14-035 Mill Creek Diversion Dam reclassified as safer by Corps

Published May 21, 2014

WALLA WALLA, Wash. – The U.S. Army Corps of Engineers Walla Walla District’s Mill Creek Diversion Dam located upstream of Walla Walla, Wash., recently received a safer “Low Urgency” classification from the Corps following a recent analysis and assessment of the dam and implementation of risk reduction measures.

The Mill Creek Diversion Dam was previously screened and classified within the Corps’ Dam Safety Action Classification (DSAC) program as “High Urgency” in January 2009. After completing a potential failure modes analysis and semi-quantitative risk assessment, the Corps recently concluded that risks due to Mill Creek Diversion Dam breach are low.

The Corps has a robust dam safety program which includes multiple dam inspections, monitoring, evaluations and risk reduction measures. The improved classification was made by Corps headquarters in Washington, DC. These classifications assist the U.S. Congress and the Corps in prioritizing funding for dam safety infrastructure improvements, which the Corps has been implementing in recent years.

The Mill Creek Flood Control Project system contains two dams—1) a “Diversion Dam” to divert water from the Mill Creek mainstem to the Bennington Lake storage reservoir, and 2) a 3,050-foot long, 120-foot high “Storage Dam” to safely hold water in Bennington Lake.

Diverting flood water to Bennington Lake during high flows helps reduce potential flood damage to the city of Walla Walla and adjacent downstream areas bordering Mill Creek, Yellowhawk Creek and Garrison Creek. The two-dam system is designed to generally limit maximum creek flows into Walla Walla to approximately 3,500 cubic feet per second (cfs) as long as the Bennington Lake storage reservoir has capacity to accept diverted flows.

While there was no evidence in 2009 to suggest an emergency situation existed at Mill Creek Diversion Dam, the Corps took action to further reduce public risk. The Corps implemented interim risk reduction measures (IRRM), which are short-term or ongoing efforts to reduce public safety risks while long-term solutions are pursued. In establishing IRRM, the prevention of loss of life is the first and foremost objective, followed by prevention of catastrophic economic or environmental losses. Risk reduction measures may be structural or non-structural.

At the Diversion Dam, completed Corps interim risk reduction measures include:

  • Increased gate capacity to Bennington Lake.
  • Stockpiled emergency supplies and equipment.
  • Revised/updated the emergency action plan.
  • Performed an overtopping and conveyance study.
  • Improved instrumentation: Additional instrumentation was installed at the Diversion Dam dike in September 2012. A toe drain was installed at the diversion dike in 2011.
  • Completed a potential failure modes analysis in September 2013.

Other interim risk reduction measures are in progress. The Mill Creek system—including both the Storage Dam and the Diversion Dam—will be advanced into a Dam Safety Modification Study starting in FY2015 to address uncertain performance of the system at high pools.

“Public Safety is our highest priority,” said Lt. Col. Andrew Kelly, Walla Walla District Commander. “We can reduce public risk, but we can’t totally eliminate risk. So we ask people to know their potential risks and what to do in an emergency.”

The U.S. Army Corps of Engineers urges people living near dams to be aware of both the benefits and risks associated with dams. While dams provide hydropower, recreation and more, it’s important that residents downstream of a dam be aware of potential consequences if the dam is suddenly breached, doesn’t perform as intended, or releases major water flows.

Information for residents living near dams is available at http://www.damsafety.org/media/Documents/DownloadableDocuments/LivingWithDams_ASDSO2012.pdf.

Additional information about the Corps dam safety program is at http://www.usace.army.mil/Missions/CivilWorks/DamSafetyProgram.aspx.

Additional information about the Mill Creek Diversion Dam is on the District website at http://www.nww.usace.army.mil/Missions/DamSafety.aspx.



Public Affairs Office

Release no. 14-035