FLOOD04209

• County: Gunnison
• Community: Gunnison
• Stream Name: Gunnison River, Tomichi Creek
• Basin: Gunnison
• Title: Floodplain Information Study
• Date: 1/12/1976
• Prepared For: Gunnison County
• Prepared By: Engineering Consultants
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ABSTRACT

This draft report presents the results of the hydrologic analysis necessary to prepare
the flood plain information study delineating the 100-year flood plain of Gunnison River and
Tomichi Creek in the vicinity of Gunnison, Colorado. This wark is performed according to
the scope of services outlined in the contract and it includes: determination af the basin and
sub-basin parameters; compilation and analysis of precipitation and peak streamflow data;
calculation of the 10-, 50-, 100-, and 500-year frequency peak discharges in the study
reach and the resulting runoff volume therefrom; and reporting of the observations of the ice
jamming problems experienced on the Gunnison River during the 1970 - 1974 period.

The drainage area above the study reach was divided into 15 sub-basins. The size,
length, slope, aspect, elevation (lower/upper), hydrolagic soil group and vegetative cover
for each sub-basin were determined.

Depth-Duration-Frequency curves for Gunnison River and Tomichi Creek water-
sheds above the study area were developed from data published in N.O.A.A. Atlas 2 and
T .P. 40. Hourly precipitation data from the Gunnison meteorological station were utilized
to check the accuracy of the data obtai ned from these curves. These curves were uti I i zed
for design storm to check the design discharges from individual sub-basins. Peak runoff in
the study area was found to be primarily a function of snowmelt. Further, it was concluded
that major storms occurring in San Juan basin did not produce corresponding results in Gun-
nison River basin.

Streamflow data for 51 gaging stations from within the Gunnison basin and in the
nearby basins were obtained and analyzed. From these records all correlations and data
extentions were performed. A total of 32 correlations of peak flow data were conducted,
and those resulting in high coefficient of correlations were used to extend the historical
records. Frequency distributions of the recorded and extended records for 18 gaging sta-
tions were performed utilizing the Log Pearson Type III method. From these distributions
the 10-, 50-, 100-, and 500-years peak flows at these locations were obtained. Hydro-
graphs associated with these peak flows were developed and used to determine the runoff
volume resulting therefrom. Results obtained by the "Uniform Technique for Determining
Flood Flow Frequencies" method for different frequencies were checked by the use of
S.C.S. triangular unit hydrograph method. Also, the impact of the Taylor Park Reservoir
on the peak flows of Gunnison River was determined.

The IOO-year design peak flows for the Gunnison River near Gunnison and To-
michi Creek at Gunnison are 8,930 cfs and 2,050 cfs respectively.

The discussion on ice jamming problems in Gunnison River was limited to his-
torical observation of the problem. These observations were conducted by the U.S.
Bureau of Reclamation and cover the 1970 - 1974 period.

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