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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PLAN OF DEVELOPMENT

HISTORY AND BACKGROUND

Hourglass Reservoir, also known as Big Beaver Reservoir, Is an off-<:hannel reservoir of Big
Beaver Creek, The reservoir Is located about nine miles south of Rustic. Colorado, In the
Roosevelt National Forest. The reservoir was originally a natural lake formed by glacial moraine.
To provide an outlet to the reservoir, a channel was cut part way through the morainal ridge
by placer mining techniques about the tum of Ihe century. An outlet pipe was installed in the
channel and an embankment placed above n. At that time, the capacity of the natural reservoir
was also increased by placing a five to ten foot-high dike on Ihe moraine. It Is believed that
little or no effort was made to compact the dike material, and that the material was glacial In
origin. Problems with the outlet works and the dam in the lale 194O's resulled In the reduced
use of the reservoir and the evenlual elimination of storage in the reservoir, Storage In
Hourglass Reservoir Is augmented by a feeder dneh and outlet works from Comanche Reservoir,
approximately one mile upstream. In 1966, the structure was rehabilitated Including:

1) Removal of the original outlet and the installalion of a 33-inch diameter reinforced concrete
outlel condun controlled by IwO parallel upstream slope control gates. A stilling basin was
also provided on Ihe downstream en:! of the outlet.

2) Reshaping portions of the dam and raising the crest elevation of the dam In those areas
necessary to establish a unllorm crest elevation.

3) Providing rlprap on a portion of the upstream face of the dam at the maximum section.

4) Constructing a primary spillway on the feeder ditch to the reservoir at the decreed gage
height of 34,5 feet.

5) Construction of a secondary spillway at the right abutmenl of the dam.

Following Ihese rehabilitation measures to the embankment and the filling of the reservoir to the
decreed gage height, adverse seepage developed at the downstream toe of the dam adjacent
to the stilling basin, At this time, the downstream toe was found to be soli and saturated, and

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