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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or rainfall alone, or a combination of those events. A review of the seasonal distribution of peak
flow data was made for the 20 streamgaging stations to assist in defining the primary source of
flooding. The results of these analyses are presented in Table 2.

A total of650 station years of record were examined and about 2/3 (435) of the annual peak
discharges occurred during June, 188 during May, and 24 during July. Only three out of650
station year events occurred outside of those three months. While floods as a result ofrainfall
alone could be important for small drainage areas, peak flow records show snowmelt is the
primary cause of peaks at the stations studied. Rainfall may have contributed some runoff to a
few of the annual peak flows, but it is very difficult to identify the amount of contribution from
available hydrologic records. During spring months, much of the precipitation that occurs as
rainfall will be retained in the snowpack. The ground is often still frozen during the spring
snowmelt season, so runoff occurs with very little infiltration. Rainfall did not result in
significant independent peak discharge events because the annual peaks occurred during the
snowmelt season.

The Hydrometeorological Branch of the National Weather Service, in cooperation with other
Federal agencies, made a major effort to identify historic thunderstorms in the Rocky Mountain
region during probable maximum precipitation (PMP) studies. The results were documented in
US Commerce Hydrometeorological Reports No. 55A and No. 49 dated June 1988 and
September 1977, respectively. They found that a few severe thunderstorms have occurred in the
mountains of Colorado, New Mexico, and Utah. However, these storms are very rare and
precipitation data for them are sparse, partially because only a limited number of rainfall gages
are located at high elevations. Thunderstorm, or local storm, rainfall is a significant factor for
small drainage areas at the extreme PMP level, but have not been observed often enough to be a
significant factor at the more frequent flood levels considered in this floodplain study.

The best technical evaluation procedure would be to determine an independent series of
exclusive snowmelt and exclusive rainfall events that could be analyzed independently and
combined on a probability basis into an all-season flood frequency curve. This is not possible so,
as a practical matter, the peak annual events that were recorded at each of the gaging stations,
whether exclusive snowmelt or with some rainfall contribution, were analyzed and their
frequency determined for application to this study. The streamflow records reflect the flood
history at those stations.

Regression Analyses

Discharges for specific frequencies are needed for performing hydraulic analyses in the ungaged
reaches that are part of this: study. Discharges that were determined for 10-, 50-, 100- and 500-
year events at each of the 20 stream gaging stations are presented in Table 3. The computer
program, Excel, was used to relate discharges for each of these events to their drainage areas.
They were subjected to regression analyses using a log-log relationship. Drainage area was
adopted as the only predictor because previous studies by GAl indicated that additional
predictors did not improve the correlation coefficient.

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