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

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

COE computations have been around for a long time and are consistent with current developments
such as design of Colorado State Highway bridges. It is recommended that the COE flood frequency
values computed for the study reach be used in the current study. The recommended COE peak flow
values for frequencies required for a detailed study pertinent to the Roaring Fork reach are shown
in Table 4. Peak flow values calculated following Bulletin 17B Guidelines are included in Table
4 for comparison purposes.

Eagle River (From Eagle to Gypsum)

The streamgage below Gypsum (944 square miles) is located downstream from the bridge on U.S,
Highways 6 and 24 at Gypsum and is ISO feet downstream from the mouth of Gypsum Creek. Flood
flow frequency computations at the gage are based on 50 years (1947-1996) of peak flow data. A
printout of those flood frequency computations is enclosed in the Appendix.

The gage is located at the downstream end of the Eagle River study reach. Consequently, flows from
Gypsum Creek do not contribute flow to the study reach. Discharges from Gypsum Creek needed
to excluded from the discharge frequency analysis. The Eagle River drainage area at the downstream
from Gypsum station was published in Water Supply papers as being 944 square miles. An
independent determination during this study indicates an area of989 square miles, but the official
USGS figure of944 square miles will be used. The drainage area of the Gypsum Creek at the mouth
basin was determined to be 136 square miles. The basin map of the drainage area is shown in
Figure 5.

Peak discharges for the Eagle River upstream of Gypsum Creek were estimated by adjusting
discharges computed at the gaging station for each frequency on a drainage area basis. The ratio of
the contributing area with and without Gypsum Creek (808/944) was raised to the 0.7 power. An
exponent of 0.5 is sometimes used as a rule of thumb for adjusting peak flows to other sites.
However, these are snowmelt peaks where runoff volume has an important influence on the results
so a factor of 0.7 was selected as a more appropriate adjustment. The equation for adjusting values
from the gaging site to the reach upstream from Gypsum Creek is: [Discharges at Gage] [(944-
136)]/944]07 = (0.90) (Discharges at Gage),

The same equation was used for adjusting values from the gaging site to the study reach upstream
of the confluence with Brush Creek. The drainage area of Brush Creek above its confluence with
the Eagle River just downstream of the Town of Eagle is approximately ISO square miles. The
adjustment factor to the gage values is therefore [(944-286)/944]0.7 = 0.78.

A plot in logarithmic paper of the 100-year peak discharges versus drainage areas (see Table 2) for
the Eagle River is shown in Figure 6. Peak flow values obtained from the curve on Figure 6
entering the small areas upstream of Gypsum Creek (808 sq. miles) and upstream of Brush Creek
(658 sq. miles) are approximately 90% of the peak values obtained by applying to the adjustment
equation to the peak flows calculated using the Pearson Type ill distribution to the gage data below
Gypsum.

6