An independent expert who examined the breach of the upper reservoir of AmerenUE’s 408-MW Taum Sauk pumped-storage project says poor construction practices and failure to meet intended design criteria led to the Dec. 14, 2005, failure of the mountaintop reservoir.
The report, by Paul C. Rizzo Associates Inc., also said it is Rizzo’s opinion that a secondary cause of the failure was inadequate attention to dam safety considerations in regard to design, operation, and management of the facility.
Rizzo’s 144-page report, commissioned by AmerenUE, said the utility found in October 2005 that plastic pipe supports housing reservoir level controls had failed and that repairs were not implemented prior to the dam failure because employees believed water level protection sensors would serve as backup protection. However, Rizzo said, those sensors were improperly installed, too high on the dam wall to be effective. Failure came during pump-back operations.
“During the morning of Dec. 14, 2005, the auto stop elevation for the second pump was not reached until overtopping had occurred and the Upper Reservoir Dike was very near to or at a failed condition,” the report said.
Water saturated fine materials in dam wall
Rizzo said the reservoir’s failure began 10 to 13 minutes after overtopping began. The report said the rock dam wall failed quickly because the quality of the material used in the dam’s original construction deviated from design specifications. Because of the fine-grained material used in the dam, water was unable to pass through the rockfill structure and saturated it, much like water saturates a sponge. An excessive amount of water collected in the dam material and at the foundation interface. That created an unsafe condition, causing the dam to fail in a rapid manner.
The breach released about 4,300 acre-feet of water, injuring three children of a park superintendent living below the dam, near Lesterville, Mo.
While the original design of the reservoir was consistent with practices of the late 1950s and early 1960s, Rizzo found the builders did not follow best construction practices available at the time. The dam was completed in 1963.
Additionally, Rizzo said construction practice in the field was not consistent with the intent of design drawings and specifications.
Overflow sensors installed at wrong elevation
The report also said design and specification of the water level instrumentation system was not sufficiently conservative. The plant had a single control system monitoring water levels that normally shut off pumps, plus a back-up system designed to shut off pumps should the other system fail.
Rizzo said overflow protection sensors should have been installed two feet below the crest of the parapet. Instead, they were found at seven inches and four inches below the crest. Had the probes been at design elevations, the overtopping might not have occurred, Rizzo said. Even given the loss of the back-up water level protection, overtopping still could have been prevented had the level control instrumentation supports not failed, the report said.
While operators and technicians for Taum Sauk (No. 2277) followed operational and inspection procedures as provided by AmerenUE, inadequate attention was paid to dam safety considerations, Rizzo said. Additionally, responsibilities for plant operation and dam safety were handled by a single individual. The report said any one person with those responsibilities might be forced to balance dam safety and operational considerations.
Dam must be rebuilt if reservoir is to be restored
The consultant concluded restoration of the upper reservoir to an operating condition cannot be achieved by simply repairing the breach. Restoration, if undertaken, would require the entire dam to be rebuilt with a completely different, more robust design.
No decision will be made about rebuilding the upper reservoir until the Federal Energy Regulatory Commission completes its investigation, the company said.
Ameren responds to report’s recommendations
St. Louis-based Ameren Corp., parent of AmerenUE, said April 7 it would respond decisively to the report. Ameren Chairman Gary Rainwater said the company is creating a dam safety program with an experienced civil engineer as chief dam safety engineer.
The program will include an updated dam inspection plan and site-specific safety and instrumentation training. The chief dam safety engineer will have the authority to conduct unannounced facility inspections and to take corrective action, including plant shutdown, when dam safety is threatened.
The company said it plans to establish an independent quality assurance team to review and strengthen operational processes and procedures throughout the generation system. Ameren also will re-examine safety procedures at all of its facilities, including emergency action plans, and evaluate the training of employees in safety and operations.