Taum Sauk walls sagged for years, papers show

~ As the reservoir settled, a concrete wall surrounding the top of the earthen dam was also gradually buckling.

ST. LOUIS -- Walls of the Taum Sauk reservoir had been slowly sinking for decades before the mountaintop dam burst last month, according to federal safety inspection documents.

By 1997, some stretches of the Southeast Missouri reservoir had sunk as much as 1.6 feet and could have settled more since then, according to Federal Energy Regulatory Commission documents obtained by the Associated Press.

As the reservoir settled, a concrete wall surrounding the top of the earthen dam was also gradually buckling. FERC sent a letter to AmerenUE on Nov. 19, 2004, telling the company that some panels in the concrete wall had shifted as much as 5 inches. If one of those panels gave way, the entire reservoir could fail, the letter said.

FERC is investigating whether the settling contributed to the reservoir's collapse, spokesman Bryan Lee said. He emphasized that it's far too early to draw any conclusions as to what caused the failure.

The reservoir burst before dawn Dec. 14, sending about 1 billion gallons of water rushing down the side of Proffit mountain. The deluge flattened wide swaths of forest before washing away the home of Jerry and Lisa Toops, injuring their three children.

FERC inspected the Taum Sauk reservoir annually and had approved it as safe every year since 1996, according to annual reports. The dam passed its most recent inspection in August. The positive report was filed one day after the dam collapsed.

AmerenUE spokeswoman Susan Gallagher said the company would not comment on safety issues at the dam while FERC is investigating the collapse.

Built in 1963, the reservoir received accolades as an engineering marvel. Ameren used two engines to pump water into the reservoir at night, filling it like a giant bowl. During the day, water was released through a channel to generate electricity.

Ameren says the dam collapsed after it was overfilled. Automatic pumps failed to turn off, causing water to flow over the top of the concrete walls. The water eroded the earthen dam on the other side, eventually causing the wall to fall and the entire reservoir to collapse under cascading pressure.

That process might have been quickened by the fact that some stretches of the wall had sunk lower than others, said James Alexander, director of the Missouri Department of Natural Resources' Dam and Reservoir Safety Program.

If the wall had remained an even height, water would have spilled over the sides in a shallower flow, he said. Instead, it poured over lower areas with more intensity and likely caused heavier erosion there, he said.

Sunken areas of the wall were also under more pressure, Alexander said, because Ameren filled the reservoir to the same level every night even as the walls sank lower. That left less leeway between the water and the top of the wall, putting a heavier burden on lower sections, he said.

Lee pointed out that the lowest sections of the wall did not collapse.

An independent inspection conducted for FERC in 2003 showed that some portions of the wall had sunk 1.6 feet by 1997. But the report noted that settling might have accelerated since then because of increasing leakage at the dam.

Massive leaking at the Taum Sauk reservoir was well-documented over the years.So much water seeped from the walls that Ameren built a culvert to divert the flow into a detaining pond.

FERC's reports show that the leaking was stable during the 1990s but had nearly doubled by 2003. The reservoir was leaking as much as 2 feet of water a day by 2004. Ameren hired an outside company to install liner in the wall and said leaking stopped after the project was finished in November 2004.

FERC sent a letter to Ameren that month saying the company should pay close attention to shifting in the reservoir's concrete wall.

The wall was built from dozens of panels hooked together through a tongue-and-groove system, Alexander said. As the wall settled, the panels became disjointed.

Alexander said many panels had gaps between them that were filled with a foam-like material that hardened after application. Lee said the foamy material was used only to stop leakage and wasn't a structural support.

FERC inspectors drove along the wall and examined it visually over the years, according to their reports. They saw signs of leakage but no major cracks or misalignment in the wall and deemed it safe. Lee said inspectors found no structural problems in the wall before it collapsed.