BOULDER CANYON PEOJEOT — NELSON 443 



up-stream from the intersections with the inclines to spillways and 

 intake towers, were 200 feet to 393 feet in length ; and the gates for 

 temporary diversion in the Nevada outer tunnel consisted of four 

 sets of Qy2-ioot by 7-foot slide gates, operated by motor-driven oil 

 pumps and hydraulic cylinders. Bulkliead gates placed at the inlets 

 of diversion tunnels were built of structural steel plates and members, 

 making a section approximately 56 feet wide, 51 feet high, and 12i^ 

 feet thick. Two hydraulic cylinders, nearly 8 feet in diameter and 

 70 feet in height, were required to lower or raise each gate in its steel 

 and concrete frame, water being released from the cylinders in the 

 lowering operations or being pumped beneath the piston heads to 

 raise the gate. Forty-two railroad cars were required to bring one 

 gate to the project, the weight of the moving parts of each gate being 

 in excess of 2,000,000 pounds, and its frame, cylinder, and other 

 operating parts another million pounds. 



The cofferdams themselves were larger than many diversion dams 

 located along the tributaries of the Colorado and were constructed 

 with greater care than numerous more permanent structures. Essen- 

 tially they were rolled earth fills whose slopes away from the river 

 were covered with heavy rock blankets. A 4-inch thickness of rein- 

 forced concrete covered the upstream face of the upper cofferdam, 

 sheet steel piling extended to rock across the upstream toe, and a 

 rubber seal at the intersections of the face paving with the canyon 

 walls and sheet piling prevented percolation of water through these 

 vulnerable locations. A massive barrier of rock was placed im- 

 mediately downstream from the lower cofferdam to protect it from 

 eddy action of the river upon its exit from the diversion tunnels. 

 After serving their purpose of diverting the river, the lower coffer- 

 dam and rock barrier were removed as otherwise they would obstruct 

 the flow from the powerhouse tailrace. 



The upper cofferdam was 480 feet long, 750 feet thick at the base, 

 and 98 feet high; the lower cofferdam 350 feet long, 550 feet thick 

 at the base, and 66 feet high, while the rock barrier was 375 feet long, 

 200 feet thick at the base and 54 feet high. The crest width of the 

 upper cofferdam was 70 feet and of each of the downstream structures 

 50 feet. Materials placed in the dams amounted to 514,616 cubic 

 yards of earth, 108,156 cubic yards of rock, and 2,394 cubic yards of 

 concrete. 



Following the skirmishes with the river in the Imperial Valley 

 and the preliminary engagements of a financial, legislative, and pre- 

 paratory nature came the eventual conflict with the Colorado at 

 Black Canyon — the supreme test of man's ingenuity when finally 

 engaged in battle with nature's forces. Arrayed on the river's side 

 were not only cloudbursts, silt, and sudden floods, but as well the 



