pattern formation in a thin layer of sediment abstract a stability analysis is performed to investigate the process which leads to the formation of bottom forms when a thin layer of sediment covers a rigid substratum subject to the oscillatory flow induced by a surface gravity wave the amplitude of the bottom perturbation is assumed to be so small to linearize the hydrodynamic problem and the flow field is determined by analytical means however nonlinear effects are significant and play a fundamental role into the morphodynamic problem when the rigid substratum is bared by the growth of the most unstable mode hence the development of the bottom profile is evaluated by a numerical approach the model results show that the wavelength of the morphological patterns which are generated when the rigid substratum is bared is longer than the wavelength of the bottom forms which appear when the rigid substratum is always covered by the sediment the results of the theoretical model are supported by some experimental observations made in our laboratory

the morphodynamics of tidal sand waves a model overview this paper reviews recent theoretical studies of sand waves which are rhythmic large scale bedforms observed in the continental shelf far from the near shore region emphasis is given to the investigations carried out in the framework of the eu research project humor first the results of linear morphodynamic stability analyses are described which allow to understand the initial behavior of the sand waves hence indications on the physical processes controlling the appearance and development of sand waves are obtained along with quantitative predictions of the wavelength of sand waves and of their migration speed then nonlinear models are described which are used to predict the equilibrium profile of sand waves and their interaction with human interventions like sand extraction or the construction of pipelines finally we discuss an analytical model which describes how the sand wave instability behaves when it is triggered locally this leads to the generation growth and expansion of a so called sand wave packet

sand ripples under sea waves part 3 brick pattern ripple formation an oscillatory flow over a cohesionless bottom can produce regular three dimensional bedforms known as brick pattern ripples characterized by crests perpendicular to the direction of fluid oscillations joined by equally spaced bridges shifted by half a wavelength between adjacent sequences a photo of brick pattern ripples is shown in sleath 1984 p 141 in the present paper brick pattern ripple formation is explained on the basis of a weakly nonlinear stability analysis of a flat cohesionless bottom subject to an oscillatory flow in which three dimensional perturbations are considered

a theoretical model of asymmetric wave ripples the time development of ripples under sea waves is investigated by means of the weakly nonlinear stability analysis of a flat sandy bottom subjected to the viscous oscillatory flow that is present

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

fabric attractors in general triclinic flow systems and their application to high strain shear zones a dynamical system approach high strain zones may deform by flow with a triclinic symmetry this paper describes triclinic flow in a reference frame where instantaneous stretching axes isa are fixed the operation of triclinic flow is described in two ways first in terms of flow and the nature of flow eigenvectors and in the second part of the paper in terms of finite strain in monoclinic flow at least one of the eigenvectors of the flow coincides with one of the isa and one or two of the eigenvectors act as attractors of foliation or lineation elements in triclinic flow some flow eigenvectors are undefined since the two largest eigenvalues controlling the flow are imaginary imaginary eigenvalues are particularly common at high kinematic vorticity and within flow with deviation of the vorticity vector of more than 20 from one of the isa strong deviation from monoclinic flow is therefore possible but this will not produce permanent foliations or lineations for triclinic flow that does produce permanent fabrics the angle between isa and the fabric is so small that it is unlikely that it can be recognised in nature a discussion of the potential application of such results within real shear zones is presented

geometry prediction for wave generated bedforms published data on wave generated bedform geometry from field and laboratory measurements are re evaluated in terms of relative fluid orbit diameter at the bed and a wave period parameter d 8t2 laboratory bedform data shows a strong dependence on these parameters however for very small values of the period parameter typically field data there is evidence that bedform length depends only on sediment grain size the disappearance of sediment bedforms at high fluid velocities apparently occurs through a decrease in bedform height improved methods for the prediction of bedform geometry under waves are provided in graphical form and computer code

direct numerical simulations of ripples in an oscillatory flow sea ripples are small scale bedforms which originate from the interaction of an oscillatory flow with an erodible sand bed the phenomenon of sea ripple formation is investigated by means of direct numerical simulation in which the sediment bed is represented by a large number of fully resolved spherical grains i e the flow around each individual particle is accounted for two sets of parameter values differing in the amplitude and frequency of fluid oscillations among other quantities are adopted which are motivated by laboratory experiments on the formation of laminar rolling grain ripples the knowledge of the origin of ripples is presently enriched by insights and by providing fluid and sediment related quantities that are difficult to obtain in the laboratory e g particle forces statistics of particle motion bed shear stress in particular detailed analysis of flow and sediment bed evolution has confirmed that ripple wavelength is determined by the action of steady recirculating cells which tend to accumulate sediment grains into ripple crests the ripple amplitude is observed to grow exponentially consistent with established linear stability analysis theories particles at the bed surface exhibit two kinds of motion depending on their position with respect to the recirculating cells particles at ripple crests are significantly faster and show larger excursions than those lying in ripple troughs in analogy with the segregation phenomenon of polydisperse sediments the non uniform distribution of the velocity field promotes the formation of ripples the wider the gap between the excursion of fast and slow particles the larger the resulting growth rate of the ripples finally it is revealed that in the absence of turbulence the sediment flow rate is driven by both the bed shear stress and the wave induced pressure gradient the dominance of each depending on the phase of the oscillation period in phases of maximum bed shear stress the sediment flow rate correlates more with the shields number while the pressure gradient tends to drive sediment bed motion during phases of minimum bed shear stress

detailed observations of the rippled surface of antarctic blue ice areas this paper presents detailed observations of the regularly rippled surface on an antarctic blue ice area near svea at five sites the wavelength of the ripples was found to be 20 24 cm while the wave height crest trough was 1 2 cm the ripple crests are generally oriented perpendicular to the direction of the strongest winds repeated measurements show that wave heights increase throughout the summer with most ablation occurring in the wave troughs this implies that traditional methods of measuring ablation such as stakes when a rod on the ice surface is used to define a mean surface height tend to underestimate total ablation because they sample only crests one site exhibited significant migration of the surface ripples of about 2 cm month 1 in the downwind direction whereas three other sites showed no significant wave movement the formation and the specific characteristics of the surface ripples are most likely caused by self amplifying interaction mechanisms between the free ice surface and the overlying turbulent atmosphere which necessarily involve spatial variations in sublimation a simple model was used to quantify the interaction between the surface ripples the airflow aloft and the sublimation rate the model is able to predict wavelengths and migration rates that are in reasonable agreement with the observations

modelling the formation of shoreface connected sand ridges on storm dominated inner shelves a morphodynamic model is developed and analysed to gain fundamental understanding of the basic physical mechanisms responsible for the characteristics of shorefaceconnected sand ridges observed in some coastal seas these alongshore rhythmic bed forms have a horizontal lengthscale of order 5 km and are related to the mean current along the coast the seaward ends of their crests are shifted upstream with respect to where they are attached to the shoreface the model is based on the two dimensional shallow water equations and assumes that the sediment transport only takes place during storms the flux consists of a suspended load part and a bed load part and accounts for the eects of spatially non uniform wave stirring as well as for the preferred downslope movement of sediment the basic state of this model represents a steady longshore current driven by wind and a pressure gradient the dynamics of small perturbations to this state are controlled by a physical mechanism which is related to the transverse bottom slope this causes a seaward deflection of the current over the ridges and the loss of sediment carrying capacity of the flow into deeper water the orientation spacing and shape of the modelled ridges agree well with eld observations suspended load transport and spatially non uniform wave stirring are necessary in order to obtain correct e folding timescales and migration speeds the ridge growth is only due to suspended load transport whereas the migration is controlled by bed load transport

on the modeling of sand wave migration 1 we describe a simple mathematical model capable of reproducing the main features of sand wave inception and growth in particular we focus on the prediction of the migration rates that sand waves undergo because of tidal and residual currents the model adequately predicts migration rates even for the cases of upstream propagating sand waves i e for sand waves which migrate in the direction opposite to that of the residual current we show that upstream downstream propagation is mainly controlled by the relative strength of the residual current with respect to the amplitude of the quarter diurnal tide constituent and by the phase shift between the semi diurnal and quarter diurnal tide constituents therefore to accurately predict field cases a detailed knowledge of the direction strength and phase of the different tide constituents is required

modeling sand wave characteristics on the belgian continental shelf and in the calais dover strait 1 the capability of the model of besio et al 2006 to predict the main geometrical characteristics crest orientation wavelength of tidal sand waves is tested by comparing the theoretical predictions with field data in particular the field observations carried out by mouchet 1990 and van lancker et al 2005 along the continental shelf of belgium are used additional comparisons are carried out against the field measurements described by le bot 2001 and le bot and trenteseaux 2004 which were carried out in an adjacent region attention is focused on the prediction of the wavelength of the bottom forms indeed the capability of a linear stability analysis to predict the occurrence of sand waves has been already tested by hulscher and van den brink 2001 and more recently by van der veen et al 2006 the obtained results show that the theoretical predictions fairly agree with field observations even though some of the comparisons suggest that the accuracy of the predictions depends on the accurate evaluation of the local current and sediment characteristics

note on sediment sorting in a sandy bed under standing water waves we present new quantitative data on the sorting of sediments on a sandy seabed under standing waves starting from a flat bed composed of a homogeneous mixture of a coarse and a fine sand with mean diameters 0 11 and 0 21 mm we observed simultaneous ripple and sand bar formation and sand sorting on the seabed over days of wave action sand bars formed with crests beneath the surface wave nodes and flat plateaus flanked by mounds beneath the antinodes bar crests were composed of sand coarser on average than 0 21 mm while the flat plateaus were covered by sand finer on average than 0 11 mm comparison with two experiments involving sand beds of more homogeneous size distributions shows that the mounds are characteristic of the motion of fine suspensions

analysis of the scale of errors in nearshore bathymetric data most studies of nearshore hydrodynamics sediment transport and morphology focus on bathymetric variability within a narrow band of spatial and temporal scales typically these studies rely on bathymetry estimates derived from field observations consisting of discrete samples in space and time with varying degrees of measurement error sampling limitations which result in aliasing and measurement errors can significantly contaminate variability at resolved scales and may lead to large errors in the representation of the scales of interest using a spectral analysis interpolation errors were analyzed for three different nearshore bathymetric data sets each of which targeted a different range of spatial scales bathymetric features that were unresolved or poorly resolved e g beach cusps introduced the potential for contamination in two of the data sets this contamination was significantly reduced using an appropriate scale controlled interpolation method leading to more accurate representations of the actual bathymetry an additional benefit of using scale controlled interpolation is that interpolation errors may be estimated independently of actual observations which allows one to design bathymetric sampling strategies that ensure that dominant scales are either resolved or largely removed finally interpolation errors corresponding to a particular sample design can be used to determine which interpolated values contribute usefully to a band limited analysis of bathymetric variability

generation and nonlinear evolution of shore oblique transverse sand bars the coupling between topography waves and currents in the surf zone may self organize to produce the formation of shore transverse or shore oblique sand bars on an otherwise alongshore uniform beach in the absence of shore parallel bars this has been shown by previous studies of linear stability analysis but is now extended to the finite amplitude regime to this end a nonlinear model coupling wave transformation and breaking a shallow water equations solver sediment transport and bed updating is developed the sediment flux consists of a stirring factor multiplied by the depth averaged current plus a downslope correction it is found that the cross shore profile of the ratio of stirring factor to water depth together with the wave incidence angle primarily determine the shape and the type of bars either transverse or oblique to the shore in the latter case they can open an acute angle against the current up current oriented or with the current down current oriented at the initial stages of development both the intensity of the instability which is responsible for the formation of the bars and the damping due to downslope transport grow at a similar rate with bar amplitude the former being somewhat stronger as bars keep on growing their finite amplitude shape either enhances downslope transport or weakens the instability mechanism so that an equilibrium between both opposing tendencies occurs leading to a final saturated amplitude the overall shape of the saturated bars in plan view is similar to that of the small amplitude ones however the final spacings may be up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller or larger in the case of alongshore migrating bars the asymmetry of the longshore sections the lee being steeper than the stoss is well reproduced complex dynamics with merging and splitting of individual bars sometimes occur finally in the case of shore normal incidence the rip currents in the troughs between the bars are jet like while the onshore return flow is wider and weaker as is observed in nature

tidal induced large scale regular bed form patterns in a three dimensional shallow water model the three dimensional model presented in this paper is used to study how tidal currents form wave like bottom patterns inclusion of vertical flow structure turns out to be necessary to describe the formation or absence of all known large scale regular bottom features the tide and topography are treated as a coupled system and the potential free instabilities in this system are investigated a linear stability analysis is used to study the dynamics of bed forms this yields information on the role of physical mechanisms such information cannot be obtained by direct numerical nonlinear simulation the basic state represents a horizontal elliptical flow veering in vertical direction over a flat bottom it is shown that in such a system modes can grow which have characteristics of both tidal sand banks and sand waves the model predictions are compared with observations and show good agreement

modeling of vortex ripple morphodynamics 1 a new morphology module has been developed in order to perform simulations of the bed form evolution under hydrodynamic forcing this module has been implemented into the numerical model dune2d which solves the reynolds averaged navier stokes rans equations in the boundary layer and resolves sediment transport over moving sand beds the morphology module is composed of a modified central scheme and a routine that simulates avalanches and has been successfully applied to the study of orbital ripples evolution ripple growth from a quasiflat bed have been simulated and the growth behavior has been compared to previous experimental measurements the simulated ripples reach an equilibrium state where energy damping processes avalanches compensate energy production processes flux of sediment these processes have been analyzed during ripple creation growth merging and annihilation furthermore sandy bed evolutions have been simulated with random initial bed forms in order to examine the influence of the bed history on the final solution and the wave energy dissipated during these evolutions

sand ripples under sea waves part 2 finite amplitude development in the present paper the authors formulate a theory to predict the time development of sand ripples characterised by small but finite amplitude under the action of surface gravity waves the theory is based on a weakly nonlinear stability analysis of a flat sandy bottom subject to viscous oscillatory flow the parameters of the problem namely the reynolds number of the flow and the reynolds and froude numbers of sediments are assumed to fall within a neighbourhood of the critical conditions determined in blondeaux 1990 the analysis can predict the actual ripple height wavelength and profile when flow separation is absent i e for the case of rolling grain ripples assuming sleath s 1984 criterion for separation the values of the relevant parameters at which transition from rolling grain ripples to vortex ripples occurs are predicted a comparison between theoretical findings and experimental data supports the validity of the present theory

self organization mechanisms for the formation of nearshore crescentic and transverse sand bars the formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow water equations and on a simplified sediment transport parameterization by assuming normally approaching waves and a saturated surf zone rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed two types of bedforms appear one is a crescentic bar pattern centred around the breakpoint and the other herein modelled for the first time is a transverse bar pattern the feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations basically the waves stir up the sediment and keep it in suspension with a certain cross shore distribution of depth averaged concentration then a current flowing with against the gradient of sediment concentration produces erosion deposition it is shown that inside the surf zone these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro morphodynamic input conditions to which of the patterns develops it is suggested that crescentic bar growth would be favoured by high energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment in intermediate conditions mixed patterns may occur

sandbar formation under surface waves theory and experiments 1 we report a combined theoretical and experimental study of sandbar formation under simple harmonic surface waves for coarse grains and weak waves an established empirical rule of bed load transport is used with an asymptotic theory for the fluid flow the surface waves are governed by potential theory and a depth linear eddy viscosity is employed in the turbulent boundary layer at the seabed the derived bed stress is used to predict the sand bed evolution laboratory experiments and corresponding numerical simulations for both high and low beach reflection are discussed for weak reflection the shear stress associated with the return current is found to be important partial simulation of a field record in cape cod bay is also described

nonlinear dynamics of sand banks and sand waves sand banks and sand waves are two types of sand structures that are commonly observed on an off shore sea bed we describe the formation of these features using the equations of the fluid motion coupled with the mass conservation law for the sediment transport the bottom features are a result of an instability due to tide bottom interactions there are at least two mechanisms responsible for the growth of sand banks and sand waves one is linear instability and the other is nonlinear coupling between long sand banks and short sand waves one novel feature of this work is the suggestion that the latter is more important for the generation of sand banks we derive nonlinear amplitude equations governing the coupled dynamics of sand waves and sand banks based on these equations we estimate characteristic features for sand banks and find that the estimates are consistent with measurements

sand ripples under sea waves part 1 ripple formation in the present paper we formulate a predictive theory of the formation of sand ripples under sea waves the theory is based on a linear stability analysis of a flat sandy bottom subject to a viscous oscillatory flow the conditions for decay or amplification of a bottom perturbation are determined along with the wavelength of the most unstable component as a function of the reynolds number of the flow and of the froude and reynolds numbers of the sediments a comparison between theoretical findings and experimental data supports the validity of the present theory an analytical solution for viscous oscillatory flow over a small amplitude wavy bottom is determined for arbitrary values of the ratio r between the amplitude of fluid displacement and the wavelength of bottom waviness previous works by lyne 1971 and sleath 1976 who considered small or large values of r are thus extended

highest natural bed forms 1 fluid flow interacts with sedimentary beds forming waves of different kinds which are of considerable practical importance since they influence significantly the near bed flow both over and below the bed sediment transport and wave height attenuation we focus here on steep bed forms capable of producing flow separation in this case the large scale vorticity generated in the phenomenon of separation rules the process of friction which appears to be practically unaffected by sediment motion under the crests of the bed forms the mean shear force due to friction is balanced by the force that bed forms exert on the flow via pressure which can be calculated from the work of gimenez curto and corniero 2002 bed forms grow until they have a height such that friction at their troughs is negligible thus ceasing the motion of fluid and sediment this condition leads to a very simple expression for the limiting steepness which compares favorably with existing observations on bed form geometry under steady open channel flow as well as under oscillatory flow

dunes and alternate bars in tidal channels a simple idealized model is proposed to predict the appearance of alternate bottom forms in tidal channels the model is based on the linear stability analysis of the flat seabed driven by tidal currents the hydrodynamics is described by means of the full three dimensional continuity and reynolds equations to quantify turbulent stresses the boussinesq assumption is introduced and an algebraic model is used for the eddy viscosity the morphodynamics is described by the exner equation and a simple sediment transport predictor when applied to tidal channels the model predicts the appearance of alternate bottom forms if the channel width is larger than a critical value this finding agrees with previous analyses however the results obtained show the existence of two different modes close to the conditions of incipient sediment motion or when the suspended load is intense the model suggests the appearance of an alternate sequence of shoals and pools the first mode characterized by a wavelength which might be comparable with the horizontal excursion of the tide however under such circumstances to provide accurate quantitative results the model should be extended to include the effects of the local acceleration and of the possible variations of the depth and width of the channel which are neglected in the analysis in all the other conditions the model predicts the appearance of a second mode presently termed tidal alternate bars this mode is geometrically similar to the first mode i e it is characterized by depositional and erosional areas which are found in an alternate arrangement but it has significantly shorter wavelengths in this case the wavelength of the bedforms scales with the water depth the physical mechanism generating tidal alternate bars appears to be the same as that generating tidal dunes and it cannot be described by means of a depth averaged approach

regularity and randomness in the formation of beach cusps abstract controversy still exists on whether rhythmic features such as beach cusps are the result of the presence of standing edge wave motions in the hydrodynamics or if they are the result of self organising processes the compatibility between the two mechanisms is here investigated through the use of a numerical model simulating the formation and development of beach cusps simulations characterised by different forcing conditions have been performed a series of random simulations each individual run simply differing from the others in the seed used in the random number generator showed the typical self organisation behaviour with the features appearing at different locations and even with slightly different spacing regular series of simulations have been run by changing the wavelength of the template superimposed by the hydrodynamic forcing and the number of cycles when the template was present results surprisingly indicate that even a minimum number of cycles with a regular forcing can deterministically induce the final shoreline configuration furthermore if the forcing template has the same wavelength as the one resulting from purely random simulations growth rates are much faster than those obtained with random conditions implications for the kind of field measurements necessary to discern which of the two mechanisms is responsible for beach cusp formation have also been considered

a note on tidally generated sand waves the process leading to the formation of sand waves in tide dominated coastal areas is investigated by means of the linear stability analysis of a flat sandy bottom subject to oscillatory tidal currents the conditions for the decay or amplification of small bottom perturbations are determined for arbitrary values of the parameters of the problem according to field observations the initial growth of sand waves requires a minimum amplitude of the tidal current even when the critical bed shear stress for the initial motion of sediment is set equal to zero moreover the minimum amplitude depends on sediment characteristics in particular the analysis shows that sand waves appear only for a sandy bottom and their growth does not take place when a coarse sediment covers the sea bed the solution procedure extends the truncation method which is often used to describe the flow generated by the interaction of bottom perturbations with the oscillatory tidal current the obtained results show that the truncation method describes the mechanism inducing the growth of sand waves but values of the parameters exist for which its results are not quantitatively accurate finally the asymptotic approach for large values of both r w hich isthe ratio between the amplitude of the horizontal tidal excursion and the wavelength of the bottom perturbations and of the stress parameter s is modified in the bottom boundary layer to describe cases characterized by values of s of order one which is the order of magnitude suggested by an analysis of field data

sediment transport in oscillatory boundary layers in cases of rippled beds and sheet flow an experimental study was focused on the process of sediment transport in unsteady flow conditions due to wave action wave induced oscillatory flow conditions near the seabed were simulated at full scale 1 1 in a new large oscillating water tunnel two sets of experiments series a and b were carried out during series a bed forms and wave cycle averaged suspended sediment concentrations were measured under sinusoidal waves series b focused on measurement of the wave cycle averaged sediment transport rates under regular and random asymmetric oscillatory flows upper shoreface conditions and was aimed at the verification of quasi steady formulas for the description of cross shore sediment transport bedform dimensions appeared to decrease considerably under the influence of wave asymmetry and wave randomness only the measured ripple dimensions under asymmetric waves series b showed good agreement with nielsen s 1979 relations for most of the series b experiments the bed was plane sheet flow the net sediment transport was directed onshore and the measured transport rates showed a strong correlation with the velocity moment an empirical quasi steady transport model is proposed which is based on the new tunnel data and other existing data sets the limitations of the quasi steady model approach became clear in the rippled bed regime and through the presence of a consistent influence of the wave period in plane bed conditions in rippled bed conditions the suspended concentration profiles followed a negative exponential distribution for most of the experiments the measured concentration decay length showed a linear relation with the ripple height in plane bed sheet flow conditions the measured suspended concentration profiles followed a negative power function the power or concentration decay parameter was constant 2 1 for a wide range of velocity conditions it is suggested that the mobile bed sheet flow layer has a strong damping effect on the mixing of suspended sediments turbulence damping

sedimentary structures sorby and the last decade henry clifton sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures together with their use as palaeocurrent indicators research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant particularly as regards the physical explanation of structures and to present significant challenges and opportunities perhaps the most pressing needs are for a better understanding of 1 bedforms in gravels silts and carbonate sediments 2 tidal and especially sand wave bedding 3 hummocky and swaley cross stratification and 4 soft sediment and dewatering structures in turbidites many sedimentary structures present a little exploited opportunity to quantify process rates and define short time periods from the rock record

observations of wave generated vortex ripples on the north carolina continental shelf 1 sand ripples with wavelengths between 0 5 and 3 m were observed on the bottom across the u s east coast continental shelf off north carolina during three side scan sonar surveys in september and december 1999 ripples were present in about 75 of the survey images in particular in regions with coarser sediments analysis of surficial sediment samples shows that median grain diameters range from 0 1 to 4 7 mm with large variations on the inner shelf over distances 1 km the observed ripple properties are consistent with wave generated vortex ripples analysis of concurrent wave observations indicates that the ripple crests were aligned perpendicular to the average direction of near bottom wave induced motions during preceding events that were sufficiently energetic to mobilize surficial sediments furthermore the ripple wavelengths proportionality to near bottom wave orbital excursions is consistent with wave formed vortex ripples these findings support the hypothesis that the observed strong attenuation of waves across the shelf resulted from form drag over large vortex ripples

investigation of triggering of poleward moving auroral forms using satellite imager coordinated observations poleward moving auroral forms pmafs are thought to be an ionospheric signature of dayside magnetic reconnection while pmafs are more likely to occur when the interplanetary magnetic field imf is southward how often pmafs are triggered by changes in solar wind parameters is still an open question to address this issue we used one of the automatic geophysical observatories all sky imagers in antarctica and the time history of events and macroscale interactions during substorms themis b and c satellites which can give solar wind measurements much closer to the subsolar bow shock than by wind or ace to examine if pmafs occurred in association with imf orientation changes we identified 60 pmafs in conjunction with themis b and c during 2008 2009 and 2011 and 70 of events show reduction of bz before pmaf onset indicating that imf southward turning plays an important role in triggering a majority of pmafs in contrast the magnitude of the imf bz reduction in omni data was smaller and the reduction occurred in a slightly smaller percentage of events 40 60 this suggests that solar wind structures that missed the l1 point or evolution of solar wind between the l1 point and themis may be important for identifying imf changes responsible for transient dayside reconnection additionally 17 pmafs that did not have substantial imf southward turnings are correlated well with foreshock events indicating that foreshock phenomena may also play a role in triggering pmafs

laboratory study of sand bed forms induced by solitary waves in shallow water 1 the surface profile of water waves propagating in shoaling water approaches the solitary waveform before wave breaking the effect of the high non linearity of solitary waves may be very significant on bed forms induced in the nearshore zone in this study experiments on bed form generation beneath solitary waves are carried out in a 10 m long flume used in resonant mode solitary waves are generated in shallow water on the background of a standing harmonic wave one solitary wave soliton propagates in each direction of the flume on the time period of the flow above an initially flat sandy bed ripples form rapidly on the bed and a strong interaction with the free surface occurs the amplitude of the soliton and the phase shift between the soliton and the harmonic wave decrease with time while the ripple amplitude increases the amplitude of the harmonic wave is not affected by the ripples the final ripple wavelength is about 1000 times the sand median diameter bars with superimposed ripples appear with bar crests being positioned beneath the nodes of the standing wave when bars form with crests beneath the antinodes of surface wave for standing waves without solitons the eulerian drift distribution in the flume is not affected by the propagation of solitons we propose an energy balance for solitons propagating in shallow water above flat beds in which a term for the dissipation due to sand ripples is introduced which defines a coefficient of interaction between solitons and ripples