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

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
I.
.
.
I.
.
.
.
.

indicates that Ridgway Reservoir would reduce the lOO-year discharge at Colona gage from
the stream gage frequency analysis value of approximately 4,370 cfs to 2,000 cfs,

Analysis of the available data and methods of 100.year flow computation is provided in the
following paragraphs, Conclusions and recommendations on appropriate approaches to
computing 100.year flows are also provided for each stream reach studied by approximate
methods,

Pleasant Vallev Creek. Gaging station records are available for Pleasant Valley Creek for a
period of 12 years from 1956 to 1967, Frequency analysis of this limited data yields an estimated
lOO-year discharge approximately 30 percent higher than the CWCB regional regression equation
analysis for the same point. Regional regression equations are based on limited data and by
definition, are developed to cover a broad area, Because of these facts, there can be substantial
error in computed flows, Documentation available for the CWCB regional regression equations
does not include information on standard estimates of error. Gage data is available only for a
very limited time period but is site specific and provides more conservative results than the
regression equation approach, Based on this, it is recommended that the results of the frequency
analysis be used to estimate 100.year discharge rates rather than regional regression equations,
Frequency analysis results for the USGS gaging station on Pleasant Valley Creek may be
extended to other locations on the creek (within the noted limits) using the following equation
from the publication Manual for Estimating Flood Characteristics of Natural Flow Streams in
Colorado (CWCB, 1976):

Qungaged = Qgaged(Aungagedl Agagedf79

Equation 2

Qung.ged = Discharge at ungaged location (cfs)
Qg.ged = Discharge at gaged location (cfs)
Aung.ged = Drainage area at ungaged location (cfs)
Ag.ged = Drainage area at gaged location (cfs)
Equation applicable for ungaged sites where drainage area ratio lies between 0.5 and 2,0

Note that Equation 2 is applicable only for ungaged sites where the drainage area is within a
factor of two of the drainage area at the gaged site, For drainage areas outside these limits a
detailed hydrologic analysis is recommended, /

Dallas Creek, A substantial period of record is available for the Dallas Creek gaging station (42
years) and it is recommended that the frequency analysis results and Equation 2 be used to
determine 100'year discharge rates rather than the regional regression equation, The gage on
Dallas Creek is located just upstream of the confluence point with the Uncompahgre River,
Based on the limits of applicability of Equation 2, discharge values may be computed for Dallas
Creek for drainage areas of approximately 48 sq mi and greater using the frequency analysis
results and Equation 2, For locations further upstream, the regional regression approach of
Equation 1 may be used but note that this equation yields higher discharge values than indicated
by the frequency analysis, Otherwise, for locations outside the limits of applicability of Equation
2, a detailed hydrologic analysis could be carried out.

5