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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

The discharges for specific: frequencies for the main stem Gore Creek stations, when plotted
versus their drainage areas, all plot higher than most of the other data points in the analysis. The
three stations are consistent with each other for each frequency. A line was drawn through those
three data points and parallel to the regression equations for each frequency. Consequently, the
slope of the relationship for the three main stem stations is the same as that for the regression
equations. This curve was used to determine discharges for specific frequencies for un gaged
reaches along the main stem of Gore Creek. The regression equations for 10-, 50-, 100- and 500-
year events, obtained using the 17 stations listed above, are presented on Figures 2 through S.
The curves that were derived for the main stem Gore Creek and their equations are also
illustrated on the figures.

Discussion of Results

The results of hydrologic analyses that are recommended for use in the Gore Creek basin
hydraulic analyses are presented in Table 4. The same computation points that were used in the
original Flood Insurance Study were adopted for this evaluation to facilitate comparison of
results. In general, the results of both studies are quite similar. Some of the specific discharges
computed for tributaries during current studies are slightly higher than those from previous
studies. Discharges computed for the main stem Gore Creek are slightly lower than previously
developed, particularly for less frequent events. The lOa-year floods computed for large areas
are about 200 cfs less with smaller differences for small areas.

During review of previous studies, it was learned that the discharges that were adopted by GAl
for the main stem were from previous studies by Hydro-Triad. Hydro-Triad used a Log-Normal
distribution in their flood frequency computations for individual gaging stations. This procedure
overstates the magnitude of infrequent floods at stations such as those in this region whose peak
flow records exhibit negative skewness. They computed a mean curve through the data points for
individual stations and an upper envelope curve that was 1.5 standard deviations above the mean
curve. Hydro-Triad recommended using the upper envelop curve to be conservative and because
they realized data on which to determine flood frequencies were limited at that time. That was
then a good decision because main stem stations would have otherwise been understated when
compared to current results.

The lOa-year discharges presented in a table in the FIS were compared to the upper envelop
curve presented by Hydro-Triad. This comparison revealed that the discharges came directly
from the Hydro-Triad report, even though this is not so stated. The additional years of data that
have become available for three main stem Gore Creek stations allows an analysis to be made
specifically for Gore Creek. These are the major reasons for differences between studies of
discharge for specific frequencies.

The slopes of the four regression equations computed for specific frequencies varied uniformly
from 0.73 at the 1 a-year event to 0.70 at the 500-year event. This is a reasonable range of slopes
since 0.7 is a rule of thumb value for a snowmelt dominated relationship and provides comfort
that the results are reasonable. The fact that 21 additional years of peak flow data are now
available also increases confidence in the results of the current study.

8