Low Sediment Concentrations Research Articles

Regulation of alpine meadow patch coverage on runoff and sediment under natural rainfall on the eastern Qinghai-Tibetan Plateau

Soil and water conservation is the most important among ecological functions of alpine meadow on the Qinghai-Tibetan Plateau. Quantitative assessment of the effects of meadow patch coverage (MPC) on the runoff and sediment processes in alpine meadow is urgent for predicting the water conservation function of this special alpine ecosystem. Here, a field experiment was conducted to determine the effects of different meadow patch coverage through runoff and sediment processes in a typical alpine meadow on the eastern Qinghai-Tibetan Plateau. The results showed that runoff in 30% and 60% MPC plots were 39.4% and 37.9% less than in 90% MPC plot, respectively. The sediment yield in 60% and 90% MPC plots were 77.9% and 82.1% less than in 30% MPC plot, respectively. The sediment concentration in 30% MPC in heavy rainfall scenario is 4.34 times and 6.51 times higher than that in light rainfall scenario and moderate rainfall scenario, respectively. Our results indicated that MPC presented a greater impact on sediment yield than runoff volume, although runoff was greatly affected by rainfall. The higher MPC followed the greater runoff coefficient and the lower sediment concentration. The influences of MPC on runoff and sediment yield in light rainfall scenario and moderate rainfall scenario were weaker than those in heavy rainfall scenario. It is necessary to maintain surface runoff while controlling soil loss for realizing the sustainability of alpine meadow ecosystems. Therefore, these findings may conducive to offer a theoretical guidance to achieve the combined goal of soil erosion regulation during alpine meadow degradation on the Qinghai-Tibetan Plateau.

Spatio-temporal distribution of microplastics in a Mediterranean river catchment: The importance of wastewater as an environmental pathway

Microplastics (MPs) are considered to be ubiquitous contaminants in freshwater ecosystems, yet their sources and pathways at the river catchment scale need to be better determined. This study assessed MP (55–5000 µm) pollution in a Mediterranean river catchment (central Spain) and aimed to identify the importance of wastewater as an environmental pathway. We sampled treated and untreated wastewaters, and raw and digested sludge from five WWTPs during two seasons. River water and sediments were sampled at three locations with different anthropogenic influences during three seasons. On average, 93% (47–99%) of MPs were retained by WWTPs. Concentrations in river water and sediment ranged between 1 and 227 MPs/m3 and 0–2630 MPs/kg dw, respectively. Concentrations strongly depended upon land-use, with pollution levels increasing significantly downstream of urban and industrial areas. Seasonality influenced the observed MP concentrations strongly. During high flow periods, higher water but lower sediment concentrations were observed compared to low flow periods. We estimate that 1 × 1010 MPs are discharged into the catchment via treated and untreated wastewater annually, which constitutes up to 50% of the total MP catchment discharge. Thus, we conclude that the wastewater system represents a major environmental pathway for MPs into Mediterranean rivers with low dilution capacity.

Impact assessment of ephemeral discharge of contamination downstream of two legacy base metal mines using environmental DNA

Mining and processing metalliferous ores can degrade the environment well beyond the footprint of the mine, particularly where on-site containment and post-mining remediation has been insufficient to prevent releases of solid and aqueous mine wastes. In this study, we investigated the potential of sediment and water chemistry coupled with environmental (e)DNA metabarcoding to evaluate discrete and cumulative ecological impacts of two legacy base metal (copper (Cu), zinc (Zn), lead (Pb)) mines (Peelwood and Cordillera) which discharge metals via ephemeral tributaries into perennial Peelwood Creek. Although the two mine streams exceeded Australian guidelines for sediment and freshwater quality for Cu, Zn and Pb, Peelwood Creek had relatively low sediment and water metal concentrations, suggesting a low potential for environmental toxicity. Although sediment and water chemistry defined the extent of biological impacts, metabarcoding showed that Peelwood and Cordillera mines had discrete impacts and Peelwood mine was the main source of contamination of Peelwood Creek. Metabarcoding showed that prokaryotes can be good indicators of metal contamination whereas eukaryotes did not reflect contamination impacts in Peelwood Creek. Metabarcoding results showed that benthic communities downstream of Cordillera mine were less impacted than those below Peelwood mine, suggesting that Peelwood mine should be considered for further remediation.

Identification of total suspended solid (TSS) in the Aceh waters from ocean color

The suspended sediment in Aceh waters was analysed from remote Sensing Reflectance of Aqua MODIS resolution 4 km. Two equations from previous studies were used to determine the magnitude and distribution of suspended sediment concentrations in Aceh waters during 2018. The results show the suspended sediment concentrations from the two equations are relatively the same even though they differ spatially (offshore and near the coast of Aceh waters). Based on Zhan et al. equation, the suspended sediment in Aceh waters have a maximum value of 0.786 mg/L and a minimum value of 0.589 mg/L. Meanwhile, based on Zhang et al. equation, the maximum and minimum suspended sediment in Aceh Waters are 0.639 mg/L and 0.271 mg/L, respectively. Based on these two equations, Krueng Kluet estuary located in the western part of Aceh has the highest suspended sediment. In contrast, Krueng Peusangan estuary in the eastern part of Aceh has a low sediment concentration. The highest suspended sediment in Aceh waters occurs in November to December and the low period is in April to September. The heavy rainfall encourages freshwater runoff and high suspended sediment in the mouth of the Aceh waters, especially in the western part of Aceh.

An Instrument With Constant Volume Approach for In Situ Measurement of Surface Runoff and Suspended Sediment Concentration

AbstractThere is an increasing demand for reliable methods to measure soil loss in order to get a better understanding of sediment movement and to develop process‐based sediment transport models. This study described the design, calibration, and practical application of an instrumented technique that automatically characterized surface flow in situ fields. The instrument consisted of a sampling assembly, a measuring assembly, and a management platform. The special design combined with a volume‐mass conversion relationship was used to determine the exact volume and mass of the sediment‐laden water, thereby allowing the implementation of real‐time measurement and synchronous calculation of runoff rate and sediment concentration. Calibration results showed that normal distributions and an average error of −3.67% were obtained for the target sediment concentrations of 2–1,000 g/L. Compared with the gravimetric method, the average error of the new technique for sediment concentration was 4.82%, and the highest precision was 98.50%. The high repeatability and accuracy of the technique suggest that it can be used with confidence to give satisfactory results in fields. A field experiment under natural rainfall showed that a grass‐covered plot had 18.94% less runoff rate and 42.92% lower sediment concentration than a bare soil plot, illustrating the impact of vegetation cover on reducing soil loss. This uniquely designed instrument provides a new approach to real‐time sediment detection in surface flow. It will be beneficial for rigorously evaluating runoff and sedimentation and for in‐depth research regarding water and soil loss processes.

Runoff-sediment relationship and erosion dynamic characteristics for two types of engineering deposits under rainfall condition

The engineering deposits produced by the increasing frequency of production and construction activities are the main source of man-made soil erosion. In this study, we examined the change of runoff-sediment relationship and erosion hydrodynamic characteristics with the engineering deposits of aeolian sandy soil and red soil, based on simulated rainfall experiments with different gravel contents (0, 10%, 20%, 30%) and rainfall intensities (1.0, 1.5, 2.0, 2.5 mm·min-1). The results showed that the sediment yield rate of the aeolian sandy soil deposits gradually increased with the duration of rainfall. The sediment yield rate of red soil deposits under 1.0 mm·min-1 rainfall intensity increased first and then gradually stabilized. Under other rainfall densities, there was a trend of fluctuation after rapid decline, the greater the rainfall intensity and the smaller the gravel content, the more intense the fluctuation. When the gravel content was 0 and 10%, there were rills erosion on the slope surface of aeolian sandy soil accumulation, and the sediment yield rate of rill development stage was 6.74-57.40 times of that of the sheet erosion stage. The erosion process of red soil deposits could be divided into two stages: the loose particle erosion and the soil-rock erosion stage. The sediment yield rate of the loose particle erosion stage was 1.05-3.49 times that of the soil-rock erosion stage. In general, the sediment yield rate of two soil deposits increased with increasing rainfall intensity. The sediment yield rate fluctuated with the increases of gravel content at 1.0 and 1.5 mm·min-1, with a decreasing trend under >1.5 mm·min-1. The sediment yield rate of aeolian sandy soil deposits was 1.45-4.14 times of that of red soil deposits under the same rainfall and gravel content conditions. During the erosion process of aeolian sandy soil deposits, the runoff-sediment relationship changed from low sediment concentration to high sediment concentration, while there was a reverse relationship for red soil deposits. During the high sediment concentration period, the increasing rate of the sediment yield rate of aeolian sandy soil deposits was 1.94-37.60 times of that of red soil deposits. For low sediment concentration period, the decreasing rate of the sediment yield rate of red soil deposits was 1.40-21.30 times of that of aeolian sandy soil deposits. In general, the runoff power was better than the runoff shear force in describing the erosion dyna-mics of these two types of deposits. The critical runoff power increased with increasing gravel content. The critical runoff power of aeolian sandy soil deposits during the rill erosion stage (0.02-0.04 W·m-2) was two times of that of the sheet erosion stage, while the critical stream power was lower than that of the red soil deposits. These results provide a scientific reference for modelling soil erosion processes for engineering deposits.

Transport characteristics of non-cohesive sediment with different hydrological durations and sediment transport formulas

River sediment transport is an important global environmental issue, for it leads to waterway sedimentation, ecological degradation, and river geomorphologic evolution, its quantification is of the highest importance for river and watershed management. However, it remains unclear what the applicability of different sediment transport formulas in sediment transport process, and what the characteristics of non-cohesive sediment transport processes with different hydrological durations. This study developed a two-dimensional (2D) surface water Flow Model (FM) coupled with a Sediment Transport (ST) model in the MIKE 21 modelling system. The characteristic difference on simulated non-cohesive sediment transport under different sediment concentrations, hydrological durations, and sediment transport formulas were quantified by calculating the bed load, suspended load, and bed level change. Results indicated that the bed load and suspended load using the Engelund and Fredsøe formula were significantly lower than the Van Rijn formula for individual rainfall simulation scenarios. With the increase in peak flow, both suspended load and bed load first increased and then decreased. In the long-term duration simulations, high sediment concentrations were accompanied by higher bed load and lower suspension load for the Engelund and Fredsøe formula, but by lower bed load and suspension load for the Van Rijn formula. The high sediment concentrations resulted in a higher erosion rate, and low sediment concentrations resulted in a higher deposition rate. The two formulas had a high degree of consistency in seasonality and the bed level changes in spring and summer were mainly sedimentary, while in autumn and winter they were mainly erosive. The Engelund and Fredsøe formula produced high riverbed erosion capacity at low discharge, but the Van Rijn formula had high erosion capacity at high discharge.

Modeling Lahars on a Poorly Eroded Basaltic Shield: Karthala Volcano, Grande Comore Island

Lahars impose significant secondary hazards on highly populated volcanic islands by remobilising volcanic ash deposits. Karthala, on Grande Comore Island, is a basaltic shield volcano with sporadic occurrence of ash-forming phreatic eruptions. In 2005, two mildly explosive episodes emplaced tephra blankets on the summit area. Heavy precipitation subsequently triggered the repetitive occurrence of small-volume secondary lahars up to 2012. These lahars impacted the settlements at the foot of Karthala, damaging roads and hundreds of houses, affecting thousands of inhabitants. This study aims at gaining insights into the effectiveness of two simple tools to reproduce the lahars’ extent in this data-poor context. To understand the physical characteristics of the lahars, we first documented the extent and characteristics of the debris deposits at the foot of Karthala volcano and in the ravines that guided the flows. Our observations suggest that the debris was emplaced by small-scale (volumes ≤105 m³), rain-triggered and predominantly low sediment concentration lahars. The spatial extent of the deposits served to calibrate and compare numerical lahar simulations from the widely used volume-limited LaharZ model with results from Q-LavHA, a recent probabilistic flow model originally developed for lava flows. Our comparison demonstrates that LaharZ outperforms Q-LavHA for small-scale lahar simulations. While Q-LavHA has features that mitigate some limitations of LaharZ, such as its ability to simulate flow bifurcations and the transition from constrained to unconstrained flow, it typically underestimates the actual inundated area. Nevertheless, both models show rather poor performance, encountering difficulties in delineating lahar flow paths due to the smooth topography of the volcanic edifice. We therefore also evaluated the potential to increase simulation accuracy by updating a 10-meter resolution Digital Elevation Model (DEM) with channel topography measurements. While this approach led to improved simulation accuracies for both models compared to using un-adapted DEMs, it did not prevent simulations to miss the hazard-prone position of settlements actually affected by the hazard. This study helps inform numerical volcanic flow simulation strategies on young and poorly dissected volcanic edifices, such as basaltic shields, and forms a basis for further developing numerical simulation tools more adapted to these particular environmental settings.

Erosion characteristics of viscoelastic anticorrosive coatings for steel structures under sand flow

Due to the continuous erosion of anticorrosive coatings on hydraulic steel structures, the coating falls off over large areas, causing exposure and rusting of the steel structure. This process directly affects the normal use of hydraulic steel structures. To study the erosion law of viscoelastic anticorrosive coatings for steel structures in a sand flow environment, three kinds of polyurethane coatings were prepared with different proportions (A:B = 1:1, 1:2 and 1:3), and accelerated physical erosion tests of the above-mentioned anticorrosive coatings for steel structures in a sand flow environment were carried out with traditional epoxy asphalt coatings as the control group. The erosion patterns of various viscoelastic anticorrosive coatings under different erosion times, sand flow velocity and sediment concentrations under sand flow were obtained. The results show that the abrasion amount of coatings is logarithmically correlated with erosion time and exponentially correlated with sand flow velocity. Under higher sediment concentrations (4.0–8.0 kg/m3), the abrasion of the coating increases with increasing sand flow velocity. Under an erosion condition of low sediment concentration (0.5–1.8 kg/m3), the abrasion of the coating undergoes a long period of slow growth. The abrasion of the polyurethane coating is lower than that of the epoxy asphalt coating, and among the tested samples, the 1:3 polyurethane coating has the least abrasion and the best erosion resistance. Based on the results of the accelerated erosion test and the existing erosion theory, a prediction model was proposed for the viscoelastic anticorrosive coating of steel structures under sand flow, and the correctness of the prediction model was verified by the experimental results. The research results have important guiding significance for reasonably predicting the durability of hydraulic steel structures.

Sediment resuspension in winter in an exceptional low suspended sediment concentration area off Qinhuangdao in the Bohai Sea

Seawater with low suspended sediment concentration (SSC) and high salinity dominates the sea area off Qinhuangdao all year round, especially in winter, which is an exceptional case since most of the coastal waters in the Bohai Sea are turbid. Being one of the most famous coastal tourist resorts in the area, it is crucial to keep the seawater around Qinhuangdao clear and unpolluted. Three bottom-mooring systems were deployed in the low SSC region from February to March 2017 for up to 22 days to reveal the processes and mechanisms of local sediment resuspension. The observation results showed that during the strongest wind of the whole deployment period, the sediment resuspension process included a turbulent resuspension event that occurred in the high turbulent kinetic energy period, and a continuous resuspension event when the bed shear stress (BSS) was larger than critical. The turbulent resuspension event was characterised by high-turbidity resuspension of sediments near the bed in the form of pulsation, with a depth of influence not more than the thickness of the bottom boundary layer. It was found to be triggered by strong turbulence driven by storm winds. In contrast the continuous resuspension could be attributed to high wave-dominated BSS. During storm winds, high concentrations of suspended sediment in the upper and middle layers appeared to lag the wind speed by about 8–10 h. In contrast, the turbulent resuspension event occurred at the same time as the strong winds. In-situ measurements from all three moorings indicate that it is the local weak sediment resuspension, even under the influence of storm winds in winter, that causes the formation of the low SSC area off Qinhuangdao.

Improvement on numerical modeling of total dissolved gas dissipation after dam

Discharge from high dam causes total dissolved gas (TDG) supersaturation, which dissipates slowly along the downstream river and induces fish bubble disease. The dissipation process is closely related to the hydraulic condition. In most TDG prediction models, the hydraulic condition is lumped in a parameter called the dissipation coefficient (k), whose determination usually contains large uncertainty. In this study, the property of k was experimentally investigated under three water depths and three flow velocities. The results demonstrated that the k value increased with v/H ratios. Based on the experimental results, an equation was proposed to calculate k value. The equation was validated by the observed data from field survey in Xiangjiaba Reservoir in the Jinshajiang River, the upstream of Yangtze River. Meanwhile, the proposed equation was compared to two typical equations, a dissolved oxygen (DO) reaeration equation and an empirical equation, using the same datasets. The comparisons indicated that DO reaeration equation was not suitable for k value estimation, particularly under shallow water conditions. The limitation of empirical equation was that the k value was determined by data from a specific case so that was not applicable to other conditions. The proposed equation showed best performance in calculating k value and hence estimating the TDG levels, especially under the condition of low sediment concentration. In addition, the proposed equation was more generically applicable compared to the existing formulas. Our findings could help to draw up governing measures to minimize the risk of supersaturated TDG and achieve river ecological restoration.

Effects of hydrodynamic on the mobility of phosphorous induced by sediment resuspension in a seepage affected alluvial channel

The phosphorus (P) mobility caused by sediment resuspension was investigated in a tilting flume, considering the important effect of upward seepage. The water level and velocity were observed during the experimental run, and water samples were collected for the measurement of sediment and P concentrations. A lower value of P and sediment concentrations occurred at the upstream end of the test section, and then a stable trend was gradually observed towards the downstream end due to the sediment resuspension and deposition within the test section. A lower P release was identified for the seepage run, accompanied with a lower sediment concentration in the overlying water. These phenomena were directly linked with the turbulent flow characteristics such as time-averaged velocity, Reynolds shear stresses, and turbulence intensity. Results show that the level of turbulence decreased with the upward seepage, indicating lower P and sediment concentrations. The turbulent length scale also decreased with the upward seepage, which led to a decrease in the energy and momentum transfer induced by the larger eddy size in the near-bed region, and eventually, a lower rate of sediment resuspension and P release with upward seepage. The results of the present study are useful for civil or hydraulic engineers since the knowledge of P transport with sediment will improve the management of contaminated sediment in seepage affected alluvial channel.

Evaluation of a virus-based control method to protect cultured oysters from the harmful dinoflagellate Heterocapsa circularisquama

This study evaluates a novel control method for harmful algal blooms (HABs) that uses natural sediment including associated viruses. In Japan, commercial aquaculture of bivalves such as oysters, pearl oysters and clams is severely threatened by blooms of the harmful dinoflagellate, Heterocapsa circularisquama. HcRNAV is a single-stranded RNA virus that specifically infects H. circularisquama. Given that high concentrations of HcRNAV remain in bottom sediment following a H. circularisquama bloom, dispersal of sediments containing HcRNAV was expected to provide a promising tool to mitigate blooms of this dinoflagellate. A laboratory experiment demonstrated the effectiveness of a) HcRNAV alone, and b) sediments with HcRNAV, in reducing H. circularisquama densities. For the first time, a field test was implemented in an active oyster farm on Sado Island, Japan in which sediment containing HcRNAV collected from nearby waters was dispersed into 15,000 L floating mesocosms during early stages of a H. circularisquama bloom. In the treatment spraying with sediment containing HcRNAV, the density of H. circularisquama decreased significantly until it reached only ~1% of the initial after five days, while that of HcRNAV increased significantly by Day 2 (one-way ANOVA, n = 3, P < .01). These results indicate that the virus + sediment treatment was very effective in mitigating the H. circularisquama bloom under field conditions. Only a very low sediment concentration achieved these results, thereby reducing potential ecological impacts. Given the high diversity and mutation potential of HcRNAV, and the variable host response, it is recommended that field application of this mitigation strategy use indigenous sediment. This study provides the first field test of viral control of HAB worldwide and will facilitate its further practical application.

Preliminary assessment of the sediment quality of Bach Dang estuary using Embryo assay of the Pacific oyster, Crassostrea gigas

AbstractThe Pacific oyster, Crassostrea gigas, was used as a bioindicator organism to investigate the sediment quality of at Dinh Vu port and Dong Bai intertidal in the Bach Dang estuary. The embryos of oyster were exposed to sediment contacts for 24 h to quantify the abnormality percentage of the development of D‐shape oyster larvae. In the sediments collected from April to July 2016, the results showed that the low sediment concentrations (0.5 and 1.0 g.L‐1) had higher development defect for oyster embryos. The development of oyster embryos had no affected because the concentration of ammonium and particulate organic carbon were low in Bach Dang estuary.

The effects of flocculation on the entrainment of fluid mud layer

Flocculation is an integral part for cohesive sediment transport processes as it will further influence other processes such as settling, deposition as well as erosion processes. To understand the impacts of flocculation on entrainment process of fluid mud with different salinities, a series of entrainment and sedimentation experiments were conducted. Entrainment experiments were conducted under constant hydrodynamic conditions driven by a shear ring on the top of an annular flume. While the sedimentation experiments were conducted within cylinders with different diameters. Results show that fluid mud samples with different salinities can form different distributions of sediment concentration in the vertical direction, which will significantly influence the entrainment rate of fluid mud. Results also show that entrainment rate of fluid mud with higher salinity (E18) is smaller than that of fluid mud with lower salinity (E4) in the upper layer of fluid mud. Sedimentation tests show that the settling rate of fluid mud sample with higher salinity is enhanced and the gelling concentration is reached at a lower sediment concentration. These two mechanisms cause the different distributions of sediment concentration for fluid mud under different flocculation states. Accordingly, the mud characteristics such as density, viscosity, yield strength, buoyancy effects and eventually the entrainment rate vary with different fluid mud samples (the initial sediment concentrations are the same). An effort has also been made to predict the entrainment rate with existing equation. The good agreement between from equation and experimental results further demonstrates that the flocculation process should be considered when calculating the entrainment rate, as accurate prediction of the entrainment rate of fluid mud requires the details of sediment concentration and rheological properties of fluid mud in the vertical direction.

Flow Resistance Equation in Sand-bed Rivers and Its Practical Application in the Yellow River

To fully reflect the effect of the flow characteristics, sidewall conditions and sediment concentrations on the bed roughness of sand-bed rivers, this study established a new flow resistance equation by introducing a comprehensive influence coefficient presented via a combination of power-function forms of the relative flow velocity, von Karman constant of sediment-laden flows and the ratio of particle size to viscous sublayer thickness. The comprehensive influence coefficient accordingly acts as a synthesized factor representing the combined effects of the flow intensity, bed material movement, energy consumption condition, and relative friction condition in the near-wall region of sediment-laden flows. Based on the field data from sediment-laden flows under scenarios of both high and low sediment concentrations, the performance of the proposed equation was validated to achieve the best accuracy in the calculation of Manning’s roughness coefficient compared with that of several previously presented flow resistance equations. Furthermore, the proposed flow resistance equation was adopted to quantify the stable channel width of the Lower Yellow River (LYR), i.e., the optimum main channel width for sediment transportation in the typical wandering reach of the LYR. The calculated stable channel width is consistent with the current river training width of the LYR, indicating that the proposed equation has great potential as a theoretical tool that can be used to support the determination of the river training strategy for the LYR.

Estimating low eroded sediment concentrations by turbidity and spectral characteristics based on a laboratory experiment.

Automatic and real-time monitoring of sediment concentrations in eroded runoff is an effective way to accurately assess soil erosion process on slopes. It is assumed that low sediment concentrations could be inferred from turbidity and spectral characteristics, which are two simple and economical observation methods, and the soil properties would affect this kind of measurements. Four kinds of soil are used to obtain water samples with low sediment concentrations (0.1-10mg/L), namely, black soil (BS), albic soil (AS), cinnamon soil (CS), and brown soil (BRS). The turbidity and spectral characteristics of the samples are measured to evaluate the relationships by calibration and validation between sediment concentrations and turbidity (method 1), sediment concentrations and reflectance with several transformations (method 2), and sediment concentrations and both indicators (method 3). The influences of soil properties on these relationships are also discussed. The linear relationship between sediment concentrations and turbidity is significant in each sample (method 1). Method 2 has a lower accuracy than method 1, in which the obtained characteristic bands and fitting models are different among samples, with a poor result for BS samples and acceptable results for the other samples. Overall, method 3 has the highest accuracy. The order of simulation accuracy from high to low is generally BS > AS > BRS > CS. The influences of soil properties are obvious and various. The effects of soil median diameter (d50) and specific surface area (SSA) on turbidity coefficients and characteristic spectral bands are not significant, but the soil organic matter (SOC) contents are. The results indicate that measurement of sediment concentration within low ranges by both turbidity and spectroscopic techniques has good accuracy in method 3, and different samples should be calibrated in application due to the effects of various soil properties.

Potential physical effects of suspended fine sediment on lotic macroinvertebrates

This study investigates the potential for physical damage caused by suspended fine sediment on gills of three macroinvertebrate species, Hydropsyche siltalai, Ephemera danica and Ecdyonurus venosus. Macroinvertebrate cadavers were exposed to three suspended sediment concentrations (control 3.5, low 83.7 and high 404.0 mg l−1) at two velocities (low 0.19 m s−1 and high 0.37 m s−1), for 6 h in a recirculating flume. Tracheal gill surfaces were subsequently examined for evidence of physical damage using scanning electron microscopy (SEM) images. Physical damage predominantly consisted of fine sediment coverage of gill surfaces, appearing as a deposited layer of sediment obscuring and potentially clogging the gill. For E. venosus, suspended sediment concentration influenced gill cover, but velocity had no significant effect. Coverage of H. siltalai gill surfaces increased significantly between low and high sediment concentrations but only at the higher flow velocity. Gill coverage of E. danica did not differ across any sediment concentration. The results were consistent with reported species sensitivities to fine sediment, despite the use of cadavers. However, we found limited evidence of physical abrasion as a direct physical effect of fine sediment under the experimental conditions used.

To Implement A Clear-Water Supply System for Fine-Sediment Experiment in Laboratories

Fine sediment transport is currently attracting increasing attentions owing to its importance in the dynamics of sediment-contaminant interaction in the fluvial environment downstream of dams, which calls for more detailed and accurate flume experiments. However, because of inaccurate loading and undesired recirculation of fine sediment in the usual short laboratory flumes, such experiments are often unrepeatable and unreliable. In this technical note, we propose a new sediment feeder, to load dry sediment sample at the inlet, and a pressurized sediment filter, to screen the sediment out at the outlet, to implement a clear-water supply system for the flumes. It can improve fine sediment experiments not only by accurate loading but also through preventing undesired sediment recirculation, which can interfere and even modify the designated upstream input conditions. These devices have been constructed and tested, shown to be practical, simple and effective. Using them together can also provide a way to reclaim all the samples of experimental sediment that are of crucial importance for repeat and multiple tests for different contamination with a given sediment without losing the prescribed composition and other properties. This implementation is especially suitable for simulating fine particle affinity contaminant transport in fluvial turbulent flows in low sediment concentrations.

Predicting the vertical low suspended sediment concentration in vegetated flow using a random displacement model

Based on the Lagrangian approach, this study proposes a random displacement model (RDM) to predict the concentration of suspended sediment in vegetated steady open channel flow. Validation of the method was conducted by comparing the simulated results by using the RDM with available experimental measurements for uniform open-channel flows. The method is further validated with the classical Rouse formula. To simulate the important vertical dispersion caused by vegetation in the sediment-laden open channel flow, a new integrated sediment diffusion coefficient is introduced in this study, which is equal to a coefficient β multiplying the turbulent diffusion coefficient. As such, the RDM approach for sandy flow with vegetation was established for predicting the suspended sediment concentration in low-sediment-concentration flow with both the emergent and submerged vegetation. The study shows that the value of β for submerged vegetation flow is larger than that for emergent vegetation flow. The simulated result using the RDM is in good agreement with the available experimental data, indicating that the proposed sediment diffusion coefficient model can be accurately used to investigate the sediment concentration in vegetated steady open channel flow.