The U.S. Army Corps of Engineers (Corps) operates 13 dams in the Willamette River Basin, referred to as the Willamette Valley Project (WVP). The authorized purposes of the dams include flood risk management, water quality, hydropower, navigation, irrigation, recreation, fish and wildlife, and municipal and industrial water supply. The dams are operated as a system to manage flood risk during major flood season (December- January). In spring, the reservoir pools are allowed to fill, capturing a portion of rainfall and snowmelt for the conservation release season (May-November). During the conservation release season, stored water is released for conservation benefits, such as minimum flows, water supply, irrigation, and power production. The dams are operated to meet minimum releases at the individual dams and as a system to meet flow targets at downstream river control points.
As part of its comprehensive dam safety program, the Corps is continuously assessing its dams to better understand dam safety risks and inform future actions. Seismic hazard and seismic performance of the dams in the Willamette Valley is a focus area for the program. Detroit Dam is located about 150 miles east of the Cascadia Subduction Zone, a megathrust fault along the Oregon Coast. The Cascadia Subduction Zone is capable of producing very large, long duration earthquakes every 300-500 years. In 2020, as a part of the comprehensive dam safety program, an updated seismic hazard analysis was completed for Detroit Dam to better understand the potential earthquake ground motions at the site. This hazard study has been used to analyze the performance of the spillway gates and found the risk to be higher than previously assessed. The performance of the spillway gates in an earthquake is a function of the potential ground motions, the height of the gates above the dam foundation, and the water level acting on the gates at the time of an earthquake.
Structural analysis of the spillway gates has shown there is a possibility for buckling of the spillway gate’s supporting arms resulting in an uncontrolled release of water from the dam. With this possibility, there is potential for devastating flooding to affect large portions of the narrow North Santiam river canyon and urban areas because Detroit Dam is located upstream of several communities including the state capital, Salem Oregon.
Although the likelihood of an extreme earthquake is low, the potential impacts of a breach of the spillway gates are high due to the large downstream population. Therefore, immediate action is warranted to reduce risk to acceptable levels.
Targeted measures (called Interim Risk Reduction Measures) are being analyzed for implementation in spring 2021 to reduce life-safety risk while issues are evaluated further. These measures include reducing the maximum conservation pool (the highest allowable level during summer) of Detroit reservoir by 5 feet. The Corps continues to evaluate the seismic performance of the spillway and other components of the dam to determine if long-term modifications or changes to operations will be necessary, including the potential for major rehabilitation or reconstruction of the spillway structures.
The pool restriction elevations from 1563.5 feet to 1558.5 feet at Detroit Damn (5-foot reduction) was assessed as an alternative based on the recommendation of the risk assessment teams and additional analysis performed by the Corps to reduce life-safety risk to tolerable levels.
The project requires review under applicable laws and regulations, including the National Environmental Policy Act (NEPA). The proposed pool restriction (Action Alternative) would begin 29 April 2021, after the Corps has received all required environmental clearances and completed the NEPA process and continue until 7 September 2021 and recur annually until a permanent solution is developed. To evaluate the impacts of implementing this interim measure, the Corps team used a Hydrologic Engineering Center (HEC) reservoir simulation model (HEC-ResSim) to determine the potential project and system-wide impacts of a pool restriction at Detroit Dam. The Corps has developed this Draft Environmental Assessment (EA) to evaluate the environmental impacts of this summer conservation pool restriction at Detroit Dam. The interim pool restriction is presented as the Action Alternative in the EA. The Corps is the lead federal agency for this EA.
This draft EA is being made available for a 15-day public review period. All comments, whether by conventional mail or email, must be received no later than the expiration date of this public notice to ensure consideration.