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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embankment appears to be adequate.

Considerable experience-based Judgement must be used In applying the results of theoretical
stability analyses, since the analyses Involve simplifying assumptions, as well as the use of
assumed geometry and soil and seepage parameters, which In themselves are not amenable
to accurate determination.

5.3.5 SEEPAGE CONDITIONS

A cnude but Informative test of seepage loss from Hourglass reservoir was made by the City
on September 11, through September 13, 1990. Both the Inflow and the outflow to the reservoir
were cut off and the reservoir level was observed. Based on the reservoir area-capacity data,
the apparent reservoir loss was calculated to be approximately 7 acre-feet per day at gage
height 24 feet. Data is not available to extrapolate reservoir losses at other gage heights.

We evaluated the effect of a partial cutoff below the dam on the quantity of seepage. The
evaluation was made for feasibility level studies. A detailed seepage evaluation was beyond the
scope of our services. Our analysis Indicates that a partial cutoff penetrating about half the
depth of the foundation soils will reduce seepage losses from the reservoir by about 50 percent.
This would involve construction of a continuous cutoff about 100 feet deep beneath the
embankment and the north side of the reservoir. A partial cutoff penetrating 90 percent of the
foundation soils will reduce the seepage by about 70 percent. We estimate that a full depth
cutoff would reduce the seepage by about 90 percent.

Our experience indicates actual seepage may vary from the values estimated by a multiple of
10 to 100. Decisions based on seepage estimates should consider this potential variability.

5.3.6
PLAN

SUMMARY OF EXISTING CONDITIONS AND RECOMMENDED INTERIM OPERATION

The results of the analyses indicate that the existing embankment has an adequate factor of
safety wtth regard to slope stability failure. However, phreatic levels in the embankment have
been fairly high during periods of reservoir operation, and the subsurface investigations indicate
that zones in the foundation materials are highly pervious. Continued, uncontrolled seepage
could result in a piping failure of the embankment and/or foundation materials. Particle

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