OVERVIEW AND HISTORY OF THE TAUM SAUK PUMPED STORAGE PROJECT

The Taum Sauk Pumped Storage Power Plant was built by Union Electric (now AmerenUE) between 1960-63 to store water for peak power generation. It consists of a lower reservoir sited along the East Fork of the Black River and an upper reservoir, connected via a largely unlined tunnel through rhyolite and granite, 800 feet higher atop nearby Proffit Mountain. Prior to failure, the upper reservoir consisted of a kidney-shaped rockfill dike approximately 70-90 ft. high, capped by a 10 ft. concrete parapet wall. It held 1.5 billion gallons (~4,600 acre-feet) when filled. The project was the largest pumped storage facility in North America when it began operations and won a prestigious engineering award just weeks prior to the failure.

The upper reservoir was formed by blasting the top of Proffit Mountain and forming the rock rubble into the embankment. This structure was lined with reinforced shotcrete to contain leakage. This liner cracked as portions of the embankment settled up to two feet due to excessive fines in the rockfill. The ongoing settlement also exacerbated seepage losses.

Prior to 1994 when the electric power industry was deregulated, the plant was operated approximately 100 days per year. Electric utilities were allowed to sell power on the spot market to other utilities at unregulated rates, making operations at the plant profitable up to 300 days a year. The plant’s massive pump-turbine units were replaced with higher efficiency models. By 2004, leaks through the cracked reservoir liner resulted in ~2% lost capacity per day, so a geomembrane liner was installed to curtail the losses. The reservoir’s water level gages, which were used to activate and shutdown the pumps, were also replaced and played a role in the failure.

At least two overtopping incidents occurred 11 weeks prior to the Dec 2005 failure, one of which was described as “Niagara Falls” by plant operators. This overflow resulted in noticeable scour of the embankment crest, requiring repairs. A less serious overtopping incident occurred a few days later, which was not directly observed. The new instrumentation conduits worked themselves loose and experienced severe deflections that registered erroneous stage levels. Although operators warned of continued use, a workaround was instituted and repairs were postponed.