16. SELECTION OF TYPE. Preliminary designs and estimates were prepared for a concrete dam. Also considered was a conventional-type powerplant and an underground-type powerplant. The final unit consists of a constant-radius concrete arch dam with a tunnel-type spillway through each abutment and an outlet works through the dam near the left abutment. The powerplant is located immediately downstream of the dam and an indoor-type structure was finally selected. A layout of the dam and powerplant is shown on figure 4.

As constructed, the crest of the dam is at elevation 3715 and the crest length is 1,560 feet. The top width (neglecting added width to accommodate roadway and walkways) is 25 feet. The dam is 710 feet high above the lowest point of the foundation, and the maximum base width is 300 feet. The radius at the axis of the dam is 900 feet. The upstream face is vertical below elevation 3300 and is curved slightly in a downstream direction above that elevation. The downstream slope of the central portion of the dam is constant at 0.31 to 1. A more detailed description of the dam is given in section 30 and the powerplant is described in section 66. The spillway and outlet works are described in sections 40 and 47, respectively.

A. RIVER DIVERSION

17. GENERAL. Diversion of the Colorado River during construction of Glen Canyon Dam was accomplished through two 41-foot-diameter concrete-lined tunnels, one located in each abutment.

The diversion tunnel through the right abutment is 2,749 feet long. Approximately 910 feet of the downstream portion of the diversion tunnel was later incorporated into the right spillway tunnel. The invert elevation at the entrance is 3137.37 which is essentially river level.

The diversion tunnel through the left abutment is 3,011 feet long. Approximately 1,085 feet of the downstream portion of the diversion tunnel was later incorporated into the left spillway tunnel. The left diversion tunnel invert is at 3170.67 which is 33.3 feet higher than the right diversion tunnel. The left tunnel entrance was set at the higher elevation so that a temporary outlet works could be installed in the left tunnel plug during a low flow season without the use of an entrance closure structure. Riverflows less than 15,000 cubic feet per second were confined to the right diversion tunnel.

The temporary outlet works (sec. 18), consisting of three 7- by 10.5-foot outlets controlled by 7- by 10.5-foot slide gates in tandem, were located in the left tunnel plug to control releases during reservoir filling. Access to the gate operating chamber was through a 5-by 7-foot access adit from the dam. A trashrack at tunnel entrance was constructed to protect the outlet works. The left diversion tunnel and outlet works are shown on figure 23, and the access adit from the dam is shown on figure 24.

In the initial design planning, the diversion tunnels were 50 feet in diameter, unlined, and approximately 2,500 feet in length. Tests performed to determine whether the sandstone through which the tunnels were bored could withstand the erosive force of sediment-laden, high-velocity flow indicated that the diversion tunnels should be lined. Accordingly, lined tunnels were specified and the diameter was reduced to 44 feet. Subsequent to the final diversion studies, the diameter of the tunnels was further reduced to 41 feet to match the final size requirement for spillway discharges. (See preceding discussion relative to use of a portion of the diversion tunnel for the spillway tunnel.)

Hydraulic model studies were made to check the alinement and elevation of the diversion tunnels with respect to the river channel. The hydraulic model also was used for preliminary investigations of the deflector buckets at the end of the tunnel spillway. The two discharge quantities used for the diversion studies were 30,000 and 65,000 cubic feet per second per tunnel. Model tests were made with the right tunnel operating singly and also with both tunnels operating. Since the intake portal of the left tunnel is about 33 feet higher than the right tunnel intake, the left tunnel would not operate singly.

The investigations showed that, in general, the tunnel alinement and grade were satisfactory for diversion flows. The curved channel downstream from the right tunnel caused some eddies in that vicinity, but when the curved channel was replaced by a straight channel, the eddies were eliminated and the flow was entirely satisfactory.

18. DIVERSION PLAN. During early construction of the dam, the river was diverted through two 41-foot-diameter concrete-lined diversion tunnels. One tunnel, with the inlet invert at elevation 3137.37, passed around the damsite through the right canyon wall. The other, with the inlet invert at elevation 3170.67, passed through the left wall. After construction of the dam and

power units had advanced sufficiently, the upstream portions of these diversion tunnels were abandoned and permanent seals, or plugs, were installed to close them (figs. 5 and 25). The downstream portions were then connected to the spillway tunnels that slope down to meet them, thereby completing the reservoir spillway facilities (figs. 5 and 25).

From practical and economic points of view, water storage had to start as soon as the water was available and sufficient construction had been achieved. During such storage, water had to be released through or around the dam to meet downstream commitments. To fulfill these conditions, some form of low-level outlet works had to be provided. Owing to the relative narrowness of the canyon, the appreciable space requirements of the penstocks, and predicted future silt levels, the inlets to the permanent river outlets in the dam had to be placed at elevation 3374 (fig. 5). This meant that a lake about 265 feet deep had to be impounded before the river outlets could release water. They therefore could not be used to make releases during the early reservoir storage period.

To provide the necessary river control for the interim period, an outlet structure was built into the left diversion tunnel plug (fig. 23.) It consisted of three 7-foot-wide by 10.5-foot-high rectangular passages in the plug, followed by the same size high-head slide gates and by 7-foot-wide by 14.5-foot-high conduits that discharged into the downstream tunnel. Bellmouth inlets were provided at the conduit entrances in the upstream face of the plug. An emergency gate and a service gate were provided one behind the other in each conduit. The gates are of the slide type (sec. 22) developed and used successfully at the Bureau's Palisades Dam for a number of years at various gate openings with heads up to 200 feet.

The tunnel plug outlet works was constructed during the low flow period in the fall and winter of 1961-62. During this construction, all of the riverflow was diverted past the damsite through the right diversion tunnel. After the construction was completed, massive steel slide gates were lowered in the right diversion tunnel entrance and a solid concrete plug installed to close it permanently. After this closure, and up to the time the lake had risen to service level, and overall construction had advanced enough, all flows past the dam were released through the higher, left diversion tunnel outlet works.

After the reservoir reached a service level and the permanent river outlet works and turbines could be operated, the tunnel outlet gates were closed and the conduits filled and sealed with concrete. The remaining section of tunnel plug was then installed to complete the plug and permanently seal the upstream part of the tunnel. The final connection to the left spillway tunnel was then made (fig. 25).

19. HYDRAULIC CONSIDERATIONS. (a) Diversion Tunnels. -Diversion floods of 5-, 10-, 25-, 50-, and 100-year frequencies are shown on figure 26. The size of the tunnels (41 feet) was determined from routing studies made using the 25-year frequency flood which has a peak flow of 196,000 cubic feet per second and a 15-day volume of 3,550,000 acre-feet. Routing of this flood through the tunnels resulted in a maximum reservoir water surface elevation of 3277 and a combined discharge of 143,000 cubic feet per second. The upstream cofferdam, designed by the contractor, had a top elevation of 3300 and the downstream cofferdam a top elevation of 3165.

(b) Temporary Outlet Works. -In order to meet downstream requirements during initial filling of the reservoir, an outlet works of sufficient capacity to pass all riverflows downstream with only temporary storage was necessary. It was determined, from routing studies, that a diversion tunnel outlet works with a capacity of approximately 33,000 cubic feet per second at reservoir elevation 3566 (410 feet of head), in conjunction with the permanent river outlets in the dam (invert elevation 3374), would satisfy all requirements. The outlet works were tested in a hydraulic model. The conclusions are presented below and further outlined in Hydraulic Laboratory Report No. Hyd-468.1 Discharge curves for the outlet works are shown on figure 27.

20. CONCLUSIONS FROM MODEL STUDIES.

(1) A satisfactory diversion tunnel outlet works was obtained with three conduits spaced 12 feet 6 inches apart and provided with regulating slide gates 7 feet wide by 10.5 feet high.

(2) Rectangular bellmouth conduit entrances with elliptically curved surfaces provided good boundary surface pressures under all operating conditions.