EQUIPMENT & SUPPLIES--Bedload samplers; use of Helley-Smith sampler In Reply Refer To: October 2, 1979 EGS-Mail Stop 412 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 79.17 Subject: EQUIPMENT & SUPPLIES--Bedload samplers; use of Helley-Smith sampler Through the years, the Geological Survey has provided leadership in the research and development of procedures for collection and analysis of water data. In the field of suspended-sediment sampling, for example, the Survey has played a dominant role in the development of standard sampling equipment and procedures that are now in use throughout the United States and in many foreign countries. The Water Resources Division feels that the Survey has a similar responsibility to show leadership in the development of bedload samplers and sampling techniques and in the reporting and interpretation of bedload information. In order to assist Division personnel in planning and programming for measurement of bedload discharge, the Quality of Water Branch periodically will issue updated guidelines on the use of the Helley-Smith sampler as new evidence regarding sampling efficiency, accuracy, and precision of the sampler becomes available. The development of the Helley-Smith sampler (a modified version of the Arnhem or Dutch sampler) in 1971 by Helley and Smith (1971) constituted the Geological Survey's first effort to develop a bedload sampler. Considerable field use of this sampler as well as field research has been conducted since it's development. Virtually all of the documented field research has been conducted at the bedload-transport research facility at East Fork River near Pinedale, Wyoming (See Emmett, 1979, copy attached). Research on the hydraulic characteristics of the original Helley-Smith sampler and a double-size version of the sampler has been carried out at our Gulf Coast Hydroscience Center at Bay St. Louis, Mississippi (Druffel and others, 1976). More recently, extensive laboratory testing of the original Helley-Smith sampler design, as well as several modifications of the original design plus other new samplers, has been started by WRD researchers at the St. Anthony Falls Hydraulics Laboratory, University of Minnesota. The purpose of this memorandum is to present revised guidelines for the use of this sampler, based largely on conclusions stated in Emmett's rcport. Please refer to the following previous memoranda issued on this subject: 1. QW Branch Technical Memorandum No. 76.04 2. WRD Memorandum No. 77.60 3. QW Branch Technical Memorandum No. 77.07 Restrictions given in the above-mentioned memoranda reflect a conservative philosophy regarding use of the Helley-Smith sampler. Because of the fact that multi-million dollar river engineering projects are being designed at least in part on the basis of models calibrated with bedload data obtained by the Geological Survey using this sampler, we will continue to maintain a conservative philosophy until we have more definitive information from the research being conducted at the St. Anthony Falls Hydraulic Laboratory on the efficiency of the sampler. Revised Conditions for use of Helley-Smith Sampler--The field- calibration tests reported by Emmett indicate that the original sampler (76.2 x 76.2 mm opening) used at the East Fork River research site had a trap efficiency of near 100% for particles in the size range 0.5 to 16 mm. There remains some question about the trap efficiency for particles in the 8 to 16 mm size class, however, because of the paucity of particles in that size range trapped in the sampler during the East Fork River studies (Emmett, 1979, p. 26). Quality of Water Branch Technical Memorandum No. 77.07 gave as an optimum condition for use of the sampler a range of median diameter of bed material (d50) rather than giving the actual size of particles moving into the sampler. In that memorandum a range in d50 of 2 to 8 mm was given as a criterion or optimum condition for use of the sampler. Use of the d50 of bed material still appears to be a logical basis for deciding whether the sampler should be used. However, on the basis of the East Fork River data and until further laboratory tests are completed, the principal criterion for most efficient use of the original Helley-Smith sampler (76.2 x 76.2 mm opening) is changed to require that the median diameter of the bed material at the location to be sampled be within the range of 1 to 8 mm. Using this range as a guide will mean that generally the size of most of the particles moving as bedload will be within the range of 0.5 to 32 mm, the size range for which we have some assurance at this time that the sampler has acceptable sampling characteristics. Until further guideliues are issued the Helley-Smith sampler with a 76.2 mm opening should not be considered for use in a) streams in which the median diameter of the bed material is smaller than about 1.0 mm. (Exception: If the median diameter is smaller than 1.0 mm but the content of material finer than about 0.25 mm is no greater than 10-15 percent, the sampler may still be used. b) streams in which the median diameter of the bed material is larger than about 8 mm. (Exception: If the median diameter is laFger than 8 mm but the content of material larger than 32 mm is no greater than about 10 percent, the sampler may still be used.) c) streams in which the quantity of fine material, particularly organic material, is great enough to clog the sampler bag. d) streams in which large particles or irregular bed configurations are likely to interfere with a good fit of the sampler to the streambed. Dune bed forms having a length:height ratio of less than about 20 may fall in this category. Sampling technique--Emmett's field-calibration studies indicated that the technique used in the collection of samples with the Helley-Smith sampler may be as significant as the actual efficiency of the sampler. Therefore, upon receipt of this memorandum and until further notice, the following sampling technique should be used. This technique, used with the standard 76.2 x 76.2 mm Helley-Smith sampler and for material in the size range specified above, will be considered a provisional method of the Geological Survey for measurement of bedload discharge. Measurements made using this method should be documented and referred to as having been collected by the provisional USGS method (for bedload sampling using the standard Helley-Smith sampler). Provisional method 1. Samples will be collected at a minimum of 20 equally-spaced sampling stations in the cross section. Variations--For wide cross sections, sampling stations should not be spaced greater than 15 meters (50 ft) apart. For narrow cross sections, sampling stations need not be closer than about 0.5 meters apart. 2. The sampling duration will be the same at each sampling station (for each sample in the cross section). The sampling duration will depend upon the rate of filling of the bag, and probably will vary from 30 to 60 seconds. If more than 60 seconds is required to collect an appreciable quantity of material, the transport rate generally is insignificant. The correct procedure would be to collect a sample at a high- velocity location in the cross section to see whether there is bedload moving and to select a sampling duration that will assure that the bag does not fill to more than about 40 percent at any sampling station (Druffel, 1976). 3. One sample will be collected at each sampling station starting from one bank and proceeding across to the other bank. Each sample will be placed in a separate bag and labeled. Return to the beginning sampling station and repeat the sampling, collecting a second sample at each of the same stations sampled during the first pass across the river. Each sample will be placed in a separate bag and labeled. 4. Depending upon the objective of the sampling program, (l) each sample will be analyzed individually, or (2) a composite of the two samples collected at each sampling station will be analyzed. It is recommended that analyses always include a determination of particle-size distribution in each sample or composite. 5. Results will be reported in terms of mass per unit time per unit width, such as pounds per second per foot (of width) or kilograms per second per meter. The unit bedload discharge reported will be a widthweighted average for the cross section. 6. If possible, the bed should be sampled to determine the size distribution of the bed material, and suspended-sediment samples should be collected so that total sediment discharge can be calculated (Hubbell, 1964, pp 7-19). Note that this is not a simple calculation. Publication of data--WRD Memorandum No. 77.60 prohibited the publication of Helley-Smith bedload data in the annual basic-data report series, thereby limiting publication to special reports. We will be studying this question again in the near future based on the current state of knowledge and would appreciate receiving recommendations from any district or project office on publication format, needed changes in publication policy, or new parameter codes needed if data are to be published in the basic-data reports. Parameter codes currently are available for sieve diameters of bedload ranging from 0.062 mm to 128 mm--codes 80226 through 80238. There also is a code for Sediment Discharge, Bedload (Tons/day), 80225. Finally, Quality of Water Branch Technical Memoranda Nos. 76.04 and 77.07 both requested that the Branch be kept informed by the districts of their use of the Helley-Smith sampler as an aid in our determination of the applicability of this sampler. To date, we have received no information from any district office. I repeat this request. Specific information about the accuracy, precision, and limits of practical applicability of any sampler is absolutely necessary before we can recommend its designation as a standard sampler. R. J. Pickering Attachment Distribution: A(Regional Hydrologist only), FOL References Druffel, L., Emmett, W. W., Schneider, V. R., and Skinner, J. V., 1976, Laboratory hydraulic calibration of the Helley-Smith bedload sediment sampler: U.S. Geol. Survey Open-File report 76-752, 63 p. Emmett, W. W., 1979, A field calibration of the sediment-trapping characteristics of the Helley-Smith bedload sampler, 96 p. Helley, E. J., and Smith, W., 1971, Development and calibration of a pressure-difference bedload sampler: U.S. Geol. Survey Open-File report, 18 p. Hubbell, D. W., 1964, Apparatus and techniques for measuring bedload: U.S. Geol. Survey Water-Supply Paper 1748, 74 p.