FLOOD05487

• County: Gunnison
• Community: Crested Butte
• Stream Name: Coal Creek
• Basin: Gunnison
• Title: Floodplain Information Report
• Date: 9/18/1992
• Prepared For: Crested Butte
• Prepared By: RBD, Inc.
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

encroachment was established by reducing the existing Coal Creek width an equal
distance from both top of existing bank locations using HEC-2 Encroachment
Method 1. Cross sections used were those obtained from a field survey by RBD
Inc.. The location of the cross sections are shown on the Floodplain delineation
sheets 4 and 5. The contour base mapping utilized on sheets 4 and 5 was
obtained from the Colorado Water Conservation Board and was used in the 1976
Floodplain Information Report on Coal Creek by Engineering Consultants, Inc..
Regrading adjacent to Coal Creek, through the Town of Crested Butte, has
occurred since the original report in 1976. The floodway delineation shown on
sheets 4 and 5 was based on the individual cross sections and not the base map
contours. The floodway delineation has been referred to on the exhibit sheets as
the Proposed Floodway only for use by the Town of Crested Butte in the possible
event this reach of Coal Creek is improved at one time. Included in the Technical
Appendix, under separate cover, are the detailed computations of these hydraulic
calculations.

The f100dway delineation and water surface elevations presented in Table 2 are
based on the HEC-2 computations assuming no reduction in the channel or
hydraulic structure conveyance capabilities, due to debris accumulation, as in the
floodplain delineation.

TABLE 2
100 YEAR FLOODWAY DELINEATION
-----------------------------------------------------------
Cross Cross Floodway Water Surface Water Surface
Section Section Width Elevation with Elevation with Diff.
Number Station (feet) Floodway out Floodway (ft.)
------------------------------------------------------------
8 27+88 N/A 8866.6 8866.6 0.0
9 29+ 75 33 8870.0 8869.3 0.7
10 32+44 31 8874.6 8873.6 1.0
11 34+ 71 29 8877.8 8877.0 0.8
12 36+51 29 8881.3 8880.4 0.9
13 39+23 29 8885.6 8884.7 0.9
14 39+90 29 8887.2 8887.0 0.2
15 40+60 N/A 8888.9 8888.3 0.6
16 42+47 N/A 8890.8 8890.9 -,1
17 42+55 N/A 8890.5 8890.6 -.1
18 43+32 N/A 8892.1 8892.1 0.0

8