Sediment Settling Research Articles

Temporal variability of hydrodynamics, sediment concentration and sediment settling velocity in a tidal creek

Measurements of flow, turbulence and suspended sediment concentration were carried out in a tidal creek that acts as a conduit for water and sediment to an intertidal salt marsh. A laser diffraction system (LISST), optical backscatter sensors (OBS) and acoustic Doppler velocimeters (ADV) were used to measure suspended sediment concentration and particle settling velocity variability throughout various tidal cycles for spring and neap conditions. Results suggest that ADV is a very useful system capable of measuring both sediment concentration and settling velocities. During neap tides, sediment in the tidal creeks is primarily in the form of flocs with a settling velocity ranging between 0.02 and 0.20mm/s and a mean particle size of 25–75μm. This population of sediment is enriched by both larger flocs and larger individual grain particles, during spring tides. The results indicate that in the tidal creek, erosion can occur only during the ebb stage of the spring tides, while during the flood tides the tidal creeks do not experience any significant erosion. On the contrary, they might act as temporary depositories for the small flocs. Sedimentation on the marsh surface occurs predominantly in the form of flocs, which seems to have the same settling velocities (0.24mm/s) both during neap–spring tides. This indicates that the neap–spring tidal cycle is controlling marsh sedimentation through sediment availability for deposition and inundation time. Spring tides provide more sediment and more time for the flocs to be deposited and thus contribute more to marsh accumulation.

Evidence for morphology-induced sediment settlement prevention on the tubular sponge Haliclona urceolus

Several mechanisms are known to assist the survival of sponges in highly sedimented environments. This study considers the potential of sponge morphology and the positioning of exhalant water jets (through the osculum) in the adaptation of Haliclona urceolus to highly sedimented habitats. This sponge is cylindrical with an apical osculum, which is common in sedimented subtidal habitats at Lough Hyne Marine Nature Reserve, Cork, Ireland. Fifteen sponges were collected, preserved (killed with the structure and morphology maintained) and then replaced in a high sediment environment next to a living specimen (at 24 m). After 5 days, the sediment settled on both living and preserved sponges was collected and dried. No sediment was collected from living sponges, while preserved specimens had considerable amounts of settled sediment on their surfaces. The amount of sediment collected on these preserved specimens was significantly linearly correlated with sponge dry weight, maximum diameter and oscula width (R2>0.70, P<0.001, df=14). Observations of flow direction (using coloured dye) through H. urceolus showed that water is drawn into the sponge on its underside and exits via a large vertically pointing osculum. Sponge morphologies (shape) have often been considered as a means of passive adaptation to a number of different environmental parameters with oscula position enabling entrained flow through the sponge in high flow conditions. However, this study shows how the combination of sponge morphology (tubular shape) and positioning of the osculum may enable H. urceolus to survive in highly sedimented environments. Similar mechanisms may also aid the survival of some deep-water sponge species with similar morphologies.

The transport, transformation and dispersal of sediment by buoyant coastal flows

Rivers provide the dominant pathway of terrigenous sediment to the ocean. The density difference between riverine and salt water as well as the density anomaly contributed by sediment have important consequences on the delivery, transport and ultimate fate of the sediment issued from the land. Most fresh water outflows produce positively buoyant plumes; however, the aggregation and settling of sediment cause the route of sediment transport to diverge from the route of fresh water flow. Sediment is trapped in frontal zones, both in estuaries and on the inner shelf, often resulting in large increases in the concentration of sediment relative to the riverine source. At high concentrations, the density anomaly due to the sediment itself contributes to the vertical stability of the flow, often increasing the trapping efficiency of the frontal zone. Sediment trapping may also occur within the wave boundary layer on the continental shelf, leading to high concentrations within a layer only on the order of 10 cm thick, but which may represent an important cross-shelf conduit of sediment. The high concentration layers formed both by frontal trapping and by wave boundary layer trapping can attain excess densities large enough to initiate down-slope motion on the continental shelf, resulting in an important cross-shelf transport process. Because of these trapping processes, the formation of high-concentration layers, and the occurrence of hyperpycnal plumes, the transport of sediment in river-influenced environments is often dominated by near-bottom fluxes rather than fluxes in the surface plume.

Fine-grained sediment lofting from meltwater-generated turbidity currents during Heinrich events

Turbidity currents generated from sediment-carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater. From experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal. This ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows. Evidence for large-scale lofting of suspended sediment from the top of giant sand- and gravel-carrying turbidity currents in the Labrador Sea comes from two independent lines of observations: (1) The first is a distinct sedimentary facies consisting of stacked, centimeter-thick graded mud layers that contain grains of ice-rafted debris (IRD) supported by the mud. Deposition of these unusual layers requires a gradedlayer‐forming process that is slow enough to allow the incorporation of IRD; this is not possible with normal mud-carrying turbidity currents. (2) The second observation is the presence of a huge abyssal sand and gravel plain in the central Labrador Sea that received its sediment from bed-load‐rich meltwater discharges from the Hudson Strait outlet of the Pleistocene Laurentide Ice Sheet. These discharges turned into turbidity currents that released rising columns of freshwater that carried fine-grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater. Deposition from these slow turbid interflows would allow the incorporation of IRD in the accumulating graded mud deposits. The IRD-spiked graded mud facies is restricted to Heinrich layers within 300 km radius of the Hudson Strait ice-stream terminus, tying the sand-carrying turbidity currents via fine-grained sediment lofting to Heinrich events. Estimated total discharge volumes of individual currents are on the order of 10 3 km 3 , supporting the notion that Heinrich ice-rafting events were times of maximum meltwater generation.

FIELD STUDY ON NEAR-BOTTOM TURBULENCE AND RESUSPENSION OF COHESIVE SEDIMENT IN THE TONE RIVER ESTUARY

Near-bottom turbulence and flux of suspended sediment were measured in the Tone River Estuary to study a relationship of resuspension and settling of cohesive sediment with turbulence. An acoustic Doppler velocimeter was deployed because of its advantage of measuring suspended sediment concentration besides turbulence by utilizing acoustic intensity data. Vertical profile of velocity and spatial distribution of suspended sediment were measured with an ADCP and water quality meters.Three phases for sediment suspension were observed: First phase was characterized by quasi-equilibrium under low Reynolds stress, second phase by resuspension of comparatively fine sediment accompanied by a growing turbidity front moving upstream, and the third phase by strong resuspension of comparatively rough sediment under strong Reynolds stress. Budget analysis of suspended sediment suggested that the settling velocity at the third phase was 10-3[m/s] while the value at the first phase was 10-5[m/s].

Multibeam bathymetry and the depositional environments of Kongsfjorden and Krossfjorden, western Spitsbergen, Svalbard

Kongsfjorden and Krossfjorden are two ice-proximal fjords on the western coast of Spitsbergen which have been surveyed using multibeam bathymetry, sub-bottom profiling and gravity coring. Central and outer Kongsfjorden is dominated by a 30 km2 outcrop of bedrock, with a thin (<10m) sediment cover. The bedrock displays a relict sub-glacial, ice-scoured topography produced during the glacial re-advances of the Weichselian (20 Ky BP) and again during the last major Holocene re-advance of the Little Ice Age (550-200 yrs BP). Drumlins and glacial flutes are common across the floor of Kongsfjorden, with lengths of 1.5-2.5 km and widths of <100 m, rising up to 10 m in water depths of <100 m. This topography is smoothed by bottom currents from the wind-driven forcing of surface waters. The flow is counter-clockwise, matching boundary layer movement under the influence of Coriolis force. Both fjords are characterized by a variable acoustic character, based on sub-bottom profile data. The deepest basins are dominated by parallel, well-laminated reflectors and an irregular-transparent acoustic character indicating the presence of Holocene-age fine-grained sediments up to 30 m thick. A parallel, irregular-transparent acoustic character with waveform morphology in inner Kongsfjorden is interpreted as moraines, originating from the 1948 and 1869 surges of Kronebreen glacier. Mass-flows are common on the flanks of topographic highs as acoustically chaotic-transparent lensoid and wedge-shaped reflectors. The sediments of outer and central Kongsfjorden are characterized by bioturbated, gas-rich homogeneous muds interpreted as being the result of the settling of fine-grained sediment and particulate suspensions.

Water masses and bottom boundary layer dynamics above a sediment drift of the Antarctic Peninsula Pacific Margin

Data from eight CTD casts and two one-year long current time series collected at 8 and 60 m above the seafloor of a sediment drift, off the Pacific Margin of the Antarctic Peninsula are presented, with special emphasis on bottom boundary layer dynamics and processes relevant to sediment settling and re-suspension. The water masses over the drift are characterized, including also a comparison with other measurements available from that region. The south-westward flow along the continental rise exhibits a strong topographic (bathymetric) control in the near-bottom current regime. A consistent mean flow deflection between an upper and lower current regime suggests that only the lower regime falls within a bottom (turbulent) Ekman layer. The bottom current regime is not energetic enough to maintain the coarse sediment fraction in suspension. The absence of evidence for a nepheloid layer justifies the assumption that most sediment was supplied to the margin during glacial periods. Two events, with peak velocities of up to 20 cm s−1, are associated with barotropic eddies shown as negative (cyclonic) mean sea level anomalies detected by ERS/TOPEX satellite altimeters. These energetic bottom current pulses may give way to episodic sediment re-suspensions of the sortable (non-cohesive) part of the sediment, thus exerting a minor role in redistributing fine sediments through the mean flow regime.

Differential rates of vertical accretion and elevation change among aerial root types in Micronesian mangrove forests

Root systems in mangrove swamps have captured the attention of scientists for decades. Among the postulated roles of root structures include a contribution to the geomorphological stability of mangrove soils through sediment trapping and binding. In this study, we used feldspar marker horizons and sediment pins to investigate the influence of three different functional root types—prop roots in Rhizophora spp., root knees in Bruguiera gymnorrhiza, and pneumatophores in Sonneratia alba—on vertical accretion and elevation change in three mangrove forests in the Federated States of Micronesia. Prop roots facilitated vertical accretion (11.0 mm year −1) more than pneumatophores or bare soil controls (mean, 8.3 mm year −1). Sediment elevation, on the other hand, increased at an average rate of only 1.3 mm year 1 across all root types, with rate differences by root type, ranging from −0.2 to 3.4 mm year −1, being detected within river basins. This investigation demonstrates that prop roots can assist in the settling of suspended sediments from estuarine waters, yet prop root structures are not as successful as pneumatophores in maintaining sediment elevation over 2.5 years. As root densities increase over time, an increase in turbulence-induced erosion and in shallow subsidence as organic peat layers form is expected in Micronesian mangrove forests.

Hydrography and cohesive sediment modelling: application to the Rømø Dyb tidal area

Estuaries act as sinks for fine-grained sediments and because of the cohesive properties of these sediments, heavy metals and nutrients tend to accumulate in estuaries. In order to quantify the erosion, transport and deposition of these pollutants, modelling of cohesive sediment dynamics is a very useful tool. However, the description of cohesive sediment dynamics through numerical analysis is a difficult task since the physical properties of cohesive sediments are of complex nature. In this paper, setup and calibration of a cohesive sediment transport model covering the period from October 20, 1999 to December 13, 1999 are described and an interpretation of the results is carried out. Further, a comparison with measured suspended sediment concentrations and bed level measurements from the modelling period is presented. A detailed bathymetry is used. This is necessary in order to describe the water movements in a realistic way. A bottom description is created in a way so that differences in erodibility of the sediment can be described. Further, a spatial differentiated description of critical shear stress, both for erosion and deposition of the cohesive sediment bottom, is made in order to describe the processes of settling and scour lag. The hydrodynamic simulation is shown to be very reliable, and therefore, it has been possible to extract key values for the area. Thus, the maximum current velocities for the Lister Dyb area are modelled to 1.2 and 0.93 m s −1 for the flood and ebb period, respectively. The tidal prism has likewise been computed to 620×10 6 m 3. The cohesive sediment transport modelling has shown that the highest sediment concentrations at a given site appear when onshore winds are prevailing. Further, it can be recognized in the results that an inward sediment transport direction is prevailing, especially after a windy period with waves has mobilized considerable amounts of sediment. A detailed investigation of the cohesive sediment's settling velocities collected in the area is used to give a site-specific description. Since the description of the settling velocity changes with temperature, several simulations using different descriptions have been carried out. These simulations have shown that this parameter is very influential for the net deposition result. Thus, an increase of the settling velocity will increase the deposition rates considerably. The modelling results and the comparisons to measured data presented in this paper show that it has been possible to calibrate the hydrodynamic model in accordance with observed values. In continuation of this, the deformation of the tidal wave has also been modelled satisfactorily. Modelling of the cohesive sediment dynamics has been more complicated and the modelling results show divergence from the measured results. However, the levels of the sediment concentrations and the overall net sedimentation pattern show accordance with observed values.

Responses of Deep-Sea Meiobenthic Communities to Sediment Disturbance Simulating Effects of Polymetallic Nodule Mining

During a benthic impact experiment (BIE) carried out during 1995–1997 by the Interoceanmetal Joint Organization (IOM) at an abyssal site in the North-East Pacific, sediment disturbance mimicking that resulting from polymetallic nodule extraction was created with a specialised device (the Benthic Disturber). The effects of the disturbance on meiobenthic communities were assessed immediately after disturbance and 22 months later. A reduction in meiobenthos abundance, observed immediately following impact, was not significant; neither were changes in composition of the meiobenthos which was dominated by nematodes and harpacticoids. The lack of any significant numerical response is probably accounted for by the moderate degree of disturbance in this study, compared with other BIE-type experiments. On the other hand, statistically significant changes in both meiofauna abundance and vertical distribution profiles in the changed sediment within the Disturber tracks were recorded. After 22 months, a significant increase in overall meiobenthos abundance was detected in that part of the test site affected by increased resuspended sediment settlement and receiving natural phytodetrital inputs. Certain taxon-specific responses on the part of nematodes and harpacticoids were noted both immediately after the disturbance and 22 months later. They were explained by the effects of sediment physical reworking and responses to phytodetrital enrichment. The results presented should aid in developing experimental designs, on both temporal and spatial scales, of future deep-sea tests aimed at assessing the scale and consequences of man-made impacts.

Branching dynamics of two species of arborescent demosponge: the effect of flow regime and bathymetry

A series of measurements were taken from 50 random examples of the arborescent sponges Raspailia ramosa and Stelligera stuposa at a number of sites and depths at Lough Hyne, Co. Cork, Ireland. Sites ranged from fast flowing habitats, experiencing little sediment settlement, to the converse where current flow was extremely slight and sediment settlement rates high. Depth intervals at which sponges were measured ranged from 6 to 24 m. Sponges of both species varied with respect to measurements taken and the growth proportions (ratios of measurements taken). The greatest morphological differences occurred between depth and current extremes. However, at the most sedimented site (24 m) branching complexity of both species actually decreased in opposition to the proposed current models. It is discussed that simple morphologies (such as singly dichotomized sponges) may be better suited to extremely sedimented regimes being more erect than larger, more complex specimens. Other biological factors that may limit branching complexity are considered and emphasis is placed upon the possible inadequacies of community or group level models of morphological adaptation.

Temporal and spatial sedimentation rate variabilities in the eastern Gotland Basin, the Baltic Sea

The present study examines sedimentation rates in the eastern Gotland Basin using a variety of methods that reveal considerable heterogeneity in the rates, both spatially and temporally. High‐resolution seismic recordings and correlation with long sediment cores indicate increased thickness of strata and higher sedimentation rates (0.75 mm a‐1) in the eastern part of the basin than in the western part (0.23 mm a‐1) since the Littorina transgression some 8000 14C years BP. This difference is apparently a consequence of a counterclockwis e near‐bottom circulation in the basin with periodically high current speeds that cause winnowing on the steep SE slope of the basin and differential settling of sediments in areas of low current speeds. On shorter time scales, recent sediment accumulation rates based on radiometric dating (210Pb) are in general twice as high as those observed 25 years ago using the same method. The higher modern rates, compared to those of the 1970s, may partly be due to increased eutrophication, as more carbon is buried in the sediment, and partly due to increased erosion in shallow water areas. However, strong lateral variations are evident. The average sediment accumulation rates vary between 119 and 340 gm‐2 a‐1 (corresponding to sedimentation rates of 2.1–2.5 mm a‐1) in the deepest part of the basin. Very high rates (6100 g m‐2 a‐1, corresponding to sedimentation rates of 30 mm a‐1) are observed on an intraslope basin site (offshore Latvia) at a water depth of only 70 m. The radiometrically determined sediment accumulation rates are up to three times higher than those estimated from average water column concentrations of suspended matter and from sediment trap flux rates. The discrepancy suggests that sedimentation in the deep basin may have a substantial contribution from near‐bottom lateral transport.

Potential long-term ecological impacts caused by disturbance of contaminated sediments:a case study.

Several submerged barges were recently removed from the Passaic River, New Jersey, USA, in two areas (areas 1 and 2) where contaminated sediments are known to exist. During removal of the single barge in area 1, elevated turbidity levels and chemical parameters were measured. Greater increases were measured in area 2, where several barges were removed. In both areas, water column concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and several metals exceeded one or more water quality criteria; turbidity levels in area 2 also exceeded regulatory criteria. Potential chemical bioaccumulation from the water column into residential aquatic receptors was estimated using standard models and assumptions. The modeled results predicted that steady-state tissue concentrations of bioaccumulative chemicals would not occur as a result of the brief increase in water column concentrations that occurred during barge removal but that metals and PCDD/Fs could bioaccumulate to levels that exceed regulatory ecological criteria during long-term sediment disturbance activities. In addition, based on some simplistic assumptions regarding settling of suspended sediments, we estimate that chemical bioaccumulation from surface sediments into the food web could result in substantial increases in PCDD/F body burdens in the benthic forage fish, mummichog. Our findings are consistent with the limited number of field studies that have measured increased body burdens of bioaccumulative chemicals following dredging. We suggest that, prior to consideration of extensive dredging as a remedial alternative for any river system, the potential significant and long-term impacts on the food web must be evaluated.

Hydrostatic settling velocity of sediments

The settling of sediments is an accelerative process in which the concentration of the main sediments zone will heavily influence settling velocity, but the explicit relationship between the concentration and settling velocity has not been reported in literature. Here a theoretical function was built for the time dependent concentration and time dependent settling velocity of sediments; then the entire settling process reflecting concentration was shown on the basis of sediments instant-settlement theory and mathematical method. Agreement of computed results and experimental data was found. Several governing parameters, including particle size, particle density, initial suspended sediments concentration and suspension height, were discussed with a series of calculated velocity curves. The research indicated that (1) the presented concentration-velocity time relationship is rational, (2) settling process of the sediments group with variation of concentration consists of acceleration stage, uniform motion stage and deceleration stage, and (3) particle size, particle density and initial suspended sediments concentration have more influence on the settling velocity than the suspension height and water temperature.

Sediment Trapping and Transport in the ACE Basin, South Carolina

A study took place during May 1998 and May 1999 to examine the processes controlling localized accumulation of fine-grained sediments in the lower Ashepoo River. This region, referred to as the Mud Reach, is an area of muddy bottom sediments bounded by fine sands. The Mud Reach is located downstream of the landward extent of the salt intrusion where an estuarine turbidity maximum commonly occurs. Tidal time-series measurements made in the Mud Reach during May 1998, when river discharge was at a 10-yr high, showed high concentrations of suspended sediment (0.05–1 g I−1) during maximum tidal current velocity with concentrations in the bottom 30 cm exceeding 70 g I−1 (fluid mud). A correlation between salinity stratification and increased suspended sediment concentration suggests that inhibited vertical mixing enhances the settling of flocculated sediments to the bed. Measurements made during May 1999 show a two-order-of-magnitude decrease in the concentration of near-bed sediments. A decrease in river discharge during the 1999 observation period of more than 100 m3 s−1 suggests that changes in the hydrography and in the supply of sediments to the system both may be important factors in the trapping of fine-grained sediments in the region. The source of sediments is likely from muddy deposits in the Fenwick Cut, a man-made section of the Atlantic Intracoastal Waterway about 2 km north of the Mud Reach that connects the Ashepoo and Edisto Rivers. The Fenwick Cut appears to be an effective area for trapping sediments where shoaling has increased by 130% in the last decade. Current measurements show that flow velocities decrease through the Cut, likely allowing for the settling of suspended particles that form the thick deposits of unconsolidated mud observed during both years.

Turbulent exchange of fine sediments in a tidal channel in the Ems/Dollard estuary. Part II. Analysis with a 1DV numerical model

In-situ high-frequency measurements were made of velocities and suspended sediment concentrations at three levels close to the bed at one location in a tidal channel during a few tidal cycles in June and August 1996, as described in a companion paper. In the present paper time series of mean velocities, suspended sediment concentrations and turbulence properties obtained are analysed with a one-dimensional numerical model. The model comprises a k– ε turbulence model that allows for the investigation of possible effects of density stratification on the vertical exchange processes. Computational results indicate that rapid settling of suspended sediment towards slack water can be explained from stratification effects. They also indicate a locally limited availability of sediment from the bed during high flow velocities. A sediment settling velocity of about 0.5 mm/s was found from fitting the computed vertical sediment fluxes to the measured ones. Attempts, using algebraic relations, to establish a possible dependence of settling velocity on suspended sediment concentration or turbulence level were not successful.

Evolution of stratigraphic sequences in multisegmented continental rift basins: Comparison of computer models with the basins of the East African rift system

This article presents a series of numerical simulations of the stratigraphic evolution of continental rift basins. We model the geomorphic and tectonic processes acting in this depositional environment, which contrasts with the traditional approach of modeling the settling of sediments, especially in marine clastic basins. What is new in our model is that it can simulate the formation of basins with linked half grabens, which is a fundamental feature of rift systems. The attributes of the numerical simulations such as the overall morphology, basin architecture, drainage systems, and stratigraphy are in good agreement with digital elevation models and reflection seismic lines of the basins of the East African rift system, specifically the Tanganyika and Malawi rifts.

Behavior of density current in rectangular settling tank

Density current in a rectangular settling tank is investigated mathematically. The basic equation consists of equation of motion for horizontal and vertical direction together with the mass conservation equation of both fluid and sediment. As a turbulence model, the standard k-e model is employed. Flow velocity profiles and sediment concentration distribution are illustrated. When the temperature of inlet suspension is warmer than that of the tank water, the suspension flows downward with the positive buoyancy. This density current increases the turbulence in its accelerating stage, which prevents the settling of sediments. On the other hand, in the case of high sediment concentration, the turbidity current accelerates rather low region in a tank, which may bring the resuspension of bottom sediment. It is also shown that the flow field is very complicated one, because the density current brings up under the developing stage of bottom boundary layer. If the sediment particle has higher fall velocity, they settle down within a short distance. In such case, the density current gives its effect on the sediment motion only in the developing stage and this effect leads to the decrease of removal ratio.

Lutocline Behavior in High-Concentration Estuary

The behavior of the lutocline as a measure of fine sediment mixing in high-concentration estuaries is briefly examined with reference to the Jiaojiang estuary in China. A lutocline mixing index is introduced and shown to correlate empirically with the rate of turbulent kinetic energy production in such a way that with increasing energy production lutocline mixing increases. Suspended sediment concentration and energy production together govern the turbulent mixing length, hence lutocline mixing, which appears to be measurably modulated by sediment settling, interparticle interaction, and cohesion. Further understanding of lutocline response to tidal forcing will require direct and long-term measurements of turbulent fluxes, especially within the zone of lutocline occurrence and in the high-concentration suspension underneath.

Settling of Fine Sediment in a Channel with Emergent Vegetation

The potential for enhanced removal of sediment from flowing water in a channel by means of settling on vegetation was studied theoretically and experimentally. Settling on vegetation was modeled as a distributed sink of sediment with vegetation spread throughout depth and length of the channel. Sediment-capture experiments were performed in a laboratory channel using model vegetation and well-sorted silt. The theory predicts that settling on vegetation has the potential to increase sediment-capture efficiency considerably, and this was demonstrated in the experiments. The enhancement of sediment removal was greater for artificial vines than for inclined strips. For the runs with flow velocities >10 mm/s the sedimentation efficiency was less than predicted. The reduced efficiency could be related to bed destabilization (which was observed in some runs), re-entrainment from the vegetation, or sediment sliding off the vegetation.