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

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Included in the Technical Appendix, under separate cover, are the detailed
computations of the hydraulic calculations.

The survey bench mark descriptions and elevations used in obtaining the cross
section data for Coal Creek were:

a. U.S. Coast and Geodetic Survey, 1934, No. Y 157 Located behind
the Old Jail House at Elevation 8908.699

b. U.S. Coast and Geodetic Survey, 1934, No. Z 157 Located at the
High School at Elevation 8885.343

As shown on sheet 3 and 4, the banks of Coal Creek upstream of Elk Avenue
cannot convey 805 cfs of flow and split flows occur. The HEC-2 computer
program utilized the split flow analysis to determine the amount of the 100 year
flow leaving the left and right banks of Coal Creek. The split flow analysis found
that between station 51 +32 and station 51 +50 (cross sections 36 and 37),43 cfs
of flow leaves Coal Creeks northern embankment (left bank). The split flow
analysis further found that from station 51 +89 to station 53+94 (cross sections
38 through 42),86 cfs of flow leaves Coal Creeks southeastern embankment (right
bank). The HEC-2 computer program automatically subtracted the split flow loss
from the 805 cfs initial flow, thus 676 cfs of flow continued downstream below
station 51 +32 (section 36) in Coal Creek.

The remaining 676 cfs of flow in Coal Creek does not leave the banks of the creek
as it is conveyed downstream, through the Town of Crested Butte. The split flows
occurring through the Town of Crested Butte are upstream of Elk Avenue.
Ponding occurs at Elk Avenue against the pedestrian bridge on the south side of
the street, but does not spill onto Elk Avenue at the Coal Creek crossing. This
study is an existing floodplain analysis, and therefore routing of 805 cfs of flow
completely through town is not a part of this analysis. If Coal Creek were
improved to convey 805 cfs of flow downstream to Elk Avenue, Elk Avenue and
various other restrictions downstream, through the Town of Crested Butte, would
cause additional flow splits due to restricted openings and low channel
embankments.

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