首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Effects of gully and channel erosion on the export of sediments are in general well understood, but the effects on carbon (C) and nitrogen (N) export remain an open question. We examined these effects and the role of flow magnitude, total wet season rainfall, catchment size and the C and N content and solubility of most probable sediment sources in a subtropical catchment. We sampled the baseflow and the rising stage of high‐flow events for one wet season and analysed 5 years of water quality data from event sampling stations. Total suspended sediment was the main variable explaining N export followed by rainfall, flow and catchment size. N was exported mainly in particulate organic form and C in dissolved form. The quality of most probable sediment source fractions explains these results and points to fractionation during transport into C and N richer and C and N poorer fractions, with travel distance ultimately determining the average quality of transported sediment for different flow magnitudes. Erosion would have caused a lower C and N concentration in sediments, a lower proportion of mineralized N, a larger proportion of dissolved organic C and a larger C : N ratios of the soluble fraction as compared with unincised grassed valleys. This would alter the rates of nutrient cycling and energy flow within and across ecosystem compartments in streams receiving this export. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

2.
Grain size effect on trace metals (cadmium, copper, lead, zinc, and iron) and total organic content distribution in various fractions (<0.063, 0.063–0.105, 0.105–0.250, 0.250–0.500, and 0.500–1.000 mm) of contaminated sediment has been studied. Selective partitioning of the studied contaminants in sediment fractions was observed, with a minimum content in the fine sand fraction of grain size 0.125–0.250 mm. Anomalously high concentrations of trace metals and organic matter content in the medium and coarse sediment fractions (>0.250 mm) was explained by the formation of large agglomerates (clusters) during the generally recommended drying procedures. These large agglomerates, formed from smaller sediment fraction particles enriched by various contaminants kept on their large specific area by adsorption forces, have been observed in photographs of the medium and coarse sediment fractions only. The formed agglomerates consist of small particles cemented either by dissolved organic matter or by sea salts present in the marine sediment. The formation of such agglomerates should be taken into consideration when conducting metal contamination studies on sediments.  相似文献   

3.
Based on data from 35 stations on the tributaries of the Yellow River, annual specific sediment yield (Ys) in eight grain size fractions has been related to basin‐averaged annual sand–dust storm days (Dss) and annual precipitation (Pm) to reveal the influence of eolian and fluvial processes on specific sediment yield in different grain size fractions. The results show that Ys in fine grain size fractions has the highest values in the areas dominated by the coupled wind–water process. From these areas to those dominated by the eolian process or to those dominated by the fluvial process, Ys tends to decrease. For relatively coarse grain size fractions, Ys has monotonic variation, i.e. with the increase in Dss or the decrease in Pm, Ys increases. This indicates that the sediment producing behavior for fine sediments is different from that for relatively coarse sediments. The results all show that Ys for relatively coarse sediments depends on the eolian process more than on the fluvial process, and the coarser the sediment fractions the stronger the dependence of the Ys on the eolian process. The Ys–Dss and Ys–Pm curves for fine grain size fractions show some peaks and the fitted straight lines for Ys–Dss and Ys–Pm relationships for relatively coarse grain size fractions show some breaks. Almost all these break points may be regarded as thresholds. These thresholds are all located in the areas dominated by the coupled wind–water process, indicating that these areas are sensitive for erosion and sediment production, to which more attention should be given for the purpose of erosion and sediment control. A number of regression equations were established, based which the effect of rainfall, sand–dust storms and surface material grain size on specific sediment yield can be assessed. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

4.
Experiments on initiation of motion of different fractions of non-uniform sediment mixtures are reported. Four sediment mixes and reference transport method to define the threshold of bed particle movement are used in this analysis. Factors controlling the initiation of motion are analyzed. It is found that relative size of different grains with respect to a median size greatly affects the initiation of movement. The incipient motion of non-uniform sediment is very much dependent on the absolute size for coarser fractions. Considering the factors controlling the initiation of motion the experimental data sets along with the other available data yield a new relationship for the calculation of critical shear stress of non-uniform sediments. A set of equations have found that all the data points fall into a single line for the finer and for coarser fractions.  相似文献   

5.
Polycyclic aromatic hydrocarbons (PAHs) in sediment from Auckland Harbour (New Zealand) are not distributed evenly throughout bulk sediment, but highly concentrated in coarser, low-density fractions. Concentrations of 24 PAHs, measured in sediment that was separated into six size fractions that were furthermore separated into two density sub-fractions by flotation in sodium-polytungstate solution (rho = 2.15 g cm(-3)), varied between 4-103 microg g(-1)dw among grain size fractions and 2-998 microg g(-1)dw for density sub-fractions. Highest PAH concentrations were measured in the low density, 125-250 microm fraction. All sediment fractions had a similar relative PAH composition, dominated by >3-ring PAHs, suggesting a common pyrogenic origin. Low density material had 10-200 times higher PAH concentrations and 10-100 times higher organic carbon (OC) content, yet differences in OC content only partially accounted for variations in PAH concentration. Low density particles contributed more than 75% of the Sigma PAH, while comprising only 3% of bulk sediment dry weight. This may have significant utility for contaminant mitigation efforts in Auckland Harbour.  相似文献   

6.
Determining sources, quantities and travel distances of eroding soil is of increasing importance given its impact on‐ and off‐site, the sediment‐associated transport of nutrients, metals and micro‐organisms and the ongoing need to provide data for soil erosion model development and validation. Many soil tracers have been developed; however, most comprise foreign materials, such as fluorescent beads and rare earth oxides, which cast doubts on the validity of tracing results given their different physical characteristics. To avoid these problems, we have investigated the potential of soil, which has been heated under reducing conditions to enhance its ferrimagnetic content, as a soil erosion tracer; while the technique has been used successfully to trace river sediment it has not been successfully applied to soil erosion studies. For a suite of 16 magnetic concentration‐dependent properties, values were found to be significantly greater, by at least one order of magnitude, after heating, both for the bulk soil and nine individual particle size fractions. Individual size fractions could be differentiated using two different magnetic properties, thus illustrating the technique's potential to provide information on particle size‐specific erosion. Soil box experiments demonstrated the potential for both in situ measurement of magnetic susceptibility and laboratory measurement of the magnetic properties of eroded sediment, to trace and quantify soil erosion. Thus, heated soil, with artificially‐enhanced ferrimagnetic properties, is successfully demonstrated to have great potential as a size‐specific, cost‐effective and representative soil erosion tracer. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

7.
Restoring belts of perennial vegetation in landscapes is widely recognized as a measure of improving landscape function. While there have been many studies of the transport of pollutants through grass filter strips, few have addressed sediment related processes through restored tree belts. In order to identify these processes and quantify their relative contribution to sediment trapping, a series of rainfall simulations was conducted on a 600 m2 hillslope comprising a pasture upslope of a 15 year old tree belt. Although the simulated events were extreme (average recurrence intervals ～10 and 50 yr), the trapping efficiency of the tree belt was very high: at least 94% of the total mass of sediments was captured. All the size fractions were trapped with a minimum Sediment Trapping Ratio (STR) of 91% for the medium‐sized fragments. Fractions < 1·3 µm and > 182 µm were totally captured (STR = 100%). Through the joint analysis of sediment budgets and soil surface conditions, we identified different trapping processes. The main trapping process is the sedimentation (at least 62% of trapped sediment mass) with deposits in the backwater and as micro‐terraces within the tree belt. Modelling results show that the coarsest size fractions above 75 µm are preferentially deposited. Joint infiltration of water and sediments has also been noticed, however, this process alone cannot explain the selective trapping of the finest fractions. We suggest that the finest fractions transported by the overland flow may be trapped by adsorption on the abundant litter present within the tree belt. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

8.
Sediment samples collected from Kaohsiung Harbor (Taiwan) were analyzed for 15 priority polycyclic aromatic hydrocarbons (PAHs) and 118 individual polychlorinated biphenyl (PCB) congeners in various particle size fractions and two density fractions. The PAHs in the low density fraction were a much more sensitive and effective indicator of the PAH source than those in the sediment as a whole. Hierarchical cluster analysis of PAH analytes in the low density sediments revealed significant differences in compositional patterns between locations and among size fractions. In the sediment samples from both study sites, the low density fraction particles had up to 155 and 150 times higher concentrations of PAHs and PCBs, respectively, than those in the corresponding high density fractions. In addition, the total toxic benzo[a]pyrene and 2,3,7,8-tetrachlorodibenzodioxin (TCDD, dioxin) equivalents in the sediment low density fractions were much higher (up to 170 and 273 times, respectively) than the corresponding high density fractions.  相似文献   

9.
The objective of this research is to investigate spatial and temporal variation in the transport of phosphate by grain size fractions of suspended sediment in two Lake Erie tributaries. A sediment-phosphate transport model is proposed to evaluate and quantify the kinetic control of dissolved phosphate by fluvial sediment. A first-order assessment of the sediment-phosphate export from Big Otter Creek and Big Creek to Lake Erie is presented. Output from this model was calculated using historical discharge and sediment data collected by Environment Canada in combination with adsorption coefficients derived from laboratory experiments for a range of sediment grain size fractions. Results of these calculations provide a quantitative interpretation of the influence of suspended sediment on phosphate dynamics in tributaries and the potential impact on water quality of receiving waters.  相似文献   

10.
《国际泥沙研究》2020,35(2):115-124
Bed-load transport plays a critical role in river morphological change and has an important impact on river ecology.Although there is good understanding of the role of the variation of river bed grain size on transport dynamics in equilibrium conditions,much less is understood for non-equilibrium conditions when the channel is either aggrading or degrading.In particular,the relative role of different grain sizes in the promotion and hindering of the transport of coarse and fine fractions in a degrading channel has yet to be investigated.The current study attempts to provide new understanding through a series of flume experiments done using uniform and graded sediment particles.The experiments revealed coarser grain-size fractions for a poorly-sorted sediment,relative to uniform-sized sediment,reduced the transport of finer grains and finer fractions enhanced the transport of coarse grains.This hinderingpromotion effect,caused by relative hiding and exposure of finer and coarse fractions,increased with bed slope and decreased with relative submergence.In particular,as relative submergence increased,the graded fractions tended towards behaving more like their unifo rm-sized counterparts.Also,the bed-load parameter of the graded fractions increased more with a rise in bed slope than observed for the uniformsized counterparts.These results revealed,for degrading channel conditions,such as downstream of a dam,bed-load equations developed for uniform bed sediment are inappropriate for use in natural river systems,particularly in mountain streams.Furthermore,changes in river bed composition due to activities that enhance the input of hill-slope sediment,such as fire,logging,and agricultural development,are likely to cause significant changes in river morphology.  相似文献   

11.
1 INTRODUCTION The particle size of sediment eroded from basins can provide basic information about erosion processes (Meyer et al., 1980), which can be divided into sheet wash sediment processes on hill slopes and fluvial sediment processes in rivers. In…  相似文献   

12.
Research over the last decade has shown that the suspended sediment loads of many rivers are dominated by composite particles. These particles are also known as aggregates or flocs, and are commonly made up of constituent mineral particles, which evidence a wide range of grain sizes, and organic matter. The resulting in situ or effective particle size characteristics of fluvial suspended sediment exert a major control on all processes of entrainment, transport and deposition. The significance of composite suspended sediment particles in glacial meltwater streams has, however, not been established. Existing data on the particle size characteristics of suspended sediment in glacial meltwaters relate to the dispersed mineral fraction (absolute particle size), which, for certain size fractions, may bear little relationship to the effective or in situ distribution. Existing understanding of composite particle formation within freshwater environments would suggest that in‐stream flocculation processes do not take place in glacial meltwater systems because of the absence of organic binding agents. However, we report preliminary scanning electron microscopy data for one Alpine and two Himalayan glaciers that show composite particles are present in the suspended sediment load of the meltwater system. The genesis and structure of these composite particles and their constituent grain size characteristics are discussed. We present evidence for the existence of both aggregates, or composite particles whose features are largely inherited from source materials, and flocs, which represent composite particles produced by in‐stream flocculation processes. In the absence of organic materials, the latter may result solely from electrochemical flocculation in the meltwater sediment system. This type of floc formation has not been reported previously in the freshwater fluvial environment. Further work is needed to test the wider significance of these data and to investigate the effective particle size characteristics of suspended sediment associated with high concentration outburst events. Such events make a major contribution to suspended sediment fluxes in meltwater streams and may provide conditions that are conducive to composite particle formation by flocculation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

13.
Sediment resuspension is an important way for shallow lake internal pollution to interact with the overlying water column,and the pollution risks are reasonably related to the retention of resuspended sediment particles in overlying water.In the current study,the settling of resuspended sediment particles was comprehensively investigated under different disturbances using five urban lake sediments.The results show that the particle size distributions of resuspended sediment from different lakes exhibited similar variations during settling with disturbance,although varied settling times were observed under static conditions.During settling with and without disturbance,sediment particle sizes were mainly within 8-63μm at the initial stage,and were<8μm in the later stages of settling.Based on these settling characteristics,the sediment particle size was divided into sand(>63μm),silt(8-63μm),and very fine silt and clay(<8μm)fractions.Kinetic analysis suggested that sediment settling for different particle sizes could be well described by the first-and second-order kinetic equations,especially when settling was disturbed(r2=0.727-0.999).The retention of resuspended sediment could be enhanced as particle sizes decreased and disturbance intensities increased.Furthermore,a water elutriation method was successfully optimized,with separation efficiencies of 56.1%-83%,to separate sediment particles into the defined three particle size fractions.The chemical compositions of sediment were found to change with different particle sizes.Typically,calcium tended to form large-size sediment,while the total contents of aluminum,iron,magnesium,and manganese showed significantly negative correlations with sediment particle sizes(p<0.01)and tended to distribute in small-size particles(e.g.,<8μm).Overall,the sediment particle size related settling dynamics and physicochemical properties suggested the necessity on determining the pollution of resuspended sediment at different particle sizes for restoration of shallow lakes.  相似文献   

14.
A method for collecting suspended sediment samples has been developed that pumps a discharge-weighted volume of water from fixed depths at four to 40 locations across a river and separates the suspended sediment in the sample using a continuous-flow centrifuge. The efficacy of the method is evaluated by comparing the particle size distributions of sediment collected by the discharge-weighted pumping method with the particle size distributions of sediment collected by depth integration and separated by gravitational settling. The pumping method was found to undersample the suspended sand sized particles (> 63 μm) but to collect a representative sample of the suspended silt and clay sized particles (< 63 μm). The centrifuge separated the silt and clay sized particles (< 63 μm) into three fractions. Based on the average results of processing 17 samples from the Mississippi River and several of its large tributaries in 1990, about 10% of the silt and clay sized material was trapped in a centrifuge bowl-bottom sealing unit containing the nozzle and consisted of mostly medium and coarse silt from 16 to 63 μm. About 74% was retained on a Teflon liner in the centrifuge bowl and consisted of sizes from 0–1 to 63 μm. About 9% was discharged from the centrifuge in the effluent and was finer than 0–1 μm. About 7% was lost during the processes of removing the wet sediment fractions from the centrifuge, drying and weighing. The success of the discharge-weighted pumping method depends on how homogeneously the silt and clay sized particles (< 63 μm) are distributed in the vertical direction in the river. The degree of homogeneity depends on the composition and degree of aggregation of the suspended sediment particles.  相似文献   

15.
Dam break flows and resulting river bed erosion can have disastrous impacts on human safety,infrastructure,and environmental quality.However,there is a lack of research on the mobility of non-uniform sediment mixtures resulting from dam break flows and how these differ from uniform sized sediment.In this paper,laboratory flume experiments revealed that coarse and fine fractions in non-uniform sediment had a higher and a lower bed-load parameter,respectively,than uniform sediments of the same size.Thus,the finer fractions were more stable and the coarser fractions were more erodible in a nonuniform bed compared to a uniform-grained bed.These differences can be explained by the hiding and protrusion of these fractions,respectively.By investigating changes in mobility of the mixed-size fractions with reservoir water levels,the results revealed that at low water levels,when the coarser fractions were only just mobile,the bed-load parameter of the finer fractions was higher than the coarser fractions.The opposite was observed at a higher water level,when a significant proportion of the coarsest fractions was mobilized.The higher protrusion of these grains had an important effect on their mobility relative to the finer grains.The transported sediment on these mixed-sized beds was coarser than the initial bed sediment,and became coarser with an increase in reservoir water level.  相似文献   

16.
C. K. Jain  I. Ali 《水文研究》2000,14(2):261-270
The effects of solution pH, sediment dose, contact time, and particle size on the adsorption of cadmium ions on bed sediments have been studied for a highly polluted river in western Uttar Pradesh, India. The role of the coarser sediment and the clay and silt fractions has been examined. The optimum contact time needed to reach equilibrium is of the order of 30 and 60 min for 0–75 μm and 210–250 μm sediment size, respectively. The extent of cadmium adsorption increases with increasing pH and adsorbent doses and decreases with adsorbent particle size. The important geochemical phases, iron and manganese oxide, provide the active support material for the adsorption of cadmium. The competitive experiments conducted in the presence of lead and zinc ions indicate that both the ions suppress the ability of cadmium to adsorb on to sediments. The Langmuir and Freundlich adsorption models were used to determine the mechanistic parameters associated with the adsorption process. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

17.
A model was developed for the prediction of the entrainment rate of non‐uniform sediment considering the movement of bedforms. Laboratory experiments were conducted to advance the formulations of the proposed model and to validate and estimate the model parameters. The model parameters were related to the hydraulic conditions of the flow and the properties of the sediment mixtures using dimensional analysis and gene expression programming. The model incorporated four parameters on its formulation, namely the Shields stress and critical Shields stress to describe the hydraulic and sediment conditions of the flow, the Kramer coefficient of uniformity to describe the grain size distribution of a particular sediment mixture, and the relative position of a particular grain size fraction to the geometric mean to describe the entrainment rate of that fraction within the sediment mixture. The proposed model provided satisfactorily predictions with a deviation less than 25% between the measured and predicted values for most of the fractions, which confirms the validity of the proposed approach and model in predicting of the entrainment rates of various fractions. The model predictions were also compared with other models available for the prediction of the entrainment rate of non‐uniform sediment. The model predictions were within the same order of magnitude of the other models’ predictions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

18.
Abstract The suspended sediment load in the middle Yellow River basin (YRB) cannot be well predicted by capacity‐based transport formulas because a large fraction of suspended sediment load is composed of wash load. This study evaluated the spatial variations of sediment rating curves (SRCs) in the middle YRB. Both power and linear SRCs were used to fit daily flow and suspended sediment concentration (SSC) historical data at 49 gauging stations throughout the middle YRB. The spatial variation in regression coefficients was investigated, and the relationship between regression coefficients and the physical characteristics of watersheds was discussed. The results indicate that SRC regression coefficients vary with drainage area and basin slope, but their responses to these parameters are remarkably different in watersheds with different underlying surfaces, which indicates the significance of sediment availability, erodibility, and grain size distribution. For power SRCs representing sediment transport in unsaturated flows, the regression coefficients are more closely correlated with the drainage area in loess regions and with the basin slope in rock mountain regions. For linear SRCs representing sediment transport in saturated flows, saturated SSCs vary with coarse (particle size > 0.05 mm) and fine (particle size < 0.01 mm) fractions in suspended sediment. The maximum saturated SSC among the different gauging stations is associated with the optimal grain size composition of suspended sediment, which has been proposed for loess regions in previous studies. This study provides theoretical support for estimating the regression parameters for sediment transport modelling, especially in ungauged basins.  相似文献   

19.
Abstract

The adsorption of lead and zinc ions on bed sediments of the River Kali in western Uttar Pradesh, India, has been studied. The role of the coarser sediment fraction (210–250 μm) in controlling metal pollution has been elucidated and compared to those of the clay and silt fractions. The parameters controlling metal uptake, viz., solution pH, sediment dose, contact time, and particle size have been evaluated. The optimum contact time needed to reach equilibrium is of the order of 45 min for both the metal ions. The extent of adsorption increases with an increase of pH. Furthermore, the adsorption of the metal ions increases with increasing adsorbent doses and decreases with adsorbent particle size. The two geochemical phases of iron and manganese oxide act as the active support material for the adsorption of the metal ions.  相似文献   

20.
Accurate estimates of N and P loads were obtained for four contrasting UK river basins over a complete annual cycle. The fractionation of these loads into dissolved and particulate, and inorganic and organic components allowed a detailed examination of the nutrient load composition and of the factors influencing both the relative and absolute magnitude of these components. The particulate phosphorus (TPP) loads account for 26–75% of the annual total phosphorus (TP) transport and are predominantly inorganic. The inorganic (PIP) and organic (POP) fractions of the TPP loads represent 20–47% and 6–28% of the annual TP transport, respectively. In contrast, the particulate nitrogen loads (TPN) represent 8% or less of the annual total nitrogen (TN) loads and are predominately organic. For dissolved P transport, the dissolved inorganic fraction (DIP) is more important, representing 15–70% of the TP loads, whereas the dissolved organic fraction (DOP) represents only 3–9% of the TP loads. The TN loads are dominated by the dissolved component and more particularly the total oxidized fraction (TON), which is composed of nitrate and nitrite and represents 76–82% of the annual TN transport. The remaining dissolved N species, ammonium (NH4-N) and organic N (DON) account for 0·3–1·2% and 13–16% of the annual TN transport, respectively. The TPN and TPP fluxes closely reflect the suspended sediment dynamics of the study basins, which are in turn controlled by basin size and morphology. The dissolved inorganic nutrient fluxes are influenced by point source inputs to the study basins, especially for P, although the TON flux is primarily influenced by diffuse source contributions and the hydrological connectivity between the river and its catchment area. The dissolved organic fractions are closely related to the dissolved organic carbon (DOC) dynamics, which are in turn influenced by land use and basin size. The magnitude of the NH4-N fraction was dependent on the proximity of the monitoring station to point source discharges, because of rapid nitrification within the water column. However, during storm events, desorption from suspended sediment may be temporarily important. Both the magnitude and relative contribution of the different nutrient fractions exhibit significant seasonal variability in response to the hydrological regime, sediment mobilization, the degree of dilution of point source inputs and biological processes. © 1998 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号