Cowlitz River Dams Built to Withstand Most Earthquakes

Editor’s Note: The following story is Part Two of a two-part series looking at the potential effects of an earthquake in Lewis County. Part One remains available at www.chronline.com and www.lewiscountywatch.com.

With 51 dams, Lewis County is ranked No. 7 out of 39 counties for the most water retention structures in the state of Washington.

Three dams on the Cowlitz River in Lewis County all pose hazards to downstream residents if they were to fail. 

The Cowlitz Falls dam, owned by Lewis County Public Utilities District, is rated 1C, meaning seven to 30 lives are at risk if it fails. 

The Tacoma Power-owned Mossyrock and Mayfield dams are both rated 1A with more than 300 lives at risk, according to the state Department of Ecology’s Inventory of Dams with data through October 2014.

“They monitor this really, really closely because they realize the responsibility that they have to maintain structural integrity under all types of conditions,” Lewis County Emergency Management Director Steve Mansfield said.

A failure at Mossyrock and Mayfield dams would cause widespread flooding in southern Lewis County and into Cowlitz County, Mansfield said. But there are plans in place if failures were to happen. 

Steve Grega, at Cowlitz Falls, and Toby Brewer, of Tacoma Power, both hold the title of chief dam safety engineer with one of their primary focuses being structural integrity for different hazards, including earthquakes.

Officials at the dams looked at the maximum ground acceleration for different “maximum credible” earthquakes in the region. Brewer said the models test for three different local earthquakes — a 6.5 magnitude quake close to the ground surface with an epicenter near Mossyrock, a 7.5 magnitude from the Juan de Fuca plate and a 9.0 magnitude earthquake originating from the offshore Cascadia fault.

Brewer said the earthquake near Mossyrock would cause the highest peak ground acceleration at 0.55g, or 55 percent of the weight of gravity.

All three dams would withstand the scenarios and maybe receive a few cracks and other slight damage.  

“As far as a whole dam washing out, it’s highly unlikely,” Grega said. 

If something did happen causing the dam, which was built in 1993, to break, Grega said less than 1 foot of water would fill Mossyrock.

But if Mossyrock or Mayfield broke, which is also highly unlikely, Brewer said it would be damaging.

“If Mossyrock were to go out it would scour the river bed from there to Longview and Toledo and some of the local towns downstream would be in a world of hurt,” Brewer said. 

The dams keep and test detailed emergency plans in case something were to happen.

Both of Tacoma Power’s dams were built in the 1960s and were designed to withstand ground acceleration at 0.15g; however, Brewer said the models and tests have proven that they can handle more than that. 

The dams regularly run scenario drills for different hazards, including earthquakes, floods, volcanic eruptions and sabotage events as required by the Federal Energy Regulatory Commission.

Operators also make daily, weekly and monthly rounds checking the structure and dam measurements. Every five years, independant consultants review all safety issues for the dams.

If changes to expected seismicity are discovered, the dams have to factor those into their plans.  But for now, experts aren’t predicting any regional earthquakes powerful enough to cause any of the three dams on the Cowlitz River to fail, Brewer said, noting that he spoke to consultants just last week.