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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2. Straight Creek (18.3 sq. mi. at Laskey Gulch, approximately the upstream limit of
the study reach, and 19.7 sq. mi. at the confluence with the Blue River),

3. Willow Creek (12.9 sq. mi. at confluence with the Blue River), and

4. Unnamed tributary of Willow Creek (0.5 sq. mi. at confluence with Willow
Creek).

Neither of the tributaries, Willow Cr. or Straight Cr. at Laskey Gulch, have a long
enough gage record is adequate for a statistical analysis. Willow Cr. had a gage that
operated for ten years, and Straight Cr. at Laskey Gulch has a gage that has operated
since 1986 (eight years of record as of 1994). The regional regression relationships are
recommended for basins over five (5.0) square miles. The two smaller tributaries, Split
Creek and Unnamed Willow Creek tributary, are too small for regression analysis. These
tributaries were analyzed using standard rainfall-runoff analysis.

Table 4 summarizes the flood frequency analysis for each gaged tributary using
equations 1-4.

Table 4. Blue River Tributaries -
Flood Frequt~ncy Analysis using Regional Regression Equations
Tributary Area QIO Qso QIOO Qsoo
Straight Cr. at Blue River 19.7 393 504 550 652
Straight Cr. at Laskey Gulch 18.3 374 478 522 619
Willow Cr. at Blue River 13.9 309 394 430 508

Flows for the Unnamed Willow Creek tributary and Split Creek were estimated using
NRCS methodology as described in "Procedures for Determining Peak Flows in
Colorado" (USDA, SCS, March 1984). Input data for this analysis consists of soil data,
24-hour rainfall depth/frequency data, storm pattern, and travel time.

Soils information was obtained from the Town of Silverthorne drainage master plan
(McLaughlin Water Engineers, 1983). Rainfall depth/frequency data was obtained from
the NOAA Atlas for Colorado ("Precipitation Frequency Atlas of the Western United
States, Volume 3, Colorado", National Oceanic and Atmospheric Administration, 1973)
using the supplemental maps for the May-November period. This May - November
period represent mostly rain events although large snowfall can occur as late as May in
Summit County. The SCS travel time method was used for estimation of runoff time.
Travel time is the sum of overland flow time, shallow concentrated flow time, and
channel flow time. According to this method, the hydrograph lag time, Tlag, is 0.6T travel.
The various flow paths were determined from USGS maps (7.5-minute quadrangles) for
each area.

Table 5 summariz~:s the basin characteristics for each tributary. The watershed areas
were subdivided into upper and lower basins, since the lower basins in each case tend to
be partially developed and the upper basin is within Forest Service boundaries. Soil
conditions are similar in both the upper and lower basins, curve numbers were adjusted
upward (TR-55, Figure 2.2) to account for the effect of development. A conservative
estimate of impervious area was made at 40 percent for the lower subbasins.