modelling of cohesive sediment transport in a tidal lagoon an annual budget abstract the annual net sediment flux for an enclosed estuary is usually calculated as a residual value in sediment budget studies based mainly on sediment core dating in this study the numerical model mike 21 mt was used to directly model the annual net transport of cohesive sediment for the lister dyb tidal area in the northern part of the european wadden sea the model was calibrated by use of a combination of field data and calibration parameters derived from existing literature the model reproduced the hydrodynamics satisfactorily as well as the concentration level and variation of suspended cohesive sediment concentration net import for the year 2002 was found to be 45 000 t of cohesive sediment corresponding to a mean tidal period import of 64 t which compares to an approximate deposition of 0 1 mg l 1 of the tidal prism gross annual transport through a control cross section of the tidal inlet was 1 million ton meaning that 4 5 of the sediment transported in suspension was on average deposited within the tidal lagoon this net transport was found to be rather constant from tidal period to tidal period thus supporting the theory that the processes of settling and scour lag tidal velocity asymmetry together with sediment floc formation are the controlling processes for import of fine grained sediment to shallow coastal plain estuaries temporary export of sediment from the area occurred during two major wind events in january and february when the maximum loss of sediment during one single tidal period was computed to be 40 000 t which is of the same order as the yearly net import of sediment this indicates that net annual import of cohesive sediment to shallow estuaries may be much more variable than inferred from core dating studies found in literature

role of flocculation and settling processes in development of the mangrove colonized amazon influenced mud bank coast of south america abstract the guianas coast downdrift northwestward of the amazon river mouth is characterized by the periodic migration of mud banks that originate from the amazon the characteristic sizes of these banks as well as their rates of alongshore migration have been estimated from remote sensing however the physical mechanisms leading to their displacement are not yet fully understood the present work is aimed at investigating a number of micro scale processes involved in the coastal and estuarine dynamics of fine sediments and expected to occur during the migration of mud banks the relative magnitudes of flocculation hindered settling and consolidation have been determined sampling of coastal mud was carried out during a field survey in 2001 that focussed on the fluid mud layer in the leading edge of a mud bank in french guiana settling column experiments were conducted under quiescent conditions for various mean sediment concentrations in the range of 2 5 110 g l 1 which is typical of mud bank concentrations the time dependent vertical profiles of suspended sediment concentration were monitored using an optical settling tank equipped with 32 pre calibrated optical sensors the corresponding settling velocities were deduced from the equation of the conservation of mass the results show that the timescales of hindered settling and consolidation processes are much larger than the timescales of mixing mechanisms such as tides or propagating waves the individual floc settling velocities are too small to counterbalance the turbulent mixing induced by breaking waves hindered settling favoured by wave action and by the high background suspended sediment concentrations is thus an overarching process in wave driven mud bank migration this pervasive hindered settling regime is characteristic of the wave exposed outer and leading edges of mud banks where active mobilization of mud assures mud bank migration as a result consolidation is theoretically precluded the experiments pointed out however that additional flocculation and differential settling should enhance sedimentation during slack water and under low wave conditions we deduce from this that enhanced settling in the inner subtidal intertidal parts of leading edges of banks during such conditions is important in the temporary sedimentation that generates gel like fluid mud patches and mud bars these low energy inner leading parts of banks form an accreted substrate colonized by mangroves which further contribute to enhanced settling and rapid sedimentation

instruments for particle size and settling velocity observations in sediment transport in this paper we describe two sensors for measurement of particle size distribution and settling velocity distribution these measurements are critical to the correct estimation of the true sediment concentration in the field as well as to evaluating models for transport rates of sediments a multi angle measurement of laser scattering is made and inverted to obtain the particle size distribution since small angle scattering is relatively insensitive to particle composition the size distribution measurements are robust and do not require particle refractive index it is shown that with a knowledge of the size distribution true particulate volume concentration can be obtained unaffected in calibration by changes in particle size distribution the data from bottom boundary layer experiments using the instrument show the presence of temporal variability in size distribution associated with the strength of forcing of the boundary layer the importance of these observations lies in the implication that historical data acquired with single parameter optical or other sensors needs to be revisited in the second instrument analyzing the observation of size distributions during settling in a settling column produces settling velocity estimates in this case the history of concentration of each size class is examined to determine the settling velocity without invoking any assumptions of settling regime settling velocity data from a field experiment off the new jersey coast fit the model wf na 0 45 10 23 a 1 2 n

can the morphology of deep sea benthic foraminifera reveal what caused their extinction during the mid pleistocene climate transition abstract over 100 cosmopolitan species of deep sea benthic foraminifera extinction group ext gp became extinct during the late pliocene middle pleistocene 3 6 0 55 ma most had elongate cylindrical tests and terminal apertures with complex modifications this study provides new hypotheses on the functions of the morphologies that characterised the ext gp and how these features could have been associated with their demise from our functional morphological analysis we infer that i their elongate cylindrical or flabelliform tests combined with fine perforations and a complex terminal apertural face are indicative of infaunal k strategists with a low rate of metabolism and ii their complex apertural faces may also have been an adaptation for gathering or processing their specific phytodetrital food we propose three alternative hypotheses for the cause of these extinctions and where possible test them using our high resolution micropaleontological and geochemical record through the last 1 07 ma in lower bathyal site md 97 2114 in the sw pacific ocean hypothesis 1 is that the ext gp species were unable to adapt to increased variability in the overall quantity or pulsed seasonality of the food supply to the sea floor and were out competed by opportunistic r strategist benthic foraminifera this is supported by the highly variable and increasing abundance of opportunistic foraminifera at our study site during the final phase of the extinction in the mid pleistocene climate transition mpt we doubt however that there was increased variability in phytoplankton productivity throughout the world s oceans sufficient to bring about the global demise of the ext gp hypothesis 2 is that lowered pco2 during increasingly severe mpt glacials which coincided with the final phases of the extinction may have caused the decline and possible loss of the ext gp s phytoplankton food source declining pco2 during neogene cooling was coeval with declining relative abundance of the ext gp and reticulofenestrid nannofossils but the final demise of this latter phytoplankton group occurred slightly later than the mpt in our study site and cannot be implicated with the extinction if this hypothesis has any validity maybe the phytoplankton group left no fossil record our third alternative hypothesis is that maybe our ext gp had much common dna which made them the selective target of pathogens that caused their extinction this does not easily explain their earlier disappearance at abyssal depths than at bathyal depths in our study region which can be accommodated by hypotheses 1 and 2

factors controlling the field settling velocity of cohesive sediment in estuaries abstract it has long been recognized that the suspended sediment concentration ssc is one of the major determinants for the flocculation of cohesive particles into sediment flocs in estuaries it is furthermore well known that the turbulent shear of the water significantly influences the flocculation process and the equilibrium settling velocity of flocculated sediment in a turbulent flow a vast number of authors have reported algorithms relating the median settling velocity w 50 to suspended sediment concentration however only a few studies have dealt with the impact of the turbulent shear in this paper expressed as the root mean square rms velocity gradient g in the water on the w 50 in situ there is a strong need to establish algorithms based on in situ measurements describing the dual impact of both ssc and g on the flocculation process and hence w 50 the present paper addresses this topic field settling velocities of suspended cohesive sediment have been measured in micro meso and macro tidal estuaries regression analyses between the w 50 ssc and g are presented it is shown that by including both g and ssc in the regression analyses a significant increase in the correlation of the description of w 50 and the controlling parameters from each area can be obtained a generic algorithm describing the data from all the investigated areas is suggested it works well within specific tidal areas but fails to give a generic description of the field settling velocity

a look at the date of snowmelt and correlations with the arctic oscillation spring snow cover across arctic lands has on average retreated 5 days earlier since the late 1980s compared to the previous 20 years however it appears that since about the late 1980s the date the snowline first retreats north during the spring has changed only slightly in the last 20 years snow has not been disappearing significantly earlier snowmelt changes observed since the late 1980s have been step like unlike the more continuous downward trend seen in arctic sea ice extent at 70 n several longitudinal segments of 10 show significant negative trends while only two longitudinal segments at 60 n show significant trends one positive and one negative these variations appear to be related to variations in the arctic oscillation ao when the springtime ao is strongly positive snow melts earlier when it is strongly negative snow disappears later in the spring the winter ao is less straightforward at higher latitudes 70 n a positive ao during the winter months is correlated with later snowmelt but at lower latitudes 50 n and 60 n a positive wintertime ao is correlated with earlier snowmelt if the ao during the winter months is negative the reverse is true similar stepwise changes since the late 1980s have been noted in sea surface temperatures and in phytoplankton abundance as well as in snow cover

model based interpretation of sediment concentration and vertical flux measurements in a shallow estuarine environment a one dimensional numerical model describing tidally varying vertical mixing and settling was used to interpret sediment concentrations and vertical fluxes observed in the shoals of south san francisco bay by two acoustic doppler velocimeters advs at elevations of 0 36 m and 0 72 m above bed measured sediment concentrations changed by up to 100 g m 3 over the semidiurnal tidal cycle these dynamics were dominated by local resuspension and settling multiple particle class models suggested the existence of a class with fast settling velocities ws of 9 0 10 4 m s 1 in spring and 5 8 10 4 m s 1 in fall and a slowly settling particle fraction ws of 1 10 7 m s 1 in spring and 1 4 10 5 m s 1 in fall modeled concentrations of slowly settling particles at 0 36 m were as high as 20 g m 3 during fall and varied with the spring neap cycle while fine sediment concentrations in spring were constant around 5 g m 3 analysis of in situ water column floc size distributions suggested that floc properties in the lower part of the water column were most likely governed by particle size distribution on the bed and not by coagulation validating our multiple particle size approach a comparison of different sediment bed models with respect to model performance sensitivity and identifiability suggested that the use of a sediment erosion model linear in bottom shear stress b e m b c was the most appropriate choice to describe the field observations when the critical shear stress c and the proportionality factor m were kept constant

a simple method for calculating in situ floc settling velocities based on effective density functions abstract we present a simple method to obtain settling velocity estimates of fine grained suspended sediment using the following 1 in situ floc volume fraction measurements 2 calculations of in situ floc mass 3 measurements of in situ floc size 4 laboratory measurements of primary particle size and 5 laboratory measurements of organic matter obtained through loss on ignition this method is based on the mass distribution over the particle size spectra floc properties are incorporated by using information concerning the primary particles and the ratio between the volume concentration and the mass concentration of the in situ measurements in this manner the changes in effective densities caused by flocculation and biological mediation are approximated this method relies solely on measurements conducted with a particle sizing instrument and on an analysis of water samples the method is applied to measurements obtained with a lisst and data from three different estuaries in different climatic zones are used the settling velocity estimates found using this method are compared to settling tube measurements and to mass settling method estimates as well as to calculations using fractal dimension methods from the literature settling velocities calculated using the method presented in this study lie within the ranges of the three other methods demonstrating that this simple method produces results within the ranges of settling tube velocities and those predicted by the more comprehensive multiparameter fractal dimension models the presented method allows for easy intercomparisons between studies by utilising instruments such as the lisst and it is believed that this method would also produce reliable settling velocities using other instruments that yield a volume distribution as well as a proxy for total volume and mass concentration e g some camera systems

the role of algae in fine sediment flocculation in situ and laboratory measurements abstract the precise interactions between organic and inorganic particles in the context of flocculation is an on going topic of research the suspended particulate matter spm found in estuaries is composed of both organic and inorganic particles with specific particle size distributions psd s these psd s are a function of the hydrodynamic conditions suspended sediment concentration ssc organic matter composition salinity and seasonal variations a field campaign was carried out in august 2015 in the turbidity maximum zone of the yangtze estuary where the spm dynamics were recorded the concentration of algae in the water column was indirectly measured through the chlorophyll a concentration cc we show that there is a strong correlation between ssc and cc in the whole water column for the whole tidal cycle additional flocculation experiments in the laboratory confirm that the largest observed flocs are predominantly organic based and that salinity alone could not induce the flocculation of the yangtze mineral particles a key parameter for the maximal floc size is the algae concentration to sediment concentration ratio when this ratio is high the d50 is high and vice versa

morphological responses of an estuarine intertidal mudflat to constructions since 1978 to 2005 the seine estuary france abstract since 1834 the seine estuary france has been the site of numerous construction projects with the aim to accommodate and secure boat traffic since 1978 the increasing of the activities of le havre port located at the mouth of the estuary has accelerated the construction work rate several dykes a bridge and new port facilities have been constructed in rapid succession modifying considerably the hydrodynamic conditions which sustain a partially vegetated sandy muddy tidal flat located in the north bank of the estuary between the new port of le havre and the normandy bridge achieved in 1995 the present study deals with the morphological evolution of this zone from 1978 to 2005 the use of a low altitude remote sensing technique combined with traditional methods of ground survey and probes allow to demonstrate the impact of human activities on sedimentary and vegetation dynamics the northern mudflat of the estuary is the most affected by these human activities which surface have reduced of 62 during the last 27 years with an intensified local erosion during the last 27 months corresponding to a loss 1 250 000 m 3 of fine grained sediment at the same time the general sanding up in the channel of the zone has caused a loss of more than 31 of the tidal prism more than three quarters of which occurred during the last three years results also establish that the response times of the sedimentary or topographic readjustment to an installation depend on the extent and the nature of the construction in fact the sedimentary readjustments to an installation can be delayed by up to 10 years in the case of the seine estuary this delay is explained by a rapid succession of construction works which may occult the effect of a single installation except in the case of a dyke built perpendicular to ebb and flood currents the impacts of these installations reach a hydro sedimentary equilibrium on the level between 1 and 7 years after their completion

morphodynamic evolution of an intertidal mudflat under the influence of amazon sediment supply kourou mud bank french guiana south america the coastal environment between the amazon and the orinoco rivers is characterised by the migration of large mud banks formed by accretion of the muddy discharge from the amazon the migration of mud in the shallow coastal waters is associated with the creation of mudflats that form a surface for the development of coastal mangroves after consolidation this study focuses on the fine scale sedimentary processes involved in the morphodynamic evolution of a consolidated mudflat and its erosion mudflats can be divided into two areas the seafront and the inner part between the seafront and the land this study highlights a link between tidal mud supply biofilm migration and increasing elevation in the latter area the migration of a biofilm through each cycle of tidal supply prevents erosion and permits the continuous accretion of the entire mudflat over several years this increase in topography is also modulated by fortnightly tidal cycles desiccation greatly impacts the mudflat s structure at a yearly scale this process plays an important role in the erosion of the seafront area under wave action by allowing the formation of mud pebbles which are progressively abraded into fluid mud supplied to the inner part of the mudflat by over wash processes this study provides a better understanding of the behaviour of mudflats on the wave exposed coast downdrift of the mouth of the amazon by describing 1 the processes involved in sediment exchanges between mudflats and mud banks 2 the mechanisms associated with the persistence of mudflats along the french guiana coast downdrift of the mouth of the amazon and 3 the processes involved in the erosion and recycling of these mudflats

settling velocity and mass settling flux of flocculated estuarine sediments new formulations are presented for the settling velocity and mass settling flux the product of settling velocity and sediment concentration of flocculated estuarine mud physics based formulae for these are developed based on assumptions of a two class floc population microflocs and macroflocs in quasi equilibrium with the flow the settling velocities of microflocs and macroflocs are related to floc size and density via the kolmogorov microscale as a function of turbulent shear stress and sediment concentration including height dependence and floc density dependence coefficients in the formulae are calibrated against an existing large data set of in situ observations of floc size and settling velocity from northern european estuaries various measures of performance show that the resulting formulae achieve an improved level of agreement with data compared with other published prediction methods the new formulae with the original calibration coefficients perform well in tests against independent measurements made in two estuaries

inssev an instrument to measure the size and settling velocity of flocs in situ abstract an instrument has been developed to observe the settling of individual flocs in turbid water in order to measure size and settling velocity spectra of estuarine cohesive suspended sediments inssev in situ settling velocity instrument is bed mounted and comprises a computer controlled decelerator chamber that collects a sample of water from which some of the suspended matter is allowed to enter the top of a settling column the settling flocs are viewed using a miniature video system subsequent analysis of video tapes can provide direct measurements of size and settling velocity of individual flocs down to 20 m from this information floc density can be estimated the main feature of the instrument is its ability to video flocs in situ irrespective of the concentration in the estuary with as little disturbance to their hydrodynamic environment as possible preliminary field testing in the tamar estuary has given useful results in flow velocities up to 0 4 m s 1 and in concentrations up to 200 mg 1 1 the video recordings show large numbers of low density flocs with multiple structures linked by fine organic fibres

in situ particle size spectra and density of particle aggregates in a dredging plume abstract an in situ laser particle sizer the lisst 100 was used to describe the spatial variation of beam attenuation coefficient in situ particle size spectra and aggregate densities in a dredging plume in the sound oresund between denmark and sweden the results proved that the above mentioned parameters varied significantly within the investigated length of the plume approximately 2 km it is shown how the small single primary particles aggregate and change the in situ particle size spectra into a slightly better sorted and coarser size distribution and at the same time how the mean density of particles aggregates decreases significantly this shift in the state in which the particles exist in the water effectively changes the optical response of the mass of particles suspended in the water it is shown that adequate correlations between mass concentrations and beam attenuation coefficients can only be obtained if parameters describing the in situ quality and state of the mass of particles e g standard deviation of the size spectra and volume concentration is included in the regression from the spatial variation in mean density and in situ particle size it was possible to calculate the spatial difference in settling velocity it was found that the difference in settling velocity was only about a factor of 1 7 because increasing in situ particle size was counter balanced by decreasing mean density furthermore the time scale of flocculation within the plume was found to be in the order of 50 min

importance of wave induced bed liquefaction in the fine sediment budget of cleveland bay great barrier reef data from a three year long field study of fine sediment dynamics in cleveland bay show that wave induced liquefaction of the fine sediment bed on the seafloor in shallow water was the main process causing bed erosion under small waves during tradewinds and that shear induced erosion prevailed during cyclonic conditions these data were used to verify a model of fine sediment dynamics that calculates sediment resuspension by both excess shear stress and wave induced liquefaction of the bed for present land use conditions the amount of riverine sediments settling on the bay may exceed by 50 75 the amount of sediment exported from the bay sediment is thus accumulating in the bay on an annual basis which in turn may degrade the fringing coral reefs for those years when a tropical cyclone impacted the bay there may be a net sediment outflow from the bay during the dry tradewind season fine sediment was progressively winnowed out of the shallow reefal waters

biological mediation of the settling velocity of bed material eroded from an intertidal mudflat the danish wadden sea abstract the equivalent settling diameter of the eroded material from erosion experiments in situ has been examined for almost two years on a temperate intertidal mudflat the erosion experiments were carried by use of a portable eromes erosion equipment after each erosion experiment the eroded material was allowed to settle for some minutes and the settling velocity distribution and hereby equivalent settling diameters were calculated using stoke s law the settling velocities were strongly dependent on the fecal pellet content of the material showing larger settling diameters with increasing contents of fecal pellets whereas velocities were not dependent on applied bed shear stress primary grain size dry bulk density organic content or chlorophyll a content of the bed material the pellets at the investigated mudflat were produced by the snail hydrobia ulvae which lives in densities up to about 300 000 indiv m 2 at the site the average settling diameter for eroded material without fecal pellets was about 20 m whereas it was about 80 m for highly pelletized material the pellets are eroded quite easily but will probably not escape from the tidal basin due to the increase in settling velocities and associated increase in settling lag and scour lag therefore it is argued that although h ulvae will tend to destabilize the surface of the mudflat the net effect with respect to the fine grained sediment budget is probably positive due to the increase in settling velocity caused by the pelletisation by the snail the temperature dependent ingestion by h ulvae causes a clear seasonality in the pellet content and hereby settling velocity of the material eroded at the mudflat this may partly explain earlier observations of seasonal variations in erosion and deposition on the mudflat

flocculated suspended sediment in a micro tidal environment abstract silt and clay particles are unstable when suspended in saline waters they coagulate into sediment flocs having settling velocities orders of magnitude higher than the individual particles constituting them in the danish wadden sea in situ analyses were carried out to obtain the natural textural composition of the suspended sediment a settling tube braystoke sk 110 was used and the analysis revealed that the suspended sediment at the investigation sites was composed of an unflocculated population coarser than about 4 5 and of a flocculated population finer than this grain size median settling velocities from 10 4 to 10 3 m s 1 were determined within the concentration range 50 1000 mg 1 1 it is concluded that suspended sediment concentration is a major determinant for the natural textural composition of flocculated sediment the variation in concentration can account for about 80 of the variance in median settling velocity turbulent mixing within the water column is found to be the limiting factor for the growth of sediment flocs

mass settling flux of fine sediments in northern european estuaries measurements and predictions abstract this study describes a new flocculation model which was based entirely on observations of settling floc spectra and turbulent shear stresses acquired primarily from a series of in situ experiments conducted in several european estuaries the synthesis identified that the key components which best quantitatively describe a floc population are the changes in the macrofloc 160 m and microfloc settling velocities ws macroem and ws microem together with how the suspended matter is distributed across each floc sub population spm ratioem the ws macroem algorithm displayed an increase in settling velocity at low shear stresses due to flocculation enhanced by shear and floc disruption at higher stresses for the same concentration the transition being a turbulent shear stress of about 0 36 n m 2 the combination of the three algorithms into a single equation to predict mass settling flux msf em estimated the total flux of the 157 measured floc samples with a cumulative error 1 and 5 mm s 1 were both in error by an average of 86 and 41 respectively a concentration settling parameterisation and the van leussen techniques under predicted the total cumulative flux by 35 and 38 respectively furthermore the analysis indicated that the various existing methods all incurred high predictive errors at times under estimating by over 70 as concentration levels rose in close proximity to the bed the findings of this study demonstrate the new manning floc settling velocity mfsv empirical model has flexibility in adapting to a wide range of both turbulent shear stress and suspended sediment concentration estuarine conditions

a study of in situ sediment flocculation in the turbidity maxima of the yangtze estuary abstract in order to improve our understandings of temporal and vertical variations of sediment flocculation dynamics within the turbidity maxima tm of the highly turbid yangtze estuary ye we deployed lisst 100c a laser instrument for in situ monitor of the sizes and concentrations of flocculated particles in a wet season field data in terms of vertical profiles of flow velocity suspended sediment concentration ssc salinity flocculated particle size distribution and volume concentration were obtained based on field works conducted at consecutive spring moderate and neap tides data analyses show that the mean floc diameters d m were in the range of 14 95 m and flocculation exhibited strong temporal and vertical variations within a tidal cycle and between spring neap cycles larger d m were observed during high and low slack waters and the averaged floc size at neap tide was found 57 larger than at spring tide effective density of flocs decreased with the increase of floc size and fractal dimension of flocs in the ye was mainly between 1 5 and 2 1 we also estimated the settling velocity of flocs by 0 04 0 6 mm s 1 and the largest settling velocity occurred also at slack waters moreover it is found that turbulence plays a dominant role in the flocculation process floc size decreases significantly when the shear rate parameter g is 2 3 s 1 suggesting the turbulence breaking force combined effects of fine sediment flocculation enhanced settling process and high sediment concentration resulted in a large settling flux around high water which can in part explain the severe siltation in the tm of the ye thus shedding lights on the navigation channel management

role of atmospheric circulation with respect to the interannual variability in the date of snow cover disappearance over northern latitudes between 1988 and 2003 1 this paper analyzes the main spatial patterns in the dates of snow cover disappearance variability over northern latitudes between 1988 and 2003 the dates of snow cover disappearance were calculated using satellite passive microwave data from the special sensor microwave imager spatial and temporal patterns were obtained using principal components analysis in the s mode we identified eight components each representing a large region characterized by homogeneous interannual variability in the dates of snow cover disappearance we found that atmospheric circulation summarized by means of teleconnection indices had an important impact on the date of snow cover disappearance for most of these regions a role is played by the arctic oscillation in western siberia the spring east atlantic west russian pattern in central siberia and the pacific north

a laboratory examination of floc characteristics with regard to turbulent shearing abstract turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters and their respective aggregate break up this study examines the inter relationships between floc characteristics over increasing turbidity 80 200 mg l 1 and turbulent shear 0 0 6 n m 2 environments by the use of a laboratory flume within which a suspension can be sheared at a controlled rate and with an unintrusive macro lens miniature video camera mounted in a viewing port on the flume channel wall the camera enables the direct simultaneous measurement of both floc size and settling velocity from which accurate estimates of floc effective density and porosity can be made measurements were made 120 s after the induced turbulence has ceased the instrument has an upper viewing turbidity limit of 210 mg l 1 and a lower resolution of 20 m the sediment was collected from the inter tidal mudflats at weir quay on the tamar estuary in devon southwest england the results indicated that increasing turbidity at low shear levels encouraged floc growth but the effect of the increasing turbulent shear 0 35 n m 2 together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process at shears up to 0 35 n m 2 the largest size and settling velocity flocs were produced at high concentrations whereas above 0 35 n m 2 disruption caused smaller flocs at higher concentrations the use of algorithms which were based either on a single floc characteristic i e size or settling velocity or a mean fractal dimension were seen not to accurately approximate the experimental data a multiple regression analysis of the experimental data produced the following formula based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions referred to as max20size mean values settling velocity 0 301 0 00337 rms of the gradient in turbulent velocity fluctuations 0 000606 spm 0 00335 floc size which proved to be the most accurate representation with an r 2 value of 0 95 a similar formula was determined for the average value of the four fastest settling flocs within each sample group max4w s this highlights the importance of modelling algorithms that are developed from data that take into account effective density variations i e simultaneous size and settling velocity measurement