ASTM_D_5388_-_21.pdf

Designation:D538821Standard Test Method forIndirect Measurements of Discharge by Step-BackwaterMethod1This standard is issued under the fixed designation D5388;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 the computation of discharge ofwater in open channels or streams using representative cross-sectional characteristics,the water-surface elevation of theupstream-most cross section,and coefficients of channelroughness as input to gradually-varied flow computations.21.2 This test method produces an indirect measurement ofthe discharge for one flow event,usually a specific flood.Thecomputed discharge may be used to define a point on thestage-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,health,and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards,Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade(TBT)Committee.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 Method3.Terminology3.1 Definitions:3.1.1 For definitions of terms used in this standard,refer toTerminology D1129.3.2 Definitions of Terms Specific to This Standard:NOTESeveral of the following terms are illustrated in Fig.1.3.2.1 alpha(),na dimensionless velocity-head coeffi-cient that represents the ratio of the true velocity head to thevelocity head computed on the basis of the mean velocity.It isassumed equal to unity if the cross section is not subdivided.For subdivided sections,is computed as follows:5(ki3ai2KT3AT2(1)where:k and a=the conveyance and area of the subsection indi-cated by the subscript I,andK and A=the conveyance and area of the total cross sectionindicated by the subscript T.3.2.2 conveyance(K),na measure of the carrying capacityof a channel without regard to slope and has dimensions ofcubic feet per second.Conveyance is computed as follows:K 51.49nAR2/3(2)3.2.3 cross-section area(A),nthe area at the water belowthe water-surface elevation that it computed.The area iscomputed as the summation of the products of mean depthmultiplied by the width between stations of the cross section.3.2.4 cross sections(numbered consecutively in downstreamorder),nrepresentative portions of a reach and channel andare positioned as nearly as possible at right angles to thedirection of flow.They must be defined by coordinates ofhorizontal distance and ground elevation.Sufficient ground1This 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 Nov.1,2021.Published January 2022.Originallyapproved in 1993.Last previous edition approved in 2013 as D5388 93(2013).DOI:10.1520/D5388-21.2Barnes,H.H.,Jr.,“Roughness Characteristics of Natural Streams,”U.S.Geological Survey Water Supply Paper 1849,1967.3For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards,Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade(TBT)Committee.1points must be obtained so that straight-line connection of thecoordinates will adequately describe the cross-section geom-etry.3.2.5 expansion or contraction loss(ho),na value iscomputed by multiplying the change in velocity head throughthe reach by a coefficient.For an expanding reach:ho5 Kehv12 hv2!(3)and for a contracting reach:ho5 Kchv22 hv1!(4)where:hv=velocity head at the respective section,andKe and Kc=coefficients.3.2.5.1 Dis