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.