Cherry Creek Dam US Army Corps of Engineers ® Water Control Plan Modification Study Omaha District

There is a significant safety concern at Cherry Creek Dam because of its location in a densely populated area that includes much of downtown Denver, and the dam’s potential for overtopping during an extreme rain event. Although the chances of overtopping or failure are very remote, the consequences, which include life loss, economic loss, and environmental damage, are what designate Cherry Creek as a high risk dam.

The U.S. Army Corps of Engineers is currently conducting a Cherry Creek Dam Water Control Plan Modification Study. The goal of the study is to establish a timeline for increased releases from Cherry Creek Dam in an extreme flooding event to reduce the potential for dam overtopping and failure as well as reducing the overall flood extent.The following will be considered in the study: (1) removing the 5,000 cubic feet per second (cfs) Denver maximum flow target to ensure releases can be made from Cherry Creek Dam, (2) increasing Cherry Creek Dam releases above the 5,000 cfs limit (up to 13,300 cfs), and (3) evaluating added impacts of Cherry Creek Dam releases to existing uncontrolled drainage flooding.

Releasing more water from Cherry Creek Dam during extreme flooding could prevent dam overtopping during some extreme events and would significantly reduce the cost of structural modifications to prevent overtopping.Additional information about the Water Control Plan Modification Study can be found at http://go.usa.gov/cQ7hP.

Results from this study will also be incorporated into the Cherry Creek Dam Safety Modification Study. Additional information about the dam safety study can be found at: www.nwo.usace.army.mil/Missions/CivilWorks/Planning/PlanningProjects/CherryCreekDSMS.aspx.

Project Location and Description

Cherry Creek Dam, completed around 1950, was built and is currently operated by the U.S. Army Corps of Engineers for the primary purpose of flood risk mitigation. In the late 1950s, recreation replaced irrigation as a project purpose. Cherry Creek Dam is an earth-fill embankment dam with an outlet structure for operational water releases.The embankment is 14,300 feet long with a maximum height of 141 feet. The dam’s outlet structure is a triple barrel concrete conduit system through which water discharges into the Cherry Creek channel located a short distance downstream and flows toward downtown Denver. U.S. Army Corps of Engineers Omaha District 1616 Capitol Ave., Ste. 9000, Omaha, NE 68102 • 888-835-5971 • www.nwo.usace.army.mil Find us on Facebook: facebook.com/OmahaUSACE • Twitter: twitter.com/OmahaUSACE • Flickr: http://www.flickr.com/OmahaUSACE YouTube: http://www.youtube.com/OmahaUSACE • DVIDS: http://www.dvidshub.net/unit/OmahaUSACE The spillway, located at the dam’s east abutment and running parallel to the alignment of the dam, is designed to minimize the likelihood of the dam overtopping in an extreme rain event. It is an open cut channel designed to transfer floodwater into the Toll Gate and Sand Creek drainage basins if the lake’s pool elevation reaches the spillway crest elevation. If Cherry Creek Reservoir rises to elevation 5647.6 feet, the spillway would flow at an estimated rate of 30,000 cfs where one cubic foot is equal to 7.5 gallons. This would cause flooding, which in some cases could be significant, in Denver, Aurora, and Commerce City. The non-damaging flow rate on Toll Gate Creek and Sand Creek is less than 2,000 cfs.

Cherry Creek Dam is managed in combination with Chatfield and Bear Creek Dams to mitigate flood risk in the Denver area. The current water control plan for Cherry Creek Dam, as well as Chatfield and Bear Creek Dams, limits releases from the outlet structure to not exceed a flow rate of 5,000 cfs at the Denver stream gage.The targeted flow at this stream gage includes the uncontrolled drainage flooding in Denver, Colorado, reservoir releases from Cherry Creek, Chatfield and Bear Creek Dams, and is imposed to prevent significant flooding in downtown Denver. What You Should Know

Because dams do not eliminate flood risk, downstream residents should become aware of the potential impacts to their family, home and community. They should also learn about their community’s evacuation notification plans and safe, efficient evacuation routes. The highest population at risk, due to dam failure, is immediately downstream of the dam along Cherry Creek and the South Platte River. Life loss potential also exists downstream of the spillway along Toll Gate Creek and Sand Creek. Denver, Arapahoe, and Adams Counties have the highest potential for life loss.

The figure on the right shows inundation mapping in the Denver area resulting from a dam breach due to overtopping. Inundation from a Cherry Creek Dam breach will extend downstream along the South Platte River into Nebraska. Recommendations for Residents

• There is risk associated with living near a dam—know your risk. • Listen for and follow instructions from local emergency management officials. • Evaluate your need to purchase flood insurance.

Inundation map showing flood boundaries for the 100-year floodplain and for the maximum conceivable flood event. Map Disclaimer: Actual areas flooded will depend on storm magnitude and Additional Information failure conditions. For more information concerning the release of inundation mapping, please contact the Omaha District using the information located on the front of this fact sheet. For additional information about dam safety and living with dams, please visit: www.usace.army.mil/Missions/CivilWorks/DamSafetyProgram.aspx www.damsafety.org/media/Documents/DownloadableDocuments/LivingWithDams_ASDSO2012.pdf

February 8, 2016* 2 *Modified from January 26, 2016 public meeting