Instead, corps spokesman Bruce Henrickson said Tuesday, the federal agency decided to change the rating of Dworshak Dam from “Unsafe or Potentially Unsafe” to “Conditionally Unsafe” after studying the risks faced by the dam.
Officials additionally have determined that the cracks in the dam and leaks don’t present an urgent risk of failure.
According to the corps’ newest analysis, the biggest risks come from earthquakes, which are rare in the region, and adverse weather events – including heavy snowpack, an abnormally wet spring and a rapid warming trend.
Dworshak Dam is 717 feet tall – only 9 feet shorter than Hoover Dam. It’s also the tallest straight-axis dam in the western hemisphere, which means that unlike the Hoover, it relies on a massive amount of concrete rather than a curved design to hold back the force of the water’s flow.
The impact of a dam failure would be huge, likely inundating the communities of Ahsahka, Orofino, Lewiston and Clarkston. In fact, Henrickson said, such a failure would likely be felt through five downstream dams – all the way to McNary Dam on the Snake River near Umatilla, Ore.
The U.S. Army Corps of Engineers began building Dworshak in 1966, creating a 53-mile long reservoir that extends into the Bitterroot Mountains. The corps has spent millions of dollars over the past five decades attempting to patch cracks in the dam and in the bedrock beneath the structure.
In 2007, an assessment of the dam showed that there was significant leaking coming from multiple sources, including previously repaired cracks, and that there was substantial pressure on the foundation. Additionally, the corps found that structures inside the dam were cracking and that some relief drains were plugged. The assessment also showed the current standards of care weren’t being met for a dam of Dworshak’s massive size.
The report prompted the corps to give Dworshak a rating of “Urgent – Unsafe or Potentially Unsafe” through the agency’s newly implemented Dam Safety Action Classification system.
That rating prompted the corps to take several steps, including installing new instruments to measure movement inside the dam. The corps also studied the friction and strain placed on gates that open up to allow water through as needed.
The corps is still working on risk reduction measures such as updating the dam’s safety emergency action plans and trying to find and repair leaks.
The dam’s new safety rating is “High Priority – Conditionally Unsafe,” a one-step improvement over the previous rating. (AP)