FLOOD04204

• Designation Number: 179
• County: Ouray
• Community: Ouray
• Stream Name: Uncompahgre River, Ouray to Dallas Creek
• Basin: Gunnison
• Title: Floodplain Information Report - Uncompahgre River, Ouray to Dallas Creek, Ouray County, Colorado
• Date: 4/1/1978
• Designation Date: 12/1/1981
• Floodplain - Doc Type: Floodplain Report/Masterplan

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In the ~ide~ valley ~he~e the West Arm exists an atte~pt has been made
to confine flows, to the p~esent newer channel, by the grading of berms
with native alluvial material. These berms provide adequate protection
during normal flows, but, consisting of permeable materisl, wate~ will
seep through during flows higher than normal and of sevetal days du~ation
and water will cover areas lower than the 50 and 100-yeat flood elevations.
Erosion of berms during major flows may flood low lying areas. This water
will not have noticeable velocity and should not be considered in the same
hazard category as the flows in the main channel. These areas have been
designated as "Areas subject to floodingbutsrenathydrauliLall ycan-
nected to the floodway."

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Drsina~e Areas Tributarv To The Uncornpah~re River

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Total Area
Sta. Identification Sq. HI.
0+00 Above Dallas Creek 150
28+60 Cedar Cr""k US
173+70 CuddiganGulch m
271+70 Coal Creek 120
358+10 Unnamed Creek 110
482+30 Cutler Creek "
589+10 Dexter Creek as
712+30 Ca"cadeCreek 75
756+80 Canyon Creek "

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Natural obstructions to floodflows within the study reach e~ist such as
brush, small trees and other st~ea",land vegetation. Some man-Tllade fEatures
such as bridges, culverts, fences and buildings exist which may act as ob-
structions by themselves or in combination with floating ttees, brush or
transported tocks cause interruption to flow. During floods these obstruc~
tions impede flood flows and cause backwater conditions that may increase
the flood heights upstream of the obsttuction and velocities do~~stream of
the obstruction. The bridge heights are such that debris has not blocked
the wate~ways previously and with the steep channel grades the velocities
carry the debriS through the openings without structural da~age to the
bridgcs.

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Portland and Cascade Creeks

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An analysis was ~ade of portland Creek and Cascade Creek in conjunc~ion
with the <lescrihed Uncompahere River study. The ~"'o creeks have doeu",,,nted
his toties of occasional flash flooding, and they represent a grca~"r hazard
to Outay ~han does the Uncompahgre RiveT. It ;Jas initlally felt that a hy-
draulic analysis of backwater profiles ;Juuld ~roduce a valid representation
of the flood ha~ards presented by the channels. As will be illustrated,
conventional procedures of hydraul1c analysts were unus"ble.

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Cascade Creek and Portland Cr~ek drain tributary watersheds of 1.3 and 2.8
aquurc ~11cs, respectively. Thc ~atcr~hcd~ lie within a Circular basin
east of Ouray which is appropdately named "The Amphitheater." The area
is encircled by jagged peaks reaching elevations of 12,700 feet which have
essentially bare rack faces. More than oae half the ",a""rsheds contain no
vegetation, whiCh combined ",ith the steep slopcs, results in extremely high
rat"s of tunoff when storlll clouds are trapped in the ",atershed 'Hea. In
the procesa of foOlling 4000 feet in elevation to the To~ of Ouray, the flow
in the creeks reaches violent velocities which car~y sand, gravel, rocks,
trees and la~ge boulders into town. This process of violent fragmentation
and erosion of the Amphitheater has, over a period of centuries, deposited
an "alluvial fan" of boulders in the Uncompahgre \'alley ~'hich resembles a
large rock dome at the mouths of the t;JO creeks. The To~~ of Ouray is con-
structed upon this "alluvial fa,," and is exposed to flo~'s which, ",hen they
reach the fan, attempt to spread out and flatten out ~nd follow circuitous
routes to the l'ncompahgre, ~'ashi"g through town along frequ..ntly ""pH'dict-
able paths. The Section "Ilistory of Flooding" ill\Jstrs{es sev"ral of the
floods of this nstu~e from the t~o creeks.

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