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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From this comparison of peak flows it could be concluded that during peak flow per-
iods the variation of these flows within the study reach are minimal. Therefore, in the hy-
draulics analysis of this study the IOO-year flows used to delineate the flood plains will be
varied within the study reaches only where applicable. The combined IOO-year flow for
both Tomichi Creek and Gunnison River will be used downstream af their confluence and im-
mediately upstream of the confluence in case of overlapping between the two floodplains.
Also, the IOO-year flows in the study reach upstream from Ohio Creek confluence with Gun-
nison River will be adjusted to reflect the adsence of Ohio Creek flows. Examples for fre-
quency distributions of peak flows of Gunnison River near Gunnison and Tomichi Creek at
Gunnison are shown in the Appendix.

SCS TRIANGULAR HYDROGRAPH METHOD

Results obtained by the "Unifon1l Technique for Determining Flood Flow Frequen-

cies" method for the different frequencies were checked by the use of S.C.S. triangular unit

hydrograph method as described in Chapter 16 of the S.C.S. National Engineering Handbook,

Section 4, Hydrology. The input necessary for this procedure include storm distribution

curve, S.C.S. runoff curve number, basin area, time of concentration, time increment for

hydrograph computation, storm depth and maximum desi red time of hydrograph base. These

parameters were computed for all 16 sub-basins within the study area.

Storm hydrographs for the Gunnison River near Gunnison, Tomichi Creek at Gunni-

son, and the five sub-basins in Tomichi Creek basin (T-l through T-5) were next computed.

Peak flows for the IOO-year frequency resulting from this analysis are presented in Table 12.

This table also shows the comparable figures obtained by the Log Pearson Type III frequency

distribution method.

Examples uti I izi ng th is procedure to compute peak discharges and an explanation

of the procedure itself is presented in the Appendix.

RUNOFF VOLUME

To obtain the volume of runoff resulting from peak flow periods it is necessary to

define the shape of the peak hydrograph generating these peak flows.

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