PROJ00090

• Contract/PO #: FS0015X
• Contractor Name: Powderhorn Metropolitan District No. 1 and No. 2
• Contract Type: Loan
• Water District: 0
• County: Mesa
• Loan Projects - Doc Type: Feasibility Study

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runoff drainage detention, raw water storage, irrigation and related uses as may be needed
throughout the year by the District.

The proposed reservoir and dam structure has been designed to provide capacity for
approximately 75 acre feet. The top 10 acre feet of volume would be reserved for storm
water detention. Therefore, the reservoir would not normally be filled to capacity and
releases would occur so as to maintain a level which would allow for approximately 10
acre feet of detention pond storage. The next 50 acre feet of active capacity would
provide for fill and releases for water augmentation. The balance of the reservoir capacity
at the lower levels would provide an operating pool of water for use within the service
area for raw water storage for water treatment, irrigation water and related uses. Some
capacity will be used for silt deposits. By maintaining an operating pool of water, it will
provide for an attractive and aesthetic condition throughout the year.

In order to provide storage for approximately 75 acre feet, the height of the dam will be
approximately 30 feet. As defined by the State of Colorado Division of Water Resources,
the dam would be classified as a small Class III jurisdictional dam. The size of the
reservoir and dam under the small dam design criteria, requires a spillway capacity
capable of passing, as a minimum, the 100-Year flood. Additionally, as described by the
U.S. Department of Interior Bureau of Reclamation Design of Small Dams publication;
in the case of snowfed perennial streams, the spillway capacity should never be less than
the peak discharge of the runoff of record that has resulted from snow melt nmoff.
Exhibit D includes a Preliminary Drainage Report for Powderhom Ski Village which
addresses the drainage basin and resultant drainage which flows through the Robbins
Reservoir site. The Drainage Report estimates that the combined runoff from snowmelt
and 100- Year storm runoff for the drainage basin results in approximately 400 cfs at the
Robbins Reservoir site, without giving consideration to any upstream detention ponds.
The ultimate development and construction of upstream detention ponds would affect the
total amount of runoff and streamflow to the Robbins Reservoir site. An amount of 500
cfs has been used for sizing and preliminary design of the spillway structure.

Robbins Reservoir and the surrounding area is owned by the Powderhom Recreation
Development Company and is within the Metro District boundary. They have the control
for the future development downstream from the reservoir for some distance and,

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