do critical stresses for incipient motion and erosion really exist the concept of critical stress for the initial motion of noncohesive sediment beds under turbulent flow conditions is reviewed observational definitions of incipient motion are many and not entirely compatible some laboratory flume observations of sediment movement suggest that no true threshold exists current understanding of turbulent fluid motion at a sediment bed suggests that some particle movement must occur at all nonzero time mean velocities a combined model for flume flow and sediment transport having no threshold is seen to explain features of data that have been previously used to support the threshold concept

the variability of critical shear stress friction angle and grain protrusion in water worked sediments the erodibility of a grain on a rough bed is controlled by among other factors its relative projection above the mean bed its exposure relative to upstream grains and its friction angle here we report direct measurements of friction angles grain projection and exposure and small scale topographic structure on a variety of water worked mixed grain sediment surfaces using a simple analytical model of the force balance on individual grains we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography the friction angle projection and exposure of single grain sizes vary widely from point to point within a given bed surface the variability within a single surface often exceeds the difference between the mean values of disparate surfaces as a result the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution rather than a single value on a given bed the crtitical shear stress distributions of different grain sizes have similar lower bounds but above their lower tails they diverge rapidly with smaller grains having substantially higher median critical shear stresses large numbers of fines trapp ed within pockets on the bed or shielded by upstream grains are effectively lost to the flow our calculations suggest that critical shear stress as conventionally measured is defined by the most erodible grains entrained during transient shear stress excursions associated with the turbulent flow this implies a physical basis for the indeterminacy of initial motion these observations suggest that transport rate shear stress relationships may be controlled in part by the increasing numbers of grains that become available for entrainment as mean shear stress increases they also suggest that bed textures and grain size distributions may be controlled within the constraints of an imposed shear stress and sediment supply regime by the influence of each size fraction on the erodibility of other grain sizes present on the bed

structure stability and transformation of contaminated lacustrine surface fine grained laminae an in situ annular flume sea carousel was deployed in hamilton harbour lake ontario to assess the structure stability and transformation of lacustrine surface fine grained laminae sfgl such laminae typify depositional lacustrine environments and occur at the sediment water interface the critical erosion thresholds erosion rates and internal friction coefficients were determined for natural undisturbed and physically disturbed sites in order to assess changes in bed stability sediment cores taken at each site were analyzed for bulk density using a ct scanner in order to provide an intercomparison of our results flocs and aggregates pumped from the flume during erosion experiments were analyzed for morphological characteristics using conventional optical microscopy and transmission electron microscopy to evaluate the mechanism and development of the sfgl and its structure a general three layer model is developed which depicts the organic flocs of the sfgl layer 1 compressing within a collapse zone layer 2 to form a consolidated bed layer 3 the sfgl was observed to be porous of low density and of high water content which possessed yield resistance due in part to binding by extracellular polymeric substances secreted by a colonized microbial population transmission electron microscopy observations showed an active biological community in the sediment promoting biostabilization time series of erosion thresholds and friction coefficients of the disturbed bed showed that the sfgl reconsolidated rapidly and increased in shear strength the artificial disturbance of the sediment resulted in incorporation of the sfgl and the collapse zone with the consolidated bed through mixing this stimulated the process of consolidation chiefly through removal of gas and breakdown of the organic fibril network reconsolidation began with the formation of a collapse zone at the surface of the disturbed sediment high friction coefficients and an increasing density with depth in the new surface layer indicated that rapid consolidation occurred after disturbance

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

residual transport of suspended material by tidal straining near sloping topography abstracttidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean essential for this process is an externally imposed horizontal density gradient typically resulting from either freshwater runoff or differential heating here it is shown that near sloping topography tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role a combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process the main finding of this study is that for a wide range of conditions suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping downslope transport may also occur however only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle la

erosion threshold of saturated natural cohesive sediments modeling and experiments 1 predicting the erosion resistance of saturated natural sediments requires taking into account cohesion which results from interactions between clay particles the current paper describes a combined experimental and theoretical examination of the threshold conditions for a mixture of clays and sands erosion threshold measured values are larger than those predicted from noncohesive models beyond the usual dependence on grain size a significant correlation between erosion threshold and porosity measurements is confirmed for heterogenous mixtures of grains with particle reynolds number lower than 5 for ph values in the range of 6 8 and under freshwater conditions a model of the erosion criterion is proposed first a cohesion force between two spherical particles is introduced into the usual erosion criterion this reveals a specific function of the cohesion force called cohesion function the force we consider is the long range van der waals interaction then multiple interactions between one particle and those surrounding it are counted and modeled on the basis of both the coordination number and the porosity finally the overall erosion threshold for the sediment bed is inferred from the average of the multiple interactions over the grain size distribution the model highlights that cohesion comes from clay particles about 2 10 6 m and can affect the entire grain size range from clays to sands by means of coordination the model relevance is assessed by comparison with experimental thresholds obtained from resuspension campaigns the results show that the proposed cohesion model offers good agreement with experimental data

inferring the mass fraction of floc deposited mud application to fine grained turbidites fine sediment deposition in the ocean is complicated by the cohesive nature of muds and their tendency to flocculate the result is disaggregated inorganic grain size digs distributions of bottom sediment that are influenced by single grain and floc deposition this study outlines a parametric model that characterizes bottom sediment digs distributions modelled parameters are then used to infer depositional conditions that account for the regional variation in the grain sizes deposited by turbidity currents on the laurentian fan sohm abyssal plain offshore south eastern canada results indicate that on the channellized laurentian fan the mass fraction of floc deposited mud increases only slightly downslope the small evolution in this fraction arises because sediment concentration and turbulent energy are associated in turbidity currents on the sohm abyssal plain however the mass fraction of floc deposited mud decreases probably as a result of lower sediment concentration at this source distal site estimates of the mass fraction of mud deposited as flocs suggest that floc deposition is the dominant mode by which sediment is lost from suspension although single grain deposition contributes more to the depositional flux in proximal areas where high energy breaks flocs and in distal areas where low sediment concentration limits floc formation it is concluded that throughout the dispersal system changes in the fraction of flocculated mud deposited from turbidity currents reflect changes in sediment concentration and energy downslope

critical conditions of incipient motion of cohesive sediments the critical condition of incipient motion of cohesive sediments was investigated in a laboratory study one hundred experimental runs were performed with sediment samples by varying the yield stress to determine the relationship between the critical condition of incipient motion and the rheological properties of the cohesive sediments the results indicate that yield stress is a factor that has a major influence on the incipient motion of cohesive sediments in addition the critical shields parameter is found to be exponentially proportional to the yield stress and inversely proportional to the median grain size the effect of yield stress on the critical shields parameter is significant for the cohesive sediments and becomes progressively weaker with increasing median grain size furthermore an empirical formula for calculating the critical shields parameter of cohesive sediments that includes a rheological term and a gravity term is proposed by introducing the yield stress according to this formula a modified shields diagram is obtained in which the values of the critical shields parameter for cohesive sediments vary within a band that contains countless curves instead of on a single line to reflect the influence of the yield stress this modification of the traditional shields curve is effective for fine sediments but the effects tend to vanish for coarse sediments as the behavior of sediments changes from cohesive to non cohesive finally potential further investigations are discussed

effects of biological activity on the entrainment of marine sediments nowell a r m jumars p a and eckman j e 1981 effects of biological activity on the entrainment of marine sediments mar geol 42 133 153 the effects of animal tracks and fecal pellet production on the critical entrainment velocity of marine sediments were examined experimentally laboratory measurements in a free surface seawater flume were made using three sediment sizes effects of three species of polychaetes and two species of bivalves were tested boundary shear velocity was calculated from the mean velocity profile in the logarithmic region of the boundary layer measurements were made with a hot film anemometer tracking doubled the boundary roughness and decreased the critical entrainment velocity by 20 ambient or free sediments were more easily entrained than fecal mounds which were restrained from movement by mucous adhesion between the fecal coils isolated pellets such as those egested by amphicteis scaphobranchiata transported readily as bedload over a cohesive sediment surface

the influences of biological activity and consolidation time on laboratory cohesive beds abstract a small annular laboratory flume mini flume was used to investigate the changes in erosion threshold and erosion rate on natural mud taken from an intertidal mud flat surface in southampton water u k mud samples were wet sieved at 2 mm to remove shells and large macrofauna and macroflora and then mixed into a turbid slurry with local estuarine water a mud bed was formed in the flume by letting the sediment deposit in still water at room temperature 20 c the bed was subjected to steady flows of increasing magnitude in a series of steps the flume was subjected to illumination on a dark light cycle of 12 hours to simulate the solar cycle subsequent erosion experiments were carried out after periods of bed consolidation of 2 6 12 24 48 and 96 hours in order to examine changes in bed stability with time this series of experiments was considered the baseline situation in which algal and bacterial binding were present at the end of each experiment the bed was fully resuspended mixed for a period of 5 minutes and then allowed to redeposit a second series of experiments was undertaken using the same sediment but in this case the bed was continuously held in darkness no photosynthesis of microphytobenthos a third series of experiments was carried out in darkness but antibiotics were added to the flume to remove the effects of bacterial activity a final series of experiments was carried out with the addition of 50 ml l 1 of sodium hypochlorite to remove all biological activity in order to examine the effects of electrochemical binding the control the results reveal that under the normal conditions of light day night an increase in sediment stability stabilization index bi of circa 3 was found over that of electrochemical bonding the bed treated with antibiotics suffered an overall stabilization of 1 5 relative to the control

use of total grain size distributions to define bed erosion and transport for poorly sorted sediment undergoing simulated bioturbation abstract flume experiments were conducted using poorly sorted beds made to resemble those glacial deposits that blanket the antarctic seafloor today flow within the flume was gradually increased over 2 cm s 1 increments up to 20 cm s 1 an initial run was made without altering the bed and two additional runs were made in which the bed was mechanically mixed in order to simulate bioturbation initial and efficient transport of a given size particle was determined using total grain size distribution data for samples taken from the residual bed from trays placed at different elevations above the floor of the flume and from the water column using this method we were able to measure the current velocities at which initial movement occurred as well as relate different modes of transport for given size ranges to current velocity without mixing the bed current velocities greater than 16 cm s 1 were required to initiate particle movement but because the bed became armored this initial movement was not reflected in the total grain size distribution of the bed with mixing a residual bed containing little silt and clay was produced at velocities of only a few centimeters per second once fines were removed from the bed sand sized material began to move by either traction or saltation a curve relating current velocity to sediment transport by bed load traction and saltation intermittent suspension and suspension is presented the results of our flume experiments demonstrate how sorted sediments are presently being derived from cohesive often overcompacted poorly sorted glacial sediments on high latitude continental shelves where measured current velocities are less than 20 cm s 1 the antarctic seafloor for example furthermore we have gained important information about the true meaning of grain size data as it relates to current velocity mean grain size sorting and skewness reflect not only current velocity but also the efficiency with which the bed was mixed therefore attempts to reconstruct paleovelocity using only these parameters may be meaningless

bottomset architecture formed in the troughs of dunes and unit bars abstract bottomsets are formed in the troughs of migrating bedforms the flow and sediment dynamics downstream of bedforms differ and consequently the thickness grain size and internal structure of bottomsets vary greatly a classification of the controls on bottomset formation is proposed related to two and three dimensional processes flow and sediment unsteadiness and pre existing deposits structures and deposits created by flow and sediment unsteadiness as well as pre existing deposits are also considered in relation to changes they can induce in subsequent trough processes the classification is illustrated by observations from laboratory flumes the modern burdekin river australia and the rock record the roaches grit england and hawkesbury sandstone australia in bottomset examples described in this paper internal variability both vertically and laterally was much greater than that within the dominant foreset component complex compound structures were created by the interaction and predominance of different bottomset controls this combined with their high preservation potential makes them a highly useful paleoenvironmental indicator bottomset variation e g changes in the abundance of mud laterally will influence the permeability heterogeneity of fluvial reservoir rocks bottomsets formed under relatively steady conditions are likely to be laterally extensive and have similar characteristics over their entire length they may act as significant barriers to vertical flow conversely bottomsets generated in unsteady regimes tend to be highly variable potentially creating conduits between cross bed sets aiding inter set permeability

a field method for determining the firmness of colonized sediment substrates abstract substrate firmness influences the erodibility remobilization and topographic expression of that substrate sediment distribution patterns remobilization of sediment and the architecture of biogenic sedimentary structures are strongly affected by the firmness and cohesiveness of the sediment given the potentially important role sediment firmness plays in different depositional settings it is important to have a consistent means of evaluating it this paper demonstrates that a modified metallurgical technique the brinell hardness test can be used to produce accurate and consistent firmness data in modern depositional settings in this method a glass or metal sphere the indentor is dropped from a fixed height into a cohesive medium the size of the indent produced is inversely proportional to the firmness of the media firmness values can be reported as a pressure exerted by the substrate kpa this method has some advantages over standard penetrometers such as ease of use portability and simplicity of equipment testing a large area and flexibility of calculation field tests show that this method is accurate if the indentation diameter is between 10 and 80 of the indentor diameter the method is inappropriate for dry unconsolidated sand and thixotropic mud it is however extremely useful for assessing the firmness of a wide range of soft to firmground sediments that are composed of clay through coarse sand

measuring the in situ erosion shear stress of intertidal sediments with the cohesive strength meter csm the shear resistance of muddy sediments directly governs the susceptibility of the sediment to erosion by tidal and wave induced currents measurements of the natural erosion shear stress are important in modelling estuarine systems and in determining the possible impact of human disturbance a second generation erosion instrument the cohesive strength meter csm designed to measure the critical erosion shear stress ocr of intertidal sediments in situ is described the csm allows measurements of small scale spatial and temporal variation of sediment stability to be made on a scale and rapidity previously unachievable new features of specialized hardware are described and the instrument is empirically calibrated in terms of an equivalent horizontal bed shear stress using quartz sand some example results from recent field trials on the konigshafen sylt romo bight are presented the erosion characteristics of this intertidal flat varied on a cm scale due to the presence of patchy diatom biofilms areas with a diatom biofilm were more stable than those without giving a biostabilization index of 6 2

suspended sediment storm modeling abstract a variety of sediment transporting storms was observed in the hebble long deployment indicating several distinct forcing mechanisms in particular some storms appear to originate locally gross and williams this issue in this paper a simple model of a locally forced storm is presented to elucidate the time dependent dynamics of erosion and deposition events in the deep sea based on these modeling efforts we make the following inferences storms exhibit peak sediment concentration before peak velocity this behavior is due to a limited amount of erodible bed sediment after near instantaneous erosion of the bed is complete fine sediment concentration in the water column drops due to dilution throughout a thickening turbulent mixed layer in the accelerating flow during and immediately after a storm event turbulence intensity above the 40 m thick momentum mixed layer can be sufficient to suspend fine particles in low concentration to elevations of 100 m above bottom or more following the storm the rate of sediment clearing depends on gravitational settling and the potential importance of additional turbulence driven deposition and aggregation mechanisms syn and post storm overthickening of the suspended sediment mixed layer relative to the ekman momentum mixed layer thickness as well as sensitivity of the post storm concentration profile to turbulence driven deposition can explain some complex features of near bed nepheloid layers in the hebble region observed by other workers validation of suspended sediment transport models is based on field observations obtained with light transmissometers transmissometer measurements of suspended sediment concentration depend in turn on calibration of instrument response to both mass concentration and particle size distribution by analyzing the temporal and spatial changes in mass concentrations with the model presented herein we calculate an operational calibration function during hebble storms this calibration function does not change by more than 30 due to particle sorting by turbulent suspension

the effects of bacteria on the flow behavior of clay seawater suspensions abstract the presence of glue like exopolymer produced by the marine benthic bacterium alteromonas atlantica present in concentrations comparable with typical marine muds can enhance the yield stress of dilute clay seawater suspensions typical of the sediment water interface by 60 this effect is inferred to be due mainly to bacterial attachment to and exopolymeric bridging between clay domains under nutrient poor conditions the relative change in the yield stress of a clay seawater suspension and by inference its erosion resistance is dependent on the availability of nutrients and the history of microbial attachment this conclusion applies regardless of the constitutive model used to describe the behavior of the suspension under shear our results are relevant to rheological studi s of fine particle suspensions in general because inevitable contamination of abiotic slurries often occurs under the very conditions nutrient poor that can result in maximal binding effects

may the shields theory be extended to cohesive and adhesive benthic sediments 1 a threshold criterion for incipient motion of cohesive adhesive sediments based on moment balance and dimensional considerations is here developed the criterion discriminates between single particles and flocs critical conditions and induces a modification to the traditional shields curve commonly adopted for noncohesive particles this modification is particularly effective for the smaller size particles but tending to vanish for the larger ones the proposed approach is validated on the basis of experiments performed on sediment cores sampled at different depths from seven lakes with different trophic conditions and organic matter content the incipient motion conditions are evaluated by means of an original procedure based on image analysis techniques which enables the entrainment between single particles and flocs to be distinguished the adhesion force which is mainly due to the biological activity at the sediment water interface is estimated following the proposed criterion and it is found to depend on the organic matter content in particular the cores sampled in the littoral photic zone are characterized by a bell shaped dependence between adhesion coefficient and organic matter content which is typical for indicators of biological activity of the littoral ecosystems

the influence of clay on the threshold of movement of fine sandy beds the erosion threshold i e surface shear strength of mixed sediment deposits has been tested under the action of unidirectional currents mean critical speeds 4 mm above the bed ranging from 12 4 cm s 1 to 18 2 cm s 1 or simulated waves mean critical periods ranging from 2 0 s to 6 7 s for near bed wave amplitudes of 0 28 0 57 m using an oscillating tray in a rectangular recirculating flume the sediment mixtures investigated consisted of angular fine grained quartz sands d50 152 5 and 215 m and cohesive estuarine mud the investigation has shown that the cohesive material increases generally the erosion threshold of sandy deposits the rate of increase is smaller for mud contents 30 clay mineral content 11 14 by dry weight and is attributed to an increase of the internal angle of friction when the clay mineral content exceeds 11 14 by dry weight then the sand particles are not in contact with each other consequently the clay component controls the erodibility of the sediment mixture finally a prolonged and intense stress history can increase considerably by a factor of 1 3 1 7 the original erosion threshold value of these sediments

stabilizing effects of bacterial biofilms eps penetration and redistribution of bed stability down the sediment profile biofilms consisting of microorganisms and their secreted extracellular polymeric substances eps serve as ecosystem engineers stabilizing sedimentary environments natural sediment bed provides an excellent substratum for biofilm growth the porous structure and rich nutrients allow the eps matrix to spread deeper into the bed a series of laboratory controlled experiments were conducted to investigate sediment colonization of bacillus subtilis and the penetration of eps into the sediment bed with incubation time in addition to eps accumulation on the bed surface eps also penetrated downwards however eps distribution developed strong vertical heterogeneity with a much higher content in the surface layer than in the bottom layer scanning electron microscope sem images of vertical layers also displayed different micro morphological properties of sediment eps matrix in addition colloidal and bound eps exhibited distinctive distribution patterns after the full incubation the bio sedimentary beds were eroded to test the variation of bed stability induced by biological effects this research provides an important reference for the prediction of sediment transport and hence deepens the understanding of the biologically mediated sediment system and broadens the scope of the burgeoning research field of biomorphodynamics

holocene sediment fluxes in the northeast atlantic from 230thexcess and radiocarbon measurements radiocarbon and 230thexcess data from six ne atlantic box cores are considered the cores form a transect from the porcupine abyssal plain over the east thulean rise to the southern end of feni drift the chronology for the cores is established from bulk sediment carbonate radiocarbon data and reveals that sections exhibiting constant accumulation rates can be identified in all the cores with rates of 3 0 3 5 cm kyr 1 on the plain through the holocene and late holocene rates of 4 3 6 6 cm kyr 1 elsewhere five out of the six cores show accumulations of more 230thexcess than is produced in the overlying water column with the greatest inventories up to 225 of production in the cores from the rise and drift a size fraction comparison between two cores from the plain and rise reveals that the higher overall accumulation rates and 230thexcess inventories in the off plain cores are due to an increased fine 5 m component fraction whereas the flux of coarser material is similar to that received on the plain this suggests that the higher fluxes of materials observed are physically rather than biogeochemically driven and also that drift formation has been continuously active in the late holocene sections of all the cores where regular accumulation is defined by the radiocarbon data are modeled first by a linear radiocarbon age depth model and second by a constant rain 230thexcess 0 model prorated for the observed core inventories these modeling approaches yield historical mass accumulation rate estimates which are generally in reasonable agreement 30 but the differences observed appear to be well organized in time rather than random

bedload transport of mud across a wide storm influenced ramp cenomanian turonian kaskapau formation western canada foreland basin abstract mudstone dominated shallow marine rocks of the kaskapau formation were deposited on a low gradient ramp that spanned the foredeep of the western canada foreland basin during the late cenomanian to middle turonian organic clay rich and silt rich mudstone accumulated on the flank of the forebulge 200 km from the western shoreline within this mudstone a large proportion of the clay minerals are organized into silt and very fine sand size aggregate grains these aggregates were produced both in the water column as marine snow and phytodetritus and also through reworking of previously deposited cohesive mud to form intraclasts the latter along with siliceous silt grains form wave and combined flow ripples graded beds scour fills and ripple tail lamination where sand size sediment comprising detrital siliceous calcareous bioclastic or phosphatic grains is present it is molded into combined flow ripples hcs gutter casts and lags thus all sediment grades indicate storm wave and current reworking of the sea floor at a distance of 200 km offshore the common occurrence of clay minerals in the form of aggregate grains organized into combined flow ripples and parallel lamination implies advective transport of clay minerals as bedload driven by combined flows across a very low gradient ramp on the distal part of this ramp latest cenomanian rocks include thin tongues of sw prograding quartz rich sandstone that was derived from an emergent forebulge this source was drowned during the early turonian eustatic rise when the sediment abruptly changed to organic rich mudstone dominated by clay mineral aggregates this compositional change was mainly a response to a sudden increase in distance to detrital sources in the west rather than a dramatic increase in water depth throughout much of early to middle turonian time the sea floor in the forebulge region lay above effective storm wave base for silt estimated at 70 m at sea level lowstands wave winnowing and erosion of the sea floor concentrated bioclastic lags at the top of siltier upward sequences lags are interpreted to correspond to falling stage lowstand and early transgressive systems tract deposits on the western margin of the basin previous studies may have substantially overestimated water depth for organic and clay rich calcareous mudstones in the western interior seaway

magma vesiculation and pyroclastic volcanism on venus abstract theoretical consideration of the magma vesiculation process under observed and inferred venusian surface conditions suggests that vesicles should form in basaltic melts especially if co 2 is the primary magmatic volatile however the high surface atmospheric pressure 90 bars and density on venus retard bubble coalescence and disruption sufficiently to make explosive volcanism unlikely the products of explosive volcanism fire fountains convecting eruption clouds pyroclastic flows and topography mantling deposits of ash spatter and scoria should be rare on venus and effusive eruptions should dominate the volume fraction of vesicles in basaltic rocks on venus are predicted to be less than in chemically similar rocks on earth detection of pyroclastic landforms or eruption products on venus would indicate either abnormally high volatile contents of venus magmas 2 5 4 wt or different environmental conditions e g lower atmospheric pressure in previous geologic history

predicting erosion resistance of muds abstract when measured or apparent yield stress of cohesive muds is assumed to reflect average particle particle bond strength in network structures critical bed shear stress required for grain by grain entrainment can be predicted from an analysis of the forces acting on component grains of geometrically flat mud beds erosion resistance is then defined in terms of cohesive yield stress as well as nominal particle size shape relative density and packing geometry predicted values of critical bed shear stress compare favorably with existing data from selected studies in which appropriate yield stress and hydrodynamic parameters have been reported

working with natural cohesive sediments summary of stabilizing and destabilizing biotic in uenceson natural cohesive sediment stabilizing destabilizingeps secretion extracellularpolymeric substances bybacteria and microphytobenthosenhances cohesion promotes occulation and hence deposition blistering trapping ofoxygen bubbles in bio lmsincreases buoyancy of thebio lm to such an extentthat it pulls away fromthe sediment sediment compaction burrowingmacrofauna increasesediment densityand hence stability pelletization theformationof faeces and psuedo faeces can enhanceerodibility increased drainage burrowand channel formationpromotes dewatering grazing organismsfeedingon intertidal ats causesphysical disturbance andresuspensionof the sediment andreduces the stabilizingin uence ofmicrophytobenthos network effects filamentous biotaramify through the sediment matrixbinding sediment particles together burrow cleaning somebenthic fauna cleantubes they inhabit inthe sediment giving riseto a localizedbenthic ux flow effects

saltation threshold on earth mars and venus new formulations valid for wide ranges of particle diameter and density and gas density are presented for prediction of saltation threshold speed for small particles a low air density wind tunnel was used to extend the range of previous investigations and to separate the effects of reynolds number and interparticle forces of cohesion the new formulations are used to predict saltation threshold for atmospheric conditions on the surface of the earth mars and venus

sediment segregation by biodiffusing bivalves the selective processing of sediment fractions sand and mud 63 mm and 63 mm median grain size by macrofauna was assessed using two size classes of inert uv fluorescent sediment fraction tracers luminophores the luminophores were applied to the sediment surface in 16 m 2 replicated plots defaunated and control and left to be reworked by infauna for 32 days as the macrofaunal assemblage in the ambient sediment and the control plots was dominated by the common cockle cerastoderma edule this species was used in an additional mesocosm experiment the diversity abundance and biomass of the defaunated macrobenthic assemblage did not return to control values within the experimental period both erosion threshold and bed elevation increased in the defaunated plots as a response to the absence of macrofauna and an increase in microphytobenthos growth in the absence of macrobenthos we observed an accretion of 7 mm sediment containing ca 60 mud image analysis of the vertical distribution of the different luminophore size classes showed that the cockles preferentially mobilised fine material from the sediment thereby rendering it less muddy and effectively increasing the sand mud ratio luminophore profiles and budgets of the mesocosm experiment under no waves no current conditions support the field data very well

erosion of soft cohesive sediment deposits erosion behavior of soft cohesive sediment deposits has been investigated in laboratory experiments such deposits are representative of the top active layer of estuarial beds an experimental procedure involving layer by layer erosion under a range of bed shear stresses b of successively increasing magnitude was utilized interpretation of the resulting concentrationtime data together with bed density profiles yielded a description of the variation of the bed shear strength s with depth as well as an expression for the rate of surface erosion in general s increased with depth and was also influenced by the type of sediment bed consolidation period and salinity the rate of erosion was found to vary exponentially with b s 1 2 in modeling estuarial bed erosion it is essential to take these characteristics of s and the rate of erosion into account

factors controlling critical shears for deposition and erosion of individual grains abstract we have quantified critical deposition and erosion shear stresses for individual fine 24 141 m natural quartz particles in a flow chamber with radial parallel flow in seawater 30 10 15 c critical erosion shear stresses as expected are slightly larger than estimates from previous studies which strove for cohesionless conditions while critical deposition shear stresses are three orders of magnitude larger than predicted by hjulstrom 1935 and equal to the stress on a particle settling at stokes terminal fall velocity manipulative experiments indicate that critical deposition shear stress of medium to coarse silt is little affected by physical chemical adhesion bed roughness or microbial adhesion while critical erosion shear stress is at least doubled by microbial adhesion or bed roughness critical stresses for individual grains of comparable size are normally distributed supporting the concept of well behaved thresholds for both erosion and deposition because the thresholds do not differ greatly in magnitude for particles of a given size organism produced and maintained features e g pits and tubes can be important in shifting local conditions from erosive to depositional and vice versa

which is the better proxy for paleo current strength sortable silt mean size ss or sortable silt mean grain diameter d ss a case study from the nordic seas the coarseness of the 10 63 m terrigenous silt i e sortable silt fraction tends to vary independently of sediment supply in current sorted muds in the world s oceans with coarser sediments representing relatively greater near bottom flow speeds traditionally the coarseness of this size fraction is described using an index called sortable silt mean size ss which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10 63 m terrigenous silt fraction where the relative weights of the individual size bins become increasingly disproportionate with respect to the actual number of grains within those size bins towards the coarse end of the size range this not only increases the absolute value of the apparent mean size within the 10 63 m terrigenous silt fraction but it may also affect the apparent pattern of relative changes in the coarseness of the sortable silt fraction along the core in addition it makes ss more prone to biases due to for example analytical errors here we present a detailed analysis of grain size distributions over three selected holocene time intervals from two complementary sediment cores jm97 948 2a md95 2011 extracted from the center of a high accumulation area along the flow path of the main branch of the atlantic inflow into the nordic seas and show that differential number based statistics which likely better describes variations in the actual coarseness of the sortable silt fraction may provide a more robust alternative to ss this article is protected by copyright all rights reserved

research on erosive properties of cohesive sediments to describe the transport of cohesive sediments it is necessary to specify the erosion flux at the bed experiments on the erosion of soft mud layers in a steady flow were performed in the delft tidal flume and in an annular flume the results were analyzed using the erosion rate function as derived by parchure and mehta it is concluded that a reasonable description is possible however the floc erosion rate appeared to be a function of the bed shear stress in one case the erosion rate function needed to be adjusted slightly in order to obtain agreement between measured and calculated suspension concentrations

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

effects of physical properties on erosional and yield strengths of fine grained sediments abstractdue to interparticle forces between clay particles in fine grained sediments erodibility and transport mechanisms for fine sediments are different from those for coarse sediments and the capability to predict the erosion resistance of fine sediments is still in question in this study sediment specimens with different kaolinite contents were prepared by mixing ground silica and georgia kaolin with tap water geotechnical tests were carried out to obtain the sediment physical properties the erosional and yield strengths of the specimens were determined through hydraulic flume experiments and rheometer tests respectively relationships among the erosional and yield strengths and the sediment physical properties were developed in dimensionless form erosional and yield strengths of fine sediments are predicted by sediment properties including clay size fraction and water content in addition a relationship is obtained to predict shields parameter as a function of the corresponding dimensionle