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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extrapolation. The curve fltting process included a least squares computation for best fit and an
upper envelope curve was drawn parallel to it 1.5 standard deviations above the mean curve.
Due to the short records, they recommended that the upper envelope curve be used in defining
peak discharges for the floodplain.

The report also included some information about rainfall and runoff from rainfall using a U.S.
Soil Conservation Service (SCS) approach and provided discharge frequency analyses for seven
small streams in Eagle County. They examined peak flow data for the entire period of record at
small area station, but failed to identify any peak flood discharge caused by runoff from rainfall.

Gingery Associates, Inc, Englewood, Colorado

Gingery Associates, Inc. (GAl) prepared a Hydrology Report for the US Department of Housing
and Urban Development, Federal Insurance Administration, that is entitled., Flood Insurance
Study for Eagle County, Colorado and is dated August 9, 1978. This report was prepared to
document the methods and procedures used to determine flood discharges for streams in Eagle
County, Colorado. In addition to Vail, discharge data for the communities of Basalt, Red Cliff,
Minturn, Eagle and Gypsum were included. The 10-,50-, 100- and 500-year frequency
discharges were determined for streams to be studied by detailed methods and only lOa-year
frequency discharges were provided for streams that were to be studied by approximate methods.

As part of their analysis of stream gaging records, the actual stage recording charts were obtained
from the USGS archives so the peak flows resulting exclusively from snowmelt could be
separated from peaks resulting from a combination of snowmelt and rainfall events. They used
the combined rainfall-snowmelt events (referred to as rainfall events in their report) in their
analysis because they represent the most severe condition. Gaging stations with 12 or more years
of record were used in the analysis. Using some short records was necessary in view of the
limited peak flow data in the region. Adopting this criteria resulted in them using 14 gages for
analysis of snowmelt and 9 gages for rainfall events.

They adopted regional skew coefficients of - 0.55 for snowmelt and - 0.25 for rainfall runoff
events. They then used the Log Pearson Type III distribution as outlined in Bulletin 17B,
Guidelines for Determining Flood Flow Frequency to calculate 10-, 50-, 100-, and 500-year
discharges at each stream gage. They used a Soil Conservation procedure outlined in US Soil
Conservation Service, Procedures for Determining Peak Flows in Colorado, dated January, 1977
for computing peak flows for drainage areas less than 2,000 acres (3.125 sq. mi.).

A regression analysis was made of the computed discharges versus physiographic and
meteorological characteristics of the basins. They examined 10 basin parameters as predictors,
but settled on drainage area alone since no significant improvement in correlation occurred as a
result of adding other basin characteristics to the regression analysis.

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