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.