Diversion at Portage Mountain Project

by Harold K. Pratt, (F.ASCE), Chief Engr.; British Columbia Hydro. and Power Authority, Vancouver, British Columbia, Canada,

Serial Information: Journal of the Power Division, 1970, Vol. 96, Issue 3, Pg. 349-368

Document Type: Journal Paper

Abstract: A 15 yr flood of 270,000 cfs was ued for the design of the diversion facilities during dam construction. The design flow was determined by extending flow records through correlation with neighboring rivers. The diversion path was an open cut intake channel, 800 ft long and 280 ft wide, leading into three 48-ft diam., horseshoe-shaped, concrete-lined tunnels, designed for free flow. Savings resulted from using depressed tunnel inverts with upshots at the outlet portals, and from the 2-ft thick unreinforced lining. The depressed inverts provided supercritical flow in the tunnels and the upshots proved beneficial in deflecting flow form the river bottom at the tunnel outlets. Diversion was accomplished by blowing a rock plug at the entrance to the intake channel and completing closure of the diversion cofferdam. The tunnel successfully carried an instantaneous peak flow of 314,000 cfs.

Subject Headings: Tunnels | River flow | Hydraulic design | Water intakes | Supercritical flow | Floods | Building design |

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