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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There are several reservoirs on the Eagle River which provide some incidental flood controL
Because it is likely that spillway facilities have been designed to evacuate flood flows in excess
of the 100-year discharge, it has been assumed that reservoir will not affect downstream 100-year
flood discharge, In actuality, there will be some flood flow attenuation due to surcharge capacity
in the reservoir and travel times from the reservoir inflow points to the spillway. Regardless, the
gage records analyzed above implicitly consider reservoir storage in a general matter. Additional
analysis of the reservoirs and basin would be needed to more accurately estimate peak
discharges.

Communities should regulate the approximate floodplains on a case-by-case basis. FEMA
requires that a detailed floodplain analysis must be performed by a professional engineer and
submitted by the developer or development proponent for development projects involving more 5
acres or more than 50 units (whichever is more restrictive).

Hvdraulic Analvsis
Hydraulic methods used for the approximate floodplain delineations shown on the Town of
Gypsum FIRM panels are not available or have not been documented by FEMA. For future
studies, an approximate 100-year flood elevation at a particular location on the river may be
obtained by using the 100-year peak flow value (as provided in this report) in conjunction with
an acceptable hydraulic analysis. Hydraulic analyses for determining 100-year water surface
elevations for approximate floodplains may be performed using FEMA's Quick-2 computer
program, the Corps of Engineers' HEC-2 computer program, Flowmaster, or other acceptable
hydraulic methods. Channel and floodplain cross-sections may be obtained from detailed
topographic mapping or from field surveys. Cross-sections obtained from USGS quadrangle
maps are not advised.

Floodplain Mappinl!:
The existing FIRMs for the Town of Gypsum depict areas subject to 100-year flooding based on
approximate technical methods, The Colorado Water Conservation Board can be contacted to
request technical assistance for approximate floodplain analyses as needed.

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