identification of permian palaeowind direction from wave dominated lacustrine sediments lodeve basin france lacustrine environments are an excellent indicator of continental palaeoclimate in particular the sedimentary record of waves in lakes may be used to constrain atmospheric palaeocirculation wave ripples have been identified in a permian lacustrine basin the salagou formation 260 250 ma lodeve basin located in the southern french massif central part of the western european hercynian mountain chain wave ripple patterns are interpreted with regards to hydrodynamics and water palaeodepth it is shown that in the case of the salagou formation wave ripple orientations were controlled by the direction of the prevailing palaeowind the late permian wind blew from between north and 20 east of north possibly over several millions of years and certainly throughout the period of deposition of about 2000 m of strata in the lodeve basin permian lacustrine sedimentation is widespread and well preserved on the earth s surface and so wave ripple data may help constrain numerical modelling of the earth s past climates especially with regards to permian times outside of desert regions

silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

southern ocean control of silicon stable isotope distribution in the deep atlantic ocean the fractionation of silicon si stable isotopes by biological activity in the surface ocean makes the stable isotope composition of silicon delta si 30 dissolved in seawater a sensitive tracer of the oceanic biogeochemical si cycle we present a high precision dataset that characterizes the delta si 30 distribution in the deep atlantic ocean from denmark strait to drake passage documenting strong meridional and smaller but resolvable vertical delta si 30 gradients we show that these gradients are related to the two sources of deep and bottom waters in the atlantic ocean waters of north atlantic and nordic origin carry a high delta 30 sisignature of 1 7 parts per thousand into the deep atlantic while antarctic bottom water transports si with a low delta si 30 value of around 1 2 parts per thousand the deep atlantic delta si 30 distribution is thus governed by the quasi conservative mixing of si from these two isotopically distinct sources this disparity in si isotope composition between the north atlantic and southern ocean is in marked contrast to the homogeneity of the stable nitrogen isotope composition of deep ocean nitrate delta n 15 no3 we infer that the meridional delta si 30 gradient derives from the transport of the high delta si 30 signature of southern ocean intermediate mode waters into the north atlantic by the upper return path of the meridional overturning circulation moc the basin scale deep atlantic delta si 30 gradient thus owes its existence to the interaction of the physical circulation with biological nutrient uptake at high southern latitudes which fractionates si isotopes between the abyssal and intermediate mode waters formed in the southern ocean

some guidelines in reconstructing ancient sea conditions from wave ripplemarks abstract the collection of substantial field data on wave ripple mark spacing wavelength steepness grain size and symmetry index from the miocene burdigalian sandstones of western switzerland has allowed apparently close reconstruction of sea conditions results indicate however that ancient wave reconstructions must be restricted at present to data from relatively steep ripple marks vfi less than 7 5 i e vortex ripples data from the flatter ripple marks post vortex types formed at high oscillatory shear stresses do not provide reasonable estimates of wave conditions penecontemporaneous overwashing and or post depositional compaction ripple formation near the destruction condition rather than the threshold of sediment movement orbital diameters far exceeding ripple spacing and superimposed unidirectional currents are all factors which seriously affect wave reconstructions evaluation of ancient wave conditions provides insight into geologically important phenomena such as local water depth and basin size

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

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

parameterization and simulation of near bed orbital velocities under irregular waves in shallow water a set of empirical formulations is derived that describe important wave properties in shallow water as functions of commonly used parameters such as wave height wave period local water depth and local bed slope these wave properties include time varying near bed orbital velocities and statistical properties such as the distribution of wave height and wave period empirical expressions of characteristic wave parameters are derived on the basis of extensive analysis of field data using recently developed evolutionary algorithms the field data covered a wide range of wave conditions though there were few conditions with wave periods greater than 15 s comparison with field measurements showed good agreement both on a time scale of a single wave period as well as time averaged velocity moments

ripple marks in quartz arenites of the hurwitz group northwest territories canada evidence for sedimentation in a vast early proterozoic shallow fresh water lake abstract supermature quartz arenites of the whiterock member deposited in the early proterozoic intracratonic hurwitz basin have a maximum thickness of 400 m and a minimum areal extent of 100 000 km2 the unit is unusual in that it is virtually devoid of other rock types and displays sedimentary structures that consist almost entirely of short spaced ripple marks and parallel stratification both formed by oscillatory flow the ripples have spacings that are consistently small average

tectonometamorphic evolution of the chuncheon amphibolite central gyeonggi massif south korea the chuncheon amphibolite part of the gubongsan group which overlies the yongduri gneiss complex is interlayered with calc silicate rock marble quartzite biotite schist and quartzofeldspathic gneiss in the central gyeonggi massif south korea metamorphic pressures and temperatures estimated from the amphibolite are 5 5 10 6 kbar and 615 714 c these p t conditions are close to those defined by the reaction curve between kyanite and sillimanite and suggest medium pressure type metamorphism of the chuncheon amphibolite for two metapelites intercalated with the amphibolite temperatures are estimated to be 607 699 c consistent with those obtained from the amphibolite on the other hand pressures estimated from these metapelites are significantly different 4 6 kbar and 9 13 kbar when rim and core compositions of garnet are respectively used these p t estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3 7 kbar during denudation rapid decompression is likely on the basis of the results of mineral chemistry phase equilibria and geothermobarometer moreover in conjunction with the occurrence of kyanite in the adjacent gyeonggi gneiss complex p t estimates of the chuncheon amphibolite and metapelite suggest a clockwise p t t path this evolutionary path may be related to the amalgamation of continents during the late proterozoic event which corresponds to the jinningian orogeny in the qinling belt of china

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

effect of velocity hiatuses in oscillatory flow on migration and geometry of ripples wave flume experiments a series of wave flume experiments was conducted to closely look at characteristics of geometry and migration of wave generated ripples with particular reference to the effect of velocity hiatuses during which the near bed flow velocity becomes much smaller than the threshold of sediment movement three types of wave patterns were generated two types for simulating waves with intervening velocity hiatuses and regular waves for comparison purposes in the former two types two different wavelengths of water waves were generated alternately in the course of a wave test the wave with a longer wavelength was set large enough to mobilize the bottom sediment whereas the wave with a shorter wavelength was set too small to mobilize the sediment the former two types were designed to be different in sequence of convexity and concavity of wave patterns the sequence with the convex concave longer wave and successive convex concave shorter wave was described as a zero up crossing wave pattern and the inverse sequence was described as a zero down crossing wave pattern the ripples developed under oscillatory flow with intervening hiatuses manifested the following characteristics in geometry and migration i the morphological characteristics of ripples namely wavelength height and the ripple steepness are unaffected by the intervening hiatuses of velocity ii the directions of ripple migration under the zero up crossing and zero down crossing wave patterns corresponded well with the directions of the flow immediately before onset of the hiatuses iii the observation of sand particle movement on the ripple surface indicated that under the zero up crossing waves the velocity hiatus prevents the entrained sediment cloud from being thrown onshore and thus the sediment grains thrown onshore are fewer than those thrown offshore as a result of the sediment movement over one wave cycle the net sediment transport is directed offshore under the zero up crossing wave pattern iv the velocity of ripple migration was highly correlated with acceleration skewness under most of the zero up crossing zero down crossing wave patterns flow acceleration skewed negative positive and ripples migrated offshore onshore

wave formed structures and paleoenvironmental reconstruction clifton h e and dingier j r 1984 wave formed structures and paleoenvironmental reconstruction in b greenwood and r a davis jr editors hydrodynamics and sedimentation in wave dominated coastal environments mar geol 60 165 198 wave formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits for a particular grain size threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow velocity limits the ratio of ripple spacing to grain size provides an estimate of the length of the near bottom orbital motion the degree of velocity asymmetry is related to the asymmetry of the bedforms though it presently cannot be estimated with confidence a plot of water depth versus wave height h h diagram provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction the assumptions and the degree of approximation involved in the different techniques impose significant constraints inferences based on wave formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences

evidence for a major change in silicon cycling in the subarctic north pacific at 2 73 ma the initiation of northern hemisphere glaciation in the subarctic north pacific at 2 73 ma was marked by an abrupt cessation of high opaline accumulation considered to result from an increased stratification of the water column that should have led to higher utilization of nutrients in the surface ocean we present a new stable si isotope based record of si utilization that is hard to reconcile with this model a drop in 30si 28si by 0 4 at 2 73 ma is coincident with an increase in bulk n isotope composition the contrasting utilization records cannot have been both caused by a hydrographic change alone excluding a change in the si n export ratio these results either imply a relative increase in silicic acid supplied to the surface waters or a change in its si isotope composition while it is impossible to distinguish between these two possibilities both imply a regional or global change in the si biogeochemical cycle potentially caused by an enhanced storage of si in the underlying deep waters of the pacific

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

spatially correlated depth changes in the nearshore zone during autumn storms acoustic reflection measurements of seabed elevation were made at stanhope beach prince edward island during three successive storms in october november 1989 the instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2 2 m bed elevation measurements were made at 30 min intervals with a range resolution of 1 mm at horizontal spacings between 22 cm and 1 5 m using an array of four acoustic sounders operating at 1 2 25 and 5 mhz it is shown that such an array can be used as 1 an interferometer to determine bedform properties and migration rates 2 a leveling device to determine changes in local bottom slope and 3 an erosion deposition gauge in the interferometric mode time domain autocorrelation and cross correlation analyses of the bottom elevations are used to estimate the wavelengths migration rates and migration directions of sand ripples and megaripples the ripple wavelengths 10 15 cm and heights 3 5 cm are comparable to measurements made by divers ripple migration rates were 1 6 cm h 1 directed offshore in the direction of the mean shore normal near bottom current and increased with increasing mean current speed evidence of megaripple like features 2 5 cm height 1 5 3 m length is found also migrating in the direction of the mean shore normal near bottom drift at speeds of about 20 cm h 1 bedload transport rates in the onshoreoffshore direction are estimated from the bedform migration rates the ripple transport rate was small an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift in contrast the large scale bedform transport appears to have been comparable to the mean current transport of suspended sediment interestingly it appears that the total bedform transport was comparable to that predicted by watanabe s 1981 net bedload transport formula provided a coefficient appropriate to irregular wave conditions is used during the third and highest energy storm event the measurements indicate transition to flat bed and generalized local erosion at a maximum rate of 8 cm h 1 this erosion event was associated with a general shoreward migration of the bar

stable silicon isotope signatures of marine pore waters biogenic opal dissolution versus authigenic clay mineral formation dissolved silicon isotope compositions have been analysed for the first time in pore waters 30sipw of three short sediment cores from the peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments the 30sipw varies between 1 1 and 1 9 with the highest values occurring in the uppermost part close to the sediment water interface these values are of the same order or higher than the 30si of the biogenic opal extracted from the same sediments 0 3 to 1 2 and of the overlying bottom waters 1 1 to 1 5 together with dissolved silicic acid concentrations well below biogenic opal saturation our collective observations are consistent with the formation of authigenic alumino silicates from the dissolving biogenic opal using a numerical transport reaction model we find that approximately 24 of the dissolving biogenic opal is re precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 mol si cm 2 yr 1 the fractionation factor between the precipitates and the pore waters is estimated at 2 0 dissolved and solid cation concentrations further indicate that off peru where biogenic opal concentrations in the sediments are high the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino silicate phases

geometry migration and evolution of wave orbital ripples at leo 15 observations of the temporal evolution of the geometric properties and migration of wave formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements six weeks of bedform observations were taken at the sandy medium to coarse sized sand leo 15 site located on beach haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar during this period six tropical storms several of hurricane strength passed to the east of the study site ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed the predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3 4 of the wave orbital diameter although orbital diameters become larger than 130 cm during the maximum wave event it is unclear if a transition to non orbital scaling is occurring ripple migration is found to be directed primarily onshore at rates of up to 80 cm day suspended transport due to wave motions calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near bottom suspended transport bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave to current velocities

time sequence observations of wave formed sand ripples on an ocean shoreface analysis of an 18 day time lapse film record of shoreface ripple development with concurrent measurements of near bottom flow and surface waves provides new insight on equilibrium bedform conditions adjustment of ripple planform to variable hydrodynamics and ripple migration behaviour the study was conducted in approximately 10 m water depth 1 km off martinique beach on the atlantic coast of nova scotia canada under low energy summer wave conditions significant wave height and peak period during the study averaged 0 7 m and 8 s respectively with extremes up to 1 7 m and 11 s during passage of three weak weather disturbances six mutually exclusive ripple types have been defined 1 short wavelength regular ripples 2 variable bifurcated ripples 3 variable terminated ripples 4 short crested ripples 5 long wavelength regular ripples and 6 chaotic ripples ripple wavelength ranged from 0 07 m to 0 24 m and displayed a strong reynolds number dependence together with other published field data the results suggest a lower limit of 0 06 m for the wavelength of wave ripples in ocean shoreface environments ripple orientation ranged through 38 and responded rapidly to changes in wave approach direction but did not conform to the orientation of the adjacent shoreline ripples were observed to migrate both on and off shore with and against the wave advance direction at rates up to 0 1 m h 1 associated with net flows other than wave induced onshore asymmetry and mass transport migration mainly of ripple types 1 and 2 occurred during the peak of storm events but showed no obvious correlation with measured near bottom flow magnitude or direction ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight crested rippie types 1 and 5 are present disequilibrium in orientation or dimensions is expressed by increasing sinuosity bifurcation and crest termination in types 2 3 4 and 6

response of sand ripples to change in oscillatory flow ripples take time to evolve to a new equilibrium state in response to a change in wave generated oscillatory flow the paper presents results from flow tunnel experiments designed to examine oscillatory flow transient ripple processes under controlled full scale laboratory conditions the experiments include study of the growth of ripples from flat bed and the evolution of existing ripples to new equilibrium ripples in response to a step change in the flow in general ripples evolve through a combination of two main processes i from a flat bed or from a bed consisting of ripples that are smaller than the equilibrium ripples through a combination of slide and merge ii from a bed consisting of ripples that are larger than the equilibrium ripples through a combination of split and merge the experimental results show that equilibrium ripple geometry is independent of initial bed morphology while the time to reach equilibrium is largely independent of the initial bed and the equilibrium ripple size the time to reach equilibrium depends strongly on the mobility number and a new empirical equation relating mobility number and the number of flow cycles to equilibrium is proposed this equation is combined with a simple exponential function for ripple height growth or decay to produce a new empirical model for ripple height evolution which gives a reasonably good overall agreement with the measurements the model is based on experiments involving one sediment size only and further work is needed to develop the model for other sand sizes

geometry and dynamics of wave ripples in the nearshore zone of a coarse sandy beach 1 extensive measurements of ripple characteristics and dynamics along with associated suspended sediment fluxes and hydrodynamic conditions were made in the shoaling and surf zones of a macrotidal coarse grained beach at sennen cove cornwall england median grain diameter 0 69 mm suborbital vortex ripples were observed during the majority of the study period with height 5 cm and length 35 cm the scale and shape of the ripples did not vary significantly as the bed shear stress increased during wave shoaling and breaking however ripple migration rates onshore directed were strongly dependent on their location relative to the breakpoint increasing from 0 1 cm min 1 under shoaling waves to 2 cm min 1 in the outer surf zone during low energy conditions farther inside the surf zone ripples persisted but migration rates slowed probably owing to the presence of the offshore directed mean flow which impedes landward migration of the ripples under low wave conditions during which measured sediment fluxes peaked around the outer surf zone and decreased through the saturated surf zone bed form transport rates under shoaling waves were of the same magnitude as net suspended sediment fluxes but at least an order of magnitude smaller in the outer surf zone under high energy conditions during which suspended sediment fluxes in the surf zone were offshore directed owing to the presence of the seaward directed mean flow bed form transport rates were several orders of magnitude smaller than suspended fluxes

analytical method for estimating palaeowave climate and water depth from wave ripple marks analytical expressions are developed for evaluating palaeodepth and palaeowave conditions from wave ripple marks palaeodepth and palaeowave climate estimates can be extremely powerful predictors in reconstructions of vertical movements related to basin subsidence local and regional ancient wave climate trends and the basin shape the analytical expressions developed herein are applied to wave ripple marks preserved in the lower marine molasse of switzerland western austria and southern germany the results suggest that in contrast to traditional concepts of an eustatic sea level change the lower marine molasse regression was a result of mainly depositional processes the subsidence rate greatly exceeded the rate of sea level fall over a wide geographical area furthermore the widening of the lower marine molasse basin to the ene is well documented by the increase of the ancient wave period and wave power calculated from wave ripple marks found in storm deposits

wave formed sand ripples at duck north carolina a recently developed acoustic multiple transducer array was utilized to measure small scale bed forms in the nearshore and inner shelf regions at duck north carolina two populations of wave formed ripples were observed short wave ripples swr with heights ranging from 3 mm to 2 cm and lengths ranging from 4 to 25 cm and long wave ripples lwr with heights ranging from 3 mm to 6 cm and lengths ranging from 35 to 200 cm the swr were only present sometimes and their presence or absence was determined by a critical value of the near bed mobility number the swr were highly dynamic sometimes flattening during wave groups and reforming over several incident wave periods the lwr in contrast were almost always present they were longer and lower relief than predicted by models or generally observed previously both swr and lwr were often observed to migrate shoreward but were rarely observed to migrate seaward the dimensions of the swr when they were present were predictable by the nielsen 1981 model or the wiberg and harris 1994 model to within approximately a factor of 2

video based observations of nearshore sand ripples and ripple migration 1 observations of o 1 m length and o 0 1 m height sand ripples at waimea bay a steep slope 0 05 pocket beach on the north shore of oahu hawaii are presented time series of ripple patterns over a 900 m2 section of the nearshore in depths of 1 2 m are obtained using time averaged video images from an argus station overlooking the bay ripples are detected during weak wave conditions in the summer or between winter swell events the ripple field exhibits narrow band wave number structure with ripple crests oriented parallel to the shoreline the ripple wavelengths vary with wave orbital displacements but they are shorter than predicted by orbital vortex ripple scaling relationships a new suspension limited model smith and wiberg 2006 traykovski 2006 in which the suborbital ripple wavelengths vary with wave period appears to describe well the observed ripple wavelength scaling lagged correlations between sequential argus images reveal area average ripple migration rates ranging from 3 3 m day 1 offshore to 4 5 m day 1 the corresponding estimated sediment transport ranges from 0 3 m3 m 1 day 1 to 0 5 m3 m 1 day 1 similar to previous estimates of cross shore transport based on observed volume changes at the subaerial beach this suggests that the recovery of beach sand following major swell driven erosion events may be accomplished in part by the shoreward migration of seabed ripples