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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5.3

spillway would carry flow, this area could erode, allowing water along the downstream toe of
the embankment.

Several past inspections by the State Engineer's Offlce have noted possible low spots on the
crest of the embankment, and that the crest may need to be regraded to slope to the upstream
side. A State Engineer's Offlce inspection report, dated July 31, 1986, noted minor spalling of
concrete in the floor of the energy dissipator, and some exposure of reinforcing steel.

GEOTECHNICAL EVALUATION

5.3.1 GEOLOGY

5.3.1.1 Geomorphology

Hourglass Reservoir is located wtthin Beaver Creek drainage, an east treading drainage on
the eastern slope of the Rocky Mountains. The site Is located approximately 311 miles
northeast of Comanche Peak. Beaver Creek drainage is a U-shaped glacial valley.
Comanche Reservoir Is located approximately II mile upstream of Hourglass Reservoir. The
valley slopes are steep in the vicintty of the reservoirs and become relatively less steep
downstream of Hourglass Reservoir which corresponds to a general widening of the valley
floor. In the vicinity of the reservoirs the surface of the valley floor is relatively flat. Beaver
Creek is located near the northern side of the valley and has eroded into the flat valley
bottom. Beaver Creek is approximately 75 feet in elevation below the reservoir. Beaver
Creek maintains a relatively flat gradient from Comanche Reservoir to a point approximately
, of a mile downstream of Hourglass Reservoir. At this location, the stream gradient
increases sharply and flows down the face of an east facing terminal moraine.

A large part of the present geomorphology is the result of glacial activity. During the last
Pleistocene Ice age. Beaver Creek drainage was occupied by a valley glacier which
advanced eastward down the drainage. During the advance, the glacier eroded and/or
mod~ied the overburden depostts and scoured the bedrock surface, eroding the more
weathered and weaker portions of the rock. The overburden deposits which were
overridden by the glacial ice were compacted by the weight of the ice. As the glacier
retreated, debris carried by the glacier was depostted in the valley either directly by the

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