OMAHA, NE -
Water managers for the U.S. Army Corps of Engineers Omaha District have extended the public comment period to December 16 for a Draft Report outlining proposed modifications to the Water Control Plan for Cherry Creek Dam during extreme flood events.
The proposed plan allows water managers to release more water from the dam to reduce the risk of overtopping and failure. Public comments on the report will now be accepted through December 16, 2016. Changes to the water control plan could still be implemented as early as January 2017.
The plan’s recommended changes are to increase releases from Cherry Creek Dam to 7,000 cubic feet per second when the reservoir pool elevation reaches 5591.8 feet NAVD88. The normal pool elevation is 5551.8 feet NAVD88 and average releases are between 10 and 100 cubic feet per second.
“Extreme rainfall events occur, which is why we must have plans in place to address them,” said Kellie Bergman, Chief of Water Management for the Omaha District. “However, it is important to note that the record reservoir elevation at Cherry Creek Dam is 25 feet below the recommended pool elevation that would trigger the higher releases proposed in a modified water control plan,” Bergman added.
The study is a result of input from public scoping meetings held in January for a study at Cherry Creek Dam to address the dam’s potential for overtopping and failure in a maximum conceivable precipitation event.
While modifying the water control plan would reduce the risk of overtopping and failure at Cherry Creek Dam during extreme flood events, it does not eliminate the risk. With a modified water control plan the dam could safely pass 22.7 inches of basin average rainfall runoff, which is two inches more than it can currently pass, but still two inches short of the maximum conceivable precipitation event.
Materials including presentations, posters, fact sheets and a video from public meetings held in September to present proposed modifications to the Cherry Creek Water Control plan and alternatives under consideration to address overtopping concerns are available at go.usa.gov/cQ7hP.
Public Comments on the Cherry Creek Water Control Plan Modification Report may be submitted via email to firstname.lastname@example.org or mailed to: U.S. Army Corps of Engineers, Omaha District; CENWO-ED-HA; ATTN: Katie Seefus; 1616 Capitol Avenue; Omaha, NE 68102-4901. Comments must be postmarked or received by December 16, 2016.
Background: The Cherry Creek Dam project was authorized in the 1940s for the primary purpose of mitigating flood risk to the downstream city of Denver from floods originating on Cherry Creek above Cherry Creek Dam. Cherry Creek Dam and Reservoir is located on Cherry Creek, 11.4 miles southeast of its confluence with the South Platte River in Aurora, Colorado.
In 2005, USACE sought to better categorize the risk at all of its dams. Cherry Creek Dam received an elevated risk rating primarily because of the dam’s large downstream population and the potential for overtopping issues during an extremely rare precipitation event.
The probable maximum flood event is the resulting flood from the maximum conceivable precipitation event and the standard used to establish a dam’s reservoir capacity. For Cherry Creek Dam, the maximum conceivable precipitation event is 24.7 inches of rainfall over a three day period in the Cherry Creek basin. Updated data models indicate Cherry Creek Dam would overtop and fail in this scenario which requires the U.S. Army Corps of Engineers to address this hydrologic deficiency. The large downstream population, which extends from the dam, through downtown Denver and along the South Platte River into North Platte, Nebraska, increases the dam’s risk level and the priority for addressing its potential for overtopping.
The current Cherry Creek Dam Safety Modification Study began in 2013 to evaluate those risks in greater detail and to assess possible options for remediating the risk in accordance with USACE policy as described in Engineering Regulation 1110-2-1156 “Safety of Dams – Policy and Procedures.”
For more information about the Cherry Creek Dam Safety Modification Study, visit http://www.nwo.usace.army.mil/Missions/CivilWorks/Planning/PlanningProjects/CherryCreekDSMS.aspx