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<br />EM 1110-2-1405
<br />31 Aug 59
<br />
<br /><;1. Having d('('ided upon a conservative value for the peak discharge rate of the unit 11.Y<irogra-ph,
<br />r-omplctc iht' ('omputation of the synthctie unit hydrograph in the manner outlined ill tIlt' following
<br />paragl'apll.
<br />
<br />23. UNIT HYDROGRAPH ADJUSTMENT. ]L'ollowing is an illustration of the procedure used in
<br />adjusting the 6,hour unit hydrograph 1\"0. I of plate No. II, derived from hydrologic records for the 1,290
<br />square,mile drainage area of the Saluda River above Chappells, S. C., to apply to the 970 square,mile
<br />drainag,' area represented by subarea No.1. (See plate No, 10.)
<br />a. The widths of unit hydrograph No. I of plate No. II were scaled at ordinates equal to ,50 and
<br />75 percent of the peak discharge, respcrtively, tbe values were plotted in plate No. 7, and linrs A-,A'
<br />and B-B' were drawn through the points parallel to Curves 'Y--50 and W-75 shown thereon.
<br />b. Diseharge ordinates of unit hydrograph :r\ o. 1, plate 1\0. 11, were reducrd in direct proportion to
<br />
<br />the drainage areas involved, C~79~~0.75) and the reduced values were plotted as bydrograph "0. 1
<br />
<br />in plate 1\"0. 12 to serve as a guide in shaping the modified unit hydrograph,
<br />c. Copffieients ep and (\, corresponding to unit hydrogTltph ~o. 1 for tl1{' 1 ,290 ~qu8rp-mil(' drainage
<br />area abovl' Chappplls, S.C., Wf're computed as follows:
<br />
<br />/.ro=47 miles (measured from map)
<br />1,=92 miles (measured from map)
<br />(U.,.)"=12,:l
<br />Ip=34 hours. (See plate 1\0, 11.)
<br />tR= 6 hours (seleeted value)
<br />:14
<br />~--"tTl to nearest hour, r:-.:.
<br />a.a
<br />
<br />Qp= 14,100 c.Ls. (See plate No. 11.)
<br />]4,100 09 fl'
<br />qp= 1290 ~l .. c. .s. sq. m!.
<br />
<br />(' tp ( . ) 34 2 8
<br />'~(I.C,)"" approximate ~ 12.:I~ . .
<br />640 Cp= Ipqp~:170. (See equat.ion :3.)
<br />
<br />(See equation I.)
<br />
<br />d. The following hypothetical case is discussed to demonstrate the use of equation 4 in adjusting
<br />the unit hydmgraph characteristics for ehanges in duration of effective rainfall. If the basic unit
<br />h,ydrograph (or hydrographs) being utilized had been for a 12-hour duration of rainfall, that is
<br />tR~:~ 12, instpad of a 6-honr unit hydrograph (tR=tr), equation 4 eould be used to adjust t!w valup of
<br />lug, tpR. of :{4 hours to the value of lag, tp, for standard unit duration, i,,; namely, tp=34-0.25X
<br />( ) 'I (' ld t I :12.5 d f . 5 ld I 10.9X:14
<br />12 -6 - ;t::.5 lOurs. t wou t len equa -2-:-101' 2.6, an qp rom equatlOn l a wou equa - --;-1'2------;----
<br />I .. . .G
<br />or 11.4 c.Ls. per square mile. These are empirical relatiollships and arc necessary adjustmf>nts (for this
<br />illust,rn.t ion) sinet' (;1 varies with duration of rainfall excess: ,,,-'hilc Cp is assumed to fC'main eonstan t.
<br />f'. Assumillg that coefficients (\ and 640 rp ('omputed for the 1290 square-mile drainage area are
<br />applicahh' to the- 970 squarf'-mile area under study, the peak discharge and lag corrf'spolldiTlg to the
<br />smaller al'<'u were computed as follows:
<br />
<br />1,,"='12 miles (measured from map)
<br />1,=fj6 miles (measured from map)
<br />
<br />13
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