Bedload Transport in Gravel-Bed Streams with Unimodal Sediment

Bedload transport in many gravel-bed streams becomes highly complicated because of the nonuniformity of the grain size and the vertical stratification of channel bed material. A new relation for computing bedload transport rates in gravel-bed streams is proposed here. In an effort to account for the variation of the makeup of the surface material within a wide range of Shields stresses, the relation employs a two-parameter approach, one related to the material in the pavement and the other to that in the subpavement layers. The mode is used to represent the grain sizes of each layer. Available bedload transport data from gravel-bed streams with unimodal sediment are used to test the accuracy of the relation. A comparison with other bedload transport relations is also considered.     

Effect of Seepage-Induced Nonhydrostatic Pressure Distribution on Bed-Load Transport and Bed Morphodynamics

Bed-load transport is commonly evaluated in the condition of a hydrostatic pressure distribution of the flow field; while this condition is reasonable for quasi-steady, quasi-uniform rectilinear flows, it cannot be satisfied in a large variety of flow conditions, i.e., near an obstacle as in the case of a bridge pier. The dimensionless Shields number, which contains the assumption of a hydrostatic pressure distribution in its denominator, therefore cannot be strictly applied to evaluate bed-load transport in all the configurations where nonhydrostatic pressure distributions are observed. In the present work, a generalization of the Shields number is proposed for the case of nonhydrostatic pressure distribution produced by groundwater flow. Experiments showing the effects of vertical groundwater flow on the bed morphodynamics are presented. The comparison between the experimental observations and numerical results, obtained by means of a morphodynamic model which employs the new formulation of the Shields number, suggests that the proposed generalization of the Shields number is able to account the effect of the nonhydrostatic pressure distribution on the bed-load transport.     

Influence of Turbulence on Bed Load Sediment Transport

This paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel. The external turbulence was generated by (1) a horizontal pipe placed halfway through the depth h; (2) a series of grids with a clearance of about one-third of the depth from the bed, and extending over a finite length of the flume; and (3) a series of grids with a clearance in the range (0.1–1.0)h from the bed, but extending over the entire length of the flume. Two kinds of experiments were conducted: plane-bed experiments and ripple-covered-bed experiments. In the former case, the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport. In the ripple-covered-bed case, the mean and turbulence quantities of the streamwise component of the velocity were measured, and the Shields parameter, due to skin friction, was determined. The Shields parameter, together with the RMS value of the streamwise velocity fluctuations, was correlated with the sediment transport rate. The sediment transport increases markedly with increasing turbulence level.     

Measurement of Coarse Gravel and Cobble Transport Using Portable Bedload Traps

Portable bedload traps (0.3 by 0.2 m opening) were developed for sampling coarse bedload transport in mountain gravel-bed rivers during wadable high flows. The 0.9 m long trailing net can capture about 20 kg of gravel and cobbles. Traps are positioned on ground plates anchored in the streambed to minimize disturbance of the streambed during sampling. This design permits sampling times of up to 1 h, overcoming short-term temporal variability issues. Bedload traps were tested in two streams and appear to collect representative samples of gravel bedload transport. Bedload rating and flow competence curves are well-defined and steeper than those obtained by a Helley–Smith sampler. Rating curves from both samplers differ most at low flow but approach each other near bankfull flow. Critical flow determined from bedload traps is similar using the largest grain and the small transport rate method, suggesting suitability of bedload trap data for incipient motion studies.     

Numerical Modeling of Bed Deformation in Laboratory Channels

A depth-average model using a finite-volume method with boundary-fitted grids has been developed to calculate bed deformation in alluvial channels. The model system consists of an unsteady hydrodynamic module, a sediment transport module and a bed-deformation module. The hydrodynamic module is based on the two-dimensional shallow water equations. The sediment transport module is comprised of semiempirical models of suspended load and nonequilibrium bedload. The bed-deformation module is based on the mass balance for sediment. The secondary flow transport effects are taken into account by adjusting the dimensionless diffusivity coefficient in the depth-average version of the k–ε turbulence model. A quasi-three-dimensional flow approach is used to simulate the effect of secondary flows due to channel curvature on bed-load transport. The effects of bed slope on the rate and direction of bed-load transport are also taken into account. The developed model has been validated by computing the scour hole and the deposition dune produced by a jet discharged into a shallow pool with movable bed. Two further applications of the model are presented in which the bed deformation is calculated in curved alluvial channels under steady- and unsteady-flow conditions. The predictions are compared with data from laboratory measurements. Generally good agreement is obtained.     

Statistically Derived Bedload Formula for Any Fraction of Nonuniform Sediment

Based on a method of combining stochastic processes with mechanics, a new bedload formula for the arbitrary kth size fraction of nonuniform sediment is theoretically developed by using a stochastic model of sediment exchange and the probabilistic distribution of fractional bedload transport rates. The relations, proposed recently by Sun, for the probability of fractional incipient motion and for the average velocity of particle motion are introduced to bedload formula. Plenty of experimental data for the bedload transport rate of uniform sediment are used to determine two constants. The theoretical bedload formula for any fraction of nonuniform sediment possesses several advantages, including a clear physical concept, a strict mathematical derivation, and a self-adaptability to uniform sediment. The formula is verified with natural data expressing the transport of nonuniform sediment under full motion in laboratory flume. The result shows that the experimental observations agree well with the predicted fractional bedload transport rates. Comparison of the theory with field data finds that the proposed formula still applies to partial transport of bedload in gravel-bed streams as long as the immobile percentage of bed material is taken into account.     

Measurements of Sediment Erosion and Transport with the Adjustable Shear Stress Erosion and Transport Flume

Soil and sediments play an important role in water management and water quality. Issues such as water turbidity, associated contaminants, reservoir sedimentation, undesirable erosion and scour, and aquatic habitat are all linked to sediment properties and behaviors. In situ analysis is necessary to develop an understanding of the erosion and transport of sediments. Sandia National Laboratories has recently patented the Adjustable Shear Stress Erosion and Transport (ASSET) Flume that quantifies in situ erosion of a sediment core with depth while affording simultaneous examination of transport modes (bedload versus suspended load) of the eroded material. Core erosion rates and ratios of bedload to suspended load transport of quartz sediments were studied with the ASSET Flume. The erosion and transport of a fine-grained natural cohesive sediment were also observed. Experiments using quartz sands revealed that the ratio of suspended load to bedload sediment transport is a function of grain diameter and shear stress at the sediment surface. Data collected from the ASSET Flume were used to formulate a novel empirical relation for predicting the ratio of bedload to suspended load as a function of shear stress and grain diameter for noncohesive sediments.     

Validity of using non-pilot subjects to represent pilots in a sustained acceleration environment

BACKGROUND: A preliminary study determined the similarities between the personality of military pilots (transport and fighter) and centrifuge subjects using the Edwards Personal Preference Schedule (EPPS). Past, similar personality studies have shown differences between military fighter vs. transport pilots, and general population vs. male and female general aviators. To use subjects in lieu of pilots in the centrifuge, they must represent the pilot characteristics of interest, for both ethical and scientific reasons. With the increase in measuring performance metrics (e.g., reaction time, tracking tasks, missile evasion) during centrifuge testing, any factor effecting performance must be explored. It is unknown whether personality effects performance. METHODS: Cluster analysis of 36 pilot and subject personality tests consisted of the Partitioning Around Medoids (PAM) program by Leonard Kaufman and Peter Rousseeuw (10) and Ward's method/K-MEANS clustering (CSS:STATISTICA). RESULTS: The clusters generated by the 36 pilots and subjects did not match the Retzlaff and Gibertini (21) clusters. Two clusters were preferred over three, and while the values of the personality variables Dominance, Exhibition, and Aggression (DOM, EXH, AGG) were similar, the pilot membership did not coincide. Subjects had basically the same cluster characteristics as pilots and did not alter the pilot cluster composition characteristics when clustered together. Females did not appear to differ from the males in the cluster analysis. Clustering did not differentiate between fighter and transport pilots using the chosen variables. CONCLUSION: These preliminary results support the hypothesis that there are no major differences in personality between fighter pilots, transport pilots, or centrifuge subjects using the EPPS.     

Comparisons of Sidewall Correction of Bed Shear Stress in Open-Channel Flows

When investigating sediment transport in laboratory open-channel flows, it is often necessary to remove sidewall effects for computing effective bed shear stress. Previous sidewall correction methods are subject to some assumptions that have not been completely verified, and different values of the bed shear stress may be obtained depending on the approach used in making sidewall corrections. This study provides a quantitative assessment of the existing correction procedures by comparing them to a new sidewall correction model proposed in this study. The latter was derived based on the shear stress function and equivalent roughness size for both rigid and mobile bed conditions, which were obtained directly from experimental measurements. The comparisons show that the Einstein correction formula and the Vanoni and Brooks method generally predict relatively lower and higher bed shear stresses, respectively, while the Williams’ empirical function leads to more scatter. This study also demonstrates that the widely used Vanoni and Brooks approach can be well approximated by a simple formula derived based on the Blasius resistance function. The sidewall effects, when removed in the different ways, would consequently affect the presentation of the bedload function. Experimental results of bedload transport, when plotted as the dimensionless transport rate against the dimensionless shear stress with the latter being corrected using the present model, exhibit less scatter than those associated with the previous procedures.     

Accuracy of Cross-Channel Sampled Sediment Transport in Large Sand-Gravel-Bed Rivers

The accuracy of cross-channel integrated sediment transport of bed material is determined with an elaborate set of field measurements in the Waal River, The Netherlands. The measurements were done during a discharge wave in the upstream part of the river, which has a bimodal sand-gravel bed. The sampling strategy should take both spatial and temporal aspects into account to obtain maximum accuracy. Presence of moving bedforms, differences in bed-sediment grain size in the cross section, and presence of preferential transport lanes dictate that at least five subsections for sampling in the cross section are necessary. The accuracy of cross-channel integrated bedload transport depends mainly on the measurement strategy. An uncertainty of <20% (bedload) and 7% (suspended load) of cross-channel integrated sediment transport is shown to be feasible if 30 samples of bedload and two vertical profiles of suspended bed-material load are taken in one subsection, provided that the cross section of the river is divided into at least five subsections. The samples in one subsection should be distributed over the length of the bed form. Changes of discharge during the measurements cause systematic differences between the subsections. To minimize this uncertainty a compromise between the spatial and temporal accuracy is necessary. Therefore, when only one vessel with instruments is available for doing the measurements, the number of sampling positions and subsections must be reduced if the rate of change of discharge is large. Based on the results a prediction method is given to estimate the feasible accuracy in the planning phase of future campaigns, and the necessary time and financial investment for that accuracy.     

Theory of nucleation with cluster loss and injection: Application to plastic deformation and irradiation

Conventional nucleation theory considers the growth and decay of clusters only by thermally activated single atom addition or subtraction. Many environments of interest, including plastic deformation and energetic particle irradiation, induce numerous dynamic processes to create a host of defects which may produce a new nucleation regime. Creation and movement of dislocations and point defects and passage of thermal and stress waves could readily destroy or remove atoms from subcritical clusters, which typically contain at most only a few tens of atoms. Displacement cascades could destroy subcritical precipitates or vacancy clusters. It is also possible that clusters are injected into the system, as with vacancy-rich displacement cascades which may act as precursors for dislocation loop or void nuclei. This article modifies the nucleation equation to account for such cluster loss and injection and solves the resulting equation analytically for the steady state. The effects of cluster loss are described by several dimensionless groupings of kinetic and thermodynamic parameters. Cluster loss is found significant only if the probability of destruction is comparable to or greater than the probability of growth by single atom capture. The effects of cluster loss are predicted to be greatest for phases which form with large critical nuclei or complex unit cells. Athermal single atom loss, as by radiation resolution in ion mixing, appreciably reduces the nucleation rate only when the rate of loss approaches or exceeds the rate of solute atom capture. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Deconvolution Technique to Separate Signal from Noise in Gravel Bedload Velocity Data

A deconvolution procedure is presented to estimate the probability density function of bedload transport velocity from noisy stationary data collected using the bottom tracking feature of acoustic Doppler current profilers (aDcps). The procedure involves the optimization of a computational summation of random variables for the instrument noise (assumed to be Gaussian with zero mean) and the spatially averaged bedload velocity within the insonified area of each acoustic beam (V). The procedure was tested on two aDcp time series, measured in two different gravel-bed rivers (Fraser River and Norrish Creek). Models generated using either a semitheoretical compound Poisson-gamma distribution or an empirical gamma distribution for V were similar and did not differ significantly from the distribution of the original data. Optimized distributions for V were highly positively skewed. The instrument noise was comparable to instrument noise for aDcp water velocity measurements, i.e., an order of magnitude greater than typical bottom tracking noise. The deconvolution procedure presented herein is generally applicable for the difficult measurement problem of determining the actual signal distribution when measurements are contaminated by noise, particularly for the case of positive-valued signal contaminated by Gaussian noise. The procedure produced the first field estimates of spatially averaged bedload velocity distribution.     

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.     

Stochastic Prediction of Sediment Transport in Sand-Gravel Bed Rivers

Classical deterministic bedload transport predictors are applied to sand-gravel bed rivers. The turbulent bed shear stress is modeled according to a probability distribution to obtain realistic bedload transport rates at incipient motion. In extending the predictors to stochastic predictors for nonuniform sediment, many parameters that represent near-bed turbulence and the particle size distribution must be chosen. The parameters that give realistic results are chosen by analyzing the results of a new experimental flume dataset with relatively large water depths. Choosing other combinations of parameters may give equal total bedload transport rates, but at the cost of large errors in fractional transport rates. Attention is given to the hiding-exposure phenomenon and a hindrance effect related to nonuniform sediment. Validation based on two independent field datasets shows that successful predictions of particle sizes near the threshold for motion are feasible using the stochastic approach, while the deterministic approach gives successful predictions well above incipient motion.     

Exponential Formula for Bedload Transport

An exponential formula that does not involve the concept of the critical shear stress is derived in this study for computing bedload transport rates. The formula represents well various experimental data sets ranging from the weak transport to high shear conditions. Comparisons of the present study are also made with many previous bedload formulas commonly cited in the literature.     

Application of random-effects probit regression models.

A random-effects probit model is developed for the case in which the outcome of interest is a series of correlated binary responses. These responses can be obtained as the product of a longitudinal response process where an individual is repeatedly classified on a binary outcome variable (e.g., sick or well on occasion t), or in "multilevel" or "clustered" problems in which individuals within groups (e.g., firms, classes, families, or clinics) are considered to share characteristics that produce similar responses. Both examples produce potentially correlated binary responses and modeling these person- or cluster-specific effects is required. The general model permits analysis at both the level of the individual and cluster and at the level at which experimental manipulations are applied (e.g., treatment group). The model provides maximum likelihood estimates for time-varying and time-invariant covariates in the longitudinal case and covariates which vary at the level of the individual and at the cluster level for multilevel problems. A similar number of individuals within clusters or number of measurement occasions within individuals is not required. Empirical Bayesian estimates of person-specific trends or cluster-specific effects are provided. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Calretinin-immunoreactivity in trigeminal neurons innervating the nasal mucosa of the rat

Trigeminal primary neuronal cell bodies were labeled by retrograde transport of Fluoro-gold (FG) from the nasal mucosa of rats. The trigeminal ganglion containing the labeled cell bodies were processed for double stain for calretinin- and tachykinin-immunoreactivities (CR- and TK-irs). Except for a few contralateral cells, all the cells that innervated the nasal mucosa (NM cells) were confined to the ophthalmo-maxillary division of the trigeminal ganglion ipsilateral to the FG application. In the dorsal two-thirds of the ganglion, NM cells formed a cluster in the rostromedial part of ophthalmo-maxillary division (the rostromedial cluster). In the ventral third, the number of cells in the rostromedial cluster markedly decreased. Instead, numerous NM cells were found in the caudolateral part of the ophthalmo-maxillary division (the caudoventrolateral cluster). CR- and TK-irs were detected in 18% and 54% of overall population of NM cells, respectively. Virtually all of CR-immunoreactive (-ir) NM cells coexpressed TK. Although the proportion of TK-ir cells, irrespective of CR-ir, was similar for both clusters, CR-ir cells were more frequent in the caudoventrolateral cluster than in the rostromedial cluster. In the dorsal 1/3 of the ganglion where all the NM cells belonged to the rostromedial cluster, only 8.4% exhibited CR-ir. On the other hand, as much as 30.1% of NM cells expressed CR-ir in the ventral 1/3 where most NM cells were found in the caudoventrolateral cluster. Trigeminal cell bodies innervating the cornea and conjunctivum were located in the rostromedial part of the ophthalmo-maxillary division.(ABSTRACT TRUNCATED AT 250 WORDS)     

Choosing the number of clusters in Κ-means clustering.

Steinley (2007) provided a lower bound for the sum-of-squares error criterion function used in K-means clustering. In this article, on the basis of the lower bound, the authors propose a method to distinguish between 1 cluster (i.e., a single distribution) versus more than 1 cluster. Additionally, conditional on indicating there are multiple clusters, the procedure is extended to determine the number of clusters. Through a series of simulations, the proposed methodology is shown to outperform several other commonly used procedures for determining both the presence of clusters and their number. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Particle Densimetric Froude Number for Estimating Sediment Transport

It has been established that for ratios of flow depth to bed particle diameter less than ten (flow on very rough boundaries) neither the Reynolds number of the solid loose particles at a stream bed nor the Shields parameter are adequate variables to predict critical flow conditions for the initiation of motion. A particle densimetric Froude number F? = U/[(s?1)gD]1/2 (where U=mean velocity, s=ratio of sediment and fluid densities, g=acceleration due to gravity, and D=characteristic diameter of bed particle) is here proposed as an alternative criterion to predict hydraulic conditions for the initiation of motion. Values of critical F? were computed after calibration with available experimental data sets. After the critical conditions for the initiation of particle motion were exceeded, transport of bed particles was established. In order to evaluate the performance of a transport equation that contains F? in sediment transport, a set of the most employed formulations to estimate bed material transport in steep slope macrorough flows were tested. The comparison of the results shows that F? can be used to accurately predict sediment discharge.