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

During the course of the study, hydrologic procedures, flood elevations,
flood boundaries, and floodway dilineations were reviewed with officials
of Colorado Water Conservation Board (Reference 43) and FEMA,

The results of the study were reviewed at the final Consultation
Coordination Officer meeting held on June 22, 2000, and attended by
representatives of FEMA, the State of Colorado, and the Town of
Silverthorne. All problems raised at that meeting, and by community
comments received November 23, 2000, have been addressed in this study.

In order to determine the peak flows for the Blue River below Dillon
Dam, the current gage record (through water year 1994) was compared to
the flood frequency values provided in the 1978 FIS report for the Town
of Silverthorne (Reference 50), by a statistical analysis of the USGS
gage (09050700) below the dam (Reference 441 using the HEC-FFA program,
It was found that there was not a significant difference between the
values so the peak flows from the 1978 FIS flood frequency values were
used. The peak flows for Willow Creek were developed using the USGS
regional regression equations (Reference 45).

Peak discharge-drainage area relationships for the Blue River, and
Willow Creek are shown in Table 1, "Surrunary of Discharges."

Water-surface elevations of floods of the selected recurrence intervals
for the Blue River were computed through use of the USACE HEC-2
step-backwater computer program (Reference 46). Water-surface
elevations of floods of the selected recurrence intervals for the Willow
Creek were computed through use of the USACE HEC-RAS step-backwater
computer program (Reference 47).

Cross sections for the backwater computations and flood plain
delineations for Blue River, Straight Creek, and Willow Creek were
obtained from topographic maps and cross section surveys. The Town of
Silverthorne Geographic Information System (GIS) (Reference 48) coverage
for topography (1994) was combined with cross sections obtained by field
survey for a complete description of the flood plain,

All bridges were field checked to obtain elevation data and structural
geometry,

Roughness factors (Manning's "n") used in the hydraulic computations
were chosen by engineering judgment and based on field observation of
the streams and flood plain areas. A Manning's Ilnll value of 0.040 was
selected for the main channels of Blue River, and Willow Creek, and the
"n" value in the overbank was determined to be 0.100.

Starting water-surface elevations for Blue River, and Willow Creek were
calculated using the slope-area method.

All elevations are referenced to the National Geodetic Vertical Datum of
1929 (NGVD). Elevation reference marks and their descriptions are shown
on the Flood Insurance Rate Maps (Exhibit 2) .

For each stream studied in detail, the boundaries of the floodway,
100-year, and 500-year floods, have been delineated using the flood
elevations determined at each cross section. Between cross sections,

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