Devil’s Kitchen Dam Auxiliary Spillway Modifications

Devil’s Kitchen Dam, a concrete gravity structure owned and operated by the U.S. Fish and Wildlife Service (USFWS), crosses Grassy Creek to form Devil’s Kitchen Lake, one of Illinois’ deepest lakes. The lake attracts visitors to the Crab Orchard National Wildlife Refuge and is a popular fishing destination, offering one of the few places in the state to find rainbow trout.

Designed by the U.S. Army Corps of Engineers and built in 1959, the dam includes a 670-foot-long concrete gravity section, 214-foot-long embankment, and 554-foot-wide uncontrolled main spillway. An uncontrolled earthen berm auxiliary spillway is located 400 feet from the dam’s left abutment.

The 540-foot-wide auxiliary spillway was designed and constructed so the dam could pass the inflow design flood, the maximum flood event the dam must withstand. However, frequent activation of the auxiliary spillway resulted in overtopping of the earthen berm, leading to erosion of the berm and downstream channel and requiring frequent maintenance. The overtopping and erosion had the potential to cause the berm’s failure, leading to possible downstream flooding.

The auxiliary spillway’s activation affected the public roadway immediately downstream of the earthen berm. This roadway provides access for maintenance and emergency operations related to the dam and the surrounding recreational areas. During activation, the roadway became impassable, limiting dam access for monitoring and inspection.

To reduce risk, the USFWS implemented measures to increase the auxiliary spillway’s safety and reliability.

Our team addressed these critical issues by evaluating and designing dam safety modifications to enhance the spillway’s resiliency and reduce its activation frequency. The project involved replacing the original earthen berm with a more robust reinforced concrete parapet with crest elevation 1 foot higher than the original berm.

The new parapet, which raises the crest of the auxiliary spillway to an elevation of 514 feet, spans 496 feet in length and reaches a maximum height of four feet. The higher crest elevation reduces the auxiliary spillway’s activation frequency, reducing the occurrences of roadway flooding and downstream channel erosion. In addition, replacing the earthen berm with a concrete parapet eliminates the risk of overtopping failure of the berm, which may result in downstream flooding. Additional riprap was placed in the approach and discharge channels to mitigate potential erosion.

The parapet was constructed on the roadway’s upstream side. To compensate for lost roadway width, the roadway was widened by four feet, and new guardrails were added to maintain safety.