WSP07002

• File Number: 8449
• Description: Platte River Studies
• State: CO
• Basin: South Platte
• Date: 2/1/1991
• Author: City of Greeley
• Title: Feasibility Study for Rehabilitation of Hourglass Dam and Reservoir Vol. 1
• Water Supply Pro - Doc Type: Publication

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4.3

WATER SUPPLY

Hourglass Reservoir has a drainage area of 0.74 square miles. Whne 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. Waler 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 runofl is also given. The
Comanche Reservoir's 11.9 square mile drainage area Is adjacent to and similar 10 the
Hourglass Reservoir 0,74 square mile drainage area. It follows that the slorage 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 operalion studies of Comanche Reservoir, based on Ihe calculated 2.900 acre-feet
per year storable inflows, The Hourglass Reservoir slorage 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.

Unfortunately such potential increased water supplies are Illusory. The reservoir operation
studies do not take probable seepage from the reservoir Into account. A test of seepage from
Hourglass Reservoir that shows a seepage loss of 3.5 or 7 acre-feet per day. Seepage losses
would be higher at higher reservoir levels. higher than that during the test. Acre-foot per day
seepage loss results in 200 acre-feet loss In one month. Since such seepage losses from
Hourglass Reservoir cannot be Identified as Increases in downstream streamflow. they represent
tolal water loss to the City.

4-7

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