PROJ00553

• Contract/PO #: FS0022X
• Contractor Name: West Anvil Water & Power Company
• Contract Type: Loan
• Water District: 0
• County: Garfield
• Loan Projects - Doc Type: Feasibility Study

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from the Una Reservoir, considering the nature of the
flood hydrographs. The spillway wi 11, therefore, be
designed for the peak inflow.

In order to establish values for both the IDF and the PMF at the

Webster Hill project site, it was decided to use prorated values

of the 500-year flood event and the PMF at the Una site, with

the adjustment being based upon the difference in drainage

areas.

3.

IDF and PMF Calculations for Webster Hill Reservoir
The 1DF for Webster Hill was estimated in two ways -- by
multiplying the 500-year flood event at the Una site
(44,000 cfs) by (1) the direct ratio of the drainage areas,
equal to 0.95, and (2) the square root of the ratio, equal to
0.97. Values were converted to cfs/sq mi (cubic feet per second
per square mile) for comparison. Similarly, the PMF at the Una
site (130,000 cfs) was prorated for the Webster Hill site.

At the Una site, the PMF of 130,000 cfs equaled 17.64 cfs/sq mi.
Prorating by the direct ratio resulted in 17 .69 cfs/sq mi at
Webster Hill; using the square root of the ratio, the value was
18.06 cfs/sq mi. Oue to the close agreement achieved by using
the direct ratio, a PMF value of 17.69 cfs/sq mi for the
drainage area of 6,983 sq mi at Webster Hill was selected. This
results in a value of 123,500 cfs for a project PMF. The
Webster Hill dam will be designed to safely pass the project PMF
by overtopping the structure with a depth of flow over the crest
of approximately 10 ft.

The spillway will be designed to pass an IDF equal to the 500-
year flood flow of 41,800 cfs at a reservoir pool elevation of
5,255 ft msl.

The term "x-year flood" is used extensively in water resources
1 iterature, and an estimate of the magnitude of such events is
necessary to estab 1 i sh the des i gn parameters of a project. The

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