acoustic backscatter intensity and radial velocity were obtained seaward of the break point over a wedge with an arc of about 45 and a radius of 200 to 400 m the acoustic intensity is proportional to some combination of the bubble density and the suspended sediment in the water well defined jets of water extending seaward from the surf zone or rip currents were observed these rip currents occurred episodically recurred aperiodically one to four times per hour or so and lasted several minutes each the water transported offshore within the rips was rich in suspended sediment and bubbles producing distinctive bright patterns that can be easily recognized in successive images of backscatter intensity from the sonar fluid velocities were estimated from the doppler shift of the sonar signal approaching 0 7 m s in some of the events simultaneously a transect of temperature density and turbidity within the rip current was obtained with conductivity temperature depth ctd and optical backscatter strength obs sensors these data describe the ejection of warm turbid water from the surf zone and provide a comparison between the acoustic intensity and the optical turbidity of the fluid the horizontal extent of the sonar measurements and the vertical resolution of the ctd obs measurements complement each other in the study of the dynamics of the nearshore environment exchange rates are not fully resolved here since only one seaward jet is contained within the field of view however these observations suggest that rip currents could cause significant exchange between the nearshore and offshore waters

beach wizard nearshore bathymetry estimation through assimilation of model computations and remote observations article i nfo a data model assimilation method called beach wizard is presented with which the nearshore subtidal bathymetry can be accurately estimated based on video derived observations of wave roller dissipation and variation of the intertidal shoreline and or radar derived observations of wave celerity using many consecutive images these observed properties are compared with numerical model results and through a simple optimal least squares estimator approach the estimated bathymetry is adjusted gradually for each image in order to improve the fit between model output and observations the key advantages of the technique are that it is based on multiple sources of information i e different remote sensors and or data products depends on only a few free parameters to which the model results are insensitive and shows good skill herein the technique is applied to a synthetic case and two sets of field data from sites at duck nc usa and egmond the netherlands the method which may be extended with observations of other properties from other sources than the three described in this paper can deliver coastal state information i e simultaneous updates of bathymetry waves and currents with high temporal and spatial resolution and can be used in conjunction with or instead of in situ measured data

topographically controlled breaking wave induced macrovortices part 2 changing geometries this is the second part of a series examining the behaviour of breaking wave induced macrovortices the first part examined theoretically general behaviours during startup conditions and gave computational results for macrovortex generation and their evolution on widely spaced breakwaters in this paper we extend and test qualitatively and quantitatively some of the basic results of part 1 in particular the initial longshore vortex transport for a wide range of geometries ranging from narrow rip current topographies to isolated breakwaters accounting for the presence of a shoreline is found to be necessary for the representation of longshore vortex transport results show vortex motion ranging from strongly offshore in the case of a narrow rip current to strongly longshore in the case of an isolated breakwater even with the significant approximations inherent in the analytical predictions numerical computations using time domain boussinesq type equations and laboratory experiments confirm the trends part 3 examines the horizontal mixing features of wave induced flows over isolated single breakwater configuration or multiple ripcurrent configuration submerged structures

rip currents mega cusps and eroding dunes abstract dune erosion is shown to occur at the embayment of beach mega cusps o 200 m alongshore that are associated with rip currents the beach is the narrowest at the embayment of the mega cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune to undercut the dune and to cause dune erosion field measurements of dune beach and rip current morphology are acquired along an 18 km shoreline in southern monterey bay california this section of the bay consists of a sandy shoreline backed by extensive dunes rising to heights exceeding 40 m there is a large increase in wave height going from small wave heights in the shadow of a headland to the center of the bay where convergence of waves owing to refraction over the monterey bay submarine canyon results in larger wave heights the large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale the strongly refracted waves and narrow bay aperture result in near normal wave incidence resulting in well developed persistent rip currents along the entire shoreline the alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95 confidence the alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95 confidence therefore it is concluded the mega cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega cusp

focused synthetic aperture radar processing of ice sounder data collected over the greenland ice sheet the authors developed a synthetic aperture radar sar processing algorithm for airborne spaceborne ice sounding radar systems and applied it to data collected in greenland by using focused sar phase corrected coherent averaging they improved along track resolution by a factor of four and provided a 6 db processing gain over unfocused sar coherent averaging without phase correction based on a point target analysis for a greenland ice sounding data set also they demonstrated that the focused sar processing reduced clutter and enabled them to identify bedrock interface returns buried in clutter using focused sar technique they processed data collected over a key 360 km long portion of the 2000 m contour line of southwest greenland to the best of their knowledge these are the first high quality radar ice thickness measurements over this key location moreover these ice thickness measurements have been used for improving mass balance estimates of the greenland ice sheet

a trend surface technique for discrimination of surf zone morphology rip current channels a major problem facing coastal nearshore studies is the conversion of survey data to precise two and three dimensional representations of complex surf zone topography in addition to inherent problems in providing accurate survey data most signi cant topographical boundaries are often delineated in a subjective manner a trend surface technique is hereby presented which provides an objective rigorous and replicable method of identifying morphological rip system channel boundaries in the surf zone using an extensive eld dataset obtained over two eld experiments including an almost complete cycle of low energy intermediate beach state evolution this technique is shown to produce accurate planimetric dimensions for a range of topographic beach state morphologies previously absent or highly descriptive in the literature formal morphometric de nitions of geometric dimensions for rip morphologies including rip channel width relief depth and cross sectional areas are provided several new aspects of rip current morphology are discussed from results of the approach the approach described here is also intended to illustrate the potential for future trend surface applications to surf zone and other geomorphological studies copyright 2003 john wiley sons ltd

an experimental study of rip channel flow a laboratory study of the flow over a bar with a single rip channel has been performed first the well known pattern of a bar circulation cell with a strong offshore directed current out through the rip channel and a weaker onshore directed return flow over the bar is documented then measurements of the three dimensional structure of the flow in the area where the rip channel the bar and the trough meet and well inside the rip channel are presented these measurements reveal that 3d effects play an important role and that a depth integrated viewpoint may not always be sufficient for predicting the flow in the near bed region particle tracking experiments illustrate the near bed flow pattern over the entire area these demonstrate how the overall trajectory pattern changes as a function of the distance of wave breaking from the bar crest for some conditions the rip current is fed from the trough and for other conditions it is fed directly from the bar both the 3d measurements and the trajectory tests show the existence of a weaker onshore directed near bed drift in the area where the rip current ceases finally in a series of sensitivity tests measurements of the rip current intensity for different wave climate and water level conditions reveal a strong correlation between the rip current intensity and the wave height both normalized

morphodynamics of a large scale rip current system at muriwai beach new zealand abstract field measurements of rip spacing nearshore morphology water surface elevation and eulerian and lagrangian flows were made in order to investigate the morphodynamics of a large scale rip current system at muriwai beach new zealand muriwai is a high energy meso tidal beach characterised by modal breaker wave heights of 2 5 m and incident wave periods of 10 15 s the monitored rip system was characterised by a 400 m long 75 m wide longshore feeder channel and a 150 m wide rip neck channel oriented obliquely to the shore and extending over a distance of almost 400 m during the experiment the beach evolved from a longshore bar trough and rip state to a transverse bar and rip configuration mean eulerian flow velocities obtained from ducted flowmeters deployed on the margin of the feeder channel and rip neck were on the order of 1 m s 1 and instantaneous flows were commonly in excess of 2 m s 1 mean lagrangian surface flow velocities extending from the base of the feeder through to the rip head were obtained by tracking rip floaters and were on the order of 0 7 m s 1 with maximums in the rip neck region of 1 4 m s 1 a distinct tidal modulation of rip current flow existed with maximum velocities occurring at low tide and minimum velocities at high tide comparison with other rip studies suggests that although the magnitude of the morphodynamic and hydrodynamic processes occurring within large scale rip systems is extreme the behaviour of these rip systems is very similar to low energy rips with much smaller spatial scales there is evidence to suggest that distinct morphodynamic scaling relationships exist between these environments

observing surf zone dispersion with drifters abstract 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 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 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 r

a mechanism for the generation of wave driven rhythmic patterns in the surf zone the coupling between topographic irregularities and wave driven mean water motion in the surf zone is examined this coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation this circulation in turn creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large scale bed forms to investigate this coupling mechanism the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases first the breaking line is considered to be fixed and second the perturbations in water depth that produce a displacement of the breaker line are accounted for the first case shows that the basic topography can be unstable with respect to two different modes a giant cusp pattern with shore attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect in the second case the varying breaker line may have a strong influence on the circulation this is clear for the giant cusp topography whose growth is totally inhibited in contrast the morphology and the growth of the crescentic pattern remains almost unchanged

swash groundwater dynamics in a sandy beach laboratory experiment abstract experiments are carried out in a wave basin in order to study the swash groundwater interaction a set of wave gages and pressure sensors are used to monitor the free surface and groundwater dynamics the study is based on the comparison of two selected cases which differ by the gravity and infragravity forcing conditions the features of wave breaking in the surf zone the presence of a standing wave attached to the beach face and the wave setup at the shoreline significant differences are observed in the response of the swash groundwater system the water table overheight appears to be controlled by the amount of infragravity energy available at the shore cyclic beach drainage processes can be observed when the water table is low significant time averaged and time resolved flows are observed into the sand depending on the swash hydrodynamics the presence of a gravity standing wave modulation attached to the shore is shown to affect swash and groundwater dynamics most of the pressure gradients observed under the swash zone are related to infiltration flows and are thus related to moderate increase of the sediment relative weight

surf zone surface retention on a rip channeled beach the retention of floating matter within the surf zone on a rip channeled beach is examined with a combination of detailed field observations obtained during the rip current experiment and a three dimensional 3 d wave and flow model the acoustic doppler current profiler observed hourly vertical cross shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm s for the most offshore sensors model calculations of very low frequency motions vlfs with o 10 min timescales typically explain over 60 of the observed variability both inside and outside of the surf zone the model calculations also match the mean rip current surface flow field inferred from gps equipped drifter trajectories seeding the surf zone with a large number of equally spaced virtual drifters the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories collecting the hourly drifter exits good agreement with the observed surf zone retention is obtained provided that both stokes drift and vlf motions are accounted for in the modeling of the computed drifter trajectories without stokes drift the estimated number of virtual drifter exits is o 80 almost an order of magnitude larger than the o 20 of observed exits during the drifter deployments conversely when excluding the vlf motions instead the number of calculated drifter exits is less than 5 thus significantly underestimating the number of observed exits

infragravity rip current pulsations 1 the origins of rip current pulsations within the infragravity frequency band 0 004 0 04 hz are determined from measurements made with arrays of colocated pressure and velocity sensors deployed on a beach with persistent rip current channels the observations indicate significant energy in cross shore infragravity velocities that varies spatially in the alongshore due to the presence of rip channels infragravity velocities are smaller within the rip channel than on the shore connected shoal owing to differences in water depth rip current pulsations at infragravity frequencies are linked to standing infragravity motions but not to the ponding and subsequent release of water by wave group pumping

double bar beach dynamics on the high energy meso macrotidal french aquitanian coast a review the french aquitanian coast is an approximately 250 km long straight low coast exposed to high energy conditions in a mesomacrotidal setting offshore wave conditions are seasonally modulated predominantly with a wnw incidence with offshore significant wave height likely to reach 10 m during winter truc vert beach representative of most of the aquitanian coast beaches commonly exhibitstwodistinctsandbarsystems theinnerbarcangothroughallthestateswithintheintermediateclassificationandusuallyexhibitsa tansversebarandripmorphology afterafewweeksoflowerenergyconditionsduringsummer theinnerbarcommonlybecomesalow tideterracewithameanwavelengthof400mandameansoutherlymigrationrateofabout2 3m day crescenticbarshavebeenreported in the literature in nontidal to microtidal settings long term persistent crescentic patterns are however exhibited at a narrow range of wavelength mean of 700m by the outer bar at the meso macrotidal truc vert beach most of the time the outer bar is inactive and stagnates as offshore waves ofhsn3 m are required to induce a significant morphological change the crescent shape varies from a symmetric shape to a strongly asymmetric shape likely to be the result of a long period of nw wave conditions a strong and rarely observed elsewhere morphological coupling between the inner and outer bars can sometimes be observed and may be the result of the combined effectsof the initial presence of a well developed outer crescentic bar and a long period of shore normal low energy conditions a synthesis of all the data available on the area combined with observations on other environments leads to a truc vert beach state model ranging from a modal double bar configuration to an occasional triple bar configuration this work also identifies knowledge gaps to be explored by further numerical and field studies in tidal double sandbar environments 2007 elsevier b v all rights reserved

understanding coastal morphodynamic patterns from depth averaged sediment concentration this review highlights the important role of the depth averaged sediment concentration dasc to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern beach cusps surf zone transverse and crescentic bars and shoreface connected sand ridges we present a formulation and methodology based on the knowledge of the dasc which equals the sediment load divided by the water depth that has been successfully used to understand the characteristics of these features these sand bodies relevant for coastal engineering and other disciplines are located in different parts of the coastal zone and are characterized by different spatial and temporal scales but the same technique can be used to understand them since the sand bodies occur in the presence of depth averaged currents the sediment transport approximately equals a sediment load times the current moreover it is assumed that waves essentially mobilize the sediment and the current increases this mobilization and advects the sediment in such conditions knowing the spatial distribution of the dasc and the depth averaged currents induced by the forcing waves wind and pressure gradients over the patterns allows inferring the convergence divergence of sediment transport deposition erosion occurs where the current flows from areas of high to low low to high values of dasc the formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features but the physical mechanisms for their migration their finite amplitude behavior and their decay can also be explored

the distribution of basal water between antarctic subglacial lakes from radar sounding antarctica s subglacial lakes have two end member geophysical expressions as hydraulically flat radar reflective regions highlighted in ice surface topography and radar sounding profiles definit

ripex observations of a rip current system rip current kinematics and beach morphodynamics were measured for 44 days at sand city monterey bay ca using 15 instruments composed of co located velocity and pressure sensors acoustic doppler current profilers and kinematic gps surveys the morphology consisted of a low tide terrace with incised quasi periodic rip channels representative of transverse bars offshore 17 m depth significant wave height and peak period ranged 0 20 3 0 m and 5 20 s the mean wave direction was consistently near 08 resulting in rip channel morphology which evolved in response to the changing wave characteristics an inverse relationship between sediment accreting on the transverse bar and eroding in the rip channel was found the spatial distribution of sediment is reflected in the background rip current flow field the mean velocity magnitudes within the rip channel transverse bars increased offshore onshore with decreasing tidal elevations and increased with increasing sea swell energy eulerian averaged flows were predominantly shoreward on the transverse bars and seaward within the rip channel throughout the experiment resulting in a persistent cellular circulation except during low wave energy the rip current spacing to the rip channel width was less than or equal to two which suggests that the rip currents are influenced by each other and that no two dimensional bar return flow should be present the vertical velocity profile on the bar indicated that the flow was predominantly shoreward the flow field within the surf zone was depth uniform except for significant shear occurring near the surface owing to stokes drift the wave induced transport hypothesis is evaluated d 2005 elsevier b v all rights reserved

radio echo sounding of pine island glacier west antarctica aperture synthesis processing and analysis of feasibility from space airborne radio echo sounding of the west antarctic ice sheet over pine island glacier was performed in the austral summer of 2004 2005 under the national science foundation s west antarctic ice sheet initiative the british antarctic survey flew its newly developed 150 mhz ice sounding radar over pine island glacier and collected approximately 35 000 km of sounding data the synthetic aperture radar sar technique was applied to process those data in order to enhance radar signatures from the bed as a matter of fact airborne ice sounding radar systems are generally affected by surface clutter returns masking the echoes of internal layers and ice bedrock transition at a large depth focused and unfocused doppler filtering sar techniques were compared and their respective advantages disadvantages were analyzed enhancement of bedrock detection at a large depth 2000 m through sar processing is demonstrated finally a simulation analysis was performed for assessing the feasibility of ice sheet sounding from space it is shown that the gain in bed detection threshold is marginal in the satellite sounding geometry airborne radar antarctica ice sounding satellite remote sensing subglacial topography synthetic aperture radar sar processing west antarctic ice sheet

transient rip currents and nearshore circulation on a swell dominated beach 1 eulerian and lagrangian measurements of nearshore circulation on a longshore uniform beach with perpendicularly incident swell waves are described both the eulerian and lagrangian data show a variable rotational part of the current field that is distinct from but coexists with that due to infragravity waves a well defined feature of the flow field includes transient rip currents which occur in varying locations and are not topographically controlled drifter trajectories show that the rips behave like a shallow plane jet consisting of a constrained neck region that flows into a spreading head region there is continuous modulation of the flow within the rip neck estimates of the length scale across the rip neck using two particle statistics are 20 30 m well defined discrete vortex features are observed at the initiation point of one of the necks and in one of the rip heads estimated dispersion values suggest that mixing is greatly enhanced outside the surf zone by the intermittent presence of the rip heads the dispersion calculated from both inside and outside the surf zone appears to show scale dependence with a power law close to 4 3 in a similar way to oceanic turbulence at much larger scales

quasi three dimensional modeling of rip current systems 1 the focus of the paper is the analysis of the flow in rip current systems generated by channels in longshore bars on a beach the horizontal variations of rip current systems are described through the use of the quasi three dimensional nearshore circulation model shorecirc model predictions are compared to laboratory measurements of waves and current velocities throughout the entire rip current system and show reasonable agreement the rips in the two channels are found to behave differently because of the depth variation across the basin it is found that higher bottom stress leads to more stable flow where the rip current meanders less and fewer eddies are generated the wave current interaction creates forcing which reduces the distance rip currents flow offshore and can lead to a slow pulsation of the rip current this pulsation is in addition to the instabilities of a jet which can also be present in rip currents the three dimensionality of the rip current system is found to have a significant effect on the overall circulation patterns index terms 4255 oceanography general numerical modeling 4512 oceanography physical currents 4546 oceanography physical nearshore processes keywords rip currents nearshore circulation numerical modeling waves

rip current review rip currents are shore normal narrow seaward flowing currents that originate within surf zone extend seaward of the breaking region rip head and can obtain relatively high velocities within the last decade there have been a significant number of laboratory and field observations within rip current systems an overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized rip current flows are partitioned into mean infragravity very low frequency vorticity and tidal contributions and it is found that each contributes significantly to the total data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths the maximum mean current occurs inside the surf zone where the maximum forcing is present owing to the dissipation of waves

mean lagrangian flow behavior on an open coast rip channeled beach a new perspective article i nfo the accepted view of rip currents is that they are an efficient mechanism for transporting material out of the surf zone previous rip current campaigns on natural beaches have focused on eulerian measurements with sparse in situ pressure and current meter arrays here for the first time spatially synoptic estimates of rip current flow patterns vorticity and lagrangian transport behavior are measured in the field using a fleet of 30 position tracking surfzone drifters during multiple rip current occurrences on an open coast beach in monterey ca contrary to the classic view shepard et al 1941 the rip current flow field consisted of semi enclosed large scale vortices that retained the drifters and resulted in a high number of lagrangian observations that are temporally and spatially repeated approximately 19 of the drifters deployed in the rip currents exited the surf zone per hour on average during the experiments the observed surf zone retention of drifters is consistent with measurements from different open coast beach rip current systems 14 at meso macrotidal truc vert france and 16 at macrotidal perranporth united kingdom the three hour average cross shore rip current velocity at monterey was 30 cm s with peak time averaged velocities of 40 60 cm s depending on wave and tidal conditions drifters that episodically exited the surf zone were transported approximately 2 surf zone widths offshore at 20 cm s published by elsevier b v

echo source discrimination in single pass airborne radar sounding data from the dry valleys antarctica implications for orbital sounding of mars the interpretation of radar sounding data from mars where significant topographic relief occurs will require echo source discrimination to avoid the misinterpretation of surface echoes as arising from the subsurface this can be accomplished through the identification of all radar returns from the surface in order to positively identify subsurface echoes we have developed general techniques for this using airborne radar data from the dry valleys of antarctica these data were collected in a single pass including taylor glacier ice covered lake bonney and an ice free area of taylor valley the pulsed radar 52 5 67 5 mhz was coherently recorded our echo discrimination techniques included a radar simulator using a digital elevation model dem to predict the location and shape of surface echoes in the radar data real and simulated echo strengths were used to calculate a signal to clutter ratio this was complemented by the cross track migration of radar echoes onto the surface these migrated echoes were superimposed on the dem and imagery in order to correlate with surface features using these techniques enabled us to identify a number of echoes in the radar data as arising from the surface and to identify subsurface echoes including a continuous reflector under the main trunk of taylor glacier and multiple reflectors beneath the terminus of taylor glacier surface based radar confirms the thickness of the glacier at three crossing points the results illustrate the importance of using complementary techniques the usefulness of a dem and the limitations of single pass radar sounding data

surf zone eddies coupled with rip current morphology 1 energetic very low frequency vlf frequencies 0 004 hz surf zone eddies szes were observed on a beach composed of shore connected shoals with quasi periodic 125 m incised rip channels at sand city monterey bay california incident waves consisted of predominantly shore normal narrow banded swell waves szes were located outside the gravity region in alongshore wave number ky spaced within the vlf band and did not appear to exist in higher frequency bands the szes were significant urms vlf 0 25 m s and constant in intensity within the surf zone shore connected shoals and rip channels and rapidly decreased offshore the alongshore and cross shore sze velocity variances were similar in magnitude vlf sze velocities were not forced by vlf surface elevations and were not well correlated with rip current flows r2 0 18 there is an indication that the szes were related to wave forcing with the szes statistically correlated with incoming sea swell wave height r2 0 49 f ky spectral estimates illustrate a strong relationship between rip channel spacing and sze cross shore velocities ky 0 008 m 1 and minimal sze alongshore velocity variation ky 0 m 1 data analysis suggests that the szes are not simply instabilities of an unstable rip current jet a simple conceptual model suggests that sze f ky spectra can be explained by the entire rip current circulation cells oscillating predominantly in the cross shore and slightly in the alongshore

the geology and mineralization of the long chieng track lct subvolcanic au ag cu pb zn deposit lao pdr abstract the long chieng track deposit contains a mineral resource of 32 mt 0 77 g t au 4 9 g t ag 0 12 cu it is described here as a subvolcanic polymetallic au ag cu pb zn deposit occurring within a folded and faulted permian age volcanosedimentary formation in the truong son fold belt of north central laos the geological sequence is interpreted to have formed in a subduction related arc environment mineralization is associated with dacitic porphyritic dykes and small stocks which were intruded into a sequence of volcaniclastics carbonaceous sediments sedimentary breccias and limestones age dating by u pb geochronology of the dacitic porphyry unit yielded an intrusion age of 279 4 ma whole rock chemical analysis demonstrates that the rock suites are classified as i type calc alkaline series with elevated levels of alkali elements mineralization at lct occurs predominantly as stockwork veins which are spatially associated with the dacitic intrusions the veins may be classified into main four types including undifferentiated vnn base metals bmv massive pyrite mpy and buck buk veins all vein types are mineralized except the buk veins cross cutting and overprinting relationships indicate that the vnn and bmv veins were formed earlier than the mpy veins the dominant sets include steep nne dipping 66 010 and steep nnw dipping 70 340 veins and shallow to moderate sse dipping veins 21 172 the majority of high gold grades are hosted within the vnn veins which predominantly dip to the sse in contrast lead and zinc are dominant in the bmv veins while the highest copper grades are mainly hosted in steep nne and nnw dipping mpy veins ore minerals include pyrite galena sphalerite chalcopyrite electrum and native gold with minor arsenopyrite and molybdenite gold occurs either as electrum or native gold grains or as inclusions in pyrite five distinct alteration phases are recognized 1 quartz adularia sulfide 2 sericite quartz pyrite carbonate chlorite 3 fe carbonate 4 epidote carbonate chlorite 5 kaolin halloysite alunite phases 1 to 4 are generally progressively zoned from the deposit centre to the periphery phase 5 is associated with late steam heated hot spring alteration and weathering and oxidation the field geology relationships suggests that the depth of formation can be estimated to be approximately 1 3 km below the paleo surface the style of mineralization and geological relationships of the lct deposit are similar to those described in other deposits formed in the transitional zone between the porphyry and epithermal environments therefore we consider that sub volcanic style is the most appropriate environment to characterize the lct deposit

low energy rip currents associated with small bathymetric variations we show for the first time that low energy waves can induce a rip current system over subtle alongshore bathymetric variations comprehensive field measurements across a rip current that morphologically migrated 12 m day through a coherent cross and alongshore array of co located pressure and velocity sensors were obtained the rip current is associated with a small bathymetric surfzone non uniformity 1 in 300 alongshore variation the circulation was kinematically non uniform for 5 of the time over the course of the 20 day experiment and was present at low tides associated with increases in rip current activity the presence of the rip current and mild sloped rip channel induce statistically significant alongshore variations in hrms wave direction directional spreading infragravity waves and very low frequency motions changes in the directional spreading are correlated with the presence of very low frequency motions influenced by the presence of the rip current

mean currents and sediment transport in a rip channel measurements of hydrodynamics and suspended sediment transport were carried out along a cross shore transect in a rip channel a tidally induced threshold for rip flow initiation was found this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth s in the rip neck with normally incident waves rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide at high tide when the rip was inactive the tendency was for weak onshore directed mean flows and or oscillatory incident waves transporting smaller amounts of sediment landward the net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip outside the surf zone transport was mainly landward due to oscillatory incident waves

high altitude radar measurements of ice thickness over the antarctic and greenland ice sheets as a part of operation icebridge the national aeronautics and space administration nasa initiated a program called operation icebridge for monitoring critical parts of greenland and antarctica with airborne lidars until icesat ii is launched in 2016 we have been operating radar instrumentation on the nasa dc 8 and p 3 aircraft used for lidar measurements over antarctica and greenland respectively the radar package on both aircraft includes a radar depth sounder imager operating at the center frequency of 195 mhz during high altitude missions flown to perform surface elevation measurements we also collected radar depth sounder data we obtained good ice thickness information and mapped internal layers for both thicker and thinner ice we successfully sounded 3 2 km thick low loss ice with a smooth surface and also sounded about 1 km or less thick shallow ice with a moderately rough surface the successful sounding required processing of data with an algorithm to obtain 56 db or lower range sidelobes and array processing with a minimum variance distortionless response algorithm to reduce cross track surface clutter in this paper we provide a brief description of the radar system discuss range sidelobe reduction and array processing algorithms and provide sample results to demonstrate the successful sounding of the ice bottom interface from high altitudes over the antarctic and greenland ice sheets

observations of nearshore crescentic sandbars 1 the temporal and spatial variability of crescentic sandbars is analyzed with hourly long term months video observations collected at four barred sites and are qualitatively compared to the temporal and spatial variability predicted by hypotheses underpinning existing approaches and models for crescentic bar formation edge wave template model linear stability analysis and nonlinear models the observations coming from the single barred beaches at duck north carolina usa and miyazaki kyushu japan and from the double barred beaches at the northern gold coast queensland australia and noordwijk netherlands show that crescentic sandbar wavelength and amplitude variations over space and time are very common for instance at any moment in time the wavelength of the smallest and longest crescentic bar can differ by a factor of 2 temporal changes in wavelength and amplitude result from merging and splitting of individual crescents causing the final configuration of a crescentic sandbar system to be very different from the initial configuration the gold coast data indicate that these intrinsically nonlinear interactions are an attempt of the crescentic bar system to self organize into a more uniform pattern as splitting is usually confined to the longest crescentic bar observed whereas merging usually combines the smallest crescentic bars into a longer bar the observed spatial and temporal crescentic bar behavior contrasts qualitatively with behavior predicted from the edge wave template model and implies that the predictive skill of linear stability models is limited nonlinear models are potentially better suited for a comparison against these field observations several suggestions to improve these models and hence to facilitate a data model comparison are made