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1.
Both the floc formation and floc breakup of cohesive sediment are affected by turbulent shear which is recognized as one of the most important parameters, and thus, on the settling and transport of cohesive sediment. In this study, the development of floc characteristics at early stage and steady-state of flocculation were investigated via a three-dimensional lattice Boltzmann numerical model for turbulence-induced flocculation. Simulations for collision and aggregation of various size particles, floc growth, and breakup in isotropic and homogenous turbulent flows with different shear stresses were conducted. Model results for the temporal evolution of floc size distribution show that the normalized floc size distributions is time-independent during early stage of flocculation, and at steady-state, shear rate has no effect on the shape of normalized floc size distribution. Furthermore, the size, settling velocity, and effective density of flocs at the non-equilibrium flocculation stage do not change significantly for shear stresses in the range 0–0.4 N m?2. The relationships between floc size and settling velocity established during floc growth stages and that during steady-states are different.  相似文献   

2.
In this paper, we study a population balance equation (PBE) where flocs are distributed into classes according to their mass. Each class i contains i primary particles with mass m p and size L p. All differently sized flocs can aggregate, binary breakup into two equally sized flocs is used, and the floc??s fractal dimension is d 0?=?2, independently of their size. The collision efficiency is kept constant, and the collision frequency derived by Saffman and Turner (J Fluid Mech 1:16?C30, 1956) is used. For the breakup rate, the formulation by Winterwerp (J Hydraul Eng Res 36(3):309?C326, 1998), which accounts for the porosity of flocs, is used. We show that the mean floc size computed with the PBE varies with the shear rate as the Kolmogorov microscale, as observed both in laboratory and in situ. Moreover, the equilibrium mean floc size varies linearly with a global parameter P which is proportional to the ratio between the rates of aggregation and breakup. The ratio between the parameters of aggregation and breakup can therefore be estimated analytically from the observed equilibrium floc size. The parameter for aggregation can be calibrated from the temporal evolution of the mean floc size. We calibrate the PBE model using mixing jar flocculation experiments, see Mietta et al. (J Colloid Interface Sci 336(1):134?C141, 2009a, Ocean Dyn 59:751?C763, 2009b) for details. We show that this model can reproduce the experimental data fairly accurately. The collision efficiency ?? and the ratio between parameters for aggregation and breakup ?? and E are shown to decrease linearly with increasing absolute value of the ??-potential, both for mud and kaolinite suspensions. Suspensions at high pH and different dissolved salt type and concentration have been used. We show that the temporal evolution of the floc size distribution computed with this PBE is very similar to that computed with the PBE developed by Verney et al. (Cont Shelf Res, 2010) where classes are distributed following a geometrical series and mass conservation is statistically ensured. The same terms for aggregation and breakup are used in the two PBEs. Moreover, we argue, using both PBEs, that bimodal distributions become monomodal in a closed system with homogeneous sediment, even when a variable shear rate is applied.  相似文献   

3.
In natural waters,exopolymers or extracellular polymeric substances(EPS) exuded by microorganisms interact with clay particles,resulting in the flocculation of clays and hence alteration to the properties of suspended cohesive sediments.To investigate and further understand how neutral EPS affect cohesive sediment transport and the final sediment yield,an experimental study was conducted on laboratory-prepared clay and guar gum(used as an analog for neutral EPS) suspensions to characterize EPS-induced flocculation and the settling velocity of resultant floes.Four different clays consisting of kaolinite,illite,Ca-montmorillonite,and Na-montmorillonite were studied to examine the influence of different layer charges on clay flocculation induced by neutral EPS.Floc size was determined by a laser particle size analyzer,and settling velocity estimated by analyzing the time-series floc settling images captured by an optical microscope.Results indicate that neutral EPS promote clay-EPS flocculation for all four clays with the particle/floc size significantly increased from～0.1-60μm to as large as～600μm.Clays’ layer charge has a profound influence on the clay-EPS flocculation.With the same floc size,the settling velocity of clay-EPS flocs is typically smaller than that of pure clay flocs,which is attributed to the reduced density of flocs caused by the EPS. However,for flocs of the same composition(e.g.pure clay or hybrid clay-EPS mixture),the settling velocity increases with size.The fractal dimension of these clay-EPS flocs estimated from settling velocity ranges from 1.39 to 1.47,which are smaller than that of pure clay flocs,indicating that these flocs are less compacted than the pure clay flocs.  相似文献   

4.
When fine particles are involved, cohesive properties of sediment can result in flocculation and significantly complicate sediment process studies. We combine data from field observations and state-of-the-art modeling to investigate and predict flocculation processes within a hypertidal estuary. The study site is the Welsh Channel located at the entrance of the Dee Estuary in Liverpool Bay. Field data consist of measurements from a fixed site deployment during 12–22 February 2008. Grain size, suspended sediment volume concentration, and current velocity were obtained hourly from moored instruments at 1.5 m above bed. Near-bottom water samples taken every hour from a research vessel are used to convert volume concentrations to mass concentrations for the moored measurements. We use the hydrodynamic model Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS) coupled with the turbulence model General Ocean Turbulence Model (GOTM) and a sediment module to obtain three-dimensional distributions of suspended particulate matter (SPM). Flocculation is identified by changes in grain size. Small flocs were found during flood and ebb periods—and correlate with strong currents—due to breakup, while coarse flocs were present during slack waters because of aggregation. A fractal number of 2.4 is found for the study site. Turbulent stresses and particle settling velocities are estimated and are found to be related via an exponential function. The result is a simple semiempirical formulation for the fall velocity of the particles solely depending on turbulent stresses. The formula is implemented in the full three-dimensional model to represent changes in particle size due to flocculation processes. Predictions from the model are in agreement with observations for both settling velocity and SPM. The SPM fortnight variability was reproduced by the model and the concentration peaks are almost in phase with those from field data.  相似文献   

5.
Based on experimentally determined floc breakup rate coefficients and aggregation rate coefficients, the relations between floc coherence, physiological state of the mixed liquor, separation characteristics of not easily settling suspended particles and technological parameters were demonstrated.  相似文献   

6.
Particles eroded from hillslopes and exported to rivers are recognized to be composite particles of high internal complexity. Their architecture and composition are known to influence their transport behaviour within the water column relative to discrete particles. To‐date, hillslope erosion studies consider aggregates to be stable once they are detached from the soil matrix. However, lowland rivers and estuaries studies often suggest that particle structure and dynamics are controlled by flocculation within the water column. In order to improve the understanding of particle dynamics along the continuum from hillslopes to the lowland river environment, soil particle behaviour was tested under controlled laboratory conditions. Seven flume erosion and deposition experiments, designed to simulate a natural erosive event, and five shear cell experiments were performed using three contrasting materials: two of them were poorly developed and as such can not be considered as soils, whilst the third one was a calcareous brown soil. These experiments revealed that soil aggregates were prone to disaggregation within the water column and that flocculation may affect their size distribution during transport. Large differences in effective particle size were found between soil types during the rising limb of the bed shear stress sequence. Indeed, at the maximum applied bed shear stress, the aggregated particles median diameter was found to be three times larger for the well‐developed soil than for the two others. Differences were smaller in the falling limb, suggesting that soil aggregates underwent structural changes. However, characterization of particles strength parameters showed that these changes did not fully turn soil aggregates into flocs, but rather into hybrid soil aggregate–floc particles. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
Flocculation settling characteristics of mud: sand mixtures   总被引：2，自引：1，他引：1  
When natural muds become mixed with sandy sediments in estuaries, it has a direct effect on the flocculation process and resultant sediment transport regime. Much research has been completed on the erosion and consolidation of mud/sand mixtures, but very little is known quantitatively about how mixed sediments interact whilst in suspension, particularly in terms of flocculation. This paper presents the settling velocity findings from a recent laboratory study which examined the flocculation dynamics for three different mud/sand mixtures at different concentrations (0.2–5 g.l?1) and turbulent shear stresses (0.06–0.9 Pa) in a mini-annular flume. The low intrusive video-based Laboratory Spectral Flocculation Characteristics instrument was used to determine floc/aggregate properties (e.g., size, settling velocity, density and mass) for each population. Settling data was assessed in terms of macrofloc (>160 μm) and microfloc (<160 μm) settling parameters: Wsmacro and Wsmicro, respectively. For pure muds, the macroflocs are regarded as the most dominant contributors to the total depositional flux. The parameterised settling data indicates that by adding more sand to a mud/sand mixture, the fall velocity of the macrofloc fraction slows and the settling velocity of microflocs quickens. Generally, a mainly sandy suspension comprising 25% mud and 75% sand (25M:75S), will produce resultant Wsmacro which are slower than Wsmicro. The quickest Wsmicro appears to consistently occur at a higher level of turbulent shear stress (τ?～?0.6 Pa) than both the macrofloc and microfloc fractions from suspensions of pure natural muds. Flocculation within a more cohesively dominant muddy-sand suspension (i.e., 75M:25S) produced macroflocs which fell at similar speeds (±10%) to pure mud suspensions at both low (200 mg l?1) and intermediate (1 g?l?1) concentrations at all shear stress increments. Also, low sand content suspensions produced Wsmacro values that were faster than the Wsmicro rates. In summary, the experimental results of the macrofloc and microfloc settling velocities have demonstrated that flocculation is an extremely important factor with regards to the depositional behaviour of mud/sand mixtures, and these factors must be considered when modelling mixed sediment transport in the estuarine or marine environment.  相似文献   

8.
This study investigates the consequences of flocculation for sediment flux in glacier‐fed Lillooet Lake, British Columbia based on density, fractal dimension, in situ profiles of sediment concentration and size distribution, and settling velocity equations presented in the literature. Sediment flux attributed to macroflocs during the late spring and summer accounts for a significant portion of sediment flux in the lake, equivalent to at least one‐quarter of the average annual sediment flux. Fine sediment is reaching the lake floor faster in flocs than occurs if settling as individual grains. This flux varies both spatially and temporally over the observation period, suggesting a link between deposition via flocculation and the properties of bottom sediments. Macrofloc flux increased through June, reached a peak during July, and then declined into August. Macrofloc flux was greatest in the distal end of the first basin, approximately 10 km from the point of inflow. Relatively high excess densities (～0·1 g cm–3 at 500 µm) for flocs in situ are consistent with a composition dominated by inorganic primary particles. Microlaminations within Lillooet Lake varves have been linked by earlier workers to discharge events, and the action of turbidity currents, emanating from the Lillooet River. While turbidity currents undoubtedly occur in Lillooet Lake, these results demonstrate flocculation as an adjunct process linking discharge, lake level, macrofloc flux, bulk density and microlaminations. In situ measurements of sediment settling velocity in glacier‐fed lakes are required to better constrain flux rates, and permit comparison between flocculation in lacustrine environments with existing studies of estuarine, marine and fluvial flocculation. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

9.
Research over the last decade has shown that the suspended sediment loads of many rivers are dominated by composite particles. These particles are also known as aggregates or flocs, and are commonly made up of constituent mineral particles, which evidence a wide range of grain sizes, and organic matter. The resulting in situ or effective particle size characteristics of fluvial suspended sediment exert a major control on all processes of entrainment, transport and deposition. The significance of composite suspended sediment particles in glacial meltwater streams has, however, not been established. Existing data on the particle size characteristics of suspended sediment in glacial meltwaters relate to the dispersed mineral fraction (absolute particle size), which, for certain size fractions, may bear little relationship to the effective or in situ distribution. Existing understanding of composite particle formation within freshwater environments would suggest that in‐stream flocculation processes do not take place in glacial meltwater systems because of the absence of organic binding agents. However, we report preliminary scanning electron microscopy data for one Alpine and two Himalayan glaciers that show composite particles are present in the suspended sediment load of the meltwater system. The genesis and structure of these composite particles and their constituent grain size characteristics are discussed. We present evidence for the existence of both aggregates, or composite particles whose features are largely inherited from source materials, and flocs, which represent composite particles produced by in‐stream flocculation processes. In the absence of organic materials, the latter may result solely from electrochemical flocculation in the meltwater sediment system. This type of floc formation has not been reported previously in the freshwater fluvial environment. Further work is needed to test the wider significance of these data and to investigate the effective particle size characteristics of suspended sediment associated with high concentration outburst events. Such events make a major contribution to suspended sediment fluxes in meltwater streams and may provide conditions that are conducive to composite particle formation by flocculation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

10.
Sediment resuspension is an important way for shallow lake internal pollution to interact with the overlying water column,and the pollution risks are reasonably related to the retention of resuspended sediment particles in overlying water.In the current study,the settling of resuspended sediment particles was comprehensively investigated under different disturbances using five urban lake sediments.The results show that the particle size distributions of resuspended sediment from different lakes exhibited similar variations during settling with disturbance,although varied settling times were observed under static conditions.During settling with and without disturbance,sediment particle sizes were mainly within 8-63μm at the initial stage,and were<8μm in the later stages of settling.Based on these settling characteristics,the sediment particle size was divided into sand(>63μm),silt(8-63μm),and very fine silt and clay(<8μm)fractions.Kinetic analysis suggested that sediment settling for different particle sizes could be well described by the first-and second-order kinetic equations,especially when settling was disturbed(r2=0.727-0.999).The retention of resuspended sediment could be enhanced as particle sizes decreased and disturbance intensities increased.Furthermore,a water elutriation method was successfully optimized,with separation efficiencies of 56.1%-83%,to separate sediment particles into the defined three particle size fractions.The chemical compositions of sediment were found to change with different particle sizes.Typically,calcium tended to form large-size sediment,while the total contents of aluminum,iron,magnesium,and manganese showed significantly negative correlations with sediment particle sizes(p<0.01)and tended to distribute in small-size particles(e.g.,<8μm).Overall,the sediment particle size related settling dynamics and physicochemical properties suggested the necessity on determining the pollution of resuspended sediment at different particle sizes for restoration of shallow lakes.  相似文献   

11.
《Continental Shelf Research》2007,27(3-4):309-321
In October 2000, a 100-year flood event in the Po River resulted in the formation of a fine-grained sediment deposit extending up to 10 km from the river mouth. Soon after this event, and for a subsequent period of 2 years, box cores were collected on a grid of stations off of the Po Delta to observe the evolution of the flood deposit. Using a process-based parameterization of the disaggregated inorganic grain size distribution, the evolution of the surficial sediment on the Po shelf since the 2000 flood has been interpreted in the context of particle flocculation dynamics. This method produces estimates of floc limit, the diameter at which the flux of single grains to the seabed equals the flux of flocs, and floc mass fraction, the amount of material deposited as flocs. Floc limit depends on the extent of flocculation in suspension, and floc mass fraction describes the extent of flocculation in the sediment. Immediately after the flood, these two parameters were high at stations located beneath the path of the flood plume as observed in satellite images. The occurrence of a highly flocculated deposit below the path of the plume leads to two hypotheses: (1) the high sediment concentration in the river flood plume produced extensive sediment flocculation in the plume and (2) post-depositional remobilization of sediment delivered from the plume to the seabed was limited in the relatively low-energy environment of the Po prodelta. Floc limit and floc mass fraction estimated from bottom sediment sampling 3 and 10 months after the initial sampling were lower, indicating that during normal discharge, flocculated fine-grained sediment from the Po River settles close to the mouth, leaving only a small amount of material in suspension in the plume for direct deposition onto the prodelta. These findings are consistent with laboratory studies of suspended sediment that show that sediment concentration and turbulent energy exert dominant control on the extent of flocculation and the loss of sediment from suspension.  相似文献   

12.
Most entrained estuarine sediment mass occurs as flocs. Parameterising flocculation has proven difficult as it is a dynamically active process dependent on a set of complex interactions between the sediment, fluid and the flow. However the natural variability in an estuary makes it difficult to study the factors that influence the behaviour of flocculation in a systematic manner. This paper presents preliminary results from a laboratory study that examined how floc properties of a natural estuarine mud from the Medway (UK), evolved in response to varying levels of suspended sediment concentration and induced turbulent shearing. The experiments utilised the LabSFLOC floc video camera system, in combination with an annular mini-flume to shear the suspended sediment slurries. The flows created in the mini-flume produced average shear stresses, at the floc sampling height, ranging from 0.01 N m−2 to a peak of 1.03 N m−2. Nominal suspended particulate matter concentrations of 100, 600 and 2000 mg l−1 were introduced into the flume. The experimental runs produced individual flocs ranging in size from microflocs of 22.2 μm to macroflocs 583.7 μm in diameter. Average settling velocities ranged from 0.01 to 26.1 mm s−1, whilst floc effective densities varied from 3.5 up to 2000 kg m−3. Low concentration and low shear stress were seen to produce an even distribution of floc mass between the macrofloc (>160 μm) and microfloc (<160 μm) fractions. As both concentration and stress rose, the proportion of macrofloc mass increased, until they represented over 80% of the suspended matter. A maximum average macrofloc settling velocity of 3.3 mm s−1 was attained at a shear stress of 0.45 N m−2. Peak turbulence conditions resulted in deflocculation, limiting the macrofloc fall velocity to only 1.1 mm s−1 and placing over 60% of the mass in the microfloc size range. A statistical analysis of the data suggests that the combined influence of both suspended concentration and turbulent shear controls the settling velocity of the fragile, low density macroflocs.  相似文献   

13.
Aggregation processes of fine sediments have rarely been integrated in numerical simulations of cohesive sediment transport in riverine systems. These processes, however, can significantly alter the hydrodynamic characteristics of suspended particulate matter (SPM), modifying the particle settling velocity, which is one of the most important parameters in modelling suspended sediment dynamics. The present paper presents data from field measurements and an approach to integrate particle aggregation in a hydrodynamic sediment transport model. The aggregation term used represents the interaction of multiple sediment classes (fractions) with corresponding multiple deposition behaviour. The k–ε–turbulence model was used to calculate the coefficient of vertical turbulent mixing needed for the two‐dimensional vertical‐plane simulations. The model has been applied to transport and deposition of tracer particles and natural SPM in a lake‐outlet lowland river (Spree River, Germany). The results of simulations were evaluated by comparison with field data obtained for two levels of river discharge. Experimental data for both discharge levels showed that under the prevailing uniform hydraulic conditions along the river reach, the settling velocity distribution did not change significantly downstream, whereas the amount of SPM declined. It was also shown that higher flow velocities (higher fluid shear) resulted in higher proportions of fast settling SPM fractions. We conclude that in accordance with the respective prevailing turbulence structures, typical aggregation mechanisms occur that continuously generate similar distribution patterns, including particles that settle toward the river bed and thus mainly contribute to the observed decline in the total SPM concentration. In order to determine time‐scales of aggregation and related mass fluxes between the settling velocity fractions, results of model simulations were fitted to experimental data for total SPM concentration and of settling velocity frequency distributions. The comparison with simulations for the case of non‐interacting fractions clearly demonstrated the practical significance of particle interaction for a more realistic modelling of cohesive sediment and contaminant transport. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

14.
The purpose of this paper is to establish a relation between a few measurable quantities (the so-called ζ potential, organic matter content, and shear rate) and the flocculation behavior of mud. The results obtained with small-scale flocculation experiments (mixing jar) are compared to results of large-scale experiments (settling column). The mud used for all experiments has been collected in October 2007 in the lower Western Schelde, near Antwerp, Belgium. From this study, it was found that the mean floc size and the Kolmogorov microscale vary in a similar way with the shear rate for suspensions with different pH and salt concentrations. The size of flocs at a given shear rate depends on the properties of the suspension, which affect the electrokinetic properties of the sediment; these can be described by means of the ζ potential. The main findings of this paper are: (1) In saline suspensions at pH = 8, the mean floc size increases when the salt concentration and the ζ potential increase. (2) For a given ζ potential, the mean floc size at low pH is larger than observed at pH = 8 for any added salt. (3) The mean floc size increases with increasing organic matter content. (4) Mud with no organic matter at pH = 8 and no added salt flocculates very little. The response of mud suspensions to variations in salinity and pH is similar to that of kaolinite. This suggests that a general trend can be established for different and complex types of clays and mud. This systematic study can therefore be used for further development of flocculation models.  相似文献   

15.
《国际泥沙研究》2016,(3):237-243
The Three Gorges Reservoir (TGR) is suffering from unexpected fine sediment deposition, to better understand the fine sediment transport processes, field measurements were conducted at the Zhongxian and Fengjie reaches. A method based on the sediment diffusion equation was proposed to measure the settling velocities using the Acoustic Doppler Velocimeter (ADV). The backscatter acoustic intensities (BSI) received from the ADV were calibrated against the sediment concentrations measured via water sampling, suggesting a linear relationship in double logarithmic coordinate system. The instantaneous sediment concentration was calculated using the derived relationship, and then the settling velocity was obtained through the proposed procedure. The settling velocities of the fine particles in the TGR were found to vary with the water depth. Most of the effective settling velocities were within the range of 0.1–10 mm/s, which were larger than those of the primary particles, indicating that the flocculation was likely to occur in the TGR. Additionally, it is suggested that the turbulent motion played an important role in the flocculation in the TGR.  相似文献   

16.
Colloid transport and distribution in the hyporheic zone   总被引：1，自引：0，他引：1  
Colloids moving from the stream into the hyporheic zone may have a negative impact on aquatic ecosystems as they are potential contaminants or carriers of contaminants. Moreover, retained colloids in the hyporheic zone could not only reduce the exchange flux between the stream and streambed but also change the conditions of the bed, affecting the habitats for aquatic organisms. Previous studies focused on the exchange flux across the sediment–water interface, but the colloid transport processes and distribution of retained colloids in the streambed have received little attention. We conducted experiments within a laboratory flume to examine these processes in a streambed driven by bedform‐induced hyporheic flow. Retained colloids measured in the bed at the end of the experiments revealed colloid retention mainly in the shallow layer of hyporheic zone (0–5 cm below the interface). The results demonstrated significant effects of particle trapping and settling on the colloid transport and distribution in the streambed. Retention leads to the formation of a colloid‐filled shallow layer in the bed. Particle paths based on model simulations showed that colloid settling in pore water modifies the direction of colloid transport and allows the colloid particles to move more deeply in the bed.  相似文献   

17.
Fluvial suspended sediment typically consists of a variety of complex, composite particles referred to as flocs. Floc characteristics are determined by factors such as the source, size and geochemical properties of the primary particles, chemical and biological coagulation processes in the water column and shear stress and turbulence levels in the stream. Studies of floc morphology have used two contrasting methods of sampling and analysis. In the first method, particles settle on a microscope slide and are observed from below using an inverted microscope. The second method uses filtration at no or low vacuum and particles deposited on the filter are observed with a microscope. Floc morphology can be quantified using fractal dimensions. The aims of the present study were to examine the effect of the two sampling methods on the fractal dimensions of particle populations, and to evaluate for each method how well the fractal dimensions at the various sampling sites reflect basin conditions. Suspended solids were collected in triplicate on inverted microscope slides and on 0·45 μm Millipore HA filters in two southern Ontario streams with contrasting riparian zones during a minor runoff event resulting from the melt of a freshly fallen snowpack. An image analysis system was used to determine area, longest axis and perimeter of particles. The morphology of the particle population of each sample was characterized using four fractal dimensions (D, D1, D2 and DK). Systematic differences in fractal dimensions obtained with the two methods were observed. For the settling method, outlines of larger particles were frequently blurred because of the distance between the focal plane (the top of the inverted microscope slides) and the plane of the particle outline. In this method, the blurring of large particles can cause an increase in the projected area and length of the particle. The effect on the particle perimeter is unpredictable because it depends on the amount of detail lost through blurring and its effect on the apparent increase in particle size. Because of blurring, D and D1 tend to be systematically lower for the settling method, whereas the net effect on D2 is unpredictable. Particle size distributions derived from settling are typically coarser because small, low density particles may remain in the water column and all particles may not deposit on the slides. This loss of fines results in systematically lower DK values for the settling method compared with the filtration method. Fractal dimensions and particle size distributions obtained with the filtration method were sensitive to and clearly indicated differences between drainage basins and between sites within each basin. These differences were explained by basin characteristics and conditions. Fractal dimensions and particle size distributions obtained with the settling method were less sensitive to drainage basin characteristics and conditions, which limits their usefulness as process indicators. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

18.
In Mediterranean mountain agroecosystems, soil erosion associated with the development of ephemeral gullies is a common environmental problem that contributes to a loss of nutrient-rich topsoil. Little is known about the influence of ephemeral gully erosion on particle size distribution and its effect on soil organic (SOC) and inorganic (SIC) carbon among different sized soil particles in agricultural soils. In this study, laboratory tests were conducted using velocity settling tube experiments to examine the effects of erosion on sediment particle size distributions from an incised ephemeral gully, associated with an extreme event (235 mm). We also consider subsequent deposition on an alluvial fan in order to assess the distribution of SOC and SIC concentrations and dissolved carbon before and after the extreme event. Soil fractionation was carried out on topsoil samples (5 cm) collected along an ephemeral gully in a cultivated field, located in the lower part of a Mediterranean mountain catchment. The results of this study showed that the sediment transported downstream by runoff plays a key role in the particle size distribution and transportability of soil particles and associated carbon distribution in carbonate rich soils. The eroding sediment is enriched in clay and silt-sized particles at upslope positions with higher SOC contents and gradually becomes coarser and enriched in SIC at the end of the ephemeral gully because the finest particles are washed-out of the study field. The extreme event was associated with an accumulation of dissolved organic carbon at the distal part of the depositional fan. Assessment of soil particle distribution using settling velocity experiments provides basic information for a better understanding of soil carbon dynamics in carbonate rich soils. Processes of soil and carbon transport and relationships between soil properties, erodibility and aggregate stability can be helpful in the development of more accurate soil erosion models. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.  相似文献   

19.
There is increasing interest in tidal wetlands as mechanisms for sustainable and long-term coastal defence. The complexities of the interaction between the deposition of suspended particulate matter (SPM) and submerged vegetation, however, is to a large extent poorly understood. Consequently, accurate parameterisation of cohesive sediment settling fluxes in these environments is a crucial requirement for the development of high-resolution numerical models of wetland morphodynamics. A novel laboratory experiment is described in which the turbulent flow structure within a canopy of the halophytic macrophyte Spartina anglica is examined, and floc characteristics quantified using a unique floc camera configuration able to measure directly the full spectral floc size (D) and settling velocity (Ws). We provide the first quantitative observations of floc characteristics from shallow (h<0.5 m), vegetated flows and investigate the potential influence that variations in vegetative density may have on flocculation, and thus depositional fluxes, in comparison to unvegetated flows.  相似文献   

20.
Field measurements have been carried out to obtain more quantitative knowledge on the longitudinal distribution of the fine-grained sediment dynamics in the Ems Estuary. Both the short-term (time scale < tidal period) and the long-term (tidally averaged) fine-grained sediment transports have been investigated. It is shown that the short-term erosion/sedimentation cycles are most characteristic and are the building blocks for the ultimate long-term transports. Herein, the macroflocs play a key role. The macroflocs had sizes in the range of 200 to 700?μm, sometimes more than 1?mm, and survived high current velocities. The settling velocities were in the range of 0.5 to 8?mm/s. Turbulence intensities and suspended sediment concentration are of utmost importance for the settling properties of the sediment, but the “flocculation ability??of the suspended sediment is shown to be of equal importance. The suspended sediment concentration is only important at low levels of turbulence. At high levels of turbulence, the limiting properties of turbulence dominate. It is demonstrated that the surface properties of the suspended sediment particles are of prime importance for the flocculation ability of the water/sediment mixture, as do the sizes of the resuspended microflocs. Special attention is given to the longitudinal distribution of the sizes and settling properties of the macroflocs as well as of the sediment transports. The flocculation ability was shown to vary almost one order of magnitude along the estuary. These higher flocculation abilities in the direction of higher salinities are responsible for the increase of the sizes of the macroflocs in the seaward direction. Therefore, notwithstanding the low suspended sediment concentrations at the seaward boundary of the Ems Estuary, large macroflocs and corresponding high settling velocities were observed in this area. It explains that, in the Ems Estuary, no distinct effect was determined of the suspended sediment concentration on the settling properties of the suspended macroflocs.  相似文献   

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