EQUIPMENT & SUPPLIES: Bedload Samplers--Guidelines for Application of the Helley-Smith Bedload Sampler
                                                  April 6, 1977


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


WRD Distribution: A, B, S, FO, PO


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.