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<br />e <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2-1405 <br />31 Aug 59 <br /> <br />d, Havinl'( dooided upon a conservative value for the peak discharge rate of the unit hydrograph, <br />r.omplctc tlH' eomputa.tion of the syntlH'tic unit hydrograph in the manner outlillC'd ill thl' following <br />paragraph. <br /> <br />23. UNIT HYDROGRAPH ADJUSTMENT. Following is an illustration of the procedure used in <br />adjusting the 6-hour unit hydrograph ]'\0. I of plate ]'\0. 11, derived from hydrologic records for the 1,290 <br />square-mile drainage area of the Saluda River above Chappells, S. G, to apply to the 970 "'llIare-mile <br />drainago area represented by subarea No.1. (See plate ]'\0. 10.) <br />a. The widths of unit hydrograph /1;0. I of plate No. II were scaled at ordinates equal to 50 and <br />75 pereent of the peak discharge, respectively, the values were plotted in plate 1\0. 7, and lillos A-A' <br />and B-B' were drawn through the points parallel to Curves W,,50 and W-75 shown thereon. <br />b. Discharge ordinates of unit hydrograph No, 1, plate No. ] 1, were reduced in direct proportion to <br /> <br />the drainal'(e areas involved, C~~=O,75) and the reduced values were plotted as hydrograph :'\0. I <br /> <br />in plato ]'\0. 12 to ""I've as a guide in shaping the modified unit hydrograph, <br />c. Co('ffi(~ients ep and (\, eorrcsponding to unit hydrog-raph No. J [01' the 1,290 squ8,rp-mile drainage <br />area abov(' Chapp<,lls, S.C,. w{'re ("omputed.as foHows: <br /> <br />l.w=47 miles (measured from map) <br />1.~92 miles (measured from map) <br />(IJ.,.)O,'~ 12.:1 <br />1.=34 hours. (See plate :'\0. 11.) <br />IR~6 hours (selected value) <br />:l4 <br />=t" to nearest hour, 5--=' <br />.a <br /> <br />Q.~ 14,100 c.l.s. (See plate 1\0. J 1.) <br />J4,100 09 f / . <br />qp~ 1290 ~J . c. .s. sq. m.. <br /> <br />(' Ip ( . ) 34 28 <br />'~(t.I...)o-:3 approXImate =12.3~ . . <br />640 e.~/pqp=370. (See equation :l.) <br /> <br />(See equation I.) <br /> <br />d. The following hypothetical case is discussed to demonstrate the use of equation 4 in adjusting <br />thC' unit. hydrograph characteristics for changes in duration of effective rainfalL If thE-' ba.sic unit <br />h)'drograph (or hydrol'(raphs) being utilized had been for a 12.h01lr duration of rainfall, that is <br />tR= 12, instead of a 6-hour unit hydrograph (tR=fT), equation 4 could be used to adjust tll(' valup of <br />lag', fpR. of :H hours to the value of lag, tp) for standard unit duration, tT,; namely, tp=34-0.25X <br /> <br />( ) 2 I (' Id h I 32.5 d f . Id I 1O.9X:l4 <br />12,-6 :::-:-,;r ,5 lOurs. I wou t en equa 12.:l or 2.6, an qp rom equatIon 5a wou equa --':l2.5- <br /> <br />or 11.4 ('.Ls. per square mile. These are empirical relatiOllships and are necessary adjustments (for this <br />illust.ration) sincr: C, varies with duration of rainfall excess; while Cp is a..c;..'mmed to !,pmain constant. <br />('. Assuming that coefficients Ct and 640 rp eomputed for the 1290 square-m.ile drainage area arc <br />applieah)" to the 970 square-mile area under study, the peak discharge and lal'( corresponding to the <br />smallC'f arpu were computed as follows: <br /> <br />1...~:l2 milos (measured from map) <br />1,~1\6 miles (measured from map) <br /> <br />13 <br />