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

origin of shore normal channels from the shoreface of sable island canada a suite of modern shore normal channels was discovered on the southern shoreface of sable island canada repetitive multibeam and sidescan surveys of these features as well as box coring and sampling showed that the depressions were perennial features of the shoreface they occurred in fine and medium sand and were most abundant in depths less than 20 m they were up to 1 m deep and 50 m wide increased in size seawards and generally followed the seabed slope the channels dominated the inner shoreface becoming wider and deeper seawards to depths in excess of 40 m there is a continuum of scales in width and depth of these channels the smallest of which are equated with gutters preserved in the geological record the internal structure of the shoreface sediments where the gutters are found is characterised by normally graded tempestites composed of cut and fill structures overlying an erosion surface thus the environmental setting internal structure and scale are similar to ancient counterparts gutters have been interpreted to result from downwelling over palaeo shorefaces during ancient storms myrow 1992b the composition of gutter infill as well as the stability evolution associated bedforms and hydrodynamic conditions under which they formed were examined to verify genetic interpretations of the ancient counterparts the multi sensor benthic lander ralph was deployed within a field of shore normal channels for 17 days during late winter 1998 ralph burst sampled flow at hourly intervals whilst imaging the seabed resulting sequential imagery revealed the genesis of a group of five gutters during storm spin down that was coincident with the formation of large scale wave ripples this was followed by infilling of the gutters and wave ripple destruction within 20 hours by longshore sand transport the gutters were located within and parallel to a larger channel 50 m wide and 0 50 m deep they were formed by coastal set up and subsequent downwelling that was triggered by the rotation of storm winds from seawards onshore wind to landwards offshore wind the gutters formed within 2 to 3 hours during spin down of a winter storm under live bed conditions of sand transport wave height and mean tidal flows were well below those of the storm peak and so they were not directly responsible for creating the gutters the mechanism we propose for the evolution of gutters supports the geological interpretations that is a storm driven downwelling event as described by swift and niedoroda 1985 however the downwelling event appeared to take place suddenly rapid acceleration in flow briefly few hours duration and within narrow corridors guided by wider and deeper channels of the shoreface high turbidity during the time of gutter formation leads us to believe that the phenomenon of inner shelf turbidity currents as suggested by walker 1985 exists and may be instrumental in channel formation

seabed stresses in combined wave and steady flow conditions on the nova scotia continental shelf field measurements and predictions abstract a tripod holding electromagnetic flowmeters at two heights within 1 m above the seabed has been deployed at two shallow sites 25 and 45 m depths on the continental shelf off nova scotia canada wave flows are comparable with the small mean flows at both sites friction velocities have been estimated from the observed spectra of vertical turbulent velocities using a modification of the dissipation method appropriate to low reynolds number conditions the results from each site show no significant change of friction velocity with height as expected for measurements from within the constant stress layer however in each case the observed friction velocities are considerably larger than would be predicted on the basis of the observed bottom roughness and the mean flows alone indicating that the wave flows were important in enhancing the friction velocity the theory of grant and madsen 1979 has been used to predict the friction velocities based on the observed mean and wave velocities and o

microstructural evidence for magma confluence and reusage of magma pathways implications for magma hybridization karakoram shear zone in nw india in the karakoram shear zone ladakh nw india miocene leucogranitic dykes form an extensive varied and complex network linking an anatectic terrane exposed in the pangong range with leucogranites of the karakoram batholith mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions and microstructures suggest anatexis was contemporaneous with shearing the network is characterized by continuous and interconnected dykes with only rare cross cutting relationships forming swarms and chaotic injection complexes where magmatic rocks cover up to 50 of the outcrop area despite this volume of magma the system did not lose continuity suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously leucogranites in this network including leucosomes in migmatites carry an isotopic signature intermediate between the two main anatectic rocks in the source suggesting efficient homogenization of the magmatic products here we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features these include i narrow tortuous corridors of fine grained minerals cutting across or lining the boundaries of larger grains interpreted to be remnants of magma filled cracks cutting across a pre existing magmatic rock ii corrosion of early formed grains at the contact with fine grained material iii compositional zoning of early formed plagioclase and k feldspar grains and quartz overgrowths documented by cathodoluminescence imaging iv incipient development of rapakivi and anti rapakivi textures and iv different crystallographic preferred orientation of early formed quartz and fine grained quartz mapping of the fine grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the s c fabric defined by pre existing magmatic grains we conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches new influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an s c fabric related to syn magmatic shearing a mixed isotopic signature resulted not from the mixing of magmas but from the micro scale interaction between new magma batches and previously crystallized magmatic rocks through local equilibration

suspended sediment transport and benthic boundary layer dynamics abstract the mellor yamada level ii turbulence closure model is used to examine the dynamics of a benthic boundary layer bbl carrying sediment in suspension using flow and sediment properties from observations in the florida straits implications for sediment transport in this and similar environments are discussed model results indicate that three size classes provide an adequate representation of the frequency distribution of a moderately well sorted sand size sediment the benthic boundary layer responds to a stabilizing suspended sediment concentration gradient by a reduction of about 45 in the bottom stress damping of turbulence and by a decrease in the slope of the velocity profile the velocity field in the lowermost portion of a neutrally stratified bottom boundary layer is governed by the relationship u u k ln z z 0 to recover the predicted velocity distribution in the sediment stratified bbl from this relationship von karman s constant k must be reduced by about 15 the model however treats k as internally invariant applying similarity theory to the model output yields u u k ln z z 0 where k k 1 ar f a is an empirically determined constant and rf is the flux richardson number in this case k may be interpreted as a modified von karman s constant increases in the angle of turning of the current vector 5 and enhancement of the ekman like nature of the current spiral are predicted these model predictions are consistent with findings from studies of stably stratified atmospheric boundary layers and sediment laden channel flows

measurement and prediction of wave generated suborbital ripples 1 experiments in a large scale wave flume using regular and irregular waves with periods between 4 s and 6 s and heights between 0 2 m and 1 55 m have examined the formation of wave generated ripples using sediment beds composed of four grain sizes d50 0 349 mm 0 329 mm 0 220 mm and 0 162 mm in a water depth of approximately 4 m estimates of wave generated ripple height and wavelength were obtained using zero down crossing analyses of bed profiles measured by acoustic means along a 4 m transect normal to the ripple crests further information pertaining to was obtained from bed images obtained using scanning sonar the analyses reported here focus on the sequence bedforms that evolved in response to stepwise increases and decreases in wave height results show that ripples for the most part are suborbital in nature and do not conform well to empirical equations used frequently to predict and values in the field on the basis of the present data two new equations for prediction of and are obtained and their use in field situations where hydrodynamic and sedimentary conditions favor development of suborbital bedforms is recommended

field observations of bedforms and sediment transport thresholds of fine sand under combined waves and currents abstract seabed video images and s4 wave current meter data collected during the build up of a moderate storm on the scotian shelf are analysed for bedform development and sediment transport threshold of fine sand under combined waves and currents as the storm built up the following sequence of bedforms was observed 1 relict wave dominant ripples with worm tubes and animal tracks during the preceding fairweather period 2 irregular sinuous asymmetrical current dominant and intermediate wave current ripples under bedload transport 3 regular nearly straight or sinuous asymmetrical to slightly asymmetrical wave dominant ripples under saltation suspension 4 upper plane bed under sheet flow conditions 5 small crest reversing transitory ripples at the peak of the storm and 6 large scale lunate megaripples which developed when the storm decayed these data also show that only single sets of asymmetrical intermediate wave current ripples will form when waves and currents are co linear the development of the crest reversing transitory ripples indicates a high energy transition stage under quasi sheet flow conditions a direct comparison of the skin friction combined shear velocity and the critical shear velocities for bedload suspension and sheet flow transport under estimated the onset of these sediment transport modes as the presence of ripples causes the shear stress to increase from ripple trough to ripple crest the ripple enhanced skin friction shear velocity must be used to determine properly the initiation of bedload transport at high transport stages the boundary layer dynamics is controlled mainly by the thickness of the bedload transport layer thus a transport related bedload shear velocity predicted based upon the sum of the grain roughness and bedload roughness has to be compared against the conventional threshold criterion to properly define the onset of suspension and sheet flow transport modes

a suspended sediment stratification correction for combined wave and current flows a simple model for the near bottom combined wave and current flow over a moveable sediment bed is presented the model is an extension of the grant and madsen 1979 combined wave and current model with moveable bed effects on the physical bottom roughness included as by grant and madsen 1982 the unsteady conservation of fluid momentum and sediment mass equations which are coupled through an eddy diffusivity closure scheme are solved for the wave and current velocity profiles along with the suspended sediment concentration and transport profiles sample runs are presented to illustrate the effect of varying wave conditions on the current the mean sediment concentration and the mean sediment transport results show that for fine to medium sands self stratification of the flow can be important during storms owing to the large amount of sediment suspended by the waves and the enhanced turbulent mixing associated with the wave current interaction the theory easily can be generalized for inclusion in a full ekman layer model

modeling bottom roughness in the presence of wave generated ripples 1 near bottom wave current and microtopography measurements have been obtained at the leo 15 site off the southern coast of new jersey the measurements are used in conjunction with a recently calibrated continental shelf bottom boundary layer model to refine estimates of bottom roughness for a rippled bed under combined wave and current flows in the presence of nearly two dimensional wave generated ripples application of the grant and madsen 1979 type bottom boundary layer model produces estimates of the bottom roughness felt by the current that are several times greater than established formulations for rippled beds under waves modifying one of the primary length scales in the eddy viscosity profiles returns estimates that are more consistent with the wave roughness results application of the roughness model in conjunction with a refined empirical ripple geometry model produces highly accurate estimates of measured shear velocities and apparent roughnesses during a moderate storm the demonstrated accuracy of this simple combined roughness and ripple geometry formulation reveals its utility when considering applications of coupled bottom boundary layer shelf circulation models

dynamics of ripples superimposed on a sand ridge on a tideless shoreface abstract the presence of ripples superimposed on an active sand ridge is the most usual situation under low to moderate energy conditions in the wave dominated tideless ebro delta coast four types of small scale bedforms were identified from the critical threshold of sediment motion to wash out conditions i small undulations with the potential role of ripple migration as an additional long term mechanism of sediment transport is addressed on the ebro delta shoreface 3d ripple migration follows the direction of sand ridge migration towards the se the respective migration rates and their differences in size support the hypothesis that a subordinate part of sand ridge migration can result from the contribution of ripple migration under low to moderate regimes suggesting that in specific environments the dynamics of small scale bedforms can play a subordinate but not negligible role in the evolution of larger bedforms

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

geometry and migration characteristics of bedforms under waves and currents part 2 ripples superimposed on sandwaves this paper presents results of a series of detailed measurements of geometric and migrating characteristics of ripples superimposed upon sandwaves under the action of combined waves and currents velocity measurements within the fluid surface wave characteristics and 3d mapping of the bottom were recorded with an acoustic doppler velocimeter adv an acoustic water level sensor and a 32 composite element array of sub aquatic acoustic sensors respectively bottom records were statistically analyzed to obtain height length and migration rates of ripples experiments examined ripple heights and wavelengths for the mobility factors as defined in eq 4 and the reynolds wave number within the ranges 10 88 and 16 103 rew 5 105 respectively measured values were compared with laboratory and field data together with semi empirical and analytical formulae from the literature good correlation was obtained when plotting measured ripple length and length in dimensionless form as a function of the reynolds wave number rew under a given hydraulic condition it was observed that ripples with different geometric characteristics may coexist at different locations over the sandwave ripple steepness is presented as a function of the shields parameter although characterized with rather large scatter finally average ripple migration speed is presented as a function of the shields parameter and the mobility number

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

geometry and grain size sorting of ripples on low energy sandy beaches field observations and model predictions there are very few field measurements of nearshore bedforms and grain size distribution on low energy microtidal beaches that experience low amplitude long period waves field observations are needed to determine grain size distribution over nearshore bedforms which may be important for understanding the mechanisms responsible for ripple development and migration additional nearshore field observations of ripple geometry are needed to test predictive models of ripple geometry ripple height length and sediment composition were measured in the nearshore of several low energy beaches with concurrent measurements of incident waves the distribution of sediment sizes over individual ripples was investigated and the performance of several models of ripple geometry prediction was tested both spatially and temporally sediment samples were collected from the crest and trough of 164 ripples the sand sized sediment was separated from the small amount generally 2 mm that was present within the sand sized fraction the ripple crests were found to be significantly coarser better sorted and more positively skewed than the troughs overall the troughs were finer than the crests but contained a greater proportion of the small fraction of sediment larger than 2 mm the field model of nielsen 1981 and the model of wiberg harris 1994 were found to be the most accurate models for predicting the wavelength of parallel ripples in the nearshore of the low energy microtidal environments surveyed the wiberg harris 1994 model was also the most accurate model for predicting ripple height temporal changes in ripple wavelength appear to be dependent on the morphological history of the bed

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

the distribution of nearshore bedforms and effects on sand suspension on low energy micro tidal beaches in southwestern australia abstract field measurements of bed morphologies hydrodynamics and sediment suspension were made on 15 low energy h sig two general trends in cross shore bedform patterns were identified among the low energy beaches the first trend found on beaches of finer sand median grain diameter of 0 24 mm consisted of a cross shore sequence from irregular ripples to cross ripples to parallel ephemeral ripples to plane bed in the direction of increasing bottom shear stress the second sequence found on beaches of medium sand median grain diameter 0 39 mm consisted mainly of parallel ripples with bifurcations that became increasingly organised into parallel ripples as water depth shallowed the wavelength of the cross ripples was found to increase linearly with increasing near bed orbital diameter while the wavelength of the ephemeral ripples was found to decrease slightly with increasing near bed orbital diameter suspended sediment concentrations were found to be greater over cross ripples and parallel ripples than over plane bed and ephemeral ripples even though bottom shear stresses were greater over ephemeral ripples co spectra of suspended sediment and cross shore currents revealed transport directions to be different over each of the ripple patterns

observations of morphological change at an ebb tidal delta abstract video observations of depth limited wave breaking patterns at an ebb tidal delta on the energetic west coast of new zealand at raglan were used to identify geomorphic features over a 5 year period the terminal lobe mouth bar channel margin linear bars and swash bars were identified and tracked over the duration morphodynamic response was related to environmental conditions by correlating observed movements with concurrent wave and tidal conditions movements occurred throughout the record with a slight tendency to occur more during the transition between seasonal forcing trends winter deltas were generally broader and extended further seaward than the summer deltas which were more cuspate the formation of a double barred ebb shoal was observed with the cross shore position of the outer bar influenced by wave conditions while the inner bar was influenced by ebb jet strength furthermore swash bars were observed to constrict the seaward extent of the main channel during large wave events which was subsequently eroded by tidal currents these observations are consistent with the present consensus that ebb shoal features are dependent upon competition between ebb jet strength and opposing waves interannual morphological changes were not significantly correlated to any particular environmental forcing suggesting that either some process or combination of processes not considered were influential or that the system might be showing signs of emergent behaviour

observed and predicted bed forms and their effect on suspended sand concentrations abstract in this paper we study the effect of bed forms on suspended sand concentrations and we compare three existing bed form predictors to field and laboratory measurements over a wide range of conditions and propose a new predictor that better collapses the measured bed form dimensions we apply the different bed form predictors to estimate the suspended sediment concentration distributions based on the 1d advection diffusion equation and the bed form enhanced shear stress which includes the flow contraction effect according to nielsen coast eng 10 1986 23 predictions of suspended sand concentration improve when including a bed form predictor interestingly it does not make much difference which bed form predictor is implemented but more whether a bed form predictor is implemented however the choice of a particular bed form predictor is more important when the bed form effect in the sand transport model is based a roughness length scale k s and thus on 2 instead of where k s is nikuradse s equivalent roughness length scale is the bed form height and is the bed form length this is the case in the van rijn van rijn l c 2000 general view on sand transport by currents and waves data analysis and engineering modelling for uniform and graded sand transpor 2000 and crosmor 2000 models z2899 20 z2099 30 z 2824 30 wl delft hydraulics delft the netherlands sand transport model for example we discuss the effect of so called long wave ripples on the suspended sand concentrations

storm dominated bottom boundary layer dynamics on the northern california continental shelf measurements and predictions measurements of near bottom velocity profiles in 85 m water depth during a storm on the continental shelf off northern california using the geoprobe tripod in december 1979 provided estimates of shear velocities u and roughness lengths z0 when the near bottom velocity profiles were logarithmic these estimates agree within 90 confidence intervals with values computed from a simple near bottom combined wave current model that includes movable bed effects the reasonably good comparison between model and profile estimates of u suggests that such models can be used to predict bed shear stresses or shear velocities under combined flows of waves and currents typical of stormy conditions on continental shelves if stratification corrections to the velocity profile resulting from suspended sediment are small the repeated occurrences of storms of similar intensities and wind velocities during the winter months off northern california suggest that the resultant high bottom stresses due to the combined effects of waves and currents are major factors in controlling the distribution of surficial sediment on the central portion of the northern california shelf

wave mechanisms responsible for grain sorting and non uniform ripple distribution across two moderate energy sandy continental shelves abstract mechanisms responsible for observed ripple geometry and grain size trends at two open coast sites are considered the field sites are two moderate energy sandy shelves on the inner continental shelf out to 50 m depth at east gippsland in southeastern australia and at pakiri in northeast new zealand both are characterised by bands of medium sediments inshore and offshore separated at 20 45 m depth by a zone with significantly coarser grain size 0 9 mm cf 0 3 mm video observations of the seabed at 126 sites on the east gippsland shelf revealed that the band was also characterised by larger wavelength ripples 1 0 m cf 0 3 m the size of the band stretching over an average of 20 km from 20 45 m depth and its presence at both field sites suggests that the observed characteristics are a long term response to physical forcing possibly in near equilibrium with modern processes numerical modelling demonstrated that grain sorting on the shelf is initiated by the presence of maxima in predicted ripple activity and seabed roughness around 35 m depth the local grain size evidently increases due to winnowing of the fines positive feedback through dependence of ripple size on grain size causes further lengthening of ripple wavelength until the pattern has evolved to become highly pronounced on the shelf and self sustaining but limited by reducing sediment mobility as grain size increases

boundary layer dynamics and sediment transport under storm and non storm conditions on the scotian shelf abstract near bed measurements of waves currents seabed responses and ripple migration rates were obtained using an instrumented tripod at a water depth of 39 m on the scotian shelf during the winter of 1992 93 these data and the grant and madsen s 1986 grant w d madsen o s 1986 the continental shelf bottom boundary layer annu rev fluid mech 18 265 305 combined flow boundary layer model are used to examine wave current interaction various sediment dynamic thresholds and sediment transport on an exposed high energy continental shelf the seabed was found to be rippled for most time of the deployment and thus the ripple enhanced combined skin friction shear velocity had to be used to determine the initiation of bedload transport under combined flows this indirectly indicates adequate predictions of the skin friction shear velocities inside the wave boundary layer by the grant and madsen model at high transport stages bedload roughness must be added to the grain size roughness to obtain a transportrelated combined shear velocity in order to predict correctly the onsets of saltation suspension and sheet flow transports otherwise the initiation of suspended load transport and the total sediment transport rates will be severely under estimated the comparison between the prediction by the grant and madsen 1986 model and the calculation of a quadratic stress law suggests that the total current shear velocity was enhanced by a factor of 2 3 due to the wave current interaction during storms while the apparent bottom roughness was increased by more than an order of magnitude the non linear coupling between waves and currents also causes a 20 increase of the combined skinfriction shear velocity inside the wave boundary layer this non linear coupling is most important when waves and currents are roughly equal in magnitude and the angles between them are less than 30 four sediment transport formulae were tested while the engelund hansen total load and yalin bedload methods did not perform well the einstein brown and bagnold formulae were found to respectively give reasonable predictions of the bedload and total load sediment transport rates under the observed combined flow conditions the predicted sediment transport direction is also in good agreement with the observed ripple migration direction the gsc sediment transport model sedtrans92 predicts that the net daily transport rates during the storms reached 822 kg m 1 day 1 and were 2 3 orders of magnitude higher than the non storm transport suggesting the dominance of storm processes in sediment transport on the scotian shelf these values are in good agreement with the results from sand tracer experiments conducted in this region we thus conclude that sedtrans92 can properly simulate boundary layer dynamics and sediment transport on continental shelves

bottom stress estimates and their prediction on the northern california continental shelf during code 1 the importance of wave current interaction abstract high quality near bottom boundary layer measurements obtained at a midshelf location 90 m water depth in the code region off northern california are described bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions june 1981 during code 1 are analyzed to obtain bath velocity profiles and mean bottom stress and bottom roughness estimates during the time period described the mean near bottom 0 997 approximately 30 percent of the time effects induced by unsteadiness from internal waves result in some degradation of the profiles 0 96 r 0 997 the rest of the time mean stress profiles indicate the logarithmic layer is approximately a constant stress layer the near bottom flow field is composed of mean currents and oscillatory currents due to well typing mean u values estimated from measurements greater than 30 cm above the bottom have magnitudes of 0 5 1 0 cm s 1 mean stress valu

effects of settling and preferential deposition of sediment on ripple roundness under shoaling waves abstract a comprehensive series of wave flume experiments were performed to examine the effects of settling of entrained sediment on ripple roundness using three sizes of well sorted quartz sand d50 0 32 0 49 0 73 mm the observed ripple profiles were closely examined with reference to ripple roundness and ripple asymmetry in order to quantify the effect of sediment settling settling time factor s w0t is ripple height w0 is settling velocity and t is wave period is newly introduced it is shown that ripples developed under shoaling waves manifest the following geometrical features pertaining to ripples 1 the measured performance of ripple roundness correlates systematically with s irrespective of grain size 2 the preferential deposition of entrained sediment on the ripple crest and the onshore side slope of each ripple is accentuated under conditions of smaller s such deposition takes place under shoaling waves when the oscillatory flow direction is reversed from onshore to offshore thereby allowing the entrained sediment to stall to some extent prior to being subjected to the subsequent peak offshore going flow thus round crested ripples ensue 3 the way in which preferential deposition occurs under shoaling waves is similar to the deposition process that occurs under combined flows on rippled beds yokokawa et al 1995 4 this reasoning led to a comparison of the ripple roundness and ripple symmetry in both flow regimes ripples formed under shoaling waves are in cases significantly round crested with marked asymmetry in contrast equally round crested ripples under combined flows yokokawa 1995 are commonly more symmetrical in profile thus combined flow ripples can be identified from the geological record using their degree of roundness and symmetry of their profile

unifying large and small wave generated ripples 1 frequently in field and laboratory studies of wave generated bed forms on beds of sandy sediments both large and small wave generated ripples lwrs and swrs are observed and in some situations may be superimposed upon one another the present paper examines relationships between measured wave conditions and the geometry of lwrs and swrs and shows that existing empirical equations used to predict the dimensions of swrs perform badly for lwrs to address this problem relationships found between ripple geometry and the wave reynolds number are used as the basis for new expressions for prediction of ripple height wavelength and steepness that improve the accuracy of existing approaches for both lwrs and swrs the present results indicate strongly that swrs and lwrs are related and their superposition in certain conditions reflects the time lag between the response of the bed to changing hydrodynamic conditions they must be considered therefore when estimating the total bed roughness in a range of practical engineering and numerical modeling applications

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

suspended sediment transport in an estuarine tidal channel within san francisco bay california abstract a recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within san francisco bay california measurements were made every 15 min over six successive flood and ebb tidal cycles they included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed near bottom pressure fluctuations vertical temperature gradient and bottom photographs additionally suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle the instrument system was retrieved during each slack water period to exchange suspended sediment sample bags while the instrument was deployed std nephelometer measurements were made thoughout the water column and water samples were collected each 1 h and bottom sediment was sampled at the deployment site size distributions of the suspended sediment samples estimates of particle settling velocity s friction velocity u and reference concentration c a at z 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles three suspended sediment particle conditions were evaluated 1 individual particle sizes in the 4 11 62 5 0 5 m size range with the reference concentration c a at z 20 cm c 2 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration c a at z 20 cm c f and 3 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress an analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables s u and c a in addition computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions

movable bed roughness in unsteady oscillatory flow a model to predict the roughness in unsteady oscillatory flows over movable noncohesive beds is presented the roughness over movable beds is shown to be a function of the boundary shear stress rather than a fixed geometrical scale as is the case for fully rough turbulent boundary shear flows over immobile beds the model partitions the roughness into two distinct contributions these two contributions are due to the form drag around individual bed forms and to the near bed sediment transport the form drag over the bed forms is treated explicitly as a function of the boundary geometry and shear stress the ripples are predicted as a function of the local skin friction and a semiempirical expression is derived using standard law of the wall arguments which gives the ripple or form roughness as a function of the boundary geometry the ripple roughness is found to be proportional to the product of the ripple steepness and height favorable comparison of the form drag model with the results of bagnold s 1946 fixed ripple study is found the value of z0 associated with intense sediment transport in oscillatory flow over a flat bed is determined from carstens et al s 1969 experiments this value is found to be 7 or 8 grain diameters an expression is derived for the roughness associated with the maximum thickness of a near bottom sediment transporting layer consistent with owen s 1964 roughness hypothesis for saltation of uniform grains in air at large values of the boundary shear stress relative to the critical value for initial sediment motion the derived expression is similar to the results of smith and mclean s 1977 unidirectional flow approach modified for oscillatory flow the total roughness model is found to compare favorably with carstens et al s 1969 data in contrast to smith and mclean s 1977 steady flow findings the results here show that when ripples are present they account for a significant portion of the boundary roughness

acoustic measurements of suspended sand on the shoreface and the control of concentration by bed roughness acoustic backscatter measurements of the concentrations of sand in suspension on the shoreface seawards of the breaking zone during a mild storm event show that sand concentrations increase initially but then rapidly decrease as the wave energy increases it is suggested that the bed roughness is a major control on suspended sand concentration and that the decrease in concentration is due to decreasing ripple steepness after the break off bed shear stress is exceeded no direct measurements of bedforms were available and the combined wave current interaction model of grant and madsen j geophys res vols 84 1979 and 87 1982 with extensions to movable bed roughness ripple dimensions from carstens et al s 1969 laboratory data u s army corps eng cerc tech mem 28 and to sediment induced self stratification glenn and grant j geophys res vol 92 1983 was used to predict the current and suspended sand concentration profile matching the measured and modeled concentrations at 2 cm above the bed the values of the resuspension coefficient 0 were found to decrease as excess skin friction increased in a manner similar to that suggested by drake and cacchione cont shelf res vol 9 1989 but were an order of magnitude larger using the much smaller ripple dimensions of nielsen j geophys res vol 86 1981 changed the model results very little significant differences were found between the time averaged suspended sand profiles and those predicted by the model sand transport fluxes have also been computed using the instantaneous products of measured suspended concentrations and currents the current being the sum of steady and wave induced components the wave induced fluxes show considerable variability from run to run but the general pattern is of a transport profile having a shoreward maximum at 5 10 cm above the bed and offshore transport in the few centimetres closest to the bed little net transport occurred above 15 cm these fluxes show a weak dependence on the wave energy becoming more shoreware and the height of the maximum shoreward transport decreasing as the wave energy increased again consistent with the influence of the ripple steepness

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

coupling bedform roughness and sediment grain size sorting in modelling of tidal inlet incision abstract a key problem in the decadal morphodynamic modelling of tidal inlet system is on the evolution of the inlet channels especially the unrealistic channel incision the present study attempts to couple two processes including the bedform roughness and sediment grain size sorting in the two dimensional depth averaged process based model to solve the problem based on the comparison of the modelling results and relevant field observations in the dutch wadden sea it is suggested that bedform roughness height predictor is a useful tool for determining spatial and temporal heterogeneous bed frictions within tidal channels bedform roughness is dominated by dune roughness in order to represent the memory of old bedforms a relaxation time method was implemented the corresponding large relaxation time 2 m2 tidal periods smooths the tidal variations of dune roughness and makes it respond to hydrodynamic changes on a longer time scale when decadal morphological modelling is carried out the model performance can be significantly improved by introducing either bedform dune roughness predictor or sediment sorting processes the mechanisms are twofold first in the deep channels where flow velocities are high large dunes develop and cause large bed drag coefficients second coarser bed sediments within the channel area induced by sorting prevent the channel incision if both effects are coupled in the morphodynamic modelling the inlet channel incision is further controlled which is not just a result of the linear superposition of both effects because sorting can further promote the development of dunes within the deep channels

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

field measurements of the suspended sand concentration profiles and fluxes and of the resuspension coefficient γ0 over a rippled bed measurements of the acoustic backscatter from suspended sand over a rippled bed and of currents were made near the crest of a ridge and runnel beach on the macrotidal north norfolk coast of the united kingdom two 12 min records were taken close to high water in a depth of 1 7 0 2 m visual inspection of the data shows that sand was intermittently suspended to heights of 20 30 cm but that these events were not always correlated with strong wave induced currents the structure of some of the suspension events suggest that they are due to vortex shedding from a rippled bed the observed time averaged concentration profiles were compared with predictions generated using glenn and grant s 1987 combined flow model the nondimensionalised concentrations showed good correspondence in the first 8 to 10 cm above the wave boundary layer supporting the use of a value of 0 74 drawn from atmospheric data for the constant of proportionality between sediment and momentum diffusivity above this height observed values of the proportionality constant tended to approach 1 0 which may have been due to a decrease in sediment settling velocity with height which is consistent with finer sand being suspended higher in the flow or alternatively to nonuniform turbulence in the vicinity of the wave ripples absolute concentrations measured 2 cm above the bed were consistent with values of o 10 4 for the empirical constant 0 in smith and mclean s 1977 pickup function concentrations within the wave boundary layer were observed to decrease much more rapidly than predicted by the model sand transport fluxes were computed for both the cross shore and alongshore components of the flow using a current profile based on the combined flow model of glenn and grant 1987 for the two records vertically integrated fluxes of 1 5 mg s cm width and 0 89 mg s cm width directed towards the shore were calculated for the wave oscillatory component of the transport the mean flow produced shoreward vertically integrated fluxes of 5 1 and 10 5 mg s cm width the uncertainty associated with these estimates is considerable 50 the vertical profile of the total flux wave oscillatory and mean for both records showed a strong shoreward transport close to the bed an offshore transport between 5 and 15 cm above the bed and a further weak onshore flux above this demonstrating the necessity for measurements of the whole concentration profile rather than a few point samples for the calculation of sand transport over a rippled bed in a wave dominated environment

measurements of storm generated bottom stresses on the continental shelf large values of bottom friction velocity u and roughness length z0 determined from burst averaged speed data taken on the continental shelf in outer norton sound alaska with the geoprobe tripod during a storm in september 1977 are correlated with extremely large values of near bottom concentration of total suspended particulate matter tsm combined wind driven and tidal currents exceeding 30 cm s at 1 m above the bottom and intense oscillatory bottom currents with maxima above 45 cm s were associated with the largest measured values of tsm at 2 m above the sea floor the values of u and z0 obtained from the law of the wall velocity depth relationship are diminished substantially throughout the storm period average reduction of 44 when the turbulence reducing effects of the vertical concentration gradient of tsm are considered the form of the latter correction was adapted from smith and mclean 1977a values of the mean u computed from the theory of grant and madsen 1979 which predicts an enhanced shear stress due to nonlinear wave current interactions compare favorably with the u values determined from the measured velocity profiles the measured values of z0 however are considerably larger than any of the estimates based on 1 the actual scales of the physical roughness elements 2 the apparent roughness of grant and madsen 1979 or 3 the thickness of the bed load layer as formulated by smith and mclean 1977a

on the relationship between bedload and suspended sand transport on the inner shelf long island new york data obtained by a near bottom profiling concentration and velocity system pcv deployed in 10 m water depth at a site 1 km from the long island coastline are used to examine links between bedload and suspended sand concentrations calculations of bedload areal concentration c are based on the empirical results of vincent et al 1981 and use the theoretical formulations of grant and madsen 1978 1979 to describe the interaction between wave and current boundary layers suspended sand concentrations were obtained directly from an acoustic concentration meter acm average suspended sand profiles cz were found to fit closely to a log linear profile cz c1 1 a loge z z1 where c1 is the sand concentration at a height z1 1 cm from the bed and a is empirically determined as 0 22 0 005 a linear correlation is observed between the areal bedload concentration c the volume of bedload per unit area of the bed and the suspended sand concentration 1 cm above the bed c1 with a correlation coefficient of 0 82 significant at the 1 level and supports the hypothesis of einstein 1950 that bedload and suspended load are related through bedload concentration it is also shown that the sedment threshold criterion of komar and miller 1973 when expressed as a ratio here called the komar ratio can be used as a useful predictor for c1 under conditions where the wave orbital currents are much greater than the mean flow these relationships offer the opportunity for the calculation of both bedload and suspended sand transport rates from measurements of the steady current velocity and wave parameters combined with information defining the surficial sediments and local bottom topography