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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movements were noted on the downstream face of the dam and along the toe. Embankment
modiflcatlons carried out In 1970 Included the construction of a blanket and chimney filler,
extension of the outlet pipe in the downstream direction approximately 72 feet, and placement
of additional fill to flatten the downstream slope from the original 2:1 (horizontal to vertical) slope
to a slope of approximately 4:1. In July, 1971, an area of heavy leakage was located near the
right abutment of the main embankment approximately 40 feet downstream of the outlet works.
At that time, it was conduded that the seepage was from water passing through the natural,
undisturbed ground of the righ! abutment. The gage height has been restricted to 32 feel since
the early 1970'5,

Surface water runoff from a severe thunderstorm in 1984 eroded a gulley several feet deep
along the left groin of the downstream face, Seepage from the left abutment was noted
entering the bottom of the gulley, The gulley was backfilled as soon as conditions permitted.
On August 5, 1985, after approximately 45 days of storage near gauge height of 32 feet. a wet
area was noted just above the outlel works and extending about 22 feet upstream at the left
abutment. At this lime, it was concluded that the seepage was the result of water flowing
through the left abutment. This water was collecting In the backfilled material placed in the
erosion gulley the previous year, Instead of being collected In the blanket filter, After
development of this wet area, the State Engineer's Office further restricted the gage height to
a maximum of 32 feet. with no more Ihan 45 days above gage height 31 feet. and no more
than 90 days above gage height 30 feet.

Historically, a seepage area has developed at the downstream toe of the embankment belWeen
about stations 114+00 and 117+00. The depth of water In the seepage area has been
observed to vary with the reservoir level, The depth of water in the seepage area reaches
about five feet when the reservoir level Is near gage height 31 feet and disappears when the
reservoir drops below about gage height 26 feet.

Seepage was also noted during the 1980 Inspection by the Corps of Engineers about 300 to
400 feet downstream of the dike portion 01 the embankment. The seepage was exiting the
morainal ridge about 30 to 40 feet below the crest of the dam, There was no evidence thaI
soil particles were being transported by the seepage.

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