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1.
An understanding of the behavior of cohesive sediment is required to solve various engineering problems such as scour around bridge elements, mitigation of soil erosion, pavement design, river bed degradation,stable channel design. Pavement foundation designers principally use the California bearing ratio(CBR)to describe the subgrade and subbase materials and their strength. Several laboratory experiments were done to study the variation in the CBR of cohesive mixtures comprised of clay-gravel m...  相似文献   

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

3.
The current study is the first step in a systematic experimental research on the erosion behaviour of sand-mud mixtures. It concerns the effect of a varying sand content and clay mineralogy on the porosity, structure, strength and permeability of artificially generated sediment mixtures. The permeability of a sediment mixture is an especially significant parameter concerning the type of erosion that occurs. It determines if the erosion of the bed is either a drained or an undrained process, respectively indicating surface erosion or mass erosion. Measurements on various mixtures concerning the consolidation coefficient and the permeability have been executed. Results show a distinct transition of behaviour between a sand-silt dominated network structure and a clay-water matrix. The occurrence of these two types of structures appears to depend on the porosity of the volume fraction of sand related to silt, which is, therefore, an important parameter concerning the type of erosion. Finally, the study provides a valuable data set that can be used as a reference for following stages of this research on the erosion behaviour of natural cohesive sediments.  相似文献   

4.
Erosion of sediment at two sites within the siliceous-clay province of the northeast equatorial Pacific is investigated by examining radionuclide, radiolarian and percent smectite profiles of three ? cores. A numerical ? model of the sediment is presented that examines the effect of various particle rain rates, erosion rates and biological mixing parameters on these profiles.230Th, because of its longer half-life, is a more sensitive tracer of erosion than231Pa. Because the input of these radionuclides into the sediment is not well defined however, conservative tracers, such as the percent smectite of the clay fraction and fossil radiolarians are used to constrain the rate of sediment recycling. Dissolution of radiolarians though can greatly affect the interpretation of the observed biostratigraphy.  相似文献   

5.
The role of mud erosion under waves in governing cohesive sediment transport in estuarial and coastal waters is well known. A laboratory study was conducted in order to elucidate the mechanism by which soft muds erode under progressive waves in a flume. Two types of cohesive sediment were used, a commercial kaolinite and an estuarial mud. Beds were formed by pouring in a pre-prepared sediment-water slurry and allowing the deposit to consolidate for a period ranging from 2 to 14 days. A multi-layered hydrodynamic model, which considers the mud to be viscoelastic, has been developed and used to evaluate the bed shear stress at the oscillating mud-water interface. The viscoelastic property of the mud has been confirmed by rheological measurements, and model results on velocity, pressure and wave attenuation verified against flume data. Concentration profiles indicate a distinct evolutionary pattern resulting in a highly stratified suspension. Just above the bed, a thin layer of fluid mud is generated. Above this layer, the suspension concentration is significantly lower. This two-layered feature of the concentration profile is related to the oscillatory response of the mud and water layers, and the associated momentum exchange and mass diffusion characteristics. An expression relating the rate of erosion to the bed shear stress in excess of bed shear resistance has been developed. Generation of fluid mud during erosion is a significant feature of the role of waves over mud.  相似文献   

6.
Sediment found in China’s Yangtze and Yellow River systems is characterized by large silt fractions. In contrast to sand and clay, sedimentation and erosion behaviour of silt and silt–clay–sand mixtures is relatively unknown. Therefore, settling and consolidation behaviour of silt-rich sediment from these river systems is analysed under laboratory conditions in specially designed settling columns. Results show that a transition in consolidation behaviour occurs around clay contents of about 10 %, which is in analogy with the transition from non-cohesive to cohesive erosion behaviour. Above this threshold, sediment mixtures consolidate in a cohesive way, whereas for smaller clay percentages only weak cohesive behaviour occurs. The settling behaviour of silt-rich sediment is found to be in analogy with granular material at concentration below 150 g/l. Above 150–200 g/l, the material settles in a hindered settling regime where segregation is limited or even prevented. The results indicate that for modelling purposes, multiple sediment fractions need to be assessed in order to produce accurate modelling results.  相似文献   

7.
《国际泥沙研究》2021,36(6):723-735
This numerical modeling study (i) assesses the influence of the sediment erosion process on the sediment dynamics and subsequent morphological changes of a mixed-sediment environment, the macrotidal Seine estuary, when non-cohesive particles are dominant within bed mixtures (non-cohesive regime), and (ii) investigates respective contributions of bedload and suspended load in these dynamics. A three dimensional (3D) process-based morphodynamic model was set up and run under realistic forcings (including tide, waves, wind, and river discharge) during a 1-year period. Applying erosion homogeneously to bed sediment in the non-cohesive regime, i.e., average erosion parameters in the erosion law (especially the erodibility parameter, E0), leads to higher resuspension of fine sediment due to the presence of coarser fractions within mixtures, compared to the case of an independent treatment of erosion for each sediment class. This results in more pronounced horizontal sediment flux (two-fold increase for sand, +30% for mud) and erosion/deposition patterns (up to a two-fold increase in erosion over shoals, generally associated with some coarsening of bed sediment). Compared to observed bathymetric changes, more relevant erosion/deposition patterns are derived from the model when independent resuspension fluxes are considered in the non-cohesive regime. These results suggest that this kind of approach may be more relevant when local grain-size distributions become heterogeneous and multimodal for non-cohesive particles. Bedload transport appears to be a non-dominant but significant contributor to the sediment dynamics of the Seine Estuary mouth. The residual bedload flux represents, on average, between 17 and 38% of the suspended sand flux, its contribution generally increasing when bed sediment becomes coarser (can become dominant at specific locations). The average orientation of residual fluxes and erosion/deposition patterns caused by bedload generally follow those resulting from suspended sediment dynamics. Sediment mass budgets cumulated over the simulated year reveal a relative contribution of bedload to total mass budgets around 25% over large erosion areas of shoals, which can even become higher in sedimentation zones. However, bedload-induced dynamics can locally differ from the dynamics related to suspended load, resulting in specific residual transport, erosion/deposition patterns, and changes in seabed nature.  相似文献   

8.
This paper discusses the growth stages,spatial structures,quantitative fitting relationships among various parameters,growth patterns and influencing factors of mud cracks by field survey,core observation and SEM analysis.The study shows that:(1) Mud crack growth can go through three stages,i.e.the syndiagenetic stage,the burial diagenetic stage(including early diagenetic stage,middle-late diagenetic stage) and the epidiagenetic stage.(2) Quantitative fitting relationships among various parameters allow a great significance to describe the spatial structure,the regional distribution and the growth environment of mud cracks.(3) Mud crack growth has three models,such as the unilateral growth model including the linear growth pattern,the curvilinear growth pattern and the bifurcation growth pattern,the multilateral growth model including the inteisectional growth pattern,the join growth pattern and the dispersed growth pattern,and the mixed growth model including the combination of any patterns listed above.(4) Modern mud crack growth usually undergoes four stages.Sand beds in sand-mud rhythmic strata can play a lubricative role on crack growth and provide enough sandy deposits for filling cracks.(5) Mud crack growth usually produces bifurcation and bifurcation angles which are mostly 120° or 90° that are related to sediment heterogeneity and released energy.(6)Factors affecting mud crack growth cover many aspects:clay content and salinity can control the number of mud cracks in different areas;terrain can control mud crack morphology;and different sedimentary cycles can control the growth patterns and filling models of mud cracks.  相似文献   

9.
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   

10.
Among rheological models for estimating the rate of dissipation of non-breaking waves in muddy seabeds, those representing viscoelastic and poroelastic behaviors are used widely. In that regard, the dependence of the wave attenuation coefficient derived from basic rheological representations of mud behavior is examined on a cursory basis. For wave attenuation due to viscoelastic muds, results based on a semi-analytical model incorporating the effects of typically thin mud layers are summarized. This and an existing model for poroelastic beds are tested against selective laboratory data. Relying on these tests, it is emphasized that fluid-like mud should be modeled as a viscoelastic fluid medium, and that only non-fluid beds can be modeled as poroelastic media. Mechanisms for energy dissipation depend on bed compactness specified by the solids volume fraction, porosity or density, and on a characteristic Péclet number defined in terms of particle size, permeability and wave frequency. Due to the role of the latter parameter, for simulation of wave attenuation the chosen rheological model for a bed of given compactness must be applicable over the expected range of wave frequency.  相似文献   

11.
Processes of soil erosion and sediment transport are strongly influenced by land use changes so the modelling of land use changes is important with respect to the simulation of soil degradation and its on‐site and off‐site consequences. The reliability of simulation results from erosion models is circumscribed by considerable spatial variation in many parameters. However, most of the currently widely used erosion models at the mesoscale are semidistributed, which leads to difficulties in incorporating a high degree of spatial information, especially land use information, so that the effects of land use changes on soil erosion have hitherto not been investigated in detail using these models. In this article, a grid‐based distributed erosion and sediment transport model is introduced, which simulates the spatial pattern of erosion and deposition rates and sediment transport processes in river channels. In this model, land use affects soil erosion through altering soil loss and influencing sediment delivery. Simulated soil erosion for events recorded in 1989 and 1996 in the Lushi basin in China was analyzed by comparing it with historical land use maps. The results indicated that even relatively minor land use changes had a significant effect on regional soil erosion rates and sediment transport to rivers. The average erosion rate increased from 1989 to 1996, after the transformation of forest to farmland. The results of the study suggest that the proposed soil erosion model can be applied in similar river basins. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

12.
This paper focuses on the importance of biophysical interactions on short-term and long-term sediment dynamics. Therefore, various biological (macrobenthos, photopigments, colloidal EPS) and physical parameters (grain size, water content, sediment stability, bed level) were determined (bi)monthly in nine sampling plots on the IJzermonding tidal flat (Belgium, 51°08′N, 2°44′E) during three consecutive years (July 2005–June 2008). Results showed that sediment stability varied on the short timescale and was directly influenced by biota, while bed level varied mainly on the long-term due to interannual variability. The short-term dynamic relationships between mud content, water content, fucoxanthin and macrobenthos density resulted in a seasonal mud deposition and erosion cycle, and directly influenced sediment stability. Moreover, macrobenthos was proven to be the most important parameter determining sediment stability. On the long-term, a shift was observed from high fucoxanthin/chla concentration, high mud content and zero to moderate densities of Corophium volutator towards low fucoxanthin/chl a and mud content and high Corophium densities, which resulted in a transition from net accretion to net erosion. However, most measured variables proved to be poor predictors for these long-term bed level changes, indicating that external physical forces, such as waves and storminess, probably were the most important factors triggering long-term sediment dynamics. Nevertheless, biota indirectly influenced bed level changes by mediating short-term changes in sediment stability, thereby influencing the erodability of the sediment. The macrobenthos, and especially the mud shrimp Corophium, was suggested as the (indirect) driving destabilising factor for the sampling plots in the IIzermonding when considering the long-term evolution.  相似文献   

13.
This study examines the particle size characteristics of hillslope soils and fluvial suspended sediments in an agricultural catchment. Samples of surface runoff and stream flow were collected periodically and analysed for the size distributions of the effective (undispersed) sediment. This sediment was subsequently dispersed and the ultimate size distributions determined. The median effective particle size of stream suspended sediment was considerably coarser than the median ultimate particle size, indicating that most of the load included a substantial proportion of aggregates. Moreover, the proportion of fine material (i.e. silt and clay) increased, and the proportion of sand-sized material decreased, with increasing discharge. This decrease in sediment size with increased flow, which is contrary to the traditional assumption of a positive discharge/particle size relationship, is thought to reflect: (i) the influx of silt and clay, predominantly the former, originating on the catchment slopes and brought to the stream by overland flow along vehicle wheelings, roads and tracks; and (ii) erosion of fine material from the channel bed and banks. During large storms, however, the proportion of sand-sized sediment increased during the rising limb of the hydrograph, as a result of the entrainment of coarser source material from the valley floor during overbank flooding. The stream suspended sediment was finer than the catchment soils and considerably finer than material eroding from the catchment slopes during storms. The degree of clay and silt enrichment in the suspended sediments was largely the result of preferential deposition of the coarser fraction during the transport and delivery of sediment from its source to basin outlet. The data from this study confirm that a significant mode of sediment transport in fluvial systems is in the form of aggregates, and that the dispersed sediment size distribution is inappropriate for determining the transportability of sediment by flow. © 1997 by John Wiley & Sons, Ltd.  相似文献   

14.
In this study, we present a novel approach to measure fundamental processes of cohesive sediment erosion. The experimental setup consists of a laboratory erosion flume (SETEG) and a photogrammetric method to detect sediment erosion (PHOTOSED). Detailed data are presented for three erosion experiments, which were conducted with a natural non-cohesive/cohesive sediment mixture at increasing sediment depths (4, 8, 16 cm). In each experiment, the sediment was exposed to a set of incrementally increasing shear stresses and the erosion was measured dynamically, pixel-based, and approximate to the process scale given the resolution of PHOTOSED. This enables us to distinguish between (i) individual emerging erosion spots caused by surface erosion and (ii) large holes torn open by detached aggregate chunks. Moreover, interrelated processes were observed, such as (iii) propagation of the erosion in the longitudinal and lateral direction leading to merging of disconnected erosion areas and (iv) progressive vertical erosion of already affected areas. By complementing the (bulk) erosion volume profiles with additional quantitative variables, which contain spatial information (erosion area, specific deepening, number of disconnected erosion areas), conclusions on the erosion behaviour (and the dominant processes) can be drawn without requiring qualitative information (such as visual observations). In addition, we provide figures indicating the spatio-temporal erosion variability and the (bulk) erosion rates for selected time periods. We evaluate the variability by statistical quantities and show that significant erosion is mainly confined to only a few events during temporal progression, but then considerably exceeds the time-averaged median of the erosion (factors between 7.0 and 16.0). Further, we point to uncertainties in using (bulk) erosion rates to assess cohesive sediment erosion and particularly the underlying processes. As a whole, the results emphasise the need to measure cohesive sediment erosion with high spatio-temporal resolution to obtain reliable and robust information. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd  相似文献   

15.
The environmental effects of oil-based drilling mud cuttings have been evaluated using all the data available from monitoring around North Sea platforms. Beneath the platforms making extensive use of oil-based drilling muds the natural sediment is buried by cuttings and the hydrocarbon concentration 250 m from the platforms may be 1000 times background, but the concentration gradient is very steep and background levels are usually reached 2000–3000 m from the platform.The extent of the biological effects appear to be greater from the use of oil-based muds than from water-based muds. Beyond the area of physical smothering the effects of oil-based mud cuttings may be due to organic enrichment or to the toxicity of the aromatic fraction of the oil. Despite the large scale of inputs, in all the fields studied the major deleterious biological effects occurred within 500 m of the platform. Surrounding the area of major impact was a transition zone in which subtle biological effects could be detected as community parameters returned to normal, generally within 400–1000 m.The shape and extent of this zone varied and was largely determined by the current regime and scale of the drilling operation. Elevated hydrocarbon concentrations were detected beyond the areas of biological effects.  相似文献   

16.
《水文科学杂志》2013,58(6):1253-1269
Abstract

Although soil erosion has been recognized worldwide as a threat to the sustainability of natural ecosystems, its quantification presents one of the greatest challenges in natural resources and environmental planning. Precise modelling of soil erosion and sediment yield is particularly difficult, as soil erosion is a highly dynamic process at the spatial scale. The main objective of this study was to simulate soil erosion and sediment yield using two fundamentally different approaches: empirical and process-oriented. The revised form of the Universal Soil Loss Equation (RUSLE), along with a sediment delivery distributed model (SEDD) and the Modified Universal Soil Loss Equation (MUSLE), which are popular empirical models, were applied in a sub-basin of the Mun River basin, Thailand. The results obtained from the RUSLE/SEDD and MUSLE models were compared with those obtained from a process-oriented soil erosion and sediment transport model. The latter method involves spatial disaggregation of the catchment into homogeneous grid cells to capture the catchment heterogeneity. A GIS technique was used for the spatial discretization of the catchment and to derive the physical parameters related to erosion in the grid cells. The simulated outcomes from the process-oriented model were found to be closer to observations as compared to the outcomes of the empirical approaches.  相似文献   

17.
Soil erosion models are essential tools for the successful implementation of effective and adapted soil conservation measures on agricultural land. Therefore, models are needed that predict sediment delivery and quality, give a good spatial representation of erosion and deposition and allow us to account for various soil conservation measures. Here, we evaluate how well a modified version of the spatially distributed multi‐class sediment transport model (MCST) simulates the effectiveness of control measures for different event sizes. We use 8 year runoff and sediment delivery data from two small agricultural watersheds (0·7 and 3·7 ha) under optimized soil conservation. The modified MCST model successfully simulates surface runoff and sediment delivery from both watersheds; one of which was dominated by sheet and the other was partly affected by rill erosion. Moreover, first results of modelling enrichment of clay in sediment delivery are promising, showing the potential of MCST to model sediment enrichment and nutrient transport. In general, our results and those of an earlier modelling exercise in the Belgian Loess Belt indicate the potential of the MCST model to evaluate soil erosion and deposition under different agricultural land uses. As the model explicitly takes into account the dominant effects of soil‐conservation agriculture, it should be successfully applicable for soil‐conservation planning/evaluation in other environments. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

18.
Abstract

Two mathematical models were used to estimate the annual sediment yield resulting from rainfall and runoff at the outlet of the Nestos River basin (Toxotes, Thrace, Greece). The models were applied to that part of the Nestos River basin (838 km2) which lies downstream of three dams. Both models consist of three submodels: a simplified rainfall-runoff submodel, a physically-based surface erosion submodel and a sediment transport submodel for streams. The two models differ only in the surface erosion submodel: that of the first model is based on the relationships of Poesen (1985) for splash detachment and splash transport, while the corresponding submodel of the second model is based on the relationships of Schmidt (1992) for the momentum flux exerted by the droplets and the momentum flux exerted by the overland flow. The degree of conformity between the annual values of sediment yield at the basin outlet according to both models is satisfactory.  相似文献   

19.
Water level, sediment heterogeneity, and plant density are important factors that determine plant growth, distribution, and community structure. In the present study, we investigated the effects of these factors on the growth and root characteristics of Carex brevicuspis. We conducted an outdoor experiment to monitor biomass accumulation and allocation, relative root distribution mass ratio, longest root length, and total N and P contents of C. brevicuspis plants. We used a factorial design with two water levels (0 cm and −15 cm relative to the soil surface, named high and low water level treatments, respectively), three sediment types (sand/clay sediment with 0–15 cm of sand and 15–30 cm of clay; mixed sediment with 0–30 cm mixture of sand and clay with 1:1 volumw ratio; and clay/sand sediment with 0–15 cm of clay and 15–30 cm of sand), and three plant densities (88 plants per m2, 354 plants per m2, and 708 plants per m2). Biomass accumulation decreased with increasing plant density and was significantly higher in the low water level and the clay/sand sediment than in the high water level and the other two sediment types. The shoot:root ratio was markedly higher in the high water level than in the low water level and decreased with increasing plant density; further, in the high water level, it was significantly lower in the sand/clay sediment than in the other two sediment types. The relative root distribution mass ratio was markedly higher in the high water level treatments than in the low water level treatments. Further, in the high water level treatments, the relative root distribution mass ratio increased with increasing plant density in the clay/sand sediment and was lower in the sand/clay sediment than in the other two sediment types. The longest root length was significantly lower in the high water level than in the low water level and increased with increasing plant density in the sand/clay sediment in the high water level. Total N content in the plants was influenced only by sediment type; on the other hand, total P content was markedly higher in the high water level than in the low water level. Our data indicate that growth of C. brevicuspis was limited by higher water level, higher density and sand/clay sediment. Plants can increase shoot:root ratio and develop shallow root system to acclimate to high water level and thus could adjust shoot:root ratio and root characteristics, e.g. decrease their shoot:root ratio and allocating more root and increasing root length to the nutrient rich layer to acclimate to conditions of higher density and sediment heterogeneity.  相似文献   

20.
Numerical models have not yet systematically been used to predict properties of fluvial terrace records in order to guide fieldwork and sampling. This paper explores the potential of the longitudinal profile model FLUVER2 to predict testable field properties of the relatively well‐studied, Late Quaternary Allier system in France. For the Allier terraces an overlapping 14C and U‐series chronology as well as a record of 10Be erosion rates exist. The FLUVER2 modelling exercise is focused on the last 50 ka of the upper Allier reach because for this location and period the constraints of the available dating techniques are tightest. A systematic calibration based on terrace occurrence and thicknesses was done using three internal parameters related to (1) the sediment erodibility; (2) the sediment transport distance; and (3) the sediment supply derived from the surrounding landscape. As external model inputs, the best available, reconstructed, tectonic, climatic and base‐level data were used. Calibrated model outputs demonstrate a plausible match with the existing fluvial record. Validation of model output was done by comparing the modelled and measured timing of aggradation and incision phases for the three locations. The modelled range of landscape erosion rates showed a reasonably good match with existing erosion rate estimates derived from 10Be measurements of fluvial sands. The quasi‐validated model simulation was subsequently used to make new testable predictions about the timing and location of aggradation and erosion phases for three locations along the Allier river. The validated simulations predict that along the Allier, reach‐specific dynamics of incision and aggradation, related to the variations in sediment supply by major tributaries, cause relevant differences in the local fluvial terrace stratigraphy. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

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