FLOOD04808

• County: Summit
• Community: Breckenridge
• Stream Name: Blue River, French, Illinois, Lehman Gulches
• Basin: Colorado Mainstem
• Title: Technical Addendum Floodplain Information Breckenridge
• Date: 10/1/1975
• Prepared For: Summit County
• Prepared By: Leonard Rice Consulting Water Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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- 12 -

Each flood hydrograph for the Blue River was routed through the
Goose Pasture Tarn. The routed flood hydrograph was added to
the base flow in the Blue River and that hydrograph was used
in the hydraulic computations to determine the lateral extent
of flooding.

Each flood hydrograph was routed through the Goose Pasture Tarn.
The routed flood hydrograph was added to the base flow in the
Blue River and that hydrograph was used in the hydraulic compu-
tations to determine the lateral extent of flooding.

Other techniques used to reduce the basic precipitation and runoff
data in understanding the hydrology of the Breckenridge area
include the Pearson Log III, Hazen and snowmelt analysis. The
three methods are statistical in nature, differing primarily in
their techniques in determining the skew coefficients. The
Pearson Log III and the Hazen methods reduce the peak discharges
recorded at the Hoosier Pass snow course station and the peak
discharge for the s~e year. The Pearson Log III Analysis was
run on a number of,streams in the area on which USGS records are
maintained. The results are shown in Table 5.

The primary drawback to the statistical methods is that the USGS gage
is located downstream from Breckenridge and encompasses a much
larger area than what would affect the town of Breckenridge. To
reduce the statistical flows it would be necessary to derive a
valid, area reduction factor. Application of the SCS technique
which defines an area reduction factor raised to the 0.8 power
results in peak flows through the town of Breckenridge for the
statistical methods to range from 560 cfs to 890 cfs.

DESIGN RUNOFF

Table 6 summarizes the flows at the various design points along
the Blue'River derived by MITCAT, which modeled the thunderstorm
condition, and Pearson Log III analysis, which was the statistical
method. Also shown is the runoff from a storm centered over
various tributaries below the Tarn summarized from Figure 2. The
design flow selected for each frequency interval has been underlined.

Using the design flows in Table 6 discharge-frequency curves were
prepared for each design point. These are shown in Figures 3
through 9. Figure 10 is the discharge-frequency curve for the
tributaries.

~ Leonard Rice Consulting Water Engineers, Inc.