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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PROJECT SUMMARY

Hourglass Dam is located on a small Beaver Creek tributary Just east of Comanche Dam, about nine
miles south of Rustic. Colorado. It is a 45 foot high. 24,000 foot long earth embankment dam, It
impounds 1,693 acre-feet of storage at water surface elevation of 9,366,5 feet (34,5 gage). The dam's
crest is at elevation 9,372 feet (40 gage).

Hourglass Dam was built around 1898. Since then ft has been enlarged, abandoned, and rehabilitated,
The last rehabilftation wor1< was done In 1967. The reservoir is currently under a Slate Engineer's 9,363
fool (31 gage) maximum water level restriction. The restriction was imposed because excessive seepage
occurs at that reservoir level. The excessive seepage saturates the downstream toe of the dam
embankment thereby reducing the slrength of the earth materials and reducing the dam's stabillly, The
maximum water level restriction reduces the useable reservoir capacity, from 1,693 10 1,434 acre feet,
a loss of 259 acre-feet of storage,

TriConsultants (formerly McCall-Ellingson & Morrill, Inc.) inftlated field investigations to determine existing
conditions at the dam and reservoir sfte under a contract wfth Greeley dated October 19, 1989,
Woodward-Clyde Consultants were used as geotechnical sub-consultants, Sfte Invesligations were
carried out at the sfte during several days beginning on October 23, 1989. A second eontracl dated
July 16. 1990, received August 13, 1990, was received from Greeley to prepare Ihis feasibilily study for
Hourglass Dam and Reservoir.

The feasibilily study included two addftional sfte vlsfts, a simplified seepage loss delermination,
consideration of seven alternatives, two informal progress meetings and further consideralion of two
proposed rehabilitation plans. Plans ranged from providing an essentially water tight liner for the entire
reservoir and dam face, to a relatively simple plan for enhancing dam stability. Feaslbillly level cost
estimales range from 40 million dollars to 500 thousand dollars.

II is clearly impractical to provide a high degree of seepage reduction at any reasonably foreseeable
water or reservoir storage capacity values, Other Greeley dam projects can probably provide reservoir
storage at lower cost. The two final plans proposed are: Option 1 - Downstream Drainage Construction
to simply enhance dam safely; and Option 2 - Partial Slurry Wall, a cut off at the dam to partially
reduce seepage and to further enhance dam safely, The feasibility level construction cost estlmales of
lhese two plans are 328.075 dollars for Option 1, and 1,346.975 million dollars for Option 2, The
options are desililned so that Option 1, the downstream drainage construction, can be done first and
Oplion 2. the partial slurry wall can be built later, wfth minimum duplication of effort. If Option 1 is built
firsl, as expected, the construction cost of Opllon 2 Is reduced to 995,500 dollars,