Apparatus and Techniques for Measuring Bedload

The need for accurate determinations of the total sediment discharge of particles of bedload size has prompted this investigation of available and possible measuring apparatus and procedures. The accuracy of measurements of sediment discharge made with trap-type samplers is affected by the variability of sampler efficiency, by the oscillatory variation of bedload discharge, and by sampler placement. Equations that were developed for determining total discharge from measured bedload discharge and measured suspended-sediment discharge are simplest if the bedload apparatus measures only the true bedload. Early bedload samplers are generally unsatisfactory. Recently developed or suggested apparatus include various improved samplers of the pressure-difference type, a pumping sampler, a magnetic sampler, acoustical instruments that measure the magnitude of the sound of particle collisions, an ultrasonic bedload sampler designed to measure and integrate electronically the concentration and velocity, and a tiltmeter designed to measure the total sediment discharge from the ground tilt that results from the passage of flow. All the pressure-difference samplers are improvements over early samplers, but none are void of the inherent shortcomings of trap-type apparatus; probably the Sphinx (Dutch) and VUV (Hungarian) samplers are the most satisfactory. The acoustical instruments are capable of measuring only the relative discharge. The ultrasonic sampler and the tiltmeter are not adequate without further development. Some new possible apparatus and means for measuring or aiding in measuring bedload discharge are small pit samplers, ultrasonic sounders, pressure transducers, and photography. A small pit sampler for measuring bedload discharge was designed to provide self-placement and portability; however, its practicability and efficiency are undetermined. Exploratory films show that by using slowmotion photography the discharge of particles larger than about pea size can be determined provided the flow is clear; however, photography generally is not practical. Ultrasonic sounders provide continuous and accurate data on bed configuration and dune movement for use in equations that were developed for computing the bedload discharge. Computations with the equations indicate that the interpretation of the sounding data needs further study. Pressure transducers placed beneath the bed surface possibly can be used to provide information on dune movement; however, their installation would be difficult. The time required for collecting data on bed configuration and dune movement throughout a cross section could be substantially reduced by using several transducers simultaneously In conjunction with an ultrasonic sounder. A modified ultrasonic sounder that provides information on the shape and velocity of large particles and a method for determining the discharge of such particles were proposed; the method seems most feasible for particles of high sphericity. 2 APPARATUS AND TECHNIQUES FOR MEASURING BEDLOAD INTRODUCTION One of the principal concerns in studies of sediment transport is the determination of the total sediment discharge of coarse sediments those particles of bedload size. Such particles do not always move as bedload but sometimes move as suspended-sediment load. Because the suspended sediment can easily be measured, the main problem is the determination of bedload the sediment that slides, rolls, or skips along in almost continuous contact with the streambed. In the past, attempts have been made to determine the bedload discharge in three general ways: By direct measurement with some type of apparatus; by definition of physical relations from which the bedload could be estimated; and by quantitative measurements of the results of some sedimentation process, such as erosion or deposition. Unfortunately, direct-measuring apparatus have been useful for only a very limited range of sediment and hydraulic conditions; the definition of physical relations has not been complete enough to estimate precisely the bedload discharge; and the quantitative measurements have supplied information only on the characteristics of the reach that was studied. As a result, no single apparatus or procedure, whether theoretical or empirical, has been universally accepted as completely adequate for the determination of bedload discharge over the wide range of sediment and hydraulic conditions in nature. The need for adequate determinations of bedload discharge prompted an investigation on the measurement of bedload. This report presents the results of a review and evaluation of the existing and contemplated apparatus and techniques that were studied as a part of the investigation. Because the majority of previous development work and study on the measurement of bedload was done outside the United States, much of the literature had to be reviewed through translations. Many of the translations of publications on early bedload samplers were furnished by the U.S. Waterways Experiment Station, the U.S. Soil Conservation Service, the U.S. Bureau of Reclamation, the National Bureau of Standards, and the University of California. Publications and other informational material on some of the recently developed measuring apparatus were kindly supplied by some foreign laboratories of the International Association for Hydraulic Research, by the U.S. Corps of Engineers, and by Mr. Sam Shulits. The publication by Smoltczyk (1955) was translated partly by Dr. A. N. Zervins and partly by Mr. Donald Steinmetz. Mr. William Thordarson of the Geological Survey and Mr. Istvan Takacs translated the articles by Juniet (1952) and Ivicsics (1956), respectively. APPARATUS AND TECHNIQUES FOR MEASURING BEDLOAD 3 SOME ASPECTS OF MEASURING SEDIMENT DISCHARGE The total sediment discharge of any size fraction includes the suspended-sediment discharge and the bedload discharge of the size fraction. Theoretically, both of these discharges can be determined either by measuring the weight of sediment that passes a section in a given time or by combining the concentration, the area of flow, and the velocity of the particles. However, in practice, the suspendedsediment discharge is usually computed by the concentration method, and the bedload discharge is usually computed by the weight method. Different methods are vised because suspended-sediment particles are transported for sustained periods of time at about the same velocity as that of the flow, and bedload particles usually are transported intermittently at velocities less than that of the flow. Thus, suspended-sediment discharge can be computed from a concentration and a water discharge (the product of particle velocity and area of flow equals water discharge), whereas the bedload cannot. Seemingly, the simplest and most practical method for measuring the weight of bedload passing a section in a given time would be to collect the sediment for a given time in some kind of portable sampler; this method has been used in most efforts to measure the bedload discharge. However, because with this method the sampler must rest on the bed, the flow pattern and the bedload discharge in the vicinity of the sampler are altered to some extent. The degree to which the bedload discharge is altered by any particular sampler depends on many factors, such as the stream velocity and depth, the magnitude of the bedload discharge, and the particle size of the bedload. For accurate bedload discharge measurements, the sampler must be designed so that the bed-load discharge is virtualy unaltered. Also, it must be designed to sample the largest and the smallest bedload particles, to accumulate and retain the bedload particles, to be stable on the bed, and to orient vertically and horizontally so that all particles have an equal opportunity for entrance. These criteria, collectively cannot be completely satisfied. As a result, samplers must be calibrated to determine their efficiencies. The efficiency of any given sampler may vary with any or all of the following factors : velocity, depth, particle size, bedload magnitude, degree of filling, and bed configuration. Inasmuch as all these factors vary in a natural stream, efficiencies are highly variable and uncertain. The bed configuration is a particularly important factor because its effect on efficiency is superimposed, through sampler orientation and stability, on the other factors. Although discussions of the effects of bed configuration on efficiency are not available from the 4 APPARATUS AND TECHNIQUES FOR MEASURING BEDLOAD literature, certain effects seem probable. If the bed relief is formed by dunes that are large relative to the sampler, the sampler location and orientation with respect to the dunes would cause the efficiency to vary radically. Thus, variability in efficiency is the least when the bed relief is low, except possibly if high velocity makes the sampler unstable. Also, if the bed relief is formed by antidunes, which are usually unstable, sampler stability, and therefore efficiency, probably will be affected adversely. Inasmuch as bed relief can be approximated roughly according to the Froude number, certain generalities about the variability of efficiency can also be made according to the Froude number. Tranquil flow over movable material generally forms, successively, ripples, dunes, a so-called transition bed, and possibly antidunes as the Froude number increases. Rapid flow generally forms a transition bed and then antidunes as the Froude number increases. Hence, in tranquil flow, sampler efficiencies might be expected to be least variable at low Froude numbers, highly variable at Froude numbers from about 0.3 to 0.6, only fairly variable at Froude numbers from about 0.6 to 0.8 (if the sampler remains stable), and highly variable at Froude numbers from about 0.8 to 1.0 and throughout the rapid-flow regime. Besides variable and uncertain efficiencies, other factors cause inacc