By Nicholas K Geranios
Associated Press
SPOKANE, Wash. — A leaking dam near Walla Walla poses an "unacceptable" risk to the public and will be specially managed until it can be repaired, the U.S. Army Corps of Engineers said.
The 67-year-old Mill Creek Storage Dam is seeping water and is especially dangerous when Bennington Lake is more than 17 percent full, the agency said Wednesday. It is typically kept just 5 percent to 10 percent full.
Mark Lindgren, engineering chief with the corps in Walla Walla, said the dam can still be used to prevent floods in the Walla Walla area, but will be more closely monitored to make sure it does not collapse.
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"We basically made the decision for this flood season to use the dam, and this one-time exposure should have a relatively low risk," Lindgren said in a telephone interview from Walla Walla.
The corps is in the process of evaluating all 610 of its dams nationwide for safety. Dams get a rating of 1 to 5, with 5 being safest and 1 being the most at risk.
The Mill Creek Dam is the only one in the Walla Walla District - which covers portions of Washington, Idaho, Oregon, Wyoming, Nevada and Utah - to be rated a 1.
That rating means the risk to public safety is unacceptable when Bennington Lake is more than 17 percent full for an extended period of time, the corps said. It requires the corps to take action to reduce risk.
The problems are that gravel and silt materials in the dam may be washing away, which promotes water seepage and could lead to collapse. The dam is also prone to earthquake damage.
The earthen dam was built in 1941 and is 120 feet high, 3,050 feet long and 800 feet wide at the base and normally contains little water. It sits above and just east of the city of Walla Walla and is used to catch water that flows from the Blue Mountains down to Mill Creek. The creek overflows about once every three years, and the reservoir was last filled in 1996.
If it collapsed, most of the water it holds back would spill into Mill Creek, but some could flood outlying portions of Walla Walla.
For now, the corps will increase monitoring of the dam while seeking money to make improvements. The cost won’t be known until any damage can be fully assessed.
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The corps is also working with Walla Walla County to create an alert system and to place emergency supplies in advance of any problems.
"While we cannot completely eliminate risk, we can reduce risk," Lindgren said.
On the Net:
Walla Walla District: http://www.nww.usace.army.mil
