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' �'-�!''�`ta <br />3� <br />Figure 3-4 shoae lag faetors required to ed.tuet aquation 3-2 for �ater- <br />eheda where the natural condition of the main channel hae been Ytiydrau- <br />lically improved. Ii the main ohannel hae not been modified, the lag <br />computed by equation 3-2 can be used. Not enough data are available, nor <br />is equation 7-2 accurste enough, to dietinguleh between the typea of <br />channel modii'ication uwde. 1he adjuetment for chennel itoprovement ie <br />made as follows. If 50 parcent of the channel has been modified from ite <br />natural condition and the future-condition curve number ie computed to <br />be 80, then the lag computed by equation 3-2 (or read from fi�{ure 5-3) <br />ie multiplied by 0.7. <br />to0 <br />M 7S <br />�8 <br />$ s° <br />� J ts <br />� <br />1 <br />.� .e .� .s .s <br />IwG RAL10R � <br />Figure 3-4.--Factc:a for edjusting leg fmm equation 3-2 or figure 3-3 <br />�hen the msin channel hee been 1�Mraulical�y improved. <br />Figure 3-5 ehrnvs leg featore for adjusting equation 3-2.if part of the � <br />•aterehed ie impervioue. If the future-condition curve number is 100 or� <br />the impervious area ie ¢ero, edjuetmante are not neaeesary. Nhen a <br />eignificent part of the waterehed ie impervioae� time of aoncentration <br />ie decreased becauee the flow paths t� the main channel are more effi- � <br />cient than under natural conditione. <br />� <br />Since figures 3-4 and 3-5 are ueed only �ith future-condition curve ? <br />nwnbera the leg factora cannot be ueed to direct�y compute the decrease <br />in lag �or time of concentration) from present conditions. To drftermine <br />the change in lag or time of concentration from present to futur� con- '9 <br />ditione, compute the preaent value �nd then� ueing the futurc-conc7ltion' <br />curve number, compute the future valae. ; <br />Mhen on�y peak diechargea from en urten xaterahed are deaired, lag doee <br />not have to be co:nputod. Peek feetora in figures b-1 and 4-2, diseusee� <br />ir� the next chapter, are uaed in the eeme msnner as the lag fectore wheA <br />urban modificatione tq a waterehed have occurred. If other procedures <br />are used to compute peake, but a tine cf concentrntion for future con- <br />ditione ie desired without making a deciiled e+.irvey to deterrrdne �he <br />