Washout. Photos of the collapsed enclosure appeared to show a wall made of fill material. (Photos by AP/Wideworld)

A contractor is stabilizing sediment and dealing with debris damming the river below a collapsed pumped-storage reservoir on a Missouri mountaintop while federal and state inspectors begin what is expected to be a months-long investigation to determine the cause of the collapse.

Initial inspections suggest that the 100-ft-high rock-filled, concrete-lined wall enclosing Taum Sauk Reservoir, near Lesterville, Mo., was comprised of rocky fill, not the granite blocks removed to create the reservoir in 1963. The collapse at 5:24 a.m. on Dec. 14 spilled about 1.3 billion gallons of water and triggered a call from Gov. Matt Blunt (R) for a comprehensive survey of Missouri’s dam safety law and a comparison with regulations in other states. The Highway Patrol also was investigating whether the failure was accidental.

AmerenUE, St. Louis, the owner of the reservoir, has contracted with Mactec Inc., Alpharetta, Ga., to begin addressing short-term issues at the site.

A 20-ft-high wave swept from the 55- acre reservoir, down the western side of Proffit Mountain, through a state park, into the Black River and finally into a lower 380-acre storage reservoir—a distance of about two miles. The flood destroyed the home of a park superintendent and injured his three children, who were hospitalized. One remains in a hospital, but is expected to recover.

The 1.5-billion-gallon reservoir is part of a 42-year-old pumped-storage peaking plant. During peak power demand, water from Taum Sauk flows through a mile-long tunnel to generate electricity at the 450-MW plant. The water is held in a lower 380-acre, 2.1-billion-gallon reservoir, then pumped back during the night. In 2004 a polyethylene liner was installed at the upper dam to stem some small leaks.

Ameren officials think something went wrong with the pumping operation and it did not automatically shut off, leading to overtopping and a 600-ft-long breach. “We do not know for certain the cause of the failure, but it looks like it may have been caused by an instrumentation failure at the reservoir,” says Gary Rainwater, chairman, president and CEO of Ameren, the parent of AmerenUE. “This is preliminary and we are in the process of bringing in experts to the site to give us a more precise determination.”

Six million tons of granite, removed from the top of Proffit Mountain to create the reservoir, were used to raise its enclosing wall, according to AmerenUE. But James Alexander, director of the Missouri Dept. of Natural Resources, says photos of the collapsed section appear to show only soil and small rocks. DNR inspectors now are on site, determining the facts for further action.

On Dec. 20, Mactec presented its plan for the work at a public meeting in Lesterville, near the reservoir. Results were not available at ENR press time, but a DNR spokeswoman says Mactec’s presentation is expected to lay the groundwork for scheduling future work. Debris from the collapse partially dammed the East Fork of the Black River and changed the flow, but drinking water and wastewater facilities were not affected, she says.

The Federal Energy Regulatory Commission and state Dept. of Conversation also are investigating. No cost estimates are available.