ASTM D3858-95(1999)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: D 3858 – 95 (Reapproved 1999)
Standard Test Method for
Open-Channel Flow Measurement of Water by Velocity-Area
Method
This standard is issued under the fixed designation D 3858; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope Applicable Methods of Committee D-19 on Water
D 4409 Test Method for Velocity Measurements of Water in
1.1 This test method covers the measurement of the volume
Open Channels with Rotating Element Current Meters
rate of flow of water in open channels by determining the flow
D 5089 Test Method for Velocity Measurements of Water in
velocity and cross-sectional area and computing the discharge
Open Channels with Electromagnetic Current Meters
therefrom (Refs (1-7) ).
2.2 ISO Standard:
1.2 The procedures described in this test method are widely
ISO 3455 (1976) Calibration of Rotating-Element Current
used by those responsible for the collection of streamflow data,
Meters in Straight Open Tanks
for example, the U.S. Geological Survey, Bureau of Reclama-
tion, U.S. Army Corps of Engineers, U.S. Department of
3. Terminology
Agriculture, Water Survey Canada, and many state and pro-
3.1 Definitions of Terms Specific to This Standard:
vincial agencies. The procedures are generally from internal
3.1.1 current meter—an instrument used to measure, at a
documents of the above listed agencies, which have become
point, velocity of flowing water.
the defacto standards as used in North America.
3.1.2 discharge—the volume of flow of water through a
1.3 This test method covers the use of current meters to
cross section in a unit of time, including any sediment or other
measure flow velocities. Discharge measurements may be
solids that may be dissolved in or mixed with the water.
made to establish isolated single values, or may be made in sets
3.1.3 float—a buoyant article capable of staying suspended
or in a series at various stages or water-level elevations to
in or resting on the surface of a fluid; often used to mark the
establish a stage-discharge relation at a site. In either case, the
thread or trace of a flow line in a stream and to measure the
same test method is followed for obtaining field data and
magnitude of the flow velocity along that line.
computation of discharge.
3.1.4 stage—the height of a water surface above an estab-
1.4 Measurements for the purpose of determining the dis-
lished (or arbitrary) datum plane; also termed gage height.
charge in efficiency tests of hydraulic turbines are specified in
3 3.2 Definitions—For definitions of terms used in this test
International Electrotechnical Commission Publication 41 for
method, refer to Terminology D 1129.
the field acceptance tests of hydraulic turbines, and are not
included in this test method.
4. Summary of Test Method
1.5 This standard does not purport to address all of the
4.1 The principal of this test method consists in effectively
safety concerns, if any, associated with its use. It is the
and accurately measuring the flow velocity and cross-sectional
responsibility of the user of this standard to establish appro-
area of an open channel or stream. The total flow or discharge
priate safety and health practices and determine the applica-
measurement is the summation of the products of partial areas
bility of regulatory limitations prior to use.
of the flow cross section and their respective average veloci-
2. Referenced Documents ties. The equation representing the computation is:
2.1 ASTM Standards: Q 5 ( ~av!
D 1129 Terminology Relating to Water
where:
D 2777 Practice for Determination of Precision and Bias of
Q 5 total discharge,
a 5 individual partial cross-sectional area, and
This test method is under the jurisdiction of ASTM Committee D-19 on Water
v 5 corresponding mean velocity of the flow normal (per-
and is the direct responsibility of Subcommittee D19.07 on Sediments, Geomor-
pendicular) to the partial area.
phology, and Open-Channel Flow.
4.2 Because computation of total flow is a summation or
Current edition approved Sept. 10, 1995. Published November 1995. Originally
published as D 3858 – 79. Last previous edition D 3858 – 90. integration process, the overall accuracy of the measurement is
The boldface numbers in parentheses refer to the references listed at the end of
this test method.
For availability of this publication, contact the International Electrotechnical
Commission, 3 rue de Varembe, CH 1211, Geneva 20, Switzerland. Available from American National Standards Institute, 11 W. 42nd St., 13th
Annual Book of ASTM Standards, Vol 11.01. Floor, New York, NY 10036.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 3858
FIG. 1 Typical Open-Channel Vertical-Velocity Curve (Modified from Buchanan and Somers)
generally increased by increasing the number of partial cross vertical-velocity curve (see Fig. 1), (2) two points (0.2 and
sections. Generally 25 to 30 partial cross sections, even for 0.8 depth below water surface), (3) one point (0.6 depth), (4)
extremely large channels, are adequate depending on the one point (0.2 depth), (5) three points (0.2, 0.6, and 0.8 depth),
variability and complexity of the flow and the cross section. and (6) subsurface (that is, just below the water surface) (see
With a smooth cross section and uniform velocity distribution, 10.9 for further description of each method.)
fewer sections may be used. The partial sections should be
5. Significance and Use
chosen so that each contains no more than about 5 % of the
total discharge. No partial section shall contain more than 10 %
5.1 This test method is used to measure the volume rate of
of the total discharge.
flow of water moving in rivers and streams and moving over or
through large man-made structures. It can also be used to
NOTE 1—There is no universal “rule of thumb” that can be applied to
calibrate such measuring structures as dams and flumes.
fix the number of partial sections relative to the magnitude of flow,
channel width, and channel depth because of the extreme variations in Measurements may be made from bridges, cableways, or boats;
channel shape, size, roughness, and velocity distribution. Where a rating
by wading; or through holes cut in an ice cover.
table or other estimate of total flow is available, this flow divided by 25
5.2 This test method is used in conjunction with determina-
can serve as an estimate of the appropriate flow magnitude for each partial
tions of physical, chemical, and biological quality and sedi-
section.
ment loadings where the flow rate is a required parameter.
4.3 Determination of the mean velocity in a given partial
cross section is really a sampling process throughout the 6. Apparatus
vertical extent of that section. The mean can be closely and
6.1 Many and varied pieces of equipment and instruments
satisfactorily approximated by making a few selected velocity
are needed in making a conventional discharge measurement.
observations and substituting these values in a known math-
ematical expression. The various recognized methods for
determining mean velocity entail velocity observations at
Buchanan, T. J., and Somers, W. P., “Discharge Measurements at Gaging
selected distances below the water surface. The depth selec-
Stations,” U.S. Geological Survey Techniques of Water-Resources Investigations,
tions may include choice of (1) enough points to define a Book 3, Chapter A8.
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
D 3858
The magnitude of the velocity and discharge, location of the headphone or registers a unit on a counting device. The count
cross section, weather conditions, whether suspended, floating, rate is usually measured manually with a stopwatch, or
or particulate matter are present in the water, and vegetative
automatically with a timing device built into the counter.
growth in the cross sections are all factors determining
6.1.3 Width-Measuring Equipment—The horizontal dis-
equipment needs. Instruments and equipment used normally
tance to any point in a cross section is measured from an initial
include current-meters, width-measuring equipment, depth-
point on the stream bank. Cableways, highway bridges, or foot
sounding equipment, timers, angle-measuring devices, and
bridges used regularly in making discharge measurements are
counting equipment. The apparatus is further described in the
commonly marked with paint marks at the desired distance
following paragraphs.
intervals. Steel tapes, metallic tapes, or premarked taglines are
6.1.1 Current Meter— Current meters used to measure
used for discharge measurements made from boats or un-
open-channel flow are usually of the rotating-element (see
marked bridges, or by wading. Where the stream channel or
Note 2) or electromagnetic types. Refer to Test Methods
cross section is extremely wide, where no cableways or
D 4409 and D 5089 for more specific information. However,
suitable bridges are available, or where it is impractical to
the equipment sections of this test method emphasize the
string a tape or tagline, the distance from the initial point on the
rotating-element meters mainly because of their present wide-
bank can be determined by optical or electrical distance meters,
spread availability and use. The operation of these meters is
by stadia, or by triangulation to a boat or man located on the
based on proportionality between the velocity of the water and
cross-section line.
the resulting angular velocity of the meter rotor. Hence, by
6.1.4 Depth-Sounding Equipment—The depth of the stream
placing this instrument at a point in a stream and counting the
below any water surface point in a cross section, and the
number of revolutions of the rotor during a measured interval
relative depth position of the current meter in the vertical at
of time, the velocity of water at that point is determined.
that point, are usually measured by a rigid rod or by a sounding
Rotating-element meters can generally be classified into two
weight suspended on a cable. The selection of the proper
main types: those having vertical-axis rotors, and those having
weight is essential for the determination of the correct depth. A
horizontal-axis rotors. The principal comparative characteris-
light weight will be carried downstream and incorrectly yield
tics of the two types may be summarized as follows: (1) the
depth observations that are too large. A “rule of thumb” for the
vertical-axis rotor with cups and vanes operates in lower
selection of proper sized weights is to use a weight slightly
velocities than does the horizontal-axis rotor, has bearings that
heavier in pounds than the product of depth (feet) times
are well protected from silty water, is repairable in the field
velocity (feet per second) (no direct metric conversion is
without adversely affecting the meter rating, and works effec-
available). The sounding cable is controlled from above the
tively over a wide range of velocities; (2) the horizontal-axis
water surface either by a reel or by a handline. The depth-
rotor with vanes disturbs the flow less than does the vertical-
sounding equipment also serves as the position fixing and
axis rotor because of axial symmetry with flow direction, and
supporting mechanism for the current meter during velocity
is less likely to be fouled by debris. Also, the rotor can be
measurements. Sonic depth sounders are available but are
changed for different velocity ranges and meters of this type
usually not used in conjunction with a reel and sounding
are more difficult to service and adjust in the field.
weight.
NOTE 2—Vertical-axis current meters commonly used are of the Price
6.1.5 Angle-Measuring Devices—When the direction of
type and are available in two sizes, the large Price AA and the smaller
flow is not at right angles to the cross section, the velocity
Pygmy meter. The rotor assembly of the type AA is 5 in. (127 mm) and the
vector normal to the cross section is needed for the correct
Pygmy is 2 in. (51 mm) in diameter. The rotor assemblies of both meters
determination of discharge. The velocity as measured by the
are formed with 6 hollow metal or solid plastic cone-shaped cups.
The small Price pygmy meter is generally used when the average depth current meter, multiplied by the cosine of the horizontal angle
in a stream cross section is less than 1.5 ft (0.5 m) and velocity is below
between the flow direction and a line perpendicular to the cross
2.5 ft/s (0.8 m/s). The large Price type meter should be used when average
section, will give the velocity component normal to the
depths are greater than 1.5 ft (0.5 m). For high velocities, the large meter
measuring cross section. A series of horizontal angles and
may be used for shallower depths. Do not change the meter if a few partial
corresponding cosine values are usually indicated as a series of
sections are outside these limits. In any case, meters should not be used
marked points on the measurement note form (standard form)
closer to the streambed than 1.5 rotor or probe diameters.
or on a clipboard. The appropriate cosine value is then read
Current meters used in the measurement of open-channel flow are
exposed to damage and fouling by debris, ice, particulate matter, sedi- directly by orienting the note form or clipboard with the
ment, moss, and extreme temperature variations, and should be selected
direction of the cross section and the direction of flow. When
accordingly. Meters must be checked frequently during a discharge
measuring in deep swift streams, it is possible to sound the
measurement to ensure that they have not been damaged or fouled.
depth but the force of the current moves the weight and meter
6.1.2 Counting Equipment—The number of revolutions of a into positions downstream from the cross section; hence, the
rotor in a rotating-element type current meter is obtained by an depths measured are too large (see Fig. 2). Measurement of
electrical circuit through a contact chamber in the meter. the vertical angle (between the displaced direction of the
Contact points in the chamber are designed to complete an sounding line and the true vertical to the water surface) is
electrical circuit at selected frequencies of revolution. Contac
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