WALLA WALLA -- The safety rating of the Bennington Lake dam has changed for the better.
Efforts by the U.S. Army Corps of Engineers coupled with further studies of the dam have resulted in an improved risk classification for the structure, said Gina Baltrusch, public information specialist with the Corps' Walla Walla District office.
Three years ago the dam had been ranked in the "urgent and compelling" category for repairs, which was the worst classification in the Corps' "Dam Safety Action Category" rating system. The system rates dams on a numerical scale of 1-5, with a rating of 1 being the least safe. The diversion dam on Mill Creek also received the same rating.
But the Corps' dam safety Senior Oversight Group recently improved the Mill Creek Storage Dam to a rating of 3, or "high priority," based on a completed Issues Evaluation Studies report and ongoing risk reduction measures, which includes increased monitoring and other steps to protect the public.
The initial 2008 rating was primarily due to potential dam seepage and "piping" which is when water moves through the earth fill and carries material out, causing sections of the dam face to sink. These problems occur when the pool behind the dam is filled above 17 percent of its capacity for an extended period of time during flood events. The reservoir is normally kept at 5 to 10 percent capacity for recreation purposes.
Corps officials have been at work to design long-term solutions. In discussing the problems in 2008, district engineers said the root causes of the problems are the age of the structure as well as the more conservative safety standards in place today as opposed to when the project was constructed in the late 1930s and early 1940s.
The lake behind the dam is named for Virgil B. Bennington, president of the Walla Walla Chamber of Commerce in the early 1900s who lobbied Congress for flood protection from Mill Creek. The lake was formally named for Bennington with the passage in 1992 of the Water Resources Development Act.
Andy Porter can be reached at firstname.lastname@example.org or 526-8318.
The U.S. Army Corps of Engineers will host a public information meeting on Aug. 17 on improved safety rating of the Bennington Lake dam. The meeting will be held be held at the Bennington Lake boat ramp parking lot beginning at 9:30 a.m.