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MACROSCOPE

"Reliably Safe"

The history of one problematic dam in Oregon teaches how not to manage risk

Douglas Larson

A New Type of Dam

The Flood Control Act of 1965 first authorized the building of the Willow Creek dam, which was to be located at a narrowing of the river, about 100 meters upstream of the Heppner city limit. To reduce the cost of the project (first estimated at $50 million) and to speed up construction, the Corps decided to use roller-compacted concrete, which had been touted in engineering journals during the 1970s as a revolutionary material suitable for, among other things, dam construction. Willow Creek thus became the first dam in the United States to be built in this way.

Ground was broken in May 1982. About 330,000 cubic meters of concrete were used, most of the material being laid down in successive layers, each 30 centimeters thick, which were then compacted with vibratory rollers. When completed six months later, the dam consisted of 160 layers, stacked to a height of nearly 50 meters, with a total length of some 550 meters. The dam's interior was accessible through a longitudinal maintenance tunnel 275 meters in length. By February 1983, construction was finished. Total cost proved to be around $35 million—well below the original price tag, especially if one takes inflation into account.

Corps engineers then proceeded to fill the reservoir and quickly discovered that water seeped through the dam at a significant rate, estimated at 11 cubic meters per minute, escaping mostly through the junctions between the concrete layers and coalescing into streamlets that cascaded down the dam face. To stem the flow, the engineers largely emptied the reservoir and injected cement grout into a series of vertical drill holes, each extending from the crest of the massive structure to its foundation. This effort, costing an additional $2 million, slowed the leakage to about 3 cubic meters per minute.

In May of 1984, while working as a Corps limnologist, I began a long-term study of the reservoir. What I found was a water body highly enriched with nitrogen and phosphorus and with dissolved and particulate organic matter, originating mostly from cattle feedlots and other agricultural sources in the watershed. As plant nutrients, these substances spurred the growth of enormous quantities of cyanobacteria (blue-green algae) and planktonic diatoms. After dying, these organisms settled to the bottom, where they decomposed along with other organic material washed in during thunderstorms and with the spring runoff from the surrounding snow-covered hills.

During summer, the reservoir became divided into two distinct zones, with warmer, less-dense water above a few meters depth and colder, denser water below. The density contrast prevented any significant circulation between the two layers. Consequently, microbial decomposition of organic matter along the bottom gradually depleted oxygen and greatly increased the carbon dioxide concentration deeper down. By mid-summer, the water that was more than a few meters below the surface became completely anoxic.

In the late summer of 1985, workers inspecting the maintenance tunnel reported a strong "rotten egg" smell, indicating the presence of hydrogen sulfide gas (which can be lethal in sufficient concentrations). Subsequent air testing detected not only H2S, but also potentially explosive methane. Corps officials issued a warning, requiring people entering the tunnel to wear gas masks or other breathing devices.

In a January 1986 memorandum, I warned my Corps colleagues about the possibility that sulfur-oxidizing bacteria in reservoir waters might produce sufficient sulfuric acid to erode the concrete and weaken the dam's structural integrity. This process could also explain the persistent leakage. (I cited several studies of concrete corrosion by H2S-derived sulfuric acid in municipal sewer systems.) I recommended that the Corps commission a thorough study of the dam by independent scientists and that it install an aeration system to replenish oxygen in the deeper waters behind the dam during summer months. On paper at least, one commercial aeration unit, costing $90,000, would be sufficient to preclude the formation of H2S and other reduced chemicals.

My superiors at the Corps approved the independent scientific review but deferred on my proposal for aeration until further study could confirm that it would help in slowing deterioration of the concrete. In the summer of 1986, I assembled a team of outside scientists to assess the situation: four highly qualified researchers from Oregon State University and the University of Washington. Their investigation, conducted during the following two years, determined that chemosynthetic bacteria living in the interstices of the roller-compacted concrete and on the surface of the dam acidified the water seeping through. Chief among these problematic organisms was Thiobacillus, a bacterium capable of oxidizing H2S to sulfuric acid. Other oxidizing bacteria converted ammonia to nitric acid. These acids, along with carbonic acid and organic acids already present in these waters, dissolved the calcium carbonate of the concrete and redistributed some of it within the dam structure. Another fraction was precipitated on the dam face, with the balance being released downstream. The scientific team estimated that, on average, 29 metric tons of cement materials were being carried away annually. This loss, they warned, could compromise the dam's structural integrity, making it unsafe.





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