FLOOD01823

• County: Garfield
• County: Mesa
• Community: Rifle, Parachute, DeBeque
• Stream Name: Colorado River
• Title: Floodplain Information Report Colorado River
• Date: 4/1/1996
• Prepared For: Garfield County / Mesa County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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5.0 IIIo'TERPRETATION OF THIS REPORT

5.1

Hyd.roIOll:ic Findinl!s

The results of the hydrologic analysis for the Colorado River mainstem are considered accurate.
They are based principally on existing approved data prepared by federal agencies. The
availability of detailed information for almost 70% of the study reacb provided accurate
relationships with the unstudied reach fIndings.

5.2 Hvdraulic Findini!S

The hydraulic methods used for this study are similar to those used in more detailed studies.
They are much more detailed than the methods used in typical approximate studies. Thc 100-
year flood depths for the previously unstudied reaches range from 19.2 feet in a medium reach
of the river to 26.5 feet near the downstream limit of the study (where the drainage area
essentially doubles at the Gunnison River confluence). The majority of the flood dcpths are
generally consistent throughout the previously WlStudied ponions of the study reach with
differences of only a few feet (18.2-26.5 feet for the lOO-year flow depths). The variation
between the 50-year and lOll-year flood depths isgeocrally less than one fool. To find flood
depths for the reaches where there has already been a detailed study, interested parties are
directed to the flood profIies in those detailed srudies.

5.3

Floodulain Delineations

The primary difficulty in presenting precise delineations of the 50-year and lOO-year floodplains
for the previously unstudied reaches was the limited quality of the available base mapping for
the unstudied r(:ache. 0f tIle Colorado River. Using USGS wpographic quadrangll: !TI.1p5 at
C\"Intour intervals 0f 20 or 40 feet to ponrny fl()()d line.' in unsurveyed area. provides Iimlted
infonnat1on. The margin for interpretation is very great. especially in medium to wide reaches
of the river where contours are widely spao:d. More detailed topographic infonnation will be
needed in areas of specifIc interest.

This study can be Uti1il.ed to prepare more detailed floodplain delineations in such areas of
specific interest. Those who are interested in greater detail have three choices for using this
study:

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They can consult the extsung detailed strn:lles. if they peroun to theIr slte. to better
ascenain the flood Slams of the site in question;
They can rely on the hydrologic determinations in this report and use other topographk
information ;!J.1Q hydraulic models; Of
They can use the depth infonnation in this repon with more detailed t0pographic
information to modify the floodplain ddineation.

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6.0 REFERE1',CES

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Federal Emerl!encv MaruJl!ement Al!enC1l; Flood Insurance Study and Flood Insurance
Rate Map date<lJanuary, 1986; Town of Rifle. Colorado.

Federal Emeri!enC1l Management Ag-enC1l; Flood Insurance Study and Flood Insurance
Rate Map dated January, 1986; unincorporated Garfield County_

Federal Emeri!enC1l Manai!emenf Al!encv; Flood Insurance Study and F]ood Insuram:e
Rate Map dated April, 1989; Town of DeBeque, Colorado.

Federal Emerf!encv Management Al?encv; Flood Insurance Study and Flood Insurance
Rate Map dated September, 1991; Town of Parachute, Colorado.

Federal Emeruncv MaruJl!emel1t AUI1Q1; Flood Insurance Study and Flood Insurance
Rate Map dated Iuly, 1992; Town of Palisade, Colorado.

Federal Emeri!encv Management AvenC1l; Flood Insurance Study and Flood Insuram:e
Rate Map dated July, 1992; City of Grand Junction. Colorado.

Federal Emervencv Manal!ement Al!eI1cv; Flood Insurance Study and Flood Insurance
Rate Map dated Ju]y, 1992; Town of Fruita, Colorado.

Federal Emerf!el1cv MaruJf!enwnt Af!eno; Flood Insurance Studies and Flood In!>urance
Rate Map dated July, 1992; unincorporated Mesa County.

J.E. , ",,(dord &: Associates: Floodplain Information Repon, Parachute Cn.:ek, Colorado
River, TO'w'n of P;:;raehme, C'..o]orado dated OcIOt>cr, ]989.

J.F. Saw and Associates. Inc.: Hydrology Report, Flood Insurance Studies in Mesa
County Colorado. dated May, 1989 and work maps from Rifle 10 DeBcque Canyon dated
October 1988.

U.S. Annv COrDS of Enl!ineers. Sacramento Disln'ct: Unpublished Hydrology Rcpon
dated 1985.

United States Geolollical SUn'e\': Water-Data Repon. Water Resources Dat:l, Colorado,
Water Year 1993.

CUTI:efir PrO!!ram: A public domain computer program, version 1.0 b)' T. S. Cor..

Chow, Ve.l1.h,: Open Channel Hydraulics. 1959.

Q1JYr; An in-house hydraulics computer program (not for pllblic domain)

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