For a release capacity at normal reservoir level can thus approach 47,000 second-feet. release of 14,000 second-feet through the power penstock system, the velocity in the power tunnel will be about 26 feet per second. Although the combined capacity through the turbines and bypass gates can exceed the 14,000 second-feet, to guarantee efficient operation of the power generating units, flows through the combined installations normally will be limited to that discharge. To best conform to the site geol 34. Selection of Outlet Works Arrangement. ogy and topography, the outlet works, powerplant, and spillway are all located at the left abutment of the dam. The outlet works arrangement was selected to serve in effecting diversion during construction as well as to fit in with the positioning of the spillway and powerplant. Limitations of the foundation geology and abutment topography made it necessary to choose each structure to best fit existing conditions. The andesite occurs only on the left abutment as an intruded mass dipping downstream, such that at the upstream toe of the dam the base of the intrusion emerges to river level, where it is underlain with river sands and gravels. The foundation at the site of the outlet works intakes, therefore, was unable to support a vertical tower type intake structure, necessitating an inclined structure placed along the rock face of the abutment. The slopes of the abutment are steep; and to avoid excessive and expensive structure excavations, unusually steep excavation slopes were specified. Some raveling of the hillside slopes is expected, and access roads for rockfall cleanup are provided. To prevent loose and blocky rock from falling from the high areas above the spillway and outlet works intake portals, a wire mesh covering anchored to the hillside was provided. 35. Description of Outlet Works and Power Penstock System. - (a) General.-The outlet works and power penstock system arrangement consists of two parallel tunnels excavated through the left abutment at river level. These tunnels served initially as waterways for diverting the river around the dam during construction, after which they were plugged at their upstream ends and adapted as penstocks to serve the outlet gates and power turbines. The tunnels are 26 feet in finished diameter and are spaced 125. 5 feet on centers. The tunnels approximately parallel the spillway tunnel, whose center is about 175 feet to the left of the left outlet tunnel. The right penstock tunnel delivers water to the powerplant turbines and to the two right outlet gates which empty into the outlet works stilling basin, while the left tunnel carries the flows to the remaining four outlet gates and to the two regulating valves, which also empty into the stilling basin. Separate intake structures controlled by fixed-wheel emergency closure gates are provided at the upstream end of the tunnels, and downstream manifold pipes deliver flows from the tunnels to the outlet control house and to the powerplant. The general design arrangement is shown on figures 53 and 54. Detail arrangement of the outlet and power penstock system components are described in the following subsections. (b) Intake Structures.--The intakes for the outlet works and power penstock tunnels are situated along the left abutment slope upstream from the dam, immediately above the portals of the tunnel entrances through which diversion flows entered the main tunnels. These intakes are spaced 110 feet on centers. Each intake consists of a rectangular semibellmouth-shaped entrance joining a transition which changes in cross section from rectangular to circular. The entrance and transition are formed in tunnel and are inclined at 450. The entrance opening is a rectangle, 22. 4 by 39. 4 feet in size, and is normal to the transition centerline. The bellmouth entrance and transition is 56 feet long; its shape changes from the rectangular opening to a 19. 25- by 27. 57-foot rectangular throat section in its upper 13.79 feet, and then varies from the throat section to a 26-foot-diameter circular section in the remaining 42. 33 feet of length. Each intake connects to the horizontal leg of its respective tunnel through a 118-foot centerline radius vertical curve. The centers of the entrance openings are at elevation 5465. 5, or about 90 feet above the inverts of the main tunnels and 32.0 feet below the minimum power head reservoir level. Each intake is provided with a 19. 67- by 28.03-foot fixed-wheel emergency closure gate which operates in slots located just downstream from the transition throat. A trashrack structure with racks placed about 32 feet ahead of the entrance opening, inclined on a 45° slope, covers each intake entrance. The two intake structures are recessed into the abutment slope in trapezoidally excavated notches so that the rack-bar surfaces are about flush with the abutment face. Figures 55 and 56 show details of the arrangement. Trashracks enclose only the lower portions of the structures opposite the Figure 56.--Looking downstream at the intake structures for the spillway tunnel, left; outlet tunnel, center; and power tunnel, right. The temporary bulkhead has been lowered in front of the outlet tunnel adit, and the initial storage of water in Palisades Reservoir has begun. 456-108-3723, November 3, 1956. intake openings, since the bulk of the flow entering the tunnels will be directed from immediately in front of the entrances. The trash-bar panels bear on inclined reinforced concrete beams which are supported on heavy lateral reinforced concrete arches. The upper supporting members of the trashrack structure consist only of inclined guide beams and supporting bents. Tracks and guides are attached to the beams to facilitate installation of the trashracks and to accommodate a trashrack rake if trash cleaning becomes necessary. Also see section 63. To prevent formation of ice around the inlet structures, an air bubbler system is provided. Air pipes are embedded in the concrete beams and arches, and nozzles to release air bubbles are placed at strategic locations to produce a uniform circulation of water in front of the structure. The fixed-wheel gates will provide emergency closure of the tunnels in the event of a failure of the penstocks or of a malfunctioning of the individual guard gates or valves installed ahead of the outlet gates and turbine wicket gates. The emergency gate closure will also permit unwatering the tunnels for inspection or for maintenance or repair of the tunnels or gates. Their closure will also relieve pressure on the guard gates or valves during sustained shutdown periods. The fixed-wheel closure gates are motivated by hydraulic hoists whose pistons are sufficiently long to close the gates with a single thrust. The gates can be raised above the normal water surface for inspection or servicing by lifting the gate one full thrust of the hoist piston, latching the gate leaf, removing a sectional length of the gate stem, and then reattaching the hoist piston to next lower stem section. When the gates are in the fully raised position, access to their undersides is possible from gate erection and service pits located near the tops of the intake structures. Each of the two hoists is mounted in a separate housing at the top of the intake structure. The hoist houses contain rooms in which the oil pumps, air compressors, and other control equipment are housed. 1 |