ASTM D5614-94(1998)

NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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Designation: D 5614 – 94 (Reapproved 1998)
Standard Test Method for
Open Channel Flow Measurement of Water with Broad-
Crested Weirs
This standard is issued under the fixed designation D 5614; 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 3. Terminology
1.1 This test method covers measurement of the volumetric 3.1 Definitions—For definitions of terms used in this test
flow rate of water in open channels with two types of method, refer to Terminology D 1129.
horizontal broad-crested weirs: those having a square (sharp) 3.2 Definitions of Terms Specific to This Standard:
upstream corner and those having a well-rounded upstream 3.2.1 boundary layer displacement thickness—the boundary
corner. layer is a layer of fluid flow adjacent to a solid surface (in this
1.2 The values stated in inch-pound units are to be regarded case, the weir crest and sidewalls) in which, due to viscous
as the standard. The values given in parentheses are for friction, the velocity increases from zero at the stationary
information only. surface to an essentially frictionless-flow value at the edge of
1.3 This standard does not purport to address all of the the layer. The displacement thickness is a distance normal to
safety concerns, if any, associated with its use. It is the the solid surface that the flow streamlines can be considered to
responsibility of the user of this standard to establish appro- have been displaced by virtue of the boundary-layer informa-
priate safety and health practices and determine the applica- tion.
bility of regulatory limitations prior to use. 3.2.2 crest—the horizontal plane surface of the weir.
3.2.3 critical flow—open channel flow in which the energy,
2. Referenced Documents
expressed in terms of depth plus velocity head, is a minimum
2.1 ASTM Standards: for a given flow rate and channel. The Froude number is unity
D 1129 Terminology Relating to Water
at critical flow.
D 2777 Practice for Determination of Precision and Bias of 3.2.4 Froude number—a dimensionless number expressing
Applicable Methods of Committee D-19 on Water
the ratio of inertial to gravity forces in free surface flow. It is
D 3858 Practice for Open-Channel Flow Measurement of equal to the average velocity divided by the square root of the
Water by Velocity-Area Method
product of the average depth and the acceleration due to
2.2 ISO Standards: gravity.
ISO 555-1973 Liquid Flow Measurement in Open
3.2.5 head—in this test method, the depth of water above a
Channels—Dilution Methods for Measurement of Steady specified elevation. The measuring head is the depth of flow
Flow—Constant Rate Injection Method above the weir crest measured at an appropriate location
ISO 3846-1989 Liquid Flow Measurement in Open Chan-
upstream of the weir; the downstream head is referenced
nels by Weirs and Flumes—Rectangular Broad-Crested similarly to the crest elevation and measured downstream of
Weirs
the weir. The head plus the corresponding velocity head is
ISO 4373-1979 Measurement of Liquid Flow in Open often termed the total head or total energy head.
Channels—Water Level Measuring Devices
3.2.6 hydraulic jump—an abrupt transition from supercriti-
ISO 4374-1990 Liquid Flow Measurement in Open Chan- cal flow to subcritical or tranquil flow, accompanied by
nels by Weirs and Flumes—Round-Nose Horizontal Crest
considerable turbulence or gravity waves, or both.
Weirs 3.2.7 nappe—the curved sheet or jet of water overfalling the
downstream end of the weir.
3.2.8 primary device—the device (in this case, the weir) that
This test method is under the jurisdiction of ASTM Committee D-19 on Water
creates a hydrodynamic condition that can be sensed by the
and is the direct responsibility of Subcommittee D19.07 on Sediments, Geomor-
secondary instrument.
phology, and Open-Channel Flow.
Current edition approved Sept. 15, 1994. Published November 1994. 3.2.9 Reynolds number—a dimensionless number express-
Annual Book of ASTM Standards, Vol 11.01.
ing the ratio of inertial to viscous forces in a flow. The pertinent
Available from American National Standards Institute, 11 West 42nd St., 13th
Reynolds number on the weir crest is equal to the (critical)
Floor, New York, NY 10036.
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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 5614 – 94 (1998)
velocity multiplied by the crest length and divided by the 7. Apparatus
kinematic viscosity of the water.
7.1 A broad-crested weir measuring system consists of the
3.2.10 secondary instrument—in this case, a device that
weir itself and its immediate channel (the primary) and a head
measures the depth of flow (referenced to the crest elevation)
measuring device (the secondary). The secondary device can
at an appropriate location upstream of the weir. The secondary
range from a simple staff gage for visual readings to an
instrument may also convert this measured head to an indicated
instrument that senses the depth continuously, converts it to a
flow rate or could totalize flow rate.
flow rate, and displays or transmits a readout or record of the
3.2.11 stilling well—a small free-surface reservoir con-
instantaneous flow rate or totalized flow, or both.
nected through a restricted passage to the head-measurement
7.2 Square-Edge (Rectangular) Broad-Crested Weir:
location upstream of the weir so that a head measurement can
7.2.1 Configuration—The square-edge broad-crested weir
be made under quiescent conditions.
as shown in Fig. 1 is rectangular in longitudinal profile and
3.2.12 subcritical flow—open channel flow that is deeper
provides a plane horizontal crest that has finite length in the
and at lower velocity than critical flow for the same flow rate;
direction of flow and extends the full width of the channel
sometimes called tranquil flow. A Froude number less than one
between vertical sidewalls. A contracted section must be
exists.
constructed as shown (see also 7.4.1.2) if the channel does not
3.2.13 submergence—a condition in which the water level
have vertical sidewalls or is wider than the desired crest. The
on the downstream side of the weir is high enough to affect the
vertical sidewalls must extend downstream of the downstream
flow over the weir and hence alter the head-discharge relation.
face of the weir a distance of at least twice the maximum head.
It is usually expressed as a ratio or percentage of downstream
Recommended limits on dimensions and geometric ratios are
to upstream head or downstream to upstream total head.
given in 7.2.5. The upstream and downstream faces must be
3.2.14 supercritical flow—open channel flow that is shal-
vertical and perpendicular to the channel surfaces, and it is
lower and at higher velocity than critical flow for the same flow
important that the upstream corner be square and sharp.
rate. A Froude number greater than one exists.
NOTE 1—High flow rates combined with floating debris may damage
3.2.15 tailwater—the water elevation immediately down-
the sharp edge; rounded-edge weirs should be considered for such
stream of the weir.
applications.
3.2.16 tranquil flow—see subcritical flow.
7.2.2 Construction Requirements:
3.2.17 velocity head—the square of the average velocity
7.2.2.1 The structure must be sturdy enough to withstand
divided by twice the acceleration due to gravity.
the maximum flow rate and must be watertight so that no
4. Summary of Test Method
measurable leakage can bypass it.
7.2.2.2 Finish—Large weirs constructed in the field should
4.1 In broad-crested weirs, the length of the horizontal crest
have a finish equivalent to that of smooth concrete. Smaller
in the direction of flow is large enough relative to the upstream
weirs, such as those in a laboratory environment, should have
head for essentially rectilinear critical flow to occur at some
a smoothness equivalent to that of rolled sheet metal.
point along the crest. This ideally permits the flow rate to be
7.2.2.3 Level—The crest must not deviate from a level plane
obtained from a single measurement of the upstream head; a
by more than 0.01 ft (2 mm) at any point or exceed a slope of
corrective coefficient must be applied in practice. This coeffi-
0.01 anywhere.
cient has been evaluated experimentally for square-edge weirs
7.2.3 Head Measurement Location—Make the head mea-
and can be determined analytically for rounded weirs.
surement at a distance of 3 h to 4 h upstream of the
max
5. Significance and Use
upstream face of the weir, where h is the anticipated
max
5.1 Broad-crested weirs can be used for accurate measure- maximum head.
ments of a wide range of flow rates, but their structural 7.2.4 Head-Discharge Relations:
simplicity and sturdiness make them particularly useful for 7.2.4.1 Basic Equations—The basic relation for the flow
measuring large flows under field conditions. rate, Q, over a broad-crested weir is, in compatible units,
5.2 Because they require vertical sidewalls, broad-crested
3 2 1 2 3 2
/ / /
Q 5 2 3 g C C Bh (1)
~ !
/ v d
weirs are particularly adaptable to rectangular artificial chan-
nels or to natural and artificial channels that can readily be
lined with vertical sidewalls in the immediate vicinity of the where:
h = measured upstream head referenced to the crest el-
weir.
evation,
6. Interferences
B = width of the weir between the vertical side-walls,
g = acceleration due to gravity,
6.1 Broad-crested weirs are not suitable for use in sediment-
C = discharge coefficient that accounts for departures
d
laden streams that are carrying heavy bed loads. However,
from ideal conditions, and
floating debris is readily passed, particularly by the rounded
C = velocity-of-approach coefficient that permits the flow
v
weir (see 7.2.1).
rate to be related to the measured head rather than the
6.2 Broad-crested weirs cannot be used beyond submer-
total head, H. Then,
gence limits because insufficient data exist to document their
3 2 2 3 2
performance. It is therefore necessary to adhere to the / /
C 5 ~H/h! 5 @~h1aV /2g!/h# (2)
v u
tailwater-level limitations described in this test method.
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D 5614 – 94 (1998)
FIG. 1 Square-Edge Broad-Crested Weir
(5) h/P < 1.6; and
where:
(6) 0.1 < L/P <4.
V = average velocity at the head-measurement location,
u
and The minimum h is recommended in order to minimize the
a = coefficient that accounts for any increase in the kinetic
effects of surface tension, viscosity, and surface roughness and
energy term caused by a nonuniform velocity distri- to avoid small heads that may be difficult to measure accu-
bution. However, in this test method, the approach
rately. The minimum h/L prevents frictional effects from
velocity is considered sufficiently close to uniform causing the point of critical flow to shift away from the
(see 7.4.1) for a to be essentially unity.
upstream end of the crest. The limitation on maximum h/P is
7.2.4.2 In the case of square-edge weirs, both C and C are intended to reduce the likelihood of upstream disturbances, and
d v
affected by the head-to-weir height ratio, h/P, so it is conve-
the remaining limitations are recommended mainly to conform
nient to combine them into a single coefficient, C; then, to the experiments from which the coefficients were obtained.
3 2 1 2 3 2 Limiting values of tailwater depth to avoid submergence are
/ / /
Q 5 2 3 g CBh (3)
~ !
/
given in 7.4.2.2.
7.3 Rounded Broad-Crested Weir:
7.2.4.3 Discharge Coeffıcient, C:
7.3.1 Configuration:
(1) The discharge coefficient is given as a function of h/L
7.3.1.1 The rounded broad-crested weir is shown in Fig. 3.
and h/P in Fig. 2, which has been adapted from ISO 3846-
As in the square-edge weir, a plane level crest of finite
1989.
streamwise length extends over the full channel width between
(2) The discharge coefficient for h/L # 0.3 is constant at
vertical sidewalls. If the channel is not rectangular or of
0.850, provided that h/P < 0.15. (For h/L > 0.4, the weir is no
suitable width, construct a contracted section as shown. The
longer truly broad crested in accordance with 4.1, since the
upstream face must be vertical and perpendicular to the
flow over the crest is curvilinear throughout.)
channel surfaces. However, the following geometric features
7.2.5 Limiting Conditions—The flow conditions and dimen-
depart from those of the square-edge weir.
sions of the square-edge weir are subject to the following
7.3.1.2 To prevent separation round the upstream corner to
limits:
a radius of at least 0.2 H , where H is the anticipated
max max
(1) h > 0.2 ft (0.06 m), or 0.1 L, whichever is larger;
maximum upstream total head.
(2) B > 1 ft (0.3 m);
(3) P > 0.5 ft (0.15 m);
NOTE 2—Sources customarily express rounded-weir dimensions in
(4) 0.1 < h/L< 1.6; terms of total head, H. Users can place them in terms of measured head,
NOTICE: This standard has either been superceded and replaced by a new version or discontinued.
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D 5614 – 94 (1998)
FIG. 2 Discharge Coefficients for Square-Edge Weirs (Dashed Portions of Curves Are Outside of the Recommended Limits)
h, by using (Eq 2) and Table 1. If H/P is limited to a maximum of 1.5 as
and relative surface roughness can be determined by methods
recommended in 7.3.5, H/h will not exceed approximately 1.06.
given in ISO 4374-1990 and in fluid mechanics texts; however,
unless the surfaces are excessively rough, it is sufficiently
7.3.1.3 The length of the horizontal part of the crest must be
accurate to use d / L = 0.003 for relatively small and smooth
at least 1.75 H , and the total length (including radius) must
*
max
weirs, as in a laboratory, and d /L = 0.004 for larger concrete
be at least 2.25 H .
*
max
weirs.
7.3.1.4 The downstream face of the rounded weir can be
7.3.4.2 The velocity-of-approach coefficient, C ,inEq1is
sloped rather than vertical; the only effect is on the tailwater
v
given in Table 1 as a function of C Bh/A , where A is the
depth necessary to avoid submergence (see 7.4.2.3).
d u u
cross-sectional area of the approach flow and is equal to B
7.3.2 Construction Requirements:
(P + h).
7.3.2.1 The watertightness an
...