interferometer studies of equatorial f region irregularities and drifts a radar interferometer technique developed at jicamarca peru and first used to study electrojet irregularities has now been used successfully to study plasma turbulence in the equatorial f region our first results have shown that the most turbulent echoes appear to come from a region that extends for tens of kilometers in altitude but for only a kilometer or less in the east west direction this slab may very well be the wall of a depleted region a plasma bubble sometimes the irregularities can be tracked as they move eastward or westward velocity profiles for the evening period obtained in this way show a strong shear with westward velocities at the lowest altitudes observed and eastward velocities above a plausible explanation for this shear is that the westward drifts are driven by electric fields produced by westward e region winds and mapped up along magnetic field lines while at higher heights where the electron density is greater the drifts are controlled by the f region dynamo driven by eastward winds

wave induced sediment transport and sandbar migration onshore sediment transport and sandbar migration are important to the morphological evolution of beaches but are not well understood here a model that accounts for fluid accelerations in waves predicts the onshore sandbar migration observed on an ocean beach in both the observations and the model the location of the maximum acceleration induced transport moves shoreward with the sandbar resulting in feedback between waves and morphology that drives the bar shoreward until conditions change a model that combines the effects of transport by waves and mean currents simulated both onshore and offshore bar migration observed over a 45 day period

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

experimental study of runup for sandy beaches under waves and tide abstract this paper describes an experimental investigation of runup for intermediate sandy beaches from a physical modelling in a wave flume with tide simulation shoreline elevation measurements are acquired over a wide range of conditions using an optical method simultaneous morphological and hydrodynamic changes are considered to determine the best parameters to predict the maximum wave runup the tidal level significantly affects the breaking conditions position height and type and the slopes of the beach present data show that the surf zone slope and the wave breaking height should be used to estimate the maximum runup the results are compared with previous formulations issued from the literature a new formula is proposed for the runup estimation for intermediate beaches

longshore variability of beach states and bar types in a microtidal storm influenced low energy environment beach classification models are widely used in the literature to describe beach states in response to environmental conditions these models were essentially developed for sandy barred to barless beaches in micro to mesotidal environments subject to moderate to high wave energy conditions and have been based on field studies over limited stretches of coast here we further interrogate the performance of the australian beach classification scheme by analysing beach states and corresponding bar types on a regional scale in a storm influenced low wave energy microtidal environment using a large and unique spatial and temporal dataset of supra and subtidal beach morphology and sedimentology the 200 km long coast of the gulf of lions in themediterranean consists of quasi continuous sandy beaches with a well developed double sandbar system all the reported classical beach stateswere observed on this coast from reflective to dissipative alongwith two more unusual states the rock platform constrained beach state which is associated with bedrock outcrops and the non barred dissipative beach statewhich ismore commonly found in large tidal range settings lidar bathymetry shows that the transitions between beach state zones are marked mainly headlands but transitions also occur progressively along stretches of continuous sandy beach the longshore distribution of beach states and associated bar types on a regional scale can be related to the variability of hydrodynamic conditions wave incidence and energy and sediment characteristics particle size however the influence of these parameters on beach state seems to be largely controlled by the geological context such as the presence of a river mouth headland or rock platform finally we assessed the ability of the parameter commonly used to characterise beach states which combines wave characteristics and sediment fall velocity to predict the observed beach states and bar types using a very large set of hydrodynamic and sedimentary data our results based on high frequency spatial sampling show that the fall velocity of the subtidal sediment coupled with wave statistics one month prior the observed beach state strongly improved the predictive power of the parameter

effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position remote sensing of wave breaker patterns clearly visible as high intensity bands in time exposure video images has become a powerful tool to obtain large scale kilometers and long term years time series of nearshore sandbar position however intensity based bar crest positions xi differ from directly measured positions xb by a time varying distance x which is of o 10 m and depends on the offshore wave height h0 the water level 0 and the bathymetry itself the effect of these parameters on x was investigated from simultaneous video observations and bathymetric surveys obtained in the double barred system at egmond aan zee netherlands and from wave model predictions assuming that the roller energy represents image intensity when the wave field over a bar was predicted to be nonsaturated xi was observed and predicted to move offshore as either 0 decreased or h0 increased under saturated conditions x only responded to changes in 0 additional model investigations showed that an increase in outer bar crest depth similar to that observed during interannual bar behavior significantly reduced the x variability at the outer bar and increased the x variability at the inner bar implications of our observational and model findings for studying sandbar position from video imagery are outlined

sediment transport associated with morphological beach changes forced by irregular asymmetric skewed waves 1 the waveshape effects on sediment transport are investigated for cross shore beach profile changes this study is based on experiments performed in the laboratoire des ecoulements geophysiques et industriels wave flume for irregular waves the interest of such experiments resides in presenting complex combinations of wave skewness and asymmetry in bed load ripple and sheet flow regimes net sediment transport rates on typical beach morphodynamics are analyzed in regard to wave skewness and asymmetry undertow and ripple occurrence onshore bar migration is mainly associated with onshore directed sediment transport whereas terrace profile and offshore bar formation correspond to offshore sediment transport as for natural beaches energetic moderate wave climates mostly induce offshore onshore sediment fluxes for a given significant wave height an increase decrease in the wave climate peak period is associated with an increase decrease in wave skewness and leads mostly to offshore onshore sediment fluxes the experiments are fully characterized by unsteady behavior consequently several conditions exhibit phase lag effects where the sediment is mobilized by the wave crest and transported by the following trough which produces a net offshore transport even for a weak undertow the presence of ripples clearly contributes to enhance this behavior an original concept due to its application to skewed asymmetric irregular waves presents the important interaction between wave nonlinearities driving the sediment fluxes the net sediment transport rate under strongly skewed waves is either offshore directed due to phase lag effects or onshore directed when the wave asymmetry is large enough both these mechanisms probably largely contribute to bar formation and migration

internal architecture and evolution of bioclastic beach ridges in a megatidal chenier plain field data and wave flume experiment beach ridges in macrotidal environments experience strong multi annual to multi decennial fluctuations of tidal inundation the duration of tide flooding directly controls the duration of sediment reworking by waves and thus the ridge dynamics flume modelling was used to investigate the impact of low frequency tidal cycles on beach ridge evolution and internal architecture the experiment was performed using natural bioclastic sediment constant wave parameters and low frequency variations of the mean water level the morphological response of the beach ridge to water level fluctuations and the preservation of sedimentary structures were monitored by using side view and plan view photographs results were compared with the internal architecture of modern bioclastic beach ridges in a macrotidal chenier plain mont st michel bay france surveyed with ground penetrating radar the experimentally obtained morphologies and internal structures matched those observed in the field and the three ridge development stages identified in ground penetrating radar profiles early transgressive late transgressive and progradational were modelled successfully flume experiments indicate that flat bioclastic shapes play a key role in sediment sorting in the breaker zone and in sediment layering in the beach and washover fans water level controls washover geometry beach ridge evolution and internal structure low water levels allow beach ridge stabilization and sediment accumulation lower on tidal flats during subsequent water level rise accumulated sediment becomes available for deposition of new washover units and for bayward extension of the beach ridges in the field low frequency water level fluctuations are related to the 4 4 year and 18 6 year tidal cycles experimental results suggest that these cycles may represent the underlying factor in the evolution of the macrotidal chenier coast at the multi decadal to centennial time scale

experimental simulation of shoreface nourishments under storm events a morphological hydrodynamic and sediment grain size analysis this study focuses on barred beach shoreface nourishments physically simulated in a wave flume the attack of a schematic storm on three different nourishments is analysed the apex and waning storm phases lead respectively to offshore and onshore sediment transports nourishments in the trough and on the outer bar feed the bar and increase wave dissipation offshore the bar acts as a wave filter and reduces shore erosion lee effect in contrast nourishment on the beach face leads mostly to shore feeding and reconstruction feeder effect with successive nourishments the beach face clearly becomes steeper and onshore sediment transport is reduced during moderate wave climates the surface grain size analysis reveals marked variations coarser sediments are sorted on the bar and the upper beach face these locations correspond to large wave dissipation zones during the storm apex

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 sand bar evolution on a natural beach waves currents and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross shore transect extending 255 m from 1 to 4 m water depth the seafloor location was measured nearly continuously even in the surf zone during storms with sonar altimeters mounted on fixed frames the crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore primarily when the offshore significant wave height exceeded about 2 m with 1 5 m of erosion near the initial location and 1 m of accretion at the final location an energetics type sediment transport model driven by locally measured near bottom currents predicts the observed offshore bar migration but not the slow onshore migration observed during low energy wave conditions the predicted offshore bar migration is driven primarily by cross shore gradients in predicted suspended sediment transport associated with quasi steady near bottom offshore flows these strong 50 cm s currents intensified near the bar crest by wave breaking are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side the feedback amoung morphology waves circulation and sediment transport thus forces offshore bar migration during storms

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

morphologic properties derived from a simple cross shore sediment transport model this paper builds on the now classical discussions by bowen 1980 and bailard 1981 on the applicability and implications of bagnold s 1963 sediment transport model to nearshore profile modeling we focus on the morphologic implications of both the strengths and weaknesses of bagnold s model isolating the transport terms that are well predicted i e mean flow terms from those that are not well predicted i e transport due to correlations between flow and sediment load we factor bagnold s model into a dimensional transport magnitude and a nondimensional term the nondimensional term describes the relative importance of transport due to undertow gravity and correlations between flow and sediment load the transport magnitude largely determines the response time of nearshore profiles for typical nearshore environments this response time was estimated to vary as a function of incident rms wave height hrms from 500 years hrms 0 5 m to 2 years hrms 3 m the relative importance of competing transport mechanisms is shown to depend strongly on the relative wave height defined as the ratio of the rms wave height to the local depth simplified nearshore transport parameterizations that are a function of this variable were derived and were interrogated for the existence and form of equilibrium profiles several differences from previously computed equilibrium profiles were noted first because the relative wave height saturates in natural surf zones equilibrium profiles converge to a relatively flat profile near the shoreline second under some situations a seaward sloping equilibrium profile may not exist third the long response times combined with unknown stability of an equilibrium profile make it difficult to assess the physical connection between theoretical equilibrium profiles and profiles observed in nature

application of prototype flume tests for beach nourishment assessment the development of underwater profiles and sand losses from the dune respectively the beach by various water level and wave conditions has been studied worldwide in large wave flumes since 1955 virtually all major results collected over the last decades are documented and described in this paper moreover to this overview results from the mast iii safe experiments carried out in the grosser wellenkanal gwk in hannover are summarized and discussed in more detail the tests were focused on beach and dune stability under given conditions with normal and raised water levels the results are concentrated on surf zone profiles beach slope and erosion furthermore dune erosion with and without barriers built into the dune face and perched surf zone by means of an underwater sill were investigated

bars formed by horizontal diffusion of suspended sediment abstract the mechanism responsible for the ubiquitous presence of convex beach profiles and shoreward migration of linear bars is examined using numerical circulation and sediment transport models the models are validated against laboratory measurements and observed natural beach cross sections while not discounting the importance of infragravity and advective horizontal circulation or bed return flow mechanisms a robust diffusive process explains the convex profile shape and bar formation in the presence of concentration gradients across the surf zone a diffusive sediment flux from high to low concentration results in the transfer of sediment outwards from the breakpoint both onshore and offshore and the subsequent formation of a diffusion bar and diffusion profile the profiles are characterised by single and double convex dome like shapes developing during shoreward migration of the bars by the diffusion mechanism the mechanism explains several phenomena observed on natural beaches including i convex beach profiles ii shoreward migration of the bar with concomitant beach accretion under narrow band swell iii reduced propensity for bar formation on low gradient fine sand beaches or under wide band wave spectra even though multiple bars are common on some low gradient beaches and iv offshore migration of the bar during periods of increasing wave height the diffusion mechanism can be dependent on orbital motion alone and as such requires no frequency selection or strong correlation between multiple processes for bar formation

nearshore sandbar migration field observations suggest that onshore sandbar migration observed when breaking wave driven mean flows are weak may be related to the skewed fluid accelerations associated with the orbital velocities of nonlinear surface waves large accelerations both increases and decreases in velocity magnitudes previously suggested to increase sediment suspension occur under the steep wave faces that immediately precede the maximum onshore directed orbital velocities weaker accelerations occur under the gently sloping rear wave faces that precede the maximum offshore directed velocities the timing of strong accelerations relative to onshore flow is hypothesized to produce net onshore sediment transport the observed acceleration skewness a measure of the difference in the magnitudes of accelerations under the front and rear wave faces is maximum near the sandbar crest the corresponding cross shore gradients of an acceleration related onshore sediment transport would cause erosion offshore and accretion onshore of the bar crest consistent with the observed onshore migration of the bar crest furthermore the observations and numerical simulations of nonlinear shallow water waves show that the region of strongly skewed accelerations moves shoreward with the bar suggesting that feedback between waves and evolving morphology can result in continuing onshore bar migration

influence of storm sequencing on breaker bar and shoreline evolution in large scale experiments abstract new large scale experiments on the influence of storm sequencing on beach profile evolution are presented the experiments comprised three sequences that commenced from the same initial beach profile each sequence consisted of two storms of different energy with each storm followed by a recovery phase a specific focus of the experiments was the influence of storm chronology as well as the influence of the recovery wave energy and duration on beach evolution the breaker bar and the shoreline are studied as indicators for the beach response both evolve towards an equilibrium location for each wave condition where the breaker bar reaches its equilibrium much faster than the shoreline overall no enhanced beach erosion due to storm sequencing is observed despite a similar cumulative wave power of the three sequences the final beach configuration of each sequence seems to be determined by the last wave condition instead of previous storms however storm sequencing is important when the beach profile has not yet fully recovered before being disrupted by the subsequent storm in this case the second storm does not necessarily cause further erosion but can result in onshore sediment transport and hence form part of the recovery the bulk onshore sediment transport as well as the shoreline recovery rate vary depending on the wave condition and the profile disequilibrium and show a maximum value for a recovery condition with an intermediate wave energy the very low energy condition that was performed for a long duration 24 hours generated a near stationary beach profile with typical features of a reflective beach

physical modeling of three dimensional intermediate beach morphodynamics experiments have been performed in a large wave tank in order to study the morphodynamics of rip current systems both accretive and erosive shore normal wave conditions were applied the beach evolving through all the states within the intermediate beach classification under the so called down state accretive and up state erosive morphological transitions results show that any prescribed change in the wave conditions drastically increases the rate at which the morphology changes the surf zone morphology tends toward a steady state when running a given wave climate for a long duration we quantitatively describe a full down state sequence characterized by the progressive evolution of an alongshore uniform bar successively into a crescentic plan shape a bar and rip channel morphology and a terrace from the analysis of a large data set of dense eulerian measurements and bathymetric surveys we depict several feedback mechanisms associated with wave driven rip current circulation wave nonlinearities and the seabed evolution at first a positive feedback mechanism drives a rapid increase in the rate of morphological change beach three dimensionality and rip intensity by the time the sandbar evolves into a bar and rip morphology a negative feedback mechanism characterized by a decaying beach change rate and an increasing beach alongshore uniformity overwhelms the former mechanism an erosive sequence characterized by both an overall offshore bar migration and an increase in beach three dimensionality is also described

mechanics of the scouring and sinking of submerged structures in a mobile bed a physical model study abstract wave induced flow velocity and turbulence may cause scouring in the seabed around coastal structures depending on the wave climate and tidal water depth a 3 d physical modelling study was conducted to investigate the possible causes of the sinking of two submerged coastal structures on the santa maria del mar smm beach spain the experimental investigation was conducted by employing a froude similarity law with a geometric scale of 1 20 and the submerged modular structure was subjected to different wave climates and tidal water levels the combinations of the significant wave height and peak period were chosen from in situ real wave conditions that were monitored during the sinking of the prototype structures linking and unlinking conditions for the modules in the structure were investigated in this study the results show that the modules sank to approximately 48 of their height at the end of the tests with storm waves and semi linking conditions of low water depth most of the experimental results were compatible with the prototype monitoring results tests with proper linking among the modules and tests with an appropriate gravel foundation resulted in a sustainable solution because they presented much less or almost no scouring and sinking

impact of meridional winds on equatorial spread f revisited 1 the impact of meridional winds on the onset and evolution of equatorial spread f esf is studied using the nrl ionosphere model sami3 esf the prevailing belief is that meridional winds have a stabilizing effect on the onset of esf in this letter it is shown that this is not necessarily the case and that meridional winds can be destabilizing we find that a wind profile with a positive gradient as a function of latitude v m 0 is a stabilizing influence on the generalized rayleigh taylor instability a wind profile with a negative gradient v m 0 is a destabilizing influence here a northward wind is positive and increases in the northward direction it is suggested that meridional wind profiles may account for in part the longitudinal and day to day variability of esf

shore and bar cross shore migration rotation and breathing processes at an embayed beach a principal component analysis pca is used to decompose data on the coupled morphodynamics of the shoreline and nearshore sandbar of a typical single barred embayed beach tairua beach new zealand dynamic patterns are classified into simultaneous modes where the bar and shoreline move at the same time and non simultaneous modes where the shore moves independently from the bar and vice versa two simultaneous modes accounting for 65 of the variance of the shoreline and barline dominate the system one mode describes inverse shoreline and sandbar cross shore migrations alongshore averaged occurring with simultaneous rotations in the same direction the other mode accounts for migration in the same direction accompanied by variations of the barline curvature similar to breathing modes previously described in embayed beach shoreline modelling studies two non simultaneous modes of lesser importance account separately for independent shoreline and barline rotations 10 to 15 of the variance explained a pca applied to the shore and sandbar behaviours modelled by four standard equilibrium models simulating shore and sandbar cross shore migrations and rotations show that these are interrelated because of a correlation between wave energy and direction shore and bar rotations are coupled partially because the shape of the bay induces a correlation of their respective drivers the wave angle of incidence and the alongshore gradient of wave energy however this correlation depends on the wave energy this in combination with different shore and sandbar response times quantified using the models also explains the independent rotations reflected by the non simultaneous modes

surf zone cross shore boundary layer velocity asymmetry and skewness an experimental study on a mobile bed 1 an experimental study on a physical model of a beach in a two dimensional wave flume was designed to investigate velocity nonlinearities in the wave boundary layer the cross shore velocity was measured in the surf zone along a vertical profile every 3mm from free stream elevation down to the still bed level the skewness and the asymmetry of the phase averaged velocity were computed at each elevation observations indicate that the free stream asymmetry transforms into bottom velocity skewness a linear experimental relation between free stream asymmetry to skewness ratio and bottom skewness to free stream skewness ratio is established a theoretical linear relationship is discussed which predicts the phase lead of the bottom velocity this phase lead is also determined by fourier analyzing the velocity time series the first two fourier components yield the same phase lead at the bed that is found to be about 30 and nearly constant over all the experiments made

bar trough generation on a natural beach mechanisms for bar trough generation are examined using velocities measured in the field applied to the bowen 1980 bailard 1981 energetics based sediment transport model measurements consist of a cross shore array of nine electromagnetic current meters spanning the surf zone and daily bathymetric surveys during a 10 day period during which two storms occurred when the bathymetry evolved from a three dimensional terrace to a well developed linear bar the model predicts bed and suspended load transport separately based on various velocity moments the velocities are partitioned into mean currents low frequency infragravity and shear instabilities 0 05 hz to determine the relative importance of various mechanisms to the total transport velocity moments are computed over 90 min intervals to resolve tidal fluctuations tidal signatures were apparent in all modes of transport predicted transport rates are integrated and compared with daily cross shore bathymetric profiles averaged over a 400 m length of beach the suspended load terms were an order of magnitude greater than bed load terms owing to the low fall velocity of the fine grain sand within the surf zone model results for this experiment indicate the dominant mechanism for bar development was sediments mobilized by the strong longshore current and incident short waves within the surf zone and transported offshore by the mean undertow and shoreward transport onshore due to short wave velocity skewness using standard coefficients the model correctly predicted the first order movement of the bar during storms but underpredicted trough development and did not always perform well during mild wave conditions

elastic anisotropy in textured hcp iron to 112 gpa from sound wave propagation measurements we report the first experimental determination of elastic anisotropy in highly textured polycrystalline hexagonal closed packed hcp iron obtained at room temperature and 112 gpa by inelastic x ray scattering ixs in a diamond anvil cell dac the compressional sound velocity vp at 50 and 90 from the c axis was determined to be vp 50 9900 200 m s and vp 90 9450 150 m s respectively the difference of 4 5 between the two velocities is of the same order of the seismic observations of the anisotropy in earth s inner core combining our new results with the equation of state of hcp iron we derive the pressure dependence of the shear velocity vs the vs values extrapolated to inner core densities are 30 higher than the prem results the extrapolated vp values are instead in fair agreement with the prem seismic model though differences in the slope remain furthermore assuming a strong texture characterized by cylindrical symmetry we provide an estimate for the single crystal elastic modulus c11 as a function of pressure while our results are in general agreement with the predominant existence of hcp iron in earth s inner core the remaining differences might be assigned to the presence of another solid iron phase light elements or liquid inclusions as well as anharmonic high temperature effects

data driven numerical simulations of equatorial spread f in the peruvian sector 3 solstice we present results from a continuing effort to simulate equatorial spread f esf using observations from the jicamarca radio observatory near lima peru jicamarca measures vertical and zonal plasma drifts along with plasma number density profiles overhead the number density profiles are used to initialize a three dimensional regional model of the ionosphere capable of simulating plasma density irregularities produced during esf conditions the vertical drifts measurements are used to drive the numerical simulation continuously neutral winds are derived from the new horizontal wind model 14 hwm 14 model and the zonal winds are scaled so as to make the zonal plasma flows at the start of the simulation agree with the isr profile measurements coherent scatter radar imagery from jicamarca is used to validate the simulation results campaign data were collected in april and december 2014 and a few events representative of low and high esf activity were selected for analysis the numerical simulations are able to reproduce the level of activity observed along with the gross features of the esf irregularities and radar plumes data from a network of hf beacons are being incorporated into the forecast analysis in order to elucidate radar plumes which sometimes appear even when the simulation fails to predict them

modeling cross shore sandbar behavior on the timescale of weeks we compare predictions of a coupled wave averaged cross shore waves currents bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration the observations span a 10 and 44 day period with onshore offshore bar migration at duck north carolina and at hasaki kashima coast japan respectively a 3 5 month period of onshore bar migration at duck and a 22 day period of offshore bar migration at egmond netherlands with best fit parameter values the modeled temporal evolution of the cross shore bed profiles agrees well with the observations model skill defined as 1 minus the ratio of prediction to no change error variances ranges from 0 50 at egmond to 0 88 for the prolonged onshore bar migration at duck localized in time and space reductions in model skill coincide with alongshore variations in the observed morphology consistent with earlier observations simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves undertow suspended sediment transport and the sandbar simulated onshore bar migration is predicted for energetic weakly to nonbreaking conditions and is due to the feedback between near bed wave skewness bedload transport and the sandbar with negligible to small effects of bound infragravity waves and near bed streaming under small waves and conditions when breaking and nonbreaking conditions alternate with the tide the sandbar is predicted to remain stationary the intersite differences in the optimum parameter values are at least partly induced by insensitivity to parameter variations parameter interdependence and errors in the offshore wave forcing

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

wave induced sediment transport and onshore sandbar migration the 25 m onshore migration of a nearshore sandbar observed over a 5 day period near duck nc is simulated with a simplified computationally efficient wave resolving single phase model the modeled sediment transport is assumed to occur close to the seabed and to be in phase with the bottom stress neglected intergranular stresses and fluid granular interactions likely important in concentrated flow are compensated for with an elevated relative to that appropriate for a clear fluid model roughness height that gives the best fit to the observed bar migration model results suggest that when mean current induced transport is small wave induced transport leads to the observed onshore bar migration based on the results from the simplified phase resolving model a wave averaged energetics type model e g only moments of the near bottom velocity field are required with different friction factors for oscillatory and mean flows is developed that also predicts the observed bar migration although the assumptions underlying the models differ the similarity of model results precludes determination of the dominant mechanisms of sediment transport during onshore bar migration

temporal and spatial scales of beach profile change duck north carolina abstract an 11 year time series of high resolution beach profile surveys made on an atlantic ocean beach was analyzed for spatial and temporal characteristics of beach profile change approximately 300 profile surveys most extending from the dune to 8 m depth were available for the analysis on each of four cross shore lines together with electronically recorded and statistically processed wave time series from a nearshore gage located seaward of the survey site the profile survey data set was analyzed for such quantities as depth change frequency of depth change general seasonal shape of the profile seasonal depth change and change in the profile produced by extreme storms the morphodynamics of an inner and outer longshore bar were also examined including depth to bar crest bar height and length and speed of bar movement onshore and offshore several properties of depth change were related to wave characteristics representative results were average profile elevation change from 4 m to 4 m was symmetric about the mean sea level shoreline the average spring and autumn profile shapes were almost identical and occurred as transitional states between the summer and winter profile shapes the depth of active profile movement within survey accuracy of about 0 025 m was 4 m for the summer and 6 m for the winter and the 10 year frequencies of maximum absolute depth change in depths of 2 4 6 and 8 m were 1 64 1 38 0 22 and 0 12 m respectively a surprising result was that typical large storms transported sand into the nearshore from the seaward end of the profile from a depth of about 6 8 m

seismic anisotropy in the earth s innermost inner core testing structural models against mineral physics predictions using an updated data set of ballistic pkikp travel time data at antipodal distances we test different models of anisotropy in the earth s innermost inner core imic and obtain significantly better fits for a fast axis aligned with earth s rotation axis rather than a quasi equatorial direction as proposed recently reviewing recent results on the single crystal structure and elasticity of iron at core conditions we find that an hcp structure with the fast c axis parallel to earth s rotation is more likely but a body centered cubic structure with the 111 axis aligned in that direction results in very similar predictions for seismic anisotropy these models are therefore not distinguishable based on current seismological data in addition to match the seismological observations the inferred strength of anisotropy in the imic 6 7 implies almost perfect alignment of iron crystals an intriguing albeit unlikely situation especially in the presence of heterogeneity which calls for further studies

storm sequencing and beach profile variability at hasaki japan abstract beach profile evolution under storm sequence forcing presents an emerging research topic that has only been investigated at a limited number of sites the occurrence and effects of storm sequencing on beach profile evolution are studied at hasaki beach japan using weekly beach profile and two hourly offshore wave measurements during the 25 year study period the supratidal beach at hasaki is subjected to long term accretion and steepening while the shoreline shows a long term oscillation in addition oscillations of the supratidal beach volume and the shoreline at semi annual and annual intervals are identified which are largely controlled by the variability of the wave height hasaki beach is subjected to frequent storms that often cluster in sequences especially during the extra tropical cyclone season january to march the majority of storms and sequences generate erosion of the beach above the low water level but some also lead to recovery despite a tendency for storms and storm sequences with larger power to cause more erosion the present data does not demonstrate increased beach erosion by storm sequences following these findings the tendency of the beach to evolve towards equilibrium and the importance of the antecedent beach morphology are demonstrated

sand transport by unbroken water waves under sheet flow conditions a model of wave induced onshore sand transport and resulting topographical changes seaward of the break point on a sloping beach is presented for the specific case in which the waves are random normally incident weakly nonlinear and relatively long the bottom boundary layer is fully turbulent and sand transport occurs as sheet flow the model consists of the following components 1 a new empirical expression for sand transport as sheet flow in which the instantaneous transport rate is directly proportional to the instantaneous bottom shear stress 2 an expresssion for the mean bottom shear stress based on a theoretical analysis of the bottom boundary layer 3 a simple representation of the shoaling wave field in which the local properties are described by linear long wave theory the probability density function of the surface displacement is gaussian and the waves conserve energy flux as they shoal and 4 the equation for conservation of sediment mass computations based on the model compare favorably with recently reported field measurements of the onshore motion of a long offshore bar

field evidence of pressure gradient induced incipient motion 1 recently models for the onshore migration of nearshore sandbars were improved by including an empirical acceleration term in the sediment transport formulation hoefel and elgar 2003 here field observations of the fluid sediment interface suggest that the success of this empirical parameter may result from unsteady forcing of sediment beds by nearshore waves the velocity and sediment observations show that under large pressure gradients present during the strengthening of onshore directed flow several centimeters of previously still sediment can be mobilized and transported onshore the onshore directed mobile bed layer rapidly settles as the wave velocity weakens these observations are inconsistent with traditional sediment transport theories shields 1936 bagnold 1966 that assume incipient motion is solely based on the force applied by bed stress but are consistent with theories that include an additional force induced by the horizontal pressure gradient madsen 1974 sleath 1999 in this paper we provide the first field evidence of incipient motion that is significantly influenced by the unsteady forcing introduced by the horizontal pressure gradient due to surface gravity waves

stability of iron crystal structures at 0 3 1 5 tpa abstract ab initio molecular dynamics simulations carried out for tetragonal and orthorhombic distortions of iron closely follow the results of static lattice electronic structure calculations in revealing that the body centered cubic bcc phase of fe is mechanically unstable at pressures of 0 3 1 5 tpa and temperatures up to 7000 k crystal structural instabilities originate in the static lattice for the bcc configuration and are consistent with recent results from both static and dynamic high pressure experiments both theory and experiment thus show that the close packed hexagonal hcp and face centered cubic fcc crystal structures of iron are those relevant to the cores of earth like planets

shoreline resilience to individual storms and storm clusters on a meso macrotidal barred beach abstract this study investigates the impact of individual storms and storm clusters on shoreline recovery for the meso to macrotidal barred biscarrosse beach in sw france using 6 years of daily video observations while the study area experienced 60 storms during the 6 year study period only 36 storms were analysed due to gaps in the video data based on the 36 individual storms and 13 storm clusters analysed our results show that clustering impact is cumulatively weak and shoreline retreat is governed by the first storms in clusters while the impact of subsequent events is less pronounced the average post storm beach recovery period at this site is 9 days consistent with observations at other beaches apart from the dominant effect of present storm conditions shoreline dynamics are also significantly affected by previous storm influence while recovery is strongly modulated by tidal range and the bar location our results reveal that not only is the storm energy important but also the frequency of recurrence storms result in greater retreat when time intervals between them are longer which suggests an interaction between short storm events and longer term evolution

cross shore distribution of longshore sediment transport comparison between predictive formulas and field measurements abstract the skill of six well known formulas developed for calculating the longshore sediment transport rate was evaluated in the present study formulas proposed by bijker bijker e w 1967 some considerations about scales for coastal models with movable bed delft hydraulics laboratory publication 50 delft the netherlands journal of the waterways harbors and coastal engineering division 97 4 1971 687 engelund hansen engelund f hansen e 1967 a monograph on sediment transport in alluvial streams teknisk forlag copenhagen denmark ackers white journal of hydraulics division 99 1 1973 2041 bailard inman journal of geophysical research 86 c3 1981 2035 van rijn journal of hydraulic division 110 10 1984 1431 110 11 1984 1613 110 12 1984 1733 and watanabe watanabe a 1992 total rate and distribution of longshore sand transport proceedings of the 23rd coastal engineering conference asce 2528 2541 were investigated because they are commonly employed in engineering studies to calculate the time averaged net sediment transport rate in the surf zone the predictive capability of these six formulas was examined by comparison to detailed high quality data on hydrodynamics and sediment transport from duck nc collected during the duck85 superduck and sandyduck field data collection projects measured hydrodynamics were employed as much as possible to reduce uncertainties in the calculations and all formulas were applied with standard coefficient values without calibration to the data sets overall the van rijn formula was found to yield the most reliable predictions over the range of swell and storm conditions covered by the field data set the engelund hansen formula worked reasonably well although with large scatter for the storm cases whereas the bailard inman formula systematically overestimated the swell cases and underestimated the storm cases the formulas by watanabe and ackers white produced satisfactory results for most cases although the former overestimated the transport rates for swell cases and the latter yielded considerable scatter for storm cases finally the bijker formula systematically overestimated the transport rates for all cases it should be pointed out that the coefficient values in most of the employed formulas were based primarily on data from the laboratory or from the river environment thus re calibration of the coefficient values by reference to field data from the surf zone is expected to improve their predictive capability although the limited amount of high quality field data available at present makes it difficult to obtain values that would be applicable to a wide range of wave and beach conditions

ab initio calculations of the elasticity of iron and iron alloys at inner core conditions evidence for a partially molten inner core abstract ab initio finite temperature molecular dynamics simulations have been used to calculate the elasticity of hcp fe bcc fe fes and fesi at core conditions the calculated compressional wave velocities are in excellent agreement with the most recent experimental data however the calculated shear wave velocities of all phases studied are significantly higher than those inferred from seismology this discrepancy can only be explained if the inner core is partially molten containing more than 8 liquid

an energetics total load sediment transport model for a plane sloping beach bagnold s energetics based total load sediment transport model for streams is used as a basis for the development of a total load model of time varying sediment transport over a plane sloping bed in both the bedload and suspended load the transport rate vectors are found to be composed of a velocity induced component directed parallel to the instantaneous velocity vector and a gravity induced component directed down slope the model is applied to idealized surfzone conditions leading to estimates of the local longshore and onshore offshore sediment transport rates as well as the equilibrium beach slope as a function of the local wave and current conditions the model is combined with a nonlinear longshore current model and spatially integrated to obtain predictions of the total longshore transport rate as a function of the incident wave conditions the results support the general form of the wave power equation except that the wave power coefficient is no longer constant but is instead a complex function of the incident wave and beach characteristics

the behaviour of a multiple bar system in the nearshore zone of terschelling the netherlands 1965 1993 abstract the behaviour of a multiple bar system in the nearshore zone of the island of terschelling the netherlands is investigated on the time scale of years using a data set of soundings the data set covers the period from 1965 to 1993 the behaviour is analysed in terms of bar crest position and in terms of morphometric parameters such as bar crest depth height width and volume two or three breaker bars are permanently present in the study area each individual bar passes through three stages during its existence 1 generation close to the shore 2 seaward migration and 3 degeneration at the outer margin of the nearshore bar zone the key factor controlling the behaviour of the inner bars is the crest depth of the outer bar three different couplings between the behaviour of the individual bars have been observed 1 bar stage changes due to the degeneration of the outer bar 2 prevention of the transition of an inner bar from the first to the second stage caused by the appearance from alongshore of a new outer bar having a small crest depth and 3 difference in mean annual seaward migration rate caused by the possible presence of a more seaward positioned bar it is hypothesized that the crest depth of the outer bar governs the relative importance of processes inducing shoreward and seaward migration in the inner nearshore bar zone and therefore controls the behaviour of the nearshore bar zone on the time scale of years

nearshore sandbar migration predicted by an eddy diffusive boundary layer model 1 we simulated the erosion and accretion of a natural beach using a wave resolving eddy diffusive model of water and suspended sediment motion in the bottom boundary layer nonlinear advection was included in this one dimensional vertical profile model by assuming that waves propagated almost without change of form flows were forced by fluctuating pressure gradients chosen to reproduce the velocity time series measured during the duck94 field experiment the cross shore flux of suspended sediment beneath each field deployed current meter was estimated and beach erosion accretion was calculated from the divergence convergence of this flux horizontal pressure forces on sediment particles were neglected the model successfully predicted two bar migration events one shoreward bar migration and one seaward but failed to predict a third seaward migration event simulated seaward sediment transport was due to seaward mean currents simulated shoreward sediment transport was due to covariance between wave frequency fluctuations in velocity and sediment concentration and was mostly confined to the wave boundary layer predicted seaward shoreward bar migration was driven by a maximum in the current generated wave generated flux over the sandbar a wave generated downward flux of shoreward momentum into the wave boundary layer contributed to shoreward sediment transport and often had a local maximum over the bar crest second order nonlinear advection of sediment mostly representing shoreward advection by the stokes drift also often had a local maximum over the bar crest together wave generated momentum fluxes and the stokes drift substantially increased shoreward transport and were essential to predictions of shoreward bar migration

relation between beachface morphology and wave climate at trafalgar beach cadiz spain abstract two years of offshore wave data and daily time exposure images from trafalgar beach a 2 km long sandy beach located on the southwest coast of spain that frequently exhibits rhythmic features were used to 1 explore the variability of the beachface morphology and 2 determine environmental conditions associated with the different morphological states the beachface morphology at three distinct alongshore sectors was analyzed and classified and five different morphological states were found that are related with the presence or absence of beach cusps and a berm 1 large beach cusps 2 small beach cusps 3 low tide terrace 4 plane beach berm and 5 plane beach the predominant beachface morphology is characterized by the presence of large beach cusps and the main wave climate consisted of offshore significant wave heights ranging from 0 5 1 m and wave periods between 4 and 12 s an alongshore variation of the morphology is found which might be related to the nearshore wave variability swan wave model results the morphologies are in some cases well correlated with the daily offshore incident wave climate described by the daily maximum significant wave height and the corresponding period particularly for the moderate to high energy wave conditions small beach cusps appear under short period waves whereas when the wave periods are longer the morphology tends to change to large beach cusps this transition only occurs if the forcing is maintained as constant for a certain duration which depends itself on the wave energy it is concluded that correlations over 90 are only found for the highest wave energy conditions or under long wave periods for the remainder it is not possible to generally correlate the beachface morphology based only on the wave forcing because the previous morphological state cannot be ignored

laboratory observations of flow and sediment transport induced by plunging regular waves 1 two parallel experiments involving the evolution and runup induced by plunging regular waves near the shoreline of a sloping bed are considered 1 a rigid bed experiment allowing direct hot film measurements of bed shear stresses and 2 a sediment bed experiment allowing for the measurement of pore water pressures as well as observation of sediment suspension and bed morphological changes both experiments utilize the same initial bed profile and wave forcing the experiments show that the mean bed shear stresses experienced onshore of incipient breaking are amplified by nearly a factor of 2 relative to prebreaking conditions whereas their corresponding turbulent fluctuations are amplified even more strongly by a factor of 5 6 the plunging processes lead to a series of vortices whose formation may be explained as the result of shear layer instability measurements show that these vortices can significantly enhance peaks in the offshore directed bed shear stresses moreover near bed pore pressure measurements indicate that these vortices cause large upward directed pressure gradients which in turn produce a corresponding series of suspended sediment plumes shoreward of the initial breaking event these findings are related to the induced morphological changes over both short and long time scales the present results are also compared and contrasted with previous experiments utilizing a similar methodology but involving plunging solitary waves

effects of horizontal pressure gradients on bed destabilization under waves we report on new experiments designed to investigate bed destabilization processes in a two dimensional wave flume physical model of a beach the mobile bed consists of non cohesive granular material of low density the wave conditions are provided by repeating a cycle of waves made of two bichromatic groups of different period the horizontal and vertical velocities are acoustically profiled vertically from free stream elevation down to the still bed level in the surf zone additional measurements of the fluid pressure at positions closely aligned horizontally and vertically in and slightly above the sediment bed are undertaken mobile bed interfaces still bed and top interface are detected via acoustic and optical methods both methods are cross compared and give similar results flow turbulence over the bed is analysed the reynolds turbulent shear stress is found negligible compared to the orbital flow induced momentum diffusion the shear stress and the horizontal pressure gradient are computed at near bed elevation and used in the bed incipient plug flow model of sleath cont shelf res vol 19 13 1999 pp 1643 1664 both the model and the measurements confirm that destabilization occurs when the non dimensional pressure gradient or sleath number exceeds the threshold value of 0 3 which is simultaneous with strong flow acceleration the near bottom fluid shear stress detected during these flow accelerations at steep wave fronts is found experimentally to be negative which is retrieved with an unsteady plug flow model this is suggesting that the fluid above the bed resists the sediment layer motion at these particular phases

the spatial and temporal variability of sand bar morphology the spatial and temporal variability of nearshore sand bar morphology is quantified using a unique data set spanning 2 years the data consist of daily time exposure images of incident wave breaking on an open coast sandy beach which may be used to infer bar morphology lippmann and holman 1989 the morphology in each image is classified into an eight state morphologic scheme in which bars are uniquely defined by four independent criteria the most frequently observed morphologies are the longshore periodic rhythmic bars observed in 68 of the data linear bars occur under highest wave conditions h s 1 78 m and are unstable mean residence time 2 days shore attached rhythmic bars are the most stable mean residence time 11 days and generally form 5 16 days following peak wave events non rhythmic three dimensional bars are very transient mean residence time 3 days eighty seven percent of transitions to lower bar types defined in text occurred one state at a time supporting our selection of the ordering of states and suggesting the suitability of a sequential morphology model transitions to higher states occurred under rising wave energy and were evenly spread among the possible higher states with more substantial changes in morphology resulting from larger wave increases this suggests that up state erosional transitions based on offshore bar migration are better described by an equilibrium model where response is better correlated with incident wave energy than with preceding morphological state time exposure images were also digitized to yield quantitative estimates of bar crest location as a function of longshore distance principal component analysis was used to decompose bar position into two dimensional linear and three dimensional longshore variable components cross shore linear bar position ranges 50 m about the 2 year mean 27 m standard deviation and dominates bar variability 74 6 three dimensional bar structure accounts for 14 of the variance 12 m standard deviation changes in incident wave height precede cross shore bar migration by less than 1 day changes in longshore variability are inversely correlated to changing wave conditions with bar morphology becoming linear rapidly during storms on time scales of less than 1 day evolution to significantly three dimensional structure typically occurs over 5 7 days following peak wave events

shorerise and bar berm profiles on ocean beaches a beach equilibrium model is developed that treats the outer shorerise portion of the profile independently from that of the inner bar berm portion the two portions are matched at the breakpoint bar the partitioning of the profile in this way is consistent with the different forcing modes on either side of the breakpoint this formulation utilizes beach profile data not previously available it is shown that both portions of the profile are well fitted by curves of the form h axm where h is positive downward and x is the positive offshore coordinate surprisingly the value of m 0 4 is nearly the same for shorerise and bar berm and does not change significantly with seasonal beach changes summer winter the principal difference between seasonal profiles is that in winter higher waves the breakpoint bar is deeper and farther offshore while the berm crest is displaced landward thus the changes in seasonal equilibria are manifested by simple self similar displacements of the bar berm and shorerise curves as a consequence of changes in surf zone width and o 1 variations in the factor a

wave bottom boundary layer processes below irregular surfzone breaking waves with light weight sheet flow particle transport the present work investigates the structure of the near bed flow below irregular surfzone breaking waves inducing light weight sheet flow particle transport the experiments are carried out in the legi flume under steady equilibrium conditions between the wave forcing and the underlying bed morphology synchronized acvp and video images provide detailed information about the mean wave and current characteristics and the coupled flow regimes across the entire wave breaking region including the outer and the inner surfzones an analysis of the impact of breaking eddies in the wave boundary layer wbl is undertaken at the beginning of the inner surfzone subsequently the intrawave variation of several contributions of the total shearing force per unit area and the net values of the reynolds stress related to phase averaged velocities are analyzed it is found that the wave component is the dominant term the turbulent reynolds stress the low frequency and the mean terms are at least 1 order of magnitude lower due to the irregular wave forcing the net values are separated into the net wave by wave reynolds stress and the wave reynolds stress averaged over the entire irregular wave sequence all these measured bed shear stress terms are then compared to estimations obtained with two different parameterized models in order to evaluate their prediction performances the values of the model parameters are discussed in comparison to those found in the literature finally the vertical profile of net reynolds shear stress exhibits a nearly constant value across the sheet flow layer

bar generating cross shore flow mechanisms on a beach random waves normally incident on a dissipative beach induce a variety of cross shore flows such as asymmetric oscillatory flow wave grouping induced long wave flow breaking induced turbulent flow and momentum decay induced undertow these flows are identified analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two dimensional case of bar generation on a beach

the condor equatorial spread f campaign overview and results of the large scale measurements during the condor campaign a number of instruments were set up in peru to support the rocket experiments in this series of papers we report on the results of the experiments designed to study the equatorial f region in this overview paper we summarize the main results as well as report upon the macroscopic developments of spread f as evidenced by data from backscatter radas from scintillation observations and from digital ionosonde meaurements in this latter regard we argue here that at least two factors other than the classical gravitational rayleigh taylor plasma instability process must operate to yield the longest scale horizontal organization of spread f structures the horizontal scale typical of plume separation distances can be explained by invoking the effect of a shear in the plasma flow although detailed comparison with theory seems to require shear frequencies a bit higher than observa tions indicate on the other hand the largest scale organization or modulation of the scattering layer cannot be explained by the shear theory and must be due to local time variations in the ionospheric drift or to gravity wave induced vertical motions using simultaneous rocket and radar data we were also able to confirm the oft quoted hypothesis that rapid overhead height variations in the scattering region over jicamarca are primarily spatial structures advecting overhead the detailed rocket radar compari son verified several other earlier results and speculations particularly those made in the plumex experiments in particular companion papers discuss and extend some of the plumex results to include the role of anomalous diffusion labelle et al this issue in the theory of equatorial spread f and to shed light upon the shallow spectral form often observed in the intermediate scale regime labelle and kelley this issue

storm driven variability of the beach nearshore profile at duck north carolina usa 1981 1991 the us army corps of engineers field research facility frf at duck north carolina has collected approximately biweekly beach nearshore profile data to 8 m depth and associated wave data since 1981 sediment budget analysis was used to examine the medium scale years to a decade variability of the beach nearshore profile from 1981 to 1991 significant changes occurred during four groups of energetic storm events during february march of 1983 1987 1989 and december 1989 each group was comprised of at least two storms within a period of less than 39 days both with hmo 4 m during each storm group offshore sediment movement caused a distinct outer bar to migrate offshore and grow in size resulting in an abrupt increase in the volume of sediment on the upper shoreface the net profile changes were much larger than the changes due to single storms and the cumulative effect of the storms can be considered as one event during these events the first storm appears to have a destabilizing effect on the profile which has insufficient time to recover before the second and subsequent storm s as a result several storms in quick succession are able to have a large impact on the morphology the intervening periods between the groups of storm events termed fairweather conditions lasted up to 4 years they are characterized by slow but steady sediment redistribution averaging 33 m3 m 1 year 1 from the upper shoreface 5 m depth toward the shore while the total sediment volume was effectively constant the onshore feed of sediment was not significantly affected by individual storms during the fairweather conditions these two processes of 1 morphologic change during groups of storm events and 2 the steady onshore feed of sediments from the shoreface during fairweather conditions appear to play an important role on medium and long term profile evolution at least at duck

on cross shore migration and equilibrium states of nearshore sandbars 1 the location of submerged sandbars in the nearshore zone changes over time in response to variability in wave conditions here cross shore sandbar migration is studied with an empirical model consisting of a differential equation relating cross shore sandbar migration to wave forcing model parameters are fitted to data sets containing multiple years of daily observed positions of five sandbars at three field sites from the fitted model parameters we determine the cross shore location of zero migration equilibrium location the stability of this equilibrium and the rate response time at which a sandbar migrates toward or away from the equilibrium location we find that for breaking waves a sandbar moves toward a stable wave height dependent equilibrium location for non to slightly breaking waves however a sandbar moves away from the equilibrium location implying that the beach profile evolves toward a state without submerged sandbars the response times can differ up to a factor three for different sandbars at a particular field site and a factor ten between sandbars at different sites for breaking waves response times are found to decrease with increasing wave height from months to days while for non to slightly breaking waves response times exceed several months in general response times remain larger than the timescale of variability in wave climate several days implying that sandbars spend most of their lifetime out of equilibrium with the wave forcing the model is able to hindcast the relevant features of cross shore sandbar migration on timescales of days to several years and outperforms a baseline prediction of no change for all sandbars when the equilibrium location is reached by both the hindcasted and actual sandbar the hindcast error depends on the accuracy of the predicted equilibrium location only and no longer on the history of accumulated errors as a result sandbars that occasionally reach their equilibrium location can accurately be predicted from the wave forcings over their entire lifespan however it is difficult to predict the exact rate of the yearly to interannual trends that are observed at two field sites because the sandbars at these sites never reach the offshore located equilibrium states associated with high waves

formation of two section cross shore profile under joint influence of random short waves and groups of long waves abstract we show that beach profiles may develop a two section almost equilibrium structure under joint impact of short wind waves and groups of long waves with duration of a few minutes and properties that are independent of wind waves the upper section of the profile is convex and follows the power law h x4 3 at small depths and in the swash zone wind waves may cause either erosion or accretion of this section but groups of long waves effectively redistribute the sediment over the convex section and practically do not alter the sediment volume the ability of short groups of highly energetic waves to regularly build up such profiles from those created by random sea waves is a principally new feature of wave coastal interaction that may have extensive consequences on the estimates of the wave induced coastal hazards

sand transport under combined current and wave conditions a semi unsteady practical model for the general purposes of morphodynamic computations in coastal zones simple formula based models are usually employed to evaluate sediment transport sediment transport rates are computed as a function of the bottom shear stress or the near bed flow velocity and it is generally assumed that the sediment particles react immediately to changes in flow conditions it has been recognized through recent laboratory experiments in both rippled and plane bed sheet flow conditions that sediment reacts to the flow in a complex manner involving non steady processes resulting from memory and settling entrainment delay effects these processes may be important in the cross shore direction where sediment transport is mainly caused by the oscillatory motions induced by surface short gravity waves the aim of the present work is to develop a semi unsteady practical model to predict the total bed load and suspended load sediment transport rates in wave or combined wave current flow conditions that are characteristic of the coastal zone the unsteady effects are reproduced indirectly by taking into account the delayed settling of sediment particles the net sediment transport rates are computed from the total bottom shear stress and the model takes into account the velocity and acceleration asymmetries of the waves as they propagate towards the shore a comparison has been carried out between the computed net sediment transport rates with a large data set of experimental results for different flow conditions wave current flows purely oscillatory flow skewed waves and steady currents in different regimes plane bed and rippled bed with fine medium and coarse uniform sand the numerical results obtained are reasonably accurate within a factor of 2 based on this analysis the limits and validity of the present formulation are discussed

a morphological model of the beach profile integrating wave and tidal influences abstract meso and macrotidal beaches constitute a significant proportion of the world s beaches however by far they have been less studied and understood than tideless beaches even though in recent times several local studies have addressed this problem few attempts to model tidal beaches exist a morphological model capable of predicting the beach profile behaviour under different wave and tide conditions is proposed it is based on the concept of the two section equilibrium beach profile and has been validated with field and laboratory data this is achieved by means of two parameters the modal tidal range and the dimensionless fall velocity tide is considered a local variable whose principal effect is the lengthening of the intertidal profile the greater the tidal range the wider the intertidal profile here defined as the surf profile the dimensionless fall velocity defines the transition from dissipative to reflective situations in beaches of any given tidal range wave height is the controlling parameter in seasonal beach changes as the wave height decreases the beach profile changes from erosive to accumulative these morphological changes in the surf and shoaling sections of the profile occur in the opposite direction whilst in the surf profile the slope of the upper part of this section becomes steeper and the concavity of whole section increases in the shoaling profile the upper part flattens resulting in a less concave section in this transition the slope break between surf and shoaling profiles here defined as the discontinuity point becomes smoother and difficult to identify the main morphological parameter of the model xo describes the length of the surf profile this parameter is capable of expressing slightly different variability in micro meso and macrotidal beaches in microtidal beaches the length of the surf profile decreases proportionally to the dimensionless fall velocity in meso and macrotidal beaches xo decreases from the dissipative to the intermediate state but increases from intermediate to reflective this results from the flattening of the lower part of the surf profile in the reflective case where a small change in tidal range generates a high stretching of the surf profile this beach morphological model is presented as a framework to understand the first order behaviour of beaches under the action of waves and tides this becomes a useful and easy to apply tool in coastal management and prediction of equilibrium beach profile under diverse conditions

physical model study of beach profile evolution by sea level rise in the presence of seawalls abstract persistent and accelerating sea level rise slr may have a significant impact on the evolution of sandy coastlines this century the response of natural sandy beaches to slr has been much discussed in the literature however there is a lack of knowledge about the impact of slr on engineered coasts laboratory experiments comprising over 320 h of testing were conducted in a 44 m l x 1 2 m w x 1 6 m d wave flume to investigate the influence of coastal armouring in the form of seawalls on coastal response to slr the study was designed to investigate the effects of contrasting types of seawalls reflective impermeable versus dissipative permeable on beach profile response to increased water levels in the presence of both erosive and accretionary wave conditions the results obtained showed that seawalls alter the evolution of the equilibrium profile with rising water level causing increased lowering of the profile adjacent to the structure under erosive wave conditions modelled profiles both with and without seawall structures in place were observed to translate landward in response to slr and erode the upper profile it was found that the erosion demand at the upper beach due to a rise in water level remains similar whether a structure is present or not but that a seawall concentrates the erosion in the area adjacent to the seawall resulting in enhanced and localised profile lowering the type of structure present dissipative permeable versus reflective impermeable was not observed to have a significant influence on this response under accretive conditions the preservation of a large shoreface and berm resulted in no wave structure interaction occurring with the result that the presence of a seawall had no impact on profile evolution a potential two step method for estimating the observed profile response to water level rise in the presence of seawalls is proposed whereby a simple profile translation model is used to provide a first estimate of the erosion demand and then this eroded volume is redistributed in front of the seawall out to the position of the offshore bar

radar and satellite global equatorial f region vertical drift model we present the first global empirical model for the quiet time f region equatorial vertical drifts based on combined incoherent scatter radar observations at jicamarca and ion drift meter observations on board the atmospheric explorer e satellite this analytical model based on products of cubic b splines and with nearly conservative electric fields describes the diurnal and seasonal variations of the equatorial vertical drifts for a continuous range of all longitudes and solar flux values our results indicate that during solar minimum the evening prereversal velocity enhancement exhibits only small longitudinal variations during equinox with amplitudes of about 15 20 m s is observed only in the american sector during december solstice with amplitudes of about 5 10 m s and is absent at all longitudes during june solstice the solar minimum evening reversal times are fairly independent of longitude except during december solstice during solar maximum the evening upward vertical drifts and reversal times exhibit large longitudinal variations particularly during the solstices in this case for a solar flux index of 180 the june solstice evening peak drifts maximize in the pacific region with drift amplitudes of up to 35 m s whereas the december solstice velocities maximize in the american sector with comparable magnitudes the equinoctial peak velocities vary between about 35 and 45 m s the morning reversal times and the daytime drifts exhibit only small variations with the phase of the solar cycle the daytime drifts have largest amplitudes between about 0900 and 1100 lt with typical values of 25 30 m s we also show that our model results are in good agreement with other equatorial ground based observations over india brazil and kwajalein

sediment transport and beach morphodynamics induced by free long waves bound long waves and wave groups new laboratory data are presented on the influence of free long waves bound long waves and wave groups on sediment transport in the surf and swash zones as a result of the very significant difficulties in isolating and identifying the morphodynamic influences of long waves and wave groups in field conditions a laboratory study was designed specifically to enable measurements of sediment transport that resolve these influences the evolution of model sand beaches each with the same initial plane slope was measured for a range of wave conditions firstly using monochromatic short waves subsequently the monochromatic conditions were perturbed with free long waves and then substituted with bichromatic wave groups with the same mean energy flux the beach profile changes and net cross shore transport rates were extracted and compared for the different wave conditions with and without long waves and wave groups the experiments include a range of wave conditions e g high energy moderate energy low energy waves which induce both spilling and plunging breakers and different turbulent intensities and the beaches evolve to form classical accretive erosive and intermediate beach states the data clearly demonstrate that free long waves influence surf zone morphodynamics and promote increased onshore sediment transport during accretive conditions and decreased offshore transport under erosive conditions in contrast wave groups which can generate both forced and free long waves generally reduce onshore transport during accretive conditions and increase offshore transport under erosive conditions the influence of the free long waves and wave groups is consistent with the concept of the relative fall velocity h wst as a dominant parameter controlling net beach erosion or accretion free long waves tend to reduce h wst promoting accretion while wave groups tend to increase the effective h wst promoting erosion

formation and development of a breaker bar under regular waves part 1 model description and hydrodynamics abstract in this work a detailed hydrodynamic model is presented which is used for the study of cross shore sediment transport and morphodynamics in two dimensions the model is described in the framework of the generally unstructured finite volume method considerable emphasis is put on those subtleties in the morphological formulation which are required to achieve mass conservation for the amount of sediment in the bed and in suspension in this first part of two the hydrodynamic description over the cross shore profile is presented the model is validated against an experiment with detailed measurements of the free surface and turbulence over a fixed breaker bar profile a test matrix covering a large interval of the surf similarity parameter is simulated and the phase lag between the breakpoint and the initiation of the setup is described the relation of this phase lag to a cross shore delay in dissipation of organised energy into turbulence is described the relation of this phase lag to the distribution of the location of maxima in bed shear stresses and magnitude of the undertow is also described furthermore processes in the hydrodynamics which will have a smoothing effect on the mean cross shore sediment transport and morphodynamic response are considered all simulations are presented for regular waves and for values of the deep water surf similarity parameter 0 in the range from 0 08 to 1 19 i e covering both spilling and plunging breakers