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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Blue River downstream of Dillon Dam

The flood flow of the Blue River will be increase by flows from several tributaries
through the study reach. Table 3 summarizes the tributaries that contribute to the Blue
River from Dillon Dam to Maryland Creek.

Table 3. Blue River Tributaries - Dillon Dam to Maryland Creek
Blue River Reach Tributary Tributary Area
below Dillon Dam
above Straight Cr.

Bushee Cr.

1.1
19.7
6.9
13.9
3.4
1.5
2.3
6.5
4.4

Basin Area
335.0
336.1
355.8
362.7
376.6
380.0
381.5
383.8
390.3
394.7

Straight Cr.

above Willow Cr.

Willow Cr.

above Hamilton Cr.

Hamilton Cr.

above Bushee Cr.

above Maryland Cr.

The FIS reports similar basin areas (rounded to the nearest square mile) ending at
Willow Creek (377 sq, mi.). The regional relationships developed by the USGS for
Summit County show a nonlinear relationship for flood discharge where:

QIO = 23.0 Ao.68s (S + 1.0l.364 (1)

Qso = 34.5 A 0.700 (S + 1.0)1.768 (2)

QIOO = 39.5 A0706 (S + 1.0{S77 (3)

Qsoo = 51.5 Ao.7ls (S + 1.0{209 (4)

In these equations, Qf is the peak discharge in cfs for flood frequency, f, A is the
basin area in square miles, and S is the'mean basin slope. Based on previous flood
frequency analysis for the FIS we estimated the mean basin slope at 0.40 for the Blue
River above Dillon Dam. Since the terrain of the study area tributaries is similar to the
basins above Dillon Dam the mean basin slope was assumed to be constant for the update
study.

A linear relationship that increases discharge proportional to basin area (10-year flood
at 5.5 cfs/sq.mi., 50-yr flood at 7.4 cfs/sq.mi., 100-year flood at 8.3 cfs/sq.mi., and 500-
year flood at 10.4 cfs/sq.mi.) can be used in place of the more complex non-linear
equations. Figure 2 shows a comparison between the linear and non-linear approaches to
scaling basin flows. As can be seen, the two methods produce similar results. It is
recommended that for ease of use within the Blue River that the linear method be used.

Blue River Tributaries
Four tributaries are included in the FIS:
1. Split Creek (1.1 sq. mi.),