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•2 for water-' <br />een t�ydrau- <br />� the leg <br />.vailable, nor <br />types of <br />�vement ie <br />fied from ite <br />computed to <br />figure 3-3) <br />� figure 3-3 <br />red. <br />part of the <br />ier ie 100 0 <br />. When a <br />mcentration <br />� more effi- <br />ion curve <br />: the decrea <br />To determin <br />future con- <br />ire-conditia <br />red, lag do <br />-2, diecuse <br />; factore • <br />proceduree <br />future con <br />�xmine the <br />3-9 <br />individuel overland componente of f].rnr� figureo 3-4 az:d 3-5 oan be used. <br />Fig�iree 3-4 end 3-S are approximations at best end have the eame limi- <br />tatione and uees ee equation 3-2 end figure 3-3. <br />ioo.�.. ;.,. ... ,. .. . .. . . <br />. . � �qm �; �, ��qq . . . . . . . . . . <br />. . . . . `e . . �d � . : � G' yC . 'ch B5 • . � . <br />� TS . :.. �� . ._ _ _.,. _ ce B° ": <br />w ' �~ � • , �T� � . . <br />�� ' cp . .,,� . <br />�� 5� :� � .. � :: : <br />� <br />� .I,•, <br />o■ <br />i <br />i. � � � . <br />. .. _ .�-:. .... <br />. .. _ .• � �. . <br />. . .i j i i . . . <br />, , <br />.� i: , .. . <br />.B ,7 .6 .5 <br />W6 lACTOA <br />Figure 3-5.--Factore for adJueting lag from equation 3-2 or figure 3-3 <br />wh�n impervioue areea onc�.r in the waterehed. <br />Cc le 3-2 <br />�atere ed of 1,000 sares hee s present-aoadition aurve number of 75, <br />�verege �aterehed alope of 4 percent� a�d t�ydraulia length of 13,200 <br />feet. Urban development ie expeated to modify about ^0 peroent of the <br />Qydraulic lengthi increase the impervious area to 40 peraent, and in- <br />ereese the runoff nurve number to 80. Compute,the present- and future- <br />oondition time of concenLration ueing the ourve nwnber method. <br />1. Preeent-oondition lsg from equation 3-2 or figure 3-3�nith CN = 75. <br />L �(]3,200)o•e(3.33 * 1)0•� = 1.45 hr <br />1,900(4)°.s <br />2• Preeent-condition time of concentration from equation 3-1. <br />T� = 1.67(1.45) = 2.42 hr <br />�• Tuture-cotdition lag. <br />�• Baeic future-condition leg with CN = B0: <br />L _ (13.200)o•e(2.5 + 1)0.� _ 1.25 hr <br />1,900(4)o.s <br />L� 1eg factor for modification of 70 percent of the 1�r��raulic length <br />r� riBux'e 3-4: 2�ydraulie-length leg factor = 0.59 <br />