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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August (DMJM, 1975). The rainfall that occurs during these months usually has a minor
effect on the runoff during the period before the peak. After the snowmelt peak, rainfall
usually increases the runoff, but rarely causes flooding (Gingery, 1978b).

There have been no serious flood problems in Silverthorne since the Dillon Reservoir
began operating in 1963. The highest discharge has been 1800 cubic feet per second,
which is less than the 10-year flood. The longest flood on record prior to the construction
of the dam was in 1918. The combined flow of the Blue River, Ten Mile Creek, and the
Snake River was 3500 cubic feet per second, just upstream from the present location of
Silverthorne. According to U.S. Geological Survey records, high flows have been
recorded during May and June of several other years on all three streams. However, no
significant damage was reported. The largest flood in the area occurred in 1965, but
Silverthorne did not feel its impact due to the mitigating effects of Dillon Reservoir
(DMJM, 1975; USGS,. 1966).

Floodwaters can breach a private road to the south of Willow Creek, causing a
ponding effect along State Highway 9.

2.4 Flood Protection Measures

Dillon Dam regulates the discharges ofthe Blue River at Silverthorne and Reservoir
located approximately 0.5 mile upstream of the southern corporate limits. The reservoir
storage significantly reduces the peak discharges of the frequent floods. It is observed in
the Blue River hydrology that, for the infrequent flood events under consideration here,
the reduction in flood peaks due to reservoir attenuation is approximately 10 to 20
percent of the peak discharge.

3.0 ENGINEERING METHODS

For the flooding sources studied in detail in the community, standard hydrologic and
hydraulic study methods were used to determine the flood hazard data required for this
study. Flood events of a magnitude which are expected to be equaled or exceeded once
on the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have
been selected as having special significance for flood plain management and for flood
insurance premium rates. These events, commonly termed the 10-,50-, 100-, and 500-
year floods, have a 10,. 2, 1, and 0.2 percent chance, respectively, of being equaled or
exceeded during any year. Although the recurrence interval represents the long-term
average period between floods of a specific magnitude, rare floods could occur at short
intervals or even within the same year. The risk of experiencing a rare flood increases
when periods greater than 1 year are considered. For example, the risk of having a flood
which equals or exceeds the 100-year flood (I percent chance of annual occurrence) in
any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the
risk increases to approximately 60 percent (6 in 10). The analyses reported here reflect
flooding potentials based on conditions existing in the community at the time of
completion of this study. Maps and flood elevations will be amended periodically to
reflect future changes.

3.1 Hydrologic Analyses