the objectives and reationale of hebble abstract hebble s precise aim is to develop and to test explicit predictions about the response of adhesive cohesive marine sediments to imposed and controlled stresses pursuit of this goal has necessitated a coordinated interdisciplinary effort to date including physical oceanographers sedimentologists radiochemists and biochemists and biological oceanographers

erosion of biofilm bound fluvial sediments sediment grains in rivers are often bound together and stabilized by bacterial films experiments and mathematical models show that sediments bound by biofilms behave like a single elastic membrane that can rip catastrophically if the river flows fast enough

critical assessment of jet erosion test methodologies for cohesive soil and sediment abstract the submerged jet erosion test jet is a commonly used technique to assess the erodibility of cohesive soil employing a linear excess shear stress equation and impinging jet theory simple numerical methods have been developed to analyze data collected using a jet to determine the critical shear stress and erodibility coefficient of soil these include the blaisdell iterative and scour depth methods and all have been organized into easy to use spreadsheet routines the analytical framework of the jet and its associated methods however are based on many assumptions that may not be satisfied in field and laboratory settings the main objective of this study is to critically assess this analytical framework and these methodologies part of this assessment is to include the effect of flow confinement on the jet the possible relationship between the derived erodibility coefficient and critical shear stress a practical tool in soil erosion assessment is examined and a review of the deficiencies in the jet methodology also is presented using a large database of jet results from the united states and data from literature it is shown that each method can generate an acceptable curve fit through the scour depth measurements as a function of time the analysis shows however that the scour depth and iterative methods may result in physically unrealistic values for the erosion parameters the effect of flow confinement of the impinging jet increases the derived critical shear stress and decreases the erodibility coefficient by a factor of 2 4 relative to unconfined flow assumption for a given critical shear stress the length of time over which scour depth data are collected also affects the calculation of erosion parameters in general there is a lack of consensus relating the derived soil erodibility coefficient to the derived critical shear stress although empirical relationships are statistically significant the calculated erodibility coefficient for a given critical shear stress has an uncertainty of several orders of magnitude this study shows that jet results should be used with caution and the magnitude of the uncertainty in the derived erodibility parameters should be carefully considered

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

a 1 d morphodynamic model for rill erosion 1 the key objective of this paper is to develop a 1 d hydrodynamic and sediment transport model namely rill1d that handles transcritical flows over abrupt changes e g formation of pool crests and width change in a single rill and also predicts changes in rill bed elevation replication of rill conditions in terms of flow and bed evolution changes necessitated the use of an enhanced tvd maccormack scheme with implementation of tseng s surface gradient method to provide an oscillation free solution over formed pool crests the model at the end of each time step calculates the flow depth velocity and bed shear stress and provides changes in bed elevation and grain size distribution within a cell the hydrodynamic performance of rill1d is evaluated by comparing its results with either analytical solutions or experimental observations from various benchmark tests with rigid bed conditions that include 1 formation of a hydraulic jump in a rectangular channel 2 steady subcritical and transcritical flow without a shock over a hump 3 formation of a hydraulic jump in a converging diverging frictionless channel and 4 flows over step pools it is shown that the enhanced tvd maccormack scheme adequately simulates transcritical flows by preserving the mass conservation and reducing the artificial numerical error the scheme also approximates satisfactorily depth and velocity for a single rill with a fixed bed consisting of steps and pools except near the pool headwalls where a backroller forms with negative velocities the sediment component of rill1d is evaluated by two field investigations of single rills with mobile beds with the field data the code is tested for its ability to reproduce measured values of sediment transport rates a sensitivity analysis is performed to assess the effects of cell size and critical erosional strength in the predictive ability of the model rill1d performs reasonably well in these simulations in terms of sediment prediction rates and fared adequately in terms of replicating rill bed morphology

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

constraining core undertones using stacked ida gravity records a search is made for oscillatory gravity signals possibly associated with core undertones in the frequency range between the diurnal and semidiurnal tidal bands ida tidal data recorded during 6 month intervals following four large earthquakes were used and the data from different stations were stacked according to spherical harmonic surface amplitude patterns with angular order l 4 the detection level of a signal in the diurnal semidiurnal frequency band was roughly 10 20 ngal but the only signal detected was a 9 ngal signal of 9 54 hr period whose origin remains unexplained

erosion and delivery of deposited peat sediment 1 erosion of deposited organic peat sediments from peat harvesting ditches was studied in both laboratory and field conditions peat sediments were placed in a recirculation flume at controlled ph levels and allowed to stabilize from 15 min to 10 d afterward different flow velocities were applied and the peat erosion was measured the erosion rate was related to the physical properties of peat the particle size and form settling velocity peat type and degree of decomposition von post the mean average settling velocity for the peat was measured to be 0 87 m h 1 the median sediment particle size was 42 m which indicates that the peat sediment is partly cohesive the deposited sediment formed a loose layer overlaid by more stabilized layers critical shear stress for the surface of the loose peat layer was 0 01 n m 2 and for the more consolidated layer was 0 059 n m 2 after a stabilization of three days the erosion coefficients determined during laboratory study varied between 3 7 10 4 and 4 1 10 6 kg m 2 s 1 the determined erosion and sedimentation rates were used to calibrate sediment erosion and deposition models a new approach was then developed for the uses of these models to simulate peat sediment delivery from field ditches the model was tested against measured values which were in accordance to observed sediment transport rates

the structure of the inner core inferred from short period and broadband gdsn data summary broadband displacement and velocity records of pkp phases are formed by the simultaneous deconvolution of the instrument responses of waveforms recorded by the short and long period channels of the global digital seismograph network revisions of standard earth models are inferred from the comparison of synthetic and observed waveforms the synthetics incorporate a q model designed for the bandwidth of the data and a source model that accounts for the variations of waveform about the focal sphere due to the effects of finiteness and complexity of the rupture process the interpretation of the data waveforms allows the position of the d cusp to be fixed at 121 f 1 and the c cusp at 154 f 2 for a surface source the cusp positions are consistent with a p velocity of 10 82 f 0 02 km s on the underside of an inner core boundary at 1216 km and a p velocity of 10 30 f 0 05 kms on its upper side resulting in a p velocity jump of aa 0 52 k 0 07 km s based on the agreement of synthetic and observed waveforms that have been diffracted beyond the ccusp there is no zone of small or negative p velocity gradient or of strong anelasticity at the bottom of the outer core second order features in the observed waveforms near the d cusp suggest that the s velocity may be nearly zero at the top of the inner core q must increase with depth in the inner core in order to satisfy the pulse width of the pkp df phase at distances greater than 150 in order to satisfy average properties of the inner core inferred from travel times and normal modes these combined results require strong gradients in p velocity s velocity and anelasticity in the upper 200 300 km of the inner core

erosion of the upper layer of cohesive sediments characterization of some properties a recent companion paper reported an experimental protocol used to analyze sediment properties this protocol identified for both freshwater and marine sediments a surface layer with specific dynamic properties critical erosion shear stresses in the range 0 025 0 05 n m 2 and a second layer with critical erosion shear stresses about ten times larger the present study compares these former results with recent work which extended the applicability domain of the shields diagram to very fine particles the surface layer is shown to consist in fine and unconsolidated sediments that behave like noncohesive material whereas the second layer is characterized as being cohesive the surface layer is mainly representative of recent deposits of suspended particles this points out the existence of a fluffy layer of fine sized particles resting near the bed with specific erosion characteristics which has to be considered separately when studying sediment properties

influence of sediment structure on erosional strength and density of kaolinite sediment beds the role of sediment pore water chemistry and the resulting particle structure in determining the erosional stability of settled cohesive sediment beds in rivers lakes and estuaries is examined kaolinite sediment is used as the surrogate sediment in this experimental investigation with the beds settled from concentrated suspensions the bed stability with respect to erosion or resuspension is measured in a laboratory flume as a function of sediment pore water chemistry the chemical properties varied are sediment ph ionic strength and natural organic matter the remolded bed sample is prepared from a sediment suspension having controlled chemical properties that is allowed to settle into the flume bed where its erosional strength and density are determined with depth in the sample different structures of settled beds are observed with changes in chemical parameters under low ph and low organic content conditions the initial suspension before settling is flocculated the resulting settled beds show strong stratification with respect to erosional strength but weak stratification of bulk density with depth on the other hand under high ph or high organic content conditions at low ionic strength the initial suspension is dispersed the resulting settled beds have lower erosional strength and weak stratification of erosional strength with depth but strong stratification of bulk density with depth this research shows that the relationship between erosional strength and bulk density of a settled bed depends strongly on the structure of the sediment particle associations as determined by pore water chemistry

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

source parameters of the 8 october 2005 mw7 6 kashmir earthquake during the last six years the national geophysical research institute hyderabad has established a semi permanent seismological network of 5 broadband seismographs and 10 accelerographs in the kachchh seismic zone gujarat with the prime objective to monitor the continued aftershock activity of the 2001 mw7 7 bhuj mainshock the reliable and accurate broadband data for the mw 7 6 8 oct 2005 kashmir earthquake and its aftershocks from this network as well as from the hyderabad geoscope station enabled us to estimate the group velocity dispersion characteristics and the one dimensional regional shear velocity structure of peninsular india firstly we measure rayleigh and love wave group velocity dispersion curves in the range of 8 to 35 sec and invert these curves to estimate the crustal and upper mantle structure below the western part of peninsular india our best model suggests a two layered crust the upper crust is 13 8 km thick with a shear velocity vs of 3 2 km s the corresponding values for the lower crust are 24 9 km and 3 7 km sec the shear velocity for the upper mantle is found to be 4 65 km sec based on this structure we perform a moment tensor mt inversion of the bandpass 0 05 0 02 hz filtered seismograms of the kashmir earthquake the best fit is obtained for a source located at a depth of 30 km with a seismic moment mo of 1 6 1027 dyne cm and a focal mechanism with strike 19 5 dip 42 and rake 167 the long period magnitude ma mw of this earthquake is estimated to be 7 31 an analysis of well developed spn and ssn regional crustal phases from the bandpassed 0 02 0 25 hz seismograms of this earthquake at four stations in kachchh suggests a focal depth of 30 8 km

biological and granulometric controls on sedimentary organic matter of an intertidal mudflat abstract a variety of measures of organic matter concentration and quality were made on samples collected from the top few mm of intertidal mudflat sediment over the course of a year in order to assess the relative importance of biological and sedimentological influences on sedimentary organic matter winter and summer were times of relatively fine grained sediment accumulation caused by biological deposition or stabilization processes and resulting in higher organic matter concentrations stable carbon isotope and br c ratios indicated a planktonic source of bulk organic matter ratios of organic carbon to specific surface area of the sediments were consistent with an organic monolayer coverage of sediment grains correction for changing grain size during the year showed no change in the organic concentration per unit surface area in spite of organic matter inputs by in situ primary production buildup of heterotroph biomass and mucus coatings and biodeposition of organic rich seston there were also no indications of changes in bulk organic quality measured as hydrolyzable carbohydrates and amino acids in response to these biological processes it is concluded that biological processes on a seasonal time scale affect the bulk organic matter of these sediments via a modulation of grain size rather than creation or decay of organic matter

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

relationships between physical geochemical soil properties and erodibility of streambanks among different physiographic provinces of tennessee usa erosion of cohesive soils in fluvial environments is dependent on physical geochemical and biological properties which govern inter particle attraction forces and control detachment rates from stream beds and banks most erosion rate models are based on the excess shear stress equation where the soil erodibility coefficient kd is multiplied by the difference between the boundary hydraulic shear stress b and the soil critical shear stress c both kd and c are a function of soil properties and must be obtained through in situ field or laboratory testing many studies have generated predictive relationships for kd and c derived from various soil properties these studies typically were conducted in watersheds within a single physiographic region with a common surficial geology and or investigated a limited number of soil properties particularly geochemical properties with widely reported differences in relationships between c and soil properties this study investigated differences in predictive relationships for c among different physiographic provinces in tennessee usa erodibility parameters were determined in the field using a mini jet test device among these provinces statistically four unique clusters were identified from a dataset of 128 sample sites and these data clusters were used to develop predictive models for c to identify dominant properties governing erosion in these clusters 16 significant physical and geochemical soil properties were identified for c prediction among these soil properties water content and passing 200 sieve soil less than 75 m were the dominant controlling prameters to predict c in addition to clay 2 m bulk density and soil pore water chemistry this study suggests that unique relationships exist for physiographic provinces that are likely due to soil physiochemical processes associated with surficial geology that determine minerology properties of the cohesive soil

a temporal and spatial study of mudflat erosion and deposition sediment elevations were measured on an intertidal mudflat in maine to determine the temporal and spatial changes in sediment volume the winter and spring months were times of greatest sediment change with erosion in winter months followed by rapid accretion in the spring in some months erosion and deposition took place during the same time interval but on different parts of the mudflat the data suggest that areas of erosion quickly become sites of accretion indicating that an equilibrium surface level is being maintained mudflats may strongly influence the adjacent estuary by acting as sediment banks that can periodically lend sediments to the particulate pool offshore we conclude that mudflats should not be thought of as steady accretionary environments but rather as more variable environments that respond to seasonal changes

data interpretation for in situ measurements of cohesive sediment erosion methods for interpreting data from in situ flume measurements of cohesive sediment dynamics are evaluated and a technique for estimating various erosion parameters using in situ measurements is proposed there is currently a lack of uniformity in analysis techniques for cohesive erosion data collected in flumes and with in situ instruments and the proposed technique resolves some of these inconsistencies the data set used in this study was derived from field experiments conducted with a straight benthic in situ flume in different aquatic environments in new zealand the experiments with stepwise increases in flow velocity revealed that peaks in the erosion rate at the beginning of each velocity step are most likely associated with heterogeneous bed structure as transient hydrodynamic effects due to the experimental procedure were found to be insignificant the field data showed an exponential decay of the erosion rate with time that is indicative of depth limited erosion these data are used to illustrate methods for the parameterization of the proposed semiempirical erosion equation taking into account the time dependency of the erosion process

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

observation of seismic core modes from a superconducting gravimeter record abstract there have been few reliable reports of observation of seismic core modes in spite of their geophysical importance in this study we analyze low frequency seismic signals from a superconducting gravimeter scg located at the esashi earth tides station japan to make observations of core modes spectral analysis of an scg record from the great macquarie ridge earthquake which occurred during the period of our test runs of a scg by means of the sompi method has yielded well resolved signals which are most likely to be the core modes 2s2 and 6s2 close investigation of the analysis results however has revealed that there may be systematic bias in their q estimates due to some methodological problems our tentative conclusion is that the q values of the core modes observed here are relatively low probably not exceeding 1000 and also that their amplitudes are larger than theoretically predicted from a spherically symmetric earth model at any rate the present results demonstrate that the scg also has excellent potential as a long period seismometer

influence of an infaunal bivalve on the erosion of an intertidal cohesive sediment a flume and modeling study a model was developed to describe the erosion of a cohesive sediment within an annular flume the model relates the rate of resuspension of sediment per unit area to the difference between the amount of sediment available for resuspension from the bed and the amount already resuspended into the water column the rate of erosion also depends on the excess free stream near bed current velocity above a critical threshold velocity the basic five parameter model describes 96 of the variation in suspended sediment concentration within the flume where natural sediment is exposed to increments in current velocity up to 43 cm s 1 equivalent to a shear stress of 1 5 pa in the absence of macrofauna the addition of a small clam macoma balthica to the experimental system led to increased levels of resuspension at all velocities this effect increased asymptotically with macoma population density and could be modeled using two additional parameters complete analysis of the resulting seven parameter model showed that the effect of the fauna was confined solely to the amount of sediment available for resuspension field data supported the laboratory determined functional relationship between macoma density and the quantity of sediment resuspended at environmentally realistic maximum current velocities and provided field based parameter estimates for modeling the erosion of in situ sediment using predicted flood and ebb tidal current velocities along a 3 5 km transect normal to the shore we determined the potential impact of macoma on the amount of sediment resuspended over a single tidal inundation we estimate that natural densities of macoma increase the amount of sediment resuspended at our sites above midtidal shore level by 0 42 kg m 2 per tide we suggest that experimental flume studies which can include field deployments combined with this parametersparse model of fauna1 impact on sediment resuspension provide a straightforward means of quantifying the complex effects of biotic components on rates of sediment erosion

biogenic structure of early sediment fabric visualized by low temperature scanning electron microscopy the accurate visualization of early biogenic sediment fabric is important in the search for modern analogues to relict fossil structure and therefore for the interpretation of past environmental conditions water may have a significant structural role within hydrated material and sample dehydration may be detrimental to the structural interpretation of natural sediments and their associated microbiota water is present in sediments as free water or in the form of bound or hydration water extracellular polymeric substances associated with microbial assemblages have a high content of hydration water and are distorted by dehydration the use of low temperature scanning electron microscopy for the visualization of water saturated sediments and associated biogenic structures is examined low temperature sem allows samples to be examined without the removal of water that is normally associated with the preparatory procedures for traditional ambient sem samples for low temperature sem are first frozen cryofixed and then examined fully hydrated and still frozen at low temperature cf 180 c on the stage of a specially adapted sem the process of low temperature sem for the examination of hydrated sediments is described and several examples of cohesive and mixed flat sediments and their associated biogenic fabrics are given detail of the internal fabric and associated microbiota can be examined by freeze fracture techniques to preserve the natural condition of the sediments

incorporating ecogeomorphic feedbacks to better understand resiliency in streams a review and directions forward decades of interdisciplinary research show river form and function depends on interactions between the living and nonliving world but a dominant paradigm underlying ecogeomorphic work consists of a top down unidirectional approach with abiotic forces driving biotic systems stream form and location within the stream network does dictate the habitat and resources available for organisms and overall community structure yet this traditional hierarchal framework on its own is inadequate in communicating information regarding the influence of biological systems on fluvial geomorphology that lead to changes in channel morphology sediment cycling and system scale functions e g sediment yield biogeochemical nutrient cycling substantial evidence that organisms influence fluvial geomorphology exists specifically the ability of aquatic vegetation and lotic animals to modify flow velocities and sediment deposition and transport thus challenging the traditional hierarchal framework researchers recognize the need for ecogeomorphic frameworks that conceptualize feedbacks between organisms sediment transport and geomorphic structure furthermore vital ecosystem processes such as biogeochemical nutrient cycling represent the conversations that are occurring between geomorphological and biological systems here we review and synthesize selected case studies highlighting the role organisms play in moderating geomorphic processes and likely interact with these processes to have an impact on an essential ecosystem process biogeochemical nutrient recycling we explore whether biophysical interactions can provide information essential to improving predictions of system scale river functions specifically sediment transport and biogeochemical cycling and discuss tools used to study these interactions we suggest that current conceptual frameworks should acknowledge that hydrologic geomorphologic and ecologic processes operate on different temporal scales generating bidirectional feedback loops over space and time hydro and geomorphologic processes operating episodically during bankfull conditions influence ecological processes e g biogeochemical cycling occurring over longer time periods during base flow conditions this ecological activity generates the antecedent conditions that influence the hydro and geomorphologic processes occurring during the next high flow event creating a bidirectional feedback this feedback should enhance the resiliency of fluvial landforms and ecosystem processes allowing physical and biological processes to pull and push against each other over time

effect of a global plume distribution on earth normal modes we show that a global distribution of small scale heterogeneities in the lower mantle induces both frequency splitting and amplitude scatter of low angular order modes these effects can be used in order to map the global distribution of small scale structures in the mantle such as plumes two independent datasets consisting respectively of lower mantle modes interaction coefficients and of observed amplitudes of core modes can then be used to constrain the low angular degrees of the spatial distribution and the mean size of heterogeneities allowing us to constrain the mantle structure at different scales if these heterogeneities are assumed to be plumes associated with hotspot activity the mean size of plumes obtained using the two datasets is then in good agreement with that obtained from geodetical or numerical convection studies

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

summary of seismological constraints on the structure of the earth s core we review the current state of the seismological constraints on core structure the evidence supports a simple two layer model of the core the fluid outer core seems to have properties which are consistent with those of a vigorously convecting region the inner core has a sharp boundary characterised by a 0 5 0 6 g cc density jump a 0 6 0 7 km s jump in p velocity and a jump in shear velocity to 2 5 km s or greater there is weak evidence for an anomalously steep shear velocity gradient at the top of the inner core finally the attenuation of both body waves and free oscillations indicate that the inner core is strongly attenuating near its surface though the data suggest that the attenuation is frequency dependent

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

the deposition and erosion of cohesive sediments determined by a multi class model sediment properties were studied experimentally using a laboratory flume and sediments sampled from both fresh and saltwater systems i e the rhone river and the adjacent continental shelf golfe du lion mediterranean sea an experimental protocol is proposed which includes the development of an analytical method based on a multi class model in order to determine the erosion and deposition fluxes of these sediments this approach highlights the presence of a fluff layer characterized by a very low critical erosion shear stress and a second layer with a significantly larger critical erosion shear stress if a global model is used i e only one class of particles is considered this distinction between the two layers cannot be made as sediment parameters are identified for the whole eroded particles including the fluff layer critical shear stresses of erosion were 0 05 0 04 and 0 025 n m 2 respectively for the first class of particles eroded at rhone 1 rhone 2 and sofi stations the second class of particles showed critical shear stress 10 20 times higher around 0 20 n m 2 the use of the multi class model allowed estimation of the erosion rates in the order of 2 5 10 6 6 2 10 5 and 2 10 6 kg m 2 s 1 for rhone 1 rhone 2 and sofi settling velocities were in the order of 0 012 0 045 mm s 1 for the first class and between 0 05 and 0 09 mm s 1 for the second class for the river experiments it is shown that the fluff layer corresponding to the first class of particles being eroded is mainly representative of recent deposits of suspended particles probably trapped in the overlying water column during sediment sampling this highlights the difficulties in sampling sediments and transporting these from field to lab and more generally into flumes and the limits of applying a global model to flume experiments when studying sediment properties

study of clay sand network structures and its effect on river bank erosion an experimental approach evolution of the riverbank morphology due to the removal and collapse of bank sediment results in substantial loss of fertile land and human property at the lower stretch of a perennial river clay sand mixed bank sediments are predominant existing literature reveals that large shear stresses are generated due to the interaction between the hydrodynamic forces and bank roughness this may lead to the dislodgement of the sediment particles from the bank surface the generation of shear stress of a turbulent flow depends on the scales of turbulent fluctuations and the clay sand network structures size shape and arrangement of clay and sand particles however information on the modulation of turbulent flow structures due to the variation of clay sand network structures of the bank sediment remains unrevealed in the present experimental investigation an attempt is undertaken to study the effect of sediment surface roughness and the associated turbulent scales of the interacting flow this is achieved by fabricating artificial banks of five different proportions of clay in the clay sand mixture in a laboratory flume scanning electron microscope sem and 3d profilometer measured the surface roughness parameters of clay sand network structures for different clay proportions information on near bank turbulence characteristics was quantified using a 16 mhz acoustic doppler velocimeter adv results from the present study show that the surface roughness of bank sediment decreases with increasing clay fraction in the clay sand network structure of the bank sediment which also lowers the associated turbulent velocity fluctuations or eddy scales of the flow furthermore small scale turbulent fluctuations decrease the sediment entrainment rate from the bank face

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

influence of cohesion on the incipient motion condition of sediment mixtures 1 previous investigations on initiation of motion and erosion of cohesive sediment mixtures have not included gravel within the mixtures experimental results on the incipient motion characteristics of cohesive sediment mixtures containing gravel are presented in this paper the sediment used consisted of fine gravel mixed with clay in proportions varying from 10 to 50 by weight and fine gravel with fine sand mixed in equal proportion with clay proportions again varying from 10 to 50 by weight the stages of the incipient condition of motion were visually identified three modes of initiation of motion of cohesive sediment mixtures were noticed namely pothole erosion line erosion and mass erosion the modes of initiation of motion changed mainly with clay percentages in the mixture its antecedent moisture characteristics and the applied shear stress analysis of the experimental data from the present study and the literature revealed that depending upon the flow and sediment characteristics the critical shear stress of the incipient motion condition in cohesive sediment mixtures can be up to 50 times larger than the critical shear stress of cohesionless sediments having similar arithmetic mean size as the cohesive sediments on the basis of dimensional considerations a relationship is proposed for the determination of critical shear stress of the incipient motion condition in cohesive sediment mixtures also containing gravel the variables namely clay percentage void ratio and unconfined compressive strength of the sediment bed are noticed to be the main parameters controlling the incipient motion condition of the cohesive sediments in the absence of cohesion the proposed relationship reduces to the critical shear stress for the incipient motion condition of the cohesionless sediment mixture having the same arithmetic mean size the condition for the threshold of erosion of cohesive sediment mixtures by the flow is thus parameterized in the present study

structure of the inner core inferred from observations of its spheroidal shear modes spheroidal modes of elastic gravitational oscillation that have an energy partition dominated by shear energy in the inner core are called core modes recordings from the ida gravimeter network of an earthquake in the tonga region on june 22 1977 have provided the first unambiguous observations of core modes they conclusively show that at low frequencies the inner core has a very high q about 3500 they also tightly constrain the ratio of the inner core radius to the mean shear velocity to be 345s with an uncertainty of 0 5

reverse engineering mother nature shale sedimentology from an experimental perspective abstract experimental study of the sedimentology of shales can take a variety of forms at its simplest one can experiment with suspensions in a glass jar and try to understand their settling behavior or one can manipulate mud in a tank or bucket to gain insights into its rheology this approach was championed over a century ago by sorby and the insights gained can be quite profound more recently tank and settling tube experiments of animal sediment interactions compaction behavior and sediment unmixing via re suspension have proven to be highly informative in spite of their simplicity flumes can be used to obtain quantitative information about depositional and erosional parameters and to generate fundamental bedforms in flume experiments however it is of critical importance that the flume be designed in a way that flocculated materials move under shear stress conditions that would be reasonable in natural environments although much flume work on muds has been conducted by hydraulic engineers the transfer of that knowledge to sedimentology is hampered by the fact that engineers and sedimentologists are interested in different though not mutually exclusive products from such experiments engineers and hydrologists are commonly concerned with quantifying fluid flow properties whereas sedimentogists are particularly interested in the sedimentary products that result from a variety of flow conditions recent sedimentologically oriented flume studies have shown that muds can form deposits at flow velocities and shear stresses that would suffice to transport and deposit medium grained sand mud suspensions are prone to flocculation and the resulting floccules travel in bedload and form ripples that accrete into beds the latter finding suggests that many laminated shales were deposited from currents rather than by settling from slow moving or still water there are many other sedimentary features in shales that can potentially be reproduced in flume studies and in the future serve to provide a quantitative basis for shale sedimentology

use of in situ flume to quantify particle flux biodeposition rates and sediment erosion for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna