FLOOD10333

• County: Eagle
• Community: Vail
• Stream Name: Gore Creek and Tributaries
• Title: Hydrology Report, Gore Creek and Tributaries
• Date: 4/5/1999
• Prepared For: Vail
• Prepared By: J.F. Sato and Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Flood Frequency Analysis Of Streamgaging Data

Flood frequency analyses of annual peak flow data were made for 9 stream gages located in the
Gore Creek basin. It is unusual to have records available from that many streamgaging stations
in a 102 square mile drainage basin. Many of those stations now have records more than 30
years in length. The location of those gages is shown in Figure 1. In addition, data for 11 gaging
stations located in Eastern Eagle County, but outside of the Gore Creek basin, were analyzed to
obtain a larger data sample and to better fill out the range of drainage areas for which data are
needed.

Previous flood frequency studies by GAl for the flood insurance study used peak flow data
through 1976. The current study used data through 1997 so 21 additional years of data were
available for the current study. The orographic effects of mountain barriers have an effect on
precipitation in the Gore Creek basin and was a consideration in selecting stations to supplement
the Gore Creek gages. The Piney and Eagle River peak flow data used in this study are all
nearby and drain the western slope of the Gore Mountain Range. Care was taken to ensure the
streams adopted for use in the regional study reflect reasonably similar orographic effects and
runoff characteristics to the Gore Creek basin.

The flood frequency analyses for individual stream gaging stations were made using the
computer program, fffreak, Version 1, that was developed by US West. The program was
designed to run using peak annual discharge values contained in their HYDRODA T A product.
The program follows procedures recommended in Bulletin 17B except for two features that were
not used in this analysis: two station comparisons and weighting of independent estimates. The
Appendix contains a printout of each of the annual series of peak discharges used for discharge
frequency computations, and a plot of each discharge frequency curve that was computed in the
study. Printout of discharge frequency computations are available upon request. A summary of
the results of this statistical analysis of 20 stations is presented in Table 1.

A plot of the standard deviations computed for these records versus their drainage areas resulted
in a wide scatter of data points. No meaningful relationship was apparent. The average value of
the standard deviations shown in Table 1 was just under 0.20 which is typical when snowmelt
floods dominate frequency distributions.

A generalized coefficient of skew of - 0.3 was adopted for all stations based on the generalized
skew map presented in Bulletin 17B (Revised September 1981 with Editorial Corrections March
1982). As shown on Table 1, the average unadjusted coefficient of skew computed from the
station records is -0.33 and the average adjusted skew is -0.28, which are both very close to the
-0.3 value that was adopted from the generalized skew map.

Seasonal Distribution of Floods

There are three potential sources that could produce runoff in the study area including
thunderstorms, general storms, and snowmelt. Floods could occur as a result of rapid snowmelt

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