response characteristics of a short range high resolution digital sonar altimeter abstract the datasonics model asa 920 digital sonar altimeter dsa is a compact high frequency 1 mhz short range 0 5 5 m underwater sonar device originally designed as an altimeter for submersibles the dsa has previously been successfully used on a bottom mounted tripod to measure changes in relative bed elevation at a point on the shoreface prior to and during a storm fixed to a rigid mounting on the seabed the dsa produces a digital output that is proportional to the transducer elevation above the bed the purpose of this report is to describe the response characteristics of the dsa tests using a flat steel plate and a flat sand bed as targets showed that the dsa is capable of resolving differences in elevation of at least 0 5 cm a laboratory calibration demonstrated that the instrument response is linear because the acoustic pulse emitted by the transducer has a finite width the dsa is incapable of delineating undulating sand beds when the bedslope taken over the beamwidth is significant this limits the instrument s use over a bed of wave generated ripples in general the altimeter does not see into the troughs of ripples when its height above the bed is more than eight times the ripple wavelength the instrument stability is good over 12 operating hours 14 days of burst sampling the indicated elevation drifted by 0 3 cm in the field the acoustic pulse is susceptible to interference by suspended particulate matter and bubbles during deployment in a water depth of 8 m on the shoreface two problems occurred pulse scattering that caused a loss of echo detection which was manifested as spikes in the record and premature echoes that obscured the actual bed surface a second altimeter with an operating frequency of 300 khz datasonics model psa 900 also gave a linear response but with a reduced accuracy of approximately 1 cm

hummocky cross stratification and megaripples a geological double standard abstract hummocky cross stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits the geometry of the cross strata sets implies a low relief oval bedform of a few meters spacing orientation is poorly defined or random hummocky cross strata sets have been attributed to the action of strong storm wave surges megaripples of 2 5 m spacing are typical responses of the inner atlantic shelf of north america to storm flows side scan sonar records have shown that megaripples on the inner shelf are commonly hummocky oval to circular in plan view and lack asymmetry box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure the megaripples am produced by combined flow storm currents these are alongcoast geostrophic flows with a mean flow component of 20 60 cm sec 1 a high frequency wave orbital current of similar or weaker intensity is present but its role in megaripple formation is subordinate to that of the mean flow the high velocity coastal margin of the flow is often jetlike and downwelling in nature and is characterize by a down current decrease in sand flux and rapid deposition of sand the bottom boundary layer of such flows is characterized by dense sediment suspensions 40 60 mgl 1 and is highly turbulent due to the interaction of the mean flow and wave orbital current components eddy diffusivity up to 200 cm2 sect at 40 cm above the bed sand transport in this coastal boundary zone is obliquely offshore but neither the density of the sediment suspension nor storm surge ebb appear to be important driving forces it is difficult to account for either modern hummocky megaripples or ancient hummocky cross stratification as responses to purely oscillatory flows combined flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past hummocky cross stratified beds are generally too thick 20 80 cm to be accounted for by in situ resuspension they are more likely to have been despoited by combined flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm s duration evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime measurements on the atlantic shelf likewi e suggest a transitionlike regime whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes but more intense turbulence and a higher suspended sand concentration the lack of normal cross bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate this characteristic may be a consequence of the complex trajectories followed by near bottom water particles in response to combined flow currents

long term sediment mobilization at a sandy inner shelf site leo 15 1 nearly 2 years of wave and current observations are used to drive a calibrated bottom boundary layer model to examine sediment transport at the long term ecosystem observatory leo 15 located off of the southern coast of new jersey the multiyear record is of sufficient resolution to categorize sediment mobilization characteristics such as storm duration frequency of occurrence seasonal trends and modes of transport at this shallow water sandy site a total of 51 sediment transport events are identified within the 2 year timeframe thirty two are categorized as winter events and wintertime sediment transport constitutes 63 of the total for the 2 year period in nearly all cases bed load transport dominates the suspended load the majority of the bed load transport is onshore due to wave asymmetries that lead to higher orbital velocities during the forward half of the wave cycle for the remainder of events a bimodal wave spectrum is suspected to reverse the orbital velocity skewness leading to offshore transport suspended sediment transport is primarily directed alongshore toward the southwest consistent with current patterns during northeasters by including bed load transport the net sediment transport for the 2 year period is directed primarily onshore and slightly alongshore toward the southwest

combined flow processes and sedimentary structures on the shoreface of the wave dominated grande riviere de la baleine delta abstract shoreface sediments exposed in raised deltaic deposits along the hudson bay coast are characterized by a thick succession of predominantly parallel laminated and small scale cross laminated fine sand ripple forms where preserved include both combined flow and wave ripple geometries hummocky cross stratification is present in lower parts of the section similar facies have been found in cores from the present day shoreface measurements of waves currents and suspended sediment concentrations in 10 m water depth conducted over a 15 day period in the fall of 1999 captured four different meteorological periods these included an intense four day storm which sustained significant wave heights over 3 m for 15 hours and a storm surge of over 1 m height fair weather conditions produce weak alongshore tidal flows but oscillatory motions generated by small local waves predominate during storms orbital motions dominate the flow but a strong inertial flow generated by wind forcing is also present comparison with a published combined flow bedform stability diagram suggests that the predominant bedforms at 10 m depth would be small 2 d wave dominated ripples and small weakly asymmetrical 3 d ripples under fair weather conditions and large 3 d ripples and upper plane bed under storm wave conditions the predominantly interbedded parallel laminated and small scale cross laminated facies is therefore interpreted to be characteristic of the upper shoreface where the seabed experiences intense oscillatory motions under all storms and even some fair weather conditions combined flow ripples would form during storm events because of the stronger wind driven mean residual current hummocky cross stratification forms in deeper water during storms where the ratio of maximum orbital velocity to mean residual current velocity remains for longer periods in the large 3 d ripple field during the waning phase of storms a numerical bed predictor model using the 15 day data set as input also suggests bimodal conditions fluctuating between wave ripples and upper plane bed at 10 m water depth this model further indicates that combined flow ripples would form rarely but tends to underestimate the threshold of sediment suspension

modelling shoreface profile evolution current knowledge of hydro sediment and morpho dynamics in the shoreface environment is insufficient to undertake shoreface profile evolution modelling on the basis of first physical principles we propose a simple panel type model to map observed behaviour the internal dynamics are determined by slope dependent wave induced cross shoreface transports while the external driving factors are lateral sediment supply and sea level rise this model concept is tested with reasonable success against the observed behaviour of the central holland coast considering two hindcast periods one covering the evolution over the last century the other the subboreal subatlantic evolution a limitation of this model is that the cross shoreface dynamics are solely steered by the variations of shoaling short waves since a variety of other wave and current dynamics may be expected to be present in the coastal boundary layer it may well be that the effects of the mechanisms and conditions which are not represented are hidden in the coefficients of the sediment transport formula this limits the accuracy of the coefficients as used and our findings should be considered as an order of magnitude estimate only indeed behaviour oriented modelling implies that generalization of results to arbitrary situations and conditions is not straightforward yet we expect that some of the conclusions are more generally applicable this concerns the substantiation of the assumption that the upper shoreface responds on a much smaller time scale than the lower shoreface and the idea that the shoreface profile is not always and everywhere in equilibrium with its forcing a worthwhile observation from the holland coast application is that the bottom slope effect on the transport is only important at geological time scales the profile evolution at the engineering time scales say 10 to 100 years is effectively quasi static in that there is no feedback between the long term averaged transport and the state of the profile this implies that at these smaller scales the profile changes can be predicted on the basis of a static sediment balance this does not mean that the gravitational downslope transport is unimportant as a physical phenomenon in coastal profile evolution it is only unimportant if a highly aggregated model like this is applied at relatively short time scales

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

evaluation of depth of closure using data from duck nc usa using 12 years of frequent high precision profiles collected on a wave dominated sandy oceanic beach to 8 m depth the characteristics and interpretation of depth of closure or the seaward limit of significant profile change is critically examined this includes evaluation of the predictive capability of d1 the seaward boundary of the littoral zone as originally defined by hallermeier 1981 hallermeier r j 1981 a profile zonation for seasonal sand beaches from wave climate coastal eng 4 253 277 depth of closure during major erosional events is usually produced by the seaward limit of offshore bar movement following the original recommendation of hallermeier 1981 d1 based on 12 h exceeded wave height and a reference depth of mean low water provides a robust limit to the observations using a 6 cm change criterion empirically the observed depth of closure is 69 of d1 although the scatter is large this scatter is partly controlled by pre event profile shape most particularly bar configuration depth of closure under accretional conditions can also be measured but it is time scale dependent as accretion is a slow steady process d1 underpredicts closure for accretional situations time interval e g annual depth of closure represents the integral effect of erosional and accretional events an important observation of the data is that depth of closure increases with time scale however the full population of time interval observations is not resolved at duck due to the measurement limit 8 m depth as time interval increases from 1 year to 8 years less cases close most non closing time interval cases coincide with the periods influenced by the most energetic wave events however time interval closures are generally deeper than the biggest event closure in the period showing that it represents more than the largest event the frequency distribution of the data suggests that most if not all of these missing data simply represent closures deeper than 8 m depth up to a 4 year time interval d1 t appears to provide a reasonable limit to the quantified observations it is an interesting result that d1 an event based approach might provide a limit to closure over periods up to 4 years as these time interval closures are generally larger than those produced by single storms therefore the general applicability of this result for more complete data sets or on rapidly evolving i e eroding or accreting coasts is uncertain the present data also suggest that on swell dominated coastlines where accretional processes are dominant d1 may underpredict closure suggesting an important limitation to this approach but as sediment would be moving onshore this may not be a practical problem therefore within the limits of the data set hallermeier s 1981 approach is found to define robust estimates of depth of closure particularly for individual erosional events this useful result is expected to find widespread application in coastal geology and engineering

spatial variability of bottom types in the lower chesapeake bay and adjoining estuaries and inner shelf abstract the spatial distributions of the bed textural and morphologic properties that influence boundary layer roughness characteristics in the lower chesapeake bay the lower portions of the york james and elizabeth rivers and the adjacent inner continental shelf were systematically mapped a high resolution fully corrected side scan sonar mapping system 100 khz was used for remote acoustic detection of bottom roughness supported by ground truthing by direct in situ observations by divers these complementary methods proved to be especially effective in detecting a wide range of roughness controlling bed surface properties at various scales fine scale variations in sediment size and associated bottom texture are considered to be the main source of heterogeneity in nikuradse skin friction roughness a wide variety of small and intermediate scale morphologic elements provide meso scale and small scale distributed form drag roughness depending upon location the distributed roughness may be either biogenic or hydrodynamically induced by currents and waves although anthropogenic roughness prevails in certain instances e g port areas in terms of particular combinations of roughness scales and types combined sonar and diver observation data allow the beds to be systematically but qualitatively classified into 10 bottom types each of which is associated with a particular type of subenvironment

shelf storm flow dynamics insight from the mesoproterozoic rampur shale central india abstract the intracratonic rift related mesoproterozoic rampur shale in kudri india formed in the marine mid and outer shelf domain it bears a dominant imprint of shore parallel storm driven flow as predicted by oceanographers for modern shelves and thus differs from other ancient storm deposits in the geological record in the rampur shale gutters are shore parallel while tool marks of various kinds are both shore parallel and shore normal and show bipolarity in both directions hydrodynamic analysis and inter relationships between the flow features suggest that the storm driven flow was shore parallel dominantly unidirectional and steady initially and that it became increasingly unsteady thereafter while also becoming dominantly shore normal during peak oscillation as storm conditions waned the gutters were filled by sediments under dominant wave influence these rampur data are consistent with observations made in modern seas and suggest that storm driven flows in general have evolved in the same way over time although their geological records may vary in details because of local constraints

cross shore mean flow in the surf zone abstract the seaward mean returnflow or undertow in the surf zone which compensates for the shoreward mass flux above the wave trough level is found to be driven by the force imbalance between the wave momentum flux on the one hand and the set up on the other hand in qualitative agreement with the model of dyhr nielsen and sorensen 1970 a model which quantifies this imbalance is shown to yield theoretical results in good accordance with experiments when the proper boundary conditions are accounted for

simulation of large scale coastal change using a morphological behaviour model abstract quantitative simulation of large scale coastal behaviour lscb is possible using a model based only on principles of sand mass conservation and geometric rules for shoreface and active sand body morphology dynamic processes are subsumed into a set of parameters that define the geometry of the active cross shore profile which in turn can represent an entire coastal cell the parameters together with a set of local behaviour rules governing adjustments at discrete points along the profile provide a simple formulation that is constrained by sediment mass balance to yield emergent numerical solutions for profile kinematics the emergent qualities of this shoreface translation model are exemplified in simulation experiments on changes to sand deposits on the southeastern australian coast and shelf during the post glacial marine transgression in another example the model is used to simulate lscb over engineering time scales at an idiosyncratic site both examples illustrate the over riding importance of small residual sand movements that accumulate through time to cause lscb in comparison to gross sand movements the residuals are too small to measure directly or predict on the basis of available theory exploratory inverse simulation provides a quantitative method for side stepping these limitations the shoreface translation model therefore is used as a tool to explore possible forms of coastal behaviour through an approach that incorporates available data along with theoretical considerations validation is not strictly possible for this type of inverse approach so the degree to which a simulation reproduces observed features in nature is a measure of its success

persistent shoreline shape induced from offshore geologic framework effects of shoreface connected ridges mechanisms relating offshore geologic framework to shoreline evolution are determined through geologic investigations oceanographic deployments and numerical modeling analysis of shoreline positions from the past 50 years along fire island new york a 50 km long barrier island demonstrates a persistent undulating shape along the western half of the island the shelf offshore of these persistent undulations is characterized with shoreface connected sand ridges sfcr of a similar alongshore length scale leading to a hypothesis that the ridges control the shoreline shape through the modification of flow to evaluate this a hydrodynamic model was configured to start with the us east coast and scale down to resolve the fire island nearshore the model was validated using observations along western fire island and buoy data and used to compute waves currents and sediment fluxes to isolate the influence of the sfcr on the generation of the persistent shoreline shape simulations were performed with a linearized nearshore bathymetry to remove alongshore transport gradients associated with shoreline shape the model accurately predicts the scale and variation of the alongshore transport that would generate the persistent shoreline undulations in one location however the ridge crest connects to the nearshore and leads to an offshore directed transport that produces a difference in the shoreline shape this qualitatively supports the hypothesized effect of cross shore fluxes on coastal evolution alongshore flows in the nearshore during a representative storm are driven by wave breaking vortex force advection and pressure gradient all of which are affected by the sfcr

temporal and spatial relationship between sediment grain size and beach profile abstract sediment samples and beach profile evolution data collected along one profile line at el puntal spit santander spain are used to analyze the spatial and temporal structure of the grain size distribution variability and its relationship with the beach profile changes standard principal component analysis pca and three way pca is applied to determine the temporal and spatial scales of variability of the data results indicate that the sediment grain size distribution varies markedly along the beach profile both spatially and temporally these variations are shown to be strongly related to morphological changes in the beach profile the spatial eigenvectors determined from the profile data and those from the sediment data exhibit similar patterns with their maxima and minima located at the same position since eigenvectors may be regarded as representative of uncorrelated modes of variability it is concluded that the spatial variability of both sediment and profile data are strongly related in particular it is shown that the location of the highest variability of grain size corresponds to that of the beach profile also different grain sizes are shown to exhibit a distinct degree of variability which leads to the conclusion that each sediment size responds to the same hydrodynamics differently the temporal eigenvectors determined from the profile and the sediment data shown a seasonal dependency however their maxima and minima are not located at the same position it is shown that this temporal shift is due to the different response of each sediment size to the hydrodynamics and in particular that the recovery of the profile starts with fine material from the bar it is inferred that models for beach profile evolution which do not take into account the sorting processes involved in the sediment transport cannot be fully succesful a master grain size sample constructed by adding all the grain samples taken over the profile is used to further examine the cross shore redistribution of the sediment the following working hypothesis is suggested for a beach profile within a physiographic unit the master grain size does not depend on time

the importance of extratropical and tropical cyclones on the short term evolution of barrier islands along the northern gulf of mexico usa abstract data are presented indicating the complexity and highly variable response of beaches to cold front passages along the northern gulf of mexico in addition to the impacts of tropical cyclones and winter storms within the past decade an increase in the frequency of tropical storms and hurricanes impacting the northern gulf has dramatically altered the long term equilibrium of a large portion of this coast a time series of net sediment flux for subaerial and nearshore environments has been established for a section of this coast in florida and to a lesser extent mississippi the data incorporate the morphological signature of six tropical storms hurricanes and more than 200 frontal passages data indicate that 1 barrier islands can conserve mass during catastrophic hurricanes e g hurricane opal a strong category 4 hurricane near landfall 2 less severe hurricanes and tropical storms can promote rapid dune aggradation and can contribute sediment to the entire barrier system 3 cold fronts play a critical role in the poststorm adjustment of the barrier by deflating the subaerial portion of the overwash terrace and eroding its marginal lobe along the bayside beach through locally generated high frequency steep waves and 4 barrier systems along the northern gulf do not necessarily enter an immediate poststorm recovery phase although nested in sediment rich nearshore environments while high wave energy conditions associated with cold fronts play an integral role in the evolution and maintenance of barriers along the northern gulf these events are more effective in reworking sediment after the occurrence of extreme events such as hurricanes this relationship is even more apparent during the clustering of tropical cyclones it is anticipated that these findings will have important implications for the longer term evolution of barrier systems in midlatitude microtidal settings where the clustering of storms is apparent and winter storms are significant in intensity and frequency along the coast

a new mechanism for seaward dispersion of midshelf sediments silt to sand size particles may be transported across middle continental shelves by currents forced by the radiation stress of variable amplitude swell with the passage of each group of waves of sufficient amplitude for the high waves in the group to resuspend bottom sediments there is a seaward bias in the return of sediments to the seafloor over a storm season with the passage of many wave groups the net seaward sediment displacement may be 1 5 km

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

sedimentology and morphodynamics of a barrier island shoreface related to engineering concerns outer banks nc usa abstract forty nine vibracores were collected from a barrier island shoreface following 12 4 years of biweekly profile surveying the sedimentologic architecture of the shoreface was linked to time series elevation change and profile shape to determine relationships between morphodynamics facies development erosional processes profile closeout and cross shore transport the modern shoreface mass which erosionally overlies a tidal inlet associated complex attains a maximum thickness of 3 4 m below the beach to middle profile before pinching out seaward between 9 and 12 m depth concave erosional surfaces overlain by cross stratified fine to medium sand and gravel make up most of the lower half of the shoreface prism below the beach through middle profile reflecting longshore trough incision and subsequent current dominated aggradation at the landward margin gravel rich laminae record episodic seaward progradation of beach surfaces over coarse inner trough settings seaward a parallel laminated fine sand facies dominates the upper part of the prism recording intermittent shoal zone buildup including trough filling under high velocity plane bed conditions similarly a bioturbated parallel laminated fine to very fine sand facies makes up the entire prism below the outermost lower ramp sector again indicating buildup under high velocity conditions however accretion of the lower ramp results from major storms that cause trough scour along landward locations and simultaneous displacement of fine sand onto the lower ramp conversely lower ramp erosion typically occurs during less energetic conditions as sediment is slowly returned shoreward causing inner shoreface buildup close spacing of major storms during some years led to net progradation of the shoreface the upper and lower limits of surveyed elevation change ule and lle repeatedly develop similar limit profile shapes over shoreface accretion erosion cycles the ule reflects accretion maxima resulting from beach bar and lower ramp buildup the lle and lower sedimentologic limit lls along the inner 250 to 300 m of the active shoreface are a product of storm trough scour down to a maximum depth of 5 5 m below the lower ramp facies the lle and lls are primarily products of less energetic wave erosion down to 5 m shoreward and 9 m offshore depths the lle closely matches the lls documenting that 90 of the shoreface prism was reworked during the 12 4 year period whereas actual ages for erosional events indicate a potential of 2 to 4 years for complete reworking of the shoreface mass textural distribution indicates net long term transport direction and loci of deposition for different sized material the coarsest material is concentrated at landwardmost locations and well sorted fine to very fine sand at seaward locations medium sand to gravel tends to remain within the trough zone even during extreme storm events the ule and lle also represent the upper and lower limits for profile closure events a location of about 4 5 m depth at 300 to 350 m from shoreline marks the boundary between inner profile and lower ramp associated closure events the boundary between trough associated and lower ramp facies and the lower ramp morphologic break all of which correspond to the juncture between longshore current and shoaling wave dominated zones the lower ramp zone of closure is a zone of seaward decreasing storm transport in which fine to very fine sand is the typical bedload material

long term changes in morphology and textural sediment characteristics in response to energy variation on shoreface connected ridges off the east frisian barrier island coast southern north sea abstract patterns of surficial sediment distribution across shoreface connected sand ridges located along the barrier island coast of the southern north sea generally show a strong affinity between textural parameters and ridge morphology comparable to that observed in other parts of the world thus the sediment of the upper shoreface and the inner ridge crest consists of fine sand water depths 12 m in order to assess the mobility of the sediments and the consistency of the observed textural patterns two high resolution multi beam bathymetric surveys were carried out along selected corridors across a ridge and trough system off spiekeroog island located along the southern north sea coast of germany one in 2003 the other in 2007 the bathymetric data of the two surveys show only small scale topographic changes these were produced mainly by local changes in the bedform patterns on the inner ridge the bathymetric surveys therefore document a remarkable morphological stability of the ridges over time periods of several years spatial and temporal variations in the distribution of standard textural parameters were assessed on the basis of 95 sediment samples recovered on the same grid in 1989 and 2005 these data show that for example the distribution of coarse sand 0 1 phi became enhanced along the axis of the outer trough while expanding onto the lower ridge flanks current measurements in fair weather demonstrate that the coarse sediments cannot be moved by the local tidal currents the observed changes in the distribution of coarse sand and in particular its expansion onto the adjacent ridge flanks must therefore be associated with wave action probably in combination with tidal currents this is supported by an assessment of the entrainment potential of coarse sand d 1 mm by typical waves during storms t 8 s h 2 5 m and fair weather t 4 s h 0 7 m the results demonstrate that the effective wave base for coarse sand during fair weather conditions is limited to a water depth of 4 8 m whereas during storms it exceeds a depth of 26 4 m the mobility of the coarse sands in the outer trough at water depths 16 m and with it the morphodynamics of the ridges as a whole is thus clearly controlled by the superimposition of storm waves on the tidal current regime

simulation of grain size abundances on a barred upper shoreface abstract using the barred upper shoreface of long island new york as an example this study provides a semi quantitative demonstration of the concept that the distribution of grain sizes in a closed depositional system is the product of the reworking and redistribution by the physical process and the availability of the sediment source in the particular case of long island the physical process is represented by the agitation of sediment grain sizes by the shoaling waves under the average fair weather wave conditions as a first order approximation a nearby subaerial morainal deposit represents the drowned glacial deposit on the shoreface as the sediment source the probability of abundance of four representative sediment grain sizes across the upper shoreface of the study site was simulated the shoaling wave field over a segment of the upper shoreface of long island was first simulated by a numerical wave and circulation model using deepwater incident waves under the conditions of the average fair weather remote ocean swells and a local storm the model results were then used to estimate sediment entrainment across the upper shoreface of the study site by using an empirical formulation developed for sediment movement under progressive waves the probability of the occurrence of each grain size on the upper shoreface is characterized by a non dimensional retention index the results indicate that the fair weather wave climate has the highest probability for sediment retention on the seabed the cross shore distribution of a grain size which is represented by the probability of abundance abundance index can be simulated by multiplying the probability of occurrence retention index as a result of the differential wave agitation with the availability grain size frequency distribution of the source sediment deposit the grain sizes representing the fine sand group have the best fit between the observed and simulated distributions suggesting a group of grain sizes that is in dynamic equilibrium with shoaling waves for the rest of the grain sizes the combined effects of shoaling waves and other secondary hydrodynamic processes are important in determining their cross shore distributions

across shelf benthic transports on the inner shelf of the middle atlantic bight during the halloween storm of 1991 instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 m and 8 m off the u s army corps of engineers field research facility at duck north carolina usa over a two week period that included the severe and prolonged halloween storm of late october 1991 the storm persisted for five days and generated waves with heights and periods of up to 6 m and 22 s although the instrumentation was destroyed current profile and suspended sediment concentration profile data were recovered from the 13 m site wind driven mean along shelf currents at 1 24 m above the bed attained speeds of nearly 0 5 m s 1 across shelf flows primarily seaward directed had speeds varying from 0 05 to 0 15 m s 1 these seaward flows intensified in association with groups of high waves total mean current and skin friction bed shear stresses were increased relative to moderate energy values by more than an order of magnitude notably the highest shear stresses occurred in association with high long period swell during the late phase of the storm after winds had turned offshore creating shoreward mean flows near the bed suspended sediment concentrations exceeded 1 g 1 1 throughout the lower meter of the water column and at elevations well above the top of the wave boundary layer net across shelf suspended sediment fluxes were strongly dominated by mean flows cospectral analyses show that important but secondary roles were also played by infragravity oscillations and by wave orbital velocities

medium term frequency distributions of cross shore suspended sediment transport rates in water depths of 3 to 9 m abstract a data set of several thousands of hours of near bed flow obtained at three cross shore positions in 3 to 9 m water depth in the multiple bar system of terschelling netherlands was used to estimate the medium term years frequency distribution of the cross shore suspended sediment transport rates induced by short waves infragravity waves and cross shore mean flows predictions of an energetics based transport model were categorised into groups of the local height over depth ratio with a width of 0 02 and were subsequently coupled to the discrete medium scale probability distribution of this ratio at all depths the estimated medium scale sediment transport rate by the short waves q ms h and mean flows q ms mf were of approximately equal magnitude and were about three times as large as that of the infragravity waves in general the medium term sediment transport rates were dominated neither by the most extreme conditions nor by day to day situations this was related to the infrequent recurrence of the most energetic events and by the predicted negligible transport rates under daily conditions in 9 m depth breaking conditions contributed to about 90 of both q ms h and q ms mf in shallower water 3 5 m non breaking conditions became increasingly important for q ms h whereas q ms mf remained fully dominated by surf zone conditions this observation as well as literature findings for water depths less than about 3 m suggest that the range of small scale conditions that contribute most to q ms h and q ms mf changes in the onshore direction from mainly breaking conditions at depths in excess of 5 7 m towards prolonged non breaking periods for q ms h and short breaking events for q ms mf on the beach

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

new instrumentation for the investigation of sediment suspension processes in the shallow marine environment abstract an instrumentation system has been developed that continuously measures flow velocity water depth and suspended sediment concentration profiles in the nearshore zone the system consists of four components 1 an array of electronic sensors including an electromagnetic current meter a surface piercing resistance wave gauge and a five element array of optical back scatterance sensors 2 a signal processing electronics unit 3 a guyed mast support structure and 4 a data acquisition system the small size of the sensors and their response characteristics make the array suitable for monitoring sediment transport to within a few centimeters of the bed under moderately large breaking waves 2 m results from an experiment on an exposed ocean beach on the coast of washington u s a indicate that sand suspension events of long duration 15 40 sec relative to the period of incident waves contribute a significant quantity of sand to the total littoral drift work is now in progress to modify the system for self contained operation on the continental shelf field investigations with the modified system will focus on the acquisition of a data set to test the validity of the suspended sediment distribution equation

modification of the quaternary stratigraphic framework of the inner continental shelf by holocene marine transgression an example offshore of fire island new york abstract the inner continental shelf off fire island new york was mapped in 2011 using interferometric sonar and high resolution chirp seismic reflection systems the area mapped is approximately 50 km long by 8 km wide extending from moriches inlet to fire island inlet in water depths ranging from 8 to 32 m the morphology of this inner continental shelf region and modern sediment distribution patterns are determined by erosion of pleistocene glaciofluvial sediments during the ongoing holocene marine transgression much of the shelf is thus an actively forming ravinement surface remnants of a pleistocene outwash lobe define a submerged headland offshore of central fire island east of the submerged headland relatively older pleistocene outwash is exposed over much of the inner continental shelf and covered by asymmetric sorted bedforms interpreted to indicate erosion and westward transport of reworked sediment erosion of the eastern flank of the submerged pleistocene headland over the last 8000 years yielded an abundance of modern sand that was transported westward and reworked into a field of shoreface attached ridges offshore of western fire island west of the submerged headland erosion of pleistocene outwash continues in troughs between the sand ridges resulting in modification of the lower shoreface comparison of the modern sand ridge morphology with the morphology of the underlying ravinement surface suggests that the sand ridges have moved a minimum of 1000 m westward since formation comparison of modern sediment thickness mapped in 1996 1997 and 2011 allows speculation that the nearshore shoreface sedimentary deposit has gained sediment at the expense of deflation of the sand ridges

hydrodynamics and sedimentation in wave dominated coastal environmentsshoreface morphodynamics on wave dominated coasts an open ocean shoreface typical of long wave dominated sandy coasts has been examined through a combination of extensive field measurements of wave and current patterns with computations of marine bedload transport and sedimentation sand transport on the upper shoreface is dominantly controlled by waves with only secondary transport by currents sand on the middle and lower shoreface as well as the inner continental shelf is entrained by storm waves and transported by a complex pattern of bottom boundary layer currents storm events have been studied and modeled for the shoreface off tiana beach long island the dominant effect of coastal frontal storms is to cause significant shore parallel bedload transport with important shore normal secondary components these storms tend to result in net offshore transport of sand removed from the beach and surf zone systems the bedload transport during a storm is convergent on the shoreface leading to accretion most accretion occurs on the upper shoreface with lesser deposits covering the middle and lower shoreface as well as the inner continental shelf longer term equilibrium can be maintained by slow return of sand up the shoreface during non storm conditions annual and geologic time scale budgets of shoreface sand transport and sedimentation yield equilibrium net accretion or net deposition the annual balance results from an integration of the event scale bedload transport patterns and morphologic responses these processes and responses have feedback mechanisms which stabilize the system over longer but not geologic time scales geologic time scale balances are controlled by relative sea level changes and relative availability of sediment supply with the event scale shoreface and transporting processes providing the mechanism to produce the changes in long term morphology and sedimentation patterns in the area of study the long term pattern is one of net shoreface erosion and the permanent loss of sand to the shelf floor

the bottom boundary layer of the bay stem plains environment of lower chesapeake bay field observations of bottom boundary layer processes in the bay stem plains environment of lower chesapeake bay depth 11 12 m were made using a bottom mounted tripod supporting electromagnetic current meters and a pressure sensor the prandtl von karman equation the inertial dissipation method and the grant and madsen wave current interaction model were applied to the field data to estimate time varying bed stress and to evaluate the likely contributions of waves and currents to total shear stress and to skin friction biogenic bed micromorphology dominated bed roughness a background signal of long period low amplitude swell was present for most of the time although these oscillations were too weak to agitate the bottom sediments interactions of the swell with tidal currents in the presence of a biogenically roughened bottom caused the hydraulic roughness height z 0 and drag coefficient c d sensed by the mean current to be significantly increased in addition internal waves with frequencies in the neighbourhood of the pycnoclinal brunt vaisala frequency period range approximately 60 100 s appeared intermittently and caused near bottom velocity fluctuations with amplitudes of over 10 cm s 1

shoreface morphodynamics on wave dominated coasts abstract an open ocean shoreface typical of long wave dominated sandy coasts has been examined through a combination of extensive field measurements of wave and current patterns with computations of marine bedload transport and sedimentation sand transport on the upper shoreface is dominantly controlled by waves with only secondary transport by currents sand on the middle and lower shoreface as well as the inner continental shelf is entrained by storm waves and transported by a complex pattern of bottom boundary layer currents storm events have been studied and modeled for the shoreface off tiana beach long island the dominant effect of coastal frontal storms is to cause significant shore parallel bedload transport with important shore normal secondary components these storms tend to result in net offshore transport of sand removed from the beach and surf zone systems the bedload transport during a storm is convergent on the shoreface leading to accretion most accretion occurs on the upper shoreface with lesser deposits covering the middle and lower shoreface as well as the inner continental shelf longer term equilibrium can be maintained by slow return of sand up the shoreface during non storm conditions annual and geologic time scale budgets of shoreface sand transport and sedimentation yield equilibrium net accretion or net deposition the annual balance results from an integration of the event scale bedload transport patterns and morphologic responses these processes and responses have feedback mechanisms which stabilize the system over longer but not geologic time scales geologic time scale balances are controlled by relative sea level changes and relative availability of sediment supply with the event scale shoreface and transporting processes providing the mechanism to produce the changes in long term morphology and sedimentation patterns in the area of study the long term pattern is one of net shoreface erosion and the permanent loss of sand to the shelf floor

maintenance of the shoreface by wave orbital currents and mean flow observations from the long island coast waves and currents were observed for a total of 36 days in august 1976 and 1977 calculations of wave transport of sediment using a bagnold equation for transport and second order stokes theory show that wave orbital current asymmetry results in landward sediment transport in 10 m or less water depth but does not significantly effect transport at greater depths calculations of sand transport using the madsen grant formulation show that during a 36 day observation period sediment in less than 10 m of water generally moved landward into the breaker and then along coast with the wave driven littoral flow the calculations show that below 10 m the generally upwelling coast parallel mean flow did not move sediment however during two days of a mild summer northeaster storm a downwelling coastal jet developed with 60 cm sec sup 1 maximum current the calculations show that if this flow had been combined with the intense wave regime of the succeeding several days sediment would have been entrained across the lower as well as the upper shoreface movement would have been offshore and alongshore such storm transport is believed to be typical of the more intense winter storms and to be responsible for the erosionalmore shoreface retreat that has caused up to a kilometer of shoreline retreat on the long island coast in the last century less

postglacial sediment deposition along a mixed carbonate siliciclastic margin new constraints from the drowned shelf edge reefs of the great barrier reef australia a seismic stratigraphy analysis was conducted at two sites hydrographers and noggin passages separated by about 540 km on the shelf edge of the central great barrier reef gbr australia we used recently available seismic and bathymetry data and a new synthesis of downhole logs and lithological petrophysical and radiometric data from cores recovered by the integrated ocean drilling program expedition 325 great barrier reef environmental changes we compared the stratigraphy of both sites identifying a full depositional sequence with deposits from at least 28 ka bp to 7 ka bp bounded by two marine flooding surfaces within this sequence each systems tract is represented by unique depositional features characteristic of the shelf edge systems despite the broad environmental and geomorphic similarities between the two sites differences in postglacial reef development were remarkable these contrasts can be explained as a result of 1 local antecedent substrate variations and 2 the interplay of shelf physiography with late quaternary sea level fluctuations which favoured changes in biological production and sediment flux as the palaeo shoreline evolved from linear to complex during intermediate sea levels during these intermediate sea levels the northern estuarine coast and its steep substrate at shelf edge locations contrasted strongly with the protected palaeo lagoons and the extensive gentle marginal terraces found at the southern central gbr this setting enhanced the regional differences in sediment transport and reef development through the last transgression the conceptual model presented here provides a broader depositional framework and improves the understanding of the main processes controlling the spatial and temporal depositional patterns on the shelf edge of mixed siliciclastic carbonate margins

rollover drowning and discontinuous retreat distinct modes of barrier response to sea level rise arising from a simple morphodynamic model we construct a simple morphodynamic model to investigate the long term dynamic evolution of a coastal barrier system experiencing sea level rise using a simplified barrier geometry the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux with barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response the modeled barrier system demonstrates four primary behaviors height drowning width drowning constant landward retreat and a periodic retreat height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea level rise on the other hand width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash driven landward migration during periodic barrier retreat the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens this periodic retreat which occurs even with a constant sea level rise rate arises when overwash rates and shoreface response rates are large and of similar magnitude we explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate barrier response can be particularly sensitive to the sea level rise rate and back barrier lagoon slope overall our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea level rise rate sediment supply or back barrier geometry

the morphodynamic effects of incident wave groupiness and tide range on an energetic beach abstract the roles played by daily tide range and incident wave groupiness in affecting the short term morphodynamic responses of an energetic beach are examined demodulation of original tide and wave time series from the moderate energy narrabeen beach australia resulted in new time series of daily tide range and a daily grouping factor which expresses the relative amplitude of the alternation between groups of high and low waves using these new time series together with time series of daily beach state and changing breaker conditions statistical analyses were performed aimed at determining the contributions made by tide and groupiness in explaining time varying beach state additional to those made by time varying wave conditions eigenvector analyses of monthly surveyed beach and surf zone profiles provided a set of time series on modes of profile configuration change which were also analyzed to determine the relative effects of waves and tide range on profile shape both tide range and groupiness are significant determinants of beach state the six beach states are reflective low tide terrace transverse bar and rip rhythmic bar and beach longshore bar trough and dissipative wright and short 1984 provided that wave conditions are within the appropriate range to favor beach states at the reflective end of the sequence spring tides will tend to favor the low tide terrace over the transverse bar and rip state higher tide ranges are also associated with more subdued bar trough topography the more accentuated bar trough profiles are developed when tide range is minimal overall higher relative incident wave groupiness favors the more dissipative states incident wave groupiness is the dominant factor determining whether the low tide terrace state or transverse bar and rip state prevails provided incident wave steepnesses permit one or the other of these states groupiness is also important in discriminating between the transverse bar and rip and rhythmic bar and beach states

the systematic contribution of transporting mechanisms to the cross shore sediment transport in water depths of 3 to 9 m the systematic contributions of short waves infragravity waves and mean flows to the cross shore sediment transport were studied with direct measurements of instantaneous near bed sediment concentrations and velocities as well as with an energetics based sediment transport model using measured time series of near bed cross shore flow as input the study was conducted at four cross shore positions in the multiple bar system of terschelling netherlands during three field campaigns each with a five week duration the data were collected in 3 to 9 m water depth during a wide range of conditions from low energy non breaking conditions to almost fully saturated breakers the height over depth ratio hs h was applied at all four measuring stations as a local conditional parameter all model predictions were grouped into hs h classes with a width of 0 02 to highlight the generality of the data the energetics approach predicted that the suspended load dominated over the bedload at all four stations both the sediment flux measurements as well as the model predictions showed that the largest contributors to the gross suspended sediment transport were made by the short waves and the undertow inducing an onshore and offshore transport respectively their contributions to the gross suspended transport were about 40 to 50 under surf zone conditions bound infragravity waves were observed and predicted to result in an offshore directed transport that was of subordinate magnitude compared to the observed and estimated magnitudes by the short waves and the undertow however as these transports almost cancelled out because of their about equal magnitude but opposite sign the suspended transport by bound infragravity waves may not be ignored and may rather paradoxically have a relatively large influence on the net suspended sediment transport hydrodynamical processes that do not seem to be of importance to the onshore and offshore sediment transport in 3 to 9 m water depth in the long run are mean flows under non breaking conditions and free infragravity motions a direct comparison between measured sediment fluxes and the model predictions suggests that simple energetics models are suitable for predicting cross shore sediment transport in 3 to 9 m water depth

cross shore migration of lunate megaripples during duck94 1 remote acoustic observations of lunate megaripple migration are presented from two storm events during the duck94 nearshore dynamics experiment for time periods during which longshore currents were weak 20 cm s during these periods significant wave orbital velocities were 50 80 cm s megaripple heights were 0 05 0 5 m megaripple horns were directed shoreward the crest to trough slope of the shoreward advancing ripple face was close to the angle of repose and migration speeds were 10 40 cm h onshore the observations also indicate that the megaripples stalled and may have begun to migrate offshore when the mean offshore flow exceeded 20 cm s during the second storm stress based bed load sediment transport models are moderately successful in predicting the observed dependence of migration velocity on measured fluid velocities separated into zero mean current infragravity wave and sea and swell wave frequency bands wave and mean current friction factors fw and fc are obtained by best fit between the predicted and observed migration velocities for two choices 3 2 and 5 2 for the stress exponent in the bed load transport part of the model net transport is computed using wave velocity amplitudes determined from both run length statistics and wave by wave improved agreement with observations is obtained for the wave by wave net transport predictions and for the wave like treatment of the infragravity band the level of agreement is relatively insensitive to the value of the best fit current and wave friction factor values for 5 2 and 3 2 are fc 4 3 10 3 and 8 0 10 3 and fw 1 7 10 2 and 4 8 10 2 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

shoreface morphodynamics of a high energy steep and geologically constrained shoreline segment in northern ireland abstract the shoreface a complex and poorly understood part of the coastal zone plays a critical role in sediment transport processes between the beach and the inner shelf this two year study examined the surface and subsurface architecture and the process response mechanisms of a high energy steep and geologically constrained shoreface segment 1 20 m depth in northern ireland fourteen sequential bathymetric surveys covering an area of 2 km2 were compared and analysed in order to investigate seabed changes and their relationship to wave and wind forcing results reveal that the shoreface is highly dynamic and complex an examination of high energy conditions and lower than average energy conditions revealed significant periods of both accretion and erosion these complex morphodynamic responses are attributed mainly to a combination of antecedent pre survey morphology differences in wind forcing and coastal surges a comparison of seabed changes over 2 yr reveals net shoreface accretion which is attributed to inner shelf to shoreface sediment transport over the same period the adjacent west strand beach showed moderate erosion the study provides information on the morphodynamics of a steep high energy embayment shoreface a coastal environment which has received little previous attention

vertical and horizontal particle transport in the coastal waters of a large lake an assessment by sediment trap and thorium 234 measurements 1 the horizontal and vertical flux of particulate material in the nearshore of southern lake michigan 0 40 m was estimated with the naturally occurring radionuclide 234th horizontal fluxes of 234th supplemented apparent vertical fluxes of 234th in the water column based on local 234th 238u disequilibria by a factor of 7 14 reinforcing the importance of lateral transport in coastal environments calculated onshore transport of particulate material across the 40 m isobath was as high as 1 1 106 kg km 1 d 1 and exceeds estimates of terrigenous riverine and bluff erosion loading estimates of onshore flux of organic carbon exceeded areal primary productivity by as much as 300 and should be considered in nearshore carbon budgets bottom tethered sediment traps placed 5 m above the bottom measured sedimentation rates that were 1 order of magnitude lower than 234th derived mass fluxes from the water column and 2 orders of magnitude lower than 234th derived mass fluxes to the lakebed we ascribe this difference to under collecting by the sediment trap either because of trap hydrodynamics or flux occurring below the trap capture plane cross shore fluxes showed a periodicity of 4 days and correlated strongly with a topographic vorticity wave that is present throughout the year in southern lake michigan the impact of this wave as a driver of bidirectional cross shore flux on biogeochemical cycling and both nearshore and offshore food webs has not yet been explicitly considered

on offshore bed load sediment transport in the coastal zone abstract laboratory data and theoretical derivations are presented for cross shore bed load transport the laboratory investigations were carried out in a wave flume with movable bed sand with d 50 0 022 cm the tests have shown inter alia that bed load transport in the surf zone is shoreward if single breaking occurs i e on a uniform slope or at a single bar and is accompanied by offshore transport seawards of the breaking point multiple transitions from offshore to onshore transport take place if the wave breaking process repeats on beds with more than one bar the theoretical considerations are based on bagnold s energetics approach the sediment transport is assumed to be controlled primarily by asymmetric oscillatory motion of water superimposed on a steady return current gravity and the critical conditions of incipient sediment movement the parameters intervening in the theoretical formulae have been identified good agreement between theory and experiments has been reached in particular for short waves and beds with no more than one underwater bar the formulae derived have been found to be acceptable for ursell numbers s 60

sediment transport under wave and current combined flows on a tide dominated shoreface northern coast of france abstract hydrodynamic circulation and sediment transport patterns were investigated on a macrotidal shoreface off the southern north sea coast of france from in situ directional wave and current measurements and through numerical modeling of sediment flux current meters equipped with wave gauges were deployed at two sites on the mid upper shoreface in approximately 5 6 m water depth during 6 and 15 days sediment flux was estimated using the sedtrans96 numerical model li m z amos c l 2001 sedtrans96 the upgraded and better calibrated sediment transport model for continental shelves comp geosc 27 619 645 based on grant and madsen s grant w d madsen o s 1986 the continental shelf bottom boundary layer ann rev fluid mech 18 265 305 combined flow boundary layer theory the hydrodynamic measurements and modeling results show that sediment on the shoreface is essentially transported alongshore in response to shore parallel tidal currents the direction of the net sediment transport is largely determined by the asymmetry of the tidal flows which can be flood or ebb dominated but during important wind speed events winds blowing in the same direction as tidal flow can significantly reinforce tidal currents or conversely can limit tidal current speeds when acting in the opposite direction although sediment transport on this macrotidal shoreface is strongly controlled by tidal processes waves can significantly enhance bed shear stresses resulting in increased sediment remobilization and hence higher sediment flux wave induced onshore sediment transport may occasionally occur but is several orders of magnitude lower than longshore transport this macrotidal shoreface is thus characterized by a hydraulic regime strongly dominated by shore parallel tidal flows that favour longshore sediment dispersal rather than shore normal transport our results imply that the water depths from which sediment may be transported shoreward to beaches is certainly much more restricted on a macrotidal shoreface such as the one bordering the northern coast of france than on micro and mesotidal shoreface environments where weaker tidal currents do not significantly limit cross shore sediment movements

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

measurements of high concentration suspended sediments using the optical backscatterance sensor abstract an optical backscatterance sensor obs was used to measure extremely high concentration suspended sediments on the amazon continental shelf as part of the amasseds project a multidisciplinary amazon shelf sediment study using a pumping system for obtaining water samples to calibrate the sensor in situ the obs output was observed to increase approximately linearly for concentrations 36 g l the obs response was nearly continuous from 0 to 320 g l making it possible to obtain continuous suspended sediment concentration profiles through fluid muds

reverse sediment transport induced by amplitude modulated waves abstract amplitude modulated waves can induce bedload and suspended load transports of sediment in the reverse direction of the wave propagation the reverse transport rate is mainly controlled by the magnitude of the bottom orbital velocity the mechanism is most effective in transporting silts and fine sands offshore but not clays the sorting characteristic may be amplified when non uniform size sediments are present in the sea bed the mechanism provides a feasible explanation for two shelf sedimentation patterns difficult to understand using conventional seaward sediment transport models

an energetics bedload model for a plane sloping beach local transport bagnold s energetics based sediment transport model for streams is used as a basis for the development of a model for the time varying transport of bedload over a plane sloping bed the sediment transport vector is found to consist of two components the velocity induced transport directed parallel to the instantaneous velocity vector and the gravity induced transport vector directed downslope the model is applied to the case of sediment transport within the surf zone for the separate cases of weak and strong longshore currents relative to the wave bore oscillatory water velocity the results suggest the bagnold s oscillatory sediment transport model which forms the basis for a number of longshore sediment transport models is only correct for the special case of weak longshore currents and near normal incident waves for an artibrary strength longshore current v and a local incident wave angle a the longshore transport rate is found to be proportional to sina v 1 sin sup 2 a v sup 3 where u is the wave induced oscillatory water velocity the model also predicts the onshore offshore sediment rate for weak longshore currents and near normal incident waves the predicted equilibrium beach slope is equal to tanf strong longshore currents and or significant local wave angle conditions however cause amore change in this equilibrium beach slope less authors bailard j a inman d l publication date 1981 03 20 osti identifier 6767919 resource type journal article resource relation journal name j geophys res united states journal volume 86 c3 research org scripps institution of oceanography university of california la jolla california 92093 country of publication united states language english subject 58 geosciences shores sedimentation coastal waters gravitation sediments silt velocity water currents currents surface waters 580500 oceanography 1980 1989

contrasting geomorphological storm response from two adjacent shorefaces shorefaces play a critical role in cross shore sediment transport between the beach and inner shelf particularly during storm conditions a comparison and examination of storm driven sedimentary changes on two adjacent shorefaces in northern ireland located only 5 km apart revealed significantly different geomorphological responses the steeper shoreface at west strand responded with extensive sediment deposition across almost the entire shoreface in contrast with the more dissipative and quasi linear shoreface at portstewart which mostly showed nearshore bar changes results from the two sites which have similar wave wind characteristics and seabed sediments suggest that i cross shore morphology ii immediately previous antecedent shoreface morphodynamic behaviour and iii the presence or lack of offshore sand appear to be the primary controls on storm driven sedimentary changes attributed to the high energy event copyright 2015 john wiley sons ltd

role of waves and tides on depth of closure and potential for headland bypassing abstract depth of closure is a fundamental concept used to define the seaward extent of a morphodynamically active shoreface at a particular temporal scale the estimation of this limit in relation to the depth in front of the bounding headlands along embayed coastlines allows questioning whether embayments often deemed closed sediment cells experience more headland bypassing than expected wave based parameterisations developed for microtidal beaches are most widely used to estimate closure depth however a re evaluation of the concept for shorefaces influenced by geological control presence of headlands and or bedrock and strong tidal currents is appropriate here we use the macrotidal embayed and high energy coastline of sw england to identify the active nearshore limits with a multi method approach that includes observations of shoreface morphology and sedimentology offshore inshore wave formulations and bed shear stress computations we identify the basal limit of significant i e 0 14 m morphological change depth of closure doc and a maximum depth of extreme bed activity and sediment transport depth of transport dot observations of doc correspond closely to the values predicted by existing formulations based on inshore wave conditions 10 15 m for the study area relative to mean low water spring water level in this case the computed dot represented by the upper plane bed transition attained under extreme conditions exceeds 30 m depth in the study area the significant implication is that even though many headlands appear sufficiently prominent to suggest a closed boundary between adjacent embayments significant wave and tide driven sediment transport is likely to occur beyond the headland base during extreme events especially at low water levels the maximum depth for significant sediment transport dot was computed across a broad wave current parameter space further highlighting that tidal currents can increase this closure depth estimate by 10 m along macrotidal coastlines representing a 30 increase compared to tideless settings this work illustrates the importance of tidal currents in depth of closure calculations and challenges the notion that embayed beaches are generally closed cells as headland bypassing may be more wide spread than commonly assumed along exposed coastlines globally

barrier island evolution middle atlantic shelf u s a part i shoreface dynamics abstract in part i of this paper near bottom fluid velocity and sediment concentration measurements from the shoreface of a middle atlantic barrier long island coast are analyzed to provide insight into the dynamics of erosional shoreface retreat in part ii these data are combined with observations of coastal stratigraphy in order to construct a model for the evolution and behavior of middle atlantic barriers wave motions on the long island shoreface tend to drive sediment onshore calculations of the onshore sediment flux driven by asymmetrical shoaling waves show that for the wave states commonly encountered on the middle atlantic shelf values become strongly positive landward of approximately 10 m water depth upper shoreface but drop to very low values shortly seaward of that isobath on the lower shoreface and inner shelf floor fair weather wave current interaction tends to cause a landward creep of sediment even though the wave orbital component of the near bottom velocity field is nearly symmetrical this movement occurs because the wave approach direction generally lies within 90 of the direction up coast or down coast of the alongshore wind driven current therefore fluid shear stress acting on the sediments tends to be reinforced during the shoreward stroke of the wave orbital motion and partially cancelled during the seaward stroke during major storms the rate of sediment transport increases by at least an order of magnitude and the role of wind driven currents becomes important much more sediment is resuspended by the wave orbital current component because storm waves are more powerful than fair weather swells but the wind driven flow component is also much stronger and is now available for transporting the resuspended sediment storm flows over the shoreface occur in distinct dynamic zones surf zone friction dominated zone transition zone geostrophic zone the zones are defined by dynamical considerations and expand or contract with the intensity of the causative wind stress during peak flow events the characteristic length scales of the friction dominated and geostrophic zones tend to correspond with the morphologic zones of the shoreface and inner shelf the storm intensified wind driven alongshore flows of the lower shoreface are frequently jet like in nature and may be upwelling or downwelling wind driven coastal flows with a strong downwelling component occur during most storms on the long island coast and are particularly important to the coastal sand budget during these events sand is entrained by the storm intensified upper shoreface circulation system of wave driven alongshore currents and rip currents and is fed into the main wind driven coastal current because of the offshore component of bottom flow sand is swept down the lower shoreface and onto the adjacent inner shelf fair weather processes may be unable to return storm deposited sand to the beach from such an offshore position the shoreface transport regime of the long island barrier coast thus consists of long periods of time months during which sand moves slowly toward the beach punctuated by short intense periods hours or days during which sand is transferred from the shoreface to the adjacent inner shelf the long term sense of movement of the shoreface whether retreating the long island case or prograding must depend on the loss or gain of sand by shoreface with respect to the inner shelf the data shows that the coastal sand budget is controlled not only by the upper shoreface cycle withdrawl of sand from the beach prism storage in the breakpoint bar and its subsequent return to the beach but by a cycle of larger spatial and temporal scale in which sand is exchanged between the shoreface as a whole and the adjacent inner shelf

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

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

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

bars bumps and holes models for the generation of complex beach topography in shallow water any two waves of the same frequency are shown to produce complex patterns of drift velocity above the sea bed if the longshore components hi h of the wave numbers of the two waves are different these steady flow patterns exhibit a longshore periodicity of wave number hi h irrespective of whether the waves propagate in the same direction say hi he positive or in opposite directions he negative the interaction of two edge wave modes is examined in detail the drift velocities are calculated and a simple sediment transport model is used to predict the beach topography that would be in equilibrium with these flow patterns as expected crescentic sand bars are produced by the special case of standing edge waves hi h intriguingly for all other cases a combination of complex transverse bars plus meandering or straight offshore bars result patterns that are surprisingly reminiscent of many published descriptions of complex rhythmic topography the extension of the model to three or more waves produces topography that appears to be very irregular although the pattern should repeat over sufficiently long distances along the beach even with only three waves these distances may be very large compared to the scale of the bars

exploring shoreface dynamics and a mechanistic explanation for a morphodynamic depth of closure using energetics based formulations for wave driven sediment transport we develop a robust methodology for estimating the morphodynamic evolution of a cross shore sandy coastal profile in our approach wave driven cross shore sediment flux depends on three components two onshore directed terms wave asymmetry and wave streaming and an offshore directed slope term in contrast with previous work which applies shallow water wave assumptions across the transitional zone of the lower shoreface we use linear airy wave theory the cross shore sediment transport formulation defines a dynamic equilibrium profile and by perturbing about this steady state profile we present an advection diffusion formula for profile evolution morphodynamic peclet analysis suggests that the shoreface is diffusionally dominated using this depth dependent characteristic diffusivity timescale we distinguish a morphodynamic depth of closure for a given time envelope even though wave driven sediment transport can and will occur at depths deeper than this morphodynamic closure depth the rate of morphologic bed changes in response to shoreline change becomes asymptotically slow linear wave theory suggests a shallower shoreface depth of closure and much sharper break in processes than shallow water wave assumptions analyzing hindcasted wave data using a weighted frequency magnitude approach we determine representative wave conditions for selected sites along the u s coastline computed equilibrium profiles and depths of closure demonstrate reasonable similarities except where inheritance is strong the methodology espoused in this paper can be used to better understand the morphodynamics at the lower shoreface transition with relative ease across a variety of sites and with varied sediment transport equations

bedform contributions to cross shore sediment transport on a dissipative beach abstract field measurements of hydrodynamics suspended sediment transport rates and bedform sediment transport rates were made in the intertidal section of a dissipative sandy beach d50 0 26 mm slope 1 80 at perranporth uk pressure transducers acoustic doppler velocimeters optical backscatter sensors and an acoustic sand ripple profiler were deployed for 12 tides measuring in a range of wave heights from 0 5 to 2 2 m water depths from 1 to 6 m and in current strengths up to 0 4 m s data were analysed in terms of the distance to shore x normalised by the surf zone width xs and spanned the region 0 4 1 5 the fractional contribution of the shoreward directed bedform transport to the total absolute transport was up to 100 and occurred broadly in the region of the breakpoint 0 5

equilibrium shoreface profiles a sediment transport approach abstract large scale coastal behaviour models use the shoreface profile of equilibrium as a fundamental morphological unit that is translated in space to simulate coastal response to for example sea level oscillations and variability in sediment supply despite a longstanding focus on the shoreface profile and its relevance to predicting coastal response to changing environmental conditions the processes and dynamics involved in shoreface equilibrium are still not fully understood here we apply a process based empirical sediment transport model combined with morphodynamic principles to provide new insight into equilibrium shoreface profile development and shape the quantitative model is based on field measurements and shows that a balance between onshore sediment transport due to oscillatory wave motion and offshore sediment transport due to gravity produces profile shapes that are similar to natural shoreface profiles from the danish north sea coast and the southeast australian coast model predictions of equilibrium profile response to the effects of climate change such as rising sea level or changing wave climates are examined implementation of the model is straightforward there is no tuning or calibration and computation times are short it is therefore easily implemented with repeated iterations to manage uncertainty

simulation of non gaussian space random functions for modeling transport in groundwater the recent developments and successful applications of covariance related stochastic methods to groundwater fields experiments suggest that further efforts in developing spatial characterization methods are warranted one area that deserves further attention is that of characterizing multimodal distributions such as fractured rock masses dolomite rocks with dissolution channels and sand shale formations traditionally used covariance functions may not be enough to fully characterize spatial continuity applications where the pattern of spatial continuity depends on the specific magnitude level of the attribute for example hydraulic conductivity alternative models based on multiple indicator covariances or mixture of populations offer greater flexibility applying such models to the case of perfect stratification a stochastic simulations is proposed which allows generating different realizations all sharing the same vertical conductivity covariance yet differing by other spatial statistics and leading to widely different transport characteristics a mere covariance based approach would have failed to distinguish between these alternatives investigation of tracer transport in bimodal stratified formations suggests a dependence of the mean advection velocity on the dispersion coefficients as a result of a mechanism that enhance longitudinal dispersion directly proportional to lateral pore scale dispersion and entails a diversion of a substantial portion of the plume into the low conductivity layers more this mechanism may not be evident when the class specific spatial structures of the conductivities are ignored less

hydrodynamic and sedimentary responses to two contrasting winter storms on the inner shelf of the northern gulf of mexico abstract results are presented from the deployment of three bottom mounted instrumentation systems in water depths of 6 9 m on the sandy inner shelf of louisiana usa the 61 day deployment included nine cold front passages that were associated with large increases in wind speed two of the most energetic cold front passages were characterized by distinct meteorological hydrodynamic bottom boundary layer and sedimentary responses and may potentially be treated as end member types on a continuum of regional cold front passages arctic surges ac storms have a very weak pre frontal phase followed by a fairly powerful post frontal phase when northeasterly winds dominate migrating cyclones mc storms are dominated by a strong low pressure cell and have fairly strong southerly winds prior to the frontal passage followed by strong northwesterly winds on the basis of measurements taken during this study ac storms are expected to have a lower average significant wave height than mc storms and are dominated by short period southerly waves subsequent to the frontal passage currents are weak and northerly during the pre frontal phase but become very strong and southwesterly following the passage sediment transport rate during as storms was not as high as during mc storms and the mean and overall direction tended to be southwesterly to westerly with low frequency flows producing easterly transport and wind wave flows producing southeasterly transport mc storms had the most energetic waves of any storm type with peaks in significant wave height occurring during both the pre and post frontal phases the wave field during mc storms tended to be more complex than during as storms with an energetic northerly swell band gradually giving way to a southerly sea band as the post frontal phase progressed currents during mc storms were moderate and northerly during the pre frontal phase but became much stronger and southeasterly during the post frontal phase shear velocity was high during both the pre and post frontal phases of the storm although sediment transport was highest following the frontal passage mean and overall sediment transport was directed southeasterly during mc storms with low frequency and wind wave flows producing northerly transport in summary the data sets presented here are unique and offer insight into the morphosedimentary dynamics of mid latitude micro tidal coasts during extratropical storms

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

holocene sediment lithofacies and dispersal systems on a storm dominated back arc shelf margin the east coromandel coast new zealand abstract holocene sediment lithofacies and dispersal systems on the east coromandel shelf new zealand are mainly characterised by an accommodation dominated regime in which autochthonous siliciclastic sediments were reworked through erosional shoreface retreat during the post glacial marine transgression 12 0 6 5 ka stabilisation of sea level at its present position ca 6 5 ka initiated onshore reworking of the autochthonous deposits into fine grained regressive barrier and shoreface sands while coarser sands remained offshore to form an erosional lag inner shelf deposit modern episodes of shoreface erosion rework fine and coarse autochthonous sands offshore and northwards into very large 0 5 2 5 m 250 1500 m submarine dunes the submarine dunes are similar in form to sand ridges on the north american atlantic shelf but with crests striking perpendicular to both the shoreline and generating flows allochthonous siliciclastic lithofacies are also important aspects of east coromandel shelf sedimentation and are transported offshore from infilled estuary systems to form very fine grained upper shoreface and muddy mid shelf sands the regional geology has a strong influence on shelf lithofacies and dispersal systems off the east coromandel coast southern shelf regions are associated with deep back arc basins which have formed major late cenozoic depocentres for sediments sourced from the taupo volcanic zone further north a shallow volcanic platform extends out across the shelf and forms a barrier to any large scale along shelf dispersal of sediments consequently lithofacies in northern shelf regions tend to be highly variable and reflect local catchment lithologies

multiple detachment model for the early rifting stage of the late proterozoic damara orogen in namibia reconstructions of the inland branch of the late proterozoic damara orogen in namibia indicate rifting and show a marked asymmetry in both sedimentary facies and structural styles from north to south early synrift sequences comprising terrigenous siliciclastics minor carbonates and local volcanics vary markedly within two initial half grabens with later subsidence postrift sequences became laterally more continuous when sedimentation coalesced they vary from carbonate dominated in the north to siliciclastic in the south this indicates sedimentation on a broad epicontinental shelf gentle open folds in the north increase in complexity southward culminating in a fold thrust belt these tectonic styles are believed to be a direct result of reactivation of early extensional faults although most did not address this point previous models for the development of damara rift structures assumed pure shear homogeneous stretching of the crust to produce the graben systems these models however fail to account satisfactorily for observed asymmetries early rifting in the inland arm of the damara orogen is proposed to have been controlled by two down to the northwest low angle detachment faults asymmetry in basin configuration was a direct consequence of this crustal thinning mechanism which we believe controlled the depositional environments this model is more appropriate in explaining the asymmetrical relations observed

biological mediation of bottom boundary layer processes and sediment suspension in the lower chesapeake bay abstract in relatively low energy estuarine environments where bottom skin friction shear stresses do not appreciably exceed the critical shear stress required for sediment transport benthic biology can profoundly affect the responsiveness of the bed to benthic flows the bottom boundary layer processes that suspend and transport fine sediments at two sites in the lower chesapeake bay eastern usa were examined in an interdisciplinary study that embraced all seasons instrumented boundary layer tetrapods were used to record time varying bed stress and suspended sediment concentrations box cores surface and profile photographs and direct observations provided data on benthic biology an annular seabed flume was used to obtain in situ estimates of the threshold shear stresses at which sediments were resuspended observed bed stresses related to semi diurnal tidal currents were in the range of 0 10 to 0 14 pa at the lower energy site but often exceeded 0 20 pa at the more energetic site these stresses were often enhanced by wind stress and waves biological effects dominated bed roughness and influenced the susceptibility of bottom sediments to physical entrainment wave or tide generated bed forms rarely were observed at either site epifaunal organisms increased the relief amplitude of the sediment water interface by more than 8 cm temporal and spatial variability in the abundance and species composition of epifauna was high at both sites making the effects of these organisms on hydraulic roughness difficult to predict application of the seabed flume showed that the critical stress at which sediment was suspended varied from 0 10 to 0 14 pa this variability is attributed to seasonal and interannual variability in biological activity the lowest critical stress values coincided with the most intense bioturbation the floor of the bay stem appears to approximate a quasi equilibrium surface with normally recurrent physical bed stresses remaining near or slightly below threshold values the bed remains active because benthic biota mix the sediment column provide bed relief and mediate the responsiveness of the bed to physical processes

field measurements of fairweather bottom boundary layer processes and sediment suspension on the louisiana inner continental shelf field measurements of bottom boundary layer and sediment transport processes were made on the louisiana inner continental shelf in spring 1992 at a depth of 15 5 m and in spring and summer 1993 at a depth of 20 5 m two different wave current boundary layer sediment transport models were applied to the measured near bed flows in addition the log profile method was applied to estimate hydraulic roughness and bed stress consistent with the results of others our measurements show that near bed flows were very weak under non storm conditions bed stresses were typically too low to resuspend bed sediments however the advection of high turbidity layers or plumes past the instrumentation apparently caused a sustained period of high suspended sediment concentration throughout the log layer in spring 1993 in the absence of wave activity or high suspended sediment concentrations boundary layer profiles showed the bed to have been hydraulically very smooth with z o 0 1 cm however wave agitation combined with increased suspended sediment concentration caused hydraulically rough conditions with z o 0 1 cm

a review and experimental assessment of equilibrium grain size and the ideal wave graded profile abstract the idealised equilibrium profile is thought of as one in which the beach profile and sediment grain size distribution across the profile are in equilibrium with the wave generated hydraulic regime field observations of a seaward fining gradient suggest that sediment of a particular size may move across the profile to a position in which it is in equilibrium with the wave and flows acting on that sediment grain two major hypotheses have been proposed to explain the selective shore normal sorting of grain sizes the hypothesis of asymmetrical thresholds under waves and the null point hypothesis although neither of these hypotheses has been fully accepted neither has been unequivocally rejected the null point hypothesis is still a central point of discussion in most studies of beach equilibrium and the idea of an equilibrium sediment size gradient has not been abandoned the predictions of a numerical model developed to test the hypothesis of asymmetrical thresholds under waves are compared to measured sediment size variations on five beaches the model is more successful at predicting the general trend of onshore sediment size increase than at predicting actual sediment sizes the results of this research indicate that a mean sediment size cannot be related simply to wave induced velocities and suggest that further exploration of a threshold or equilibrium hypothesis will not provide a great deal more insight into the processes responsible for sediment sorting

swash zone sediment transport modes abstract the result of field experiments designed to investigate the relative proportions of bedload and suspended load are described the ratio of bedload to suspended sediment load in the swash zone is examined in both swash and backwash on four beaches by measuring the amounts collected in a sediment trap bedload transport is found to dominate the backwash the relative proportions of bedload and suspended load change over the tidal cycle with increasing bedload dominance at low tide the total amount of sediment transported as swash and backwash is noticeably greater at high tide than at low tide more sediment is transported on the flood tide than on the ebb

storm related sedimentation influenced by coastal configuration in the stratigraphic record of a tectonically active shelf upper pleistocene le castella terrace italy abstract analysis of patterns of coastal circulation and sediment dispersal is an essential step for the study of controlling factors influencing the long term dynamics of coastal systems modern settings offer the possibility to monitor relevant parameters over relatively short time spans however geological examples complement this perspective by providing a time averaged record where longer trends and stratigraphically significant processes can be evaluated this study investigates the shallow marine deposits of le castella terrace upper pleistocene southern italy to document how patterns of circulation influenced by coastline configuration can affect the preserved millennial scale depositional record of a progradational shoreline system the regressive portion of the le castella terrace deposits developed during a relative sea level highstand and falling stage consists of a progradational wedge mainly composed of redistributed skeletal particles of a coeval shallow water carbonate factory preservation of the morphology of the paleocoastline and abundant current related sedimentary structures allow reconstruction of the predominant sediment dispersal dynamics responsible for the formation of this sedimentary wedge facies and paleocurrent analysis indicate offshore and alongshore sediment transport modes consistent with coastal circulation driven by storms normally incident to the shoreline and a sharp change in coastline orientation this coastal inflection influenced circulation patterns causing flow separation and eddy formation in the lee of the curved coastline syndepositional tectonic deformation also affected the architecture of the preserved deposits controlling the nucleation and development of a clinostratified body and determining localized lateral stratigraphic variability this study illustrates how transient but recurrent circulation patterns associated with changes in coastal orientation and related to high energy storm events can leave a predominant signature in the stratigraphic record of microtidal shallow marine successions

predicting ripple roughness and sand resuspension under combined flows in a shoreface environment ripple measurements and flow and sediment dynamical data obtained from the shoreface of the middle atlantic bight using instrumented tripods were analyzed to evaluate various predictors of ripple geometry and roughness ripple roughness controls on sand resuspension and suspended sediment concentration profiles under combined waves and currents were also evaluated the limited observation of sand ripples in the field indicates that the grant and madsen 1982 method overestimates ripple roughness while the nielsen 1981 method tends to under predict ripple roughness a modified ripple predictor is thus proposed based on the grant and madsen method and it is shown to give reasonable predictions under the present experiment conditions this modified ripple prediction along with wave current and suspended sediment concentration data recorded by the tripods were then brought into the combined flow bottom boundary layer model of grant and madsen 1986 and the modified rouse equation of glenn and grant 1987 to calculate sand resuspension coefficient 0 and to predict suspended sediment concentration profiles it was found that under low energy fair weather conditions sand ripples are in the equilibrium range and ripple roughness increases with the bed shear stress this causes strong vortex activity close to seabed and thus higher resuspension coefficient reference concentrations are moderate due to this high resuspension coefficient even though bed shear stresses are low under moderate storm conditions ripple break off occurs and ripple roughness will decrease with bed shear stress this reduces the vortex activity and hence sand resuspension coefficient 0 the combination of this moderately high bed shear stress and reduced but still moderate ripple roughness favours sand suspension and produces the highest reference concentration for the encountered experimental conditions as bed shear stress is further increased ripples are nearly washed out and sand resuspension coefficient is further decreased approaching the previously suggested constant value of 1 3 10 4 this corresponds with the lowest reference concentration despite the high bed shear stress suspended sediment concentrations predicted by the modified rouse equation using this time variable resuspension coefficient and properly calculated bottom boundary layer parameters are reasonable compared to the field measured concentration profiles

geologic framework of the northern north carolina usa inner continental shelf and its influence on coastal evolution abstract the inner continental shelf off the northern outer banks of north carolina was mapped using sidescan sonar interferometric swath bathymetry and high resolution chirp and boomer subbottom profiling systems we use this information to describe the shallow stratigraphy reinterpret formation mechanisms of some shoal features evaluate local relative sea levels during the late pleistocene and provide new constraints via recent bedform evolution on regional sediment transport patterns the study area is approximately 290 km long by 11 km wide extending from false cape virginia to cape lookout north carolina in water depths ranging from 6 to 34 m late pleistocene sedimentary units comprise the shallow geologic framework of this region and determine both the morphology of the inner shelf and the distribution of sediment sources and sinks we identify pleistocene sedimentary units beneath diamond shoals that may have provided a geologic template for the location of modern cape hatteras and earlier paleo capes during the late pleistocene these units indicate shallow marine deposition 15 25 m below present sea level the uppermost pleistocene unit may have been deposited as recently as marine isotope stage 3 although some apparent ages for this timing may be suspect paleofluvial valleys incised during the last glacial maximum traverse the inner shelf throughout the study area and dissect the late pleistocene units sediments deposited in the valleys record the holocene transgression and provide insight into the evolutionary history of the barrier estuary system in this region the relationship between these valleys and adjacent shoal complexes suggests that the paleo roanoke river did not form the albemarle shelf valley complex as previously proposed a major fluvial system is absent and thus makes the formation of this feature enigmatic major shoal features in the study area show mobility at decadal to centennial timescales including nearly a kilometer of shoal migration over the past 134 yr sorted bedforms occupy 1000 km 2 of seafloor in raleigh bay and indicate regional sediment transport patterns between capes hatteras and lookout that help explain long term sediment accumulation and morphologic development portions of the inner continental shelf with relatively high sediment abundance are characterized by shoals and shoreface attached ridges and where sediment is less abundant the seafloor is dominated by sorted bedforms these relationships are also observed in other passive margin settings suggesting a continuum of shelf morphology that may have broad application for interpreting inner shelf sedimentation patterns

role of morphologic feedback in surf zone sandbar response several aspects of feedback mechanisms associated with surf zone sandbar response have been characterized using bathymetric surveys sampled approximately monthly over a 16 year period at the army corps of engineers field research facility north carolina the measured bathymetry was alongshore averaged and modeled by the superposition of two gaussian shaped sandbars on an underlying planar slope a third half gaussian shaped bar represented steepening at the shoreline the rms error between the measured bathymetry and the profile model was 0 10 m estimated over 322 different surveys the model explained 99 of the profile variance that remained after first removing the linear cross shore trend from each observed profile bar response which was extracted from the modeled profiles was compared to a local hydrodynamic forcing variable was defined as the ratio of the wave height to water depth evaluated at bar crest locations at low values of i e nonbreaking conditions bars migrated onshore and their amplitude tended to decay at high values of i e breaking conditions bars migrated offshore with relatively little change in amplitude the transition between onshore and offshore migration occurred at a value of that was consistent with the onset of wave breaking bar migration was associated with a stabilizing feedback mechanism which drove bar crests toward an equilibrium position at the wave breakpoint however we observed that the rate of bar response showed no reduction for any nonzero choice of indicating that bars never reached equilibrium systematic bar amplitude decay was observed under nonbreaking conditions bar amplitude decay could drive farther away from breaking conditions allowing further bar amplitude decay this is a destabilizing feedback mechanism potentially leading to bar destruction

short term changes in the morphodynamic states of beaches and surf zones an empirical predictive model a time series of 6 5 years of daily observations of beach states and wave characteristics from southeastern australia was analyzed with the aim of determining the degree to which time varying beach state can be explained and predicted the six commonly occurring beach states dissipative longshore bar trough rhythmic bar and beach transverse bar and rip low tide terrace and reflective discussed earlier were related by means of discrete discriminant analyses to the parameter hb wst where hb is breaker height ws is the fall velocity of the beach sand and t is peak breaker period using a total of 1545 cases two values were used in the analyses the immediate value occurring on the day the particular state was observed and a weighted mean value expressing recently antecedent conditions the immediate value made a negligible contribution to explaining day to day beach state observations however the antecedent conditions showed a strong relationship and provide a successful means of prediction by examining cases where the time derivatives of both state and were near zero it was possible to define the equilibrium conditions associated with each state directions of change erosional or accretionary are predicted in terms of departures of from the equilibrium value appropriate to the beach state prevailing at the time change begins