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<br />.
<br />
<br />.
<br />
<br />Colorado. 111 in r,','~rr,r,e S-curve is 61",.." "n pla~" 13. l.ag tim"" ",,,r,, b"~,,d
<br />
<br />27, rL> <level oping dcsigLl fl~oJ" (.ll€ lj:lI'~'.niQu~ portion of (11" b"Rin~
<br />
<br />on Kn valuus rnngingfro,"O.O::itoD,06anrl\.,,,r,,d<Olcrminedhy fie Idin-
<br />
<br />were n~gigl\ed ~n lafiltr<.itioll nILe of 0.02 inch p~r hOllr. It m;JY SC~'"
<br />
<br />spections,,,,dC(Jmparbol\swithothe....ar<"'s. Unithydro[(raphs"r"tabul.1ted
<br />
<br />cont r~dictory ~" ""y that inHI trotion mny occur 0[1 ilnpcrviolls sur faces
<br />
<br />onPlaCe$ 1/,a,,<1 15.
<br />
<br />but st"rli~s of largo con"-rete parking arer,s h"v~ shown that 100 percent
<br />
<br />runoff cannot be "twined bec"u~o' of "."ltc. PQrcol"ti~t throllgh joints ;and
<br />
<br />26. Loss Allalysi5. Whe.n con9id~ring rainfall i~filtration 1055"S \lnd~r
<br />
<br />other tli"-c.ontin,,ities in an othenJb~ impc~vious surface.
<br />
<br />urbani"ed COll~iti~"~, the sub-areas "c.rc separated into two parts _
<br />
<br />the pervious portion and the i",pcrviousportion. The pervious port1on
<br />
<br />28. Infilt"ration rat~" for thc. pe.rvious (lre,'s of the study area were
<br />
<br />is that part (If the devcl(lped ar"a such as l,.",ns, gardens, andoth er
<br />
<br />bas~d On fi~ld oo~ervations and losses used by the Omah~ District On
<br />
<br />planting ~p"c"s ~"sceptible to infiltration after dev"lopment ha" t.'k<lo
<br />
<br />'Watcr~hcds Ilcar the st"dy. A constant loss rate of 2..30 inch per hour
<br />
<br />pl,,~_~. Th~ im!,crviou$ portion is that !'art of the ",,,,a covered by .(lof",
<br />
<br />"as adoptcd for use in this study.
<br />
<br />patios, side..~ll<:s, dr1.,c,,'ayr., alleys, "nd ~treets. IllGpecrion of aHial
<br />
<br />photographs have shown thar the degree of imperviousness may vary from
<br />
<br />29. Prcqueney Analysis_ The lack of adequate stream flow records in thc
<br />
<br />about 20 percent in rhe caSe of lo'W-density residential areas to about
<br />
<br />study area necessitated adoption of "Manual for Estimating F10 odChar"crer-
<br />
<br />90 percenr in areas "here business-commercial land use predominates.
<br />
<br />istics of Natural-Flo'W Strea'1lS in Colorado," by the U.S.G.S. for deter-
<br />
<br />Table 3 is a listing of imp"rvious area perce.ntaees that are considered
<br />
<br />mining discharge-frequency r ,lationships. The manu~l is applicable only
<br />
<br />typical for a given land use.
<br />
<br />to natura1-f1o" s~reams. Therefore, another ,""thod 'Was used for fre-
<br />
<br />quency analysis for urban ar"3s. Rainfall fro", Tec~ni"al Paper No. 40
<br />
<br /> TABLE 3
<br /> LAJ'lOUSE , IMPERVIOUSNESS
<br />LOII'DENSI1''f REgIDENTIAl. " "
<br />MEDIUHllENSITYRESIDENlIAL 25 35
<br /> ..
<br />HIGH DENSITY REStDE:'lTIAL 30 "
<br />BUS1NESS -COMMlCRCTAL " "
<br />U(:~T I~IJJSTRL\L '.5 65
<br />HEAl'\" INDUSTRIAL 50 "
<br /> -ll-
<br />
<br />'W"suscdandappliedtoeachurbanarea'sunithydrograph. Theresultant
<br />
<br />peak discharges "ere plot red on log-probability paper and resulted in
<br />
<br />a disch~rge frequency Telari,.nship for "..cll urbdn a.ea. An example of
<br />
<br />urbnn are" rainf" 11 amounts usell in t!,is ~tudy is shown on Pl<'~o;: 16.
<br />
<br />30. Hood,. of tl.;,s).f',llar<ld FrcQ\1<i'nsy_ Th. 10. 50, 100 and 50(}-year peak
<br />
<br />fl",,,o are reQulr"d for the flood insurar.c"stlldy. l'h"y were cbtai""d for
<br />
<br />'lOn-urban "rea" by us in;; the lJ _ S. G. S. r~~r~ss ~o" equ.1lions. Equ"t ions
<br />
<br />-12-
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