442 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 19 3 5 



direction of the canyon workings, and the possible large fluctuations 

 of the river's flow. 



A final decision was made to drive two diversion tunnels through 

 each canyon wall around the dam and powerhouse site, erect an 

 earthfiU cofferdam downstream from the inlet portals and another 

 upstream from the outlet portals, place steel bulkhead gates at the 

 inlet portals of the two tunnels farther from the river (termed the 

 " outer diversion tunnels "), build plugs in the two tunnels closer to 

 the river (termed the "inner diversion tunnels"), and construct 

 plugs in the outer diversion tunnels — the one on the Nevada side to 

 contain high pressure slide gates for temporary use during the filling 

 of the lower portion of the reservoir. 



The downstream two-thirds of the diversion tunnels were to be 

 used later in the operation of the reservoir, the outer ones becoming 

 the outlets for the spillways and the inner ones containing the pen- 

 stock headers from the upstream intake towers. Plugs were, there- 

 fore, placed in the diversion tunnels immediately upstream from the 

 intersections with the inclined tunnels to the spillways and to the 

 towers. As previously mentioned, the inner diversion tunnels also 

 contained plugs for outlet works at the downstream ends of the 

 pipe headers. 



Control of the river consisted in turning it through the diversion 

 tunnels by means of temporary dikes, building the cofferdams 

 behind these protecting dams, shutting off the flow through the 

 inner diversion tunnels and the outer Nevada diversion tunnel in 

 periods of low river flow while temporary dams were dumped across 

 the tunnel inlets and outlets, and constructing concrete plugs in these 

 three tunnels, the outer Nevada one being equipped with gates. The 

 bulkhead at the inlet of the outer Arizona tunnel was lowered, di- 

 verting the river flow through the outer Nevada tunnel under 

 control of the slide gates, thus commencing storage in the reservoir. 

 A concrete plug was then built in the Arizona outer tunnel. 



After the reservoir has risen to the lower gates in the intake 

 towers, and one of the outlet systems is ready for operation, the 

 river control will be taken over by the needle valves in the outlet 

 works. The bulkhead at the inlet of the Nevada outer tunnel will 

 be lowered by remote control, as the gate sill will be under at least 

 265 feet of water, and the slide gates in the plug closed for the last 

 time. The openings through the plug will then be filled with concrete 

 and grouted. 



Each of the four diversion tunnels was excavated to 56-foot di- 

 ameter — as high as a 4-story building — and lined with a 3-foot 

 thickness of concrete. The total length of the tunnels was approxi- 

 mately 3 miles. Plugs placed in diversion tunnels, immediately 



