FLOOD10080

• County: Mesa
• Community: Grand Junction
• Stream Name: Colorado River
• Basin: Colorado Mainstem
• Title: Flood Hazard Information
• Date: 11/1/1976
• Prepared For: Grand Junction, Mesa County
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

FL.OOD DAMAGE PREVENTION

In order to provide emergency protection
to the Connecticut Lakes area, levees weN)
comtructed by iocal interests on the south
bank of the Colorado Ri,'er downstream from
the Grand Avenu~bridge,lAwdikeshaveaiso
been constructed on the north bank of the
Colorado River upstream from the Grand
Avenue bridge in the Riverside Park area.
During the 1957 flood. a levee was constructed
along the east bank of the Gunnison River by
the U,S, Atomic Energy Commission to afford
protection 1.0 its installation. These works
however, are not adequate to contain the 100:
year flood, which would be considerably
larger than the 1957 flood event.
Most of the reservoirs in the basins of the
Colorado and Gunnison Rivers above the study
area are operated for water C<lnservation
purposes and conseq\lently provide only
incidental flood protection to the Grand
Junction area, In the Gunnioon Rivel'
drainage, however, regulations for j'lint
conservatlon.floodcontroloperati'lnofPaonia
Reservoir hll.\'e been prepared by the Corps of
Engineers and the Bureau of Reclamation.
The conservation o[)Cration of Blue Mesa
Resen'oir provides substantial flood control
benefits,butformalopcrating,'ey,ulationsfor
fle>od control have nol bcon prepardl. The
incidental storage. limited flood control space.
>r"ns-m'luntain div<'rsiuns, and irrigation
useshaveservedandwillprobablycontinuelo
serve in reducing p<'ak flows for very large
fiocdsin the study r'lach.
Flood plain re!l;ulations have bccn adopted

?j' M~a County. Designated flood plains.
!ncludmglowhazardandfloodwa,\'s~oncs.arc
mcorpor~ted into county ,..oning maps.
AppheallOns for new deyelopmentor revision
ofexistingdevelopmentilitheseare",.mustbe
approved by the County Commission and
sueh new or revised development issubjc<:t to
provisionsdesignedtoprehibit:
a. Storage of materials that are floatablc
or may be detrimental to human. animal. or
plant life.
b. Disposal of garbage Or other solid
waste materials.
c. Residential use.
d. Creation or dcposition of additional
debris.
e. Placement of fill. installation ofstruc-
tures, Or storage of materials that could
adversely affeet floodflow and possiblyeause
additional flooding,
The City of Grand Junction zoning
ordinance provides that, if proposed conslruc-
ti'ln ma,,- be damaged by flooding. 'lr may
cause existing huilding.~ to be damagc<l by
flooding, the pcrmit for such stl'ucturesmust
be approved in writing by the City En"incer.
If the City ~~n!;ineer will not approve a
lo<.:ation for building beca,,"eor its flooding
potential. !1C must state his r~asnno to the
applicant in writing, The applicant may have
reeour,e w the Citj. Council to grant the
~"'rmit. b'-'I o"ly af!<!-r con~itl<!-r"t.iM lw the
City Plan"inl(' and Zoninl/;Commission.'

FUTURE FLOODS

100- AND SOO-YEAR FLOODS
The 100-yearflood is one with II. peak flow
m~nitude that has a I percentchanec of
bemgequallc<lorex{'('eded in a,*-givcnyear,
and a lrequencyofoccurrene{'ofaboutonce in
100year,;on the long.term average. Similarly.

the 5()l)..y..ar flood h""aO.Zpercentchanceof
beinl' equalled or exceeded in any given year,
and a frequency ofoccurreneeofaboutonec in
500 year5 on the long-term avera~, As
previouoly indicated, sn'lwmelt runoff from

,

I
I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

tne drainage basins of the Colorado and
Gunnison Ri,'ers and convecti"e type cloud.
burst storm runolf lrom the drainage basins
of the tributary streams create the most
severe fle>odconditions in the study area.
The unit hydrographs for Leach Creek.
lIoriwn Drive Channel. and Lewis Wash were
de,'eloped by using the Snyder technique and
data from several similar nearby basins with
recorded thunderstorm run'lff. Regional
snowmelt flood envelope cun-es lor the
ColoradoandGunnisonRiver5weredoveloped
using flow-discharge frequenc,,'data.
Snowmelt flows of the Colorado and Gun-
nison Rivcrs at Grand Junction were
developed from frequency curves for those
streams above Grand Junction. Based on
available data, the 1921 flood was selected as
being most representati,'e for combinc<l
runoflfromthetworivers.andthe5tandard
project flood was determined to ha,'e a
frequeneyofZSOyears(50percentlargerthan
the 1921 flood). To establish standard project
flows on the Colorado River, a 150 percent

\'alue of lhe 1921 floodflo\\"s at Palisade was
det~rmincti t\nd then r~d\lCed by 8.000 cubic
feet l)('r H'l'ond to r~fl~ct the effect of
upstream ,'e,;en'oil's, for standard proj~d
flows on the Gunnison Ri\'~r, 150 percent of
the 1921 floorlflo\\' al Gr:\Od Junetion was
establish~d and then divided into runoff above
Rnd lx-lo\\' Blue Me,a R~"e"\'oir (55 and 45
percent, re,;pecti"c1y), Blue Me,a Reservoir
was completed in 1965, Runoff above the
reservoir was Comp\lted asa ratio'lf the 1921
flows Rnd adjusted for p,'esent condition..
Reservoir "el"a,,," wco'e wade so that
downstream channel capadties would n'lt be
exceed~d and assuwinl(' maintenance 01
minimum powe,' pool le"el. Downstream
runoff was then added tollrr;,'e at p~sent
standard p"oject flow at Grand Junction.
FloW's in the two rivrrs were ~oml:iined for
total standard project snowmelt flows at
GrandJun~tio". The 100.year flood Hent WM
established as an 89 pe,'eent value of the
standard project event_ The resulli"g
flooclfl'lWS are shown in Table 3,

TABLE 3

PEAK FLOWS
100. AND 500.YEAR FLOODS

Location
Abo"~ mouth of Gunnison River
Below mouth of Gunnison l(iver
At Grand Junctioll
At H Ro3d
At Independence Ranchmens Ditch

Stream

Colorado River
Colorado HiveI'
Gunnison River
uachCreck
Hori;'.on Drive
Channel
Lewis Wash

At 1-70

Peak Flow
",
lOO-Year SOO.Year
Flood Flood

63.000 82.000
8~,llOO 107,000
20,000 25.000
1,800 4,200
GOO 1.500
1,41)f1 3,800

,