FLOOD00328

• County: Eagle
• Community: Eagle
• Stream Name: Eagle River
• Title: Hydrology Report Roaring Fork and Eagle Rivers
• Date: 12/1/1997
• Prepared For: Eagle County
• Prepared By: J.F. Sato & Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The firm of Simons, Li and Associates performed a hydrologic study for a "Garfield County
Unincorporated Areas Flood Insurance Study" (January 3,1986). This study was for a number of
streams in the County including the lower Roaring Fork River. It also separated streamflow data
into snowmelt and rainfall events. Some of those peaks were determined using USGS strip charts.
Those rainfall and snowmelt events were analyzed separately and composite curves were developed.
These studies included some drainage areas larger than the areas in this study.

The COE studies indicated above were summarized in the "Pitkin County Colorado, and
Incorporated Areas Flood Insurance Study" (June 4. 1987), Data from a large number of gaging
stations were analyzed, The gages were located in Pitkin, Summit, Eagle, Garfield, Mesa, Delta, and
Gunnison Counties. The streamflow data were separated into snowmelt and rainfall peaks and
analyzed separately.

Snowmelt peaks from a total of 40 gages and rainfall from 25 gages were analyzed to obtain
individual and regional statistical parameters of mean, standard deviation, and skew for the two types
of peaks. They found that in numerous instances, the rainfall peaks were of very small magnitude
and several streams did not show any significant peaks caused by rainfall. The two types of
distributions were combined to obtain the overall peak discharge-frequency relationship for each
stream, For ungaged streams, the values of regional statistical parameters were used and discharge,
drainage area curves were developed for specific frequencies. Those studies were mostly for smaller
drainage areas then the ones in this current study.

A summary of the results of pertinent previous flood frequency studies that have been made for the
Roaring Fork River are presented in Table 1.

Eagle River

The previous flood study of the Eagle River most pertinent to the reach included in this LMMP Task
Order is the FIS for the Town of Gypsum dated March 16,1986. Peak discharge series recorded at
the gage below Gypsum (09070000) was used in this study to determine peak flows for different
frequencies. The FIS for Eagle County, unincorporated areas, May 1980 includes other peak
discharge frequency data for the Eagle River as shown on Table 2,

III. NEW HYDROLOGIC STUDIES FOR THIS "LMMP" TASK ORDER

General

I.F. Sato and Associates performed flood frequency analyses for the Roaring Fork River gages at
Aspen and at Glenwood Springs and for the Eagle River gage downstream from Gypsum. These
analysis were made using the computer program, FFFREAK. Version 1, that was developed by US
West. The program was designed to run using peak discharge values contained in their CD
HYDRODAT A product. The program follows procedures recommended in Bulletin 17B except for

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