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WSP12441
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Last modified
1/26/2010 4:15:20 PM
Creation date
1/26/2007 11:20:28 AM
Metadata
Fields
Template:
Water Supply Protection
File Number
8220.105.H
Description
Water Projects - Navajo - Operation Studies
State
CO
Basin
Colorado Mainstem
Water Division
5
Date
10/1/1998
Author
DOI-BOR
Title
Outlet Works Discharge Capacities - Navajo Dam - CRSP - Colorado-New Mexico - Technical Memorandum Number Nav-8130-TM-98-1 - DOI-BOR - 10-01-98
Water Supply Pro - Doc Type
Report/Study
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<br />001660 <br /> <br />size (either clamshell or bonneted slide gates), with a discharge coefficient of 0.96 instead of the <br />assumed value of 0.724, the maximum discharge capacity of the main outlet works could be <br />increased by about 11 percent, or by about 450 ft!/s at reservoir elevation 6085 (from 4,110 ft!/s <br />to 4,560 ft!/s). Extensive modifications would probably be required for the existing stilling basin <br />to provide adequate energy dissipation for the larger flows. <br /> <br />B. Auxiliary Outlet Works <br /> <br />The auxiliary outlet works was originally provided for diversion and care of the river during <br />construction of the dam and closure of the main outlet works. The auxiliary outlet works is <br />located to the right of the main outlet works, with the downstream portion located beneath the <br />spillway centerline. The intake is a box-type structure with trashracks on the top and three sides, <br />and includes a circular bellmouth entrance to a 7-foot-diameter concrete-lined pressure tunnel, <br />with an intake sill at elevation 5775.0. Metal guides and seals are provided onthe face of the <br />bellmouth for the installation of stop logs, if required. The pressure tunnel extends about 730 feet <br />between the intake structure and a gate chamber, which contains 4- by 4-foot tandem outlet guard <br />and regulating gates and associated transitions. The downstream discharge tunnel has a modified <br />horseshoe shape, with a width of 6 feet and a height of 8 feet, to maintain free flow conditions for <br />the design discharge. The downstream tunnel is about 850 feet long, and discharges into the <br />spillway chute through the spillway floor. The invert ofthe downstream tunnel exit was originally <br />fonned to the trajectory of the flow at maximum head with minimum friction losses. The <br />corresponding flow velocities in the discharge tunnel are very high under maximum head, and <br />cavitation was a concern. This resulted in a design recommendation to limit operation of the <br />auxiliary outlet works to reservoir levels below elevation 5920.0, except for short periods when <br />inspection or emergency repairs are necessary for the main outlet works [2]. <br /> <br />The existing DOC for Navajo Dam [7] indicates the 4- by 4-foot regulating gate should not be <br />operated at openings greater than 47-1/2 inches (or 3.95 feet), to minimize the possibility of <br />cavitation damage to the concrete surfaces of the downstream tunnel, and operation at heads in <br />excess of 150 feet (elevation 5920.0) should be avoided as far as possible. According to the <br />DOC, operation of the auxiliary outlet works between reservoir elevations 5920.0 and 5990.0 <br />requires the downstream tunnel to be inspected at frequent intervals (at least every 3 days). Any <br />cavitation damage must be repaired by approved methods before resuming releases. The DOC <br />also indicates the auxiliary outlet works should not be used when the reservoir is above elevation <br />5990.0, except during an extreme emergency. During such emergency periods, the regulating <br />gate should be closed every few hours to inspect the downstream tunnel for cavitation damage. <br /> <br />A discharge curve for the auxiliary outlet works is shown on Drawing No. 711-0-447 (revised <br />1967, included as Appendix A). Although this curve reflects the elevation 5920.0 flow restriction <br />for operation of the auxiliary outlet works, a maximum discharge capacity of approximately 1,780 <br />fi3/s is indicated (by a dashed line) for a maximum reservoir water surface at elevation 6101.6. <br />Hydraulic loss computations for the auxiliary outlet works are summarized in the Technical <br />Record of Design and Construction (see Appendix B), and result in the following discharge <br /> <br />5 <br />
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