Suspended Sediment Particle Size Research Articles

Summary of Experiments and Influencing Factors of Sediment Settling Velocity in Still Water

Sediment deposition significantly impacts soil erosion processes, consequently influencing the geographical morphology and surrounding environments of reservoirs and estuaries. Given the intricate nature of sediment deposition, it is imperative to consolidate and analyze existing research findings. Presently, studies on sediment settling velocity primarily employ theoretical, laboratory, and field experimentation methods. Theoretical approaches, rooted in mechanics, examine the various forces acting on sediment particles in water to derive settling velocity equations. However, they often overlook external factors like temperature, salinity, organic matter, and pH. Although laboratory experiments scrutinize the influence of these external factors on sedimentation velocity, sediment settling is not solely influenced by individual factors but rather by their collective interplay. Field observations offer the most accurate depiction of sediment deposition rates. However, the equipment used in such experiments may disrupt the natural sedimentation process and damage flocs. Moreover, measurements of sediment particle size from different instruments yield varied results. Additionally, this paper synthesizes the impact of suspended sediment concentration, particle size, shape, temperature, salinity, and organic matter on sediment settling velocity. Future research should focus on innovating new laboratory observation methods for sediment settling velocity and utilizing advanced scientific and technological tools for on-site measurements to provide valuable insights for further investigation into sediment settling velocity.

In-Stream Laser Diffraction for Measuring Suspended Sediment Concentration and Particle Size Distribution in Rivers: Insights from Field Campaigns

This study evaluates the laser in situ scattering and transmissometry (LISST) instrument LISST-SL2, a laser diffraction instrument for suspended sediment sampling in rivers, with concurrent physical measurements of suspended sediment concentration (SSC) and particle size distribution (PSD) as well as velocity measurements by an acoustic Doppler current profiler (ADCP). We collected 136 LISST-SL2 samples along with 61 physical samples for SSC measurement, of which 24 physical samples included PSD measurement during 2018–2020 from 11 sites in Washington state and Virginia. An effective density is required to convert the measured volumetric SSC by the LISST-SL2 into a reported mass SSC, and by default the LISST-SL2 assumes a value of 2.65 g/mL. From our data set, we computed effective densities (mass SSC/volumetric SSC) that ranged from 0.5 to 5.4 g/mL, with a best-fit value of 2.05 g/mL. Additionally, the LISST-SL2 was not able to measure the finest sediment sizes in suspension, which affects the resulting PSD. Therefore, we propose some adjustments of the LISST-SL2 data with a supporting physical sample to account for these effective density and PSD issues. When doing so, we were able to reduce the root-mean square relative error (RMSRE) to 18% from 117% for SSC, and to 26% from 78% for PSD. LISST-SL2 velocities were generally higher than ADCP velocities with a 21% RMSRE. Our results and guidance will allow for more accurate sampling by the LISST-SL2, which has potential for studying spatial and temporal variation of suspended sediment characteristics in rivers.

Inferring Suspended Sediment Carbon Content and Particle Size at High Frequency From the Optical Response of a Submerged Spectrometer

AbstractReal time monitoring of suspended sediment (SS) carbon content and particle size information is essential to understand SS transport mechanisms and processes. However, limited in situ methods are available, and manual and unattended sampling makes high frequency observation challenging. Based on evidence that optical measurements of SS concentration are dependent on particle size and composition, we hypothesize that SS carbon content and mean particle size can simultaneously be inferred from the absorbance data measured with a submerged spectrometer at high frequency. With the aim to predict both parameters, we investigate if global calibrations can be obtained and used instead of site‐dependent local calibrations. To test this, we created a laboratory data set using a tank setup, and an in situ field data set. Sediment samples varying in composition were collected and used in a tank setup to obtain global (all sites grouped together) calibrations between absorbance, particle size and carbon content. Two sites were instrumented to collect in situ field data, to generate local (site‐specific) calibrations and to validate the global calibrations obtained in the laboratory. A procedure to account for the effect of SS concentration when using the global calibration to predict is also presented. Our findings indicate that global calibrations can accurately predict from in situ absorbance readings, with a median absolute error of 17.3 μm, versus 10.5 μm for a local calibration. Local calibrations are indispensable to reliably predict carbon content, with a median absolute error of 1.3%.

The interplay of suspended sediment concentration, particle size and fluid velocity on the rapid deposition of suspended iron oxide particles in PVC drinking water pipes

The inner walls of drinking water pipes are often expected to be clean and controlled surfaces to assure safe water access to the public. However, these surfaces are typically contaminated with particulate materials and biofilms that eventually degrade water quality. While water utilities place significant efforts in identifying and flushing material deposits from compromised pipes, the development of effective preventive strategies is still limited by the lack of knowledge about material accumulation processes. The aim of this paper is to examine the interplay between suspended sediment concentration, particle size and fluid velocity and the attachment of suspended iron oxide particles in PVC drinking water pipes. For that purpose, a series of short experiments were completed, whereby water amended with a known concentration of selected and stable iron oxide particles was introduced in a full-scale pipe loop laboratory over a short period of time and both turbidity and suspended sediment concentration (SSC) were measured at the inlet and outlet of the pipe loop. Results showed that a selected fraction of the injected particles with sizes above a specific threshold in the range of 4.6 to 6.8 µm had not reached the downstream section of the pipe loop, but instead remained attached to the pipe walls. In addition, exponential decay of the SSC was observed to occur along the pipes and to cause most of the sediments to accumulate in the upstream section of the pipe loop. The research improved our understanding of the physical processes of particulate material accumulation in DWDSs, and it will help water utilities forecast and monitor material accumulation and discolouration potential.

Response of nephelometric turbidity to hydrodynamic particle size of fine suspended sediment

Turbidity is used as a surrogate for suspended sediment concentration (SSC), and as a regulatory tool for indicating land use disturbance and environmental protection. Turbidity relates linearly to suspended material, however, can show non-linear responses to particulate organic matter (POM), concomitant with changes in particle size distribution (PSD). In the paper the influence of ultra-fine particulate matter (UFPM) on specific turbidity and its association with POM in suspended sediment are shown for alpine rivers in the Southern Alps of New Zealand. The approach was two-fold: a field-based investigation of the relations between SSC, POM, and turbidity sampled during event flow; and experimental work on hydrodynamic particle size effects on SSC, POM, PSD, and turbidity. Specific turbidity changes over event flow and are sensitive to increasing proportional amounts of sand, UFPM, and POM in suspension. Furthermore, the UFPM is the size fraction (<6 μm) where POM increases. The implications of the current study are that the slopes of turbidity-SSC relations are undesirable in locations that may be dominated by cyclic release of POM or distinct pulses of fine-grained material. At locations where the turbidity-SSC slopes approximate 2, the POM proportion is usually <10% of the total suspended load. However, when turbidity-SSC slopes are <1 this is likely caused by high amounts of side-scatter from UFPM concomitant with higher proportions of POM. Thus, the use of turbidity as a proxy for determining SSC may have serious consequences for the measurement of representative suspended sediment data, particularly in locations where POM may be a significant contributor to overall suspended load.

Hydrodynamic characteristics and channel morphodynamics at a large asymmetrical confluence with a high sediment-load main channel

Previous studies of confluences have concentrated mainly on confluences of small streams, confluences with simple geometric configurations, and confluences of curving rivers. However, the confluences of natural rivers, especially large confluences of high sediment-load rivers, generally have complex geometric confluent boundaries and various incoming flow and sedimentation conditions. In this study, several field measurements of the three-dimensional velocity components, water surface elevation, bed topography, and suspended sediment particle sizes reveal the complex hydrodynamic characteristics and bed morphology of the asymmetrical confluence of the high sediment-load Yellow River and its clear tributary, the Fen River. The results show that the water surface morphology of this confluence is elevated in the stagnation zone, mixing interface, and the opposite side of the tributary, whereas the converse is true at the downstream junction corner. The geomorphologic features of the bed show a small scour hole and a long narrow scour belt under conventional hydrological conditions, or a larger scour hole under hydraulic conditions when the value of the momentum flux ratio is much greater than one. In contrast to the confluences of small streams and curving rivers, this confluence does not exhibit continuous flow separation or helical motion, but has pronounced regions such as a stagnation zone, flow acceleration and deflection areas, a shear layer/mixing interface, and a flow recovery zone. The position and size of these regions vary with the momentum flux ratio. The particle size of suspended sediment at the confluence is normally distributed, and the changing sedimentation process corresponds to the scour and siltation state of the bed. In general, the particle size of the suspended sediment is small in the stagnation zone and the downstream channel near the tributary, but large in the acceleration zone and shear layer. These results are helpful to better understand the hydrodynamic and morphodynamics characteristics of confluences with complex geometric configurations and junction conditions.

Field investigation of flow and bed erosion/deposition at the confluence of the Yellow River and Fen River

In order to study the characteristics of flow structure and bed erosion/deposition at the confluence of the Yellow River and the Fen River, the three-dimensional velocity components, water depth, water level, and suspended sediment particle sizes of the typical sections were measured in field to analyze the bed morphology, the flow field structure, suspended sediment distribution, and the variation of the bed erosion or deposition. The results showed that the pattern of the flow field at the confluence essentially conforms to the conceptual model, but represented a relatively complex structure, owing to the dual influence of the convergent flows and the tanglesome bed topography. The spatial distribution of suspended sediment was affected by the flow velocity and eddy turbulence. The particle size of suspended sediment was approximately normal distribution. The median particle size ranged from 17.19μm to 140μm. The suspended sediment at the confluence center was mainly composed of upstream transfer sediment and resuspended local sediment near the bed surface. The riverbed morphology was highly complex with frequent changes in erosion and deposition. And there was a strip-shaped sand ridges at the center of the channel and a long-narrow scouring belt near the tributary side. In addition, the bed configuration was similar in the dry and normal season. Due to the sediment deposition, the bed elevation was higher in dry season than that in normal season. During the flood season, the washed bed was stated in a dynamic flattening period, as well as the sand ridge disappeared and the scour belt became shallow.

Occurrence of anammox on suspended sediment (SPS) in oxic river water: Effect of the SPS particle size

Anammox is a newly discovered nitrogen transformation process. However, its role in nitrogen removal in fresh water is far from understood. Here, we hypothesized that anammox could occur on suspended sediment in oxic river water. To test this hypothesis, simulation experiments with a nitrogen stable (15N) isotopic tracer technique were conducted to study the occurrence of anammox on suspended sediment (SPS) in oxic river water, and the effects of the SPS particle size, including <20 μm, 20–63 μm, 63–100 μm, 100–200 μm, and <200 μm (original SPS) size fractions, were investigated. The results showed that anammox occurred in oxic water with SPS due to the existence of low oxygen microsites around/on SPS, and the anammox rate was even higher than the denitrification rate. The anammox rate increased with the SPS concentration, and it was negatively correlated with the particle size and was positively correlated with the organic carbon content of SPS (p < 0.05). The 29N2 produced by anammox in a system containing 1.0 g L−1 SPS with a particle size below 20 μm was 0.27 mg-N/m3·d, which was 5.3 times higher than that produced with a particle size of 100–200 μm. The anammox rate was significantly positively correlated with the anammox bacterial abundance (p < 0.01), and Ca. Brocadia was the dominant species. This study suggests that the SPS in oxic water may be a ‘hotspot’ for the anammox process and that its role in nitrogen removal should be considered in future studies.

High Platform Elevations Highlight the Role of Storms and Spring Tides in Salt Marsh Evolution

We combine sea level records and repeat lidar surveys at 8 sites in the United Kingdom and the United States to explore controls on marsh accretion. We compare marsh elevations relative to sea level as well as lidar-derived marsh accretion rates to simple 0-dimensional settling simulations in order to explore constraints on suspended sediment concentration and particle size. We find that the marsh platforms examined occupy a narrow range of elevations in the upper tidal frame, situated between Mean High Tide $MHT$ and the Observed Highest High Tide $OHHT$. Under sinusoidal tidal forcing, common in marsh accretion models, marshes at these elevations are never inundated, highlighting the inadequacy of sinusoidal forcing in numerical models of salt marshes. Forcing the model with year-long tidal records, deposition rates follow hyperbolic contour lines when expressed as a function of sediment concentration and median grain size. We also observe that when using a median sediment grain size $D_{50} = 50 \mu m$ and sediment concentrations derived from satellite data, modeled deposition rates are much lower than when using field data. We find that the deposition of coarse, concentrated sediment is necessary for platforms in the upper tidal frame to withstand sea level rise, suggesting a strong dependance on infrequent high-deposition events. This is particularly true for marshes that are very high in the tidal frame, making accretion increasingly storm-driven as marsh platforms gain elevation. Finally, we reflect on the capacity of marshes to regenerate after erosion events within a context of changing sediment supply conditions.

Evaluation of a simple, inexpensive, in situ sampler for measuring time‐weighted average concentrations of suspended sediment in rivers and streams

AbstractThe accurate measurement of suspended sediment (&lt;200 μm) in aquatic environments is essential to understand and effectively manage changes to sediment, nutrient, and contaminant concentrations on both temporal and spatial scales. Commonly used sampling techniques for suspended sediment either lack the ability to accurately measure sediment concentration (e.g., passive sediment samplers) or are too expensive to deploy in sufficient number to provide landscape‐scale information (e.g., automated discrete samplers). Here, we evaluate a time‐integrated suspended sediment sampling technique, the pumped active suspended sediment (PASS) sampler, which collects a sample that can be used for the accurate measurement of time‐weighted average (TWA) suspended sediment concentration and sediment particle size distribution. The sampler was evaluated against an established passive time‐integrated suspended sediment sampling technique (i.e., Phillips sampler) and the standard discrete sampling method (i.e., manual discrete sampling). The PASS sampler collected a sample representative of TWA suspended sediment concentration and particle size distribution of a control sediment under laboratory conditions. Field application of the PASS sampler showed that it collected a representative TWA suspended sediment concentration and particle size distribution during high flow events in an urban stream. The particle size distribution of sediment collected by the PASS and Phillips samplers were comparable and the TWA suspended sediment concentration of the samples collected using the PASS and discrete sampling techniques agreed well, differing by only 4% and 6% for two different high flow events. We should note that the current configuration of the PASS sampler does not provide a flow‐weighted measurement and, therefore, is not suitable for the determination of sediment loads. The PASS sampler is a simple, inexpensive, and robust in situ sampling technique for the accurate measurement of TWA suspended sediment concentration and particle size distribution.

Influence of live microbes on suspended sediment concentration in coastal ecosystem

Microbes present in both fresh and saline water associate with suspended sediments by secrete extracellular polymeric substance to survive under stressful environmental conditions. Deposition of these bio-contaminated sediments affects the bed ecosystem. When re-suspended, they further affect water quality and health of both human as well as aquatic life. In this study, laboratory experiments are conducted in an annular flume to understand the interaction between suspended sediments attached with microbes (biosediment) and without microbes (non-biosediment) at different mix ratios (Mr = 0:1, 0.25:0.75, 0.50:0.50, and 1:0) and water salinity (S = 0, 1, 2, 5, 10, and 35 g/L). Results show that, rate of deposition increases with an increase in biosediment concentration for any water salinity and mix ratio. In saline water, presence of microbes on suspended sediments induces more deposition until water salinity of 5 g/L and thereafter deposition reduces. Presence of 25% and 50% of biosediment in water increases deposition up to a critical salinity of 1.72 g/L and 2.60 g/L, respectively. Suspended sediment particle size increases with an increase in biosediment concentration and decreases with an increase in water salinity. Fractal dimension results show that, when salinity increases, fragile non-biosediment and dense biosediments are retained in suspension. This study shows that presence of live microbes on suspended sediments affects their depositional behaviour.

Flow class analyses of suspended sediment concentration and particle size in a mixed-land-use watershed

Knowledge gaps remain concerning fundamental suspended sediment physical processes/relationships, such as particle size class dynamics and hydroclimatic variability. Streamwater grab samples were collected four times per week (Oct. 2009–Feb. 2014) at nested-scale gauging sites (n = 5), representing contrasting dominant land use practices. Streamflow was monitored in situ. Grab samples were analyzed for total suspended sediment concentration and mean particle size using laser particle diffraction. Comparisons were performed of suspended sediment parameters corresponding to different streamflow classes (i.e. 20th, 40th, 60th, 80th, and 99th percentile flows). Average suspended sediment concentrations displayed a decreasing trend from the predominately agricultural headwaters to the urbanized mid-watershed, and a subsequent increase to the suburban lower watershed. Results indicated significant (p < 0.05) differences in concentrations corresponding to different flow classes, with concentrations at more urban sites displaying greater “sensitivity” to streamflow variability. Significant (p < 0.05) differences between concentrations at different sites were found, but concentrations became progressively more similar (p > 0.05) at higher flows. Mean particle size results displayed significant differences (p < 0.05) between flow classes at every site. Notably, results showed a decrease in particle size during progressively higher flows, despite expectations based on stream velocity/competence relationships. Significant (p < 0.05) spatial differences in particle size were found between sites, specifically for flows within the 20th and 40th percentile flow class. However, the spatial pattern was weakened at higher flows (60th, 80th, and 99th percentile flow classes) as sites displayed greater statistical similarity. Collectively, results highlight the compounding influences of streamflow variability and land use practices on suspended sediment regimes; and considering unexpected results regarding relationships between particle size and flow, emphasize the need for continued research concerning particle size dynamics.

Influence of suspended sediment characteristics on the bioaccumulation and biological effects of citalopram in Daphnia magna

The influence of suspended sediment (SPS) characteristics on the bioavailability of the antidepressant citalopram (CIT) was investigated in the cladoceran Daphnia magna. The bioaccumulation, swimming behaviours, psychological indices, and oxidative stress were examined. The CIT bioaccumulation were altered in the presence of SPS, such that the body burden of CIT decreased as the concentration of SPS increased and as the organic carbon content of SPS (foc) increased; moreover, the body burden of CIT increased as SPS particle size increased. All the biomarker activities of D. magna were markedly induced at a CIT exposure concentration of 10 μg/L. However, the biological effects of CIT did not depend on the body burden of CIT as SPS concentration increased, while the swimming activities and oxidative stress were significantly enhanced by SPS concentration at 1 g/L. The influences of SPS particle size and foc on the activities of swimming and physiological indicators were mainly associated with the CIT bioaccumulation, while foc in SPS was more substantial than particle size. In addition, the antioxidant activities decreased as foc increased and were significantly strengthened at particle sizes of 30–60 μm. The impacts of different SPS characteristics on the adsorption and desorption capacity of CIT and the ingestion habits of D. magna were the main reasons for the variations in CIT body burden and biological effects. According to the results obtained in this study, the SPS characteristics should be considered in the risk assessment of contaminants in natural aquatic environments.

Spatiotemporal variability of suspended sediment particle size in a mixed-land-use watershed

Given existing knowledge gaps, there is a need for research that quantitatively characterizes spatiotemporal variation of suspended sediment particle size distribution (PSD) in contemporary watersheds. A five-year study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed study design, comprising five gauging sites partitioning the catchment into five sub-watersheds. Streamwater grab samples were collected four times per week, at each gauging site, for the duration of the study period (Oct. 2009–Feb. 2014). Samples were analyzed using laser particle diffraction. Significantly different (p<0.05) suspended sediment PSDs were observed at monitoring sites throughout the course of the study. For example, results indicated greater proportions of silt at site #5 (65%), relative to other sites (41, 32, 29, and 43%, for sites #1–#4, respectively). Likewise, results showed greater proportions of sand at sites #2 and #3 (66 and 68%, respectively), relative to other sites (57, 55, and 34%, for sites #1, #4, and #5, respectively). PSD spatial variability was not fully explained by hydroclimate or sub-watershed land use/land cover characteristics. Rather, results were strengthened by consideration of surficial geology (e.g. supply-controlled spatial variation of particle size). PSD displayed consistent seasonality during the study, characterized by peaks in the proportion of sand (and aggregates) during the winter (i.e. 70–90%), and minimums during the summer (i.e. 12–38%); and peaks in the proportion of silt particles in the summer (i.e. 61–88%) and minimums in the winter (i.e. 10–23%). Likely explanations of results include seasonal streamflow differences. Results comprise distinct observations of spatiotemporal variation of PSD, thereby improving understanding of lotic suspended sediment regimes and advancing future management practices in mixed-land-use watersheds.

Dynamics of suspended sediment concentration, flow discharge and sediment particle size interdependency to identify sediment source

Spatiotemporal behavior of sediment yield is a key for proper watershed management. This study analyzed statistical characteristics and trends of suspended sediment concentration (SCS), flow discharge (FD) and sediment particle sizes using data from 24 gage stations scattered throughout the United States. Analysis showed significant time- and location-specific differences of these variables. The median values of SSC, FD and percentage of particle sizes smaller than 63µm (P63) for all 24 gage stations were found to be 510.236mgl−1 (right skewed), 45.406m3s−1 (left skewed) and 78.648% (right skewed), respectively. Most of the stations exhibited significant trends (P<0.001) in daily SSC (18 stations; one increasing and 17 decreasing), FD (19 stations; seven increasing and 12 decreasing), and P63 (15 stations; five increasing and 10 decreasing) as well. Further, 46% of the stations exhibited significant trends in all three variables. The wash load significantly contributed (79.085±11.343%) to sediment load recorded at the gage stations. Results of the study can be used for developing best watershed management practices which may call for local or regional planning based on natural (i.e., precipitation amount, type and erosivity, watershed area, and soil erodibility) and human-affected (i.e., land use and hydraulic structures and water resources management) factors governing the study variables.

Improving understanding of mixed-land-use watershed suspended sediment regimes: Mechanistic progress through high-frequency sampling

Given the importance of suspended sediment to biogeochemical functioning of aquatic ecosystems, and the increasing concern of mixed-land-use effects on pollutant loading, there is an urgent need for research that quantitatively characterizes spatiotemporal variation of suspended sediment dynamics in contemporary watersheds. A study was conducted in a representative watershed of the central United States utilizing a nested-scale experimental watershed design, including five gauging sites (n=5) partitioning the catchment into five sub-watersheds. Hydroclimate stations at gauging sites were used to monitor air temperature, precipitation, and stream stage at 30-min intervals during the study (Oct. 2009–Feb. 2014). Streamwater grab samples were collected four times per week, at each site, for the duration of the study (Oct. 2009–Feb. 2014). Water samples were analyzed for suspended sediment using laser particle diffraction. Results showed significant differences (p<0.05) between monitoring sites for total suspended sediment concentration, mean particle size, and silt volume. Total concentration and silt volume showed a decreasing trend from the primarily agricultural upper watershed to the urban mid-watershed, and a subsequent increasing trend to the more suburban lower watershed. Conversely, mean particle size showed an opposite spatial trend. Results are explained by a combination of land use (e.g. urban stormwater dilution) and surficial geology (e.g. supply-controlled spatial variation of particle size). Correlation analyses indicated weak relationships with both hydroclimate and land use, indicating non-linear sediment dynamics. Suspended sediment parameters displayed consistent seasonality during the study, with total concentration decreasing through the growing season and mean particle size inversely tracking air temperature. Likely explanations include vegetation influences and climate-driven weathering cycles. Results reflect unique observations of spatiotemporal variation of suspended sediment particle size class. Such information is crucial for land and water resource managers working to mitigate aquatic ecosystem degradation and improve water resource sustainability in mixed-land-use watersheds globally.

Turbid releases from Glen Canyon Dam, Arizona, following rainfall-runoff events of September 2013

ABSTRACTWildman RA Jr, Vernieu WS. 2017. Turbid releases from Glen Canyon Dam, Arizona, following rainfall-runoff events of September 2013. Lake Reserv Manage. 00:1–6.Glen Canyon Dam is a large dam on the Colorado River in Arizona. In September 2013, it released turbid water following intense thunderstorms in the surrounding area. Turbidity was >15 nephelometric turbidity units (NTU) for multiple days and >30 NTU at its peak. These unprecedented turbid releases impaired downstream fishing activity and motivated a rapid-response field excursion. At 5 locations upstream from the dam, temperature, specific conductance, dissolved oxygen, chlorophyll a, and turbidity were measured in vertical profiles. Local streamflow and rainfall records were retrieved, and turbidity and specific conductance data in dam releases were evaluated. Profiling was conducted to determine possible sources of turbidity from 3 tributaries nearest the dam, Navajo, Antelope, and Wahweap creeks, which entered Lake Powell as interflows during this study. We discuss 4 key conditions that must have been met for tributaries to influence turbidity of dam releases: tributary flows must have reached the dam, tributary flows must have been laden with sediment, inflow currents must have been near the depth of dam withdrawals, and the settling velocity of particles must have been slow. We isolate 2 key uncertainties that reservoir managers should resolve in future similar studies: the reach of tributary water into the reservoir thalweg and the distribution of particle size of suspended sediment. These uncertainties leave the source of the turbidity ambiguous, although an important role for Wahweap Creek is possible. The unique combination of limnological factors we describe implies that turbid releases at Glen Canyon Dam will continue to be rare.

Assessment of flow field and sediment flux at alpine desanding facilities

ABSTRACTThis paper deals with flow field and sediment flux measurements at alpine desanding facilities. 3D flow velocities and turbidity were recorded and water samples were taken at three alpine desanding facilities. The samples were evaluated regarding suspended sediment concentration (SSC) and particle size distribution (PSD) in the laboratory. SSC was correlated with turbidity and reliable correlations were found for two facilities. The applied instrumentation and methods proved to be appropriate to assess flow field and sediment fluxes. The results show that the flow field is inhomogeneous in large parts of the basins and that the presence of tranquilizing racks has a strong influence on the flow. PSD revealed a refinement of the mean particle size in streamwise direction. The mass-related trapping efficiency of the desanding facilities was estimated based on calculated sediment fluxes and compared to two different trapping efficiency definitions. The results are briefly compared with a current design guideline.

Coupled Nitrification-Denitrification Caused by Suspended Sediment (SPS) in Rivers: Importance of SPS Size and Composition.

Suspended sediment (SPS) is ubiquitous in rivers, and SPS with different particle sizes and compositions may affect coupled nitrification-denitrification (CND) occurring on SPS significantly. However, there is no related research report. In this work, 15N isotope tracer technique was adopted to explore the CND in systems containing SPS (8 g L-1 and 1 g L-1) collected from the Yellow River with various particle sizes, including <2, 2-20, 20-50, 50-100, and 100-200 μm. The results showed that the CND occurred on SPS and the CND rate was negatively related to particle size; both nitrification and denitrification rate constants increased with decreasing SPS particle size. For instance, SPS (8 g L-1) with a particle size below 2 μm had the highest 15N2 emission rate of 1.15 mg-N/(m3·d), which was 2.9 times that of 100-200 μm. This is because SPS with a smaller particle size had a larger specific surface area and a higher organic carbon content, which is beneficial for bacteria growth. Both the nitrifying and denitrifying bacteria population were positively correlated with CND rate (p < 0.05). Different from the 15N2 production, 15N2O emission rate did not decrease with increasing SPS particle size. For the system containing 8 g L-1 SPS, 15N2O emission rate reached the highest of 1.05 μg-N/(m3·d) in the 50-100 μm SPS system, which was 17.5 times that of 100-200 μm. Similar results could be found from the system with 1 g L-1 SPS. This is due to the fact that the oxygen concentration at the SPS-water interface increased with SPS particle size, and the oxygen conditions might be most suitable for the production of N2O in the 50-100 μm system. This study suggests that SPS size and composition play an important role in nitrogen cycle of river systems, especially for the production of N2O.

Response of suspended sediment particle size distributions to changes in water chemistry at an Andean mountain stream confluence receiving arsenic rich acid drainage

AbstractAcid drainage is an important water quality issue in Andean watersheds, affecting the sustainability of urban, agricultural, and industrial activities. Mixing zones receiving acid drainage are critical sites where changes in pH and chemical environment promote the formation and dissolution of iron and aluminum oxy/hydroxides. These particles can significantly change the speciation of toxic metals and metalloids throughout drainage networks via sorption, desorption, and settling processes. However, little is known about the behavior of particle size distributions (PSDs) in streams affected by acid drainage and their relationship to metal speciation. This work studied: (a) the PSDs for a wide range of mixing ratios found at a fluvial confluence affected by acid drainage, and (b) the response of PSDs and arsenic speciation to environmental changes found when the particles approach complete mixing conditions. The confluence between the Azufre River (pH ~ 2, high concentration of dissolved metals) and Caracarani River (pH = 8.6, low concentration of dissolved metals) was used as a representative model for study. Field measurements show a bimodal PSD with modal diameters of ~50 and 300 μm. At shorter distances from the junction, the smaller modes with smaller particle volumes were dominant across the stream cross‐sections. A systematic shift towards larger particle sizes and larger particle volumes occurred downstream. The analysis of laboratory PSDs for Azufre/Caracarani mixing ratios between 0.01 and 0.5 (pH from 6.2 to 2.3) showed a bimodal trend with ~15 and 50 μm characteristic diameters; larger particles formed at pH&gt;4. When particle suspensions were transferred in laboratory experiments from very low pH to full mixing conditions (pH ~ 2.8 and mixing ratio ~ 0.25) particle sizes varied, and the dissolved arsenic concentration decreased. The observed reaction kinetics were slow compared to the time scale of advective transport, creating opportunities for engineered controls for arsenic. This work contributes to a better understanding of the chemical‐hydrodynamic interactions in watersheds affected by mining, and identifying opportunities to improve water quality at points of use.