late pleistocene early holocene fluvial facies and depositional processes in the fehmarn belt between germany and denmark revealed by high resolution seismic and lithofacies analysis in the narrow offshore border zone between germany and denmark 550 km of high density 3 5 khz subbottom seismic reflection profiles were recorded within a 70 km 2 area in order to reconstruct the seismic stratigraphy of late pleistocene to early holocene lacustrine and fluvial environments using detailed line drawings seismic facies analyses and a hierarchy of bounding surfaces a depositional unit was recognized and subdivided into subunits 4 a oblique parallel 4 b mound oblique tangential 4 c sigmoid oblique 4 d and 4 e shingled and parallel the base of this seismic facies association defines a wide u shaped valley with well defined scours and in the valley sides steps are located above deep steep dipping reflections stratigraphic control was available from 32 coring sites 5 to 12 m deep vibrocores subunit 4 b represents coarsening up silt and sand and samples from subunit 4 d show fining up fine sand silt and clay the seismic facies association is proposed to have formed by a fluvial event of short duration some time in the period between 10 3 c 14 ka bp and 9 0 c 14 ka bp subunits 4 a to 4 e represent gradually decreasing flow power a peak flow initiated the fluvial event after which water discharge and level fell rapidly subsequently the normal background discharge from the baltic sea area dominated the flow style reflections beneath the step like valley side with high dip angles are interpreted as faults this tectonic activity resulted in subsidence in the analysed area and could possibly have influenced the fluvio dynamic development the seismic stratigraphic succession reveals a high resolution record of sediments in this area in particular the stepwise uncovering of the morphology of the subunits preserved in high resolution seismic facies associations is proposed as a useful tool in modelling the dynamic development of the near sea floor environment less

fauna and flora in submarine early holocene lake marl deposits from the southwestern baltic sea studies of plant and animal macrofossils have been carried out on early holocene lake marls from the southwestern baltic the records have been dated to the interval from c 8500 to 7800 14c years bp by correlating pollen assemblages to an onshore radiocarbon dated pollen diagram the sediments contain from 5 to 81 carbonate and only few macroscopic remains of wetland and terrestrial taxa the plant and animal mainly mollusc macrofossils reflect deposition at the outer edge of the belt with submerged macro limnophytes in the lower littoral perhaps at water depths at 4 7 m the ratio between the shells and opercula of the snail bithynia tentaculata points to some post mortem disturbance but it can be concluded that the water was highly alkaline and mesotrophic

late weichselian and early holocene shore displacement in the central baltic sea abstract the glacial unloading and resulting isostatic land uplift have together with hydro isostasy eustatic rise of sea level and geoidal changes ruled the shore displacement of fennoscandia within the baltic basin periods of damming and shifts of outlet further complicated the shore displacement during the period studied ca 12 2 8 4 ka bp information on the baltic history was derived from shoreline observations and studies on sediment cores from lakes formerly directly affected by the baltic these cores were analyzed with respect to pollen diatoms and lithology to determine sequences of baltic sediments and dated by a combination of radiocarbon dates and biostratigraphy the studied areas of southeastern sweden mainly central gotland and the oskarshamn area eastern smaland display a similar course of shore displacement the late weichselian period shows a regression of a slightly decreasing rate between ca 12 2 and 10 3 ka bp then followed a fast drop 25 m of the baltic s water level the final drainage of the baltic ice lake during the next ca 700 years the yoldia stage a regression of ca 17 m occurred this period is mainly characterized by freshwater diatoms in the baltic but at ca 10 ka bp there is evidence of a shortlasting 100 150 year period with brackish water the ancylus transgression of ca 11 m began ca 9 6 ka bp and culminated ca 9 3 ka bp it was followed by a fast regression of 8 10 m thereafter the rate of regression decreased shoreline gradients showed that the fastest shoreline tilting occurred around 10 ka bp thus ca 2 ka after deglaciation

the evolution of the mecklenburg bay environment in the holocene in the light of multidisciplinary investigations of the sediment cores abstract multi proxy studies of the sediment cores and acoustic profiles from mecklenburg bay display a more detailed picture of the evolution of the environment in the holocene studies of diatoms geochemistry grain size and radiocarbon dating clearly reveal the age and character of freshwater and marine transgressions and regressions after the final regression of the baltic ice lake a local lake and swamp environment was recorded by peat gyttja sediments this environment existed until the ancylus lake transgression dated ca 10 500 cal bp during the ancylus lake period the area remained a shallow water lake transitional isolated from the main baltic basin the first slightly brackish marine inflows confirmed by the appearance of planktonic mesohalobous and halophilous diatoms were identified at the beginning of the initial littorina sea stage and dated ca 8800 cal bp the boundary between the ancylus lake and the initial littorina sea is marked by increasing contents of biogenic silica and iron the initial littorina sea stage was characterized by increasing mesohalobous and polyhalobous diatom forms and decreasing oligohalobous species the initial littorina sea environment was recorded as a slightly brackish reservoir sporadically affected by weak marine inflows the littorina transgression appeared in the mecklenburg bay at ca 7500 7700 cal bp and is reflected by the domination of polyhalobous diatoms increasing mean grain size biogenic silica and calcium and magnesium contents the initial littorina sea was a 1500 year long period in mecklenburg bay existing until the littorina transgression the main littorina transgression first appeared in mecklenburg bay around 7700 cal bp and after a few hundred years at around 7200 cal bp in the arkona basin

an experimental study of flow bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models the nature of flow sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size density sediment under equilibrium and non equilibrium aggradational degradational conditions and compared with theoretical models during each experiment water depth bed and water surface elevation flow velocity bed shear stress bedload transport and bed state were continuously monitored equilibrium uniform flow was established with a discharge of about 0 05 m3 s 1 a flow depth of about 0 01 m a flow velocity of about 0 81 0 88 m s 1 a spatially averaged bed shear stress of about 1 7 2 2 pa and a sediment transport rate of about 0 005 0 013 kg m 1 s 1 i e close to the threshold of sediment transport such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment pebble clusters bedload sheets and low lying bars were ubiquitous in the experiments heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits aggradation was induced by 1 increasing the downstream flow depth flume tilting and 2 sediment overloading tilt induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross stratified deposit with downstream dipping pebbles pseudo imbricated and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate these differences between pre and post aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles imbricated whereas the equilibrium flow and sediment transport conditions remained relatively unchanged degradation was induced by 1 decreasing the downstream flow depth flume tilting and 2 cutting off the sediment feed tilt induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions the cessation of sediment feed produced degradation and armour development a reduction in the mean water surface slope and flow velocity an increase in flow depth and an exponential decrease in bedload transport rate as erosion proceeded a bedload transport model predicted total and fractional transport rates extremely well when the coarse grained or bedform trough areas of the bed are used to define the sediment available to be transported a sediment routing model midas also reproduced the equilibrium and non equilibrium flow conditions total and fractional bedload transport rates and changes in bed topography and texture very well

structural properties of mobile armors formed at different flow strengths in gravel bed rivers differences in the structure of mobile armors formed at three different flow strengths have been investigated in a laboratory flume the temporal evolution of the bed surfaces and the properties of the final beds were compared using metrics of surface grain size microtopography and bed organization at both grain and mesoscales measurements of the bed condition were obtained on nine occasions during each experiment to describe the temporal evolution of the beds structured mobile armors formed quickly in each experiment at the grain scale 1 45 mm 9 ds50 17 mm where ds50 is the median surface particle size surface complexity decreased and bed roughness increased in response to surface coarsening and the development of the mobile armor particles comprising the armor also became flow aligned and developed imbrication at a larger scale 100 200 mm the surface developed a mesoscale topography through the development of bed patches with lower and higher elevations metrics of mobile armor structure showed remarkable consistency over prolonged periods of near constant transport demonstrating for the first time that actively transporting surfaces maintain an equilibrium bed structure bed structuring was least developed in the experiments conducted at the lowest flow strength however little difference was observed in the structural metrics of the mobile armors generated at higher flows although the range of transport rates studied was limited the results suggest that the structure of mobile armors is insensitive to the formative transport rate except when rates are low 0 03 where is the dimensionless shear stress

measuring and defining bimodal sediments problems and implications gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions these can be quantified in several ways giving wide differences in derived statistics and calculations problems associated with measuring summarizing and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition no single index of bimodality serves all purposes alternatives are suggested measured near bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume this has implications for the abrupt downstream transition often found from gravel to sand bed

darss sill as a biological border in the fossil record of the baltic sea evidence from diatoms abstract biostratigraphical and palaeoecological analyses of cores along a transect from femer belt to the arkona basin reveal that north sea waters began to enter the western baltic sea between 8600 and 8400 calibrated years bp studies of diatoms indicate that mecklenburg bay was characterised by slightly brackish water conditions between 8400 and 8000 cal years bp at around 8000 cal years bp increasing salinity is indicated by a strong dominance of the diatoms paralia sulcata and dimeregramma minor some centuries later another diatom assemblage appeared and became dominant in mecklenburg bay this assemblage includes hyalinella lateripunctata and pravifusus hyalinus species typical of shallow water areas along the atlantic coast today at this time the first marine molluscs made their appearance the oldest shell of a marine mollusc found in our material is dated to 7600 cal years bp the associated assemblage that includes adult specimens of the gastropod aporrhais pespelicani indicates higher salinities than today during the littorina sea stage a marine diatom flora with p sulcata catenula adhaerens and d minor crossed the darss sill and became widely distributed in the arkona basin pomeranian bay and the baltic sea proper in contrast taxa indicative of the hyalinella lateripunctata p hyalinus assemblage are only found west of the darss sill in femer belt and mecklenburg bay apparently the darss sill threshold has been acting as an important salinity border from around 7800 cal years bp until today

model experiments on mobile paved gravel bed streams laboratory experiments which model streams with poorly sorted gravel beds at reduced scale reveal that pavement is a mobile bed phenomenon it acts to decrease the inherent difference in mobility between large and small grains by overrepresenting the percentage of large grains exposed to the flow pavement can coexist with the motion of all available grain sizes because motion is sporadic so that at any given time only a small percentage of surface grains are actually in motion particles in the immediate subpavement are only occasionally moved and deeper particles are hardly ever moved the experiments were conducted at constant discharges they provide accurate quantitative models of two field streams in that they obey the same bedload law as field data both sediment feed and recirculating configurations were utilized the only systematic difference between the two is in the behavior of the finer grains

size selective entrainment of bed load in gravel bed streams entrainment of mixed size gravel bed material was studied in nine reaches of three high power streams in scotland and norway paired measurements of at a point shear stress estimated from velocity profiles and bed load transport by hand held sampler were made they extended to very high values 400 n m 2 3 5 kg m x s x analyses of maximum bed load diameter mean bed load diameter transport rates of individual size fractions and tracer pebble movements all show some dependence of threshold shear stress for entrainment on absolute particle size despite strong relative size effects precise equal mobility of all sizes was approached in the data set with the highest shear stresses and transport rates size selective transport in the streams studied is also indicated by clear downstream and downbar reductions in surface sediment size over distances too short for abrasion to be significant

a late pleistocene lacustrine transgression in the fehmarn belt southwestern baltic sea in order to reconstruct the late pleistocene seismic stratigraphy 550 km of high density 3 5 khz subbottom seismic reflection profiles were recorded within a 70 km 2 area in the narrow offshore border zone between germany and denmark a depositional unit 3 was analysed for its seismic facies association and in the central study area mound oblique channel reflection free shingled hummocky broken and parallel associations occurred the rims of the association shows mound oblique hummocky and minor parallel seismic facies stratigraphic control was available from 32 coring sites in the central study area unit 3 represents rhythmic layers of silt with fine sand and clay in an overall fining upward 3 to 5 m thick sequence at the rim of the basin the unit comprises fine sand silt and rhythmic layers of silt with clay and fine sand unit 3 is proposed as having formed partly in contact with during the transgression of a lake the baltic ice lake some time between 10 8 10 0 c 14 ka bp after 10 0 c 14 ka bp the water body stagnated when contact with the baltic basin was cut off analysis of base level of the subunits implies tectonic activity resulting in subsidence of the central study area which could possibly have influenced the transgressive development less

mean flow and turbulence structure over fixed two dimensional dunes implications for sediment transport and bedform stability detailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed two dimensional dunes in a laboratory channel laser doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength the density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters which are assessed using higher moment measures and quadrant analysis these flow field maps illustrate that 1 the time averaged downstream and vertical velocities agree well with previous studies of quasi equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest 2 the maximum root mean square rms of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell 3 the maximum vertical rms values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream extending almost to the next crest 4 positive downstream skewness values occur within the separation cell whereas positive vertical skewness values are restricted to the shear layer 5 the highest reynolds stresses are located within the zone of flow separation and along the shear layer 6 high magnitude high frequency quadrant 2 events ejections are concentrated along the shear layer kelvin helmholtz instabilities and dominate the contribution to the local reynolds stress and 7 high magnitude high frequency quadrant 4 events occur bounding the separation zone near reattachment and close to the dune crest and are significant contributors to the local reynolds stress at each location these data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation magnitude and downstream extent of the flow separation zone and resultant shear layer furthermore the origin of dune related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting the fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the kelvin helmholtz instabilities which are both greater for dunes than ripples these instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates

paraglacial barrier lagoon development in the late pleistocene baltic ice lake southwestern baltic abstract marine geological investigations including shallow seismic sedimentological and stratigraphical studies were carried out in fakse bay southwestern baltic seismic sequence analytical methods were used supplemented by detailed lithological descriptions pollen analysis and 14c dating of core material detailed correlations between sediment facies units and seismic sequences resulted in the establishment of four depositional sequences i glacial related deposits ii lagoon pond and freshwater coastal deposits iii lake bog lake deposits and iv marine sub littoral deposits most attention has been paid to sequences ii and iii showing the development of a transgressive freshwater barrier beach ridge and a following regression pollen spectra indicate allerod and younger dryas chronozone ages 11 500 10 000 14c yrs b p for the development of a sheltered lagoon pond basin behind the transgressive freshwater barrier beach ridge the freshwater coastal deposits are interpreted as the southwesternmost marginal deposits of the baltic ice lake and represent the transgression maximum this transgression maximum reached a level of about 13 m below the present sea level the final drainage of the baltic ice lake resulted in the development of a sheltered basin behind the former beach ridge a fluctuating groundwater level here favoured the deposition of alternating lake gyttja and bog peat deposits in the central part of the basin pollen analysis and 14c dating reveal that the lake deposits correspond to high groundwater levels associated with the ancylus lake 9000 yrs b p and the littorina sea 7000 yrs b p transgressions while the bog sediments represent the low water stands in the early holocene

critical shear stress of natural sediments the critical shear stress of individual fractions ci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture for strongly bimodal sediments ci increases with grain size an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes ci to deviate from size independence in the direction of unisize shields values a quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of ci is observed and to predict the variation of ci with grain size in bimodal sediments because properties of sediment grain size distributions other than bimodality appear to have little influence on ci the trends presented here for 14 unimodal and bimodal sediments may be quite general

late pleistocene and early holocene paleogeography of the darss sill area southwestern baltic abstract today about 73 of the water exchange between the baltic and the north sea takes place through the darss sill area in hydrographic terms the darss sill is a major morphological unit separating the deeper areas of the mecklenburg bay and the arkona basin from each other it is characterized by an average water depth of 16 m the main bathymetric features are the gedser reef with a water depth of less than 10 m and the kadet channel which has a maximum water depth of 32 m according to previous authors there are two different ice marginal lines between falster denmark and the darss peninsula germany the recent kadet channel is regarded as a large glacial valley cutting the ice marginal lines joint danish and german investigations in the darss sill area 1989 1991 indicate the existence of only one marginal line in this region which is represented by a till cropping out in the area between falster and the german coast the formation of the marginal line ca 13 500 bp caused a damming of a pre existing subglacial drainage system in the gedser reef area southeast of the recent kadet channel glaciofluvial drainage occurred towards the ne through a deep channel into the baltic ice lake immediately after the retreat of the ice during the baltic ice lake transgression the channel was filled by sandy sediments from sources east of the investigated area this accumulation process resulted in the blocking of the channel at the time of the baltic ice lake highstand maximum at this time occasional overflow in the kadet channel area must have started the main erosion and deepening of the kadet channel must however be ascribed to the ancylus lake overflow

turbulence and secondary flow over sediment stripes in weakly bimodal bed material longitudinal stripes are a common bed form in heterogeneous alluvial sediments and consist of periodic spanwise variations in bed texture and elevation that are aligned parallel to the mean flow direction this paper quantifies mean and turbulent flow structures over self formed sediment stripes in a weakly bimodal sand and gravel mixture turbulence anisotropy generates two secondary circulation cells across the channel half width which produce a cross stream perturbation in boundary shear stress the interaction between this flow structure and the selective transport of bed material generates spanwise sediment sorting that is symmetrical about the centerline finer sediments are entrained from regions of high shear stress transported laterally by the secondary flow and deposited in regions of lower shear stress lateral changes in bed texture further enhance the near bed secondary flow which provides a positive feedback mechanism for stripe growth in bimodal sediments at shear stresses just above the entrainment threshold stripes may replace lower stage plane beds at higher shear stresses the coarser sediment becomes more mobile and the stripes are replaced by flow transverse bed forms read more http ascelibrary org doi abs 10 1061 asce 0733 9429 1999 125 5 463

field measurements of secondary currents in straight rivers basic knowledge of turbulent structures in open channel flows are now available from accurate experimental data that were obtained using a laser doppler anemometer in laboratory flumes many of these experiments have however been conducted at moderate reynolds numbers therefore it is not yet clear whether such laboratory data of open channel turbulence can be applied to river turbulence at high reynolds numbers in this study velocity measurements of the biwako sosui river in kyoto and the aichi irrigation channel in nagoya have been conducted by making use of three component electromagnetic flow meters multicellular secondary currents were evident in a wide river whereas large scale free surface secondary currents associated with velocity dip phenomena were generated in a narrow river the features of such secondary currents in rivers could be explained by theory of turbulence and also they coincided well with the laboratory experiments and numerical calculations

early holocene history of the baltic sea as reflected in coastal sediments in blekinge southeastern sweden integrated palaeoecological studies of two fiord sediment sequences in the province of blekinge se sweden covering the time span 11 000 5000 cal bp reveal the timing and the environment for the ancylus lake littorina sea transition 9800 8500 cal bp the first ingression of saline water into the baltic sea through the danish straits occurred earlier than formerly assumed new evidence particularly mineral magnetic and palaeobotanical analyses demonstrate that on the general trend of the eustatically caused littorina transgression several minor fluctuations of the water level can be identified between 8500 and 5000 cal years bp a distinct regression phase around 8 100 cal bp is correlated with the greenland ice core cold event dated to 8200 ice core years bp this is described as a regional climatic catastrophe for the baltic sea region the coastal stratigraphy is compared with the offshore stratigraphy earlier studied a tentative shore displacement curve for early and middle holocene is presented

flow velocity profiles in gravel bed rivers velocity measurements carried out in a rectangular laboratory flume having a gravel bed are presented the measurements are carried out for four different bed shapes characterized from different concentrations of coarser elements and for the two conditions of small and large scale roughness the analysis showed that the dean profile is usable to establish the velocity profile for both hydraulic conditions for small scale hydraulic condition an indirect estimate procedure of the coefficients of dean profile is proposed

the gravel sand transition flume study of channel response to reduced slope this paper reports flume experiments with a bimodal gravel sand bed and sediment feed designed to investigate river channel response to declining slope in successive runs an initial near equilibrium channel was subjected to progressively bigger reductions in flume slope this simulated a series of sites along a concave profile approaching a local base level and undergoing a transition from gravel to sand bed it also models the effects of a rise in base level on a fixed site slope reduction led to immediate increase in depth and reduction in velocity and shear stress hence progressive aggradation deposition was size selective and led to the following sequence of bed texture change with increasing severity of slope reduction 1 sand in lee of gravel clusters 2 elongated sandy patches and thin streaks 3 wider sand ribbons covering gravel completely and developing ripples there was feedback from the gradual bed surface changes to hydraulic conditions near bed velocity increased while depth shear stress and roughness height decreased these feedbacks were more pronounced with a bigger reduction in slope these results support the view that the gravel sand transition along river channels with a local base level control is assisted by positive feedback between changes in bed surface texture and near bed flow

topographically induced upwelling off eastern australia abstract a high resolution numerical study of an idealized western boundary current flow over variable topography is presented with application to the east australian current eac the results indicate that alongshelf topographic variations off australia s east coast cause the eac to accelerate over the narrowing continental shelf near cape byron this acceleration is sufficient to hinder the geostrophic adjustment in the bottom boundary layer bbl which would usually cause the eac driven bbl to shut down consequently a region of persistent high bottom stress was established off cape byron which was responsible for driving an upwelling bbl it is shown that the enhanced vertical mixing associated with a low richardson number flow beneath the eac reduced the local stratification consequently the burger number is decreased resulting in a long shutdown timescale of the bbl which enables a nearshore thermal to be established and maintained such fronts are commonly observed in the region as a p

salinity changes in the central baltic sea nw europe over the last 10000 years we attempt a reconstruction of salinity levels of the central baltic sea based on diatom assemblages the isotopic composition of organic matter and sedimentological expression of anoxia over the last 10 000 years we use the data to investigate the dependence of salinity levels on climate evolution and isostasy changes in salinity of surface and deep waters were most pronounced from 8400 to approximately 5000 cal bp density stratification between salty deep and fresher surface waters caused the frequent development of anoxic conditions and deposition of laminated sediments on large parts of the sea floor in the central baltic sea and dramatic changes in organic carbon accumulation rates from 5000 to 3100 cal bp the salinity of the basin decreased oxygenation of deep sea floors was improved and fertility of the sea surface was significantly reduced this is reflected by low accumulation rates of organic carbon in bioturbated sediments since 2800 cal bp salinity rose again and anoxic periods were more common even though the major steps in environmental evolution in the baltic sea coincide with known patterns of climatic change of the north atlantic realm over the last 10 000 years we find no conclusive evidence for synchronous changes or linear responses on submillennial timescales however we note that major variations in our salinity records agree with temporal patterns of reconstructed summer warmth and winter precipitation in southern scandinavia both types of record suggest that climate in the mid holocene was far from stable our data also confirm that climate evolution over the late holocene had significant impact on environmental conditions in the baltic sea

cellular secondary currents in straight conduit an investigation of cellular secondary currents i e three dimensional flow patterns is very important in hydraulic engineering because these currents might cause three dimensional sediment distributions and bed configurations such as sand ribbons in straight rivers it is however fairly difficult to measure secondary currents of water flows even by making use of hot film anemometers because the velocity of secondary currents is within 5 of the mainstream velocity thus on the basis of the hypothesis that the existence of free surface may not be an essential cause of secondary currents the present study has investigated experimentally the turbulent structure of their currents in air conduit considering an essential interaction between secondary currents and bedform longitudinal ridge elements were attached onto both lower and upper bottoms of the conduit which simulated the longitudinal ridges and troughs of river bedform i e sand ribbons all three components of the velocity were measured accurately by hotwire anemometers the structure of secondary currents was examined through the equations of mean flow vorticity and mean flow energy

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

early holocene brackish and marine facies in the fehmarn belt southwest baltic sea depositional processes revealed by high resolution seismic and core analysis holocene stratigraphy is established from 3 5 khz seismic reflection data and a number of vibro cores across the narrow fehmarn belt between germany and denmark using detailed line drawings texture seismic and lithofacies analyses in a selected part of the study area a distinct depositional unit 5 was identified the unit was subdivided into sub units 5a to 5j due to a different seismic texture and facies pattern the base of the unit 5 is generally smooth but with step like parts which are interpreted to result from tectonism in each sub unit the seismic facies association includes mound oblique seismic facies and within channel deposits sigmoid shingled and parallel facies the sub unit represents offset stacked lagoonal and spit deposits deposited under conditions of progressive sea level rise sub units 5a to 5h show a change in depositional style from fluvial domination to gradually more wave influenced thus the dynamic evolution of the shallow marine sub sea floor environment and changes in depositional conditions are revealed by high resolution seismic facies associations

on the entrainment of sediment and initiation of bed defects insights from recent developments within turbulent boundary layer research sediment entrainment and the initiation of bed defects has commonly been ascribed to the impact of high velocity sweeps upon mobile sand beds results of visualization experiments suggest that these sweep impacts are grouped and may define patches of entrainment upon a mobile bed that may be wider than an individual sweep impact additionally the presence of longitudinal ribs of sediment generated by sweep impacts may stabilize the position of sweeps and low speed streaks these observations are interpreted in the light of recent boundary layer research which suggests the formation of multiple hairpin shaped vortices within turbulent flows these are postulated to generate multiple sweeps that manifest themselves as entrainment patches on a mobile bed these features of the turbulent boundary layer and its modification by sweep generated sediment ridges can be used to propose a model for bed defect formation and the subsequent development of current ripples

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small