Four Idaho, Washington dams plus McNary Levee System reclassified as safer by Corps of Engineers
Dworshak Dam reclassified from 2012 “Moderate Risk” to “Low Risk” following second Issue Evaluation Study;
other dams and levee system reclassified safer after Corps risk assessments
WALLA WALLA, Wash. – Four U.S. Army Corps of Engineers Walla Walla District dams in Idaho and Washington state, plus the McNary Levee System in southeast Washington, have been reclassified by the Corps as safer “Low Risk” dams and dam-related levees within the Corps’ nationwide Dam Safety Action Classification (DSAC) system. The Corps operates more than 700 dam nationwide and has an ongoing and aggressive Dam Safety Program to reduce public risk and environmental damage.
Reclassified “Low Risk” dams include Dworshak Dam near Orofino, Idaho; Mill Creek Storage Dam near Walla Walla, Wash.; Ice Harbor Lock and Dam near Burbank, Wash.; and Lower Monumental Lock and Dam near Kahlotus, Wash.
The McNary Levee System, also known as the “Tri-Cities Levee System,” was also reclassified as “Low Risk.” It’s located upstream of McNary Lock and Dam at the confluence of the Columbia and Snake Rivers. The dam-related levee system was constructed as part of the McNary Lock and Dam project and is comprised of Richland Levees, Kennewick Levees, and Pasco Levees.
“Public safety is our highest priority, and safety is a daily focus at our dams,” said Lt. Col. Damon Delarosa, Walla Walla District Commander. “We can’t completely eliminate risk, but we can reduce risk of dams and dam-related levees by continuing to inspect, assess, maintain or upgrade the dams we manage”
The Corps’ Dam Safety Program includes monitoring and periodic inspections of Corps dams, identifying and managing risk, short-term and long-term risk reduction measures, and prioritizing dam safety actions on a nationwide basis. Corps dams are classified, in increasing order of safety, as Very High, High, Moderate, Low, or Normal Risk or Urgency. The Corps has used this risk-informed DSAC process since 2007. Dam Safety Action Classifications can change as the Corps evaluates potential concerns and impacts on downstream residents, communities and environment.
“Low Risk” means for confirmed and unconfirmed dam safety issues, the combination of life, economic or environmental consequences with the likelihood of dam failure is low to very low. Reclassifications of the four dams and one levee system were recommended to the Corps Headquarters’ Dam Safety Officer in Washington, DC by the Corps’ risk assessment teams, the Walla Walla District, and Corps Headquarters’ Dam Senior Oversight Group (DSOG).
The Corps recently reclassified Dworshak Dam near Orofino, Idaho, as a safer “Low Risk” dam after completing a Phase 2 Issue Evaluation Study (IES). The Phase 2 IES updated the consequence estimates, gathered additional data, and performed further analysis of the dam’s performance during a seismic event. Analysis showed the probability of a dam breach due to a seismic event is remote.
Previously, Dworshak had been initially classified in October 2007 as “High Risk” because of engineering unknowns related to structural stability and foundation seepage of concrete gravity sections. In May 2012, Dworshak was classified as a safer “Moderate Risk” dam based on a Phase 1 Issue Evaluation Study, which improved the Corps’ understanding of conditions at the dam. That Phase 1 IES confirmed the robustness of the dam design and historical performance of the structure, and that potentially significant failure modes are a result of rare seismic events.
The McNary Levee System was recently reclassified as “Low Risk” after the Corps performed a risk assessment including a potential failure mode analysis (PFMA). Remaining risks are associated with potential internal erosion of embankment foundation materials and potential overtopping of embankments during infrequent flood events.
The Mill Creek Project near Walla Walla, Wash., includes both the Mill Creek Diversion Dam and the Mill Creek Storage Dam. Mill Creek Storage Dam was recently reclassified as “Low Risk” after a Corps risk assessment and potential failure mode analysis. Remaining risk is potential overtopping and erosion of the diversion dike during an extreme flood event and internal erosion of the Storage Dam embankment and foundation due to sustained high pool events. Since the Diversion Dam was reclassified in 2014 as “Low Risk,” the entire Mill Creek Project is now considered “Low Risk.”
Lower Monumental Lock and Dam on the lower Snake River is being reclassified “Low Risk,” pending a memorandum from Corps headquarters Dam Safety, after a Corps risk assessment. Remaining risks are primarily due to potential structural instability due to a significant seismic event, and embankment overtopping during an extreme flood event.
Ice Harbor Lock and Dam on the lower Snake River was reclassified as “Low Risk” in 2016. Remaining risk is primarily due to the potential for breach due to overtopping of the dam during an extreme flood event.
“Dam safety is a shared responsibility of area residents, local emergency management, and the Corps of Engineers,” added Lt. Col. Delarosa. “Residents living near dams should know their risk and know their role in dam safety, and talk with their local emergency management office.”
More information about the Walla Walla District Dam Safety Program, including fact sheets about each District Dam and the McNary Levee System, is on the District website at http://www.nww.usace.army.mil/Missions/Dam-Safety/.