An engineering firm has determined that instrumentation failure and human error were the primary causes of the Taum Sauk Reservoir Dike failure on Dec. 14, 2005, in Missouri. The owner is awaiting a federal response before considering any rebuilding effort.

Flood. Instrumentation failur, human error and construction deficiencies caused breach.

At about 5:24 a.m. a 700-ft long section of the concrete-faced, rock-filled, kidney-shaped 6,562-ft-long dike failed due to overtopping. The resulting breach spilled about 1 billion gallons of water from the 55-acre impound down the western slope of Profitt Mountain through Johnson’s Shut-Ins State Park to the Black River, near Lesterville, Mo. The torrent injured three people.

The 84-ft-high dike is part of a 43-year-old pumped-storage peaking powerplant owned by St. Louis-based AmerenUE, a unit of Ameren Corp. During peak electrical demand, water flows from the reservoir on top of the 1,540-ft tall mountain, through a mile-long tunnel inside the mountain to generate power at the 450 MW plant. During off-peak hours, the water is pumped up from a 2.1-billion gallon lower reservoir.

After the accident, Ameren hired Paul C. Rizzo Associates Inc., Monroeville, Pa., to investigate. Rizzo’s April 5 report notes that a liner was installed and the instrumentation and control system was upgraded at the dike during a 2004 outage, but it concludes that “the design and specification of the instrumentation system were inadequate from a dam safety perspective.” The report also notes that inadequate initial design of instrumentation supports led to field changes, which led to support failure and faulty readings. Misplaced probes also caused problems. “These three items combined to allow the overtopping of the reservoir during the pump-back cycle,” the report says.

Rizzo’s report also takes issue with the dike’s construction, noting that it was consistent with general, but not best practices of the late 1950s and early 1960s. It would not be adequate today because of “excessive fines within the rockfill, inadequate foundation preparation, lack of compactive effort during rockfill placement, and the use of the parapet wall to store water on an everyday basis,” the report says.