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Transcript for Challenge at Glen Canyon, segment 03 of 11
Efforts were made to control this unanticipated rise of Lake Powell by operating the power plant at full capacity, thus releasing twenty-eight thousand cubic feet per second of water. But it was not enough as inflow into Lake Powell rose quickly to over ninety thousand cubic feet per second in early June and the left spillway gates had to be opened. It was understood that these spillways would probably suffer some erosion by a physical process called cavitation. Cavitation occurs when high velocity flows are thrown upward by some small obstruction. This causes a partial vacuum which produces vapor cavities in the water. These unstable cavities then collapse, sending intense shock waves against the concrete. At first small pieces, then larger pieces of concrete are literally pounded out. After one hole has formed, a leapfrog action tends to promote the start of another on down through the tunnel in stairstep fashion.
It was realized, too, that most of the damage would probably occur at the elbow section where the spillway levels out. After only four days operation, inspectors found that cavitation had indeed been active at Glen Canyon. A photo taken by one of the inspectors disclosed holes in the concrete lining twenty feet wide and up to four feet deep. The spillways would have to be used, but the left one would carry most of the excess flow, thus preserving the right spillway for any future need. To reduce spillway use, wooden plywood flashboards four feet high were added to the spillway gates, and the outlet tubes were opened to bypass an additional seventeen thousand cubic feet per second of water.
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