EQUIPMENT & SUPPLIES: Bedload Samplers--Guidelines for Application of the Helley-Smith Bedload Sampler April 6, 1977 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 77.07 Subject: EQUIPMENT & SUPPLIES: Bedload Samplers--Guidelines for Application of the Helley-Smith Bedload Sampler Quality of Water Branch Technical Memorandum No. 76.04, dated October 15, 1975, transmitted guidelines for qualifying bedload discharge data collected with the Helley-Smith bedload sampler. WRD Memorandum No. 77.60 further restricted the publication of bedload data in annual basic-data reports. The cautious tone of these two memorandums was due to a lack of definitive information on the performance of the Helley-Smith sampler under a wide range of hydraulic conditions. Although adequate information still does not exist and probably will not be available for some time to come, recent field experience with the sampler, including the results of several special studies, has provided enough information on its sampling characteristics that further guidelines for its application can be specified. A description of the sampler and a summary of what we know about its performance so far are contained in the attachment to this memorandum. On the basis of the information provided in the attachment, it is evident that the present version of the Helley-Smith sampler (3-inch x 3-inch or 6-inch x 6-inch nozzle with 3.22 area ratio and 0.25-mm mesh triangular-shaped bag) cannot be regarded as having the same order of accuracy as suspended-sediment sampling equipment. Therefore, programs involving use of the sampler should not be undertaken unless (l) there is a clear necessity for bedload data, and (2) hydraulic and sedimentologic conditions throughout the data-collection period are such that the sampler is suitable for routine use. The following guidelines should be adhered to in assessing whether or not use of the sampler is appropriate: 1. Optimum conditions for use of the Helley-Smith sampler are when the median diameter of the bed material (D50) is between 2 and 8 mm (very fine to fine gravel), the range of bed material sizes is relatively narrow, and bed forms, flow depths, and velocities permit controlled placement and retrieval of the sampler. 2. Use of the Helley-Smith sampler ordinarily is unsuitable in a. streams where the median diameter of bed material is smaller than about 0.5 mm. b. streams where the median diameter of bed material that is subject to movement is larger than about 64 mm. c. streams where the composition of the bed is irregular and large particles are likely to interfere with a good fit of the sampler to the streambed. d. streams where the predominant bed form or the composition of the bed provide the potential for scooping up bed material when the sampler is retrieved. In general, if the median diameter of the bed material is less than about 105 mm and a significant portion of coarse material is not present, or if dunes having length-height ratios of less than about 20 are common, the potential for scooping is high. e. streams where the quantity of organic debris is large enough to promote clogging of the sampler bag. In the event that use of the Helley-Smith sampler is deemed appropriate, provisions should be made to independently determine bedload discharges by several of the most appropriate computational procedures so as to obtain as broad a base as possible for judging the validity of the results obtained with the sampler. Because of the lack of more definitive information, the sampling efficiency of the Helley-Smith sampler always should be assumed to be 1.0 unless specifically contrary data suggest a better value. If reasonable agreement between the several independent methods for estimating bedload discharges cannot be achieved, or if differences cannot be rationally reconciled, bedload discharges should not be disseminated or published, except in reports written specifically to describe the characteristics of the Helley-Smith sampler. In any event, the restrictions placed on the publication of Helley-Smith bedload-transport data in our annual basic data reports through WRD Memorandum No. 77.60 will remain in effect until further notice. Although these guidelines limit application of the Helley-Smith sampler, they seem prudent at this time. You will be notified as additional information becomes available on the applicability of the sampler or as sampler modifications are shown to be desirable. As with the advent of any new instrument, we must lean heavily during its evaluation on field trials and experience. In order to help us further evaluate the accuracy and precision of this sampler, as well as the limits of its practical application, you are requested to furnish this office any information you may have obtained to date on its accuracy, precision, or applicability-- even a qualitative appraisal will be appreciated. Please send your comments to the Chief, Quality of Water Branch. R. J. Pickering Attachment WRD Distribution: A, B, S, FO, PO Attachment Hydraulic Efficiency and Sampling Efficiency of Helley-Smith Bedload Sampler April 1977 by W. W. Emmett and D. W. Hubbell By design, the Helley-Smith sampler is a bedload sampler; that is, it is intended to collect sediment particles that are moving along the bottom, or close to the bottom, by rolling, sliding, or bouncing (actually, those particles moving within 0.25 ft. of the bed). It is not intended as a bed-material sampler; that is, it is not a sampler for collecting samples of the stationary material of which the stream bottom is composed. In the transport process, individual bed-material particles are lifted from the bed and set into motion. After traveling a short distance, they again come to rest. Thus, the particles that make up the bedload progress downstream through alternating periods of movement and rest. When a particle actually is in motion it is part of the bedload; when it is not moving it is simply bed material. Obviously, there is an intimate relationship between the bedload and the bed material; but, even though the same particles are present in both entities, the size distribution of the bedload ordinarily is finer than that of the bed material. To obtain bed- material samples, standard samplers such as the US BM-54, US BMH-53, or US BMH-60, or acceptable procedures such as pebble counting (Wolman, 1954), should be used. To date, data suitable for objectively assessing the performance of the standard Helley-Smith sampler have been collected in five separate studies. Two of these studies were concerned primarily with the hydraulic efficiency of the sampler (that is, the ratio of the mean velocity of flow discharged through the sampler to the mean velocity of flow that would have passed through the area occupied by the nozzle entrance had the sampler not been there). Three studies have dealt directly with the sampling efficiency of the sampler (that is, the ratio of the mass of bedload collected by the sampler during any given period of time to the mass of bedload that would have passed through the area occupied by the nozzle entrance in the same period of time had the sampler not been there). Results from the two studies of hydraulic efficiency are combined in USGS Open-File Report 76-752, "Laboratory Hydraulic Calibration of the Helley-Smith Bedload Sediment Sampler" by L. Druffel and others (1976). The results from the three sampling efficiency studies, along with pertinent hydraulic and sediment data, are summarized in Table 1. The most extensive data set, by far, is available from the East Fork River near Pinedale, Wyoming, where a bedload trap) has been constructed in the bottom o# the stream across its entire width. Some of the hydraulic and bedload transport data from this site are presented in the report, "Bedload Measurements, East Fork River, Wyoming," by Leopold and Emmett (Proc. Nat. Acad. Sci., v. 73, no. 4, pp 1000-1004, April 1976). manuscript t describing the field-determined sediment-trapping characteristics of the Helley- Smith sampler currently is in preparation by Emmett. In the East Fork River there was a very wide range of particle sizes in the bed material and in transport as bedload. On the average, only 10- 15 percent of the bedload is finer than about 0.4 mm, and much of the bedload is relatively coarse. Also, because of coarse material in the stream bottom, the bed is either flat or has long, low dunes and is fairly resistant to local scour around a foreign object placed on the bed. Data from the Colorado State University (CSU) flume study is reported on pages 11-13 of the USGS Open-File Report "Development and Calibration of a Pressure-Difference Bedload Sampler", by E. J. Helley and W. Smith (1971). The bed material used in that study was a natural river sand, predominantly in the very coarse sand range (1-2 mm), and the bed usually was formed into low amplitude dunes that were typically soft and readily scoured. The study on the Rio Grande Conveyance Channel near San Acacia, New Mexico, covered only a narrow range of hydraulic conditions and transport rates. The bed material was predominantly fine to medium sand. At the time of study, water temperatures were near 0!C and the bed was flat and hard so that little, if any, scour occurred around the sampler. Results from the three sampling efficiency studies show totally divergent effects. The Pinedale data indicate that the sampler has an overall sampling efficiency of 92 percent. A detailed analysis of efficiency for each size category of particles indicates a near perfect agreement in the dominant size category of 0.5 to 1.0 mm, about a 90 percent efficiency for particle sizes greater than 1.0 mm, and an overefficiency for particle sizes less than 0.5 mm. With particle sizes smaller than 0.5 mm, the efficiency increases as the size decreases (down to a size that readily passes through the mesh) and reaches about 200 percent for the 0.125 to 0.25 mm size range. The high efficiencies probably result partly because the sampler collects some fine sand in suspension and such material is not measured by the bedload trap. The CSU data indicated that for coarse (.5-1.0 mm) to very coarse (].02.0 mm) sand, when the bed is moderately soft, the sampler consistently gives results that are too high. More importantly, however, the study showed that because of the weight of the sampler, it frequently settled into the bed and tended to scoop up bed material when it was retrieved. This action produced erroneous results that varied according to the extent of the scooping. At San Acacia the sampling efficiency was excessively low, and the study confirmed the conclusion in Open-File Report 76-752 that bedload particles about equal in size to the mesh openings of the bag can plug the openings and lead to erratic and perhaps drastic reductions in the sampler's ability to accept and retain bedload.