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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5.3.1.3 Overburden

The above mentioned geologic map IOOlcates the valley In which the reservoir Is located
contains glacial deposits of the Pinedale and Bull Lake Glaclatlons. The thicknesses of
these deposits are not known. Based on geomorphic evidence, It Is estimated that the
thickness of overburden deposits may be on the order to 150 to 200 feet. These deposits
consist of glacial outwash, which includes Interbedded sand, gravel, and occasional cobbles
with beds of fine sand, silt, and clay. Based on exposures near the dam outlet works and
the material encountered in the borings, the upper approximately 40 feet of outwash is
primarily sand and gravel with occasional cobbles. In exposures near the inlet to the
reservoir, this upper outwash Is stratified In 1 to 4-foot-thick beds; some contain cross-
bedding indicative of braided channel fills and bars. Below this level, the outwash is
generally finer grained. An interbedded layer of silt and fine sand about 10 feet thick was
observed in the outwash near the dam outlet works.

Ground and lateral moraines mantle most of the bedrock on the valley slope along the
south side of the reservoir. The type of moraine is distinguished by the method of
deposttion. The lateral moraine Is deposited along the edge of the ice sheet which the
typically thinner ground moraine Is deposited beneath the Ice sheet. The moraines generally
consist of poorly sorted silt to boulder-sized material. Boulders range up to 10 feet In size.
Several relatively small outcrops of precambrian rock were observed in the moraine.

5.3.1.4 Earthquake Potential

In recent studies completed by the U.S. Department of the Interior, Geologic Survey,
estimates of horizontal acceleration in bedrock are less than 4 percent gravity and
approximately 4 percent gravity with 90 percent probabiltty of not being exceeded in a 10
and 50 year time period, respectively. A horizontal acceleration of between 6 and 9 percent
gravity wtth 90 percent probability of not being exceeded is estimated for a time period of
250 years. The maximum magnitude used for the project seismic zone was 6.1

In the 1981 publication "Earthquake Potential in Colorado', the magnttude of the maximum
credible earthquake for the Eastern Mountain Province, which includes the project area, is
estimated between magnitude 6.0 and 6.75.

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