large scale bedforms sediment distribution and sand mobility in the eastern north sea off the danish west coast abstract two side scan sonar surveys were carried out 9 months apart in a 24 km2 off thorsminde at the danish west coast were water depth is 12 18 m sediment samples were systematically collected in the same area using a 250 m sampling grid the north sea study area is exposed to strong wind induced coastal currents and high waves and thereby characterized by significant sediment transport the seabed data were used to analyse the large scale dynamical bedforms found in the area the mobility of the bedforms the areal variation of the seabed mean grain size and the grain size variation across the large bedforms the compilation of side scan sonar mosaics revealed very large bedforms with a mean wavelength of approximately 500 m and heights of 1 3 m these bedforms indicate a north going net sediment transport caused by the jutland coastal current large dunes l 10 100 m and smaller bedforms were also recorded as well as large shoreface connected ridges with a spacing of 3 4 km the bedform configuration appears to be relatively stable only small changes having been observed between the two surveys even though a 100 yr cyclone and several storms occurred between the two surveys the small changes observed also indicate a net northerly sediment transport the surface sediments reveal a remarkably high positive correlation between the mean grain size and the position across the very large flow transverse bedforms from coarse sand in the troughs to fine sand at the crest and lee side slope of the bedforms

flow and sediment transport induced by tide propagation 2 the wavy bottom case 1 a model is formulated to describe the flow field locally induced by the propagation of a tidal wave and to evaluate the sediment transport the three dimensional reynolds averaged momentum equations are considered and coriolis terms are retained since they affect the tide propagation turbulent reynolds stresses are described by means of the boussinesq assumption and the eddy viscosity is assumed to be an assigned function of the distance from the sea bed the effects of wind waves on sediment transport are taken into account and both the bed load and the suspended load are computed a comparison of the obtained results with laboratory data and field measurements shows that the model provides a reliable and accurate description of both the velocity field and sediment transport and does not require high computational costs

sea ripple formation the heterogeneous sediment case abstract ripple formation beneath sea waves is analyzed both by experimental and analytical means when the bottom is made up of a mixture of sands an oscillatory flow is obtained in a closed duct by the oscillations of two rigidly connected pistons located at the ends of the duct the amplitude and period of the oscillations can be continuously varied a fixed tray located at the bottom of the duct and filled with different types of sediments allows ripple formation to be observed the presence of graded sediments is found to have a stabilizing effect and causes longer ripples to appear moreover a selective sediment transport is observed and quantified which tends to pile up the coarse grains at ripple crests leaving the fine ones in the troughs as in the companion paper the theory is based on a linear stability analysis of a flat sandy bottom subject to an oscillatory flow because of the presence of a mixture a modified version of exner equation is used and an hiding factor should be inserted in the sediment transport rate formula the flow regime in the bottom boundary layer is assumed to be turbulent the conditions for ripple appearance are determined along with their wavelengths as they form good agreement is found between experimental data and theoretical findings

tide turbulence and suspended sediment modelling in the eastern english channel abstract the present paper is concerned with modelling fluid and suspended sediment dynamics in a tide dominated environment the procedure consists of a one dimensional vertical model driven by an oscillatory horizontal pressure gradient derived from a two dimensional vertically integrated tidal model the vertical model includes two linearised momentum equations for the horizontal velocity components and a series of advection diffusion equations for concentrations of suspended sediment of specific size turbulence generated at the seabed is computed with the aid of a two equation closure describing the time space evolution of the turbulent kinetic energy k and of the dissipation rate of the turbulent kinetic energy e standard k e model a mixed type bottom boundary condition for the sediment concentration equations is adopted to take into account downward fluxes at times of decelerating flow and slack waters the model is applied to the eastern part of the english channel the tidal currents turbulent kinetic energy and the total suspended sediment load predicted by the model are compared with field data collected in two sites the vertical structure of these flow properties is fairly well predicted by the present model better results are found at the measuring point located farther from the coastline where advective terms can be reasonably neglected

the role of suspended load transport in the occurrence of tidal sand waves tidal sand waves are dynamic bed patterns which are formed by the complex interaction between hydrodynamics sediment transport and geomorphology field data from the southern north sea reveal that sand waves are absent where suspended load transport is the dominant transport mode in order to understand the mechanisms responsible for the absence of sand waves we study the influence of suspended load transport on the formation of tidal sand waves with a numerical process based geomorphological model delft3d model simulations are presented in which the vertical eddy viscosity and sediment diffusivity are both spatially and temporally variable k e turbulence model first it is shown that the preferred wavelength of sand waves for a relatively large grain size increases by the inclusion of suspended sediment while for a relatively small grain size the flat bed is stable and no sand waves evolve second it is shown that suspended load transport causes the suppression of long sand waves resulting in a finite range of wavelengths that experience growth finally by varying flow velocity amplitude and grain size critical conditions for sand wave formation are found i e conditions for which sand waves are marginally generated

tidal sand wave formation influence of graded suspended sediment transport 1 the influence of graded suspended sediment transport on the formation of tidal sand waves is investigated hereto in the framework of a linear stability analysis we examine the time development of small amplitude perturbations superimposed to a flat heterogeneous seabed in a shallow tidal sea dominated by one tidal constituent the heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom as well as in suspension is considered the active layer concept is employed and hiding exposure effects are taken into account the mechanisms giving rise to the sediment sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension on the other hand the formation of sand waves is quantitatively substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture in particular the graded suspended sediment transport largely affects the contribution to the amplification rate related to the sorting process this leads to the growth of bed forms characterized by a smaller wavelength compared with that predicted for a uniform sediment when the mean grain size of the sediment mixture is relatively coarse on the other hand longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size moreover a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case a comparison of the theoretical predictions with field observations seems to support the model even though it cannot be considered conclusive

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

a mechanism for the formation and maintenance of shore oblique sand ridges on storm dominated shelves a remarkable set of shore oblique sand ridges occupies the inner portion of the storm dominated middle atlantic and south atlantic bights off the east coast of the united states of america a mechanism for present day formation and growth of these distinctive geological features follows from a linearized analysis of the stability of alongshore periodic topographical perturbations based on a depth averaged fluid dynamical model with a simple representation of sediment transport the analysis indicates exponential growth of shore oblique features as a result of offshore deflection of storm driven alongshore flows at ridge crests which leads to a convergence of sediment flux because the effective carrying capacity decreases with increasing distance offshore the analysis indicates a set of conditions required for ridge growth model solutions for topography under conditions favorable for ridge growth are strikingly similar to the observed sand ridges

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

horizontal and vertical sediment sorting in tidal sand waves modeling the finite amplitude stage the bed of coastal seas displays a large number of rhythmic bed features of which sand waves are relevant to study from an engineering perspective sediments tend to be well sorted over these bed forms which is so far poorly understood in terms of modeling of finite amplitude sand waves using delft3d we employ bed stratigraphy and consider four different grain size classes which are normally distributed on the phi scale the standard deviation sortedness is then varied whereas the geometric mean grain size is kept constant the results show that typically the crests of sand waves are coarser than the troughs residual flow causes net sedimentation on the lee side of the crest and consequently the general sorting pattern is distorted since larger grains experience a larger settling velocity they are deposited on the upper lee slope whereas the smaller grains are found on the lower lee slope due to sand wave migration also the internal structure of the sand wave is revealed which follows the same sedimentation pattern as the lee slope surface these results qualitatively agree with sorting patterns observed offshore the sorting processes lead to longer wavelengths and lower wave heights as a function of standard deviation this relates to the dampening effect of suspended sediment transport for fine grains finally it appears that the modeled wave heights fall in the same range as observations in the north sea these results are valuable for e g predicting the morphological response after engineering activities and determining suitable aggregates for sand extraction

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

a parameterization of the wavelength of tidal dunes the idealized model of besio et al on the formation of sand waves and sand banks journal of fluid mechanics 2006 557 1 17 is used to predict the wavelength of tidal dunes sand waves generated by tidal currents in estuaries and shallow seas the predictions are then analysed and a formula is proposed to estimate the wavelength of tidal dunes as a function of the parameters of the problem the wavelength of the dunes is found to increase when the water depth is increased and or the strength of the tidal current is decreased on the other hand the size of the bottom material if medium sand is considered and the tidal ellipticity are found to have a relatively small influence on the length of the bottom forms the formula proposed provides results which are consistent with field observations of different authors copyright 2011 john wiley sons ltd

environmental setting morphology and volumetric evolution of the middelkerke bank southern north sea abstract the middelkerke bank a tidal sandbank located in the southern north sea on the belgian continental platform has been the object of a multidisciplinary and international research project the morphological and hydrodynamical setting of the study area is presented a complete side scan sonar coverage of the bank allowed a detailed analysis of the morphology of the bank and of its superimposed bedforms large dunes cover the flanks and summit of the middelkerke bank they have a constant orientation a height ranging from 0 5 to 5 m a wavelength from 75 to 150 m and in most cases an asymmetrical profile the slopes of both lee and stoss flanks are very low small and medium dune fields occur on the entire bank and in the adjacent swales they are 2 d and present a wavelength ranging from 1 to 15 m flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank bathymetric surveying of the middelkerke bank was carried out to assess the importance of volumetric variations of the bank mass caused by seasonal factors results show that changes in the bank volume can be quite important the bank s volume decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself volumetric variations of the total bank volume in a winter summer period can range along certain sections of the bank from 21 to 26 despite these large seasonal volumetric variations the bank shows a long term dynamic equilibrium as proven by the obtained evolution trend

bed load transport on the shoreface by currents and waves tide driven bed load transport is an important portion of the net annual sediment transport rate in many shoreface and shelf environments however bed load transport under waves cannot be measured in the field and bed load transport by currents without waves is barely measurable even in spring tidal conditions there is consequently a strong lack of field data and validated models the present field site was on the shoreface and inner shelf at 2 to 8 5 km offshore the central dutch coast far outside the surfzone where tidal currents flow parallel to the coast bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13 18 m with fine and medium sands the near bed flow was measured over nearly a year and used for integration to annual transport rates an empirical bed load model was derived which predicts bed load transports that are a factor of n5 smaller than predicted by existing models however they agree with laboratory data of sand and gravel transport in currents near incipient motion the damped transport rates may have been caused by cohesion of sediment or turbulence damping due to mud or biological activity the annual bed load transport rate was calculated using a probability density function pdf derived from the near bed current and orbital velocity data which represented the current and wave climate well when compared to 30 years of data from a nearby wave station the effect of wave stirring was included in the transport calculations the net bed load transport rate is a few m 2 year this is much less than predicted in an earlier model study which is partly due to different bed load models but also due to the difference in velocity pdf the annual transport rate is very sensitive to the probability of the largest current velocities

experiments on bar formation in a straight flume 2 graded sediment laboratory experiments have been carried out in a large laboratory flume using a strongly bimodal sediment mixture in a range of flow conditions such that the initially flat bed of the flume became unstable enhancing the development of alternate bars in order to elucidate the effect of grain sorting the present experiments are compared with a previous set of experiments performed in the same flume under similar hydraulic conditions but using a nearly uniform sand with a mean geometric grain diameter equal to that characterizing present experiments the comparison suggests that sediment heterogeneity may appreciably affect bed form characteristics the development of smallscale ripples and mesoscale dunes sediment waves tended to be inhibited thus allowing a decrease in flow resistance owing to the bimodal character of the adopted mixture at low values of the bed shear stress a condition of partial transport was attained for which only the finer grained portion of the mixture was observed to move while the coarsergrained fractions remained essentially immobile throughout the experimental run however complete mobilization of all size fractions was observed to occur in runs carried out at higher slopes i e at higher bed shear stress in order to generate an alternate bar pattern selective transport of individual grain size fractions coupled with the characteristic bar topography pattern led to an intense longitudinal sorting which accreted the coarser particles on bar crests furthermore bar migration caused through scour and fill a significant vertical sorting as a consequence of these sorting processes and in accordance with previous experimental observations bar height turns out to be invariably damped with respect to uniform sediment experiments the trend exhibited by the wavelengths is less clear and suggests that in the present experiments sorting effects do not enhance the clear shortening of bar wavelengths typically observed in other series of flume experiments carried out with weakly bimodal mixtures

influence of bed roughness on dune and megaripple generation richards 1980 conjectured that megaripples of the continental shelf are due to a seabed instability in the presence of seabed ripples towards a better understanding of this megaripple formation process we use a morphodynamical numerical model to investigate the bed roughness influence on the bed load induced linear stability of a sandy bed we find that the linearly most amplified mode shifts from a dune mode grain roughness towards a megaripple mode large bed roughness then we discuss how megaripples could be generated over a flat bed and over dunes

grain sorting effects on the formation of tidal sand waves a model is developed to investigate the process which leads to the formation of sand waves in shallow tidal seas characterized by a heterogeneous sea bed composition the main goal of the analysis is the evaluation of the effects that a graded sediment has on the formation of the bottom forms and the investigation of the sorting process induced by the growth of the bottom forms the analysis is based on the study of the stability of the flat bed configuration i e small amplitude perturbations are added to the flat bottom and a linear analysis of their time development is made for an oscillatory tidal current dominated by one tidal constituent the results show that the graded sediment can stabilize or destabilize the flat bottom configuration with respect to the uniform sediment case depending on the standard deviation sigma of the grain size distribution and on the ratio r over cap between the horizontal tidal excursion and the water depth for moderate values of r over cap i e values just larger than the critical value for which the sediment is moved and sand waves appear the presence of a sand mixture stabilizes the flat bed on the other hand for large values or r over cap the mixture has a destabilizing effect in both cases the effect that a sand mixture has on the stability of the flat bed configuration is relatively small moreover for moderate values of r over cap the fine fraction of the mixture tends to pile up at the crests of the bottom forms while the coarse fraction moves towards the troughs for large values or r over cap the grain size distribution depends on the value of sigma the results are physically interpreted and provide a possible explanation of the apparently conflicting field observations of the grain size distribution along the sand wave profile carried out in the north sea

significance of wind driven rain wind splash in the erosion of blanket peat abstract wind splash is a process in which wind and rain combine to cause soil erosion in upland britain the conditions necessary for wind splash erosion are relatively common and frequently occur in locations where blanket peat is an important land cover a typical location is moss flats north pennines northern england uk wind splash processes were monitored intensively at this site over 3 months using a circular configuration of mass flux sediment samplers and meteorological data logged from an on site automatic weather station maximum peat flux rates were measured between south southwest and west northwest directions in association with relatively moderate intensity frontal rainfall typically 4 6 mm h 1 wind splash processes operate in any direction due to changeable synoptic weather patterns windward peat fluxes were typically 2 13 times greater than those recorded at leeward orientations spatial patterns of erosion are reflected in the wider landscape through the development of small scale erosional landforms peat hags which frequently display preferred orientations within the range of maximum peat flux it is suggested that wind splash may be a more important process of peat erosion than hitherto reported in uk upland areas

on the formation of sand waves and sand banks a fully three dimensional model is proposed for the generation of tidal sand waves and sand banks from small bottom perturbations of a flat seabed subject to tidal currents the model predicts the conditions leading to the appearance of both tidal sand waves and sand banks and determines their main geometrical characteristics a finite wavelength of both sand waves and sand banks is found around the critical conditions thus opening the possibility of performing a weakly nonlinear stability analysis able to predict the equilibrium amplitude of the bottom forms as shown by previous works on the subject the sand wave crests turn out to be orthogonal to the direction of the main tidal current the present results also show that in the northern hemisphere sand bank crests are clockwise or counter clockwise rotated with respect to the main tidal current depending on the counter clockwise or clockwise rotation of the velocity vector induced by the tide only for unidirectional or quasi unidirectional tidal currents are sand banks always counter clockwise rotated the predictions of the model are supported by comparisons with field data finally the mechanisms leading to the appearance of sand waves and sand banks are discussed in the light of the model findings in particular it is shown that the growth of sand banks is not only induced by the depth averaged residual circulation which is present around the bedforms and is parallel to the crests of the bottom forms a steady drift of the sediment from the troughs towards the crests is also driven by the steady velocity component orthogonal to the crests which is present close to the bottom and can be quantified only by a three dimensional model while the former mechanism appears to trigger the formation of counter clockwise sand banks only the latter mechanism can give rise to both counter clockwise and clockwise rotated sand banks

the palaeo valley infilling glaciogenic sarah formation an example from rahal dhab palaeo valley saudi arabia the sarah formation is a glaciogenic sedimentary unit deposited along the gondwana margin during the latest ordovician ice age and represents a major hydrocarbon reservoir in northern saudi arabia large scale glacial palaeo valleys cut into the qasim formation and were infilled by the sarah formation post glacial transgression in the earliest silurian resulted in the deposition of the qusaiba shale member and associated organic rich basal source rocks which cap the sarah formation infilled palaeo valleys this unique setting makes the sarah formation an important emerging exploration target in saudi arabia this paper focuses on the facies and depositional architecture in seismic scale outcrops of the sarah formation in north western saudi arabia the rahal dhab palaeo valley provides a 100 km long dip oriented cross section which has been covered by 24 vertical sections sedimentary architectural analyses at metre to kilometre scale and by three cored shallow boreholes in the rahal dhab palaeo valley the sarah formation was deposited in a proglacial setting that ranged from marginal marine to offshore pro delta and is made up of three units i sarah sandstone ii sarah shale and iii uqlah the present study shows the relationships between these three units and architectural controls on reservoir quality in this system this paper contributes to the regional understanding of the sarah formation and the new depositional model of the rahal dhab palaeo valley provides an outcrop reservoir analogue for hydrocarbon exploration in adjacent areas this article is protected by copyright all rights reserved

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

investigation of a self organization model for beach cusp formation and development a recent numerical investigation of self organization werner and fink 1993 suggests that the feedback process between currents and sediment response can result in self organized patterns and can be used to predict beach cusp formation and spacing a similar model based on self organization is tested here in order to understand the processes occurring during beach cusp formation and development to evaluate the sensitivity toward the parameters used and to examine how the model might relate to field observations results obtained confirm the validity of the self organization approach and its capacity to predict beach cusp spacing with values in fair agreement with the available field measurements with most of the input parameters primarily affecting the rate of the process rather than the final spacing however changes in the random seed and runs for large numbers of swash cycles reveal a dynamical system with significant unpredictable behavior cusp spacing tends to change with time and cusp regularity shows large long term variations cusps are found to be accretionary in the swash zone and in agreement with most observations mean flows are horn divergent over developed topography simulations over nonplanar slopes characterized by the presence of preexisting nonrhythmic or cuspate features have been performed results indicate that preexisting large amplitude cusps are destroyed if their spacing is substantially different from that expected under self organization and that the final spacing is consistent with that predicted by the model for an equivalent plane beach these findings support the hypothesis that self organization is a robust mechanism for beach cusp formation

nearly pure sorting waves and formation of bedload sheets bedload sheets are coherent migrating patterns of bed material recently observed both in flume studies and in field streams with beds of coarse sand and fine gravel this newly recognized feature is inherently associated with the heterogeneous character of the sediment and consists of sorting waves with distinct coarse fronts only one or two coarse grains high the question of the formation of bedload sheets poses an interesting and peculiar stability problem for the grain size distribution sorting waves are essentially two dimensional migrating perturbations associated with variations of this distribution we show that their growth is strictly associated with grain sorting in fact the latter gives rise to perturbations of bedload transport which drive small perturbations of bottom elevation the amplitude of which scales with grain size the sorting wave also induces spatial variations of bottom roughness and consequently alters the fluid motion which conversely exerts a spatially varying stress on the bed the feature of bedload sheets which allows them to be distinguished from dunes over beds with coarse sand or fine gravel is then the fact that sorting is the dominant effect controlling their growth rather than being a relatively small perturbation of the mechanism which gives rise to dunes in the case of uniform sediment the requirement that perturbations should not alter the sediment budget leads to an integral condition which gives rise to an integro differential mathematical problem with the help of recently developed bedload relationships suitable for mixtures as well as appropriate modelling of turbulent channel flow over a bed with spatially periodic perturbations of bottom elevation and roughness we are able to derive a general dispersion relation which can be readily solved in terms of undisturbed size densities in the form of sums of dirac distributions perturbations are found to be unstable within a range of wavenumbers depending on the relative roughness and froude number we show that when the effects of perturbations of bottom elevation are neglected the unstable region corresponds to the range of conditions where the bottom stress leads bottom roughness a range distinct from that which characterizes the formation of dunes this result is given a physical explanation which depends crucially on the deviation from equal mobility of different grain sizes in the surface layer the effect of perturbations of bottom elevation is however not negligible when the bottom roughness is fairly large compared to depth in the latter case perturbations of bottom elevation and of bottom roughness are equally important and gravel sheets are not easily distinguished from small amplitude dunes comparison with the field observations of whiting et al 1985 1988 is satisfactory insofar as the bedload sheet mode is unstable under the conditions of the experiments and the predicted wavelengths fall within the experimental range the laboratory observations of kuhnle southard 1988 on the other hand appear to fall within a range of bottom roughness where the observed bedforms do not exhibit features unambiguously distinct from those of small amplitude dunes

a linear stability analysis of tidally generated sand waves a linear stability analysis is carried out to examine the initial stage of sand wave growth under tidal flows and the occurrence of a preferred length scale the fact that these bedforms typically have length scales small compared to the tidal excursion is exploited by adopting an asymptotic approach to solve the hydrodynamic part of the problem i e to find the hydrodynamic response to an initially small bed perturbation this method is shown to have important advantages over previously used methods since it allows an exploration of the complete sand wave regime whereas other methods fail for short sand waves and in general it is also more accurate it is found that the selection of a preferred length scale depends mainly on only two parameters the bed slope coefficient and the ratio of friction velocity to eddy viscosity whereas there appears to be almost no dependence on the water depth

comparison between predicted and observed sand waves and sand banks in the north sea for the first time a prediction model of regular morphological patterns on the seabed was tested against observations of sand wave and sand bank occurrence in the entire north sea the model which originates from first physical principles predicts this occurrence via two dimensionless parameters on the basis of the water depth the tidal velocity amplitude the level of zero intercept above the seabed z0 and a viscosity variation parameter alt epsilon the latter two quantities were varied in a number of predictions for the entire north sea whereas for the first two local values were used the range of realistic values of alt epsilon and z0 was large enough to let these two parameters distinguish between the possible combinations of bed forms as is shown in the shallower southern bight of the north sea the results were more sensitive to variations in z0 than in alt epsilon a slightly more detailed approach focused on sand waves only in the southern north sea and estimated local values for z0 using depth information quantification of the results showed that the model was able to predict the contours of the sand wave patches but it could not account for the absence of the bed features within this area the type of bed deposit partly explains the smaller scale variation the work confirms the validity of the theoretical bed form prediction model and verifies the hypothesis that the large scale seabed features are formed as free instabilities of tide topography interactions