Designation: D5614 − 94 (Reapproved 2014)Standard Test Method forOpen Channel Flow Measurement of Water with Broad-Crested Weirs1This standard is issued under the fixed designation D5614; 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.1. Scope1.1 This test method covers measurement of the volumetricflow rate of water in open channels with two types ofhorizontal broad-crested weirs: those having a square (sharp)upstream corner and those having a well-rounded upstreamcorner.1.2 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.1.3 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:2D1129 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 Standards:3ISO 555-1973 Liquid Flow Measurement in OpenChannels—Dilution Methods for Measurement of SteadyFlow—Constant Rate Injection MethodISO 3846-1989 Liquid Flow Measurement in Open Chan-nels by Weirs and Flumes—Rectangular Broad-CrestedWeirsISO 4373-1979 Measurement of Liquid Flow in OpenChannels—Water Level Measuring DevicesISO 4374-1990 Liquid Flow Measurement in Open Chan-nels by Weirs and Flumes—Round-Nose Horizontal CrestWeirs3. 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 boundary layer displacement thickness— the bound-ary layer is a layer of fluid flow adjacent to a solid surface (inthis case, the weir crest and sidewalls) in which, due to viscousfriction, the velocity increases from zero at the stationarysurface to an essentially frictionless-flow value at the edge ofthe layer. The displacement thickness is a distance normal tothe solid surface that the flow streamlines can be considered tohave been displaced by virtue of the boundary-layer informa-tion.3.2.2 crest—the horizontal plane surface of the weir.3.2.3 critical flow—open channel flow in which the energy,expressed in terms of depth plus velocity head, is a minimumfor a given flow rate and channel. The Froude number is unityat critical flow.3.2.4 Froude number—a dimensionless number expressingthe ratio of inertial to gravity forces in free surface flow. It isequal to the average velocity divided by the square root of theproduct of the average depth and the acceleration due togravity.3.2.5 head—in this test method , the depth of water above aspecified elevation. The measuring head is the depth of flowabove the weir crest measured at an appropriate locationupstream of the weir; the downstream head is referencedsimilarly to the crest elevation and measured downstream ofthe weir. The head plus the corresponding velocity head isoften termed the total head or total energy head.3.2.6 hydraulic jump—an abrupt transition from supercriti-cal flow to subcritical or tranquil flow, accompanied byconsiderable turbulence or gravity waves, or both.3.2.7 nappe—the curved sheet or jet of water overfalling thedownstream end of the weir.1This 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 1994. Last previous edition approved in 2008 as D5614 – 94 (2008).DOI: 10.1520/D5614-94R14.2For 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.3Available 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 States13.2.8 primary device—the device (in this case, the weir) thatcreates a hydrodynamic condition that can be sensed by thesecondary instrument.3.2.9 Reynolds number—a dimensionless number express-ing the ratio of inertial to viscous forces in a flow.The pertinentReynolds number on the weir crest is equal to the (critical)velocity multiplied by the crest length and divided by thekinematic viscosity of the water.3.2.10 secondary instrument— in this case, a device thatmeasures the depth of flow (referenced to the crest elevation)at an appropriate location upstream of the weir. The secondaryinstrument may also convert this measured head to an indicatedflow rate or could totalize flow rate.3.2.11 stilling well—a small free-surface reservoir con-nected through a restricted passage to the head-measurementlocation upstream of the weir so that a head measurement canbe made under quiescent conditions.3.2.12 subcritical flow—open channel flow that is deeperand at lower velocity than critical flow for the same flow rate;sometimes called tranquil flow.AFroude number less than oneexists.3.2.13 submergence—a condition in which the water levelon the downstream side of the weir is high enough to affect theflow over the weir and hence alter the head-discharge relation.It is usually expressed as a ratio or percentage of downstreamto upstream head or downstream to upstream total head.3.2.14 supercritical flow—open channel flow that is shal-lower and at higher velocity than critical flow for the same flowrate. A Froude number greater than one exists.3.2.15 tailwater—the water elevation immediately down-stream of the weir.3.2.16 tranquil flow—see subcritical flow.3.2.17 velocity head—the square of the average velocitydivided by twice the acceleration due to gravity.4. Summary of Test Method4.1 In broad-crested weirs, the length of the horizontal crestin the direction of flow is large enough relative to the upstreamhead for essentially rectilinear critical flow to occur at somepoint along the crest. This ideally permits the flow rate to beobtained from a single measurement of the upstream head; acorrective coefficient must be applied in practice. This coeffi-cient has been evaluated experimentally for square-edge weirsand can be determined analytically for rounded weirs.5. Significance and Use5.1 Broad-crested weirs can be used for accurate measure-ments of a wide range of flow rates, but their structuralsimplicity and sturdiness make them particularly useful formeasuring large flows under field conditions.5.2 Because they require vertical sidewalls, broad-crestedweirs are particularly adaptable to rectangular artificial chan-nels or to natural and artificial channels that can readily belined with vertical sidewalls in the immediate vicinity of theweir.6. Interferences6.1 Broad-crested weirs are not suitable for use in sediment-laden streams that are carrying heavy bed loads. However,floating debris is readily passed, particularly by the roundedweir (see 7.2.1).6.2 Broad-crested weirs cannot be used beyond submer-gence limits because insufficient data exist to document theirperformance. It is therefore necessary to adhere to thetailwater-level limitations described in this test method.7. Apparatus7.1 A broad-crested weir measuring system consists of theweir itself and its immediate channel (the primary) and a headmeasuring device (the secondary). The secondary device canrange from a simple staff gage for visual readings to aninstrument that senses the depth continuously, converts it to aflow rate, and displays or transmits a readout or record of theinstantaneous flow rate or totalized flow, or both.7.2 Square-Edge (Rectangular) Broad-Crested Weir:7.2.1 Configuration—The square-edge broad-crested weiras shown in Fig. 1 is rectangular in longitudinal profile andprovides a plane horizontal crest that has finite length in thedirection of flow and extends the full width of the channelbetween vertical sidewalls. A contracted section must beconstructed as shown (see also 7.4.1.2) if the channel does nothave vertical sidewalls or is wider than the desired crest. Thevertical sidewalls must extend downstream of the downstreamface of the weir a distance of at least twice the maximum head.Recommended limits on dimensions and geometric ratios aregiven in 7.2.5. The upstream and downstream faces must bevertical and perpendicular to the channel surfaces, and it isimportant that the upstream corner be square and sharp.NOTE 1—High flow rates combined with floating debris may damagethe sharp edge; rounded-edge weirs should be considered for suchapplications.7.2.2 Construction Requirements:7.2.2.1 The structure must be sturdy enough to withstandthe maximum flow rate and must be watertight so that nomeasurable leakage can bypass it.7.2.2.2 Finish—Large weirs constructed in the field shouldhave a finish equivalent to that of smooth concrete. Smallerweirs, such as those in a laboratory environment, should havea smoothness equivalent to that of rolled sheet metal.7.2.2.3 Level—The crest must not deviate from a level planeby more than 0.01 ft (2 mm) at any point or exceed a slope of0.01 anywhere.7.2.3 Head Measurement Location—Make the head mea-surement at a distance of 3 h to 4 hmaxupstream of theupstream face of the weir, where hmaxis the anticipatedmaximum head.7.2.4 Head-Discharge Relations:7.2.4.1 Basic Equations—The basic relation for the flowrate, Q, over a broad-crested weir is, in compatible units,Q 5 ~2/3!3/2g1/2CvCdBh3/2(1)D5614 − 94 (2014)2where:h = measured upstream head referenced to the crestelevation,B = width of the weir between the vertical side-walls,g = acceleration due to gravity,Cd= discharge coefficient that accounts for departures fromideal conditions, andCv= velocity-of-approach coefficient that permits the flowrate to be related to the measured head rather than thetotal head, H. Then,Cv5 ~H/h!3/25 @~h1αV2u/2g!/h#3/2(2)where:Vu= average velocity at the head-measurement location,andα = coefficient that accounts for any increase in the kineticenergy term caused by a nonuniform velocity distribu-tion. However, in this test method, the approachvelocity is considered sufficiently close to uniform (see7.4.1) for α to be essentially unity.7.2.4.2 In the case of square-edge weirs, both Cdand Cvareaffected by the head-to-weir height ratio, h/P, so it is conve-nient to combine them into a single coefficient, C; then,Q 5 ~2/3!3/2g1/2CBh3/2(3)7.2.4.3 Discharge Coeffıcient, C:(1) The discharge coefficient is given as a function of h/Land h/P in Fig. 2, which has been adapted from ISO 3846-1989.(2) The discharge coefficient for h/L ≤ 0.3 is constant at0.850, provided that h/P 0.4, the weir is nolonger truly broad crested in accordance with 4.1, since theflow over the crest is curvilinear throughout.)7.2.5 Limiting Conditions—The flow conditions and dimen-sions of the square-edge weir are subject to the followinglimits:(1) h 0.2 ft (0.06 m), or 0.1 L, whichever is larger;(2) B 1 ft (0.3 m);(3) P 0.5 ft (0.15 m);(4) 0.1 h/L 1.6;(5) h/P 1.6; and(6) 0.1 L/P 4.The minimum h is recommended in order to minimize theeffects of surface tension, viscosity, and surface roughness andto avoid small heads that may be difficult to measure accu-rately. The minimum h/L prevents frictional effects fromcausing the point of critical flow to shift away from theupstream end of the crest. The limitation on maximum h/P isintended to reduce the likelihood of upstream disturbances, andthe remaining limitations are recommended mainly to conformto the experiments from which the coefficients were obtained.Limiting values of tailwater depth to avoid submergence aregiven in 7.4.2.2.7.3 Rounded Broad-Crested Weir:7.3.1 Configuration:7.3.1.1 The rounded broad-crested weir is shown in Fig. 3.As in the square-edge weir, a plane level crest of finiteFIG. 1 Square-Edge Broad-Crested WeirD5614 − 94 (2014)3streamwise length extends over the full channel width betweenvertical sidewalls. If the channel is not rectangular or ofsuitable width, construct a contracted section as shown. Theupstream face must be vertical and perpendicular to thechannel surfaces. However, the following geometric featuresdepart from those of the square-edge weir.7.3.1.2 To prevent separation round the upstream corner toa radius of at least 0.2 Hmax, where Hmaxis the anticipatedmaximum upstream total head.NOTE 2—Sources customarily express rounded-weir dimensions interms of total head, H. Users can place them in terms of measured head,h, by using (Eq 2) and Table 1.IfH/P is limited to a maximum of 1.5 asrecommended in 7.3.5, H/h will not exceed approximately 1.06.7.3.1.3 The length of the horizontal part of the crest must beat least 1.75 Hmax, and the total length (including radius) mustbe at least 2.25 Hmax.7.3.1.4 The downstream face of the rounded weir can besloped rather than vertical; the only effect is on the tailwaterdepth necessary to avoid submergence (see 7.4.2.3).7.3.2 Construction Requirements:7.3.2.1 The watertightness and finish requirements for therounded weir are the same as those for the square-edge weirgiven in 7.2.2.1 and 7.2.2.2.7.3.2.2 Level—The crest of the rounded weir must be levelwithin the slope of 0.001.7.3.3 Head Measurement Location—Measure the head at adistance of 3 H to 4 Hmaxupstream of the upstream face of theweir.7.3.4 Head-Discharge Relations:7.3.4.1 For rounded-edge weirs, the discharge coefficient,Cd,inEq 1 is associated with frictional effects along the crestand may be expressed in terms of boundary layer growth asCd5 @1 2 ~2δ*/L!~L/B!#@1 2 ~δ*/L!~L/h!#3/2(4)where:δ*= boundary-layer displacement thickness.The value of δ*⁄L as a function of Reynolds number (see3.2.9) and relative surface roughness can be determined bymethods given in ISO 4374-1990 and in fluid mechanics texts;however, unless the surfaces are excessively rough, it issufficiently accurate to use δ*/ L = 0.003 for relatively smalland smooth weirs, as in a laboratory, and δ*⁄ L = 0.004 forlarger concrete weirs.FIG. 2 Discharge Coefficients for Square-Edge Weirs (Dashed Portions of Curves Are Outside of the Recommended Limits)D5614 − 94 (2014)47.3.4.2 The velocity-of-approach coefficient, Cv,inEq 1 isgiven in Table 1 as a function of CdBh/Au, where Auis thecross-sectional area of the approach flow and is equal toB(P + h).7.3.5 Limiting Conditions—The flow conditions and dimen-sions of the rounded weir are subject to the f