Estimating instantaneous concentration of suspended sediment using acoustic backscatter from an ADV

The measurement of instantaneous sediment concentration remains a challenging task. In this study, a three-step procedure is proposed to estimate instantaneous sediment concentration using acoustic backscatter from Acoustic Doppler Velocimeter (ADV). The influences of acoustic noise and particle diameter on an ADV's performance was first tested in the laboratory, then the three-step procedure was verified based on field measurements in the Zhongxian and Fengjie reaches in the Three Gorges Reservoir. The first step involves reconstructing the backscatter signal time series. Due to contamination from both the noise floor and spurious spikes, the denoising-despiking method was applied instead of the traditional velocity-despiking methods, and this approach performed well based on spectrum ana-lysis. The second step involves calibrating the sediment concentration against the backscatter signal. A linear relation, whose slope and intercept were calibrated to be dependent on particle diameter, is proposed in double logarithmic coordinates. The third step involves calculating the instantaneous sediment concentration using reconstructed instantaneous backscatter based on the proposed relation. The accuracy of the proposed method was evaluated through consideration of the concentration spec-trum and sediment flux, indicating that the proposed three-step procedure is effective for the mea-surement of instantaneous sediment concentration.     

Numerical modelling of the suspended sediment transport in Torres Strait

One-dimensional vertical and three-dimensional fine-resolution numerical models of sediment transport have been developed and applied to the Torres Strait region of northern Australia. The one-dimensional model, driven by measured waves and currents, was calibrated against measured suspended sediment concentrations using a sequential data assimilation algorithm. The algorithm produced a good match between model and data, but this was achieved only by allowing some temporal variability in parameter values, suggesting that there were underlying uncertainties in the model structure and forcing data. Implications of the assimilation results to the accuracy of the numerical modelling are discussed and the need for observational programmes having an extensive spatial and temporal coverage is highlighted. The three-dimensional sediment model, driven by modelled waves and currents, simulates sediment transport over the shelf during the monsoon and trade-wind seasons covering 1997–2000. The model predicts strong seasonal variability of the sediment transport on the shelf attributed to seasonally varying hydrodynamics, and illustrates significant inter-annual variability of the sediment fluxes driven by extreme events. The developed model provides a platform for testing scientific hypothesis. With additional calibration, including uncertainty analysis, it can also be used in a management context.     

Using ADV backscatter strength for measuring suspended cohesive sediment concentration

Laboratory experiments were conducted at two institutes to reveal the relationship between acoustic backscatter strength and suspended sediment concentration (SSC). In total, three acoustic Doppler velocimeters (ADVs) with different frequencies (5, 10 and 16 MHz) were tested. Two different commercial clays and one natural sediment from Clay Bank site in the York River were checked for acoustic responses. The SSCs of selected sediments were artificially changed between a selected low and a high value in tap or de-ion water. Each ADV showed quite different backscatter responses depending on the sediment type and SSC. Not all devices had a good linear relationship between backscatter strength and SSC. Within a limited range of SSC, however, the backscatter strength can be well correlated with the SSC. Compared with optical backscattering sensor (OBS), the fluctuation of ADV backscatter signals was too noisy to be directly converted to the instantaneous changes of SSC due to high amplification ratio and small sampling volume. For the more accurate signal conversion for finding the fluctuation of SSC, the ensemble average should be applied to increase the signal-to-noise ratio. There are unexpected responses for the averaged backscatter wave strength: (1) high signals from small particles but low signals from large particles; and (2) two linear segments in calibration slope. These phenomena would be most likely caused by the different gain setting built in ADVs. The different acoustic responses to flocculation might also contribute somewhat if flocs are tightly packed. This study suggests that an ADV could be a useful instrument to estimate suspended cohesive sediment concentration and its fluctuation if the above concerns are clarified.     

A new approach for linking event‐based upland sediment sources to downstream suspended sediment transport

In this study, we proposed a new approach for linking event sediment sources to downstream sediment transport in a watershed in central New York. This approach is based on a new concept of spatial scale, sub‐watershed area (SWA), defined as a sub‐watershed within which all eroded soils are transported out without deposition during a hydrological event. Using (rainfall) event data collected between July and November, 2007 from several SWAs of the studied watershed, we developed an empirical equation that has one independent variable, mean SWA slope. This equation was then used to determine event‐averaged unit soil erosion rate, Q_S/A, (in kg/km²/hr) for all SWAs in the studied watershed and calculate event‐averaged gross erosion E_ea (in kg/hr). The event gross erosion E_t (in kilograms) was subsequently computed as the product of E_ea and the mean event duration, T (in hours) determined using event hydrographs at the outlet of the studied watershed. Next, we developed two linear sediment rating curves (SRCs) for small and big events based on the event data obtained at the watershed outlet. These SRCs, together with T, allowed us to determine event sediment yield SY_e (in kilograms) for all events during the study period. By comparing E_t with SY_e, developing empirical equations (i) between E_t and SY_e and (ii) for event sediment delivery ratio, respectively, we revealed the event dynamic processes connecting sediment sources and downstream sediment transport. During small events, sediment transport in streams was at capacity and dominated by the deposition process, whereas during big events, it was below capacity and controlled by the erosion process. The key of applying this approach to other watersheds is establishing their empirical equations for Q_S/A and appropriately determining their numbers of SWAs. Copyright © 2011 John Wiley & Sons, Ltd.     

Use of a single-bowl continuous-flow centrifuge for dewatering suspended sediments: Effect on sediment physical and chemical characteristics

The use of a single-bowl continuous-flow centrifuge (CFC, Sharples-Pennwalt Model AS-12) for dewatering suspended sediment from large volumes of river water is evaluated. Sediment-recovery efficiency of 86-91 per cent is comparable to that of other types of CFC units. The recovery efficiency is limited by the particle-size distribution of the feed water and by the limiting particle diameter that is retained in the centrifuge bowl. The limiting particle diameter, using the parameters for this study (bowl radius = 10.5cm; bowl length = 71.1 cm; rotational velocity = 16000 r min^?1; flow rate = 2 L min^?1, and an assumed hydrated particle density = 1.7 gm cm^?3), is 370 nm. There seems to be no particle-size fractionation within the centrifuge bowl—the median particle size was the same at the top as at the bottom. Particle electrophoretic mobility plays some role in fractionation of particles within the centrifuge. The mobility ranged from ?1.19 to ?2.01 × 10^?8 m² V^?1 s^?1, which is typical of clays coated with organic matter, the charge of which is partially neutralized by divalent cations and iron. Contamination by trace metals and organics is minimized by coating all surfaces that come in contact with the sample with either FEP or PFA Teflon and using a removable FEP Teflon liner in the centrifuge bowl. Because of the physical and chemical factors affecting particle fractionation within the centrifuge, care must be exercised in interpreting the environmental consequences of particles collected by continuous-flow centrifugation.     

Theoretical/numerical model for the transport of non-uniform suspended sediment in open channels

In this paper, we address the transport of multi-disperse suspended sediment mixtures in open channels, via the use of the two-fluid model. To that end, we extend previously developed frameworks for the dilute and non-dilute transport of suspended sediment. Within the scope of the Reynolds-averaged Navier-Stokes (RANS) equations, these modeling frameworks comprise mass and momentum equations for both phases (water and sediment). Here, we calculate the distribution of total volumetric concentration of sediment using two approaches: (1) by considering the mixture as represented by a single size; we call this approach Partial two-fluid model for uniform sediments (PTFMU); and (2) by combining the volumetric concentration of the sediment corresponding to several particle size classes; we call this approach Partial two-fluid model for non-uniform sediments (PTFMNU). In the second approach, we propose a methodology for the computation of the overall velocity of the disperse phase as a function of the velocities of each size class. k-ε type closures to account for the turbulence in the carrier phase (water) are applied. We also consider the coupling between the two phases through the drag force. Velocities of the carrier and disperse phases, and concentrations for each sediment class size are numerically solved by integrating the differential equations over control volumes. In order to validate our models, we compare numerical results to experimental data of Einstein and Chien [H.A. Einstein, N. Chien, Effects of heavy sediment concentration near the bed on velocity and sediment distribution, MRD sediment series report, University of California, Berkley, 1955] and Taggart et al. [W.C. Taggart, C.A. Yermoli, S. Montes, A. Ippen, Effects of sediment size and gradation on concentration profiles for turbulent flow, Massachusetts Institute of Technology, 1972]. Results of mean velocity of the carrier phase are in close agreement with the experimental data. For the prediction of sediment concentrations, we observe that there is a difference in the results using the two approaches mentioned above. We additionally obtain values of the Schmidt number needed to improve the agreement between predictions of the distribution of suspended sediment and the experimental data, and discuss the effect of sediment size and increasing sediment concentration on the values of the Schmidt number.     

A discontinuous finite element suspended sediment transport model for water quality assessments in river networks

Suspended sediment plays an important role in the distribution and transport of many pollutants (such as radionuclides) in rivers. Pollutants may adsorb on fine suspended particles (e.g. clay) and spread according to the suspended sediment movement. Hence, the simulation of the suspended sediment mechanism is indispensable for realistic transport modelling. This paper presents and tests a simple mathematical model for predicting the suspended sediment transport in river networks. The model is based on the van Rijn suspended load formula and the advection–diffusion equation with a source or sink term that represents the erosion or deposition fluxes. The transport equation is solved numerically with the discontinuous finite element method. The model evaluation was performed in two steps, first by comparing model simulations with the measured suspended sediment concentrations in the Grote Nete–Molse Nete River in Belgium, and second by a model intercomparison with the sediment transport model NST MIKE 11. The simulations reflect the measurements with a Nash‐Sutcliffe model efficiency of 0.6, while the efficiency between the proposed model and the NST MIKE 11 simulations is 0.96. Both evaluations indicate that the proposed sediment transport model, that is sufficiently simple to be practical, is providing realistic results.     

Magnitude‐frequency characteristics of effective discharge for suspended sediment transport,Fraser River,British Columbia,Canada

This study analyses archival discharge and sediment concentration data (1965–1988), monitored by Water Survey of Canada, to examine suspended sediment transport rates and their relationship to effective discharge (Q_eff) based on daily discharge duration curves. Effective discharge was determined as the mid‐point of the discharge class transporting the greatest portion of the suspended sediment load (hence class‐based Q_eff). Results showed that the concept of effective discharge was applicable to the Fraser River basin where the average class‐based Q_eff occurred during 8·4% of the study period with individual values ranging from 0·03% to 16·1%. The durations of effective discharge classes ranged from 0·02% to 19·6% while the transport of 50% of total sediment loads ranged from 3% to 22% with an average of 14% of the time. Equations for predicting the class‐based Q_eff in the Fraser River basin from bankfull discharge and drainage area are presented. The observed variations among stations in sediment‐discharge regimes based on subjectively selected 20 discharge classes, seem to reflect the influence of sediment controlling factors such as geology, physiography, catchment size and land use practice in the basin. Future directions of research on applications of the effective discharge concept are explored. As a solution to the problem of lack of an objective method for determining the effective discharge, the effective discharge should be determined from event based assessments of sediment transport (event‐based Q_eff), avoiding any subjectivity in the selection of number of discharge classes used for its determination. In conclusion, it is proposed that continued use of the conventional method of determining Q_eff should cease. Copyright © 1999 John Wiley & Sons, Ltd.     

A two-dimensional model for suspended sediment transport in the southern branch of the Rhine–Meuse estuary,The Netherlands

For the southern branch of the Rhine–Meuse estuary, The Netherlands, a two-dimensional horizontal suspended sediment transport model was constructed in order to evaluate the complicated water quality management of the area. The data needed to calibrate the model were collected during a special field survey at high river runoff utilizing a number of techniques: (1) turbidity probes were used to obtain suspended sediment concentration profiles; (2) air-borne remote sensing video recordings were applied in order to obtain information concerning the spatial distribution of the suspended sediment concentration; (3) an acoustic probe (ISAC) was used to measure cohesive bed density profiles and (4) an in situ underwater video camera (VIS) was deployed to collect video recordings of the suspended sediment. These VIS data were finally processed to fall velocity and diameter distributions and were mainly used to improve insight into the relevant transport processes, indicating significant erosion of sand from the upstream Rhine branch. For quantitative calibration of the model, the data from the turbidity profiles were used. Sedimentation and erosion were modelled according to Krone and Partheniades. The model results showed a good overall fit to the measurements, with a mean absolute error of 18 per cent (standard fault = 1 per cent), corresponding to concentrations of about 0·020 (upstream) to 0·005 kg m⁻³ (downstream). The overall correlation between observed and simulated suspended sediment concentrations was 0·85. The remote sensing video recordings were used for a qualitative calibration of the model. The distribution pattern of the suspended sediment on these photos was reproduced quite well by the model. However, a more accurate calibration technique is needed to enable the use of aerial remote sensing as a quantitative calibration method. Copyright © 1999 John Wiley & Sons, Ltd.     

A geomatics‐based approach for the derivation of the spatial distribution of sediment transport processes in periglacial mountain environments

Within this paper modern techniques such as satellite image analysis and tools provided by geographic information systems (GIS) are exploited in order to extend and improve existing techniques for mapping the spatial distribution of sediment transport processes. The processes of interest comprise mass movements such as solifluction, slope wash, dirty avalanches and rock‐ and boulder falls. They differ considerably in nature and therefore different approaches for the derivation of their spatial extent are required. A major challenge is addressing the differences between the comparably coarse resolution of the available satellite data (Landsat TM/ETM+, 30 m × 30 m) and the actual scale of sediment transport in this environment. A three‐stepped approach has been developed which is based on the concept of Geomorphic Process Units (GPUs): parameterization, process area delineation and combination. Parameters include land cover from satellite data and digital elevation model derivatives. Process areas are identified using a hierarchical classification scheme utilizing thresholds and definition of topology. The approach has been developed for the Kärkevagge in Sweden and could be successfully transferred to the Rabotsbekken catchment at Okstindan, Norway using similar input data. Copyright © 2008 John Wiley & Sons, Ltd.     

In-channel surficial fine-grained sediment laminae. Part II: Chemical characteristics and implications for contaminant transport in fluvial systems

Samples of surficial fine-grained laminae (SFGL) were collected in three south-western Ontario rivers. Each sediment sample was subjected to a sequential extraction procedure designed to partition particulate metals (Cd, Pb, Cu, Zn) into five operationally defined fractions: (1) exchangeable; (2) bound to carbonates; (3) bound to Fe-Mn oxides; (4) bound to organic matter; and (5) residual. Particulate phosphus was sequentially extracted from the sediment samples into three fractions: (1) non-apatite inorganic P; (2) apatite P; and (3) organic P. The major accumulate phases of trace metals in SFGL are carbonates, Fe-Mn oxides and organic matter. The content of NAIP in SFGL ranged from 17 to 38% of total particulate P. Compared with suspended and bed sediments, levels of P and trace metals in SFGL were lower at the study sites. A conceptual overview of physical, chemical and biological processes influencing formation of SFGL and the potential role of this fine-grained sediment for contaminant transport in fluvial systems is presented.