The Tale of Two Spillways

ABSTRACT ONLY - They were the best of spillways, but they were deficient spillways, it was the age of risk reduction.
Crab Orchard Dam and Devils Kitchen Dam are high hazard dams, owned and operated by the U.S. Fish and Wildlife Service, located on the Crab Orchard National Wildlife Refuge in southern Illinois. Although these dams are located less than 6 miles apart and were both built in the mid 1900's, the dams are polar opposites. Crab Orchard Dam is an earthen embankment dam with a concrete service spillway and Devils Kitchen Dam is a concrete gravity dam originally constructed with an earthen berm auxiliary spillway. In the past year, the spillways at both dams have undergone significant dam safety rehabilitation and modifications, which included construction of a concrete overlay of the service spillway at Crab Orchard Dam, and a concrete parapet control section at the auxiliary spillway of Devils Kitchen Dam.
The paths that lead to these projects are both unique and filled with interesting inspection findings, questions about the original design, past modifications leading to unintended consequences, and concerns over the integrity and performance of the structures. At Devils Kitchen Dam, information in early reports lead to the uncertainty if the earthen berm was intended to act as a fuse-plug spillway; apparent changes in hydrologic conditions resulted in more frequent activation of the auxiliary spillway; and the structural integrity of the auxiliary spillway was questioned after a recent activation. These questions and concerns, among others, ultimately led to modifying the auxiliary spillway to not only improve the integrity of the spillway, but also reduce its frequency of activation.
At Crab Orchard Dam, the past installation of fish screens resulted in unintended hydraulic consequences during a significant rainfall event, leading to their removal; potential instability of the spillway was identified during certain seismic conditions; and concrete deficiencies were observed during a detailed concrete inspection. The spillway was consequently modified and rehabilitated with a reinforced concrete overlay, which was designed to account for limitations with lowering the reservoir during construction.
The story of both spillways illustrates prime examples of using information collected over the entire history of the structures to make risk informed decisions to improve the performance and condition of these spillways.