PROJ00492

• Contract/PO #: C153364
• Contractor Name: Beck, R. W. and Associates
• Water District: 0
• County: Garfield
• Bill Number: SB 81-439
• Loan Projects - Doc Type: Feasibility Study

5,271
3,470
3,675
3,970
7,765

72, 000
38,960
41,510
45,880
84,200

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lies above EI 9000. The portion of that peak flow attributable to snowmelt is
approximately 25,000 cfs and the major portion of the snow pack is located at
elevations above 9000 feet. The typical May 1 snowpack water equivalent for
that basin above 9,000 feet is 13 inches. The 72-hour PMP for that basin as
estimated froDl HMR49 is 6.3 inches. The estimated 500-year flood peak is
25,400 cfs and the ratio of the PMF to the 500-year flood is 2.83. The simi-
larity between the spring runoff hydrographs on the Colorado and Gunnison
Ri vel'S is illu;~trated in Fig. II-5 by the 1957 flood hydrographs.

4. Des:lgn Flood Estimates for
Oth,~r Gunnison River Projects

Des~~gn flood estimates for USBR projects located on the Gunnison
(16)(20)
River compare to those for the TM.-County Project estimated by Beck

as follows:

ProJect

TM.-Count:,
BI ue Mesa
Morrow Poj~nt
Crystal
Daninguez

Drainage Area (mi 2)

Estimated Design Flood
Peak Flow (cfs)

This canparison is shown graphically on the USER Peak Discharge Curve
(Fig. II-6).

5. PMF Calculation for Una Reservoir

The peak flow for the PMF was calculated in two slightly different
ways and then the res ults of the two methods were averaged. The average was
then checked for reasonableness by three other estimation methods.

The two primary estimation methods are based on: (1) adjustment of
the portion of the Tri-County PMF peak flow attM. butable to snownelt by the
direct ratio of the Una versus the Tri-County drainage areas above EI 9000;
and (2) adjustment of the snownelt peak by the direct ratio of the average

II-14