FLOOD07328

• County: Summit
• Community: Summit County and Unincorporated Areas
• Basin: Colorado Mainstem
• Title: FIS - Summit County and Unincorporated Areas
• Date: 8/9/2001
• Prepared For: Summit County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

The hydraulic analyses for Blue River Middle Branch and Cucumber and
Illinois Gulches were computed using the USACE HEC-2 computer program
(Reference 33). Starting water-surface elevations were determined by
the slope-area method or predetermined elevations.

Cross-section and geometric data, such as culvert sizes, inverts, bridge
dimensions, and roadway elevations, were obtained from a variety of
sources. Information from the original Flood Insurance Study for the
unincorporated areas of Summit County (Reference 41), served as a base.
Additionally, cross sections were surveyed specifically for this
restudy, particularly in areas where the topography was known to have
changed. Restudies of the flood plain had been conducted for recent
developments in the Town of Breckenridge. Several short sections of
river had been surveyed for projects such as pipeline crossings or new
bridge construction. This information was also incorporated into the
restudy. Finally, the 1980 Breckenridge Town Topographic Mapping was
used in areas where it was known that conditions changed between the
time of the original study and production of the Breckenridge Town
Topographic Mapping,

To determine channel and overbank roughness coefficients (Manning's "nil
values), an assessment of the conditions along each channel was made.
In areas where channel alterations had not been made, the general, main
channel "n" values ranged between 0.0035 and 0.0050. Overbank "n"
values ranged from 0.0350 to 0.1000,

The Flood Insurance Study for the Town of Breckenridge. Colorado
(Reference 42), was being updated concurrently with this restudy, These
studies are in agreement with each other.

Table 1, "Summary of Discharges," Table 2, "Floodway Data,lI and
Exhibit 1, "Flood Profiles, 1 were revised to reflect changes as a result
of the restudy,

Elue River

This study was revised on August 9, 2001, to show the effects of
detailed hydraulic analyses for the following streams: 31ue River
within the corporate limits of the Town of Silverthorne; and Willow
Creek, from west of the upstream corporate limits of the Town of
Silverthorne to its confluence with the Blue River {beyond the
downstream corporate limits).

Those areas studied by detailed methods were chosen with consideration
given to all proposed construction and forecasted development through
1998.

The hydrologic and hydraulic analyses for this study were performed by
Simons and Associates, Inc., for the Federal Emergency Management Agency
(FEMA) , under Contract No, EMD97-CO-0031. This study was completed in
October 1998,

Community base map selection and the identification of streams requiring
detailed study was accomplished in a meeting attended by representatives
from FEMA, the study contractor, the Colorado Water Conservation Board,
and the Town of Silverthorne on February 6, 1997. Summit County also
provided assistance in gathering information for this report.

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