Designation: D5129 − 95 (Reapproved 2014)´1Standard Test Method forOpen Channel Flow Measurement of Water Indirectly byUsing Width Contractions1This standard is issued under the fixed designation D5129; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.ε1NOTE—Editorial corrections were made throughout in July 2015.1. Scope1.1 This test method covers the computation of discharge(the volume rate of flow) of water in open channels or streamsusing bridges that cause width contractions as metering de-vices.21.2 This test method produces the maximum discharge forone flow event, usually a specific flood. The computed dis-charge may be used to help define the high-water portion of astage-discharge relation.1.3 The values stated in inch-pound units are to be regardedas standard. The values given in parentheses are mathematicalconversions to SI units that are provided for information onlyand are not considered standard.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D1129 Terminology Relating to WaterD2777 Practice for Determination of Precision and Bias ofApplicable Test Methods of Committee D19 on WaterD3858 Test Method for Open-Channel Flow Measurementof Water by Velocity-Area Method2.2 ISO Standard:4ISO 748 Liquid Flow Measurements in Open Channels—Velocity-Area Measurements3. Terminology3.1 Definitions—For definitions of terms used in this testmethod, refer to Terminology D1129.3.2 Definitions of Terms Specific to This Standard:3.2.1 alpha (α)—a velocity-head coefficient that adjusts thevelocity head computed on basis of the mean velocity to thetrue velocity head.3.2.2 area (A)—the area of a cross section, parts of a crosssection, or parts of bridges below the water surface. Subscriptsindicate specific areas as follows:Ai= area of subsection i,Aj= area of piers or piles that is submerged,A1= area of total cross Section 1 (see Fig. 1), andA3= gross area of Section 3.3.2.3 conveyance, (K)—a measure of the carrying capacityof a channel cross section, or parts of a cross section, and hasunits of cubic feet per second or cubic metres per second.Conveyance is computed as follows:K 5*1.486nAR2/3where:n = the Manning roughness coefficient,A = the cross-section area, ft2(m2), andR = the hydraulic radius, ft (m).*in SI units = 1.01This test method is under the jurisdiction of ASTM Committee D19 on Waterand is the direct responsibility of Subcommittee D19.07 on Sediments,Geomorphology, and Open-Channel Flow.Current edition approved Jan. 1, 2014. Published March 2014. Originallyapproved in 1990. Last previous edition approved in 2008 as D5129 – 95 (2008).DOI: 10.1520/D5129-95R14E01.2This test method is similar to methods developed by the U.S. GeologicalSurvey and described in documents referenced in Footnote 5.53For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at

[email protected] For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http://www.ansi.org.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1FIG. 1 Definition Sketch of an Open-Channel ContractionD5129 − 95 (2014)´12The following subscripts refer to specific conveyances forparts of a cross section:Ka,Kb= conveyances of parts of the approach section toeither side of the projected bottom width of thecontracted section (see Fig. 2). Kdis always thesmaller of the two,Kd= conveyance at the upstream end of the dikes,Ki= conveyance of subsection i,Kq= conveyance of the part of the approach sectioncorresponding to the projected bottom-width, andKT= total conveyance of cross section.3.2.4 depth (y)—depth of flow at a cross section. Subscriptsdenote specific cross section depths as follows:y1= depth of flow in Cross Section 1 (approach section), andy3= depth of flow in Cross Section 3 (contracted section).3.2.5 eccentricity (e)—a measure of the symmetry of thecontraction in relation to the approach channel.3.2.6 friction slope (Sf)—the energy loss, hf, divided by thelength of the reach, L.3.2.7 Froude number (F)—an index to the state of flow in achannel. In a rectangular channel, the flow is tranquil orsubcritical if the Froude number is less than 1.0 and is rapid orsupercritical if it is greater than 1.0.3.2.8 head (h)—static or piezometric head above an arbi-trary datum. Subscripts indicate specific heads as follows:hf= head loss due to friction, andhs= stagnation-surface level at embankment face.3.2.9 hydraulic radius (R)—is equal to the area of a crosssection or subsection divided by its wetted perimeter.3.2.10 length (L)—length of bridge abutment in direction offlow. Subscripts or symbols identify other lengths as follows:Ld= length of dikes,Lw= distance from approach section to upstream side ofcontraction,u = length of projection of abutment beyond wingwalljunction, andx = horizontal distance from the intersection of the abut-ment and embankment slopes to the location on up-stream embankment having the same elevation as thewater surface at Section 1.3.2.11 wetted perimeter (P)—is the sum of the hypotenuseof a right triangle defined by the distance between adjacentstations of the cross section and the difference in bed eleva-tions.3.2.12 width (b)—width of contracted flow section. Sub-scripts denote specific widths as follows:bd= offset distance for straight dikes, andbt= width of contracted flow section at water surface.3.3 Symbols:3.3.1 flow contraction ratio = m.FIG. 2 Definition Sketch of an Eccentric ContractionD5129 − 95 (2014)´133.3.2 coeffıcients—C = coefficient of discharge,C = coefficient of discharge for base condition,n = Manning roughness coefficient, andk = discharge coefficient adjustment.4. Summary of Test Method4.1 The contraction of a stream channel by a bridge createsan abrupt drop in water-surface elevation between an approachsection and the contracted section under the bridge that can berelated to the discharge using the bridge as a metering device.A field survey is made to determine distances between andelevations of high-water marks upstream and downstream fromthe contraction and the geometry of the bridge structure. Thesedata are used to compute the fall in the water surface betweenan approach section and the contracted section and selectedproperties of the sections. This information is used along withdischarge coefficients, determined by extensive hydraulic labo-ratory investigations and verified at field sites, in a dischargeequation to compute the discharge, Q.5. Significance and Use5.1 This test method is particularly useful to determine thedischarge when it cannot be measured directly by some type ofcurrent meter to obtain velocities and with sounding weights todetermine the cross section.5.2 Even under the best conditions, the personnel availablecannot cover all points of interest during a major flood. Theengineer or technician cannot always obtain reliable results bydirect methods if the stage is rising or falling very rapidly, ifflowing ice or debris interferes with depth or velocitymeasurements, or if the cross section of an alluvial channel isscouring or filling significantly.5.3 Under the worst conditions, access roads are blocked,cableways and bridges may be washed out, and knowledge ofthe flood frequently comes too late. Therefore, some type ofindirect measurement is necessary. The contracted-openingmethod is commonly used on valley-floor streams.6. Apparatus6.1 The equipment generally used for a “transit-stadia”survey is recommended. An engineer’s transit, a self-levelinglevel with azimuth circle, newer equipment using electroniccircuitry, or other advanced surveying instruments may beused. Standard level rods, a telescoping, 25-ft (7.62 m) levelrod, rod levels, hand levels, steel and metallic tapes, tag lines(small wires with markers fixed at known spacings), vividlycolored flagging, survey stakes, a camera, and ample notepaper are necessary items.6.2 Additional equipment that may expedite a survey in-cludes axes, shovels, a portable drafting machine, a boat withoars and motor, hip boots, waders, nails, sounding equipment,two-way radios, ladder, and rope.6.3 Safety equipment should include life jackets, first aidkit, drinking water, and pocket knives.7. Sampling7.1 Sampling as defined in Terminology D1129 is notapplicable in this test method.8. Calibration8.1 The surveying instruments, transit, etc., should havetheir adjustment checked, possibly daily when in continuoususe or after some occurrence that may have affected theadjustment.8.2 The standard check is the “two-peg” or“ double-peg”test. If the error is over 0.03 ft in 100 ft (0.091 m in 30.48 m),the instrument should be adjusted. The two-peg test and how toadjust the instrument are described in many surveying text-books. Refer to manufacturers’ manual for the electronicinstruments.8.3 If the “reciprocal leveling” technique is used in thesurvey, it is the equivalent of the two-peg test between each oftwo successive hubs.8.4 Sectional and telescoping level rods should be checkedvisually at frequent intervals to be sure sections are notseparated. A proper fit at each joint can be checked bymeasurements across the joint with a steel tape.8.5 All field notes of the transit-stadia survey should bechecked before proceeding with the computations.9. Procedure9.1 To obtain reliable results, the site selected should be onewhere the geometry of the bridge is close to one of the standardtypes or modified types described in Section 11. If a desirablesite cannot be found, other methods, such as the slope-areamethod, may yield better results.9.1.1 The channel under the bridge should be relativelystable. Because the amount of scour at the time of the peakflow cannot be determined, do not use this test method atcontractions on sand channels. Avoid contractions where largescour holes have formed because the coefficients presentedherein do not apply.9.1.2 The fall, ∆h, is the difference in the computed watersurface elevation, between Sections 1 and 3, and is not to beless than 0.5 ft (0.15 m). It is defined by high-water marks.9.1.3 The fall should be at least four times the friction lossbetween Sections 1 and 3. Therefore, avoid long bridgesdownstream from heavily wooded flood plains.9.2 The approach section, Section 1, is a cross section of thenatural, unconstricted channel upstream from the beginning ofdrawdown. Locate Section 1 one bridge-opening width, b,upstream from the contraction to be sure it is upstream from thedrawdown zone. For a completely eccentric contraction, onewith no contraction on one bank, locate Section 1 twobridge-opening widths upstream because such a contraction isconsidered as half a normal contraction. Section 1 includes theentire width of the valley perpendicular to the direction of flow.9.2.1 When water-surface profiles are level for some dis-tance along the embankment or upstream from the contraction,ponded approach conditions may exist. Even so, survey anapproach section because under some conditions, the approachvelocity head just balances the friction loss.D5129 − 95 (2014)´149.3 The contracted section, Section 3, is the minimum areaon a line parallel to the contraction. Generally, the section isbetween the abutments. When abutments of a skewed bridgeare parallel to the flow, Section 3 is still surveyed parallel to thecontraction even though the minimum section is actuallyperpendicular to the abutments. An angularity factor (see13.3.1) adjusts the surveyed section to the minimum section.9.3.1 The area, A3, is always the gross area of the sectionbelow the level of the free water surface. No deductions aremade for areas occupied by piles, piers, or submerged parts ofthe bridge if they lie in the plane of the contracted section.9.3.2 The mean velocity, V3, is computed using the grossarea, A3.9.3.3 The conveyance, K3, is computed with the area ofpiles, piers, or submerged parts deducted from the gross area.9.3.4 The wetted perimeter used to compute the hydraulicradius, R, will include the lengths of the sides of the piles,piers, or bridge surfaces in contact with the water.9.4 Water-surface levels for Sections 1 and 3 must bedetermined as described below; otherwise, the discharge coef-ficients will not be applicable.9.4.1 At Section 1, develop a profile on each bank near theends of the section from high-water marks in the vicinity. Ifthere are not marks in these areas and a large degree ofcontraction exists, draw a profile of marks along the upstreamface of the embankment. If this profile is level for much of thedistance along the embankment, assume this elevation is thesame as that of Section 1.9.4.2 For Section 3, obtain water-surface elevations alongthe downstream side of the embankment adjacent to theabutments regardless of the location of Section 3.9.4.3 Compute water-surface elevations at Sections 1 and 3as the average of the elevations on each bank.9.4.4 The one exception is an opening with a high degree ofeccentricity. In this area, determine the elevation of Section 3from marks on the contracted side only and use this elevationto compute both the area of Section 3 and fall between Sections1 and 3.9.5 Complete details of the bridge geometry should beobtained so that both plan and elevation drawings can be made.Determine wingwall angles and lengths, lengths of abutments,position and slopes of the embankments and abutments,elevation of roadway, top width of embankment, details ofpiers or piles, and elevations of the bottom of girders or beamsspanning the contraction. Use a steel tape for most linealmeasurements rather than scaling distances from a plan.Pictures of the upstream corners of both abutments should betaken. Note which of the four types of contractions theconstriction is.10. Basic Computations10.1 The drop in water-surface level between an upstreamsection and a contracted section is related to the correspondingchange in velocity. The discharge equation results from writingthe energy and continuity equations for the reach between thesetwo sections, designated as Sections 1 and 3 in Fig. 1.Q 5 CA3Œ2g S∆h1α1V122g2 hfD(1)where:Q = discharge,C = coefficient of discharge,A3= gross area of Section 3, this is the minimum sectionbetween the abutments and is not necessarily at thedownstream side of the bridge,∆h = difference in elevation of the water surfac