PORTLAND, Ore --
The Portland District found no significant environmental impacts associated with reducing Detroit Dam’s Reservoir maximum pool elevation by five feet to curb the risk of a large earthquake causing the spillway gates to buckle.
Based on the Finding of No Significant Impact (FONSI), the district commander signed the Interim Risk Reduction Measure Plan, which means Detroit’s reservoir maximum fill level will top out at 1558.5 feet. With the lack of rainfall and a predicted low water year, the lake level is not expected to reach the new maximum pool elevation this summer.
In late 2020, the District completed an earthquake hazard analysis of Detroit Dam. With a better understanding of seismic affects, the study found the dam’s overall risk assessment to be higher than the previous evaluation, completed in 2018. Structural analysis of the spillway gates in a simulated earthquake exposed the possibility of the gate’s supporting arms collapsing. The uncontrolled release of water could cause devastating flooding to downstream populations – including Oregon’s capital city, Salem. The District’s dam safety office recommended immediate action to restrict the pool elevation under the Interim Risk Reduction Measures umbrella.
“The Corps has a public safety responsibility when a project has known safety issues. The risk reduction measure is an important action in managing the seismic risks associated with Detroit Dam,” said Ross Hiner, the Portland District dam safety program manager. “There is no immediate threat to the dam’s stability or safety.”
The dam was designed and constructed in the 1940
’s and 50 's prior to a modern understanding of earthquake hazards in the Pacific Northwest.
“As we learn more about earthquake performance in future studies, we will be determining whether physical remediation or other long-term measures will be required in order to re-establish the full operational range and benefits of Detroit Dam,” said Hiner.
As required by the National Environmental Policy Act (NEPA), the Corps conducted an Environmental Assessment (EA) to evaluate the proposed IRRM’s overall impacts. The final EA can be found here: https://go.usa.gov/xHAeE.
Comments received during the 15-day public comment period of the environmental assessment process can be found in Appendix C of the final EA. A recording of the public engagement meeting on 22 March 2021 can be found here: the following link: &feature=youtu.be
For more information about the Detroit Dam IRRM plan visit: https://www.nwp.usace.army.mil/DetroitRiskReduction
Portland District’s 150th Birthday: Portland District is one of the nation’s most comprehensive and diverse civil works programs and has been supporting the people of Oregon and southwest Washington since 1871. Throughout its 150-year history, the District has been operating locks and dams along the Columbia River, managing flood risks in the Willamette Valley and Rogue River Basin, maintaining Oregon's coastal waterways for navigation, and leading the Nation in hydropower generation. The team of more than 1,400 civil servants manage these missions all while ensuring equal attention is paid to environmental protection and restoration, fish and wildlife enhancement, and world-class recreation opportunities.