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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The PMP for Hourglass Reservoir was taken from NOAA Hydrometerological Report No. 55A.
The U.S. Army Corps of Engineers HEe-l computer program was used to produce the inflow
hydrographs. The general storm was found to produce largest maximum probable flood, PMF.
The 6-hour PMP of 10.66 inches Is the most severe. The resulting PMF hydrograph has a peak
discharge of 1772 cubic feet per second at 1.45 hours, and a volume of 3981 acre-feet. The
computer printout used Is presented In Appendix

5.4.2 FLOOD ROUTING

Aood routing was done with a starting elevation of 9366.5 feet, (34.5 gage) the crest of the side
channel spillway. The outlet gates were assumed to be closed. The spillway rating curve
developed and used is the U.S. Arrrrf Corps of Engineers Phase I Dam Inspection Dam
Inspection Report. The maximum storage In the reservoir is 1889 acre-feet for a 6-hour storm.
Peak stage 9329.2 feet (37.2 gage) at 3.5 hours. This leaves a freeboard of 2.8 feet for the
dam at maximum reservoir stage.

These results clearly Indicate that the pnesent side channel spillway, and the emergency spillway
at the right end of the dam, have a combined capacity to discharge the PMF. The two
spillways require minor improvements to present potential excessive rebuilding costs in the event
that they do function.

5.4.3 OUTLET WORKS

The existing outlet works are apparently in good condition. It Is concluded that the 33-inch
concrete pipe should be extended 70 feet to accommodate additional earth fill at the
downstream toe of the dam. The existing outlet works are apparently In good condition.

The outlet conduit extension will not reduce the capacity of the outlet works. Since the present
outlet works can discharge 165 cubic feet per second of water surface elevation 9,372 (40
gage), the present outlet works can discharge the upper five feet of reservoir capacity of 400
acre-feet in about 30 hours. This compares favorably to the State Engineer's requirement that
the upper five feet of storage must be discharged at not less than one foot per day.

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