FLOOD10330

• County: Summit
• Community: Silverthorne
• Stream Name: Blue River, Straight Creek, Willow Creek
• Title: Flood Insurance Restudy, Technical Support Data Notebook, Silverthorne, Summit County, Colorado
• Date: 1/16/1998
• Prepared For: Silverthorne
• Prepared By: Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Creek is a piedmont area with a well defined stream channel and poorly defined
overbanks. Statistics for each model are summarized in Table 9.

Table 9. FrS Update Streams

Sections
Stream Length Modeled Old! New Bridges Tribs
(feet) Interp. Survey
Blue River 35243 108 38 70 7 5
Straight Cr. 2882 61 48 13 5 0
Willow Cr. 8867 128 106 22 4 0

The grade of the stream channels is steep and the flow regime tends toward critical
flow. Analysis of the water surface was conducted using the HEC-2 program for the
Blue River in subcritic:al mode. The tributaries, which are steeper than the Blue River,
were analyzed using the HEC-RAS program in mixed regime mode.

The Manning's "n'" for the streams was estimated at 0.040 for the main channel and
0.100 for the floodplains. The majority of the sections in each stream were modeled as
simple riverine cross-sections with a main channel and adjacent overbank floodplain. In
a few cases on the Blue River, a section would contain a shallow island that was
submerged at flood stage. This type of cross section was modeled using a horizontal
variation in roughness (the HEC-2 NH method). The channel roughness values are the
same as previously used in the FrS, the values also agree with measurements of stream
channel roughness made by S&A (1986, the study included reaches of the Blue River).
Overbank areas primarily consists of dense riparian vegetation (typically willows) except
where land developme:nt has cleared the ground.

The downstream boundary condition for each stream was determined from a normal
depth rating. The most downstream cross section was used with estimated "n" value for
the channel and floodplain. Table 10 summarizes information used in development of the
downstream rating for each stream.

Table 1 O. FrS Update Downstream Rating Information
Roughness
Channel Over-
bank
0.100
0.100
0.100

Stream

Section
ID
1
o
o

Energy Grade

Blue River
Straight Cr.
Willow Cr.

0.040
0.040
0.040

0.00806
0.0166
0.0308

Bridges were modeled within the streams. Within the Town of Silverthorne, there are
seven bridges across the Blue River, five bridges across Straight Creek, and four bridges
across Willow Creek. The 1-70 crossing of the Blue River was not included in the Blue
River model. This bridge basically spans the entire valley of the Blue River and has little
influence on the water surface profile during flood flow. Table 11 summarizes bridge
characteristics.