PROJ00235

• Contract/PO #: C153633
• Contractor Name: Greeley, City of
• Contract Type: Loan
• Water District: 3
• County: Larimer
• Bill Number: HB 91-1006
• Loan Projects - Doc Type: Feasibility Study

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4,3 WATER SUPPLY

Hourglass Reservoir has a drainage area of 0,74 square miles. While this small drainage area
contributes runoff into the reservoir, a major part of the water stored in the reservoir is water
released from Comanche Dam, Water is carried to Hourglass Reservoir through a short channel
extending from the Comanche Dam Hourglass outlet.

The water supply study done by Morrison-Knudsen Engineers, Inc. (MKE) as a part of the
feasibility study for rehabilitation of Comanche Dam and Reservoir of 1986, indicates an average
annual inflow of 6,250 acre-feet per year for the period 1950 through 1983, An average storable
flow of 2,900 acre-feet per year based on April through June runoff is also given, The
Comanche Reservoir's 11.9 square mile drainage area is adjacent to and similar to the
Hourglass Reservoir 0,74 square mile drainage area, It follows that the storage flow from the
Hourglass Reservoir drainage is about 6.2 percent of that of the Comanche Reservoir drainage
or 180 acre-feet per year, There are no records of Hourglass Reservoir inflow from Comanche
Dam,

MKE made operation studies of Comanche Reservoir, based on the calculated 2,900 acre-feet
per year storable inflows, The Hourglass Reservoir storage capacity was not taken into account
in these studies. A rudimentary analysis of the MKE operation studies shows that a safe yield
of 530 acre-feet per year, or an annual empty reservoir yield of 430 acre-feet per year is
theoretically possible using the full capacity of Hourglass Reservoir,

The Option 1 potential 430 acre-feet of safe yield from Hourglass Reservoir, up to its full
capacity, will be subject to operating criteria, Such water would have to be released
downstream prior to accumulation of seepage losses as inflows diminish below seepage loss
rates in the fall, Option 2 would provide a degree of seepage reduction in order to facilitate
storing the potential 530 acre-feet of safe yield,

4.4 SEDIMENT

Sediment yield above Hourglass Reservoir is estimated from the Colorado Land Use Commission
sediment yield map of 1974, This information is based on surveys of reservoir sediment
accumulations of 25 years and longer, Much of the information was obtained from small
reservoirs with small drainage basins,

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