FLOOD04244

• County: Ouray
• Stream Name: Uncompahgre River
• Basin: Gunnison
• Title: Hydrology Addendum to FIS Ouray County Unincorporated Areas
• Date: 7/3/1985
• Prepared For: Ouray County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdroloeic Analvsis
Table 1 lists the streams and reaches for which approximate or detailed floodplain boundary
information is contained in the July 3, 1985 PIS, Detailed analysis was carried out only for the
Uncompahgre River south of the community of BllllllS and consisted of log-Pearson Type ill
statistical analysis of 54 years of streamflow records, Discharge rates for the Uncompahgre
River are presented in the PIS for the 10., 50-, 100- and 500.year events,

Approximate analysis of flood hazards was carried out for the Uncompahgre River north of the
community of'DallM and reaches of Dallas Creek, Pleasant Valley Creek and Cow Creek.
Hydrologic analyses and discharge rates for approximate flooding sources are not documented by
FEMA in the 1985 PIS. As previously discussed, Colorado statutes require that this information
be provided in order for floodplain studies to be "designated" and thus made legally usable for
floodplain regulation, This section of the report documents 100.year discharge values developed
for approximate stream reaches using various methodologies and sources of data, An analysis of
the various approaches is presented and recommendations made,

100.year discharge rates for approximate flooding sources are presented in Table 2 for up to three
methods of computation:

L Regional Regression, Regional regression equations for river basins in Colorado have been
developed by the CWCB and are documented in Guidelines for Determining JOG-Year
Flood Flows for Approximate Floodplains in Colorado (CWCB, January 1999), Regression
equations presented in this publication compute 100-year peak discharge rates and are based
upon data from floodplain studies previously designated by the CWCB, Regression
equations have been developed for nine major river basins within the state and a number of
subbasins, The appropriate equation from these guidelines for the Uncompahgre River and
its tributaries within Ouray County is as follows:

QIOO = 180,8(A)0.578
A = Drainage area in square miles (2<A<8,OOO sq mi)
QIOO = 100-year peak discharge in cfs

Equation I

2, Frequency Analysis, Stream gage frequency analysis has been carried out for USGS gaging
station locations where periods of record were judged to be of adequate length to produce
meaningful results, Those locations are listed in Table 2, Note that only minimal periods of
record were available for Cow Creek and Pleasant Valley Creek (18 and 12 years
respectively) while substantial periods of record were available for Dallas Creek (42 years)
and the Uncompahgre River at Colon a (82 years),

3, US ACE Ridgway Reservoir Analysis, Ridgway Reservoir is operated to minimize flooding
on downstream reaches of the Uncompahgre River and has the ability to significantly reduce
peak flood flows, The Ridgway Dam and Reservoir Uncompahgre River Colorado Water
Control Manual (V,S, Army Corps of Engineers, Sacramento District, November 1996)

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