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<br />, Chapter 4: r;raphical Peak Discharge method
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<br /> Thi, chapter ~t, the Graphical Peak DiscllarKe I.,
<br /> method for ''',.putilll( ""ak di""haJ"l(e from rural alld
<br /> urball areas. "he Graphie,a1 method was develo""d ..
<br /> from h)'dn>j(I' oh analy...., u,illg TR.20, "Compuler
<br /> Pro!(l"alll for Pl't>j~cI Fonnulation-Hydrology" (SCS
<br /> 1983). The pt'ak QI'chlU'!:t' equatioll used is ..
<br /> lip = quAmQF p IEq. 4-11 -
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<br /> where
<br /> 'Ip = peak dischargt' (cM: ./
<br /> qu = unit pt'ak dischal'j(e (c,mlin):
<br /> Am = drainage area (mi2):
<br /> Q = runoff (in); alld , . 11 11 ..
<br /> Fp = polld and swamp acljustment factor. ..,,,,,11 ('I, htllll
<br /> The input requirements for the Graphical method are Flp,", ~&l.-V..ri~tion of I.'''' (or P and 01.
<br /> as follow" (1) T, (hr), (2) drain3j(e area (mi2). (3)
<br /> appropriate rainfall di,tribution (I. IA. ii, or III), (4) Table .j.1.-I. ,'alu8 for runoff cun-e numben
<br /> 24.hour rainfall lin), alld (5) C:>1. If pond and swamp
<br /> arl!a$ are ~pread thruug-hout the wate~hed and are Cun'e I. Cun'f' I,
<br />( not con~iderpd :n the T(' l'Omputation. an adjul"tment numbt-r (in) nwnber (in)
<br />fnr pond and ~wamp area~ i~ al:->c) needed.
<br /> 40 ;\.000 70 0.857
<br /> 41 2.X;8 71 0.1117
<br /> 1 42 2.762 7'l 0.778
<br /> Peak discharge computation 43 2.651 73 0.740
<br /> 44 2..>15 74 0.703
<br /> For a selected rainfall fr"'l"cn"y, the 24.hour rainfall -15 2..144 75 0.667
<br /> 41) 2.34ll 76 0.632
<br /> (P) is obtained from appelldix R or more detail"" 47 ~2-"'~ 77 0.597
<br /> local precipilation mal'" C:>1 alld lotal runoff (Q> for 411 2.167 78 0.s64
<br /> the wate""hed are computed acconlinJ( 10 the 49 2.01<2 i9 0,532
<br />, methuds outlined in chapter 2. The C:>1 is u:;cd to 50 2.000 SO 0.500
<br /> delennine the initial abstraction (I,.) fnlm table 4.1. 51 I.!l'~ HI 0.469
<br /> I,.JP i, then computed. 52 1.H46 Il2 0.439
<br /> 5:1 1.774 ll3 0.410
<br /> If the computed IjP rdtio i~ outl"ide the r"d.nJ.,-re shown 54 1.704 !14 0.381
<br /> 55 1.6:16 ll5 0.353
<br /> in exhIbit 4 (4.1, 4.IA, 4.11. anrl 4.111) for the rainfall 66 1.571 !l6 0.326
<br /> distribution or intt'~:.;t. then the IimitinJ{ vaJue 57 1.509 87 0.299
<br /> ,hould be u_1. If the ratio fall, betwl",n the IimitinJ( 56 1.448 Il8 0.:.'7:1
<br /> values, u:;c linear int"I'(l<>lation. Fil(Url' 4.1 iIIu'trates 59 1.390 H9 0.247
<br /> the :;cn,ilivity of I,.JP to eN alld P. 60 1.333 !IO 0.222
<br /> 61 1.279 91 0.198 \
<br /> Peak discharge per H(luare mile IK'r inch of runolT 62 1.226 92 0.174
<br /> (qui i, obtained fnJm exhibit 4-1, 4.IA. 4.11, or 4-111 6.1 1.175 93 0.151 \
<br /> by usinl( T, (chapter ;l), rainfall di,tribution type, and 64 1.125 94 0.128 '.
<br /> [JP r"dtio. The pond and swamp adjustment factor is 65 1.077 95 0.105
<br />~- obtained from table 4.2 (nJund~d to the nearest table 66 1.030 96 0.083
<br />~J value). Use worksheet 4 ill appt'nclix D to aid in 67 0.985 97 0.062
<br /> 66 0.941 98 0.041
<br /> computing' the pt'ak cli;charge u,inl{ the Graphical 69 O,H99
<br /> method.
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<br />(21()'VI-TR.55. Second Ed., June 19&1)
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