FLOOD05072

• Designation Number: 78
• County: Mesa
• Community: Fruita
• Stream Name: Colorado River and Tributaries
• Basin: Colorado Mainstem
• Title: Flood Hazard Information - Colorado River and Tributaries, Fruita, CO
• Date: 11/1/1976
• Designation Date: 9/1/1977
• Floodplain - Doc Type: Floodplain Report/Masterplan

Flooding on the tributary washes draining
from the Book Cliffs and Roan Plateau is
caused by intense rainfall from cloudburst
storms', but definitive informationondoud-

burot floods on the,e streams is very limited.
Predpitation records for the Frultaareashow
the following I-day rainfall amounts (actual
storm durations unknown):

Dale

Amoun!
(Inches)
1.08
1.42
1.20

Oclober13,1957
Augus131,1963
June24,1969

In the study area, 100-year floods on the
Colorado River would be caused by ~nowmelt
and S(){)..year floods would be cauood by

snowmelt augmented br rain, On the
tributary washes, 100- and 500-year floods
would result from cloudbur~t ~torms,

FLOOD HISTORY
The Colorado River has a long history of
snowmelt flooding, but detailed information
on flood evenl. in the study area is
unavailable.Thc earlicst Colorado River flood
known in the Fruita arcaoeeurred in June-
July 1884. Other floods on that stream were
recorded in 1917,1920,1921,1935,1952,and
1957. In general,the 1884 flood is considered
the most severe known in thc Fruitaarca. It

~ultedfromrapidme1tingofadeepsnow-
pack and concurrent heavy rains. Cloudburst
storms are known to have occurred over the
tributary drainage basins and reportedly
causcd the washes to overflow many times,
but, duc to the rural nature of the re~ion,
definitive data On these flood events are not
available,

FLOOD DAMAGE PREVENTION

Most of the reservoir. in the basins of the
Colorado and Gunnison Rivers above Fruita
areop"rated for water conservation purposes
and eonooqucntly provide only ineidental flood
protection to the Pruita area, In the Gunnison
River drainage, however, regulations for joint
conservation-flood control operation Qf Paonia
Reservoir have been prepared by the CorpsQf
Engineers and the Bureau of Reclamation.
The conservation operation of Blue Mesa
Reser"oir provides substantial flood control
benefits, butformaloperatingregulation~for

flood control have not been prepared. The
incidental storage, Iimitcd flood eontrol spaee,
trans-mountain diversions, and irrigation
uses have served and will probably continue to
serve In reducing peak flows for very large
floods in tbestudy reaeh.

Flood plain regulations have bcen adopted
by Mesa County. Designated flood plains,
inc1uding low hazard and floodway zones, are
incorporated into county zoninJl: maps.
Applications for new dcvelopmcntor re\'ision

'S;m"ltan<o""noodin.o:onallt',o\rib"t.,;""p,"b~b]ywou]d I'IOt<><<",d"""'\oo,mall .n....I."""tord""dbu,.,.,
,torn".

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ofexistingde"c1opmentinthe~eareasmustbc
appro,'ed br the Count;' Commission, and
such new Or rcvised development is subject to
pfO\'Isions designed to prohibit:
a. StoraJl:e of materials that are floatable
Or ma,'bcdetrimental to human, animal, Or
plant life.
b. Disposal of garbage 0" other solid
waste material~.
c. Residential use.

d. Creation 01" deposition of additional
debris.
e. Placement 01 fill. installationofstruc-
tures, or stornge of materials that could
ad"er~ely affect floodflol\' and pos.~ibly cause
additionalfloodinl<.
Fruita doeo not have a zoning-ordinance.
The community is Intheproee~sofadopting
~ubdivi~ion regulations, but the regulations
under consideration do not address flood
hazardsperse,

FUTURE FLOODS

100- AND SOO-YEAR FLOODS
The lOO-yearflood is one witha peak flow
magnitude that has a 1 percent ehance of
bcingequalled or exceeded in anrgiven year,
and a frequenc;' of occurrence of about once in
100yearson the long-term average, Similarly,
the ::;OO-year flood h~s a 0.2 percent chance of
being equalled orexceede<l in any given year,
and a frequency of OCCllTrenCe of about once in
500 years on the long-term average. As
previously indicated, snowmelt runoff from
the Colorado River drainage basin and
convective type eloudburst storm runoff from
the drainage basins of the tributarv washes
create the most Severe fiood condition. in the
study area.
The unit hydrographs for Big Salt Wash,
Little Salt Wash, and Reed Wash were
derived by the S-I<raph method,utilizingan S-
graph developed lor Rifle Creek at Rifle. A
regional snowmelt flood envelope curve for
the CoJorado River was developed using flow-
diseharge frequency data.
Snowmelt flows at Fruita were developed
from frequency eurves for the Colorado and
Gunn;son Riwrs above Grand Junction. Based
on a"ailablc data, the 1921 flood was selected
as being most representaliw for combined
runoff from the two rivers, and the standard

project flood was determined to have a
frequeneyof25Oyears(50percentlargerthan
the 1921 flood). To establish standard project
flows On the Colorado River, a 150 percent
value o! the 1921 floo(lnowsatPalisade was
determined and then reduced by S,OOO cubic
feet per second to reflect the effect of
upstream reservoirs. For standard project
flows on the Gunnison River, 150 percentQf
the 1921 f1oodflow at Grand Junction was
establi~h>dandthendividcdintorunofrabov"
and below Blue Mesa Reservoir (55 and 45
percent, respectively). Blue Mesa Reservoir
WaS completed in 1965. Runoff a\xlve the
reservoir was computed asa ratio of the 1921
flows and adjusted for present conditions.
Reservoir releases were made so that down-
stream channel capacities would not be
exceeded and assuming maintenance ofmin-
imum power pOOl level. Downstream runoff
was then added to arrive at present standard
project flow at Grand Junetion, Flows in the
two rivers were combined for standard
project flow at Frulta, The loo-year flood
e"ent was estabJished as an 89 percent value
of the standard project event. The resulting
f100(1nows areshowll i"TaLle3, p<<g,,6.

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