FLOOD09803

• County: Summit
• Community: Cripple Creek
• Stream Name: Colorado River
• Basin: Colorado Mainstem
• Title: Floodplain Management Study Cripple Creek
• Date: 9/1/1992
• Prepared For: CWCB
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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t lrne of con[erlt~ation values were com~'llted from an ern~)erical equatjofl
i f! S (I i I C (I n S E' t~ \/ a t j (I n S E' t- vie e En 3 j nee t" i rl 8 F i f:.l I d iT1 a n L:. a! ~~ C hap t e t- ;2,
I f1terpjE'd fOt" srlli::.l i t"LH" a I wat~.?t.sherjs.

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-rc=[L^CI.8((1(\OO/CN)-9)^O.7J/1140(Y^O.5)
Tc= tirne of corlcentration in hours
L= flow ler'lgttl in feet
eN= t-u~off cut"ve number
y= average watershed slope

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Y~100CI/A
y= average watershea siope In percent
A= drainagE: area in square feet
I ~':: C ontour- i ntE.'t-va.! in feet
c::= t (I t~. i C (I n t (I u. t" 1 e r::3 t: h j n fee t

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11:; was determined at a rneetins on Jan. 6f i~~~ ttlat the Rational
Formula (Q=CIA) would be useful to the city in Nlaking site speCific
pea~( discharge computations. The!'"efore the TR-20 dischargE vaiues
oeternline,j for" t~ljs study couid be used to develop necessa~y runoff
coe+'fic!t.nts ~c: ValUE!!::.) for- local use..

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S'rlce t~le BeS 24 ~Iour type II rainfal I distritrution was used irl TR-Z0,
iT should also be used to determine rairlfaI I inter:sity values (I).
f'r1ost of the appl ic:ation of the r~ationai f'ot~roula wi jl be for" sma! I
(j~-ajr!a8e aXf?a.s '..Jhpre tt'le time of concentr"ation i~l le':;.s tha.n :~:O
IT:irlutes. Therefore a rainfal I duratjofl of 30 minutes was selected f(.v
tet2rmirlirlg I. l'he 8~eate5t 30 minu'~e interlsity ir"clm the BCS 24 hour-
type II rail1fal I dlstributiofl produces 37.8 % (If t~le total 24 hour
t-ainfal I dept~. Ttlis 30 minute r~infal I depth was determined for eacri
storm frequency of interest and then converted to inches per ~Iour fOt-
IjS~ as I, see Table 2.

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Tf!e rational formula was rearranGed (C=Q/IA) to ((Impute C values usiG8
previollsly determined Q and A vaiues from TR-20 data, and I values
previously discussed. A tHelationship between C values and curve
nl_Frlbet~s (eN) ft-orn TF:--;2:0 was estab i i shed. eN va! ll€'S fOt~ the TF\-20
watersheds reflect weighted average CNs for a variety of land uses and
~.oils within each water~shed. The cr\lS 8enet~al1y did not e;'~ceed the low
:::(15. Since the t"'a.tional equation may be used fOt" smallet~ individual
land us-es. and -::;.(lils, such 3.S a. city block of cOfl1rnet~cial buiidinss, tne
C.~s ftHom TR-2(1 may not be sufficiently site specific. TtlEt-efore for
iarld uses with eNs greater than the low 80s, a table (Table 14-1) from
tti2 Ill"'landbook of App lied Hydt~o J 08yl1 by' Ven Te Chol.v was usef'u J in
selecting C values for rnore site specific types of land uses.

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