ASTM_D_5388_-_93_2013.pdf

Designation:D538893(Reapproved 2013)Standard 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 the standard.The SI units given in parentheses are forinformation only.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 Method3.Terminology3.1 Definitions:3.1.1 For definitions of terms used in this test method,referto Terminology D1129.3.2 Definitions of Terms Specific to This Standard:NOTESeveral of the following terms are illustrated in Fig.1.3.2.1 alpha()a dimensionless velocity-head coefficientthat represents the ratio of the true velocity head to the velocityhead computed on the basis of the mean velocity.It is assumedequal to unity if the cross section is not subdivided.Forsubdivided 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 crosssection indicated by the subscript T.3.2.2 conveyance(K)a measure of the carrying capacity ofa channel without regard to slope and has dimensions of cubicfeet per second.Conveyance is computed as follows:K 51.49nAR2/3(2)3.2.3 cross-section area(A)the area at the water below thewater-surface elevation that it computed.The area is computedas the summation of the products of mean depth multiplied bythe width between stations of the cross section.3.2.4 cross sections(numbered consecutively in downstreamorder)representative of a reach and channel and are posi-tioned as nearly as possible at right angles to the direction offlow.They must be defined by coordinates of horizontaldistance and ground elevation.Sufficient ground points mustbe obtained so that straight-line connection of the coordinateswill adequately describe the cross-section geometry.3.2.5 expansion or contraction loss(ho)in the reach iscomputed by multiplying the change in velocity head throughthe reach by a coefficient.For an expanding reach: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.1,2013.Published January 2013.Originallyapproved in 1993.Last previous edition approved in 2007 as D5388 93(2007).DOI:10.1520/D5388-93R13.2Barnes,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 States1 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 DiscussionThe values of the coefficients can rangefrom zero for ideal transitions to 1.0 for Ke and 0.5 for Kc forabrupt changes.3.2.6 fall(h)the drop in the water surface,in ft(m),computed as the difference in the water-surface elevation atadjacent cross sections(see Fig.1):h 5 h12 h2(5)3.2.7 friction loss(hf)the loss due to boundary friction inthe reach and is computed as follows:hf5L Q2K1K2(6)where:L=length of reach,feet(metres),andK=conveyance at the respective section.3.2.8 Froude number(F)an index to the state of flow inthe channel.In a prismatic channel,the flow is tranquil orsubcritical if the Froude number is less than unity and a rapidor supercritical if it is greater than unity.The Froude number iscompu