New wall eases flooding fears at Tacoma's biggest waste-treatment plant
The News Tribune, By Stacia Glenn
New flood wall aims to protect Tacoma’s Central Wastewater Treatment Plant from flooding into the Puyallup River
The $9 million project was completed in late spring
Flooding at the plant could cost $10 million in damaged equipment and see millions of gallons of untreated waste dumped into Commencement Bay
The question of what would happen if Tacoma’s Central Wastewater Treatment Plant flooded was nearly answered in 2009 when a giant storm threatened the $1 billion facility abutting Puget Sound.
The mayor declared a civil emergency. City workers were diverted from regular duties to help stack 17,000 sandbags around the treatment plant on the Tideflats. Nearly 5 inches of rain fell in two days, forcing the evacuation of 26,000 Pierce County residents and the closing of three lanes of Interstate 5 in Fife.
In the end, the nearby Puyallup River came within 6 inches of topping its banks but didn’t flood.
Still, plant officials knew it was time to look at protecting the facility from flooding before a storm damaged $10 million in electrical equipment and allowed millions of gallons of untreated wastewater to flow into Commencement Bay.
“The whole premises of this project started from the ... fact that this is the single most critical component in the city’s infrastructure,” division manager Geoffrey Smyth said recently.
“We started looking at it from a long-term perspective on how to protect the plant.”
THE CENTRAL WASTEWATER TREATMENT PLANT WAS BUILT IN 1952 AND IS VALUED AT $1 BILLION.
The answer was a $9 million flood wall that wraps 2,700 feet around the outside of the plant at 2201 Portland Ave. The wall is on the opposite side of the facility because studies showed the biggest flood threat would come from the Puyallup River flooding upstream and running through the Tideflats and into the plant on the nonriver side.
Officials had bandied the idea about for years but didn’t have the money to build it. That’s where Councilman Ryan Mello came in.
Mello sits on the Pierce County Flood Control Zone District board and chairs the city’s Infrastructure, Planning and Sustainability Committee, so he was familiar with the threat and the need for flood protection at the treatment plant.
He secured a $6 million pledge from the Flood Control Zone District. Another $3 million came from the city’s utility customer rates. The city fronted most of the cost and the district repays them $1 million each year.
If the plant was flooded, officials say, it would short out $9 million to $10 million worth of equipment and effectively shut down the facility for months.
That means millions of gallons of waste would flow into Commencement Bay and the Puyallup River, causing irreparable harm to the environment.
“After all our hard work to protect the bay, it’s not palatable,” Smyth said.
It also would hurt the economy, because business at the Port of Tacoma would be halted during the cleanup.
Officials considered building a levee around the plant but didn’t have enough property to build on. Poor soil conditions ruled out a concrete flood wall, which left builders with steel.
Along the wall are five floodgates, two of which are raised manually and three that are raised automatically by water pressure. One of the manual gates is over a railroad track.
The floodgates, designed by Houston-based FloodBreak, were the first and the highest in the western United States.
The plant has two pumps to ensure that wastewater doesn’t spill into the Puyallup River if surface water inside the plant begins to rise. In case of heavy rain, the surface water is pumped out below one of the floodgates.
“We’re basically creating an island, shutting off all utility from the outside area,” assistant division manager Eric Johnson said.
A major challenge to construction was driving sheet piles into the ground within 12 inches of 23 utilities, including a sanitary sewer line and a water pressure main.
The project was finished on schedule and under budget.
That, coupled with several unique construction components, helped the city and contractor IMCO General Construction win the Northwest Construction Consumer Council’s distinguished project award for a public project under $10 million.
Despite several flood warnings in Pierce County and on the Puyallup River over recent weeks, the treatment plant hasn’t been threatened.
A storm Nov. 17 knocked out power at the plant for 20 minutes, causing treated wastewater to flow into the lower Puyallup River at Cleveland Way and at the Foss Waterway by Dock Street.
The treated sewage was waiting to be pumped 2 miles out to Commencement Bay when the power went out.
It’s unclear how much water was released into the river but Johnson said it was unrelated to the flood wall project.
The flood wall was built to withstand a 500-year storm. Staff members put together a 46-page plan specifically for the plant, telling workers what to do in case of a major flood.
“There would almost have to be a levee failure for the plant to be threatened,” Smyth said.
The plant has twice faced the threat of high water since it was built in 1952, the most recent being the 2009 storm that did not flood the facility.
In February 1996, the Puyallup River swelled to its greatest height on record at the time. City workers flocked to put sandbags around the facility but some water seeped in, doing minimal damage.
The storm did $34.5 million in damage in the county, including $15.5 million for dikes and levees and $14.5 million for roads.
“There was a lot of uneasiness involved with really large flooding events,” said Johnson, adding that everyone at the plant rests easier now in the face of big storms.
Stacia Glenn: 253-597-8653