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GENERAL PROCEDURE FOR GAGING STREAMS Current-meter measurements In the making of a discharge measurement the cross section is divided into 2o.-1l0 partial sections, and the area and mean velocity of each is determined separately. A partial section is a rectangle whose depth is equal to the sounded depth at a meter location (a vertical) and whose width is equal to the sum of half the distances to the adjacent verticals. At each vertical the following obser"ations arc made: (1) The dis- tance to a reference point on the bank, (2) the depth of tlow. and (3) the velocity as indicated by current meter at one or two points in the "ertieal. These points are at either the 0.2 and 0.8 depths (two-point method) or the 0.6 depth (one-point method) from the water surface. The average of the two velocities, or the single velocity at 0.6 depth, is taken to bo the mean ,'elocity in the ,'ertical. The discharge in each partial section is computed as the product of mean "eIocity times depth at the "ertieal times the sum of half the distances to adjacent verti- cals, The sum of the discharges in all the par- tial scctions is the total discharge of the stream. The measurement can be made by wading the stream with a current meter which slides on a gradnated depth rod as shown in fignres 2 and (), or it. can he made from a supporting structure snch as a bridge (see fig. 7), cableway, or boat, the IllPter heing suspend(.f1 hy a {'a hIp. The Price current meter is nscd to obselTe ,-elQ{'ity, except in shallow depths where the pygmy current meter is nsed. The I'otor on both these meters has a ,'ertieal axis and six cone- shaped cups, Each meter is individually cali- brated in the rating tlun", at the National Bu- reau of Standards. Fignre 8 shows a velocity- azimuth-depth assembly, which has been in use since 1958, primarily on large rivers and in estuarine studies. The-sf' methods and the asso(.jated f'CJuip- ment h'1\'e been de,'eloped by the Geologieal ~l11'Yey over a period of 60 yeal'Fl. Satisfactory rpsults have been obtained in measul'inA" dis- charges l'ang:ing from the trickle of a small stream to the 7,500,000-cfs tlow of the .\mazon HiYeI'. Methods and eqnipment nserl in making discharg'c l1le:lS1l1'pnwnts hy the rlllT(>nt~metpr 7 method are described in detail in book 3, chap- ter A8, of this series, by Buchanan and Somers,' Indirect discharge measurements During floods, it is frequently impossible or impractical to measure the peak discharges when they occur, because of conditions beyond control. Roads may be impassable; structures from wbich cnrrent- meter measurements might have been made may be nonexistent, not suitably located, or destroyed; knowledge of the flood rise may not be a"ailable sufficiently in advancc to pennit reaching tbe site near the time of the peak; the peak may be so sharp that a satisfac- tory cnrrent-meter measurement could not be nlade eYell with un engineer present at the time; the flow of dehris or i~e may be such as to pre- vellt use of a CUl'l'ent. meter; or limitations of }wl'sonnel might make it impossible to obtain direct ll1eaSlll'emellt~ of high-stage discharge at llumerous locutions during a short flood period. Consequently, mauy peak discharges mnst be determined afte,' the passage of the flood by indirect m~thods snch as slope-area, contract~d- opening, flow~oYer-dam, 01' flow-throu~h-('ul- "ert, rather than hy direct current-meter meas- llrement. To e,'aluate the accuracy of indirect methods, comparisons have been made at every opportu- nity. "'hen it has been possible to compare peak d,is('hal'p.-e computed hy indil'e<'t means with peak discha1'l:re measured hy ClU'l'ent meter or othpl' dil'ect means, the agreement, ill g-eneral, has snpported confidence in the auxiliary methods. Indirect measurements make llse of the t'llC'l'g-y equatioll for <.'ompnt-ing discharg-e. The speeific eqnations differ for ditl'erent types of flow, s1Iell as open-channel flow, flow O\'er dams, and flow through culverts. These equations re- late the discharge to the water-surface profiile and the geometry of the channel. A field sUl'\'ey is made after the flood to determine the location and elevation of high-water marks and the geometry of the channel. I BllI'hllnHll. T. .T.. amI SOIlll'l"ll. W. P.. Dis('hnl"~e me:umr(!. 1lll'lIts at g'llg'iug' statloHs: ti.K GcoJ. Hllr"l'r 'l'l't'hlliqu('s \\'att'I".Il.I'Sullr('('l'l In"" hook ::, l'hap. .\~. 1I11111lh. datn.