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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METHOD 1 - UTILIZING STREAM FLOW REGRESSION RELATIONS FOR THE
MOUNTAIN REGION

From the reference titled "Estimation of Natural Streamflow Characteristics in
Western Colorado", a summary of regression relations for various mountain
regions and related regression models is provided. For the Coal Creek Basin, the
parameters of drainage area and overall basin slope were required in the
regression model. The overall basin slope was determined by averaging twenty
five equally spaced grid slopes within the Coal Creek Basin. Given the basin size
of 21 square miles and the average basin slope of 0.385 ftlft, the regression model
yielded a 100 year peak flow in Coal Creek of 536 cis.

METHOD 2 - FLOOD FLOW FREQUENCY ANALYSIS FITTING THE LOG-
PEARSON TYPE III DISTRIBUTION

Stream flow records available for Coal Creek include the period 1943 - 1946.
These records were measured at a U.S.G.S. gaging station upstream from the
Town of Crested Butte. The gaging station had a drainage area of approximately
8.5 square miles upstream. The reference manual utilized was "Guidelines for
Determining Flood Flow Frequency" Bulletin # 17B. The 1943 - 1946 gage data
was adjusted on a straight line proration basis for the entire 21 square mile Coal
Creek Basin. The two additional years of field measured data for 1978 and 1979
was combined with the gaging data and the data base for the updated Log-
Pearson Type III Distribution was achieved. The Flood Flow Frequency Analysis
computer model was run with six years of data and the model yielded a 100 year
peak flow in Coal Creek of 815 cis.

METHOD 3 - UTILIZING ADJACENT DRAINAGE BASIN INFORMATION

To verify that 815 cis of flow as derived in Method 2 was reasonable or required
adjustment, various other basins were selected in the central and southwest
mountain areas of Colorado with longer gage records and better flow projections.
For the selected streams, plus Coal Creek at Crested Butte, the multiple linear
regression analysis computer program Minitab was utilized to obtain a best fit line
through the data. The three variables used in the analysis were basin slope, basin
area, and discharge per square mile. The variables used were based on snow
melt peaks for elevations above 9000 feet. Basins with somewhat similar
orientation, a variety of basin slopes but not extreme slopes, and a substantial area
above 10,000 feet in elevation, were utilized. The resultant regression equation
yielded a 100 year peak flow in Coal Creek, of 805 cis.

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