Why Portland's drainage system failed to stop this week's flooding

Flooding in Portland's Pearl District on Dec. 7

PORTLAND, Ore. - Parts of Portland looked more like a lake this week after the city saw near record rainfall.

On Monday, Portland's third wettest day ever, the Pearl District flooded as manholes overflowed.

The brown water they spewed was around 90 percent storm runoff and 10 percent sewage, according to the city's Bureau of Environmental Services (BES).

Similar scenes played out throughout the city this week as creeks and drains overflowed and people had to take sometimes desperate measures to avoid being stranded and stay out of the muck.

So how did it happen in a city with a massive and expensive drainage system?

To find out, KATU's On Your Side Investigators talked with Bill Ryan, chief engineer for BES.

"This was a greater storm than what the system is designed to manage," Ryan said.

The city paid $1.4 billion for a Big Pipe sewage and drainage system that started operating in 2011.

Ryan said it can handle storms that have a 1 in 25 chance of happening in a given year.

But the storm on Monday was much bigger.

"The storm was so big we're calling it a 100-year storm," said Ryan. "It's essentially a storm that has a 1 in 100 percent chance of occurring any given year."

Portland was hit with another storm in that category this year on Halloween.

Both overloaded the system.

"Yesterday," said Ryan, "we put over 250 million gallons through the treatment plant."

And he said we may just have to get used to the risk.

"Yes, we could've designed something to handle 100-year storms, something that's only gonna happen once every 100 years," said Ryan. "It would've cost four to five times that $1.4 billion price tag. Certainly four times what we're paying now for sewer rates would not be considered affordable by anybody."

Linc Mann, a spokesman for BES, said they're still analyzing what happened this week and won't know exactly how much storm run-off and sewage overflowed until Friday.

BES has issued a combined sewer overflow advisory that may last through Friday.

Sewage started spilling into the Willamette River on Monday and BES says you should avoid any contact with it from the Sellwood Bridge to the Columbia River.

A notice from the agency says, "During very heavy rain storms, some combined sewage can overflow to the Willamette River and Columbia Slough. That is what the system is designed to do and that is the way it operates."

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