FLOODGATE 13


Even today the project seems enormous. In the late 1920s, the engineering firm of Stone & Webster designed and built Bagnell Dam and Lake of the Ozarks. At the time of its creation, Lake of the Ozarks was the nation's largest man-made reservoir: 129 miles long with an additional 80 linear miles of tributaries and long coves; 1,300 miles of shoreline; 95 square miles of surface area; 650 billion gallons of impounded water. Bagnell Dam would be 2,543 feet long, 148 feet high, with an electrical generating capacity of 201,000 horsepower and an annual average output of 400 million kilowatt hours.

Every aspect of the dam construction had to be worked out in detail. Tens of thousands of mathematical equations were computed on slide rules. Nothing could be left to chance. The lake itself would serve as a giant storehouse of energy for the generating turbines--as dependable as gravity, and as sure as rain.

The Osage River was the source of all the lake water. The Osage drained an area of 14,000 square miles. Even in the driest years it never ceased to flow. Indeed, the Osage was as well known for its sudden and spectacular floods as for its periods of diminished flow. Bagnell Dam would be designed not only to hold back the river, but to release excess flow in any flood, even the greatest imaginable flood. This was critical, for the worst thing that could happen was to have the headwaters overtop the dam.

Just how much water could theoretically come down the Osage valley was an educated guess. Mathematical formulas were developed that took into account the total drainage area, the type of terrain and ground cover, and the heaviest amount of sustained rainfall. The first two parameters could be quantified, but the last--the heaviest rainfall amount--was an unknown variable. Government weather records had only been kept for forty years prior to the 1920s; the extent of earlier floods was largely conjecture based on piecemeal historical accounts and scanty geological evidence. A margin for error would have to be factored into the formula. The dam subsequently was designed for thirteen floodgates, each capable of releasing 101,000 gallons of water per second.

The spillway section of Bagnell Dam. The steel latticework of the floodgates can be seen above the white cascading water. Each floodgate weighs 27 tons and is lifted by the moveable gantry cranes seen above the dam structure. Tarp covers the fifth floodgate from the right while workmen sandblast and repaint it.

During the 1929-1931 construction phase, Bagnell Dam was framed for all thirteen floodgates. But at nearly the last minute there came a change. Some officials within Union Electric Light and Power Company--owner of the dam--objected to the idea of thirteen floodgates. It was an unlucky number, they said. Many skyscrapers built at the time did not have a thirteenth floor--at least on the building directories--for the very same reason. So why should Bagnell Dam be forever jinxed with thirteen floodgates?

So the engineers were consulted. They took out their slide rules and worked the numbers again. Could the dam do without a thirteenth floodgate? Was it safe to reduce the flow rate by 101,000 gallons per second? Well, yes...probably. After all, the maximum design flood was theoretical. And the margin of error was...reasonable.

The thirteenth floodgate was never installed. The spillway opening where it would have hung was walled in with concrete. It was not needed in 1943 when the Osage River rose to record high levels. The dam was not overtopped--the twelve existing floodgates handled the flow. And Bagnell Dam has experienced few problems in its 80 year history. Perhaps that history would have been far different had there been floodgate 13. Or is the real test is yet to come?

 

Copyright © 2011 by Michael Gillespie. All rights reserved.