FLOOD02706

• County: Statewide
• Basin: Statewide
• Title: Paleohydrologic Techniques used to define the Spatial Occurrence of Floods
• Date: 1/10/1990
• Prepared By: Elsevier Science Publishers
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Fla. 12. Approximate area of cODtributilll runofflO the July 12, 1976 flood on Sweetwater Creek at mouth ne;lnol!lvo.
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data combined with paleoflood info}mation
can be used to define the hydrolOlliGf'CJions to
be used in regional flood-frcquenci ;~udics.
This study further indicates that realorpt flood
estimating procedures currently in USt~eed to
compute rainfall-produced flood esti tes lor
only that part of the basin below about 300 m
in Colorado. Regional rainfall fiOod-frtQuency
relations for the foothills area of th~outh TABLE 2
Platte River basin in Colorado hav~ ntly Comparison of flood n
intervals for Clear Cree
been developed (Jarrett, 1987). This analysis _
only used drainage areas below 2440 IT~( a con- Rl!Qlr- FJooclllischal'l
servative elevation selection) rather _~n the !C"ce .
total basin drainage area. Resional f1;od-frt- .Ihterval Foothills anall
Quency equations provide more rel~ -'e csli- )y) Stalion Rear'
mates of both common and rare flools (Jar- ~ sion
relt, 1987; Jarrell and Costa, 1988). ~en the ~
drainage area below 2400 m, rathe~~a~ till '~'IO 72 64
total drainage area, was used in the..fSIonsl ,'SO ISI 183
flood-frequency relations,the standardFrroro! :~oo 199 270
estimate for the relations improvedJi..olD 14. '_00 354 606
to 44%. Regional flood-frequency 'illationS- ;'U.s. Federal Emel'JenC
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of rainstorms where rainfall data are sparce.
Improved estimates of the cOlltributina drain-
age area will benefit developmellt of and users
of envelope curves of maximum floods (ct. Fii.
4) and provide valuable information to those
who use raillfall-runoff modelina methods to
assess flood hydrology.

Regiona/flood-frequency relatiOllS

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Investigations of flood potential based on
rainfall-runoff modelina techniques fail to ad-
equalely describe the flood hydrology of footh.
iU and mountain streams; generally, these
techniques significantly overestimate flood
m3&nitudes (Jarrett, 1987). Investigators have
assumed that the total basin area contributes
runoff during rainstonns. However, in the
foothills of Colorado noods arc caused by in-
tense short-duration thunderstorms limited to
areas below 2300 m (Figs. 4 and S; Table I).
Analysis of streamflow and precipitation

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FAX NO. 2385034

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supported by pale
that the 1976 Big-
recurrence interv.
years.
Use of paleoh)
limited only to 10\
is useful for anall
Rainfall-runoff IDe
in Colorado to de1
for flood-hazard s
ues were calculi
models rather thar
because ":.. the l
puted by these me
reliable to prediC1
events ..... (U.S. F
ment Aaency, 198
results from rain fa
gional analysis bas,
portant because it
magnitude-freque:
these flood-disch3J
of flood-hazard stu
ment and desian 01
Flood character
fall-runoff analysis
shown for Oear C:
(Table 2). Values I
timates, the relioD.
estimate of the sta'
(considered the be

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