FLOOD08558

• County: Eagle
• Community: Gypsum
• Stream Name: Eagle River
• Basin: Colorado Mainstem
• Title: Hydrology Addendum FIS - Town of Gypsum, Eagle County
• Date: 10/1/2002
• Prepared For: CWCB
• Prepared By: Montgomery Watson Harza
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hvdrolol!:ic Analvsis
The Town of Gypsum FIS presents both detailed and approximate floodplain delineations.
Detailed delineations were developed using standard hydrology methods and are mostly well
documented. The profiles in the FIS for Eagle River and Gypsum Creek were verified to match
the basin floodplain elevations shown on the FIRM map dated September 30, 1983.

The FIS provides detailed discharges for both Eagle River and Gypsum Creek. These figures are
shown in Table I.

Table 1.

Flooding Source
Ea Ie River
G sum Creek

Drainage Area Description
at west Town ofG sum limits
at mouth

Summar of Dischar es iven in FIS re ort

Drainage
Area (mi2)
944
100

100-year
Discharge (cfs)
8,223
1,950

The CWCB regional regression equation methodology is fully described in a publication by the
CWCB entitled "Guidelines for Determining 100-year Flood Flows for Approximate Floodplains
in Colorado". The CWCB regression equation for this region is shown in Table 2. For the two
streams that are not shown in the FlS and are included in the designation, Hardscrabble Gulch
and Unnamed Tributary, the CWCB Regional Regression Equations were used.

Sub-
Re ion ID

Sub-Re ion Name
Upper Colorado River

I to 950

COL-I

For the equation in Table 2, the QIOO is the 100-year peak flow in cubic feet per second, and A is
the upstream (contributing) drainage area in square miles. The regression equations are only valid
for the range of drainage areas as shown. Calculation worksheet is shown in the appendix. The
100-year peak discharge for each of the undeveloped approximate floodplains is shown in Table
3.

a e ummary 0 a cu a e ISC arl!:es
Drainage IOO-year
Flooding Source Drainage Area Description Area (mi2) Discharge (cfs)
Hardscrabble Gulch south and east of Gypsum Creek 6 293
Unnamed Tributary west of Gypsum I 85

T bl 3 S

fC I I t d D' h

Peak flow records are available at USGS streamflow gages on the Eagle River and Gypsum
Creek within the Town. However, since the FIS performed a detailed study of the discharges
from Eagle River and Gypsum Creek gages, further analysis was not included in this report.

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