ASTM D5129-95(2003)
(Test Method)Standard Test Method for Open Channel Flow Measurement of Water Indirectly by Using Width Contractions
Standard Test Method for Open Channel Flow Measurement of Water Indirectly by Using Width Contractions
SIGNIFICANCE AND USE
This test method is particularly useful to determine the discharge when it cannot be measured directly by some type of current meter to obtain velocities and with sounding weights to determine the cross section.
Even under the best conditions, the personnel available cannot cover all points of interest during a major flood. The engineer or technician cannot always obtain reliable results by direct methods if the stage is rising or falling very rapidly, if flowing ice or debris interferes with depth or velocity measurements, or if the cross section of an alluvial channel is scouring or filling significantly.
Under the worst conditions, access roads are blocked, cableways and bridges may be washed out, and knowledge of the flood frequently comes too late. Therefore, some type of indirect measurement is necessary. The contracted-opening method is commonly used on valley-floor streams.
SCOPE
1.1 This test method covers the computation of discharge (the volume rate of flow) of water in open channels or streams using bridges that cause width contractions as metering devices.
1.2 This test method produces the maximum discharge for one flow event, usually a specific flood. The computed discharge may be used to help define the high-water portion of a stage-discharge relation.
1.3 The values stated in inch-pound units are to be regarded as the standard. The SI units given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 5129 – 95 (Reapproved 2003)
Standard Test Method for
Open Channel Flow Measurement of Water Indirectly by
Using Width Contractions
This standard is issued under the fixed designation D 5129; 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 3.2 Definitions of Terms Specific to This Standard:
3.2.1 alpha (a)—a velocity-head coefficient that adjusts the
1.1 This test method covers the computation of discharge
velocity head computed on basis of the mean velocity to the
(the volume rate of flow) of water in open channels or streams
true velocity head.
using bridges that cause width contractions as metering de-
3.2.2 area (A)— the area of a cross section, parts of a cross
vices.
section, or parts of bridges below the water surface. Subscripts
1.2 This test method produces the maximum discharge for
indicate specific areas as follows:
one flow event, usually a specific flood. The computed dis-
charge may be used to help define the high-water portion of a
stage-discharge relation.
A = area of subsection i,
i
1.3 The values stated in inch-pound units are to be regarded
A = area of piers or piles that is submerged,
j
as the standard. The SI units given in parentheses are for
A = area of total cross section 1 (see Fig. 1), and
information only.
A = gross area of section 3.
1.4 This standard does not purport to address all of the
3.2.3 conveyance, (K)—a measure of the carrying capacity
safety concerns, if any, associated with its use. It is the
of a channel cross section, or parts of a cross section, and has
responsibility of the user of this standard to establish appro-
units of cubic feet per second or cubic metres per second.
priate safety and health practices and determine the applica-
Conveyance is computed as follows:
bility of regulatory limitations prior to use.
*1.486
2/3
K 5 AR
n
2. Referenced Documents
2.1 ASTM Standards:
D 1129 Terminology Relating to Water where:
n = the Manning roughness coefficient,
D 2777 Practice for Determination of Precision and Bias of
2 2
A = the cross-section area, ft (m ), and
Applicable Methods of Committee D19 on Water
R = the hydraulic radius, ft (m).
D 3858 TestMethodforOpenChannelFlowMeasurements
3 *in SI units = 1.0
of Water by Velocity-Area Method
The following subscripts refer to specific conveyances for
2.2 ISO Standard:
parts of a cross section:
ISO 748 Liquid Flow Measurements in Open Channels—
Velocity-Area Measurements
K,K = conveyances of parts of the approach section to
a b
3. Terminology
either side of the projected bottom width of the
3.1 Definitions—For definitions of terms used in this test
contracted section (see Fig. 2). K is always the
d
method, refer to Terminology D 1129.
smaller of the two,
K = conveyance at the upstream end of the dikes,
d
K = conveyance of subsection i,
This test method is under the jurisdiction of ASTM Committee D19 on Water
i
and is the direct responsibility of Subcommittee D19.07 on Sediments, Geomor- K = conveyance of the part of the approach section
q
phology, and Open-Channel Flow.
correspondingtotheprojectedbottom-width,and
Current edition approved June 10, 2003. Published August 2003. Originally
K = total conveyance of cross section.
T
approved in 1990. Last previous edition approved in 1999 as D 5129 – 95 (1999).
3.2.4 depth (y)—depth of flow at a cross section. Subscripts
This test method is similar to methods developed by the U.S. Geological
Survey and described in documents referenced in Footnote 5.
denote specific cross section depths as follows:
Annual Book of ASTM Standards, Vol 11.01.
Available from American National Standards Institute, 25 W. 43rd St., 4th
Floor, New York, NY 10036.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D 5129 – 95 (2003)
data are used to compute the fall in the water surface between
y = depth of flow in cross section 1(approach section),
an approach section and the contracted section and selected
and
properties of the sections. This information is used along with
y = depth of flow in cross section 3(contracted section).
discharge coefficients, determined by extensive hydraulic labo-
3.2.5 eccentricity (e)—a measure of the symmetry of the
ratory investigations and verified at field sites, in a discharge
contraction in relation to the approach channel.
equation to compute the discharge, Q.
3.2.6 friction slope (S )— the energy loss, h, divided by the
f f
length of the reach, L.
5. Significance and Use
3.2.7 Froude number (F)—an index to the state of flow in a
channel. In a rectangular channel, the flow is tranquil or
5.1 This test method is particularly useful to determine the
subcritical if the Froude number is less than 1.0 and is rapid or
discharge when it cannot be measured directly by some type of
supercritical if it is greater than 1.0.
current meter to obtain velocities and with sounding weights to
3.2.8 head (h)—static or piezometric head above an arbi-
determine the cross section.
trary datum. Subscripts indicate specific heads as follows:
5.2 Even under the best conditions, the personnel available
cannot cover all points of interest during a major flood. The
engineer or technician cannot always obtain reliable results by
h = head loss due to friction, and
f
direct methods if the stage is rising or falling very rapidly, if
h = stagnation-surface level at embankment face.
s
flowing ice or debris interferes with depth or velocity measure-
3.2.9 hydraulic radius (R)—is equal to the area of a cross
ments, or if the cross section of an alluvial channel is scouring
section or subsection divided by its wetted perimeter.
or filling significantly.
3.2.10 length (L)—length of bridge abutment in direction of
5.3 Under the worst conditions, access roads are blocked,
flow. Subscripts or symbols identify other lengths as follows:
cableways and bridges may be washed out, and knowledge of
the flood frequently comes too late. Therefore, some type of
L = length of dikes, indirect measurement is necessary. The contracted-opening
d
L = distance from approach section to upstream side of
method is commonly used on valley-floor streams.
w
contraction,
u = length of projection of abutment beyond wingwall
6. Apparatus
junction, and
6.1 The equipment generally used for a “transit-stadia”
x = horizontal distance from the intersection of the abut-
survey is recommended. An engineer’s transit, a self-leveling
ment and embankment slopes to the location on
level with azimuth circle, newer equipment using electronic
upstream embankment having the same elevation as
circuitry, or other advanced surveying instruments may be
the water surface at section 1.
used. Standard level rods, a telescoping, 25-ft (7.62 m) level
3.2.11 wetted perimeter (P)—is the sum of the hypotenuse
rod, rod levels, hand levels, steel and metallic tapes, tag lines
of a right triangle defined by the distance between adjacent
(small wires with markers fixed at known spacings), vividly
stations of the cross section and the difference in bed eleva-
colored flagging, survey stakes, a camera, and ample note
tions.
paper are necessary items.
3.2.12 width (b)—width of contracted flow section. Sub-
6.2 Additional equipment that may expedite a survey in-
scripts denote specific widths as follows:
cludes axes, shovels, a portable drafting machine, a boat with
oars and motor, hip boots, waders, nails, sounding equipment,
two-way radios, ladder, and rope.
b = offset distance for straight dikes, and
d
b = width of contracted flow section at water surface.
6.3 Safety equipment should include life jackets, first aid
t
3.3 Symbols: Symbols: kit, drinking water, and pocket knives.
3.3.1 flow contraction ratio = m.
3.3.2 coeffıcients— 7. Sampling
7.1 Sampling as defined in Terminology D 1129 is not
applicable in this test method.
C = coefficient of discharge,
C8 = coefficient of discharge for base condition,
8. Calibration
n = Manning roughness coefficient, and
k = discharge coefficient adjustment.
8.1 The surveying instruments, transit, etc., should have
their adjustment checked, possibly daily when in continuous
4. Summary of Test Method
use or after some occurrence that may have affected the
adjustment.
4.1 The contraction of a stream channel by a bridge creates
an abrupt drop in water-surface elevation between an approach 8.2 The standard check is the “two-peg” or“ double-peg”
section and the contracted section under the bridge that can be test. If the error is over 0.03 ft in 100 ft (0.091 m in 30.48 m),
related to the discharge using the bridge as a metering device. theinstrumentshouldbeadjusted.Thetwo-pegtestandhowto
A field survey is made to determine distances between and adjust the instrument are described in many surveying text-
elevationsofhigh-watermarksupstreamanddownstreamfrom books. Refer to manufacturers’ manual for the electronic
the contraction and the geometry of the bridge structure.These instruments.
D 5129 – 95 (2003)
8.3 If the “reciprocal leveling” technique is used in the 9.3.4 The wetted perimeter used to compute the hydraulic
survey, it is the equivalent of the two-peg test between each of radius, R, will include the lengths of the sides of the piles,
two successive hubs. piers, or bridge surfaces in contact with the water.
9.4 Water-surface levels for sections 1 and 3 must be
8.4 Sectional and telescoping level rods should be checked
determined as described below; otherwise, the discharge coef-
visually at frequent intervals to be sure sections are not
ficients will not be applicable.
separated. A proper fit at each joint can be checked by
9.4.1 At section 1, develop a profile on each bank near the
measurements across the joint with a steel tape.
ends of the section from high-water marks in the vicinity. If
8.5 All field notes of the transit-stadia survey should be
there are not marks in these areas and a large degree of
checked before proceeding with the computations.
contraction exists, draw a profile of marks along the upstream
face of the embankment. If this profile is level for much of the
9. Procedure
distance along the embankment, assume this elevation is the
9.1 To obtain reliable results, the site selected should be one
same as that of section 1.
wherethegeometryofthebridgeisclosetooneofthestandard
9.4.2 For section 3, obtain water-surface elevations along
types or modified types described in Section 11. If a desirable
the downstream side of the embankment adjacent to the
site cannot be found, other methods, such as the slope-area
abutments regardless of the location of section 3.
method, may yield better results.
9.4.3 Compute water-surface elevations at sections 1 and 3
9.1.1 The channel under the bridge should be relatively
as the average of the elevations on each bank.
stable. Because the amount of scour at the time of the peak
9.4.4 The one exception is an opening with a high degree of
flow cannot be determined, do not use this test method at
eccentricity. In this area, determine the elevation of section 3
contractions on sand channels.Avoid contractions where large
from marks on the contracted side only and use this elevation
scour holes have formed because the coefficients presented
to compute both the area of section 3 and fall between sections
herein do not apply.
1 and 3.
9.1.2 The fall, Dh, is the difference in the computed water
9.5 Complete details of the bridge geometry should be
surface elevation, between sections 1 and 3, and is not to be
obtainedsothatbothplanandelevationdrawingscanbemade.
less than 0.5 ft (0.15 m). It is defined by high-water marks.
Determine wingwall angles and lengths, lengths of abutments,
9.1.3 The fall should be at least four times the friction loss position and slopes of the embankments and abutments,
elevation of roadway, top width of embankment, details of
between sections 1 and 3. Therefore, avoid long bridges
downstream from heavily wooded flood plains. piers or piles, and elevations of the bottom of girders or beams
spanning the contraction. Use a steel tape for most lineal
9.2 The approach section, section 1, is a cross section of the
measurements rather than scaling distances from a plan.
natural, unconstricted channel upstream from the beginning of
Pictures of the upstream corners of both abutments should be
drawdown. Locate section 1 one bridge-opening width, b,
taken. Note which of the four types of contractions the
upstreamfromthecontractiontobesureitisupstreamfromthe
constriction is.
drawdown zone. For a completely eccentric contraction, one
with no contraction on one bank, locate section 1two bridge-
10. Basic Computations
opening widths upstream because such a contraction is consid-
ered as half a normal contraction. Section 1 includes the entire 10.1 The drop in water-surface level between an upstream
width of the valley perpendicular to the direction of flow.
section and a contracted section is related to the corresponding
change in velocity.The discharge equation results from writing
9.2.1 When water-surface profiles are level for some dis-
theenergyandcontinuityequationsforthereachbetweenthese
tance along the embankment or upstream from the contraction,
two sections, designated as sections 1 and 3 in Fig. 1.
ponded approach conditions may exist. Even so, survey an
approach section because under some conditions, the approach
V
velocity head just balances the friction loss.
Q 5 CA Œ2gSDh1a 2 hD (1)
3 1 f
2g
9.3 The contracted section, section 3, is the minimum area
on a line parallel to the contraction. Generally, the section is
where:
between the abutments. When abutments of a skewed bridge
Q = discharge,
are parallel to the flow, section 3 is still surveyed parallel to the
C = coefficient of discharge,
contraction even though the minimum section is actually
A = gross area of section 3, this is the minimum
perpendicular to the abutments. An angularity factor (see
section between the abutments and is not
13.3.1) adjusts the surveyed section to the minimum section.
necessarily at the downstream side of the
9.3.1 The area, A , is always the gross area of the section
bridge,
below the level of the free water surface. No deductions are
Dh = difference in elevation of the water surface
made for areas occupied by piles, piers, or submerged parts of
between sections 1 and 3,
the bridge if they lie in the plane of the contracted section. 2
V
a ⁄ ;2g = weighted average velocity head at 1,
1 1
Q
9.3.2 The mean velocity, V , is computed using the gross
3 V = average velocity, ⁄A1 , and
...
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