a phase resolving cross shore sediment transport model for beach profile evolution abstract a phase resolving wave transformation module is combined with an intra wave sediment transport module to calculate the on offshore sediment transport rates the wave module is based on the boussinesq equations extended into the surf zone the vertical variation of the mean undertow and the intra wave sediment concentrations are calculated the net sediment transport rates are calculated and the equation for conservation of sediment is solved to predict the beach profile evolution the results of the present paper showed that the undertow contribution to the sediment transport rates is not dominating in all parts of the surf zone even for eroding beaches suggesting that other contributions should not be neglected the present model also showed that for the same offshore wave energy the time series of the oscillatory motion is important and that the effect of wave groups cannot be disregarded

observations of wave induced pore pressure gradients and bed level response on a surf zone sandbar horizontal and vertical pressure gradients may be important physical mechanisms contributing to onshore sediment transport beneath steep near breaking waves in the surf zone a barred beach was constructed in a large scale laboratory wave flume with a fixed profile containing a mobile sediment layer on the crest of the sandbar horizontal and vertical pore pressure gradients were obtained by finite differences of measurements from an array of pressure transducers buried within the upper several centimeters of the bed co located observations of erosion depth were made during asymmetric wave trials with wave heights between 0 10 m and 0 98 m consistently resulting in onshore sheet flow sediment transport the pore pressure gradient vector within the bed exhibited temporal rotations during each wave cycle directed predominantly upwards under the trough and then rapidly rotating onshore and downwards as the wave front passed the magnitude of the pore pressure gradient during each phase of rotation was correlated with local wave steepness and relative depth momentary bed failures as deep as 20 grain diameters were coincident with sharp increases in the onshore directed pore pressure gradients but occurred at horizontal pressure gradients less than theoretical critical values for initiation of the motion for compact beds an expression combining the effects of both horizontal and vertical pore pressure gradients with bed shear stress and soil stability is used to determine that failure of the bed is initiated at non negligible values of both forces

post storm beach and dune recovery implications for barrier island resilience abstract the ability of beaches and dunes to recover following an extreme storm is a primary control of barrier island response to sea level rise and changes in the frequency and or magnitude of storm surges whereas erosion of the beach and dune occurs over hours and days it can be years to decades before the beach and dune are able to recover to their pre storm state as a consequence there are numerous descriptions of near instantaneous beach and dune erosion due to storms the immediate onshore transport of sand and the initial phases of beach and dune recovery following a storm but a paucity of data on long term beach and dune recovery a combination of previously published data from galveston island texas and new remotely sensed data from santa rosa island florida is used in the present study to quantify the rate of dune recovery for dissipative and intermediate beach types respectively recovery of the dune height and volume on galveston island was observed within two years following hurricane alicia 1983 and was largely complete within six years of the storm despite extensive washover in contrast the dunes on santa rosa island in northwest florida began to recover four years after hurricane ivan 2004 and only after the profile approached its pre storm level and the rate of vegetation recovery regrowth was at a maximum results show that complete recovery of the largest dunes in height and volume will take approximately 10 years on santa rosa island which suggests that these sections of the island are particularly vulnerable to significant change in island morphology if there is also a change in the frequency and magnitude of storm events in contrast the areas of the island with the smallest dunes before hurricane ivan exhibited a rapid recovery but no further growth in profile volume and dune height beyond the pre storm volume and height despite continued recovery of the largest dunes to their pre storm height a change in storm magnitude and or frequency is a potential threat to barrier island resilience particularly for those sections of the island where dune recovery has historically taken the longest time further study is required to determine how and why dune recovery varies for the dissipative and intermediate beaches of galveston island and santa rosa island respectively

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

divergent response of an intertidal swash bar this paper examines the processes responsible for the morphodynamics of an intertidal swash bar at skallingen denmark during seven successive storms one with a large surge of 3 02 m dnn during this period a subtidal bar migrated landward onto the foreshore and continued to migrate across the intertidal zone as a swash bar the onshore migration of the inner subtidal bar resulted from the erosion of sediment from the upper foreshore and dune ramp during the large storm surge that was transported seaward causing the landward displacement of the bar through accretion on the landward slope the magnitude and direction of suspended sediment transport within the intertidal zone and more specifically at and close to the crest of the swash bar varied with the ratio of both the significant hs and average havg wave heights to the water depth hcr at the swash bar crest the local depth minimum the transition between onshore and offshore suspended sediment transport was associated with the average wave of the incident distribution breaking on the swash bar crest havgh 0 33 while the onshore directed transport was largest at infragravity frequencies sediment resuspension was best explained by the skewed accelerations under the surf bores offshore transport was dominated by the cross shore mean currents undertow that developed when the significant wave of the distribution broke on the swash bar crest hsh 0 33 and weakened as the average wave of the distribution started to break at the crest havgh 0 33 and the surf zone approached saturation in contrast to subtidal bars the swash bar at skallingen exhibited a divergent behaviour with respect to the cross shore position of the breaker zone migrating onshore when the average wave broke seaward of the crest and migrating offshore when the average wave broke landward of the crest copyright 2006 john wiley sons ltd

sediment suspension under waves and currents time scales and vertical structure field measurements of the vertical structure of near bed suspended sediment concentrations were obtained from arrays of fast response optical backscatter suspended solids sensors to examine the time dependent response of sediment resuspension to waves and currents and the constraints imposed by bedforms data were recorded from both a nonbarred marine shoreface and a barred lacustrine shoreface under both shoaling and breaking waves significant heights of 0 25 1 50m peak periods of 3 and 8 s and in water depths of 0 5 5 0 m sediment concentrations are positively correlated with increasing elevation above the bed but lagged in time the time lag varies directly with separation distance between measurement locations and inversely with the horizontal component of the near bed oscillatory velocity both the presence of wave groups and the settling velocities of the sediment particules in suspension influence the temporal changes in concentration at a given elevation sediment concentrations appear to respond more slowly to the incident wind wave forcing with distance away from the bed as a result of two factors 1 the sequential increase in concentration induced by a succession of large waves in a group and ii the relative increase in finer sediments with smaller settling velocities bedforms interact with the near bed horizontal currents to impose a distinct constraint upon the timing of suspension events relative to the phase of the fluid motion and therefore the vertical structure of the suspended sediment concentration at a range of time scales the near bed concentrations appear to be strongly dependent upon the vertical convection of sediment associated with the ejection from the wave boundary layer of separation vortices generated in the lee of ripple crests concentration gradients in the presence of vortex ripples are large as are the correlation between concentrations measured at different elevations within the fluid

using nonlinear forecasting to learn the magnitude and phasing of time varying sediment suspension in the surf zone the time dependent response of sediment suspension to flow velocity was explored by modeling field measurements collected in the surf zone during a large storm linear and nonlinear models were created and tested using flow velocity as input and suspended sediment concentration as output a sequence of past velocities velocity history as well as velocity from the same instant as the suspended sediment concentration was used as input this velocity history length was allowed to vary the models also allowed for a lag between input instantaneous velocity or end of velocity sequence and output suspended sediment concentration predictions of concentration from instantaneous velocity or instantaneous velocity raised to a power up to 8 using linear models were poor correlation coefficients between predicted and observed concentrations were less than 0 10 allowing a lag between velocity and concentration improved linear models correlation coefficient of 0 30 with optimum lag time increasing with elevation above the seabed from 1 5 s at 13 cm to 8 5 s at 60 cm these lags are largely due to the time for an observed flow event to effect the bed and mix sediment upward using a velocity history further improved linear models correlation coefficient of 0 43 the best linear model used 12 5 s of velocity history approximately one wave period to predict concentration nonlinear models gave better predictions than linear models and as with linear models nonlinear models using a velocity history performed better than models using only instantaneous velocity as input including a lag time between the velocity and concentration also improved the predictions the best model correlation coefficient of 0 58 used 3 s approximately a quarter wave period of the cross shore velocity squared starting at 4 5 s before the observed concentration to predict concentration using a velocity history increases the performance of the models by specifying a more complete description of the dynamical forcing of the flow including accelerations and wave phase and shape responsible for sediment suspension incorporating such a velocity history and a lag time into the formulation of the forcing for time dependent models for sediment suspension in the surf zone will greatly increase our ability to predict suspended sediment transport

observations of morphological change and sediment transport on a steep gravel beach abstract observations of morphological change and swash sediment loads concurrent with measurements of water level waves and currents were obtained during a spring to spring tidal cycle on a steep macrotidal gravel beach in southwest england the wave conditions experienced during the field experiment were relatively calm and the significant inshore wave height and period averaged 0 4 m and 6 s respectively incident frequency water motions dominated the hydrodynamics and the phasing between water surface elevation and cross shore and alongshore currents was inconsistent with standing edge wave motion morphological change was directly related to maxima in shoreline elevation and four modes of berm adjustment identified regression and rollover in response to falling and rising water levels respectively and removal and reformation during and after storms respectively swash processes transported substantial amounts of sediment at rates comparable to sand beaches and the net transport under modal wave conditions was estimated to be 1 kg per swash onshore per unit metre beach width steep hydrodynamic gradients induced by the abrupt slope breaks of the step and berm are one of the main causes of the large morphological variability characteristic of gravel beaches

field measurements of intertidal bar evolution on a high energy beach system nearshore bars play a pivotal role in coastal behaviour helping to protect and restore beach systems particularly in post storm conditions examination of bar behaviour under various forcing conditions is important to help understand the short to medium term evolution of sandy beach systems this study carried out over a nine week period examines the behaviour of three intertidal bars along a high energy sandy beach system in northwest ireland using high frequency topographic surveys and detailed nearshore hydrodynamic modelling results show that in general there was onshore migration for all the bars during the study period despite the variability observed between bars which was driven mostly by wave dominated processes under the prevailing conditions migration rates of up to 1 83 m day 1 and as low as 0 07 m day 1 were observed during higher wave energy events the migration rates of the bars decelerated in their onshore route however under lower wave energy conditions they quickly accelerated maintaining their shoreward migration direction tidal influence appears to be subordinate in these conditions being restricted to moderating the localized wave energy at low tides and in maintaining runnel configurations providing accommodation space for advancing slip faces the study highlights the intricate behavioural patterns of intertidal bar behaviour along a high energy sandy coastline and provides new insights into the relative importance of wave and tidal forcing on bar behaviour over a relatively short time period copyright 2016 john wiley sons ltd

effects of wave shape on sheet flow sediment transport 1 a two phase model is implemented to study the effects of wave shape on the transport of coarse grained sediment in the sheet flow regime the model is based on balance equations for the average mass momentum and fluctuation energy for both the fluid and sediment phases model simulations indicate that the responses of the sheet flow such as the velocity profiles the instantaneous bed shear stress the sediment flux and the total amount of the mobilized sediment cannot be fully parameterized by quasi steady free stream velocity and may be correlated with the magnitude of local horizontal pressure gradient or free stream acceleration a net sediment flux in the direction of wave advance is obtained for both skewed and saw tooth wave shapes typical of shoaled and breaking waves the model further suggests that at critical values of the horizontal pressure gradient there is a failure event within the bed that mobilizes more sediment into the mobile sheet and enhances the sediment flux preliminary attempts to parameterize the total bed shear stress and the total sediment flux appear promising

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

discrete particle model for sheet flow sediment transport in the nearshore fully three dimensional discrete particle computer simulations of high concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments a simple impulse momentum approach explains simulation results and forms the basis for adding an acceleration related term to widely used energetics sediment transport formulae transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores simulations integrate f ma and a corresponding set of equations for the torques for each sphere normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point respectively particle interactions are both inelastic and frictional pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag added mass and buoyancy forces on particles transport properties of the simulated granular fluid assemblage are robust to large variations in material properties of the particles simulated transport rates agree with available experimental data for unsteady transport of coarse sands the mode of bed load motion dispersion of bed load particles and particle segregation by size and density are qualitatively consistent with available particle scale observations of bed load transport of natural particles

the influence of swash based reflection on surf zone hydrodynamics a wave by wave approach abstract a detailed understanding of the behaviour of waves in the nearshore is essential for coastal engineers as these waves cause beach erosion coastal flooding and damage to coastal structures significantly the influence of reflected waves is often neglected in surf zone studies although they are known to influence wave properties and circulation in the nearshore in this paper a phase resolving model is rigorously applied to model conditions from the prototype scale bardexii experiment in order to examine and assess the influence of swash based reflection on surf zone hydrodynamics at both the individual wave and time averaged timescales surface elevation is separated into incoming and outgoing signals using the radon transform and a crest tracking algorithm is used to extract incident and reflected wave properties it is found that on steep beaches tan 1 9 the swash based reflection the reflection generated in the swash during the backwash contributes significantly to the intrawave variability of individual wave properties such as the wave height to water depth ratio through the generation of quasi nodes antinodes system for expressed with individual wave heights variations up to 25 and 40 are obtained for the modelled regular and irregular wave tests whereas it reaches 15 when it is based on the significant wave height the outgoing wave field induced hydrodynamics is also found to affect time averaged parameters undertow and horizontal velocity skewness the undertow is mainly strengthened particularly in the shoaling region where the outgoing component dominates over the contribution from the incoming wave field offshore of the bar an onshore directed flow streaming close to the bed is also generated under the outgoing wave field and is suspected to help in stabilising the bar position this along with the influence of the outgoing wave field on the horizontal velocity skewness and the presence of quasi standing waves suggests a complex contribution of the hydrodynamics induced by swash based reflection into sediment transport rates and nearshore bar generation migration

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

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

a surfzone morphological diffusivity estimated from the evolution of excavated holes downslope gravity driven sediment transport smooths steep nearshore bathymetric features such as channels bars troughs cusps mounds pits scarps and bedforms downslope transport appears approximately as a diffusive term in the sediment continuity equation predicting changes in bed level with a morphological diffusivity controlling the rate of seafloor smoothing despite the importance of surfzone sediment transport and morphological evolution the size of the downslope transport term in nearshore models varies widely and theories have not been tested with field measurements here observations of the infill of large excavated holes in an energetic inner surf zone provide the first opportunity to infer the morphological diffusivity in the field the estimated diffusion coefficient is consistent with a theoretical bedload morphological diffusivity that scales with the three halves power of the representative bed shear stress

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

modes of cross shore sediment transport on the shoreface of the middle atlantic bight the mechanisms responsible for onshore and offshore sediment fluxes across the shoreface zone seaward of the surf zone were examined in a 3 year field study the study was conducted in the southern part of the middle atlantic bight in the depth region 7 17 m using instrumented tripods supporting electromagnetic current meters pressure sensors suspended sediment concentration sensors and sonar altimeters the observations embraced fairweather moderate energy swell dominated and storm conditions cross shore mean flows ranged from near zero during fairweather to 20 cm s 1 during the storm oscillatory flows were on the order of 10 cm s 1 during fairweather and 100 cm s 1 during the storm suspended sediment concentrations at about 10 cm above the bed were 5 kg m 3 during the storm three methods were applied to evaluate the relative importance of incident waves long period oscillations mean flows and gravity in effecting shoreward or seaward sediment flux 1 an energetics transport model was applied to instantaneous near bottom velocity data 2 higher moments of near bottom flows were estimated and compared and 3 suspended sediment fluxes were estimated directly from the instantaneous products of cross shore velocity and suspended sediment concentration the results show that measurable contributions were made by all four of the processes most significantly mean flows were seen to dominate and cause offshore fluxes during the storm and to contribute significantly to onshore and offshore flux during fairweather and moderate energy incident waves were in all cases the major source of bed shear stress but also caused shoreward as well as seaward net sediment advection low frequency effects involving wave groups and long period waves made secondary contributions to cross shore sediment flux contrary to expectations low frequency fluxes were just as often shoreward as seaward whereas cross correlations between suspended sediment concentration and the instantaneous near bottom current speed were high and in phase under storm conditions they were weak and out of phase during fairweather conditions this suggests that simple energetics models are probably inadequate for predicting fairweather transport of suspended sediment

mechanisms controlling the complete accretionary beach state sequence accretionary down state beach sequence is a key element of observed nearshore morphological variability along sandy coasts we present and analyze the first numerical simulation of such a sequence using a process based morphodynamic model that solves the coupling between waves depth integrated currents and sediment transport the simulation evolves from an alongshore uniform barred beach storm profile to an almost featureless shore welded terrace summer profile through the highly alongshore variable detached crescentic bar and transverse bar rip system states a global analysis of the full sequence allows determining the varying contributions of the different hydro sedimentary processes sediment transport driven by orbital velocity skewness is critical to the overall onshore sandbar migration while gravitational down slope sediment transport acts as a damping term inhibiting further channel growth enforced by rip flow circulation accurate morphological diffusivity and inclusion of orbital velocity skewness opens new perspectives in terms of morphodynamic modeling of real beaches

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

profile response of a lacustrine multiple barred nearshore to a sequence of storm events profile change in a lacustrine multiple barred nearshore was investigated over the ice free season of 2001 2002 at burley beach on the southeastern shore of lake huron in order to identify the feedback mechanisms between the pre existing morphology and the wave forcing and the consequence of those feedbacks to the behaviour of the nearshore environment the characteristics of the offshore wave field were monitored using a falmouth scientific combined 3d acm wave recorder and pressure transducer supplemental wave data were downloaded from a 3 m discus buoy operated by the national data buoy center 75 km to the nwof the study site the three nearshore bars were in a quasi equilibrium state through a large part of the ice free season with dramatic changes occurring during relatively moderate storm events that followed much larger storms in late october a comparison of the incident wave field with changes in the nearshore profile through canonical correlation analysis indicates that the morphology responds to the distribution of the significant root mean square rms and average wave heights between surveys the threshold between bar decay and onshore bar migration and growth is associated with the onset of breaking of the rms wave at the bar crest hrms hcr 1 c0 3 0 4 the threshold between onshore and offshore migration is associated with the onset of breaking of the average wave at the bar crest havg hcr 1 c0 3 0 4 coincident with complete dissipation of the significant wave over the lakeward slope of the bar hs hcr 1 n0 6 inshore wave data collected during an instrumented study at the same site revealed that the middle and inner bars remained at the threshold of onshore and offshore migration over a wide range of offshore significant wave heights 0 8 to 2 4 m prior to the october storms this self organised equilibrium is a result of changes to the incident wave distribution through breaking on the outer bar it is concluded that the prediction of bar response requires an understanding of the feedback between the bar and the local wave distribution in addition to an understanding of the feedback associated with the profile as a whole d 2005 elsevier b v all rights reserved

morphodynamics of intertidal bars in wave dominated coastal settings a review abstract intertidal bar systems are ubiquitous features on wave dominated beaches in coastal settings with a significant 1 m tidal range depending primarily on the wave conditions and the tidal range and to a lesser extent on the nearshore gradient they can assume a variety of forms slip face bars represent the most pronounced and dynamic intertidal bar morphology and are generally found on their own around the mean high tide level they usually form low on the intertidal beach after storm induced beach erosion and develop into a berm under prolonged calm wave conditions low amplitude ridges and sand waves represent multiple bar morphologies the bars occur across the entire intertidal profile and they remain present throughout the year multiple intertidal bars tend to be rather subdued and relatively static especially sand waves and their origin remains unclear the morphological response of intertidal bars to changing wave conditions is largely forced bars build up and migrate onshore under calm waves and are flattened and may migrate offshore during storms the morphological response is however significantly affected by relaxation time effects and morphological feedback particularly on beaches with multiple intertidal bars despite their morphological differences the intertidal bar types exhibit pronounced similarities in their morphodynamics sediment transport processes and morphological response are principally controlled by the tidal water levels on the beach because these together with the offshore wave energy level and the beach morphology determine the type intensity and duration of the wave processes operating on the cross shore profile it is the dominant importance of tidal water level variations and wave processes in shallow water depths swash and surf zone bores rather than wave height variability and deeper water wave processes breaking and shoaling waves that constitutes the main difference between intertidal and subtidal bar morphodynamics

dynamics of multiple intertidal bars over semi diurnal and lunar tidal cycles north lincolnshire england multiple intertidal bars are common features of wave dominated sandy beaches yet their short term 1 month and small scale 1 km morphology and dynamics remain poorly understood this study describes the morphodynamics of multiple intertidal bars in north lincolnshire england during single and lunar tidal cycles under two contrasting conditions first when significant wave height was 0 5 m and second when significant wave height frequently exceeded 1 m the relative importance of swash surf and shoaling processes in determining morphological change was examined using detailed field observations and a numerical model the beach featured four intertidal bars and both cross shore and longshore bar morphology evolved during the field investigation particularly under medium to high wave energy conditions numerical modelling suggests shoaling processes are most common on the seaward two bars under calm wave conditions hs 0 5 m and that surf zone processes become more common during neap tides and under more energetic hs 0 5 m conditions surf processes dominate the inner two bars though swash influence increases in a landward direction the numerical modelling results combined with low tide survey data and high resolution morphological measurements strongly suggest changes in the intertidal bar morphology are accomplished by surf zone processes rather than by shoaling wave or swash processes this is because shoaling waves do not induce significant sediment transport to have any morphological effect whereas swash action generally does not have enough scope to act as the swash zone is much narrower than the surf zone it was found however that the absolute rate of morphological change under swash action and surfzone processes are of similar magnitudes and that swash action may induce a significant amount of local morphological change when the high tide mark is located on the upper bar making this process important for bar morphodynamics copyright 2007 john wiley sons ltd

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

breaker turbulence and sediment suspension in the surf zone field measurements of fluid velocities and suspended sediment concentrations were used to investigate the relative role of coherent vortices related to wave breaking in suspended sediment dynamics the measurements were obtained from a barred intermediate type beach and the instrument rig was exposed to a range of wave types during the experiment including shoaling waves and both spilling and plunging breakers the time scale of the observed breaker vortices was 5 hz and thus in the frequency range of wave orbital motion coherent breaker vortices were separated from wave orbital motions and incoherent higher frequency turbulence using a threshold magnitude for the vertical velocity component plunging breakers displayed the largest root mean square rms and maximum vertical velocities the latter reaching approximately 1 m s 1 these measurements were made at an elevation of 0 15 m above the bed given the observed scale of the breaker vortices and the phase match between suspended sediment concentrations and fluid velocities the turbulence injected into the water column by the plunging breakers played a significant role in suspending bed sediment on the other hand the role of coherent vortex structures in suspending sediment under spilling breakers and shoaling waves is more limited rms vertical velocities under the spilling breakers and shoaling waves in this experiment were respectively a factor 2 and 4 smaller than for plunging breakers different phase relationships were observed between the horizontal fluid velocity and vortex induced sediment suspension for each of the wave types which may contribute to an explanation of net offshore and onshore sediment transport observed during episodes of beach erosion and recovery respectively

hydrodynamics and sediment transport within the inner surf zone of a lacustrine multiple barred nearshore abstract it has become increasingly recognized that the typically unexpected behavior of the nearshore profile over a sequence of storm events is a response to strong feedback between the antecedent morphology and the incident wave field this paper systematically examines the suspended sediment transport and morphodynamics within the surf zone of a lacustrine multiple barred upper shoreface a range of significant wave heights h s and local relative wave heights h s h 1 h water depth are considered in order to describe and identify the limits to the feedback mechanisms that can dominate the nearshore response data from collocated concentration and velocity sensors indicate that the direction and magnitude of suspended sediment transport varies with the degree of local wave energy dissipation and the cross shore variation in these relationships depends partly on the local bed slope this dependence indicates the presence of a strong self organization at the scale of the individual bar within the inner surf zone these bar scale relationships are constrained by the shoaling transformations which occur over the outermost bar for ranges of incident wave height defined locally e g intermediate waves h s 0 4 to 0 9 m storm waves 0 9 to 1 4 m the innermost and middle bars remain in a state of self organized equilibrium for intermediate range waves this equilibrium is a consequence of wave height reduction over the outer bar through breaking and the generation of secondary harmonic waves for the storm wave range the equilibrium is maintained through a resonant amplification of discrete edge wave modes with the bars forced to a position coincident with the surface elevation antinodes these upper shoreface scale feedback mechanisms are weakened particularly for intermediate wave heights following threshold storms capable of moving the outermost bar offshore and lower on the profile this allows the middle and inner bars to be forced directly by variations in the offshore incident waves thus the bars respond in a cyclic manner the bar cycle

field observations of intra wave sediment suspension and transport in the intertidal and shallow subtidal zones abstract sandy beaches are highly dynamic due to the transport of sediment by waves and currents while beach erosion is mainly driven by cross shore currents beach recovery is driven by the onshore directed transport of sediment by incoming short waves this latter process is complex and often inaccurately predicted by morphodynamic models this is partly because of the omission of the effects of bed form induced and surface generated turbulence on sediment pick up in these models in this paper the phase relationship between the oscillatory flow turbulence and suspended sediment concentration was investigated based on field measurements obtained beneath shoaling waves spilling breakers and surf bores wave averaged and intra wave variability in turbulence and suspended sediment concentration were analysed on a wave by wave basis where individual waves were extracted and then grouped according to relative wave height beneath shoaling waves small but steep wave ripples were shown to affect the phasing of suspended sediment concentration and to cause an offshore directed short wave suspended sediment flux beneath spilling breakers and surf bores sediment was mainly stirred under the wave crest and the short wave suspended sediment flux was onshore directed based on the analysed data several mechanisms were suggested to cause sediment stirring friction between the wave orbital motion and the seabed penetration of surface generated turbulence towards the seabed increased bed shear stress at the wave front due to acceleration skewness and during conditions with strong undertow wave current interactions

seasonal scale nearshore morphological evolution field observations and numerical modeling abstract a coupled waves currents bathymetric evolution model delft 3d is compared with field measurements to test hypotheses regarding the processes responsible for alongshore varying nearshore morphological changes at seasonal time scales a 2001 field experiment along the beaches adjacent to grays harbor washington usa captured the transition between the high energy erosive conditions of winter and the low energy beach building conditions typical of summer the experiment documented shoreline progradation on the order of 10 20 m and on average approximately 70 m of onshore sandbar migration during a four month period significant alongshore variability was observed in the morphological response of the sandbar over a 4 km reach of coast with sandbar movement ranging from 20 m of offshore migration to over 175 m of onshore bar migration the largest seasonal scale onshore migration event observed in a natural setting both observations and model results suggest that in the case investigated here alongshore variations in initial bathymetry are primarily responsible for the observed alongshore variable morphological changes alongshore varying incident hydrodynamic forcing occasionally significant in this region due to a tidal inlet and associated ebb tidal delta was relatively minor during the study period and appears to play an insignificant role in the observed alongshore variability in sandbar behavior at kilometer scale the role of fully three dimensional cell circulation patterns in explaining the observed morphological variability also appears to be minor at least in the case investigated here

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

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

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

calculating shoreline erosion potential using nearshore stratigraphy and sediment volume outer banks north carolina 1 despite the acknowledged influence of coastal geological framework on the behavior of beaches and barrier islands and a wealth of geological and bathymetric observations from the inner shelf quantitatively connecting those observations to shoreline behavior has been difficult nearshore geologic and morphologic variability described by recent research is not well represented by conventional geologic parameters such as mean grain size and shoreface slope used in most shoreline change models we propose that total nearshore sediment volume as calculated to a continuous seismic reflection surface provides a flexible and robust metric for use in the prediction of shoreline change this method of determining the volume of sediment in the nearshore accounts for three dimensional sandbar morphologies and heterogeneous seafloor sediments the decadal scale shoreline change rate for northeastern north carolina is significantly correlated to the volume of sediment in the nearshore when a geologically defined base is used in volume determinations suggesting that the shallow stratigraphic framework of transgressive coasts is an important influence on decadal shoreline behavior nearshore sediment volume was overestimated when an arbitrary depth constant baseline was used and was not correlated to decadal shoreline change this implies that a volume metric which accounts for both framework geology and variable seafloor morphology better represents the geologic character of the shoreface and may help to improve existing models of shoreline change an empirical model of regional shoreline erosion potential demonstrates the importance of incorporating nearshore sediment volume shallow framework geology and surface morphology when predicting seasonal to decadal shoreline evolution

seascape level variation in turbulence and wave generated hydrodynamic signals experienced by plankton abstract plankton exhibit diverse and dramatic responses to fluid motions and these behaviors are likely critical for survival and fitness fluid motions can be generated by organisms or by physical processes including turbulence and surface gravity waves physical processes vary geographically in their intensity and generate hydrodynamic signals experienced by plankton as fluid forces on their sensory receptors in this synthesis we review how turbulence and waves vary in space the scales and statistics of their motions and the forces exerted on plankton we then quantify the hydrodynamic signals produced by turbulence and waves in four seascape types surf zones inlets and estuaries the continental shelf and the open ocean using published dissipation rates wind and wave data from buoys and observations from two coastal sites in massachusetts usa we relate these geographic patterns in signals to the observed behaviors of example species and to the forces sensed by typical plankters with different receptor types turbulence generated shears are largest in the surf zone inlets and estuaries while wave generated accelerations are larger offshore as a result each seascape exhibits some range of combined shears and accelerations that is distinct these signals generate forces on plankton that vary among habitats and with plankton size and swimming speed spatial patterns in fluid forces create a potential mechanism for dispersing larvae to distinguish habitats by their hydrodynamic signatures however turbulence can be strong in all seascapes and may cause widespread interference in signaling among predators and prey plankton with a single receptor type could identify nearshore habitats while those with multiple receptor types potentially could distinguish inshore vs offshore seascapes or decode signals produced by physical processes and by other organisms

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

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

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

a numerical empirical approach for evaluating morphodynamic processes on gravel and mixed sand gravel beaches abstract pedrozo acuna et al pedrozo acuna a simmonds d j otta a k and chadwick a j 2006 on the cross shore profile change of gravel beaches coastal engineering 53 4 335 347 presented a numerical empirical investigation of the processes that control sediment transport in the swash zone on steep gravel beaches this was based on a sensitivity analysis of a sediment transport profile model driven by a highly non linear boussinesq model lynett p j wu t r and liu l f p 2002 modelling wave run up with depth integrated equations coastal engineering 46 89 107 which was compared to near full scale measurements performed in the gwk flume in hanover in this paper we have extended our analysis to compare these earlier results with those relating to a mixed sediment gravel and sand beach the parametric sensitivity analysis also incorporates a discussion of the effects of acceleration about which there is much debate the sensitivity analysis suggests that fluid acceleration can contribute to the onshore movement of sediment that causes steepening of initially flat beach faces composed of coarse sediment however acceleration alone cannot be the cause of the observed berm growth during the gwk tests instead a complex balance of processes is responsible for the profile evolution of coarse grained beaches with no single dominant process

cross shore sediment transport partitioning in the nearshore during a storm event 1 vertical profiles of fluid velocity and sediment concentration have been collected across the surf zone during the sandyduck 97 field experiment analysis from data collected during a 3 day storm event shows that in agreement with preceding observations the mean component of cross shore sediment transport throughout the water column and across the surf zone is almost uniformly in the offshore direction similarly the wave coherent component is generally onshore directed but does on rare occasions exhibit offshore transport the infragravity component in these conditions while exhibiting a definite offshore bias is generally a negligible component thus net cross shore transport is predominately a balance between the wave coherent component and the mean component in observations that stress the importance of the near bed region to cross shore sediment transport it is shown that this balance is a function of elevation above the bed the increasing importance of the wave coherent component as the bed is approached can lead to situations where the net transport direction shifts from being directed offshore higher in the water column to onshore directed near the bed

onshore sandbar migration in the surf zone new insights into the wave induced sediment transport mechanisms we present a novel process based morphodynamic model which includes transport processes due to both velocity and acceleration skewness and a new formulation for intrawave motions that successfully simulates observations of onshore sandbar migration results confirm findings of previous studies in which each process was considered separately and in which sediment transport was computed from the observed water motion however our results indicate that accounting for the joint action of both velocity and acceleration skewnesses causes major improvement of the modeled onshore bar migration and is essential to accurately model the evolution of the entire cross shore bottom profile when compared with observations we also demonstrate that the morphodynamics in the shoaling zone are dominated by velocity skewness bed shear stresses while sediment transport induced by acceleration skewness pressure gradients controls the morphodynamics in the inner surf zone

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

formation and evolution of multiple intertidal bars on macrotidal beaches application of a morphodynamic model a numerical model is presented to simulate the formation and evolution of intertidal bar morphology dridges and runnelst cross shore sediment transport is modeled using a sediment transport shape function which is a mathematical formulation that expresses the volumetric sediment transport rate as a function of cross shore location for energetic wave conditions the shape function predicts offshore sediment transport in the surf zone and onshore transport outside the surf zone for calm conditions the shape function predicts onshore transport in the surf zone and zero transport outside the surf zone the model is forced by measured wave tide conditions and the time integrated cross shore sediment transport rate is obtained by advecting the shape function up and down the beach with changing tide levels morphodynamic feedback is included in the model specifically the occurrence of relatively flat beach sections troughs leads to the subdivision of the beach profile into distinct cross shore sediment transport cells each forced by its own wave breaking conditions surf zone sediment transport in the troughs is considered zero and the troughs thus effectively serve as sediment transport barriers according to the model and in agreement with field observations intertidal bars form and migrate onshore under the influence of low wave conditions while offshore bar migration and flattening of the morphology occurs under high wave conditions the model supports the contention that ridge and runnel topography has a surf zone origin but the ridges are not breakpoint bars in sensu stricto d 2004 elsevier b v all rights reserved

cross shore suspended sediment transport in the surf zone a field based parameterization existing cross shore sediment transport models for two dimensional surf zone bathymetries almost invariably predict offshore directed sand transports and bar migrations during storm conditions however onshore directed suspended sediment fluxes and associated nearshore bar migration were observed during recent field experiments on a gently sloping beach on the danish north sea coast field measurements of suspended sediment flux obtained during three experiments on two different beaches are used to parameterize the observed fluxes this parameterization predicts suspended sediment transport due to incident waves and undertow across bars in two dimensional surf zones first a non dimensional sediment flux index is formulated which describes the tendency towards net onshore or offshore transport and the strength of that tendency the non dimensional formulation circumvents the problem of measurement inconsistencies due to varying elevations of sediment concentration sensors relative to the bed the index is found to depend upon the undertow velocity the incident wave skewness and the cross correlation between orbital velocity and sediment concentration however some of these parameters are difficult to predict particularly in barred surf zones and therefore the independent variables are recast in terms of a set of more easily obtainable parameters the sediment flux index depends on a combination of the following non dimensional bed shear stress the shields parameter relative water depth wave orbital velocity relative wave height and bed slope finally a formulation of suspended sediment transport across bars is obtained by linking the flux index with a parameterization of the sediment concentration distribution in the water column these concentrations are found to depend on non dimensional bed shear stress relative wave height and water depth the formulation predicts a tendency for onshore directed sediment transport due to incident waves on gently sloping beaches and or with large bed shear stresses on steeply sloping beaches and or in the inner part of the surf zone there is a tendency towards offshore sediment transports due to the undertow

cross shore sand transport by infragravity waves as a function of beach steepness two field data sets of near bed velocity pressure and sediment concentration are analyzed to study the influence of infragravity waves on sand suspension and cross shore transport on the moderately sloping sand motor beach 1 35 the local ratio of infragravity wave height to sea swell wave height is relatively small hig hsw 0 4 most sand is suspended during negative infragravity velocities and qig is offshore directed the infragravity contribution to the total sand flux is considerably larger and reaches up to 60 during energetic conditions on the whole hig hsw is a good indicator for the infragravity related sand suspension mechanism and the resulting infragravity sand transport direction and relative importance

short term morphodynamics of an intertidal bar on megatidal ebb delta the morphodynamics of an intertidal bar located on an ebb delta of a megatidal inlet system in normandy france was examined during four short experiments under low to high energy wave conditions and spring or neap tide contexts although there have been numerous studies on ebb tidal bar morphology and on the processes affecting such bars these concern only microtidal or mesotidal settings and are mainly based on observations no detailed work involving hydrodynamic conditions embracing the whole tidal cycle has been carried out so far on the mechanisms of onshore migration of these coastal accumulation features especially along macro and megatidal coasts in this study the morphological response of the bar was coupled with the residence times and intensities of each hydrodynamic process swash surf shoaling waves over a representative bar cross shore profile and sediment transport patterns deduced from fluorescent tracer the results show that the bar migrated exclusively shoreward during moderate to storm conditions hs 0 7 m at the bottom of the bar seaward slope during low energy wave conditions no bar movement was observed swash action was not the dominant process mainly due to its duration which did not exceed 8 min in the course of a semi diurnal tidal cycle the movement of sediments and the bar morphology is induced mainly by surf processes and their associated currents the increase in the significant wave height disturbed the general mean current behaviour which was parallel to the bar crest during low energy conditions under surf conditions mean flows are directed onshore with an absence of bed return flow this allows sediment to be transported towards the bar crest causing a landward bar migration an offshore directed cross shore current not favourable for bar migration is recorded with larger water depths and no breaking waves shoaling conditions this study also highlights the influence of the tidal water level fluctuations in such a large tidal range setting which induce a rapid shift in processes such as bed shear and sediment re working across the shore normal profile these conditions explain the relative low rate of migration of the bar over the delta platform about 33 m year this rate is much less than those reported from micro mesotidal environments calculations of bed shear stress also show that the mean currents alone are not responsible for onshore bar movement notwithstanding the megatidal character of the field site

wind effects on shoaling wave shape near the shore cross shore winds strongly affect the location of the break point and the breaking wave height from casual observation from the beach wind direction onshore or offshore and speed also appear to affect wave shape i e skewness and asymmetry although as of yet this effect has not been quantified near the shore the effect of wind on shoaling wave shape is investigated with laboratory experiments using monochromatic waves and onshore directed wind wind increases the shoaling wave energy at discrete multiples of the primary frequency and has a significant effect on the wave shape at both a deeper and shallower shoaling locations at the shallower location the ratio of wave energy at 2 times the primary frequency to the primary frequency is also a function of wind speed indicating interaction between the wind and the nonlinear wave shoaling process nearshore wave models do not account for these wind effects incorrect predictions of third order velocity moments wave shape believed to control wave driven sediment transport would result in incorrect beach morphological evolution predictions

medium term variations of bar properties and their linkages with environmental factors at hasaki japan beach profile data obtained every weekday for 15 years along a 427 meter long pier were analysed by complex empirical orthogonal function analysis the seaward bar migration was represented by the first mode and the migration frequency and the temporal variation of the bar amplitude were estimated with the complex time coefficient of the first mode although the seaward bar migration frequency which is the reciprocal of the duration time of the bar migration cycle was relatively constant before 1993 and ranged between 1 3 to 0 8 cycles year 0 7 to 1 25 years cycle it gradually decreased after 1994 and reached 0 1 cycle year 10 years cycle at the end of 1996 subsequently it gradually recovered to the frequency before 1993 the bar amplitude had a similar long period fluctuation it was large before 1993 but small from 1995 to 1997 and then returned to the pre 1993 value the bar migration frequency was weakly correlated with the bar amplitude and the offshore wave energy flux

source to sink evolution of lignin composition in the madre de dios river system with connection to the amazon basin and offshore while lignin geochemistry has been extensively investigated in the amazon river little is known about lignin distribution and dynamics within deep stratified river channels or its transformations within soils prior to delivery to rivers we characterized lignin phenols in soils river particulate organic matter pom and dissolved organic matter dom across a 4 km elevation gradient in the madre de dios river system peru as well as in marine sediments to investigate the source to sink evolution of lignin in soils we found more oxidized lignin in organic horizons relative to mineral horizons the oxidized lignin signature was maintained during transfer into rivers and lignin was a relatively constant fraction of bulk organic carbon in soils and riverine pom lignin in dom became increasingly oxidized downstream indicating active transformation of dissolved lignin during transport especially in the dry season in contrast pom accumulated undegraded lignin downstream during the wet season suggesting that terrestrial input exceeded in river degradation we discovered high concentrations of relatively undegraded lignin in pom at depth in the lower madre de dios river in both seasons revealing a woody undercurrent for its transfer within these deep rivers our study of lignin evolution in the soil river ocean continuum highlights important seasonal and depth variations of river carbon components and their connection to soil carbon pools providing new insights into fluvial carbon dynamics associated with the transfer of lignin biomarkers from source to sink

performance of a sonar altimeter in the nearshore abstract a 1 mhz sonar altimeter with automatic gain control is shown to provide accurate estimates of the distance between the instrument and the seafloor laboratory experiments indicate that distance estimates degrade slightly when the bottom is rough or sloped and when sediment is suspended in the water column results from field tests both within and seaward of the surf zone show some degradation owing to a combination of suspended sediment and bubbles bed undulations and perhaps the dynamic nature of the sand bottom under waves seaward of the surf zone the bottom can be located within 2 cm nearly continuously whereas inside the surf zone the bottom can be located only intermittently and the accuracy decreases to 3 cm a 300 m long cross shore transect of 16 altimeters was deployed from the shoreline to about 4 m depth for 3 months in summer fall 1994 near duck nc results show that the altimeters are robust and can usually provide estimates of the seafloor position every few minutes even in the surf zone during large storms

eulerian measurements of horizontal accelerations in shoaling gravity waves laboratory and field measurements of suspended sediment in the nearshore suggest that fluid accelerations are an important factor in sediment transport by oscillatory waves here eulerian accelerations of the cross shore velocity are calculated from measurements of velocity obtained by an array of bottom mounted electromagnetic flow meters spanning a natural surf zone large shoreward accelerations of brief duration are associated with the steep front faces of both near breaking and breaking waves weaker offshore accelerations of longer duration occur during passage of the more gently sloped rear faces the acceleration field is thus strongly skewed in the shoreward direction power spectra and bispectra indicate as expected that statistics of the acceleration field are significantly influenced by high frequency motions but are rather insensitive to surf beat

a behavior oriented dynamic model for sandbar migration and 2dh evolution 1 a nonlinear model is developed to study the time dependent relationship between the alongshore variability of a sandbar a t and alongshore averaged sandbar position xc t sediment transport equations are derived from energetics based formulations a link between this continuous physical representation and a parametric form describing the migration of sandbars of constant shape is established through a simple transformation of variables the model is driven by offshore wave conditions the parametric equations are dynamically coupled such that changes in one term i e xc drive changes in the other i e a t the model is tested on 566 days of data from palm beach new south wales australia using weighted nonlinear least squares to estimate best fit model coefficients the model explained 49 and 41 of the variance in measured xc and a t respectively comparisons against a 1 d horizontal 1dh version of the model showed significant improvements when the 2dh terms were included 1dh and 2dh brier skill scores were 0 12 and 0 42 respectively onshore bar migration was not predicted in the 1dh model while the 2dh model correctly predicted onshore migration in the presence of 2dh morphology and allowed the bar to remain closer to shore for a given amount of breaking providing an important hysteresis to the system the model is consistent with observations that active bar migration occurs under breaking waves with onshore migration occurring at timescales of days to weeks and increasing 2dh morphology while offshore migration occurs rapidly under high waves and coincides with a reduction in 2dh morphology citation splinter k d r a holman and n g plant 2011 abehavior orienteddynamicmodelforsandbarmigrationand2dh evolution j geophys res 116 c01020 doi 10 1029 2010jc006382

alongshore momentum balances in the nearshore the one dimensional time averaged over many wave periods along shore momentum balance between forcing by wind and breaking waves and the bottom stress is examined with field observations spanning a wide range of con ditions on a barred beach near bottom horizontal currents were measured for 2 months at 15 locations along a cross shore transect extending 750 m from the shoreline to 8 m water depth the hourly averaged bottom stress was estimated from observed currents using a quadratic drag law the wave radiation stress was estimated in 8 m depth from an array of pressure sensors and the wind stress was estimated from an anemometer at the seaward end of a nearby pier the combined wind and wave forcing integrated over the entire cross shore transect is balanced by the integrated bottom stress the wind stress contributes about one third of the forcing over the transect analysis of the momentum balances in different cross shore regions shows that in the surf zone wave forcing is much larger than wind forcing and that the bottom drag coefficient is larger in the surf zone than farther seaward consistent with earlier studies

cross shore sediment transport and morphological response on a macrotidal beach with intertidal bar morphology truc vert france a 3 week field investigation was conducted on a macrotidal beach with intertidal bar morphology with the aim of quantifying cross shore sediment transport rates and beach response cross shore transport rates were estimated in four different ways 1 net transport from bed level changes 2 measured suspended sediment fluxes 3 sediment fluxes associated with migrating wave ripples and 4 total load transport predicted using the bailard bailard j a 1981 an energetics total load sediment transport model for a plane sloping beach journal of geophysical research 86 10938 10954 model excellent qualitative agreement was found between the different measures under all recorded wave conditions with onshore offshore sediment transport and beach accretion erosion observed under low to moderate high wave conditions the agreement is due to the strong correspondence between the direction of mean cross shore flows across the intertidal bar surface and that of the sediment transport significant onshore mean flows u 0 1 0 2 m s 1 were persistently observed under low wave conditions and these were strongest in shallow water depths h 1 m these flows are attributed to the weak three dimensional water circulation in the surf zone associated with the intertidal bar rip morphology and caused onshore bar migration under high wave conditions on the other hand strong mean cross shore flows u 0 2 0 4 m s 1 due to the bed return flow were consistently directed offshore and resulted in bar destruction under low wave conditions vertically integrated suspended fluxes ripple transport rates and predicted total transport fluxes were remarkably similar but when the fluxes are integrated over a complete tidal cycle they are considerably smaller than that derived from bed level changes based on low tide surveys

sandbars in motion nearshore surf zones are turbulent places yet despite the apparently chaotic nature of the sea surface in these areas sandbars often show complex patterns with an intricate structure in their perspective stive and reniers discuss recent efforts to understand how these patterns form they highlight the report by hoefel and elgar who consider a new transport mechanism based on flow acceleration within the waves this mechanism should help to improve the predictability of sandbar structures