The team discovered that the high flow in 2011 dislodged a manhole cover, one of many covers that provides access to the drainage system under the concrete slab of the spillway chute. The system, designed to drain normal water seepage under the spillway, was overwhelmed by the additional water that passed through the missing cover. Dam safety experts evaluated the covers to determine their ability to remain in place during future spillway releases. It was concluded that under extreme operating conditions—significantly larger than the 2011 flood—dislodged manhole covers would allow water under the chute and force the displacement of a portion of the slab. Known as hydraulic jacking, this condition could lead to a failure of the spillway to perform as designed and would reduce USACE’s ability to control high water releases from the reservoir.
Potential solutions could include thickening the spillway slab with various types of concrete to make the drainage system redundant, redesigning and constructing a new drainage system that relocates the manholes outside of the spillway slab, retrofitting the manhole covers to make them more resilient, or a combination of these measures.