Field methods for measurement of fluvial sediment

Methodological sensitivity of morphometric estimates of coarse fluvial sediment transport

Motion pictures taken at Duck Creek, a clear stream 6.5 m wide and 35 cm deep near Pinedale, Wyoming, provide detailed, quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them. Bed shear stress was approximately 6 Pa (60 dynes cm−2), which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material; and transport was almost entirely as bedload. The displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones, and rarely by brief sliding of large, angular ones. Entrainment was principally by rollover of the larger particles and liftoff of the smaller ones, and infrequently by ejection caused by impacts, whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones. The displacement times averaged about 0.2−0.4 s and generally were much shorter than the intervening repose times. The collective motions of the particles were characterized by frequent, brief, localized, random sweep-transport events of very high rates of entrainment and transport, which in the aggregate transported approximately 70% of the total load moved. These events occurred 9% of the time at any particular point of the bed, lasted 1–2 s, affected areas typically 20–50 cm long by 10–20 cm wide, and involved bedload concentrations approximately 10 times greater than background. The distances travelled during displacements averaged about 15 times the particle diameter. Despite the differences in their dominant modes of movement, the 8–16 mm particles typically travelled only about 30% slower during displacement than the 2–4 mm ones, whose speeds averaged 21 cm s−1. Particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely, with diffusivities of 4.6 and 0.26 cm2 s−1, respectively. The bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler. The 2–4 mm particles were entrained 6 times faster on finer areas of the bed, where 8–16 mm particles covered 6% of the surface area, than on coarser ones, where they covered 12%, even though 2–4 and 4–8 mm particles covered practically the same percentage areas in both cases. The 4–8 and 8–16 mm particles, in contrast, were entrained at the same rates in both cases. To within the statistical uncertainty, the rates of distrainment balanced the rates of entrainment for all three sizes, and were approximately proportional to the corresponding concentrations of bedload.

Bedload transport of fine gravel observed by motion-picture photography

A major challenge for geomorphologists is to scale up small-magnitude processes to produce landscape form, yet existing approaches have been found to be severely limited New ways to scale erosion and transfer of sediment are thus needed This paper evaluates the concept of sediment connectivity as a framework for understanding processes involved in sediment transfer across multiple scales We propose that the concept of sediment connectivity can be used to explain the connected transfer of sediment from a source to a sink in a catchment, and movement of sediment between different zones within a catchment: over hillslopes, between hillslopes and channels, and within channels Using fluvial systems as an example we explore four scenarios of sediment connectivity which represent end-members of behaviour from fully linked to fully unlinked hydrological and sediment connectivity Sediment-travel distance – when combined with an entrainment parameter reflecting the frequency–magnitude response of the system – maps onto these end-members, providing a coherent conceptual model for the upscaling of erosion predictions This conceptual model could be readily expanded to other process domains to provide a more comprehensive underpinning of landscape-evolution models Thus, further research on the controls and dynamics of travel distances under different modes of transport is fundamental Copyright © 2014 John Wiley & Sons, Ltd

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Sediment connectivity: a framework for understanding sediment transfer at multiple scales

Transport of Gravel and Sediment Mixtures

Temporal variations in bedload transport rates that occur at a variety of timescales, even under steady flow conditions, are accepted as an inherent component of the bedload transport process. Rarely, however, has the cause of such variations been explained clearly. We consider three data sets, obtained from laboratory experiments, that refer to measurements of bedload transport made with continuously recording bedload traps. Each data set is characterized by a predominant low-frequency oscillation, on which additional higher-frequency oscillations generally are superimposed. The period of these oscillations, as isolated through the use of spectral analysts, ranged between 0·47 and 168 minutes, and was associated unequivocally with the migration of bedforms such as ripples, dunes, and bars. The extent to which such oscillatory behaviour may be recognized in a data set depends on the duration of sampling and the length of the sampling time, with respect to the period of a given bedform.



Several theoretical probability distribution functions have been developed to describe the frequency distributions of (relative) bedload transport rates that are associated with the migration of bedforms (Einstein, 1937b; Hamamori, 1962; Carey and Hubbell, 1986). These distribution functions were derived without reference to a sampling interval. We present a modification of Hamamori's (1962) probability distribution function, generated by Monte Carlo simulation, which permits one to specify the sampling interval, in relation to the length of a bedform. Comparisons between the simulated and observed frequency distributions, that were undertaken on the basis of the data described herein, are good (significant at the 90 per cent confidence level). Finally, the implications that temporal variability, which is associated with the migration of bedforms, have for the accurate determination of bedload transport rates are considered.

Temporal variations in bedload transport rates associated with the migration of bedforms

Experimental results show that radioactive tracer techniques provide a feasible means for applying Lagrangian techniques to the observation of sediment transport processes in natural rivers and lab...

Sand Transport Studies with Radioactive Tracers

One significant facet of the disposal of radioactive wastes in alluvial streams pertains to the influence that natural sediments have on the transport and dispersion of radioactive wastes. The sorption of radioactive wastes by clay, silt, and sand (particularly clay-coated sand) is a common phenomenon; hence, the transport and dispersion of waste can be affected to a significant extent by the transport and dispersion of the individual sediment particles. To study the transport and dispersion of bed-material particles (sand), an experiment was conducted in the North Loup River near Purdum, Nebr. In the experiment, sand labeled with radioactive iridium 192 was released from a line source on the bed of the river. After release, the movement of the tracer particles downstream was observed by monitoring the streambed with a sled-mounted scintillation detector. In addition, the vertical distribution of the tracer particles in the bed was observed by monitoring core samples of the bed material. The amount of activity and the number of tracer particles used in the experiment were determined from detailed calibrations in which field conditions were simulated. The tracer particles varied within a fairly narrow range about a median fall diameter of 0.305 mm. The size of the tracer particles was selected so that they would be slightly coarser than the dominant size range of the bed material. Longitudinal traverses with the radiationdetection equipment at approximately daily intervals and extending over a 13-day period defined the dispersion of tracer particles along the channel. The traverses also indicated that some of the tracer particles were buried as a concentrated deposit for about 7 days. Presumably, these tracer particles were deposited in an unusually deep dune trough and were not reexposed until a similarly deep trough passed by. The temporary burial of the tracer particles demonstrates that deposits of contaminated particles in an alluvial stream can remain undisturbed for relatively long periods of time and then suddenly be released. A concentration-distribution function, which is based on probability theory, was derived to characterize the dispersion process. The parameters of the function are related to the dispersion time and distance. Significant characteristics of the function are that the variance and the mean of the distribution are proportional to time and that for large times the maximum concentration varies inversely with the square root of time. Comparable observed and theoretical distributions agreed well except near the origin and at the downstream tail. The explanation for the differences is based on certain discrepancies between physical features of the experiment and the corresponding assumptions on which the theoretical distribution function is based. The major discrepancy is that the theory applies only to a group of tracer particles having identical transport characteristics, whereas the tracer particles did not all have the same characteristics. In addition to defining the dispersion of particles, the observed distribution curves provided information for computing the bedmaterial discharge. The computation of discharge was made with a continuity-type equation, which states simply that the discharge is the product of the mean velocity of the particles and the cross-sectional area through which they move. Although a direct comparison of the computed discharge and a measured discharge could not be made, the computed discharge compared favorably with the discharge computed by the modified Einstein procedure. Although the concentration-distribution function was derived for the initial conditions of a vertical-plane source, it can be extended to more complex conditions by performing appropriate mathematical operations. In addition, the function can be applied to real situations if the constants in the concentrationdistribution function and the depth of the zone of particle movement in the bed are known. When the bed configuration consists of dunes or ripples, the depth of the zone of particle movement can be estimated from longitudinal profiles of the bed surface. However, until the relations between hydraulic and sediment parameters and the constants in the theoretical distribution function are defined, the constants can be evaluated only by experiment. INTRODUCTION AND ACKNOWLEDGMENTS The potential benefits of nuclear industries to mankind depend on the extent to which safe, reliable, and economically feasible means are found to dispose of radioactive wastes. Radioactive wastes present a unique disposal problem because, unlike most other industrial wastes, they cannot be rendered biologically harmless by any of the conventional treatment techniques. One current disposal practice is to release wastes into the environment. The U.S. Geological Survey, in cooperation with the Division of Reactor Development of the Atomic Energy Commission, is engaged in investigations related to the management of radioactive Cl C2 TRANSPORT OF RADIONUCLIDES BY STREAMS wastes that are released into natural streams. One of these investigations deals with aspects of the influence of fluvial sediment in the disposal process. The study reported herein is one facet of that investigation. At the present time (1962), about 95 percent of the low-level, short-lived liquid waste that is released into the environment is disposed of in waterways such as streams, lakes, and reservoirs (Nace, 1959). If a waterway is used for the disposal of radioactive wastes, the possibility exists for the activity to exceed safe levels in and along the waterway. The concentration of activity in a waterway at any time and place depends on a variety of factors which often operate in conjunction with one another. For instance, radionuclides initially dispersed by turbulent diffusion to concentrations far below the maximum permissible concentration may be taken up and concentrated to many thousand times the ambient water concentration (Robeck and others, 1954) by aquatic biota or fluvial sediment. Unfortunately the various factors and processes that tend to concentrate and disperse activity are neither understood well nor easily evaluated. Additional complications are introduced by the exchange of radionuclides between the various transporting media. The exchange processes are usually complex and depend on chemical and physical factors related to the radioactive waste effluent, the transporting media, and the flow. One of the major transporting media in natural waterways is sediment (Sayre and others, 1963). Abundant evidence indicates that the uptake of radioactive waste products by sediment is a common phenomenon. Fine sediment (clay and silt) tends to have a much higher sorption capacity per unit weight than does sand; however, Kennedy (1963) has shown that some stream sands, particularly those containing clayimpregnated rock fragments, may have relatively high sorption capacities. According to Kennedy, most of the sorption capacity of suspended sediment is due to the clay fraction, but most of the sorption capacity of the bed material may be due to sand and silt, because those components constitute a very high percentage of the bed material. Because of the potential for high amounts of exchange, the distribution of activity in the waterway may be governed to a significant extent by the rates at which the individual contaminated sediment particles are transported and dispersed. If flow conditions induce high concentrations of contaminated sediment, the level of activity within the body of the flow could exceed a safe level. If flow conditions induce low transport rates or deposition of contaminated sediment, activity could build up in and along the channel to an unsafe level. Deposits might consist of sand particles, flocculated fine materials, organic sediments, or unflocculated fine materials that have penetrated into the bed. Hazardous conditions can also arise as a result of exposure of contaminated sediment through flooding and fluctuating water levels. To evaluate the various potential sediment-induced hazards in waterways, the characteristics of the sediment and the rates of transport and dispersion of the different fractions of the sediment load must be considered. Ordinarily, the total sediment load is divided according to the size and source of the sediment into the bed-material load and the fine-material load. The bed-material load is that part of the total sediment load derived from the bed material; consequently, it is composed of particles whose sizes are similar to those of bed-material particles. The bed-material load also includes some suspended sediment in addition to all the bedload. The fine-material load is that part of the total sediment load composed of particles whose sizes are finer than those of particles present in the bed material. Generally, particles of the fine material are in suspension almost continuously and are transported at almost the same velocities as the fluid particles; however, the average velocities of bed-material particles, because of a process of alternate movement and temporary storage in the bed, are much lower than those of fluid particles. Because the transport mechanisms associated with the two kinds of sediment loads are different, the ways in which they are dispersed are also different. This report describes a field experiment, conducted in the North Loup River near Purdum, Nebr., in which the dispersion and transport of bed-material particles were studied by means of radioactive-tracer techniques. In the experiment, sand particles that were labeled with a radioactive nuclide were released on the riverbed. As the tracer particles moved and dispersed downstream with the natural bed-material load, their distribution along the channel was determined with radiationdetection equipment. Because the application of radio active-tracer techniques in sediment studies is relatively new, part of the repo

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Transport and dispersion of labeled bed material, North Loup River, Nebraska

Cyclic variations in bed load discharge at a point, which are an inherent part of the process of bed load movement, complicate calibration of bed load samplers and preclude the use of average rates to define sampling efficiencies. Calibration curves, rather than efficiencies, are derived by two independent methods using data collected with prototype versions of the Helley‐Smith sampler in a large calibration facility capable of continuously measuring transport rates across a 9 ft (2.7 m) width. Results from both methods agree. Composite calibration curves, based on matching probability distribution functions of samples and measured rates from different hydraulic conditions (runs), are obtained for six different versions of the sampler. Sampled rates corrected by the calibration curves agree with measured rates for individual runs.

New Approach to Calibrating Bed Load Samplers

Even during constant, or virtually constant, flow conditions, bed load transport rates are known to vary over a range of temporal scales. Previous studies have emphasized that, in order to provide an accurate estimate of the mean bed load transport rate at a given point, either many samples must be collected repeatedly or the sampling time must be long. We systematically evaluate errors associated with at-a-point bed load sampling during steady flow conditions in the presence of dunes and provide an indication of the way in which sampling should proceed in order to minimize sampling errors. The analysis is undertaken using laboratory data and a theoretical probability distribution function. It is assumed that bed load samplers have an efficiency of 100% and that the flow conditions and the configuration of the primary bed forms remain constant during the sampling period. In the presence of dunes, sampling should be accomplished using (1) the shortest practicable sampling time for each sample, (2) short, equally spaced time intervals between samples, and (3) as long a sampling period as is necessary to ensure that at least one primary bed form has passed the sampling point. Such procedures will provide information about high- and low-frequency fluctuations in at-a-point bed load transport rates and a reasonably reliable estimate of the mean bed load transport rate. Only about 21 sequential samples are required to estimate the mean bed load transport rate at any given point. However, because the sampling time generally is short in comparison with the time that it takes a dune to pass the sampling point, between 50 and 100 samples are needed to define both high- and low-frequency at-a-point rate variations. A knowledge of the prevailing bed configuration is necessary in order to ensure that sampling proceeds in accordance with these requirements.

David W. Hubbell

Papers

Temporal variations in bedload transport rates associated with the migration of bedforms

Sand Transport Studies with Radioactive Tracers

Transport and dispersion of labeled bed material, North Loup River, Nebraska

New Approach to Calibrating Bed Load Samplers

At‐a‐Point Bed Load Sampling in the Presence of Dunes