Median Settling Velocity Research Articles

Suspended sediment properties in the Lower Mekong River, from fluvial to estuarine environments

The Mekong river is one of the largest rivers in the world, which flows through six countries of Southeast Asia (China, Myanmar, Laos, Thailand, Cambodia and Vietnam). Its hydro-sedimentary regime is changing rapidly, as a consequence of a regional shift of land use (agriculture, road, etc.), damming, sand mining and climate changes, among others. This study assesses the behavior of particles transported in suspension in the Lower Mekong River (LMR), along approximately 1700 km from fluvial to estuarine environments. Suspended sediment properties were estimated, simultaneously with hydrodynamic conditions, during three field campaigns. In addition, further investigations were performed in the laboratory to assess the structures of particles (flocculated or not), their capacity to flocculate (and the impacts on siltation), under a wide range of sediment concentration (20–30,000 mg.L−1). This study confirms that suspended sediment transported in the LMR are predominantly (75% by volume) flocculi (or freshly eroded soils aggregates), with median aggregated particle size in the range 10–20 μm and median settling velocity of the order of 0.01–0.1 mm s−1. These flocculi are robust under the hydrodynamic conditions (turbulence and suspended sediment concentration – SSC) existing in the LMR. Laboratory investigations reveal the existence of a threshold sediment concentration (400 mg.L−1), beyond which flocculation and sedimentation increase of orders of magnitudes. Thus, concentration that exceeds this threshold might promote the formation of so-called fluid mud layers. Because of the nonlinear response of flocculation and sedimentation with SSC and considering the ongoing changes at a regional scale in the LMR, higher occurrence of fluid mud layers in the fluvial upstream waterbodies might be anticipated, and a lower occurrence in estuaries and alongshore where the concentration decrease. The geomorphology could be impacted, with an over-siltation in dams and an exacerbated erosion of the muddy-mangrove coast.

Seasonal, Spring‐Neap, and Tidal Variation in Cohesive Sediment Transport Parameters in Estuarine Shallows

AbstractNumerical models for predicting sediment concentrations and transport rely on parameters such as settling velocity and bed erodibility that describe sediment characteristics, yet these parameters are rarely probed directly. We investigated temporal and spatial variation in sediment parameters in the shallows of San Pablo Bay, CA. Flow, turbulence, and suspended sediment data were measured at sites located at 1 and 2 m below mean lower low water (MLLW) from November 2013 through April 2015, supplemented by monthlong periods in 2011, 2012, and 2016. Maximum current velocities were 0.40–0.47 m s-1 at these depths; the strongest currents decreased to 0.27–0.34 m s-1 during neap periods. Winters 2013–2014 and 2014–2015 experienced strong drought conditions, limiting the potential for seasonal impact on sediment conditions during this experiment. Despite this, the more storm‐influenced site showed clear changes during the winter: the roughness parameter decreased from 10−4 to 10−5 m, from hydrodynamically rough to smooth conditions, and bed erodibility increased by an order of magnitude. Median settling velocity was 2.05·10−4 m s-1; it varied twofold within a tidal cycle, decreasing as current velocity grew during flood and ebb. This tidal control on floc size affected settling velocity on the spring‐neap timescale, possibly driving a spring‐neap oscillation in erodibility. Our findings highlight variation in sediment dynamics that is commonly ignored in numerical models and the need for field observations to ground truth ongoing modeling efforts.

Effects of attached bacteria on organic aggregate settling velocity in seawater

We investigated whether attached bacteria affect the settling velocity of organic aggregates via modifications of the physical properties of aggregates, including density, porosity, and morphology. Model aggregates, prepared by mixing 2 different polysaccharides (fucoidan and chitosan), were incubated in coastal seawater passed through either 0.8 μm (AGG0.8) or 0.2 μm (AGG0.2) filters. After incubation for 48 h, AGG0.8 were much more densely (3.2- to 10.1- fold) colonized by bacteria than AGG0.2. Based on median settling velocities (W50), as determined by laser in situ scattering and transmissometry, the W50 of AGG0.8 was lower (1.6- to 4.5-fold) than that of AGG0.2 for a size class of 62 to 119 μm. Stokes model analyses indicated that this reduction in W50 could be largely attributed to the higher porosities of AGG0.8 (0.932−0.981) than those of AGG0.2 (0.719−0.929). Our results support the notion that the modification of aggregate structure by attached bacteria (porosity enhancement) can be an important factor controlling the settling velocity of marine particles.

Global importance, patterns, and controls of dissolved silica retention in lakes and reservoirs

Lentic water bodies (lakes and reservoirs) offer favorable conditions for silica (SiO2) burial in sediments. Recent global estimates suggest that (1) lentic SiO2 trapping is a globally important SiO2 flux, and (2) through reservoir construction, humans have dramatically altered river dissolved SiO2 (DSi) transport and coastal DSi delivery. However, regional to global scale patterns and controls of DSi removal in lentic systems are poorly constrained. Here we use 27 published lake and reservoir DSi budgets to develop insights into patterns and controls of lentic DSi retention and to develop a new, spatially explicit, global model of lentic DSi removal called SiRReLa (Silica Retention in Reservoirs and Lakes). In our analysis, lentic DSi removal (kg SiO2 yr−1) was significantly and positively related to DSi loading (P < 0.0001; r2 = 0.98), and DSi removal efficiency was significantly and positively related to water residence time (P < 0.0001; r2 = 0.68). In addition, DSi settling rates were, on average, 6.5‐fold higher in eutrophic systems than in non‐eutrophic systems (median settling velocities: 11.1 and 1.7 m yr−1 for eutrophic and non‐eutrophic systems, respectively; P < 0.01). SiRReLa, which incorporates these insights, performed quite well in predicting both total DSi removal (kg SiO2 yr−1; Nash Sutcliffe Efficiency (N.S.E) = 0.88) and DSi removal efficiency (% Si removed; N.S.E. = 0.75), with no detectable bias in the model. Global application of SiRReLa confirms that lentic systems are important sinks for DSi, removing 89.1 Tg DSi yr−1 from watersheds globally, roughly 19–38% of all DSi inputs to surface waters. Small lakes and reservoirs (<50 km2) were critical in the analysis, retaining 81% (72 Tg DSi yr−1) of the globally retained total. Furthermore, although reservoirs occupy just 6% of the global lentic surface area, they retained approximately 35% of the total DSi removed by lentic systems. Regional hot spots for lentic DSi removal were identified and imply that lentic systems can remove the vast majority of DSi across a large fraction of Earth's land surface. Finally, a sensitivity analysis indicates that future improvements in DSi trapping and transport models should focus on improving estimates of DSi input to surface waters.

Factors controlling the field settling velocity of cohesive sediment in estuaries

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.

Sediment eddy diffusivity and selective suspension under waves and currents on the inner shelf

Sediment diffusivity and effective settling velocity, we, (or equivalently grain size) of near-bed suspended sand was inferred from observed concentration profiles. Concentration data were obtained at 20-m depth off Dounreay, Scotland, and at 13-m depth off Duck, North Carolina, USA. These data accommodate different dynamic conditions (from wave-dominated at Dounreay to wind-driven current-dominated at Duck) and different sediment properties (median size of bed sediment ranging from 120 to 290 μm). Regression of observed concentration profiles using the Rouse-type diffusion equation yielded the Rouse parameter P=we/κu*, where κ is von Karman’s constant, u* is the characteristic shear velocity, and we is the effective settling velocity. Linearly increasing eddy diffusivity extended several times higher than the classical wave boundary layer and was not observed to strongly change slope at the top of the wave boundary layer. Instead, u* within several 10s of cm of the bed was nearly equal to wave-current shear velocity (u*cw) or current shear velocity (u*c) depending on whether conditions were wave- or current-dominated. For the Rouse parameter, it was found that u* ≈ u*c for u*c > ws and u* ≈ u*cw for u*c < ws, where ws is the median settling velocity of the bed sediment. The effective settling velocity in suspension (we) was, in turn, evaluated from P using independent estimates of u*cw and u*c. The inferred size of suspended sediment varied in response to forcing conditions. For an inverse Rouse number u*sf/ws near one, where u*sf is the skin-friction shear velocity, the representative grain size of suspended sediment approached the median size of bed sediment. A similar trend was seen for u*cw/ws or u*c/ws near two, depending on whether conditions were wave or current dominated. With increasing (or decreasing) u*/ws, the grain size of suspended sediment also increased (or decreased), suggesting selective suspension of sediment particles as a function of flow strength.

Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 345:51-62 (2007) - DOI: https://doi.org/10.3354/meps07006 Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf C. F. Jago1,*, G. M. Kennaway2, G. Novarino2, S. E. Jones1 1School of Ocean Sciences, University of Wales Bangor, Marine Science Laboratories, Menai Bridge, Anglesey LL59 5AB, UK 2Departrment of Zoology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK *Email: c.f.jago@bangor.ac.uk ABSTRACT: The settling and resuspension of carbon-rich suspended particulate matter (SPM) are important determinants of pelagic and benthic productivity, and the fate of primary production, in tidally stirred shelf seas. The physical and biological properties of SPM were measured over 5 wk of a declining spring plankton bloom, dominated by the flagellate Phaeocystis, in stratified and mixed waters of the Irish Sea, NW European shelf. SPM consisted of slow-settling, small flocs with a modal settling velocity of 2 × 10–3 to 4 × 10–3 mm s–1 in long-term suspension and fast-settling, large flocs with a modal settling velocity of 2 × 10–2 to 2 mm s–1, deposited as benthic fluff at slack waters and resuspended by tidal currents; flocs reached 500 µm in diameter and settled at up to 20 mm s–1. Larger flocs were composed of living Phaeocystis and other scavenged material. Floc size and settling velocity of SPM scaled on the weight percentage in SPM of arabinose, a photosynthate produced by Phaeocystis. Over a 5-fold range of arabinose content, there was a 3 order of magnitude range of median settling velocity. Larger flocs in mixed waters, with high Phaeocystis content, were stronger than smaller flocs in stratified waters with low Phaeocystis content. Resuspended flocs in mixed waters were stronger than flocs in long-term suspension. SPM concentration in mixed waters halved during the period of observation, with most of this reduction occurring in Week 1. We conclude there was net sedimentation of Phaeocystis-enriched SPM as benthic fluff despite frequent resuspension. KEY WORDS: SPM · Phytoplankton · Phaeocystis · Flocs · Settling velocity · Shelf seas Full text in pdf format PreviousNextCite this article as: Jago CF, Kennaway GM, Novarino G, Jones SE (2007) Size and settling velocity of suspended flocs during a Phaeocystis bloom in the tidally stirred Irish Sea, NW European shelf. Mar Ecol Prog Ser 345:51-62. https://doi.org/10.3354/meps07006 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 345. Online publication date: September 13, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

Aggregate settling velocity of combined sewage overflow

In order to better model the fate and transport of material in a combined sewer overflow (CSO), hydrologic and geochemical measurements were made during a storm event in Flushing Bay, NY, USA. Particle size, total suspended solids concentrations, and CSO solid density are used to calculate the settling velocity of aggregates of the CSO material. Roughly half of the material that is discharged from the outfall sinks rapidly to the bottom, while the other half forms a turbid surface plume in fresher water. The dry solid density of the CSO material was 1589 kg m −3 and the volume fraction of solid material to total aggregate volume ranged from 0.1 to 1.0. The settling velocity estimates of aggregated material in the surface plume ranged from about 0.5 to 9.0 mm s −1 with a median settling velocity of 2.9 mm s −1. A strong inverse relationship between the size of the aggregates and the aggregate density resulted in a fractal dimension of the aggregates of 2.3.

Characteristics and sedimentation of initial effluent discharged from excavated levee ponds for channel catfish

Most channel catfish are produced in excavated levee ponds that are constructed with fixed drains extending from the deepest part of the pond through the levee. When these ponds are drained, sediment accumulated in and around the drainpipe inlet is discharged with the first flush of water, resulting in a high initial solids concentration in the effluent. This study was conducted to evaluate (1) the quality and settling characteristics of initial pond effluent from excavated ponds with fixed internal drains and (2) the spatial and temporal changes in water quality as the first flush of catfish pond effluent flowed through a 1-m wide vegetated drainage ditch. Chemical and settling characteristics of effluent samples collected at the discharge point during the first 20min of discharge were determined. The initial discharge was very high in total suspended solids, volatile suspended solids, total nitrogen, and total phosphorus. The initial solids concentration was 900 times higher than that of the bulk pond water. After 15min, poorly consolidated sediment from around the drain inlet had been discharged, and effluent quality was identical to the bulk pond water. The median-settling velocity of solids decreased from 0.7cm/s at the start of pond draining to 0.03cm/s after 15min. After initial effluent plumes had flowed 150–200m downstream, nearly all solids, nitrogen, and phosphorus in the initial effluent were removed and concentrations of those substances were lower than in bulk pond water. The duration of poor water quality of initial effluent from catfish ponds with internal drains was brief (ca. 10min) and discharged solids settled rapidly after flowing through 150–200m of vegetated ditch extending from pond effluent outfalls.

Variation in the Settling Velocity of Suspended Particulate Matter in Shallow Lakes, with Special Implications for Mass Balance Modelling

AbstractMeasurements of sedimentation were combined with water samples to calculate settling velocity of suspended particulate matter (SPM) in lakes. The study sites were open water stations and enclosures in Lake Erken (Sweden) and Lake Balaton (Hungary). Settling velocities were found to vary considerably both inside and outside the enclosures. Within sites, the differences between 25th and 75th percentiles of measured settling velocities of SPM were two‐ or three‐fold. Median settling velocities of SPM ranged from around 0.5 m/d in the enclosures of Lake Erken to more than 8 m/d in the open water of Lake Balaton. Special relevance was attributed to flocculation, which is known to be affected by, e.g., SPM concentration and turbulence. Even though not directly measured, the less turbulent environment inside the enclosures was suggested to explain the low settling velocity compared to the open water environment. Settling velocity apparently correlated with water current speed (r2 = 0.66; n = 12). Stepwise multiple linear regressions were used to relate the variability in settling velocity of SPM to the variability of possible controlling factors in a number of data subsets. In most cases, one variable describing the total amount of settling material (e.g., SPM) and one variable reflecting the composition of settling material (e.g., total phosphorus) were chosen. The use of suspended solids concentration to predict settling velocity in mass balance models was discussed. It was found that the mean slope between SPM and vSPM was close to 0.1 (m4d–1 g–1). (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Settling velocities of fine suspended particles in the Changjiang Estuary, China

Point-sampled suspended sediment concentration profiles (grain size largely<32 μm) were measured in August 1978, April 1990, and May 1990 in the Changjiang Estuary. They were selectively fit with the Rouse equation in order to calculate the median settling velocities w s,50 of fine suspended particles in the Changjiang Estuary, East China Sea. Calculated settling velocities ranged from 0.4 to 4.1 mm s −1. Furthermore, w s values increased with the mean concentration C ̄ of sediment in suspension. The best-fit equation for the field settling velocity of fine particles in the Changjiang Estuary can be expressed by the power law: w s=2.37 C ̄ 0.84 ( r 2<0.3).

Dynamics of suspended particles in coastal waters (southern North Sea) during a spring bloom

Measurements of suspended particle concentration, size and settling velocity were made at a shallow site in the southern North Sea during a spring phytoplankton bloom. The site was characterised by strong differences in surface and near-bed residual flows; therefore particle processes in each layer are effectively decoupled as long as the water column is stratified. Four distinct energetic events during the observation period caused variation in the characteristics and behaviour of the particle population: (1) moderate spring tides with low wave activity; (2) strong winds, increased wave activity; (3) strong spring tides; (4) weak neap tides. During Event 1 weak tidal resuspension occurred, median particle diameter was relatively large, but median settling velocities of both chlorophyll and total SPM were low. During the higher energy Events 2 and 3 there was resuspension of relatively small, high-density particles producing high median total SPM settling velocities but low median particle diameter. In addition, a phytodetrital fluff layer, characterised by high chlorophyll settling velocity, was resuspended and dispersed during storm conditions (Event 2). During calm, weak neap tides (Event 4) there was negligible resuspension and enhanced particle settling and deposition, particularly in the phytodetritral component of the particle population, allowing rapid replenishment of the benthic fluff layer. This work indicates the relatively rapid rate at which fluff layers can be formed and dispersed, and highlights the need for high frequency measurements. The range of contrasting physical conditions over which the data-set was collected makes it an ideal candidate for parameterising and validating suspended sediment dynamics models.

Settling velocity characterization of aquacultural solids

A top-loading settling column is described and used to characterize the settling properties of the solids in the discharge water from a commercial rainbow trout production facility. Mass-based and phosphorus-based settling curves are presented. The median settling velocity on a mass-basis for the settleable solids was 1.7 cm s −1. The median settling velocity for the settleable phosphorus was 1.15 cm s −1. Manually stripping fecal material from rainbow trout resulted in settleable solids with a median settling velocity of 0.7 cm s −1. Examination of the settling velocity curves show that halving the overflow rate (OFR) from 2 to 1 cm s −1 changes the removal efficiency from 0.61 to 0.73, an increase of about 20%. Halving the OFR again to 0.5 cm s −1 increases the removal efficiency to 0.81, an improvement of about 11%. Settling characteristics of aquacultural solids will vary from facility to facility. The methods described in this paper can be used to perform a similar type of analysis at other aquacultural sites, which may be growing other species under different management regimes.

Particulate metals and their settling velocities in the Humber Estuary, UK

Abstract Concentrations of particulate Fe (Fe P ), Mn (Mn P ), Zn (Zn P ) and Ni (Ni P ) have been determined in suspended particulate matter (SPM) from the mouth of the Humber Estuary and its coastal waters during five cruises in 1994–1995. The particulate metal concentrations were inversely related to salinity, although the relationship was less pronounced at times of high fluvial input and increased tidal amplitude. During each tidal cycle, water samples were collected using a quasi in situ Settling Velocity Tube (SVT). Five settling fractions of SPM were retrieved from the tube as a function of time and the median settling velocities ( w s 50 ) determined for SPM and for each particulate metal. At the mouth of the Humber w s 50 for SPM was in the range 1 to 200 μm s −1 , whereas the SPM from the coastal waters had a value of 40 μm s −1 . Concurrent measurements of in situ particle diameters of SPM, using a laser reflectance particle sizer, gave values varying between 44 and 166 μm, with the largest particles being detected on flood tides in winter 1995 and summer 1995. The values of w s 50 for the particulate metals covered two orders of magnitude, with the highest values being for Mn P (270 μm s −1 ), Ni P (110 μm s −1 ) and Zn P (130 μm s −1 ) during dynamic conditions in spring 1995, whereas in summer 1995 all the values of w s 50 were in the range 1–3 μm s −1 . In some cases, particulate Ni P and Zn P had lower values of w s 50 than the bulk SPM. The results suggest a differential transport of metals based on their settling velocities and their preference to associate with various components of a particle assemblage. The results are discussed in the context of the implications for the refinement of coupled hydrodynamical–geochemical models for SPM and particulate metals.

A comparison of in situ techniques for estuarine floc settling velocity measurements

An Intercomparison Experiment carried out in the turbidity maximum of the Elbe estuary aimed to determine the relative performance of a number of methods of measuring the settling velocity of estuarine flocs. These comprised several Owen Tubes, with different sampling protocols, side withdrawal tubes, a settling box, and two in situ video systems. There were significant differences between the results which may partly relate to small-scale spatial and temporal patchiness in the turbidity field. Owen Tubes generally give settling velocities an order of magnitude smaller than the direct video measurements, which may indicate that the tubes disrupt flocs on sampling. Between Owen Tubes different methods of calculating the results may lead to different median settling velocities, especially at low concentrations. However, a well-controlled sampling protocol with the settling tubes gave consistent results. The direct video methods appear to give comparable results, and need to be evaluated further.

Measurements of in situ settling velocities in the Elbe estuary

Analysis of in situ settling velocities of suspended mud flocs in estuaries is difficult to carry out because of the very fragile nature of the sediment flocs. Therefore, a number of different analytical methods have been developed. The Owen tube was developed in the late 1960s, and has been essential to the further development of more sophisticated methods to measure in situ settling velocities. There are a number of problems connected with the use of the Owen tube: possible floc breakage during sampling, flocculation in the tube during analysis caused by differential settling, and secondary flows in the tube after withdrawal of subsamples. Nevertheless, the Owen tube is one of the few instruments generally available that can be used in all kinds of meteorological and hydrographical situations, covering suspended-sediment concentrations from tens to thousands of mg-dm −3. Therefore, it is still relevant to carry out Owen tube analyses for comparison with results obtained by newly developed methods such as the in situ video technique. Another thing that makes the settling tube convenient is its inexpensiveness, making it one of the few apparatuses that one can afford to possess in duplicate. Thus, the settling tube can be tested against itself to evaluate its ability to reproduced the results when analyses are carried out simultaneously on duplicate samples. Investigations carried out with two different Owen tubes in the Danish Wadden Sea suggest that the differences between median settling velocities are typically within about 20% of one another. This is a small difference compared with those that normally occur when results of different instruments are compared.

Determination of settling velocity in the Elbe estuary using quisset tubes

A version of the Owen tube with reduced flow disturbance and wider operating concentration range —UWB QUISSET—is described. Quasi in situ settling velocity distributions of suspended particulate matter (SPM) were determined using these during an intercalibration exercise in the Elbe estuary in June 1993. Systematic variation was observed, in response to tidal resuspension, advection and redeposition. The observed variation in median settling velocity was in good agreement with that obtained by other workers using different settling tubes. Systematic differences in absolute values of up to an order of magnitude between the various techniques may have been caused by differences in tube design, sample handling or raw data interpretation.

Settling velocity determination using the BIGDAN settling tube and the Owen settling tube

The BIGDAN tube for measuring the in situ settling velocity of suspended particulate matter (SPM) is introduced, and its handling and subsampling method described. BIGDAN was conceived for measurements in the marine environment where the water current is weak and the SPM concentration low. BIGDAN is an alternative to the instruments of the well-known Owen tube type generally used in estuaries or in rivers. Recommendations for use of the Owen tube are given and the results of some fundamental tests presented. Finally it is recommended to quantity the results of settling tests not only by the SPM's median settling velocity, but also by mass portions of settling velocity fractions.

Fractionation and measurements of settling velocities of suspended matter using an owen tube

In June 1993 an intercomparison programme was carried out in the zone of maximum turbidity of the Elbe estuary. A number of methods for characterizing estuarine floc size and measuring the settling velocity of suspended matter were compared. This study presents data of field measurements obtained with an Owen settling tube of the type Braystoke SK 110. Differences in the results (median settling velocities, concentration of suspended particulate matter) of the various types of settling tubes are discussed. They are considered to be dependent on SPM patchiness and on handling of the instruments. The content of organic material in flocs increases towards the slower settling fractions. Alternative uses of the Owen tube are discussed.

A robust in situ settling velocity box for coastal seas

A new instrument for measuring the in situ settling velocity able to withstand the harsh conditions common in coastal waters is introduced. This settling box traps a parcel of fluid and measures the decaying concentration with an array of optical backscatter probes; output from these four sensors is then combined to produce a single settling velocity distribution. Preliminary deployments have been undertaken in the river Elbe and have produced encouraging results. Once the box is closed, an initial period of turbulent fluctuations is followed by settling and a smooth decrease in concentration. Settling velocity distributions recorded at transitions from flood/ebb tides are distinct from those at ebb/flood transitions; median settling velocity was found to increase with sediment concentration.