FLOOD00209

• Designation Number: 543
• County: El Paso
• Community: Unincorporated El Paso County
• Title: Flood Insurance Study - Pitkin County, CO, and Incorporated Areas
• Date: 9/1/1988
• Designation Date: 3/1/2001
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Castle Creek. Gaging station records are available for the USGS gaging station Castle Creek
Above Aspen, Co. (station no. 09074800) for a period of 25 years from 1970 to 1994. Frequency
analysis of this data yields an estimated l00-year discharge of 620 cfs at the gaging site which
has a drainage area of 32.2 sq mi. The CWCB regional regression equation for the Upper
Colorado River Subregion (Equation 1) yields a computed loo-year discharge for the gaging site
of 982 cfs. This value is substantially higher than the frequency analysis result for the same site.
Gage data also exists for a location farther downstream for the period 1913-1919. The seven
years of available data was judged too short a time period though to yield meaningful results and
no frequency analysis was carried out for this site.

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 for a significant time period
for Castle Creek and is site specific. Based on this, it is recommended that the results of the
frequency analysis be used to estimate l00-year discharge rates rather than regional regression
equations. Frequency analysis results for the USGS gaging station on Castle Creek may be
extended to other locations on the creek using the following equation from the publication
. Manual for Estimating Flood Characteristics of Natural Flow Streams in Colorado (CWCB,
1976):

Qungaged = Qgaged(Aungag..,/ Agaged)o.79

Equation 2

Qungaged = Discharge at ungaged location (cfs)
Qgaged = Discharge at gaged location (cfs)
Aungaged = Drainage area at ungaged location (cfs)
Agaged = 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.

Hunter Creek. Gage records of significant length exist for two locations on Hunter Creek. The
USGS gaging station, Hunter Creek Above Midway Creek, Near Aspen Co. (station no.
09073700) has 16 years of data collected between 1965 and 1980. A second USGS gaging
station, Hunter Creek Near Aspen, Co. is located further downstream and has 36 years of
recorded data collected between 1950 and 1999. Frequency analyses have been carried out for
these two sites and compared to the results of regional regression analysis. A comparison of
these results is provided in Table 3. At the more upstream gaging site, gage records indicate a
substantially higher discharge than regression analysis. At the downstream location, the results
of the two analyses are very similar.

Because the gaging station data is of significant length, is site specific and indicates higher
discharge rates than the CWCB regional regression equation, it is recommended that the results

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