Equilibrium Near-Bed Concentration of Suspended Sediment

A new approach is presented for calculating the equilibrium near-bed concentration of suspended sediment in an alluvial channel flow. It is formulated from the balance between bed sediment entrainment and suspended sediment deposition across the near-bed boundary. The entrainment flux is determined making use of a turbulent bursting outer-scale-based function and the flux of deposition by the product of near-bed concentration and hindered settling velocity of sediment. A number of flume data records in the literature are analyzed to calibrate and verify the present approach. The observed near-bed concentrations for the data records are obtained by first isolating the suspended load transport rate from the observed total load transport rate using Engelund and Fredsoe's bed-load formula and then equating the suspended load transport rate to the shape integration of Dyer and Soulsby. The present approach is shown to perform satisfactorily compared to the results of data analysis. It is found that the near-bed concentration is evidently dependent on sediment particle size in addition to the Shields parameter due to skin friction. This finding seems to challenge previous relationships that simply represent the near-bed concentration as empirical functions of the purely skin-friction-related Shields parameter.     

Monitoring Suspended Sediment Dynamics Using MBES

This technical note describes use of a multibeam echosounder (MBES) to quantify the dynamics of suspended sediment in a large open channel. A methodology is detailed that uses the backscatter magnitude from the MBES water-column data to adjust the magnitude of sonar returns for the various sonar settings, spatially and temporally average the data to account for the random nature of acoustic backscatter from the suspended sediment, and calibrate the processed data with direct samples. A case study of flow at the confluence of the Rio Paraná and Rio Paraguay, Argentina, where there is a distinct turbidity difference along the mixing interface of the two flows, is used to demonstrate the unique capabilities of MBES to quantify sediment concentrations and dynamics within the water column.     

Deposition Properties of Fine Sediment

This paper describes depositional properties of the fine portion (D<0.075?mm) of the surficial bed sediments taken from the Upper St. Clair River. The experiments were conducted using the circular flume facility at the Canada Center for Inland Waters in Burlington, Canada. Tests were conducted under the same initial conditions, which involved mechanical mixing following the application of a very high shear stress that resulted in a solids suspension of 200 g/L. Deposition was allowed to occur under five different shear stresses. The deposition rate and the floc size were monitored using a laser based Malvern particle size analyzer. The results were used to estimate the settling velocity as a function of applied shear stress and lapsed time for the selected shear stress.     

Hydraulic Resistance Induced by Deposition of Sediment in Porous Medium

The deposition of the instantaneously released sediment into a gravel matrix and the hydraulic resistance induced by the rapid siltation are investigated herein. The experimental results reveal that the depositional patterns of the sediment are governed by the gravel-sediment size ratio, the amount of sediment released, and the seepage flowrate. The observations also indicate that the stable stage of sediment deposition is reached shortly after the occurrence of sudden slump. A regression relation is developed to quantify the stable-stage hydraulic resistance with the major governing factors. Given the hydraulic resistance, one can use the siltation equation to evaluate the hydraulic conductivity of the silted porous medium. With such information, planning of the engineering alternatives to meet the seepage flow requirements is made possible.     

Suspended Sediment Concentration Profiles in Wave-Current Flows

Improved expressions to predict time-averaged and horizontally averaged vertical suspended sediment concentration profiles (C? profiles) in wave and current (w-c) conditions are required to advance our present ability to predict rates of noncohesive sediment transport in the marine environment. In the present paper, a new expression for predicting C? profiles has been calibrated and tested using measurements of waves, currents, and C? profiles obtained in a laboratory basin for a broad range of w-c conditions. Good agreement between observed and predicted C? profiles is demonstrated.     

Theory of Fine Sediment Infiltration into Immobile Gravel Bed

A theoretical model is developed to describe the process of fine sediment infiltration into immobile coarse sediment deposits. The governing equations are derived from mass conservation and the assumption that the amount of fine sediment deposition per unit vertical travel distance into the deposit is either constant or increases with increasing fine sediment fraction. Model results demonstrate that fine sediment accumulation decreases rapidly with depth into coarse substrate initially void of fine sediment, which is consistent with experimental observations that significant fine sediment infiltration occurs to only a shallow depth. Comparisons of the theory with flume data indicate that the model adequately reproduced the weighted-averaged fine sediment fraction values from experiments. An early model developed by Sakthivadivel and Einstein for fine sediment infiltration is in part based on the generally incorrect assumption that intragravel flow remains constant following fine sediment infiltration. Applying a correction to the Sakthivadivel and Einstein model based on alternate hypothesis that introgravel flow is driven by a constant head gives similar results as the proposed model.     

Fluctuations of Suspended Sediment Concentration and Turbulent Sediment Fluxes in an Open-Channel Flow

A complex problem of turbulent-sediment interactions in an open-channel flow is approached experimentally, using specially designed field experiments in an irrigation canal. The experimental design included synchronous measurements of instantaneous three-dimensional (3D) velocities and suspended sediment concentration using acoustic Doppler velocimeters (ADV) and a water sampling system. Various statistical measures of sediment concentration fluctuations, turbulent sediment fluxes, and diffusion coefficients for fluid momentum and sediment are considered. Statistics, fractal behavior, and contributions of bursting events to vertical fluxes of fluid momentum and sediment are evaluated using quadrant analysis. It has been found that both turbulence and sediment events are organized in fractal clusters which introduce additional characteristic time and spatial scales into the problem and should be further explored. It is also shown that Barenblatt’s theory of sediment-laden flows appears to be a good approximation of experimental data.     

Modeling Nonuniform Suspended Sediment Transport in Alluvial Rivers

Problems and difficulties in modeling sediment transport in alluvial rivers arise when one uses the theory of equilibrium transport of uniform sediment to simulate riverbed variation. A two-dimensional mathematical model for nonuniform suspended sediment transport is presented to simulate riverbed deformation. Through dividing sediment mixture into several size groups in which the sediment particles are thought to be uniform, the nonuniformity and the exchange between suspended sediment and bed material are considered. The change of concentration along the flow direction, size redistribution, and cross-sectional bed variation can then be described reasonably well by the model. In simulating the flow field with big dry-wet flats, moving boundary problems are solved very well by introducing a so-called finite-slot technique. Verification with laboratory data shows that the model has a good ability to simulate channel bed variations. Last, the model was applied to a real alluvial river system. Variables such as water level, sediment concentration, suspended sediment size distribution, and riverbed variation were obtained with encouraging results.     

青山湖水体总悬浮物浓度的高光谱遥感反演

利用2009年11月9日城市湖泊青山湖实测的水体高光谱遥感数据和同步水质采样数据,在分析水体固有光学特性的基础上,采用单波段、两波段比值和一阶微分3种算法进行了湖泊水体总悬浮物浓度(TSS)遥感反演研究.结果表明,单波段、两波段比值和一阶微分模型相关系数均较好,688 nm处一阶微分线性回归模型相关系数R<'2>为0.932 8,RMSE为2.896 1 mg/L,为最佳的TSS实测光谱遥感反演模型,可以用于青山湖水体TSS浓度的反演.该研究可为水质指标大面积的卫星遥感反演研究提供基础.     

Lateral Variations in Suspended Sediment Concentration over Dunes

A series of laboratory flume experiments were conducted to characterize lateral variations of suspended sediment over dunes. Forty experimental runs were made using flow depths of 0.328 and 0.127?m in a 1.2-m-wide channel. The Froude number was 0.47 and the median diameter of the bed material was 0.52?mm. Point samples of suspended sediment and depth-integrated values calculated using acoustic backscatter data were collected simultaneously at two lateral positions at spacings of 0.41, 0.20, 0.10, and 0.05?m. Root mean square differences between the paired samples increased from 30 to 94% and from 20 to 45% of the mean concentration for point samples and depth-integrated samples, respectively, as the lateral spacing between samples was increased. Lateral variations in the concentration of suspended sediment should be considered when designing sediment sampling strategies over dune beds.     

Turbulent Effects on the Settling Velocity of Suspended Sediment

The mean settling velocities of suspended sediments in turbulence have been examined. The settling velocities in a flume are directly measured by using an acoustic Doppler velocimeter. The results indicate the same trend as previous work in homogeneous isotropic turbulence. In addition to the flume experiment, the numerical experiments were conducted in the velocity field of homogeneous isotropic turbulence simulated by Kraichnan’s technique. The experimental and numerical results show that at high turbulence intensity the relative settling velocity increases with the increasing relative turbulence intensity regardless of the Stokes number. At intermediate turbulence intensity, it seems that the settling data bifurcate, i.e., the particles at the large Stokes number tend to be slowed, whereas the settling velocity of particles is increased at the small Stokes number.     

Preventing Sediment Deposition in Inverted Sewer Siphons

Laboratory experiments were carried out to determine minimum self-cleansing velocities for preventing deposition of sediment in upward sloping and vertical pipes of inverted siphons. Tests were made using sediment sizes of 0.78 and 4.3 mm in a pipe of 150 mm diameter for eight angles of inclination between 0 and 90°. The criterion for self-cleansing conditions was defined as the minimum velocity needed to prevent the formation of deposits on the invert of the pipe. For a given sediment concentration the self-cleansing velocity was found to be a maximum at pipe slopes between about 22.5 and 45°. Analysis of the forces acting on sediment particles in inclined pipes led to the development of a formula for predicting self-cleansing velocities taking account of pipe size, sediment size, sediment concentration, and pipe slope.     

Rate of Deposition of Fine Sediment from Suspension

Standard depth-integrated models of sediment dynamics predict that concentrations of suspended fine sediment should decay at a characteristic rate that is controlled by the particle settling velocity and the depth of the water. In contrast, a model which resolves the processes of settling and dispersion in the water column has suggested that this decay rate should be independent of the settling velocity, and is controlled by dispersive processes in the water column. By revisiting the problem of sediment dispersion and settling following a point release of material, we resolve this discrepancy and confirm that depth-integrated models capture the correct physical behavior.     

Unified View of Sediment Transport by Currents and Waves. II: Suspended Transport

The problem of suspended sediment transport in river and coastal flows is addressed. High-quality field data of river and coastal flows have been selected and clustered into four particle size classes (60–100, 100–200, 200–400, and 400–600?μm). The suspended sand transport is found to be strongly dependent on particle size and on current velocity. The suspended sand transport in the coastal zone is found to be strongly dependent on the relative wave height (Hs/h), particularly for current velocities in the range 0.2–0.5?m/s. The time-averaged (over the wave period) advection–diffusion equation is applied to compute the time-averaged sand concentration profile for combined current and wave conditions. Flocculation, hindered settling, and stratification effects are included by fairly simple expressions. The bed-shear stress is based on a new bed roughness predictor. The reference concentration function has been recalibrated using laboratory and field data for combined steady and oscillatory flow. The computed transport rates show reasonably good agreement (within a factor of 2) with measured values for velocities in the range of 0.6–1.8?m/s and sediments in the range of 60–600?μm. The proposed method underpredicts in the low-velocity range (<0.6?m/s). A new simplified transport formula is presented, which can be used to obtain a quick estimate of suspended transport. The modeling of wash load transport in river flow based on the energy concept of Bagnold shows that an extremely large amount of very fine sediment (clay and very fine silt) can be transported by the flow.     

Energy Model to Predict Suspended Load Deposition Induced by Woody Debris: Case Study

Large woody debris (LWD) has been used repeatedly to create aquatic habitats. This study attempts to quantify and predict geomorphological changes induced by LWD. Six cylindrical bundles of LWD were anchored in a stream in central Ohio, and bed elevations were monitored for up to seven months. A model was developed to predict deposition downstream of the LWD. Sediments accumulated immediately downstream of the LWD structures. The average accumulation depth 0.25 m downstream of the LWD was 0.10 m. At 1.25 m downstream, accumulation depth averaged 0.07 m. The model to predict the sediment deposition had R2 values of 0.87–0.77, respectively, at the two downstream locations. The most important terms in the model were the Froude number and bankfull depth. An advantage of this model was the use of easily measurable variables including average bankfull velocities, depth, and cross-sectional area. This fact will facilitate the use of the model in field settings. Suggestions for future improvements to the model include calibration/validation in different streams, inclusion of a temporal variable, and sediment characterization.     

Analysis of Suspended Sediment Transport in Open-Channel Flows: Kinetic-Model-Based Simulation

This paper presents a comprehensive analysis of suspended sediment transport in open channels under various flow conditions through a kinetic-model-based simulation. The kinetic model, accounting for both sediment-turbulence and sediment-sediment interactions, successfully represents experimentally observed diffusion and transport characteristics of suspended sediments with different densities and sizes. Without tuning any model coefficients, the nonmonotonic concentration distribution and the noticeable lag velocity with a negative value close to the wall are reasonably reproduced. Examination of flow conditions typical of suspension dominated rivers shows that the conventional method may overestimate or underestimate unit suspended-sediment discharge, depending on the Rouse number, sediment size, as well as shear velocity. The error may be less than 20% for dp<0.5?mm and might exceed 60% for dp>1.0?mm under typical flow conditions where shear velocity ranges from 1.0?to?12.5?cm/s and flow depth ranges from 1.0?to?5.0?m.     

Effect of Shallowness on Sediment Deposition in Recirculation Zones

Various hydrodynamic and sediment deposition experiments were conducted on a purpose-built, wide, shallow water table using a range of incoming flow and bed roughness conditions, including five flow rates, different inflow sediment concentrations, and different artificial roughness coefficients generated by four transverse string intervals. A recirculation zone was created behind a 0.5-m-wide backward-facing step on the left side of the table. Surface velocities, sediment concentrations, and processes and features of sediment deposition in recirculation zones were measured. Experimental results indicate that (1)?as the value of the shallow stability parameter increased from 0.033 to 0.47, the length of the recirculation zone decreased to less than half, but the corresponding area and volume of sediment deposition increased about four times, and (2)?it is the strength of the mixing layer rather than the size of the recirculation zone that dominates the process of sediment deposition as well as the final deposition feature.     

Discharge and Suspended Sediment Transport during Deconstruction of a Low-Head Dam

In March of 2003, the 43?m wide, 2.2?m high St. Johns Dam (Sandusky River, Ohio) was breached to lower the water level in the reservoir. In November of the same year, the dam was removed in an effort to restore aquatic habitat and connectivity in the river. During both the breach and the dam removal, high resolution time series of discharge and suspended sediment concentrations were monitored 200?m downstream of the dam. Discharge and suspended sediment during the breach were not discernible from background values. In contrast, the dam removal resulted in a peak suspended sediment concentration of 59?mg/L and a peak discharge of 33.5?m3/s, which returned to background levels of 19?mg/L and 1.5?m3/s, respectively, approximately 8?h after the removal. The floodwave during the removal attenuated by 50% at the City of Fremont, 53?km downstream, illustrating the diffusive nature of the channel and the limited risk of flooding downstream. Levels of suspended sediment and discharge during the removal were comparable to subsequent discharge events. Spatial distributions of turbidity in and upstream of the dam pool and archived turbidity data from the City of Tiffin, 13?km downstream of the dam, suggest that sediments stored in the impoundment did not statistically enhance turbidity up to 2 years after the removal. Generally, the removal had a minor impact on water quality and posed no risk to public safety or to downstream aquatic habitats.     

Transport and Deposition of Suspended Soil Colloids in Saturated Sand Columns

Understanding colloid mobilization, transport, and deposition in the subsurface is a prerequisite for predicting colloid-facilitated transport of strongly adsorbing contaminants and further developing remedial activities. This study investigated the transport behavior of soil-colloids extracted from a red-yellow soil from Okinawa, Japan. Different concentrations of suspended-soil colloids (with diameter <1??μm) were applied, at different flow velocities and pH conditions, to 10-cm long water-saturated columns repacked with either Narita (mean diameter D50 = 0.64??mm) or Toyoura (mean diameter D50 = 0.21??mm) sands. The transport and retention of colloids were studied by analyzing colloid effluent breakthrough curves (BTCs), particle size distribution in the effluent, and colloid deposition profiles within the column. The results showed a significant influence of flow velocity: Low flow velocity caused tailing of colloid BTCs with higher reversible entrapment and release of colloids than high flow velocity. The finer Toyoura sand retained more colloids than the coarser Narita sand at low pH conditions. The deposition profile and particle size distribution of colloids in the Toyoura sand clearly indicated a depth-dependent straining mechanism. By fitting colloid transport models (one-site and two-site models) to the colloid effluent breakthrough curves, transport and deposition of colloids in Narita sand at low pH were best described by a one-site attachment-detachment model, whereas colloid transport and deposition in Toyoura sand at low pH were better captured by a two-site attachment, detachment, and straining model. The coupled effects of solution chemistry, colloid sizes, and medium surface properties have a dominating role in particle-particle and particle-collector interactions in colloid transport and deposition.