FLOOD07531

• County: Ouray
• Community: Ouray
• Stream Name: Uncompahgre River
• Basin: Gunnison
• Title: Floodplain Information Report
• Date: 4/1/1978
• Prepared For: Ouray County
• Prepared By: A & S Consultants
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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In the ~lde~ valley where the West Arm exists Sn atte~pt has been made
to confine flows, to the present newer channel. by the grading of berms
with native alluvial material. These berms provide adequate protection
during normal flows, but, consisting of permeable materisl, water will
seep through during flows higher than normal and of several days duration
and water will cOVer areas lower thsn the 50 and 100-year 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 flooclingbut are not hydrauliLsll ycon-
nected to the flood",,,y."

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Drsina~e Areas Tributsrv To The Uncornpah~re River

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Total Area
Sra. Identification Sq. Hi.
0+00 Above Dallas Creek 150
28+60 Cedar Cr""k US
173+70 Cuddigan Gulch m
271+70 Coal Creek 120
358+10 Unnamed Creek 110
482+30 Cutler Creek "
589+10 nexter Creek as
712+30 Cascade Creek 75
756+80 Canyon Creek "

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Natural obstructions to floodflows within the study reacb exist such as
brush,smalltreesandotherstreamlandvegetation. Someman-Tlladefeatures
such as bridges, culverts, fences and buildings exist ",hich may act aa ob-
structions by themselves or in combination with floating trees, brush or
transported rocks 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 obstruction and velocities do~~stream of
the obstruction. The bridge heights are such that debris has not blocked
the waterways previously and with the steep channel grades the velocities
carry the debris through the openings without structural da~age to the
bridges.

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Portland and Cascade Creeks

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An analysis was ~ade of portland Creek and Cascade Cr~ek in conjunction
with thp <lescribed Uncompahere River stlldy. Th~ t",o creeks have dOCllm~at~d
histories of o~casional flash flooding, and they repr~sent a gr~ater hazard
to Outay th~n do..s th.. Un~ompahgr" Rlv..r. It ",as initlally felt that a hy-
draulic analysis of ba~kwate~ profiles wuuld ~rodu~.. a valid repr..s..ntation
of the flood ha~~rds pres..ntcd by the ch3nnels. As will be illustrat..d,
conventional procedures of hydraulic analysis were unus"b1e.

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Cascade Creek and Portland Cr~~k drain tributary watersh..ds of 1.3 and 2.8
squure ~iles, respectively. The ~ntcr~hed~ lie within a circular basin
east of Ouray whieh is appropdately named "The Amphitheater." The area
is ~ncircled by jagged peaks reaching elevations of 12,700 fcet which have
e"a~ntially bare rock faces. Mote than one half the ",aursheds contain nO
vegetation, whiCh combined with the steep slopes, result~ in ~xtremely high
rate~ of tunoff when storlll clouds are napped in the ",atershed 'Hea. In
the process of falling 4000 feet in elevation to the To~ of Ouray, the f10",
in the creeks teaches violent velocities which carry sand, gravel, rocks,
trees and large bouldera into town. This pro~ess of violent ftagmentation
and erosion of the Amphitheater has, over a period of centuries, deposited
an "alluvial fan" of boulders in the Uncompahgre \'alley ~'hich resembl~s a
large rock dome at the mouths of the two creeks. The To~~ of Ouray is eon-
struet~d upon this "alluvial fan" and is exp,,~ed to flo~'s which, ",hen they
reach the fan, attempt to spread out and flatten out ~nd follow circuitous
routes to the l'ncompahgre, ~'ashing through town along frequ<,ntly ""pH'dl~t-
able paths. The Section "History of Flooding" ill\Jstra{~S s"v"ral of the
floods of this nstute from the t~o creeks.

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