PROGRAMS AND PLANS--Policy and Guidelines for the Collection and Publication of Bedload Data In Reply Refer To: WGS-Mail Stop 415 July 24, 1990 OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 90.08 Subject: PROGRAMS AND PLANS--Policy and Guidelines for the Collection and Publication of Bedload Data One of the fundamental features of bedload movement is the extreme variation of the transport rate even when the flow is constant. The theory of dune movement indicates that an individual bedload sample may contain a mass of sediment which ranges from zero to four times that expected from the actual average bedload of the stream (Hubbell, 1987). Field data have shown that both spatial and temporal variability are as large, or larger than predicted by this theory (Emmett, 1980, Carey, 1985, Pitlick 1988). Thus, an individ-ual bedload sample may give no more than the roughest estimation of the actual average bedload transport. Individuals measuring bedload should be well aware of this variability and of the potential inaccuracy of their data. The following policy and the attached Guidelines for the collection, publication, and storage of bedload data were developed after much discussion within WRD and supersede previous policy and guideline statements given in the Office of Water Quality Technical Memorandum Nos. 76.04, 77.07, 79.17, and 80.07, as well as WRD Memorandum No. 77.60. A brief summary of the WRD policy issues is given in this memorandum and detailed Guidelines for the collection and publication of bedload data are given in the attachment. This policy applies to all bedload data being collected except those for projects investigating bedload-sampling, equipment, or analytical procedures. POLICY Acceptable Conditions It is the WRD policy that samples can be collected wherever physical conditions will permit sample collection. Acceptable physical conditions are defined in the attached Guidelines. Sampler Selection a. Sampler type: Samplers should be fabricated exactly to design specifications because it has been shown that relatively minor variations in construction can cause significant differences in sampling efficiency. Examples of minor variations that affect sampling efficiency include the thickness of the nozzle wall and locating the frame mount above the bottom of the nozzle. Schematics of the Helley-Smith and FIASP samplers are shown in Figure 1 of the Guidelines. Working drawings of the samplers shown in Figure 1 can be obtained from the Federal Interagency Project in St. Anthony Falls, Minnesota. The sampler type must be recorded as part of the basic data associated with any bedload measurement. b. Bag mesh size: Field personnel are to use their own judgment in the selection of the proper mesh size. Mesh sizes normally used are 0.25, 0.5, 1.0, and 2.0 mm with the 0.25 mm size being the most common. Mesh size must be recorded as part of the basic data associated with the bedload measurement. c. Nozzle expansion ratio: The WRD endorses the use of the 1.40 expansion ratio nozzle, tentatively recommended by the Technical Committee of the Federal Interagency Sedimentation Subcommittee, but will continue to accept data obtained with the standard 3.22 expansion ratio nozzle. d. Nozzle size: The nozzle should be at least two times the size of the largest particles likely to be in motion. Sampling Procedure a. Particle-size analyses: It is mandatory that particle-size analyses of the bedload and suspended load are made at all bedload sampling sites until sufficient analyses have been obtained to define the particle size distribution at the site for the flow in question. Bed material size analyses should also be made, if at all possible. b. Tetherlines: The use of tetherlines, often called stay-lines, is strongly recommended when bedload samples are collected using a cable- suspended sampler. c. Cross-sectional procedures: The U.S. Geological Survey Open-File Report 86-531 lists three cross-sectional procedures that can be used. They are the Single Equal Width Increment (SEWI) method, the Multiple Equal Width Increment (MEWI) method, and the Unequal Width Increment (UWI) method. The SEWI method actually involves collecting 2 samples at each of 20 verticals and is the most commonly used procedure. Sampling procedures should consider spatial (cross-sectional) variations and temporal (at-a-point) variations. Because knowledge of where bedload has occurred in the past does not necessarily imply where bedload will occur in the future, it is unlikely that pre-judgment will allow an investigator to substitute spatial concerns for temporal concerns, or vice versa. Unless sufficient data have been previously collected at the site to ensure that spatial and temporal variations are consistent enough to pre-select an appropriate sampling procedure, enough samples must be collected at each vertical to establish the mean and range of values to be expected, and at enough verticals to define the cross sectional distribution. It is the responsibility of the field personnel to select the procedure that is optimal for the local conditions. d. Sampling time: Although a sampling time of less than 10 seconds should be avoided, the sample bag should never be filled to more than about half full. The sampling time should not be so long that a significant amount of clogging of the bag occurs. e. Sample compositing: Individual bedload samples can be (1) analyzed individually, or (2) composited into one or more samples for analyses. Until the sampling variability for the site is understood, all samples should be analyzed individually. Sampler efficiency Sediment-trapping efficiency is the amount of material trapped in the sampler relative to the amount of material that would have passed through the space occupied by the sampler nozzle were the sampler not in place. Bedload data stored in WATSTORE should not be adjusted for sediment-trapping efficiency. Documentation Bedload data may be stored in WATSTORE and published in the annual data reports. The data will be qualified, however, by storing with each data set the mandatory information given in Table 1 of the Guidelines. Districts should review bedload data stored in WATSTORE prior to the issuance of this memo-randum and enter as much of the mandatory and optional information as possible. In order to gain general knowledge on the subject, it is recommended that anyone who programs, plans, or executes bedload- sampling efforts should become familiar with all references given in the attached Guidelines. Questions and/or comments concerning this new policy should be directed to the Chief, Office of Surface Water, 415 National Center, Reston, Virginia 22092. Charles W. Boning Chief, Office of Surface Water Attachment (see note below) WRD DISTRIBUTION: A, B, FO, PO Note: The attachment to this memorandum is long and complex, containing both figures and tables. It does, however, include important information, mandantory requirements, and matters of policy. If not otherwise available, a copy of this attachment should be obtained from the Office of Surface Water.