Inspectors shocked to find reservoir wall was composed of 'rubble'

ST. LOUIS -- Inspectors searching for the cause of a reservoir breach that sent a billion-gallon torrent of water streaming down the side of an Ozark mountain were dumbfounded by what they saw: the retaining wall wasn't built with granite.

Instead, the broken portion of the wall -- 70 to 80 feet high and about two football fields wide -- appeared to consist entirely of soil and smaller rock. For decades, it was assumed that granite was the main material keeping water in the reservoir.

"We were shocked," James Alexander, director of the Missouri Department of Natural Resources' Dam and Reservoir Safety Program, said Thursday.

At least two homes and several vehicles were swept away, and three children were critically injured when the upper reservoir of the Taum Sauk Lake hydroelectric plant in southeast Missouri breached before daybreak Wednesday. Most of the 50-acre reservoir was emptied in about 12 minutes.

The injured youngsters were children of Johnson's Shut-Ins State Park superintendent Jerry Toops and his wife, Lisa, whose home was ripped from its foundation.

A spokesman at the St. Louis hospital where the children were being treated for hypo-thermia said 5-year-old Tanner remained in critical condition Thursday night. His 3-year-old sister Tara and 7-month-old brother Tucker were upgraded from serious to fair condition and removed from intensive care.

The breach apparently occurred after an automated system malfunctioned and pumped too much water into the reservoir. A backup system that should have caught the problem also apparently failed, said Gary Rainwater, chairman and chief executive of St. Louis-based utility AmerenUE.

Inspectors from AmerenUE and the state were assisting the Federal Energy Regulatory Commission, which oversees the plant, with the investigation.

FERC officials did not return phone calls Thursday night seeking comment. Asked to respond to Alexander's comments, AmerenUE issued a statement that read, "We will address this as part of our investigation."

If a large part of the retaining wall consisted mostly of soil and smaller rock, it likely was doomed once too much water was pumped into the reservoir, said Charles Morris, an environmental engineering professor at the University of Missouri-Rolla. Soil-based retention walls will erode when overtopped, he said.

Leaks have occurred over the years at the reservoir, which was completed in 1963. AmerenUE installed a plastic liner about a year ago to limit the leaks.

While inspectors might not have been able to predict the collapse, they would not have been as dismissive of the leaks if they had known part of the wall was made of fill, not granite, Alexander said.

"If I would have known that, yeah, that would be more of a concern," he said.

Information provided by AmerenUE showed 6 million tons of granite was removed to level the top of 1,590-foot-tall Proffit Mountain, and workers used the removed stone to build a sloping retaining wall 90 feet tall and covering an area the equivalent of 30 football fields.

The company said in addition to granite, the reservoir was lined with concrete and asphalt.