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

In a letter dated March 5, 1985 from Mr. Larry Lang to Mr. Doug Gore at FEMA,
Region VIII, the Colorado Water Conservation Board concluded that the
appropriate range for a 100 year flow in Coal Creek at Crested Butte should be
750 to 800 cfs. Based on the findings in Method 3 above and the findings in 1985
by the Colorado Water Conservation Board, a 100 year peak flow value of 805 cfs
for Coal Creek at Crested Butte was selected for this study.

IV. HYDRAULIC ANALYSIS

FLOODPLAIN DELINEATION

The hydraulic analysis included computation of the water surface profile of Coal
Creek through the Town of Crested Butte. Water surface profiles were determined
using 49 channel cross sections, and the Corps of Engineers HEC-2 computer
program. Results of the 100 year flood computations from HEC-2 are shown in
Table 1. . The 100 year flood profiles and cross section locations are shown on
sheets 8 through 10. The cross sections used were obtained from a field survey
by RBD Inc. in May 1992. The location of the cross sections are shown on the
Floodplain delineation sheets 3 through 5. The contour base mapping utilized on
sheets 2 through 5 was obtained from the Colorado Water Conservation Board
and was also 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
floodplain delineation shown on sheets 3 through 5 was based on the individual
cross sections and not the base map contours. New base mapping of Coal Creek
through Crested Butte would need to be obtained in order for the cross section
water surface elevations to match the base mapping contours.

The water surface elevations presented in Table 1 are based on the HEC-2
computations assuming no reduction in the channel or hydraulic structure
conveyance capabilities due to debris accumulation. Peak runoff flows, through
the Town of Crested Butte, begin between 4 and 5 P.M. and remain until dark
each day of the runoff. Debris can accumulate every day that the peak flow
exceeds that of the previous day. Because the peak runoff flows begin between
4 and 5 P.M. and last until dark, we have assumed that the Town of Crested Butte
maintenance staff keep the debris clear of obstructions during the high flow
periods each summer. Therefore we have modeled Coal Creek through the Town
of Crested Butte assuming no debris accumulation in the channel or at hydraulic
structures. A channel roughness coefficient of 0.038, through the Town of
Crested, was utilized in the hydraulic computations. The value of 0.038 was
confirmed by actual field measurements upstream of the Maroon Street bridge on
June 15, 1978, according to the Memorandum from the Colorado Water
Conservation Board to the Town of Crested Butte dated August 24, 1978.

4