The staff of the Missouri Public Service Commission concluded that too much water was pumped into the upper reservoir but both loose and improperly set water level sensors failed to alert staff. It was added that the plant was customarily operated with an insufficient margin of safety.

The Commission’s report says: ‘The breach was entirely avoidable in that the company knew for over two months that the water level sensors wee unreliable, as they had broken free from their anchoring system, but unaccountably failed to make repairs.’

In addition, the emergency sensors were set too high to be effective.

The report added: ‘The failure was a management failure in that Ameren had organised the operation of its plants and the performance of maintenance, repair and improvement activities at its plants in such a way that overall direction was lacking and crucial information was not shared.’

The Commission commented that investigation and analysis of the failures may permit Ameren to avoid a similar incident at another of its facilities.

The Commission’s evidentiary hearing ran from July to August, and a report was required by mid-October. The hearings took testimony from 13 witnesses and received 60 exhibits.

The plant was built in 1963 and stored water from Black river. On 14 December 2005, the dam breach reportedly resulted in the upper reservoir draining in about 30 minutes, causing flooding of the Johnson’s Shut-Ins State Park.

In August this year, the US Federal Energy Regulatory Commission (FERC) issued a conditional approval for Ameren Services to start work on the rebuild of the upper reservoir. FERC placed conditions on the rebuild, including approval of the designs and schedule, and a requirement to meet all parties involved. In addition, FERC wanted a reforestation plan, reduced effects on wetlands, and re-opening limited recreational facilities at the lower reservoir.

In its report following the hearing, the Commission said none of the cost of the incident or rebuild, either direct or indirect, will be carried by ratepayers. It also wants to check the financial accounting for the rebuild, and require a single point of supervisory authority on site for the construction work.

The Commission also wants Ameren’s internal policies and procedures to reflect its demands, and requires a ‘whistleblower”’ programme to be implemented. Ameren must also appoint a system-wide safety officer, and report in 90 days on its plan to implement the recommendations.

In February, Ameren submitted plans and an environmental report to FERC to rebuild the upper reservoir, which will involve construction of an RCC dam. It hoped to begin construction work this year and bring Taum Sauk back into service in 2009.

Ameren was not immediately available for comment.

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